Interference Management in 5G and Beyond Network: Requirements, Challenges and Future Directions

Received January 22, 2021, accepted April 4, 2021, date of publication April 15, 2021, date of current version
May 14, 2021.

Digital Object Identifier 10.1109/ACCESS.2021.3073543
Interference Management in 5G and Beyond

Network: Requirements, Challenges
and Future Directions
MARAJ UDDIN AHMED SIDDIQUI 1 , FAIZAN QAMAR 2 , FAISAL AHMED3 ,
QUANG NGOC NGUYEN 4 , (Member, IEEE), AND ROSILAH HASSAN 2 , (Senior Member, IEEE)
1 Faculty of Engineering and Information Sciences, School of Electrical, Computer and Telecommunications Engineering, University of Wollongong, Wollongong,
NSW 2522, Australia
2 Centre for Cyber Security, Faculty of Information Science and Technology, Universiti Kebangsaan Malaysia (UKM), Bangi 43600, Malaysia
3 Software Engineering, Faculty of Social Sciences, Narva College, University of Tartu, 20307 Narva, Estonia
4 Department of Computer Science and Communications Engineering, Faculty of Science and Engineering, Waseda University, Tokyo 169-8050, Japan
Corresponding authors: Quang Ngoc Nguyen (quang.nguyen@fuji.waseda.jp) and Rosilah Hassan (rosilah@ukm.edu.my)
This work was supported in part by the Network Communication Technology (NCT) Research Groups, FTSM, Universiti Kebangsaan
Malaysia, in part by the Dana Impak Perdana UKM under Grant DIP-2018-040, in part by the Fundamental Research Grant Scheme under
Grant FRGS/1/2018/TK04/UKM/02/17, in part by the Fujitsu-Waseda Digital Annealer FWDA Research Project and Fujitsu Co-Creation
Research Laboratory, Waseda University (Joint Research between Waseda University, Japan, and Fujitsu Laboratories), and in part by the
School of Fundamental Science and Engineering, Faculty of Science and Engineering, Waseda University.
ABSTRACT In the modern technological world, wireless communication has taken a massive leap from
the conventional communication system to a new radio communication network. The novel concept of Fifth
Generation (5G) cellular networks brings a combination of a diversified set of devices and machines with
great improvement in a unique way compared to previous technologies. To broaden the user’s experience, 5G
technology provides the opportunity to meet the people’s potential necessities for efficient communication.
Specifically, researchers have designed a network of small cells with unfamiliar technologies that have
never been introduced before. The new network design is an amalgamation of various schemes such
as Heterogeneous Network (HetNet), Device-to-Device (D2D) communication, Internet of Things (IoT),
Relay Node (RN), Beamforming, Massive Multiple Input Multiple Output (M-MIMO), millimeter-wave
(mm-wave), and so on. Also, enhancement in predecessor’s techniques is required so that new radio is
compatible with a traditional network. However, the disparate technological models’ design and concurrent
practice have created an unacceptable intervention in each other’s signals. These vulnerable interferences
have significantly degraded the overall network performance. This review article scrutinizes the issues of
interferences observed and studied in different structures and techniques of the 5G and beyond network. The
study focuses on the various interference effect in HetNet, RN, D2D, and IoT. Furthermore, as an in-depth
literature review, we discuss various types of interferences related to each method by studying the state-
of-the-art relevant research in the literature. To provide new insight into interference issue management
for the next-generation network, we address and explore various relevant topics in each section that help
make the system more robust. Overall, this review article’s goal is to guide all the stakeholders, including
students, operators, engineers, and researchers, aiming to explore this promising research theme, comprehend
interferences and their types, and related techniques to mitigate them. We also state methodologies proposed
by the 3rd Generation Partnership Project (3GPP) and present the promising and feasible research directions
toward this challenging topic for the realization of 5G and beyond network.
INDEX TERMS Interference, 5G and beyond (B5G), HetNet, relay node, D2D, IoT.
I. INTRODUCTION other simultaneously. The explosive growth in user data

In future wireless communication, it is expected that hun- demand with a 1000-fold increase in user density triggers
dreds of different devices and users communicate with each the need to explore the broader horizon of the radio link to
accommodate future requisites with a more comprehensive
The associate editor coordinating the review of this manuscript and approach effectively [1]. An enormous amount of research
approving it for publication was Ting Yang . was conducted and still being performed in various sections
This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/
68932 VOLUME 9, 2021
M. U. A. Siddiqui et al.: Interference Management in 5G and Beyond Network: Requirements, Challenges and Future Directions
of radio communication. Researchers from all parts of the as HetNet [16]. It is a network that supports high reliability
communication and networking societies, from academia with greater capacity of the users, less power consumption,
to industrialists and vendors to operators, have actively minimum end-to-end delay, enhanced spectrum, and energy
participated in making this theme possible [2]. This phe- efficiencies [17].
nomenon leads to enormous growth in higher throughput, The next-generation network is considered a multi-tier
larger capacity, and lower latency in the air (up to 1ms) and HetNet cellular network because it supports various wireless
in a cellular network (end-to-end delay less than 10ms) [3]. devices in each tier [18]. Each tier consists of small gadgets
Besides, this tendency is predicted to be increased drasti- to heavy machine type wireless equipment and sends data
cally in the next decade. The combination of advancement request directly to its respective small base station (SBS),
in predecessor’s technologies and innovative work in wire- which requires low power for transmission and minimized
less transmission shows a tendency to fulfill user’s requisite energy consumption [19]. However, in a multi-tier 5G cellular
and next-generation cellular objectives i.e., 5G and beyond, network, the simultaneous operation of many small cells is
or sometimes referred to as Sixth- Generation (6G) net- severely damaged by the different types of interferences that
works [4]. Typically, the massive expansion in traffic is due limit the user’s quality of experience and reduce the current
to a rise in demand for high-definition applications and ser- 5G expectations [20]. The heavy interference issues are due
vices, social networking, gaming, online surfing, concurrent to the novel architecture, the wireless medium’s broadcast
operation of a diverse set of machine type communication, nature, and complex coordination of low-power small cells.
and so on. It is expected to provide high Quality of Services Also, the heterogeneity structure and newer modulation and
(QoS) requirements of real-time and heavy data applications multiple access techniques introduce unique types of inter-
with secure connections [5]. ference. Therefore, the management, mitigation, and can-
By virtue of matching extensive cellular data require- cellation of Interferences play a critical part in the current
ments and supporting massive machine type communication 5G mobile communication [21].
and internet services, 5G new radio is supposed to provide Furthermore, due to previous mobile generation’s limita-
expected high efficiency. The 5G is divided into two fre- tions, such as congestion in the below 6 GHz frequency band
quency ranges (FRs), i.e., FR1 < 6 GHz and FR2 > 24 GHz and the need for higher bandwidth, an mm-wave frequency
(mm-wave band) [6]. It is observed that FR1 is almost fully band has been introduced in current 5G networks [22].
occupied and has limited resources to be utilized for the It offers a large untapped frequency band that can increase
current 5G network, whereas FR2 is the frequency range the system capacity by 100 times compared to a conventional
where most of the spectrum untapped, and it can be easily homogeneous network, especially the 4G LTE network [23].
exploited for future cellular communication [7]. To satisfy the It relies highly on directional transmission and minimizes
user’s requirements of cellular technology, various types of the heavy isotropic path loss. There are also prominent dif-
techniques were introduced, such as Massive Multiple Input ferences in path loss of mm-wave transmission for cellular
Multiple Output (M-MIMO) [8], Heterogeneous Network users in Line-of-Sight (LOS) and Non-Line-of-Sight (NLOS)
(HetNet) [9], Cognitive Radio (CR) [10], Ad-hoc net- coverages [24]. However, mm-wave sinusoid size is concise
work [11], and so on. These methods are a joint venture of and the small amplitude wavelength does not penetrate over
previous and novel wireless technologies [12]. a long distance and is badly affected by the scattering, fad-
In the conventional cellular system, it is observed that ing, refraction, propagation losses, and blocked by various
the available technologies, with the scarcity of the spec- concrete-based building structures [25]. It is also positively
trum and high power consumption, do not satisfy the future exposed to the reflection and refraction phenomenon due
data demand of the users and environmental standards [13]. to heavy glass materials. Besides, the atmospheric absorp-
Technological enhancement in the traditional single-cell tion characteristics at the extremely high-frequency range
network due to several limitations is impracticable. This (30-300 GHz) and the signal attenuation in heavy rain limit
phenomenon leads to innovative design and wireless com- the mm-wave transmission’s propagation distance. In partic-
munication techniques that help accomplish user’s require- ular, in an urban coverage area, users with an mm-wave spec-
ments in the next decade. Therefore, an ultra-dense mobile trum in ultra-dense network encounter penetration, reflection,
network is expected to accommodate a vast number of users and interference issues. Therefore, it is very critical to con-
with maximum throughput, twice the spectral efficiency, and duct proper channel modeling of each mm-wave spectrum
better power consumption compared to the traditional cellular before the practical deployment [26]. Moreover, the shorter
network [14]. According to the wireless network technologies wavelength and simultaneous operation of small cells within
analysis, the next-generation cellular network is expected to a macro cell induce more troublesome for the cell edge users.
deliver each user with ultra-reliable connectivity via multiple The reason is that they are devastated vigorously by the
devices anywhere and anytime [15]. Thus, to dispense user interference of MBS downlink signals and inter-cell interfer-
necessitate and better network performance, the researchers ence besides Self-Interference (SI) due to the single antenna
suggest a network’s densification by deploying small cell duplexer technique [27]. This review helps to provide new
nodes such as micro-, pico-, femto-, and relays. The inter- insight into interference management for the next-generation
communication of all small cells within a macro cell is known network by addressing and exploring various factors and
VOLUME 9, 2021 68933

relevant topics. Each section helps make the system more a modification of a signal in a disruptive way. Interference
robust. To the best of the author’s knowledge, there is no such is different from noise, which can be anything that disturbs
work in literature that focuses on the comprehensive study the useful signal [28]. Noise can be due to temperature,
on interference issues for these four main 5G research areas, loud machine voice signals, gamma rays, impurities, and so
i.e., HetNet, RN, D2D, and IoT. The topic discussed in this on. Hence, it is not interference; nevertheless, interference
study is about the Interference management in 5G and beyond degrades a network’s performance and user experience. In the
network, as summarized in Fig. 1. next-generation cellular network with multiple low power
nodes, random deployment, and frequency re-use scenarios,
the system causes severe interference issues [29]. Therefore,
the major challenge of a novel design for a new and feasible
spectrum of current 5G cellular systems is interference.
Several researchers discussed the issues of interference and
devised various techniques to mitigate the different types of
interferences either by coordination, cancellation, or simul-
taneously. However, the most impactful and inevitable inter-
ference in a multi-tier cellular system is Inter-cell interference
(ICI). Inter-cell interference coordination (ICIC) was initially
implemented as an only solution of a multi-low power BS
network. It is separated into coordination methods such as
ICIC and enhanced ICIC (eICIC), where the only differ-
ence is that the eICIC technique reduces the complicated
coordination overhead observed in conventional ICIC [30].
In a practical scenario, the Macro BS coordination generates
interference and other management information besides each
frequency resource block. This information exchange with
neighboring BS via the X2 backhaul interface [31]. Several
studies have been proposed and performed on interference
coordination, avoidance, and cancellation. However, a proper
FIGURE 1. Interference management in 5G and beyond network.
and robust technique to cancel the interference then drop
The rest of the paper is organized as follows: Section II down the noise level is still demandable in the research
explains the motivation of this work. Section III begins with community. Therefore, this article investigates and highlights
the classification of interferences, whereas section IV-A com- the issues of interferences observed in different structures and
prehensively discussed HetNet and various types of interfer- techniques of the current 5G network, in which we mainly
ences in HetNet. After that, it discusses some recent related focus on HetNet, RN, D2D, and IoT to gain new insights into
work on interference approaches. Similarly, discussion about designing efficient 5G and beyond as well as 6G networks in
RN is exploited in section IV-B, where it briefly described the near future.
the two major interferences that are always available and
reduced the performance capacities, i.e., self-relay interfer- III. CLASSIFICATION OF INTERFERENCES
ence and inter-relay interference. Additionally, this section In a small cell wireless cellular network, multi-tier inter-
provides in detail most of the solutions to mitigate these ferences are predetermined due to each low power node’s
interferences in literature. Next, section IV-C demonstrates specific attributes [32]. It generates and receives continuously
the D2D technique with relevant interference problems. unwanted signals from various nearby sources. It is known
Moreover, it furnishes some techniques proposed by various that the HD (Half Duplex) mode limits a radio communica-
researchers on the reduction of interferences. Section IV-D tion network’s performance since it is transmitted or received
contributes to IoT with hindrances that decrease its potential at the same frequency. Different from HD, FD (Full Duplex)
communication capabilities with relevant methods mentioned transmission mode transmits and receives signals simultane-
in several related studies conducted on IoT interferences. ously on the same frequency [33], [34] and is supported by
Section V provides a discussion about the various methodolo- a multi-antenna system that enhances the network capacity
gies that have been proposed by the 3rd Generation Partner- and minimizes the round trip data delivery time. However,
ship Project (3GPP) to deal with the 5G interference. Then, though HD shows the capabilities to avoid interferences and
future enhancements in available technologies are shown in provide quality signal strength [35], the delay in transmit-
section VI. In the end, the study is concluded in section VII. ting and receiving a signal and inefficient use of spectrum
makes it undesirable for new radio wireless communication.
II. RESEARCH MOTIVATION AND CONTRIBUTION In contrast, the FD supports higher throughput with lower
In cellular communication, interference is termed as the addi- latency and efficient use of spectrum [36]. Also, it enhances
tion of an undesired signal in an actual signal. Basically, it is the ergodic capacity [37] and the network secrecy [38];
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however, its performance is extremely descended due to

