Abstract
The noise statistics of optically demodulated signal of the recently proposed modulation format, namely differential polarization-phase-shift keying, are analyzed. It is found that, in the linear regime, the Gaussian approximation is reasonably accurate in estimating the BER. In the nonlinear regime, the noise of the demodulated signal is dominated by the effect of nonlinear phase noise and obeys chi-squared statistics with a degree of freedom of one. Gaussian approximation underestimates the BER in the nonlinear regime. A two-step BER estimation method accounting for both linear beat noise and nonlinear phase noise is proposed. The effectiveness of this two-step BER estimation method at all power levels has been established by comparison with direct error counting.
Highlights
Direct detection differential polarization-phase-shift keying (DPolPSK), called differential Jones-vector-shift keying (DJSK) has recently been proposed for high-spectral efficiency optical communications [1]
The underlying mechanism for the effectiveness of the Gaussian approximation in the linear regime and its failure in the nonlinear regime are explained by understanding the noise statistics in both regimes
Since the optical demodulator and receiver for DPolPSK is the same as that for differential phase-shift keying (DPSK), it is somewhat surprising that the Gaussian approximation works well for DPolPSK because it has been found to be not applicable for DPSK BER estimation when balanced receivers are used [3]
Summary
Direct detection differential polarization-phase-shift keying (DPolPSK), called differential Jones-vector-shift keying (DJSK) has recently been proposed for high-spectral efficiency optical communications [1]. It is a constant-intensity modulation format that encodes information on both the polarization and phase of lightwave. DPolPSK differs from the conventional differential polarization-shift keying (DPolSK) in that DPolSK encodes. Dynamic polarization control is not required at the receiver This is accomplished by using multilevel detection. This receiver simplification is especially important for wavelength-division multiplexing (WDM) systems, where dynamic polarization control should be performed on a per channel basis since the SOP at each wavelength is generally different. In [1], the conventional Gaussian approximation for BER estimation using demodulated signal is found to fairly accurate in the linear regime, but generally underestimate the BER in the nonlinear regime. A two-step BER estimation method is proposed to accurately estimate the BER in all the regimes
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