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.
翻译:智能反射表面(IRS)是扩展无线信号覆盖范围和支持高性能通信的一个很有希望的技术。通过明智地调整大量被动反射元素的反射系数,IRS可以修改无线传播环境,以有利于信号传输。不同于以前大多数不考虑IRS之间任何合作的工程,我们在这项工作中提议并研究一个合作性双IRS的多输入多输出传输系统(MIMO)通信系统,该系统的复杂程度独立于IRS元素的数量。此外,我们通过联合优化两个合作型IRS的传输变换矩阵和被动波形变换数据矩阵的反射系数来调查能力最大化问题。虽然上述问题是非电解码和难以解决的,但我们通过利用LOS频道的可移动特征来改变和简化原始问题。然后我们开发了一个新的低空间相兼容性算法,其复杂性独立于IRS组件的数量。我们分析双IRS援助的系统的能力缩放顺序,在不优化的传输双流流变式S-S-S-S的自动变换能力矩阵中,通过合作性S-递增的系统来大大提升了S-MA变换系统。