Spectral Efficiency of Coded Phase Shift Keying for Fiber Optical Communication

Several optical modulation and detection schemes are compared by computing their spectral efficiencies over a simple channel model. It is found that quadrature phase shift keying (QPSK) can be substantially more bandwidth efficient than either direct-detection on-off keying (DD-OOK) or binary phase-shift keying (BPSK). However, the energy gains of QPSK with these detectors are diminished as compared to optimal detection. The degradation is due to an abrupt behavior in spectral efficiency when non-coherent and hard detection is used. To test the theory, Reed-Solomon and low-density parity-check (LPDC) codes are designed and evaluated. The codes generally behave in a manner similar to that predicted by the theory. It is further shown that neither multiple-symbol differential detection nor decision-feedback detection helps much when using good codes.