Optimal power allocation in STAR-RIS for CoMP transmission

Simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is a new kind of metasurface which can transmit and reflect incident signals to users located at both sides of the surface. At each element of STAR-RIS, two coefficients are used to adjust the phase shifts and amplitudes of the transmitted and reflected signals. Thus, the overall power needs to be split between transmission and reflection coefficients for the sake of energy conservation. In this letter, we focus on the STAR-RIS aided downlink coordinated multipoint (CoMP) transmission and drive a close-form expression for the optimal power allocation when maximal ratio transmission (MRT) is used. Simulation results show that although this expression is only a approximate result, using it can also achieve the same bit error rate (BER) performance as that provided by the optimal power allocation.