---------- Forwarded message ---------- From: w w <biscuitk...@gmail.com> Date: Thu, May 7, 2009 at 1:10 PM Subject: Re: [Discuss-gnuradio] BPSK Demodulator (i.e. Receiver) Award/Challenge To: Ed Criscuolo <edward.l.criscu...@nasa.gov>
Ed, I was wondering if you know how to implement or have examples in GNURadio to decode the various enCoding schemes like NRZ-l, NRZ-M, Bi-phase-L, Bi-Phase-M....etc? I've looked and don't recall seeing NRZI to NRZ decoder blocks....can you point me the right direction? Is this part of the spacecraft ground station project? If so, shouldn't these blocks be moved into the trunk? Sorry for the 20 questions. Cheers, Jody On Thu, May 7, 2009 at 11:11 AM, Ed Criscuolo <edward.l.criscu...@nasa.gov > wrote: > René Fléron wrote: > >> I seriously doubt that making a regular BPSK receiver that goes >> all-the-way, >> i.e. recovers the transmitted data has ever been done. We haven't found >> any >> trace of anything be it commercial, research or DIY. >> > > NASA regularly uses BPSK as one of the supported client services > provided by their TDRSS relay satellites. > > There is a reason most people use differential-BPSK instead of > plain BPSK. Plain BPSK is easy to generate but much more > difficult to receive and decode properly. Without any kind > of absolute phase reference between the transmitter and > receiver, there is a 180-degree phase ambiguity when > recovering a binary bitstream. This results in a bitstream > that can be either what you want, or its ones-compliment. > This requires additional processing AFTER the BPSK demodulation > to resolve the ambiguity. > > One technique involves looking for a specific sync pattern > (or its compliment) in the bitstream in order to decide > if you need to invert the bitstream. > > Another (better) technique is to use a form of coding (such as NRZI) > that depends on transitions rather than the absolute bit values. > This the technique we prefer. > > In order to create a BPSK demodulator block, all that's necessary > is to recover the phase of the signal. This is easily accomplished > as atan(Q/I). Note that it is very important to use a four-quadrant > form of the arctan function to demodulate correctly. > > Once you have the demodulated sample stream, proceed as usual, > recovering the bitstream with an M&M clock recovery block, > followed by a bitslicer block, followed by an NRZI to NRZ decoder > block. There are examples of this in the Spacecraft Groundstation > Project in CGRAN. > > @(^.^)@ Ed > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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