Group Delay

The raised cosine filter blocks in the commfilt2 library implement realizable filters by delaying the peak response. This delay, known as the filter’s group delay, is the length of time between the filter's initial response and its peak response. The filter blocks in this library have a Filter span in symbols parameter, which is twice the group delay in symbols.

For example, the square-root raised cosine filter whose impulse response shown in the following figure uses a Filter span in symbols parameter of 8 in the filter block. In the figure, the initial impulse response is small and the peak impulse response occurs at the fourth symbol.

Implications of Delay for Simulations

A filter block’s group delay has implications for other parts of your model. For example, suppose you compare the symbol streams marked Symbols In and Symbols Out in the schematics shown on the Filtering page by plotting or computing an error rate. Use one of these methods to make sure you are comparing symbols that truly correspond to each other:

  • Use the Delay block to delay the Symbols In signal, thus aligning it with the Symbols Out signal. Set the Delay parameter equal to the filter’s group delay (or the sum of both values, if your model uses a pair of square root raised cosine filter blocks). The following figure illustrates this usage.

  • Use the Find Delay block to find the delay between the two signals and add that delay using the Delay block.

  • When using the Error Rate Calculation block to compare the two signals, increase the Receive delay parameter by the group delay value (or the sum of both values, if your model uses a pair of square-root raised cosine filter blocks). The Receive delay parameter might include other delays as well, depending on the contents of your model.

For more information about how to manage delays in a model, see Delays.