Dear NarChai,
Usually a cells max coverage area is defined based on the max TA it can serve without one TimeSlot TS assigned to one MS overlapping over the next TS. This TA value is 63 usually can be restricted to a lower value bu RACH TA Filter. The Value to 63 corresponds to roughly 35 km. Thats the theoratical coverage area of a standard standalone cell in open clear terrain. But we can extend this limit by using extended cell to 121km configuration this increase requires you to sacrifice half the Time slots so a radio now serves 4 users. each user has two TS and the extra TS gives Ms another 156 bits after the original 63 bits to advance its transmission so even of the burst is received in the next TS duration the BTS is still able to decode it as in extended range cells only alternate TS are used.