The prospects of using a reconfigurable intelligent surface (RIS) to aid wireless communication systems have recently received much attention. Among the different use cases, the most popular one is where each element of the RIS scatters the incoming signal with a controllable phase-shift, without increasing its power. In prior literature, this setup has been analyzed by neglecting the electromagnetic interference, consisting of the inevitable incoming waves from external sources. In this letter, we provide a physically meaningful model for the electromagnetic interference that can be used as a baseline when evaluating RIS-aided communications. The model is used to show that electromagnetic interference has a non-negligible impact on communication performance, especially when the size of the RIS grows large. When the direct link is present (though with a relatively weak gain), the RIS can even reduce the communication performance. Importantly, it turns out that the SNR grows quadratically with the number of RIS elements only when the spatial correlation matrix of the electromagnetic interference is asymptotically orthogonal to that of the channel vector towards the intended receiver. Otherwise, the SNR only increases linearly.

翻译：使用可重新配置的智能表面(RIS)来帮助无线通信系统的前景最近受到很大关注。在不同的使用案例中,最受欢迎的情况是,RIS的每个要素都以可控的阶段变换方式散布接收信号,而不增加其功率。在以前的文献中,这种设置是通过忽视电磁干扰来分析的,这种干扰是由外部来源的不可避免的波浪组成的。在这封信中,我们为电磁干扰提供了一个具有实际意义的模型,可用于作为评价RIS辅助通信的基准。该模型用来表明电磁干扰对通信性能有不可忽略的影响,特别是当RIS的大小扩大时。当存在直接联系时(虽然收益相对微弱),RIS甚至可以降低通信性能。重要的是,事实证明,只有在电磁干扰的空间相关性矩阵与预定接收器的通道矢量的微弱或微量化时,SNR才会与RIS的成反调。 否则,SNR只是线性增长。