With the increasing requirements for the capability and flexibility of the optical communication system, the coherent technology is attracting more and more attention in the optical transmission area. To realize the optical coherent receiver, a main technical challenge is how to synchronize the local oscillator and carrier laser. Compared to the conventional electric coherent receiver, to synchronize the frequencies of optical coherent receiver, the following challenges are faced:
Based on the above challenges, we synchronize frequencies via frequency offset estimation and compensation in the digital domain and propose a novel frequency offset estimation method. This estimation range covers +/-5GHz and the estimation error is within +/-30MHz. The frequency synchronization is realized by modulating a reverse frequency offset on the received digital signals.
When a frequency offset exists, the phase of the demodulated digital signals will linearly increase or decrease with time and the change slope is the frequency offset. The basic idea of this solution is described as follows: firstly, pre-decide current symbol by the estimated phase of the previous symbol and the estimated frequency offset. Secondly, estimate the phase of current symbol by the received signal and the pre-decided data. Thirdly, calculate the phase difference between current symbol and previous symbol. Finally, the frequency offset is the phase difference divided by symbol duration. This scheme is called as “Pre- decision-based Angle Differential Estimator” (PADE).
For details, please refer to:
Optical Fiber Communication Conference and Exposition, February 24-28, 2008, San Diego, California, USA.（OFC2008） Paper: OWT4
Tao Zhenning: email@example.com
Li Lei: firstname.lastname@example.org
Wu Jianming: email@example.com
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