Unexpected multirate processing in AWGN block

Problem Description: I have a communication system that works in multirate processing mode. My transmitted signal passes through an AWGN channel block, like that:

The AWGN has the following parameters:

The signal transmitted has 28 samples per bit. Therefore, it has a sample rate 28 times bigger than the bit rate.

Expected behavior: My expected behavior was each sample of the transmitted signal has corrupted by AWGN noise. However, the noise is added each bit period, instead each sample of the signal transmitted.

The following picture shows the signal after AWGN channel:

As you see, the noise is added at a different rate from the transmited signal.

Futhermore, The AWGN block is colored yellow, indicating that block works in a multirate processing. I have another Simulink communication system that AWGN block works correctly (they have same parameters). In that system, AWGN block has red color, that is, it has unique rate.

Question: How can I set the AWNG block to work in a unique rate processing. In other words, how can I set the AWNG block to add noise in every single sample of the transmited signal?

PS: I'm using matlab 2019a, on Linux.

Answer 1

[Rubem Pacelli]

I solved the problem. Some blocks has an inherited sample time by default. Specifically, de delay block has a sample time equal to -1 by default. This value means the sample time is inherited from the rest of system. The AWGN block doesn't have a sample time parameter. So I think this block inherited the sample time from the system too.

When I set the sample time of the delay block, for some reason, the AWGN block works correctally and this color turns red, as you see in the following figure:

So i think if you leave the sample time parameters of some block inherited (equals to -1) the AWGN get messy.