Intelligent reflecting surface (IRS) has been recognized as a powerful technology for boosting communication performance. To reduce manufacturing and control costs, it is preferable to consider discrete phase shifts (DPSs) for IRS, which are set by default as uniformly distributed in the range of $[ - \pi,\pi )$ in the literature. Such setting, however, cannot achieve a desirable performance over the general Rician fading where the channel phase concentrates in a narrow range with a higher probability. Motivated by this drawback, we in this paper design optimal non-uniform DPSs for IRS to achieve a desirable performance level. The fundamental challenge is the \textit{possible offset in phase distribution across different cascaded source-element-destination channels}, if adopting conventional IRS where the position of each element is fixed. Such phenomenon leads to different patterns of optimal non-uniform DPSs for each IRS element and thus causes huge manufacturing costs especially when the number of IRS elements is large. Driven by the recently emerging fluid antenna system (or movable antenna technology), we demonstrate that if the position of each IRS element can be flexibly adjusted, the above phase distribution offset can be surprisingly eliminated, leading to the same pattern of DPSs for each IRS element. Armed with this, we then determine the form of unified non-uniform DPSs based on a low-complexity iterative algorithm. Simulations show that our proposed design significantly improves the system performance compared to competitive benchmarks.

翻译：摘要：智能反射表面（IRS）已被公认为一种提升通信性能的强大技术。为降低制造与控制成本，IRS宜采用离散相位偏移（DPS），现有文献中默认将其设置为在$[ - \pi,\pi )$范围内均匀分布。然而，在信道相位具有更高概率集中于窄范围的一般莱斯衰落场景中，此类设置无法达到理想性能。受此缺陷启发，本文为IRS设计了最优非均匀DPS以实现理想性能。若采用传统IRS（各反射元件位置固定），根本挑战在于不同级联源-元件-目的地信道间可能存在的相位分布偏移。该现象导致每个IRS元件的最优非均匀DPS模式各不相同，从而在IRS元件数量较大时造成高昂的制造成本。受新兴流体天线系统（或可移动天线技术）启发，我们证明：若每个IRS元件的位置可灵活调整，上述相位分布偏移可被意外消除，使各IRS元件具有相同的DPS模式。基于此，我们通过低复杂度迭代算法确定了统一非均匀DPS的形式。仿真结果表明，与竞争基准方案相比，所提设计显著提升了系统性能。