Wideband Beamforming for Near-Field Communications with Circular Arrays
arxiv(2024)
摘要
The beamforming performance of the uniform circular array (UCA) in near-field
wideband communication systems is investigated. Compared to uniform linear
array (ULA), UCA exhibits uniform effective array aperture in all directions,
thus enabling more users to benefit from near-field communications. In this
paper, the unique beam squint effect in near-field wideband UCA systems is
comprehensively analyzed in both the distance and angular domains. It is
rigorously demonstrated that the beam focal point only exists at a specific
frequency in wideband UCA systems, resulting in significant beamforming loss.
To alleviate this unique beam squint effect, the true-time delay (TTD)-based
beamforming architecture is exploited. In particular, two wideband beamforming
optimization approaches leveraging TTD units are proposed. 1) Analytical
approach: In this approach, the phase shifters (PSs) and the time delay of TTD
units are designed based on the analytical formula for beamforming gain.
Following this design, the minimum number of TTD units required to achieve a
predetermined beamforming gain is quantified. 2) Joint-optimization approach:
In this method, the PSs and the TTD units are jointly optimized under practical
maximum delay constraints to approximate the optimal unconstrained analog
beamformer. Specifically, an efficient alternating optimization algorithm is
proposed, where the PSs and the TTD units are alternately updated using either
the closed-form solution or the low-complexity linear search approach.
Extensive numerical results demonstrate that 1) the proposed beamforming
schemes effectively mitigate the beam squint effect, and 2) the
joint-optimization approach outperforms the analytical approach in terms of
array gain and achievable spectral efficiency.
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