Capacity Bounds on Doppler OFDM Channels

2026-02-04

AI Summary

This work addresses the performance degradation in low Earth orbit (LEO) satellite communications caused by residual Doppler-induced frequency offsets, which introduce structured channel uncertainty and limit achievable rates. The authors model the Doppler-OFDM channel as a MIMO block-fading process parameterized by an unknown scalar, and propose an implicit pilot superimposed coding scheme that integrates subspace alignment with successive interference cancellation. Under near-coherent and high-SNR conditions, this approach closely approaches the channel capacity. By formulating and solving a dual problem, they derive a tight capacity upper bound and demonstrate that the proposed method achieves near-optimal transmission performance with low computational complexity.

Technology Category

• Planning, Routing, and Scheduling: Scheduling under Uncertainty
• Search and Optimization: Mixed Discrete/Continuous Search
• Constraint Satisfaction and Optimization: Distributed CSP/Optimization

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Web performance, measurement, and characterization
• Security and Privacy: Large-scale security measurements
• Economics, Online Markets and Human Computation: Incentives in network design for Web infrastructures and ecosystems

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.

Problem

• Doppler effect
• channel uncertainty
• capacity bounds
• LEO satellite
• residual frequency offset

Innovation

• Doppler OFDM
• structured channel uncertainty
• subspace nesting
• capacity bounds
• superposition coding