DDA-Net: Accurate TDD Channel Estimation via Deep Unfolding the Doppler-Delay-Angle Representation of Channel Signals

Abstract

In TDD massive MIMO systems, channel estimation under sparse frequency-hopping pilots is challenging: each snapshot captures only one narrow pilot block that hops across frequency, with tens of milliseconds between adjacent snapshots. Finite-window leakage and off-grid effects weaken the ideal Doppler-delay-angle (DDA) sparsity, limiting both classical sparse recovery and purely data-driven approaches lacking an explicit structured transform-domain model. We propose DDA-Net, a model-driven 3D deep unfolding network for joint multi-snapshot channel state reconstruction. DDA-Net integrates an ADMM-based formulation with a closed-form data-consistency update that avoids tensor inversion, a lightweight Doppler-domain learned prior, and delay oversampling to mitigate basis mismatch. It consistently outperforms strong baselines across three channel settings from 3GPP TR 38.901: UMa-NLOS, UMi-NLOS, and CDL-B. Ablations confirm that window-level 3D processing and explicit Doppler modeling yield target-dependent improvements. With minimal target-domain fine-tuning, the UMa-pretrained model surpasses both its zero-shot version and counterparts trained from scratch with the same number of target-domain samples. The Doppler-domain design proves consistently superior to its time-domain equivalent, with a wider margin after fine-tuning. These results demonstrate that combining exact physical data consistency with a learned DDA-domain prior is an effective and sample-efficient approach to channel state acquisition under sparse frequency-hopping pilots.