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Chapter 2 Introduction to Digital Commun... > 2.2 Review of digital transmission t... - Pg. 52

52 Introduction to CDMA Wireless Communications the Shannon limit are presented in Section 2.7 while the performance of an ideal digital communication system, as defined by Shannon theory, is discussed in Section 2.10 and the chapter is summarized in Section 2.9. 2.2 Review of digital transmission theory The transmission theory deals with the physical layer of the communication system and defines the channel to transport the information from source to destination. The system comprises of three main components: the transmitter, the channel and the receiver (see Figure 2.1). We will assume that the source outputs a sequence of binary digits, processed by the source encoder to remove any redundancy in the data so that the information is represented by the minimum number of binary digits. The channel encoder adds a certain redundancy to the information that can be used by the receiver to detect or correct errors which occur due to signal corruption by the channel additive noise and interference. The digital modulator maps the discrete signal into appropriate signal waveforms that are sent to the destination through wire or wireless channel. The transmitted waveforms are frequently corrupted by a Gaussian distributed additive noise, and interference from other transmitters operating within the frequency spectrum. In mobile communication systems, there are two other sources which contribute to the waveform corruption. The first source is due to the fact that there is no line of sight between transmitter and receiver antennas so that the transmitted waveforms would be reflected and diffracted by surrounding obstacles, creating multiple transmission paths for Information source Source encoder Channel encoder Transmitter Digital modulator Channel Receiver Destination Source decoder Channel decoder Digital demodulator Figure 2.1 Block diagram of a digital communication system.