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Femtocells are low power access point, based on the mobile technology in order to provide reliable coverage in home and office environment by connecting to the cellular operator's network through a broad band connection. The operator core network contains a management system which provides services to the femtocell, ensuring that the services experienced by the users are secure, of high quality and can coexist with the signals from other femtocells and outdoor network. The processing mechanisms are simple which consumes less power and provides cheap communication. In this paper, a complete review of the femto cells is undertaken by throwing light on its attributes, network architecture and issues.
Cellular technology is one of the fastest growing technologies in the world and depending upon its promotions its data traffic is expected to increase by 10 times between 2008 and 2013book. Internet is following with 1.6 billion users worldwide. The voice services of cell phones were merged with data services of internet in 3G services. Acceleration was observed in both, the broadband data consumed and the generated revenues. Operators are working to decrease the cost per bit of delivered data for achievement of higher profit margins. Cellular technology is not only used by people on move but also by the persons at home or workplace for privacy although networks are not designed for solid residential services. Using a macro cell for the delivery of acceptable residential coverage is not an effective solution from both coverage and economic perspective. Poor penetrating ability through structures like walls due to higher operating frequencies of 3G is another reason for subscriber dissatisfaction. The adoption of complex coding and modulation schemes to achieve higher data rates brings greater performance penalties to 3G, LTE and WI-Max.
Deployment of microcell in high capacity locations like offices and city centres was justified by revenue generated but when it comes to home the problem was still there as the deployment of separate base stations in homes and small offices seemed impossible just a few years back. Nowadays industry is focussing on better residential coverage for capturing market share. A complete solution of indoor coverage issue is achieved with the help of femto cell network design. By deliverance of better services to end users and increased revenues to operators, femto cells are gaining importance in industry as part of future network architectures.
Femto cell is a low power wireless access point which operates on standard wireless protocols like GSM, LTE and Mobile Wi-Max, to communicate with a wide range of mobile enabled devices, which enable femto cells to provide services to billions of existing mobile devices that users can access from almost any location as part of a wide-area network. . By operating in spectrum licensed to the service provider, femto cells provide assured quality of service without any harmful interference but making efficient use of their spectrum. Along with coverage improvement it also creates extra network capacity by serving a greater number of users with high data rates. Femto cells are installed by the customers however they continue to adjust themselves over time as the customer and operator pleases to maximise performance and reliability. The operator is always able to create or deny service to individual femtocells or users. This control is maintained whether the femtocell is owned by the receiver or end user.
The radiated power and user capacity supported by a femto cell varies from one class to another. Class 1 emerged first and is best known which transmits 20dBm of power and supports 4-8 voice calls and data services and is considered best for small homes or enterprise. Class 2 may be viewed as an evolution of picocell technology to satisfy 8-16 users and provides coverage for area larger as compared to class 1 by radiating higher power about 24dBm. Performance of third class is better than the first two but it's not used frequently.
Most air interfaces included in ITU-R IMT family have active programmes to develop standards for femtocells. These include:
3GPP standards for Home Node-B, which is a WCDMA femtocell. Both FDD and TDD options are likely and a TD-SCDMA variant is also planned.
3GPP standards for Home Node-B, which is an LTE femtocell. Both FDD and TDD options are envisaged.
3GPP2's programme for femtocells for cdma2000, cdma2000 1x, HRPD, 1x EV-DO and UMB.
WiMAX Forum's programme for WiMAX femtocells based on IEEE standards.
III. NETWORK ARCHITECTURE
Femtocells will be deployed by the large number of home users and enterprises in their premises. To accommodate hundreds of thousands of femto cells which are connected through internet, the network architecture should meet the following requirements:
Scalable to the large number of femto cells units without disturbing the existing network.
Seamless connectivity with any existing legacy operator infrastructure.
Since femtocells will be installed in the owner's
premises, they should be easily configurable.
Femto network architecture should be fully standardised to accommodate multi-vendor network. At least the following main functional elements should be standardised :
FEMTOCELL ACCESS POINT (FAP) is same as Base Station (BS) like in macro network. Its functionality is almost same as BS like communicating with Mobile Station and making a connection with Mobile Network Operator (MNO) Core Network (CN) .
