Signal strength-AP vs. CPE in Wireless Service Providers http://www.dslreports.com/forum/r16688222 en Tue, 10 Nov 2009 18:05:30 EDT Tue, 10 Nov 2009 18:05:30 EDT Re: Signal strength-AP vs. CPE http://www.dslreports.com/forum/remark,16698203 lutful : This is what I meant in my last post: the thought somehow never crossed my mind that Fresnel effect could be different for AP-CPE versus CPE-AP :)

Here is a simple RSSI/SNR formula using absolute magnitudes of the negative loss dB numbers:

For half-duplex or TDD WISP radios we use, these numbers do not change depending on which way we are transmitting:

CONSTANT = AP_ANT_GAIN - AP_CABLE_LOSS + CPE_ANT_GAIN - CPE_CABLE_LOSS

RSSI/SNR1 for AP->CPE: AP_TX + CPE_RX + CONSTANT - PATH_LOSS1
RSSI/SNR2 for CPE->AP: CPE_TX + AP_RX + CONSTANT - PATH_LOSS2

PATH_LOSS1 = PATH_LOSS2 if fading was similar (measurement taken at almost the same time) and if Fresnel effect was same both ways.

So, observed difference is mainly due to radio TX/RX characteristics: (CPE_TX - AP_TX) + (AP_RX - CPE_RX)

Since TX spec difference between radios are a few dB, the dominant effect on RSSI/SNR seems to come from differences in RX sensitivity. :)]]> http://www.dslreports.com/forum/remark,16698203 Sun, 13 Aug 2006 11:57:13 EDT Re: Signal strength-AP vs. CPE http://www.dslreports.com/forum/remark,16698155 LLigetfa : Anyway... back on topic.
Aaron,
Antennas have different characteristics and the terrain can also throw you a curve. One should never assume, as you found out, the RSSI on the other side. You would not be the first nor will you be the last to waste time aligning an antenna on one of the side lobes.]]> http://www.dslreports.com/forum/remark,16698155 Sun, 13 Aug 2006 11:42:11 EDT Re: Signal strength-AP vs. CPE http://www.dslreports.com/forum/remark,16698093 LLigetfa :
said by lutful See Profile :

Never thought that Fresnel impact could be different on AP-CPE versus CPE-AP.
I do not want to bring into question your expertise in radio propagation as my own expertise will not go unscrutinized. I have no formal education in the theories of radio propagation, only life experiences.

Wikipedia, in describing the fresnel zone, states:
To maximise receiver strength you need to minimise the effect of the out of phase signals. To do that you must make sure the strongest signals don't bump into anything - they have the maximum chance of getting to the receiver location.
In my mind then it stands to reason that obstructions near the transmitter where the signal is strongest, would have a larger impact. If you were to experiment with a light source and a fresnel lense or anything that effects the light beam, you would see the effect of the lens or obstruction in relation to the distance from the source.

said by lutful See Profile :

Otherwise, any difference in RSSI should only be due to TX power and RX sensitivity, not EIRP, since AP/CPE cables and antenna gains are the same. :)
You seem to have lost me on that logic. How can you assume AP/CPE cables and antenna gains are the same? I would not make that assumption.

The only point I was trying to make is that conditions are not symmetrical and so each direction needs to be considered separately. As you point out, RX sensitivity also needs to be factored and TX power adjusted to suit.]]> http://www.dslreports.com/forum/remark,16698093 Sun, 13 Aug 2006 11:27:42 EDT Re: Signal strength-AP vs. CPE http://www.dslreports.com/forum/remark,16695925 lutful : Never thought that Fresnel impact could be different on AP-CPE versus CPE-AP.

Otherwise, any difference in RSSI should only be due to TX power and RX sensitivity, not EIRP, since AP/CPE cables and antenna gains are the same. :)

In his case, the AP radio seems to have lesser specs than the CPE radio?]]> http://www.dslreports.com/forum/remark,16695925 Sat, 12 Aug 2006 22:27:04 EDT Re: Signal strength-AP vs. CPE http://www.dslreports.com/forum/remark,16692117 LLigetfa : There are several factors to consider.

Disparity in IERP between AP and CPE. In most cases the AP EIRP varies from the CPE. The EIRP on the AP is one-setting-for-all whereby radio power may need to be adjusted on the CPE.

LOS. The fresnel zone near the AP is almost certainly clear given that it would be high on a tower but at the CPE it could be very different. Obstructions near the transmitter (CPE) tend to scatter the signal worse than obstructions near the receiver.]]> http://www.dslreports.com/forum/remark,16692117 Sat, 12 Aug 2006 08:03:15 EDT Re: Signal strength-AP vs. CPE http://www.dslreports.com/forum/remark,16691365 Diddy1 : I was just looking for something I hadn't thought of. Guess there really isn't any other way other than to check at the AP for signal quality. Cabling is good, I just didn't aim it to get a good signal at the AP, only a good RX signal.
Aaron]]> http://www.dslreports.com/forum/remark,16691365 Sat, 12 Aug 2006 01:18:02 EDT Re: Signal strength-AP vs. CPE http://www.dslreports.com/forum/remark,16690900 BurstNET : This can be caused by interference or bad cabling as well.

SMA]]> http://www.dslreports.com/forum/remark,16690900 Fri, 11 Aug 2006 23:35:19 EDT Signal strength-AP vs. CPE http://www.dslreports.com/forum/remark,16688222 Diddy1 : How are you guys ensuring a good signal strength at the AP? What I have is a client, my first install, that has -70db at their CPE, but the AP is recieving at -85db at times. So, obviously I did not align the antenna correctly to ensure solid signal at the AP. So, the only way I can think of to ensure a good signal both ways, is to log into the AP from the subscribers CPE once installed? Is there any tips to ensure a better rx at the AP end?
Aaron]]> http://www.dslreports.com/forum/remark,16688222 Fri, 11 Aug 2006 15:19:07 EDT