Hi,
I am trying to use the bladeRF (115 FPGA) as a receiver for the FAA's Next Gen Flight Information Service-Broadcast system. They transmit at 978MHz, and use phase coherent BFSK modulation with a symbol rate of 1.04167MS/s and a deviation of +/- 312.5 KHz. Also used is byte interleaving and a R-S FECcode. The RTCA publishes the officlal spec for the system in document RTCA DO-282B. I found what I believe is a draft copy of it on line and have used it so far in my design as to buy a full copy as a non-RTCA member is $650.00 (ouch!).
At this point I have written C code running on a PC under Ubuntu that uses the bladeRF. My code is debugged and can successfully demodulate the signal and obtain symbol lock. So far my tranmitting source has been a vector signal generator, programmed to match the modulation specs that continuosly transmits the FIS-B 36 bit sync code. My demodulator/symbol timing recovery code uses the bladeRF to downconvert and then sample the IF. Those samples feed a complex FFT and by sliding the FFT window over a block of samples, I can make bit decisions and correlate against the sych code. If I don't get a correlator score of zero, I slide the window on sample to the right and try again. So far so good.
I live fairly close to Reagan National Airport and have an outdoor deck on the 13th floor of a condo building. So, to continue testing, I "replaced" the VSG with an antenna. My code no longer achieves synch lock and I am trying to elminate possible causes. The spec I have says the receiver needs to work with a receive signal strength of -91dBm at the antenna input. With the bladeRF set with the LNA at high gain (6dB) and its two VGA's both set at a gain of 30dB I can achieve synch lock down to a VSG output level of -60dBm. So, by the spec, I'll need more gain than the bladeRF can provide by itself and this might be one of the reasons why, with an antenna (it give 5dB of gain), I can't lock on to the signal. However, as close as I am to what I believe is a transmitter at the airport, I think I should be at a range from the tranmitter such that I don't need maximum sensitivity. But, the other reason may be that the sych bit sequence I've been using from the spec has been changed.
The synch sequence I am using is the binary equivalent of the hex value 153225B1D with the left most bit of the code tranmitted first. So, here are my questions:
1. Does anyone of a final copy of DO-282B to verify from it that my receiver is looking for the right sync code? In my spec it is provided in para 2.2.3.2.1.
2. Has anyone else used a bladeRF to receive the FIS-B (not the TIS-B) service? And if so, what gain setting did you use on the bladeRF? Did you also need to use an external LNA?
I also have all the necessary code written to pack bits in to bytes, de-interleave them, de-code the R-S FEC and parse the resultant bytes in to the spec's message format so I am very close here! If anyone would like to cooperate on getting this done, I'd be open to that too.
Thanks,
Ray
FIS-B ADS-B Receiver
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Re: FIS-B ADS-B Receiver
With those gains cranked all the way up, I'd actually expect to see your samples clipping.
Have you already saved off some real IQ samples via the bladeRF-cli or your software? I'd say it'd definitely be worth the time to do some post-analysis on them, perhaps for a few gain settings, to determine the magnitude of the IQ values you're seeing and perhaps to massage code using these real-world samples as input...
Have you already saved off some real IQ samples via the bladeRF-cli or your software? I'd say it'd definitely be worth the time to do some post-analysis on them, perhaps for a few gain settings, to determine the magnitude of the IQ values you're seeing and perhaps to massage code using these real-world samples as input...
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Re: FIS-B ADS-B Receiver
I have looked at both the raw sample values and the FFT amplitudes of those transformed samples and nothing seems to be over-ranging. Given the service's spec of -91dBm sensitivy (at input to antenna) and the fact that my demo code now only works with a VSG amplitude >= -60dBm, I suspect I need more gain prior to the bladeRF's quandrature down converter. I found a 17dB gain LNA from Mini Circuits (nicely packaged module, 5V input and sma connectors) for $40. So one is on order . . . a cheap experiment to potentially resolve the issue. We'll see.
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Re: FIS-B ADS-B Receiver
Oooh, nice find! Mind linking to its datasheet page here? Please do keep us all in the loop about that experiment.rayrv9a wrote:I found a 17dB gain LNA from Mini Circuits (nicely packaged module, 5V input and sma connectors) for $40. So one is on order . . . a cheap experiment to potentially resolve the issue. We'll see.
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Re: FIS-B ADS-B Receiver
Here you go: http://www.minicircuits.com/MCLStore/Mo ... 1153343855 This one is spec'ed only to 960 MHz but judging by the gain curves, it should work at my center frequency of interest, 978 MHz.
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Re: FIS-B ADS-B Receiver
What is the sample rate you're setting, and what is the bandwidth you're using?
You had mentioned a little over 1MHz symbol rate - I am guessing there is an RRC filter applied as well? What is the channel bandwidth or excess bandwidth of that filter?
You should be able to achieve lock at -91dBm output from your VSG. With a signal of -60dBm from a VSG and the gains all up, you should still see a very hot signal.
In fact, if you don't turn on the modulation, what are the levels of the counts coming from the ADC when the VSG is set to -60dBm output?
Brian
You had mentioned a little over 1MHz symbol rate - I am guessing there is an RRC filter applied as well? What is the channel bandwidth or excess bandwidth of that filter?
You should be able to achieve lock at -91dBm output from your VSG. With a signal of -60dBm from a VSG and the gains all up, you should still see a very hot signal.
In fact, if you don't turn on the modulation, what are the levels of the counts coming from the ADC when the VSG is set to -60dBm output?
Brian
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Re: FIS-B ADS-B Receiver
I am sampling at 31.25 MS/s which gives me 30 samples per symbol period of 0.96 microseconds.
Correct on the root raised cosine filter in the VSG but I am not sure of the channel BW. I am using a 7MHz IF BW with in the VGA-filter IF block of the bladeRF.
I've never looked at the A/D output values without modulation turned on but will have a look when I can get the VSG in my hands again and will let you know.
Correct on the root raised cosine filter in the VSG but I am not sure of the channel BW. I am using a 7MHz IF BW with in the VGA-filter IF block of the bladeRF.
I've never looked at the A/D output values without modulation turned on but will have a look when I can get the VSG in my hands again and will let you know.
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Re: FIS-B ADS-B Receiver
"What is the sample rate, symbol period, and IF bandwidth utilized in the bladeRF setup for this communication system? Furthermore, how does it impact the A/D output values when modulation is turned on?"rayrv9a wrote: ↑Mon Feb 09, 2015 1:33 pm I am sampling at 31.25 MS/s which gives me 30 samples per symbol period of 0.96 microseconds.
Correct on the root raised cosine filter in the VSG but I am not sure of the channel BW. I am using a 7MHz IF BW with in the VGA-filter IF block of the bladeRF.
I've never looked at the A/D output values without modulation turned on but will have a look when I can get the VSG in my hands again and will let you know.