Technical issues for 40G transport such as sensitivity, chromatic dispersion(CD) and polarization dispersion(PD) have been solved, enabling commercial deployment of 40G technology.
Most of the technical challenges associated with 40G DWDM are already widely understood, as challenges from the 10G world taken to more demanding levels. In some cases, the tighter tolerances associated with driving a 40G pulse of light down a narrow fiber requires a different response than 10G. As the following pages explain, all technical challenges have been successfully met.
- OSNR Sensitivity
40G signals occupy four times the bandwidth of signals with 10G line rates. Therefore, an optical receiver suitable for 40G utilizes a four times larger pass band. The broader pass band collects four times the optical noise of a standard 10G receiver. This explains why 40G receivers are inherently much more sensitive to optical noise and require a four times (or 6 dB) higher optical signal to noise ratio (OSNR).
Countermeasures: Enhanced forward error correction (EFEC) filters out errors that occur during transmission and reduces the sensitivity to optical noise. While only high performance 10G line cards take advantage of this technology, it is a must for 40G line cards. Furthermore, alternative modulation formats reduce the required optical bandwidth and therefore also the sensitivity to optical noise.
- Chromatic Dispersion
Compensation of chromatic dispersion was also an issue during the introduction of 10G in the late nineties. Since then dispersion compensation modules have been developed that guarantee the proper compensation of chromatic dispersion. Now, when moving from 10G signals to 40G signals, the sensitivity against chromatic dispersion increases by a factor of 16. This leads to issues with NRZ modulation, the standard for 10G DWDM. While 10G NRZ signals tolerate up to 1000 ps/nm chromatic dispersion with an acceptable penalty, 40G NRZ signals can tolerate only about 60 ps/nm. This is just too low for any practical use of 40G.
Countermeasures: New modulation formats help to reduce the influence of dispersion and so make 40G transmission feasible. In addition the dispersion compensation scheme along the line can be optimized to facilitate robust transmission of 10G as well as 40G traffic. In addition, tunable dispersion compensation modules can adjust the chromatic dispersion at the receiver site.
- Polarization Mode Dispersion
Polarization mode dispersion (PMD) is the most critical impairment for 40G transport.
Alternative modulation formats and PMD compensation enable 40G transmission on most installed fibres.
Polarization Mode Dispersion (PMD) is inherent to DWDM transmission systems and occurs when the two different polarizations of the transmitted light signal propagate with slightly different velocities through the fiber (see Figure 2). This effect can be caused by production related fiber imperfections or geometrical asymmetries as well as by mechanical stress, temperature drifts or fiber movements. Also, most of the optical components of the DWDM system itself contribute slightly to the PMD of the transmission system. While affecting a small number of 10G lines, PMD is a major limiting factor for 40G, and can restrict the
distance over which light can be transmitted over a fiber. The following guidelines apply:
- High PMD (possibly > 0.5ps/km1/2) must be assumed for fiber manufactured before 1995. Older fiber was produced with more significant imperfections than today, and was also laid in the ground and routed around corners with more stress than is now acceptable. In addition, splices connecting fibers were of a lower quality standard. For these reasons, fiber installed prior to 1995 is generally considered to be of an unknown quantity for PMD, and not suitable for 40G transmission.
- For modern fibers (manufactured since 1995), PMD is usually within acceptable limits (< 0.2ps/km1/2) for 40G transmission. Some level of PMD can be expected in 10-20% of these newer fibers, with longer transmission lengths more likely to be affected.
- For fiber laid in the ground today, PMD is usually negligible (< 0.1ps/km1/2). Fiber manufacturers are now aware of the problems caused by minute asymmetries and have modified their processes accordingly. Fiber is also laid in the ground with more care and awareness of PMD than previously.
- Air fiber is particularly vulnerable to PMD, with stresses likely when fiber moves in the wind and during switching in the high voltage network. For this reason, air fiber – even when manufactured recently – is of questionable use for transporting 40G.
So how severe a problem is PMD for 40G transport? A studyfound that 69% of the company’s fiber can be used for 40G transmissionup to 600 km without mitigating measures such as PMD compensation.
For 10G transmission, the figure climbs to 89%.
