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Unformatted text preview: 5.12 Overall Design Considerations 347 Note from Figure 5.34 that the NRZ system is not sensitive to the excess local chromatic dispersion. This is because the NRZ system essentially operates in the linear regime. Note also that the DM soliton system can achieve considerably higher transmission distances than NRZ and RZ systems for all values of the excess anoma- lous chromatic dispersion. Thus, DM soliton systems are superior to these systems over virtually all kinds of dispersion-managed fiber spans. We saw in Section 5.7.4 that (unchirped) RZ systems have a smaller PMD penalty than NRZ systems. Chirped RZ, or DM soliton systems, have an even smaller PMD penalty and thus are more suitable for transmission rates of 40 Gb/s and above, from the PMD perspective as well. 5.12 Overall Design Considerations We have seen that there is an interplay of many different effects that inﬂuence the system design parameters. We will summarize some of these effects in this section. In addition, two key issues in this regard, (1) the trade-off between higher bit rates per channel versus more channels, and (2) whether to use bidirectional or unidirectional systems, will be discussed in Chapter 13. 5.12.1 Fiber Type Among the many issues facing system designers is what type of fiber should be deployed in new installations. This very much depends on the type of system that is going to be deployed. For single-channel systems operating at very high bit rates (10 Gb/s and above) over long distances, DSF is the best choice. However, DSF makes it much harder to use WDM for upgrading the link capacity in the future, primarily due to four-wave mixing, and thus is not a practical choice for most links. For WDM systems, the choice of fiber type depends on the distance and bit rate per channel. DSF is clearly a bad choice. If the system is not chromatic dispersion limited, then standard single-mode fiber is the best choice because such a system is least susceptible to degradation from nonlinearities. As the distance and bit rate increase in future upgrades, the system will eventually become chromatic dispersion limited (for example, over 600 km at 2.5 Gb/s), and chromatic dispersion compensation must be incorporated into the system. For WDM systems operating at high bit rates over long distances, NZ-DSF provides a good alternative to using standard single-mode fiber with dispersion compensation. If the residual dispersion slope after chromatic dispersion compensation is the main problem, you can use reduced slope fiber, such as Lucent’s TrueWave RS fiber. 348 Transmission System Engineering On the other hand, if nonlinearities are the significant problem, large effective area fiber, such as Corning’s LEAF, can be used. For terrestrial systems, NZ-DSF fiber with positive dispersion in the 1.55 μ m band can be used in order to be able to upgrade the system to use the L-band wavelengths. For submarine systems, NZ-DSF with negative dispersion fiber can be used in order to avoid modulation instability.with negative dispersion fiber can be used in order to avoid modulation instability....
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- Spring '09
- Wavelength, Bit rate, chromatic dispersion, NRZ systems, Transmission System Engineering, chromatic dispersion compensation