To ease answering, I'll add more of the conversation in this
Thanks so much for your advice. I have
read the PAPR OFDM document and set the higher RX and TX gain at
both transmitter and receiver. But I still cannot demodulate the
Yes, because it's intrinsically harder to demodulate 16QAM
compared to QPSK, as I tried to explain.
So my question is do I need to change
some parameters or adding some algorithms to control the USRP to
intelligently receive different signal with different power.
These signals do *not* have a different power per se. SNR should
be the same.
Has anyone achieved such function before?
Or we can not use the USRP to receive the
Of course you can! It's really just a question of choosing
transmission parameters so that things work
I actually don't exactly know how USRP
works to receive the signal and convert it to the complex
You pretty much said it: USRPs (in general) are direct mixing
complex baseband receivers. There's a lot of literature on how
I actually also tested the signal carrier
system, but 16QAM still not works.
I'm afraid you'll have to dig into what noise is, how it affects
demodulation, and where symbol errors come from. This is theory
that you'll need to know when modifying/implementing digital
On 02/18/2017 05:22 PM, Marcus Müller
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 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
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