Understanding Handoff/Handover in Cellular Networks

This page describes the concept of handoff or handover, a crucial process in cellular communication. Handoff (or handover, the terms are used interchangeably) refers to the seamless transfer of a mobile connection from one network resource to another without interrupting an ongoing voice or data call.

This commonly occurs when a mobile device is moving, often at vehicular speed, while actively using a wireless cellular connection.

Factors Causing Handoff/Handover in a Wireless System:

Several factors can trigger a handoff:

1. Signal Strength/Power: A primary driver.
2. Speed of the Device/Mobile: Faster movement necessitates more frequent handoffs.
3. Weaker Signal from Serving Base Station & Stronger Signals from Neighboring Base Stations: As the mobile moves away from its current base station, nearby stations may offer a better signal.
4. Bit Error Rate (BER): A high BER indicates poor signal quality, prompting a search for a better connection.
5. Interference from Adjacent Channels: Interference can degrade signal quality, leading to a handoff.

Let’s examine the types of handoff/handover with respect to a GSM (Global System for Mobile Communications) system:

1. Handoff within the Same BTS (Base Transceiver Station): Transfer from one channel to another within the same cell site.
2. Handoff from One BTS to Another BTS (within One BSC - Base Station Controller): Moving the connection to a different cell site, but still under the control of the same BSC.
3. Handoff from One BSC to Another BSC (within One MSC - Mobile Switching Center): Transferring the connection to a completely different BSC, but the same MSC is managing both.

This process is typically network-initiated, operating in either an open-loop or closed-loop mode. The trigger is usually a drop in the received power from the mobile device at the base station below a certain threshold. This indicates the RF link is no longer strong enough to deliver the required BER or Eb/No (energy per bit to noise power spectral density ratio). The power estimation can be reported by the mobile device itself, or the base station can request the mobile to report its power level.

Types of Handoff: Soft vs. Hard

There are two main types of handoff: soft and hard, differentiated by the connection status during the transfer to the target resource.

• Hard Handoff - Break Before Make: In this type, the connection with the source channel/BTS/BSC is completely broken before a connection is established with the target channel/BTS/BSC. There’s a brief interruption in service during the handover.

• Soft Handoff - Make Before Break: In this type, the connection with the source channel/BTS/BSC is retained for a period of time while the connection with the target channel/BTS/BSC is being established. This allows for a smoother transition, and the user is less likely to experience any noticeable glitch or interruption in service. This results in a better user experience.