Research On Precoding And Related Techniques In Multi-User MIMO Systems

With the development of diverse communication services, data and multimedia services have surpassed the traditional voice counterpart to be the major carrier of wireless communication. The huge network data and video to deliver induced by the information explosion pose great challenge on the high-data-rate reliable communication. As the constraints of channel environment and limited spectrum resources in the wireless communication system, to improve the validity and reliability of wireless communication system requires revolutionary technology in the physical layer. Multiple Input Multiple Output (MIMO) technology has become a key physical layer technology of the fourth generation mobile communication system, since it can greatly improve transmission rate of wireless communication systems without increasing the bandwidth and the transmission power.With respect to the Single-User MIMO technology, the Multi-User MIMO technology not only provides the multiplexing gain and divisity gain, but also the multi-user gain, i.e., the sum rate scales with number of active users. Meanwhile, an inevitable problem, which is how to cancel the multi-user interference, is introduced. Theoretically, multi-user precoding is the best technology to solve this problem, but the existing precoding technology has two main problems as follows:1) Only considered the interference within the cell, however, the4G wireless communication system use the same frequency networking, the adjacent cell interference is unavoidable and will seriously affect system performance;2) the precoding is heavily depedent on the integrity of the channel state information (CSI) at the transmitter and the perfect CSI is hard to acquire in practical. Under the support of National Science and Technology Major Project of China (2011ZX03001-007-03) and the ZTE cooperation forum in production, study and research, precoding and related technology in the multi-user MIMO system are deeply investigated in the dissertation. The main research work and innovative achievements are as follows:(1) A new interference-aware precoding algorithm is proposed which is an enhancement to SLNR precoding scheme for downlink multi-stream MU-MIMO channel:1) Analyzing the existing other-cell interference-aware precoding form four aspects, including system framework, design principles, configuration limitations, performance advantages and disadvantages, system performance degrades seriously when the transmitter sends multiple data stream simultaneously;2) for multi-stream MU-MIMO, using the inter-cell interference plus noise covariance matrix for interference suppression and applying GSVD algorithm to get the novel precoder, balancing the Signal to Interfernce Noise Ratio (SINR) between the various subchannels;3) Analyzing the SINR gap between subchannels for indicating the reason that BER performance increase and the simulation results illustrate that the proposed algorithm improves the system BER performance especially in case of multiple data streams at the expense of the smaller system capacity.(2) A robust linear precoding is proposed for the limited feedback system:1) According to the system’s duplex mode, how to obtain the CSI at the transmitter and the factors that affect the integrity of obtained CSI is introduced, this paper fouses on the frequency duplex division (FDD) system, namely the limited feedback system, due to the extensive usage of this system;2) In the FDD system, we established the system channel model which introduceed the feedback delay and quantization error, then a brief analysis of the basic properties of this channel model are introduced;3) The robust MMSE liner precoding problem formulation is established based on the MMSE criterion and limit of transmit power, we use the channel model in2) and lagrange multiplier method to solve this problem to get closed-form solution, simulation results validate the performance of the algorithm;4) Calculating the MSE of this system based on the robust MMSE linear precoding obtained in3), to further enhance system performance, two users selection approaches are proposed based on decremental and incremental selection algorithms.(3) A robust non-linear precoding is proposed for the limited feedback system:1) The robust MMSE-THP non-liner precoding problem formulation is established based on the MMSE criterion and limit of transmit power and structure of THP, we use the channel model in (2) and lagrange multiplier method to solve this problem to get closed-form solution;2) Cholesky decomposition is introduced to reduce computational complexity of robust MMSE-THP non-linear precoding;3) Considering the impact of the instantaneous channel magnitude information (CSI) on the system performance, the structure of THP is modified where a channel magnitude normalization module is added at the receiver, the system performance can be further enhanced because eliminating the impact of the lack of instantaneous CMI;4) Calculating the system MSE expressiong, then theoretical analysis the impact of quantization error and feedback delay on the system performance, simulation results can futher verify the theoretical conclusions.(4) A receiver filter and robust non-linear precoding jointly design scheme is presented for the limited feedback system:1) Iterative algorithm is introduced to design transceiver when the transmitter can get the perfect CSI;2) In the FDD system, the receiver can get perfect CSI while the transmitter obtain the quantized and delayed CSI, iterative algorithm is not applicable any more. Firstly, we set the non-linear precoding as fixed value, calculating the optimal MMSE filter expression, then substituting this expressiong into system MSE expression, as well as considering the limit of transmit power and THP structure to establish the problem formation, using lagrange multiplier method and geometric mean decomposition to solve this problem.