Event-Based Beam Tracking With Dynamic Beamwidth Adaptation in Terahertz (THz) Communications

Terahertz (THz) communication will be a key enabler for next-generation wireless systems. While THz frequency bands provide abundant bandwidth and extremely high data rates, their effective operation is inhibited by short communication ranges and narrow beams, thus, leading to major challenges pertaining to user mobility, beam alignment, and handover. In particular, there is a strong need for novel beam tracking methods that consider the tradeoff between enhancing the received signal strength via increasing beam directivity, and increasing the coverage probability by widening the beam. In this paper, a multi-objective optimization problem is formulated with the goal of jointly maximizing the expected rate and minimizing the outage probability subject to transmit power and overhead constraints. Subsequently, a novel parameterized beamformer with dynamic beamwidth adaptation is proposed. In addition to the precoder, an event-based beam tracking approach is introduced that efficiently prevents outages caused by beam misalignment and dynamic blockage while maintaining a low pilot overhead. Simulation results show that the proposed beamforming scheme improves average rate performance and reduces the amount of outages caused by the brittle THz misalignment process and the particularly severe path loss in the THz band. Moreover, the proposed event-triggered THz channel estimation approach enables connectivity with minimal overhead and reliable communication at THz bands.

I. Introduction

Afundamental characteristic of next-generation wireless 6G networks is the migration towards higher frequency bands, namely the terahertz (THz) band (0.1–10 THz).1 Wireless communication links at the THz frequency bands benefit from an abundant bandwidth which enables extremely high data rates (in the order of Tbps) that are essential for future 6G services like extended reality (XR) [1] or digital twins [2]. However, unleashing the true potential of THz frequency bands necessitates overcoming key THz challenges, stemming from the channel’s uncertainty. Particularly, two major factors that restrain the communication at THz bands are the high path loss and the molecular absorption effect [3], [4]. More specifically, such factors can potentially increase the channel attenuation by more than 20 dB when migrating from a carrier frequency of 30 GHz up to 300 GHz. To compensate the effect of these phenomena, a very narrow beam (so-called pencil beam) is needed to focus the power towards the receiver [5]. Nonetheless, the communication reliability is at risk when using a narrow beam due to potential blockages and beam misalignment. Indeed, even slight changes in target direction (within a few degrees or less) can result in communication outages, especially in dynamic use cases. While this phenomenon can affect mmWave communication, it becomes substantially more pronounced in (sub-)THz systems. Due to the extremely high path loss, as well as the molecular absorption caused by water vapor, the THz band is more suitable for indoor environments with shorter ranges (≤ 20 m), lower levels of humidity, and thus stronger communication links [3]. While indoor environments may be more favorable, the reliability of THz links remains affected by beam misalignments resulting from changes in the micro-mobility of users [6], [7]. Henceforth, investigating the tradeoff between the pathloss compensation and the mitigation of beam misalignment is substantially crucial for the deployment of THz bands [8]. Indeed, the optimal tradeoff adjustment could ultimately lead to the delivery of reliable and robust THz links in dynamic environments, a fundamental necessity for 6G services like XR [1].1

The frequency range 100 – 300 GHz is typically referred to as the sub-THz band, while the unique properties of the THz band are observed above 275 GHz. However, in this work the term THz is used to refer to the overall range 0.1 – 10 THz.