THz Communications: A Catalyst for the Wireless Future

Angeliki Alexiou, Sergey Andreev, Gabor Fodor, Tadao Nagatsuma
2020 IEEE Communications Magazine  
A s the wireless world moves toward 6G, radio Tbit/s communications and the supporting access and backhaul network infrastructures are expected to become a predominant technology trend. However, certain severe limitations affect the capability of future wireless communications systems to meet the joint requirements of high data rate, near-zero latency, and high spectral and energy effi ciency. In this context, utilizing THz frequency bands for wireless transmissions, as an extension to optical
more » ... i ber, is a promising enabler to bridge this gap and provide ubiquitous high-speed Internet access beyond 5G. Moreover, an increasing number of mobile and fi xed users in the private and industry sectors will require hundreds of Gbit/s for connectivity to or between cell towers (backhaul) or between cell towers and remote radio heads (fronthaul). In such scenarios, critical parameters, apart from the data rates in the order of Tbit/s, are the communications range and the achievable spectral and energy effi ciency at reasonable capital and operational expenditures. Utilizing the THz frequency bands for access and backhaul connectivity brings unique and novel challenges that make it necessary to rethink several conventional communications and networking mechanisms. The root cause of these challenges is the ultra-wideband and highly directional nature of THz radio links and other THz communications peculiarities, in terms of signal and antenna design, channel and interference modeling, and hardware constraints. The fundamentally diff erent structure of radio interference due to narrow beams calls for a thorough characterization and detailed modeling of interference. Building on past propagation and channel modeling studies, contributions of the line-of-sight and non-line-of-sight refl ected and scattered components should be considered, as should be the inherent molecular noise, misalignment impairments, and blockage probability. Medium access control and radio resource management protocols need to operate with pencil beams and must therefore be based on radically new principles. Fast handover procedures need to incorporate the time required for discovery, localization, and tracking functionalities. The Tbit/s data rates create signifi cant challenges in transceiver processing, which include algorithm and architecture design as well as hardware implementation. Motivated by the potential of THz technologies to shape the future of wireless communications, this feature topic seeks to identify the critical technology gaps and the feasible enablers in terms of baseband processing. These include radio frequency frontend and antenna design; propagation and channel modeling; waveforms, signals, and coding; beamforming and (ultra-massive) MIMO; as well as resource management and medium access control schemes. Most importantly, this feature topic aims to shed light on the potential accelerators or show-stoppers in the adoption of THz communications as viewed by the various stakeholders, both incumbents and newcomers. In doing so, it provides insights on prototyping, implementation challenges, and standardization and regulatory matters in the THz regime. The fi rst article, "A Holistic Investigation of Terahertz Propagation and Channel Modeling toward Vertical Heterogeneous Networks," by Kürşat Tekbıyık, Ali Rıza Ekti, Güneş Karabulut Kurt, Ali Görçin, and Halim Yanikomeroglu, presents a recently proposed vertical heterogeneous network architecture for backhaul/fronthaul in the case of a large number of small cells of diff erent communications technologies, including geostationary Earth orbit and low Earth orbit satellites and networked flying platforms, along with terrestrial communications links. THz communications are proposed to enable this heterogeneous system concept. As THz links offer large bandwidth, leading to ultrahigh data rates, this solution is suitable for backhauling and fronthauling of small cells that support numerous applications from inter-satellite links to in-vivo nanonetworks. Accurate channel modeling is considered key for the feasibility of such system concepts. To this end, measurement campaign fi ndings are reported to reveal the true potential of THz communications in vertical heterogeneous network architectures. The article entitled "THz Radio Communication: Link Budget Analysis toward 6G," by Kari Rikkinen, Pekka Kyösti, Marko E. Leinonen, Markus Berg, and Aarno Pärssinen, assesses and quantifi es the potential of THz radio by considering antenna and radio frequency hardware technologies and radio propagation challenges, by means of link budget evaluations. The achievable or required noise fi gure, transmit power, and antenna gain are the main parameters of interest. From the RF viewpoint, the observed bottlenecks occur in the generation of suffi ciently high transmit power and low noise with the support of very high antenna gains. For example, the authors analyze the case of 300 GHz frequency to support 100 Gbit/s data rate at 30 GHz bandwidth on 10 m link distance for diff erent kinds of devices. The article entitled "IEEE 802.15.3d: First Standardization Eff orts for Sub-Terahertz Band Communications toward 6G" by Vitaly Petrov, Thomas Kürner, and Iwao Hosako, presents the standardization activities within IEEE 802.15.3d, an amendment to 802.15.3 established to facilitate standardization of consumer wireless communications in the sub-THz frequency band. IEEE 802.15.3d addresses switched point-to-point connectivity with data rates of 100 Gbit/s and higher at distances ranging from tens of centimeters up to a few hundred meters. Target applications and usage scenarios and key design principles for the physical and the medium access layers are presented, along with initial performance results. These results indicate substantial improvements over fi fth-generation wireless systems, thus paving the way toward sixth generation THz networks.
doi:10.1109/mcom.2020.9269507 fatcat:abnk4uh6s5fq7aefzofcd6wreq