Impact of inter-symbol interference on optical DQPSK systems performance evaluation using equivalent differential phase

The statistics of ldquoequivalentrdquo differential phase (EDP) of differential quadrature phase-shift keying (DQPSK) systems is assessed numerically. It is shown that the EDP statistics is approximately Gaussian for a large range of system parameters. Thus, performance evaluation methods based on a Gaussian approximation for the EDP may be used. However, both the mean and variance of the Gaussian distribution associated with each symbol depend on the transmitted symbol sequence, showing that the impact of inter-symbol interference (ISI) on the bit error ratio should not be neglected. Indeed, ISI may lead to an inaccuracy exceeding 1 dB on the required OSNR when not accurately taken into account.