Communications Blockset™

General CRC Syndrome Detector

Detect errors in input data frames according to generator polynomial

Library

CRC sublibrary of Error Correction and Detection

Description

The General CRC Syndrome Detector block computes checksums for its entire input frame. The block's second output is a vector whose size is the number of checksums, and whose entries are 0 if the checksum computation yields a zero value, and 1 otherwise. The block's first output is the set of message words with the checksums removed.

The block's parameter settings should agree with those in the General CRC Generator block.

You specify the number of checksums the block calculates for each frame by the Checksums per frame parameter. If the Checksums per frame value is k, the size of the input frame is n, and the degree of the generator polynomial is r, then k must divide n - k*r, which is the size of the message word.

This block supports double and boolean data types. The output data type is inherited from the input.

Example

Suppose the received codeword has size 16, the generator polynomial has degree 3, Initial states is [0], and Checksums per frame is 2. The block computes the two checksums of size 3, one from the first half of the received codeword, and the other from the second half of the received codeword, as shown in the following figure. The initial states are not shown in this example, because an initial state of [0] does not affect the output of the CRC algorithm. The block concatenates the two halves of the message word as a single vector of size 10 and outputs this vector through the first output port. The block outputs a 2-by-1 binary frame vector whose entries depend on whether the computed checksums are zero. The following figure shows an example in which the first checksum is nonzero and the second checksum is zero. This indicates that an error occurred in transmitting the first half of the codeword.

Signal Attributes

The General CRC Syndrome Detector block has one input port and two output ports. All ports allow frame-based binary column vectors only.

Dialog Box

Generator polynomial: A binary or integer row vector specifying the generator polynomial, in descending order of powers.
Initial states: A binary scalar or a binary row vector of length equal to the degree of the generator polynomial, specifying the initial state of the internal shift register.
Checksums per frame: A positive integer specifying the number of checksums the block calculates for each input frame.

Algorithm

For a description of the CRC algorithm as implemented by this block, see Cyclic Redundancy Check Coding in Communications Blockset™ User's Guide.

References

[1] Sklar, Bernard. Digital Communications: Fundamentals and Applications. Englewood Cliffs, N.J., Prentice-Hall, 1988.

[2] Wicker, Stephen B., Error Control Systems for Digital Communication and Storage, Upper Saddle River, N.J., Prentice Hall, 1995.

Pair Block

General CRC Generator