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Help me with constellation diagram plotting of received signal

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Haftom

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i am working on constellation diagram plotting of received signal, but i have got rotated constellation due to frequency offset. Is there any one who can help me to correct this offset please?
 

Re: constellation plot

what is the your modulation type? more info plz
it may be because of many reasons for example freq offset or sampling in middle of symbols
put your plot here
 

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    Haftom

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Re: constellation plot

i am testing it using 4QAM signal. to get the center frequency FFT is used but the result is shifted frequency (the signal has 100Hz center frequency but the result is 120Hz).This offset in frequency rotates the constellation plot .how can i get accurate center frequency extraction from the signal? or how can i correct the offset frequency?.(i have tested the plot by giving 100Hz center frequency it plots perfect but i need to get the frequency for unknown signals, if the frequency calculated is not exact it rotates).Thank you for your help
 

Re: constellation plot

Send known data, find the amount of rotation, from this find the frequency offset. Then, for subsequent unknown data, multiply this frequency with the with incoming carrier and filter our unwanted frequency. Thus you rotate is back to correct value
- b
 

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