Femtocell

In telecommunications, a femtocell—originally known as an Access Point Base Station—is a small cellular base station, typically designed for use in a home or small business. It connects to the service provider’s network via broadband (such as DSL or cable); current designs typically support 2 to 4 active mobile phones in a residential setting, and 8 to 16 active mobile phones in enterprise settings. A femtocell allows service providers to extend service coverage indoors, especially where access would otherwise be limited or unavailable. The femtocell incorporates the functionality of a typical base station but extends it to allow a simpler, self contained deployment; an example is a UMTS femtocell containing a Node B, RNC and in some cases GPRS Support Node (SGSN) with Ethernet for backhaul. Although much attention is focused on WCDMA, the concept is applicable to all standards, including GSM, CDMA2000, TD-SCDMA, WiMAX and LTE solutions.

For a mobile operator, the attractions of a femtocell are improvements to both coverage and capacity, especially indoors. Providing a better service to end-users in turn reduces churn. There may also be opportunity for new services and reduced cost. The cellular operator also benefits from the improved capacity and coverage but also can reduce both capital expenditure and operating expense.

Femtocells are an alternative way to deliver the benefits of fixed-mobile convergence. The distinction is that most FMC architectures require a new (dual-mode) handset which works with existing unlicensed spectrum home/enterprise wireless access points, while a femtocell-based deployment will work with existing handsets but requires installation of a new access point that uses licensed spectrum.

Overview and benefits

Operating mode

Femtocells are sold by a Mobile Network Operator (MNO) to its residential end-users or enterprise customers. A femtocell is typically the size of a residential gateway or smaller, and connects into the end-user's broadband line. Integrated femtocells (which include both a DSL router and femtocell) also exist. Once plugged in, the femtocell connects to the MNO's mobile network, and provides extra coverage in a range of typically 30 to 50 meters for residential femtocells (depending on the existing coverage and output power - usually 20 mW which is five times less than a WiFi router). From an end-users' perspective it is plug and play, there is no specific installation or technical knowledge required - anyone can install a femtocell at home.

The end-user must then declare which mobile phone numbers are allowed to connect to his/her femtocell, usually via a web interface provided by the MNO . This only needs to be done once. When these mobile phones arrive under coverage of the femtocell, they switch over from the macrocell (outdoor) to the femtocell automatically. Most MNOs provide means for the end-user to know this has happened, for example by having a different network name appear on the mobile phone. All communications will then automatically go through the femtocell. When the end-user leaves the femtocell coverage (whether in a call or not), his phone hands over seamlessly to the macro network. Femtocells require specific hardware, so existing WiFi or DSL routers cannot be upgraded to a femtocell.

Once installed in a specific location, most femtocells have protection mechanisms so that a location change will be reported to the MNO. Whether the MNO allows femtocells to operate in a different location depend on the MNO's policy. In any case international location change of a femtocell is not possible.

Benefits for end-users

The main benefits for an end-user are the following:

- "5 bar" coverage when there is no existing signal or poor coverage

- Higher capacity, which is important if the end-user uses data services on his/her mobile phone

- Depending on the pricing policy of the MNO, special tariffs at home can be applied for calls placed under femtocell coverage

- For enterprise users, having femtos instead of DECT or WiFi dual mode phones enables them to have a single phone, so a single contact list etc

Architectures

Home Node B

In May 2008, the 3GPP completed a feasibility study of femtocell network architectures. Architectures including Cellular Base Station, Collapsed Stack and UMA/GAN were evaluated. As a result, the 3GPP is pursuing a new Home Node B (or HNB) reference architecture which builds on elements from both the Collapsed Stack and UMA/GAN approaches.

As the 3GPP completes the formal standard towards at the end of 2008, vendors and operators will migrate to support this new architecture for 3G femtocells.

Note the 3GPP refers to 3G femtocells as Home Node Bs (HNBs).

Cellular base station

One approach for a femtocell is to use the traditional base station architecture. In this case, the femtocell is a base station, connecting to the core network using a standard interface; for example, a WCDMA Node B connecting to a RNC via a backhaul connection (the Iub). The slight difference from a typical base station deployment is that the backhaul would be carried over broadband ("Iub over IP") which may have quality & security concerns. A more significant drawback of this architecture is that standards-based base station controllers are designed to support only a limited number of high-capacity base stations, not large numbers of simple ones. This architecture was previously referred to in the literature as a picocell deployment and is one in which a base station controller is introduced to provide the necessary support to the numerous small pico-head base stations.

