Dear Safar,
LOL criteria can be given as below
UL -> OL
L < LOL - LOLHYST (Where L is path loss and is equal to BSTXPWR - RXlev)
OL -> UL
L > LOL + LOLHYST (Where L is pathloss and is equal to BSTXPWR – Rxlev).
In Locating algorithm when measuring basic Rxlevel, FBOFFSP is used to compensate between different frequency bands and hence a FBOFFS of 6 would mean that you are telling Locating that your BCCH is 6 dB better than your 1800 (or the OL band).
Hence when the call will return from OL to UL, the SS Downlink will be measured on 1800, and then 900 SS will be predicted (by subtracting FBOFFS) and then the criteria will be evaluated.
In case you dont want to use FBOFFS, then you can include the BCCH frequency of the cell in its own Active MF LIST. In this way cell will be measuring its own 900 strength (as it is in neighbor list) and no FBOFFS (Frequency band compensation) will be needed as you exactly know the BCCH SS.
In some cases when your antenna allows different tilts for different bands, you can keep your OL uptilted than your UL so that good OL coverage makes it keep traffic longer (increasing its utilization). But this may also increased DCR as it will tend to increased call dragging while on OL.
I hope it helps.