Research On Key Technologies For OFDM Mobile Communication Systems

Having a set of characteristics such as robustness against frequency-selective fading channels, high bandwidth efficiency, and expandability, OFDM (Orthogonal Frequency Division Multiplexing) has been proposed to be a potential candidate for the next generation mobile communication system. As a breakthrough in the field of wireless communication, MIMO (Multiple Input Multiple Output) can improve system capacity and bandwidth efficiency without the expense of bandwidth and TX power. The combination of these two techniques, MIMO-OFDM, shows even higher performance. Several channel estimation techniques for OFDM and MIMO OFDM systems are the focuses of this dissertation, and the methods to reduce the PAPR (Peak to Average Power Ratio) of OFDM system based on BCM (Block Coding Modulation) and signal space expansion, and a high-speed single-carrier system scheme based on the signal processing method in OFDM system are also studied. Several characteristics of wireless communications fading channel and various effects of radio propagations are analyzed in detail. Small-scale fading of mobile radio channel is specifically discussed. Effect of Doppler frequency shift on mobile channel is also presented. The performance of OFDM and MIMO OFDM systems in broadband wireless communications are simulated, and the effects varying the system parameters such as the guard interval and the number of carriers are testified.Based on the research of pilot subchannel frequency response in transform domain, adaptive channel estimation methods, based on pilot-aided and transform-domain interpolation, are proposed for OFDM and MIMO OFDM systems, by adjusting the energy parameter according to the SNR (Signal-to-Noise power Ratio) of receiver in order to improve the filtering effect. A novel pilot-aided channel estimation algorithm using transform-domain interpolation and the design of the optimal pilot sequences are presented for MIMO OFDM system. At the same time, a modified channel estimation method for MIMO OFDM system based on transform-domain processing is also presented. The algorithms give a band suppression filter to restrain the interference of noise in channel estimation. The performance effect on channel estimation for OFDM and MIMO OFDM systems by different energy coefficient in transform-domain and the energy coefficient under the different SNR are also analyzed. We prove that dynamically selecting energy coefficient can significantly improve the performance of channel estimation. Simulation results show that the novel adaptive method achieves good filtering performance, has better performance than the fixed energy coefficient method and almost nears the ideal channel estimation.A combined channel estimation method of time-frequency-selective fading channels in OFDM systems is proposed. The time-varying channel is modeled as an AR (Auto Regressive) process and a low-dimensional KALMAN filter based on pilots is used to estimate the AR parameters, and the ML (Maximum Likelihood) algorithm is adopted to track the time-varying channel fading factors. The KALMAN estimator explores the time-domain correlation of the channel, and a MMSE (Minmum Mean Square Error) combiner which explores the frequency-domain correlation is used to refine the KALMAN estimates. The presented solution can reduce the complexity of the high-dimensional KALMAN filter and track the channel both in frequency and time domain. The simulation results show that the proposed channel estimator can effectively compensate for noise and ICI (Intercarrier Interference).