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Re: [Discuss-gnuradio] FSK modulation/demodulation for Universal Access

From: Andy Walls
Subject: Re: [Discuss-gnuradio] FSK modulation/demodulation for Universal Access Transceiver
Date: Thu, 26 May 2016 14:31:21 -0400

On Thu, 2016-05-26 at 11:04 -0400, address@hidden

> Date: Thu, 26 May 2016 11:03:55 -0400
> From: Olivier Goyette 

> Hi everyone,
> I'm currently doing an internship at school and i'm working with USRP N210
> and GNUradio. It's been a month since I began working with these 2 tools,
> so excuse me if I may sound noob, but I really need some help. I've been
> asked to start thinking about developing an UAT for civil aviation. I've
> read a ton of standards and papers concerning this new technology, so I
> won't relate it all over here, but the main thing about it is that it uses
> FSK mod/demod for the information to be transmitted and received.
> I'm currently stuck on a problem that's been haunting me for the last 2
> weeks and that's why i'm asking for your guidance. When I run the mod/demod
> on simulation(attached #1), everything works fine. I'm sending
> *abcdefghijklmopqr
> *and on the receiving side i got the same thing.

Isn't simulation great. ;)

>  Now the problem is that
> when i'm trying to send it via the USRP with a cable between RX and
> TX(attached #2), i only get gibberish on the receiving side !

The real-world always ruins things!

> What have I done wrong ? I don't get why it's not working.

Three things:

1. On Tx you only appear to be using 1 sample/symbol.  That doesn't let
you do any pulse shaping.

2. On Rx you appare to be using only 1 sample/symbol, which makes symbol
clock recovery nearly impossible.

3. On Rx you're not performing symbol clock recovery  (M&M clock
recovery block or PFB clock recovery block).

4. On RX you're performing neither coarse nor fine frequency

5. Your USRP is unhappy with your configuration of the USRP blocks:
"The hardware does not support the requested Tx sample rate:
Target sample rate: 0.000001 Msps  (<- 1 sample per second?!)
Actual sample rate: 0.195312 Msps  (<- 195.312 ksps is the slowest the USRP can 
go apparently)"

> Thank you for your time, your help will be more than welcome

As Marcus said, yo can look at Nick Foster's gr-air-modes to see how he
handles Mode S/ADS-B (which is ASK/OOK).

You can also look at Nick's gr-ais, which is a GMSK receiver (GMSK is
GMSK with a modulation index of 0.5). 

This set of class notes talks about receiver timing synchronization:

Also fred harris' article "Let's Assume the System is Synchronized" is
good reading about receiver synchronization (although not specifically
geared towards FSK):


> Olivier

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