interferences. Therefore, a robust and concrete interfer-
ence mitigation scheme in FD transmission is required to
deliver significant results for the practical future mobile net-
work. The most common interferences associated with radio
networks are self-, adjacent channel-, intra-, and inter-cell
interference. Nonetheless, the mobile network is not lim-
ited to only these interferences. Each network is affected
by interferences endured by their respective deployment and
transmission scenario. Researchers in various literature well FIGURE 3. Intra-cell and Inter-cell interference.
discuss the prominent interferences in cellular communica-
tion, and some notable studies are as follows.
C. INTRA-CHANNEL INTERFERENCE
A. ADJACENT CHANNEL INTERFERENCE Intra-channel interference is vastly observed in multi-low
The adjacent channel interference causes when the desired power cells within a macro cell mobile network [41].
signal is interfered with by the adjacent frequency band It is mostly occurring in an mm-wave HetNet dis-
(channel) in the same coverage area, as shown in Fig. 2. tributed architecture-based scenario. This network relies on
When the transmitter of small BS or macro BS while trans- self-backhauling using the mm-wave where each small cell
mitting on a channel leaks its energy and signal added to BS communicates and forwards the backhaul information to
the frequency adjacent to that band [39]. The core reason the nearest BS. Then, this multi-hop mm-wave link delivers
for adjacent channel interference is the imperfect filtering of the data to the gateway. This leads to fast switching and
a receiver, allowing the nearby frequencies to leak into the restricts delay in transmission besides propagation losses.
passband. However, it can be consist of a passband filter’s Nevertheless, intra-channel interferences induced and affect
tangible design that conducts only a required frequency to the useful signal. It is considered a significant issue for
pass through it. Therefore, a reliable interference mitigation HetNet deployment; therefore, a robust mitigation scheme is
technique helps to reduce the impact of adjacent channel undoubtedly required to minimize the interference signal to
interference. the ground noise level.
D. INTER-CHANNEL INTERFERENCE
The inter-channel interference causes when two separate fre-
quency bands (channel) are causing interference with each
other, as shown in Fig. 4 In a HetNet system, multiple wireless
and digital communication devices operating in a close range.
Due to physical proximity, the transmitter of a high-power
signal interferes with a weak signal receiver known as
inter-channel interference. In a live network, inter-channel
interference is sometimes unavoidable, even when mobile
devices’ channels are thousands of megahertz (MHz). A prac-
FIGURE 2. Adjacent channel interference.
tical example would be a wireless device such as GPS, Wi-
Fi, and Bluetooth operating at different channel BW; how-
ever, the wide range of transmitting and receive powers of
B. INTRA-CELL AND INTER-CELL INTERFERENCE
these devices creates various interference issues such as inter-
Inter-cell interference (ICI) is one of the significant causes channel interference [42].
of the degradation of network operation. When users of the
two neighboring cells attempt to simultaneously use the same
frequency band, they received interference signals with the
actual message signal [40]. Moreover, the users at the cell
edges are badly affected by the ICI because the user received
a signal from its cell macro BS with high power and its neigh-
boring cell due to the frequency re-use factor. Fig. 3 shows
that macro BS connects directly to the users in the close range
and requires low power for the transmission. Due to simul-
taneous uplink and downlink transmission of many user’s
actual data interfere with other signals. The distortion caused
by the user to the additional equipment within the same cell
is called Intra-cell interference. FIGURE 4. Inter-channel interference.
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E. INTER-SYMBOL INTERFERENCE
In the propagation of a signal, when one or more symbols
interfered with other symbols, signal distortion occurred is
formed as Inter-symbol interference (ISI). It is caused by the
phase as well as amplitude dispersion in the channel. It is
observed due to multipath propagation, and OFDMA is a
pertinent example [43].
F. INTER-CARRIER INTERFERENCE
In Orthogonal Frequency Division Multiplexing (OFDM),
each channel divides into many subcarriers, and the signals
are orthogonal to each other. Thus, orthogonality reduces
the importance of the guard band, which enhances spectral FIGURE 5. Cross-link interference.
efficiency. However, due to the frequency offsets, the signal

caused by the adjacent beams of the BS of the same cell
lost the orthogonality among the subcarriers, resulting in
or the neighboring cell as shown in Fig. 6. The essence of
inter-carrier interference [44].
beamforming is that it is compatible with both BS and mobile
terminal.
G. INTER-NUMEROLOGY INTERFERENCE
Multiple numerologies are one of the novel 5G wireless
communication features. Multiple numerologies like 15, 30,
60, 120, 240 kHz provide essential flexibility for several
devices’ various services. For instance, lower subcarriers,
signals are more suitable for massive machine type commu-
nication (mMTC). They supported many connected devices
with low power and within the same bandwidth (BW).
Similarly, higher numerologies for ultra-reliable low latency
communication (URLLC) being their subcarrier spacing is
up to 240 kHz, and it provides a shorter symbol duration.
Also, intermediate numerologies are productive for enhanced
mobile broadband (eMBB) services for higher throughput
and significant BW. However, use different subcarrier signals FIGURE 6. Inter-beam and multi-user interference.
for other applications will generate inter-numerology inter-
ference in the network due to non-orthogonality between the J. MULTIPLE ACCESS INTERFERENCE
multiplexed signals/subcarriers [45].
In many different small cell cellular networks, the same
frequency resource blocks carried BS transmitters for simul-
H. CROSS-LINK INTERFERENCE taneous transmission of useful information. The interference
When neighboring cells transmit signals in different direc- signals are induced in the transmitted signal among the trans-
tions simultaneously on the same or partially overlap- mission of multiple radio channels in the desired message
ping time-frequency resources, it is known as cross-link signal [48]. Multiple access interference is also described as a
interference, as shown in Fig. 5. Usually, two types of combination of multi-user and co-channel interference (CCI).
cross-link interference were observed: transmission-point to
transmission-point and UE (User Equipment)-to-UE. The K. MULTI-USER INTERFERENCE
strength of interference power is of wide range, and in some Multi-user interference occurred when multiple users trans-
cases, it is much larger than the desired signal [46]. mit their respective requests of information at once to the
closest BS within the same cell (as shown in Fig. 6). Usually,
I. INTER BEAM INTERFERENCE this kind of interference is mostly observed in the radio net-
Beamforming is a novel technology used in modern cellular work, where multiple connections are active in each other’s
communication. It is a technique that identifies the best route chorus and close vicinity. As new radio, the current 5G is a
and provides an optimum throughput to a particular user in group of various uncoordinated small cells, and it is foreseen
a specified direction. This approach is essential to compen- to adjust substantial users with higher bandwidth and lower
sate for the attenuation losses in transmitting the message battery dissipation [49].
signal, especially in mm-wave communication. BS generates
multiples of narrow beams of RF energy in all directions of IV. INTERFERENCES IN 5G NETWORKS
the coverage area. However, the space division of multiple This section discusses various interference effects in HetNet,
beams introduces inter-beam interference (IBI) [47]. It is RN, D2D, and IoT. It explains various types of interferences
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related to each method by studying the state-of-the-art rele- limits the long-distance transmission of a signal but delivers
vant researches in the literature. high data transfer speed [54]. Therefore, researchers and
industrial operators devised a solution that backhaul traffic
A. HETEROGENEOUS NETWORK is transferred through to a given gateway via multi-hop links
The simultaneous operation of varied types of low power by using mm-wave frequency spectrum [55]. This method
BS transmitting and receiving a signal simultaneously from results in the easiness of the user’s handoff issue in small cells
more than 100s of antennas is performed to satisfy the current because the macro BS only performs the management task.
5G HetNet technology demand. It is a network designed It transmits the information for controlling the user handover
to support the concurrent operation of multiple technolo- in low-power cells. Whereas small BSs are solely responsible
gies. The researchers suggest the unconventional HetNet for user data transmission.
system’s concept to meet the continuous growing user data The most concerning issue in the FD technique is SI.
requisite [50]. Therefore, it presented the HetNet wireless Numerous SI cancellation methods have been proposed,
network’s idea and believes that the novel HetNet architec- such as analog or digital interference cancellation that can
ture has the potential to fulfill the future demand of both deliver up to 120 dB SI cancellation and trigger the FD
the continuously increasing number of wireless devices and communication over the HetNet structure [56]. Despite that,
the request for higher data rates. Several types of research the continuous transmission of several small cells and multi-
were performed for the optimum use of the HetNet structure. hop transmission of data to the user as of direct link and
In [51], authors proposed that an ultra-dense HetNet cellular backhaul traffic to gateway created massive congestion issues
network could deliver excellent result when a macro base with destructive interferences, handoff failures, and mobility
station (BS) transfers data directly to the users in its proximity management problems [57].
that requires low transmission power and provide energy The co-existence of 4G and 5G networks introduces two
efficiency. Whereas hand off the data to the small cell BSs stages of deployment in the current 5G wireless commu-
for the distant users. It triggers to maximize the coverage nication i.e., non-standalone (NSA) and standalone (SA)
area than the actual capacity of macro BS and alleviate the modes [58]. The SA mode is considered for the future release
dead zone coverage areas [52]. However, the users at cell of 3GPP and it is solely based on new packet core architec-
edges served by the small BS are severely interfered with ture instead of relying on 4G Evolved Packet Core (EPC).
by intra-cell and inter-cell interference that badly affect the Whereas NSA 5G network is an ongoing 3GPP standardiza-
user’s performance. Additionally, recent research shows that tion process to implement the draft standard commercially
about two percent of global CO2 emissions are generated by with the existing 4G cellular network. The concurrent oper-
the information and communication industry. Since energy ation of both the technologies in the initial NSA wireless
efficiency is the essential part of current 5G wireless com- network enhances spectral efficiency, reduces the 5G network
munication, power management of the devices and BSs in cost, and provides seamless coverage yet exposed to user
designing the new radio 5G HetNet mobile network. Thus, mobility and results in radio link failure and handover
it is considered as an eco-friendly system is referred to as a probability.
green energy communication network [53]. Table 1 illustrated A viable and durable combined interference coordination,
fictions of a diverse set of small cells and a macro cell in management, or mitigation scheme as shown in Fig. 7 needs
HetNet. to be addressed to manage these issues.
TABLE 1. Specification of different elements in HetNet. 1) INTERFERENCES IN HetNet

