Handover process and practical considerations

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.

Practical handover considerations

1. High- and low-speed users – vehicles pass through small cells quickly, causing many handoffs; pedestrians may never need one. Handoff strategy must handle both.
2. Umbrella cell approach – large high-power umbrella cells (on tall towers) serve fast users, while microcells beneath serve slow users. Speed is estimated from signal fading rate; fast users are moved to the umbrella cell, reducing handoffs and MSC load.

 ┌──────── Umbrella cell (fast users) ────────┐
 │   (◯)    (◯)    (◯)    (◯)  microcells     │
 └────────────────────────────────────────────┘

3. Difficulty in obtaining new cell sites – in cities, operators often add capacity by placing new cells at existing sites, so mixed macro/micro planning and careful power levels are needed.
4. Cell dragging – a slow user with a strong LOS signal moves far into the neighbouring cell without handoff because the signal stays above threshold, causing interference. Careful threshold and coverage setting reduces it.
5. Handoff time and margin – 1G took about 10 s for handoff, needing Δ of 6–12 dB; 2G with MAHO needs only 1–2 s and Δ of 0–6 dB.
6. Soft handoff (CDMA) – the mobile connects to several base stations at once and the best signal is chosen, reducing dropped calls.
7. Intersystem handoff – when moving between MSCs or networks, extra signalling and billing (roaming) issues arise.
8. Prioritising handoffs – guard channels and queuing of handoff requests reduce forced termination.