Resend this mail for some comments. Thanks. On Tue, Mar 27, 2012 at 9:29 PM, Alex Zhang <[email protected]> wrote:
> 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 <[email protected]> wrote: > >> On Sat, Mar 24, 2012 at 8:52 PM, Alex Zhang <[email protected]> >> 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.* > > -- Alex, *Dreams can come true – just believe.*
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