Analysis and Optimization of an Intelligent Reflecting Surface-assisted System with Interference

In this paper, we study an intelligent reflecting surface (IRS)-assisted system where a multi-antenna base station (BS) serves a single-antenna user with the help of a multi-element IRS in the presence of interference generated by a multi-antenna BS serving its own single-antenna user. The signal and interference links via the IRS are modeled with Rician fading. To reduce phase adjustment cost, we adopt quasi-static phase shift design where the phase shifts do not change with the instantaneous channel state information (CSI). Maximum Ratio Transmission (MRT) is adopted at the two BSs to enhance the receive signals at their own users. First, we obtain a tractable expression of the ergodic rate. Then, we maximize the ergodic rate with respect to the phase shifts, corresponding to a non-convex optimization problem. We obtain a globally optimal solution under certain system parameters, and propose an iterative algorithm based on parallel coordinate descent (PCD), to obtain a stationary point under arbitrary system parameters. Finally, we numerically verify the analytical results and demonstrate the notable gains of the proposed solutions. To the best of our knowledge, this is the first work that studies the analysis and optimization of the ergodic rate of an IRS-assisted system in the presence of interference.