KEYWORDS: Modulation, Signal detection, Signal to noise ratio, Signal processing, Optical engineering, Computer programming, Digital signal processing, Wavelength division multiplexing, Sensors, Transmitters
Three soft-input-soft-output (SISO) detection methods for dual-polarized quadrature duobinary (DP-QDB), including maximum-logarithmic-maximum-a-posteriori-probability-algorithm (Max-log-MAP)-based detection, soft-output-Viterbi-algorithm (SOVA)-based detection, and a proposed SISO detection, which can all be combined with SISO decoding, are presented. The three detection methods are investigated at 128 Gb/s in five-channel wavelength-division-multiplexing uncoded and low-density-parity-check (LDPC) coded DP-QDB systems by simulations. Max-log-MAP-based detection needs the returning-to-initial-states (RTIS) process despite having the best performance. When the LDPC code with a code rate of 0.83 is used, the detecting-and-decoding scheme with the SISO detection does not need RTIS and has better bit error rate (BER) performance than the scheme with SOVA-based detection. The former can reduce the optical signal-to-noise ratio (OSNR) requirement (at BER=10−5) by 2.56 dB relative to the latter. The application of the SISO iterative detection in LDPC-coded DP-QDB systems makes a good trade-off between requirements on transmission efficiency, OSNR requirement, and transmission distance, compared with the other two SISO methods.
DP-QDQ is a partial-response shaping modulation based on 16QAM, which has been proposed in our previous research paper. In this paper, the power spectral density (PSD) of the DP-QDQ signal is analyzed and compared with DP-16QAM, DP-64QAM, and DP-256QAM signals, and the simulation setup is presented. The analysis results show the DP-QDQ signal has high spectral efficiency. In addition, the optical coherent DP-QDQ system is discussed in compared with DP-16QAM, DP-64QAM, and DP-256QAM systems in terms of the tolerance to optical signal to noise ratio (OSNR), transmission distance at 100G/s by simulations. It is revealed that DP-QDQ not only achieves high spectral efficiency, but also obtains a good compromise of the performances on spectral efficiency, OSNR tolerance and transmission distance.