# Investigation of a novel structure for 6PolSK-QPSK modulation

- Yupeng Li
^{1, 2}Email author, - Ming Li
^{1, 2}, - Jiawei Han
^{1, 2}and - Tingting Han
^{1, 2}

**2017**:66

**DOI: **10.1186/s13638-017-0860-0

© The Author(s). 2017

**Received: **23 January 2017

**Accepted: **4 April 2017

**Published: **12 April 2017

## Abstract

Benefiting from the high spectrum efficiency and power efficiency, 6PolSK-QPSK (6-ary polarization-shift keying quadrature phase-shift keying) is a promising modulation format in coherent optical communication. We proposed a novel structure to generate the 6PolSK-QPSK with two dual-drive MZMs. Simulation results show that the proposed structure can generate 6PolSK-QPSK effectively and the performance is nearly the same to the traditional one.

### Keywords

6PolSK-QPSK DDMZM Coherent optical communication## 1 Introduction

In this paper, we proposed a novel structure for 6PolSK-QPSK modulation based on dual-drive MZM (DDMZM), which can reduce the complexity and cost of the transmitter. Simulation results show that the proposed structure is effective to generate the 6PolSK-QPSK, and the performance is nearly the same to the traditional one.

## 2 Theoretical analysis

*V*

_{RF1}and

*V*

_{RF2}ports are driven by the electrical signal to change the phase of optical signal.

*V*

_{d1}and

*V*

_{d2}ports are used to set the phase deviation between upper and lower phase modulator. Because RF

_{1}and RF

_{2}of the DDMZM can be adjusted independently, the DDMZM has high degree of freedom. In fact, a DDMZM can be used to generate any high-order modulation theoretically [10]. Based on the high degree of freedom of DDMZM, we can utilize it to generate the 6PolSK-QPSK.

As showed in Fig. 3, the first step is encoding nine bits to two symbols. When the first bit is 0, the subsequent bits are encoded to two DP-QPSK symbols; if the first bit is 1, the subsequent coding method is decided by the second bit and, then, PS-QPSK or DP-QPSK are encoded with different sequence according to the second bit.

The DDMZM is suitable to generate the points in the constellation above with two independent RF signals.

where *V*
_{
π
} is the voltage to make the optical signal get *π* phase shift in the phase modulator. We assume four basic phase shift for the modulation, *P* = (*π*/4, 3*π*/4, 5*π*/4, 7*π*/4), the green arrows in Fig. 4 represent the basic phase shift in *P*, and all the points in the constellation can be generated with the four basic phase shift. Because the maximum phase difference in *P* is 3*π*/2, the *V*
_{pp} of the driven signal is 3*V*
_{
π
}/2.

The optical signal *E*
_{in} is split into two parts *E*
_{1} and *E*
_{2}, both initial phases of *E*
_{1} and *E*
_{2} are 0, and with different *V*
_{RF}, the phase of *E*
_{1} and *E*
_{2} is belong to basic phase shift *P*. After combing the *E*
_{1} and *E*
_{2}, the corresponding point is obtained at the output of DDMZM. For example, if the *E*
_{1_phase} = *π*/4 and *E*
_{2_phase} = 3*π*/4, we can get the upper red point in DP-QPSK, and when *E*
_{1_phase} = *π*/4 and *E*
_{2_phase} = *π*/4, we can get the upper right blue point in PS-QPSK.

In order to get the four basic phase shift, the driven signals should have four different levels to form the points in DP-QPSK or PS-QPSK. With the coding rules, we can get the 6PolSK-QPSK at the output.

In addition, the DDMZM can work at different bias point, such as peak, null, and quad. For different bias point, the initial phase shift is different between upper branch and lower branch; the coding needs minor adjustment. We take null points for example, which means the two branches in DDMZM have phase difference *π*, so when the *V*
_{RF1} = *V*
_{
π
}/4 and *V*
_{RF2} = *V*
_{
π
}/4, with the phase difference *π*, the output is the zero point in PS-QPSK.

## 3 Simulation results and discussion

*V*

_{pp}= 3

*V*

_{ π }/2; the PBS and PBC are used for split and combine the different polarization of the light signal. Both linewidths of lasers used as signal laser and local laser are 0Hz, so the influence of frequency deviation and phase noise can be eliminated. The wavelength of the lasers is 1550 nm. The 6PolSK-QPSK is combined with the ASE (amplified spontaneous emission) noise to adjust the OSNR (optical signal-to-noise ratio) in the back to back simulation system. The simulation symbol rate is 10GBd, because each 6PolSK-QPSK symbol contains 4.5 bits; the total data rate is 45 Gb/s.

Coherent detection is used to recover the signals. The local light is combined with the signal light in two 2 × 4 90° Hybrids and converted to electrical signals with four balanced photo-diodes (BPD), obtaining I-part and Q-part signals in both *X* and *Y* polarization, respectively. The four signals *R*
_{XI}, *R*
_{XQ}, *R*
_{YI}, and *R*
_{YQ} are stored and decoded according to the rules above.

In the simulation, we assume all the parts of the system are ideal, and evaluate the performance of the proposed structure with the EVM and BER.

^{−3}when OSNR >18 dB. The results show the performance of the two transmitters are nearly the same; the proposed structure even has a little better performance.

## 4 Conclusions

We proposed a novel structure to generate the 6PolSK-QPSK, and a series of simulations were taken to show its performance. EVM and BER are measured to evaluate the performance of the proposed 6PolSK-QPSK system. The results show the proposed structure has similar performance with the traditional one. Furthermore, the RZ systems are researched and the results show that the RZ pattern can improve the system performance significantly. The proposed structure is an effective way to reduce the cost of 6PolSK-QPSK transmitter with the simple modulators.

## Declarations

### Funding

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).

### Authors’ contributions

YL is the main writer of this paper. He proposed the main idea, completed the simulation, and analyzed the result. ML and JH assisted the theory research. TH assisted the simulation. All authors read and approved the final manuscript.

### Competing interests

The authors declare that they have no competing interests.

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## Authors’ Affiliations

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