Research On Key Techniques In Broadband Wireless Communication Systems

With the development of video, audio and digital communication techniques, people pay more and more attention to wireless communication systems. Broadband, packetized, synthetic and personalized digital communication systems are expected. However, many technical problems encountered make it difficult to come into reality. This dissertation makes a deep research on synchronization, channel estimation and space-time coding techniques for broadband wireless communication systems. The main research results are listed as follows:1. On the research of synchronization for single input single output orthogonal frequency division multiplexing (SISO-OFDM) systems, two new training symbols are designed and the corresponding time and frequency synchronization schemes are proposed. The scheme based on the new training symbol weighted by PN sequence eliminates the effect of the repeated sequences on timing synchronization, thus improving the accuracy of the timing offset estimator significantly. In addition, it enlarges the frequency acquisition range with no loss in accuracy by iterative frequency offset estimation. The scheme based on the new training symbol, which is symmetric conjugate in time domain and has good autocorrelation in frequency domain, utilizes the characteristics of time domain and frequency domain to accomplish the time and frequency synchronization. Compared with the traditional schemes, it can achieve a significantly smaller mean square error of timing and frequency offset estimation and provide a wider frequency acquisition range, up to the OFDM bandwidth.2. On the research of synchronization for multiple input multiple output orthogonal frequency division multiplexing (MEMO-OFDM) systems, two new training symbols are designed and the corresponding time and frequency synchronization schemes are proposed. In distributed MIMO-OFDM systems, the training symbols of n transmitters are weighted by n PN sequences which have good cross-correlations. When the delays and the frequency offsets between the transmit antennas and receive antennas are different, they can be derived respectively. In centralized MIMO-OFDM systems, the training symbols of all transmitters are the same symbol which is symmetric conjugate in time domain and PN sequence in frequency domain. When the delays and the frequency offsets between the transmit antennas and receive antennas are same, they can be estimated very accurately. In addition, an amplitude weighted least squares estimation (AWLSE) of sample frequency offset is presented for MIMO-OFDM systems. It performs well in both gauss channels and multipath fading channels.3. On the research of time synchronization for orthogonal frequency division multiplexing access (OFDMA) uplink systems, a simple and efficient estimator of timing offset is proposed. It can estimate all the users' timing offsets simultaneously and provide a fast acquisition of synchronization with a low complexity, suiting for both the interleaved OFDMA and the sub-band based OFDMA.4. On the research of channel estimation for MIMO-OFDM systems, two simple and efficient algorithms are proposed. Based on the frequency division multiple (FDM) circular pilot sequences, the chain-weighted-averaging least squares (LS) channel estimation obtains the temporal channel estimation using the LS algorithm and improves the estimation performance by weighted averaging. The circular pilot structure and chain weighted averaging effectively decrease the effect of carrier edges, thus improving the estimation performance. The iterative channel estimation with noise reduction obtains the temporal channel response by discrete Fourier transform (DFT) based channel estimation algorithm, and improves the estimation performance by the iterative channel estimation with noise reduction. It can effectively eliminate the effect of inter carrier interference (ICI) and channel noises. Furthermore, it avoids the inversion of a large matrix, thus can be easily realized.5. On the research of space-time coding, a new non-full rank space-time trellis code (NFR-STTC) is designed. It reduces the order of modulation by sacrificing the diversity. Compared with the full rank space-time trellis code (FR-STTC) with the same transmission rate, it can provide a better performance and a lower realization complexity.