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Chapter 14:
Quadrature Amplitude Modulation (QAM) in CATV Optical Transmitters
The goal of this chapter is to provide an introduction to community access television (CATV) quadrature phase-shift keying (QPSK) modulators. QPSK modulators are used as building blocks for I and Q modulation schemes in modulator—demodulators (modems). The quadrature amplitude modulation (QAM) Modem baseband (BB) output is up-converted into RF and transmitted over fiber by linearized transmitters, as will be reviewed in Chapter 16, to the subscriber's home. By reviewing the basics of QAM and modulation impairments, the motivation behind modem calibration methods, RF and optics linearization methods, and implementations would be much clearer. System requirements from a fiber-to-coax CATV receiver converter will be discussed and their effects on the data quality will be examined. This chapter presents a review of a QPSK modulator, and then Chapter 15 provides a review of QAM modems, including a review of MPEG-framing Reed—Solomon coding M-ary modulators, equalizers, and BER (of an M-ary QAM). Chapter 16 provides a review of predistortion methods and linearization methods. Chapters 15 and 16 also provide the final structure and block diagram of the fiber CATV transmission side. The receiving side is explored, and the general concept of CATV receive modems and timing recovery is provided. Using this, an HFC FTTx receiver designer is able to identify and rectify any degradation on QAM transport, resulting from an RF-circuitry problem. Based on Chapter 9, RF receiver relations are further explored.
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