HetNet is a promising technology for the current 5G radio
communication, which suffers from various types of interfer-
ences [59]. This ultra-dense small cell system is extremely
devastated by the inter-cell, intra-cell, adjacent-channel,
self, inter-channel interferences because of unsystematic and
unorganized network design. It is observed from development
on ultra-dense small cell networks that ICI in the current
5G HetNet can be increased twice compared to the con-
HetNet is based on the random deployment and uncoordi- ventional cellular network design [60]. Consequently, ICI’s
nated nature of the small cells. It is critical and challenging to intensiveness in the current 5G multi-cell low power BS is
directly forward the backhaul traffic to each small cell node’s very high, and the features posed by ICI in its predeces-
given gateway. Since the current 5G cellular network touches sor’s mobile technologies somehow vary with the new radio
the untapped region of the mm-wave frequency band that gen- 5G network. Therefore, an advanced ICI management and
erates tremendous data transfer rate in Gbps and is expected mitigation technique would be developed for next-generation
to overcome the limitations. It supports frequency re-use cellular technology. Besides, cancellation of all other vulner-
for backhaul links and exceptional data transferring speed able interferences to maintain high QoS and fairness among
than a conventional system. The mm-wave (i.e., 1mm-10mm) the users in a cellular network.
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FIGURE 7. 5G cellular HetNet structure.
Due to the concurrent operation of numerous small cells inhabits the femto BS coverage area, it lies close to the macro
of and within these cells, multiple machine types or small BS coverage area. The femto user creates cross-tier interfer-
gadgets or smartphones communicate with each other in a ence with a macro BS. Usually, in the cross-tier downlink,
macro BS in the HetNet environment causes co-tier, cross-tier interference is received when macro BS at a distant location
interferences (as shown in Fig. 8) [61]. These interferences to the macro user but close to the femto access point received
are mostly observed in a large gathering where many users interference [63]. As macro BS power is very low and the
require higher throughput, such as heavy data applications, closest femto BS is in a close radius, it will generate high
internet surfing, downloading and uploading photos, videos, downlink interference to the macro user.
and so on. Those interferences that are precisely involved
within the multi-tier heterogeneity structure of mobile 4) CONTROL CHANNEL INTERFERENCE
network are as follows, Control channel carries scheduling and synchronization
information for downlink and uplink data channels besides
2) CO-TIER INTERFERENCE HARQ ACK/NAK information for uplink data channel [64].
Co-tier interference is observed when both users reside in the It holds the information over a physical channel. While
same network tier. In this case, an uplink transmission femto transmitting information over a physical channel, different
user belongs to one cell could be the reason for interference to user’s control may get interfered with each other. As car-
an adjacent femto BS i.e., in the closest range of femto user. rying the control data on the same physical medium will
Femto-cell transmission coverage short-area approximately induce interferences and the user can be received packets in
50m. Therefore, it is unavoidable where multiple femto BS unsynchronized order and data corrupted. Hence, the negative
installs, and an ample number of users access the link [62]. acknowledgment sends back to the serving BS and data of a
particular user re-schedule.
3) CROSS-TIER INTERFERENCE
In this scenario, users belong to dissimilar network tiers. 5) RESEARCH WORK OF HetNet IN LITERATURE
Macro user exists in the femto access point to generate The authors in [65] propose an approach based on self-
uplink interference to that femto BS. Similarly, when a user organized HetNet that focuses on maximizing the QoS. It is
68938 VOLUME 9, 2021

FIGURE 8. Co-tier & cross-tier interferences cellular HetNet structure.
based on game theory that effectively implemented the mut- are assigned different frequency channels. As a result, ICIC
ing technique of cell time transmission. Therefore, it pro- prominently decreased, and the aggregate data rate of the
vides good QoS and authenticates users. Besides, in [66], cell reached 100% and a 20% reduction in outage proba-
the authors explored the issue of small cells (SCs) with bility. Thus, simulations confirmed that the FFR procedure
network flying platforms (NFP) within the consideration improved the performance of the HetNet cellular system.
of all possible limitations. They execute two methods; the With the advent of mm-wave and microwave frequency
first is an association between SCs and NFPs to reduce bands, the network operators can implement multi-antenna
total interference while targeting each SC throughput via cellular phones and provide higher bandwidth with seamless
HBI/MTI. The second form associates NFPs with SCs decay coverage for each user. The 3GPP organization is working to
the system’s overall interference in an account of the total completely implement access and backhaul transmission of
sum rate by MTILBLS. The simulation results advocate that cellular traffic on FD wireless mode, but frequency reuse and
the proposed algorithm produced impressive results. How- multi-numerology configuration activate intra-channel inter-
ever, in [67] authors discussed improving the SINR level by ference. Therefore, in [69] authors proposed an impactful
reducing interference. To rectify the affair, the SFR approach, solution for intra-channel interference in mm-wave backhaul
besides the non-uniform SBS distribution, was furnished. connection in the HetNet system. It furnished an orthog-
Also, uniform and non-uniform distribution scenarios were onal channel allocation, efficient mm-wave access point
considered for expressions of coverage probabilities. deployment, and optimal power transmission allocation for
Consequently, simulations addressed that the suggested reducing complexities of mesh backhaul links. Consequently,
model delivered quality results by using the SFR scheme. the Ray-tracing based simulation shows that the anticipated
Similarly, SFR with a non-uniform SBS setup increases method reduces interference up to 250m from mm-wave
coverage probability due to fewer interference problems. GW. The authors in [70] proposed a PA-based interfer-
Moreover, increment in MBS and SBS densities leads to a ence alignment (IA) scheme and coordinated beamforming
harmful effect on coverage probability, but decreasing the (PA-IA-CB). This scheme was devised for power allocation
SINR threshold value provides better coverage because of and interference mitigation issues for the downlink NOMA-
the increased number of associated users. The investigators MIMO technique in HetNet. Hence, the simulation results
in [68] address inter- and intra- channel interference in the validate the proposed concept’s performance based on the
existence of femtocells within the macro cell. It presented system sum-rate and outage probability at different SNRs
the FFR (Fractional Frequency re-use) technique and divided compared to the traditional method (MIMO-OMA). In [71],
the cell into the edge and central region. The method imple- the authors focus on managing cross-tier interference by
mented as the central region users i.e., inner region, has given deploying small moving cells. They designed a mechanism
the same frequency sub-band, whereas outer region users in which S-UEs’ interference channel is signified by the
VOLUME 9, 2021 68939

dominant channel space followed by applying ZFBF in the in the femtocell network. In [78], investigators focused on
dominant channel space. Therefore, the simulation results improving beamforming vectors and power allocation for
proved that the proposed technique is efficient enough for security rate enhancement by worst-case formulation in
M-UEs and S-UEs. two-tier multi-antenna relay-assisted HetNet. Additionally,
In [72], authors raise the concern of multi-tier interference researchers devised an interference nulling strategy to min-
in HetNet uplink transmission. It designed a prioritized radio- imize CCI. Hence, it concludes that the proposed method
access scheme that relies on frequency hopping procedures. showed high reliability to accomplish secure communication
The advantages of the suggested scheme that the SC-user from experimental results.
has low BER performance and high radio-access priority. In [79] authors explored objectives are to increase net-
Whereas MC-user received relatively poor performance but work performance based on energy efficiency and through-
surpassed conventional FD. As a result, the proposed FH put alongside a reduction in the interferences. It formulated
method assured higher anti-interference and spectral effi- particular algorithms for a possible combination of eICIC
ciency than traditional FH. In [73], the authors proposed a parameters. Thus, the Relaxed-ABS algorithm removes inter-
plan, a sub-channel assignment for SWIPT-NOMA in the ference via resource allocation parameters such as subframes
HetNet downlink case with imperfect channel state infor- allocation and average airtime. To validate an algorithm’s
mation (CSI). The yield of both techniques is the improve- performance, simulation results prove that Relaxed ABS
ment of EE of the cellular network. Thus, numerical results achieve a prominent gain in energy efficiency. Nonetheless,
simplified as the proposed algorithm delivers quality per- further modification in the method by blending eICIC and
formance based on numbers of PUs/FUs, average EE of Coordinated Multipoint (CoMP) improved overall system
low power cells compared to conventional NOMA. Whereas performance. Since 5G and beyond expected to overcome
in [74] authors proposed a novel real-time dynamic user (UE) the energy consumption issues thus, several numerical and
association algorithm for multi small cell networks. It devel- practical analysis has been performed by explorers for green
ops a location-based interference mitigation algorithm. The communication. In another paper [80], researchers address
algorithm helps to mitigate co-tier and cross-tier interfer- the shortcomings of power consumption and CCI among the
ences. Consequently, computational results proved that the small cells in a dense HetNet 5G system. They apply a novel
furnished algorithm leads to better performance compared design of ICR that relies on low power cell on/off switch-
to the other algorithms. Researchers added that their results ing algorithm. Furthermore, to curtail UE’s measurements’
are applicable in a real wireless network environment for total quantity, network adjacency matrix (NAM) with the
current 5G systems. In [75] authors worked on interference interference coordination rate (ICR) calculation method is
mitigation and network capacity enhancement by investigat- merged. Consequently, the unconventional algorithm brings
ing old IA schemes. An RCIA algorithm was proposed for excellent power efficiency to the SBS. Besides, it decreases
the Multi-cell MIMO downlink HetNet. The outcome shows CCI between small cells with lower traffic loss compared to
that the suggested scheme delivers excellent performance in conventional switching strategies.
spectral efficiency compared to the TDMA technique. Table 2 shows the summary of the research work of HetNet
Furthermore, the authors addressed the case of in the literature discussed in this section.
interferences, i.e., co-tier and cross-tier in the two-tier
femtocell (FC) network, by introducing a hybrid dynamic B. RELAY NODE
ICIC scheme [76]. To gain each user utility and maximizes A few years back, a promising technology for next-generation
radio resources, two algorithms have been conceived i.e., wireless communication was introduced, now known as an
FBS-level and FMS-level. The proposed schemes’ promi- RN [81]. 3GPP presented this concept in Rel-10 to solve
nent services are Low computational complexity of the performance issues such as slower data rate, more signif-
FMS-level and distributed functionality of the FBS-level. icant delay, weaker signal, higher interference (self, inter-
To evaluate the performance of suggested techniques, the PF cell, intra-cell), and so on RN is the least expensive low
and Hungarian scheduler were used. Consequently, the PF power BS that provides enhanced coverage, greater capacity
scheduler with the recommended approach gives excellent for the users, and increased throughput, especially at the
performance compared to static and semi-static ICIC meth- cell edges. In 4G technology, relays help to give a reason-
ods. Also, it delivers high cell-edge user quality and capacity able data rate to the users in those areas where macro BS
and fairness among all the users in the macro cell. Whereas did not support acceptable throughput [82]. RN was ini-
in [77], the examiner raises an issue of interference in shared tially introduced as a switching BS that extends Macro BS’s
spectrum resources. They developed a modified firefly algo- coverage zone and alleviates the hardware installation cost.
rithm (FA), i.e. discrete firefly algorithm (DFA) to reduce However, the academic researchers and industrial experts
the co-tier and cross-tier interferences and ideal resource performed extensive researches. They advised that RN can
allocation in the shared spectrum HetNet. The experimental be applied in cellular communication to enhance throughput,
results proved that the DFA scheme performed better than energy efficiency, system capacity besides decreasing power
the random resource allocation technique. Also, it minimizes consumption. The relay channel communication provides
the interferences if users utilize the same resource blocks higher data rate coverage to the cell edge users for mobile
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TABLE 2. Summary of research work of HetNet in literature.
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TABLE 2. (Continued.) Summary of research work of HetNet in literature.
broadband networks to satisfy advanced RN objectives [83]. to the macro BS. This extension in the RN would help to
Thus, the future network, which is a combination of different minimize the complexities of high power BS.
sizes of small cells and RNs networks and usage of the mm- Furthermore, the communication between the user and the
wave frequency band, offers a proliferation of throughput and RN takes place at the same frequency as the communication
increases energy and spectral efficiency for both cell-center between the RN and the backhaul link; it is termed as in-band.
and cell-edge users. Also, multi-hopping through various Contrarily, suppose the backhaul communication performs
nodes reduces power consumption as well as delay [84]. at a different frequency than the carrier signal utilized by
Moreover, the RN is also expected to be served as full macro the user for uplink (UL) and downlink (DL). In that case,
BS at the physical layer, but its range is limited as compared the RN is known as out-band [85]. However, out-band relays
68942 VOLUME 9, 2021

