Communications Blockset™

Align Signals - Align two signals by finding delay between them

Library

Utility Blocks

Description

The Align Signals block aligns a signal with a delayed, and possibly distorted, version of itself. The block is particularly useful when you want to compare a transmitted and received signal to find the bit error rate, but do not know the delay in the received signal.

The input port labeled s1 receives the original signal, while the input port labeled s2 receives the delayed version of the signal. The two input signals must have the same sample times. The block calculates the delay between the two signal, and then

Delays the first signal, s1, by the calculated value, and outputs it through the port labeled s1 del.
Outputs the second signal s2 without change through the port labeled s2.
Outputs the delay value through the port labeled delay.

See Computing Delays in the Communications Blockset™ online documentation for more information about signal delays.

The block's Correlation window length parameter specifies how many samples of the signals the block uses to calculate the cross-correlation. The delay output is a nonnegative integer less than the Correlation window length.

As the Correlation window length is increased, the reliability of the computed delay also increases. However, the processing time to compute the delay increases as well.

You can make the Align Signals block stop updating the delay after it computes the same delay value for a specified number of samples. To do so, select the Disable recurring updates check box, and enter a positive integer in the Number of constant delay outputs to disable updates field. For example, if you set Number of constant delay outputs to disable updates to 20, the block will stop recalculating and updating the delay after it calculates the same value 20 times in succession. Disabling recurring updates causes the simulation to run faster after the target number of constant delays occurs.

Tips for Using the Block Effectively

Set the Correlation window length parameter sufficiently large so that the computed delay eventually stabilizes at a constant value. If the computed delay is not constant, you should increase Correlation window length. If the increased value of Correlation window length exceeds the duration of the simulation, then you should also increase the duration of the simulation accordingly.
If the cross-correlation between the two signals is broad, then Correlation window length should be much larger than the expected delay, or else the algorithm might stabilize at an incorrect value. For example, a CPM signal has a broad autocorrelation, so it has a broad cross-correlation with a delayed version of itself. In this case, the Correlation window length value should be much larger than the expected delay.
If the block calculates a delay that is greater than 75 percent of Correlation window length, the signal s1 is probably delayed relative to the signal s2. In this case, you should switch the signal lines leading into the two input ports.
If you use the Align Signals block with the Error Rate Calculation block, you should set the Receive delay parameter of the Error Rate Calculation block to 0 because the Align Signals block compensates for the delay. Also, you might want to set the Error Rate Calculation block's Computation delay parameter to a nonzero value to account for the possibility that the Align Signals block takes a nonzero amount of time to stabilize on the correct amount by which to delay one of the signals.

Examples

See theComputing Delays section of Communications Blockset User's Guide for an example that uses the Align Signals block in conjunction with the Error Rate Calculation block.

See Setting the Correlation Window Length, on the reference page for the Find Delay block, for an example that illustrates how to set the correlation window length properly.

Dialog Box

Correlation window length: The number of samples the block uses to calculate the cross-correlations of the two signals.
Disable recurring updates: Selecting this option causes the block to stop computing the delay after it computes the same delay value for a specified number of samples.
Number of constant delay outputs to disable updates: A positive integer specifying how many times the block must compute the same delay before ceasing to update. This field appears only if Disable recurring updates is selected.

Algorithm

The Align Signals block finds the delay by calculating the cross-correlations of the first signal with time-shifted versions of the second signal, and then finding the index at which the cross-correlation is maximized.