# Data-Aided Secure Massive MIMO Transmission under the Pilot Contamination Attack

## TL;DR

This paper proposes a data-aided secure transmission scheme for massive MIMO systems under active pilot contamination attacks, revealing that reducing user signal power can enhance security and achieving logarithmic secrecy rate scaling with antennas.

## Contribution

It introduces a novel data-aided secure downlink transmission method that exploits eigenspace separation to combat active eavesdropping in massive MIMO systems.

## Key findings

- Separation of user and eavesdropper signals in eigenspaces when antennas grow large.
- Reducing user signal power can improve security against strong active attacks.
- Secrecy rate scales logarithmically with the number of antennas in single-user scenarios.

## Abstract

In this paper, we study the design of secure communication for time division duplex multi-cell multi-user massive multiple-input multiple-output (MIMO) systems with active eavesdropping. We assume that the eavesdropper actively attacks the uplink pilot transmission and the uplink data transmission before eavesdropping the downlink data transmission of the users. We exploit both the received pilots and the received data signals for uplink channel estimation. We show analytically that when the number of transmit antennas and the length of the data vector both tend to infinity, the signals of the desired user and the eavesdropper lie in different eigenspaces of the received signal matrix at the base station provided that their signal powers are different. This finding reveals that decreasing (instead of increasing) the desired user's signal power might be an effective approach to combat a strong active attack from an eavesdropper. Inspired by this observation, we propose a data-aided secure downlink transmission scheme and derive an asymptotic achievable secrecy sum-rate expression for the proposed design. For the special case of a single-cell single-user system with independent and identically distributed fading, the obtained expression reveals that the secrecy rate scales logarithmically with the number of transmit antennas. This is the same scaling law as for the achievable rate of a single-user massive MIMO system in the absence of eavesdroppers. Numerical results indicate that the proposed scheme achieves significant secrecy rate gains compared to alternative approaches based on matched filter precoding with artificial noise generation and null space transmission.