Hand off and its strategy in GSM

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

Basic handoff idea

• Each BS watches signal level. A handoff threshold is set a little above the minimum usable level:

Δ = Pr,handoff − Pr,minimum usable

• If Δ is too large, there are unnecessary handoffs; if too small, the call may drop before handoff completes.
• The system checks that the drop in signal is not just momentary fading, so it averages the signal over time (dwell time).

 Signal
   |  BS1 signal          BS2 signal
   |\                        /
   | \______ handoff ______/
   |   \    threshold    /
   |----\--------------/----- minimum usable level
   +------------------------------ distance
   BS1           A   B          BS2
   (handoff made between A and B)

Handoff strategy in GSM: mobile assisted handoff (MAHO)

GSM uses Mobile Assisted Handoff and hard handoff (break-before-make).

1. The BSC sends the MS a list of neighbour cells (BCCH allocation list).
2. In idle TDMA time slots, the MS measures the received level (RXLEV) of up to 32 neighbours, and RXLEV and RXQUAL (bit error quality) of the serving cell.
3. The MS reports the six strongest neighbours plus serving cell values to the BTS on the SACCH about every 480 ms.
4. The BTS adds its own uplink measurements and timing advance (distance).
5. The BSC decides when to hand off, based on level, quality, distance or traffic load, and selects a target cell.
6. The target channel is reserved, the MS is commanded to switch, it sends access bursts on the new channel, and the old channel is released.

Because the MS does the measuring, handoff decisions are faster (about 1–2 s) than in network-controlled handoff of first-generation systems.

Types of GSM handoff

Prioritizing handoffs

• Guard channels: some channels are reserved only for handoff requests.
• Queuing of handoff requests: a request is queued while the mobile is still in the overlap area.
• Umbrella cells are used for fast-moving users to reduce the number of handoffs.