Network Evaluated Handover

Description

Network Evaluated Handover (NEHO) is a fundamental mobility management concept in cellular networks, particularly defined in 3GPP specifications for GSM, UMTS, and LTE. It refers to the handover procedure where the decision to initiate a handover from one cell to another is made by the network infrastructure based on measurement reports it receives from the User Equipment (UE). The process is network-controlled. The UE's role is primarily to perform radio measurements on the serving cell and neighboring cells as instructed by the network (through measurement control messages) and to send these measurement reports back to the network entity (e.g., the Base Station Controller (BSC) in GSM, the Radio Network Controller (RNC) in UMTS, or the eNodeB in LTE).

Upon receiving these reports, the network entity (the decision point) processes the data. It applies algorithms and configurable parameters (like handover margins, hysteresis, and thresholds for signal strength (RxLev) and quality (RxQual)) to determine if a handover is necessary and, if so, to which target cell. The evaluation considers factors such as the received signal level from the serving cell falling below a threshold while the signal from a neighbor cell is sufficiently stronger, or the quality of the serving cell link degrading. The network also has a broader view, potentially considering network load, cell capacity, and subscriber priorities, which the UE cannot assess.

Once the decision is made, the network orchestrates the entire handover execution. It reserves resources on the target cell, sends a handover command to the UE (e.g., a HANDOVER COMMAND in GSM, RADIO BEARER RECONFIGURATION in LTE), and coordinates with the core network to switch the user plane path. The UE follows the command to detach from the old cell and synchronize with the new one. This centralized control allows for optimized network-wide mobility management, load balancing, and consistent policy enforcement, but it relies on timely and accurate measurement reporting from the UE.

Purpose & Motivation

NEHO exists to provide centralized, network-optimized control over mobility, which is crucial for managing radio resources efficiently, maintaining call quality, and ensuring overall network stability. In early cellular systems like GSM, the network (BSC) had full control due to the limited intelligence and processing power of early mobile terminals. This approach allowed operators to implement complex handover algorithms tailored to their network topology, frequency planning, and traffic management goals from a central point.

It solves the problem of uncoordinated mobility decisions that could arise if UEs acted autonomously. A purely UE-centric decision (MEHO) might lead to "ping-pong" handovers between cells or handovers to cells that are already congested, degrading overall network performance. NEHO allows the network to consider global parameters like cell load, available capacity, and interference levels, enabling features like load-based handover and directed retry. This centralized intelligence is essential for features like hierarchical cell structures (macro/micro cells) and for enforcing operator policies on subscriber roaming and service continuity. While later technologies introduced more UE-assisted and network-controlled paradigms (like in LTE and 5G), the core principle of NEHO—where the network is the final decision authority based on collected data—remains a cornerstone of managed cellular mobility.

Key Features

• Handover decision is made by the network entity (BSC, RNC, eNodeB, gNB)
• UE provides measurement reports as instructed by network measurement control
• Network applies configurable algorithms and thresholds (hysteresis, offsets) to evaluate reports
• Enables network-wide optimization (load balancing, traffic steering)
• Supports complex cell structures and hierarchical networks
• Allows for policy-based handover control (e.g., subscriber priority, service type)

Evolution Across Releases

Defining Specifications