  Perfect channel state information (CSI) is challenging to obtain due to the
limited signal processing capability at the intelligent reflection surface
(IRS). In this paper, we study the worst-case robust beamforming design for an
IRS-aided multiuser multiple-input single-output (MU-MISO) system under the
assumption of imperfect CSI. We aim for minimizing the transmit power while
ensuring that the achievable rate of each user meets the quality of service
(QoS) requirement for all possible channel error realizations. With
unit-modulus and rate constraints, this problem is non-convex. The imperfect
CSI further increases the difficulty of solving this problem. By using
approximation and transformation techniques, we convert this problem into a
squence of semidefinite programming (SDP) subproblems that can be efficiently
solved. Numerical results show that the proposed robust beamforming design can
guarantee the required QoS targets for all the users.
