On the Transmitter-Based Preprocessing for 2-D OFDM-CDMA Forward-Link Systems Over Time-Varying Rayleigh Fading Channels

Transmitter-based preprocessing is investigated for 2-D orthogonal frequency-division multiplexing code-division multiple-access (OFDM-CDMA) forward-link systems for improving performance and shifting signal processing complexity from a mobile unit to a base station. Preprocessing schemes that are based on zero forcing (ZF) with power normalization, minimum mean square error (MMSE), and ZF with multiuser water filling (ZF-MWF) criteria are jointly investigated with 2-D spreading pattern optimization and multiuser scheduling from an information-theoretic viewpoint. Numerical results show that 1) the performance of preprocessing is quite sensitive to the 2-D spreading pattern for SNRs of interest, for example, 20% degradation on the sum data rate is observed for MMSE preprocessing if the spreading pattern is not properly selected; 2) ZF-MWF may substantially outperform the other two criteria depending on the SNRs; and 3) multiuser scheduling provides a significant performance improvement on the system sum data rate.