Re: [Discuss-gnuradio] OFDM results.
Bob McGwier wrote: The USRP is not in the same league with the USRP2. The on board oscillator is much better but external oscillators will make it even better. So the answer is a big yes, the USRP2 will be better. Actually, I need to disagree again here :) The master oscillator on the USRP1 is actually exceptionally clean. It may be as much as 20ppm off in frequency and will drift with temperature, but it is very clean down to 10 Hz offsets. It is amazing what you can do with a cheap oscillator as long as you don't make it tunable or temperature compensated. As soon as you make an oscillator temperature compensated or voltage controlled, as we need to in the USRP2, you have to go to much greater lengths (and expense!) to get comparable phase noise performance. So the USRP2 will have roughly the same phase noise performance as the USRP1. The USRP2 will be lockable to an external oscillator, but that will only affect phase noise at less than 100 Hz offsets. Matt ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
Matt Ettus wrote: Jeff Brower wrote: Bob- In your sixteen QAM and other figures I see two effects. Notice just the slightest hint that arcs through the top four constellation points in the 16 QAM is not straight. This curvature is caused by nonlinearity. Your result almost surely can NOT be clock jitter. If you had a lot of clock jitter, the pictures would look much worse. Notice the dispersion gets larger as your proceed away from the origin. This is almost surely phase noise in some oscillator. "Phase noise" of an oscillator to me means jitter. Can you clarify? Do you mean a sinusoid oscillator that has some issue with shape; i.e. non-linearity? Thanks. I have to disagree with Bob here. I think that you are seeing some form of distortion. To me there is no doubt about the nonlinear distortion as you can see curvature in the edges of the "square". If you have amplitude components modulating the signal coming into the DBS-RX mixer, it will not just spread them in angle. I do take your point about the constellation points being symmetric rather than more elongated it might just be compression effects. Back off will answer the question. In a QAM pattern, gaussian noise will look like uniformly-sized round balls. Phase noise will look like ovals which are longer in the "rotation direction" than they are along the line to the center of the constellation, proportionally so for the outer points than the inner ones. ISI, frequency-selective fading and ICI will, over time, tend to look round as well. Distortion like nonlinearities in the transmitter will mostly affect the outer constellation points. This is a little more complicated in an OFDM system rather than straight single-carrier QAM, but that still looks like what you are seeing. Also, 31 subcarriers is a very small number for OFDM, so the affect is more like on plain old QAM. I would suggest backing the transmit power off a bit further. Matt ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
Bob- > Good to hear from you again. I am distinguishing betwen clock recovery > operations in the receiver and the oscillator feeding the down > conversion mixers. Even though there is some commonality in the sources > for these two DIFFERENT things on the USRP, implementation factors of > the clock recovery, how well the DBS-RX takes the up conversion of the > oscillator by MANY factors to make the downconversion mixer sources, > etc. are almost enough different to be independent noise sources. > > I believe the clock recovery is working just fine or these pictures > would look like crap. The jitter of the constellation increasing with > increasing distance from the origin is VERY indicative of angular phase > noise in the downconversion oscillators in (say) the DBS-RX. Ok thanks for your explanation. Also I found a Wikipedia page on "phase noise" that even had a formula to convert to jitter. I understand what you are saying now. -Jeff > Jeff Brower wrote: > > Bob- > > > > > >> In your sixteen QAM and other figures I see two effects. > >> > >> Notice just the slightest hint that arcs through the top four > >> constellation points in the 16 QAM is not straight. This curvature is > >> caused by nonlinearity. > >> > >> Your result almost surely can NOT be clock jitter. If you had a lot of > >> clock jitter, the pictures would look much worse. Notice the > >> dispersion gets larger as your proceed away from the origin. This is > >> almost surely phase noise in some oscillator. > >> > > > > "Phase noise" of an oscillator to me means jitter. Can you clarify? Do > > you mean a > > sinusoid oscillator that has some issue with shape; i.e. non-linearity? > > Thanks. > > > > -Jeff > > ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
