Exploiting Time Asynchrony in Multi-User Transmit Beamforming

In this paper, we analyze the benefits of intentionally adding timing mismatch in the downlink transmit beamforming for wireless transmission. Transmit beamforming enables the so-called space-division multiple access (SDMA), where multiple spatially separated users are served simultaneously. The optimal beamforming vectors can be found to minimize the average transmit power under each user’s Quality-of-Service (QoS) constraint. We show that intentionally adding timing offsets between the transmitted signals can significantly reduce the average transmission power compared with the conventional optimal beamforming method while providing the same QoSs for users. The frequency-selectivity in communication channels provides the opportunity to exploit intelligent design for performance improvement. The frequency-selectivity is limited in environments with line-of-sight links or little scattering. In such environments, we propose the use of intentional time delays to induce frequency-selectivity that can be exploited. We provide three different methods exploiting the artificially induced frequency-selectivity which improve the performance with a computational complexity similar to that of the optimal synchronous beamforming. We derive the expressions for the achievable rates using the proposed methods and then provide efficient algorithms to solve the minimum power optimization. We show analytically and numerically that our proposed methods can provide the same QoS while serving more users, utilizing a fewer number of transmit antennas and using reduced power compared with the conventional beamforming methods.