Abstract

Recently, return-to-zero (RZ) differential phase-shift keying (DPSK) modulation at 20 Gbit/s line rate was indicated as a strong candidate for upgrading non-slope matched undersea transmission links [1]. Here, we assess the performance of a typical non-slope matched transoceanic submarine link using 20 Gbit/s RZ-DPSK. Details of the system setup, numerical modelling and experiments will be given in the conference. First, we optimize the system performance versus the pre- and post-compensation dispersions and the launch signal power for different pulse duty cycles - 50%, 33% and 20% - through single-channel transmission simulations. Next, we compare the accuracy of the existing Q-factor models for RZ-DPSK - conventional Q of the received electrical signal (Q_ei), amplitude Q (QA) [2], differential phase Q (QA<P) [2], and modified differential phase Q (QA<p,mod) [2] - versus the actual bit-error rate (BER) obtained from direct error counting in single-channel simulations. An example is given in Fig. 2 for the 33% duty cycle pulses. We observe that in the considered parameter range QA<P is the most reliable performance indicator for the 50% duty cycle and QA0,mod offers the highest accuracy for the 20% duty cycle, whereas for the 33% duty cycle the two phase Qs compete with one another at giving the most reliable performance prediction. The results also indicate that the shorter duty cycles achieve the higher BER margin, with a BER of 10~⁹ relating to a transmission distance of approximately 6000 km for the 50% duty cycle, and 8000 km for the 33% and 20% duty cycles. The degree of accuracy of the BER estimates through the various Q models is explained by the deviations of the actual probability density functions (PDFs) for the relevant quantities of the received signal from the approximations (Gaussian or modified Gaussian) underlying the pertinent models. In particular, inspection of the PDFs for the received current and differential phase reveals that the kurtosis of the distributions is larger than the value of three of the corresponding Gaussian approximations, and this " non-gaussianicity" increases with decreasing pulse width. The dependence of the BER estimates on the signal power is also investigated. Next, we analyze the BER performance of wavelength-division multiplexed (WDM) transmission at 50 GHz channel separation (0.4 bit/(s Hz) spectral efficiency). Figure 3 (left) shows the BER of a channel centred at 1553nm versus the transmission distance for the 50% pulse duty cycle as an example. We observe that the BER margin is improved by the use of large pulses. Indeed, for instance, at a transmission distance of 6000 km the directly computed BER would be of the order of 10~⁷ for the 50% duty cycle, whereas at the same distance the BER is 2.5 x 10~⁴ for the 33% duty cycle. The WDM system performance assessed through numerical simulations is compared to the experimental results. For the measurements at 50 GHz channel spacing, the results in Fig. 3 (right) are shown for a 1553 nm centred channel. The duty cycle of the pulses is 50%. Comparing the curve relative to 50 GHz channel spacing with the numerical results, it can be seen that there is a fair agreement at the considered distances. Fig. 1 Transmission system setup. retical BERs and directly computed BER versus distance. Right: distributions of received curren and differential phase, actual and approximations Fig. 3 WDM transmission. Left: theoretical BERs and y computed BER versus distance. Right: measured E R versus distance, In conclusion, we have assessed the performance of a typical non-slope matched transoceanic submarine link using 20Gb/s channel rate and RZ-DPSK modulation with different duty cycles. Through comparison with direct error counting, we have also demonstrated the limitations of the available numerical approaches to the BER estimation for RZ-DPSK. The numerical results have been confirmed by experiments, and indicate that 2 0 G b / s RZ-DPSK transmission is a feasible technique for the upgrade of existing submarine links.

© 2007 IEEE