Wednesday, March 08, 2006

2,4 Ghz DSM radios

First from Wessie vd Westhuizen:

DSM 2,4 Ghz radio legality

Just spoke to Bob Skinner, so you can take this as the OFFICIAL SAMAA standpoint.

The Spektrum system DSM radios are 100% approved, and you are covered insurance-wise if you are using these systems.

SAMAA reccomends the following :

That Clubs have a GREEN area on their peg boards, where DSM users can place their peg, to indicate that wide spectum users are flying. (this is their only must-have provisio, Club managers take note)

That no more than 4 DSM systems are in use at the same time at one field, as the range of these systems is influenced by the number of users.

That's it.
From a HOBBY SHOP and FLYER point of view, we encourage the DMS systems to be used for PARK FLYER applications only.


Now a technical view from Mike vos:

I have a lot of telecom experience in the 2.4GHz band...or shall we say headaches or maybe even nightmares.

Tom, you made a good comment…do you need a peg for your Blue Tooth? What is that ­ I thought someone mentioned the colour green. I think rather use red. Or polka dot or stars, because you will see more of that…..maybe green was the right colour….The Grass…oh so painful.

The 2.4GHz band is known as the ISM band. Set aside for Industrial, Scientific and Medical apparatus generating noise. Any device in this band must accept the noise levels of the band ­ not protection will be provided.

Although it sounds quite romantic the way these sets select an open channel and avoid the interference consider the full picture before you sell your 35Mhz on next week's hot specials. Equipment like these radios, wireless video cams, WiFi, BlueTooth other wireless links and network equipment could not be properly managed by the regulators so they said go manage yourself. It is something like opening all 6 lanes on the N1 (Pta to Jhb), yes North Bound and South bound and you go pick your "free slot" and get to the other side. You might just get there. But size is going to matter.

Consider this: The radios probably transmit something like 100mW, similar to WiFi equipment. (the legal approved ones ­ not the ones people put up with large grid pack antennas making them illegal in power output) These devices look to each other like noise. So the more radios the more noise and thus distance comes down. Maybe this is where the park flyer caution comes in…stay close. Now….Switch on your Microwave Oven! 1000 Watt!! 10 000 times the power, slap bang in the 2.4GHz band. The pie is going to be nice and warm you are going to buy now just after you pick up the pieces. There are illegal links running many kilometres and ICASA can't close them all down, they pop up everywhere. So with 4 pegs on the green board….what about the link running over the club's flying field drowning your radio's signal when you pass through it.

To go back to Tom's comment. Why even put up the green space? Why stick to a club's flying field? Any one can switch on his Bluetooth, WiFi router or Microwave oven without telling the neighbours.

So be careful ­ this band is VERY much FULL of INTERFERENCE but only looks like noise and as long as you can see a bit of signal you maybe ok. I will certainly not try it out just yet. The radios do seem to have great features.

I just think that after the first couple of pilots are going to try those on planes this group is going to make nice entertaining reading, please add pictures and video….uhm, will that mean then just 3 planes in the air? Just do not mail it with Bluetooth then only 2 planes can fly.

1 comment:

Anonymous said...

the major difference between stocastic process on a single channel and what DX6 spread spectrum does is that it sends the same signal over multiple channels many hundreds of times. A vote is taken for each "chip" to determine if that bit was "on" or "off". It is allowed to have a HUGE NOISE threshold upon which each "chip" lives. This is why you can have gargantuan number of Bluetooths (or should it be BlueTeeths??), video cameras, microwave generators, cell phones, etc all operating in the same relative band and still get the signal through. There is no one channel (or even two for the 2 Rx units in the DX6) that gets the signal. There are many 1000's of readings at different frequencies that are making up the pulse width of the throttle channel (for example).

So the entropy in Shannon's theories are distributed against many 1000's of signal channels in the hopes of getting a good "vote" as to whether a servo channel was at 1.6 milliseconds or not. The error coding use of the time domain to expand Shannon's information content as the attempt to decrease the S/N of the process.

That being said, how does this affect the performance of the DX6. I personally had my doubts when I saw the first prototype come to our field in Silicon Valley (where we have enough RF noise to insure at least one crash per day from a "locked up" radio). When I heard Paul Beard say it was just 100 mW total power, I could not believe it could control 6 channels with a 3 millisecond response time. But his helicopter flew. I talked to Cypress and other "building block" manufacturers and there response was: "you are only getting 6 channels out??? You should be getting a thousand channels out of 1024 bit resolution! What are you doing wrong?"

Well, it took some "adjustment" to the standard Shannon principles to understand that we are trading "time" for "channel bandwidth", but it all works out. So I was happy with the "theory" part of the operation. Then I ran a few experiments. I got some 3 watt 2.4 GHz video channels, some Bluetooth "test units" that go way over the legal power limit, a couple of microwave generators, and a few homemade "goodies" to sweep the frequency band in the 1-2 watt power envelope.

What I got out of this testing (done late in the day when no self-respecting flyer would be in the air), was that I can put a noise platform in the 2.4 GHz band that is 50 times the signal level that I calculated the DX6 was giving to the Rx at roughly 100 feet. The Rx hooked to 4 servos worked just fine. No glitches!!

What did happen when I got closer to the Rx with my signals was that the response time went way down (like in the 1/2 second range). Apparently when you have enough threshold noise, is that the "voting" process for "chips" slows way down ---- or could stop altogether if you hit the antenna with direct 500mv baseband signals. The "voting" process would say that it could not decide that a "signal" with proper ECC was present and just "hold the last signal" or whatever the failsafe was programmed to do.

So what kind of power levels are we talking about here? My best guess from the readings I got was that I would need a 500 Watt microwave oven signal that was sweeping the band of interest at a distance of 3 meters. My God, at that power level, the iron pigment paint on your foamy might start to get hot!

So the bottom line that my testing found (and I was a dis-believer at the start) is that the noise level from everyday equipment located near your flying field will not be a problem. What I found to be a problem was Rx and Tx antenna orientation. You can mess up the DX6 by having poor antennae orientation and shielding. It only took a 3" square of carbon fiber panel held behind the Rx unit at a 15 degree angle to shut the unit down hard. This is why Spektrum doesn't want you to put massive glow motors anywhere near the Rx unit. This is why Monokote with metal fleck in it is such a "no-no". If you mess up the Rx antenna reception, then all that fancy work to remove interference noise is down the drain: multipath signal reception or antenna shielding will put you in "failsafe" everytime.

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