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Re: FCC elevates hams to primary status, 2400-2402 MHz



>Most Part 15 users are not trained nor have the equipment required to
>detect interference. They know their widget doesn't work, but not
>why.

Here's where hams can do some public service by applying their technical
knowledge and training to actual problems in their own neighborhoods.

>The real point was that irresponsible manufacturers can still produce
>their widgets under Part 15 rules without regard to how much
>interference their device creates because the regulations are so lax.

I agree that the Part 15.247 rules could have been better. But
remember they're about 20 years old, and nobody knew as much about
this stuff as we do now.

If the FCC's intention was to maximize the chance that these devices
could share spectrum without coordination, they made several mistakes:

First, they used the wrong definition of spread spectrum process
gain. They specified a lower limit on the ratio of spread to unspread
bandwidth when they should have specified an upper limit on the user
bits/sec/Hz that the modulation and coding scheme may attain.

802.11b drove a truck through this loophole, resulting in a 11 Mb/s
modulation scheme with 0.5 bits/sec/Hz. This is hardly "spread
spectrum" by most people's use of the term, nor is it a signal that
can tolerate much co-channel interference.

Second, they did not mandate automatic power control.  Fortunately,
the maximum power output of most WiFi devices are well below the 1W
regulatory maximum. A typical 802.11b card puts out 20-50 mW, with a
few (e.g., Cisco Aironet) putting out 100mW.

Third, they did not mandate the use of forward error correction to
minimize the transmit power required for a given link at a given
speed, e.g., by limiting the Eb/No that a receiver may require to
operate.

Fourth, they did nothing to encourage the development of "cooperative"
interference management, especially among different types of devices.
While WiFi has a CSMA/CA multiple access scheme to keep WiFi stations
from colliding with each other, this does not prevent mutual
interference between WiFi, Bluetooth and cordless telephones.  WiFi
also could have defined an ad-hoc minimum-energy relaying scheme,
although it wouldn't make much sense without automatic power control.

To be sure, many of these techniques have only recently become
feasible in mass-market products. Hopefully the experience with the 5
GHz band will be better.

--Phil



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