Jamming a terahertz wireless link

The study investigates whether highly directional terahertz (THz) wireless links—often presumed more secure due to narrow beams and high-gain antennas—are vulnerable to intentional jamming. With growing bandwidth demands and anticipated migration to the 100 GHz–10 THz regime, THz systems leverage wide bandwidths for high data rates and directional antennas to overcome path loss. However, unlike conventional RF systems where jamming is well explored, THz links have unique characteristics: strong directivity necessitating accurate jammer aiming at the receiver, and broadband receivers that may admit out-of-band interference. The authors pose the question of how these features affect jamming efficacy against noncoherent on-off keying (OOK) links standardized in IEEE 802.15.3d. They examine static line-of-sight Alice–Bob links with a malicious jammer (Mallory) transmitting at an angle to Bob, exploring single-tone jamming at and away from the carrier (beat jamming) and modulated broadband jamming, and assess how envelope detection influences vulnerability and countermeasure viability.

Prior works established THz links as promising for high data rate communication and considered physical-layer security benefits of directional beams and leaky-wave antennas. However, the literature focused mainly on eavesdropping risks and secure transmission strategies, not on active jamming in THz bands. Jamming has long been studied at lower frequencies, typically assuming narrowband receivers and omnidirectional or less directive links, with known attack/defense taxonomies. Millimeter-wave/THz studies addressed interference and SINR with directional antennas and blocking, but did not analyze how noncoherent envelope detection and broadband receivers change the jamming threat model. The IEEE 802.15.3d standard includes noncoherent (envelope-detection) OOK receivers intended for low-cost short-range devices, suggesting relevance of noncoherent interference models seen in some UWB systems. This work fills the gap by experimentally characterizing THz jamming above 100 GHz, highlighting phenomena (e.g., beat jamming) not typically encountered in narrowband low-frequency systems.

Threat model and link: Alice (Tx) communicates with Bob (Rx) over a static line-of-sight, noncoherent OOK link using main lobes; Mallory (jammer) is static, aims at Bob at an angle θ (20–45°), coupling via Bob’s side lobes. Carrier and data: Alice transmits 1.12 Gbps OOK at 197.5 GHz. Bob detects with a waveguide-coupled zero-bias Schottky diode (envelope detector), baseband filtered to 0.1 MHz–6 GHz. Measurements include spectra, eye diagrams, real-time BER, and received power. Experimental setup: Alice’s Tx uses two slightly detuned 1535 nm DFB lasers and a photoconductive antenna (photomixing), with optical modulation via a LiNbO3 Mach–Zehnder modulator driven by a pulse pattern generator (PRBS length 2^7−1), amplified by EDFA. Mallory’s jammer uses a frequency multiplier chain (×16) driven by a 12.2–12.8 GHz RF oscillator; for broadband jamming, the RF is modulated by a double balanced mixer (≤500 MHz) driven by a PPG. Antennas: WR5.1 conical horns. Distances: Alice–Bob 210 mm; Mallory–Bob 280 mm. Lenses: Alice uses 75 mm and 120 mm; Mallory uses 75 mm. Jamming scenarios:

• Single-tone, center-frequency jamming: Mallory transmits a CW tone at νM=νA, varying angle and power to couple into Bob’s side lobes; BER and received power are recorded. A jamming efficiency metric e is defined, normalizing BER degradation between unjammed BER and a failure limit BERLimit=1e−3 (FEC threshold), to quantify attack efficacy.
• Decision threshold study: For noncoherent OOK, the eye diagram asymmetry under jamming is examined by sweeping Bob’s decision threshold and measuring BER vs SINR, comparing fixed vs optimized threshold.
• Beat jamming (detuned single-tone): Mallory sweeps νM from 197.6 to 201.6 GHz (and wider ranges in other tests) while Alice remains at 197.5 GHz, measuring spectral interference centered at Δν=|νA−νM| and corresponding BER.
• Modulated jamming: Mallory transmits 0.5 Gbps OOK noise-like data while sweeping carrier from 197.5 to 205 GHz, comparing to single-tone jamming at the same power. Analytical models:
• Coupling analysis via Friis transmission equation to relate Alice/Mallory received powers at Bob, incorporating antenna gains in the main and side-lobe directions and distances.
• Envelope-detection interference model showing that the detected power includes: (i) baseband data term; (ii) an interference term equal to Alice’s baseband multiplied by cos(2πΔνt), yielding spectral replicas centered at ±Δν with bandwidth equal to Alice’s; (iii) noise. For modulated jamming, an added DC term proportional to Mallory’s baseband exists.
• SINR computation by integrating signal, interference, and noise spectra over the main lobe bandwidth B, enabling semi-analytical BER prediction using an empirical BER–SINR fit for noncoherent OOK with non-midpoint decision threshold.
• Channel capacity impact assessed via normalized capacity C=log2(1+SINRjammed)/log2(1+SNRunjammed). Data acquisition: BER via real-time BERT without post-processing; spectra via spectrum analysis of baseband; eye diagrams via oscilloscope. Mallory’s angle constrained by optics; power levels reported relative to Alice’s transmit power (e.g., Mallory often ≥10 dB stronger nominally to compensate Bob’s side-lobe reception).

