To ease answering, I'll add more of the conversation in this separate mail:
You wrote: > 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 signal. 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? yes. > Or we can not use the USRP to receive the QAM signal. 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 number. You pretty much said it: USRPs (in general) are direct mixing complex baseband receivers. There's a lot of literature on how these work! > 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 transceiver systems! Best regards, Marcus On 02/18/2017 05:22 PM, Marcus Müller wrote: > > Hi Shangqing, > > > I think you forgot to mention that you've already gotten answers to > this on usrp-users: > > I 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? > 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 symbols. >> >> 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 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. > > Best regards, > Marcus > > > > On 02/18/2017 04:23 PM, Zhao Shangqing wrote: >> >> Hi all, >> >> >> 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. >> >> >> Best regards, >> >> Shangqing >> >> >> >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> Discuss-gnuradio@gnu.org >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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