5672 - How to add noise to QAM signal

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

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

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