Explain the Handoff in 3G Systems.

Answer:
Handoff in 3G systems:

3G systems consist of CDMA 2000 and WCDMA

1. Handoff in CDMA 2000: Handoff in CDMA 2000 is similar to IS95
2. Handoff in WCDMA: In WCDMA, the handoff procedure is different. Here, different set of pilots are maintained and the active set corresponds to the pilot channels being used for completing the call.

In General: In 3G systems, soft handover and its variants are used.

1. Soft Handover refers to the process by which a mobile user is in communication with multiple candidate cell sites before finally deciding to transfer data through one of them.
2. Power control is closely related to Soft Handover.
3. With Soft Handover, a conditional decision is made whether to handoff or not.
4. In Soft Handover, a mobile user is temporarily connected to more than one Base Station at the same time. Whenever a mobile user enters a region in which the received signal levels from two adjacent Base Station are comparable within some threshold level of each other, the mobile user enters the Soft Handoff condition in which it is connected to both the Base Station.
5. The mobile user remailns in this state until the received signal level from one Base Station clearly predominates. Then the mobile user is assigned exclusively to that Base Station to continue its operation.
6. In Soft Handoff state, the transmissions from the mobile user arriving at both Base Stationare forwarded to Mobile Switching Center which estimates the quality of two signals & selects the better one.
7. In general, the Soft Handoff procedure involves several cell sites. A controlling primary serving cell site coordinates the addition or removal of other cell sites to the ongoing call during Soft Handoff direction message to indicate the pilot channels to be used or removed as part of the soft Handoff process.
8. After handoff, the primary cell site is also changed. The signalsfrom multiple cell sites are combined either in Base Station Controller or Mobile Switching Center and processed as a single cell.

There are 3 types of soft Handoff:

1. Softer Handoff: In softer handoff or intersector Handoff, the mobile user communicates with two sectors of the same cell at the same time.
2. Soft Handoff: In Soft Handoff or intercell Handoff, the mobile user communicates with two (or three) sectors at different cells at the same time. A 3-way soft handoff may end by first dropping one of the sectors of any Base Station and becoming a two way soft handoff. A soft handoff uses more network resoursces than softer handoff.
3. Soft-Softer Handoff: In this, the candidate sector for handoff includes two sectors from same cell & a third sector from a neighbouring cell.