Clustering with Multiple Receiving Antennas and Imperfect CSI in Downlink CoMP Systems
The impact of inter-cell interference in the downlink can be limited by allowing cooperation among base stations (BSs). Coordinated multi point (CoMP) schemes with joint processing (JP) assumes that BSs share data and channel state information (CSI) to serve the user equipments (UEs) by using joint precoders. By assuming perfect CSI and sharing of UE data among all the BSs in the network, CoMP provides a huge gain with respect to a baseline non-cooperative scheme. Although CoMP-JP is a promising technique, many practical constraints make its implementation still challenging. CSI at the BSs is subject to imperfections due to either limited bandwidth available for feedback in frequency division duplex (FDD) systems or noise on channel estimation in time division duplex (TDD) systems. Then, latency and throughput constraints on the backhaul links do not allow the sharing of data among all the BSs. To deal with these issues, clusters of BSs are organized and JP is implemented within the cluster. To improve fairness among the UEs, dynamic clustering has been proposed, where clusters of BSs change over time adapting to the network conditions. However, most of the works on CoMP-JP with clustering focus on systems where UEs are equipped with only one antenna, although already with LTE-Advanced UEs may be equipped with up to eight antennas. Hence, it has been recognized the importance of developing algorithms for CoMP-JP that explicitly takes into account these additional UE capabilities: clusters should be organized by considering that multiple antenna UEs can either use an interference rejection combiner (IRC) to partially suppress the residual inter-cluster interference (ICI) or be served by means of a multi-rank transmission. In this work we consider a downlink homogenous scenario and propose an algorithm to perform jointly dynamic clustering and UE scheduling. We assume rank-1 transmission and UEs suppress the ICI by using IRC. By assuming a maximum cluster size of 3 BSs, we show that by increasing the number of receive antennas a considerable gain is achieved by using IRC at the UEs. However, the gain of dynamic clustering with respect to SCP falls down as the benefits of IRC are more important with SCP, where the ICI is higher with respect to CoMP. Then, we consider a TDD system where CSI at the BSs depend on pilot sequences sent by the UE. In such a scenario we show that if the channel changes slowly in the time and in the frequency domain, performance close to the perfect CSI case can be achieved. If we increase either the Doppler frequency or the delay spread of the channel, there is a decrease in the gain achieved by CoMP with respect to SCP.