Re: [Discuss-gnuradio] Tutorial on BPSK bursts

[Andy Walls]

On Sat, 2016-03-05 at 12:00 -0500, address@hidden
wrote:
> Date: Sat, 5 Mar 2016 00:46:12 -0500
> From: Tim K <address@hidden>

> 
> It's not terrifically designed, but you might find my PSK31 repo
> helpful.
> According to Balint though, the PLL is supposed to go after clock
> recovery,
> not before.
> 
> https://github.com/tkuester/gr-psk31
> 
> - Tim
> 
> On Fri, Mar 4, 2016 at 8:56 PM, Jesse Reich <address@hidden> wrote:
> 
> > So I've been playing with GNU Radio for about 2 months and trying to
> learn
> > about signals and signal processing in general. Mostly out of
> curiosity
> > that has sprung up from my profession. I am an Aerospace engineer by
> > training and I work as a satellite ground systems engineer for the
> > Cospas-Sarsat program.
> >
> > I've started with what I thought should be an easy task with
> aspirations
> > of more complex tasks. The first task, decode a BPSK signal
> modulated on a
> > 406 MHz carrier. The signals consist of a 160 ms carrier,  24 sync
> bits and
> > then 120 data bits at 400 bits/sec. I'm just trying to decode and
> write the
> > bits. Sounded to me to be an easy problem to tackle. Unfortunately I
> can't
> > seem to even get off of the ground. Does anyone know of any
> tutorials that
> > would be good.
> >
> > I've tried the gnuradio tutorials and honestly they don't really
> seem to
> > help. Any help would be greatly appreciated.
> >
> > Thanks,
> > Jesse


Hi Jesse:

See the attached, disasterous flowgraph:
a) the top half is a simulated EPIRB transmitter and noisy channel
b) the bottom half is an EPIRB PSK receiver I winged together

The receiver probably needs tweaking on the M&M clock recovery
parameters.  Also the receiver implementation ends at the recovered,
manchester encoded bits.  You have to decode the bits and frame them
yourself from that point.  Tim K probably has good stuff in his github
repos to do that.

The C/S T.001 stuff isn't your normal BPSK, so the all-in-one PSK demod
blocks in GNURadio won't work well.  (Heck, the costas loop block gets
confused on the 160 ms of unmodulated carrier.)

The C/S T.001 BSPK modulation has an unusual constellation:

(1.0          , 0.0          ): unmodulated carrier
(cos( 1.1 rad), sin( 1.1 rad)): '1' symbol
(cos(-1.1 rad), sin(-1.1 rad)): '0' symbol

This makes it easy to lock on to with a PLL with a very sluggish
response.  That is what I did in the attached GRC.

Regards,
Andy

epirb.grc
Description: application/xml