Handover

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Handover (or handoff) is the process of moving an active call or data session from one radio channel to another so the service continues without dropping. In satellite links, it means transferring control from one earth station to another without interruption. The wording varies by region: handover is common in British and European contexts, while handoff is used in American contexts.

There are two main kinds of handover:
- Inter-cell handover: the source cell and the target cell are different, which may mean moving from one cell site to another as the user travels. This keeps the call alive as you leave one cell’s coverage and enter another.
- Intra-cell handover: the user stays in the same cell but switches to a different channel, usually to get a clearer signal.

Handover can also be classified by how it happens:
- Hard handover: the system switches channels quickly, so the call uses only one channel at a time. It’s simple and cheap, but if the handover fails the call can be dropped. Reconnecting to the old cell is sometimes possible but not guaranteed.
- Soft handover: the call is kept on multiple channels across several cells at once, and a new connection is made before the old one is released. This reduces the chance of dropping the call and improves reliability, especially in areas with weak signals. The trade-off is more complex phone hardware and more network resources, which can reduce overall capacity.

Which technology uses which handover type varies:
- Analog and many older digital systems typically use hard handover.
- CDMA-based technologies (2G and 3G) use soft or softer handovers, which helps counter interference issues like the near–far problem.

How a handover is decided:
- Each cell keeps a list of neighbor cells that could be targets for a handover. This “neighbor list” is created using computer tools and field measurements or propagation models.
- While a call is in progress, the system monitors signal strength, quality, and other measurements on the current channel. If the numbers can’t meet the needed quality, a handover may be requested by the mobile device or by the base station, and the system selects the best target from the neighbors.
- In some systems (notably CDMA), a target can be chosen from outside the neighbor list to avoid interference from very strong nearby signals (the near–far problem).

In CDMA, a mobile phone in soft handover uses a rake receiver to process signals from several cells at once. The phone maintains an active set of cells whose signals are being used for the call. If a new cell provides a strong signal, it joins the active set.

Challenges and other cases:
- Handover can fail if no suitable channel is available or if interference makes it hard to maintain the call.
- There are inter-technology handovers too, such as moving a call between GSM and UMTS, or from cellular to Wi‑Fi. Standards exist to enable these transitions.
- Sometimes it’s better to prioritize a handover over a new call request to prevent dropping an ongoing call, a concept known as handover prioritization.

In short, handover keeps your conversation going as you move, balancing reliability, complexity, and network capacity with the right choice of technique and targets.