Large intelligent surface (LIS) has gained momentum as a potential 6G-enabling technology that expands the benefits of massive multiple-input multiple-output (MIMO). On the other hand, orthogonal space-division multiplexing (OSDM) may give a promising direction for efficient exploitation of the spatial resources, analogous as what is achieved with orthogonal frequency-division multiplexing (OFDM) in the frequency domain. To this end, we study how to enforce channels orthogonality in a panel-based LIS scenario. Our proposed method consists of having a subset of active LIS-panels coherently serving a set of users, and another subset of LIS-panels operating in semi-passive mode by implementing a receive and re-transmit (RRTx) process. This results in an inter-symbol interference (ISI) channel, where we characterize the semi-passive processing required to achieve simultaneous orthogonality in time and space. We then employ the remaining degrees of freedom (DoFs) from the orthogonality constraint to minimize the semi-passive processing power, where we derive a closed-form global minimizer, allowing for efficient implementation of the proposed scheme.

翻译：大智能表面（LIS）作为潜在的6G赋能技术，正逐步扩展大规模多输入多输出（MIMO）的优势。另一方面，正交空分复用（OSDM）可能为高效利用空间资源提供有前景的方向，类似于正交频分复用（OFDM）在频域中取得的成效。为此，我们研究了如何在基于面板的LIS场景中强制实现信道正交性。所提出的方法包括：让一部分主动LIS面板相干地服务于一组用户，而另一部分LIS面板通过执行接收与再传输（RRTx）过程以半被动模式运行。这会产生一个符号间干扰（ISI）信道，我们在此刻画了实现时域和空域同步正交所需的半被动处理过程。随后，我们利用正交性约束中的剩余自由度（DoFs）来最小化半被动处理功率，并推导出闭式全局最小化解，从而能够高效实现所提出的方案。