Some blocks, such as the ones listed below, can introduce delays and hence bit errors in the Error Rate Calculation Block:
*Digital demodulators
*Convolutional interleavers or deinterleavers
*Viterbi Decoder block
*Buffering, downsampling, derepeating, and similar signal operations
*Explicit delay blocks, such as Integer Delay and Variable Integer Delay
*Filters
You need to have the correct Receive Delay defined in the Error Rate Calculation Block. The documentation page titled "Manipulating Delays" in the "Modeling Communication Systems" section of the Communication Systems Toolbox documentation contains useful information on that topic. This page can be seen online at the following address:
<http://www.mathworks.com/help/toolbox/comm/ug/fp5686.html>
For more information on how to use the Find Delay block to compute delay, refer to:
<http://www.mathworks.com/help/toolbox/comm/ref/finddelayblock.html>
Some models require you not only to compute delays but also to manipulate them. For example, if a model incurs a delay between a block encoder and its corresponding decoder, the decoder might misinterpret the boundaries between the code words that it receives and, consequently, return meaningless results. More generally, such a situation can arise when the path between paired components of a block-oriented operation (such as interleaving, block coding, or bit-to-integer conversions) includes a delay-causing operation. To avoid this problem, you can insert an additional delay of an appropriate amount between the encoder and decoder. For more information, see models available from our web site, at the link above.
