Parity Check Encoded QPSK for Rate Adaptation in Coherent DSP-Based Receivers

An ultrashort rate-1/2 parity check (PC) code is used to switch the net data rate of a coherent quadrature phase shift keying (QPSK) transmission system to the equivalent of a binary phase shift keying (BPSK) system. Simulation results of PC encoded-QPSK show an improvement in receiver sensitivity of 0.8 dB over BPSK at the soft-FEC threshold of BER = 2×10
^-2
. In addition, the code allows establishing an absolute phase reference at the receiver; i.e., differential coding is not required.