Accurate Beam Training for RIS-Assisted Wideband Terahertz Communication

Terahertz (THz) communications have been widely considered as one of the promising technologies for future 6G wireless systems. In order to cope with the high path loss in THz systems, reconfigurable intelligent surface (RIS) composed of low-complexity reflecting elements can be deployed to generate directional beams. In order to acquire the direction of user equipment (UE) to send directional beams, the acquisition of accurate channel state information (CSI) is very important. Beam training is widely utilized to acquire the CSI. However, existing beam training schemes have not taken the wideband beam split effect into consideration, so the beam training accuracy decreases a lot in wideband scenarios. To solve this problem, in this paper, we propose an analytical beam training framework in RIS-assisted wideband THz communication systems. Specifically, we first propose a power distribution pattern (PDP) based direction estimation scheme, where the exact value of the received power is utilized to analytically calculate the UE direction. Then, we design the analytical codebook for the proposed framework based on the inherent parameters of the wideband THz system. Simulation results show that the proposed framework can achieve the near-optimal achievable rate performance with a lower beam training overhead than existing schemes.

I. Introduction

Terahertz (THz) communication is considered as one of the promising technologies to satisfy the high data rate requirement for future 6G systems. It is capable of providing tens of GHz of bandwidth, which is much wider than the bandwidth that millimeter communication can provide for 5G systems [1], [2], [3], [4]. However, due to the high frequency of THz band, the THz signals suffer from a severe path loss [5], which limits the coverage of THz signals. Moreover, significant attenuation happens when THz signals are blocked by obstacles such as buildings and trees. Fortunately, reconfigurable intelligent surface (RIS) has been proposed to overcome the above two problems [6], [7], [8], [9], [10]. A RIS is composed of numerous low-cost reflecting elements, each of which can reflect the incident signal with a particularly designed phase shift or amplitude. The THz RIS can be realized based on metal-oxide-semiconductor-based chip tiles or electron gas structure [11], [12]. By designing the frequency-independent phase shift of each element, a directional beam with high array gain can be generated to overcome the high path loss of THz signals. In addition, deploying RIS in communication systems can provide extra reflecting paths to solve the blockage problem when the line-of-sight (LoS) path from base station (BS) to user equipment (UE) is blocked, even if the phase estimation is inaccurate [13], [14], [15], [16], [17].