In this article, a novel vector network analyzer (VNA)-based ultrawideband (UWB) channel sounder using radio-over-fiber (RoF) techniques is presented. Benefiting from the usage of optical fiber cable, the measurement range and the dynamic range are significantly increased compared with the conventional coaxial-based VNA system. Specifically, using RoF increases the dynamic range to a maximum of 112 dB at 30 GHz for the back-to-back connection with an optical fiber cable of length 300 m. Moreover, a state-of-the-art phase compensation scheme using optical circulators is proposed for the first time. The novel scheme renders the channel sounder immune to stochastic phase changes in the optical fiber cable due to thermal changes and mechanical stress, thus permitting the remoting of virtual antenna arrays. The proposed channel sounder is experimentally validated in back-to-back measurements, an anechoic chamber, and practical indoor scenarios. The indoor channel measurements are conducted using a virtual uniform rectangular array (URA) at the millimeter-wave (mm-wave) band from 26.5 to 30 GHz. The measured results demonstrate the developed channel sounder's capability to perform UWB large-scale antenna array measurements with a long measurement range.
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).
Coaxial cables are vital to a wide range of electronic systems. Phase accuracy in coaxial cables is sensitive to the inevitable cable bending in actual usage and temperature change in the environment. For phase-critical applications, e.g. multi-antenna channel sounding, phase accuracy is fundamental to obtain trustworthy measurement results. A technique to track and remove random phase change in coaxial cables due to cable effect is presented. The phasestabilizing technique provides a general phase stabilizing framework via introducing a bi-directional transmission on the cable. The principle of the phase stabilizing scheme is explained and its effectiveness is experimentally validated in a back-to-back measurement.
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