Equalization and multiuser detection for space time block coding based transmit diversity (STD) in frequency selective channels

The 3Gpp WCDMA TDD (time division duplex) system employs a small spreading factor (less than 15). This implies that the multipath components cause intersymbol interference (ISI) and multiuser interference at the receiver. Hence, equalization and multiuser detection has to be done at the receiver to combat the interference. Previous work on multiuser interference cancellation for space time block codes only considered single path channels. Similarly, previous work on equalization for space time codes, only addresses space time trellis codes and not space time block codes. The problem of equalization and multiuser detection of space time block codes in multipath fading channels has not been addressed before. We derive the receiver structure for the above. We apply this receiver to space time block coding based transmit diversity (STTD) applied to 3Gpp WCDMA TDD system. In particular we derive the zero forcing (ZF-STTD) and the minimum mean squared error (MMSE-STTD) equalizer/multiuser detector for STTD encoded 3Gpp WCDMA TDD system. We show that for dual antenna transmission using STTD encoding, a ZF-STTD decoding receiver at the mobile gives between 1.0-3.0 dB gain over single antenna transmission, depending upon the multipath channel model. Thus STTD encoding gives significant diversity gains over single antenna transmission for the WCDMA TDD systems.