Exploiting Time Asynchrony in Multi-User Transmit Beamforming

Abstract

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 spacedivision 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. We provide three different methods exploiting the time asynchrony 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 outperform the conventional optimal transmit beamforming.

Asynchronous Transmission for Multiple Access Channels: Rate-Region Analysis and System Design for Uplink NOMA

“…where equality is achieved by τ = 0 [27]. The achievable rate pairs for the NOMA scheme using SIC are similarly obtained by inserting η 0 = 1.…”

“…Particularly, for the r.r.c. pulse shape η τ = 1 − β/4 + β/4 cos(2πτ ), which is minimized at τ = 0.5 [27]. The rate pairs for the NOMA scheme using SIC coincide with the corner points of the pentagon capacity-region and the sum-rate is calculated as:…”

“…• In this work, we assume flat fading channels, applicable to scenarios with no scattering such as satellite communication [37]. In [27], it is shown that for high scattering environments, the effect of asynchronous transmission is negligible. However, as the power of the dominant path compared with the scattering paths is increased, the effect of asynchronous transmission is intensified.…”

Detection of symbol‐asynchronous uplink NOMA with imperfect time offsets

“…The work in [28] studied the MUD for the asynchronous multiple access system with perfect time offsets, where time offsets between users were treated as an additional resource in the multiple access system. Works in [29, 30] focused on the downlink asynchronous NOMA system, where the authors in [29] considered the outage probability under imperfect channel information, and the authors in [30] optimised the average transmit power under the downlink transmit beamforming for wireless transmission. We are motivated to design the MUD algorithm for the uplink asynchronous NOMA system with imperfect time offsets, and users arrive at the BS with equal power.…”

Downlink Asynchronous Non-Orthogonal Multiple Access Systems with Imperfect Channel Information

“…It has been demonstrated that ANOMA outperforms NOMA in terms of the throughput performance, the power consumption, etc. Moreover, the time asynchrony has also been exploited in multi-user transmit beamforming [5].…”