Hetnet/Small Cells: Wiseharbor
Hetnet/Small Cells: Wiseharbor
Hetnet/Small Cells: Wiseharbor
However, these measures alone are insufficient in the most crowded environments and at cell edges where performance can
significantly degrade. Operators are also adding small cells and tightly-integrating these with their macro networks to spread
traffic loads, widely maintain performance and service quality while reusing spectrum most efficiently.
One way to expand an existing macro-network, while maintaining it as a homogeneous network, is to “densify” it by adding more
sectors per eNB or deploying more macro-eNBs. However, reducing the site-to-site distance in the macro-network can only be
pursued to a certain extent because finding new macro-sites becomes increasingly difficult and can be expensive, especially in
city centres. An alternative is to introduce small cells through the addition of low-power base stations (eNBs, HeNBs or Relay
Nodes (RNs)) or Remote Radio Heads (RRH) to existing macro-eNBs. Site acquisition is easier and cheaper with this equipment
which is also correspondingly smaller.
Small cells are primarily added to increase capacity in hot spots with high user demand and to fill in areas not covered by the
macro network – both outdoors and indoors. They also improve network performance and service quality by offloading from the
large macro-cells. The result is a heterogeneous network with large macro-cells in combination with small cells providing
increased bitrates per unit area. See Figure 1.
Heterogeneous network planning was already used in GSM. The large and small cells in GSM are separated through the use of
different frequencies. This solution is still possible in LTE. However, LTE networks mainly use a frequency reuse of one to
maximize utilization of the licensed bandwidth.
In heterogeneous networks the cells of different sizes are referred to as macro-, micro-, pico- and femto-cells; listed in order of
decreasing base station power. The actual cell size depends not only on the eNB power but also on antenna position, as well as
the location environment; e.g. rural or city, indoor or outdoor . The HeNB (Home eNB) was introduced in LTE Release 9 (R9). It
is a low power eNB which is mainly used to provide indoor coverage, femto-cells, for Closed Subscriber Groups (CSG), for
example, in office premises. See Figure 2.
Specific to HeNBs, is that they are privately owned and deployed without coordination with the macro-network. If the frequency
used in the femto-cell is the same as the frequency used in the macro-cells, and the femto-cell is only used for CSG, then there is
a risk of interference between the femto-cell and the surrounding network.
The Relay Node (RN) is another type of low-power base station added to the LTE R10 specifications. The RN is connected to a
Donor eNB (DeNB) via the Un radio interface, which is based on the LTE Uu interface. See Figure 2. When the frequencies used
on Uu and Un for the RN are the same, there is a risk of self interference in the RN. From the UE perspective the RN will act as
an eNB, and from the DeNB’s view the RN will be seen as a UE. As also mentioned, RRHs connected to an eNB via fibre can be
used to provide small cell coverage.
Introducing a mix of cell sizes and generating a heterogeneous network adds to the complexity of network planning. In a network
with a frequency reuse of one, the UE normally camps on the cell with the strongest received DL signal (SSDL), hence the border
between two cells is located at the point where SSDL is the same in both cells. In homogeneous networks, this also typically
coincides with the point of equal path loss for the UL (PLUL) in both cells. In a heterogeneous network, with high-power nodes
in the large cells and low-power nodes in the small cells, the point of equal SSDL will not necessarily be the same as that of equal
PLUL. See Figure 3.
A major issue in heterogeneous network planning is to ensure that the small cells actually serve enough users. One way to do that
is to increase the area served by the small cell, which can be done through the use of a positive cell selection offset to the SSDL
of the small cell. This is called Cell Range Extension (CRE). See Figure 4.
A negative effect of this is the increased interference on the DL experienced by the UE located in the CRE region and served by
the base station in the small cell. This may impact the reception of the DL control channels in particular.
A number of features added to the 3GPP LTE specification can be used to mitigate the above-mentioned interference problem in
heterogeneous networks with small cells:
ICIC has evolved to better support heterogeneous network deployments -- especially interference control for DL control
channels. Enhanced ICIC (eICIC) was introduced in LTE R10. The major change is the addition of time domain ICIC, realized
through use of Almost Blank Subframes (ABS). ABS includes only control channels and cell-specific reference signals, no user
data, and is transmitted with reduced power. When eICIC is used, the macro-eNB will transmit ABS according to a semi-static
pattern. During these subframes, UEs at the edge, typically in the CRE region of small cells, can receive DL information, both
control and user data. The macro-eNB will inform the eNB in the small cell about the ABS pattern. See Figure 6.
ICIC is evolved in LTE R11 to further enhanced ICIC (feICIC). The focus here is interference handling by the UE through inter-
cell interference cancellation for control signals, enabling even further cell range extension.
eICIC and feICIC are especially important when Carrier Aggregation (CA) is not used.
Further enhancements regarding heterogeneous network and small cells are coming in future 3GPP Releases. At the time of
writing, Release 12 is still in the process of being formulated with some features in the study phase and others, such as work on
interference management for neighbour TDD cells, dual connectivity between the macro cell and small cells, mobility planning
within hyper-dense environments and advances in carrier aggregation combinations already in the normative phase
(specifications).
Future updates to this paper will include what has been achieved with the completion of Release 12 during the second half of
2014.
Further reading
A new HetNet paper by Keith Mallinson, WiseHarbor; August 18, 2014
"...Coordinating the low-power layer of small cells with the macro network improves performance across the entire
network while also further boosting efficiencies in spectrum use and power consumption, automating network
configuration and optimization. The upcoming 3GPP Release 12 (due to be frozen September 2014) standardises various
capabilities in these developments including dual connectivity, small cell on/off and 256 QAM..." ...Read the full article
TR 36.806 Evolved Universal Terrestrial Radio Access (E-UTRA); Relay architectures for E-UTRA (LTE-Advanced)
TR 36.808 Evolved Universal Terrestrial Radio Access (E-UTRA); Carrier Aggregation; Base Station (BS) radio transmission and
reception
TR 36.814 Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA physical layer aspects
TR 36.815 Further Advancements for E-UTRA; LTE-Advanced feasibility studies in RAN WG4
TR 36.823 Evolved Universal Terrestrial Radio Access (E-UTRA); Carrier Aggregation Enhancements; UE and BS radio transmission
and reception
TR 36.826 Evolved Universal Terrestrial Radio Access (E-UTRA); Relay radio transmission and reception
TS 22.220 Technical Specification Group Services and System Aspects; Service requirements for Home Node B (HNB) and Home
eNode B (HeNB)
TS 36.101 Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception
TS 36.212 Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding
TS 36.213 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures
TS 36.216 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer for relaying operation
TS 36.300 Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-
UTRAN); Overall description; Stage 2 (R8, R10, R11)
TS 36.423 Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP) (R8, R10, R11)
From macro to small cells: enhancements for small cells in 3GPP, by Matthew Baker