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Hand off in GSM.
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  • If a user walks around with a wireless station, the station has to move from one access point to another to provide uninterrupted service. Moving between access points is called roaming. The term "handover" or "handoff" can also be used for the same.
  • The handoff procedure should be completed while the MS is in the overlap region.
    • As the MS moves towards the edge of the BS coverage, the signal strength and quality begins to deteriorate.
    • The signal from a neighboring BS (the new BS) becomes stronger than the signal from the serving BS (the old BS). Additionally, the new BS receives a stronger signal from the MS than that received by the old BS.
    • The conversation needs to be handed over to the new BS before the link between the old BS and the MS becomes unusable. Otherwise, the call is lost.
  • Three issues need to be considered for handoff management:
  1. Handoff detection
  2. Channel assignment
  3. Radio link transfer

1.  Handoff detection:

  • To initiate a handoff, two issues must be considered:
    • Who initiates the handoff process?
    • How is the need for handoff detected?
  • When to effect the handoff must be based on measurements of the links made at the MS, at the two BSs, or both.
  • It is obvious that the measurements can be made at either the MS or the BSs, but the decision to effect the handoff must be made either by the network or by the MS.
  • Handoff detection is based on radio link measurement. It determines the need for handoff and the target or new channel for transfer.
  • The propagation between the base station and the MS is made up of the direct line-of-sight path and scattering paths caused by reflections from or diffraction around buildings and terrain.
  • Thus, the signal received by the MS at any point consists of a large number of generally horizontally traveling uniform plane waves.

2. Channel assignment:

  • Handoff requests and initial access requests compete for radio resources.
  • At a busy BS, call attempts that fail because there are no available channels are called blocked calls.
  • Handoff requests for existing calls that must be turned down because there are no available channels are called forced terminations.
  • It is generally believed that forced terminations are less desirable than blocked call attempts.
  • Several channel assignment strategies have been developed to reduce forced terminations at the cost of increasing the number of lost or blocked calls.

3. Radio Link Transfer:

  • After the detection of handoff and assignment of channel, radio link of a mobile user has to be transferred from one base station to the other.

After this process handoff management is complete.

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