Precoding Techniques For Multi-user MIMO Wireless Communication Systems

With the rapid development of wireless communication, the forthcoming mobile communication systems are expected to provide a wide variety of services through high data rate wireless channels. To meet the demands of future wireless communication systems within a limited spectrum resource, MIMO has become the key technology due to its ability of achieving higher spectrum efficiency.The precoding scheme for the downlink of multi-user MIMO wireless communication system is an important branch of MIMO techniques. It plays an important role on improving the performance of actual communication systems. Taking advantage of the channel state information at base station, the multi-user precoding schemes preprocess the transmitted signals to combat the co-channel interference, and thus effectively improve the system performance. The multi-user precoding technique has become a hot issue for the MIMO wireless communication system.This dissertation gives an in-depth study on the precoding techniques for multi-user MIMO systems. The main contributions of this dissertation are as follows:(1) An improved precoding scheme based on signal-to-leakage-noise ratio maximization is proposed for the multi-user quasi-orthogonal space time block coded MIMO (MU-QSTBC-MIMO) system. In this scheme, a new objective function is defined, and the effect of MRC receiver is taken into account, when considering the power of co-channel interference. By using an iterative optimization approach and choosing the complex scalars properly, a simple solution of the optimization problem is achieved, and the symbols can be decoded separately at each receiver. This proposed precoding scheme is universal to the MU-QSTBC-MIMO system with arbitrary4x4QSTBC codeword. Simulation results demonstrate that this proposed precoding scheme is more robust to the number of co-channel user; it minimizes the co-channel interference more efficiently and therefore achieves better performance.(2) A leakage-based nonlinear precoder is proposed for the MIMO broadcast system, wherein each mobile user may receive multiple data streams. The precoding matrices are designed by extending the Tomlinson-Harashima precoding (THP) algorithm into the stream domain for inter-stream interference (ISI) mitigation, and maximizing the signal-to-leakage--plus-noise ratio for inter-user interference (IUI) minimization. Both the ISI and the IUI are mitigated at the Base Station transmitter. Since the centralized THP is used for precoder design, this precoder achieves a very simple receiver at each user, and only a power scaling factor is needed to be additionally transmitted to the receiver. Furthermore, the noise enhancement is avoided, which makes the system power more efficient. Moreover, it is proved that this proposed precoder supports the multiple data streams within one user equally. Simulation results demonstrate that, even without considering the ordering problem, this proposed scheme can eliminate the interference efficiently and achieve better bit error rate (BER) performance.(3) An improved precoding scheme based on signal-to-leakage-noise ratio (SLNR) maximization is proposed for the downlink of multi-cell multi-user MIMO system. In this scheme, whitening filter is used at each receiver for other-cell interference suppression, and the influence of whitening filter is considered at the transmitter for precoder design. This proposed precoding scheme can therefore eliminate the multi-user interference and the whitened other-cell interference simultaneously. Furthermore, a novel QR-decomposition-based algorithm is derived to find the optimized precoding matrix for each user. Using this proposed SLNR precoding scheme, the system sum rate capacity is improved significantly with lower computational loads. Simulation results demonstrate that the proposed SLNR precoding scheme can suppress the impact of the other-cell interference efficiently.