Adaptive Space-Time-Frequency Processing Based On OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) is one of the core technique of mobile communication system in the future. It has much better frequency-spectrum efficiency and ability of bating multi-path interference. It can not only increase the system capacity but also satisfy communication demand of multimedia.Shadow effect and multi-path interference can be restrained by applying space-time processing to OFDM system. However, it is unpractical to receiver because the size and cost of system are greatly enhanced by increasing the number of antenna.Interleaving is used to obtain time diversity in a digital communications system without adding any overhead. The purpose of interleaving is to randomize the burst channel errors. This can greatly improve the performance of the error-correcting codes. In fact, it is difficult to restrain a long continuous burst channel error using the traditional error correction codes and block interleaving because channel errors caused by the mobile wireless channel are usually burst in nature.Adaptive modulation can greatly improve the frequency-spectrum efficiency and promote system capacity. However, it is necessary for adaptive modulation to include some spending bits that apprize the modulation mode of the transmitted signal in the receiver. Besides, the modulation information require be updated in the receiver, which will increase much more spending bits.In order to restrain the multi-path fading of wireless communication system and promote the system capacity and frequency-spectrum efficiency, the paper investigate the key issue of space-time-frequency processing, adaptive interleaving and adaptive modulation of multi-user diversity in multiple-input and multiple-output orthogonal frequency division multiplexing (MIMO-OFDM)system based on small-scale fading of OFDM system.The innovation works and results are summed up as follow. 1. A model of frequency diversity based on Space-Time Block Coded Orthogonal Frequency Division Multiplexing(STBC-OFDM)systems is proposed. Without increasing the number of transmit antennas, the model promote the system BER performance and reduce the complexity of the system utilizing frequency diversity.2. In order to restrain the effect of a long continuous burst channel error in mobile wireless channel, an adaptive interleaving model for MIMO-OFDM systems is proposed. The model could better exploit the instantaneous channel variation by rearranging the transmitted symbols according to the quasi-instantaneous channel SNR in a OFDM frame. The result shows that the performance of the proposed model is better than the traditional block interleaving. In addition, the model have a good restrain to a long continuous burst channel error.3. Considered multi-user diversity and adaptive modulation, an variable-rate algorithm is investigated based on frequency selective fading channel. An model of adaptive multi-user diversity based on random beamforming and adaptive modulation in a MIMO-OFDM system is established. An resource allocation algorithm and a simplified algorithm of adaptive multi-user diversity combining random beam-former and linear QR receiver are proposed. Simulation results show that the algorithm can make the system performance approach that of eigen-beamforming system when the user number is small. Multi-user diversity with antenna allocation can obtain better performance than that of eigen-beamforming system. Finally, adaptive subchannel coded modulation can improve system throughput and BER performance. In addition, its system structure is simplified using subchannel grouping method at the expense of a negligible performance degradation.The dissertation consists of eight chapters.Chapter one is introduction, in which development history and status of mobile communication, space-time processing and adaptive technique of OFDM system are summarized. After that the research content of this dissertation are proposed. Chapter two is the fundamental knowledge involved in this dissertation. The knowledge of matrix algebra is introduced. The model and capacity of fading channel, interleaving and waterfilling are described.In Chapter three, the basic theory of OFDM is described. The implement principle of OFDM using FFT is analyzed. The principle of eliminating ISI using cyclic prefix and the effect of it is discussed. the peak-to- average ratio in OFDM system, and two methods of decreasing the peak-to- average ratio are introduced, which are called amplitude limited method and coding method. At the end of this chapter, the advantage and disadvantage of OFDM is given.In chapter four, statistics of mobile channels in OFDM systems based on small-scale fading is investigated in theory, and MMSE of channel estimator is deduced. The performance of OFDM systems is analyzed through simulation. In chapter five, an model of space-time frequency based on OFDM system is investigated. The diversity gain and the performance of wireless communication system can be improved by increasing the number of transmitted or received antenna. However, it is unpractical to receiver because the size and cost of system is greatly enhanced by increasing the number of antennas. At first, an model of STC-OFDM system is investigated. The system capacity and performance of STC-OFDM system are analyzed under different channel state information(CSI). A frequency diversity model for STBC-OFDM systems is proposed. Under the same SER performance, the number of transmitted antenna is not required to be increased because of using frequency diversity. In the end, the upper bound of PEP of the proposed scheme is analyzed, and the coding and diversity gain is derived.In chapter six, an adaptive interleaving model for MIMO-OFDM systems is investigated. Interleaving is used to obtain time diversity in a digital communications system without adding any overhead. The purpose of interleaving is to randomize the burst channel errors. This can greatly improve the performance of the error-correcting codes, which is often good at correcting random errors. In fact, the traditional method of error-correcting codes and interleaving technique don't adapt to the channel burst errors because the channel errors produced by mobile wireless channel are burst. An adaptive interleaving model for MIMO-OFDM systems is proposed, which could better exploit the instantaneous channel variation by rearranging the transmitted symbols according to the quasi-instantaneous channel SNR in the OFDM frame. The purpose is to break up a long burst channel error so that overall bit error probability is reduced. The algorithm of adaptive interleaving is deduced. The upper bound of PEP of the MIMO_OFDM systems is investigated. The code gain and diversity gain of the proposed model are given.In chapter seven, an model of adaptive multi-user diversity based on MIMO-OFDM system is investigated. Each carrier of OFDM system use different modulation according to CSI. The optimal modulation can obtain the maximal frequency-spectrum efficiency. This chapter apply the Variable-rate method to the frequency selective fading channel using multi-user diversity and adaptive modulation technique. An model of adaptive multi-user diversity based on random beamforming and adaptive modulation MIMO-OFDM system is established. An resource allocation algorithm and simplified algorithm of adaptive multi-user diversity combining random beam-former and linear QR receiver are proposed. The system BER performance is deduced when the system throughput is maximal and the BER of system is minimum.In chapter eight, a brief summary of the dissertation is given. The suggestion for future research related to the adaptive space-time processing of OFDM system is put forward.