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Unformatted text preview: 4.3 Spectral Efficiency 251 allow a small clipping probability (a few percent), which substantially reduces the power requirement while introducing only a small amount of signal distortion. 4.2.2 Applications of SCM SCM is widely used by cable operators today for transmitting multiple analog video signals using a single optical transmitter. SCM is also being used in metropolitan-area networks to combine the signals from various users using electronic FDM followed by SCM. This reduces the cost of the network since each user does not require an optical transmitter/laser. We will study these applications further in Chapter 11. SCM is also used to combine a control data stream along with the actual data stream. For example, most WDM systems that are deployed carry some control information about each WDM channel along with the data that is being sent. This control information has a low rate and modulates a microwave carrier that lies above the data signal bandwidth. This modulated microwave carrier is called a pilot tone. We will discuss the use of pilot tones in Chapter 8. Often it is necessary to receive the pilot tones from all the WDM channels for monitoring purposes, but not the data. This can be easily done if the pilot tones use different microwave frequencies. If this is the case, and the combined WDM signal is photodetected, the detector output will contain an electronic FDM signal consisting of all the pilot tones from which the control information can be extracted. The information from all the data channels will overlap with one another and be lost. 4.3 Spectral Efficiency We saw in Chapter 2 that the ultimate bandwidth available in silica optical fiber is about 400 nm from 1.2 m to 1.6 m, or about 50 THz. The natural question that arises is, therefore, what is the total capacity at which signals can be transmitted over optical fiber? There are a few different ways to look at this question. The spectral efficiency of a digital signal is defined as the ratio of the bit rate to the bandwidth used by the signal. The spectral efficiency depends on the type of modulation and coding scheme used. Todays systems primarily use on-off keying of digital data and in theory can achieve a spectral efficiency of 1 b/s/Hz. In practice, the spectral efficiency of these systems is more like 0.4 b/s/Hz. Using this number, we see that the maximum capacity of optical fiber is about 20 Tb/s. The spectral efficiency can be improved by using more sophisticated modulation and coding schemes, leading to higher channel capacities than the number above. As spectral efficiency becomes increasingly important, such new schemes are being invented, typically based on proven electrical counterparts. 252 Modulation and Demodulation One such scheme that we discuss in the next section is optical duobinary mod- ulation. It can increase the spectral efficiency by a factor of about 1.5, typically, achieving a spectral efficiency of 0.6 b/s/Hz....
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- Spring '09