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Tech Topic 23: Femtocells1

Since the introduction of Third Generation (3G) mobile services, there has been a rapid increase in the demand to combine mobile and Internet technologies, thereby giving fast and reliable access to the Internet via mobile devices. The availability of 3G networks, enhanced mobile devices and flat rate near-unlimited data plans came together to produce a tipping point in the consumer adoption of mobile data services. This rapid growth has been exhibited in the quantity of mobile broadband data consumed and the revenues derived from this data. With the migration to Fourth Generation (4G) technologies - as discussed in our last Tech Topic (see Tech Topic #22 on Long Term Evolution or LTE) - this trend is widely forecast to continue.

Traditionally, mobile service was primarily about services to users on the move - those traveling between homes and offices via public or private transport and where traditional communication services were not available in any other way. Increasingly, one of the most important and largest components of mobile traffic demand has been for services delivered to stationary end users rather than just mobile users. In other words, mobile devices are increasingly being used when subscribers are not on the move, typically within a building or within a home or work place, and are increasingly replacing the traditional fixed public telephone system.

Therefore, wireless providers are constantly looking to extend wireless coverage inside buildings to provide enterprise solutions to all of their customers. Delivering high-quality mobile services inside buildings is a difficult challenge for the macro network due to the attenuation of the radio signals. This problem is exacerbated for wireless service providers that use higher frequency bands for their transmission medium where radio-frequency signals attenuate more rapidly inside concrete or steel structures. Higher data rates are dependent on the quality of the signal at the receiver; therefore it is important to extend the signaling capabilities of the macro-network into more confined radio environments.

The purpose of this topic is to take a quick look at one portion of the growing mobile services domain – that of femtocells. Femtocells make it possible to extend the radio characteristics of the macro-cellular network while at the same time capitalizing on the terrestrial broadband backhaul network.

Apart from being able to extend network coverage where none previously existed, femtocells have the potential to play a significant disruptive role in Fixed Mobile Substitution (FMS)2. Wireless service is no longer just a luxury but rather is a requirement for a steadily growing number of people. Any mall, hotel, or other public structure that cannot offer wireless service stands to lose business, and yet it is precisely these places that have the potential for the worst coverage.

The question then to be answered is how to get cellular service where there seems to be a “hole,” or lack of service coverage in a particular area within the network footprint3. The growing demand for mobile wireless service has been driving service providers and network operators to expand their networks at an unprecedented pace. However, as traffic demand increases in urban areas, it is becoming more difficult for network coverage to reach every user within the potential geographical coverage area. There are many areas such as tunnels, indoor stadiums, elevators, and even homes where coverage is of poor quality or does not exist at all. It is precisely these areas where there are a significant number of potential subscribers, making it crucial not only to extend coverage but network capacity in terms of network access as well. Simply adding conventional macro-category cell sites to an existing network is not always practical or economical. Each new cell site requires a significant amount of time, money and resources to bring it "On Air.” Real-estate and zoning laws also make it difficult, sometimes even impossible, to put a cell site in a desired location.

A specific and important subset of this problem involves the extension of mobile services to specific residential buildings and commercial spaces. For example, subscribers may desire ubiquitous wireless coverage throughout their homes and the “On-Street” coverage or power level is not adequate to provide sufficient coverage in all the important areas of certain premises. In order to solve this problem, IP-Access4, one of the prominent in-building equipment vendors, came up with a solution called "FEMTOCELL" which met the primary objective of providing services without disturbing or significantly modifying the existing coverage of the legacy network infrastructure.

What is a Femtocell?

A femtocell is a low-power access point, based on mobile cellular technology, providing wireless voice and broadband services to customers with a limited range within a home or in an office environment.

As shown in Figure 1, femtocells connect to the mobile operator’s network facilities via a standard consumer broadband connection, such as DSL, cable or fiber. Data to and from the femtocell is carried over the Internet - or at least, over an Internet technology-based network provided by an Internet Service Provider. The wireless subscriber connects to the femtocell via the normal cellular service technologies just as if he/she were using a conventional macro-cellular network connection.

