CSI Feedback Under Basis Mismatch: Rate-Splitting Transform Coding for FDD Massive MIMO

TL;DR

This paper addresses basis mismatch in FDD massive MIMO channel feedback, proposing a rate-splitting transform coding scheme that improves performance and reduces complexity.

Contribution

It introduces a practical architecture separating long-term basis feedback from short-term coefficient quantization, with a closed-form error expression and optimal rate split analysis.

Findings

Near-optimal performance demonstrated on correlated Gaussian and COST2100 channels.

Robustness to update overhead and significant complexity reduction compared to deep learning methods.

Identifies a phase transition threshold for basis updates.

Abstract

In frequency division duplex massive multiple-input multiple-output systems, downlink channel state information must be fed back within a limited uplink budget. While transform coding with Karhunen-Loeve transform and reverse water-filling is rate-distortion optimal for Gaussian channels, its performance is limited by basis mismatch between the user and base station. We analyze this mismatch and propose a practical architecture separating long-term basis feedback from short-term coefficient quantization. Using a random vector quantization, we derive a closed-form end-to-end mean square error expression. This allows us to characterize the optimal rate split and identify a phase transition threshold for basis updates. Simulations on correlated Gaussian and COST2100 channels demonstrate near-optimal performance, robustness to update overhead, and significant complexity reduction compared to deep-learning-based autoencoders.