On Thu, Feb 23, 2006 at 08:42:32PM -0800, Martin Usher wrote: | >But it is clear that we need more power/range than what current | >802.11x can offer.
For the record, 802.11x is just one (or a few, to be more accurate) implementations of spread spectrum. There are many others. The Spektrum stuff is the only commercial SS recreational R/C implementation out that there that I'm aware of, and it doesn't use any of the 802.11x standards. (I'm pretty sure that Futaba already sells industrial R/C equipment that uses spread spectrum, but I don't know the specifics.) As for power/range, the power limit in the 2.4 GHz band is 1 watt in the US. That is more than enough to get two miles of range, especially if we use a somewhat directional antenna on the TX. (A dipole with a screen behind it to shield the operator would be a nice start, and at 2.4 GHz wouldn't even be too large.) (The Spektrum DX6 TX only uses 0.1 watt if I recall correctly, and the antenna provides very little gain.) | Apart from range (limited) the real killer with something like 802.11 is | latency. You probably won't notice it with just one or two people flying | but if you had a contest with a couple of dozen then the transmitters | would share the bandwidth by waiting their turn. There are multiple implementations of spread spectrum. Not all involve `waiting their turn'. Many, including the one used by the Spektrum DX6 stuff, can transmit all at once and the RX can pick it's signal out of the resulting mash of noise. And even if we were using 802.11b (the WiFi standard, 11 Mb), the latency would probably be unnoticible by even the best pilots, even with dozens of planes using it at once. (As long as people weren't trying to steam video back from their planes over the same WiFi connections, I guess.) | The present PCM system works extremely well for what it is -- its cheap, | its quite reliable and its economical on spectrum. ... but it can be completely knocked out by another TX on the same channel. Or a pager 10 signal KHz away could desense your RX so much that it can't hear yours. It's only reliable if there's no interference. PCM handles interfence as well as can be expected, via appropriate use of keep-position and failsafes, but either way, when there's interfence, *you don't have control*, PPM or PCM. | The problem is that there are lots of other users contending for band | space who could easily move to modern radio technologies but are | reluctant to give up "their" slice of spectrum. `Easily' is relative. Buying new hardware is expensive. How would you like it if the FCC said we were losing the 27, 72 and 75 MHz R/C bands in two years, and we were all to migrate to something else, probably 2.4 GHz spread spectrum? And a lot of these places that would give up their spectrum would migrate to ... cell phones. Which would provide some beneifts over their current systems, but they'd have to pay per minute to use it. They're not going to go easily. | So the public uses like R/C and wireless networking tend to be | crammed into little corners of the spectrum that nobody else wants Nobody else wants? You're kidding, right? Spectrum is gold. Some is more golden than others, but it's all gold. And as for the 2.4 GHz band, the only reason it's not pure gold is that it's already very popular, and even if the government outlawed it's use tomorrow, it's use wouldn't stop. | (the 2.4GHz band used by most wireless network equipment is open | because its a set of frequencies that are absorbed by water | molecules -- not that good for communication but excellent for | microwave ovens). Yes, microwave ovens use 2.45 GHz, and yes, water will absorb radiation at this wavelength. However, this does not mean that 2.4 GHz is not that good for communication, unless you're trying to communicate to a submarine (and they use megawatts and ELF signals for that -- 76 Hz or so. Not KHz, not MHz, but Hz.) Attenuation due to the atmosphere at 2.4 GHz is *negligible* -- about 0.01 dB/mile. (Citation? http://www.syngress.com/book_catalog/283_CSWAN/sample.pdf.) Even in heavy rain (4 inches/hour), the attenuation is only 0.08 dB/mile -- utterly insignifigant to us, and how often do you fly in heavy rain anyways? In fact, 2.45 GHz was chosen because it's *not* the frequency where water absorbs the most of the signal. This is a benefit because you want your food cooked all the way through, not just at the surface. (Citation: http://www.lsbu.ac.uk/water/microwave.html.) | TV band and the VHF TV bands for sale -- so the chances of us getting a | slice for free is, I'd guess, relatively slim. True, on both accounts. Big bucks. | I'd love to move to spread spectrum. Just not at microwave frequencies. Microwave frequencies would be great. 3.25 cm antennas? What's not to love? At that size, you could even have a RX that has multiple antennas, and it just uses the one with the best signal at any instant. Yes, getting the antenna out of a carbon fiber boom might be tricky, but that's easily solved by moving the first RF stage out of the boom entirely -- a little button you mount on the outside if your plane, and the antenna (or two) comes out of that. It also lets you easily have a directional antenna on your TX, giving you better range and/or using less power. (Granted, you don't want a very high gain antenna, as it won't be aimed carefully at your plane, but 6 dBi of gain would probably be just fine.) The path loss is larger at this frequency (due to the smaller receiving antenna and it intercepting less of the signal) but this is easily compensated for with more sensitive receivers. Any external interfernce is reduced by a similar figure, so it's not really a big deal. | If we could get our hands on just one redundant UHF TV channel that | would keep us happy for a very long time. Yes it would. And since our data rates are relatively low, we don't even need that much bandwidth, even for 100+ simultaneously flying airplanes in the same place. But 6 MHz of bandwidth could easily be worth over a *billion* dollars. (Reference: http://www.njtelecomupdate.com/lenya/telco/live/tb-NSAW1116879481137.html -- 60 Mhz of spectrum worth $28 billion, so 10% of that should easily be worth over a billion.) That would be way cool if the AMA could buy us 6 MHz of spectrum in an upcoming auction, but that would make even the new headquarters in Muncie seem awfully cheap. I'd love to see us get 1 MHz of bandwidth dedicated to SS R/C use (and being SS, there's no need to break it up to air and ground frequencies.) But I just don't see it happening. -- Doug McLaren, [EMAIL PROTECTED] -- Never do tomorrow what you can do the day after tomorrow -- RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. Email sent from web based email such as Hotmail and AOL are generally NOT in text format