Figure 1. Femtocell Application

Figure 1. Femtocell Application5

A femtocell is not a micro-cellular application but rather it is a mini base station application for short range indoor purposes only. Some might refer to it as a picocell that connects to a broadband connection via a router and a modem in homes or small offices. Nevertheless, it is a single channel compact size transceiver solution, that is a flexible, multi-user solution used to extend wireless coverage into venues such as offices, homes, arenas, shopping malls, stores, and campuses. It also serves the purpose of providing enough capacity for multiple users (usually up to 3 users simultaneously per channel), and provides the network operators an edge to enhance coverage without costly build-outs and extensions of the macro-network.

Figure 2 below shows the extent of femtocell connectivity to an operator’s network.

Figure 2. Basic Femtocell Network

Figure 2. Basic Femtocell Network6

Femtocell Attributes7

The Femto Forum8 has created the following set of attributes - all of which are necessary for a device to qualify as a femtocell:

A femtocell is a low-power wireless access point, incorporating all of the following characteristics:

  • Uses mobile technology: Femtocells use fully standardized wireless protocols for over-the-air transmission with mobile devices, including mobile phones and a wide range of other mobile-enabled devices. Qualifying standard protocols include GSM, WCDMA, LTE, Mobile WiMAX, CDMA and other current and future protocols standardized by bodies such as the 3GPP, 3GPP2 and the IEEE/WiMAX Forum that collectively comprise the technologies included in the ITU-R definition of IMT9. The use of these protocols allows femtocells to provide services to several billion existing mobile devices worldwide and to provide services that users can access from almost any location as part of larger wide-area networks.
  • Operates in licensed spectrum: By operating in spectrum licensed to the service provider, femtocells allow operators to provide assured Quality of Service (QoS) to customers over the air, free from harmful interference but also making efficient use of their spectrum.
  • Generates coverage and capacity: As well as improving coverage within limited geographical areas, femtocells also create extra network capacity, serving a greater number of users with high data-rate services. This differs from simple repeaters or “boosters” that may only enhance the coverage area but do not necessarily enhance network capacity.
  • Uses Internet-grade backhaul: Femtocells backhaul their data over Internet-grade broadband connections, including DSL and cable, using standard Internet protocols. This may be over a specific Internet Service Provider’s network, over the Internet itself or over a dedicated link.
  • Permits low prices: The large volumes projected for femtocells allow substantial economies of scale, driving efficiencies in manufacturing and distribution in a manner similar to other consumer electronics and with pricing projected to be comparable with access points for other wireless technologies.
  • Managed by licensed operators: Femtocells only operate within parameters set by the licensed operator. While operators have a high degree of experience to ensure automatically that femtocells operate at power levels and frequencies that are unlikely to create interference with other networks, the limits on these parameters are always set by operators, not by the end users; hence it requires frequency planning by the operators and not by the femtocell owner.
  • Self-organizing and self-managing: Femtocells can be installed by the end customer. They are set up to operate with high performance according to the local and network-wide conditions regarding radio, regulatory and operator policies, with no need for intervention by the customer or operator. Their operating parameters can be adjusted over time as the customer, operator or regulator needs evolve to maximize performance and reliability.

Femtocell Standards

Most air interfaces included in the global ITU-R IMT family have recognized standards for femtocells. These include:

  • 3GPP standards for home Node-B, which is a wideband code division multiple access (WCDMA) femtocell. Both frequency division duplexing (FDD) and time division duplexing (TDD) options are established and a time division synchronous code division multiple access (TD-SCDMA) variant is also planned.
  • 3GPP standards for Home eNode-B, which is a Long Term Evolution (LTE) femtocell. Both FDD and TDD options are provided.
  • 3GPP2’s program for femtocells for cdma2000, cdma2000 1x and 1x EV-DO.10
  • WiMAX Forum’s program for WiMAX11 femtocells based on IEEE standards.
In all cases femtocell standards support deployments in all of the existing licensed spectrum bands in which macro-cellular networks operate.

