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

TL;DR

This paper introduces a dynamic hybrid beamforming approach for near-field secure communications using large-scale antenna arrays, exploiting spherical wave properties to enhance security and data rates in NFC scenarios.

Contribution

It proposes a novel near-field dynamic precoding scheme with artificial noise that improves security and performance without requiring eavesdropper CSI, leveraging NFC's unique spherical wave characteristics.

Findings

Achieves approximately 20% higher rate capacity than existing precoders.

Effectively disrupts eavesdroppers without their CSI.

Enhances security in both angular and distance dimensions.

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.