non-overlapping channels

[stevech]

The IEEE, long ago, decided to set a convention of channel numbers in 2.4GHz (ISM) band.

Channel Numbering
1, 2, 3, ... 11 for the region encompassing the US, to match US FCC regulations. These channels are on 5MHz centers, although 5MHz wide transmissions never became popular, especially as IEEE 802.11b came along. Nor 10MHz transmissions. Today's chipsets (most) can operate in 5 or 10MHz, and do so for IEEE 802.11j which is 4.9GHz in Japan, and public safety-only in the US. That '11j exists is why hardware exists for use in the US, by public safety, though usage is sparse for reasons too messy to discuss here.

Occupied channel bandwidth
An IEEE 802.11b transmission is about 20MHz wide. That's several channel numbers. So in the US, we talk about ideally people using only channels 1, 6 and 11, so that the transmissions don't overlap (much). IEEE 802.11b uses direct sequence spread spectrum (DSSS) modulation of the transmitted signal. This was state of the art back then.

OFDM
Comes OFDM, as the alternative to DSSS, and specified by IEEE for 802.11g. This signal is about 16.5MHz wide, narrower than DSSS. So rather than non-overlapping 1, 6 and 11, one could talk about four non-overlapping channel numbers.

Type Certification
The FCC doesn't define these channel numbers, but does define the band start/end frequencies and how much RF power equipment is permitted to "spill" out of the band (this is called the spectral mask). If equipment is shown to comply, it gets an FCC Type Certification registration number. It's supposed to be illegal to sell a product that isn't type certified (and this includes other unlicensed bands, and licensed bands).

WiFi
WiFi is a private industry trade association. To use that WiFi logo, one must be a WiFi alliance member and pay dues, etc. Supposedly, WiFi established transmission signal quality measures to assure a goal error rate, and receiver sensitivity and error rate vs. signal strength, on the other end. Supposedly gear is tested for compliance before being granted use of the WiFi logo (copyright/trademark). So a real product needs first, FCC Type Certification and optionally second WiFi certification so the product can be branded WiFi. Methinks the latter isn't stringently enforced.

So the point here is that with OFDM in '11g/n, the channel overlaps seem to differ from '11b, and ye ole 1, 6 and 11. Also, band-edge "spill-over" is easier to control/minimize with OFDM. Also noteworthy that 2.4GHz IEEE channels 12-14 are not to be used in the US due to possible interference to navigation systems just above 2.5GHzx.

Lastly, it's interesting to recall than unlicensed operation (as in WiFi), is permitted under FCC regulations, Part 15. Here you'll find all the emissions limits and power restrictions vs. antenna directionality. Ah but, FCC Part 90 allows licensed users to operate in the same 2.4-2.5GHz band, with very high power limits. And a few some do, such as air-to-ground video links for public safety, when they can't afford 6GHz equipment. And too, amateur radio operators (HAMS) are permitted very high power under FCC Part 97 licensed operator rules. But HAMS don't do so much, due to interference to/from unlicensed WiFi users.

Legally, unlicensed Part 15 users of the band are "secondary" and have no recourse to interference from licensed Part 90 or 97 operators. But, like 27MHz Citizen's Band, the FCC doesn't police 2.4GHz much at all.

Just a bit of minutia for you.

 

[Mark Uhde]

Useful stuff, just one note - the 12-14 ban:

1. Isn't technically true (only 14 is actually banned, but the restrictions on 12 and 13 are too strict for vendors to risk complying).

2. Has nothing to do with navigation systems, and everything to do with communication systems. Starting with channel 14, Globalstar occupies the spectrum. In the US and Canada Globalstar, corporate jerks they are, are so well protected that 12 and 13 are heavily restricted (in practice banned) to protect them. In Japan, Globalstar's ownership of 14 isn't even accepted so it's legal (though in practice not used since OFDM is banned meaning only 802.11b rates are allowed and it overlaps 13 so 1-5-9-13 is the correct channel plan in Japan).

What's really sickening about Globalstar is that they don't NEED the channel 14 space and now they're trying to get FCC approval to take ownership of 12-14 and sell licenses to use channel 14! And presenting this as some great humanitarian thing for the good of public Wi-Fi. All while providing NO benefit over a FREE OPEN 1-5-9-13 channel plan legal in the rest of the world (except for 802.11b which needs a bit wider channel with guard band).

What we need is an FCC that quits over-protecting Globalstar's space and loosens emissions standards into the bottom of their band to make operating in 12/13 legal on the same level it is in the rest of the world, enabling a 1-5-9-13 channel plan. Sadly, knowing the FCC they're probably allow Globalstar to take over a chunk of valuable unlicensed spectrum without thinking twice about it. Who knows, they may end up deciding to "protect" Globalstar's new space by giving them channels 10 and 11 for their exclusive licensed use also. Not that that would really matter, but it'd force everyone to move to a 1-5-9 channel plan which would create a mess of overlapping channel issues for awhile.

P.S. To clarify operating on 13 would have NO harmful affect on Globalstar. Even 14 would be minimal enough they'd be happy to sell licenses to operate on it and 14 is partially INSIDE their space.

 

[stevech]

Because Globalstar received its FCC license in 1995, I recall that the restrictive emissions mask for ch. 12-14 in the US pre-dates Globalstar. But post-Globalstar, no doubt the pressure increased.

I chatted with a Globalstar sales guy at a recent trade show. As you may know, Globalstar shut down for quite a while, in/out bankruptcy, etc. To me, having worked on a Globalstar-next project in the late 90's, the inter-satellite link scheme is an engineering marvel. But Iridium and Globalstar predictably failed on the Sat-phone business model. Today, DoD $$$ (er, yours and mine) keeps Iridium on the air.

 

[Mark Uhde]

Because Globalstar received its FCC license in 1995, I recall that the restrictive emissions mask for ch. 12-14 in the US pre-dates Globalstar. But post-Globalstar, no doubt the pressure increased.

I chatted with a Globalstar sales guy at a recent trade show. As you may know, Globalstar shut down for quite a while, in/out bankruptcy, etc. To me, having worked on a Globalstar-next project in the late 90's, the inter-satellite link scheme is an engineering marvel. But Iridium and Globalstar predictably failed on the Sat-phone business model. Today, DoD $$$ (er, yours and mine) keeps Iridium on the air.

I don't believe Globalstar ever shut down. They were having massive issues with premature satellite failure, to the point the service only worked for a few minutes every hour in most locations, but I don't believe it was ever shut down.

During that time you could also get an unlimited calling plan for $30/month. But it only worked for less than an hour a day

I'm not sure if they're back up to full operations or not yet, but I know they've been launching new satellites and service is at least partially restored.