WiFocus: Bandwidth-Aware Beam Focusing in Wideband Sub-Terahertz Wireless Networks

Next-generation wireless communications above 100 GHz promise significantly higher data rates, enabled by the wide swaths of available spectrum. To overcome the severe path loss in this regime, large-scale antenna arrays are required to close the link. As a byproduct of high carrier frequencies and large electrical sizes, the near-field region extends to meter-scale distances, allowing transmitters to focus energy directly onto receivers. However, despite this potential, conventional phased-array systems suffer from severe beam split, which causes frequency-dependent divergence of the beam and effectively limits the usable bandwidth at the receiver - even in line-of-sight scenarios. To address this challenge without resorting to costly hardware solutions, this paper introduces WiFocus, a novel phase manipulation technique that redistributes power more uniformly across frequency, thereby enhancing channel capacity. We show that a strategic adjustment of the beam focal point yields a closed-form, low-complexity, one-shot solution that achieves near-optimal performance. Extensive simulations and D-band over-the-air measurements confirm the effectiveness of the proposed technique, demonstrating average achievable channel capacity within 8.8% of the theoretical upper bound, a 50.0% improvement over conventional focused beams, and a 25.4% gain over the state-of-the-art phase manipulation method.