Intelligent reflecting surface (IRS) is a promising technology to extend the wireless signal coverage and support the high performance communication. By intelligently adjusting the reflection coefficients of a large number of passive reflecting elements, the IRS can modify the wireless propagation environment in favour of signal transmission. Different from most of the prior works which did not consider any cooperation between IRSs, in this work we propose and study a cooperative double-IRS aided multiple-input multiple-output (MIMO) communication system under the line-of-sight (LoS) propagation channels. We investigate the capacity maximization problem by jointly optimizing the transmit covariance matrix and the passive beamforming matrices of the two cooperative IRSs. Although the above problem is non-convex and difficult to solve, we transform and simplify the original problem by exploiting a tractable characterization of the LoS channels. Then we develop a novel low-complexity algorithm whose complexity is independent of the number of IRS elements. Moreover, we analyze the capacity scaling orders of the double-IRS aided MIMO system with respect to an asymptotically large number of IRS elements or transmit power, which significantly outperform those of the conventional single-IRS aided MIMO system, thanks to the cooperative passive beamforming gain brought by the double-reflection link and the spatial multiplexing gain harvested from the two single-reflection links. Extensive numerical results are provided to show that by exploiting the LoS channel properties, our proposed algorithm can achieve a desirable performance with low computational time. Also, our capacity scaling analysis is validated, and the double-IRS system is shown to achieve a much higher rate than its single-IRS counterpart as long as the number of IRS elements or the transmit power is not small.

中文翻译：

---

LoS信道下的双IRS辅助MIMO通信：容量最大化和缩放

智能反射面（IRS）是一种有前途的技术，可以扩展无线信号覆盖范围并支持高性能通信。通过智能地调​​整大量无源反射元件的反射系数，IRS可以修改无线传播环境，以利于信号传输。与大多数以前没有考虑过IRS之间的合作的工作不同，在这项工作中，我们提出并研究了在视线（LoS）之下的协作式双IRS辅助多输入多输出（MIMO）通信系统传播渠道。我们通过联合优化两个协作IRS的发射协方差矩阵和无源波束成形矩阵来研究容量最大化问题。尽管上述问题是非凸的，并且难以解决，我们通过利用LoS通道的易处理特征来转换和简化原始问题。然后，我们开发了一种新颖的低复杂度算法，其复杂度与IRS元素的数量无关。此外，由于协作式无源波束成形，我们针对渐近大量的IRS元素或发射功率，分析了双IRS辅助MIMO系统的容量缩放阶数，这明显优于传统的单IRS辅助MIMO系统双反射链路带来的增益和从两个单反射链路获得的空间复用增益。大量的数值结果表明，通过利用LoS信道特性，我们提出的算法可以在较低的计算时间下获得理想的性能。还，