FEMTO GATEWAY (FGW) provides femtocell control, security and interfacing with MNO CN elements. A single femto gateway can supports a large number of FAPs . FGW is composed of Security Gateway (SeGW) and Media Gateway (MGW). The use of FGW is optional in the standard.
FEMTO MANAGEMENT SYSTEM (FMS) manages both the FAP and FGW. Its two main functional elements are FAP Management system (FAP-MS) and FGW Management system (FGW-MS). It is also used for configuring FAP via Fm interface (new interface introduced in the femto forum reference model).
Typically we have two architectural choices whether to use a RAN based solution in which femto network integrates with macro network as being a part of RAN or to use CN based solution in which femto network integrates with macro network as a CN element.
The following section will describe the femtocell network architecture for CDMA 1x.
A. CDMA 1x Femto reference model
First consumer femtocells were offered by CDMA operators these deployments were for CDMA 1x RTT technology (1x RTT is the version of CDMA 2000 which supports high data rates). Architectural choice for CDMA was of CN based solution which uses IMS . The reference diagram for CDMA 1x is shown in fig.
FAP is used for communicating with the mobile station. It also set up a circuit (voice and data) with the core network.
FIG 1.1 CDMA 1x Femto Architecture
Femto Gateway (FGW) authenticates the FAP and manages the secure layers over the broadband IP link . In FGW we have Security Gateway, It provides secure communication between FAP and CN. IP Packets between FAP and CN are encapsulated in the IPSec tunnel which is provided by Fx3 between FAP and FGW .
Telephony Application Server (TAS) provides functions equivalent to MSC in macro network. It provides processing as well as control for calls and other services.
FAP communicates with CN and TAS using Fx1 and Fx2 interfaces.
MAP Femto interworking function (MFIF) provides internetwork signalling between FAP, IP Multimedia system (IMS) and CDMA core network.
IV. ISSUES AND CHALLENGES
The band allocated to femto cells is already in use by the existing macrocells so there is a high probability of interference and health issues but quite different from those of macro cells due to their low range and power levels.
The limits in the question are based on the associated energy absorption inside the human body, measured in terms of specific absorption rate. It is recognised fact that radio frequency radiation does heat up local tissue, although the actual temperature rise depends on the rate at which heat is conducted away directly or by the flow of blood. Vendors are designing their products to fully comply with the guide lines for human exposure to electromagnetic emissions issued by International Commission on Non-Ionising Radiation Protection (ICNIRP).
Lub over IP
Each femtocell is connected to the RNC via the standard lub interface. The lub protocol stack is encapsulated within IP signalling. The first main concern is about whether RNC can support tens of thousands of node-base stations that would be deployed in the femtocell network. Moreover, despite being a standard interface, the lub has only vendor specific features which are suitable only for equipment manufacturers with an installed RNC base.
Signals produced by femtocells over the air interface are identical to those produced by conventional base station. However the interference will be different to those of macrocells due to their small coverage area and this interference is controlled by automated means instead of manual planning. Table shows a good comparison between different aspects of both.
FEMTOCELL VS. MACROCELL 
Cell phone power consumption
Femtocell to Macrocell Interference: Femtocell can cause interference both on the UL and DL. For example, a femtocell installed near a window of a residence can cause significant DL interference to the handsets outside the house. On the UL, the home handsets that are served by a certain femtocell can cause significant interference to the macrocells. Thus femtocell transmit power must be adjusted according to the macrocell power measured in the surrounding areas. In practice, any macrocell users who encounter interference would be switched to a different channel and would avoid outage from the femtocells.
Femtocell to Femtocell Interference: With the deployment of femtocell in adjacent buildings or apartments, it becomes relevant to consider the co-channel interference. Due to use of repeaters and macrocells interference, the interference levels can reach up to -65 dBm but femtocells are found to adopt well to deliver coverage over the desired areas with good performance in the presence of very strong co-channel interference resulting in high spectrum efficiency.
