7 Defining Features of Terahertz (THz) Wireless Communications Systems

 

We have looked at the Spectrum beyond 5G and 6G in earlier posts (links at the bottom of this post). Walid Saad, Professor at Virginia Tech recently shared his paper on LinkedIn. In the post he says:

Operating at sub-Terahertz/Terahertz frequencies (from 0.1 THz onwards) is being touted as a cornerstone of future wireless systems (6G and possibly beyond). In our recent work, we provide a very comprehensive vision on how wireless systems will look like if they are deployed at those frequencies.

We particularly identify seven defining features that are unique to THz wireless systems and that allow those systems to host a plethora of functions, including a synergistic integration of communications and sensing/imaging. We also shed light on the technologies and tools (e.g., AI and learning) needed to enable transformative research in this space, we define prospective use cases of such THz systems, and we provide recommendations for this general area

The paper can be downloaded from arXiv here. Quoting from the paper:

By leveraging this fundamental examination of the THz properties, we identify and provide a comprehensive treatment of the seven unique features that will define future THz wireless systems. Subsequently, we examine the behavior and needs of each feature, and we propose opportunistic techniques that harvest peculiar THz benefits. Hence, these benefits maximize the overall system performance and could potentially elevate THz wireless systems to another level. The seven unique characteristics that we envision to be the defining features of future THz-based wireless systems are shown on the left-hand of Fig. 2 and discussed next:

1) Quasi-opticality of the THz band: The EM properties of the THz band, by virtue of its quasi-opticality lead to distinct communication challenges. Most importantly, the molecular absorption effect is seen as a limiting factor to the propagation of THz waves. Nonetheless, we will underscore how this quasi-opticality is a double-edged sword for the foreseen joint communication and sensing paradigm. For instance, the molecular absorption opens up various sensing opportunities that are not found at other frequency bands. However, this comes at the expense of shorter communication ranges. We will also highlight the other sensing functionalities offered by this quasi-opticality.

2) THz-tailored architectures: Deploying wireless THz systems requires accounting for a higher density of SBSs, a shorter communication range, the ability to deliver multiple functions (sensing, communication, imaging), and a set of unique channel conditions. These factors urge adopting more opportunistic THz-tailored network architectures that can exploit the advantages of THz systems. As such, we particularly emphasize the importance of adopting cell less architectures, as well as their accompanying challenges and opportunities. Furthermore, we highlight the pivotal role of reconfigurable intelligent surfaces (RISs) in THz networks, their holographic capability due to the small THz footprint, their massive sensing elements, and the near-field communication opportunities and challenges.

3) Synergy of THz with lower frequency bands: Communication systems at THz frequencies will be deployed in a radio spectrum that is already highly populated with sub-6 GHz and mmWave technologies. In that sense, THz systems are expected to have a certain level of synergy (cooperation and seamless co-existence) with the lower frequency band wireless technologies. For instance, certain use cases, such as immersive remote presence, could opportunistically use all available wireless frequencies to deliver the target end-to-end experience. Thus, we underline the strategies that enable the co-existence of THz frequencies with mmWave and sub-6 GHz bands, services, and infrastructure. We further point out how this synergy across the different frequency bands opens the door for exciting opportunities for both communication and sensing functionalities.

4) Joint sensing and communication systems: Owing to the quasi-optical nature of THz bands, a harmonious fellowship of high-rate communications and high-resolution sensing can be formed. Consequently, we accentuate the role of joint communication and sensing systems for future THz wireless networks. Particularly, we emphasize the effectiveness of mutual feedback between the sensing and communication functionalities that can improve the overall system performance. Naturally, adopting such configurations can help transform wireless networks into a new generation of versatile systems that can offer multiple functions to their users thus opening the door for novel services and use cases to be used at the THz band.

5) PHY-layer procedures: The spatially-sparse and low rank THz channel imposes distinct challenges on the PHY-layer procedures such as wireless channel estimation and initial access. To overcome these challenges, we propose novel channel estimation techniques that bring to light the role of generative learning networks in predicting the full THz channel state information (CSI). Furthermore, we highlight the role of sensing in ensuring an enhanced initial network access for THz devices.

6) Spectrum access techniques: Conventional access schemes adopted in previous wireless generations cannot be directly applied to THz frequency bands due to hardware constraints and the unique nature of the THz propagation environment. Consequently, we examine possible spectrum access techniques that are suitable for THz systems. Particularly, we discuss the benefits that can be reaped from the concept of orbital angular momentum (OAM) given the quasi-optical nature of THz systems. We also explore the role of non-orthogonal mutliple access (NOMA) in THz systems. In fact, we will see how the synergy between NOMA and THz bands is strengthened by the natural adoption of RIS architectures at those frequencies.

7) Real-time network optimization: 6G services such as XR, holography, and digital twins necessitate an end-to-end (E2E) co-design of communication, control, sensing, and computing functionalities which to date have been attempted with limited success. Nevertheless, the inherent coupling of communication and sensing in THz makes it plausible to hypothesize that a joint co-design of the aforementioned aspects will be feasible and necessary. In that sense, we scrutinize the networking challenges particular to THz systems. Subsequently, we examine novel algorithmic approaches and techniques that can be used to optimize THz networks thus allowing them to meet the stringent requirements of beyond 5G applications. In particular, we explore the potential of artificial intelligence (AI), particularly, the emerging concepts of generalizeable and specialized learning, as well as meta-learning in optimizing the resources of the highly varying and non-stationary THz channel.

After providing a panoramic exposition of the seven defining features of THz systems, we conclude with an extensive overview of the prospective use cases. In particular, we examine the challenges and open problems of promising THz enabled use cases. We further underscore different ways to exploit the aforementioned defining features in each application. The four major 6G use cases for THz systems are shown in the right hand-side of Fig. 2.

The paper is available here.

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