Design and performance analysis of a new multiuser OFDM transceiver system

This dissertation proposes a novel multiuser OFDM transceiver, called the precoded multiuser (PMU-) OFDM. The PMU-OFDM system has approximately multiaccess interference (MAI)-free property. Hence, there is no need to use sophisticated multiuser detection. Moreover, we proposed a code design to greatly mitigate the MAI due to time offset and carrier frequency offset (CFO) effects. More specifically, if we use either symmetric or anti-symmetric Hadamard-Walsh codewords in PMU-OFDM, the system can be much more robust to time offset and CFO effects. As a result, simple transceiver design can be used in the PMU-OFDM system.; We also propose a Hadamard-Walsh code based multicarrier (MC-) CDMA system that achieves zero MAI over the frequency-selective fading channel. The proposed MAI-free MC-CDMA system has the additional advantage that it is robust to the CFO effect. It is also shown that the MAI-free property allows us to estimate the channel of each user separately and the system can perform channel estimation more easily. Owing to the MAI-free property, every user can enjoy a fully channel diversity gain to improve the bit error performance.; Furthermore, we propose two techniques to improve the performance of multicarrier systems. One is an algorithm for bit and power adaptation. The proposed algorithm has a faster convergence speed and a smaller dynamic range of the equalized noise variance than the bit swap algorithm. The other is a symbol-by-symbol maximum likelihood (ML) detection scheme. The proposed ML detection scheme outperforms the symbol-by-symbol minimum distance detection scheme by up to 2 dB in a crosstalk noisy environment.; Finally, a novel full rate space-time block code that diagonalizes the channel matrix is proposed to simplify detection complexity. Our method for channel diagonalization consists of two steps. First, we consider a code structure that can transform the channel matrix to a circulant one. Then, the circulant channel matrix can be diagonalized using DFT and IDFT in the transceiver. The proposed scheme not only diagonalizes any channel matrix with an even number of transmit antennas, but also guarantees that the real and imaginary parts of each received symbol are attenuated equally.