The current 3G and 4G cellular technologies cant support high data rate demands of the voice and video applications and end up providing poor coverage indoor. Customer dissatisfaction due to dropped calls and time-consuming downloads in high density metropolitan hubs were the major concerns of the service providers. A low cost solution for this problem is deployment of Femtocells in bandwidth demanding areas. The system capacity and network coverage can be increased with the use of Femtocells , which are small base stations connected to DSL or internet cable and are installed in residential or business environments. These Femtocells provide high-quality network access to indoor users, while simultaneously reducing the load of the whole system. In this seminar, architecture of Femtocells , the basic working, and its applications will be covered. The advantages of Femtocells over other networks and the technical issues in implementation of Femtocells will be discussed.
Introduction
Femtocells are small devices that can be installed in home or premises to increase the coverage capacity indoors. These femtocells are deployed in the area of very low coverage to provide the high voice and data services to the mobile devices that are assigned to femtocells initially. These femtocells are connected to mobile operator's core network through Internet via DSL or broadband modem. Femtocells which include both a DSL router and femtocells are called . 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.
These are also called small cells, as their coverage is very less compared to microcells (200 Km), picocells (200m), whereas Femtocells limits themselves to the range of (10m). As shown in the below figure, the femtocell deployed in home can support from 3 to 16 mobile devices. These mobile devices are operated by femtocells. The voice and data of these mobile devices is transmitted through the Femtocells network to the Mobile operators Network. As the backhauling is carried out through internet, which in turn reduces the data load from macrocell and hence increases its efficiency
Source: EMF Series Projects with Collaborations of WHO
Femtocells provide all the services such as circuit switched and packet switched services by using different architectural models. The one model is based on 3GGP standard called SIP/IMS model and the other legacy network model is based on 3GPP2 Standard. These two models are described further in more detail. Inspite of many advantages of femtocells, there are several technical issues in implementation of femtocell like handover of device, interference of cells in network and synchronization of femtocells with the network which will be dealed with in more details.
Basic Working of Femtocells: While deploying Femtocell, User declares the mobile devices that will be using the coverage area of Femtocell which is mostly done through web interface of Mobile Network operator. These defined mobile devices are when outside the coverage area of femtocell, they use the coverage of Macrocell, but as soon as the mobile device comes in the coverage are of Femtocell, the overall control of mobile device will be transferred to femtocell. The voice and data of this mobile device will be backhauled through internet to mobile operator network. The overall communication will be carried out by femtocell., and hence providing better coverage area indoors. This process of transferring the control of device from macrocell to femtocell is known as handover.
Basic Working of Femtocell
These Femtocells uses different architectures based on the Technological standards followed by Femtocells. The WCDMA uses Iuh Architecture while CDMA2000 uses SIP/IMS architecture. This network architecture share common network components.
Network Architecture Components:
Femtocell Access Point (FAP)
Security Gateway (SeGW)
Femtocell Device Management System (FMS)
Femtocell Access Point: Femtocell Access Point is a key component of femtocell architecture. These are small access points that are deployed in user location. There functions are similar to that of base station and base station controller of macrocell network. These femtocells provide connection between user equipment and mobile operator's network. There are different types of FAP's available some FAP's are plug and play devices, which can be connected to the broadband routers directly. These FAP's are also useful for prioritizing the mobile devices depending on the data being transferred by them.
Femtocell Access Points
For example, if the call is being made by any user equipment and simultaneously the song is being uploaded by any other user equipment under the same femtocell coverage area. The Mobile device with voice data will have higher priority over the device where song uploading is taking place.
Security Gateway: As the whole backhauling of data is carried over Internet, it becomes necessary to transfer the encrypted data over a secured connection to mobile operator's network and protect the mobile devices from security breach. These security Gateway is also used to authenticate all the mobile devices that are allowed to use femtocell services. When any initially defined mobile device comes under the coverage of femtocell network it is first authenticated before allowing it to use femtocell services. The encryption of data and signaling is carried out using standard Internet protocol such as IPSEC and IKEv2.
The security gateway is a network node that secures the Internet connection between femtocell users and the mobile operator core network. It uses standard Internet security protocols such as IPSec and IKEv2 to authenticate and authorize femtocells and provide encryption support for all signaling and user traffic.
