chap1-notes

chap1-notes - 1 Digital Data Communication and Storage...

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Unformatted text preview: 1 Digital Data Communication and Storage Digital communication systems include cell phones, dig- ital television via satellite or cable, digital radio, Wi-Fi and WiMax, cable modem, and so on. Additional examples include digital data storage de- vices, including magnetic (hard") disk drives, magnetic tape drives, optical disk drives, and f ash drives. In the case of data storage, information is communi- cated from one point in time to another rather than one point in space to another. Each of these examples F t into the digital communica- tion framework of C. Shannons A Mathematical Theory of Communication (1948) 1 This framework is depicted in the f gure above whose various components are described as follows. source and user (or sink): The information source may be originally in analog form (e.g., speech or music) and then later digitized, or it may be originally in digital form (e.g., computer f les). The user of the information may be a person, a com- puter, or some other electronic device. 2 source encoder and source decoder: The encoder is a processor which converts the information source bit sequence into an alternative bit sequence with a more e cient representation of the information, i.e., with fewer bits. This operation is often called compression , which can be lossless (e.g., for computer data f les) or lossy (e.g., for video, still images, or music, where the loss can be made to be imperceptible or acceptable). The source decoder is the encoders counterpart which recovers the source sequence exactly (lossless) or approx- imately (lossy). channel encoder and channel decoder: The chan- nel encoder converts its input into an alternate sequence (e.g. of codewords) which possesses redundancy whose role is to provide immunity from the various channel im- pairments. 3 The ratio of the number of bits that enter the channel encoder to the number that depart from it is called the code rate , denoted by R , with < R < 1 . The channel decoder is to recover the original data from the noisy/distorted sequence of codewords. modulator and demodulator: The modulator con- verts the channel encoder output bit stream into a form that is appropriate for the channel. For example, for a wireless communication channel, the bit stream must be represented by a high-frequency signal to facilitate transmission with an antenna of reasonable size. channel: The channel is the physical medium through which the modulator output is conveyed, or by which it is stored. Our experience teaches us that the channel adds noise, distortion, and interference. 4 For our purposes, the channel is modeled as a proba- bilistic device, and examples will be presented below....
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chap1-notes - 1 Digital Data Communication and Storage...

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