Channel capacity of WDM systems using constant-intensity modulation formats

In conclusion, constant-intensity modulation eliminates the effects of both self phase modulation (SPM) and cross phase modulation (XPM), potentially increasing the limits to spectral efficiency in WDM systems. In the linear regime at high SNR, the spectral efficiency of constant-intensity modulation is 1.1 bit/s/Hz larger than half the Shannon limit. In practice, the improvement yielded by constant-intensity over unconstrained modulation formats depends on the relative strengths of XPM and FWM. A representative 10 span WDM system shows an improvement from 2.3 to 2.8 bit/s/Hz.