  Integrated sensing and communication (ISAC) has been regarded as a key
technology for 6G wireless communications, in which large-scale multiple input
and multiple output (MIMO) array with higher and wider frequency bands will be
adopted. However, recent studies show that the beam squint phenomenon can not
be ignored in wideband MIMO system, which generally deteriorates the
communications performance. In this paper, we find that with the aid of
true-time-delay lines (TTDs), the range and trajectory of the beam squint in
near-field communications systems can be freely controlled, and hence it is
possible to reversely utilize the beam squint for user localization. We derive
the trajectory equation for near-field beam squint points and design a way to
control such trajectory. With the proposed design, beamforming from different
subcarriers would purposely point to different angles and different distances,
such that users from different positions would receive the maximum power at
different subcarriers. Hence, one can simply localize multiple users from the
beam squint effect in frequency domain, and thus reduce the beam sweeping
overhead as compared to the conventional time domain beam search based
approach. Furthermore, we utilize the phase difference of the maximum power
subcarriers received by the user at different frequencies in several times beam
sweeping to obtain a more accurate distance estimation result, ultimately
realizing high accuracy and low beam sweeping overhead user localization.
Simulation results demonstrate the effectiveness of the proposed schemes.