Jeff: Good to hear from you again. I am distinguishing betwen clock recovery operations in the receiver and the oscillator feeding the down conversion mixers. Even though there is some commonality in the sources for these two DIFFERENT things on the USRP, implementation factors of the clock recovery, how well the DBS-RX takes the up conversion of the oscillator by MANY factors to make the downconversion mixer sources, etc. are almost enough different to be independent noise sources. I believe the clock recovery is working just fine or these pictures would look like crap. The jitter of the constellation increasing with increasing distance from the origin is VERY indicative of angular phase noise in the downconversion oscillators in (say) the DBS-RX. Bob Jeff Brower wrote: Bob- In your sixteen QAM and other figures I see two effects. Notice just the slightest hint that arcs through the top four constellation points in the 16 QAM is not straight. This curvature is caused by nonlinearity. Your result almost surely can NOT be clock jitter. If you had a lot of clock jitter, the pictures would look much worse. Notice the dispersion gets larger as your proceed away from the origin. This is almost surely phase noise in some oscillator. "Phase noise" of an oscillator to me means jitter. Can you clarify? Do you mean a sinusoid oscillator that has some issue with shape; i.e. non-linearity? Thanks. -Jeff ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
The USRP is not in the same league with the USRP2. The on board oscillator is much better but external oscillators will make it even better. So the answer is a big yes, the USRP2 will be better. Bob Per Zetterberg wrote: -Original Message- From: Bob McGwier [mailto:[EMAIL PROTECTED] Sent: den 6 juni 2008 09:23 To: Per Zetterberg Cc: discuss-gnuradio@gnu.org; 'Per Zetterberg' Subject: Re: [Discuss-gnuradio] OFDM results. In your sixteen QAM and other figures I see two effects. Notice just the slightest hint that arcs through the top four constellation points in the 16 QAM is not straight. This curvature is caused by nonlinearity. Your result almost surely can NOT be clock jitter. If you had a lot of clock jitter, the pictures would look much worse. Notice the dispersion gets larger as your proceed away from the origin. This is almost surely phase noise in some oscillator. Bob Thanks! New question: Would the phase noise get smaller in USRP2 where we can use an external reference, given that we use a high precision external reference ? ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
Jeff Brower wrote: Bob- In your sixteen QAM and other figures I see two effects. Notice just the slightest hint that arcs through the top four constellation points in the 16 QAM is not straight. This curvature is caused by nonlinearity. Your result almost surely can NOT be clock jitter. If you had a lot of clock jitter, the pictures would look much worse. Notice the dispersion gets larger as your proceed away from the origin. This is almost surely phase noise in some oscillator. "Phase noise" of an oscillator to me means jitter. Can you clarify? Do you mean a sinusoid oscillator that has some issue with shape; i.e. non-linearity? Thanks. I have to disagree with Bob here. I think that you are seeing some form of distortion. In a QAM pattern, gaussian noise will look like uniformly-sized round balls. Phase noise will look like ovals which are longer in the "rotation direction" than they are along the line to the center of the constellation, proportionally so for the outer points than the inner ones. ISI, frequency-selective fading and ICI will, over time, tend to look round as well. Distortion like nonlinearities in the transmitter will mostly affect the outer constellation points. This is a little more complicated in an OFDM system rather than straight single-carrier QAM, but that still looks like what you are seeing. Also, 31 subcarriers is a very small number for OFDM, so the affect is more like on plain old QAM. I would suggest backing the transmit power off a bit further. Matt ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
Bob- > In your sixteen QAM and other figures I see two effects. > > Notice just the slightest hint that arcs through the top four > constellation points in the 16 QAM is not straight. This curvature is > caused by nonlinearity. > > Your result almost surely can NOT be clock jitter. If you had a lot of > clock jitter, the pictures would look much worse. Notice the > dispersion gets larger as your proceed away from the origin. This is > almost surely phase noise in some oscillator. "Phase noise" of an oscillator to me means jitter. Can you clarify? Do you mean a sinusoid oscillator that has some issue with shape; i.e. non-linearity? Thanks. -Jeff > Per Zetterberg wrote: > > Dear all, > > > > I am experimenting with OFDM between two USRPs. Unfortunately, I am not yet > > able to master the gnuradio framework so I have made my own implementation. > > The results are given in the link below. > > > > http://www.s3.kth.se/~perz/usrp/OFDM_results.pdf > > > > > > How does this compare with the gnuradio OFDM implementation ? > > > > What is the cause of the problems. Clock jitter ?, non-linearities ? > > > > > > BR/ > > Per Zetterberg ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
RE: [Discuss-gnuradio] OFDM results.
