Performance of DPSK signals with nonlinear phase noise for systems with small number of fiber spans

When the dependence between linear and nonlinear phase noise is taken into account, the exact error probability of DPSK signals with nonlinear phase noise is derived analytically for a fiber system with finite number of fiber spans. For the same mean nonlinear phase shift, the SNR penalty is reduced with the number of fiber spans. The discrepancy between the exact error probability and independence approximation increases with the number of fiber spans.