Since femtocells and femto-GWs communicate over public infrastructure instead of operator installed links. Communication services, voice and data, must provide adequate level of security to carry messages to their destinations without modification- intentional or unintentional. Lack of security for a femtocell system would mean that the third party will be able to observe, capture and analyze the data streams that flow in and out of the femtocell access point (FAP) either over the radio frequencies used or via the data link that carries IP connections to the core network. To provide safe communication, authentication and encryption must be done on the radio link and IP backhaul.
E. Bandwidth Issues
Internet service providers (ISPs) provide asymmetric bandwidth to broadband users, with more bandwidth available on the downlink than the on the uplink. Hence, the number of simultaneous users of a femtocell might be limited.
In order to provide seamless femto services to the users mobility to and from the macro network is one of the core issues. When a femtocell user arrives at home unambiguous handoff should be made i.e., seamless to the user as in macrocell networks.
When a femtocell handset arrives at home, services should be provided by the femtocell to the user not by the macro network, even if macrocell is providing better signal then femtocell. Typically, the handset searches for other frequencies in case the current macrocell signal is weak.
In the femto-to-macro handover case, this can be easily done by expanding the neighbour list which includes not only the radio characteristics of the neighboring macrocells, but also their full identity. In the auto-configuration case, FAP detects the macrocell identity and then uses it during the handover procedure.
In the macro-to-femto handover case it is impossible for the macrocell to know unambiguously the identity of the femtocell to which it want to connect because there can be hundreds of femtocells in the area covered by a single macro cell. Furthermore, if femtocells were to be known by identity in the macrocells' network, this would equate to an extremely heavy and dynamic configuration effort. Beacons can be radiated by femtocells to tell the handset about its presence.
With the improvement in indoor coverage femtocell makes it possible for high quality voice, data and video sessions to flow efficiently, securely and reliably and this growth in data applications is also accompanied by substantial growth in data revenue, helping to offset the decline in voice and date revenues in many markets and to enable continued overall industry growth. They operate at low power levels according to the parameters assigned. Subscribers of femto cells enjoy a wide range of potential services like special call tariffs on all the calls made or received on the femtocell and home automation and security control along with higher data rates and quality coverage. Users are also entertained with far higher usage limits or even uncapped data applications. The use of DSL, cable or fibre, for the purpose of connectivity to the core network is very impressive as a resource is directly reused for which user has already paid beside the increased efficiency of service delivery for mobile operators. The processing required for a femtocell is reduced as compared to a macrocell base station primarily due to reduced capacity and a reduced need to equalise the multipath channel. This is dominated by the reduced signal processing needs, especially for the receiver where functions such as correlators and Rake receivers have to operate at high speeds. So a reduced processing power is required which decreases the total cost per bit.
Some internet service providers' plans charge on the basis of bandwidth used. Asking the subscriber to pay additional charges for the additional capacity consumed for connecting femtocells to the core network may not be acceptable by the user.
B. Quality of Service
When femtocells are used to their full capacity, there might be a reduction in call quality/internet browsing if huge files are downloaded.
C. Operational Requirement
Femtocells work only for a particular cellular operator. So, everyone at home may require to use the same operator and have to change their numbers which may affect their social life.
D. Interface Problems
Not all cellular operators are on IP multimedia system (IMS), which might become the interface for the femtocells.
Cellular service providers are dependent on broadband services which are beyond their control. Any problem in the internet facility may bring a complete failure of cellular coverage inside homes also. So, troubleshooting might become a problem.
The motivations for femtocells include the need to provide better voice and data coverage to increasing home users. Femtocells achieve these goals by a combination of frequency reuse, better protection against interference by operating at parameters assigned by operator. Low power operation reduces health issues, similar to existing home and office devices, helping to insure that the devices are acceptable by the users when widely deployed and providing other benefits as reduced interference, increased mobile transmit power and increased battery life. Handover from femto-to-macro is a core issue as this is a necessary procedure for service continuity, while in the macro-to-femto case the macro network signal may still be good enough to provide service indoors. Femtocell market has significant challenges and massive potential challenges. There is a huge need for huge investments to produce solutions, and there are no guarantees over the success of solution from technical and economic standpoint.