Elsevier

Physical Communication

Volume 25, Part 2, December 2017, Pages 319-322
Physical Communication

Full length article
Investigation of quadrature imbalance compensation algorithm for coherent 6PolSK-QPSK

https://doi.org/10.1016/j.phycom.2017.02.004Get rights and content

Abstract

6PolSK-QPSK is a promising modulation format in optical fiber communication. Because of the damage suffered during the transmission and reception, a series of algorithms are needed to be adopted to recover the original data. We proposed a novel quadrature imbalance compensation algorithm based on the data statistical properties. Simulation results show that the quadrature imbalance can be well compensated with the proposed algorithm.

Introduction

In order to get large transmission capacity of optical fiber communication system, advanced multilevel-modulation formats are attracting great interests  [1], [2], [3]. 6PolSK-QPSK (6-ary Polarization-Shift Keying Quadrature Phase-Shift Keying), benefiting from its advantage in both spectral efficiencies (SE) and power efficiencies (PE)  [4], [5], has attracted large attention of researchers all over the world  [6], [7]. 6PolSK-QPSK makes full use of polarization and phase of the optical signal without decreasing the Euclidean distance. Coherent detection is an appropriate way to receive the 6PolSK-QPSK and recover the information in both polarizations. Because the optical signal will be damaged during the transmission in optical fiber, such as chromatic dispersion (CD), polarization mode dispersion (PMD), and quadrature imbalance (QI), clock jitter, frequency deviation, phase noise caused by the imperfect transmitter and receiver, a series of digital signal processing (DSP) algorithms compensation are necessary to recover the original signal. Quadrature imbalance is the first step of the whole compensation, for the QI will influence the subsequent procedures. QI is mainly caused by the imperfections of 90° optical hybrid and photodetectors in the receiver and the unequal level driven electrical signals in the transmitter, which shows as the I-part and Q-part receiving signals in each polarization are not orthogonal to each other, as a result the system performance will be significantly degraded.

Several DSP compensation algorithms for the QI have been proposed in the DP-QPSK system  [8], [9], such as GSOP (Gram–Schmidt Orthogonalization Process), EF (Ellipse Fitting), QI-CMA, and LOP (Löwdin Orthogonalization Process). For 6PolSK-QPSK, the compensation algorithms need to be modified because the amplitudes of the received signals are not constant.

In this paper, we proposed a novel blind algorithm used for QI compensation, the new algorithm is based on the statistical properties of the received signals. Simulation results show that the new algorithm is effective to compensate the damage caused by QI, after the compensation, the original signals are well recovered.

Section snippets

Theoretical analysis

The theory and procedure of the new method to compensate the QI is shown below.

Because the X polarization and the Y polarization have the same structure, we take the X polarization for an example. Fig. 1 shows the procedure to compensate the QI, the SXI and SXQ are the two parts of the received signal in X polarization after combining the signal light ESX and the local light ELOX, and convert to the electrical signal. Thus SX=[SXIRe{ESXELOX}SXQIm{ESXELOX}].

Because of the imperfections in

Simulation results and discussion

In order to verify the performance of the proposed algorithm, we did some simulation researches with Matlab. Fig. 2 is the simulation setup. At the transmitter side, the original data is encoded to four data streams according to the coding principle  [4], [5], 9 bits are mapped into two consecutive symbols, the DP-IQ modulator is driven with the encoded signals to generate the 6PolSK-QPSK, which contains two IQ modulator used for X and Y polarization modulation respectively. As the inset shown

Conclusion

We proposed a novel algorithm to compensate the QI in 6PolSK-QPSK coherent system. And a series of simulations were taken to show its performance. We measured the EVM and BER performance in imperfect 6PolSK-QPSK system with and without compensation, the results show the proposed algorithm can compensate the damage caused by QI completely, the compensated system performs as well as the ideal system.

Acknowledgments

This work is supported by the China Scholarship Council (201608120030), the Doctor Fund of Tianjin Normal University (52XB1505), the Doctor Fund of Tianjin Normal University (52XB1506) and National Natural Science Foundation of China (No. 11404240).

Yupeng Li was born in Shandong Province, China, in 1987. He received the B.S. degree in communication engineering from University of Jinan (UJN), Shandong, China, in 2009, and received the Ph.D. degree in Communication and information system from Beijing University of Posts and Telecommunications (BUPT), Beijing, China, in 2015. Now, he is working in the college of Electronic and communication engineering, Tianjin Normal University (TJNU), Tianjin, China, as a lecturer.

His current research

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Yupeng Li was born in Shandong Province, China, in 1987. He received the B.S. degree in communication engineering from University of Jinan (UJN), Shandong, China, in 2009, and received the Ph.D. degree in Communication and information system from Beijing University of Posts and Telecommunications (BUPT), Beijing, China, in 2015. Now, he is working in the college of Electronic and communication engineering, Tianjin Normal University (TJNU), Tianjin, China, as a lecturer.

His current research interests include advanced modulation scheme and coherent optical communication system.

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