> -Original Message- > From: Bob McGwier [mailto:[EMAIL PROTECTED] > Sent: den 6 juni 2008 09:23 > To: Per Zetterberg > Cc: discuss-gnuradio@gnu.org; 'Per Zetterberg' > Subject: Re: [Discuss-gnuradio] OFDM results. > > In your sixteen QAM and other figures I see two effects. > > Notice just the slightest hint that arcs through the top four > constellation points in the 16 QAM is not straight. This > curvature is caused by nonlinearity. > > Your result almost surely can NOT be clock jitter. If you > had a lot of clock jitter, the pictures would look much > worse. Notice the dispersion gets larger as your proceed > away from the origin. This is almost surely phase noise in > some oscillator. > > Bob > Thanks! New question: Would the phase noise get smaller in USRP2 where we can use an external reference, given that we use a high precision external reference ? ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
In your sixteen QAM and other figures I see two effects. Notice just the slightest hint that arcs through the top four constellation points in the 16 QAM is not straight. This curvature is caused by nonlinearity. Your result almost surely can NOT be clock jitter. If you had a lot of clock jitter, the pictures would look much worse. Notice the dispersion gets larger as your proceed away from the origin. This is almost surely phase noise in some oscillator. Bob Per Zetterberg wrote: Dear all, I am experimenting with OFDM between two USRPs. Unfortunately, I am not yet able to master the gnuradio framework so I have made my own implementation. The results are given in the link below. http://www.s3.kth.se/~perz/usrp/OFDM_results.pdf How does this compare with the gnuradio OFDM implementation ? What is the cause of the problems. Clock jitter ?, non-linearities ? BR/ Per Zetterberg ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
> where is the inaccuracy in sampling frequency exactly happening? > In USRP? Yes - the sampling frequency is inaccurate, because the crystal on the USRP is not oscillating at exactly 64 MHz. FYI, the samples in the plot are actually not from an USRP (I didn't record them myself) - the inaccuracy is about 74 PPM. With samples from my USRP I found that the sampling frequency is off by about 14 PPM, and in some samples that Jens Elsner recorded with his USRP I found an inaccuracy of about 9 PPM. Depending on the number of subcarriers and constellation density, 9 PPM can be enough to make the constellation a little more cloudy, if the sample rate is not corrected. Regards, Andreas signature.asc Description: OpenPGP digital signature ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
Hi Andreas, where is the inaccuracy in sampling frequency exactly happening? In USRP? Regards, Hoo Chang Shin. - Original Message From: Andreas Müller <[EMAIL PROTECTED]> To: discuss-gnuradio@gnu.org Cc: [EMAIL PROTECTED] Sent: Wednesday, June 4, 2008 12:48:00 PM Subject: Re: [Discuss-gnuradio] OFDM results. Hello Per > What is the cause of the problems. Clock jitter ?, non-linearities ? > No idea what could be the problem in your case, but maybe try also plotting the phase of all symbols in one OFDM symbol vs the offset from the central carrier. I'm currently working on DAB and I found some problems by doing this, e.g. in [1] there is a frequency dependent phase offset due to an inaccurate the sampling frequency. Regards, Andreas [1] http://people.ee.ethz.ch/~andrmuel/tmp/phase-vs-subcarrier.png ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] OFDM results.
Hello Per > What is the cause of the problems. Clock jitter ?, non-linearities ? > No idea what could be the problem in your case, but maybe try also plotting the phase of all symbols in one OFDM symbol vs the offset from the central carrier. I'm currently working on DAB and I found some problems by doing this, e.g. in [1] there is a frequency dependent phase offset due to an inaccurate the sampling frequency. Regards, Andreas [1] http://people.ee.ethz.ch/~andrmuel/tmp/phase-vs-subcarrier.png signature.asc Description: OpenPGP digital signature ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
