Low Complexity Algorithm In Down-Line Mu-Mimo System

Multiple-Input Multiple-Output (MIMO) system is one of the important technologies in wireless communication system. It will not only improve the channel capacity, but also rise the Ratio of Signal and Noise (SNR). The IEEE802.11n(Wi-Fi),3GPP LTE, WiMAX and HSPA+have contained MIMO system. In general, MIMO system has three technologies, that is Pre-coding technology, Spatial multiplexing technology and Diversity coding technology. Pre-coding technology can adjust pre-coding matrices of users adaptively to suppress channel fading, to reduce system’s Bit Error Ratio (BER) and to improve system capacity.Different from the traditional one-single-user MIMO pre-coding technology, Multi-User MIMO (MU-MIMO) pre-coding technology has to process multiple parallel data stream, and to ensure removement of Co-Channel Interference (CCI). MU-MIMO pre-coding technology could be divided into pre-coding based on channel decomposition and optimal pre-coding.In this paper, we have discussed downlink optimal pre-coding technology. Downlink optimal pre-coding model is a fractional optimization problem with one quadratic inequality constraint. As we know, the current popular approach for solving the downlink optimal pre-coding model is to optimize only the numerator of the fractional objective function (that is, ignore its denominator), which, of course, causes great errors. In this paper, we manage to transform the original problem into a quadratic optimization problem with two constraints equivalently, by moving the denominator to a constraint. Then we solve the new problem accurately by semi-definite programming relaxation method and rank-one matrix-decomposition technique. Simulation results show that the errors caused by our algorithm are far less than those of the current approach, that is the new approach is effective.