Joint Processing/ Reception in Small-Cell Environment

Low power Remote Radio Units (RRUs), giving rise to low-power small cells, are considered to be one of the key components to increase the capacity of cellular networks in dense areas with high traffic demands, especially in future heterogeneous network

deployments [38, 188]. eNodeB 3 eNodeB 2 400m UE 3 UE 1 UE 5 500m Core Network (EPC)

High-speed back-haul eNodeB 1 UE 4 UE 2 RRU 2 RRU 1 Fibre Fibre UE 6 UE 7 UE 8 Sector 1 Sector 3 Sector 2 Heterogeneous Network

130 In consolidation with macro-cells, small-cells can improve both the coverage and capacity of cell-edge users, being the focus of this research and hotspots by exploiting the spatial reuse of spectrum [188]. However, massive deployment of small-cells, can lead to significant co- channel interference in HetNets, especially for cell-edge users, interference management of which has been scarcely discussed in literature [188, 189]. Figure 5-12 present a heterogeneous deployment, incorporating uplink CoMP and low power RRUs within the coverage of a high power macro node to achieve intra-site joint reception. In the presence of CoMP, uplink transmission can acquire significant gain from macro reception diversity by coordinating low power RRUs with macro eNodeBs [50]. In particular low power RRUs can be deployed in the cell-edge to form small-cells, providing better reception than an eNodeB in scenarios with poor channel conditions. The coverage of a cell in downlink depends on the downlink transmit power [50], but for the uplink, the best link depends on the lowest path loss, where the cell-edge UEs in uplink transmission will realize a better link when a low power RRUs are available.

Hence at the edge of the low power cell (edge of the small-cell), user always gets better uplink channel conditions when attached to the low power RRU. This is because it is more closer to the low power RRU and thus lower path loss than when attaches to macro eNodeB. As illustrated inFigure 5-12, UE 8 in sector 1 can be served by both eNodeB 1 and RRU 1. Similarly joint reception can be achieved for UE 5 in sector 3 which is served by the cell- edge RRU 2 and at the same time served by its respective eNodeB 3. Therefore when intra- site uplink CoMP is applied, the uplink can be received and combined simultaneously from the macro and the low power RRUs thus increasing the uplink gain from macro reception diversity. Table 5-3 represents the base station specifications in the simulated heterogeneous network.

Table 5-3: Base station configurations in the heterogeneous network

Tx. Power Antenna configuration

Height

Macro-cell 46dBm (40W) 15dBi, Sectored 25m

Small-cell 5dBm

(0.0031W)

2dBi, Omni directional

7m

Two scenarios have been compared, distinguished by the number of small cells deployed inside the target sector to demonstrate the performance trend. In both cases macro eNodeB using joint reception with and without receiver beamforming was used to compare the achieved throughput. The measured characteristics are shown in Figure 5-13 and Figure 5-14.

132 The simulation is performed by defining a RoI (target sector) in which the eNodeBs, RRUs and UEs are positioned and it is only in this area where UE movement is performed. The basic simulation cell layout setup consists of 7 eNodeBs with 3 RRUs inside the target sector. The RRUs are deployed in pre-defined positons (locations known) inside the target sector.

The simulation assumes in average 10 users per sector as expected in typical deployments. The model also utilises full buffer traffic which indicates users have an unlimited amount of

data to transmit. Figure 5-14 presents the normalized throughput characteristic measured under these conditions for a heterogeneous network with three small cells.

Figure 5-15: JR in Heterogeneous Network Comparison

It can be observed that by increasing the number of small-cells the overall throughput of the macro cell will significantly increase, even though it is expected to also observe increased interference in the absence of a suitable interference mitigation algorithm. When the number of small-cells is increased up to 3 inside the macro-grid (target sector) the interference

134 towards the RRUs and eNodeBs increases, degrading the overall throughput by averagely 10- 15%. By using receiver beamforming interference is shown in Figure 5-14 to be minimised allowing the overall throughput to be further enhanced. However for a network with only one small-cell (Figure 5-13) in the target sector, with receiver beamforming used in the eNodeB, the cell-edge throughput shown only a moderate increase limited to 0.06 (7%). However by using the receiver beamforming technique to reduce the interference in this scenario achieves significant 0.452 (57%) throughput improvement as shown in Figure 5-14. By observing Figure 5-15 it is easier to compare how the interference increases when the number of small- cells increased in the target sector and how the interference can be significantly mitigated by using the receiver beamforming technique. Thus, it is clear that for a heterogeneous network with a higher number of small-cells, use of adaptive antennas with receiver beamforming can provide better interference mitigation.

5.6

Summary

This chapter presented a novel cooperative uplink Inter-Cell Interference (ICI) mitigation algorithm based on joint reception performed at the base station by using receiver adaptive beamforming. The concept of smart adaptive receiver antennas, using antenna reciprocity, is adopted to enhance the CS/BF and joint reception algorithms. It is assumed that eNodeBs have the knowledge of UE distribution throughout the network, where it allows for joint scheduling decisions for individual mobile users to be made based on the information received from different cells. As a result, uplink transmission for a user is scheduled according to the known positions of other users in order to minimise ICI. This is enhanced in operation by an advanced receiver beamforming technique that in corporation has the potential to improve the overall uplink capacity. Therefore the benefits of receiving uplink signals from a larger number of antennas, in different geographical locations, are investigated

using the proposed uplink joint reception and coordinated scheduling techniques. The eNodeB global view at the central office is exploited to effectively allocate transmission across the whole network.

Compared to CS/BF presented in the previous chapter, joint reception with receiver adaptive beamforming shows a 76% and 52% improvement in cell edge throughput for 1 x 2 and 2 x 4 antenna configurations respectively. In addition, the proposed joint reception scheme demonstrated an increase in the cell-edge spectral efficiency in average by 60% compared to cooperative scheduling. Investigation progressed further by simulating a high-speed UE mobility scenario, where it was observed that CS/BF increases the overall spectral efficiency by 68% and joint reception with beamforming by two times compared to non-CS/BF scheme for a 2x4 antenna configuration. Finally a heterogeneous network environment was considered, initially with only one small-cell in the target sector, demonstrating a modest impact on the cell-edge performance of 7% increment in the cell-edge throughput and receiver beamforming interference compensation. Increasing the number of small-cells to 3, a significant improvement of 57% can be observed in throughout with a parallel decrease in interference. For a heterogeneous network with a higher number of small-cells, use of adaptive antennas for joint reception can provide even better interference mitigation.

Therefore, the proposed joint reception with receiver beamforming scheme for interference mitigation can significantly improve the overall performance of future next generation networks exploiting high-speed fibre backhauls.

Chapter 6