Securing Angularly Dispersive Terahertz Links with Coding

With the large bandwidths available in the terahertz regime, directional transmissions can exhibit angular dispersion, i.e., frequency-dependent radiation direction. Unfortunately, angular dispersion introduces new security threats as increased bandwidth necessarily yields a larger signal footprint in the spatial domain and potentially benefits an eavesdropper. This paper is the first study of secure transmission strategies on angularly dispersive links. Based on information theoretic foundations, we propose a transmission strategy that channelizes the wideband transmission in frequency, and performs secure coding across frequency channels. With model-driven evaluations and over-the-air experiments, we show that the proposed method exploits the properties of angular dispersion to realize secure wideband transmissions, despite the increased signal footprint and even for practical irregular beams with side lobes and asymmetry. In contrast, without the proposed cross-channel coding strategy, angularly dispersive links can suffer from significant security degradation when bandwidth increases. In addition, we find that the security degradation due to bandwidth increment for angularly dispersive links is secondary compared to other factors including the selected secrecy rate or the directivity of the link. Nonetheless, we find that a higher angular dispersion level, i.e., a larger angular spread with the same bandwidth, results in a higher security degradation as bandwidth increases.