Effect in Cell Coverage Area
In coverage limited cells the frequency hopping may increase the cell coverage area because of the frequency diversity gain, but since the BCCH time slot doesn’t hop, the increased coverage area is relevant only for the ongoing calls that have been successfully established and are allocated a hopping TCH.
According to the simulations, the non-hopping signalling channel (BCCH / SDCCH) has a better performance than a non-hopping TCH but a worse performance than a hopping TCH channel. Therefore, the cell coverage area could be increased, but not according to the full FH gain, but by considering the performance of the BCCH time slot.
In RF FH case, the whole BCCH carrier is non-hopping. Thus, the frequency diversity gain should be considered as a quality gain in the cell border area rather than the gain increasing the cell service area.
Effect of Mobile Speed
The frequency diversity gain for the fast moving mobiles is not significant. The movement as itself causes the same gain which is lost from the frequency diversity gain. Therefore, the fast moving mobiles get the same gain than the slow moving ones, the gain just comes more or less from the moving as itself.
In GSM, the speed of Power Control (PC) is slow. When moving fast, the PC cannot follow anymore the slow fading dips so efficiently. Therefore, the fast moving mobiles might loose in PC gain. Also the Handover (HO) performance may be degraded with high speed.