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

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] FSK modulation/demodulation for Universal Access Transceiver
Date: Thu, 26 May 2016 21:05:28 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

Hi Olivier,

thanks for the reply! Ah, I was confusing UAT and ADS-B then, sorry!

You should really be sending this email to the address@hidden list - with the graphs attached, it's much easier for the others to help!
Just a question: the time_dom.png indeed looks like there's something wrong. Is the upper signal display TX, the lower display RX? In that case, make sure you have proper pulse shaping enabled. This looks like you have samples_per_symbol=1. Also, yes, it's a good idea not to drive frontends with their maximum amplitude, but that can easily remedied with a multiply_const block with e.g. 0.7.

Best regards,

PS: Amazing project!

On 26.05.2016 18:32, Olivier Goyette wrote:
Thank you for your reply.

I'm aware of gr-air-modes but the thing is that UAT and ADS-B are not working on the same medium. UAT is using 978 MHz carrier and ADS-B is using 1090 MHz. ADS-B messages for UAT are not using the same message format so, i'll need to interpret them using another format, that's why i'm starting from zero.

Concerning RX and TX for time and frequency domain, by looking at the time domain at TX, there seem to be clipping?? How do I fix it ? I've tried to play with the gain but it doesn't seem to be working..

2016-05-26 11:59 GMT-04:00 Marcus Müller <address@hidden>:
Hi Olivier!

Nice to meet you.
So, first off, a hint: GRC has a "screen capture" menu / toolbar button that lets you just export your GRC canvas, so you don't have to make a screenshot of your whole dual-desktop screen (by the way, cropping that would have worked, too ;) ).

UAT == ADS-B transmitter, right? Have you seen [1], and are you aware of Nick Foster's gr-air-modes, an ADS-B receiver?

So, we'd love to help you, but "only gibberish" is hardly a good error description. Does the RX signal in time domain look OK, or is there very little amplitude, or values close to 1, indicating clipping? What about frequency domain, does the RX signal resemble the TX signal?

Then: UHD specifically tells you there is something wrong with how you've configured things. You will need to fix that first.

Best regards,

[1] https://www.youtube.com/watch?v=NSLqRXyxiBo

On 26.05.2016 17:03, Olivier Goyette wrote:
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. 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 !

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

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


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