Channel Estimation Of MIMO-OFDM System

More and more people realize that MIMO (multiple-input multiple-output) and OFDM (orthogonal frequency division multiplexing) are the key techniques of the B3G mobile communications and broad band wireless LAN. The MIMO systems use multiple antennas at both the transmitter and receiver to transmit signal in the same frequency band at the same time can significantly increase the channel capacity. The OFDM systems distribute the high-rate data stream to many low-rate data steams that are transmitted in a parallel way over many sub channels. Thus ISI (inter symbol interference) can be handled. By using OFDM and MIMO transmission techniques, the MIMO-OFDM system can achieve high spectral efficiency, which makes it an attractive candidate for high-data-rate wireless applications. To detect the transmit signal, the channel information of every sub carriers in every channels between the antennas is a precondition. Therefore, channel estimation is the key issue in MIMO-OFDM technique.In this paper, we investigate two channel estimation methods with training sequences. One is based on LS, the other based on LMMSE. The LS method is low complexity and easy to realize, but it is sensitive to the noise. System's performance degrades quickly when the SNR is low. The LMMSE method provides a better accuracy than the LS method, but the complexity is high relative. To reduce the complexity of LMMSE method, we propose a low rank LMMSE method based on SVD which performance close to the LMMSE method when SNR is low and the complexity is lower.For the purpose of tracing the change of channel, we investigate frequency interpolate method which estimate the channel information of pilot sub carriers. And then we introduce server interpolate algorithm to calculate channel information of the other sub carriers. For 802.11n system, the pilots are use to estimate and correct the phase error which is caused by synchronization error especially.Lastly, the performance of method above are evaluated and compared based on the result of computer simulation of an IEEE802.11n base band system.