L11Gallager - 6.450 Introduction to Digital Communication...

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Unformatted text preview: 6.450 Introduction to Digital Communication October 16, 2002 MIT, Fall 2002 Lecture 11: Introduction to Channels and PAM 1 Introduction In the first lecture, we discussed the general block diagram of a communication sys- tem which is repeated below. We discussed the reasons for separating a communication system into source coding and channel coding layers, connected by a binary interface. Subsequently, we studied source coding, starting with discrete sources, and then show- ing that analog sources can be layered into sampling, followed by quantization, followed by discrete source coding. In studying the general problem of sampling, we studied the representation of waveforms by orthonormal expansions. Input- Source Encoder- Channel Encoder ? Channel Source Decoder Channel Decoder Binary Interface Output Figure 1: Separation of source and channel coding. Today we will start to consider the channel coding layer. The channel encoder converts a sequence of bits into a waveform for transmission over an analog channel, preferably as efficiently as possible i.e. , at as high a rate as possible. The channel decoder performs the inverse operation, recovering the transmitted bits with as low an error rate as possible, in the presence of noise introduced by the channel. The channel coding is often viewed as being separated into two layers, one called digital coding and the other called modulation as shown in Figure 2. Usage is by no means uniform here. The word modulation alone is often used as a synonym for both the function of discrete coding and modulation, and the two functions are usually designed and implemented as a single physical unit. Often, also, the two functions are functionally intertwined into a single function called coded modulation . In learning about channel encoding, however, it will be useful at first to separate these two functions. We have shown the channel above as a one way device going from source to destination. Usually, however, communication goes both ways, so that a physical location can send data 1- Discrete Encoder- Modulation ? Channel Discrete Decoder Demodulation Binary Interface Figure 2: Separation of encoding into discrete coding and modulation. to another location, and also receive data from that remote location. A physical device that both encodes data going out over a channel and also decodes data coming in from the channel in the opposite direction is often called a modem (for modulator/demodulator). To a good approximation, however, the communication in the two directions can usually be viewed as independent. The decoding at one location is of course matched to the type of encoding done at the other, but we can separate this from the encoding and decoding in the opposite direction; thus, as in Figures 1 and 2, we consider communication in only a single direction....
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This note was uploaded on 10/07/2009 for the course ENSC 5210 taught by Professor Daniellee during the Spring '08 term at Simon Fraser.

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L11Gallager - 6.450 Introduction to Digital Communication...

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