In this letter, we consider a double-active-intelligent reflecting surface (IRS) aided wireless communication system, where two active IRSs are properly deployed to assist the communication from a base station (BS) to multiple users located in a given zone via the double-reflection links. Under the assumption of fixed per-element amplification power for each active-IRS element, we formulate a rate maximization problem subject to practical constraints on the reflection design, elements allocation, and placement of active IRSs. To solve this non-convex problem, we first obtain the optimal active-IRS reflections and BS beamforming, based on which we then jointly optimize the active-IRS elements allocation and placement by using the alternating optimization (AO) method. Moreover, we show that given the fixed per-element amplification power, the received signal-to-noise ratio (SNR) at the user increases asymptotically with the square of the number of reflecting elements; while given the fixed number of reflecting elements, the SNR does not increase with the per-element amplification power when it is asymptotically large. Last, numerical results are presented to validate the effectiveness of the proposed AO-based algorithm and compare the rate performance of the considered double-active-IRS aided wireless system with various benchmark systems.

翻译：本文考虑一种双有源智能反射面（Double-Active-IRS）辅助的无线通信系统，其中两个有源智能反射面被合理部署，通过双反射链路辅助基站（BS）与给定区域内多个用户的通信。在固定每个有源IRS单元放大功率的假设下，我们提出了一个速率最大化问题，需满足反射设计、单元分配及有源IRS部署位置的实际约束。为求解这一非凸问题，我们首先获得最优的有源IRS反射系数与基站波束成形方案，进而基于此采用交替优化（AO）方法联合优化有源IRS单元分配与部署位置。此外，我们证明了在固定每个单元放大功率时，用户接收信噪比（SNR）随反射单元数量的平方渐近增长；而在固定反射单元数量时，当每个单元放大功率趋于无穷时，SNR不会随之增加。最后，数值结果验证了所提基于交替优化算法的有效性，并将所考虑的双有源IRS辅助无线系统的速率性能与多种基准系统进行了对比。