Research On Techniques Of Multi-user MIMO With Limited Feedback

Multi-input Multi-output (MIMO) is one of the key techniques in the next generation of wireless communications. By realizing spatial multiplexing and multi-user diversity, it could enhance the transmitting rates and stability of the system greatly. On the other hand, MIMO systems need to feedback the channel state information, which will occupy some frequency bands and decrease the efficiency. Techniques of multi-user MIMO with limited feedback could save the feedback resource efficiently and reasonably on the premise of the transmitting performance. Therefore, the techniques could be used in the next generation of wireless communication to meet the requirement of various wireless multimedia, wireless networks and mobile computing equipments, and be an important research project.The paper firstly models the multi-user MIMO system with limited feedback, and then the system performance indicators used in the paper are introduced. Moreover, this paper presents the key aspects of the system, which including optimal design of transmission performance with limited feedback to maximize the system transmitting rates, optimal cross-layer design of limited feedback combined system transmitting rates and stability and feedback schemes in the user sides. In the research of optimal design of transmission performance with limited feedback, the paper proposes a low complexity feedback scheme by introducing a modified precoding codebook based on orthogonal beamforming and a user scheduling scheme to enhance the system transmitting rates. Meanwhile, the paper proposed a two-stage feedback scheme by separating the feedback into the scheduling stage and the precoding stage, where orthogonal beamforming and zero-forcing beamforming are used respectively. As a result, the derivation of the low bound on transmitting rates and the simulation results show that the scheme could enhance the transmitting rates while decrease feedback quantity.In the research of cross-layer design of limited feedback, the optimization combining system transmitting rates and stability is studied. The paper builds a cross-layer model based on limited feedback, where using users' estimation of transmitting rates as CQI and modifying the common used utility functions for cross-layer. Simulation shows that, compared with former models, the new model could enhance the sum transmitting rates on the premise of the stability of the system. Furthermore, the sum rates approach those in optimal design of transmission performance, which just aims at maximizing the sum rates.Lastly, the feedback schemes in the user sides are studied. The paper proposes a new distribution scheme of reference signal and relative channel estimation method, which is used for MIMO systems with 8 transmitting antennas in LTE, where 3 reference signals are used per antenna to realize the channel estimation. Meanwhile, the paper finds a new fast codebook matching method. When a codeword has been compared, some unselected codes can be proved useless and removed. As a result, the matching times and computational complexity are reduced.Consequently, the paper has deep researches on several areas of multi-user MIMO system with limited feedback according to the techniques mentioned in 3GPP LTE conference with several papers and patents finished. More researches combined with multi-cell MIMO system and CDMA techniques are required with the development of mobile communication and applications.