The emerging concept of extremely-large holographic multiple-input multiple-output (HMIMO), beneficial from compactly and densely packed cost-efficient radiating meta-atoms, has been demonstrated for enhanced degrees of freedom even in pure line-of-sight conditions, enabling tremendous multiplexing gain for the next-generation communication systems. Most of the reported works focus on energy and spectrum efficiency, path loss analyses, and channel modeling. The extension to secure communications remains unexplored. In this paper, we theoretically characterize the secrecy capacity of the HMIMO network with multiple legitimate users and one eavesdropper while taking into consideration artificial noise and max-min fairness. We formulate the power allocation (PA) problem and address it by following successive convex approximation and Taylor expansion. We further study the effect of fixed PA coefficients, imperfect channel state information, inter-element spacing, and the number of Eve's antennas on the sum secrecy rate. Simulation results show that significant performance gain with more than 100\% increment in the high signal-to-noise ratio (SNR) regime for the two-user case is obtained by exploiting adaptive/flexible PA compared to the case with fixed PA coefficients.

翻译：新兴的全息大规模多输入多输出（HMIMO）概念，得益于紧凑密集布置的成本高效辐射超原子，即便在纯视距条件下也能增强自由度，从而为下一代通信系统带来巨大的复用增益。现有文献大多聚焦于能量效率和频谱效率、路径损耗分析以及信道建模，而其在安全通信领域的扩展尚待探索。本文从理论层面刻画了具有多个合法用户和一个窃听者的HMIMO网络的保密容量，并综合考虑了人工噪声和最大最小公平性。我们构建了功率分配（PA）问题，并采用逐次凸逼近和泰勒展开进行求解。进一步研究了固定PA系数、非完美信道状态信息、阵元间距以及窃听者天线数量对保密和速率的影响。仿真结果表明，在双用户场景的高信噪比（SNR）区域，相较于固定PA系数，采用自适应/灵活PA可实现超过100%的显著性能增益。