Terahertz channel performance in ULEO satellite-to-ground communications.

The exponential growth in satellite data traffic demands communication systems exceeding current microwave capacity limitations, while the terahertz (THz) frequency band (0.1-10 THz) offers unprecedented bandwidth potential with superior weather resilience compared to optical systems, particularly when combined with ultra-low Earth orbit (ULEO) satellite deployments below 300 km altitude. This paper presents a comprehensive performance evaluation for ULEO-THz satellite-to-ground communications, analyzing three distinct transmission architectures: direct satellite-to-ground (S2G), satellite-relay-ground (SRG) forwarding, and satellite-to-high-altitude base station (S2H) with fiber backhaul. Our analysis leverages altitude-resolved atmospheric propagation models validated using yearlong meteorological data from four high-altitude stations in Tibet and Qinghai, China. It incorporates frequency-dependent atmospheric absorption using ITU-R standards, free-space path loss with curved atmospheric modeling, and regional atmospheric variations to derive total channel path loss, available bandwidth capacity, and bit error rate (BER) performance under both AWGN and Weibull fading conditions across multiple THz frequencies. Results demonstrate that direct S2G transmission at lower THz frequencies achieves optimal practical performance with maximum available bandwidth under QPSK modulation, while SRG suffers prohibitive cumulative losses from multiple hops, and S2H is rendered impractical for long-haul links by substantial electro-optical conversion and fiber transmission losses.