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Reconfigurable intelligent surface (RIS) has emerged as a cost-effective andpromising solution to extend the wireless signal coverage and improve theperformance via passive signal reflection. Different from existing works whichdo not account for the cooperation between RISs or do not provide full spacecoverage, we propose the marriage of cooperative double-RIS with simultaneouslytransmitting and reflecting RIS (STAR-RIS) technologies denoted as RIS/STAR-RISunder correlated Rayleigh fading conditions to assist the communication in amassive multiple-input multiple-output (mMIMO) setup. The proposed architectureis superior since it enjoys the benefits of the individual designs. Weintroduce a channel estimation approach of the cascaded channels with reducedoverhead. Also, we obtain the deterministic equivalent (DE) of the downlinkachievable sum spectral efficiency (SE) in closed form based on large-scalestatistics. Notably, relied on statistical channel state information (CSI), weoptimise both surfaces by means of the projected gradient ascent method (PGAM),and obtain the gradients in closed form. The proposed optimization achieves tomaximise the sum SE of such a complex system, and has low complexity and lowoverhead since it can be performed at every several coherence intervals.Numerical results show the benefit of the proposed architecture and verify theanalytical framework. In particular, we show that the RIS/STAR-RIS architectureoutperforms the conventional double-RIS or its single-RIS counterparts.

Cooperative RIS and STAR-RIS assisted mMIMO Communication: Analysis and Optimization