Communications Blockset

Computing the Error Rate

The Error Rate Calculation block, in the Comm Sinks library, compares the original message from the Bernoulli Random Binary Generator block with the recovered message from the Viterbi Decoder block, and produces error statistics. The Error Rate Calculation block produces a length-three vector whose elements are:

• The error rate, which is the quotient of the next two quantities below
• The total number of errors, that is, comparisons between unequal elements
• The total number of comparisons that the block made

Since the block's Output data parameter is Port, the block sends its error statistics to the output port. At the output port, a Display block receives and displays the error statistics.

Delay in Received Data

One subtlety of the Error Rate Calculation block's configuration is the Receive delay parameter. This parameter is nonzero because a given message bit and its corresponding recovered bit are separated in time by a nonzero amount of simulation time. The Receive delay parameter tells the block which elements of its input signals to compare when checking for errors.

Tip    If you build your own model using the Error Rate Calculation block and get an error rate close to .5 for binary data, then you should check whether the Receive delay value is the correct delay for the model.

In this case, the Receive delay value is 49 samples, which is one more than the Traceback depth value (48) in the Viterbi Decoder block. The extra one-sample delay comes from the initial delay in the Buffer block. Because the Buffer block must collect two scalar samples before it can output one vector, its first meaningful output occurs at time 1 second, not time 0.

Decoding the Convolutional Code

Displaying the Error Rate