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Optical Networks - _Chapter 4 Modulation and Demodulation_47

Optical Networks - _Chapter 4 Modulation and...

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4 chapter Modulation and Demodulation O ur goal in this chapter is to understand the processes of modulation and demodulation of digital signals. We start by discussing modulation, which is the process of converting digital data in electronic form to an optical signal that can be transmitted over the fiber. We then study the demodulation process, which is the process of converting the optical signal back into electronic form and extracting the data that was transmitted. Mainly due to various kinds of noise that get added to the signal in the trans- mission process, decisions about the transmitted bit (0 or 1) based on the received signal are subject to error. In this chapter, we derive expressions for the bit error rate introduced by the whole transmission process. Subsequently, we discuss how the bit error rate can be reduced, for the same level of noise (more precisely, signal-to-noise ratio) by the use of forward error-correcting codes. We also discuss clock recovery or synchronization, which is the process of recovering the exact transmission rate at the receiver. With this background, in the next chapter, we will tackle transmission system engineering, which requires careful attention to a variety of impairments that affect system performance. 4.1 Modulation The most commonly used modulation scheme in optical communication is on-off keying (OOK), which is illustrated in Figure 4.1. In this modulation scheme, a 1 bit is 245
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246 Modulation and Demodulation 1 0 1 1 0 1 Binary data NRZ format RZ format Figure 4.1 On-off keying modulation of binary digital data.
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