Research On Power Allocation And Codes Technique Of Improving MIMO System Capacity

One of the most important topics in wireless communication system is how to realize high speed and high quality communications under the limited transmitting power and bandwidth. Power Allocation(PA) and Space Time Codes(STC) are the key techniques of the Multiple-Input Multiple-Output(MIMO) to increase spectral efficiency and improve performance. The effect of PA is dynamicly matching the characteristics of channel fading so as to increase the channel capacity. The aim of STC lies in combining the codes structure and array diversity, accordingly increasing the capacity of wireless communication systems and giving diversity gain and coding gain to them. It can also provide higher spectral efficiency than traditional single-antenna systems. These techniques are intended to provide new methods to improve the performance of communication systems.In this paper, by analyzing the characteristics and models of the MIMO channels, the principles of space-time codes are expounded. The design criteria of space-time codes of quasi-static flat-fading channels are given, and the expressions of MIMO channel capacity are derived. Two kinds of PA algorithm are analyzed, including the average allocation and water filling. According to the known and unknown channel state information on the receiver, the coding for coherence detection includes Space-Time Trellis Coding(STTC), Layered Space-Time Coding(LSTC) and Space-Time Block Coding(STBC). And the coding for non-coherence detection includes Unitary Space-Time Coding(USTC) and Differential Space-Time Coding(DSTC). Then the comparisons between them are presented. Simultaneously, the examples and simulation results are presented. As an example, the Space-Time Block Codes with optimal power allocation are analyzed by exploring the model of STC. At the view of tradeoff between efficiency and reliability, the Semi-orthogonal Algebraic Space-Time(SAST) Codes is discussed, and the related simulation results are also given. For frequency selective fading channels, the combination of space-time coding technique and orthogonal frequency division multiplexing(OFDM) technique are investigated. Analysis shows that STC-OFDM can provide robust ability in frequency selective fading channels.Based on the above, there are two new points proposed in this paper. One is that a new simple PA scheme for transmit antennas is presented, when the channel state information is unknown at the transmitter. It selects the antennas randomly by gold section theory and allocates power averagely. The simulation results show that the new scheme is better than the ordinary average allocation algorithms when the SNR is low. Moreover, it has the advantages of low complexity, convenient for using in practice. The other is that an improved Space-Time Coding is suggested which is based on the layered structure and codeword redundancy check, as we name it as Embedded Check Layered Space-Time(ECLST) Codes. It combines the advantages of LSTC and STBC. Since the performance of tradeoff is taken into account, which balances the efficiency and the reliability of wireless communication systems. At the receive antennas, the embedded check component codes improves the reliability of decoding algorithm. The new approach has higher spectrum efficiency and higher code rate than STBC. Simulation results show that the proposed approach outperforms LSTC at the ability of combating fading and realizes the reliable communication.