They key to spectrum management is to use as little power as possible to achieve the desired result. Anything beyond this is just pollution.
For Canopy you want ~16dB above the noise floor (10dB to get 2X plus 6dB margin). So for example if your noise floor is -78 (in the middle of the desert?) your target is -62. Getting as many of the SM's at this level at all co-located AP's should be your goal. SM's that are weaker will need to have bigger antennas installed or be elevated for better LOS. Some links are just too far and they have to live with what they get or find another provider. If your noise floor creeps above -70, you are going to start seeing issues because it is difficult to get links at -54 at any measurable distance. Those links that fall short will start to modulate from 2X to 1X between 16dB and 10dB margin. From 9dB to about 6dB they will stay at 1X. At 6dB and below they will start re-registering. If you only have one 900AP, running wide open isn't going to hurt you (but may be a problem for another 900MHz user). If you have additional AP's on the same tower (or nearby) you will start interfering with yourself and/or others by hitting the tower with too much signal which will affect your weaker links. Hope that helps ~Jerry From: [email protected] [mailto:[email protected]] On Behalf Of Erik Anderson Sent: Thursday, June 13, 2013 1:08 PM To: WISPA General List Subject: Re: [WISPA] Strange problem with Canopy 9000APC On 6/13/2013 1:38 PM, David Hannum wrote: He strongly urged me not to even worry about balancing. He says run everything wide open all the time - all SM's and all AP's. Now, I'm not disputing you - I don't know enough too. I'm just relaying what the Cambium engineer told me. No worries (about difference in theology). I am not trying to change anyone's mind, argue, or cause contention: just trying to help. Sorry if anyone thinks otherwise: that is why I am more of a lurker than poster. ALWAYS keep the AP at full power. Always, always, always. Well, I guess if you are using it for a PTP not PTMP and you are causing interference with other equipment then... anyway, if an AP is used as an AP, ALWAYS. But the SM... no. Through Cambium training, we were told differently: but I was not there for that training. Quite honestly, it makes a lot of sense to turn down a radio if the UPLINK strength at the AP is reading -30 to -55. Think about it for a minute from an RF prespective. Your receiver is receiving multiple transmissions. On some, it must increase the gain to make it "sound right." On others, it must decrease the gain to make it "sound right." These frequencies are preset to communicate (900 mhz band) and fine tuned to pick up the quiet sounds. Will the much stronger RF signal, over time, destroy the transceivers ability to hear the quiet sounds? Yes. That is what RF does, and why the microwave oven is so effective. There is an interesting study out there about the effects of long term RF exposure on the structural integrity of buildings. These units are not powerful and not going to damage buildings, but over time, they will damage sensitive electronics. Immediate problems, no. Long term, yes. Support for every other product on the market tells you to turn down a transmitter if receive is better than -50. A Canopy radio may be better than other radios, but we have repeatedly seen that the receiver desenses over time (way before the documented MTBF rate). For the SM, it does not matter if the receive is hot. It will never need to listen to the quiet sounds. The receiver can desense for that client and you will not have an impact. We test every radio before it is deployed/redeployed. This is due to RF sensitivity, and ones that perform worse are marked for use in locations with a strong signal only. Really, you should turn down the SM if the AP receive is that hot. 9 months is fast for burning out an AP. As I said in the first email, I am very suspicious that your problem is the power supply this time. But, I have seen the receivers on the APs get more and more desensitized (16 months is fastest) to the point the truck was rolling all the time for that AP, and a radio swap instantly fixed the numerous problems we were seeing.
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