Hi, ah! OK, yeah, that's unfortunate. So, the easy thing about OFDM: as soon as you know where your symbol starts, all you need to do is apply an FFT – and you're done. (you will still have to "equalize" your reception, but that is a point-wise multiplication at the output of the FFT.)
So, if I was to recommend a methodology for solving this: Start with building a fixed-lag correlator to find the beginning of symbols. You're not the first person to have to do that: The gr-dab module [1] contains one such, that you'd only have to parameterize to use the length and sizes of OFDM symbols in WiMaX instead of these in DAB+. (I actually recommend looking at gr-dab's receiver; it's a nice display of how to demodulate OFDM downlinks.) You'll be getting output that you can threshold to get the beginnings of symbols, then you just start taking FFTs at these positions, and in those, you look for the preamble symbols to get the equalizer taps. Best regards, Marcus [1] https://github.com/kit-cel/gr-dab On Wed, 2019-11-20 at 15:19 +0200, Eitan Hetzroni wrote: > hello, > it is an ofdm system, but i could not get to tweak the ofdm_Rx example to > find the preamble in the transmission i was looking to capture. > > On Wed, Nov 20, 2019 at 3:16 PM Müller, Marcus (CEL) <muel...@kit.edu> wrote: > > Hi Eitan, > > > > please keep replies on the list! > > I don't think there's a readymade solution for you. > > > > I'm not quite sure what you mean with "resemble OFDM": could you > > elaborate? I really thought that IEEE802.16 is a pure OFDM system, so > > this comes as a surprise. In which way is it not? > > > > Best regards, > > Marcus > > > > On Wed, 2019-11-20 at 15:13 +0200, Eitan Hetzroni wrote: > > > thanks for the rapid reply, > > > it does resemble ofdm, however as a beginner in the field i was hoping to > > > find a readymade chain to tweak with? > > > > > > > > > On Wed, Nov 20, 2019 at 3:11 PM Müller, Marcus (CEL) <muel...@kit.edu> > > > wrote: > > > > Hi Eitan, > > > > > > > > it's been a while since I've looked into IEEE802.16, but that's a very > > > > regular OFDM system, isn't it, with a fixed OFDM symbol as short > > > > preamble (for the downlink bursts), and a 2-symbol preamble for the > > > > start of frame? > > > > > > > > A simple correlation detector might do in that case; a bit nicer on the > > > > acquisition speed per computational effort might be a fixed-length > > > > correlator that detects the position of cyclic prefixes and could be > > > > applied to a whole burst or even frame to get a good timing estimate > > > > before you start decoding OFDM symbols. > > > > > > > > Best regards, > > > > Marcus > > > > > > > > PS: Your email signature is kinda funny on a publicly archived mailing > > > > list :D > > > > > > > > > > > > On Wed, 2019-11-20 at 14:43 +0200, Eitan Hetzroni wrote: > > > > > Hi, > > > > > any solution to the reception (rx) or tx of wimax using gnuradio? > > > > > could not find any proper chain that finds the preamble. > > > > > > > > > > The information transmitted is intended only for the person or entity > > > > > to which it is addressed and may contain confidential and/or > > > > > privileged material. Any review, retransmission, dissemination or > > > > > other use of, or taking of any action in reliance upon, this > > > > > information by persons or entities other than the intended recipient > > > > > is prohibited. If you received this in error, please contact the > > > > > sender and delete all copies of the message. > > > > > > The information transmitted is intended only for the person or entity to > > > which it is addressed and may contain confidential and/or privileged > > > material. Any review, retransmission, dissemination or other use of, or > > > taking of any action in reliance upon, this information by persons or > > > entities other than the intended recipient is prohibited. If you received > > > this in error, please contact the sender and delete all copies of the > > > message. > > The information transmitted is intended only for the person or entity to > which it is addressed and may contain confidential and/or privileged > material. Any review, retransmission, dissemination or other use of, or > taking of any action in reliance upon, this information by persons or > entities other than the intended recipient is prohibited. If you received > this in error, please contact the sender and delete all copies of the message.
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