A transmitter receives channel impulse response signals derived from impulse signals sent from two or more receivers, each impulse signal being sent from one of the receivers to the transmitter through multiple propagation paths. Downlink waveforms for the two or more receivers are calculated in a way so as to increase a weighted sum-rate under a total power constraint, the downlink waveforms being determined based on time-reversed channel impulse response signals and initial virtual uplink power allocation coefficients. Updated virtual uplink power allocation coefficients are determined based on the downlink waveforms, and downlink power allocation coefficients are determined based on the downlink waveforms and the virtual uplink power allocation coefficients.