The cellular system depends on the radio signal received by a mobile station throughout the cell and on the contours of signal strength emanating from the base station of two adjacent cells j and i.
As from the above discussion, the signal strength goes down as moves away from the base station. The difference in received power is a function of distance. Whereas the mobile station moves away from the base station of the cell, the signal strength weakness, and at some point, a phenomenon known as handoff occurs. This implies a radio connection to another adjacent cell. j
As the mobile station moves away from j cell and gets closer to the cell i. assuming that pj(x) and pi(y) represents the power received at the mobile station from the BSj and BSi, the received signal strength at the mobile station can be represented by curves and the variation can be expressed by the empirical relations. While the distance X1 from the received signal of BSj is approximately close to zero and the signal strength at the mobile station can be primarily attributed to BSi. While, at distance X2, the signal from BSi is minor. To receive and interpret the signals correctly at the mobile station, the received signal must be at a given minimum power level pmin and the distance X3 and X4 have two such points for BSj and BSi, respectively.
Therefore that, between points X3 and X4, the mobile station is going to be served by either BSj and BSi, and therefore the choice is left to the service provider and the underlying technology. where the mobile station have a radio link with BSj and continuously moving towards away BSi then at some point it has to be connected to the BSi and the change of such linkage from BSj to BSi is known as a handoff. Therefore regions X3 and X4 indicate the handoff area.
Where the performs handoff depends on many factors.One of the considerations is that the handoff should not take place too quickly to make the MS change the BS too frequently. If the Mobile station moves forth and back between the overlapped area of two adjacent cells due to underlying terrain or intentional movements.