Practical Considerations in Direct Detection Under Tukey Signalling

TL;DR

This paper explores a novel Tukey signalling scheme that introduces controlled ISI to enhance data rates in direct detection systems, leveraging waveform robustness and efficient detection methods to outperform traditional IMDD approaches.

Contribution

It introduces a practical Tukey signalling method with controlled ISI, enabling higher data rates and robustness in direct detection systems compared to conventional intensity modulation.

Findings

Maximum throughput of 200 Gb/s at 50 Gbaud with 8,4-SQAM

Tukey scheme outperforms IMDD by approximately 55 Gb/s

Robustness to nonlinear distortions and efficient detection methods

Abstract

The deliberate introduction of controlled intersymbol interference (ISI) in Tukey signalling enables the recovery of signal amplitude and (in part) signal phase under direct detection, giving rise to significant data rate improvements compared to intensity modulation with direct detection (IMDD). The use of an integrate-and-dump detector makes precise waveform shaping unnecessary, thereby equipping the scheme with a high degree of robustness to nonlinear signal distortions introduced by practical modulators. Signal sequences drawn from star quadrature amplitude modulation (SQAM) formats admit an efficient trellis description that facilitates codebook design and low-complexity near maximum-likelihood sequence detection in the presence of both shot noise and thermal noise. Under the practical (though suboptimal) allocation of a 50% duty cycle between ISI-free and ISI-present signalling segments, at a symbol rate of 50 Gbaud and a launch power of -10 dBm the Tukey scheme has a maximum theoretically achievable throughput of 200 Gb/s with an (8,4)-SQAM constellation, while an IMDD scheme achieves about 145 Gb/s using PAM-8. Note that the two mentioned constellations have the same number of magnitude levels and the difference in throughput is resulting from exploiting phase information under using a complex-valued signal constellation.