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Chapter 9. Modulation Schemes - Pg. 249

CHAPTER 9 Modulation Schemes 9.1 Introduction The digital signals that are developed in the process of transmitting voice, data, video, and signaling information are generated at low data rates. These data rates, typi- cally 1 to 50 kbps, are so low in frequency that their transmissions directly from the transmitter to the receiver would require antennas that are thousands of meters long. Furthermore, the signals from one transmitter would interfere with signals from other transmitters if they all use the same frequency band. Therefore, baseband signals (see Chapter 4) are modulated onto a radio frequency carrier for transmission from the transmitter to the receiver [2,3]. The radio environment at 800 2000 MHz (used for mobile communications) is hostile. We must therefore select modulation schemes that are robust. In addition to the modulation schemes we must also choose encoding algorithms that improve the performance of the system [9,10]. We introduced modulation in Chapter 4. In this chapter we focus primarily on