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. |
