Hybrid Access Mode

Description

Hybrid access mode is a configuration for Home NodeB (HNB) and Home eNodeB (HeNB) femtocells, introduced in 3GPP Release 9. It allows a femtocell to serve both its Closed Subscriber Group (CSG) members and non-CSG users, effectively acting as a hybrid between a closed CSG cell and an open access cell. This mode is defined in specifications such as TS 22.220, TS 23.830, and TS 33.107. The key characteristic is that a hybrid cell broadcasts a CSG Identity (CSG ID) but does not broadcast a CSG Indicator, which distinguishes it from a closed CSG cell that broadcasts both. This broadcast behavior signals to UEs that the cell is accessible to all, but with preferential treatment for CSG members.

From an architectural perspective, hybrid access mode operates within the Home NodeB (HNB) or Home eNodeB (HeNB) subsystem, which includes the H(e)NB, the Home NodeB Gateway (HNB-GW) or HeNB Gateway (HeNB-GW), and the core network. The H(e)NB is a small base station typically deployed in homes or small businesses to improve coverage and capacity. When configured for hybrid access, the H(e)NB's access control mechanisms differentiate between CSG members and non-members. CSG members are UEs whose subscription includes the CSG ID of the hybrid cell; they receive priority access and may enjoy better QoS or preferential admission control. Non-CSG members are allowed to access the cell but may be subject to lower priority, resource limitations, or different charging. The H(e)NB broadcasts system information including the CSG ID, which UEs use to determine their membership status. UEs that are members can use the cell as a CSG cell, while non-members treat it as a normal cell.

The operation of hybrid access mode involves several key procedures. When a UE attempts to access a hybrid cell, it reads the broadcast information. If the UE is a member of the CSG, it may initiate access with the CSG identity, and the network can grant it CSG member privileges. If the UE is not a member, it can still access the cell as a normal cell, but the network may apply different policies, such as lower scheduling priority or restricted services. The H(e)NB and core network entities, such as the Mobility Management Entity (MME) or Serving GPRS Support Node (SGSN), enforce these policies based on subscription information and local configuration. The access control is managed through the CSG list stored in the UE and the network's subscription database (e.g., HSS).

Hybrid access mode provides flexibility for operators and users. It allows a femtocell owner to share their cell with guests or passersby while still prioritizing their own devices. This can improve overall network coverage and capacity, especially in areas with high traffic. It also enables new business models, such as offering differentiated services to non-members. However, it introduces complexity in terms of access control, interference management, and charging. The 3GPP specifications define the necessary signaling and procedures to handle these aspects, ensuring that hybrid cells can be integrated seamlessly into macro networks.

In summary, hybrid access mode is a feature that enhances the utility of femtocells by making them accessible to a wider range of users while preserving the benefits of CSG membership. It is a key enabler for small cell deployments in heterogeneous networks, contributing to improved user experience and network efficiency.

Purpose & Motivation

Hybrid access mode was created to address the limitations of both closed and open access modes for femtocells. Closed access mode, where only CSG members can access the cell, provides exclusive benefits but can lead to poor user experience for non-members who are nearby and could benefit from the improved coverage. Open access mode, where the cell is accessible to all, maximizes coverage but does not offer any preferential treatment to the owner or specific users. Hybrid access mode strikes a balance by allowing both groups to access the cell while giving priority to CSG members. This solves the problem of underutilized femtocells in closed mode and provides a way to offer differentiated services. It also helps operators manage interference and load more effectively by allowing non-members to use the cell when resources are available, rather than being forced to connect to potentially weaker macro cells. The introduction in Release 9 was motivated by the growing deployment of femtocells and the need for more flexible access control mechanisms to support diverse deployment scenarios and business models.