Some Thoughts On DJI Controller Signal Interference 

     Since getting my DJI P3P I have spent a lot of time reading about all the losses of signal and drones falling out of the sky for no good reason in many cases.  This has caused me to be extremely cautious, when it come to flying my P3P.  I have had several very successful flights, in my front yard, as I was starting to get the feel of the controls.  The last time I took it out, I did all the preflight stuff, put it on the ground and walked back about 50 feet from the P3P.  I noticed that at this small distance the DJI GO app was showing loss of signal!  Now how can that possibly be?  I walked toward the P3P and about half way there is was connected and everything was work just fine.  I walked back away and found that about 40 feet away, I would get loss of signal.  I walked in a circle, keeping about 50 feet away, and there were only a couple of spots around the circle where I had a good connection to the P3P.  There was no way I was going to start the props and leave the ground this time out.

     Fast forward a couple of days later.   Put the P3P on the kitchen counter, setup the Controller on the dining room table and fired it all up.  Everything worked perfectly.  So, took the P3P outside, did the compass setup, placed it on the ground and walked away.  Got about a 100 feet away and still had a good signal, everything was working as expected.  I had done nothing to change the difficulty with connection that I was having a couple of days ago, so what in the heck had changed in the past two days.  I took it up a few feet and out about 400 feet, everything worked perfectly.  At this point I was pleased to be able to fly it, but still afraid of having loss of signal and see it fall to the ground.  So shut it down and brought it back into the house.  Time to do some research and try to find out what was going on the day I had loss of signal at a very short distance.

     Weather really turned nasty, have had lot of time to spend reading on the PhantomPilots discussion pages.  I stumbled across a very nice discussion on LightBridge communications by user  "Phamtoming" where he did a lot of analysis of how the P3P handled communications between the Controller and the P3P.  His discussion and spectrum analysis pictures of the LightBridge signals. That made the light come on!  I have a pretty extensive Electronics Lab, right in my work shop and one of the key instruments in my shop is an HP 8595E, fully loaded Spectrum Analyzer.  So, why not look at all the various signals that are coming from the pile of equipment that makes flying the P3P possible.

     I put the DJI Controller and P3P on the bench, a couple of feet apart.  I ran a coax cable from the SA input  to a small Agilent 355D VHF Attenuator.  I fashioned a 3" antenna and pushed it into a right angle BNC connector attached to the input of the attenuator.  (3 inches is about a full wavelength at 2.4 GHz)  I now have a nice vertical antenna that I can place near the various pieces of equipment that I am going to test for 2.4 GHz signal output.  And if necessary I can attenuate any signal up to 120db if necessary to keep it within range.

     Here is the lineup of equipment that I plan on looking at the output signal on.  The DJI Controller, the P3P, the Samsung 9.7" TAB A tablet, and my Samsung ATIV S Neo, which is my trusty Windows 8.1 Cell Phone.  All of these devices are what I have with me when I go out to fly the P3P, and they all use the 2.4 GHz band in one way or another, so I think I have all my bases covered, at least to start with.  As I suspected that something was interfering with the signal coming back from the P3P to the controller, I wanted to see just exactly what each of the devices was doing on the 2.4 GHz spectrum.

     So my plan is to do a 1 minute or so movie, showing what each piece of equipment is putting out on 2.4 GHz.
The first movie will be to establish the Baseline for the upcoming measurements with the Spectrum Analyzer.  I will do it with all of  the above mentioned  2.4GHz equipment turned off.

But, before the first movie, take a look at what the 2.4GHz band looks like.  I found the following on the Internet, and think you can see how easy it would be to have interference between signals in the WiFi, BlueTooth, and LightBridge spectrum.  It is a pretty small and crowded piece of spectrum, with a lot going on from many sources all the time.


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         Please watch the movies in their entirety, they are all less than 2 minutes in length.        

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I do see a few low level signals show up from time to time, they are broadcasts from my in house WiFi router.

      

Next is with only my Cell Phone WiFi turned on.

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     WiFi on Cell Phone is turned on.  Cell phone is about 18" from controller.  About how it would be if I had it in my shirt pocket, where I always carry it.  Spectrum Analyzer antenna about 12" behind the controller.  Is the same for all the measurements shown on the rest of the videos.

 

      

Next is with only my Cell Phone BlueTooth turned on.

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      WiFi turned off, BlueTooth only turned on.  It puts out a pretty good train of pulses.  Study the pictures, back up on the page, to get a better idea of what the WiFi and BlueTooth Band Plans look like.

