I think you forgot to mention that you've already gotten answers to
this on usrp-users:
And Kevin wrote:
Anyway, yes, 16 QAM is of course a lot more prone to noise than
QPSK, when using the same average power. (that's kind of logical
– the 16 QAM constellation points are a lot closer together than
the four QPSK points)
With any modulation, you'd set the RX gain as high as possible
without clipping – and that is a real problem with OFDM. I
recommend you google "OFDM PAPR". So, you'd use the same RX gain
for QPSK as for QAM modulation.
You'd of course also use the highest possible distortion-free
TX gain, for both modulations, again, the same.
So, what you see is exactly what you've learned in digital comms
101 – the closer constellation points are, the more likely it is
you get a symbol error. What did you expect?
keep in mind that in case of QPSK, the receiver does not need
to correct for amplitude. The receiver only needs to correct
the phase distortion caused by the channel since all
information is encoded in the phase of the QPSK symbols.
in 16-QAM modulation, information is encoded in both amplitude
and phase of the 16-QAM symbols. Now receiver needs to correct
both amplitude and phase distortion caused by the channel.
I am not sure if the receiver algorithm can handle QAM.
So, instead of asking the same question here, I'd recommend you
address what you did not understand in the answers you've gotten
this far! It'll make it much easier for us to help you.
Sadly, you haven't addressed any of the points we made – neither
that the average power is, in contrast to what you claim, the same
for QAM and QPSK, nor the additional hardness in demodulating these
On 02/18/2017 04:23 PM, Zhao Shangqing
I am using Gnuradio and USRP X300. I am implementing OFDM
communication using 16 or 64 QAM modulation scheme. I am
using the example of OFDM given by Gnuradio, and
just changing the payload modulation to 16QAM. I cannot
correctly demodulate all the samples, but QPSK works fine. I
know QAM has the different signal power. I wish to know if I
wish to demodulate the M-QAM signal, how to control the
receiving power to calibrate different signals to different
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