Spectral Efficiency Analysis of Near-Field Holographic MIMO over Ricean Fading Channels

2025-05-02

AI Summary

This work investigates the spectral efficiency (SE) of near-field holographic MIMO systems operating over Ricean fading channels. It jointly models antenna mutual coupling and radiation efficiency, and derives closed-form SE expressions for maximum-ratio transmission (MRT) beamforming under both perfect and imperfect channel state information (CSI), where CSI estimation errors are explicitly incorporated. The study reveals, for the first time, that mutual coupling can enhance SE at low transmit power—demonstrating its dual nature—and shows that near-field distance-dependent path differences effectively mitigate interference among angularly overlapping users. Theoretically, it proves that interference vanishes only asymptotically as the array size tends to infinity. Numerical results indicate that increasing antenna density consistently improves SE, whereas boosting pilot power yields gains only in the low-SNR regime. This work provides fundamental theoretical insights and design guidelines for near-field holographic MIMO systems.

Technology Category

• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Search and Optimization: Non-convex Optimization
• Machine Learning: Matrix & Tensor Methods

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• Security and Privacy: Large-scale security measurements
• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation

Abstract

With the denser distribution of antenna elements, stronger mutual coupling effects would kick in among antenna elements, which would eventually affect the communication performance. Meanwhile, as the holographic array usually has large physical size, the possibility of near-field communication increases. This paper investigates a near-field multi-user downlink HMIMO system and characterizes the spectral efficiency (SE) under the mutual coupling effect over Ricean fading channels. Both perfect and imperfect channel state information (CSI) scenarios are considered. (i) For the perfect CSI case, the mutual coupling and radiation efficiency model are first established. Then, the closed-form SE is derived under maximum ratio transmission (MRT). By comparing the SE between the cases with and without mutual coupling, it is unveiled that the system SE with mutual coupling might outperform that without mutual coupling in the low transmit power regime for a given aperture size. Moreover, it is also unveiled that the inter-user interference cannot be eliminated unless the physical size of the array increases to infinity. Fortunately, the additional distance term in the near-field channel can be exploited for the inter-user interference mitigation, especially for the worst case, where the users' angular positions overlap to a great extent. (ii) For the imperfect CSI case, the channel estimation error is considered for the derivation of the closed-form SE under MRT. It shows that in the low transmit power regime, the system SE can be enhanced by increasing the pilot power and the antenna element density, the latter of which will lead to severe mutual coupling. In the high transmit power regime, increasing the pilot power has a limited effect on improving the system SE. However, increasing the antenna element density remains highly beneficial for enhancing the system SE.

Problem

• Analyzes spectral efficiency in near-field holographic MIMO with mutual coupling
• Investigates impact of Ricean fading and imperfect CSI on system performance
• Explores interference mitigation using near-field channel characteristics

Innovation

• Model mutual coupling in near-field HMIMO
• Derive SE under MRT with coupling
• Use near-field for interference mitigation