Interference reduction for multiaccess multicarrier transceivers in mobile environments

Restricted until 25 Mar. 2010.High quality broadband services in mobile environments is essential for the next generation wireless communication system. Multiuser OFDM systems provide promising multiaccess techniques for high data rate transmission applications. However, in mobile environments, the Doppler spread effect or the carrier frequency offset (CFO) tend to destroy orthogonality among neighboring subcarriers and also among different users resulting in intercarrier interference (ICI) and Multiaccess interference (MAI) that severely degrade the performance of the system. Without efficient MAI and ICI suppression techniques in a mobile environment, one may resort to more complicated equalization and multiuser detection (MUD) techniques. As a result the receiver design for mobile multiuser OFDM systems is not simple. However, judicious code design in the transmitter of the multiuser OFDM systems, can either directly suppress MAI and ICI or help lower the complexity of multiuser detection techniques so that a low cost receiver is possible.; The code design problem to reduce MAI among multiple users is extensively studied in this work. First, the performance of a precoded multiuser OFDM (PMU-OFDM) system in a fast time-varying channel is evaluated and the criterion for the codeword design to suppress MAI and ICI is derived. In particular, it is proven that the use of M=2 even or the M=2 odd codewords of the M Hadamard-Walsh (HW) codes reduce the MAI due to the Doppler spread effect to a negligible amount. Later, the performance of two orthogonal codes for an MAI-free multicarrier CDMA (MC-CDMA) system in a CFO environment is studied in detail. It is shown that a proper subset of HW or carrier interferometry (CI) codewords achieves an MAI-free MC-CDMA system in the presence of CFO. Finally, it is demonstrated that employing CI codes for a fully-loaded MC-CDMA in a CFO environment leads to a sparse cross-correlation channel matrix that can be utilized to lower the complexity of the maximum likelihood, the parallel interference cancellation (PIC) and the decorrelating multiuser detectors