Browsing by Author "Yeh, Chia-Yi"

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    Feasibility of Passive Eavesdropping in Massive MIMO: An Experimental Approach
    (2018-04-18) Yeh, Chia-Yi; Knightly, Edward
    Massive MIMO systems have the potential for preventing passive eavesdropping as the signal transmitted by a large antenna array becomes highly focused. Prior works showed that passive eavesdropping becomes negligible when the number of BS antennas approaches to infinity in independent Rayleigh channel from a secrecy rate perspective. However, in practical massive MIMO systems, the number of BS antennas is in the order of a hundred, not infinity. Also, channels in the real world are not ideally independent. Furthermore, secrecy rate does not directly indicate whether a transmission can be decoded by the eavesdropper in practical wireless transmission. In this work, our analysis is based on real channel measurements from a 96-antenna ArgosV2 BS in 2.4 GHz band indoor environment with a LOS component. Instead of the asymptotic behavior, we focus on how the increasing number of BS antennas affect passive eavesdropping. Also, we propose to use the SNR difference between the intended user Bob and the eavesdropper Eve as a metric to determine how resistant to passive eavesdropping a system is. From our analysis based on real channel measurements, we find that increasing the number of antennas at the BS improves the ability of preventing passive eavesdropping, and a 96-antenna BS has the potential to prevent passive eavesdropping in the indoor LOS environment with careful power control. However, compared to the independent Rayleigh channel, indoor LOS environment is less passive-eavesdropping resistant under the same number of BS antennas. Furthermore, the marginal benefit of increasing an antenna in the indoor LOS environment decreases much faster than in the independent Rayleigh channel scenario.
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    Line-of-sight and non-line-of-sight links for dispersive terahertz wireless networks
    (AIP Publishing LLC, 2021) Ghasempour, Yasaman; Amarasinghe, Yasith; Yeh, Chia-Yi; Knightly, Edward; Mittleman, Daniel M.
    Despite the rapidly growing interest in exploiting millimeter and terahertz waves for wireless data transfer, the role of reflected non-line-of-sight (NLOS) paths in wireless networking is one of the least explored questions. In this paper, we investigate the idea of harnessing these specular NLOS paths for communication in directional networks at frequencies above 100 GHz. We explore several illustrative transmitter architectures, namely, a conventional substrate-lens dipole antenna and a leaky-wave antenna. We investigate how these high-gain directional antennas offer both new challenges and new opportunities for exploiting NLOS paths. Our results demonstrate the sensitivity to antenna alignment, power spectrum variations, and the disparity in supported bandwidth of various line-of-sight (LOS) and reflected path configurations. We show that NLOS paths can, under certain circumstances, offer even higher data rates than the conventional LOS path. This result illustrates the unique opportunities that distinguish THz wireless systems from those that operate at lower frequencies.
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    Security of Angularly Dispersive Terahertz Links
    (2022-01-07) Yeh, Chia-Yi; Knightly, Edward
    Angularly dispersive links are characterized by frequency-dependent radiation direction. In practice, this property manifests from wide bandwidths, as are expected in the terahertz (THz) regime, and from antenna structures such as the leaky-wave antenna (LWA). To date, angular dispersion has been shown to enable beam steering and path discovery, both are critical for establishing directional THz links. While angular dispersion provides new opportunities for THz communications, it also introduces new security threats. Namely, with angular dispersion, to send a wider band transmission from the transmitter Alice to the receiver Bob necessarily expands the spatial footprint of the transmission, potentially aiding an eavesdropper Eve. This thesis presents the first security study of THz angularly dispersive links using LWAs via a mix of analytical models and over-the-air experiments. In the first part of the thesis, I consider the threat of a same-distance Eve in the line-of-sight (LoS) scenario and study the unique security properties of angularly dispersive links. I show via both models and experiments that the LWA’s angle-frequency coupling leads to non-uniform secrecy capacity across sub-channels yielding advantages to an eavesdropper at edge frequencies. Yet, because different frequencies emit energy at different angles, the eavesdropper is thwarted from easily intercepting an entire wideband transmission. The experiments diverge from the analytical model in that the model underpredicts the eavesdropper’s advantage at angles smaller than the target user and subsequent asymmetric performance across angles. Nonetheless, both the model and measurements show that increasingly wide bandwidth and correspondingly wide beams have only a modest marginal security penalty. Next, I study secure coding strategies for angularly dispersive links via two representative secure coding strategies, termed I-SCADL (Independent Secure Coding for Angular Dispersive Links) and J-SCADL (Joint SCADL), with the former must code each frequency channel independently while the latter allows joint coding across frequency channels. I show that, due to angular dispersion, the independently-coded strategy, I-SCADL, results in a notable insecure region expansion both angularly and radially as the transmission band widens, whereas the joint coding strategy, J-SCADL, can effectively alleviate the secrecy degradation with increasing bandwidth as it exploits the a priori known non-uniformity across the frequency channels. The experimental results further demonstrate the advantage of J-SCADL over I-SCADL under beam asymmetry and irregularities as J-SCADL can preserve secrecy when Eve receives strong side lobes only in a subset of frequency channels. Nonetheless, for angularly dispersive links, even with J-SCADL, we find the insecure area expands with bandwidth due to the associated emission angle difference. Yet, we also find that the insecure area growth due to increasing bandwidth is significantly smaller compared to other factors including a wider single-tone beamwidth or a higher secrecy coding rate, suggesting that the security concern for angularly dispersive links under larger bandwidth is minor as long as proper secure coding strategy, such as J-SCADL, is employed.