In the last decade, major device, communication and networking feats have led to the development and commercialisation of millimeter-wave (mmWave) wireless technology. Today, wireless local area networks operating in the Industrial, Scientific and Medical (ISM) 60 GHz frequency band and orchestrated by the IEEE 802.11ad and the new IEEE 802.11ay are a reality. Similarly, the fifth generation (5G) cellular wide area networks operating in the licensed Frequency Range 2 (FR2) between 24.25 GHz and 52.6 GHz (and soon to be extended to 71 GHz) are already deployed in several countries. Higher data-rates (approaching 20 Giga-bits-per-second or Gbps) and lower latencies (approaching few milliseconds) are some of the promises of mmWave technologies to enable long-awaited applications including immersive augmented and virtual reality, the tactile Internet, and autonomous unmanned networks, among others, all within different contexts, from entertainment to education to remote work telepresence. Moreover, besides communications, the mmWave spectrum has also enabled exciting applications in other fields, from precise localisation and sensing, to the extraction of body features for security applications.
Despite the mmWave spectrum spanning the frequency range between 30 GHz and 300 GHz, all the aforementioned commercial technologies and the majority of the research solutions explored to date are for systems operating under 100 GHz. However, this is changing. Today, there are several major academic and industry research initiatives world-wide aimed at developing wireless solutions in the upper mmWave band (100 GHz — 300 GHz) and the terahertz band (300 GHz — 10 THz). The US National Science Foundation Spectrum Innovation Initiative, the Semiconductor Research Corporation (SRC) and DARPA Communication and Sensing at Terahertz frequencies (ComSenTer), multiple European projects funded by the Beyond 5G track of the Horizons 2020 program, and several industry-led efforts (e.g., Nokia, Samsung, Huawei) are just a sample set of a growing pool. In addition, there is even a standard above 275 GHz, namely, the IEEE 802.15.3d approved in 2017, which supports the point-to-point connectivity with data rates of 100 Gb/s and higher at distances ranging from tens of centimeters to a few hundred meters. When moving to such high frequencies, not only there are larger contiguous bands for ultra-broad band communication and networking systems, but electromagnetic radiation interacts with the environment in a more intimate manner, i.e., at the molecular level, giving a whole new meaning to what wireless sensing means.
Of course, such exciting opportunities come with several challenges, some of which are common for any communication system above 30 GHz, while some of which just become more critical as we move up in the spectrum. MmWave and THz signals attenuate quickly over distance, so the radios have to focus their power via highly directional, electronically steerable beams. But, aligning the beams and maintaining the link between devices during obstruction and mobility are the fundamental barriers toward reliable mmWave/THz connectivity. Thus, higher directionality and sensitivity to blockages and mobility of mmWave/THz waves, compared to traditional cellular and Wi-Fi ones, require rethinking various fundamental aspects of networked system design and deployment. The challenges and opportunities span the entire network and device stack; so novel solutions are needed for modeling channels accurately, beamforming and steering efficiently, mitigating interference effectively, accessing channels efficiently, managing mobility and association dynamically, building transport layer that works under outages effortlessly, and building applications that utilise the network resources efficiently.