Optimized hybrid QPSK/8QAM for CO-OFDM transmissions

In optical communications, a high spectral efficiency can be realized by applying high order modulation formats such as 8QAM, 16QAM and 64QAM. However, depending on the system's requirements (bandwidth, performance and transmission distance), the maximum spectral efficiency may not be achievable with the regular 2
^m
-array QAM formats. In this case, a hybrid modulation format, such as QPSK/8QAM, can provide an effective solution. In this work, we deliver the optimum design for single channel coherent optical orthogonal frequency division multiplexing systems with hybrid QPSK/8QAM modulation format. We also discuss a simple but effective strategy for applying hybrid QAMs for long-haul optical communications without considering sophisticated bit and power loading algorithms developed for wireless communications.