Collapsed stack

More common architectures collapse some of the network functionality into the base station ("collapsed stack" or "Base Station Router"), not just the base station itself (Node B or BTS) but also the controller (e.g., RNC) and enable local radio resource control. This would then connect back to the mobile operator core at a higher point (e.g., Iu interface for WCDMA) for central authentication and management. This addresses the scalability concerns above, as the resource is located locally. The original Access Point Base Station followed this architecture but also incorporated the core MSC/GSN functions of authentication, control and switching.

Collapsed stack with UMA backhaul

A variant of the above is to use GAN/EGAN Unlicensed Mobile Access (UMA) standards. In this case, the UMA/GAN client is integrated into the femtocell. UMA/GAN protocol provides the connection to the mobile core, tunneling the Iu protocol. This approach uses UMA/GAN's existing security, transport and device management capabilities.

UMA/GAN is an attractive option for operators to leverage their investment in the UMA Network Controller to support applications beyond femtocells, including dual-mode handsets/WiFi or fixed line VoIP with terminal adapters.

The approach for UMA-based femtocells differs from a dual-mode handset approach where the UMA client is integrated in the device. In the former system the terminal is not affected and the air-interface is still standard - the UMA client is incorporated in the femtocell.

SIP or IMS

The final, and most sophisticated structure is to move to a full IP-based architecture. This approach was used in the original Access Point Base Station. In this case, even more functionality is included within the femtocell, and the integration to the core is done using an IP-based technology, e.g. SIP, IMS or H.323. Many operators believe they will eventually transition their core networks toward an IMS and SIP-based infrastructure and these solutions are also viewed positively. SIP-based approaches also hold the promise of cost-effective support for large-scale deployments .

Air interfaces

Although much of the commercial focus seems to have been on UMTS, the concept is equally applicable to all air-interfaces. Indeed, the first commercial deployment is the cdma2000 Airave. Femtocells are also under development for GSM, TD-SCDMA, WiMAX and LTE. The LTE study groups have identified femtocells ("Home eNode B") as a priority area.

Issues

Although claims are made that Femtocells could be a panacea for straightforward system deployment, there are a number of complications that need to be overcome.

Interference

The placement of a femtocell has a critical effect on the performance of the wider network, and this is one of the key issues to be addressed for successful deployment.

Without unique spectrum for the femtocell 'underlay network', or very careful spectrum planning in the wider network, there is a concern that femtocells could suffer from severe interference problems. For example, in a femtocell handover between a macrocell network to a home femtocell access point, there are limitations in the standards which must be taken into account. This includes a limitation in the number of adjacent cell sites - typically 16 - for which the mobile unit can scan, measure, and then pass to the RAN handover algorithm (for 2G and 3G standards, for example).

Further, if a single frequency CDMA system is being operated, where the macro and femtocell network use the same frequency band (a typical situation for many operators who licensed only one 3G frequency band), then the power control algorithms of the macro cell and femtocell can create interference , where for example a mobile unit increases its transmit power to the femtocell as part of the 'near-far' power control inherent in CDMA systems, while it is within the coverage area of a macro unit. The resultant high power transmitter in the macro field acts as an interferer since the frequency is shared.

Finally, there is the issue of coverage area, where in high-rise accommodation, femtocell users on different floors can create interference to other users. There are several partial solutions to this problem, but the primary way to prevent interference is to use a different frequency for the femtocell coverage, particularly for CDMA deployments. Partial solutions include using the mode-2 fixed power option available in the 3G configuration parameters, which would prevent the mobile unit power from increasing and causing interference, though there is an obvious performance trade-off if this approach is used.

Many vendors are reported to have developed sophisticated algorithms to address the problem, and modelling by carriers indicates this is viable. As such, the trials now in place are designed to test these techniques and to determine to what degree interference is a problem and under what circumstances. In his paper for 'PIMRC 07', Claussen describes the UMTS femtocell/macrocell interference problem and concludes that to manage the interference that "Essential requirements such as autoconfiguration and public access" are needed. In this case 'public access' means that all deployed femtocells using the same frequency (ie. of the same operator) would need to allow anyone to access the femtocell; there are obvious backhaul issues with this if the user is paying for the DSL or Cable backhaul connection. It is suggested in the paper that this could be offset by low cost calls. However, customer surveys have shown that public access are a major drawback that makes femtocells uninteresting for users . Furthermore, other alternatives such as "hybrid access" showed that femtocell owners can enjoy improved coverage and QoS while minimising the impact to the macrocell users. However, the impact of a CSG or hybrid femtocell onto users in a neighbouring house is still to be determined. Also, femtocells based on OFDMA technology are currently under scrutiny and seem capable of reducing interference thanks to the use of Interference Management techniques based on frequency-time resource allocation.

