Description
Hard Handover (HHO), also known as 'break-before-make' handover, is a fundamental mobility procedure in cellular networks where a User Equipment (UE) transitions its connection from one cell (the source) to another (the target) by first disconnecting from the source cell and then establishing a connection to the target cell. This process inherently involves an interruption in the active radio link. The handover is 'hard' because there is no period during which the UE is simultaneously connected to both cells. The entire process—including measurement reporting, decision-making, execution, and radio resource re-establishment—is controlled by the network (network-controlled handover).
The technical execution of a hard handover follows a defined sequence. First, the UE, under network command, performs measurements on the serving cell and neighboring cells. These measurements (e.g., signal strength, quality) are reported to the network. Based on pre-configured thresholds and algorithms, the network's Radio Resource Control (RRC) layer decides a handover is necessary. The network then prepares resources on the target cell (e.g., allocates a traffic channel). It then sends a 'Handover Command' message to the UE, instructing it to leave the current channel and tune to the new channel on the target cell. Upon receiving this command, the UE disconnects from the source cell, synchronizes with the target cell, and performs a random access procedure to establish the new link. Finally, a 'Handover Complete' message is sent to the network via the new cell.
Key components involved include the UE's measurement unit, the Base Station (Node B, eNodeB, gNB) of both source and target cells, and the controlling network entity (BSC, RNC, or in 5G, the gNB-CU). The interruption time, though brief (typically tens to a few hundred milliseconds), is a critical performance metric. During this time, user data packets may be lost or delayed, which can impact the quality of real-time services like voice calls or video streaming. To mitigate this, protocols like packet forwarding from the source to the target node can be used to reduce data loss.
HHO is contrasted with Soft and Softer Handovers used in CDMA-based systems like UMTS, where the UE maintains simultaneous connections during the transition ('make-before-break'). In GSM, LTE, and 5G NR (in most frequency bands), hard handover is the standard procedure due to the nature of the radio access technology (FDMA/TDMA/OFDMA). In 5G, while the core handover principle remains, it has been optimized with faster signaling and dual-active protocol stacks to minimize interruption time, but it still follows the break-before-make paradigm.
Purpose & Motivation
Hard Handover exists as the foundational method for maintaining call and session continuity as a user moves through a cellular network. Its primary purpose is to seamlessly transfer an ongoing communication session from one radio cell to another without requiring user intervention, thereby enabling mobility. The 'break-before-make' approach was a necessary design choice for early cellular systems like GSM, which used Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). In these technologies, a UE's radio can only be tuned to one frequency/time slot at a time, physically preventing a simultaneous connection to two cells.
The problems it solves are cell boundary management and interference avoidance. As a UE moves to the edge of a cell, signal quality degrades. HHO allows the network to proactively command the UE to a stronger cell before the connection drops, improving call drop rates and overall service quality. It also allows the network to balance traffic load between cells and manage radio resources efficiently. The network-centric control allows for optimized decisions based on a global view of network conditions.
While later technologies introduced soft handover for improved reliability at the cost of greater resource consumption, HHO remained relevant due to its simplicity, efficiency in spectrum usage (no simultaneous channels occupied), and suitability for non-CDMA air interfaces. Its evolution has focused on minimizing the inherent interruption time through faster signaling, better synchronization between cells, and protocol optimizations, ensuring it remains viable for even low-latency 5G applications.
Key Features
- 'Break-before-make' operation: source connection is released before target connection is established
- Network-controlled decision based on UE measurement reports
- Involves a brief but unavoidable interruption in data transmission
- Standard handover type for GSM, LTE, and 5G NR (in non-DC scenarios)
- Requires radio resource allocation in the target cell prior to execution
- Uses explicit Handover Command and Handover Complete messaging
Evolution Across Releases
Defined as the standard handover procedure for GSM and the initial UMTS/WCDMA specifications for inter-frequency and inter-system handovers. In the UMTS context, it was used alongside soft handover, specifically for scenarios where simultaneous transmission was not possible, such as when changing frequency or radio access technology (e.g., UMTS to GSM). The core break-before-make process and associated signaling were firmly established.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |
| TS 32.404 | 3GPP TR 32.404 |
| TS 32.405 | 3GPP TR 32.405 |
| TS 32.863 | 3GPP TR 32.863 |