A Design Framework that Unifies 6G Modulation Schemes for Double Selectivity

TL;DR

This paper introduces a unified design framework for doubly-selective channel modulation schemes, showing they are all special cases of a complex Hadamard-modulated pulse train family, enabling flexible waveform coexistence and consistent performance.

Contribution

It unifies various existing modulation schemes into a single waveform family based on complex Hadamard matrices, facilitating flexible multi-waveform transceiver design.

Findings

All proposed schemes are instances of a single waveform family.

The waveform family achieves full diversity with limited delay-Doppler support.

Enables flexible multi-waveform coexistence with a common transceiver architecture.

Abstract

There is significant recent interest in designing new modulation schemes for doubly-selective channels with large delay and Doppler spreads, where legacy modulation schemes based on time-frequency signal representations underperform. Multiple modulation schemes, e.g., in the delay-Doppler, chirp, time-sequency, and other domains, have been proposed in the literature for this purpose, with varying implementation details. In this letter, we establish that all previously proposed modulation schemes for doubly-selective signaling are instances of a single family of complex Hadamard-modulated pulse trains. When the delay and Doppler spread of the doubly-selective channel is limited to a certain support, all modulation schemes in this waveform family offer equivalent, full diversity achieving performance with no symbol fading and low channel estimation overhead. The existence of this waveform family also enables flexible, multi-waveform co-existence -- allowing a common transceiver architecture to generate multiple waveforms in the family, that may each be flexibly allocated to different users and services.