As the world warms up to the idea of millimeter wave (mmWave) communication and fifth generation (5G) mobile networks, realization slowly dawns that the data rate, latency, throughput, and other performance metrics that are used to assess a new wireless communication technology will not be enough to support the demands of envisioned futuristic applications. Thus there is an eagerness to further climb up the frequency ladder to use the large swathes of available spectrum in the 0.1 - 10 THz band which is expected to act as the key technology enabler to fulfill the requirements of the sixth generation (6G) wireless communication and possibly even beyond. Channel measurement and modeling are crucial to the design and deployment of future wireless communication systems and researchers across the world are putting their best foot forward to accelerate the process. The present article presents comprehensive assimilation of research efforts in the context of THz channel sounding. A detailed overview of the current channel sounding techniques is first introduced followed by their relevance to THz band channel measurement. An in-house novel channel sounder developed for THz band measurement is also briefly introduced in this context. The paper next provides elaborate dissemination of various measurement campaigns in the band of interest followed by the modeling techniques that are available in the literature and are being adopted for the THz band. Post the description of different challenges and future research directions in the context of sounding, measurement, and modeling the article is concluded.

In this paper, a newly developed 300 GHz channel sounder is presented followed by a detailed description of test measurements and the subsequent results obtained to validate its working. An elucidation of the high-resolution double-directional channel measurement in a typical conference room scenario precedes the comparison of the results obtained for the current campaign to that obtained from an earlier campaign at 60 GHz for a similar setup. It is observed that a similar number of clusters for both the bands under investigation for all the transmitter (Tx) and receiver (Rx) positions are obtained from the generated power spectra. Any deviation is theorized to be caused either due to the usage of lower measurement bandwidth in the 60 GHz campaign or due to limited elevation expanse in the 300 GHz measurement. To identify the interacting objects (IOs) causing the clusters, environment-embedded angular power spectra (APS) and ray tracing simulations are used. The large-scale parameters (LSPs) are also evaluated for both campaigns. It is observed that signal propagation in the terahertz (THz) band is dominated more by the line-of-sight (LoS) path compared to the millimeter wave (mm-wave) band (below 100 GHz). The results are also compared with other similar results from the literature.

This paper presents a multipath component (MPC) parameter estimator that uses a double-directional (D-D) channel sounding system in WiGig millimeter-wave bands. It employs low-cost commercial-off-the-shelf (COTS) RF transceivers capable of beam steering within an azimuth angle range of ±45°; the transmit power is approximately 31 dBm, equivalent isotropic radiated power (EIRP), including a total antenna gain of approximately 19 dBi. The half-power beam widths in the azimuth and elevation planes are 6° and 45°, respectively. A fast D-D channel acquisition in 360° full azimuth range can be achieved by developing a 4×4 multiple-input-multiple-output (MIMO) time division-multiplexing (TDM) scheme that enables simultaneous use of four antenna arrays at both sides of the transmitter and receiver. The beam switching for all combinations (48×48=2304) with 12 beams at each array requires less than 200 ms, excluding optional procedure related to data storage. A high-resolution MPC extraction method was developed from the D-D angular delay power spectra obtained using the sub-grid CLEAN algorithm, which is applicable when phase coherence among multiple snapshots obtained via the beam-switching measurement cannot be guaranteed. Furthermore, the operation of the developed system was validated via a measurement at 58.32 GHz in an office floor of a university laboratory. From the data analysis, the cluster properties and the corresponding scattering processes were identified.

In order to utilize higher frequency bands above 6 GHz, which is an important technical challenge in fifth generation mobile systems, radio propagation channel properties in a large variety of deployment scenarios should be thoroughly investigated. The authors' group has also been involved in a fundamental research project aimed at investigating multiple-inputmultiple-output (MIMO) transmission performance and propagation channel properties at microwave frequency above 10 GHz from 2009 to 2013, and since then they have been conducting measurement and modeling activities for high frequency bands. This paper aims at providing comprehensive tutorial of a whole procedure of channel modeling; multi-dimensional channel sounding, propagation channel measurement, analysis, and modeling, by introducing the developed MIMO channel sounders at high frequency bands of 11 and 60 GHz and presenting some measurement results in a microcell environment at 11 GHz. Furthermore, this paper identifies challenges in radio propagation measurements, and discusses current/future channel modeling issues as future works.

This paper developed a 60-GHz millimeter-wave cluster channel model for various office environments and investigated the impact of cluster characteristics on wireless data transmission. We conducted double-directional channel measurement campaigns in a university laboratory office environment to develop the channel model using a custom-developed channel sounder with an angular resolution of 6° and a delay resolution of $\mathrm {2.5~ns}$ at $\mathrm {58.32~GHz}$ for the measurements. The super-resolution multipath parameter estimation algorithm extracts multipath components from the measured data with a delay resolution of $\mathrm {0.1~ns}$ and an azimuth resolution of 0.1°, and the $K$ -PowerMeans algorithm identified multipath clusters. Based on the conventional 3rd Generation Partnership Project (3GPP) model, a scenario-categorized site-general model, we attempted to develop a highly accurate cluster model that can more accurately reproduce small-scale fading fluctuations in site-specific environments. The large- and small-scale parameters obtained from the measured data were extracted and compared with the 3GPP map-based hybrid channel model, a quasi-deterministic channel model. The results revealed strong site dependency in the inter-cluster and intra-cluster properties. Moreover, we assessed the impact of different channel parameters on system evaluation using single-user and multi-user multiple-input- multiple-output channel capacities.