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Implementing 4-ASK on NI-USRP 2920 - DC offset problem

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Muhammed Ashraf Hussien

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I am implementing a 4-ASK Tx Rx using the NI USRP 2920 SDR

The problem is that the symbol map for the 4-ASK is as follows (This is the implementation of NI Modulation toolkit) :
0+0i for 00 symbol
0.333+0i for 01 symbol
1+0i for 10 symbol
0.6667+0i for the 11 symbol

This means we have a DC offset for the signal. I found that when I received the signal there is some sort of a non linearity in this DC offset that caused a faulty symbol detection (since the constellation is totally smeared in a non uniform way from the beginning of the packet to it's end) find attached image for more clarification.
4-ask.jpg
My questions are as follows :
1 - Why did this happen ? is it the DC offset that made saturation for the amplifiers ? kindly help me with an explanation

2 - for an Amplitude shift keying modulation , do I have to remove the DC offset before RF upconversion ? and if I removed it , in this case the 2-ASK will be similar to BPSK.

3 - I am using the Maximum Likelihood detection for converting symbols to bits , but this acts like a various threshold , it worked fine with the 2-ASK with a DC offset , but for the 4-ASK as shown in the image attached it didn't work because of the various thresholding. Any suggestions ?

Thanks in advance
Best Regards
Muhammed Ashraf
 

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