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Unformatted text preview: ELEC300U: A System View of Communications: from Signals to Packets Lecture 09a • Codewords and Hamming Distance • Error Detection: parity • Singlebit Error Correction ELEC300U 1 Some content taken with permission from material developed for the course EECS6.02 by C. Sodini, M. Perrot and H. Balakrishnan There’s good news and bad news… The good news: Our digital modulation scheme usually allows us to recover the original signal despite small amplitude errors introduced by the components and channel. An example of the digital abstraction doing its job! The bad news: larger amplitude errors (hopefully infrequent) that change the signal irretrievably. These show up as bit errors in our digital data stream. ELEC300U 2 Some content taken with permission from material developed for the course EECS6.02 by C. Sodini, M. Perrot and H. Balakrishnan Channel coding Our plan to deal with bit errors: We’ll add redundant information to the transmitted bit stream (a process called channel coding ) so that we can detect errors at the receiver. Ideally we’d like to correct commonly occurring errors, e.g., error bursts of bounded length. Otherwise, we should detect uncorrectable errors and use, say, retransmission to deal with the problem. Digital Transmitter Digital Receiver Channel Coding Error Correction Message bitstream bitstream with redundant information used for dealing with errors redundant bitstream possibly with errors Recovered message bitstream ELEC300U 3 Some content taken with permission from material developed for the course EECS6.02 by C. Sodini, M. Perrot and H. Balakrishnan More good news, bad news… • Good news: theoretically it is possible to transmit information without error at any rate below the limiting rate C • Bad news: the proof doesn’t tell how to construct the appropriate errorcorrecting code for a given R and C! Shannon’s Noisy Channel Coding Theorem: Given a noisy channel with channel capacity C then for any ε > 0 and R < C, for large enough N, there exists a code of length N and rate ≥ R and a decoding algorithm, such that the maximal probability of block error is ≤ ε . ELEC300U 4 Some content taken with permission from material developed for the course EECS6.02 by C. Sodini, M. Perrot and H. Balakrishnan Error detection and correction Suppose we wanted to reliably transmit the result of a single coin flip: Further suppose that during transmission a singlebit error occurs, i.e., a single “0” is turned into a “1” or a “1” is turned into a “0”....
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 Fall '08
 RossMurchandAmineBermak
 Hamming Code, Error detection and correction, Parity bit, H. Balakrishnan, M. Perrot, C. Sodini

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