Optical DPASK and DQPSK: a comparative analysis for linear and nonlinear transmission

The performance of two multilevel modulation formats, differential-phase-amplitude shift-keying (DPASK) and differential quadrature-phase-shift keying (DQPSK), is compared for linear and nonlinear channels. For DPASK, the optimal extinction ratio is derived for both linear and nonlinear transmission. It is shown that for single-channel transmission, DQPSK outperforms DPASK in linear channels, whereas DPASK achieves better performance when nonlinear impairments are present.