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Spread Spectrum R/C System

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There have been some rumors the last year or so that the R/C makers would offer a CDMA/spread spectrum hobby R/C system. This technology promises to eliminate interference from other pilots and other sources. And much more. The only caveat is that they will have to use the ISM band's 2.4Ghz spectrum.

From the recent tease ads, it looks like modelavionics is close to offering something they call "Wi-Fli": http://www.modelavionics.com/WiFLi/default.asp

I think the CDMA encoding scheme is a great way to go. And the 2-way transceiver design opens up some convenient telemetry opportunities for us. But, I fully expect that the current FCC Part 95 rules will need to be amended before they can sell the system for unlicensed model R/C control use. That may take awhile.

I am disappointed by their Rx sensitivity spec. It is only -90dBm. I would prefer -100dBm or better. Also, the Tx power is only 0.1W, where as 1W, via the spread spectrum rules, is legally available for unlicensed use (or so I believe). This is possible because the average power of an authentic spread spectrum signal is essentially zero watts.

So, how many of you {that have 2.4Ghz RF experience} are comfortable operating your R/C model using a microwave signal? That is, do you feel that this new technology will offer good performance with the propagation and multipath issues that are common with a microwave RF system? {Fixed LOS operation is a piece of cake compared to our fast moving models} Would you feel confident enough to be one of the early adopters?

I'll start by saying that I will take a wait and see position before acting. I've seen too much performance variations with wireless data modems, cell phones, and 2.4Ghz video gear. Enough so I will stay on the side lines for now. But, my fingers are crossed that it is found to work more reliably than what we use today (which works pretty good as it is).

What are your predictions? What is your performance comfort level if they use 2.4Ghz? How much will you pay for the Tx module and the matching Rx?

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My biggest concern on 2.4Ghz is interference.

Everything is on 2.4Ghz these days.

Wireless Networks, Wireless Phone, Data Modems, our video equipment and so much more.

Personally I'm starting to think of going back down to 900Mhz for my next wireless phone simply because everything else if moving up. ;-)

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Me too !

If it is allocated its own bit of the band then it should be good. We dont need it here in the UK though as we have 459mhz.



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I would be very interested in getting such gear. Mainly because it will allow me to use compact high gain antennas on the ground and dramatically increase RC range (given that antenna continues to track the aircraft). If bandiwdth of RC channel is 10 KHz - this means sqrt(5MHz/10Kz) ~ 20 times longer range than video connection (just from narrower band). If I bump the power output to legal 1W limit - 50-100 miles range can be expected.

Interference with incoming 2.4 GHz carrier will be the biggest problem - so one has to move video TX/RX to another band.



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Good points Cyber-Flyer. I had pencil'd some range estimates the other day, which I can share in case anyone is interested.

I consider the MaxStream Spread Spectrum RF data modem module to be the gold standard for these sort of apps. At 2.4Ghz, it has 50mW of Tx power, -102dBm Rx sensitivity at 19.2kbits (-105dBm at 9.6Kbits), and a advertised fixed line-of-sight range of 3 miles with dipoles.

Normalizing this to the Wi-Fli's 100mW @ -90dBm (15Kbits) suggests that you will realize about 1.1 mile of range with the new R/C Spread Spectrum system. My math is simple: 3 miles reduced by 4 (-12dB) then bumped by 1.5 for the extra Tx power = 1.1 miles.

The Wi-Fli is advertised at 1.2 miles, so the math seems to work. But, the numbers are for fixed LOS applications and should be derated a bit for reliable mobile use. So, the 1.2 mile range would probably be under ideal circumstances.

Bumping the power to 1 watt would probably stretch you to 3 to 5 miles in clear line-of sight conditions (100mW range normalized with +10dB gain). Adding the amp may be as easy as hacking in a MMIC if the transceiver's antenna isolator/circulator is rated for the extra power. Of course adding the high gain antenna on the ground would stretch your mileage even further. I predict that 12-20 miles would be possible if you used a +12dBd antenna and kept out of the clouds. ;)

It will be interesting to see how the new Spread Spectrum R/C systems perform when it comes to reliable range. The variables are all over the map, so there may be some issues to overcome.

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Ok, I could add such a long posting here you would need a cup of coffee to

stay up late enough to read it.


I took a good look at the Wi-Fli system from modelavioncs. It does have some

properities I like, but it has many more I don't like . For a number

of reasons mind you.

+The antenna connection on the back of the RF module, it is in a

really bad place. You can no longer lay your transmitter down on it's back

without fatiguing the antenna connection. I personally don't want to have my

transmitter sitting around standing straight up, it is a recipe for

disaster. The transmitter can fall over onto the stick/switches or back onto

the antenna causing further stress to the antenna connection and the pins

that the R/F module plugs into.

+The Wi-Fli receiver has no external antenna. Without the ability to

have an external antenna you greatly limit the location of the receiver.

Some people think that balsa/plastic doesn't attenuate a 2.4ghz signal much,

it may or may not. From my personal experience it does effect the signal.

(Plus you are really outta luck if you have carbon fiber in your fuselage. )

+Model Avionics claim to be the first to offer a

production/commercial spread spectrum transmitter. They should check a

little before they make a claim like that. We (AUAV) have had a commercially

available Spread Spectrum 8 Channel R/C System available for at least 6

months and it was available in prototype/beta format long before that.