In another paper, Ho and Claussen identify the pre-requisite for auto-configuration of the femtocell power level in order to reduce interference - though in Claussen's first paper the algorithm requires knowledge of the macrocell transmit power, which would require the operator to configure the femtocells centrally, and line-of-sight distance to the femtocell, which requires knowledge of where the femtocell is installed. In his second paper, Ho highlights the issue of increased network traffic due to handover messages between the macrocell and femtocell.

The 3GPP meeting reported that: "To the extent investigated so far co-channel deployment is feasible for open access. For closed access, analysis conducted so far indicates that co-channel deployment is feasible if adaptive interference mitigation techniques are used. Further work is required to summarise the trade-off between HNB performance and the impact on the macro layer and to determine whether an acceptable tradeoff can be identified".

A number of companies are using the approach of using the femtocell as a mobile phone (UE) in order to measure, synchronise and build a neighbour list of nearby base stations. From this information, power levels, spreading codes and other parameters can be determined and resolved in order to avoid interfering with existing infrastructure.

Spectrum

Crucially, access point base-stations operate in licensed spectrum. As licensed spectrum allocation is made to operators on a fee basis, deployment of equipment must meet the strict requirements of the licenses. To make best use of spectrum, operators use frequency and cellular planning tools to optimise the best coverage for a given amount of spectrum. The introduction of access point base stations using licensed spectrum that are sold directly to the customer has implications for frequency and cellular planning, since an unexpectedly located access point base station could interfere with other closely-located base stations.

Access control

There is also the related issue of what happens when a neighbor's mobile appliance attaches to the network using another neighbor's femtocell, or how that can be prevented from occurring.

Lawful interception

Access point base stations, in common with all other public communications systems, are, in most countries, required to comply with lawful interception requirements.

Equipment location

Other regulatory issues relate to the requirement in most countries for the operator of a network to be able to show exactly where each base-station is located, and for E911 requirements to provide the registered location of the equipment to the emergency services. There are issues in this regard for access point base stations sold to consumers for home installation, for example. Further, a consumer might try to carry their base station with them to a country where it is not licensed. Some manufacturers (see Ubicell) are using GPS within the equipment to lock the femtocell when it is moved to a different country; this approach is disputed, as GPS is often unable to obtain position namely indoors because of weak signal.

Network integration

From an operational or deployment perspective, one of the key areas that needs to be considered is that of network integration. A conventional cellular network is designed to support a relatively small number (thousands, tens-of-thousands) of base stations, whereas a femtocell deployment of millions of consumer access points requires a different architecture to support this scaling. The issue of increase in network traffic as a result of co-channel macrocell / femtocell deployment is discussed in the paper by Ho and Claussen.

Emergency calls

Access Point Base Stations are also required, since carrying voice calls, to provide a 911 (or 999, 112, etc.) emergency service, as is the case for VoIP phone providers in a small number of jurisdictions. This service must meet the same requirements for availability as current wired telephone systems. There are several ways to achieve this, such as alternative power sources or fall-back to existing telephone infrastructure.

Quality of service

When using an Ethernet or ADSL home backhaul connection, an Access Point Base Station must either share the backhaul bandwidth with other services, such as Internet Browsing, Gaming Consoles, set-top boxes and triple-play equipment in general, or alternatively directly replace these functions within an integrated unit. In shared-bandwidth approaches, which are the majority of designs currently being developed, the effect on Quality of Service may be an issue.

The uptake of femtocell services will depend on the reliability and quality of both the cellular operator’s network and the third-party broadband connection, and the broadband connection's subscriber understanding the concept of bandwidth utilization by different applications a subscriber may use. When things go wrong, subscribers will turn to cellular operators for support even if the root cause of the problem lies with the broadband connection to the home or workplace. Hence, the effects of any third-party ISP broadband network issues or traffic management policies need to be very closely monitored and the ramifications quickly communicated to subscribers.

A key issue recently identified being active Traffic shaping by many ISPs on the underlying transport protocol IPSec. UK-based femtocell authority Epitiro have recently provided significant publicly available research and insight into many of these IP-focused QoS issues. A femtocell deployment guide from Epitiro is available for download here.

