A new differential modulation for coded OFDM with multiple transmit antennas

A new differential modulation scheme is presented for coded orthogonal frequency-division multiplexing (OFDM) systems with multiple transmit antennas in frequency- and time-selective channels. In contrast to an earlier scheme that involves differential modulation in space and time (ST) across two code matrices, the new scheme performs it in space and frequency (SF) within only one code matrix. As such, the shortest coherence time of the channel that can be handled is reduced, which makes the differential SF scheme more resistant to fast fading. Along with a suitable spectral encoder and interleaver, both the differential ST and SF schemes offer joint spatio-spectral diversity and coding gain. Numerical simulation confirms that SF is indeed more resistant to time-selective fading than ST; however, the former also suffers some performance loss caused by frequency-selective fading and is preferred only when fast fading is prevalent.