need more resource blocks since two channels are required

for the access and backhaul links. Due to limited spectrum
resources, this technique is completely undesirable, and in-
band relay has been the focal point of 3GPP and future
wireless communication [86].
In the mm-wave HetNet design, RN’s efficient use plays
an integral part in transmitting a signal from Macro BS and
small cells BS. RN in full-duplex (FD) transmission mode
is considered to perform far better than (half-duplex) HD
mode. However, many techniques had been devised to use HD
mode to resolve resource wastage and support extensive data
demand for the billions of devices for future wireless com-
munication, such as two-way relaying (TWR) [87]. However, FIGURE 9. (a) Co-tier Ideal FDR (b) FDR with SI.
HD relaying immensely increases the transmission and algo-
rithmic complexities of the network. Whereas FD relaying of the amplifying aspect at the RN, where there is no need for
(FDR) provides fewer complexities at the MAC layer, greater residual SI cancellation.
spectral efficiency, and well-organized resource management Fig. 9(a). FDR supports the concurrent transmission and
and supports various resource allocation techniques for effi- reception of a signal, resulting in a broadcast from the primary
cient use of resources without loss of throughput in unfa- BS in which multiple access phases are synchronized. Ideal
vorable propagation channel conditions. This cause massive relaying that delivers precisely the output of its input signal is
congestion due to a large number of assorted devices, hid- realistically unachievable. In Fig. 9(b), the FDR experiences
den terminal, signal penetration due to the structure of the strong SI from its antenna receiver node, whereas interfer-
high rise buildings and large advertising objects such as ences from other sources would be a part of a transmitted
billboards [88]. However, FD and multi-tier HetNet induce signal.
various interference issues. The most prominent interferences
in in-band FDR are self-, inter-relay, and inter-cell interfer- 3) INTER-RELAY INTERFERENCE
ences [89], [90]. In a multi small cell and relays network within a macro cell,
FD transmission mode increases throughput, yet it experi-
1) INTERFERENCES IN RELAY NODE ences inter-relay interferences (IRI). The relay collects the
Interferences are a critical and unavoidable part of a cellular source data, but it interferes with the other relay’s signal. This
network. In wireless communication, it is dealt with in two phenomenon is mostly observed at the cell edges when two
ways: coordination or cancellation techniques. The interfer- neighboring cells use the same set of frequencies. Due to the
ences observed in RN are described below with related works. RN’s high transmission power, the relays are affected by the
interferences between them. Sometimes during the bench-
2) RELAY SELF INTERFERENCE test, it is observed that the low power transmission of RN
The wireless transmission technique FD is capable of receiv- can still disturb the 133 neighboring RN. And this problem
ing a signal and transmitting it simultaneously on the same drastically lowers the user’s data and network quality. In [94],
channel. This dramatically enhances the spectral efficiency the authors suggested the IRI mitigation technique for the
up to 2 times more than HD relaying (that uses two channels). two-hop relay. They proposed a method in which the IRI
The most promising technology of a current 5G cellular cancellation was executed at the RN to minimize the detec-
network is the mm-wave spectrum. For massive data rates, tion complexity at the destination point. Results proved that
ultra-reliable low latency, and minimum transmit power, the recommended way delivers better performance than the
mm-wave is supposed to be an efficient technique for wireless existing schemes.
front and backhaul links. However, due to its limited trans- In a current 5G radio network, a complex combination of
mission length, higher spectrum bands (above 24GHz) are several small cells with FDR supports the user’s demand, but
badly exposed to propagation losses and fading issues. There- other sources’ disturbance reduces the user’s experience [95].
fore, standardization authorities and academic researchers Several types of interference can be possible in a single RN
believe that FDR is the most viable option to expand the cov- (as shown in Fig. 10). Therefore, an efficient interference
erage limit [91]. The implementation of FDR with mm-wave cancellation scheme minimizes latency to less than one mil-
signals induced SI. It is the interference caused explicitly by lisecond and provides higher throughput at multiple Gigabits
the transmitter to the receiver antenna located in the same per second (Gbps) to support next-generation applications
equipment. This should be reduced to a minimum, at most highly desirable.
below the receiver’s noise power level, such as in [92],
the authors designed mm-wave end-fire FDR antennas for 4) RESEARCH WORK OF RELAY NODE IN LITERATURE
SI suppression. Similarly, In [93], the authors proposed the Various studies have been conducted on the management
SC-FDE-based FDR method and showed an optimum result and mitigation of relay interference since its introduction in
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FIGURE 10. IRI, ICI, RN, and D2D links in RN.
mobile networking. Few research methods to rectify RSI, IRI, As the next-generation network is considered to connect
and other interferences in RN are discussed below. billions of devices within a single-tier network, it is predicted
In [96], the authors proposed a scheme EH-FD-NOMA that the capacity and QoS will be 100 times higher than
system with amplify and forward FDR to examine the per- the traditional network. But, the malfunction in the hard-
formance of a system and take measures for SI. The Monte ware equipment would decay the performance of the system.
Carlo simulation is used to describe the numerical outcomes. Therefore, in [99], the authors analyzed the performance
Therefore, results show that implementing PB with high limitation due to disturbance in the transmission of a signal
transmit power and SI cancellation for FD will improve because of faulty hardware in radio communication devices
throughput and outage performance of near and far users. and imperfect SI cancellation in IBFD decode and forward
In [97], the authors furnish a framework to assess FDR’s system. The Monte Carlo simulation was performed to cor-
ergodic capacity in vehicle communication. Both CCI and roborate numerical results. As a consequence, the simulation
SI were taken into consideration at the destination and results achieved accurate expression for the system OP and
relay, respectively. Precisely, the calculation performed on evaluated the influence of RSI and hardware impairment
the vehicular-to-vehicular radio cooperative communications on system performance to overcome inadequacies of earlier
with FD non-generative AF relaying system with indepen- related works. In [100], the authors examine the IRI issue in
dent non-identically distributed (i.n.i.d) Nakagami-m fading the CSS TPSR protocol. It focuses on MLC signals for IRI
channels. The closed-form expression of the ergodic capacity mitigation, whereas simulation results validate the suggested
was also presented. The outcomes confirmed that system per- method’s BER performance. In conclusion, an innovative
formance decreased while the increase in SI at the relay and scheme helps to mitigate IRI in FD transmission even in
CCI at the destination. Additionally, the suitable placement severe IRI environments. In [101], the authors investigated
of the relay is suggested precisely in the middle of the source the scrambling technique to remove IRI. A novel analytical
to the destination link. The results proved that when the user model presented for a tri-sectorized Hexa shape mobile net-
is closer to the receiver side, ergodic capacity decreases; work. The effect of the scrambling approach on two relays of
thus, the system’s performance degrades. However, in [98] a sector interference coordination has been discussed. There-
authors worked to inspect the impact of in-band FDR of fore, the outcome concludes that capacity and throughput
amplifying and forward mode with imperfect transceiver such are significantly enhanced and the efficiency of access link
as damage in electronic component alongside the effect of validated based on SINR and data rate.
residual SI. The numerical and simulation results demonstrate Several investigators and industry hardware experts per-
that both SI and hardware impairments significantly impact formed numerical, simulation, and test bench tasks on HD
system performance. Hence a robust optimal power allocation and FD transmission performance measurements. And they
technique offered that effectively upgraded system perfor- advised that FD transmission with relaying technique in
mance. Nonetheless, when the network is badly affected by the next-generation mobile communication is considered to
SI and hardware malfunction, the proposed method could not be the optimum technique. Besides, the FD mode tends to
guarantee expected network efficiency. Therefore, there is support the expected demand for billions of wireless elec-
room for an advanced solution is desirable. tronic devices. Nonetheless, few of the researchers worked to
68944 VOLUME 9, 2021

provide the same results with unconventional techniques. multi-cell network with a joint BF method to mitigate SI,
Like it is presumed that the current 5G wireless communi- IRI, and RDI. In particular, all three interferences effects
cation network is disastrously affected by the interferences. can be removed by relaxing zero-forcing conditions of sev-
Thus, in [102] authors, demonstrated IRI due to succes- eral null space dimensions implementing IA. It is given that
sive transmission of source and relay to another relay and the multi-dimensions can be adjusted in a single dimension
destination links in a network. To fulfill the requirement, where the interference mitigation techniques can be utilized.
two-relay pair selection relies on available CSI in a net- The MIMO FDR transmission is achievable by effectively
work that is equipped with a cluster of HD buffer-aided canceling the SI, IRI, and RDI and it is further enhanced
relays, buffer-aided multi-antenna source, and a destination. by the PA algorithm to offer close to twice the standard
In global CSI a linear precoding process and CSIR phase HD capacity. In [108], the authors focused on analyzing FD
alignment strategy are utilized by the source to remove interferences with filter and forward-based multiple relays
IRI. Numerical results have clearly shown that proposed cellular system.
methods refined performance based on average end-to-end Two strategies are provided for the filter model based on
rate and outage probability. Whereas, traditional non-ideal SINR enhancement with a controlled transmission power of
SFD-MMRS technique with IRI is prominently decreased. relay. The results verified by the simulation test and proved
In [103], the authors proposed an unconventional virtual that the FD-FF relay network shows higher reliability than
FD buffer aided relaying protocol is furnished, and it is the HD-FF scheme regarding attainable rate. It also described
categorized by exploiting several SR links. The results indi- SOS-based filter design techniques that were obtained and
cate that IRI mess commendably suppressed via interference their efficacy is given numerically. The articulators in [109]
cancellation at the relay with many antenna elements. Results discussed the performance of dual-hop energy harvesting
verified by simulation test described that the novel virtual relaying networks under the time switching process in the
FDR technique delivers minimum latency and sustain suffi- presence of CCI over Nakagami-m fading channels. This
cient outage probability than existing schemes. Another work examination explicit the network with and without a direct
demonstrated the IRI disturbance and proposed a new virtual link. Hence, the expression of data rate and the average bit
FD cooperative NOMA framework in [104]. A relay station error probability is evaluated analytically. Numerically cal-
selection algorithm designed with adaptive IRI management culated analytical results matched with equivalent simulation
and outage probability of a suggested scheme is statisti- outcomes justify the effectiveness of the presented analy-
cally derived. Thus, simulation outcomes confirmed the more sis. The results conclude that the system performs best on
excellent performance of the relay station selection algo- throughput for optimum values of two parameters, such as α
rithm. Significantly, the proposed framework outperforms the and greater values of η. Next, the average bit error probability
traditional HD NOMA framework. Researchers worked on diminishes with quality channel conditions.
the mitigation of both interferences related to relay, i.e., IRI Moreover, nullifying the average bit error probability once
and RSI. Subsequently, in [105], authors investigated RSI and the separation between relay and CCIs increases. Hence,
IRI for FDR. They are using oblique projection and orthogo- the results proved that a direct path system performs much
nal projector techniques to nullify relays interferences. This better than without a direct path. Similarly, the explorer
technique’s unique advantage is that if RSI and IRI channels in [110] considered secrecy outage performance of decode-
are unknown, it can still be adopted. Thus, the proposed and-forward cognitive relay network in the shadow of CCIs.
method does not degrade SIR under high transmit power. All channels face Rayleigh fading channels and all nodes
Once the RSI and IRI are canceled, the FDR mode can be are furnished with a single antenna. Closed-form secrecy
implemented with full potential for next-generation commu- outage probability has been derived from measuring the
nication. Though in [106], the authors provide quality results performance. The Monte Carlo simulation was devised to
for future cellular communication. The authors suggested a validate analytical results, where opportunistic scheduling
framework of several small cell-based macro cell networks was utilized at the destination. Thus, results corroborate that
with FD mm-wave backhaul links to boost a spectrum’s the employment of several destinations may enhance system
efficiency and attain larger multiple access. An FD mm-wave secrecy performance. However, an increase in CCIs could
precoding scheme is offered that enhances spectral efficiency minimize the security of the network.
and cuts down interferences simultaneously. By using a Table 3 shows the summary of the research work of relay
three-step precoding design, vastly degrades SI, IRI, BS-UE, nodes in the literature discussed in this section.
and RS-UE interferences. In conclusion, the suggested frame-
work improves cellular network overall throughput, a large C. DEVICE-TO-DEVICE
number of user support, enhances coverage area, and high It is predicted that 100 billion electronic devices can be
data rates for each user with minimum latency. connected through wireless links by 2030 [111]. This expo-
In [107], the authors discussed the degradation of network nential growth in the quantity of the user equipment causes
performance due to various interferences. A combination of an increment in the frequency spectrum usage. In contrast,
multi-tier small cell structure with FDR transmission mode the current spectrum lacks the availability of free resource
produces SI, IRI, and RDI. The MIMO FDR assisted a blocks. Therefore, the next generation has all the ingredients
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TABLE 3. Summary of research work of relay node in literature.
68946 VOLUME 9, 2021

