Antenna Selection for a Simultaneously Transmitting and Reflecting Surface Assisted Communication System

Simultaneous transmitting and reflecting surface (STARS) can simultaneously transmit the signals to a user in the transmission space (T-user) and reflect the signals to a user in the reflection space (R-user) to improve diversity and extend coverage in all directions, unlike intelligent reflecting surfaces (IRS), which can only reflect the signals. Similarly, transmit antenna selection (AS) is a low hardware complexity technique in which the transmitter selects one antenna based on channel conditions and harnesses diversity benefits. We focus on less explored transmit AS for a STARS assisted communication system, which can improve the coverage, reduce the cost and power consumption. Considering both independent phase shift model and coupled phase shift model, we develop different AS rules that jointly optimize the antenna index at the access point and the transmission and reflection coefficients of the STARS to maximize the rate achieved by the T-user subject to the minimum rate achieved by the R-user. Different from the AS rules for the IRS assisted systems, these rules optimize both transmission and reflection phase shifts while ensuring coupled conditions are met. We show that the proposed AS rules increase the T-user rate significantly compared to random phase shift rules. Furthermore, we show that the T-user rate increases significantly as the number of elements at the STARS increase and the rate-outage of the R-user decreases by one to two orders of magnitude by increasing the number of antennas at the access point.