Differential space-time modulation

Space-time coding and modulation exploit the presence of multiple transmit antennas to improve performance on multipath radio channels. Thus far, most work on space-time coding has assumed that perfect channel estimates are a vailable at the receiver. In certain situations, however, it may be difficu lt or costly to estimate the channel accurately, in which case it is natura l to consider the design of modulation techniques that ao not require chann el estimates at the transmitter or receiver. We propose a general approach to differential modulation for multiple trans mit antennas based on group codes. This approach can be applied to any numb er of transmit and receive antennas, and any signal constellation, We also derive low-complexity differential receivers, error bounds, and modulator d esign criteria, which we use to construct optimal differential modulation s chemes for two transmit antennas. These schemes can be demodulated,vith or without channel estimates. This permits the receiver to exploit channel:est imates when they are available. Performance degrades by approximately 3 dB when estimates are not available.