TABLE 3. (Continued.) Summary of research work of relay node in literature.
to accommodate the massive stipulation. The existing 5G novel D2D communication can be conducted on a licensed
mobile network is a combination of various methods that help (in-band D2D) and unlicensed (out-band D2D) spectrum.
to achieve the expected demand. The mm-wave frequency In-band D2D is sub-divided into underlay and overlay modes.
band exploration creates the possibility to fulfill the future In in-band underlay mode, the D2D users utilize the same
user’s requisite. With many other techniques, D2D commu- spectrum resources of cellular users; however, in in-band
nication is also a promising 5G network [112]. overlay mode, pre-defined channel resources from the cel-
D2D can be useful for the direct connection among nearby lular spectrum are allotted to D2D communication [114].
disparate devices from heavy electrical machines to smart Nonetheless, the in-band overlay mode can only be deployed
electronic instruments without utilizing the core network. in particular locations and conditions. Due to the scarcity
It can also access both cellular licensed and unlicensed of spectrum resources, this category would not justify the
spectrum. It could be considered a substitute for cellular 5G and beyond requirements. Moreover, to find the suitable
communication for services such as mobile traffic handover, candidate for D2D massive research has been performed and
content sharing in meetings, data transmission, vehicle com- all suggested that in-band D2D is reliable, as it maintains a
munication, public gatherings, and so on. The fast switching high-security level, improves spectral efficiency, and can be
among nearby electrical and electronic components deliv- re-used cellular device resources. Therefore, most of the work
ers minimum delay, low transmit power, lower propagation is performed on the underlay in-band D2D mode. However,
losses, and larger throughput besides maximum spectral and many researchers favor the unlicensed band for D2D users
energy efficiencies. D2D can also be utilized for future with either Bluetooth or Wi-Fi direct [115]. As a result, inves-
energy-efficient wireless communications [113]. The basic tigators said that it is viable to operate an out-band D2D con-
D2D structure in multi-tier HetNet can be seen in Fig. 11. figured system for short-range communication using either
According to the research community in 2003, the num- Wi-Fi Direct or Bluetooth i.e., unlicensed spectrum. On the
ber of connected users to the network was 6.3 billion, other hand, long-range transmission used a licensed spec-
though the sum of related smart gadgets per user was 0.08. trum such as WLAN, LTE-A, WiMAX, and so on However,
It reached 4 times more connected devices than its previ- the licensed spectrum can and will create interference issues
ous number in 2015. Furthermore, in 2020, the connected between cellular and D2D users. As technologies improve at
automated components will touch the number of 50 billion, a quick pace and help to craft the cellular network in a more
whereas connected devices per person are going up to 7. The refined shape, recent radio communication advancements
VOLUME 9, 2021 68947

FIGURE 11. D2D communication in multi-tier HetNet.
suggest that D2D users may share the unlicensed spectrum still, the discussion is limited to interferences. Therefore,
of LTE-A in the same access level within the same network. the authors broadly discussed recent work conducted on the
Therefore, it intelligently exploits the overcrowded licensed mitigation of interference in D2D. The eminent interferences
spectrum assets, whereas the unlicensed spectrum is managed observed in D2D communication are classified into a net-
via coordinated radio resources (CRR) [116]. Nonetheless, work domain, i.e., co-tier, and frequency domain, i.e., cross-
data between several D2D users and mobile networks are tier. [120]. The D2D co-tier interference is observed when a
severely obstructed by the interferences. D2D user interferes with another D2D user in the same tier.
In an OFDM-OFDMA system, when the same set of subcar-
riers are allotted to multiple D2D users, co-tier interference
1) INTERFERENCES IN D2D
is received from the transmitting device to a nearby receiving
The advent of D2D communication introduces a new tier
device. On the other hand, cross-tier interference is generated
in the current 5G HetNet cellular system known as the
between the cellular user and D2D user. In the co-existence of
device tier. The device tier involves D2D communication, and
D2D and cellular users, the victim of interference is different
devices are randomly distributed in a network. The addition of
because of the direction of frequency resource transmission
a device tier in a cellular network significantly enhances the
(as shown in Fig. 12).
throughput, spectrum efficiency, and battery life of wireless
Scenario 1: In in-band underlay mode and both users trans-
equipment. However, the major challenges involved in the
mitting in the uplink direction, the cellular user interfering
implementation of the D2D link are security and interfer-
with the D2D receiver. In contrast, the D2D transmitter signal
ences [117]. In the D2D link, the simultaneous connection
interferes with the MBS.
between 100s of incongruent devices and two-way communi-
Scenario 2: When D2D re-used downstream resources of
cation can potentially increase interferences. Also, D2D users
licensed spectrum, D2D transmitter data interfere with cellu-
could be exposed to data encryption issues. The transmitting
lar downlink user and MBS high power signal interfere with
data is not encoded correctly due to any hardware or software
D2D receivers.
malfunctioning and the intermediate device easily hacks the
information [118]. This could be a possible result in the loss
of private and highly confidential details. Thus, D2D com- 2) RESEARCH WORK OF D2D IN LITERATURE
munication greatly improves system performance, though For handling the interference issues in D2D communication,
it is poorly exposed to security issues. Nonetheless, D2D the most appropriate efforts will be discussed as follows.
transmission is highly desirable for the current 5G cellular In [121], the authors worked on the mitigation of inter-
communication as it takes lower power to transmit from one ference between cellular and D2D users. The resource
hop to another and increases the system capacity [119]. management and power control technique was presented
Many researchers exploited the security problems and for multicast D2D communication within uplink mobile
the interferences in D2D enabled cellular networks. Here, networks to reduce interference. The sum throughput
68948 VOLUME 9, 2021

FIGURE 12. D2D interferences in 5G HetNet.
optimization problem’s formulation as per the required power D2D and cellular networks due to the sharing of cellular user
and SINR levels were furnished. To simulate the proposed resources in HetNet considered. Also, a distributed resource
scheme, a Monte Carlo simulation was applied for the perfor- allocation algorithm based on matching theory proffer to
mance measurement. Therefore, simulation results show that decay the impact of interferences and enhance network per-
the recommended technique performs much better because it formance. The simulation results advised that the proposed
achieved higher throughput and reduces overall power con- algorithm helped achieve network performance by up to
sumption compared to conventional schemes. Furthermore, 93.7% and drastically reduced interferences among D2D
in [122], the authors presented a new idea of utilizing time and cellular users. Alternatively, the articulators investigate
and space division in resource allocation. Then the study the outage performance of D2D communication underlie
was conducted on the resource management issue in D2D cellular networks by using BF and IC techniques besides
communication underlying mm-wave technology. A time slot an M-antenna BS in [125]. The research shows that D2D
scheduling algorithm based on vertex coloring is presented. users communicate via a two-way DF relaying and statistical
Hence, the proposed method helped gain higher throughput results validate that the relay-assisted network’s outage per-
per time slot impressively, around 12.5%. Additionally, sim- formance can deliver excellent results than the conventional
ulation results assist to understand that side lobe interference case without extra power usage. The results also suggest
is a serious threat in the down gradation of the network. that when the separation between D2D and cellular users is
However, it can control side lobe interference if properly far enough, the outage probability inclines zero. In [126],
choose a suitable threshold. the authors demonstrated topological interference manage-
In [123], the authors considered a process to characterize ment (TIM) for D2D communication when devices are
intra-cell interference in the D2D underlay network. The unaware of CSI with other devices in the surrounding yet
suggested PCP process to model the inhomogeneous and the network topology can cancel the interference observed.
spatially correlated distribution of MTs. Besides, it creates Therefore, the simulation results confirmed that descending
a novel approach to Euler Characteristics (EC) for approx- in signal interference when matrix rank reduction ability is
imated intractable nearest neighbor distribution function. applied. The results showed that the implemented scheme
In consequence, the research findings are outperforms all existing techniques.
a) EC and RFT framework helps to identify hotspots for In [127], authors worked on two modalities, i.e., interfer-
D2D communication. ence management and cancellation projected to AAF relay in
b) Point processes with repulsion/attraction property can the D2D network and a cooperative D2D network in which
be utilized for robust spatial modeling of MTs/BSs. one of the D2D pair acts as a relay to cellular transmission.
c) The numerical inference was conducted to detect clus- The yield of the approaches leads to the conclusion that the
ters of MTs with spatial correlation. cellular user throughput increased by using cooperative relay
d) Threshold ‘u’ of the jaunt plays a vital role in man- D2D compared to underlay D2D link. Additionally, the exam-
aging cluster size for D2D communication, coverage iners conducted a comparison test between the proposed
probability, frequency re-use, and mobile user’s level method with compress and forward relay protocol, where
of interference. the suggested method decay the BER by 9 dB and outage
In [124], the authors discussed the problem of interfer- ratio downsized to 0.002 when D2D count equals 20 pairs.
ences due to resource sharing. An unconventional design of In [128], the authors performed a test on interference mini-
spatially spreading of nodes in HetNet via PPP to explore mization to maintain the target sum-rate due to the problem in
the impact of co-tier and cross-tier interferences between assigning mobile user resource blocks to D2D pairs. A novel
VOLUME 9, 2021 68949

