Research On Limited Feedback In Coordinated Multipoint Transmission Systems

CoMP (Coordinated Multi-Point Transmission and Reception) can manage inter-cell interference through multi-cell cooperative scheduling and joint processing by sharing channel information or user data information among coordinated cells, thus it can improve system performance especially for edge users. CoMP is one of the key technologies in the LTE-A system to solve the problem of inter-cell interference. The performance improvement of CoMP requires coordinated base station to obtain CSI (channel state information) through feedback as accurately as possible. In FDD (frequency division duplexing) system, the CSI of transmitter is obtained through finite-rate feedback links. The application of limited feedback in CoMP system can improve system performance with affordable feedback overhead.In this paper, we first analyze system capacity of CoMP system based on limited feedback in different codeword selection schemes, we obtain theoretical upper bounds of system capacity of CoMP system based on limited feedback in JCS (joint selection scheme) and ICS (independent selection scheme) respectively, we compare the feedback overhead and selection complexity of JCS and ICS quantitatively. Generally speaking, JCS has better performance, but the selection complexity of JCS is too high, ICS has better flexibility and scalability and much lower selection complexity, but it exists phase ambiguity (PA) problem in ICS which degrade system performance. Simulation results show that the practical system capacity is close to the theoretical system capacity upper bound both for JCS and ICS, which has validated the availability and guidance of the theoretical system capacity upper bounds.Subsequently, we propose an adaptive bits allocation scheme in CoMP system based on limited feedback. We first analyze the global CDI (channel direction information) quantization accuracy of the users and give the approximate closed-form expression of the normalized global CDI quantization gain, we then propose the adaptive bit allocation scheme to allocate bits between quantizing the per-cell CDIs and the per-cell PAs under the total number of feedback bits constraint, aiming at maximizing the normalized average global CDI quantization gain. Simulation results show that the proposed bit allocation scheme significantly improves the average sum-rate of the CoMP system compared with the conventional equal bit allocation scheme. Finally, we design a codeword selection scheme based on quantization error collection, in which the quantization error of each step is collected by the previous step. Simulation results show that the proposed codeword selection scheme can achieve better tradeoff between selection complexity and system performance.