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Knowledge of spatial profiles of propagation channels holds significant importance in channel modeling, particularly in the subterahertz (sub-THz) frequency band spanning from 100 GHz to 300 GHz. The directional scanning scheme (DSS) offers a straightforward and cost-effective means to achieve such purpose. However, the resulting spatial profiles are subject to the directional characteristics of the employed antenna (e.g., half-power beamwidth and sidelobes). The virtual antenna array (VAA) method, i.e., mechanically relocating a single antenna to specific spatial locations to construct a VAA, can achieve significant enhancement of the spatial resolution and signal-to-noise ratio (SNR) through array signal processing. However, VAA requires phase-coherent measurements among virtual array elements, presenting a challenge to the channel-sounding implementation at sub-THz bands due to the highly sensitive phase at 1 mm level of the wavelength. In our previous work, we introduced a phase-coherent channel sounder capable of stabilizing the phase of the channel-sounding system. Leveraging this technology, we implement and validate the first 300 GHz VAA using an extremely large antenna array setup comprising 1200 array elements in the field measurements in this work. The measured results demonstrated the effectiveness of the VAA scheme, by offering significantly improved SNR and spatial resolution compared with the traditional DSS scheme.

Subterahertz (sub-THz) frequencies provide huge untapped bandwidth for high data rates in wireless communications and high-precision sensing applications. Understanding radio channel characteristics at sub-THz frequency bands is important to identify potential applications. This letter investigated the frequency dependence, spatial consistency, and impact of blockage at the W-band. Employing the vector-network-analyzer-based virtual antenna array channel sounder, several wideband measurements covering three frequency bands, eight spatial locations, with or without the blockage at the W-band were performed. The experimental analysis demonstrated that channel characteristics have good frequency consistency and a strong correlation with the spatial environment. The above research is of great significance for achieving fast beam alignment and channel prediction in different spatial locations.

A novel millimeter-wave massive MIMO system using asymmetric transceiver, i.e., unequal number of transmitting (Tx) and receiving (Rx) radio frequency chains, is expected to maintain the advantages of conventional fully digital beamforming architectures, but partly reduce the implementation cost and power consumption. However, uplink and downlink radio channels may become non-reciprocal due to the different dimensions of Tx-Rx antenna arrays. In this paper, we analyze the reciprocity of radio channels observed by practical antenna patterns with different beamwidths. Two metrics are leveraged to measure spatial channel reciprocity based on 142 GHz outdoor channel measurement data and 28 GHz indoor ray-tracing simulation data. The power angular spectrum reciprocity of uplink and downlink radio channels does not hold when the beam pattern of base station Tx array becomes much narrower than that of Rx array. Meanwhile, it becomes increasingly likely that in the extreme case (e.g., significant beamwidth difference between Tx and Rx beam patterns), pronounced angle reciprocity can still be observed in the sparse channels with less multipath components.

Sub-terahertz (Sub-THz) technology, as one of the key candidates for the six generation (6G) systems, has attracted increasing attention from academia and industry, due to its promise to unleash vast amounts of new frequency spectrum. Sub-THz system designs pose unique and more challenging circumstances compared to traditional communication systems. These challenges arise from the demanding propagation conditions, limited availability of commercial radio frequency (RF) components, the need for high-gain and beam-steerable antennas that are highly integrated at both ends of the communication link, short-range communication scenarios, and the requirement for extreme data rates. Therefore, it is crucial to assess the performance of radio devices in realistic propagation channels in sub-THz communication systems. In this work, we present the concept, challenges, and enabling solutions for achieving sub-THz radio channel emulation. Moreover, we experimentally demonstrated the reconstruction of the measured propagation channels at 140 GHz with a commercial radio channel emulator in the laboratory. The developed dynamic fading channel replay concept and experimental validation procedure allows initial tests of future sub-THz communication devices.

This letter presents the analyses of a single-user beamspace multiple-input–multiple-output (MIMO) on measured indoor and outdoor channels at 142 GHz. The rank is evaluated under different antenna sizes, number of beams, and thresholds. We assume a total power constraint at the transmitter that results in a decrease in a signal-to-noise ratio as the link distance increases. When using spatial multiplexing, the indoor and outdoor sites demonstrate an average capacity gain of 2× and 1.5× at link distances below 60 m. Also, the rank for our measured 142 GHz channels is comparable to that at 60 GHz channels but significantly lower than the rank at 5 GHz channels reported in the literature. We also found that at 142 GHz, the indoor and outdoor sites have median ranks of 3.0 and 1.7 for the small-antenna case, and 4.9 and 2.4 for the large-antenna case assuming a rank threshold of 20 dB. The indoor site has a rank higher by 1.8 than the outdoor site, regardless of the antenna size. The rank decreases by only 20% and 15% for indoor and outdoor scenarios when the beam density is halved, allowing a significant reduction in implementation complexity of the beamspace MIMO without remarkably reducing the rank.

Sub-terahertz (sub-THz) (i.e., 100–300 GHz) communication is envisaged as one of the key building blocks for future communication systems due to its vast unexploited bandwidth. Knowledge of the radio channel characteristics is key to the design and development of new radio systems and air interfaces. Reliable channel sounding is essential to build accurate and realistic channel models. The virtual antenna array (VAA) has been a popular channel sounding strategy to obtain accurate directional characterization due to its low-cost and simple system implementation. However, this concept has not yet been realized for sub-THz bands in the state-of-the-art works due to difficulty in accurate phase control. The measurement range has been rather limited at sub-THz due to significant signal loss, especially in the radio frequency (RF) cables, compared to microwave or millimeter-wave frequencies. In this article, we focus on vector network analyzer (VNA)-based channel sounders, highlighting frequency extension with sub-THz frequency extenders, measurement range extension with radio-over-fiber (RoF) schemes, and angular resolution improvement by VAA implementation with a phase-compensation scheme. These techniques enable and enhance sub-THz channel characterization. The performance of the proposed long-range phase-compensated sounder is also experimentally demonstrated by the VAA-based channel measurements at 100 GHz in an indoor scenario.

