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Re: [Discuss-gnuradio] Tutorial on BPSK bursts


From: Jesse Reich
Subject: Re: [Discuss-gnuradio] Tutorial on BPSK bursts
Date: Tue, 08 Mar 2016 21:04:36 +0000

Thank you much Andy. I'll take a look and give it a try tonight when I get home. 

Is the BPSK that is used unusual because of the 1.1 rad phase shifts and is the related to what I've heard that the signal still has power in the pure carrier throughout the message?

Also, do you know if the T.001 beacons use differential BPSK or not? 

I was able to capture a file sink of a PLB transmission last night so I'll have to give it a try.  But the frequency I saw seemed unusual; around 406.008 MHz. I believe it should be at 406.037 based on the beacon I was using. 

Thanks again,
Jesse 

On Tue, Mar 8, 2016 at 3:47 PM Andy Walls <address@hidden> wrote:
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

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