Handover process and types of handover

Handoff (handover) is the process of transferring an ongoing call from one channel or base station to another without interrupting the call, when the mobile moves into a different cell or the signal quality falls.

Handoff process

1. The base station (or mobile) continuously monitors received signal strength (RSSI) and quality.
2. When the signal from the serving cell falls to a handoff threshold (Pr,handoff), a handoff is initiated.
3. Neighbouring base stations measure the mobile's signal (1G locator receiver) or the mobile measures neighbours' signals and reports them (MAHO in 2G).
4. The MSC selects the best target cell, finds a free channel there and instructs the mobile to retune.
5. The call is switched to the new base station before the signal reaches the minimum usable level (Pr,min); otherwise the call drops.

 Power   BS1 signal             BS2 signal
   │ \                         /
   │   \ ← Pr,handoff          /
   │     \  Δ               /
   │       \ ← Pr,min     /
   └──────────────────────────── distance
   BS1          handoff         BS2

The margin Δ = Pr,handoff − Pr,min must be chosen carefully: too large causes unnecessary handoffs, too small causes dropped calls.

Types of handover with applications

1. Hard handover ("break before make") – the old link is released before the new one is set up; the mobile changes frequency or time slot. Used in FDMA/TDMA systems: AMPS, GSM, LTE.
2. Soft handover ("make before break") – the mobile is connected to two or more base stations at the same time on the same frequency, and the MSC selects or combines the best signal. Used in CDMA systems: IS-95, CDMA2000, WCDMA. Fewer dropped calls.
3. Softer handover – between two sectors of the same base station in CDMA; signals combined at the base station.
4. Intra-cell handover – change of channel within the same cell due to interference; used in GSM.
5. Inter-cell (intra-MSC) handover – between cells under the same MSC.
6. Inter-MSC / intersystem handover – when the mobile moves to a cell controlled by another MSC or system (e.g. 4G to 3G when LTE coverage ends, or roaming).
7. Mobile-controlled, network-controlled and mobile-assisted (MAHO) handover – based on who measures and decides; MAHO is used in GSM, where the mobile reports neighbour cell power levels and the network decides.