In telecommunications, a femtocell is a small, low-power cellular base station, typically designed for use in a home or small business. A broader term which is more widespread in the industry is small cell, with femtocell as a subset. It typically connects to the service provider's network via the Internet through a wired broadband link (such as DSL or cable); current designs typically support four to eight simultaneously active mobile phones in a residential setting depending on version number and femtocell hardware, and eight to sixteen mobile phones in enterprise settings. A femtocell allows service providers to extend service coverage indoors or at the cell edge, especially where access would otherwise be limited or unavailable. Although much attention is focused on WCDMA, the concept is applicable to all standards, including GSM, CDMA2000, TD-SCDMA, WiMAX and LTE solutions.
The use of femtocells allows network coverage in places where the signal to the main network cells might be too weak. Furthermore, femtocells lower contention on the main network cells, by forming a connection from the end user, through an internet connection, to the operator's private network infrastructure elsewhere. The lowering of contention to the main cells plays a part in breathing, where connections are offloaded based on physical distance to cell towers.
Consumers and small businesses benefit from greatly improved coverage and signal strength since they have a de facto base station inside their premises. As a result of being relatively close to the femtocell, the mobile phone (user equipment) expends significantly less power for communication with it, thus increasing battery life. They may also get better voice quality (via HD voice) depending on a number of factors such as operator/network support, customer contract/price plan, phone and operating system support. Some carriers may also offer more attractive tariffs, for example discounted calls from home.
Femtocells are an alternative way to deliver the benefits of fixed–mobile convergence (FMC). 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 the installation of a new access point that uses licensed spectrum.
Many operators worldwide offer a femtocell service, mainly targeted at businesses but also offered to individual customers (often for a one-off fee) when they complain to the operator regarding a poor or non-existent signal at their location. Operators who have launched a femtocell service include SFR, AT&T, C Spire, Sprint Nextel, Verizon, Zain, Mobile TeleSystems, T-Mobile US, Orange, Vodafone, EE, O2, Three, and others.
In 3GPP terminology, a Home NodeB (HNB) is a 3G femtocell. A Home eNodeB (HeNB) is an LTE 4G femtocell.
Theoretically the range of a standard base station may be up to , and in practice could be , a microcell is less than two kilometers wide, a picocell is 200 meters or less, and a femtocell is in the order of 10 meters, although AT&T calls its product, with a range of , a "microcell". AT&T uses "AT&T 3G MicroCell" as a trademark and not necessarily the "microcell" technology, however.
Overview and benefits
Operating mode Femtocells are sold or loaned by a mobile network operator (MNO) to its residential or enterprise customers. A femtocell is typically the size of a residential gateway or smaller, and connects to the 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. From a user's perspective, it is plug and play, there is no specific installation or technical knowledge required—anyone can install a femtocell at home.
In most cases, the user must then declare which mobile phone numbers are allowed to connect to their femtocell, usually via a web interface provided by the MNO. This needs to be done only 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 a way for the 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 user leaves the femtocell coverage (whether in a call or not) area, their 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 depends on the MNO's policy. International location change of a femtocell is not permitted because the femtocell transmits licensed frequencies which belong to different network operators in different countries.
Benefits for users The main benefits for an end user are the following:
"5 bar" coverage when there is no existing signal or poor coverage Higher mobile data capacity, which is important if the end-user makes use of mobile data on their mobile phone (may not be relevant to a large number of subscribers who instead use WiFi where femtocell is located) 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 ("cordless" home) phones enables them to have a single phone, so a single contact list, etc. Improved battery life for mobile devices due to reduced transmitter–receiver distance The battery draining issue of mobile operators can be eliminated by means of energy efficiency of the networks resulting in prolongation of the battery life of handsets New applications and services can be created to enhance user experience or provide additional features: In Connected car case the use of Femtocells has been proposed as a safety feature (cf. patent application EP2647257B1 by Valentin A. Alexeev)
Femtocells can be used to give coverage in rural areas.
