Coordinated multi-point transmission with imperfect CSI and other-cell interference

Coordinated Multi-Point (CoMP) transmission for LTE-Advanced systems promises enhanced throughput and coverage performance, especially for cell edge users. However, the performance of CoMP systems heavily depends on the feedback quality and channel imperfections. In this paper, we investigate the impact of quantized and delayed channel state information (CSI) on the average achievable rate of joint transmission (JT) and coordinated beamforming (CBF) systems. We derive closed-form expressions and accurate approximations on the expected sum rate of CoMP systems with imperfect CSI assuming small-scale Rayleigh fading, pathloss attenuation, and other-cell interference (OCI). Furthermore, for analytical tractability, we employ a moment matching technique that approximates the distributions of the received desired and interference signals. Based on our analytical framework, we show that CBF and JT to multiple users are more sensitive to CSI imperfections than single-user JT and we identify switching points and optimal operating regimes for each scheme. Furthermore, we propose an adaptive multimode transmission technique that switches between CoMP schemes to maximize the sum rate. Finally, the proposed approximate framework enables us to identify key system parameters, such as feedback resolution, delay, pathloss, and transmit SNR for which CoMP becomes a judicious choice of transmission strategy as compared to non-cooperative transmission.