Hi Sangqing,
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. Thanks! Marcus On 02/18/2017 05:43 PM, Zhao Shangqing wrote: > > Hi Marcus, > > > 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 explanation. > > > 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? > > > Best regards, > > Shangqing > > > ------------------------------------------------------------------------ > *发件人:* Discuss-gnuradio > <discuss-gnuradio-bounces+zsqdtc=hotmail....@gnu.org> 代表 Marcus Müller > <marcus.muel...@ettus.com> > *发送时间:* 2017年2月18日 10:22 > *收件人:* discuss-gnuradio@gnu.org > *主题:* Re: [Discuss-gnuradio] Power control for QAM demodulate > > > 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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