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Femtocell

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A femtocell—originally called an Access Point Base Station—is a scalable, multi-channel, two-way communication device extending a typical base station by incorporating all of the major components of the telecommunications infrastructure. A typical example is a UMTS access point base station containing a Node-B, RNC and GSN, with only an Ethernet or broadband connection (less commonly, ATM/TDM) to the Internet or an intranet. Application of VoIP allows such a unit to provide voice and data services in the same way as a normal base station, but with the deployment simplicity of a Wi-Fi access point. Other examples include CDMA-2000 and WiMAX solutions.

The main benefits of an Access Point Base Station is the existential simplicity of ultra low cost, scalable deployment. Design studies have shown that access point base stations can be designed to scale from simple hot-spot coverage through to large deployments by racking such units into full-scale base-stations.

The claimed attractions for a cellular operator are that these devices can increase both capacity and coverage while reducing both capex and opex.

Access Point Base Stations are stand-alone units that are typically deployed in hot-spots, in-building and even in-home. Variations include attaching a Wi-Fi router to allow a Wi-Fi hot-spot to work as backhaul for a cellular hotspot, or vice versa.

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, while a femtocell-based deployment will work with existing handsets.

As a result, Access Point Base Stations must work with handsets that are compliant with existing RAN technologies. The reuse of existing RAN technologies (and potentially re-use of existing frequency channels) could create problems, since the additional femtocell transmitters represent a large number of interference sources, potentially resulting in significant operational challenges for existing deployments. This is one of the biggest areas that femtocells must overcome if they are to be successful.

Access Point Base Stations typically rely on the Internet for connectivity, which can potentially reduce deployment costs but introduces security risks that generally do not exist in typical cellular systems.

History

In 2002, a group of engineers at Motorola in Swindon, UK, started a skunkworks team, called the AFG, to develop new technologies. Some of their major achievements included the world's smallest full-power UMTS base station, one of the first demonstrations of television to mobile, and the invention and development of the access point base station.

By 2005, a number of companies were looking to focus on this market, including Airwalk, ip. Access, RadioFrame Networks and Ubiquisys. By 2007, the idea had become more mainstream, with a number of companies publicly demonstrating systems at 3GSM, and operators announcing trials. However, to date, no large-scale deployments have been made.

In 3Q/2007, Sprint Nextel started 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 well as system manufacturers, semiconductor companies have announced chip-level products to address this application. Analog Devices has discussed the subject, while picoChip claims significant commercial traction.

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 without unique spectrum for the femtocell 'underlay network', or very careful spectrum planning in the wider network, femtocells can 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. For example, there is a limitation in the number of adjacent cell sites - typically 16 - for which the mobile unit can scan for, 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 utilise 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, whilst 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 accomodation, femtocell users on different floors can create interference to other users. In a 'open femtocell system', where the femtocells are not managed, this can be impossible to control, leading to users camping-on to the wrong femtocell, and closely located femtocells interfering with each other leading to loss of service. In a 'closed femtocell system', the femtocell units would need to be remotely spectrum managed, turning off or reducing the power of some user's femtocells in order to perserve the performance of neighbouring femtocell units. It is not clear how this would play with customers who discover that the femtocell they have installed is disabled by the operator and that all calls go through a neighbours unit - all in order to prevent interference. There are several partial solutions to this problem, but primarily the only way to prevent interference is to use a different frequency for the femtocell coverage, particularly for CDMA deployments. The partial solutions include utilising 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.

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 issues6 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 basestation with them to a new location. This could cause problems if the device were carried to a region or 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 location or country1.

Network integration

From an operational or deployment perspective, one of the key areas that still needs to be considered is that of network integration. The conventional cellular network is designed to support a relatively small number (thousands, tens-of-thousands) of basestatations. A femtocell deployment of millions of access points will require a different architecture to support this scaling. Some people have proposed UMA architecture approaches2, while others propose SIP architectures. In one of the original Motorola Access Point Base Station designs produced by the AFG, the unit only supported packet switched calls and thus contained an SGSN instead of an MSC. This architecture either utilised packet-switched calls or used a VoIP client on the phone (c.f. 'Fring'). Packets were carried through to the backhaul Ethernet/ADSL connection, thus removing the need for typical cellular core network functions and making it simple to deploy.