Femtocell Device Management System: As there is large number of femtocells, it is necessary to manage the devices and operation of all the femtocells, this is done by femtocell device management system. It is resides in operator's network. The FDMS is use to configure the different devices available and manage the operation of each device with respect to other from the operator's core network. This plays a key role in initiatilisation and activation of femtocells when deployed for the first time and continues providing it services of updating and configuring newly available services. For managing such large number of femtocells specific architecture called clustering and load balancing is used. The basic standard used by femtocell network management system is TR-069.
As shown in below figure, depending on the functionality, the FSM is classified further in two parts.
1. Automatic Network Planner: It is use to plan the allocation of carrier frequency for femtocell. It executes the Frequency Reuse algorithm, RF Planning algorithms and configures the best RF to the femtocell avoiding the interference with neighbouring femtocell.
2. Device Manager: Unlike Automatic Network Planner, it is associated with femtocell devices at user end. Its basic functions are error detection and management in femtocells devices, remote configuration and diagnostic, upgrading the software versions on the devices, collecting the performance information in particular.
Femtocell Service Management System
Architectural Models:
SIP/IMS Network Model
Legacy Network Model
1. SIP/IMS Network Model: In SIP/IMS model, when the call is made from the mobile devices, the signaling and encrypted data is carried out from femtocell to the IMS network architecture via security gateway and then forwarded to PSTN network. The following are the important components of architecture.
Femtocell Access Point (IMS Client)
SIP/IMS Core Network
Femtocell Convergence Server (FCS)
Legacy Network Model
This network does all the call routing and signaling functions. The Voice data from the femtocell access point is converted over RTP and then transmitted to femtocell convergence server. The 3G signaling is converted to IMS signaling. The nodes of SIP/IMS network consist of Home subscriber subsystem which provides the information of subscriber, Call signaling control function, manages all the signaling functions, Media gateway controller which connects to Legacy Network. The other most important component is femtocell convergence server which is an application server and it connects to MSC(mobile switching center) in legacy network using an IS-41 network interface and is connected to CSCF using standard ISC interface.
Femtocell Convergence Server also acts as MSC for mobile core network. It also conducts handover between femtocell and macrocell. When a mobile device moves from femtocell coverage to macrocell network coverage, macrocell to macrocell hand off mechanism is used and hence femtocells receive messages same as they are received when macrocell to macrocell handoffs takes place.
Below is the complete description of network blocks in SIP/IMS Architecture.
SIP/IMS Network Model
2. Legacy Network Model: This is the simple network model compared to SIP/IMS model as it allows the use of already existing mobile operators network.
The three important components of Legacy network model are
Femtocell Access Point
Femtocell Network Gateway (FNG)
Security Gateway
This model connects to mobile operator's network directly through FNG (femto network gateway). These femto network gateway connects the actual FAP's using standardize 3GPP Iuh interface to the Legacy network. FNG acts as a mobile radio network controller for femtocells. In this model the handoff is carried out by MSC of core network. In this model support of active handoff is given through the legacy MSC. When the femtocell moves from femtocell coverage to macrocell network the handoff mechanism is carried in the way similar to that between radio network controller and MSC, using the Iu interface. The legacy network model is used by 3GPPstandards for UMTS femtocells.
Legacy Network Model
The FCS and FMS mentioned above plays a very important role in setting up a call as act as mediator between the femtocell and mobile operator core network. The packet data services are provided by network components such as SGGSN/GGSN in UMTS and PDSN in CDMA femtocells. Femtocells are connected directly to SGGSN while for connecting to PDSN, FNG acts as bridge between them.
Advantages:
1. Good Coverage and increased data capacity: For the good coverage and data transfer capacity, the ratio of signal to Noise should be high enough to sustain the attenuation that occurs when a signal is transmitted from macrocell to the receiver. As the data rate of voice signal is 10kb/s compared to that of data traffic which is in Mbps, the requirement of signal strength for voice traffic is also less compared to data traffic. As the use of Smart phones is substantially increase from last few years, these high data rates could not be gained due to high attenuation that takes place during transmission of signal between transmitter and receiver. As the signal attenuation is caused mainly due to shadowing, interference from other transmitters and path loss causing the decaying of signal. The signal decay is given as D=A.d-α A is the constant loss, d is distance between transmitter and receiver and α is the decay constant. So in order to minimize the path loss d should be decreased.
