Interference cancellation with diagonalized maximum likelihood decoder for space-time/space-frequency block coded OFDM

Recently, space-time block coded OFDM (ST-OFDM), that applies space-time block codes (STBC) to OFDM, has been proposed. Space-frequency block coded OFDM (SF-OFDM) has also been proposed, where the block codes are formed over the space and frequency domain. ST-OFDM and SF-OFDM achieve good performance over multipath fading environments and fast fading environments, respectively. For systems with two transmit antennas, orthogonal conditions are required to separate the received signals: in ST-OFDM, the frequency responses of two consecutive OFDM symbols are almost identical; in SF-OFDM, the frequency responses of two adjacent subcarriers are almost identical. In practical fading environments, however, these orthogonality conditions sometimes cannot be satisfied. In those environments, the received signals cannot be well separated and the performances are degraded. Recently, a new zero-forcing (ZF) diagonalized maximum likelihood decoder (DMLD) was proposed for the space-time block coded single carrier QPSK system to maintain the orthogonality of STBC under fast fading environments and flat fading environments; the channel separation in DMLD is performed by the ZF algorithm using two receive signals at time index 2n, 2n+1 (STC) or two subcarriers (SFC). The matrix generated after the channel separation is not an identity matrix, but a matrix proportional to an identity matrix. We show that ST/SF-OFDM with DMLD outperform ST/SF-OFDM in terms of bit error rate (BER).