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Multiple transmit-and-receive antennas can be used in orthogonal frequency division multiplexing (OFDM) systems to improve communication quality and capacity. In this paper, an improved channel estimation for MIMO-OFDM system is proposed by exploiting a new method of pilot insert by which pilots are inserted into different carriers at different time. The method can estimate the CSI of all frequencies, so it will enhance the ability to resist frequency selective fading. Then it regards the results of interpolation as pre-estimation and takes different Averaged Channel Estimation in the Time-domain (ACE-T) on different channel conditions to get final channel estimation. The effectiveness of the new technique is demonstrated through the simulation of an OFDM system with two-transmit and two-receive antennas.

In this paper, we propose a new frequency synchronization algorithm for orthogonal frequency division multiplexing (OFDM) system by combining PN sequences with cyclic prefix. It is the improvement of Tufvesson's algorithm. In the new algorithm, the frequency offset is estimation with two steps. First a small step (span of the correlation) is used to estimate frequency offset which will result in a larger estimation range but less precise. After the frequency offset has been compensated, the second estimation using cyclic prefix is conducted which is a large step. The new algorithm has large estimation range and the estimation is more precise. Performance of the new estimator improved than that of the old one in AWGN channel and multipath fading channel.

This paper deals with the chip-level equalization technique for large area synchronized-code division multiple access (LAS CDMA) system communications. In the downlink of LAS CDMA, frequency-selective multi-path channel often destroys the orthogonality of spread codes at receivers between users and introduces inter-path interference (IPI), inter-symbol interference (ISI) and multi-path access interference (MAI). The conventional RAKE receiver is limited in suppressing IPI and MAI. Chip equalization is an efficient tool to restore the orthogonality of the user signals. So we focus on a new chip equalization scheme of receiver based on three criterions, and proposed the adaptive implementation algorithms. Simulation results show improved performance as compared with the RAKE receiver for the CDMA downlink.