Adam Conway's Posts (6)

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Labor Day Weekend Quadcopter Build


My 5 year son executed a fantastic Labor Day build, so I thought I would share... 

Frame: Mead Heavy Weight construction paper in "X"

Autopilot: "authentic" purple APM 

GPS: Yup, that is the white thing.

Props: Orange

This quad also advances the state of the art by including a feather "to see how fast the wind is blowing" - no joke, that is what he said.  I may try that on my quad.

Anyhow, this made me laugh and I thought I would share, for those in the US have a nice labor day weekend - outside the US, have a nice weekend too.

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The Case for WiFi in Drones

3689529736?profile=originalIn full disclosure I work at a company that builds WiFi Access Points, routers etc for enterprises and I just put a hotspot on a drone for fun

It is hard to imagine now, but Ethernet as we know it today (802.3) was not the obvious choice for wired performant connections in 1996, many competing technologies like ATM, Token Ring, RS485, 802.4 etc seemed like more reliable choices, but because Ethernet was fast and cheap (among other reasons) it won out - since then it is hard to imagine a new technology displacing it.  
Like Ethernet, WiFi is in the process of subsuming all adjacent technologies (802.15.4, Bluetooth, etc) and for all connections beyond the WAN (which will likely be LTE) it has the ability to be lower power, have better range (within reason) and perform better.  This is not because WiFi is necessarily better spec'd - in fact it is unnecessarily complicated - it is because the R&D dollars are focused on it rather than competing technologies.  A modern 3x3:3 802.11n chip has massive DSP resource and with .11ac we are seeing double the compute inside the mac/baseband.  In addition we are seeing huge improvements in frame detection, phased antenna arrays and RF front ends which has the consequence of increasing link budget (link budget is roughly analogous to signal strength).
Okay, so where am I going with this: as a community we should be looking to use WiFi to consolidate our communication paths with our drones rather than 900MHz telemetry, 2.4GHz remote, 5.8GHz analog.  Only fully integrated products like the Parrot AR drone use WiFi and I think that is a mistake.  Here are a few reasons why WiFi makes sense:
  • We put a digital RX with -95 db sensitivity inches away from a video transmitter transmitting at 23 dBm (in my case, or if you go illegal 36dBm) - this desensitizes our RX significantly reducing link budget.  While this is one example, putting a high power transmitter next to a high sensitivity receiver is a bad idea always and we do it 3 times on drones.
  • We need to deal with varying ranges and qualities of TX/RX pairs, complicated antenna and ground station setups.
  • Analog video is incredibly susceptible to multi-path, limiting quality and antenna types.  Don't we all want HD?
  • Wouldn't it be convenient to just have one communication path? with maybe a remote as a backup.
Because there are naysayers (correctly so) out there, I thought I would point out common arguments against WiFi:
  • WiFi requires a much higher SNR than 802.15.4 - this is true, but the quality of receivers and transmitters along with techniques MRC overcomes much of this and gives you a link that has a much lower duty cycle making it less susceptible to interference and provides superior error checking. A distance of 2-4km is totally practical with WiFi assuming good radios and modest antennas (we do a lot further at my company without too much issue)
  • Using WiFi requires IP - yup… but does anybody think this isn't inevitable for us?
  • WiFi is unreliable - okay, this needs a 2 part answer - WiFi is actually extremely reliable… it offers very good error checking (CRC) and will do 9 retries before dropping a frame at which point the higher level protocol will also send retries.  On the flip side, we are often used to the crappy WiFi found in an Acer laptop (not picking on Acer, just mentioning a random pc vendor) where the radio is placed poorly, the analog front end is poorly designed and the antennas are in odd positions - remove those limitations and buy a $20 card from a reputable manufacturer and you will find superior performance.
  • WiFi is really hard to integrate, especially when you need connections beyond serial speeds - I actually dont have an argument here.  Most WiFi cards today use a soft MAC which means that most of the radio runs in software - replicating that in Arduino is infeasible.  This is why the Arduino Yun uses a linux and a SoC to deliver WiFi.  The obvious fix would be to put Autopilot on a linux based platform like OpenWRT, Android or Raspberry Pi
I don't think that it is a question of whether WiFi will become prevalent on drones, it is only a question of when.  In the meantime I am working on my WiFi drone, I have 4G and WiFi on mine, Telemetry and Video are the next pieces to get working.  I will update as I progress. 
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Bad news for 2.4GHz


It looks like AT&T is looking to use 2.3GHz bands reserved for Sirius radio.  Those in the WiFi space know that Terrestrial repeaters of Sirius radio signals are a major source of outdoor interference - but those repeaters only existed in dense cities.  Now with that spectrum moving to 4G Basestations we will see much more interference in on open areas (like those we like to fly in). This will likely raise the noise floor and reduce the range and reliability of communications in 2.4 GHz.  72 MHz anyone? 

source engadget

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XBee range calculations

I originally posted this as a discussion but given the new guidelines for posting, I thought I would post it as blog. Also anybody who saw this before will find that I discovered a few important things (like the minimum SNR for zigbee). The spreadsheet that I created I left this as an editable Google Docs sheet so please don't save as you play with it.

With this spreadsheet you will find that in some cases it would be possible to transmit 10km, with the right antennas further. This assumes an XBee PRO 2.4 GHz.

There are two important caveats you should know:
1) I am not an engineer, I only play one on TV - seriously though, I am in marketing, and I haven't derived an equation in a decade - so check my work if you feel like it. That said, I do work at a WiFi company, so at least I know how RF is supposed to work.

2) Digi is not forthcoming with the radio properties of their XBee module, this isn't surprising; with radios it is rare to publish this sort of information. I found the best information in IEEE research papers outlining Zigbee. the XBee could be worse or better than stated.

Anyhow, enjoy the spreadsheet.
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