We are installing some shorter 7-8Ghz licensed shots 4 and 6 ft antennae for max throughput and availability. All of the ITU-R performance summaries result is -37 to - 40 +_ 5 dB with the exception of one which has some fresnel incursion -46 +- 9 dB. I know linkplanner has been traditionally conservative but why doesn't the rcv power show better accuracy (+- 2 for example)?
The Predicted Receive Power is the predicted receive power and associated tolerance level at each end of a link.
The tolerance is the sum of two components, a fixed value which is dependent on the equipment performance over temperature, and a variable value which is proportional to the amount of Excess Path Loss.
When using adaptive modulation the receive power shown is the maximum for the link, which corresponds to the lowest selected modulation mode and Maximum Power.
Excess Path Loss is the extra amount of loss caused by obstructions in ~60% of the first Fresnel zone. If the Excess Path Loss is zero, the link is Line Of Sight.
Licensed microwave links (in your case, 7-8 GHz) should have no Excess Path Loss, because licensed microwave links should always be deployed as Line Of Sight.
Links with Excess Path Loss (links with obstructions in the first Fresnel zone) have wider tolerance values than Line Of Sight links, since it's much harder to predict the nLOS (near LOS) or NLOS (Non LOS) performance of a product that was designed to always be deployed as LOS.
I hope this helps!
Dave
The default tolerance on a product which is often +/- 5 dB as Dave says is dependent on equipment tolerances, where both ends of the link contribute. The transmitter and receiver each individually usually only have tolerances of 2 or 3 dB (to account for unit to unit production tolerances as well as temperature), however this results in an overall tolerance of 5 dB for the link as we have to allow for the transmitter being either high or low on its tolerance and the receiver being the opposite. In the majority of cases we would expect a link to come up within 2 or 3 dB tolerance, but we have to allow for the more unusual circumstances (e.g installation at the edges of the temperature ranges etc) as well when quoting the tolerances.
Regards,
Rachel