2016 Loughborough Antennas & Propagation Conference (LAPC)

A Flexible Printed Millimetre-Wave Beamforming

Network for WiGig and 5G Wireless Subsystems
Ardavan Rahimian, Akram Alomainy, and Yasir Alfadhl
School of Electronic Engineering and Computer Science
Queen Mary University of London
London E1 4NS, UK
a.rahimian@qmul.ac.uk

Abstract—This paper presents a novel millimetre-wave (mm- a large number of independently operating channels, wireless
wave) array beamforming network (BFN) design, analysis, and backhauling, and point-to-point wireless communications. The
implementation based on the Rotman lens antenna array feeding, intended mm-wave band provides high-capacity; in the order of
intended for operation in the unlicensed 60-GHz frequency band multiplies of Gbps data rate; low-latency, and also noticeable
for the potential employment in the fifth-generation (5G) cellular immunity to interference, and although the band is vulnerable
communications and WiGig technology. The primary objective of to the oxygen absorption and also rain attenuation, it is in fact
the work is to thoroughly discuss the advanced cellular network, beneficial to the wireless RF links since it further attenuates the
and to further develop a flexible radio frequency (RF) component interference between the neighboring connections, effectively
based on the polyethylene terephthalate (PET) substrate using
resulting in the small cell network isolation increase, as well as
the in-house inkjet materials printing process, for the mm-wave
beam steering concept verification. The RF evaluation modeling
greater frequency reuse factor, and also high-capacity RF links
demonstrates the appropriateness to develop a high-performance along with interference mitigation and multipath suppression
and well-established design for the WiGig and 5G systems, along [i.e., based on the directional antenna systems with high-gain;
with the analysis of the RF characteristics. The CST Microwave since this operating factor is inversely proportional to its half-
Studio and MATLAB software are employed in order to conduct power beamwidth (i.e., the angle between the –3 dB points of
the modelling and full-wave electromagnetic (EM) simulations. the main lobe according to the peak effective radiated power),
and therefore, these types of RF antennas are mostly associated
Keywords—5G network; beamforming network; beam steering; with narrow beamwidth with properly aligned beams] [1]. The
flexible electronics; mm-wave component; Rotman lens; WiGig. high-performance links hold much potential to be employed as
the line-of-sight (LoS) wireless backhaul with the quasi-static
I. INTRODUCTION AND BACKGROUND time-varying RF channel characteristics among small cell units
In order to maintain the advanced cellular communication with the same spectrum reuse for a successful RF transmission.
systems sustainability, and to effectively manage the potential Apparently, this is due for each of the cell node to combine its
network resources, it is vital to develop the high-performance data with that received from other cell nodes before forwarding
infrastructures for the next generation of the intelligent wireless it to the cell aggregation point, and to provide flexible network
solutions and services deployment. Emerging components are centralisation for the dynamic adaptation of the cost-efficient
implanted across the frequency band; therefore, for the ultimate backhaul routes; as a major critical part of the 5G infrastructure
systems enhancements, it is crucial to effectively rethink of the with new spatial processing techniques and polarisation exploit
necessary fundamental improvements that are needed for the [2, 3]. The potential for 60-GHz RF links intended for the case
intended wireless communication systems and channels in the of the small cells in urban environments on the order of 200 m,
propagation media; in response to the dynamic environmental is thoroughly studied through simulations and measurements;
demands to both extend the range and coverage area. Novel RF in order to dispel some common myths on the band practicality
system design and enhanced link planning are of key parts of for the backhaul deployment (i.e., atmosphere and rain effects),
the cellular development for the high-performance 5G systems. and to confirm the feasibility to deliver significant increase in
Therefore, in order to meet the enormous growth demands for the network capacity for flexible 5G small cell scenarios, in the
the mobile broadband, mm-wave frequencies, and particularly ultra-dense networks (UDNs) with the full-duplex functionality
the 60-GHz frequency band, offer potential advantages to boost (i.e., the simultaneous radio transmission and reception) [4–7].
the 5G cellular network capacity. The Federal Communications The remainder of the investigation is organised as follows.
Commission (FCC)- and Office of Communications (Ofcom)- In Section II, the overview and thorough discussion of the 5G
approved 60-GHz band (i.e., 57-64 GHz); as the governmental cellular communications is provided. In Section III, the design
regulatory agencies for broadcasting and telecommunications; of the proposed printed Rotman lens BFN component, which is
provides up to the 7-GHz of license-free continuous spectrum, realised based on the inkjet fabrication method, is presented,
resulting in the provision of short-range multi-gigabit services along with the evaluation of the performance. To the best of the
deployment, including: Wireless Gigabit Alliance (WiGig) and authors’ knowledge, this is the first attempt to implement such
high-definition (HD) video streaming using the IEEE 802.11ad a flexible array beamformer using this implementation process.
standard; high-speed wireless data centre (WDC) connectivity, The paper is then concluded in Section IV.
and in general, data centre network (DCN) link processing with

