[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Discuss-gnuradio] FSK modulation/demodulation for Universal Access
Re: [Discuss-gnuradio] FSK modulation/demodulation for Universal Access Transceiver
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
> *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.
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
> 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):