Reconfigurable intelligent surfaces (RISs) have recently emerged as a promising technology that can achieve high spectrum and energy efficiency for future wireless networks by integrating a massive number of low-cost and passive reflecting elements. An RIS can manipulate the properties of an incident wave, such as the frequency, amplitude, and phase, and, then, reflect this manipulated wave to a desired destination, without the need for complex signal processing. In this paper, the asymptotic optimality of achievable rate in a downlink RIS system is analyzed under a practical RIS environment with its associated limitations. In particular, a passive beamformer that can achieve the asymptotic optimal performance by controlling the incident wave properties is designed, under a limited RIS control link and practical reflection coefficients. In order to increase the achievable system sum-rate, a modulation scheme that can be used in an RIS without interfering with existing users is proposed and its average symbol error rate is asymptotically derived. Moreover, a new resource allocation algorithm that jointly considers user scheduling and power control is designed, under consideration of the proposed passive beamforming and modulation schemes. Simulation results show that the proposed schemes are in close agreement with their upper bounds in presence of a large number of RIS reflecting elements thereby verifying that the achievable rate in practical RISs satisfies the asymptotic optimality.

中文翻译：

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关于可重构智能表面 (RIS) 的优化：无源波束成形、调制和资源分配

可重构智能表面 (RIS) 最近已成为一种有前途的技术，它可以通过集成大量低成本和无源反射元件为未来的无线网络实现高频谱和能源效率。RIS 可以操纵入射波的特性，例如频率、幅度和相位，然后将这种操纵波反射到所需的目的地，而无需进行复杂的信号处理。在本文中，在实际 RIS 环境下分析了下行链路 RIS 系统中可实现速率的渐近最优性及其相关限制。特别是，在有限的RIS控制链路和实用的反射系数下，设计了一种可以通过控制入射波特性来实现渐近最优性能的无源波束形成器。为了提高可达到的系统总和率，提出了一种可以在RIS中使用而不干扰现有用户的调制方案，并渐近地推导出其平均误码率。此外，在考虑所提出的无源波束成形和调制方案的情况下，设计了一种联合考虑用户调度和功率控制的新资源分配算法。仿真结果表明，在存在大量RIS反射元素的情况下，所提出的方案与其上界非常吻合，从而验证了实际RIS中的可实现速率满足渐近最优性。提出了一种可以在RIS中使用而不干扰现有用户的调制方案，并且它的平均误码率是渐近导出的。此外，在考虑所提出的无源波束成形和调制方案的情况下，设计了一种联合考虑用户调度和功率控制的新资源分配算法。仿真结果表明，在存在大量RIS反射元件的情况下，所提出的方案与其上界非常吻合，从而验证了实际RIS中的可实现速率满足渐近最优性。提出了一种可以在RIS中使用而不干扰现有用户的调制方案，并且它的平均误码率是渐近导出的。此外，在考虑所提出的无源波束成形和调制方案的情况下，设计了一种联合考虑用户调度和功率控制的新资源分配算法。仿真结果表明，在存在大量RIS反射元素的情况下，所提出的方案与其上界非常吻合，从而验证了实际RIS中的可实现速率满足渐近最优性。