This article outlines the key challenges of mass femtocell deployment as they relate to RF. However, to start off we need to establish a common baseline of understanding by defining what a femtocell is and why such widespread interest in femtocells exists. To avoid confusion we will leverage the definition of a femtocell — or home base station — as provided by the Femto Forum chairman, Simon Saunders, at the 2nd International Conference on Home Access Points and Femtocells. (The Femto Forum is a non-profit organization committed to the standardization and adoption of femtocell technology whose members include leading femtocell players such as Continuous Computing, picoChip and RadioFrame Networks.)

The Femto Forum defines a femtocell as a low-power domestic access point that exhibits the following attributes:

• Uses mature mobile technology
• Operates within licensed spectrum
• Generates additional coverage and capacity within the cellular network
• Leverages the consumer's broadband connection to use internet-grade backhaul
• Is a low-price device
• Is managed fully by the operator

The definition from the Femto Forum provides a very pragmatic set of parameters for identifying what is, and what is not, a femtocell. For example, a WiFi access point is clearly not a femtocell since it does not operate in licensed spectrum and does not leverage mature mobile technology. However, for the RF designer, a handful of key technical parameters are needed to supplement this femtocell definition. These include:

• Output power is expected to be less than 0.1 Watts
• Radio signal strength in the 10 dBm range (see Figure 1)
• Capacity to support roughly four to eight simultaneous users, with most initial implementations focused on four users

Now that we have a good heuristic for defining a femtocell, it is important to note why exactly an operator is interested in deploying femtocells. The primary interest is being driven by mobile operators with 3G networks that are facing adoption issues due to the "3C" problem of coverage, capacity, and churn. From a coverage perspective, the majority of 3G networks are deployed in higher frequency spectrum, which reduces in-building penetration and ultimately leads to a coverage gap. This reality, combined with the fact that consumers utilize 3G services indoors almost 70 percent of the time, results in a poor overall user experience, and, in turn, increased subscriber churn. Finally, macro network capacity continues to be an expensive challenge. In comparison to macro-network capacity expansion, a femtocell roll-out is cheaper and more efficient as the demand comes specifically from consumers lacking sufficient coverage or requiring additional capacity.

In addition to providing a compelling solution to the 3C issues of 3G, femtocells provide a platform for delivering supplementary services beyond what is available on existing 3G networks—for example, a "triple-screen" play where the same service is available seamlessly across the handset, the television, and the personal computer. It is the combination of a solution to the 3C problem and the provision of supplementary services that is expected to drive forward operators' business cases for femtocell deployment.

Despite all their promise, femtocells are not without technical and economic challenges. The problems include a lack of architectural standards, RF network design and impacts, and probably most importantly, cost. These challenges are interdependent and the remainder of this article addresses the femtocell obstacles.

The first issue addressed is the impact of a mass femtocell deployment on the RF environment and the subsequent need for an approach that minimizes interference. Simply put, there is a high level of discomfort concerning what will happen to the RF environment in a mass femtocell deployment. The management of overlapping cells is provided for and proven in the suite of 3G standards, but will this be sufficient when there is the potential for millions of small cells in a compact environment? Adaptive radio control mechanisms do exist, but will they provide for the type of user experience required to deliver on the femtocell promise?

The consumer will want to utilize the femtocell throughout the home and remain on the femtocell as long as possible prior to handover in order to take advantage of favorable tariff plans, so it is not clear yet what will happen when there is acceptable macro coverage in parts of the home. If one increases the power of the femtocell, will it create interference dead zones that could affect macro network users? These are very real and challenging issues that will require further study and innovation.

The fact is that every femtocell manufacturer has a relatively effective solution to managing interference, and these solutions are being proven in real-world deployment scenarios (e.g., Samsung Ubicell in the Sprint Airave trials). The next phase for dealing with the RF interference issue completely is two-fold. First, a level of comfort needs to be provided to the operator community beyond the eager assertions of femtocell providers. This comfort will come from more extensive modeling and simulation efforts to demonstrate to operators the degree (or lack thereof) of impact a femtocell deployment will have in various demographics (i.e., urban, suburban, etc). The modeling effort is already being taken up within the Femto Forum and is expected to provide a substantial benefit to the overall femtocell community. Second, despite having workable solutions for managing interface, there remains a need for an optimal femtocell approach. Femtocell vendors have started to realize that solving this problem is not a key differentiation point for their products and thus are beginning to collaborate on defining a "best practices" approach for the entire community in order to address RF interference management.

