Ofdm System Under Time-varying Channel, Channel Estimation And Performance Analysis Study

Due to its high spectral efficiency and robustness against frequency selective fading, Orthogonal Frequency Division Multiplexing (OFDM) has become a promising transmission technique for the B3G (Beyond Third Generation) system. The basic principle of OFDM is to divide a wideband frequency channel into several overlapping subchannels. Despite spectral overlap, the data in subchannels can be perfectly separated at the receiver as long as the subchannels are orthogonal. In OFDM systems, however, it must keep orthogonal there to maintain the characteristic of OFDM. However, the orthogonality of OFDM is destroyed in fast-fading channel, which bring to the ICI (Inter Carrier Interference). Therefore, to do research for the influence of ICI is very important to B3G system.This paper is foused on the estimation of time-varying multi-path Rayleigh channel in OFDMA (Orthogonal Frequency Division Multiple Access) systems, and studies the influence of ICI in vector OFDM systems.In the topic of channel estimation, this paper considering that in OFDMA systems, the pilot symbols in groups are inserted in the sub-carrier in the same distance at the transmitter. In the receiver of multi-user system, the LS (Least Square) algorithm is proposed. It only takes the ICI caused by non-pilots for error, which improves the precision greatly, and doesn't make the complexity increase. Compared to the way, in which the pilot symbols are inserted discretely, the method in this paper to the channel estimation improves the performance by 1.9dB.Iterative channel estimation in the OFDMA system based on the distributing of physical layers of WiMAX (World Interoperability for Microwave Access), is also proposed in this paper. It has considered design of a computationally efficient channel estimation algorithm for OFDMA systems with high mobility. After using the LS and DFT (Discrete Fourier Transform) -based channel estimation, in order to overcome the drawback of DFT-based channel estimation, the proposed algorithm considers an iterative channel update and moving average scheme without optimal MMSE (Minimal Mean Square Error) weighting and thus the complexity does not increase. Simulation results show that the proposed algorithm can improve the performance of the DFT-based channel estimation, and compared to the performance assuming that the receiver knows the channel frequency response, it get worst only by 1.1dB.This paper considers iterative receivers for convolutionally coded vector OFDM in single-antenna systems. Vector OFDM provides more signal space diversity with increasing vector length. As considering that vector OFDM improves the performance of OFDM, this paper uses the algorithm in time-varying Rayleigh channel, to research the influence of ICI caused by Doppler frequency offset. Simulation results show that the proposed algorithm has a better capacity than OFDM systems with ICI.