RE: [Discuss-gnuradio] No BPSK modulator?

[Tom Rondeau]

> -----Original Message-----
> From: address@hidden [mailto:discuss-
> 
> 2007/2/27, Eric Blossom <address@hidden>:
> > On Tue, Feb 27, 2007 at 05:07:31PM -0500, Tom Rondeau wrote:
> > > > -----Original Message-----
> > > > From: address@hidden
> [mailto:discuss-
> > > > address@hidden On Behalf Of Trond
> Danielsen
> > > > Sent: Tuesday, February 27, 2007 10:46 AM
> > > > To: gnuradio mailing list
> > > > Subject: [Discuss-gnuradio] No BPSK modulator?
> > > >
> > > > Hi,
> > > >
> > > > Is there a reason why there is no "ordinary" n-PSK modulator and
> > > > demodulator available, only the differential one? If not, I'll
> modify
> > > > the existing one and submit a patch.
> > >
> > > It's not so much that there isn't a non-differential BPSK modulator,
> it's
> > > that there is no demod for it. Basically, we have to be able to
> determine
> > > the phase orientation of the incoming signal in some way to know what
> our
> > > phase is. In BPSK, this is simple, and this was the original reason
> why
> > > gr_correlate_access_code_bb had a d_flip variable in it, so that when
> the
> > > phase inversion was detected, it would decode it properly.
> > >
> > > However, this was never a good solution and ended up hurting us
> because
> > > sometimes we'd trigger this flip with random data, which would screw
> up the
> > > packets. This was the reason the SSH sessions would crash and why
> Johnathan
> > > made the original fix in the whitener.
> > >
> > > The solution to this is to use the correlator to detect the phase, but
> let
> > > the next guy down the line understand what to do with it. We probably
> want
> > > to send a control signal out to set the phase derotation and trigger
> the
> > > access code found state at the same time, specifically, in the simple
> framer
> > > state machine.
> >
> > You could use bit 2 of the output stream of
> > gr_correlate_access_code_bb to encode the "I've seen the inverted
> > pattern".  Then anybody who cared could look at it and take action if
> > they wanted to, but it wouldn't interfere with any existing code.
> >
> > Eric
> >
> 
> Thank you all for anwering. What I am really looking for is just BPSK,
> so that should simplify things a bit. I intent to demodulate the BPSK
> modulated message from GPS satellites, therefore the interest in
> not-differential PSK modulation.

I was under the same opinion, which was why d_flip was in
correlate_access_code originally, but that screwed some stuff up down the
chain.

I think whatever new solution we use should be general for non-differential
MPSK and not just BPSK.

> To make things a bit clearer: correlate_access_code is in reality a
> preamble detector, or am I missing something?

The access code, which is not only used to identify the start of a MAC
frame, but it can also identify the network being used.

Tom