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 process of transferring the channels between two cells is called Hand-Off. These Handoff Strategies in Mobile Communication not only involves identifying a new base station, but also requires that the voice and control signals be allocated channels associated with the new base stations. This post includes Mobile and Wireless Communication Notes on various Handoff Strategies in Mobile Communication in detail.
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Processing handoffs is an important task in any cellular radio system. Handoffs must be performed successfully and as infrequently as possible and be imperceptible to the user. In order to meet these requirements, system designer must specify an optimum signal at which to initiate a handoff.
This margin is given by cannot be too large or too small. If is large unnecessary handoffs which burden the MSC may occur and if is too small, there may be may be insufficient time to complete a handoff before a call is lost to meet these conflicting requirements. Figure 7.18 is given which explains the different steps of handoff required to meet.
Demonstration of Handoff Strategies
Following figure demonstrates the case where a handoff is not made and the signal drops below the minimum acceptable level to keep the channel active. This dropped call event can happen when there is an expansive delay by the MSC in assigning a handoff or when the threshold is set too small for the handoff time in the systems. Excessive delay may occur during high traffic conditions or due to no channels are available on any of the nearby base stations.
In deciding when to handoff, it is important to ensure that the drop in the measured signal is not due to momentary fading and the mobile is actually moving away from the serving base station. But when there is a drop in the measured signal level due to momentary fading and not due to the actual moving mobile away from the base station, that is situation if the handoff of a call takes place then such handoffs must be prevented by ensuring that the base station monitors the signal level for a certain period of time before the handoff is initiated.
Factors on Which Handoff Depends
The time taken in deciding about handoff depends on various factors like:
(a) Transmitted Signal Strength: If the signal strength is fluctuating very rapidly then no. of handoffs (Forced handoff) increases.
(b) Vehicle speed: The length of time needed to decide if a handoff is necessary, depends on the speed at which the vehicle is moving. If the slope of the short term average received signal in a given time interval is steep, the handoff should be made quickly.
The time over which a call be maintained with in a cell, without handoff is called dwell time. This time is dependent on various factors like propagation, interference, distance between and other varying effects.
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