algorithm technique was presented, and results conclude that CD, MD, and SUE users while ensuring minimum QoS of
if the interferences are uniform, the algorithm takes poly- the macro-cellular user in the existence of channel estimation
nomial time. Similarly, in [129], authors focus on the inter- error. A Lagrangian duality transform was also implemented
ference problem in small cell and D2D user’s co-existence to reframe the actual power control problem. Consequently,
under the mm-wave transmission. A Stackelberg game-based the given algorithm helped provide higher system perfor-
interference control process with full frequency re-use in mance by increasing the separation between CD and SUE.
D2D link underlaid mm-wave small cell network proposed. In [136], the authors inspected the reliability of eICIC, focus-
The simulation test was performed to verify the presented ing on the SE in HetNet. The investigators studied the results
technique and results proved that the scheme obtains higher of allocation of spectrum resources to MUEs by using D2D
throughput and SINR level in full frequency re-use mode. to assist the delivery of downlink data when ABS is active to
In [130], the author performed energy optimization in the overcome transmission discontinuity. The process is called
intra-cell and inter-cell interference constraints in D2D com- D2D-eICIC, i.e., a heuristic algorithm. The network sum-
munication. A multi-cell joint optimization approach was rate via simulation results proved that the proposed technique
examined in both the interferences to increase the energy performs far better than traditional eICIC.
efficiency of cellular and D2D links. Two selected algorithms, Table 4 shows the summary of the research work of D2D
i.e., MCJPC and SCIPC, deliver optimum energy efficiency in the literature discussed in this section.
besides greater scalability.
In [131], the authors performed an analysis of coverage D. INTERNET OF THINGS
probability in D2D. The Nakagami-m fading procedure is Generally, the term IoT can be seen as building upon the con-
used for the study of the performance. The numerical results cept of a Wireless Sensor Network (WSN) [137]. They can
show a decrease in coverage probability degrades the link sense data, gather it, and afterward send it to the network via a
quality. Also, a rise in threshold the coverage reduces; there- sink (also known as gateway). These low-power devices (sen-
fore, interference can be alleviated. With the simultaneous sors) are essentially miniaturized radio communication units
use of licensed and unlicensed spectrum in D2D assisted with some physical quantity measurement capabilities.
wireless links, vehicular communication will be a key feature The new radio’s evolution in the current 5G HetNet archi-
in 5G and later 6G wireless networks. In [132], the authors tectural system has made it possible to fulfill the connec-
discussed the interference analysis for vehicle-assisted com- tivity demand of many IoT devices and their applications.
munication at mm-wave 28 GHz frequency band. Due to To meet future IoT fundamentals, new, highly desirable
massive vehicles roaming on the road and high mobility of parameters, such as security, ultra-reliability, lower round trip
vehicles, they may induce interferences and regular packet delay, the massive connection of unparalleled devices, and
loss error. The analysis confirmed that PEP relies on chan- throughput, should be optimized for a perfect user experience
nel non-linearity and multipath clusters. More multipath and with minimal disturbances [138]. IoT transforms the world’s
delay in arrival time more chances of interferences. communication network via the connection of a diverse
Apart from interference cancellation and suppression range of physical objects and handles low-power electronic
techniques, many researchers choose to investigate the components that connect one another through the Internet.
interference management method in D2D assisted wireless Energy efficiency is one critical concern in future IoT-enabled
networks. It is because an interference management approach wireless communication. The massive connectivity of small
minimizes the statistical and computational intricacies and to large scale objects and machines will emit an enormous
effectively reduces the interferences. Thus, in [133], the amount of energy in the environment. According to Erics-
authors observed interference management in D2D commu- son [139], approximately 28 billion smart wireless devices
nication within multi-tier cellular HetNet. An interference will connect through the internet worldwide by 2021, and
management scheme is presented in which only D2D UEs solely 15 billion devices belong to M2M and consumer types
in an achievable set can re-use the resources of cellular of equipment. IoT is a disruptive technology that enables
links. Hence, the proposed method decay interferences and an environment to utilize the internet in everyday human
maintain QoS. In [134], the authors exploited the usefulness life, business, and industrial applications. According to the
of D2D communication with the SWIPT energy harvesting academic explorer, billions of wireless electronic instruments
method with a target to gain the advantages of three essen- and heavy machines can run on massive IoT applications, for
tial enabling technologies of the 5G and beyond networks. instance, smart homes and buildings, intelligent grid systems,
Different algorithms such as time and power splitting SWIPT agriculture monitoring and maintenance, smart cities infras-
implemented to compared it with existing procedures. In con- tructure, and so on [140].
clusion, the results proved that the underlay D2D in HetNet In general, IoT applications and services are productive
with shared spectrum resources interferences are more vul- where updates are required on the cloud and perform indoor
nerable than traditional HetNet. In [135], the authors iden- office or smart home surveillance, including things like
tified an issue of cross-tier interference for underlay D2D an automatic door lock to garage gate opening on arrival.
users in HetNet assuming imperfect CSI. They have used In this small-scale environment, IoT can be quickly and
Fractional programming (FP) to evaluate the ideal power for effectively deployed with lower end-to-end cost, less energy
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TABLE 4. Summary of research work of D2D in literature.
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TABLE 4. (Continued.) Summary of research work of D2D in literature.
consumption, and greater scalability [141]. In comparison, notably fostered mobile services and delivered quality IoT
IoT has many critical applications, for instance, industrial device internet services. In the early era, 4G showed far better
automation, smart vehicular system (autonomous vehicles), results than competing technologies, such as WiMAX, BLE,
advanced traffic control system, the remote clinical system and so on, despite the 5G and beyond is expected to solve
for monitoring and operation, massive aeronautical commu- the future challenges and shortcomings of 4G and current
nication system updates, and so on required high reliabil- 5G technology. According to the researchers, effective uti-
ity, security, safety, dedicated end to end connection with lization of both LTE-A and 5G technologies will reduce the
ultra-reliable lower latency for stringent setups and min- inadequacies of new connectivity of wireless interfaces for
imal vulnerability to end-users. Because in case of short IoT applications [143]. The current 5G network structure
delay or any failure in transmission would result in dev- is largely based on the 4G cellular interconnected system’s
astating consequences. Additionally, researchers have some foundation. Also, it has the potential to furnish ultra-reliable
other constraints; for instance, high energy emission degrades and massive connectivity to IoT devices. It is expected to
the environmental atmosphere. Therefore, the manufacturing deliver the user with transmission speed up to a maximum of
labs should perform the proper assessment of radio various 10 Gbps, whereas 4G only supports up to 1Gbps. However,
devices’ transmission power to avoid severe health and global IoT devices are susceptible and unable to tolerate the high
warming issues. power of interfering signals.
Moreover, by deploying HetNet infrastructure in the
multi-tier 5G radio network, IoT systems may achieve 1) INTERFERENCES IN IoT
high-quality connectivity and services over a sizeable geo- In the next-generation co-tier and cross-tier HetNet IoT net-
graphical land with economic feasibility (Fig. 13). In 5G work, the interferences are unavoidable. The unlicensed ISM
HetNet IoT, a massive number of devices are connected via band is used for IoT services to provide several physical
wireless links [142]. The predecessor mobile technology 4G devices to properly tap the spectrum to adhere to short-range
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FIGURE 13. Multi-tier 5G IoT network.
radio communication conditions and regulations properly. avoiding Warden Willie. The results conclude that Willie’s
Hence, the simultaneous usage of a free license frequency unpredictability at the background noise and increase in
band of smart wireless instruments undergoes interference interference favors Alice’s security by considering noise.
constraints [144]. These interferences are harmful to the Moreover, researchers investigate the experimental valida-
effective communication and operation of several uncoordi- tion for interference statistics in IoT in a particular area.
nated devices. In IoT, it is critical to characterize the interfer- In [147], the authors performed statistical analysis on unli-
ence as the devices are of different design and specifications, censed 863 MHz to 870 MHz frequency bands in different
and not every gadget is coordinated. Some of the recent locations in Aalborg, Denmark. As a result, data measured
related work on interference issues are demonstrated in the at five other sites and confirmed the heavy-tailed nature
following. of interference in IoT communications. Whereas in [148]
authors examined wireless power transmission (WPT) aided
2) RESEARCH WORK OF IoT IN LITERATURE by an unmanned aerial vehicle (UAV) in the context of
In [145], the authors discussed interference management and the IoT. The examiner considered interference mitigation of
capacity enhancement of two-tier IoT networks. The sole both WP-IoT and UAV dynamic charging policy. Numeri-
purpose of their investigation is to control the cognitive cal results described that the proposed method effectively
interference effect on the macro cell besides maintaining curtail the loss of data packets and enhances energy effi-
the femtocell’s optimal capacity. Two novel schemes have ciency besides mitigating the network’s interference. Also,
been proposed to remove the interference effect. i.e., inter- the proposed technique performed much better than bench-
ference nulling based cognitive IA and partial mental IA mark schemes. On the other hand, a proliferation in data
scheme. Consequently, numerical results described that sug- rate is observed in D2D communication when the interfer-
gested methods improve the small cell capacity without dete- ence process is fixed in urban IoT applications by intro-
riorating performance to the primary users. Besides energy ducing a smart context-aware cognitive transmission strategy
efficiency and interference limitations in the novel network where D2D and LTE communication uses the same channel
design, another threatening concern is security. Simultaneous resources [149]. In [150], the authors eliminated NB-IoT
transmission of data signals in close-range leak information interference in the LTE network. The researchers presented
into another useful signal. The leakage of data results in the a probabilistic framework, and numerical results validate
loss of critical and highly secure communication as well as that the recommended algorithm outclasses the conventional
privacy. counterparts in spectrum efficiency, computational complex-
According to the researcher, it is an immediate concern ity, and estimation accuracy. Also, the UEs and BS in LTE
in the real-world scenario; therefore, in [146], the authors systems safeguarded from NB-IoT interference.
presented a covert wireless communication that prevents In recent research, the guard band concept was introduced
the user’s information to expose in front of the opponent in [151], where authors discussed the interference in the
users. A new method was developed in which a transmit- coexistence of LTE UEs and several IoT terminals to a single
ter Alice communicates with a receiver Bob covertly by LTE BS in multiple access uplink. In the co-occurrence and
VOLUME 9, 2021 68953

multiple access scenarios, the high-power LTE BS signal effectively manages the multi-interference with limited
interferes with the IoT receiver. To reduce the interference, knowledge of the PU system in the CSA model. The affinity
LTE offered the guard bands method in IoT. As a result, propagation (AP) algorithm idea was presented to compre-
the guard bands are not stringent to minimize the strong hend the clustering module. Consequently, the AP algorithm
interference in IoT receivers from the LTE signal. Hence delivers more excellent performance in the clustering of
system performance is compromised. Similarly, in [152], multi-interference signals compared to other algorithms. Due
the authors analyze the guard band’s interferences in the to the enormous connectivity of IoT devices and simultane-
NB-IoT system’s co-existence and the LTE network. Results ous communication, interference problem limits the potential
conclude that the LTE uplink signal with 15 kHz SCS of link connectivity and resources. Therefore, researchers
interferes with the NB-IoT uplink signal with 3.75 kHz addressed the interference mitigation procedure by con-
SCS. In [153] authors suggested various models such as sidering the 3D antenna radiation pattern of the dipole
SIR, LIBA, and an epidemic for interference diffusion. The antenna [159]. Additionally, an antenna selection method is
simulation yield infected and susceptible node congregates furnished to decide a suitable antenna pattern at the trans-
to zero, while in each diffusion, the number of replaced mitter. The simulation results authenticate that the recom-
nodes converges to the total number of underlying diffu- mended scheme delivers high performance in scenarios such
sion sets. In [154], the authors proposed an Interference as ground to air and air to air. In particular, when IoT devices
Management (IM) scheme called interference steering (IS) are well-distributed in the surface and spatial domain besides
in IoT. The results proved that IS outperformed all other the height of aerial equipment is higher.
existing IM techniques in the context of spectral efficiency. Table 5 shows the summary of the research work of IoT in
Besides, an ICI problem that IoT terminals received is dis- the literature discussed in this section.
cussed in [155]. A relay-assisted communication approach in
which a relay selection algorithm is designed helps expand V. PROPOSED METHODOLOGIES BY 3GPP
device throughput besides spectrum resources availability. This review article discussed several recent works on
The novel scheme assists in delivering optimum signals in interference mitigation and numerous new techniques that are
the positively ICI-affected area. By applying the suggested in progress on interference removal. Additionally, research
scheme, results described that a substantial number of IoT communities, universities, related industries, and main stan-
nodes that served simultaneously increases up to 17.07%. dardization bodies work together to deliver the future network
At the same time, system throughput reached up to 43.96%. demand by overcoming the various wireless cellular network
The interference characteristics are changed in 3D environ- obstacles. The 3GPP groups have become the principal stan-
ments in comparison with the existing terrestrial IoT systems. dard development authority for cellular systems and are con-
Research on aerial IoT devices was conducted based on an tinually evolving through mobile generations. Considering
analysis and the mitigation of interference [156]. A cross- the 5G and beyond HetNet system, 3GPP recently contributed
dipole antenna setting at the transmitter, either on the z or various methods and procedures to mitigate different types of
y-axis, depends on the receiver setting offered. Also, a 3D interferences under development for next-generation mobile
topology channel model centered on the device location was communication.
created. The results outcome explained that the cross-dipole
antenna method outperforms the process that uses only the A. PILOT CONTAMINATION
z-dipole antenna. The orthogonal uplink pilot signal supports the user to
Due to UEs’ extensive growth, the management and guess the channel in the same cell. For efficient resource
support of high data rates in a mass transit system is also management, the same pilot sequence could be reused by
critical. Consequently, in [157], the authors demonstrated the the neighboring cell user. The frequency reuse has shown
mIoT-small cell network’s deployment in the metropolitan great strength for the efficient use of the available spectrum
city bus system. The investigators raised the concern of the band; nonetheless, it is severely affected by the inevitable
dynamic interference problem that makes the resource allo- CCI [160]. Since massive multiple-input-multiple-output
cation challengeable in mIoT-mSC. It severely affects the (M-MIMO) is one of the 5G multi-cell network’s main
coverage area and capacity of a cellular network. The authors enabling technology. In M-MIMO TDD mode, the pilot sym-
offered LSTM networks that forecast city bus location and bols are highly exposed to contamination due to the reuse
then identify interference patterns between mSCs within a of resources and introduce ICI. The interference effect in a
road section. It also dispensed the Threshold Percentage signal decreases the achievable rates and overall efficiency
Dependent Interference Graph (TPDIG) algorithm for city of the spectrum. Extensive literature work dedicated to elim-
buses mounted with mSCs. Moreover, a comparative analysis inating ICI with a primary focus on reducing pilot symbol
of resource allocation using TPDIG, TIDIG, and GPSDIG contamination in channel estimation.
is provided based on interference; the results advised that to These techniques have helped reduce the bit error
avoid the interference’s deterministic behavior, interference rate (BER) issue and enhance a spectrum’s efficiency.
percentage is used over time-based. In [158], authors studied Earlier, most of the researchers believed and considered
the architect of iterative receiver for the SU system that non-orthogonal signals as the only source of pilot issues.
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TABLE 5. Summary of research work of IoT in literature.
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TABLE 5. (Continued.) Summary of research work of IoT in literature.
Other sources of pilot intrusions have recently been rec- Various interference minimization schemes have been
ognized, for instance, hardware impairments distortion. developed for the collaborative operation of cellular users
Subsequently, a non-reciprocal transceiver is highly vulner- and aerial vehicles scenario. Such as [164] the researchers
able to interference because of the RF chain’s internal clock analyzed and understood the performance of M-MIMO, 3D
design [161]. Also, the pilot symbol interference coordination beamforming, closed-loop power control in the presence of
in in-band FD (IBFD) M-MIMO networks is assumed to be UAVs in mobile networks. Likewise, in articles [165], [166],
complex due to pilot reuse factor 1 and resulted in SI. Thus, multi-beam UAV for cellular uplink and aerial-ground ICIC
a robust algorithm is required to completely mitigate pilot design for UAV uplink signal proposed respectively to more
contamination in IBFD M-MIMO systems [162]. effectively eliminate the strong interferences. In contrast,
a cooperative NOMA technique has been presented further
B. UNMANNED AERIAL VEHICLES
to enhance the ICIC design performance in [167]. However,
none of the work was performed on the cellular downlink
Cellular-connected UAVs will play an important role in 5G
transmission interference issues with aerial machines.
and beyond. The 3GPP standardization shows a keen inter-
est in UAVs for future wireless communication deployment.
Currently, the 3GPP member’s community practice various C. BS IDENTIFICATION
designs and protocols for UAVs’ proper deployment in the The coordination of all MBSs and the users associated with
coming practical scenario. The promising feature of cellular- them is essential to avoid ICI. The mutual coordination of all
based UAVs is that it allowed large coverage area spatially MBSs with their neighboring cells via the backhaul X2 inter-
that delivers real-time video surveillance anytime and any- face was presented. However, the identification and trans-
where. Nevertheless, frequency reuse is essential due to the mission of interferer BSs over the air are critical, though the
scarcity of resources, and sharing the spectrum between reference signal’s advancement and variation in low transmis-
UAVs and UEs induces ICI and downgrades the system sion power can easily avoid the interference problem [168].
performance [163]. In dense mmWave networks, a study has been conducted
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to avoid inter-BS interference by choosing a downstream for cellular-vehicular-to-everything (C-V2X) communica-

