CoMP Implementation Challenges. CoMP as a major enabler is envisioned for future mobile network generations. However, to reap the true performance gains, several limiting factors have been identified that require rectification. The extent to which performance measures of CoMP are compromised by any the following elements depend on the mode and deployment scenario at hand. Cooperation between the TPs in different sectors of the same base station does not require additional backhaul. Also the implemen- tation of this mode of cooperation is relatively simple since the scheduling and processing functions required for CoMP can easily be upgraded and are currently co-located [8]. In the case of inter-site CoMP however, base station cooperation requires con- stant exchange of control signalling over the air interface and the backhaul. User measurements of the channel state information (CSI), preprocessed or quantised user data and user control data need to be shared amongst TPs synchronously over the X2 [6]. The long-term and short-term reporting of CSI are used for the purpose of Hand Over (HO) preparation and link adaptation, respectively. This will consequently require a backhaul capacity of around a few Mbps to 4 Gbps and approximate backhaul latencies of no more than around a few millisec- onds. Large backhaul latencies will result in channel ageing [7] in which case, the outdating of the reported CSI will result in performance degradation of CoMP. To cater for this stringent backhaul requirement incurred by CoMP, base band processing unit (BBU) pooling offered by Cloud Radio Access Network (C-RAN) is identified as the most suitable candidate [13] [14]. Current backhaul latencies range from 100 Microseconds to about 20 milliseconds. Various factors that xx- xxxxxxx such delays can be either network technology related or stem from the choice of backhaul technology used. Processing delays introduced by network nodes and switching equipment are amongst examples of the former. The X2 backhaul connections such as conventional or millimetre microwave, multi-hop and fibre optic connections each contribute to latencies experienced in inter-site base station cooperation [6]. Another necessity for low backhaul latencies for CoMP to be feasibly realised is to avoid the limiting effect of such delays on the Hybrid Automatic Repeat Request (HARQ) that CoMP will need to be implemented with. To control the volume of feedback and control signalling traversing the X2, efficient feedback compression schemes...
