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Phantom 3 Lightbridge Frequencies

Discussion in 'Pro/Adv Discussion' started by Phamtoming, Aug 23, 2015.

  1. Phamtoming

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    Recently, I have taken a little time to profile the RF systems for the Phantom 3 "Lightbridge" system.

    I have unlocked the application installed on my mobile device to be able to select all 32 Channels that Lightbridge offers. It should be noted that the channels outside the 2.4 Ghz unlicensed ISM band have been locked out by the more recent versions of the application to prevent illegal operation outside the ISM spectrum. Using a Rhode & Schwarz DDF007 receiver I was able to get a pretty good idea of how the system functions within the RF spectrum.

    The Phantom 3 Advanced and Professional seem to be using two separate RF link systems. One system for control of the aircraft which operates in the 2.4 Ghz ISM band (exclusively) for control uplink, using spread spectrum and another system that can operate anywhere from 2.28-2.6 Ghz which is a GFSK (Gaussian frequency-shift keying) digital video signal for downlink of the video image. Both uplink and downlink systems appear to be using MIMO (multi-in-multi-out) diversity antenna systems with an RF output power collectively at ~750 mw for each system.

    Below is a list of frequencies the GFSK video downlink system uses corresponding the the DJI "Channel" number. These are 10 Mhz wide channels with the center frequency being listed and 5 Mhz being occupied on either side of center.

    CH 1: 2.285 Ghz
    CH 2: 2.295 Ghz
    CH 3: 2.305 Ghz
    CH 4: 2.315 Ghz
    CH 5: 2.325 Ghz
    CH 6: 2.335 Ghz
    CH 7: 2.345 Ghz
    CH 8: 2.355 Ghz
    CH 9: 2.365 Ghz
    CH 10: 2.375 Ghz
    CH 11: 2.385 Ghz
    CH 12: 2.390 Ghz
    CH 13: 2.405 Ghz
    CH 14: 2.415 Ghz
    CH 15: 2.425 Ghz
    CH 16: 2.435 Ghz
    CH 17: 2.445 Ghz
    CH 18: 2.455 Ghz
    CH 19: 2.465 Ghz
    CH 20: 2.475 Ghz
    CH 21: 2.485 Ghz
    CH 22: 2.495 Ghz
    CH 23: 2.505 Ghz
    CH 24: 2.515 Ghz
    CH 25: 2.525 Ghz
    CH 26: 2.535 Ghz
    CH 27: 2.545 Ghz
    CH 28: 2.505 Ghz
    CH 29: 2.565 Ghz
    CH 30: 2.575 Ghz
    CH 31: 2.585 Ghz
    CH 32: 2.595 Ghz

    As you can see, many of these channels are clearly outside the 2.4 Ghz ISM band, which is why they are locked out of the application. I have attached some photos of the spectrum analysis of these systems.

    Image 23 Depicts the video downlink signal (10 Mhz wide peak)within the 2.4 Ghz ISM band with the controller FHSS signal peaks on either side.

    Image 09 Depicts the controller operating within the 2.4 ISM band on the left (spread spectrum) and the video downlink frequency (1o Mhz wide) operating above the 2.4 ISM band in one of the "upper channels" on the Lightbridge system.

    Image 16 Depicts the video signal operating just outside the 2.4 Ghz ISM spectrum with the controller occupying peaks spread across the 2.4 ISM band.

    Image 22 Depicts the controller signal hopping across the 2.4 Ghz ISM band with the video downlink channel not shown.

    Image 23 Depicts the video downlink operating inside with 2.4 Ghz ISM band along with the controller's frequency hopping activities.
     

    Attached Files:

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  2. Michael Collins

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    How do we unlock our systems to run such tests also?
     
  3. Phamtoming

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    However, keep in mind operating outside the 2.4 Ghz ISM band is illegal unless you are a licensed HAM (for the lower 2.3-2.4 Ghz channels) and operating above is quite simply against FCC regulations. Use at your own risk. :)
     
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  4. Raph

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    Thanks for the write up. Can you please explain how the control uplink works? It only uses 1 frequency? What if there is interference on that channel?
     
  5. west drone

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    what are the channels above used for here what in the states?
     
  6. Recce2

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    In your opinion, how much has the Lightbridge System been degraded by the loss of the extra channels?
    Thanks
     
  7. SENC

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    Great information, thanks for sharing. There was a rather lengthy discussion of this a while back (look for the more channels hack thread, I think) with some additional analysis that yours validates and adds to, in my opinion. I think it is particularly useful to confirm that control of the phantom only occurs in the ISM spectrum - it helps lessen the temptation to use other frequencies when the only gain is video and not control.

    @rrmccabe @BudWalker @tomtnt @n6vmo @N017RW @hidaven
     
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  8. Darmie

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    Great information! Now, amy word best ch to use for best range considering stock cut antenna length?
     