Spectrum accuracy

To meet FCC/RA spectrum mask requirements, Access Point Base Stations must generate the RF signal with a high degree of precision, typically around 50 parts-per-billion (ppb) or better. To do this over a long period of time is a major technical challenge, since meeting this accuracy over a period longer than perhaps 12 months requires an ovenised crystal oscillator (OCXO). These oscillators are generally large and expensive, and still require calibration in the 12-to-24 month time frame. Use of lower-cost temperature-compensated oscillators (TCXO) provides accuracy over only a 6-to-18 month time frame. Both depend on a number of factors.

The solutions to this problem of maintaining accuracy are either to make the units disposable/replaceable after an 18-month period and thus keep the cost of the system low, or to use an external, accurate signal to constantly calibrate the oscillator to ensure it maintains its accuracy. This is not simple (broadband backhaul introduces issues of network jitter/wander and recovered clock accuracy), but technologies such as the IEEE 1588 time synchronisation standard may address the issue, potentially providing 100-nanosecond accuracy (standard deviation), depending on the location of the master clock. Also, Network Time Protocol (NTP) is being pursued by some developers as a possible solution to provide frequency stability. Conventional (macrocell) base stations often use GPS timing for synchronization and this could be used to calibrate the oscillator. However, for a domestic femtocell, there are concerns on cost and the difficulty of ensuring good GPS coverage.

Standards bodies have recognized the challenge of this and the implications on device cost. For example, 3GPP has relaxed the 50ppb precision to 100ppb for indoor base stations in Release 6 and has proposed a further loosening to 250ppb for "Home NodeB" in Release 8.

Handover

In order to ensure that the user gets the best data rate out of the system, the mobile appliance must somehow know to connect to the femtocell when within range, even if there is still sufficient signal from, for example, an external macrocell base station. Forcing the mobile appliance to do this, while preventing your neighbor's mobile appliance from doing the same, is quite a challenge. In addition, handoff from the femtocell to the wider area macrocell and back again is potentially quite complex.

Controversy

The impact of a femtocell is most often to improve cellular coverage, without the cellular carrier needing to improve their infrastructure (cell towers, etc). This is net gain for the cellular carrier. However, the user must provide and pay for an internet connection to route the femtocell traffic, and then (usually) pay an additional one-off or monthly fee to the cellular carrier for the privilege of obviating their network shortcomings.[1]

Deployment

Femtocell shipments are still relatively low, but are expected to grow significantly in the next year. Research firm Berg Insight estimates that the shipments will grow from 0.2 million units in 2009 to 12 million units worldwide in 2014.

Within the United States, the most significant deployment to date (Jan. 2010) is that by Sprint Nextel. This started in 3Q/2007 as a limited rollout (Denver and Indianapolis) of a home-based femtocell built by Samsung Electronics called the Sprint Airave that works with any Sprint handset. As of 17 August 2008, Airave has been rolled out on a nationwide basis. Other operators in the United States have followed suit. In January 2009 Verizon rolled out its Wireless Network Extender, based on the same design as the Spint/Samsung system. AT&T has announced femtocell testing as a limited rollout in Raleigh and Charlotte under the name “3G MicroCell” as of 28 October 2009. As of 17 December 2009 this service is also available in parts of Georgia and South Carolina. The equipment is made by Cisco Systems and forwards all data and text messages as well as voice data.

In Asia, several service providers has rolled out Femtocell networks. In Singapore, Starhub rolled out its first nation-wide commercial 3G Femtocell services, though the uptake is low, while Singtel's offering is targeted at small medium enterprises. In 2009, China Unicom announced its own Femtocell network. NTT DoCoMo in Japan launched their own Femtocell service on the 10th November 2009.

In July 2009 Vodafone released the first Femtocell network in Europe, the Vodafone Access Gateway provided by Alcatel-Lucent. Other operators in Europe have followed since then, with SFR in France with femtocells provided by Ubiquisys and Optimus Telecomunicações, S.A. in Portugal.

A number of operators have had field trials in 2008 and 2009, including Telefonica/O2, Softbank, TeliaSonera, and Vodafone.

MagicJack, a company that sells VoIP devices for consumer use, announced in January of 2010 that it will be marketing an inexpensive femtocell that will allow any GSM cell phone to make calls over its VoIP network without service from a traditional cellular telecom provider.

See also

• microcell and picocell (compare nanocell)
• Wireless access point
• Cellular repeater

External links

• Femto Forum web pages
• University of Texas: Overview of the technical and business arguments for femtocells
• ThinkFemtocell: Technical detail, business case and analysis
• BBC News: Home Cells Signal Mobile Change
• Reuters: Video news and femtocell demonstration
• [2]