Acceptable Cell

Description

An Acceptable Cell is a concept in 3GPP specifications that defines a cell on which a User Equipment (UE) can camp to obtain limited service when no Suitable Cell is available. The conditions for a cell to be considered acceptable are specified in the relevant technical specifications (e.g., TS 25.304 for UTRAN, TS 36.304 for E-UTRAN, TS 38.304 for NR). These conditions typically include: the cell is not barred, the cell is not reserved for operator use, and the UE is not prohibited from accessing the cell based on its access class or other restrictions. The cell must also meet certain signal quality thresholds, though these are less stringent than those for a suitable cell.

The primary purpose of an acceptable cell is to enable the UE to attempt emergency calls and, in some cases, receive limited services such as Earthquake and Tsunami Warning System (ETWS) or Commercial Mobile Alert System (CMAS) notifications. The UE can camp on an acceptable cell only when it cannot find a suitable cell, which would provide normal service. This ensures that the UE can still make emergency calls even in areas where only restricted cells are available, such as during network congestion, partial network outages, or when the UE is in a limited service state.

The process of cell selection and reselection involves the UE scanning for cells and evaluating them against the criteria for suitable and acceptable cells. The UE first attempts to find a suitable cell; if none is found, it may camp on an acceptable cell. The specific conditions for an acceptable cell are defined in the specifications and include checks on the cell's barring status, the UE's access class, and the cell's signal strength and quality. For example, in E-UTRAN, a cell is considered acceptable if it is not barred, not reserved, and the UE is not in a forbidden tracking area, and the cell selection criteria (S-criteria) are met.

In NB-IoT (Narrowband IoT), the definition of an acceptable cell was updated in Release 19 to exclude NB-IoT from the statement that the UE can always attempt emergency calls on an acceptable cell. This reflects the fact that NB-IoT is not designed for emergency calls, and thus the UE may not be able to make emergency calls on an acceptable NB-IoT cell. This change aligns with the specific characteristics of NB-IoT, which is optimized for low-power, low-data-rate applications and may not support voice services.

The concept of an acceptable cell is crucial for maintaining basic connectivity in adverse conditions. It allows the UE to stay connected to the network for emergency services and receive critical alerts, even when normal service is not available. This enhances the overall robustness and reliability of the network, ensuring that users can always reach emergency services when needed. The evolution of the definition over releases has been minimal, with the main change being the exclusion of NB-IoT from emergency call support in Release 19.

Purpose & Motivation

The Acceptable Cell concept exists to ensure that User Equipment (UE) can obtain limited service, particularly emergency calls, when no Suitable Cell is available. This is critical in scenarios such as network congestion, partial network failures, or when the UE is in a location with only restricted cells. Without this concept, a UE might be unable to make emergency calls in such situations, which could have life-threatening consequences.

Historically, as cellular networks evolved, the need for a fallback mechanism became apparent. Early networks might not have had such a concept, but with the introduction of more complex cell selection criteria and the need to support emergency services universally, the Acceptable Cell was defined. It addresses the limitation that a UE might not always find a cell that provides full service, but it should still be able to access emergency services.

The concept also supports regulatory requirements for emergency call support. Many countries mandate that emergency calls must be possible even without a SIM card or when the UE is in a limited service state. The Acceptable Cell ensures that the UE can camp on a cell that allows emergency calls, thus complying with these regulations. Over time, the definition has been refined to exclude technologies like NB-IoT that do not support emergency calls, ensuring that the concept remains relevant and accurate.