The reconfigurable intelligent surface (RIS), which consists of a large number of passive and low-cost reflecting elements, has been recognized as a revolutionary technology to enhance the performance of future wireless networks. This paper considers an RIS assisted multicast transmission, where a base station (BS) with multiple-antenna multicasts common message to multiple single-antenna mobile users (MUs) under the assistance of an RIS. An equivalent channel model for the considered multicast transmission is analyzed, and then an optimization problem for the corresponding channel capacity is formulated to obtain the optimal covariance matrix and phase shifts. In order to solve the above non-convex and non-differentiable problem, this paper first exploits the gradient descent method and alternating optimization, to approach the locally optimal solution for any number of MUs. Then, this paper considers a special case, which can obtain the global optimal solution, and shows the sufficient and necessary condition for this special case. Finally, the order growth of the maximal capacity is obtained when the numbers of the reflecting elements, the BS antennas, and the MUs go to infinity.
This paper adopts a novel reflection amplifiers surface (RAS) to suppress the co-channel interference in the spatial domain. The RAS can reflect and amplify the electromagnetic wave with phase shifts by designing the reflection coefficients, which enables it more flexibly reconfigure the wireless propagation environment, and even suppress interference channel gain. In this paper, a transmitter and an interferer send the desired signal and interference to the receiver, respectively, and a RAS is placed to suppress the unknown interference. First, we design the reflection coefficients for optimizing the interference suppression ratio, and prove that when the number of reflection amplifiers is greater than the number of antennas at the interferer, the interference can be perfectly suppressed. Next, a capacity maximization problem is formulated to design the optimal reflection coefficients, and an iterative algorithm based on fractional programming and the convex-concave procedure is proposed to obtain the solution for this problem. Moreover, the closed-form expression of the maximal capacity is obtained in the strong interference power case. In addition, this paper shows the upper and lower boundaries of the maximal capacity and discusses what kind of the channel conditions achieve the upper and lower boundaries. Lastly, the above results are generalized to the multiple interferer scenario.
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