  High-frequency wideband cellular communications over mmWave and sub-THz offer
the opportunity for high data rates. However, it also presents high path loss,
resulting in limited coverage. High-gain beamforming brought by the antenna
array is essential to mitigate the coverage limitations. The conventional
phased antenna arrays (PAA) cause high scheduling latency owing to analog beam
constraints, i.e., only one frequency-flat beam is generated. Recently
introduced joint phase-time array (JPTA) architecture, which utilizes both
true-time-delay (TTD) units and phase shifters (PSs), alleviates analog beam
constraints by creating multiple frequency-dependent beams for scheduling
multiple users at different directions in a frequency-division manner. One
class of previous studies offered solutions with "rainbow" beams, which tend to
allocate a small bandwidth per beam direction. Another class focused on uniform
linear array (ULA) antenna architecture, whose frequency-dependent beams were
designed along a single axis of either azimuth or elevation direction. This
paper presents a novel 3D beamforming design that maximizes beamforming gain
toward desired azimuth and elevation directions and across sub-bands
partitioned according to scheduled users' bandwidth requirements. We provide
analytical solutions and iterative algorithms to design the PSs and TTD units
for a desired subband beam pattern. Through simulations of the beamforming
gain, we observe that our proposed solutions outperform the state-of-the-art
solutions reported elsewhere.
