Eavesdropping in Massive MIMO: New Vulnerabilities and Countermeasures

Massive multiple-input and multiple-output (massive MIMO) has the potential to thwart passive eavesdropping as the signals transmitted by large antenna arrays become highly focused. Indeed, the impact of passive eavesdropping has been shown to be negligible when the base station (BS) antenna size approaches infinity for Rayleigh channels. In this paper, we experimentally explore eavesdropping in massive MIMO incorporating real-world factors including a limited BS antenna array size, potential correlation in over-the-air channels, and adaptation of modulating and coding schemes (MCS) over a discrete and finite set. Using a 96-antenna ArgosV2 BS, we first explore scaling the array size and identify eavesdropper (Eve) advantages due to channel correlation. We next identify the "MCS saturation regime" as a vulnerability even with high SNR due to limited MCS levels, and thereby demonstrating the need for power control as a counter-strategy, especially considering Eve's advantages in the over-the-air channels. We further demonstrate Eve's gain in optimizing her position, not only via being nomadic and searching for the most favorable position, but also via exploiting predictable line-of-sight (LoS) positional vulnerabilities. Specifically, we demonstrate Eve's advantage by simply sharing the elevation angle with Bob in the LoS scenario. Finally, we examine how Eve's advantage due to channel correlation scales with more eavesdropping antennas.