Optical Time-Frequency Packing: Principles, Design, Implementation, and Experimental Demonstration

TL;DR

This paper explores the application of time-frequency packing in fiber-optic communications, demonstrating its potential to significantly enhance spectral efficiency through advanced detection, adaptive equalization, and coding techniques.

Contribution

It introduces a novel adaptive equalization and detection algorithm for TFP in fiber optics and experimentally validates the approach with high spectral efficiency over multiple transmission distances.

Findings

Achieved high spectral efficiency close to theoretical limits.

Demonstrated effective adaptive compensation of impairments.

Validated system performance with experimental fiber-optic transmission.

Abstract

Time-frequency packing (TFP) transmission provides the highest achievable spectral efficiency with a constrained symbol alphabet and detector complexity. In this work, the application of the TFP technique to fiber-optic systems is investigated and experimentally demonstrated. The main theoretical aspects, design guidelines, and implementation issues are discussed, focusing on those aspects which are peculiar to TFP systems. In particular, adaptive compensation of propagation impairments, matched filtering, and maximum a posteriori probability detection are obtained by a combination of a butterfly equalizer and four 8-state parallel Bahl-Cocke-Jelinek-Raviv (BCJR) detectors. A novel algorithm that ensures adaptive equalization, channel estimation, and a proper distribution of tasks between the equalizer and BCJR detectors is proposed. A set of irregular low-density parity-check codes with different rates is designed to operate at low error rates and approach the spectral efficiency limit achievable by TFP at different signal-to-noise ratios. An experimental demonstration of the designed system is finally provided with five dual-polarization QPSK-modulated optical carriers, densely packed in a 100 GHz bandwidth, employing a recirculating loop to test the performance of the system at different transmission distances.