100G WDM Heralds the Ultra-Broadband Era

Driving Force

A 100G high-speed core router is 60 percent more efficient than a 10G router. The 100G router simplifies management, consumes less power, and is highly integrated. 100G routers and data services are the driving force behind the growth of 100G WDM. Today, 100G transport has become the focus of attention for leading telecom operators and vendors worldwide.

Key 100G Technologies

A 100G WDM system needs to

• be applicable to a long-haul backbone network with a radio repeater transmission distance of more than 1500 km
• be applicable to a metro core network with a radio repeater transmission distance of up to 300 or 400 km
• support a wavelength spacing of 50 GHz
• be compatible with existing WDM systems so that it does not affect them or put them at risk
• have chromatic dispersion (CD) tolerance and polarization mode dispersion (PMD) tolerance equivalent to or superior to 10G WDM
• support cascading of multiple reconfigurable optical add-drop multiplexers (ROADMs), for example, more than ten ROADMs
• reduce cost for a large-scale application. The price of one 100G WDM system is less than that of ten 10G WDM systems.

To meet these requirements, 100G WDM must have a cutting-edge optical modulation scheme and forward error correction (FEC) with high coding gain on the line side, CFP transceiver modules on the client side, and integrated optoelectronic chips.

The optical modulation scheme on the line side is key to the performance of a 100G WDM system. 100G optical modulation is currently based on quadrature phase-shift keying (QPSK). A combination of QPSK, FDM, PM-QPSK, and OFDM is necessary for 100G optical modulation. The Optical Internetworking Forum (OIF) has recommended using polarization mode QPSK (PM-QPSK) in long-haul 100G transmission. An OTU4 signal is divided into four subsignals, each with a baud rate of 28 to 32 Gbit/s. Each two of the four subsignals are modulated with differential QPSK and two output QPSK signals are modulated again with PM-QPSK. In this way 100G PM-QPSK optical signals are generated.

Because subsignals with baud rates of 28 to 32 Gbit/s have a narrow optical spectrum, they can support 50 GHz wavelength spacing and transmission through multiple OADMs.

When the same modulation format is used, a 100G WDM system requires 10 dB higher OSNR tolerance than a 10G system. The PMD and CD tolerances in a 100G system are one tenth and one hundredth that of 10G, respectively. Therefore, advanced technologies must be used to ensure the feasibility of a 100G system. Optical coherent detection and balanced optical receiving can improve OSNR tolerance by approximately 6 dB.

Polarization of PM-QPSK signals varies randomly after long-haul transmission, and the local optical oscillator at the receiver receives optical signals with frequency and phase differences. Therefore, high-speed digital signal processing (DSP) is the best solution to address these issues. High-speed DSP technology is also central to 100G transmission.

After DSP, a 100G WDM system has significantly improved tolerance to CD and PMD. CD tolerance is more than 40,000 ps/nm, and PMD tolerance is more than 30 ps. Dispersion compensation fibers are therefore unnecessary for the transmission links. This not only mitigates non-linear effects but also improves line OSNR.

It is difficult to develop ASIC chips for high-speed ADC (above 50 GSs/s) and for high-speed DSP. It is also difficult to integrate optical components and reduce power consumption. These are the most difficult issues in developing and commercializing 100G equipment.

ZTE 100G WDM Solution

Through the joint efforts of the IEEE, ITU-T, and OIF, 100G standards have been drafted. Mature 100G standards pave the way for widespread 100G deployment. ZTE has rolled out an industry-leading 100G WDM solution to meet the need for large-scale commercial 100G. ZTE’s 100G WDM system supports 80 channels in the C-band, and the same 50 GHz channel spacing is used in both 10G and 40G WDM systems. The transmission capacity of ZTE’s solution is up to 8 Tbit/s, ten times the capacity of a 10G WDM system. This meets the increasing demand for data services and ensures a longer equipment lifecycle.

ZTE’s 100G WDM system uses PM-QPSK modulation, coherent demodulation, and electronic equalization and compensation. An optical receiver can tolerate CD of up to 50,000 ps/nm and PMD greater than 30 ns. This means that CD and PMD do not need to be considered in 100G deployment, and CD compensation modules (DCMs) for fiber links do not need to be used. The engineering and OAM for 100G WDM equipment is simplified.

ZTE has developed its own 100G ASIC chips for coherent optical detection. The 100G ASIC chips are manufactured using a cutting-edge 40 nm CMOS process. This process allows the system to be highly integrated, consume less power, and have better signal processing capability that a 90 nm or 65 nm CMOS process. Operators can reduce power supply in the equipment room, become environmentally friendly, and save on OAM costs.

ZTE’s 100G WDM system uses industry-leading soft decision forward-error correction (SD-FEC) to lower OSNR tolerance and improve transmission capability and distance. SD-FEC with an overhead of 18 to 20 percent recommended by OIF allows for a net coding gain of up to 10.5 dB. High-speed DSP technology is also used to enhance transmission capacity for low cost. A 100G WDM system can transmit over more than 1500 km without electronic regeneration. Such transmission capacity is close to that of a 40G WDM system.

ZTE has been preparing for 100G commercialization by cooperating with operators to test its 100G WDM products. Good results have been achieved. With the world’s best high-performance 100G WDM products, ZTE offers the most competitive 100G solution. ZTE’s 100G solution features

• 96-channel ultralarge capacity to help operators eliminate the bandwidth bottleneck
• 1500+ km long-haul transmission without electronic regeneration
• advanced DSP technology for improving tolerance to CD and PMD. The transmission distance can extend to 2500 km without CD and PMD compensation. This saves investment and  makes the system easier to maintain.
• large-capacity electrical cross-connect at ODU0, ODU1, ODU2, ODU3 and ODU4. This can provide network convergence nodes with flexible traffic grooming.
• hybrid 10G/40G/100G transmission and smooth upgrade from 10G to 40G and 100G.

The ultra-broadband era is approaching fast, and operators worldwide are overhauling their optical networks. According to data released by OVUM, the 100G WDM market was launched in 2012 and is gradually entering a stage of large-scale commercialization. 100G WDM will almost certainly replace existing 10G WDM and edge out 40G WDM. ZTE is poised to offer industry-leading 100G solutions for the ultra-broadband era.