The transition to higher frequency bands, such as millimeter-wave (mmWave) and terahertz (THz), will be capitalized in the long term for future wireless communications. One of the 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. 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 a multi-band channel similarity measure of desired beam directions extracted from radio channels, which is obtained via filtering propagation paths by different beam patterns at different frequencies. Measurement-and 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 a line-of-sight (LoS)-dominated scenario and even in a non-LoS scenario. Finally, we discuss the challenges associated with exploiting spatial channel similarity.

The sixth-generation (6G) mobile communication is envisioned to be an all connected radio system, which covers the vertical expansion from ground-based networks to spaceborne networks, and the horizontal expansion from ultra-short range to ultra-long range communications. As a result, the number of radio devices and associated test cases and scenarios will grow exponentially, making field trials (i.e., testing in real deployment scenarios) unfeasible due to the cost and time concerns. A digital twin (DT) concept can be adopted to reproduce the field trials, enabling 6G radio testing and development in controllable and repeatable laboratory conditions where the radio devices and network should operate as if they would in real-world conditions. This article investigates DT-enabled 6G radio testing. First, the concept of DT for radio testing is introduced, including information on how DT can capture the radio environment with unique propagation features. Next, three state-of-the-art emulation setups of DT for radio testing are explained. The article also covers challenges introduced by new 6G radio technologies for the DT-enabled 6G radio testing, including potential solutions to tackle those challenges.

Sub-Terahertz (sub-THz) (i.e., 100–300 GHz) communication is envisioned as one of the key components for future beyond fifth-generation (B5G) communication systems due to its large untapped bandwidth. Sub-THz channel measurements are essential for building accurate and realistic sub-THz channel models. Virtual antenna array (VAA) scheme has been widely employed for radio channel sounding purposes in the literature. However, its application for the W-band (i.e., 75–110 GHz) has been rarely discussed due to system phase instability issues. To tackle this problem, a long-range phase-compensated vector network analyzer (VNA)-based channel sounder at the W-band is proposed. First, the back-to-back measurement of the developed channel sounder is carried out with the presence of cable bending, where the initial phase variation beyond $180^{\circ }$ range due to cable effects can be well corrected to within $10^{\circ }$ range with the proposed phase-compensation scheme, clearly validating its effectiveness. To examine how well it works in practical deployment scenarios, the proposed channel sounder is then employed for channel sounding with two measurement distances, covering both near-field (with a line-of-sight (LoS) distance of 7.3 m) and long-range (with a LoS distance of 84.5 m) cases. Based on the measured data, a high-resolution channel parameter estimator is applied to extract the channel multipath parameters for the large-scale VAA at the W-band, both in the near-field and long-range scenarios, respectively. The high-resolution algorithm was extended to support virtual arrays composed of both omnidirectional antenna and directive antenna in this work. The conventional directional scanning scheme (DSS) measurement is adopted as the reference measurement to validate the effectiveness and robustness of the developed channel sounder. In the end, to demonstrate the state-of-art channel sounding capabilities of the developed channel sounder, ultra-wideband (UWB) channel measurements at 104.5 GHz with 11 GHz bandwidth using the VAA scheme are conducted in a hall scenario with the measurement range up to 58 m with omnidirectional antennas, and the channel parameters are extracted using the validated high-resolution channel parameter estimator for channel modeling purposes.

The sixth-generation (6G) communication system will leverage the evolving multiple-input-multiple-output (MIMO) technology, referred to as gigantic MIMO. It is envisioned to accommodate arrays with over one thousand antennas for low and mid frequency bands, and with up to thousands of antennas for high frequency bands such as THz. With such a substantial number of antenna elements, the gigantic MIMO technology has the potential to unlock new use cases and applications with extreme requirements. Radio channel characterization plays a crucial role in the design, optimization, and performance evaluation of these radio systems, making it highly desirable for advancing the deployment of the gigantic MIMO technology. This article discusses key challenges and potential solutions for radio channel characterization of gigantic MIMO systems, including channel modeling approaches, experimental platform development i.e., channel sounder design, and radio channel parameter estimation. Further, a channel measurement campaign based on a virtual antenna array (containing 2400 antenna elements) and a corresponding ray tracing simulation are performed for a practical indoor scenario as examples to characterize a realistic gigantic MIMO channel.

Due to the large amount of unused and unexplored spectrum resources, the so-called sub-Terahertz (sub-THz) frequency bands from 100 to 300 GHz are seen as promising bands for the next generation of wireless communication systems. Channel modeling at sub-THz bands is essential for the design and deployment of future wireless communication systems. Channel measurement is a widely adopted method to obtain channel characteristics and establish mathematical channel models. Channel measurements depend on the design and construction of channel sounders. Thus, reliable channel sounding techniques and accurate channel measurements are required. In this paper, the requirements of an ideal channel sounder are discussed and the main channel sounding techniques are described for the sub-THz frequency bands. The state-of-the-art sub-THz channel sounders reported in the literature and respective channel measurements are presented. Moreover, a vector network analyzer (VNA) based channel sounder, which supports frequency bands from 220 to 330 GHz is presented and its performance capability and limitation are evaluated. This paper also discussed the challenge and future outlook of the sub-THz channel sounders and measurements.