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.
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.
The secrecy performance of forward link multi-user massive multiple-input multiple-output (MIMO) system is investigated in this paper. The base station (BS) equipped with a large number of transmit antennas sends mutually independent datas to multiple single-antenna legitimate users simultaneously. A passive multi-antenna eavesdropper is placed in the scenario to intercept the signals. We consider the case that the number of legitimate users and eavesdropper antennas increases proportionally with the number of BS antennas. Maximum ratio transmission (MRT) is used as linear precoder at the BS. Lower bounds on the ergodic achievable rate of legitimate users and upper bounds of the eavesdropper are derived. By minimizing the upper bounds on the achievable rates of the eavesdropper while satisfying the rate constraints of legitimate users, we present the corresponding optimal radiated power scaling (RPS) factor. In the limit of an infinite number of BS antennas, we prove that the optimal RPS factor converge to a different constant limit with that of the case in which the number of legitimate users and eavesdropper antennas is finite. We show that when the number of BS antennas grows large, the secrecy performance is mainly determined by the ratios between the number of BS antennas, legitimate users and eavesdropper antennas. A number of simulation results are presented to visualize the analysis.
Physical-layer secure transmissions require that the legitimate parties Alice and Bob have certain signal advantages over the eavesdropper Eve. Unfortunately, it is still unknown what advantages can be guaranteed realistically in practice. This letter shows that hybrid massive MIMO can be exploited to the advantage of Alice for this purpose. Specifically, it allows Alice to use large antenna arrays that stealthy eavesdroppers cannot afford in many practical situations. To realize this advantage, an efficient transmission scheme is developed with hybrid massive MIMO, random dumb antenna selection, and channel reciprocity-based signal randomization techniques. It can secure the transmission against multiple stealthy eavesdroppers. Transmission security is analyzed assuming eavesdroppers with sufficient side knowledge and channel estimation capabilities. Simulations are conducted to verify the superior performance.
Massive Multiple-Input Multiple-Output (MIMO) has become a crucial enabling technology for 5G and beyond, providing previously unheard-of increases in energy and spectrum efficiency. It is still difficult to guarantee secure communication in these systems, particularly when it comes to passive eavesdroppers whose base station is unaware of their channel state information. By taking advantage of the inherent randomness of wireless channels, Physical Layer Security (PLS) offers a promising paradigm; however, its efficacy in massive MIMO is heavily reliant on resource allocation and transmission strategies. In this work, the performance of secure transmission schemes, such as Maximum Ratio Transmission (MRT), Zero-Forcing (ZF), and Artificial Noise (AN)-aided beamforming, is examined when passive eavesdroppers are present. This work will use extensive Monte Carlo simulations to assess important performance metrics such as energy efficiency, secrecy outage probability, and secrecy sum rate under different system parameters (e.g., number of antennas, Signal-to-Noise Ratio (SNR), power allocation). The results aim to provide comparative insight into the strengths and limitations of different PLS strategies and to highlight open research directions to design scalable, energy-efficient, and robust secure transmission techniques in future 6G networks.
Wireless channels are vulnerable to eavesdroppers due to their broadcast nature. One approach to thwart an eavesdropper (Eve) is to decrease her SNR, e.g., by reducing the signal in her direction. Unfortunately, such methods are vulnerable to (1) a highly directional Eve that can increase her received signal strength and (2) Eve that is close to the receiver, Bob, or close to the transmitter, Alice. In this paper, we design and experimentally evaluate Multipath Multicarrier Misinformation to Adversaries (M3A), a system for Alice to send data to Bob while simultaneously sending misinformation to Eve. Our approach does not require knowledge of Eve’s channel or location and, with multipath channels, randomly transforms Eve’s symbols even if Eve is located one wavelength-scale distance from Bob (approximately 10 cm) or if Eve is located between Alice and Bob in their direct path (Eve is approximately 1/3 closer to Alice). In particular, our approach is to move each of Eve’s received symbols (over time and across subcarriers), to an independently random transformation as compared to Bob, without Alice or Bob knowing Eve’s location or channel. We realize this by modulating Alice’s per-subcarrier beamforming weights with an i.i.d. random binary sequence, as if Alice had a separate antenna array for each subcarrier, and could randomly turn antennas in each array on and off. We implement M3A on a real-time Massive MIMO testbed, and show that M3A increases Eve’s bit error rate up to more than two hundredfold compared to beamforming, even if she is positioned approximately a wavelength away, whether above, below, or beside Bob. Finally, to ensure reliability at Bob, we show that with M3A, Bob’s bit error rate is approximately an order of magnitude lower than achieved with prior work.
In this paper, we investigate secure and reliable transmission strategies for multi-cell multi-user massive multipleinput multiple-output (MIMO) systems in the presence of an active eavesdropper. We consider a time-division duplex system where uplink training is required and an active eavesdropper can attack the training phase to cause pilot contamination at the transmitter. This forces the precoder used in the subsequent downlink transmission phase to implicitly beamform towards the eavesdropper, thus increasing its received signal power. We derive an asymptotic achievable secrecy rate for matched filter precoding and artificial noise (AN) generation at the transmitter when the number of transmit antennas goes to infinity. For the achievability scheme at hand, we obtain the optimal power allocation policy for the transmit signal and the AN in closed form. For the case of correlated fading channels, we show that the impact of the active eavesdropper can be completely removed if the transmit correlation matrices of the users and the eavesdropper are orthogonal. Inspired by this result, we propose a precoder null space design exploiting the low rank property of the transmit correlation matrices of massive MIMO channels, which can significantly degrade the eavesdropping capabilities of the active eavesdropper.
In this paper, a secure communication model for cognitive multi-user massive multiple-input multiple-output (MIMO) systems with underlay spectrum sharing is investigated. A secondary (cognitive) multi-user massive MIMO system is underlaid within a primary (licensed) multi-user massive MIMO system. A passive multi-antenna eavesdropper is eavesdropping upon either the primary or secondary transmission. To this end, physical layer secure transmissions are provisioned for both primary and secondary systems by exploiting linear precoders and artificial noise (AN) generation. The excess degrees-of-freedom at the primary massive MIMO base-station are exploited to transmit the AN sequence by using random AN shaping matrices for the purpose of degrading the signal decodability at the eavesdropper. Achievable asymptotic secrecy rates for infinitely many base-station antennas are derived, and these secrecy rates are shown to be independent of the eavesdropper channels. Moreover, for infinitely many secondary base-station antennas, the secondary system becomes independent of the primary system allowing it to operate at its maximum transmission power without disturbing the primary system.