Privacy

There is a potential privacy issue with femtocells, in that a mobile appliance will camp-on to the femtocell when close by. Since the target market is in-business (SOHO) or at-home femtocell deployment, the operating company can potentially determine when a person is at home or at the office. On the one hand, this raises all sorts of interesting new marketing opportunities, such as learning more about a person's habits in order to improve targeted marketing, whilst, on the other hand, it has the potential to infringe privacy; the debate has yet to play out in this market.

Emergency calls

Access Point Base Stations are also required, since carrying voice calls, to provide a 911 (or 999, or 112) emergency service, as is the case for VoIP phone providers6. 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 utilising an Ethernet or ADSL home backhaul connection, an Access Point Base Station must either share the backhaul bandwidth with other services, such as Internet, games machines, 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 QoS does not appear to have been discussed.

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 timeframe. Use of lower-cost temperature-compensated oscillators (TCXO) provides accuracy over only a 6-to-18 month timeframe. 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 accuracy3. 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)4, depending on the location of the master clock. Conventional (macrocell) basestations often use GPS timing for synchronization and this could be used to calibrate the oscillator1. However, for a domestic femtocell, there are concerns on cost and the difficulty of ensuring good GPS coverage.

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, whilst 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.

Architectures

Access Point Base Station - True access point

The architecture of the original Motorola AFG access point base station had two variants. In the first variant, considering for a moment only 2G/3G deployments, all the core functions including HLR, AuC, EID, GSN and BSC/Transcoder (or RNC) were contained within the access point base station (now called a femtocell). The only connection from the equipment was a Layer 2 (Ethernet, DSL, etc.) link carrying packet voice calls or Internet Protocol VoIP traffic (c.f. the 3G 'Gi' interface), and potentially configuration and management information - for example, using SNMP. Since authentication takes place locally within the femtocell, there are options to control access by local terminal or remotely via a AAA server in an analogous manner to WiFi access points. This architecture favours fixed-line or WiFi operators since they can leverage their core.

Access Point Base Station - UMA core

In the second variant, the HLR (and/or GSN) functionality resides elsewhere in the network and is under the direct operation and control of the operator. This second model mimics the current cellular architecture and is standardised in the GAN/EGAN Unlicensed Mobile Access standards. Care must be taken when extending control to other units outside the femtocell in order not to 'trombone' the data or control signal paths back into the network, as this can potentially lead to unacceptable latency, bandwidth and scalability problems. The majority of Femtocells in the market utilise this architecture. This architecture favours cellular operators since they can leverage their core.

UMA - Dual-mode

The architecture of Unlicensed Mobile Access (UMA) - Dual-mode, uses different connection technology for the macro environment to that used in the femtocell environment. For example, a dual-mode mobile unit with standard 2G/3G cellular and Bluetooth (or WiFi) connectivity would utilise cellular connectivity in the macro environment and Blutooth/WiFi connectivity in the femtocell environment. In this case the 'femtocell' is in fact a standard WiFi access point, with an IP connection to a GSN implementation in the network to allow hand-over between macro and femtocell environments (see the GAN/EGAN Unlicensed Mobile Access standards). There are many benefits to this approach including, low-cost access points and reduced interference problems (since the macro and femtocell levels operate on different bands, and the femtocells themselves, being WiFi access points, also have multiple bands to choose from). The disadvantage of dual-mode is the additional complexity, and power requirement, in the handset, though this is gradually being addressed. In this instance the 'femtocell' is not an 'Access Point Base Station', as originally envisaged, but is typically a WiFi Access Point.

UMA - Terminal Adapters

A UMA terminal adapter is a unit that typically has a wireless 2G/3G connection to a cellular operator on one side, and a WiFi/Bluetooth wireless connection on the other side. The purpose of such units is to provide a WiFi access point capability, whilst utilising a cellular backhaul network. In this instance the 'femtocell' is not an 'Access Point Base Station', as originally envisaged, but is typically a wireless gateway bridge between two or more standards.


Market

Equipment to chipset mapping

Cellular equipment vendor to femtocell vendor partnerships

Reported market estimates

Target product and service costs

References

Equipment

Software stacks

Chips and reference designs

Industry body

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