Fully-Passive versus Semi-Passive IRS-Enabled Sensing: SNR Analysis

TL;DR

This paper compares the SNR performance of fully-passive and semi-passive IRS-enabled sensing systems, revealing that fully-passive IRS can achieve significantly higher SNR growth with increasing elements, especially in large-scale setups.

Contribution

The paper derives asymptotic SNR expressions for both fully-passive and semi-passive IRS sensing systems under different channel conditions, highlighting the superior scaling of fully-passive IRS.

Findings

Fully-passive IRS SNR scales as N^4, semi-passive as N^2.

Fully-passive IRS outperforms semi-passive when N is large.

Numerical results confirm theoretical asymptotic analysis.

Abstract

This paper compares the signal-to-noise ratio (SNR) performance between the fully-passive intelligent reflecting surface (IRS)-enabled non-line-of-sight (NLoS) sensing versus its semi-passive counterpart. In particular, we consider a basic setup with one base station (BS), one uniform linear array (ULA) IRS, and one point target at the BS's NLoS region, in which the BS and the IRS jointly design the transmit and reflective beamforming for performance optimization. By considering two special cases with the BS-IRS channels being line-of-sight (LoS) and Rayleigh fading, respectively, we derive the corresponding asymptotic sensing SNR when the number of reflecting elements $N$ at the IRS becomes sufficiently large. It is revealed that in the two special cases, the sensing SNR increases proportional to $N^2$ for the semi-passive IRS sensing system, but proportional to $N^4$ for the fully-passive IRS sensing system. As such, the fully-passive IRS sensing system is shown to outperform the semi-passive counterpart when $N$ becomes large, which is due to the fact that the fully-passive IRS sensing enjoys additional reflective beamforming gain from the IRS to the BS that outweighs the resultant path loss in this case. Finally, numerical results are presented to validate our analysis under different transmit and reflective beamforming design schemes.