Hi Tom, Thanks for the reply. In the ofdm_sync_pn.py, I see that a matched filter is used, after the timing metric is obtained based on the correlation of the two halves of the preamble. I understand this matched filter is trying to find the end of the plateau of the metric and get the smooth peak. But I am a little confused with the length of this matched filter. In the code, the length of this matched filter is set as equal to cp_length. But in T.M.Schmidl's paper(page 1615), the plateau length is equal to the cp_length minus the channel impulse response length. I am thinking, without counting the channel response length, can we get the peak accurately, especially in multipath environment?
On Sun, Mar 25, 2012 at 12:40 PM, Tom Rondeau <t...@trondeau.com> wrote: > On Sat, Mar 24, 2012 at 8:52 PM, Alex Zhang <cingular.a...@gmail.com> > wrote: > > Hi Gurus, > > > > I just want to make sure how the current gnuradio ofdm exampel is > > doing synchronization. > > According to T. M. Schmidl and D. C. Cox, "Robust Frequency and > > Timing Synchonization for OFDM," IEEE Trans. Communications, vol. > 45, no. > > 12, 1997. > > When, estimating the carrier frequency offset at the receiver, if the > phase > > difference between the two halves of the 1st training symbol is > guaranteed > > to be less than PI, then the frequency offset estimate can derived by > > Phi/(Pi*T). In this situation, the even PN sequencies of the second > training > > symbol would not be needed. Otherwise, the actual frequency offset would > be > > > > Phi/(Pi*T) + 2*z/T > > and the z can be estimated by some optimization algorithm, using both of > > the training symbols. Also, the paper mentioned that the odd frequencies > of > > the second training symbol can be used to measure the sub-channels. > > > > However, I find that only one training symbol is generated to act as > > preamble at the ofdm transmitter. And on the receiver, it seems that only > > one preamble is used to estimate the timing peak and the frequency > offset. > > Is the current implementation assuming that the frequency is less than > PI? > > Or anything I missed? > > > > Looking forward to your input! > > -- > > > > Alex, > > Dreams can come true – just believe. > > > Alex, > > Take a look at the presentation we put together here: > http://gnuradio.org/redmine/projects/gnuradio/wiki/Wireless > > It explains the synchronization process. Basically, the single > preamble is made up of two identical sections, so the correlation is > done between those two sections to get the timing and fine frequency > estimate. Since this preamble is known, we also use it to handle the > coarse frequency (number of bins) offset. > > Tom > -- Alex, *Dreams can come true – just believe.*
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