A Tutorial on Terahertz-Band Localization for 6G Communication Systems

Terahertz (THz) communications are celebrated as key enablers for converged localization and sensing in future sixth-generation (6G) wireless communication systems and beyond. Instead of being a byproduct of the communication system, localization in 6G is indispensable for location-aware communications. Towards this end, we aim to identify the prospects, challenges, and requirements of THz localization techniques. We first review the history and trends of localization methods and discuss their objectives, constraints, and applications in contemporary communication systems. We then detail the latest advances in THz communications and introduce THz-specific channel and system models. Afterward, we formulate THz-band localization as a 3D position/orientation estimation problem, detailing geometry-based localization techniques and describing potential THz localization and sensing extensions. We further formulate the offline design and online optimization of THz localization systems, provide numerical simulation results, and conclude by providing lessons learned and future research directions. Preliminary results illustrate that under the same transmission power and array footprint, THz-based localization outperforms millimeter wave-based localization. In other words, the same level of localization performance can be achieved at THz-band with less transmission power or a smaller footprint.

I. Introduction

Localization is the process of estimating the position and orientation of a target, which is vital for a variety of applications, including location-aware communications [1], autonomous driving [2], industrial Internet of Things (IoT) [3], and tactile Internet [4]. Over the years, a plethora of localization techniques has been proposed. These techniques utilize different signal or measurement types that include ultrasound, visible light, radio frequency (RF), inertial measurements, and hybrid signals [5]. Among these modalities, RF signals are widely used because of their ubiquity in current wireless communication systems, where abundant cellular and wireless local area network (WLAN) infrastructures provide added value to user-oriented services and network management [6].