     

 Next is with my Tablet WiFi turned on.

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    I am quite surprised to see how small the WiFi signal is from the tablet.  The Spectrum Analyzer antenna is much closer to the tablet than the Cell Phone in my shirt pocket.

      

Next is with my Tablet BlueTooth turned on.

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     Surprised to see how much larger the BlueTooth signal is on the Tablet, compared to it's WiFi signal.

      

Next is with my DJI Controller turned on.

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     This big signal is coming from the Controller WiFi signal, it is polling the spectrum, looking for the P3P.
Every now and then you can see a smaller signal pop up in different places on the spectrum.  That could be the Spread Spectrum LightBridge signal looking to lock on to the Video System in the P3P.  Just guessing, but it is definitely coming from the Controller.

      

Next is with my DJI Controller and the P3P turned on.

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     Finally a complete picture of how the Controller looks locked on to the P3P.  Note the 10 MHz wide signal with the modulation showing on top.  That is the LightBridge video system signal, coming from the P3P.   I am looking at a spectrum that starts at 2.4 GHz and ends at 2.5 GHz.  That makes the division on the screen 10 MHz wide.  Clearly we are seeing the LightBridge Video signal from the P3P.  The other signal you see is the polling of the WiFi communications system between the Controller and P3P.  That is the data stream that carries information to and from the P3P, so the P3P know what you are telling it you want to do.  And in return, gives feedback to the controller showing it is following your commands.  The LightBridge Video signal is nice and steady and firmly locked in it's position.

      

Next is with Tablet & Phone with both having WiFi & BlueTooth turned on.

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     Finally time to get to the bottom of what I think is happening between my devices,  that is disrupting communications between the Controller and the P3P.  So I did a lot of tests, turning WiFi and BlueTooth on and off on both the tablet and cell phone, while moving the P3P to different locations in my house.  What you are going to see in this video is a complete breakdown of communications between the Controller and P3P.  I ended up with the controller in the basement and the P3P in the furthest 2nd floor bedroom, just as far away as I could get it in the house.  If I had to guess, I would say about 50 feet, with the signal going through a lot of walls on the way there and back.
    The first thing you should notice is that the LightBridge signal is considerably less in amplitude, as is the WiFi control signals coming back from the P3P.   Watch as the LightBridge signal channel location keeps changing, toward the end of the video it does settle down a bit.  The video on the screen of the Controller Tablet was breaking up in the early part of the movie, and the WiFi communications between the Controller and P3P was lost and regained several times along the way as well.

      

Next is with only the Controller and P3P on, all WiFi & BlueTooth turned off.

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      Here we are, about 5 minutes later looking at the same signals coming to and from the P3P.  Everything is working perfectly, great pictures and able to get command information to and from the P3P with excellent signal strength and no disruption of the signal.  The only thing that has changed is that I do not have any WiFi or BlueTooth turned on, either Tablet or Cell Phone.
    During the 5 minutes between the last two videos, I systematically turn on and off first WiFi and then BlueTooth on the Tablet and Cell phone.  And I finally found the culprit that is disruption my communications!  It is BlueTooth ! ! ! !   Every time I turn on my Cell Phone BlueTooth, standing near the controller, the signal between the Controller and the P3P gets disrupted as you saw in my previous video.  If I back away from the Controller, 5 to 6 feet, the signal becomes stable again and the connection to the P3P is again solid.  Interesting that the BlueTooth signal from the Tablet, when is right on top of the Controller antennas, is not causing the same problem.

     Some guessing here, as the Tablet does not disrupt the signals to the P3P, my thought are that internally in the Tablet there must be some kind of communications between the BlueTooth connected signal and the WiFi signal.  Since they are both operating in the same band, and from the same frequency source, that would make some kind of sense that one does not disrupt the other.  But again, just guessing!
  
      So from all of this I conclude that my Cell Phone, on BlueTooth, is blocking the signal from the P3P to the Controller.  Now I am wondering just how many of those who have had problems flying their DJI Drones, carry a cell phone, and have BlueTooth turned on!  Could be that my particular phone is the  exception, but I have proven, at least to me that I will never fly with EITHER WiFi or BlueTooth turned on anywhere near my DJI Controller when out enjoying flying my P3P.  From now on, my Tablet & Cell Phone will be in "Airplane" mode, BEFORE I start the Controller and the P3P!

      In conclusion, first off I am not any kind of an expert in these matters, just a many year retired Electronics Technician in many fields of study, who likes to play around with all things electronic!  Will be 80 years old in April, still having fun!

73...George - WB8BGY

 

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