802.11n/MIMO Power Limit question
Last Post: August 9, 2010:
Have the EIRP calculations changed for MIMO systems?
Say I was previously limited to 20 dBm output on a pre- 802.11n device.
So now, I want to replace said device with an "n" AP where each radio chain in the 2x2 device is putting out 20 dBM. As far as the regulations go, am I still running 20 dBm, or am I running at 20 + 20 = 23 dBm?
If the answer is 23 dBm, do I now have to turn down my power output on both radios so that the total is still 20 dBm?
In reading the FCC Part 15.247 it does say "output power at the intentional radiator," which I read as antenna. I'm no expert, but it seems to me that if you have separate radio chains on separate attenae in your AP, you should be fine with both chains running at your max power. Again no expert, but that's the way I read it.
IR=Intentional Radiator, would be everything up to but not including the antenna.
EIRP=Equivalent Isotropically Radiated Power, Would include the antenna and is whats actually transmitted into the air.
Eek, I guess that partially explains my low score on the standards section of the test. Thanks for the info Mike. I'll have to reread the FCC doc with that shift in focus now.
Sorry, I'm confused on what the answer is. If the 2 antennas has to equal 20dB, does that mean that the coverage is less than using 1 antennas?
I can't find any answers to this question even though I'm sure it's a simple answer.
What do the rules say?. What do various Regulatory Agnecies (e.g. FCC) say about this?
In older diversity devices, only one antenna at a time was transmitting. Now that multiple antennas will be transmitting simultaneously on the same frequency, have the rules changed?
Just called the FCC?s office of Engineering and Technology and am waiting for a callback.
In the meantime, found the following:
The third slide seems to suggest composite power.
Am sure GT and Devin will be able to talk to their engineering depts and find out.
I've just this minute come up against this very issue. Our vendor of choice's 11n APs have a maximum transmit power of 13dBm on 2.4Ghz which when compared with the 18dBm of their ABG kit seems very low. I've emailed their support team for clarification.
Thanks a lot for that link. They have several other documents on that site that are also relevant and interesting. One of the ATCB documents spoke of more infomration on the FCC site, but I did not find it yet.
I've noticed that too, even on non-"n" equipment. It might be due to signals showing up out of band (ie below channel 1 or above channel 11). Several mfg's (older) chip designs had no dynamic power control built in - you just got what the chip put out. If manufacturers had control of the power output in firmware, they might be able to make a more marketable device.
It will be interesting to get some real details on this subject.
I've also seen admonitions, [i]without clarification[/i], that "these should not be [b]beam forming[/b] systems".
From everything I've seen in the last few days, it looks like DFS is more of a bellyache for mfg's than MIMO.
I think its as much PoE as EIRP that creates these additional limitations (beyond the UNI band restrictions). Does MIMO technically result in combined gain afa EIRP is concerned (its not a single radiator)? The Cisco 1140s and 1250s reduce their TxP by 3dB when in 40Mhz operation, and the 1140s are further reduced by HT data rates if you look at the output of the "show controller" command. The 1250s originally required 802.3at or proprietary methods to power both radios. Cisco subsequently released code allowing the 1250s to be af-compatible, but now their HT rate maxes out at MCS7. Here's hoping MIMO makes up the difference.