Low-Probability-of-Intercept Cellular and Wi-Fi Communications

Problem Description

This project tackles the challenge of enabling low-probability-of-intercept (LPI) communications in real-world 5G and WiFi systems. Existing physical-layer LPI techniques often rely on custom hardware or are incompatible with billions of deployed user devices, limiting their practicality. The key vulnerability arises because eavesdroppers exploit the same pilot signals as legitimate receivers to estimate the channel and decode data. When pilots and data symbols experience identical effective channels, both legitimate users and eavesdroppers can successfully demodulate transmissions. The central problem, therefore, is how to prevent eavesdroppers from decoding data—without modifying user devices or requiring knowledge of eavesdropper channels.

Challenges

Several challenges complicate this objective. First, any new technique must be fully compatible with existing WiFi and 4G/5G devices, meaning no changes to user hardware or software can be assumed. Second, the system must ensure that intended receivers continue to observe aligned pilot and data channels, while forcing eavesdroppers to observe misaligned ones. This requires careful precoder design under practical constraints such as limited transmit power and a finite number of antennas. Finally, the solution must be effective against both out-of-network eavesdroppers (with no channel knowledge available) and in-network eavesdroppers (with partial channel knowledge), while balancing the tradeoff between security and communication throughput.

Approach / Methodology

We propose Spatial Pilot Perturbation (SPP), a novel MIMO-based precoding technique. The core idea is to apply different precoders to pilot and data symbols within the same transmission frame. For intended receivers, the precoders are designed so that pilots and data experience identical compound channels, enabling correct decoding. For eavesdroppers, however, the pilot and data experience mismatched compound channels, disrupting channel estimation and preventing demodulation. SPP introduces two key design elements: a transmission vector, optimized as if no eavesdroppers exist, and a perturbation vector, chosen from the nullspace of the legitimate user’s channel to distort eavesdropper reception. The approach generalizes to multi-user MIMO, single versus multiple eavesdroppers, and scenarios with or without eavesdropper CSI. Importantly, SPP requires no modifications to user devices and leverages the standard pilot structures already present in 5G and WiFi systems.

Main Results

We validated SPP through both numerical simulations and real-world experiments on 5G and WiFi software-defined radio testbeds. Results demonstrate that, even without any knowledge of eavesdroppers, SPP reduces eavesdropping rates to ≤0.2% in 5G and ≤0.9% in WiFi, with only a 10–18% reduction in throughput. Further experiments show that when limited eavesdropper CSI is available, the error vector magnitude (EVM) gap between users and eavesdroppers increases by several dB, providing additional security. Overall, SPP is shown to be highly effective, backward-compatible, and practical for deployment in existing wireless systems, marking an important step toward real-world adoption of physical-layer LPI techniques.

Publications

• SPP: Achieving low-probability-of-intercept cellular and Wi-Fi communications via MIMO-based spatial pilot perturbation [PDF]
  P. Yan*, M. Afshari*, and H. Zeng
  to appear in IEEE Transaction on Wireless Communications, 2025.