  Millimeter-wave (mmWave) and terahertz (THz) communications with hybrid
precoding architectures have been regarded as energy-efficient solutions to
fulfill the vision of high-speed transmissions for 6G communications.
Benefiting from the advantages of providing a wide scan range and flat array
gain, the uniform circular array (UCA) has attracted much attention. However,
the growing bandwidth of mmWave and THz communications require
frequency-independent phase shifts, which can not be perfectly realized through
frequency-independent phase shifters (PSs) in classical hybrid precoding
architectures. This mismatch causes the beam defocus effect in UCA wideband
communications, where the high-gain beams could not form at non-central
frequencies. In this paper, we first investigate the characteristics of the
beam defocus effect distinguishing itself from the beam split effect in uniform
linear array (ULA) systems. The beam pattern of UCA in both frequency domain
and angular domain is analyzed, characterizing the beamforming loss caused by
the beam defocus effect. Then, the delay-phase-precoding (DPP) architecture
which leverages the true-time-delay (TTD) devices to generate
frequency-dependent phase shifts is employed to mitigate the beam defocus
effect. Finally, performance analysis and extensive simulation results are
provided to evaluate the effectiveness of the DPP architecture in UCA systems.

Signal Processing