This paper presents the first vector network analyzer (VNA)-based sub-Terahertz (sub-THz) phase-compensated channel sounder at 220-330 GHz using radio-over-fiber (RoF) techniques that could enable long-range phase-coherent measurements. The optical cable solution enables long-range channel measurements at sub-THz bands, since it can effectively minimize the cable loss. This paper also proposes a novel phase compensation scheme to stabilize the phase variations introduced by optical fiber of the channel sounder to enable its application in multichannel/antenna measurements. This proposed channel sounder is validated in back-to-back measurements under two optical cable conditions, i.e., with presence of thermal changes and mechanical stress. The phase variation introduced by the cable effects in the system is shown to be over 400 • in 220-330 GHz, compared to 15 • at 220-288 GHz and 37 • in 288-330 GHz after compensation, respectively, demonstrating the robustness and effectiveness of the developed channel sounder in practice. The developed system, which has a dynamic range of 106.7 dB, can support measurement range up to 300 m (limited by the optical cable length in our system and subject to over-the-air signal transmission loss in practical environment).

Sub-THz, i.e. frequency range 100 -300 GHz, frequency bands have attracted huge interest in recent years for beyond 5G communication and high data-rate applications, due to its large available frequency bandwidth. In this work, we presented two vector network analyzer (VNA) based channel sounder systems, operating at the frequency range of 75-110 GHz and 220-330 GHz, respectively. The focus is on the channel sounder structure, link budget as well as system calibration performance. Moreover, we presented a simple wideband directional measurement in a rich scattering laboratory scenario with the two discussed channel sounders. The results showed that similar dominant multipath components (MPCs) can be observed in the two frequency bands, yet many more weak MPCs can be seen at 100 GHz, compared to results at 300 GHz.

With the increasing demand for high bandwidth wireless communication systems, and with a congested spectrum in the sub-6 GHz frequency bands, researchers have been looking into exploration of millimeter wave (mmWave) and sub-terahertz (sub-THz) frequency bands. Channel modeling is essential for system design and performance evaluation of new wireless communication systems. Accurate channel modeling relies on reliable measured channel data, which is collected by high-fidelity channel sounders. Furthermore, it is of importance to understand to which extent channel parameters are frequency dependent in typical deployment scenario (including both indoor short-range and outdoor long-range scenarios). To achieve this purpose, this paper presents a state-of-art long-range 28 GHz and 300 GHz VNA-based channel sounder using optical cable solutions, which can support a measurement range up to 300 m and 600 m in principle, respectively. The design, development and validation of the long-range channel sounders at mmWave and sub-THz bands are reported, with a focus on their system principle, link budget, and back-to-back measurements. Furthermore, a measurement campaign in an indoor corridor is performed using the developed 300 GHz system and 28 GHz channel sounding systems. Both measured channels at the 28 GHz and 300 GHz channels are shown to be highly sparse and specular. A higher number of Multi Path Components (MPC) are observed for the 28 GHz system, while the same main MPC are observed for both systems.

This paper presents a novel and cost-effective vector network analyzer (VNA)-based phase-compensated channel sounder operating in the frequency range of 10-50 GHz using radio-over-fiber (RoF) techniques, which can support multilink/channel long-range phase-coherent measurements. The optical cable enables long-range channel measurements with a dynamic range of 115.7 dB at 30 GHz (for the back-to-back connection). The phase compensation scheme is utilized for stabilizing the inherent phase variations introduced by the optical fiber of the channel sounder to enable its application in multichannel/antenna measurements. A novel optical delay line and combiner scheme is proposed and implemented to separate the signals, thereby saving the port resource on the VNA for multi-link/channel measurements. The proposed channel sounder is validated in back-to-back measurements under two optical cable conditions, i.e., with the presence of thermal changes and mechanical stress. The phase change could be maintained within 3 • at 10-30 GHz and 7 • in 30-50 GHz, respectively, compared to the over 80 • phase variation introduced by the cable effects at 10-50 GHz, demonstrating the robustness and effectiveness of the developed channel sounder in practice. With the proposed optical delay line and combiner scheme, multiple channels can be measured simultaneously with minimal VNA ports, significantly reducing the measurement cost and time for multi-link channel measurements. Two multi-link indoor channel measurements are conducted and analyzed using a virtual uniform rectangular array (URA) at 28 GHz bands. The results demonstrate the capability of the proposed channel sounder to perform high-fidelity long-range ultrawideband multi-link channel measurements.

The transition to higher frequency bands, e.g., millimeter-wave (mmWave) and terahertz (THz), will be capitalized on the long term for future wireless communications. One of challenges relates to rapid establishment of mmWave/THz links with low beam training overhead due to highly directional transmission. A promising solution is to take advantage of the coexistence of sub-6 GHz, mmWave, and THz wireless networks and to use out-of-band spatial information for enabling fast beam search. The success depends on the spatial similarity of radio channels across different frequency bands. In this article we promote a feasibility study of low-frequency spatial channel information assisted high-frequency beam search from a radio channel point of view. We develop multi-band channel similarity measure of desired beam directions extracted from radio channels, which are obtained via filtering propagation paths by different beampatterns at different frequencies. Measurementand ray-tracing-based evaluations across multiple frequencies and environments are performed, which prove the usability of out-of-band information aided beam search strategy in line-ofsight (LOS) dominated scenario and even in non-LOS scenario. Finally, we discuss the challenges associated with exploiting spatial channel similarity.