Doubt on if this integrator out is correct?

I have been designing a GMSK modulator and within that I encountered an issue within the integrator side. The transitions are very sharp. I have seen ideal curves to be different from this. This input to this block is bernoulli binary. Can someone just tell me if the scope output is correct or what all chnages to be made to achieve a proper one?

[Aravind]

Hi @Souparnika K S,

Could you share your Simulink model and other related files including the expected output and incorrect output? This will help in identifying and debugging the issue.

[Souparnika K S]

Hi, Thank you for your response. I have attached the simulink file of GMSK modulator.

I have referenced the below model and its corresponding outputs. I have got similar outputs for my model as well. But I have a doubt within the outptu of integrator region since I gave a modulus of range [0,1], so it got limited within that range I guess. Can that be a reason for all the sharp transitions within the integrator output. I would like to know if the approach in my model is right and if the outputs are coming out properly.

[Souparnika K S]

The below shown is the reference output of I (real) and Q (imaginary) part.

The below shown is the scope output of I and Q I got, when I correlate with this, I think the I Q part is coming out properly since the waves are kind of similar.

Answer 1

[Aravind]

Hi @Souparnika K S,

From your question, it seems you are curious about whether the sharp transitions in the integrator section are typical for a GMSK modulator.

Upon reviewing your model, it appears you have implemented an integrator manually and are using a modulus operation to keep the integrator's output within the range [0,1]. This modulus operation is responsible for the sharp transitions you observe, and it alters the I (real) and Q (imaginary) parts of the modulated signal slightly.

I recommend using the built-in "Discrete Time Integrator" block in Simulink and setting the "Integrator method" parameter to "Accumulation: Forward Euler", instead of your custom integrator. In a GMSK modulator, a modulus operation is unnecessary in the integrator. By switching to the "Discrete Time Integrator" block, the I (real) and Q (imaginary) outputs of the modulated signal should be correct.

For more details on the "Discrete Time Integrator" block, you can refer to the MathWorks documentation: https://www.mathworks.com/help/releases/R2024a/simulink/slref/discretetimeintegrator.html.

I hope this helps clarify your question.

[Souparnika]

Hi, Thank you for your response.

I tried using discrete time integrator, but the modulated output is getting abrupted. So, I tried a different method for integrator avoiding mod block. In the new design, I used an add block, delay block and a saturation block to make it in the range of 2*pi and -2*pi to get the phase information. And then multiplied it with pi*h/Tb, where h is the modulation index, Tb is the bit duration.

I am attaching the model file for the reference. Could you tell me if you find any insight from the new design.