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 2016 Loughborough Antennas & Propagation Conference (LAPC)

II. 5G CELLULAR NETWORK ANALYSIS

In the upcoming and evolving advanced 5G standards, the
radio access networks (RANs) are undergoing a paradigm shift
[8, 9]. Cellular systems are envisioned to inherently provide
much higher data rates, along with delivering state-of-the-art
enhancements [10, 11] in order to improve the wireless channel
capacity, and to ensure wider system spectrum efficiency with
a minimum possible bit error rate (BER), for the highly reliable
RF link planning, and the high-performance maintenance [12].
The 5G cellular communication networks will operate based on
the multi-tier architecture, and will incorporate the high-speed
backhaul, advanced physical (PHY) layer technologies, such as
large-scale multiple-input multiple-output (MIMO) antennas,
and intelligent three-dimensional (3D) beamforming [13, 14].
The 5G cellular architecture will consist of the energy-efficient
network units, including: RF base stations (BSs), access points
Fig. 1. The 5G-centric UDN topology based on the mm-wave links [20].
(APs), relay stations (RSs), nodes, and cells (i.e., for efficient
provision in terms of the required high data rates per unit area). In the primary HetNet cellular system with APs, the small
The network units will execute based on the specified resource cell data from the core network is further obtained via wireless
allocation (RA), cell association (CA), coordinated multipoint backhaul, based on the antenna systems that are responsible for
(CoMP; dynamic coordination and scheduling among multiple transmitting the mm-wave backhaul signals to the RF Aps. The
separated network points), and link adaptation (LA; modulation linear transmit processing is then applied at the backhaul hub,
and channel coding rate matching according to the link quality) in order to deliver user data to the small cell APs using wireless
strategies and mechanisms; in order to guarantee an acceptable backhaul links, based on the QoS requirements of its small cell
system throughput and quality of service (QoS) for the mobile users, which can be fed back to the core network [23]. Hence,
terminals and cell-edge users [15]. The multi-tier segments will the small cell network units are able to provide ubiquitous high
be provided with the flexible capacities and characteristics, and data rate services across the whole HetNet system, in order to
will form the heterogeneous networks (HetNets) topology. The offload huge volumes of data and a large number of users from
HetNets will mainly consist of a large number of the RSs, and macrocells to small cells; based on utilisation of the efficient
the low-power small cells (e.g., pico and femtocells) deployed transmit processing, and the hybrid beamforming systems [24].
inside the macrocell radiowave propagation range; in order to The high-speed wireless transmission link between the small
potentially improve the network coverage and system capacity, cell and the network controller should also provide the required
and also to reduce the RF transmit power in both the uplink and flexibility in order to handle the obstacles and reflected signals;
downlink transmissions, in the 5G cellular network ultra-dense based on the robust model which preserves orthogonality in the
areas with higher data rate demands [16, 17]. This is due to the operating RF domain through partitioning a highly frequency
short distance between the user equipments (UEs) and the low- selective channel to a group of nonselective narrowband ones.
power BSs in the small cell that results in notably reductions in Hence, the modified system transmission parameters should be
the radiation exposure from a UE connected to a BS, which has accurately designed for high-throughput network performance,
similar RF transmit power characteristics as with the UEs [18]. and high-capacity and reliable network signalling is required in
The hybrid versatility of the small cell will then enable the CA support of data and signalling exchanges among the APs which
to conduct the precise RA; in order to ensure that the loss rate consequently enhances the overall throughput based on the RF
due to interference will not dominate the gain rate due to higher planning and CoMP transmission techniques; as part of the 5G
spatial reuse (i.e. the total number of accommodated concurrent UDN with the optimised resource scheduling and interference
transmissions) in the HetNet-UDN (i.e. Fig. 1) range supported management policies. Hence, deployment of the efficient links
by the multi-gigabit high-capacity mm-wave channels [19, 20]. in support of the coordination and signalling among the small
It is also worth noting that as part of the HetNet paradigm, the cell nodes based on the mm-wave beamforming antennas are of
mm-wave radio technologies will be integrated into traditional crucial importance in order to obtain the necessary channel link
cellular systems to introduce additional frequencies that can be reliability in terms of the system threshold gain. Therefore, the
utilised in densely urban areas for the purpose of flexible short- radiowave propagation should then be utilised to establish the
and medium-range full-duplex communications. The flexibility high-efficient RF links for the short-range HetNet backhauling
of the 5G networks will also depend on the advanced wireless infrastructures. In the link deployment, the primary segments,
backhaul protocols and front-ends, with self-transmitted signal including the transmitting-, and receiving-end, and the channel
cancellation and smart RF spectrum sharing mechanisms [21]. should be precisely developed and analysed. At the UE level,
The wireless backhauling operates based on the intermediate V-band integrated switched-beam systems are considered for
network which incorporates the radio links between the RAN the advanced low-cost and low-power module deployment in
and the cellular core network, and the cell gateway provides the the 5G autonomous devices. Furthermore, at the AP-level, mm-
connectivity for small cells to the backhaul, as an aggregation wave systems with the beam steering solutions are then needed
point based on the communication. Therefore, the mechanism to provide both the extended range and angular coverage, along
results in the cell scalability improvement, and in the network with the flexibility to enable the spatial multiplexing, as well as
signalling load, along with the system interface reductions [22]. the RF system interference and AP displacement mitigation.

