MIMO-THP System with Imperfect CSI

Abstract

In recent years, it was realized that designing wireless digital communication systems to more efficiently exploit the spatial domain of the transmission medium, allows for a significant increase of spectral efficiency. These systems, in general case, are known as Multiple Input Multiple Output (MIMO) systems and have received considerable attention of researchers and commercial companies due to their potential to dramatically increase the spectral efficiency and simultaneously sending individual information to the corresponding users in wireless systems. In MIMO channels, the information theoretical results show that the desired throughput can be achieved by using the so called Dirty Paper Coding (DPC) method which employs at the transmitter side. However, due to the computational complexity, this method is not practically used until yet. Tomlinson Harashima Precoding (THP) is a suboptimal method which can achieve the near sum-rate of such channels with much simpler complexity as compared to the optimum DPC approach. In spite of THP's good performance, it is very sensitive to erroneous Channel State Information (CSI). When the CSI at the transmitter is imperfect, the system suffers from performance degradation. In current chapter, the design of THP in an imperfect CSI scenario is considered for a MIMO-BC (BroadCast) system. At first, the maximum achievable rate of MIMO-THP system in an imperfect CSI is computed by means of information theory concepts. Moreover, a lower bound for capacity loss and optimum as well as suboptimum solutions for power allocation is derived. This bound can be useful in practical system design in an imperfect CSI case. In order to increase the THP performance in an imperfect CSI, a robust optimization technique is developed for THP based on Minimum Mean Square Error (MMSE) criterion. This robust optimization has more performance than the conventional optimization method. Then, the above optimization is developed for time varying channels and based on this knowledge we design a robust precoder for fast time varying channels. The designed precoder has good performance over correlated MIMO channels in which, the volume of its feed back can be reduced significantly. Traditionally, channel estimation and pre-equalization are optimized separately and independently. In this chapter, a new robust solution is derived for MIMO THP system, which optimizes jointly the channel estimation and THP filters. The proposed method provides significant improvement with respect to conventional optimization with less increase in complexity.