# RIS-Assisted Cooperative Time-Division Multiple Access
## 1 Introduction  
With the evolution of wireless communications toward 5G-advanced, terrestrial wireless communications have integrated other communications such as satellite communications, IoT networks, and symbiotic ambient backscatter communications for high spectral efficiency and ubiquitous access to massive devices in high-density areas [1; 2; 3]. Together with integrated communication systems, reconfigurable intelligent surfaces (RISs) have recently been recognized as a promising technology for future 6G communications [4; 5], RISs integrate a large number of passive reflecting elements with low hardware cost and energy consumption [6; 7; 8]. With digital reconfigurable and programmable metasurface technology, RISs can adeptly modify the amplitude and phase of the incident signal and reflect it constructively, improving spectrum efficiency and coverage capability [9; 10; 11]. Due to their significant advantages, RIS-aided communication systems have been intensively investigated with numerous related research endeavors. The authors in [9; 10] jointly optimized the transmit beamforming at the base station (BS) and passive beamforming at the RIS to minimize the total transmit power at BS in multiple-input single-output (MISO) downlink systems. This study showed that a RIS-aided MIMO system achieved the same performance as a massive benchmark MIMO system with significantly reduced active antennas/RF chains. In [9], the authors focused on continuous phase shifts at reflecting elements, while in [10], they focused on discrete phase shifts at reflecting elements considering practical implementation. In [12], the authors demonstrated that RIS can establish a virtual line-of-sight (LoS) between BS and user equipment (UE), resulting in enhanced coverage and signal-to-noise ratio (SNR) in mmWave systems. In the study presentedin [13], the authors devised energy-efficient designs that encompass both the allocation of transmit power and the determination of phase shifts for the passive reflecting elements. These designs were developed while adhering to individual link budget guarantees for the users. The author in [14] optimized precoding at BS and phase shift values at RIS to balance the trade-off between spectral efficiency and energy efficiency. The works mentioned above addressed intricate phase optimization challenges that pose computational intensity and real-time implementation difficulties. Crucially, these algorithms relied on precise channel state information (CSI) for the link from BS to UE across the RIS. There is much research on channel estimation in RIS-assisted systems, but it is worth noting that channel estimation still has many difficulties and issues for practical implementation, such as its accuracy, computational complexity, and overhead [15; 16; 17; 18; 19; 20]. The random phase shift-based opportunistic scheduling can be utilized as an intriguing alternative approach to overcome the abovementioned issues in RIS-assisted communications [21; 22; 23; 24; 25; 26]. In other words, BS dynamically serves the UE experiencing the most favorable instantaneous channel condition at any given moment under the given random phase shift on the RIS, not focusing on optimizing the phase shift on the RIS. In [24], the authors analyzed the sum capacity by incorporating the fading effects of both direct links (i.e., between BS and UE) and reflection links (i.e., between BS, RIS, and UE). This capacity was expressed as a function of both the number of users and the number of meta-atoms. The authors in [25] demonstrated that the throughput of the RIS-assisted opportunistic communication system asymptotically converged to the optimal beamforming-based throughput when the number of users was large. In [26], opportunistic scheduling was used to increase energy-efficient performance while turning on/off using multiple RISs spatially distributed to serve wireless users.