Introduction to Coordinated Multipoint (CoMP)

continuous improvement of spectral efficiency is necessary.

While approaching the theoretical limits of channels within a single-cell, attention has yet again turned towards mitigating interference caused by neighbouring cells. In legacy cellular networks, inter-cell interference has been regarded as an ever-present limit on the overall capacity of the network. However, Long Term Evolution Advanced (LTE-A) systems propose to overcome this limitation by introducing a group of techniques that offer to decrease, eliminate or possibly even harness inter-cell interference by coordinating the operation of multiple sites.

This promising feature which includes different levels of coop-eration is referred to as Coordinated Multipoint (CoMP), see

Figure 20. DL CoMP is referred to as CoMP transmission, and uplink UL CoMP is referred to as CoMP reception, though both offer significant gains, the two impose different challenges and require dedicated solutions. Since a user’s uplink signal inher-ently reaches the receiver antennas of multiple cells, the partial exploitation of this signal was already introduced in the soft handover feature of third generation systems. Downlink CoMP was first introduced in fourth generation LTE-A systems [32][33], thereby imposing new challenges, including the re-quirement for fast backhaul communication among cooperat-ing transmission sites, which was the focus of the study.

Figure 20.CoMP technique categories

While the transmission of a single UE inherently reaches mul-tiple eNBs in UL CoMP, DL CoMP necessitates mulmul-tiple trans-mitters to coordinate over the backhaul [34] prior to transmis-sion. Additionally, DL CoMP introduces a requirement for eNBs to have detailed and up to date Channel State Information (CSI) [32] of the mobile channels. This CSI, and possibly even user plane data has to be shared among transmitting nodes over the backhaul. This imposes strict latency and capacity re-quirements for the backhaul, which is not met in many LTE de-ployments.

An overview of the different standardized topological scenar-ios [33] is given by Lee et al. [35], see Figure 21.

Coordinated Multipoint

Intra-site Inter-site

intra-eNB Inter-eNB

Downlink Uplink

Coordinated Scheduling / Beamforming

Joint Processing

Dynamic Cell Switching

Joint Transmission Coordinated

Scheduling

Joint Processing

Dynamic Cell Switching

Joint Reception

Figure 21.Intra-site, inter-site intra-eNB and inter-eNB CoMP scenarios.

If the cooperating cells are the sectors of the same eNB, then it is referred to asintra-site CoMP. This scenario has no extra backhaul requirements as it requires no communication among different nodes; however, it fails to address the issue of inter-ference at the cell edges between different sites, where spectral efficiency is the worst.

If the antennas participating in the cooperative transmission are at geographically separate locations, then it is referred to as inter-site CoMP; it offers a huge potential in increasing cell edge spectral efficiency. Though the cooperating transmitting antennas may be at different sites, they do not necessarily be-long to different eNBs. A scenario where one eNB controls mul-tiple Remote Radio Heads (RRH, sometimes remote radio units) forming a Distributed Antenna System (DAS) [36] is an inter-site intra-eNB scenario. This eNB may also be referred to as the baseband unit, baseband hotel, or equipment hotel.

The architecture can be called Cloud Radio Access Network (C-RAN), or baseband pooling [37]. The RRHs may be low power pico cells or high power macro cells. The low power RRHs may each have different cell IDs or may share the same cell ID, thereby in effect forming multiple small cooperating cells or one large cell. This is a topology which is different from already deployed legacy infrastructures. It requires high capacity con-nections with guaranteed low latency between the eNB and the RRHs, which are referred to as fronthaul. Such a topology eas-ily enables any level of CoMP cooperation, as it is resolved within a single eNB.

Remote Radio Head

eNB eNB

Inter-site intra-eNB CoMP Intra-site

CoMP

Inter-eNB CoMP

Inter-site inter-eNBCoMP is the cooperation among indi-vidual eNBs. This scenario offers improved cell edge perfor-mance at possibly any cell edge since it could be used to coor-dinate any neighbouring eNBs [39]. The eNBs are intercon-nected with the X2 interface, which has no capacity or latency guarantees [40]. CoMP demands low latency coordination, therefore, its applicability is limited in legacy network topolo-gies without dedicated solutions [34].

In the case of inter-eNB UL CoMP, the reception points are not co-located; therefore, the received signals are sampled and forwarded to a single Evolved Node B (eNB). This implies a high data rate, thus requiring very high capacity links [38].

Diehm and Fettweis [41] quantify the negative impact of back-haul signalling delay on the performance of the scheduling. The combined decoding at the eNB is performed based on all of the received signals. Since an UL Hybrid Automatic Retransmis-sion Request (HARQ) retransmisRetransmis-sion has to occur strictly 8 ms after the initial failed transmission, the necessity of a retrans-mission has to be signalled back to the UE strictly 4 ms after the initial failed transmission [42]. Whether a retransmission is required can only be precisely determined after the combined decoding. This unavoidably implies transporting large volumes of data within a strict delay requirement. Consequently, either the HARQ process has to be delayed or the applicability of UL CoMP is limited in inter-eNB scenarios [38]. However, the in-tention of the study was to show that inter-eNB DL CoMP does not necessarily require the transfer of large amounts of data within a strict time frame.

Since CSI describes the momentary conditions of fluctuating channels, it applies only for a brief period. Therefore, in DL in-ter-eNB cases, this CSI along with control plane information has to be shared among the cooperating nodes via the backhaul network with low latency. It is critical for the efficiency of CoMP transmission that the CSI sharing procedure is completed within a few milliseconds, or preferably within one Transmis-sion Time Interval (TTI), which is 1 ms, as mentioned by Irmer et al. [39] and described by Biermann et al. [34]. Cili et al. [43]

calculate that for a low mobility scenario the performance of the

selected variant of CoMP starts degrading after 5 ms of CSI de-lay, whereas in a high mobility scenario, CSI delays of over a single millisecond cause severe degradation due to the short channel coherence times. Furthermore, as described later on, in some versions of DL CoMP, not only control plane signalling but also the user plane data has to be synchronously available for transmission at multiple nodes; this may imply a large vol-ume of data traffic. The backhaul signalling procedure de-scribed by Choi et al. [44] requires first CSI and then a large amount of data to be moved over the X2 which is then pro-cessed within a single millisecond. Biermann et al. [45] de-scribe the extent to which the topology of the backhaul links limits feasibility.