Research On The Estimation Of Channel In OFDM System

The sharp increase of demand for wireless communication service promotes the new technology being used in wireless wideband digital communication systems. Orthogonal Frequency Division Multiplexing (OFDM) has been proved to being not only good at combating the Inter-Symbol Interference (ISI) caused by time disperse of radio channel, but also converting the complex channel equalization in time domain into simple form in frequency domain. For this, the research aiming to apply OFDM to system of wireless broadband digital communication has received more and more attention in recent years. In OFDM system, the coherent demodulation requires the Channel Status Information (CSI) being known by the receiver through channel estimation, which makes the channel estimation a key technology in the OFDM system. This dissertation focuses on the study of channel estimation in OFDM system. The main innovations of the dissertation are as follows:(1) An improvement is proposed to make the conventional linear interpolation filter more accurate. The least number of pilots is studied when it is used to recover the channel information in frequency domain. A novel frequency-domain interpolation algorithm in pilot-aided OFDM systems is proposed. The algorithm is simple with few pilots. The mean squared error (MSE) of the proposed algorithm is proved to drop exponentially as the Signal-to-Noise Ratio (SNR) increases.(2) When the fading channel has a long time multipath delay and the OFDM system is with small number of subcarriers, a novel channel estimation base on TSK fuzzy network in frequency domain is proposed. The algorithm is also used in the channel estimation for the Space-Time Block Coding (STBC) Multiple-Input Multiple-Output (MIMO) OFDM systems. When there are enough pilots in system, the performance of the proposed algorithm is close to that of using minimum mean-square-error (MMSE) without computation of the inverse matrix. And the nonlinear character of the algorithm improves the performance when there are short of pilots in OFDM systems.(3) Novel blind channel estimation is proposed by using linear pre-coding and subspace tracking techniques. The combining of the subspace tracking techniques with linear pre-coding OFDM system can decreases the influence of the varying data distribution, varying channel and noise and tracks the variation of channel and make the algorithm more robust.(4) Novel blind channel estimation of OFDM system is proposed by combining different modulation schemes on adjacent subcarriers. The algorithm is simple and fast because of its deterministic calculation.(5) A distribution space of channel transfer function in frequency domain is built up by exploiting the finite-alphabet property of the symbols. Semi-blind channel estimation for OFDM systems is proposed by using the distribution space of channel transfer function. In the algorithm, a reference is first calculated by one pilot OFDM symbol at the initialization. Then the channel estimation can be realized by updating both the distribution space and the reference. The performance of the algorithm is close to that of ideal MMSE, while the algorithm avoids the inverse matrix calculation, which makes computation linear and easy to realize. Such algorithm is also proposed to be used in STBC MIMO-OFDM systems.