Handoff in Mobile Connections

In cellular communications, handoff (also called handover) is the process of transferring an active call or data session from one cell in a cellular network to another, or from one channel to another. In satellite communications, it is the process of transferring control from one earth station to another. Handoff is essential for preventing loss of service or interruption to a caller or data session user as they move through the network.

Situations for Triggering Handoff

Handoffs are triggered in any of the following situations:

• Mobility − When a subscriber moves out of coverage of one cell and enters the coverage area of another cell, a handoff maintains service continuity. The tasks performed by the first cell are transferred to the new cell.

• Capacity limitations − Each cell has a predefined capacity limit. When the number of users reaches maximum capacity, handoff transfers some calls to adjacent cells, provided subscribers are in overlapping coverage areas.

• Cell hierarchy management − Handoff occurs when transferring duties between macrocells and microcells. For example, a traveling user may be handed off from a large cell to a smaller microcell when they stop moving, optimizing network resources.

• Interference mitigation − Handoffs help resolve interference issues when calls use the same frequency for communication.

Types of Handoffs

There are two primary types of handoffs based on connection management:

• Hard Handoff − An actual break in connection occurs while switching from one cell to another. The radio link to the existing cell is broken before establishing a link with the target cell. This follows a "break before make" policy and is typically used for inter-frequency handoffs.

• Soft Handoff − At least one radio link is maintained when links are added and removed. This ensures no service interruption during handoff. It follows a "make before break" policy and is commonly used in co-located sites.

Mobile Assisted Handoff (MAHO)

Mobile Assisted Handoff (MAHO) is a technique where mobile devices assist the Base Station Controller (BSC) in transferring calls to another BSC. This approach is used in GSM cellular networks.

In traditional systems like AMPS, handoff is solely managed by the BSC and Mobile Switching Centre (MSC) without mobile device participation. However, in GSM networks using MAHO, when a mobile station is not using its allocated time slots for communication, it measures signal quality from nearby base stations and reports this information to the serving BSC. The BSC then makes informed handoff decisions based on this real-time signal quality data.

Conclusion

Handoff is a critical process that ensures seamless connectivity as mobile users move between cells or when network conditions change. The choice between hard and soft handoff depends on the specific network technology and requirements, with modern systems increasingly favoring techniques that minimize service interruption.