Increased Spectral efficiency
Femtocell overcomes this issue as the transmitter is installed in the home/premises and the receiver that will be the mobile devices will also reside at very short distance from the transmitter. Hence decreasing the distance between transmitter and receiver, will decrease the attenuation of signal and hence will avoid the signal to noise ratio to degrade. As low power is required by femtocells to operate, which eventually increase the battery life of mobile device. Mobile device will require very less power to be transmitted and hence more number of mobile devices can be used in small coverage area of femtocell. This increase in the number of mobile devices increases the overall spectral efficiency.
2. Offloading Macrocell: As the back hauling of data is carried out by femtocell the load on the macrocell is comparatively reduce. The whole control of data and all the data transfer of a mobile device take place through the femtocell. The backhauling data through Internet provides better capacity to mobile device, simultaneously reducing the data uploaded directly to Macrocell radio network. Hence the mobile base station can provide god coverage and capacity to other mobile devices that are not under the coverage area of Femtocell. The mobile base station can provide coverage to more number of mobile users in its cell area. This will be advantageous to service provider as well as subscriber. Subscriber can enjoy high data capacity and coverage area and simultaneously reducing the overall burden on the macrocell and hence improving macrocell reliability.
3. Self Organising: The Femtocell can be easily installed by non technical user. It has to be 'plug and play'. Femtocells automatically get configured to available network environment. Any operational changes even after installation are detected and the femtocell device gets updated. Femtocell are capable of detecting and managing fault during operation. It is self configuring, self optimizing, error detecting and rectifying device. They basically work on self organizing algorithms which are executed by device manager.
4. Cost effective: It has been observed that 70% of total data transfers take place indoors. In order to provide good coverage indoor it is necessary for service providers to install more base station as the number of mobile users is increasing. Installing the macrocell basestaton is very costly and requires huge infrastructure. Installing macrocell will not be efficient way to increase the coverage indoors, as there is 20db loss of signal due to the infrastructure, fading etc. Femtocell deployments will costs above $1000/month in site lease, and additional costs for electricity and backhaul. Hence installing femtocell will reduce the operating and maintaining cost, providing the good coverage capacity indoors.
5. Win-Win Model: Due to poor coverage indoor, causing interruption in services results in customer churn and hence customers look forward to different service providers. Implementing femtocell will be beneficial to providers as it offloads the data traffic from macrocell and user can get added services provided by its provider when in the coverage area of femtocell, hence creating win-win situation for both the providers and subscribers.
Challenges:
As there are many advantages and used of femtocells there have also been some technical challenges faced while deployment of femtocellls.
1. Synchronization: Femtocell synchronization is very important in accurate implementation of femtocells. In order to provide uninterrupted service to subscriber the basestation and femtocells should be very accurately synchronized.
Handsets should be accurately synchronized with the frequency of basestation.
To provide reliable handover it is necessary that femtocell should be synchronized with the basestation network, otherwise the difference in frequency can cause handover failure.
Sychronization reduces the interference which can in turn increase the quality of service.
There are different ways of synchronizing the femtocell to the network, they are described in detail as follows:
1. Femtocell Synchronization from Internet: Femtocells can be synchronized by using the internet connection with network operator. The network operator's clock servers send the timing information to the FAP via internet in the form of packets. The protocols such as precision time protocol, network time protocol and IEEE1588 is used. The operation is Master slave based model, in which master is the network operator's clock which sends the timing details to the slave (Access points). The main issue with this type of synchronization is that the packets can get delayed depending on the traffic on the channel. As the timing information would be transmitted frequently and need to be highly precise, this may lead to increase in bandwidth consumption.
2. Femtocell synchronization via GPS: Collecting timing information from GPS receivers which can be embedded in femtocells. It is the low cost way of synchronization. The assistance data is sent from the macrocell of the adjacent cell to femtocell which helps to provide the sufficient timing information. The problem with this way of Synchronization is the attenuation factor. The attenuation will increase in case of femtocells as it resides inside the building.
3.Using Adjacent macrocell for Synchronization: The synchronization information can be obtain by the macrocell, as the femtocells have to always exchange information for handover. This could only pose problem when the coverage of macrocell is less and signal could not be reach the femtocell network.
