THz Channels for Short-Range Mobile Networks: Multipath Channel Behavior and Human Body Shadowing Effects

Technical Summary: THz Channels for Short-Range Mobile Networks

Problem Statement
The terahertz (THz) band (0.1–10 THz) is identified as a critical enabler for sixth-generation (6G) mobile networks, promising ultra-high data rates (>100 Gbps) and low latency (<1 ms) through the utilization of tens of GHz transmission bandwidths. However, the deployment of fully mobile and reliable THz wireless access faces significant hurdles. Unlike point-to-point fixed links, mobile scenarios are plagued by extreme spreading losses, interaction losses, and the fragility of communication links due to human body shadowing (HBS). While highly directional communication is necessary to overcome path loss, it complicates the maintenance of stable links when line-of-sight (LoS) is blocked. Furthermore, the scarcity of viable multipath signals in non-line-of-sight (NLoS) regions, exacerbated by reflection losses and surface roughness, challenges the implementation of spatial multiplexing and diversity techniques. Existing channel models often lack generalizability across diverse environments, and HBS modeling frequently relies on simplified geometries that fail to capture the dynamic nature of human-induced blockages at THz frequencies.

Methodology
This paper employs a dual-pronged approach combining empirical channel sounding and motion capture-integrated modeling:

1. Multipath Channel Characterization: The authors utilized a custom-developed double-directional high-resolution channel sounder operating at 300 GHz. The system employs angle scanning with 26 dBi rectangular horn antennas to capture channel impulse responses in time, frequency, and space domains. Measurements were conducted across four distinct environments: two conference rooms of different scales, a corridor, and an open square. The study focuses on extracting multipath clusters and analyzing their power delay profiles, cluster counts, and relative power compared to the LoS path.
2. Human Body Shadowing (HBS) Modeling: To address dynamic blockage, an integrated measurement system was developed to simultaneously acquire 300 GHz channel responses and human motion data. A motion capture (MoCap) system generated human body point clouds, which were synchronized with a continuous wave (CW) signal measurement setup. The authors proposed a ray-tracing (RT) based HBS prediction model that utilizes a "human-shaped screen." This screen is derived by projecting the human point cloud onto a plane perpendicular to the LoS. The model treats the human body as a diffracting edge, incorporating Uniform Geometrical Theory of Diffraction (UTD) to account for diffraction and creeping effects, rather than relying on simple rectangular or cylindrical approximations.

Key Contributions and Results

• Environment-Dependent Multipath Characteristics:

  • Cluster Distribution: The study reveals that the number and strength of multipath clusters are highly sensitive to the environment size. Corridors and medium conference rooms yielded an average of eight clusters, while large conference rooms had six, and open squares had only four.
  • Power and Feasibility: In indoor environments, approximately 40% of multipath clusters possessed a relative power greater than −10 dB, suggesting feasibility for multi-stream transmission. In contrast, this proportion dropped below 10% in open square environments, indicating that multipath utilization is significantly less viable in large, open outdoor spaces.
  • Propagation Mechanisms: Single-bounce reflections were the dominant mechanism aside from the direct wave. In NLoS conditions, diffracted waves were largely unobservable, and obstructions like tree leaves caused losses exceeding 10 dB.
  • Capacity Implications: Even without passive reflecting surfaces (PRS), a 4×4 multi-beam MIMO configuration in LoS scenarios could achieve an average channel capacity exceeding 9 bps/Hz at a 10-meter separation with a 20 dB SNR. With PRS, this capacity could reach up to 18 bps/Hz.

• Advanced HBS Modeling:

  • Human-Shaped Screen: The proposed model, which uses a human-shaped screen derived from MoCap data, demonstrated superior accuracy compared to conventional rectangular screen models.
  • Validation: In an office scenario validation, the proposed model reduced the bias error in predicting the fading pattern envelope by a factor of five compared to the rectangular model. This improvement is attributed to the model's ability to accurately capture diffraction paths and phase stationarity points.
  • Predictive Capability: The results suggest that RT-based simulations using this specific HBS model can serve as a reliable, computationally efficient alternative to full-wave electromagnetic simulations, enabling real-time link context prediction for proactive path scheduling.

Significance and Claims
The paper asserts that its findings provide essential insights for the design of robust, high-data-rate, short-range THz communication systems anticipated in future 6G networks. By systematically analyzing channel behavior across multiple environments using a consistent setup, the work highlights the necessity of incorporating environmental factors into system design, particularly for spatial multiplexing and diversity techniques. Furthermore, the introduction of a motion capture-based approach for HBS measurement and prediction addresses a critical gap in the literature, offering a method to enable proactive path scheduling and mitigate link degradation caused by human blockage. The authors conclude that while THz bands offer transformative potential, realizing reliable mobile access requires a unified understanding of static multipath behaviors and dynamic blockage modeling, paving the way for future system-level evaluations and the integration of technologies like Intelligent Reflecting Surfaces (IRS) and near-field communications.