  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.
