I. Introduction
Recently, multichip differential phase-shift keying (MC-DPSK) detection has been proposed in optical fiber transmission systems to improve the performance of the DPSK signals. The transmitter for the MC-DPSK modulation is identical to that of a standard DPSK; however, its receiver is applied with multiple consecutive chips which simultaneously extract differential phase encoding of a DPSK signal [1], [2]. By employing a soft-detection method, the MC-DPSK detection can perform better than a standard DPSK. Also electrical equalization has been proved to be an effective approach to overcome distortions due to chromatic dispersion (CD) and polarization-mode dispersion (PMD) [3]–[5]. It has been shown that the combination of MC-DPSK detection and maximum likelihood sequence estimation (MLSE) can further improve the CD and PMD tolerances [6], in which branch signals from multiple chips are jointly processed by a joint MLSE equalizer to recover the original bit sequence.