transmission coordination scheme and decision made on the tion [179]. In fast-moving vehicles, mobility management
schedule of packet delivery based on SINR level [169]. Few and channel estimation are two potential issues for better
authors discussed the inter-BS beam coordination process vehicular communication performance. However, the pro-
to increase the network capacity by providing coordination posed technique helped to reduce Doppler shifts by increas-
algorithms among the several BSs [170]. In contrast, spec- ing the number of demodulation reference symbols per
trum pooling has been investigated by using a centralized frame. Besides, rising in demodulation of the reference sig-
beam coordination technique [171]. Although these algo- nal sequence randomization for heavy interference reduces
rithms have helped to transfer channel information efficiently, high vehicular density [180]. 3GPP’s C-V2X also specified
the framework’s viability extensively relies on the latency of protocols for short-range vehicle-to-vehicle (V2V) commu-
the information exchanged and more work needed to assess nications PC5 [181]. In this regard, an air interface called
the practical effects. sidelink/PC5 was utilized for direct contact between vehicles.
Along with wide-area vehicle-to-network (V2N) communi-
D. CONTROL CHANNEL
cation (allows automobiles to communicate with BS). In an
Control channels are part of uplink and downlink data trans-
Intelligent transportation system (ITS), end-to-end transmis-
mission. It carries controlling information of the UEs; for
sion latency and viability are exposed in V2X technologies
instance, the physical uplink control channel (PUCCH) con-
and the safety of traffic and bandwidth efficiency of applica-
veys information about CQI, ACK/NAK, and the HARQ
tions is suffered to react to environmental changes [182].
mechanism. The control channels play an integral role in
channel estimation, signal synchronization, cell searching,
G. NARROW BEAM LINK ACQUISITION
and so on. However, the separation between these uplink and
The large antenna arrays and mm-wave transmission
downlink control channels is crucial to avoid. Thus, 3GPP
increases the importance of narrow and focused beams. The
proposed that MBS’s power intensity should be less than
mm-wave transmission is highly directional and sensitive to
the given threshold power and significantly minimize the
the misalignment of beams. Therefore, the researchers and
interference changes [172].
standardization industries flourished new approaches such as
Many studies have recently been devoted to the data and
Beamforming and Beamsteering [183],. The proposed tech-
control channel interferences when small cells are deployed
niques helped avoid the interference phenomenon in the mas-
in a macro cell overlay mode network [173], [174]. In the con-
sive MIMO system and provide dedicated narrow beams to
trol channel context, many of these researches have identified
each user yet regular large multiple beams from the antennas
coverage hole’s existence when femtocells are configured in
produce IBI. Also, narrow beams are highly vulnerable in
a macro-cell overlay wireless network. To reduce the effect
establishing a connection between D2D users and MBSs,
of control channel interferences, 3GPP suggested that femto-
particularly in high mobility. To mitigate the IBI issue and
cells should be operated in only closed access mode, i.e., each
reduce sidelobe level amplitude tapering method has been
femtocell construct and maintain UEs list that is permissible
investigated [184]. Some investigators advised that standard
to access the services [175].
forms of zero-forcing can also provide excellent results in
E. RECEIVER DESIGN suppressing interference in the desired direction [185], [186].
To suppress the inter-user and intra-cell interferences at the But both are dependent on the adjustment of amplitude exci-
user terminal, the 3GPP proposed the technique called Code tation level over the antennas for proper beam formation.
Word Level Interference Cancellation (CW-IC). The standard Table 6 shows the summary of the proposed methodologies
development authority further added that ICI could also be by 3GPP discussed in this section.
minimized with Minimum Mean Square Error-Interference
Rejection Combining (MMSE-IRC) in an ultra-dense mul- VI. FUTURE RESEARCH WORK
tiple small cell network methodologies [176]. Much of The parallel operation of HetNet, RN, D2D, and IoT
research work has lately been conducted on the user-terminal technologies in the current 5G wireless communication net-
interference avoidance in multi-user FD MIMO systems. work improves the system’s functioning and makes future
It has been primarily focused on the mitigation of inter-user technologies more concrete and reliable. It also enhances
interference by proposing Eigen-BF [177] and non-linear the user’s experience and efficiency of the cellular network
processing [178] to attain maximum transmission rate. Nev- effectively. The future challenges of the discussed topics are
ertheless, both techniques have extremely relied on the summarized below.
eigenmode of the channel, and terminal antenna element
spacing also significantly impacts the performance of the A. HETEROGENEOUS NETWORK
transmission. In mm-wave enabled HetNet system of 5G and beyond,
maintaining the link quality is a crucial challenge, especially
F. CELLULAR-V2X with the vehicular user. Because the mm-wave are highly
The 3GPP introduced two modes, i.e., BS scheduled susceptible to blockages of the natural and environmental
and direct scheduled in release 14 of the LTE standards sources or any other moving heavy vehicles, in cellular
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TABLE 6. Summary of proposed methodologies by 3GPP.
communication, transmission orthogonality of sub-carriers by using the computationally effective algorithm in the pres-
can be lost when movement multipath is present. There- ence of multi-carrier interference in the HetNet system. The
fore, many investigations have been performed to mitigate framework consists of fractional programming, game theory,
vehicular cellular user interferences. For instance, in [187], and learning tools to handle GEE’s limitations. The numerical
the authors presented a unique idea for the mm-wave signal test proven that imposing minimum rate constraints reduce
reliability and overall delay problem by deploying machine the GEE performance, however, it completely supports the
learning algorithms to predict blockage efficiently and deliv- minimum rate for communication for all users in a multi-cell
ers handoff to adjacent BS without interrupting the connec- network.
tion. The simulation results proved that the proposed method
provides optimum results. However, the technique was devel- B. RELAY NODE
oped for a single user and could be challenging for multi-user In conventional railway systems, they are still using previous
systems. Similarly, due to the simultaneous transmission of cellular technologies that are undesirable for future demand
licensed and unlicensed spectrum users, the BS performs con- of data rate and QoS. Therefore, an advancement in the
tinuous, seamless switching. The multiple switching between railway structure and new technologies leads to smart train
different technologies regularly introduces interference, networks. Modern railway networks are required to meet
fading, and SNR problems. the upcoming demand of higher throughput users, massive
To minimize the issue, a feed-forward neural network connectivity of various wireless devices, scalability, and reli-
technique was implemented for system reliability and seam- ability. An RN deployment in railways would be smart for
less coverage [188]. In [189], the authors proposed a future moving platform scenarios [192]. Therefore, an FD
collusion-resistant spectrum auction-based KNN algorithm RN in trains or mass communication systems in urban and
for interference avoidance to implement a dynamic spec- rural areas is exciting for the researchers. However, the fast-
trum allocation technique. The simulation results show moving platform creates a blockage, continuous discon-
that the suggested algorithm effectively solved the problem nection, multipath and multi-user interferences, and other
and highly recommendable for implementation. Moreover, pertinent issues. A robust scheme is demandable. In [193],
an essential feature of the 5G and beyond network is NFV. the authors proposed an ML(Machine Learning) based selec-
The 5G cellular system uses it to create a more flexible tion technique in hybrid multi-hop networks that adaptively
environment by partitioning the network function from its choose each relay’s forwarding technique. The minimum
hardware components. Nonetheless, the virtualized network transmit power for all transmitting nodes has been derived,
of NFV causes unauthorized intrusion or data leakage [190]. and the method is considered appropriate for the ultra-dense
It is a severe security threat, and to furnish secure processing network. Hence, the suggested algorithm provides excellent
of every data, a suitable machine learning mechanism is results, but the node’s location and relay residual power are
highly desirable. Similarly, in [191], the authors presented an key selection factors and any small error will create chaos in
approach to enhance system global energy efficiency (GEE) the network. Additionally, an algorithm avoided the high FD
68958 VOLUME 9, 2021

transmission complexity which is a major concern for future fully connected Quadratic Unconstrained Binary Optimiza-
interference-oriented networks. tion (QUBO) problems much faster than classical computing,
DA can realize various potential service-oriented applica-
C. DEVICE-TO-DEVICE tions for the customized service-oriented optimization prob-
In D2D, a new interference management method is lems, including enhancing smart mobility services with high
required for directional interference in the current mm-wave QoS, realizing virtualization and softwarization for smart
5G network for multiple D2D links. Besides, 5G multi-tier cities with lower energy consumption, and enabling various
HetNet with D2D communication, management of interfer- Real-time services which provide information to the users
ences, and resource allocation problems for underlay shared instantly. In this way, DA enables various IoT-based applica-
spectrum with cellular users are more challenging than exist- tions with optimal efficiency to realize potentially unexplored
ing methods. The management and mitigation of interfer- future network services.
ences among low-power wireless sensors, small cells, and Besides, some other aspects where research is immensely
macro-cell mobile links, and D2D links are still challeng- required are storage capacity for Bigdata, coming from var-
ing and have space for further enhancement. Because, level ious IoT sources, and security. Typically, due to the massive
and conditions of interferences are different in different exchange of data between nodes with the help of co-existence
tiers of HetNet due to several access limitations (such as technologies, security is one major concern for future com-
public, transport, private, and so on) [194]. Privacy and munication that protects user data and other applications
authentication are essential concerns in self-governing D2D from various other devices’ illegal involvement. To improve
communication. A lot of critical information shared over the maximum coupling, coverage enhancement is critical to
cellular networks could be corrupted, hacked, or misplaced. hold effectively tactile internet and multimedia applications.
Therefore, this is still an open topic for future concerns. Similarly, backup security is also required for high-level user
D2D communication is considered a coordinated reception data safety and to avoid any intrusion from hackers [199].
and transmission of various devices, and a moving object Current supervised and unsupervised ML techniques are sig-
creates instability in the D2D link. Hence an agile scheme nificantly exposed to detect the attacks due to oversampling
that maintains network stability with high profile security is and insufficient training data. Therefore, a backup security
essential. Also, a robust method is demandable for D2D com- solution with the ML method is demandable to provide secure
munication where cellular network coverage is unavailable. IoT connections. Another future concern in IoT is a more
In [195], authors discussed various D2D enabled threats in the concrete system required for nodes to perform underlay dif-
HetNet system, for instance, resource scheduling, power and ferent operating situations with a decrease in energy while
interference management, spectral efficiency, and so on. The maximizing network and cloud systems [200]. The Time of
Distributed Artificial Intelligence Solution (DAIS) algorithm Flight (ToF) imaging approach is exclusively using in a prac-
was proposed, and simulation results proved that the sug- tical world that provides depth maps in real-time. Most of the
gested method delivers low computational work and greater work performed on hardware modification and complex algo-
spectrum efficiency. For future improvement, the researchers rithms is costly to rectify the problem. In [201], the authors
considered implementing the same Belief-Desire-Intention presented an encoder-decoder neural network technique,
(BDI) intelligent agents with extended capabilities (BDIx) which significantly reduces the reflectance and provides good
framework with a more robust AI scheme to evaluate system results for the real world scenario. Nonetheless, the suggested
performance in a more complex environment by introducing method failed to cancel the Message Passing Interface (MPI)
mobile UEs. Besides advancing stochastic geometry-based when objects are very close to the camera.
methods, they are required to create a more uncertain envi-
ronment for the downlink and uplink coverage and low power VII. CONCLUSION
consumption for UAVs, D2D, and cellular UEs enabled For next-generation communication, it is expected that bil-
HetNet system [196]. lions of devices communicating with each other require
higher throughput while maintaining the spectrum frequency
D. INTERNET OF THINGS and QoS for each user. Due to the proliferation of data
An advanced and intelligent 5G IoT network is essential demand and the rapid growth in the numbers of users and
that can process a high volume of data with less power electronic devices directed towards the ultra-dense commu-
consumption, lower latency, and good connection reliabil- nication network, the 5G and beyond network can also be
ity among various devices as billions of devices are inter- termed as a multi-tier HetNet cellular network. Consequently,
linked and a large amount of traffic using by all connected the network performance from the simultaneous operation
gadgets via the Internet. Hence, a stringent management of multi-small cells within a multi-tier network is heavily
scheme in IoT to handle enormous automated equipment is degraded with various interferences. We then investigated
needed [197]. In such a challenging context, Digital Annealer the interference in the multi-tier HetNet cellular network.
(DA), a quantum-inspired technology applying the concept Furthermore, this study broadly discussed several types
of simulated annealing for combinatorial optimization prob- of interferences classified according to their deployment
lems, would be a promising solution [198]. By solving and propagation characteristics, such as co- and cross-tier
VOLUME 9, 2021 68959