• Single-tone center-frequency jamming (νM=νA=197.5 GHz): • Jamming efficacy depends on Mallory’s coupling into Bob’s antenna pattern; BER degradation tracks Bob’s simulated radiation pattern as Mallory’s angle varies (20–45°). Up to θ≈43–45°, BER increases over no-jamming baseline. • At angles θM<27°, where Bob’s main lobe edge is accessible, Mallory achieves 100% jamming efficiency (complete disruption) at sufficient power. • At a fixed θ≈22° (Bob’s side-lobe ≈17 dB below boresight), increasing Bob’s received power by only ~0.25 dB raises jamming efficiency to ~50%; an increase of ~0.75 dB fully jams the link (e≥1). Example powers: Mallory 10.7 dBm vs Alice −10.5 dBm nominal. • Envelope detection creates asymmetric eye degradation: only the ‘1’ level broadens; ‘0’ remains largely unaffected. Optimal decision threshold shifts lower as interference increases; fixed thresholds perform notably worse than per-point optimized thresholds. • Capacity reduction: increasing interference lowers normalized capacity C computed from measured SINR.
• Beat jamming (νM≠νA): • Even a single-tone jammer detuned from Alice produces an interference band centered at Δν with bandwidth equal to Alice’s data bandwidth, due to multiplication in the envelope detector. Thus, the interference cannot be filtered without removing data when |Δν|<B. • Measured BER vs |Δν| is oscillatory, matching a SINR-based model: worst degradation at Δν≈0 and at Δν≈B/2. With B corresponding to 1.12 Gbps OOK, model parameters Ac=1, Am=0.17, baseline SINR≈19 dB, pulse width τ≈0.8929 ns reproduce data. • For |Δν|>B (interference lying outside Bob’s effective data band), BER impact becomes negligible. Consequently, broader transmission bandwidths (higher data rates) enlarge the window of vulnerability to beat jamming.
• Modulated jamming (0.5 Gbps OOK jammer): • More damaging than single-tone at the same transmit power: achieves e≥1 across 197.52–200.5 GHz, fully destroying the link. For νM>201.5 GHz, where single-tone jamming is ineffective, modulated jamming still attains e≈0.75. • Analytical expression reveals a DC interference term from Mallory’s modulation at the detector output, enabling disruptive effects even when νM is far from νA, provided Bob’s detector is sensitive at νM (here 140–220 GHz). Tailoring jammer bandwidth to overlap Alice’s baseband maximizes disruption.
• Practical insight: The vulnerability stems from noncoherent envelope detection at THz and the broadband nature of receivers, enabling both on-frequency and detuned (beat) or modulated jamming to be highly effective while potentially reducing detectability when detuned.

The findings directly address the core question of THz link susceptibility to jamming. Despite high-gain directional antennas, THz links with noncoherent envelope detection are highly vulnerable when a jammer can aim into receiver side lobes. Small increases in received power suffice to induce major BER degradation, and the asymmetric effect on ‘1’ bits forces dynamic threshold adjustments that an adaptive jammer can counter by varying power. Crucially, broadband detection enables beat jamming: a detuned single-tone generates in-band interference centered at the beat frequency with bandwidth matching the data, maximizing disruption at Δν≈0 and Δν≈B/2 and expanding vulnerability with higher data rates. Modulated jamming is even more potent due to a DC interference component, allowing attacks from further detuning as long as the receiver detects the carrier. These results imply that directivity alone does not confer security against active interference in THz systems. The capacity impact measured via SINR corroborates substantial throughput loss under jamming. The work motivates reconsideration of PHY designs and countermeasures for THz links. Countermeasures discussed include increasing Alice/Bob transmit power and antenna directivity to improve desired-link coupling relative to side-lobe coupling, monitoring received power/spectra/eye-diagram asymmetry for attack detection, and employing interference-robust techniques such as low-weight channel coding emphasizing ‘0’ bits, binary pattern modulation, chirp spread spectrum, or phase-domain spreading. However, reactive or intermittent jammers and advanced modulations require further investigation to assess resilience and detection strategies.

This work presents the first experimental study of jamming above 100 GHz on a noncoherent OOK THz link. It introduces and validates the concepts of single-tone jamming, beat jamming (detuned single-tone exploiting envelope detection), and modulated jamming, showing that minimal increases in received power (~0.25–0.75 dB) can substantially or completely disrupt links, that BER degradation is maximized at Δν≈0 and Δν≈B/2, and that modulated jamming extends disruptive capability even when the jammer’s carrier is far from the intended link. The key vulnerability arises from broadband, noncoherent envelope detection, making filtering ineffective and allowing detuned attacks with reduced detectability. The study quantifies the enhanced susceptibility of broadband THz receivers and underscores that directionality alone is insufficient for jamming resistance. Future research directions include assessing coherent and higher-order modulation schemes, multi-carrier systems, and dynamic/reactive jammer models; designing detector and front-end architectures less susceptible to envelope-based interference; and developing robust PHY/MAC-layer anti-jamming and detection techniques tailored to THz systems.

The experiments assume a simplified, static threat model: a constant, non-reactive jammer at fixed angles with line-of-sight propagation and controlled laboratory distances. The study focuses on a single noncoherent OOK modulation with envelope detection (zero-bias Schottky diode) and specific hardware (photomixer source, multiplier-chain jammer), which may limit generalizability to other receiver architectures (e.g., coherent, multi-carrier, higher-order modulations). Angular positioning was constrained by physical optics; the broadband modulator had limited 500 MHz bandwidth. Results pertain to short-range links within the detector band (140–220 GHz). The impact of multipath, mobility, adaptive beamforming, and reactive/on-off jammers was not explored and warrants future work.