Subarray-Based Near-Field Beam Training for 6G Terahertz Communications

Terahertz (THz) communications have become an important technology for achieving 6G networks that require higher data rates. To overcome severe signal attenuation in THz communications, highly directional beamforming technique using extremely large-scale antenna array (XL-array) is widely used at the base station (BS). Since the channel exhibits near-field characteristics at higher frequencies and wider antenna apertures, the BS must perform near-field beam training to obtain distance information as well as angle information of the channel. A major issue of conventional near-field beam training is the considerable beam training latency due to the large overhead incurred by angle and distance domain search. To address this issue, we propose a subarray-based near-field beam training that estimates the user location by performing frequency-dependent beam training on partitioned array antennas. In the first step, BS simultaneously searches multiple directions by generating multiple frequency-dependent beams using phase shifters (PSs) and true time delays (TTDs). In the second step, BS estimates the user angle and distance information by exploiting the extracted directions from subarrays. Numerical results show that the proposed scheme achieves 28% improvement in the data rate and 99% reduction in the latency.