Research On Limited Feedback Technology In MIMO System

Multiple-input multiple-output technology(MIMO) can improve the system capacity and the bit error performance without additional spectrum resources. The performance gain of MIMO depends on how much the transmitter knows the channel state information(CSI). In a time division duplexing(TDD) system, the channel state information can be achieved by the reciprocity of the channel. While in a frequency division duplexing(FDD) system, CSI usually is achieved by limited feedback. A typical implementation of limited feedback is both the transmitter and the receiver having a same codebook. Based on the channel estimation, the receiver chooses the optimal codeword from the codebook and sends the index of the codeword to the transmitter via a feedback link.This paper focuses on the limited feedback technology in MIMO system. Firstly, we introduce the solutions of capacity in multiple antennas system, the strategy of precoding at transmitter and the scheme of signal detecting at receiver. Then we study the criterion of codebook design and codeword selection in a limited feedback system. Analysis of the property and simulation of the performance are given for common codebooks. Results show that the system performance is improved when the scale of antennas and the size of codebook increase. For codebooks with better performance, there is not any construction method for codebook design, and they can only be obtained by exhaustive search via computer. The scale of the search increases exponentially with the number of antennas and the size of the codebook. Meanwhile, the required storage resources for codebook and computing resources for searching codewords increase seriously.Targeted at the compromise between the system performance and the resources for storage and computing, this paper proposes a cyclic shift codebook whose codewords are generated by cyclically shifting a group of mutually unbiased vectors. On the aspect of storage, every element of the codeword comes from quaternary alphabet and needs only 2 bits to store. On the aspect of complexity, element of the codeword is real number or imaginary number with simple form, and there is a cyclic shift relation among the codewords, which will be used to reduce the searching complexity at the receiver greatly. Meanwhile, two modes for codeword searching are given, aiming at balancing the required hardware resources and processing time. For beamforming, spatial multiplexing and multi-user MIMO scenario, we propose the corresponding methods for codebook design and codeword searching. Besides, the computational complexities are analyzed respectively. Analyses and simulation results under different systems show that the cyclic shift codebook can achieve comparable performance gain against traditional codebooks while the storage requirement and computational complexity are reduced significantly.