In intelligent reflecting surface IRS assisted multiple input multiple output MIMO systems, a strong line of sight LoS link is required to compensate for the severe cascaded path loss. However, such a link renders the effective channel highly rank deficient and fundamentally limits spatial multiplexing. To overcome this limitation, this paper leverages the large aperture of sparse arrays to harness near field spherical wavefronts, and establishes a deterministic deployment criterion that strategically positions the IRS in the near field of a base station BS. This placement exploits the spherical wavefronts of the BS IRS link to engineer decorrelated channels, thereby fundamentally overcoming the rank deficiency issue in far field cascaded channels. Based on a physical channel model for the sparse BS array and the IRS, we characterize the rank properties and inter user correlation of the cascaded BS IRS user channel. We further derive a closed form favorable propagation metric that reveals how the sparse array geometry and the IRS position can be tuned to reduce inter user channel correlation. The resulting geometry driven deployment rule provides a simple guideline for creating a favorable propagation environment with enhanced effective degrees of freedom. The favorable channel statistics induced by our deployment criterion enable a low complexity maximum ratio transmission MRT precoding scheme. This serves as the foundation for an efficient algorithm that jointly optimizes the IRS phase shifts and power allocation based solely on long term statistical channel state information CSI. Simulation results validate the effectiveness of our deployment criterion and demonstrate that our optimization framework achieves significant performance gains over benchmark schemes.

翻译：在智能反射面（IRS）辅助的多输入多输出（MIMO）系统中，需要强视距（LoS）链路来补偿严重的级联路径损耗。然而，此类链路会导致有效信道高度秩亏，从根本上限制了空间复用能力。为突破这一限制，本文利用稀疏阵列的大孔径来捕获近场球面波前，并建立了一个确定性部署准则，将IRS策略性地置于基站（BS）的近场区域内。该部署利用BS-IRS链路的球面波前特性来工程设计去相关信道，从而从根本上解决了远场级联信道中的秩亏问题。基于BS稀疏阵列与IRS的物理信道模型，我们表征了级联BS-IRS-用户信道的秩特性与用户间相关性。进一步推导出闭合形式的有利传播度量，揭示了如何通过调节稀疏阵列几何构型与IRS位置来降低用户间信道相关性。由此产生的几何驱动部署规则为创建具有增强有效自由度的有利传播环境提供了简单指导。该部署准则所催生的有利信道统计特性，使得低复杂度的最大比传输（MRT）预编码方案成为可能。这为基于长期统计信道状态信息（CSI）联合优化IRS相移与功率分配的高效算法奠定了理论基础。仿真结果验证了所提部署准则的有效性，并表明我们的优化框架相较于基准方案实现了显著的性能增益。