Joint Precoding for RIS-Assisted Wideband THz Cell-Free Massive MIMO Systems

Reconfigurable intelligent surface (RIS)-aided terahertz (THz) cell-free massive multiple-input-multiple-output (mMIMO) networks have been envisioned as a prospective technology for future 6G networks. However, due to the ultrawide bandwidth and the frequency-dependent characteristics of RISs, beam-split effect has become an unavoidable obstacle. To compensate the severe performance degradation caused by beam split effect, we introduce additional time delay (TD) layers at both access points (APs) and RISs. Accordingly, we propose a joint precoding framework at APs and RISs to fully unleash the potential of the considered network. Specifically, we first formulate the joint precoding as a nonconvex optimization problem. Then, given the location of unchanged RISs, we adjust the TDs of APs to align the generated beams toward RISs. After that, with knowledge of the optimal TDs of APs, we decouple the optimization problem into three subproblems of optimizing the baseband beamformers, RISs and TDs of RISs, respectively. Exploiting multidimensional complex quadratic transform, we transform the subproblems into convex forms and solve them under alternate optimizing framework. Numerical results verify that the proposed method can effectively mitigate beam split effect and significantly improve the achievable rate compared with conventional cell-free mMIMO networks.