Research On Channel Estimation Techniques Of Wireless OFDM Systems In Fast Time-varying Channel

Because of its rapid transmission of data, easy realization in countering interference of multi-path and high utilization of frequency spectrum, OFDM has received an increasing great interest in the radio communication and has been widely used in many fields, such as digital audio broadcasting (DAB), digital video broadcasting (DVB), wireless local area networks (WLAN) and power line carrier communication, etc. OFDM will be one of the most important researches for the next generation mobile communication system. However, future wireless systems work at higher frequency, higher rate data transmission, and faster terminal movement, the high-speed railway now under construction in China runs at a velocity over 350 km/h as illustrative examples, making the channels to be rapid variant. Under the fast fading of channel, the orthogonality among subcarriers caused by Doppler shift will be destroyed, resulting in inter-carrier interference (ICI) and performance degradation.Adapting to the characteristic of fast fading channel in the future radio communication systems fore mentioned, a new iterative joint channel estimation and symbol detection algorithm is proposed in this paper. The algorithm utilizes the optimality of Kalman filter in tracking the time-varying channel, combined with the estimation and tracking of decision-feedback channel, jointly performs the time domain channel estimation and frequency domain channel estimation. The algorithm reduces the complexity of the traditional Kalman filter estimation method greatly, and tracks the channel information effectively at the same time. Besides, since the correlation of the frequency domain response of the channel decreases, when the interval between two adjacent subcarriers increases over frequency-selective fading channels, the estimate values of Kalman filter is corrected in the algorithm proposed. Furthermore, the complexity of algorithm is reduced and it can achieve a good performance on channel estimation even when the channel fluctuates rapidly.The phase noise introduces the common phase error (CPE) and ICI to OFDM systems. It is especially important to cancel the ICI in fast fading channel. This paper presents a novel technique which can jointly perform the channel and phase noise estimation to track rapid-varying channels when considering the phase noise in OFDM system. By utilizing the optimal comb pilots and a linear minimum mean square error (LMMSE) estimator, the algorithm can jointly perform the channel and phase noise estimation using a modified basis expansion model (BEM). And the proposed scheme can offer accurate channel information.Recently, the algorithms of estimation and equalization of time-varying channel attract more and more attention. And it is very effective to use the method of basis expansion as the model of time-varying channel on channel estimation. This paper also proposes a modified channel estimation method using discrete prolate spheroidal sequences basis expansion model, based on the traditional complex exponential basis expansion model. Then the paper presents a reduced-complexity maximum-likelihood sequence estimation receiver suitable for this method of channel modeling. A two-step approach is adopted where in the first step we estimate the channel using DPS-BEM and adaptive algorithm. Then using the estimated channel from the first step, a Viterbi detector is used to estimate the information sequence. This scheme can reduce the computational complexity and estimate the channel information accurately.