**1. Introduction**

232 Optical Communication

2002.

[DiBEG] DiBEG. http://www.dibeg.org/.

[3GPP 2002] 3GPP TS 25.141, 3rd Generation Partnership Project. http://www.3gpp.org/,

[ARIB] ARIB STD-B31. http://www.arib.or.jp/english/html/overview/ov/std\_b31.html.

Wavelength division multiplexing (WDM) technology for fiber optic transmission systems has been developed and introduced in order to make use of approximately 60 THz bandwidth that is offered by silica optical fibers "in [1,3]". In recent years total information carrying capacity of transmission systems was increased for the account of channels number, channel spacing and per channel bit rates. However in this case must take into the account total amount of optical power coupled into a fiber. Coupled power increase resulting in additional transmission impairments caused by nonlinear optical effects (NOE) and its combination with linear distortion mechanisms "in [4,6]". Generally it leads to distorted transmission in some channels of fiber optic transmission system (FOTS) or even to complete failure of system's channels. It means that informative signals cannot be detected on the other end of the fiber with a required error probability. This reduces system's total carrying capacity and channel's data throughput. Consequently, another system's total transmission capacity increment solution must be found out.

Currently one of the most intensively studied system's total transmission capacity increment solutions is the increasing of system's channel spectral efficiency "in references [7,11]". Actually it is more efficient utilization of available bandwidth. It means that more informative bits are transmitted using one hertz from available frequency band. It ensures that a smaller number of channels must be used to transmit the same amount of informative bits. Channel's spectral efficiency can be increased in three different ways. The first one, the reduction of used system's channel spacing "in [2]". This means that a larger number of transmission channels can be allocated in available frequency band. The second one, the increase of per channel bit rate maintaining previously used channel spacing values for separation of transmission channels "in [10]". And finally the third one is the combination of pervious two ways.

© 2012 Bobrovs and Ivanovs, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Bobrovs and Ivanovs, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Obviously that it is easier to achieve a larger channel's spectral efficiency if for optical signal modulation and coding some of novel modulation formats are used. This novel (or advanced) modulation formats provide narrower optical signals spectrum or multilevel encoding schemes that ensure more bits per one symbol than it is in traditional modulation formats, for example, on – off keying (OOK) with non – return to zero (NRZ) encoding format (NRZ – OOK) "in reference[10]". Maximal spectral efficiency, which can be obtained with traditional OOK modulation formats, is about 0.4 bit/s/Hz "in [2]". "It has been reported in ref. [7,8]" that using such novel modulation formats as quadrature amplitude modulation (16 – QAM particularly) and orthogonal frequency – division multiplexing(OFDM) together with polarization division multiplexing (PDM) technique it can be achieved SE larger than 6 bit/s/Hz and even reaches 7 bit/s/Hz.

Study object of this chapter is optimal mixed WDM system configuration that provides lowest in system's channels detected signals BER values. This developed mixed WDM system's model is offered for the future design of backbone optical networks and can be considered under the concept of next generation optical network (NGON). Chosen optical signal modulations formats and per channel bit rates, according to authors' thoughts, are the most appropriate and probable at this moment. It was concluded after careful evaluation of current state of optical telecommunication networks, their possible and the most likely development strategy and trends in the future.
