-

In mobile cellular communication systems, when a mobile station moves out of the coverage
area of serving cell and enters to the coverage area of an adjacent cell, the channel which is used
by without interruption of service. This process which is done without user intervention, is called
Handoff or Handover. Proper decision for handoff and fast execution of it, has considerable effect on
preservation of call quality and reduction of network load.
A well known algorithm for handoff is "Averaging and Hystersis". In choosing averaging
window length and hystersis margin for this algorithm, trade-off between mean number of
handoffs, delay in handoff and received signal quality must be considered. It is shown that using
more hystersis and less averaging improves signal quality and rreduces sensitivity to variations in the
speed of mobile station. But this method increases sensitivity to variations in the standard deviation
of shadow fading. To reduce this sensitivity, changing hystersis margin proportional to the
standard
deviation
of
shadow
fading
IS
recommended.
Using path loss slope, an algorithm for simultaneous reduction in delay and mean number
of handoffs is peresented. When a mobile station approaches to (recedes from) a base station, the
slope of averaged redeived power is positive (negative). Varying hysteresis margin by this slope,
makes handoff easier(harder) to the base station for which mobile station is approaching (receding). Thus delay and mean number of handoffs reduce simultaneously.
Simulations show that when a mobile station
moves on the cell boundaries, repeated handoffs to the base stations of that cells occur. Care must
be taken to avoid overlap between high traffic streets and cell boundaries, in order to reduce
total number of handoffs in the network.