5672 - How to add noise to QAM signal

[Stefan_Bieder]

Hi everybody,

I use the 5672 PXIe generator for creating a QAM signal with an user defined constallation set. Since I need continuous streaming I take advantage of the OSP of the 5672. I simply send the low-rate IQ - information symbols and the extensive oversampling and pulse-shaping is performed using the onboard signal processing unit. Everything works well.

Now, I want to add white gaussian noise to my signal. For my physical understanding, I would have to add the noise samples to the signal AFTER oversampling and pulse-shaping. Since this operation is done in hardware, I can not go this way. What I am doing right now is as follows: I simply add the noise samples to my low rate IQ-information symbols (BEFORE oversampling and pulse-shaping).

Has anyone an idea, how to overcome this wrongdoing I am aware of?

Thanks for all replies

Stefan

[Andy_Hinde]

Hi Stefan,
How do you know this approach is incorrect? As I read through your email, I was going to suggest adding the white noise prior to the PXIe-5672 OSP, at the lower IQ Rate in software.

What is your final output IQ sample rate? Once the IQ data is written to the PXIe-5672 onboard memory, it flows through the OSP and directly to the analog output. You would have to use an analog noise generator and a combiner to add the noise at this point. There is no way to add the noise in firmware after the output of the OSP and before the signal is upconverted to IF (and passed to the PXI-5610 RF Upconverter).

Thanks,
Andy Hinde
National Instruments

[Stefan_Bieder]

Hi Andy,

the information symbol rate (which means the IQ rate BEFORE oversampling and pulse shaping) is about 150 kSymbols / sec. As a benefit of this pretty low rate, we are able to generate information symbols continiously including all scrambling and coding operations.

Actually, your suggestion seems to be the only solution to add white noise to the signal. From my understanding, adding the noise to the information symbols results in loss of the "white" (uncorrelated) property of the noise due to the filtering operation (pulse-shaping).

Maybe for next generation of signal generator devices of NI: It would be a great feature of the OSP to add impairments like noise to the signal for purpose of testing the performance of receiver devices. Actually, this is the application we run.

Thanks for your help

Stefan

[Yardov]

Guten Morgen Stefan,

During NI week 2008, I saw this product from DAQTron (NI alliance member):
http://daqtron.com/3601.php

You might want to use this to add the impairments you need. Hopes it helps,

[Stefan_Bieder]

Thanks Gerardo,

I hope, you are doing fine.

The product you suggested seems to be very powerful. Actually, I think it would be sufficient, using a kind of DSP / FPGA for adding impairments to the baseband signal. I do not know whether the FPGA of the 5442 has some free gates (space) left for this task. In my opinion, this would be an interesting feature of the RF-generators. Some stand-alone vector signal generators do have this feature. What do you think?

Best greetings

Stefan

[Yardov]

Thanks Stefan,

The FPGA on the 544x is not exposed to the user for custom programming. We do have the PCI-5640R that is a LabVIEW RIO target to do custom FPGA programming. This can open any application you can immagine. Wish you were here for NI Week and can talk more about this vision. There are upcomming product that might help us. I am also going to feed this feedback so that we can steer the products into the correct direction.

Best,