I'm taking the mailing list back into the people who receive your
email (it's always a good idea to reply to the list in whole, not
to the individual people), so that someone who has a compact
example can help you. I'd first of all encourage you to
investigate where the problem exactly lies; does it look like
synchronization fails, or is it really just an SNR problem that
hinders successful equalization, or is maybe the equalizer not up
to your channel?
A specific description of your setup (carriers,
bandwidth/sampling rate, …) would be helpful.
On 02/18/2017 05:43 PM, Zhao Shangqing
Thanks so much for your recommendation. Yes, it's my
mistake, I just noticed your suggestions and post the same
question here. Sorry about that. Again, thanks for your
I have read the document about the "OFDM PAPR". I
modified the Tx and Rx gain at both transmitter and
receiver but still haven't got the correct demodulation.
In addition, I also test the signal carrier communication
system, rather than OFDM, 16 QAM still not works. At the
receiver, I have corrected the phase distortion. But I
don't know how to correct the amplitude offset. I know 16 QAM is more like to get
errors, but actually, I cannot even get one correct
symbol. If you can provide me some existing gnuradio
examples about this?
I think you forgot to mention that you've already gotten
answers to this on usrp-users:
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
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,
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?
And Kevin wrote:
Also 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
However, 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.
So 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
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 finer-grained constellations.
On 02/18/2017 04:23 PM, Zhao
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 powers.
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