lutful
Premium
join:2005-06-16
Ottawa, ON
 ·TekSavvy Solutions..
1 edit | reply to LLigetfa
Re: Signal strength-AP vs. CPE
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. |
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LLigetfa
join:2006-05-15
Fort Frances, ON
| reply to 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. |
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LLigetfa
join:2006-05-15
Fort Frances, ON
| reply to lutful
said by lutful  :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 :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. |
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lutful
Premium
join:2005-06-16
Ottawa, ON
·TekSavvy Solutions..
| reply to LLigetfa
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? |
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LLigetfa
join:2006-05-15
Fort Frances, ON
| reply to Diddy1
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. |
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