In cellular communication, the process of moving from one cell to another while maintaining an active communication session is known as handover or handoff. This mechanism is crucial for providing seamless communication experiences to mobile users as they move through different coverage areas. Here's a high-level description followed by detailed mechanisms:
High-Level Description
The handover process ensures that a mobile device's connection is transferred from one cell to another without interrupting the ongoing call or data session. This is vital in mobile communications to maintain continuous service as users move, drive, or change location. The process involves measuring signal strengths, deciding when to handover, and then executing the handover to a new cell while ensuring the communication continuity.
Detailed Mechanisms
Handover Initiation:
- Signal Strength Measurement: Mobile devices constantly measure the signal strength of their current cell and neighboring cells. This information is used to assess the need for a handover.
- Thresholds and Events: Handover decisions are triggered when the measured signal strength crosses predefined thresholds or when specific events occur. These thresholds and events are designed to identify when the connection to the current cell becomes inadequate or when a neighboring cell can provide a better connection.
Handover Decision:
- The decision to initiate a handover is made by the network, taking into account various factors including signal strength, cell load, user speed, and Quality of Service (QoS) requirements.
Handover Execution:
- Preparation: Once a handover decision is made, the network prepares the target cell to accept the handover. This involves resource allocation in the target cell to ensure the continuity of the service.
- Switching: The mobile device is then instructed to switch its frequency or radio access technology (RAT) to connect to the target cell.
- Completion: After the switch, the mobile device establishes communication with the new cell. The old cell is informed of the successful handover, and the resources previously allocated to the device are released.
Types of Handover:
- Intra-system(Intra RAT) Handover: Occurs within the same cellular system (e.g., from one LTE cell to another LTE cell).
- Inter-system(Inter RAT) Handover: Involves switching between different types of networks (e.g., from an LTE network to a WiMAX or WiFi network).
- Hard Handover: Involves breaking the connection with the current cell before connecting to the new cell.
- Soft Handover: The device simultaneously connects to both the old and new cells before finally transferring to the new cell.
The seamless execution of handovers is a key aspect of modern cellular networks, enabling users to move freely while using mobile services without experiencing call drops or significant interruptions in data sessions.