Single-user and multiuser adaptive maximal ratio transmission for Rayleigh channels

Abstract

In maximal ratio transmission, the base station adjusts the complex weights of its transmit antennas to compensate for downlink channel gains in order to produce signal reinforcement-diversity-at a desired mobile that may have only a single antenna. To make the method adaptive, the complex weights are obtained from the instantaneous complex gains in the uplink; however, delay and frequency offset between uplink measurements and downlink transmission reduce the correlation between the gains in the two directions. This paper provides an analysis of the effects of gain decorrelation and of the number of antennas, and an assessment of the effect of fading when the method supports multiple users. It demonstrates that large reductions in downlink transmit power are achievable, and it examines simple transmission models to see how well they support the technique.