Types of Femtocell12

Various types of femtocells are available on the market. Although individual standards differ in their definitions, the following broad classes can be identified, though these are neither exclusive nor prescriptive:

  • Class 1: This is the class of femtocells that emerged first and is currently best known. Femtocells in this class deliver a similar transmit power and deployment view as do Wi-Fi access points (e.g. typically 20 dBm of EIRP or less) for residential or enterprise application. Each delivers typically 4-6 simultaneous voice calls per channel plus data services in an open access fashion to authorized subscribers. Typically, this class of device is installed by the end user.
  • Class 2: Somewhat higher power (typically 24 dBm of EIRP) to support longer range or more users (about 6 – 12). May need to be installed by the operator but end user can also deploy with the operator’s assistance.
  • Class 3: Still higher transmit power level for longer range to provide a larger coverage foot print for more users. Typically operators might deploy these outside in an open access mode to authorized subscribers. This class of femtocell could be deployed indoors, for example in public buildings, for localized capacity, outdoors in built-up areas to deliver distributed capacity, or in rural areas to meet specific coverage needs.

What a Femtocell is not.

It is easy to confuse femtocell installations and equipment with other similar devices, therefore it is instructive to contrast femtocell behavior with several others, specifically Wi-Fi access points, repeater/ signal boosters, cordless telephones, and conventional cellular base stations.

Wi-Fi Access Points. While these also provide wireless broadband access to portable devices, there are important differences. Wi-Fi almost always operates in unlicensed or license-exempt spectrum. This means that an operator is unable to guarantee any service quality over the air since interfering devices can legally appear close to any given user. Most Wi-Fi devices operate in the 2.4 GHz or 5.0 GHz frequency bands, where only three or six non-overlapping channels are available, so the potential to avoid interference is limited. In contrast, femtocells operate in licensed spectrum and thereby ensure that an operator is in control of every transmitting device and can manage interference in order to deliver an appropriate quality of service to each user. Wi-Fi access points and client devices all transmit around 100 mW (20 dBm) of power, which does not change even when the device is close to the transmitting antenna, thus increasing the chances of interference and battery drainage to the device and intermodulation products. By contrast, in cellular technology both the mobile device and femtocell continually adjust their transmit power, a procedure called “power control,” to minimize or maximize the required transmit power. This allows the necessary control of transmit power to deliver adequate quality of service, increases the number of users that can access a given channel, reduces the co-channel interference, and significantly increases battery life. Wi-Fi has been extremely successful in delivering Internet or private Local Area Network connectivity to laptops. In contrast, while femtocells can serve the increasing numbers of mobile users with cellular-based data services / connectivity either embedded or by “dongles,” they are designed primarily to serve the much larger numbers of personal devices including newer laptops. Recently, Wi-Fi has been incorporated in other mobile devices to serve ‘dual-mode’ services (example; Wi-Fi and cellular services); however, the number of such devices is small compared with cellular phones. So while there may be a market for such devices, it is not expected to address the huge potential user base addressable by femtocells.

It should be noted that there is no device that can be considered a “Wi-Fi Femtocell”.

Repeaters / Signal Boosters. Repeaters are bi-directional amplifiers, used to extend the range and increase coverage area for systems including cellular networks. Repeaters operate by receiving signals from a base station cell site via an external antenna and then amplifying and retransmitting the same signal via radio frequency cables or antennas placed within the area for coverage improvement. They are a useful tool in many networks and can certainly improve coverage, but there are many differences from femtocells. Repeaters require meticulous installation by professionals to guarantee good results. For example, the external and internal installations require proper location and configuration to deliver the appropriate coverage while being isolated from each other to avoid feedback. The gain of the amplifier in the repeater requires proper adjustments to confine it within controlled limits since too little power will fail to deliver adequate coverage improvement, or alternatively too much power will cause feedback and oscillation with the repeater amplifier; thus, degrading or denying service for all the users. By contrast, femtocells are typically “zero-touch” devices that require no professional skills for setup.

As well as amplifying the signals from the mobile devices, repeaters necessarily inject some additional noise into the base station receiver when relaying signals from the mobile users. This limits the number of repeaters that can be deployed in a given area within the network, typically to a handful per cell site, since more will unacceptably degrade the base station performance for all other users in a given area. Therefore, repeaters must have donor sites, while femtocells can operate in a completely isolated area provided broadband backhaul connectivity is available.