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 2016 Loughborough Antennas & Propagation Conference (LAPC)

III. MM-WAVE BEAMFORMING NETWORK ANALYSIS

The field of microwave/mm-wave beamforming networks
incorporate mainly two types of array feeding systems, namely
circuit-based beamformers, and lens-based beamformers [25].
Generating multiple beams using an array, along with having
wide bandwidth and RF beam steering capability, are of crucial
importance for 5G wireless subsystems to have control over the
amplitude and phase at each element of the array. The Rotman
lens [26] is an attractive and high-performance feeding network (a) (b)
due to its low-cost, reliability, and wide scanning capabilities.
Fig. 2. The flexible 60-GHz PET-based Rotman lens BFN: (a) simulated device
It avoids the complexities of phase shifters to steer a beam over with four beam (input) ports, eight dummy ports (terminated), and eight array
wide angles, and has proven itself to be a useful beamformer (output) ports; (b) fabricated lens (i.e., 43.86 mm × 38.12 mm) using the copper
for electronically scanned arrays (ESAs). It is a true-time-delay nanoparticle ink (Intrinsiq Materials) based on the inkjet fabrication process.
(TTD) component, and produces beam steering independent of
The fabricated Rotman lens component is realised based on
the frequency, and is therefore, capable of wideband operation
the flexible substrate using the in-house Materials Processing
[27]. The Rotman lens beamforming network, as a constrained
Lab’s Fujifilm Dimatix DMP-2831 materials inkjet printer with
lens, in which an EM wave is guided along constrained paths
the copper nanoparticle ink. The process has been conducted in
upon the design equations which generates multiple RF beams
order to potentially demonstrate that wideband flexible lenses
with special magnitude and phase characteristics [28], is able to
are now feasible for the RF subsystems. The fluid waveform is
provide the required functionalities for the advanced wireless
used which has been adjusted according to the firing frequency,
subsystems. A Rotman lens BFN is a parallel plate device used
voltage, drop spacing, and printhead temperature; according to
to feed an array. It has a carefully chosen shape and appropriate
the viscosity (i.e., in terms of the centipoise) of the copper ink;
length transmission lines to produce a wave-front across the
in order for the printing process to be appropriately calibrated.
output that is phased by the time-delay in the RF transmission
A number of Rotman lens beamforming components operating
[29]. Each lens input port will produce a distinct beam that is
at mm-waves based on different topologies, have been reported
shifted in angle at the output. The design of the Rotman lens is
in the available literature for a number of applications, using
controlled by a series of equations that set the focal points and
the different realisation and fabrication technologies, including:
array positions. The inputs include the desired scan angle of the
microstrip [33], rectangular waveguide [34], low-temperature
array, the operating frequency, the number of beams and array
co-fired ceramic [35], silicon [36], system-on-package [37],
elements, and the spacing of the elements. Also, the geometry
substrate integrated waveguide [38], and micromachining [39].
optimisation along with the phase error and coupling analysis
are considered while designing the Rotman lens, as thoroughly This work has been carried out as an attempt to propose a
discussed in [30]. The EM simulations regarding the design of novel and high-performance flexible Rotman lens beamformer,
the Rotman lens for operation at the 60-GHz frequency band and to assess the feasibility of the BFN device realisation using
has been carried out, and the device’s output characteristics, the inkjet technology, for the potential RF module integration
including the beam to array coupling amplitudes and insertion within the 5G mm-wave subsystems. Figs. 3 and 4 indicate the
and return losses, the array phase division behaviour, and the S-parameters’ magnitudes and phases, for beam ports one and
current distributions are presented for the operating frequency. three activated, respectively. The magnitudes have a low-ripple
The lens has been simulated based on the 5×8 configuration, in (i.e., less than 6 dB) over the frequency range, along with an
which the mm-wave Rotman lens incorporates five input beam almost linear phase behaviour across the array elements. The
ports and eight output array ports, suitable for an eight-element parameters confirm the wideband performance of the device. It
antenna array, along with eight dummy ports to absorb the EM is worth noting that beam port one, along with beam port five,
energy, and to reduce the reflections, along with the scan angle determine the minimum efficiency of the device, since they are
of ±30˚ with the half-wavelength spacing. Furthermore, a high- symmetrical on both ends of the device, and are placed on the
resistivity substrate has been selected based on the Mitsubishi furthest distance from the central focal point. Because of the
Papers flexible PET with dielectric constant of 3.2, substrate intrinsic symmetrical structure of the Rotman lens beamformer,
thickness of 0.14 mm, and loss tangent of 0.022. The designed the performance of beam port two is theoretically identical to
Rotman lens has the structure that connects the beam contour the performance of port four. Moreover, the excited beam port
to the array contour for the optimum power distribution across three exerts the maximum device efficiency into the lens, since
the elements with linear phase progression among the adjacent it is placed on the central focal point of the RF component. The
elements. The coordinates of the lens-layer has been initially device efficiency of the lens can be expressed as the sum of the
extracted using a modified script, as in [31], and the data has absolute squares of the output array ports’ linear transmission
been imported into the simulator for the further realisation. The coefficients (i.e., S6,3 to S13,3). Therefore, the device efficiency
EM simulations have been carried out, in order to thoroughly is determined based on the obtained insertion losses, according
analyse the performances of the device. The RL-BFN is shown to the activated beam ports. The simulated results also confirm
in Fig. 2, which mainly incorporates the copper layers with the the high-performance operation of the proposed Rotman lens,
thickness of 1 µm, for both the top and ground planes of the further presenting the conformity of the behaviour according to
device. The lens has been deployed based on the PET substrate, the generalised Rotman lens theory. Fig. 5 presents the surface
for both the simulations, and the realised component, based on current distributions, based on the individually activated beam
the in-house inkjet printing fabrication method, as in [32]. ports, for the progressive RF operation at the mm-wave band.

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 2016 Loughborough Antennas & Propagation Conference (LAPC)

of mm-wave beamforming, as the enabling technology. Hence,

multiple beams, without the need for phase shifters, have been
generated, based on the proposed RF device with the wideband
operation, and also the wide angle scanning capabilities. All the
generated mm-wave beams can be used simultaneously or can
be switched at high rates, which results in the pattern diversity
for potential applications such as the advanced MIMO systems,
in which an extra radio at each beam port can be integrated for
the hybrid module development. The presented beamforming
network along with the demonstrated method of realisation,
(a) can be extended into a multilayer RF structure using the inkjet
printing based on different materials, to develop the layers, as
the separate sections of the intended RF component, as in [40].

(b)
Fig. 3. Simulated S-parameters of the Rotman lens BFN for operation at 50–70
GHz for beam port one active: (a) S-parameters’ magnitudes incorporating the
return loss, adjacent beam losses, and insertion losses; (b) linear progressive
phase distributions across the array (output) ports of the mm-wave lens.
(a) (b)

(a)

(c) (d)

(b)
Fig. 4. Simulated S-parameters of the Rotman lens BFN for operation at 50–70
GHz for beam port three active: (a) S-parameters’ magnitudes incorporating the
return loss, adjacent beam losses, and insertion losses; (b) linear progressive
phase distributions across the array (output) ports of the mm-wave lens.

IV. CONCLUSION
In this contribution, a wideband 60-GHz Rotman lens array (e) (f)
beamformer has been designed and analysed, based on the PET Fig. 5. Simulated surface current distributions of the 60-GHz Rotman lens BFN
substrate, employing the inkjet fabrication process, along with based on the flexible PET substrate, for the excited beam ports: (a) port one; (b)
the thorough discussion of the 5G networks and the importance port two; (c) port three; (d) port four; (e) port five; (f) EM distribution range.

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 2016 Loughborough Antennas & Propagation Conference (LAPC)

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