A Transition Metric in Polar Co-ordinates for MLSE of a Complex Modulated DML

We propose a metric for MLSE-Viterbi differential decoding of complex modulation of directly modulated lasers (CM-DML) that reports SNR gains of 1.8 dB at BER=10
^-3
on a simulated PAM4 signal with a typical linewidth enhancement factor α = 4.