Dynamic Precoding for Near-Field Secure Communications: Implementation and Performance Analysis

TL;DR

This work tackles secure wireless transmission in near-field MU-MISO systems employing extremely large antenna arrays. It introduces a dynamic hybrid beamforming architecture with artificial noise that leverages spherical near-field wavefronts to secure communication at the exact legitimate-user locations without requiring eavesdroppers' CSI. The approach combines a low-complexity SVD-based analog precoder with a dynamic AN-aided ZF baseband precoder, and provides closed-form insights into average SINR, secrecy capacity, and secrecy outage, including multi-path extensions and secrecy maps. Numerical results at 28 GHz show about 20% higher secrecy rate compared with ZF and WMMSE precoders, and demonstrate secure transmission across angular and distance dimensions thanks to near-field focusing. The work paves the way for practical, low-comomplexity secure NFC in ELAA-enabled 6G networks, with future directions toward MU-MIMO scenarios and multi-path secrecy analyses.

Abstract

The increase in antenna apertures and transmission frequencies in next-generation wireless networks is catalyzing advancements in near-field communications (NFC). In this paper, we investigate secure transmission in near-field multi-user multiple-input single-output (MU-MISO) scenarios. Specifically, with the advent of extremely large-scale antenna arrays (ELAA) applied in the NFC regime, the spatial degrees of freedom in the channel matrix are significantly enhanced. This creates an expanded null space that can be exploited for designing secure communication schemes. Motivated by this observation, we propose a near-field dynamic hybrid beamforming architecture incorporating artificial noise, which effectively disrupts eavesdroppers at any undesired positions, even in the absence of their channel state information (CSI). Furthermore, we comprehensively analyze the dynamic precoder's performance in terms of the average signal-to-interference-plus-noise ratio, achievable rate, secrecy capacity, secrecy outage probability, and the size of the secrecy zone. In contrast to far-field secure transmission techniques that only enhance security in the angular dimension, the proposed algorithm exploits the unique properties of spherical wave characteristics in NFC to achieve secure transmission in both the angular and distance dimensions. Remarkably, the proposed algorithm is applicable to arbitrary modulation types and array configurations. Numerical results demonstrate that the proposed method achieves approximately 20\% higher rate capacity compared to zero-forcing and the weighted minimum mean squared error precoders.

Figures (10)

• Figure 1: The illustration of a downlink MU-MISO system with multiple eavesdroppers, where $k$ represents the $k$th time slot.
• Figure 2: The illustration of Users and Eavesdroppers, where: (a) Fixed users and eavesdroppers positions; (b) Same direction for User 2 and Eavesdropper 2.
• Figure 3: The top view of beampatterns at $z=0.55d_F$, where: (a) Beampattern of the precoder for User 1; (b) Beampattern of the precoder for User 2.
• Figure 4: The side view of beampatterns, where: (a) Beampattern of the precoder for User 1 at $x=-0.3d_F$; (b) Beampattern of the precoder for User 2 at $x=0.2d_F$.
• Figure 5: Constellation diagram of received signals at legitimate users and eavesdroppers relying on the proposed algorithm, where the SINR for legitimate users is $15$ dB, and the number of total time slots is $K=800$. (a) User 1, 8-PSK; (b) User 2, 16-QAM; (c) Eavesdropper 1, chaotic constellation; (d) Eavesdropper 2, chaotic constellation.