Pilot Spoofing Attack on the Downlink of Cell-Free Massive MIMO: From the Perspective of Adversaries

TL;DR

This work addresses the vulnerability of downlink training in cell-free mMIMO to pilot spoofing attacks (PSAs) and derives a closed-form expression for the per-user downlink rate under PSA. It proposes a min-max power allocation framework for adversarial APs to minimize the maximum achievable rate, and simultaneously analyzes an alternative attack where adversaries transmit precoded random interference during downlink data transmission. The results show that downlink data-phase interference can be more damaging than PSAs in downlink training, and provide algorithmic tools (SOCP/SCA with bisection) to optimize adversarial power under practical constraints. The findings underscore the need for protecting uplink/downlink channel information and developing countermeasures to secure cell-free mMIMO against active, colluding adversaries.

Abstract

The channel hardening effect is less pronounced in the cell-free massive multiple-input multiple-output (mMIMO) system compared to its cellular counterpart, making it necessary to estimate the downlink effective channel gains to ensure decent performance. However, the downlink training inadvertently creates an opportunity for adversarial nodes to launch pilot spoofing attacks (PSAs). First, we demonstrate that adversarial distributed access points (APs) can severely degrade the achievable downlink rate. They achieve this by estimating their channels to users in the uplink training phase and then precoding and sending the same pilot sequences as those used by legitimate APs during the downlink training phase. Then, the impact of the downlink PSA is investigated by rigorously deriving a closed-form expression of the per-user achievable downlink rate. By employing the min-max criterion to optimize the power allocation coefficients, the maximum per-user achievable rate of downlink transmission is minimized from the perspective of adversarial APs. As an alternative to the downlink PSA, adversarial APs may opt to precode random interference during the downlink data transmission phase in order to disrupt legitimate communications. In this scenario, the achievable downlink rate is derived, and then power optimization algorithms are also developed. We present numerical results to showcase the detrimental impact of the downlink PSA and compare the effects of these two types of attacks.

Figures (7)

• Figure 1: Illustration of the downlink PSA, (a) Malicious APs carry out channel estimation during the uplink training phase; (b) Malicious APs send the beamformed downlink pilot sequences to users.
• Figure 2: Downlink achievable sum rate versus the number of adversarial APs, where $M=128$ and $K=4$.
• Figure 3: Downlink achievable sum rate versus the number of legitimate APs, where $N$ varies and $K=4$.
• Figure 4: Downlink achievable sum rate versus the transmit power of adversarial APs, where $M=128$, $N=32$, and $K=4$.
• Figure 5: Downlink achievable sum rate versus the number of legitimate APs, where $\mu_{\rm dp}$ varies, $N=64$ and $K=4$.