To reap the promised gain achieved by distributed reconfigurable intelligent surfaces (RISs)-enhanced communications in a wireless network, timing synchronization among these metasurfaces is an essential prerequisite in practice. This paper proposes a unified framework for the joint estimation of the unknown timing offsets and the RIS channel parameters, as well as the design of cooperative reflection and synchronization algorithm for the distributed multiple-RIS communication. Considering that RIS is usually a passive device with limited capability of signal processing, the individual timing offset and channel gains of each hop of the RIS links cannot be directly estimated. To make the estimation tractable, we propose to estimate the cascaded channels and timing offsets jointly by deriving a maximum likelihood estimator. Furthermore, we theoretically characterize the Cramer-Rao lower bound (CRLB) to evaluate the accuracy of this estimator. By using the proposed estimator and the derived CRLBs, an efficient resynchronization algorithm is devised jointly at the RISs and the destination to compensate the multiple timing offsets. Based on the majorization-minimization framework, the proposed algorithm admits semi-closed and closed form solutions for the RIS reflection matrices and the timing offset equalizer, respectively. Simulation results verify that our theoretical analysis well matches the numerical tests and validate the effectiveness of the proposed resynchronization algorithm.
翻译:为了从无线网络中分布可重新配置的智能表面(RIS)增强的通信中收获所承诺的收益,这些元表面之间的时间同步是实际中一个必不可少的先决条件。本文件提出一个统一框架,用于联合估计未知的时间偏移和RIS频道参数,以及设计分布的多RIS通信的合作反射和同步算法。考虑到RIS通常是一个被动装置,信号处理能力有限,无法直接估计RIS链接每个跳跃的单个时间偏移和频道增益。为了使估算可可引力,我们提议通过得出最大可能性估测器,共同估计连锁通道和计时抵消时间。此外,我们理论上将Cramer-Rao较低约束带参数(CRLB)定性为评估这一估测器的准确性。考虑到TRIS通常是一个被动装置,因此,在RIS和目的地联合设计一个高效的重新同步算法,以补偿多重时间偏移。基于主要最小化框架、拟议的Cramer-Rao(CRLB)级算法和拟议的模拟结果校正校正校准结果,并分别接受我们的拟议平式矩阵校正的校正校正校正和校正校正校正的校正校正校正、校正校正校正的校正结果。