Problem with Sampling Rate (Modulation)

[eltee]

Hey there,

I am trying to communicate between the software defined radio USRP-2920 and a He-100 which is a communication device for CubeSats.

So at the moment I just modulate a message (Message -> binary -> waves) with a FSK modulation.

My Problem is that the He-100 has a Baudrate (Symbol rate) of 9600 and the carrier frequency is supposed to be 436675kHz which is extremely high. My samples/symbols is 8 because 8 bits form a symbol.

If I choose these parameters for the symbol rate and the carrier frequencies as input parameters for the modulation I get an error:

The sample rate has to be twice as high as the carrier frequency to fulfill the Nyquist criteria.

But that is impossible for me because the parameters for the communication (Baudrate and carrier frequency) are fixed because these are the parameters of the He-100.

Can anyone help me? I do not have such a good background knowledge about Modulation.

Thank you.

[DesasterMaster]

Hi eltee,

on wich value have you set the sample rate?

Maybe this tutorial might help you?

https://www.researchgate.net/file.PostFileLoader.html?id=545b5550d039b12d7c8b4567&assetKey=AS%3A2721...

[eltee]

Actually I am right now only simulating, so I used a "MT FSK Modulator" and a "MT Upconvert Baseband block".

For these blocks I do not need the sampling rate as input so I did not set a value for the sampling rate

That's a good link. Thanks. But it actually doesn't really explain how the sampling rate, symbol rate and the samples per symbol are chosen correctly.

My problem is that I do not exactly know which of these values has to be the same as the values fixed for the transceiver I want to communicate with.

And even if I choose the highest possible sampling rate which is 25MSamples/s how is it possible to fulfill this Nyquist criteria? The carrier frequency is 430MHz which is far too high. (25 << 430)

[DesasterMaster]

Yes, these blocks do'nt need a sample rate, but the whole setup of the USRP will need one.

The highest frequency the USRP-2920 can handle is 2.2 GHz.

Could you provide us with your VI to have a better look what you are doing?

[eltee]

Here is my vi. As I said it is just a first try to simulate the communication. So I have to apologize because it looks a littte bit messy.

With the parameters:

carrier frequency = 440MHz

symbol rate = 9600 Hz

it does not work because of the Nyquist criteria

With other parameters like for example:

carrier frequency = 500kHz

symbol rate = 200kHz

it works fine

[DesasterMaster]

By the way, have to already taken a look at the USRP Examples?

They give you quite a good entry point and you can build up your VI on them.

You can access the NI-USRP examples from the following locations:

• From the Start menu at Start»All Programs»National Instruments»NI-USRP» Examples.
• From the LabVIEW Functions palette at Instrument I/O»Instrument Drivers» NI‑USRP»Examples

[eltee]

Yes I already looked at the examples but they do not help me to choose the correct parameters to communicate

[DesasterMaster]

Thanks for sharing your code with us.

I think you shoud go through the Hands On Guide to lern some basics of using the USRP.

A Hands-on Introduction to the NI USRP