On 8/22/2013 4:09 PM, Steve Barnes wrote:
But Mike that is the Rub. All things are never the same. 900 is dirty and Susceptible to so much noise and reflection because the signal does not die as quick. I understand the "Theory" but still have a hard time understanding how a slower carrier wave (900MHz) can carry the same Data as 5800MHz carrier wave but I know that it could in a vacuum. The issue is we don't live in a vacuum.
The "carrier" frequency has no impact on data-carrying capacity. Shannon's Law dictates that the capacity of a channel to carry information is a function of its bandwidth and its signal to noise ratio. If it is 10 MHz wide from 902 to 912, or 10 MHz from 5800 to 5810, it's still 10 MHz. And if the SNR is the same, the usable capacity is the same.
The issue of vacuum relates to things that make a path worse than the theoretical free space attenuation would dictate. Take the 60 GHz band (57-64 GHz). It has a primary allocation for satellite-to-satellite use. Now there's your vacuum! It's unlicensed because oxygen absorption at 60 GHz is around 14 dB/km, so anything done down here at the surface is unlikely to reach a satellite. It's thus great for high-speed WLAN use, like WiGig. And the FCC last week raised the power limit for outdoor point-to-point use to 82 dBm, provided the antenna gain is 51 dB (derated 2 dB for each dB of lower gain that the antenna has). This will allow huge bit rates because it's 7 GHz wide, but range at normal atmospheric pressure is going to be very limited.
900 GHz is nice in wooded areas because it gets through foliage much better than higher frequencies, but in many places it's already congested with meter readers and other devices. Those, plus the limited bandwidth, are more likely to limit real-world performance than anything else. A 6 GHz TVWS channel will do as well as 6 GHz on higher frequencies, though. Better, actually, if you can get a big enough antenna. But lower frequencies tend to need bigger antennas. Maybe those old TV antennas we used to all have before cable will make a comeback. ;-)
-- Fred R. Goldstein fred "at" interisle.net Interisle Consulting Group +1 617 795 2701
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