  True-time-delay (TTD) lines are recently applied inside Terahertz (THz)
hybrid-precoding transceiver to acquire high beamforming gain against beam
squint effect. However, beam tracking turns into a challenging puzzle where
enormous potential beam directions bring about unacceptable overhead
consumption. Frequency-scanning-based beam tracking is initially explored but
still imperfect in previous studies. In this paper, based on TTD-aided hybrid
precoding structure, we give an enhanced frequency-scanning-based tracking
scheme. Multiple beams are generated and utilized simultaneously via several
subcarriers for tracking at one timeslot. The squint beams' angular coverage at
all subcarriers can be flexibly controlled by two different subcarrier-angular
mapping policies, named forward-pairing and backward-pairing. Then multiple
physical directions can be simultaneously searched in one timeslot for lower
overhead consumption. Besides, closed-form searching radius bound, parameter
configuration and interferences are theoretically analyzed. Furthermore, we
provide the coupled codebook design for TTDs and phase shifters (PSs), with
joint consideration of both beamforming and tracking. Analytical and numerical
results demonstrate the superiority of the new frequency-scanning-based
tracking scheme and beamforming codebook.
