G and beyond wireless communication technologies are expected to utilize communications bands in the millimeter wave and terahertz (0.1-10 THz) frequency bands. THz-band communications can help unleash a broad range of wireless applications, owing to the large available bandwidth. However, the large electrical size of the antennas needed to overcome the path loss results in a large near-field distance. Operating in the near-field, the link budget cannot be determined by the Friis path loss equation, and a novel link budget mechanism must be formulated. In this paper, a simple methodology that modifies the Friis link budget for the near-field is presented and validated with experimental measurements. Most importantly, it is shown that the communication channel faces asymmetrical path losses when the transmitter and receiver are not designed reciprocally. This novel feature can affect MAC and networking strategies, as well as adaptive link design for uplink/downlink channels.

Terahertz-band (100 GHz-10 THz) communication is a promising radio technology envisioned to enable ultra-high data rate, reliable and low-latency wireless connectivity in next-generation wireless systems. However, the low transmission power of THz transmitters, the need for high gain directional antennas, and the complex interaction of THz radiation with common objects along the propagation path make crucial the understanding of the THz channel. In this paper, we conduct an extensive channel measurement campaign in an indoor setting (i.e., a conference room) through a channel sounder with 0.1 ns time resolution and 20 GHz bandwidth at 140 GHz. Particularly, the impact of different antenna directivities (and, thus, beam widths) on the channel characteristics is extensively studied. The experimentally obtained dataset is processed to develop the path loss model and, subsequently, derive key channel metrics such as the path loss exponent, delay spread, and K-factor. The results highlight the multi-faceted impact of the antenna gain on the channel and, by extension, the wireless system and, thus, show that an antenna-agnostic channel model cannot capture the propagation characteristics of the THz channel.

Terahertz (THz)-band (0.1–10 THz) communication will be key in enabling high speed wireless links due to the wide available bandwidths. At THz frequencies, the path-loss is governed by high spreading loss due to small antenna apertures and by molecular absorption loss due to water vapor. The latter also determines the available transmission bandwidth, which shrinks with distance. Modulations that consider the high propagation loss and the distance-dependent bandwidth are needed to fully exploit the THz channel’s bandwidth. Using a hierarchical constellation to simultaneously service users at symbol rates, Hierarchical Bandwidth Modulation (HBM) leverages molecular absorption to increase aggregate data rates in a broadcast system while offering flexibility to receivers experiencing high path loss. This paper introduces HBM and evaluates its performance. The symbol error rate performance for a 4/M-QAM HBM system is derived and verified using simulations. These results are used to define the design constraints for an HBM system: the HBM functional region and transition region. The functional region is verified using an experimental testbed for ultrabroadband communications. The results show that with proper design HBM successfully achieves its goal to exploit the distance-dependent characteristics of the THz channel, to spatially multiplex users, and to increase the system capacity.

Terahertz (THz) band (0.1 THz to 10 THz) communication is envisioned as a key technology to satisfy the demand for Terabit-per-second (Tbps) links in the sixth generation (6G) wireless systems and beyond. Significant progress within different device technologies is finally closing the so-called THz technology gap. However, there are notable limitations relating to the efficiency and reliability of THz devices. In order to overcome the challenges, innovative ways of digital signal processing, as well as waveform design techniques, need to be considered. In this context, to simultaneously overcome the limitations due to peak to average power ratio (PAPR) and reduce the effective symbol error rate (SER), both while using a high-order modulation scheme for ultrabroadband THz communication, the utilization of m-ary amplitude phase shift keying (M-APSK) is proposed. After optimizing the constellation with the number of rings as a constraint, the performance of the scheme is compared with M-ary quadrature amplitude modulation (M-QAM) and M-ary phase-shift keying (M-PSK) in terms of SER and PAPR. As a proof of concept, experimental results are provided to demonstrate the performance of the proposed scheme in the 120-140 GHz band, achieving bit-rates of up to 50 Gbps on a single-carrier, single-channel tens-of-meters-long link. This constellation will serve as a building block for multi-carrier modulations able to reach the 1 Tbps goal.

Terahertz (THz)-band (0.1 THz to 10 THz) communication is envisioned as a key technology to meet the demand for faster, more ubiquitous wireless communication networks. For many years, the lack of compact, fast and efficient ways to generate, modulate, detect and demodulate THz-band signals has limited the feasibility of such communication systems. Recently, major progress within different device technologies is finally closing the so-called THz gap. For the time being, communication testbeds have been developed at sub-THz frequencies, i.e., at or near the boundary with millimeter-wave communication systems. Nonetheless, higher carrier frequencies and their associated bandwidth are needed to meet the demand for much higher data rates. In this paper, the TeraNova platform, i.e., the first integrated testbed for ultra-broadband wireless communications at true THz-band frequencies, is presented. The system consists of a transmitter and a receiver based on Schottky-diode frequency multiplying and mixing chains able to up & down-convert an information-bearing intermediate frequency (IF) signal up to 40 GHz-wide between 1 and 1.05 THz, i.e., the first absorption-defined transmission window above 1 THz. Guided by the experimental characterization of the THz channel in terms of path-loss and noise, tailored framing, time synchronization, channel estimation and single- and multi-carrier modulation techniques are implemented in software and realized by a state-of-the-art arbitrary waveform generator and a digital storage oscilloscope at the transmitter and the receiver, respectively. Experimental results are presented herein to highlight the opportunities and challenges to unleash the potential of the THz band.