Analysis of 40G transmission technology

With the growth of various data services, the current network is evolving at a higher speed, and 40G transmission is gaining more and more attention as a new technology. At present, mainstream routers have already provided 10G optical interfaces. The data devices of some manufacturers even have commercial 40G optical interfaces and have been used for small-scale applications. This means that the mass commercial era of 40G has gradually arrived. Industry surveys show that the fourth quarter of 2005 will be the starting point for the official commercial start of 40G transmission equipment, and will reach a certain scale in 2006.

With 40G transmission technology, it will bring many obvious benefits to operators:

– Same bandwidth, lower hardware costs. Since the current optoelectronic device process has matured and the quality is more reliable, the 40G commercial has the necessary premise. The same is 40G capacity, the number of devices is roughly 1/4 of the four 10G optical interfaces.

– A more compact physical design, with the same number of components, 40G transmission equipment of the same capacity can occupy about 60% less space than 10G equipment.

– More efficient bandwidth resource utilization. Take the current mainstream new dense wavelength division multiplexing equipment as an example. The frequency spacing of the 10G channel has been 50 GHz, the spectral density is 0.2 bit/Hz, and the 40G wave with a frequency interval of 100 GHz. The sub-multiplexers spectral density can be as high as 0.4 bit/Hz, and the efficiency is doubled.

– Lower maintenance cost. For WDM equipment, since the maintenance workload is linearly related to the number of configured channels, maintaining 40G equipment of the same capacity is only equivalent to maintaining 10G WDM equipment. /4.

– Another significant advantage of operators using 40G transmission technology is the ability to interoperate faster with mature 40G data devices.

Lucent Technologies pioneered the introduction of LambdaUnite® MSS with the 40G optical interface and ultra-large capacity, ultra-long transmission distance Dense Wavelength Division Multiplexing in the early years of 2002. System LambdaXtremeTM Transport. And has maintained a leading position in the field. In particular, the LambdaXtremeTM Transport, a 40G ultra-large capacity Dense Wavelength Division Multiplexing Platform, condenses a large number of the latest lightwave technologies, enabling up to 64 40G channels to simultaneously achieve a multiplex section transmission distance of 600 to 700km in a common commercial fiber. Up to now, it has successfully carried out verification tests on the existing networks of many world-renowned operators, with a maximum transmission distance of more than 700km.

In all the design of ultra-large capacity Dense Wavelength Division Multiplexing (OFDM) systems with 40G path rate, it can meet the operator’s existing network setting conditions to obtain the same transmission distance as 10G or 2.5G systems, while ensuring the transmission quality. challenge. In order to achieve this, we need to think completely about the system, and boldly introduce some key new technologies, such as Raman amplifier, zero-return modulation technology, differential phase shift keying coding, dynamic tuning dispersion compensation, etc., to the maximum extent. Overcoming the effects of loss, dispersion, and nonlinearity caused by fiber optic cables.

Raman amplifier technology has been extensively studied since the 1980s. Although its development and application speed slowed down in the early 1990s, with the maturity of high-power semiconductor lasers and the requirements for high-speed transmission technology, By the end of the 1990s, regained attention, and this technology was especially important for the long-distance transmission of 40G. Since the 40G signal has a larger spectral width than 10G, it is also affected by optical noise. It is well known that all amplifiers bring some noise, and as a distributed amplification Raman amplifier, the spontaneous emission noise is much smaller than that of the erbium-doped fiber amplifier that has been commercially available before it. In addition, Lucent Technologies also uses a unique two-way Raman pumping mechanism that greatly reduces spontaneous emission noise while minimizing nonlinear effects in the fiber (eg, four-wave mixing and mutual bit positioning). Modulation, etc.) The impact on the WDM system. Figure 1 shows the amplification of the signal obtained with a Raman amplifier at the receiving end.


Figure 1: Signal power curve after amplification of a reverse-pumped Raman amplifier

Modulation technology is another important guarantee for long-distance transmission of 40G channels. At present, most of the signals use a non-return-to-zero (NRZ) modulation method, which can reduce the spectral width of the signal. However, due to the large duty ratio, the interval between the front and rear pulses is small, and it is easy to overlap. Causes crosstalk between codes. The duty cycle of the return-to-zero code (RZ) is usually only 34% to 67% of the ordinary non-return-to-zero code, which opens the interval between adjacent pulses and greatly improves the peak power based on the average energy of the signal. It provides a higher optical signal-to-noise ratio for the receiver and also increases the resistance to delay caused by polarization mode dispersion in the fiber. Lucent Technologies uses a return-to-zero code modulation technique (CSRZ) called carrier suppression, which minimizes the spectral broadening caused by modulation while preserving all the advantages of return-to-zero codes. Figure 2 is an eye diagram of a 40G signal transmitted after 800km using CSRZ.


Figure 2: 40G signal after 800 km transmission

Dynamically tunable dispersion compensation is used to maximize the effects of dispersion on receivers of 40G signals. Figure 3 shows the effect of dispersion on the long-distance transmission of signals. Due to the different propagation speeds of different spectral components, the pulse spreads after a longer distance and causes the front and back pulses to overlap each other. Ordinary Dense Wavelength Division Multiplexing (OFDM) systems use centralized dispersion compensation to obtain compensation for multiple paths simultaneously using negative dispersion fibers. Since the dispersion curve of negative dispersion fibers cannot be completely matched with the dispersion in the actual cable, it will inevitably result. The compensation for individual paths is not perfect, and the high-speed 40G signal is much more sensitive to the over-compensation and under-compensation of the dispersion than the lower rate. Therefore, separate dispersion compensation for each 40G path is required before the receiver. Optimized for optimal transmission. The dynamic dispersion compensation is actually to control the period of the fiber grating by temperature, thereby changing the degree of dispersion compensation.


Figure 3: Effect of dispersion on signal transmission

Differential Phase Shift Keying (DPSK) is a very special type of encoding that uses the phase difference between the front and back pulses to distinguish between 1 or 0, unlike traditional OOK (on-off keying), with or without pulse energy. Make a difference. An optical receiver designed with differential phase shift keying technology can correctly distinguish signals with very low optical power and obtains approximately 3 dB of additional sensitivity than ordinary OOK. By using this special coding technology, LambdaXtremeTM Transport can simultaneously transmit 64 40G high-speed channels over 2000 km without the need for expensive electrical relay equipment.

As the leader of 40G high-speed transmission technology, Lucent Technologies has signed a supply agreement for 40G dense wavelength division multiplexing equipment with multiple operators, and can simultaneously support the current mainstream rate 10G signal and 40G signal mixture on the same platform. The transmission provides operators with a smooth transition from the current 10G service to the upcoming 40G service era.