MATLAB: Am I unable to find the delay in the model with the Find Delay block in the Communications Blockset 2.5 (R13)

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I am using the Find Delay block to find the delay in my model. However, when I place this block in my model and make the appropriate connections, I cannot find the delay. The delay value varies, and never settles to a final value.

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This problem occurs when a delay-inducing block (for example, the CPM decoder) is concatenated with a multi-rate block, such as the Viterbi decoder.

The delay induced by all the blocks preceding the Viterbi decoder must be a multiple of N, where the Viterbi decoder is operating on a rate K/N code. If this is not done, then when the Viterbi decoder presents its output, the delay will include a fractional sample, and the bit error rate results will be incorrect.

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MATLAB: Do I receive high bit error rates in the model when I have little or no noise in the model using the Communication Blockset

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.

MATLAB: How to compute the delay applied to the Delay block inside the viterbi decoder subsystem in the demo “dvbt_sim.mdl” in Communications Blockset 3.3 (R2006a)

The total delay being introduced in the Inner Interleaver and the Inner Deinterleaver subsystems due to the Buffer blocks is :-

2176-to-756 = 756 (in single-tasking mode)

756-to-9072 = 9072

9072-to-756 = 0 (as 756*12=9072)

756-to-2176 = 2176

which totals 12004.

We then take mod(12004,2176) where 2176 is the input to the Viterbi Decoder, this gives us 1124. For 4/3 decoding, this is a total delay of 1124*3/4 = 843 samples.

With a traceback depth of 136, the Viterbi decoder also adds a further delay of 136, bringing the total delay to 843+136 = 979.

Now to align the actual codewords before feeding into the Convolutional Deinterleaver we have to add in an extra delay of 1632-979 = 653 samples.

Note that the “584+69” is an artifact of an earlier version of the demo.

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Related Solutions

MATLAB: Do I receive high bit error rates in the model when I have little or no noise in the model using the Communication Blockset

MATLAB: How to compute the delay applied to the Delay block inside the viterbi decoder subsystem in the demo “dvbt_sim.mdl” in Communications Blockset 3.3 (R2006a)

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