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Frequency offset estimation and compensation for digital optical coherent receiver

Abstract

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:

  1. Wider frequency offset: the frequency offset of the laser may reach +/-5GHz.
  2. Higher synchronization accuracy: after synchronization, the frequency offset must be less than +/-50MHz.
  3. Restriction of laser technology: now the commercial available laser does not support precise frequency control.
  4. Lower hardware complexity: the data rate of the optical transmission reaches tens of Gbit/s, so the frequency must be synchronized by low complexity.

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.

Solution

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

Technical points

  1. Feed back the estimated frequency offset and apply the frequency offset in the phase difference estimation to improve the estimated precision and range.
  2. Estimate the phase difference by using pre-decision method to reduce the computing complexity.

Synoptic diagram

pade

Effect

  • For 43Gbit/s QPSK system, realize full-range (+/-5GHz) and high-precision (+/-30MHz) frequency offset estimation and compensation by processing digital signals.
  • Remove the requirement of laser frequency control and make a coherent receiver by commercial available lasers.
  • Replace the optical domain frequency synchronization by frequency compensation in digital domain to reduce cost and size of the receiver.

Contact information:

Tao Zhenning: taozn@cn.fujitsu.com

Li Lei: lilei@cn.fujitsu.com

Wu Jianming: jianming.wu@cn.fujitsu.com