The molecular absorption effect is the defining channel impairment of the terahertz (THz) band (0.1-10 THz) and, simultaneously, a potential resource for secure and covert transmission. Stemming from Han, Gao, Yang, and Jornet's IEEE Wireless Communications article 'Molecular Absorption Effect: A Double-Edged Sword of Terahertz Communications' (2023), the problem asks how to fully exploit the THz spectrum despite - and by means of - molecular absorption, through three linked open objectives: (1) a precise molecular absorption model for the entire spectral windows, including the 'flat' low-absorption regions that existing curve-fitting methods systematically under- and over-estimate, together with an altitude-dependent characterization of the spectral windows across communication scenarios; (2) cooperative hybrid visible-light/THz system designs exploiting the two bands' complementary weather sensitivities; and (3) physical-layer-security schemes for the two insecure zones left open by directivity- and absorption-based methods - immediately around the transmitter and around the legitimate receiver - for which widely-spaced antenna arrays exploiting range-dependent spherical-wave propagation are a leading candidate. A 2024 follow-up by the same group advanced objective (3) by proving far-field multi-antenna techniques and frequency diverse arrays ineffective and demonstrating a widely-spaced-array scheme, but its own results show that both insecure zones persist.

Background

Terahertz (THz) communications aim to exploit the 0.1-10 THz band for sixth-generation (6G) wireless links with tens-of-GHz bandwidths. A distinctive physical obstacle in this band is the molecular absorption effect: gas molecules along the propagation path - overwhelmingly water vapor, whose absorption is about six orders of magnitude stronger than that of the next contributor, oxygen - resonantly absorb and re-emit THz radiation, producing both a frequency-selective absorption loss (which grows exponentially with distance in a homogeneous medium) and an additional self-noise. Because the absorption coefficient depends on frequency, distance, and environmental parameters such as pressure, temperature, and gas composition (a property often called triple selectivity), the THz spectrum is fragmented into absorption peaks separated by spectral windows: frequency intervals of several GHz in which absorption is weak and where transmission is recommended. Two reference treatments exist. The ITU-R recommendation P.676 (current revision P.676-13, 2022) is the standard model of gaseous attenuation but covers only up to 1 THz. The comprehensive model of Jornet and Akyildiz (2011) covers the whole 0.1-10 THz band, including an absorption-noise model, by curve-fitting the absorption coefficient with asymmetric line shapes; this fit captures the absorption peaks accurately but misestimates the 'flat' low-absorption regions between peaks - precisely where the spectral windows lie. The window structure also varies strongly with altitude: water-vapor density decreases roughly exponentially with height, the source article reports molecular absorption decreasing by about two orders of magnitude per kilometer of altitude (with oxygen becoming the dominant absorber in the stratosphere), so windows widen and gradually merge as altitude increases. On the exploitation side of the 'double-edged sword', high antenna directivity and the distance-dependence of absorption can serve physical-layer security (PLS) - the design of transmissions whose secrecy rate, i.e., the rate reliably decodable by the legitimate receiver minus that decodable by an eavesdropper, is positive. Absorption-aided THz schemes include distance-adaptive absorption peak modulation (DA-APM), which hides covert signals inside absorption peaks, and SIC-free receiver artificial noise (RAN), in which the receiver self-jams with a waveform that the legitimate user survives without self-interference cancellation (SIC) thanks to THz-induced temporal broadening. A frequency diverse array (FDA) - an array with small frequency offsets across its antennas, producing a range-varying beam pattern - was proposed to separate a legitimate user and an eavesdropper at similar angles but different ranges; a widely-spaced array (WSA) instead places the two terminals in the near field of a physically large array, so that the range dependence of spherical-wave propagation distinguishes them. Finally, visible light communication (VLC) transmits data on modulated visible/infrared light and has complementary weather behavior to THz: VLC is immune to water-vapor absorption but scattered by clouds, dust, and other particles, whereas THz is weakened by humidity but robust against such particles; free-space optical (FSO) links behave similarly to VLC in this respect. Within this landscape, the source article analyzes how the molecular absorption effect shapes THz system design across terrestrial, aerial/space, sea-surface, and nano-scale scenarios, and identifies three unresolved obstacles to fully exploiting the THz spectrum, which together form the problem.

Problem Statement

The problem, as posed in Section VI of Han, Gao, Yang, and Jornet (2023), is to remove the three obstacles that keep the molecular absorption effect from being fully exploited in THz communications, under the common objective of making maximal use of the 0.1-10 THz spectrum and of absorption itself across communication scenarios (terrestrial, aerial/space, sea surface, nano-scale):

  1. Spectral-window modeling. Develop a molecular absorption model that is accurate over the entire spectral windows - including the 'flat' low-absorption regions between peaks where transmission actually occurs - removing the systematic under- and over-estimation of existing curve-fitting methods (which capture absorption peaks via asymmetric line shapes but misfit the flat regions); and deliver an altitude-dependent characterization of the spectral windows across different communication scenarios, since the water-vapor distribution that dominates absorption changes strongly with altitude (absorption falling by roughly two orders of magnitude per kilometer) and the window structure changes accordingly.

  2. Hybrid VLC/THz cooperation. Design cooperative hybrid visible-light and THz communication systems that exploit their complementary sensitivities - THz weakened by water vapor but robust against air particles such as clouds, sand, and dust; VLC immune to water vapor but vulnerable to particle scattering - to hold a transmission-quality metric (for example an achievable rate or an outage probability) at or above an explicitly stated threshold in every weather condition studied, where the studied conditions include at least a humidity-dominated regime and a particle-scattering regime.

  3. Security in the proximal zones. Devise physical-layer-security schemes for the two insecure zones left open by directivity- and absorption-based methods: immediately around the transmitter, where the transmitter-eavesdropper path gain is too large for absorption-based protection, and around the legitimate receiver, where the eavesdropper channel is highly correlated with the legitimate channel so that angle-dependent directivity and range-dependent absorption cannot create a capacity difference. Widely-spaced antenna communications exploiting the range-dependent pattern of spherical-wave propagation are identified as a leading candidate requiring further analysis and development.

Each of the three objectives is independent and may be resolved on its own while the others remain open, but the task as posed is fully resolved only when all three obstacles are closed. An objective counts as resolved only through the artifact it names and a demonstration of its acceptance condition: for objective 1, a window-region-accurate absorption model together with an altitude-dependent characterization of the windows across the stated communication scenarios; for objective 2, a cooperative VLC/THz design together with a demonstration that its stated transmission-quality threshold is met in every weather condition studied; and for objective 3, a physical-layer-security scheme for each of the two insecure zones, with a positive secrecy rate demonstrated over a stated range of transmitter-eavesdropper and legitimate-receiver-eavesdropper geometries. Analysis alone, or a scheme that closes only one of the two insecure zones, does not resolve objective 3.

References

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