Intelligent reflecting surface (IRS) has recently received significant attention in wireless networks owing to its ability to smartly control the wireless propagation through passive reflection. Although prior works have employed the IRS to enhance the system performance under various setups, the fundamental capacity limits of an IRS aided multi-antenna multi-user system have not yet been characterized. Motivated by this, we investigate an IRS aided multiple-input single-output (MISO) broadcast channel by considering the capacity-achieving dirty paper coding (DPC) scheme and dynamic beamforming configurations. We first propose a bisection based framework to characterize its capacity region by optimally solving the sum-rate maximization problem under a set of rate constraints, which is also applicable to characterize the achievable rate region with the zero-forcing (ZF) scheme. Interestingly, it is rigorously proved that dynamic beamforming is able to enlarge the achievable rate region of ZF if the IRS phase-shifts cannot achieve fully orthogonal channels, whereas the attained gains become marginal due to the reduction of the channel correlations induced by smartly adjusting the IRS phase-shifts. The result implies that employing the IRS is able to reduce the demand for implementing dynamic beamforming. Finally, we analytically prove that the sum-rate achieved by the IRS aided ZF is capable of approaching that of the IRS aided DPC with a sufficiently large IRS in practice. Simulation results shed light on the impact of the IRS on transceiver designs and validate our theoretical findings, which provide useful guidelines to practical systems by indicating that replacing sophisticated schemes with easy-implementation schemes would only result in slight performance loss.

翻译：智能反射面（IRS）因其通过无源反射智能控制无线传播的能力，近期在无线网络中受到广泛关注。尽管已有工作在不同场景下利用IRS提升系统性能，但IRS辅助的多天线多用户系统的基本容量极限尚未被刻画。受此启发，我们考虑采用容量可达的脏纸编码（DPC）方案和动态波束成形配置，研究了IRS辅助的多输入单输出（MISO）广播信道。我们首先提出一种基于二分法的框架，通过最优求解一组速率约束下的和速率最大化问题来刻画其容量区域，该框架同样适用于零迫（ZF）方案的可达速率区域刻画。有趣的是，严格证明表明：若IRS相移无法实现完全正交信道，则动态波束成形能够扩大ZF的可达速率区域；然而，由于通过智能调整IRS相移可降低信道相关性，所获得的增益变得微乎其微。这一结果表明，采用IRS能够降低对动态波束成形的需求。最后，我们分析证明：在实际中当IRS规模足够大时，IRS辅助ZF所实现的和速率能够逼近IRS辅助DPC的性能。仿真结果揭示了IRS对收发机设计的影响，并验证了我们的理论发现，这表明用易于实现的方案替代复杂方案仅会导致轻微性能损失，从而为实际系统提供了有益指导。