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Chapter 4 Modulation and Demodulation > 4.3 Spectral Efficiency - Pg. 251

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