Open Access Mode

Description

Open access mode is a configuration option for Home NodeB (HNB) and Home eNodeB (HeNB) within 3GPP networks. In this mode, the H(e)NB functions as a standard NodeB or eNodeB, providing public access to any user equipment (UE) without restrictions. Unlike closed subscriber group (CSG) cells, which are limited to a specific set of subscribers, an open access H(e)NB does not broadcast a CSG Indicator or CSG Identity. This means that any UE can camp on and receive services from the cell, subject to normal network policies.

The architecture of an open access H(e)NB is integrated into the macro network. It connects to the core network via the same interfaces as a regular base station, such as the Iuh interface for HNB or the S1 interface for HeNB. The H(e)NB Gateway (HNB-GW or HeNB-GW) may be used to aggregate traffic from multiple H(e)NBs, but in open access mode, the cell behaves like any other cell in the network. The lack of CSG broadcasting simplifies mobility procedures because UEs do not need to check CSG subscription before accessing the cell. This makes open access H(e)NBs suitable for deployments where operators want to extend coverage or capacity in public areas, such as shopping malls or airports, without restricting access.

Key components include the H(e)NB itself, which manages radio resources and user plane traffic, and the core network elements like the Mobility Management Entity (MME) and Serving Gateway (SGW) for LTE, or the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) for UMTS. In open access mode, the H(e)NB does not perform CSG-based access control, so the network does not need to maintain a CSG subscription list for the cell. This reduces signaling overhead and simplifies network management. However, operators may still apply quality of service (QoS) and policy controls as they would for any macro cell.

The operation of an open access H(e)NB involves standard procedures for cell selection, handover, and bearer establishment. UEs discover the cell through normal cell search procedures and can initiate attach or handover without CSG checks. The H(e)NB broadcasts system information that includes the PLMN identity and other parameters, but omits the CSG Indicator and CSG Identity. This ensures that UEs not supporting CSG or not subscribed to any CSG can still access the cell. The network can differentiate between open access and CSG cells based on the absence of CSG information in the broadcast.

In summary, open access mode provides a flexible deployment option for H(e)NBs, allowing them to serve as public cells. It is particularly useful for improving coverage and capacity in high-traffic areas without the complexity of managing CSG subscriptions. The mode has been supported since Release 9 and continues to be relevant in later releases, with the definition remaining consistent after a wording clarification in Release 12.

Purpose & Motivation

Open access mode was introduced to allow Home NodeB (HNB) and Home eNodeB (HeNB) to operate as normal cells, addressing the need for improved coverage and capacity in public spaces without the restrictions of closed subscriber groups (CSGs). Before this mode, H(e)NBs were primarily deployed in closed access mode, limiting access to a specific set of users, such as a household or enterprise. This limited their utility in public areas where operators wanted to offload traffic from macro cells or extend coverage.

The motivation for open access mode stems from the desire to leverage the low-cost, plug-and-play nature of femtocells and small cells for public deployments. By allowing any subscriber to access the cell, operators can enhance network performance in hotspots like shopping centers, stadiums, and airports. It also simplifies network planning and mobility management because UEs do not need to be aware of CSG subscriptions, reducing signaling and potential access failures.

Open access mode solves the problem of limited applicability of H(e)NBs in public scenarios. It enables operators to deploy small cells that integrate seamlessly with the macro network, providing additional capacity and coverage without the overhead of managing CSG lists. This is particularly important in heterogeneous networks (HetNets) where small cells complement macro cells. The mode has been standardized since Release 9 and remains a key feature in subsequent releases, ensuring interoperability and consistent behavior across different network implementations.