Re: RE: 300mw?

On Apr 14, 2004, at 8:13 PM, <[email protected]> wrote:

> Hmm
>
> - On your first example with the STA, most of the low cost radios do 
> not have very good RX levels so the increases power from the AP
> would help increase the reach in some cases and with the better RX 
> levels on our radios this helps hear the TX from the remotes
> better. Again the system needs to be setup correctly but there any 
> many times where the higher power will help.

To a point, yes.

> - Next "Larger beam widths invite more interference", yes if not setup 
> correct. But assuming you need to cover 120* and assuming
> there is interface, a higher power radio using a lower gain antenna 
> will HELP a setup like this, because less noise gain from the
> antenna that reaches the RX of the radio and balancing the system. 
> Again its all about the design.

"less noise gain from the antenna"?   How is this possible?  (Most) 
Antennas are passive, so they'll pass, if anything, a lower SNR toward 
the radio's rx chain than what they receive.

I say most, because phased arrays can actually generate a higher SNR, 
because noise, being non-coherent (by definition) won't "add" 
coherently, white the signal (including interference, natch) will.

> - Also I agree a higher gain radio does reduce the total EIRP, but 
> this not a bad thing. For example a 300mW radio can have a max
> antenna gain of 15.6dBi which works out very good for the example I 
> listed below. Again I am not saying use a 300mW radio all the
> time which is why we also have 100mW and 200mW again adding more tools 
> to what people need AND are able to design system correctly.
>
> - One last point the high power radios fill a void where many people 
> are using AMPS and do no need to, these radios offer a lower
> cost solutions to fill the void.

The very fact that commercial radios > 20dBm exist made this true.   
The delta from 100mW to 200mW was 3dBm.   400mW is another 3dBm,
but the FCC limits any antenna used with this (at this point, mythical) 
25dBm card to 11dBi or less, unless PTP, in which case the limit is 
18dBi.

The issue is that your 300mW cards are limited to 21dBi antennas (not 
15.6dBi, as you claim, please see the table I sent.  You get 6dBi for 
free, and I think you forgot to add it in.).  A lower power card can 
generate more EIRP with more antenna gain.   This results in better 
range for two reasons:

	1) the higher EIRP will result in more power to excite the remote 
receiver
	2) the higher directivity (antenna gain) will capture more RF energy 
(bigger ear) as well as (and this is important) reduce off bore-sight
		interference (unless your antenna has really high side-lobes, but low 
side-lobes bring their own hell when used with 802.11.)

Your 120 degree antenna is 'open' to a whole lot of potential 
interferers, and there will always be more of them than you.   if 
they're co-channel, your receiver will set CCA, and you'll "pend"... a 
LOT.   If they're off-channel (even on an adjacent channel), then they 
will be seen as interference, and your range will suffer (because your 
SINR will be (a lot) lower.

This was the lesson at Vivato, btw.

Jim

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