The next primary femtocell challenge involves an operator's ability to assert and prove control over its spectrum utilization. The majority of business models assume the consumer owns the femtocell, which opens up the possibility of things like moving the femtocell from the intended location, powering off the femtocell "at will" by the consumer, and potential tampering with the device itself. The femtocell provides the operator extended capacity and coverage, but also necessitates the release of control by the operator — not something that operators are keen to do without proper constraints in place.

However, a combination of three features is expected to help operators manage this situation. The first is the use of Global Positioning System (GPS) to track femtocell location and ensure use in the proper spectral domains. The second is the integration of subscriber identity module (SIM) card functionality into the femtocell device, as the SIM card provides a proven method for authentication, as well as delivering embedded security capabilities for taking action in the case of improper utilization. Finally, there is the evolution of an enhanced Operations, Administration, and Management (OA&M) system for provisioning of femtocells as well as for providing an efficient method for demonstrating control of the femtocell to regulatory officials. The OA&M systems are being based on an extension of the TR-069 protocol from the Digital Subscriber Line (DSL) Forum, which provides a proven mechanism for the management of large-scale device deployments.

The final femtocell challenge is the lack of architectural standards, which impacts the design of all aspects of the femtocell, including RF. At last check there were roughly thirteen different proposed femtocell architectures (see Figure 2). Fragmentation to this degree hampers not only interoperability, but also economies of scale needed to ensure the price points demanded by operators. From an RF perspective, it becomes unclear exactly what parameters or control will be provided by the network versus the femtocell device, and specifically which protocols will be utilized to support the RF subsystem. In addition, the architecture variations are driven by a demand to support multiple types of radio technologies including W-CDMA, CDMA2000, WiMAX, LTE, and so on. One might wonder how there can be this much variation among architectures and at the same time a rapid traversal down the cost curve.

Fear not, for as they say, "Where there's a will there's a way" —and in this case there is strong and growing will for femtocells to succeed. For example, the thirteen different architectures can be categorized into two main categories. These categories include existing wireless standards and session-centric approaches. The wireless standards approach preaches heavy re-use of existing wireless applications and protocols to provide macro-network-equivalent service rapidly. This approach leverages proven methods for handover and call control, but does not easily support the supplementary services necessary to validate the femtocell business case.

The session-centric approach looks to leverage either an advanced softswitching network or the IP multimedia subsystem (IMS), both based on the Session Initiation Protocol (SIP). This approach provides a future-proof platform for service innovation, but lacks a proven model for call control and handover, and is hindered by the limited number of IMS deployments. The growing belief in the market place is that initial deployments will be based primarily on the existing wireless standards approach with an eventual evolution to a session- or IMS-based system.

So what does this mean to the femtocell designer? Two key requirements emerge from the lack of a consolidated architecture. The first is remote software upgradeability, as operators will want the flexibility to upgrade existing deployed femtocells as they evolve their core network strategy. Remote software upgradeability enables the entire system to evolve without post-sales technical intervention, including the RF subsystem. In addition, it requires the femtocell manufacturer to either establish in-house expertise on a wide range of applications and protocols, or else partner with a key technology vendor to ensure the ability to evolve rapidly with changing operator requirements.

The second key requirement is the support of different RF technologies. Whether it is W-CDMA or CDMA2000, all femtocell providers want to address the entire market, not just a subset. This necessitates innovation around reducing the cost to support disparate radio technologies. Software defined radio (SDR) may prove to play a substantial role in solving this issue while also reducing cost. Whatever the solution, though, it needs to address specific femtocell requirements, especially with regard to processor performance and memory limitations.

The dynamic femtocell market continues to charge ahead as it promises to solve the 3C challenges of coverage, capacity, and churn that are hampering 3G adoption, as well as deliver innovative supplementary services to the consumer. We are at a critical time in the femtocell technology evolution path, however. Providing a comprehensive understanding of the RF implications of a femtocell deployment and subsequent solution is absolutely critical to mass operator adoption. In addition, architectural harmonization will need to occur to ensure interchangeability and ultimately drive down costs.

The good news is that these challenges are being met through a combination of aggressive work within leading femtocell vendors' organizations as well as communal ecosystem efforts within the Femto Forum. A new technology like femtocells provides not only an interesting value proposition to the consumer and operator, but also an exciting platform for innovation by engineers and marketers alike. Ultimately it is innovation and wide-scale collaboration that will drive the benefits of femtocells to the consumer.

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