Beam Squint Mitigation in Wideband Hybrid Beamformers: Full-TTD, Sparse-TTD, or Non-TTD?

Abstract

Beam squint poses a fundamental challenge in wideband hybrid beamforming, particularly for mmWave and THz systems that demand both ultra-wide bandwidth and high directional beams. While conventional phase shifter-based beamformers may offer partial mitigation, True Time Delay (TTD) units provide a fundamentally more effective solution by enabling frequency-independent beam steering. However, the high cost of TTD units has recently driven much interest in Sparse-TTD architectures, which combine a limited number of TTDs with a higher number of conventional PSs to balance performance and cost. This paper provides a critical examination of beam squint mitigation strategies in wideband hybrid beamformers, comparing Full-TTD, Sparse-TTD, and Non-TTD architectures. We analyze recent Non-TTD approaches, specifically the scheme leveraging the wideband beam gain (WBBG) concept, evaluating their performance and cost characteristics against TTD-based solutions. A key focus is placed on the practical limitations of Sparse-TTD architectures, particularly the often-overlooked requirement for wideband PSs operating alongside TTDs, which can significantly impact performance and implementation cost in real-world scenarios, especially for ultra-wideband applications. Finally, we conduct a cost-performance analysis to examine the trade-offs inherent in each architecture and provide guidance on selecting the most suitable hybrid beamforming structure for various fractional bandwidth regimes.

Figures (5)

• Figure 1: Wideband hybrid beamformer leveraging (a) conventional phase-shifters (PSs) without deploying TTDs (Non-TTD), (b) one TTD per antenna element (Full-TTD), and (c): sparse TTDs together with a block of PSs (Sparse-TTD)
• Figure 2: The hierarchical structure of the article.
• Figure 3: (a) Power density versus carrier frequency in a multi carrier wideband system. (b) Representation of beam gain in the angular and spectral domains when no beam squint mitigation is utilized. (c) Comparison of beam squint removal leveraging Full-TTD MRT vs. Non-TTD WBBG mechanisms.
• Figure 4: (a): Performance versus fractional bandwidth for various beamformers and two setups including mmWave with $f_0=28$ GHz and $N=64$ antennas, and sub-THz with $f_0=140$ GHz and $N=256$ antennas. We consider $M = 16$, $\tilde{\gamma}_0 = 0$ dB, and $\theta_{\mathrm{u}}=60^\circ$. Depending on the design parameters, the Sparse-TTD beamformer's achieved performance lies at some point within the upper and lower bounds shown as the green-shaded region. (b): Cost trend versus operating fractional bandwidth. The regions where Non-TTD-WBBG, Sparse-TTD-NBBG, and Full-TTD-NBBG beamformers are more cost-effective are highlighted in orange, blue, and green, respectively.
• Figure 5: Beam squint mitigation in wideband hybrid beamformers considering the cost-performance trade-off. The normalized performance and cost benefits in each of the wideband and ultra-wideband scenarios are also visualized.