Re: [Discuss-gnuradio] QAM demod Error in GRC

[Josh Blum]

udadidd wrote:
> Hi Josh
> I am trying to implement this qam8_demod()
> I did not follow this statement that u have mentioned in one of ur previous
> mails "you just have to run a packet receiver on each different phase
> orientation of the constellation"
>
> Can u plz explain me the same?
> I have an understanding that the demod structure follows the same as that of
> dqpsk/d8psk.
> This is how i see the constellation of the demod
> http://old.nabble.com/file/p26314919/qam8_eye.png qam8_eye.png 
> I really need to implement this and need some suggestion on this

Well, when you receive a qam signal, and lock to it, you still have some 
unknown phase offset. Lets suppose qam8 has two possible phases: 0 and 
180 degrees. Since you don't know which phase is correct, you can look 
for the packet header in the received signal at 0 degrees and 
simultaneously at 180 degrees. Whichever phase offset yields a valid 
packet is the winner. Of course, this all assumes that you are framing 
your data with some kind of packet headers.

Anyone else have some qam demodulation ideas to share?

-josh