Countermeasures: Regenerating the optical signal using shorter optical paths is one possibility, but results in additional equipment costs and additional operational and maintenance effort. A far more affordable solution is to reduce the sensitivity of the 40G transmission system, either by using alternative modulation formats which are less susceptible to polarization effects or by integrating PMD compensation mechanisms into the WDM system. Compared to alternative modulation formats with still limited PMD tolerance, PMD compensation techniques can completely cancel the effect of PMD up to much higher values.
In summary, PMD is a challenge to be respected, but definitely not feared. Network operators with a modern fiber plant should not expect major problems or significant extra costs.
- Modulation Formats: What Works for 40G?
The NRZ (Non-return to zero) modulation format is the standard format for optical signal transmission up to 10G. However, NRZ lacks the necessary dispersion tolerance to accommodate 40G, making it unsuitable for this higher transmission rate. Accordingly, the DWDM industry is now moving away from NRZ and is looking for new alternative modulation formats for 40G applications. From a physical point of view, NRZ is a very basic modulation technique where light is simply switched on and off to encode the binary information onto the transmitted light. In addition to this so-called amplitude modulation, the phase or frequency of the transmitted light can also be modulated, with the various combinations of these elements resulting in a wide variety of possible modulation schemes.
Choosing a modulation scheme which best fits all requirements is a technological challenge. The modulation format has to not only improve performance for 40G transmission, it also has to be ready for commercial deployment and support interworking of the new 40G lines with 10G in the same DWDM system. Currently, two possible modulation methods, CS-RZ and duo-binary, are commercially available. Two others, DPSK and DQPSK, are under development.
4.1 CS-RZ (Carrier Suppressed Return to Zero)
CS-RZ modulation utilizes a return to zero modulation. It also engineers the phase of the optical signal in a way that the average optical signal power is reduced by one half. Therefore, the 40G signal is much less sensitive to fiber nonlinear effects and provides higher robustness against transmission impairments. It further provides improved OSNR performance and slightly improved dispersion tolerance compared to NRZ. However, CS-RZ has a fundamental limitation: it can’t operate on a 50 GHz grid necesary to support 80 channel DWDM systems. This limits its use to DWDM systems operating on a 100 GHz grid used to carry 40 channels.
4.2 Duo-binary
The traditional NRZ modulation format is not suited for 40G transmission. Duo-binary modulation offers seemless integration of 40G technology into existing 10G networks
Duo-binary modulation (also called Phase Shaped Binary Transmission, or PSBT) also engineers the phase of the optical signal and reduces the average optical signal power by one half compared to standard NRZ signals. However, due to the phase coding, the optical bandwidth of the signal is also reduced significantly. This leads to some major advantages for the transmission of 40G. First, the CD tolerance increases from 60 ps/nm to approximately 160 ps/nm. Second, the PMD tolerance increases.
Third, 40G channels utilizing the duo-binary modulation format can be operated over a 50 GHz grid, which is standard for DWDM backbone networks. A DWDM system using duo-binary for 40G modulation even supports any combination of 10G/40G wavelengths without the need for wavelength sub-bands (not possible with other modulation formats).
4.3 DPSK and DQPSK
In contrast to amplitude modulation techniques such as NRZ and duobinary, DPSK (Differential Phase Shifted Keying) and DQPSK (Differential Quadruple Phase Shifted Keying) code the bit information directly into the phase of the optical light without touching the amplitude. This technique provides a 3dB higher OSNR sensitivity and higher transmission tolerance against system impairments. In particular, DQPSK, due to the fact that it reduces the line rate by 50% while keeping the full data rate,
provides significantly enhanced tolerance against chromatic dispersion and PMD. In contrast to DPSK, DQPSK can operate over a 50 GHz spacing, which is mandatory for DWDM backbones. However, phase coding techniques require much more complex and technically sophisticated transponders and receivers compared to amplitude modulation, driving up costs for 40G equipment. Furthermore, a lack of mature components for DPSK/DQPSK makes a commercial deployment unlikely in the near
future. When they become available, these formats will likely replace today’s modulation technology – unless, of course, another modulation format emerges in the meantime Based on these facts, For example, when DPSK is ready for commercial use, administrators can simply plug in the new transponder. It’s an advantage of having a DWDM solution that is future proof, both in terms of capacity and performance.
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