Fig. 3. Numerical model of a long-haul coherent transmission system implementing DSP
The physical properties characterize the S-SMF; attenuation α = 0.2 dB, dispersion D = 16.75 ps/nm/km, dispersion slope = 0.075 ps/nm2/km, and nonlinearity coefficient γ = 1.2 km-1W-1. A 16 dB gain Erbium-doped fiber amplifier (EDFA) with a noise figure of 4 dB is used for optical amplification to compensate for power loss. On the receiver side, the incoming in-phase and quadrature-phase signals in the x and y polarization states are detected by a homodyne coherent polarization and phase diversity receiver. The receiver comprises two 90o optical hybrids and four pairs of balanced PIN photodetectors. The local oscillator (LO) laser for coherent detection operates at a frequency, power, and linewidth of 1550 nm, 10 dB, and 0.1 MHz, respectively. After coherent detection, the received signal undergoes analog-to-digital conversion by four samples per symbol down-sampling method at the reference symbol rate. The DSP module processes it after that.
Digital Signal Processing Module
The DSP module, implemented with MATLAB, comprises segmental blocks for polarization multiplexing, carrier phase estimation, linear dispersion compensation, and nonlinearity compensation. The constant modulus phase algorithm implements an adaptive finite impulse response (FIR) butterfly equalizer to demultiplex the dual-polarized signal. The Viterbi-and-Viterbi joint-phase estimation algorithm is adopted to account for relative phase error between the transmitter laser and the LO laser, which arises due to their non-zero laser linewidth (Viterbi & Viterbi, 1983). The DBP-based nonlinearity techniques are implemented by the SSFM method based on the Weiner-Hammerstein model where the nonlinear part is computed at the midpoint of each step with two surrounding virtual linear blocks . We use the step sizes given in Table 1 for the 10-step SSFM calculation. The DSP linear blocks have been characterized in Table 2.
Table 2: Characterization of DSP linear blocks