Method and apparatus for algorithm on flexible square-QAM coherent detection

Abstract

In Software defined elastic optical networks, modulation format and constellation size may be flexibly modified. As a result, digital signal processing (DSP) algorithm should be compatible with different modulation schemes or readily reconfigurable at the optical coherent receiver. Therefore we propose a novel cascaded adaptive blind equalizers based on decision-directed modified least mean square (DD-MLMS) algorithm for polarization separation and carrier phase recovery. The algorithm is square quadrature amplitude modulation (QAM) independent so that it could be applied in the elastic optical systems. The 28 Gbaud polarization multiplexing quadrature phase shift keying (PM-QPSK) and PM-16QAM back-to-back transmission is demonstrated. The results show that the performance is very close to the general algorithm but with a benefit of the reduced operation complexity. We transmit the 8×240 Gb / s PM-16QAM wavelength division multiplexing (WDM) signal over 1200 km standard single mode fiber (SSMF) based on the proposed blind equalization with a bit error ratio (BER) less than 2×10−2.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This Application claims the benefit of U.S. Provisional Application 61 / 757,991 filed on Jan. 29, 2013, the entirety of which is incorporated herein by reference.BACKGROUND OF THE INVENTIONI. Introduction[0002]For 100 Gb / s and beyond optical transmission systems, the flexible and adaptive bandwidth, known as elastic bandwidth, enables to efficiently distribute data according to the needed capacity and transmission length. Consequently the software defined elastic optical networks are becoming more and more attractive and important at present [1]-[4]. In this software defined networking (SDN), modulation format and constellation size may be flexibly modified. In that case, digital signal processing (DSP) must be compatible with different modulation schemes or readily reconfigurable at the receiver. Polarization multiplexing quadrature phase shift keying (PM-QPSK) is proposed and commercially available for 100-Gb / s optical transmission syste...

AI Technical Summary

Benefits of technology

The DD-MLMS algorithm achieves comparable bit error rate (BER) performance to conventional methods while reducing complexity, supporting flexible deployment across QPSK and 16QAM formats, and enabling transmission lengths of up to 1200 km with a BER less than 2×10−2 in 8×240 Gb / s PM-16QAM wavelength division multiplexing systems.

Problems solved by technology

However, CMA is not well compatible with 16QAM because high order QAM no longer presents constant symbol amplitude and equalized signal error cannot approach to zero.

The common problem is that the algorithm needs re-configuration and re-initialization so they are not flexible for the dynamic modulation format deployment.

But it is unrelated to carrier phase so that cannot be applied for carrier recovery.

Although conventional DD-LMS algorithm is compatible with all kinds of modulation formats, it is too sensitive to phase error.

Besides, carrier phase is blindly recovered simultaneously because phase error is reserved in the cost function.

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