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Reconfigurable intelligent surface (RIS) has become a focal point ofextensive research due to its remarkable "squared gain". However, achieving asubstantial beamforming gain typically requires a significant number ofelements, which leads to a non-negligible overhead that forwards the coherentphase shift to the RIS. Different from previous works, which primarily focus onthe information transmission phase, we consider the phase delivery overheadduring the phase-shift delivery phase to explore the trade-off betweenperformance and overhead. To reduce the phase delivery overhead via the controllink, we introduce a hybrid phase shift mechanism, encompassing both thecoherent and fixed phase shifts. Specifically, a beamforming problem isformulated for maximizing the throughput. In light of the intractability of theproblem, we develop an alternating optimization-based iterative algorithm bycombining quadratic transformation and successive convex approximation. To gainmore insights, we derive the closed-form expression of the number of elementsadopting the coherent phase shift in the large signal-to-noise ratio region.This expression serves as a valuable guide for the practical implementation ofthe RIS technology. Our simulation results conclusively demonstrate theeffectiveness of the proposed algorithm in achieving a favorable trade-offbetween throughput and overhead. Furthermore, the introduction of the hybridphase shift approach significantly reduces phase delivery overhead whileconcurrently enhancing the system throughput.

How Much Time is Required for Phase Shift Delivery in RIS-Aided Wireless Systems?