  9. Phamtoming

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    No, the control link uses the entire 2.4 Ghz ISM spectrum. It is a frequency hopping signal that covers from ~2.406 - 2.476 Ghz continuously. The signal occupies the entire band and when you are also using that band to do your video downlink on, the controller seems to skip that portion of the band in order to eliminate interference. I haven't dug too deep into the chipset used for the transmitter yet, but it almost appears adaptive to help eliminate interference...although it was clearly hopping over WiFi AP broadcast beacons in the area. It may just adapt to the video downlink signal of the Phantom. I think Photo #10022 attached to my original post demonstrates what the signal is doing nicely. If you look at the waterfall display (which represents what the transmitter is doing int he band over TIME, you can see the hop sequence from the controller doing it's "shift" across the band. Of course the controller could be using Chirp Spread Spectrum or another protocol but it appears to be FHSS from what I could tell.

    https://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum
     
    #9 Phamtoming, Aug 23, 2015
    Last edited: Aug 23, 2015
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  10. Phamtoming

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    I haven't hooked and antenna analyzer up to any of the antennas yet to measure the return loss, but if you're sticking to channels within the ISM band it really shouldn't make much of a difference in regard to the antenna. The antennas on the Phantom and Controller seem to be pretty broadbanded. Where channel selection will make a difference is if you have a lot of WiFi or cordless phone signals concentrated on one particular side of the ISM band, you can go to the other "side" so you have less RF to compete with. Remember changing the "channel" seems to ONLY affect the VIDEO DOWNLINK side, as the controller is FHSS and will hop everywhere in the band.
     
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  11. Phamtoming

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    Yes, changing channels seems to only move the operating frequency of the video downlink. In field tests, I have found that the "clean" channels don't seem to really offer too much more in terms of overall range of the video downlink side of things unless I'm close to a strong source of interference from WiFI, etc. Changing the channel ONLY changes the VIDEO side of the system...your control signal will continue to hop across the ISM band only.
     
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  12. Phamtoming

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    Not too much. If you were in a very heavily saturated RF environment (like a room with several WiFi AP's around you, etc., ) it would likely degrade the quality of your video downlink, but overall it doesn't seem to make a huge difference in field tests where "auto" channel selection is implemented in a average suburban environment.

    Loss of the additional "channels" has NO effect on the control link side of things, it just gives you fewer clean places to put your video downlink signal. If you had an environment where most ALL the 2.4 ISM band was occupied by WiFi, etc., not being able to escape the ISM band would indeed effect how well you could see the video downlink as distance away from the video downlink transmitter increased.

    Also remember from the control uplink perspective side of things, just because the 2.4 ISM RF environment where the operator is standing is "clean" that changes dramatically for the Phantom when it flies over a plethora of 2.4 Ghz sources. The Phantom relies on being synchronized to your controllers hop sequence in order to overcome all the noise at 2.4 ISM.
     
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  13. Youngbill

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    Interesting stuff.....thank you.
     
  14. Shammyh

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    Third one to the party.

    http://www.phantompilots.com/posts/485242/

    Looks like your results mirror mine and the other guy who also ran it through a frequency analyzer.

    I was too lazy to post photos though, so kudos for that!
     
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  15. ianwood

    ianwood Taco Wrangler
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    Nice work! Were you able to confirm it being FHSS vs. DEST/FASST?

    Another interesting thing to note is that the reason the control channel can coexist with the video downlink in the same frequency range is through the use of time division multiplexing.

    What I wonder about the use of TDM is when two Lightbridge units come into range of each other, do they broadcast sync frames and align with each other?
     
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  16. Phamtoming

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    Looks good Shammyh. It was nice to check my research against yours and see we're on the same page! That makes me feel better about accuracy of the data.

    I have some much better photos of the spectral analysis, but I need to edit my GPS location out of them before posting (hence me only posting the photos of the spectral analysis where the parameter entry screen is still shown..lol)
     
  17. Phamtoming

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    Yes, it would be interesting to see if they took each other into account when deciding how to set up the TDM. I did note that the system I evaluated didn't seem to try to avoid the high power WiFI AP's near me and just hopped over them probably hoping the speed of the hops and error tolerances would overcome the RF interference.

    For the sake of flying several Phantoms in the same area this would be interesting to know, especially if the controllers were far enough apart not the see each other, but definitely see the 10 Mhz video signals coming from the other aircraft with altitude.

    I wonder if my controller would see the video peak from a "foreign Phantom" and adjust itself to that as well as my video stream. This is where the "out of band" channels might come in handy.

    What is even more remarkable is that the Phantom is selective enough to still see the FHSS control signal WHILE TRANSMITTING video right in the middle of that band. I am wondering if the video signal reduces sensitivity of the ISM band control receiver when running ont he "legal" channels.

    Theory says putting the video signal as far from the ISM band would make for best performance for both video downlink and control uplink, but honestly I haven't seem much performance differences during live field tests.
     
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  18. Harleymhs

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    Thanks Guys, I just installed the hack on my IOS. Got all the channels now.. Ill see what happens now! Thank you! All good replies here!
     
  19. flyNfrank

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    Had you been on vacation or something that you recently just came back? It's hard to imagine that you spent all the time and effort in this when it was already available on some of the websites.

    Had I been in your shoes, it would have been a big blow to me to finish the details and then find out it already existed. I'm saying this because this really is a big deal mod. I'm sure I would have gotten a little sick over it.

    If will make any difference, and regardless of anything completed before hand, I do appreciate your time and efforts you put into this. Now, with saying that, if you were able to say dissect the P3's .DAT files completely, or figure out how to open up the settings to adjust speed, or the rpm's the props spin, or adjust the descend rate, ect, you would NO DOUBT be the all time HERO.

    Either way.....thanks again!
     
  20. Harleymhs

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    I copied the config file to my IOS device from itunes,. But when I go to ADD file it doesnt see the file, I enabled the hidden files so what I did was dragged the file into the files in itunes in the pilot app files, its there but I cant see it.. If I want to remove it can you tell me how to do this if I want to go back to the stock app .. Like I said its there and the MOD is working but for some reason the file is still hidden in itunes.. How would you remove the file so I can go back to the app with NO mod? Thanks for your help if u can.,