Unless a repeater can be monitored or maintained remotely, repeaters may add an operational cost in addition to the hefty deployment cost. On the other hand, femtocells can be monitored and maintained remotely from the access for the network or by the network’s wireless Base Station Controller. Lastly, repeaters tend to be far more expensive than femtocells even before the cost of professional installation services are included in the pricing structure.

Cellular Base Station. Femtocells do share much in common with conventional mobile base stations, producing almost indistinguishable signals over the air in order that standard hand-sets can be used unmodified. However, there are significant differences. Femtocells have limited capacity that is suitable for a single domestic installation while conventional base stations must serve tens or hundreds of users. Base stations are therefore professional products, with costs to match, while femtocells are consumer products, produced in volume to meet consumer pricing expectations. Femtocells are designed to work with Internet-grade backhaul, typically DSL or cable, while base stations operate over dedicated backhaul, such as leased lines or microwave links. As a result, femtocells do not have the same interfaces as standard base stations; the interface is optimized to reduce the bandwidth requirements, while also increasing the level of security to protect traffic which may be routed over the Internet rather than a dedicated network. Base stations must be planned and optimized by professional network design engineers while femtocells employ standardized network interfaces and are relatively simple to install. Lastly, femtocells of course radiate at substantially lower power levels than a base station, being typically around 10,000 times lower. So while a femtocell does have some similarities with a base station, it is both much more and much less and the two should not be confused.

On the other hand, if the end user is to be happy with the quality of service, it is essential for the femtocell backbone to have a broadband connection, such as T1, E1, FiOS, or cable Internet. DSL can also be used in areas where other broadband connections are not available. In short, the deployment of a femtocell will enhance the mobile network coverage and increase the dedicated capacity either at the edge or within the cellular networks resulting in faster throughput.

From the carrier perspective, a femtocell requires frequency and network planning for its successful deployment because there are issues of interference as well as the proper set up of the Base Transceiver Stations (BTS) in the local Mobile Switching Center on the radio side. In addition, the service provider must provision the network services that are provided to the femtocell via the Internet connection.

Also, a proper neighbor list must be created for proper roaming and hand-off connections from one femtocell to another femtocell or for transitions to the macro-cellular network and vice versa.

Customers of femtocells.

All of the major mobile service providers have femtocell applications. For example, Sprint, AT&T, and T-Mobile have some femtocell service offerings, but femtocell growth has not been as robust as expected.

Conclusion.

Femtocells are low-power wireless access points used for the home and office. They operate in licensed spectrum to connect standard mobile devices and support all common air interfaces used by operators worldwide.

For additional Information, the Femtocell Forum was founded as the industry body to give consumers and operators an opportunity to learn more about products in this field. See the Forum’s web site at http://www.femtoforum.org/ for more information.

Definition of terms used above:

- T1: A North American and Japanese digital primary rate telephone multiplexing system that combines 24 channels of digitally coded speech or other subscriber data, at 64 kb/s for each channel, with an 8 kb/s synchronization bit stream (the framing bits or F bits) into a 1.544 Mb/s bit stream. T-1 was historically a trade name of one manufacturer, although it is now often used for compatible products regardless of manufacturer. The synonym DS-1 is often used in standards and other documents.

- E1: A communication line that was developed by European standards that multiplexes thirty voice channels and two control channels onto a single communication line. The E1 line uses 256 bit frames and transmitted at 2.048 Mbps.

- Fios: The popular name for Verizon's digital TV, Internet and phone technology package.

- DSL: A digital subscriber line transmits digital information, usually on a copper wire pair. Although the transmitted information is in digital form, the transmission method is usually an analog carrier signal (or the combination of many analog carrier signals) that is modulated by the digital information signal.

- Coverage Holes: Coverage holes are portions of radio coverage in a wireless system where the customer cannot receive radio signals due to low radio signal levels or distorted radio signals. Coverage holes are commonly caused by multipath fading where multiple signals are combined in such a way that the signal levels cancel causing reduced signal levels, or areas where the received signals are attenuated below acceptable levels.

