Research On Channel Estimation And Adaptive Algorithm Technologies In OFDM Systems

Orthogonal frequency division multiplexing (OFDM) has been widely used in wide-band wireless communication. And the channel estimation and adaptive allocation algorithm is the key technology in OFDM system. The selected topic of this doctoral thesis is coming from the National Natural Science Foundation of China and other projects, which has important theoretical value and wide application prospect.In this paper, we focus on discuss how to reduce the impact of frequency selective fading and time selective fading in multi-path channel to suppress the system of inter-carrier interference (ICI). Therefore, it is critical to improve the performance of the OFDM system. On this basis, this article has mainly completed the following innovative scientific research achievements.Because classical channel estimation techniques based on frequency domain is too complex. But if we use optimal rank reduction arithmetic, the accuracy of the channel estimation is decreased very quickly. In this paper, based on the fraction taps channel approximation (FTCA) channel model, we proposed a new channel estimation algorithm that combine the advantage of both FTCA model and traditional LMMSE channel estimation algorithm. Simulation results show that the BER performance is proved to effectively approximate the LMMSE channel estimation and at the same time reduce the complexity of the original estimation algorithm. Moreover, the BER performance of the proposed channel estimation is far better than interpolation algorithm of the comb pilot LMMSE channel estimation.In order to solve the problem of the inter carrier interference (ISI) of the OFDM system in rapidly time-varying environment. In this paper, a kind of basis expansion model (BEM) based on the FTCA model is proposed in order to obtain the channel estimation in rapidly time-varying channel. The model is called FTCA-BEM in this paper. As FTCA-BEM model can well approximate the traditional Jake’s model. The algorithm can be used to estimate the wireless communication channel even if its Doppler frequency is rather high. In this paper, the optimal pilot sequence corresponding to those algorithms is also discussed to improve the performance of the channel estimation.In MIMO-OFDM system, the attenuation of different sub-carrier wave of different space channel is not the same. The channel gain of each sub-carrier can be obtained by channel estimation at the receiver. This can be feed back to the transmitter. The bits power distribution is done adaptively according to the channel gain. And then the goal of optimal system performance is achieved. A new adaptive algorithm with changed step size coefficient is introduced. An adaptive V-BLAST OFDM system is introduced in which propose a memory variable step iteration "water-fill" algorithm. The only thing we need to know is the SNR information of each sub-carrier channel. And then the system can be optimized without pre-coding and shaping. As a result, the information of feedback is reduced. By using this algorithm, the system has better performance and low computational complexity. And the convergence rate is speeded.Finally, we summarized the research work of this paper. And we prospect the further research direction and main content.