Operators are constantly looking for new ways to increase revenues and decrease costs as traditional voice services continue to offer reduced margins due to increased competition and regulatory constraints. Facing this challenge, one technology that has received a great deal of press over the past year is the femtocell. A femtocell is a very small base station. So small, in fact, that it can be placed in a customer's residence. So it was no surprise that the main standards body, the ETSI 3GPP, recently decided to adopt the name "Home Node B" in its ongoing standardization activities to describe femtocells.
The implementations of femtocells provide significant benefits that help operators considering the investment. The most common benefits result in:
- Off-loading macro radio network facilities
- Improving coverage locally in a cost-effective manner
- Implementing home-zone services to increase revenue
The purpose of this article is to identify the issues surrounding this new technology, describe end-to-end architecture alternatives, and elaborate on their market potential.
Femtocell Characteristics
Femtocells, like macro base stations, connect "standard" phones to the operator's network by a physical broadband connection which is most likely xDSL and cable. Since femtocells will operate in the residential space and users may move between an indoor and outdoor environment, a handover mechanism between the macro network and the femtocell is required. The capacity of such cells must be adequate to address a typical family use model supporting two to four simultaneous voice calls. Femtocells also must handle data. Typically, broadband technologies supply more throughput as compared to current or evolving 2G/3G standards, i.e., EDGE or HSPA resulting in no network bottlenecks. For 2G femtocells, mobile broadband data, such as that from a PC/laptop, can be handled from a widely deployed WLAN interface. Femtocells allow operators to offer innovative pricing plans to consumers at reduced or flat rates competitive with fixed line plans.
Another important characteristic of femtocells is their ability to control access. The two most common access types are open and closed. In an open access scenario, any terminal/subscriber is allowed to communicate with the femtocell. In this way, the femtocell usage resembles that of a macro cell. In a closed access scenario, the femtocell serves a limited number of terminals/subscribers that are subscribed to the given base station. In this case, the base station is perceived as being deployed for private usage.
Femtocell Issues
Like any new technology, femtocells come with issues that need to be resolved for them to become a successful, widely deployed technology. Femtocells need to be scalable and integratable. They also must be robust enough to handle interference and support synchronization. Finally, femtocells must meet all regulatory requirements.
Scalability and Core Network Integration
Today's networks rely on centralized operations support systems (OSS), billing/business support systems (BSS), and operations, administration, maintenance and provisioning (OAM&P) systems to perform the behind-the-scenes functions required to cost-effectively operate a large network. The bulk of these systems were designed around the concept of a centralized, hierarchical, intelligent network model rather than a distributed, peer-to-peer network designed to work closely with intelligent devices that originate as much content as they consume.
Integration of the femtocell model, therefore, comes with its own set of unique challenges. In traditional 3G networks, radio network controllers (the 3G network entity responsible for control of all Node Bs) communicate with the base transceiver station (Node B) over dedicated high-speed facilities that rely on the protocol known as Iu-b. In contrast, to control costs, femtocell designs typically use IP connections, often the Internet, for access to the network.
This creates a network design (and potentially, engineering) challenge for the service provider: How to cost-effectively design, build, operate and monetize the distributed, peer-to-peer network? Clearly, this concept is far removed from traditional network design considerations. First, because it assumes the deployment of thousands of femtocell nodes, existing back room systems must be modified to accept and integrate them with full functionality. Failure to do so is a show-stopper. Second, because femtocells reside on customers' premises, their management processes must be built around worst case scenarios of device availability. Femtocells don't necessarily enjoy access to uninterruptible power nor to central office-quality maintenance as traditional base stations do. This assumption must be designed into the service logic. Third, security considerations must be taken into account to ensure that femtocells are properly authenticated before being granted network access and to guard against the potential for network intrusion.
Finally, given the potential size of the market and the economics that will derive from it, standardized interfaces must be put into place to facilitate the involvement of third party manufacturers and to embrace what is rapidly becoming an open development environment.
Handling Interference
Interference is expected to be a significant issue for femtocell deployments based on wideband technologies such as WCDMA. This is either because initial operator deployments will use the same frequency for both the femtocell and the macro networks or due to the proximity of femtocell base stations in dense urban areas.
Two interference scenarios are anticipated. Femto-to-femto interference and Femto-to-macro interference.
The study of femto-to-macro interference from a recent study [1] has shown the existence of macrocell "dead zones" caused by the downlink interference from the femtocell which prevents visiting user equipment access in the dead-zone. To complicate the issue further, the size of the dead zone in relation to the home-NodeB coverage area will depend on the proximity of the femtocell site to the macrosite.
1. Dead Zone definition
Three solutions to combat interference have been proposed:
- The use of different frequencies for macro and femto networks if available
- A decreased femtocell power to reduce the effect of dead zones
- Implementing 2G as a fallback in dead zones to mitigate interference
Synchronization
Femtocells, like any other cellular technology, require synchronization. In general, three types of synchronization might be required, namely, frequency, time and phase. For example, for UMTS 3G/HSPA FDD type of deployments, frequency type of synchronization is required. Femtocells will be deployed with a packet type of backhauling interface (typically Ethernet/IP). Synchronization methods for Ethernet/IP networks are either the well-known NTP protocol or the newly standardized IEEE 1588 standard.
Regulatory issues
Another issue not to be overlooked, is the potential regulatory impact that femtocell deployments may introduce. Femtocells use licensed frequencies that radiate at a very low power in a controlled, residential environment. It is likely they will not require a license from the local authority, as macro base stations do. An additional regulatory issue will arise from the relationship between the femtocell operator and the broadband services operator. There are two possible scenarios. One is that the broadband operator is unaware of the existence of a femtocell operator. Conversely, the broadband operator and femtocell operator may have made some sort of agreement (or they are the same operator). In the latter case, there might arise regulatory concerns due to possible unfair competition (non replicated offer from competition).
Femtocell Network Architecture
A typical femtocell network architecture is depicted in Figure 2.
2. The Femto Architecture
Femtocell base stations, located on customer's premises, are backhauled through the customer's standard broadband connection, i.e., xDSL or cable. Since the traffic between the customer's premises equipment and the operator's network is traversing a public network it needs to be secured through a tunneled connection (typically an IPsec tunnel) that is terminated to a security gateway at the edge of the operator's network.
Approaches to Femtocell Deployment
Today there are four basic design models for deployment and integration. The first is the IP-based Iu-b interface (3GPP Rel.5). The second is a SIP-based approach (Iu/A Interface). Third, relies on the use of unlicensed spectrum in a technique known as Unlicensed Mobile Access (UMA). A fourth approach, which takes into account emerging IMS standards (IMS VCC), will be briefly described as well.
Iu-b BSC/RNC-Based Femto cell Deployments
In the Iu-b model, femtocells are fully integrated into the wireless carrier's network and treated like any other remote node in the network. The Iu-b protocol has a number of responsibilities including the management of common channels, common resources, and radio links along with configuration management, including cell configuration management; measurement handling and control; TDD synchronization; and error reporting. In Iu-b configurations, mobile devices access the greater network and its services via the Node B link, and femtocell are treated as traditional base stations.
SIP-Based Femtocell Deployments
As a central element in IP-based access and transport networks for signaling functions, SIP also stands to play a role in femtocell deployment. A SIP client, embedded in the femtocell, uses SIP to communicate with the SIP-enabled mobile switching center (MSC). The MSC performs the operational translation between the IP SIP network and the traditional mobile network.
3. The integrated BSC/RNC approach
