Delta-Delay-Phase Precoding Based Wideband Near-Field Beamfocusing

This paper focuses on exploring the wideband beam split effect in the near-field region under the adoption of practical true time delay (TTD) elements. We formulate the design of the wideband hybrid analog precoder as a joint optimization problem taking into account the hardware limitations of TTDs. To tackle this non-convex problem, we propose a greedy beamfocusing (GB) approach with two stages utilizing the delta-delay-phase precoding (DDPP) architecture in the near-field. The proposed approach designs the hybrid precoder while accounting for TTDs' limitations in its first stage. In the second stage, this approach adapts the resulting time delay values of TTDs to enable operation of the DDPP architecture in the near-field region. Numerical simulations depict that the state-of-art beamfocusing schemes in the near-field can realize only low-resolution beamsteering under the adoption of practical TTDs. In contrast, our proposed beamfocusing approach can achieve continuous beamsteering with an average array gain over users higher than 0.9 at all angles.