Candidate Cell for Energy Saving

Description

In 3GPP network energy saving (NES) frameworks, a candidate cell is a cell that can provide coverage for the service area of an original cell when that original cell is switched off or placed into an energySaving state. This concept is central to Energy Saving Management (ESM) as defined in specifications such as TS 28.310 (Energy Efficiency of 5G) and TS 32.551 (Energy Saving Management). The candidate cell acts as a fallback or compensating cell, ensuring that coverage and capacity requirements are still met while the original cell reduces its energy consumption.

The architecture relies on the network management system (NMS) or element manager (EM) to coordinate energy-saving actions. When an original cell is a candidate for energy saving (e.g., during low traffic periods), the management system evaluates whether neighboring cells—the candidate cells—can absorb the traffic and provide sufficient coverage. If so, the original cell may be switched off or put into a dormant state. The candidate cell then serves the affected user equipments (UEs). This process is governed by policies and algorithms that consider factors such as load, coverage, QoS requirements, and energy efficiency metrics.

Key components include the Energy Saving Management function, which can be centralized (in the NMS) or distributed (in the base station). The candidate cell must be able to handle the additional load and maintain service continuity. The decision to switch off a cell is typically based on traffic patterns, and the candidate cell must be capable of providing the necessary radio coverage. In multi-RAT (Radio Access Technology) scenarios, candidate cells may belong to different RATs, such as LTE or NR, allowing for inter-RAT energy saving.

The concept has evolved with the introduction of 5G and network slicing, where energy saving must consider slice-specific requirements. The candidate cell concept ensures that energy efficiency gains do not compromise service level agreements (SLAs). It is also relevant for self-organizing networks (SON), where automated neighbor relation (ANR) and mobility robustness optimization (MRO) interact with energy saving decisions to avoid coverage holes or excessive handovers.

In summary, the candidate cell is a fundamental enabler for green networks, allowing operators to reduce operational expenditure (OPEX) and carbon footprint while maintaining seamless service. Its implementation requires careful coordination between management plane and control plane functions, and it is standardized across multiple releases to ensure interoperability.

Purpose & Motivation

The candidate cell concept exists to address the growing need for energy efficiency in mobile networks. As traffic demands fluctuate, especially with diurnal patterns, many cells are underutilized during certain periods, leading to wasted energy. Traditional networks kept all cells active to guarantee coverage, resulting in high power consumption. The candidate cell mechanism allows operators to dynamically switch off underutilized cells and rely on neighboring cells to provide coverage, thereby reducing energy consumption without sacrificing service quality.

Historically, energy saving was managed manually or with simple rules, but as networks became denser and more complex, automated and standardized approaches were needed. 3GPP introduced Energy Saving Management (ESM) in Release 10, defining the candidate cell as a key element. This allowed for coordinated switch-off and switch-on procedures, ensuring that coverage and capacity are maintained. The concept also supports the integration of renewable energy sources and helps operators meet regulatory targets for carbon reduction.

With the advent of 5G and network slicing, the candidate cell concept has been extended to consider slice-aware energy saving, where the candidate cell must support the same slices as the original cell. This ensures that service continuity is maintained for all tenants. The candidate cell thus plays a critical role in the transition towards sustainable, energy-efficient mobile networks.