Performance Analysis of Physical Layer Security: From Far-Field to Near-Field

TL;DR

This paper analyzes the secrecy performance of near-field and far-field communications using capacity and power metrics, revealing how antenna array size and channel models influence security and introducing the concept of depth of insecurity.

Contribution

It provides closed-form expressions and asymptotic analysis for secrecy capacity and power requirements in near-field and far-field scenarios, including a novel security metric called depth of insecurity.

Findings

Near-field expands secure transmission areas, especially with eavesdroppers aligned with the receiver.

Increasing transmit antennas improves secrecy performance and reduces insecurity depth.

Secrecy capacity and power requirements follow specific scaling laws, conserving energy.

Abstract

The secrecy performance in both near-field and far-field communications is analyzed using two fundamental metrics: the secrecy capacity under a power constraint and the minimum power requirement to achieve a specified secrecy rate target. 1) For the secrecy capacity, a closed-form expression is derived under a discrete-time memoryless setup. This expression is further analyzed under several far-field and near-field channel models, and the capacity scaling law is revealed by assuming an infinitely large transmit array and an infinitely high power. A novel concept of "depth of insecurity" is proposed to evaluate the secrecy performance achieved by near-field beamfocusing. It is demonstrated that increasing the number of transmit antennas reduces this depth and thus improves the secrecy performance. 2) Regarding the minimum required power, a closed-form expression is derived and analyzed within far-field and near-field scenarios. Asymptotic analyses are performed by setting the number of transmit antennas to infinity to unveil the power scaling law. Numerical results are provided to demonstrate that: i) compared to far-field communications, near-field communications expand the areas where secure transmission is feasible, specifically when the eavesdropper is located in the same direction as the intended receiver; ii) as the number of transmit antennas increases, neither the secrecy capacity nor the minimum required power scales or vanishes unboundedly, adhering to the principle of energy conservation.