- BTS: The radio part of a wireless network (typically cellular or PCS) that includes the transmitters and receivers, antennas and tower that is used to communicate with mobile radios. A BTS is connected to a base station controller (BSC).

- HLR: The home location register is a part of a communication network (typically cellular or PCS) that holds the subscription and other information about each subscriber authorized to use the wireless network.

- VLR: A visitor location register is a database part of a wireless network (typically cellular or PCS) that holds the subscription and other information about local or visiting subscribers that are authorized to use the wireless network.

References:

  1. www.femtoforum.com date: 01/12/2011
  2. http://www.femtocellbook.com/ date: 01/12/2011
  3. http://www.femtoforum.org/femto/ date: 01/12/2011
  4. Saunders, Simon; Carlaw, Stuart; Giustina, Andrea; Bhat, Ravi Raj; Rao, V. Srinivasa; and Siegberg, Rasa. Femtocells: Opportunities and Challenges for Business and Technology, John Wiley and Sons, Ltd. Publication, The Atrium, Southern Gate, Chichester, West Sussex, PO198SQ, United Kingdom, 2009, Pg. 5-8.
  5. http://www.ipaccess.com/ date: 01/21/20011.

 

 


1 This Tech Topic is co-authored with Mr. Clifford Gonsalves, a senior engineer in the Public Safety and Homeland Security Bureau of the Federal Communications Commission. Cliff can be reached at 202-418-2488, or by electronic mail at clifford.gonsalves@fcc.gov. Cliff’s significant contributions to this topic are sincerely appreciated..

2 Fixed Mobile Substitution, usually abbreviated to FMS, is the use of a personal communications device instead of a fixed, wired, POTS telephone.

3 Note that this does not apply to larger scale areas such as rural areas where coverage may not be available.

4 See http://www.ipaccess.com/femtocells/femtocells.php, as of 01/12/2011.

5 See AIRWAVE User Guide from Sprint, http://shop.sprint.com/en/services/airave/index.shtml, as of 01/12/2011.

6 See http://www.realwireless.biz/femtobook/, as of 01/12/2011.

7 Saunders, Simon; Carlaw, Stuart; Giustina, Andrea; Bhat, Ravi Raj; Rao, V. Srinivasa; and Siegberg, Rasa, Femtocells: Opportunities and Challenges for Business and Technology, A John Wiley and Sons, Ltd. Publication, The Atrium, Southern Gate, Chichester, West Sussex, PO198SQ, United Kingdom, 2009, Pg. 5-8.

8 See Femto Forum at http://www.femtoforum.org/femto/, as of 01/12/2011.

9 International Mobile Telecommunications, comprising IMT-2000, usually known as 3G, and IMT – Advanced, which is known as 4G.

10 CDMA2000 (also known as IMT Multi Carrier (IMT MC)) is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B. All are approved radio interfaces for the ITU's IMT-2000.

11 WiMAX – Worldwide Interoperability for Microwave Access is a telecommunications protocol that provides fixed and fully mobile internet access. IEEE 802.16m is the current revision.

12 Saunders, Simon; Carlaw, Stuart; Giustina, Andrea; Bhat, Ravi Rah; Rao, V. Srinivasa; and Siegberg, Rasa, Femtocells: Opportunities and Challenges for Business and Technology, A John Wiley and Sons, Ltd. Publication, The Atrium, Southern Gate, Chichester, West Sussex, PO198SQ, United Kingdom, 2009, Pg. 5-8.

13 EIRP - Effective Isotropic Radiated Power.

14 Repeaters receive signals from an intended site/tower which is called a donor site.

15 Typical Base Station Transmit Power is 50 dBm versus 20 dBm for a typical femtocell.

16 See AT&T website: http://www.wireless.att.com/learn/why/3gmicrocell/, as of 01/12/2011;
See Sprint website: http://shop.sprint.com/en/services/airave/index.shtml, as of 01/12/2011;
See T-Mobile website: http://news.softpedia.com/news/T-Mobile-Joins-Ubiquisys-and-Huawei-for-Femtocell-Testing-129778.shtml, as of 01/12/2011.