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Wireless System for UAV Video Feed by Mike Everest

Q: What I am wanting to do is extend the range of some a cameras with built in WiFi (a GoPro 3 for example) to something of the order of ~200m.

The difficulty lies with that fact that these cameras act as an (unconfigurable) WPA2 access point and I would like to connect to the access point via WiFi from a tablet. The stock range of the setup is <5m. 

I believe I need to setup something like a transparent WiFi repeater. I am aware that WiFi repeaters are usually not the preferred method for network infrastructure solutions but given the low bandwidth requirements it may be satisfactory?

The GoPro has an internal chip antenna an the output power is roughly 20-40 mW. 

The complication is that I need to be able to talk to the camera via WiFi as I am using a Android tablet/smartphone that cant use a USB WiFi module or Ethernet port; using a PC would have been far simpler but doesnt have the battery life or daylight visibility

In terms of using a MikroTik as a repeater, would this only require one MikroTik device (I.e. a SXT Lite2) or would I need two and have to bridge them via Ethernet?


A: The GoPro, then, is your primary challenge.  It will make absolutely no difference what kind of antenna you use on the ‘receiver’ end because you have to deal with the maximum limit allowed legally in Australia – that limit is 36dBm (or about 4 watts) of EIRP.

Here’s some rule-of-thumb calculations: 

1. free space loss (the reduction in signal as it get’s further from the antenna) over 200m is about*:
a. For 2.4GHz,  86 dB
b. For 5.8 GHz, 94 dB

* There are lots of free space loss calculators around – just enter ‘free space loss calculator’ into your favourite search engine to find some!

2. Maximum legal output from the antenna for both bands is 36dBm

3. If your receiver is 200m away, then the available signal is:
a. For 2.4GHz, 36 (tx at source) – 86 (fsl) = -50dB
b. For 5.8GHz, 36 – 94 = -58dB 

4. Assuming that the GoPro has a unity antenna (essentially approximate ‘point source’) then to get any reasonable signal, that radio will need a receive sensitivity of about:
a. -60 for 2.4GHz
b. -68 for 5.8GHz

Typical receive sensitivity for production wifi chipsets these days is around -90 to -92, so I think that you are well in the right place for the link from you TO the remote.  

Note that we assume a noise floor LOWER than the receive sensitivity – i.e. little or negligible activity by other wireless systems in the area.  This may or may not be true ‘on the day’ – you should try to inform yourself about what to expect in every operational case!

In fact if receive sensitivity is indeed -90, then you will be able to manage up to -80 signal level for a useful link, which works out to about:
a. About 6Km for 2.4GHz
b. About 2.5Km for 5.8GHz

The other way around is more important though – you want to make sure that you have a stronger signal because from you to the device only requires control data (commands, connection setup etc), and therefore you will want at least 20dB over the noise floor (or RX sensitivity) and so you want to deliver at least -72 dBm to the near-end radio receiver.  Doing the same calculations in reverse: 

1. FSL over 200m:
a. 86 (2.4GHz)
b. 94 (5.8GHz)

2. Transmit power of GoPro = 40mW, 16dBm plus 1dBi antenna = energy output of antenna = 16

3. Available signal at the receiver end (near side):
a. 2.4GHz: 16 – 86 = -80
b. 5.8GHz: 16 – 94 = -88

4. In order to deliver -72dB to the radio receiver, you need antenna gain of:|
a. 8dBi (2.4GHz)
b. 16dBi (5.8GHz)

Once again, this is quite achievable (assuming ideal conditions) – going for a higher gain antenna will offer some elbow-room in case of some background noise or interference from other nearby activity, or for longer range.

Answer to your questions about using the same device to also connect your local controller/monitor system are not as straightforward as it may seem.  The problem is that wireless is a shared medium, thus your android tablet ends up ‘competing’ for airtime with the goPro.  On a wireless channel, only one device is abloe to send data at any goven time – multiple transmissions effectively cancel each other out, and no data transfer is possible in these cases referred to as data ‘collission’) Since wifi is a decentralised protocol (i.e. there is no ‘controller’ system that dictates which device is allowed to transmit at any given time) the way that network nodes avoid simultaneous transmitting is by first ‘listening’ to the airwaves when that node has data to transmit: if some other node is detected as transmitting, then the device will wait until the transmission has ended before proceeding to send.

The problem is that although a suitably designed AP system will be able to detect both transmissions by GoPro and Tablet, the tablet is not able to detect transmissions by the goPro (after all, that is why we are even having this discussion! ;-) and the goPro will almost certainly be unable to detect transmissions by the Android.  Therefore, it is highly likely (in fact almost certain) that there will be occasions when both devices attempt to transmit data at the same time.  When that happens, the transmission burst is aborted, and it will start over.  The end result is that network throughput degrades significantly - depending on the nature of the traffic, maximum throughput can be degraded by more than 95%.

Since your primary application is streaming video, I expect that the chance of collision is so high as to be considered 100% certain that it will cause you problems!

Therefore, I recommend that a better solution would be to operate the ‘receiver’ unit as a stand-alone client connecting to the GoPro over distance, and then use the Ethernet socket on that device to connect a small AP to connect the android tablet (and many other near devices if you need to – or want to ;) 

Regarding product selection, based on the above calculations, SXT would be suitable for both bands, as it has a 14dBi antenna (2.4GHz) or 16dBi for 5GHz model:

If you want some higher gain for better performance, then consider Sextant for the 5GHz band, or Groove with 20dBi panel for the 2 GHz band:

For a local AP, I suggest RB951-2n as a low cost unit suitable for that application, or 951Ui which also supports PoE-out on ether5 so that you only need 1 power supply for both devices:

I hope this info helps to answer your questions, and to help you wrap your mind around the general considerations for this kind of thing.

Further questions, comments, suggestions, are welcome!


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