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In this paper, we study an intelligent reflecting surface (IRS)-assistedsystem where a multi-antenna base station (BS) serves a single-antenna userwith the help of a multi-element IRS in the presence of interference generatedby a multi-antenna BS serving its own single-antenna user. The signal andinterference links via the IRS are modeled with Rician fading. To reduce phaseadjustment cost, we adopt quasi-static phase shift design where the phaseshifts do not change with the instantaneous channel state information (CSI). Weinvestigate two cases of CSI at the BSs, namely, the instantaneous CSI case andthe statistical CSI case, and apply Maximum Ratio Transmission (MRT) based onthe complete CSI and the CSI of the Line-of-sight (LoS) components,respectively. Different costs on channel estimation and beamforming adjustmentare incurred in the two CSI cases. First, we obtain a tractable expression ofthe average rate in the instantaneous CSI case and a tractable expression ofthe ergodic rate in the statistical CSI case. We also provide sufficientconditions for the average rate in the instantaneous CSI case to surpass theergodic rate in the statistical CSI case, at any phase shifts. Then, wemaximize the average rate and ergodic rate, both with respect to the phaseshifts, leading to two non-convex optimization problems. For each problem, weobtain a globally optimal solution under certain system parameters, and proposean iterative algorithm based on parallel coordinate descent (PCD) to obtain astationary point under arbitrary system parameters. Next, in each CSI case, weprovide sufficient conditions under which the optimal quasi-static phase shiftdesign is beneficial, compared to the system without IRS. Finally, wenumerically verify the analytical results and demonstrate notable gains of theproposal solutions over existing ones.

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