Ee 229b Error Control Coding Spring 2005
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چکیده
Suggested reference for convolutional codes : “Fundamentals of Convolutional Coding” by Rolf Johannesson and Kamil Zigangirov, IEEE Press, 1999. An encoder for a binary block code takes a block of information bits and converts it into a block of transmitted bits (a codeword). A binary convolutional encoder takes a stream of information bits and converts it into a stream of transmitted bits, using a shift register bank. Redundancy for recovery from channel errors is provided by transmitting more bits per unit time than the number of information bits per unit time. Maximum likelihood decoding can be done using the Viterbi algorithm; other decoding algorithms such as SOVA (soft output Viterbi algorithm) and the BCJR algorithm are also commonly used. In practice the information stream is of finite duration and one typically appends a few termination bits to the input stream to bring the shift register bank back to the all zeros state, so that the convolutional code is in effect used as a very long block code. Often convolutional codes are used as inner codes with burst error correcting block codes as outer codes to form concatenated codes. Errors in Viterbi-like decoding algorithms for convolutional codes tend to occur in bursts because they result from taking a wrong path in a trellis. The burst error correcting capability of the outer code is used to recover from such burst error patterns in the decoding of the inner code. See Section 15.6 on pp. 760 -761 of the text of Lin and Costello (2nd edition) for a short discussion of this idea.
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تاریخ انتشار 2005