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handoff procedures

  • The process of transferring a call from one base station to another when a user’s radio signal becomes weaker at the first and strongest at the second base station. “Weaker” and “stronger” is quantified by a signal threshold level, which is above the minimum signal level for acceptable voice communication.
  • Selecting this threshold level is critical to ensure
    • Unnecessary hand offs do not occur
    • Call dropping does not occur
  • Minimum acceptable signal level is between -90dbm and -100dbm, a slightly stringer signal is consider as handoff threshold. This margin is given by, ∆ which is 0 to 6dbm. ∆Cannot be too small or too large. If ∆ is too large, unnecessary handoffs which burden the MSC may occur, and if ∆ too small there may be insufficient time to complete the handoff before a call is lost due to weak signal conditions. Thus ∆ has to be selected carefully.
  • In diagram of improper handoff it is shown that handoff is not made and the signal drops below the minimum acceptable level to keep the channel active. This dropped event happens when excessive time is taken by MSC to assign handoff or when ∆ is very small.
  • To decide when to handoff it is important to ensure that the drop in signal level is not due the momentary fading but it is due to mobile moving away from base station. In order to ensure that base station monitors the signal level for a certain period of time before handoff is made.
  • There are four types of hand off
    • Hard hand off
    • Soft hand off
    • Mobile assisted hand off
    • Intersystem hand off
A. Hard hand off: Frequency channels are taken back by the older base station before new connection is established. This is break before make type of handoff. In this frequency channels are utilized properly but chances of drops are more.
B. Soft hand off: Frequency channels are taken back by the older base station after new connection is established. This is break after make type of handoff. This is reliable but frequency channels are not utilized properly.
C. Mobile Assisted hand off: In this a mobile scan signal strength of all base stations near to it and continuously reports the results of these measurements to the serving base station. Whenever signal strength of its current base station decreases and becomes less than the power received from the neighbouring cell, it initiates handoff.
D. Intersystem hand off: This hand off occurs when the user moves from one MSC to another MSC i.e. from one cellular system to another cellular system while on a voice call. When mobile signal becomes weak in a given cell and the MSC cannot find another cell within its system to which it can transfer the call in progress intersystem handoff is implemented.

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