Reconfigurable Intelligent Surfaces (RISs) have been recently considered as an energy-efficient solution for future wireless networks due to their fast and low-power configuration, which has increased potential in enabling massive connectivity and low-latency communications. Accurate and low-overhead channel estimation in RIS-based systems is one of the most critical challenges due to the usually large number of RIS unit elements and their distinctive hardware constraints. In this paper, we focus on the uplink of a RIS-empowered multi-user Multiple Input Single Output (MISO) uplink communication systems and propose a channel estimation framework based on the parallel factor decomposition to unfold the resulting cascaded channel model. We present two iterative estimation algorithms for the channels between the base station and RIS, as well as the channels between RIS and users. One is based on alternating least squares (ALS), while the other uses vector approximate message passing to iteratively reconstruct two unknown channels from the estimated vectors. To theoretically assess the performance of the ALS-based algorithm, we derived its estimation Cram\'er-Rao Bound (CRB). We also discuss the downlink achievable sum rate computation with estimated channels and different precoding schemes for the base station. Our extensive simulation results show that our algorithms outperform benchmark schemes and that the ALS technique achieves the CRB. It is also demonstrated that the sum rate using the estimated channels always reach that of perfect channels under various settings, thus, verifying the effectiveness and robustness of the proposed estimation algorithms.
翻译:最近,人们将重新配置的智能表面视为未来无线网络的一个节能解决方案,原因是其快速和低功率配置,这增加了促成大规模连通和低纬度通信的潜力。在基于RIS的系统中,准确和低管的频道估计是最重要的挑战之一,因为其通常有大量的RIS单元元素及其独特的硬件限制。在本文件中,我们侧重于将具有RIS动力的多用户多用户多输入单一输出(MISO)的通信系统连接起来,并提议一个基于平行因子分解的频道估计框架,以展开由此形成的连锁频道模型。我们为基础站和RIS之间的频道以及RIS与用户之间的频道提供了两种迭代估计算法。一个是交替最小方(ALS),而另一个是使用矢量传递到从估计的矢量中解析出两个未知的渠道。从理论上评估基于ALS-Exmex的算法的功能,我们根据平行因素分解的CBound(CRBB Bound)估算出一个平行的轨迹,因此,我们用可实现的轨道和CSAximal的计算结果。因此,我们还讨论了我们所测算的可实现的可实现的轨道。