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We analyze by simulation the performance of the differential quadrature phase-shift keying (DQPSK) modulation format in dispersion-managed multi-span WDM optical systems at 43 Gb/s, with 50 GHz channel spacing, in the presence of fiber nonlinearity. We show that the use of maximum-likelihood sequence-estimation (MLSE) receivers significantly helps to increase tolerance to dispersion map design. We also show that the use of an MLSE algorithm which takes into account the correlation of samples allows to improve system performance, partially mitigating the impact of fiber non-linearity. Some of the results presented here hint at a possible use of similar techniques in the context of coherent systems as well.