Low-Complexity Channel Estimation and Orthogonal Precoding for Downlink Delay-Doppler Domain Multiple Access

The orthogonal delay-Doppler division multiplexing (ODDM) modulation has been widely acknowledged as a potential candidate for supporting ultra-reliable wireless communications under high-mobility scenarios. Nevertheless, the downlink transmission utilizing ODDM modulation remains an open problem due to the high complexity of prior channel estimation designs and complicated multi-user interference in the delay-Doppler (DD) domain. To address this issue, the channel estimation and data detection for downlink DD domain multiple access (DDMA) are investigated in this paper. The fast Fourier transform (FFT) interpolation is first presented for estimating each equivalent channel delay tap. To further promote the channel estimation accuracy, an orthogonal matching pursuit (OMP)-enabled scheme is also developed, where the normalized delay time and Doppler shift for each virtual path are determined separately to reduce the computational complexity. To ease the data detection at the user equipment (UE) side, we propose the orthogonal precoding for data symbols to provide the Gaussian distribution as the prior knowledge thanks to the central limit theorem, which can support the highly efficient interference cancellation without sharing the constellation information among UEs. Simulation results confirm the excellent performance of the proposed downlink DDMA schemes, where comparable reliability with optimal interference cancellation by sharing the knowledge of constellation among all UEs while the flexibility and efficiency can be guaranteed.