By Debjani De, Architect, Ravi Raj Bhat, Director Engineering & V. Srinivasa Rao, Architect
The initial euphoria surrounding femtocells has given way to pragmatism in order to identify and knock down real-world deployment barriers such as RF interference, business cases for consumers/operators/vendors, scaling the technology to support millions of Femtocell Access Point (FAP) units worldwide, enabling “plug-and-play” solutions, etc.
In a previous paper, Femtocell Network Architecture and Signaling Protocol Options [1], we had listed various signaling protocol options to realize a femtocell network solution at the Fa interface, illustrated in Figure 1. 3GPP and Femto Forum worked closely to identify tunneled-Iu as the preferred signaling protocol solution for Universal Mobile Telecommunication System (UMTS) networks. To this end, Femto Forum members worked closely with 3GPP to define the Iuh (Iu home) interface between a FAP and a Femto Gateway (FGW).
This paper focuses on signaling protocol solutions identified and defined to scale femtocells to millions of units of deployment worldwide. In particular, the paper describes and analyzes the control and data plane operations of a femtocell network with respect to the Iuh interface and protocols. Note that throughout the paper, the terms FAP and Home NodeB (HNB) are used interchangeably, as are the terms FGW and Home NodeB Gateway (HNB-GW).
Figure 1: Femtocell Reference Model (Source: Femto Forum)
IUH Interface Overview
Figure 2 illustrates the tunneled-Iu or Iu-over-IP approach for UMTS networks using existing UMTS protocols. In fact, most first generation UMTS Femtocell solutions will implement the signaling protocol illustrated in Figure 2. While this is an evolutionary and the least intrusive means of adding femtocell capability to the network, it does have certain limitations:
- The current Iu interface is not scalable to support millions of HNBs (one for each femtocell-capable home/enterprise) that are going to attach to core network (CN) elements such as the Mobile Switching Center (MSC). The SS7 emulation provided by Signaling Connection Control Part (SCCP) running over Message Transfer Part 3 User Adaptation (M3UA) protocol using underlying Internet Protocol (IP) networking does not provide enough addressing capability (i.e., point codes) to support millions of HNBs.
- In a femtocell network, because an HNB is a customer premise device it is possible that it will be powered on only when the femto subscriber wants to use femtocell services. This means that an HNB may dynamically join and depart from the network, similar to the subscriber’s user equipment (UE). So, we need an HNB registration and de-registration procedure in addition to UE registration and de-registration, which is not supported by existing UMTS protocols.
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