Fast and Accurate Terahertz Beam Management via Frequency-Dependent Beamforming

Terahertz (THz) communication is envisaged as an attractive way to attain abundant spectrum resources for 6G wireless communications. One main difficulty of the THz communications is the severe attenuation of signal power caused by the high diffraction and penetration losses and atmospheric absorption. To compensate for the severe path loss, a beamforming technique realized by the massive multiple-input multiple-output (MIMO) has been widely used. Since the beamforming gain is maximized only when the beams are appropriately aligned with the signal propagation paths, acquisition of accurate beam directions is of paramount importance. A major issue of the conventional beam management schemes is the considerable latency being proportional to the number of training beams. In this paper, we propose a THz beam management technique that simultaneously generates multiple frequency-dependent beams using the true time delay (TTD)-based phase shifters. By closing the gap between the frequency-dependent beamforming vectors and the desired directional beamforming vectors using the TTD-based signal propagation network called intensifier, we generate very sharp training beams maximizing the beamforming gain. From the numerical results, we demonstrate that the proposed scheme achieves more than 70% reduction in the beam management latency and 60% increase in the data rate.

I. Introduction

Recently, terahertz (THz) communications have received much attention to alleviate spectrum bottleneck and support high data rates for 6G wireless communications [2], [3]. Using the abundant spectrum resource in the THz frequency band ( THz), THz communications can support immersive mobile services such as digital twin, metaverse realized by XR devices, and high-fidelity mobile holographic displays [4], [5]. Well-known drawback of the THz communications is the severe attenuation of the signal power caused by the high diffraction and penetration losses and atmospheric absorption [6]. To deal with the problem, a beamforming technique realized by the massive multiple-input multiple-output (MIMO) has been widely used [7], [8], [9], [10]. Since the beamforming gain is maximized only when the beams are properly aligned with the signal propagation paths, the base station (BS) needs to acquire the accurate channel information in a form of angle-of-arrivals (AoAs) and angle-of-departures (AoDs). The process to acquire the AoAs/AoDs associated with the paths between the BS and the mobile and then send the directional beams to the acquired directions is collectively called beam management [11], [12]. In general, the beam management of 5G NR consists of two steps: 1) beam sweeping and 2) beam refinement. In the beam sweeping step, the BS sequentially transmits the training beams carrying the reference signal and the mobile reports the index of the beam corresponding to the highest reference signal received power (RSRP) to the BS. After that, in the beam refinement step, the BS narrows down the direction of the mobile by sending multiple pilot signals (e.g., channel state information reference signal (CSI-RS)) to the direction obtained from the beam sweeping [13].