Capacity Bounds on Doppler OFDM Channels

TL;DR

This paper investigates the capacity limits of Doppler-affected OFDM channels in LEO satellite systems, proposing new transmission schemes and deriving bounds to improve data rates under residual frequency uncertainties.

Contribution

It introduces a novel channel model for Doppler effects, derives capacity bounds, and proposes a practical superposition scheme with subspace alignment for improved rates.

Findings

SN scheme achieves near-optimal rates

Capacity bounds are characterized in high-SNR regimes

Proposed method shows low complexity in simulations

Abstract

Low Earth orbit (LEO) satellite systems experience significant Doppler effects due to high mobility. While Doppler shifts can be largely compensated, residual frequency uncertainty induces a structured form of channel uncertainty that can limit achievable rates. We model this effect using a block-fading channel of the form $ \mathbf{H} = \mathbf{F} + s \mathbf{G} $, where $s$ is an unknown scalar random parameter. We first study this model in a general $N\times N$ MIMO setting. For this channel, we derive achievable rate lower bounds based on explicit transmission schemes and capacity upper bounds using a duality approach. We study Gaussian signaling and propose a practical superposition scheme with subspace alignment (SN) and successive interference cancellation, where a coarse-layer stream serves as an implicit pilot for decoding refined-layer data. We characterize asymptotic capacity in the near-coherent and high-SNR regimes, and show via Doppler-OFDM simulations that the proposed SN scheme achieves near-optimal rates with low complexity.