When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. This handoff operation not only involves identifying a new base station, but also requires that the voice and control signals be allocated to Channels associated with the new base station.
- Identifying a new base station.
- Re-allocating the voice and control channels with the new base station
Handoff must ensure that the drop in the measured signal is not due to momentary fading and that the mobile is actually moving away from the serving base station.
Running average measurement of signal strength should be optimized so that unnecessary handoffs are avoided.
- Depends on the speed at which the vehicle is moving.
- Steep short term average -> the hand off should be made quickly
- The speed can be estimated from the statistics of the received short-term fading signal at the base station
Dwell time: the time over which a call may be maintained within a cellwithout handoff
Dwell time depends on
- Once a particular signal level is specified as the minimum usable signal for acceptable voice quality at the base station r receiver a slightly stronger signal level is used as a threshold at which a handoff is made
- Minimum usable signal for acceptable voice quality (-90dBm to -100dBm)
Figure 5.5 Handoff Threshold
Handoff marginΔ=Pr,handoff−Pr,minimumusablecannot be too large or too Small.
- If is Δtoo large, unnecessary handoffs burden the MSC
- If isΔtoo small, there may be insufficient time to complete handoff before a call is lost. As shown in fig 5
- If a mobile moves from one cellular system to a different cellular system controlled by a different MSC.
- Handoff requests is much important than handling a new call.
Various methods for handoff
- One method for giving priority to handoffs is called the guard channel concept, whereby a fraction of the total available channels in a cell is reserved exclusively for handoff requests from ongoing calls which may be handed off into the cell
- Queuing of handoff requests is another method to decrease the probability of forced termination of a call due to lack of available channels.