Study On Limited Feedback Technology In MIMO System

Multiple input multiple output (MIMO) technology, which could provide diversity gain and spatial multiplexing gain while can also achieve SDMA and interference cancellation, will improve capacity and performance of wireless communication system to meet the growing wireless communication business requirements. So the next-generation wireless communication system is bound to use this technology. The tremendous advantage of MIMO depends largely on whether the transmitter can achieve the channel state information (CSIT) to do precoding. However, the actual wireless communication system is limited feedback system and the issue of limited feedback CSI is needed to solve. This work focused on how to feedback and use partial CSI in limited feedback MIMO system and the contents of the work are listed as follows:This dissertation discusses the impact of CSI on the MIMO system. MIMO system models and signal processing in different CSI circumstances are aggregated. The physical meaning of precoding is analysisd in the perspective of beamforming.Then the codebook-based feedback is studied. A comprehensive study of Grassmann codebook indicated two disadvantages of it. One is that it is limited to a specific channel and the other is that it does not include any information for power allocation. To overcome these disadvantages, a power allocation methods which is called 'forcing equal power waterfilling'(FEPW) is proposed. Combined with FEPW, Lloyd algorithm is suggested to design codebook containing power and direction information for channels of any statistic distribution.After that, the dissertation focuses on opportunistic beamforming and provides some improvements to hurdle the bottleneck of this technology, whose capacity is too small in sparse network. Overall, the improvements include three points: one is the combination of antenna selection and antenna combining with opportunistic beamforming while not increasing any feedback overhead, another is the transmitter shifts between a single beam and multi-beam mode to overcome the main problem of noise and multi-beam interference according to the SNR, while the other is a method named 'partial random opportunistic SDMA with beam selection' (POSDMA_S). This method utilizes the channel coherence of time to appropriate degree of random beam freedom to improve the system capacity while ensuring the beam selection gain does not decrease rapidly with user number and subcarrier number.Finally, the application of MIMO limited feedback in the cognitive radio system is researched. A class of beamforming in the null space of primary user channel is proposed. The performance of opportunistic beamforming in the null space of primary user channel is simulated and analysised. Such technology minimizes the interference to the primary user as much as possible and ensures the capacity of the cognitive network.