Reconfigurable intelligent surface (RIS) has become a focal point of extensive research due to its remarkable "squared gain". However, achieving a substantial beamforming gain typically requires a significant number of elements, which leads to a non-negligible overhead that forwards the coherent phase shift to the RIS. Different from previous works, which primarily focus on the information transmission phase, we consider the phase delivery overhead during the phase-shift delivery phase to explore the trade-off between performance and overhead. To reduce the phase delivery overhead via the control link, we introduce a hybrid phase shift mechanism, encompassing both the coherent and fixed phase shifts. Specifically, a beamforming problem is formulated for maximizing the throughput. In light of the intractability of the problem, we develop an alternating optimization-based iterative algorithm by combining quadratic transformation and successive convex approximation. To gain more insights, we derive the closed-form expression of the number of elements adopting the coherent phase shift in the large signal-to-noise ratio region. This expression serves as a valuable guide for the practical implementation of the RIS technology. Our simulation results conclusively demonstrate the effectiveness of the proposed algorithm in achieving a favorable trade-off between throughput and overhead. Furthermore, the introduction of the hybrid phase shift approach significantly reduces phase delivery overhead while concurrently enhancing the system throughput.

翻译：可重构智能表面（RIS）因其显著的“平方增益”而成为广泛研究的焦点。然而，实现显著的波束赋形增益通常需要大量单元，这导致通过控制链路向RIS转发相干相位偏移时产生不可忽略的开销。与先前主要关注信息传输阶段的研究不同，我们考虑相移交付阶段中的相位交付开销，以探索性能与开销之间的权衡。为了通过控制链路减少相位交付开销，我们引入了一种混合相移机制，包括相干相移和固定相移。具体而言，我们构建了一个用于最大化吞吐量的波束赋形问题。鉴于该问题的复杂性，我们结合二次变换和逐次凸近似，开发了一种基于交替优化的迭代算法。为获得更深入的见解，我们推导了大信噪比区域中采用相干相移的单元数量的闭式表达式。该表达式为RIS技术的实际实现提供了有价值的指导。仿真结果最终证明了所提算法在实现吞吐量与开销之间的有利权衡方面的有效性。此外，混合相移方法的引入显著降低了相位交付开销，同时提高了系统吞吐量。