Besides that, they aren't even taking orders yet.

+They don't offer their system on 900mhz and it looks like they have

no plans to offer it on 900mhz. This forces current 2.4 video users to

900mhz video equipment since they are using the entire ISM band from

2,400mhz to 2,483.5mhz.

+No ability to toggle the transmitter in receive mode. This is very

useful when being used for flying UAV's so that once the airplane is

switched to autonomous mode the modem on the ground can then receive

telemetry data back from the airplane (Lat/Lon, temperature, pressure,

humidity, etc..).


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Obviously we think that Digital Spread Spectrum is viable for UAV usage. It

provides a secure method of remotely controlling a UAV with little/no chance

of having the UAV commands over-ridden by an outside source. The RF can

still be jammed as with any RF based system, the stronger signal wins.

Dave spent a lot of time and money designing AUAV's Spread Spectrum Control Link with the specific intent of it being used with UAVs. It provides a totally

different band that eliminates the problem of sending a UAV on a sortie only

to have it shot down by some kid with a park flyer in his backyard that is on the same 72mhz channel that your UAV is on. Going to ham channels is no better because who knows who is going to key up on 50 or 53mhz and lock out your signal.

Our system was thought up long before BPL (Broadband over Power Lines) was

brought up in this country. Now that BPL is an issue everyone is running to the 2.4 ISM band for their new control system. This will limit 2.4gHz because of the noise on the band. But the AUAV Digital Spread Spectrum is license free, and is available now on both 2400 and 900. Very shortly the 900 band will be emptying out as everyone moves to 2.4 and 5.8.


I wanted to highlight a number of features of AUAV's Digital Spread Spectrum

Control Link.

+We use transceivers, in the transmitter and in the airplane. This

allows for telemetry to be sent back at given intervals or continuosly once

a UAV is put into autonomous mode. The UAV can be switched back into manual

mode at any given time.

+We offer systems on 900 and 2400mhz. At both 9600bps and 19,200bps

in both bands. This allows for greater control resolution and more telemetry


+Our receivers have much higher RF sensitivity, our 900 mhz system

has a -110dBm sensitivity, the 2400mhz system has -105dBm.

+Both transceivers are equipped with external antenna connections,

this in itself allows for much greater range.

+If even more range is desired we can install amplifiers. Power

output can be increased from 140mW for 900mHz to 500mW or even to 1 full

watt. On the 2400mHz band the power can easily be amplified from 50mW to

800mW. Generally no amplification is needed though with the amazing receiver


+The 2400mHz version of our DSSS Control system uses the upper part

of the ISM band allowing channel 1 (2413mHz) of the 2.4 video band to be

used without causing any interference. We have run Blackwidows 3 Watt video

transmitter on channel 1 within inches of our DSSS receiver. It caused

NO glitches in our Spread Spectrum Receiver at either the transmitter

or the receiver.


We have flown a number of airplanes using Digital Spread Spectrum and have

even flown a helicopter using DSS. Details and photos can be found at


Matthew Klarich

Autonomous Unmanned Air Vehicles

Palmetto, FL


Edited by mklarich

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Your radio system sounds great. I too am not satisfied with the standard 72Mhz systems for UAV control, mainly due to the range limitations and lack of two way communication. It would be nice to avoid another set of radios to do the telemetry. I would be very interested in purchasing a 900Mhz system if they are for sale. Can you post specs such as receiver weight and power requirements ? And price.


Steve Marshall


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Val, how far do you want it to go?? We can transmit a full watt at either 900 or 2.4ghz plus we are using less bandwidth then video. Not to mention antenna gain, technically I believe it is 1 watt overall strength antenna gain included but.....


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I definitely think both systems have some great ideas going for them.

If I understand Wi-Fli correctly, your are still using your existing radio. Only replacing the frequency module. I think this is a nice feature that makes their product directed more to a mass market.

The AUAV system would seem more focused towards the UAV market where one might no longer care so much about having all the bells and whistles of your 9Z radio for instance.

I'll take one of each! ;-)

Edited by mikep

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Mike, exactly. We started developing this radio system long before BPL came up so we never intended it for mass market. I like the ability to drop a new freq. module in your radio and go but I still don't like where that antenna connection is on the back of their module. That module wasn't built to support that kind of load and those pins especially aren't meant for that kind of abuse as long and spindly as they are. Just my 2 cents.

When BPL came up a couple months we took a 9C and decoded the signal coming out of the buddy port. We then sent that through a spread spectrum 2.4 modem and to one of our receivers. Basically it does the exact same thing that WiFli does. We just weren't comfortable cramming everything into a module case and putting the stress on those pins. We can easily make a box that would sit next to you and you would run a cord from your buddy port. If great interest is shown we can have a case made that will attach to the back of your transmitter via some other method then relying on the module pins.



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Val, how far do you want it to go??

I wonder how my answer will help you with your answer???

Edited by cyber-flyer

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We can easily make a box that would sit next to you and you would run a cord from your buddy port. If great interest is shown we can have a case made that will attach to the back of your transmitter via some other method then relying on the module pins.

Personally I would probably go more for a solution like that. For one I imagine the production cost would be lower. Second, I actually do like my bells and whistle on my radio. :-)



Edited by mikep

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Val, with the stock rubber duck antennas that come with the modems we have successfully controlled at 2 miles at approx 500 feet. Never tried further but we can do testing if you so desire.


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