interferences in HetNet and homogeneous and control chan- [14] X. Ge, S. Tu, G. Mao, and C. X. Wang, ‘‘5G ultra-dense cellular
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P. Sroka, K. Wesolowski, D. Lister, and I. Thibault, ‘‘3GPP C-V2X the B.S. degree in electronic engineering from the
and IEEE 802.11p for vehicle-to-vehicle communications in highway Sir Syed University of Engineering and Technol-
platooning scenarios,’’ Ad Hoc Netw., vol. 74, pp. 17–29, May 2018. ogy, Karachi, Pakistan, in 2010, and the M.Eng.St.
[181] IEEE Standart for Information Technology, IEEE Standard 802.11 p- degree in telecommunications engineering from
2010, IEEE Computer Society, 2010. the Faculty of Information Science and Tech-
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vey,’’ Comput. Netw., vol. 169, Mar. 2020, Art. no. 107093. NSW, Australia, in July 2013. He is currently
[183] M. D. Foegelle, ‘‘New and continuing measurement challenges for 5G serving as a Network Engineer for Orient Energy
mmwave and beamforming technologies,’’ in Proc. 13th Eur. Conf. Anten- Systems Pvt. Ltd., Karachi. His research interests
nas Propag. (EuCAP), 2019, pp. 1–5. include interference management, millimeter-wave communication, massive
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[186] M. Majidzadeh, A. Moilanen, N. Tervo, H. Pennanen, A. Tolli, and FAIZAN QAMAR received the B.E. degree in
M. Latva-Aho, ‘‘Partially connected hybrid beamforming for large electronics from Hamdard University, Karachi,
antenna arrays in multi-user MISO systems,’’ in Proc. IEEE 28th Annu. Pakistan, in 2010, the M.E. degree in telecommu-
Int. Symp. Pers., Indoor, Mobile Radio Commun. (PIMRC), Oct. 2017, nication from NED University, Karachi, in 2013,
pp. 1–6. and the Ph.D. degree in wireless networks from
[187] S. Sonmez, I. Shayea, S. A. Khan, and A. Alhammadi, ‘‘Handover the Faculty of Engineering, University of Malaya,
management for next-generation wireless networks: A brief overview,’’ Kuala Lumpur, Malaysia, in October 2019. He is
in Proc. IEEE Microw. Theory Techn. Wireless Commun. (MTTW), vol. 1, currently serving as a Senior Lecturer for the Fac-
Oct. 2020, pp. 35–40.
ulty of Information Science and Technology, Uni-
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versiti Kebangsaan Malaysia (UKM), Malaysia.
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He has more than nine years of research and teaching experience. He has
Adv. Wireless Commun. (SPAWC), Jul. 2019, pp. 1–5. authored and coauthored numerous ISI & Scopus journal articles and IEEE
[189] F. Zhao and Q. Tang, ‘‘A KNN learning algorithm for collusion- conference papers. He is also serving as a reviewer in more than 15 high rep-
resistant spectrum auction in small cell networks,’’ IEEE Access, vol. 6, utation journals with different publishers, such as IEEE, Elsevier, Springer,
pp. 45796–45803, 2018. Wiley, and Hindawi. His research interests include interference management,
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tems: Challenges and machine learning approaches,’’ Int. J. Mach. Learn. D2D communication, and quality of service enhancement for future wireless
Cybern., vol. 12, no. 2, pp. 1–47, 2020. networks.
68964 VOLUME 9, 2021

FAISAL AHMED received the bachelor’s degree 2013 admission to the Graduate School of Global Information and Telecom-
in computer engineering from the Usman Insti- munication Studies (GITS), Waseda University. He was a recipient of young
tute of Technology, Karachi, Pakistan, the master’s author recognition awarded by the International Telecommunication Union
degree in electrical engineering from the Blekinge for the young first-author of highly potential research under 30 years old,
Institute of Technology, Karlskrona, Sweden, and and actively contributed to many ITU-T standardization activities related
the Ph.D. degree from the Tallinn University of to Information-Centric Networking (ICN) and Future Internet architecture.
Technology (Tal Tech.), Estonia. His Ph.D. thesis Besides, he had experience with editorial and conference organizations.
deals with the design of energy prediction methods He has served as an Organizing Committee Member (OCM) for the Inter-
combined with transient computing techniques for national Conference on Computer Science and Software Engineering 2020,
the implementation of energy-efficient in the con- the 3rd International Conference on Computer Science & Cloud Computing
text of the Internet of Things (IoT). He has published various articles on the 2020, and the Asia Pacific Society for Computing and Information Tech-
web of science and one is an Oxford press. After his Ph.D., he moved to nology (APSCIT) Annual Meeting 2019, as well as a Session Chair for
Sweden and joined Ericson as a Team Leader of the Network Infrastructure IEEE ICCC 2017. He has been a Guest Editor and a Keynote Speaker with
(EEINFRA). In 2019, he joined the University of Tartu in the capacity of a the 2nd International Conference on Artificial Intelligence and Advanced
Junior Lecturer at the Computer Science Department. Manufacturing (AIAM 2020), Shanghai, China, and the IEEE International
Conference on Artificial Intelligence and Computer Applications (ICAICA
2021), Dalian, China, June 2021. He has also served as the sole and lead
Editor of a book entitled Congestion Control: Design, Applications and
Protocols (Nova Science Publishers, March 2021). In addition, he is also
QUANG NGOC NGUYEN (Member, IEEE) a Topic Editor of Electronics, Switzerland, and a Series Editor of Atlantis
received the B.Eng. degree in information Highlights in Engineering and Advances in Engineering Research (WoS).
technology, Honor the Computer Science Pro-
gram conducted in English from the Posts
and Telecommunications Institute of Technology ROSILAH HASSAN (Senior Member, IEEE)
(PTIT), Hanoi, Vietnam, in 2012, and the M.Sc. received the degree in electronic engineering
and Ph.D. degrees from Waseda University, Tokyo, from Hanyang University, Seoul, South Korea,
Japan, in 2015 and 2019, respectively. He became the M.E.E. degree in computer and communication
a one of the youngest faculty members of PTIT. from Universiti Kebangsaan Malaysia (UKM),
He has been a Research Associate with Waseda Malaysia, in 1999, and the Ph.D. degree in mobile
University, since 2018. He is currently an Assistant Professor with the communication from the University of Strath-
Faculty of Science and Engineering, Waseda University. He has hosted and clyde, U.K., in 2008. She worked as an Engineer
participated in many research projects, including the collaborative EU-Japan with Samsung Electronic Malaysia, Seremban,
research projects funded by the European Commission’s Horizon 2020 Pro- Malaysia, in 1997. She is currently an Associate
gramme (e.g., the Green ICN and 5G! Pagoda projects), and the Ministry Professor with UKM. She is also a Senior Lecturer with UKM for more
of Internal Affairs and Communication (MIC) of Japan. His research has than 20 years. She is also with the Centre for Cyber Security, Faculty of
also been funded by the Ministry of Economy, Trade and Industry (METI) Information Science and Technology (FTSM), UKM. She is also the Head of
of Japan, Waseda University, and Fujitsu Ltd. and Fujitsu Lab. His research the Network Communication Technology (NCT) Laboratory in her Faculty.
interests include network design, green networking, future Internet, wireless Her research interests include mobile communications, networking, the IoT,
communication systems, beyond 5G (B5G)/6G, the Internet of Things (IoT), ICT, 4IR, and big data. She has had experience as an External Examiner
sensor networks, data science, artificial intelligence (AI), applied intelli- for Ph.D. and master’s for both national and international levels. She is
gence, the Internet of Vehicles (IoV), combinatorial optimization/multi- also an active member of IEEE, WIE, Malaysia Society for Engineering
objective optimization (MOO), network slicing, information systems, game (MySET), Malaysian Board of Technologists (MBoT), and IET. She is
theory, and technology standardization. He is also a Senior Member of the also a reviewer of several IEEE conferences and national and international
European Alliance for Innovation (EAI). Also, he has been a reviewer of top journals. She had supervised to completion to date seven Ph.D. students and
journals in the field of communications and computer engineering as well as 12 ongoing Ph.D. students. She also had experience as the Deputy Director
computer science, e.g., IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS of Academic Entrepreneurship and Innovation for seven years at University.
(JSAC), IEEE INTERNET OF THINGS JOURNAL, Computer Science Review, IEEE Her task is to organize the compulsory course for 13 faculties with more than
ACCESS, Sensors (Basel, Switzerland), and IEEE Communications Standards 5000 students.
Magazine. He was the sole Awardee of Asia Special Scholarship for Fall
VOLUME 9, 2021 68965

Interference Management in 5G and Beyond Network: Requirements, Challenges and Future Directions

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Received January 22, 2021, accepted April 4, 2021, date of publication April 15, 2021, date of current version

May 14, 2021.

Interference Management in 5G and Beyond

I. INTRODUCTION other simultaneously. The explosive growth in user data

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however, its performance is extremely descended due to

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efficiency. However, due to the frequency offsets, the signal

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TABLE 1. Specification of different elements in HetNet. 1) INTERFERENCES IN HetNet

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FIGURE 7. 5G cellular HetNet structure.

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FIGURE 8. Co-tier & cross-tier interferences cellular HetNet structure.

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TABLE 2. Summary of research work of HetNet in literature.

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TABLE 2. (Continued.) Summary of research work of HetNet in literature.

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need more resource blocks since two channels are required

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FIGURE 10. IRI, ICI, RN, and D2D links in RN.

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TABLE 3. Summary of research work of relay node in literature.

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TABLE 3. (Continued.) Summary of research work of relay node in literature.

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FIGURE 11. D2D communication in multi-tier HetNet.

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FIGURE 12. D2D interferences in 5G HetNet.

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TABLE 4. Summary of research work of D2D in literature.

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TABLE 4. (Continued.) Summary of research work of D2D in literature.

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FIGURE 13. Multi-tier 5G IoT network.

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TABLE 5. Summary of research work of IoT in literature.

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TABLE 5. (Continued.) Summary of research work of IoT in literature.

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to avoid inter-BS interference by choosing a downstream for cellular-vehicular-to-everything (C-V2X) communica-

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TABLE 6. Summary of proposed methodologies by 3GPP.

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