Analysis of Large Capacity Transmission Technology for 100G OTN

With the rapid development of the Internet in recent years, the number of Internet users, application types, bandwidth needs, and so on have all experienced explosive growth. Taking China as an example, the annual growth rate of the Internet traffic in the next four or five years is expected to reach 60 ~ 70%, the total bandwidth of the backbone transmission network will increase from 64 Tbit/s to about 150 Tbit/s, or even more than 200 Tbit/s. With the implementation of the "Broadband China-optical network City" Plan and the powerful drive of new bandwidth applications such as mobile Internet, Iot, and cloud computing, the transmission network is urgently needed to have a higher capacity. At present, a 40g wdm transmission system has been deployed on the backbone network. However, the explosive development trend of service capacity shows that 40G transmission will be stretched in the future when carrying multiple services, therefore, the demand for 100GWDM ultra-high speed transmission technology is imminent.

Key requirements for 100G OTN

To upgrade the long-distance transmission system to the 100 Gbit/s OTN system, the following requirements must be met:

1) supports 50 GHz channel spacing;

2) dispersion (CD) margin ± 700 ps/nm;

3) polarization modulus dispersion (PMD) margin of 10 ps (DGD average value );

4) It can be smoothly upgraded in the existing DWDM network and OTN network;

5) there is no major crosstalk cost for existing DWDM channel signals.

100 Gbit/s OTN system upgrade requirements must meet all of the above requirements, currently can through coherent detection function of 100G PM-QPSK modulation mode to meet these requirements.

100 key technical features of Gbit/s PM-QPSK

A. OSNR Performance Improvement

The PM-QPSK with coherent detection function is more sensitive than binary (OOK) with approximately 6dB of optical signal-to-noise ratio. The capacity of 100 Gbit/s is 10 times that of 10 Gbit/s. Therefore, the modulation scheme of 100 Gbit/s needs to provide 10 dB performance higher than that of 10 Gbit/s OOK code type. The key advantage of Coherent Detection is that the optical wave phase information can be transferred to the digital field, so it can use the powerful Electronic Dispersion Compensation (EDC) capability to clear signal distortion at a very low cost. Therefore, through the use of 100 Gbit/s PM-QPSK and EDC, coherent detection technology can be improved by 6 dB (compared with the direct detection of OOK); the use of high encoding gain FEC can be improved by 2 to 3 dB; because the transmission cost of CD and PMD is reduced, 1-2 dB is improved. In this way, the total improvement can reach 9-11 dB, making the 100 Gbit/s PM-QPSK close to the 10 Gbit/s OOK system light signal-to-noise ratio sensitivity. This means that 100 Gbit/s can reach the current 10 Gbit/s system transmission distance in applications.

B. dispersion (CD) margin

Modem chip with EDC function, without external Tunable Dispersion Compensator. The total amount of chip dispersion compensation is determined by the two factors of the Finite Impulse Response (FIR) Self-adaptive filter: the number of beats and the delay of beats. The deployment of 10 Gbit/s DWDM mainly uses the Dispersion Compensation Optical Fiber (DCF) to limit the residual dispersion to 10 Gbit/s OOK receiver tolerances (typically +/-400 ps/nm ), in this range 100 Gbit/s PM-QPSK EDC is easy to achieve.

C. polarization modulus dispersion (PMD) margin

An EDC modem chip can also be used for the compensation of PMD. One of the key aspects of PMD compensation is that it is necessary to track the changes of High-Speed polarization dynamics on the network very quickly. This is very different from the dispersion compensation, because the dispersion changes are relatively static (the change volume is very slow and very small), usually caused by the temperature change of the optical fiber.

D. Phase Modulation

Phase Modulation is widely used in 40G systems and further promotes the maturity of this technology in G systems. The information carried by optical carriers can be doubled by the use of the technology. The combination of the technology with polarization multiplexing reduces the baud rate of G signals to about 25 Gbaud/s, therefore, it can be used in an OTN system with 50 GHz interval and also reduces the requirement of signals for nonlinear tolerance of optical fibers.

E. polarization multiplexing

The two polarization states of optical signals are orthogonal to each other to realize that two channels of information are carried on the same optical carrier, which reduces the bit rate by half. Polarization multiplexing is a mature technology that has been applied in 40G systems, and G has become an indispensable technology. Polarization multiplexing only requires some relatively simple passive devices for the transmitter. The difficulty lies in the decoding of the receiver. However, with the continuous maturity of Coherent Detection Technology, polarization decoding and multiplexing can be easily processed in electrical domains.