Standardised architectures
The standards bodies have published formal specifications for femtocells for the most popular technologies, namely WCDMA, CDMA2000, LTE and WiMAX. These all broadly conform to an architecture with three major elements:
The femtocell access points themselves, which embody greater network functionality than found in macrocell basestations, such as the radio resource control functions. This allows much greater autonomy within the femtocell, enabling self-configuration and self-optimisation. Femtocells are connected using broadband IP, such as DSL or cable modems, to the network operator's core switching centres. The femtocell gateway, comprising a security gateway that terminates large numbers of encrypted IP data connections from hundreds of thousands of femtocells, and a signalling gateway which aggregates and validates the signalling traffic, authenticates each femtocell and interfaces with the mobile network core switches using standard protocols, such as Iuh. The management and operational system which allows software updates and diagnostic checks to be administered. These typically use the same TR-069 management protocol published by the Broadband Forum and also used for administration of residential modems.
The key interface in these architectures is that between the femtocell access points and the femtocell gateway. Standardisation enables a wider choice of femtocell products to be used with any gateway, increasing competitive pressure and driving costs down. For the common WCDMA femtocells, this is defined as the Iuh interface. In the Iuh architecture, the femtocell gateway sits between the femtocell and the core network and performs the necessary translations to ensure the femtocells appear as a radio network controller to existing mobile switching centres (MSCs). Each femtocell talks to the femtocell gateway and femtocell gateways talk to the Core Network Elements (CNE) (MSC for circuit-switched calls, SGSN for packet-switched calls). This model was proposed by 3GPP and the Femto Forum. New protocols (HNBAP and RUARUA is a Sigtran type protocol used to carry Radio Access Network Application Part (RANAP) signalling over the IP based Iu-h interface. Like that of the other Sigtran adaptation protocols, RUA also uses the services of SCTP (Stream Control Transmission Protocol) on top of IP. [RANAP User Adaptation]) have been derived; HNBAP is used for the control signaling between the HNB and HNB-GW while RUA is a lightweight mechanism to replace the Signalling Connection Control Part (SCCP) and M3UA protocols in the Radio Network Controller (RNC); its primary function is transparent transfer of RANAP messages.
In March 2010, the Femto Forum and ETSI conducted the first Plugfest to promote interoperability of the Iuh standard.
The CDMA2000 standard released in March 2010 differs slightly by adopting the Session Initiation Protocol (SIP) to set up a connection between the femtocell and a femtocell convergence server (FCS). Voice calls are routed through the FCS which emulates an MSC. SIP is not required or used by the mobile device itself. In the SIP architecture, the femtocell connects to a core network of the mobile operator that is based on the SIP/IMS architecture. This is achieved by having the femtocells behave toward the SIP/IMS network like a SIP/IMS client by converting the circuit-switched 3G signaling to SIP/IMS signaling, and by transporting the voice traffic over RTP as defined in the IETF standards.
Air interfaces Although much of the commercial focus seems to have been on the Universal Mobile Telecommunications System (UMTS), the concept is equally applicable to all air-interfaces. Indeed, the first commercial deployment was the CDMA2000 Airave in 2007 by Sprint.
Femtocells are also under development or commercially available for GSM, TD-SCDMA, WiMAX and LTE.
The H(e)NB functionality and interfaces are basically the same as for regular High Speed Packet Access (HSPA) or LTE base stations except few additional functions. The differences are mostly to support differences in access control to support closed access for residential deployment or open access for enterprise deployment, as well as handover functionality for active subscribers and cell selection procedures for idle subscribers. For LTE additional functionality was added in 3GPP Release 9 which is summarized in.
Issues
Interference The placement of a femtocell has a critical effect on the performance of the wider network, and this is the key issue to be addressed for successful deployment. Because femtocells can use the same frequency bands as the conventional cellular network, there has been the worry that rather than improving the situation they could potentially cause problems.
Femtocells incorporate interference mitigation techniques—detecting macrocells, adjusting powerX. Kang et al. ``Price-based resource allocation for spectrum-sharing femtocell networks: a Stackelberg game approach , in IEEE Journal on Selected Areas in Communications, vol. 30, no. 3, pp. 538-549, Apr. 2012. and scrambling codes accordingly. Ralph de la Vega, AT&T President, reported in June 2011 they recommended against using femtocells where signal strength was middle or strong because of interference problems they discovered after widescale deployment.de la Vega, Ralph , All Things D video interview 38 minutes in, June 24, 2011, accessed June 28, 2011. This differs from previous opinions expressed by AT&T and others.
A good example is the comments made by Gordon Mansfield, executive director of RAN Delivery, AT&T, speaking at the Femtozone at CTIA March 2010: