Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Hi Prateek, yes - I sampled another radio station (this time at 1,5 GHz -> broader subcarriers) and the "jitter" was significantly reduced. I implemented deinterleavingi, channel coding, and descrambling - but so far no success in demodulating. Yesterday I wrote a sender to check my receiver but I

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Achilleas, thanks for your comments. I am beginning to like this problem, as it has extended my knowlegde of OFDM tremendously - thanks everybody. Nevertheless I'd like to see it solved. We have ruled out pretty much every effect - I'm beginning to suspect that the transmitter actually sends sh

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

(comments inlined) On Wed, Apr 05, 2006 at 02:02:23PM -0400, Robert McGwier wrote: > Yes. Here is what you are missing: > > Let us concentrate (as does your nice animated gif) on one channel in > the OFDM. Yes, what's depicted are all decoded symbols in one OFDM frame. > Let us suppose you have

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Jens, I would like to elaborate on some of your comments. I am using the following notation: N fft size (2048) G guard interval (504) v0 fractional frequency offset f0=v0 T absolute frequency offset tau0 fractional time offset t0=tau0 T absolute timne offset To sum up: Frequency offset result

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Yes. Here is what you are missing: Let us concentrate (as does your nice animated gif) on one channel in the OFDM. Let us suppose you have a variable delay into the signal after its onset. This will happen with probability one because your clock and the transmitter clock will not be the sam

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

> Thank you very much for the reference. This was a very nice link to > Mostofi's work. The following paper is also pertinent to Jen's thinking > on the guard interval offset being irrelevant. It is not of course: > > *Y. Mostofi*, D. Cox and A. Bahai, "Effect of Timing Synchronization > Er

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Thanks for the reply! I'm really struggeling here... > Yes, I think this is a critical step. small letters are time domain and > capital letters are frequency domain. > > x(n) ---> X(k) > FFT > > x(n-n') -> e^(j*2*pi*n'*k/N) * X(k) > FFT > > for one OFDM sy

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Achilleas, my estimate is based on frequency domain correlation. Just count the offset of the maximum (o). Then estimate the fractional offset (fo) by taking the value left (l) and right (r) of the maximum and calculate fo = (r-l)/(r+l). To compensate substract the resulting frequency offset (o+f

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Prateek Dayal wrote: On 4/4/06, *Robert McGwier* <[EMAIL PROTECTED] > wrote: http://www.cds.caltech.edu/~yasi/publications.html Also for frequency offset, what really matters is not the frequency offset in

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

On 4/4/06, Robert McGwier <[EMAIL PROTECTED]> wrote: Jens Elsner wrote:> Achilleas,>> thanks for taking the time. My goal is to implement COFDM in gnuradio,> DAB is a nice start.>> To your points: Time sync is not a problem in OFDM - the guard interval takes care of > that. Nice property. Try p

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Jens, I used a complete ad-hoc method for automatic frequency estimation: tracing the minimum ratio of the power of DC over the total power of the remaining channels, averaged over all symbols in a frame. Your estimate seems quite good! I atatch the .m file I have not tried anything with the

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Jens Elsner wrote: Achilleas, thanks for taking the time. My goal is to implement COFDM in gnuradio, DAB is a nice start. To your points: Time sync is not a problem in OFDM - the guard interval takes care of that. Nice property. Try playing along with the "start_resamp" value - it will

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

I guess I was a little bit too quick with my answer... In more detail: > 2) You have done frequency correction manually: did you do this by > looking at the DC subcarrier (no power transmitted here) in the > received signal prior to OFDM demod? > Frequency synchronization in OFDM is very crucial

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Achilleas, thanks for taking the time. My goal is to implement COFDM in gnuradio, DAB is a nice start. To your points: > I got interested in this discussion and looked at the > standard briefly and at your matlab code. > I have a couple of initial points; I will spend more time sometime this >

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Jens, I got interested in this discussion and looked at the standard briefly and at your matlab code. I have a couple of initial points; I will spend more time sometime this week on your code: 1) where in the standard it says that this is pi/4 DQPSK ? I only saw that the modulation used is DQP

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Further more this means that the demodulated symbol (differences!) should always have phase 0, pi/4, 2pi/4, 3pi/4 (plus a *constant* offset of pi/8). But this offset is varying. Am I right? And why should the transmitter introduce a varying offset? The problem is somewhere in the demodulation ch

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Bob, thanks for the reply. This is exactly what I am doing: [...] dem = data ./ data_l; [...] A point-wise division by the last OFDM symbol. This also restores amplitude, but has the same effect on the phase. I do not control the receiver, my data is from a radio station. I am only trying t

Re: [Discuss-gnuradio] OFDM demodulation problem / example video

Jens: This pi/4-DQPSK. That means New-symbol * (complex conjugate (Old-symbol)) is pi/4 modulo pi/2. Are you taking this into account on both the transmitter and the receiver and it in all of the bins before your inverse fft provides the time domain signal to transmit would be my best