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by colanderman13 hours ago |
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upvoteby riedel7 hours ago|
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At least in Europe the 868 Band is is in contrast to 2.4 allowed only for low duty cycle applications that do not actually occupy the channel for more than 1% afaik (given space multiplexing). I also remember also that the free to us band was quite narrow by design (we built sensor nodes bit banging a PHY transceiver that were in the grey area of unenforced rules, 20 years ago .l)
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upvoteby sandos6 hours ago|
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Nobody uses the 1% bands afaik, there are 10% duty cycle bands.
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upvoteby nerdsniper11 hours ago|
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What issues does it create for others to use too narrow of a bandwidth? Why “should” the FCC care if someone is only using a small portion of the spectrum that would otherwise be fine fr them to use?

Thanks for educating us!

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upvoteby colanderman10 hours ago|
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Spectral power density is the primary concern.

The legal power limit in these bands is 1 W. If you spread that out over 500 kHz, that signal is weaker than background noise at any given frequency for anyone more than about a city block away. (Give or take many factors.)

But, if you compress that 1 W into, say, 12.5 kHz (typical for FM voice), your signal is now detectable (and will interfere with other, possibly licensed, users) at over 6 times the distance.

There are probably other factors. For example, it's not legally sufficient to simply reduce your power by a corresponding factor. I suspect it may simply be the FCC's goal to reduce conflict between users by mandating spread-spectrum technologies for unlicensed use.

Note also that 47 CFR 15.247(e) [1] gives a spectral power limit which corresponds approximately with the 1 W max / 500 kHz min specified in (b)(3) and (a)(2).

Final side note – https://docs.fcc.gov/public/attachments/FCC-02-151A1.pdf is interesting reading as to how the current form of 15.247 came to be. Specifically it changed the rule from specifically DSSS to digital modulation generally, which in turn allowed the transition from 802.11b (DSSS) to 802.11g (OFDM) on 2.4 GHz.

[1] https://www.ecfr.gov/current/title-47/part-15/section-15.247...

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upvoteby codys10 hours ago|
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The idea with either requiring very wide band or frequency hopping on the 900Mhz band is to make it so that usages of the 900Mhz band 1. are tolerant to some loss (ie: by temporary collision) and 2. don't collide continuously (by using wide band or frequency hopping).

It's a mechanism to try to make the 900Mhz band more useful to uncoordinated users.

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