2. Femtocell Security: Security plays a key role in femtocell management system, as whole data is carried over the Internet. The femtocell security have been classified in two types as-
1. User Privacy- As the complete transfer of subscriber information(voice and data) is carried out over internet while backhauling, the transmitted data should be protected against security breach. Some denial of service attacks that increase the burden on the system by creating dummy and fake user s can caused the authorized users to be deprived of services and coverage.
2. Fraud users: Some unauthorized users can enjoy the facility of femtocell services by hacking the femtocell and leading to customer dissatisfaction due to unusual bills. Also they can misuse the available customer information. Hence in order to avoid these scenarios following measures were taken.
Protocols such as IPSec and extensible authentication protocol were used. Security can also be provided by continuously authenticating the femtocell service users and always ensuring that femtocell area does not increase the physical coverage area.
3. Interference: The major challenge in femtocell deployment is interference due to the same use of frequency by neighboring femtocell or the macrocell of the area.
Causes of Interference in Femtocells:
1. Random deployment of femtocells: Unlike antennas the femtocells are installed randomly without any central controlling unit that will govern the deployment of femtocells in the specific area. Femtocells can be easily installed by anyone, which can cause the ad hoc installation of femtocells ultimately increasing the probability of interference between the femtocells and base station. As the Device manager will not know the frequency allotted to the basestation in which the femtocell will be deployed.
2. Reuse of Cellular spectrum: As the bandwidth of spectrum available is less, some frequencies are reused by other cell which is not adjacent to the current cell in order to increase the spectral effeciency. This is other main cause for interference.
3. Restricted Users: In order to allow the femtocell coverage to limited number of mobile devices, the other mobile devices of same providers face coverage issues in the area near to femtocells.
Types of Interference:
Femtocell to Macro cell Interference: The interference which takes place between the femtocell and its base station is called femto-macro interference. This is caused due to restriction on number of users in one femtocell. It causes interference while uplink as well as downlink of the mobile device that are not authorized to femtocell.
Femtocell-Macrocell Interference
For example consider a femtocell using frequency f1 which is the same frequency that of macrocell network. Hence this will cause interference leading to more consumption of signal power by the femtocell and hence giving better coverage to femtocell authorized devices and causing macrocell to give poor coverage poor coverage to the mobile devices not under femtocell coverage area. Hence due to interference the non femtocells authorized user will be deprived of the services provided by the service provider. The decrease in coverage area and data capacity is directly proportional to distance between the macrocell and the mobile devices, hence the devices near to edge of the cells will suffer maximum from problems such as call disconnection, no coverage etc. Also if number of mobile devices increases in that area can lead to severe coverage issues due to already existing femtocell and increase in number of user equipments which require high coverage.
Macrocell-Femtocell Interfernce
Downlink Interference: Consider the femtocell deployed in home. Any active femtocell handset at the edge of femtocell coverage area will also start receiving the signal power from macrocell which will result in overloading of macrocell and hence less signal power will be received by the macrocell handsets.
Uplink Interference: Now the macocell handset which doesnot have access to the femtocell is in the coverage area of femtocell. The handset is calling and hence receiving the full signal power. This may affect the femtocells mobile devices which are also on call and on edge of femtocell coverge area causing the call dropping.
Femtocell-Femtocell Interference: Due to increase in number of femtocells and its deployment in random fashion can cause two neighboring femtocells to interfere with each other. The femtocell which has maximum signal power reception cannot act as the only femtocell in the area due to limited user access.
Femtocell Femtocell Interference
Mitigation of Interference:
Adaptive power control: In this mitigation way the femtocells have added feature in which they continuously monitor the received signal power from the macrocell and compares it against the total power spectral density of the macro and femto cell downlink channel. If the power received is much higher compared to that received by macro cell handsets then it automatically lower down the signal power consumption.
Intelligent Carrier Frequency Allocation: Avoid the use of same frequency within the area of adjacent cells. The better way to use this is spectrum division. The spectrum is divided and classified based on the frequencies that will be used by femocell and the other that would be used by macrocell in particular area in order to avoid the interference between the cells and femtocells. As shown in figure the spectrum is divided into free available frequency bands and that used for femtocells and also includes frequency spectra for macrocell. Also frequency convergence server use some frequency reuse algorithm in order to avoid the allocation of same frequency in nearby cells.
Mobile Phone Uplink Power Limits: When the macrocell handset makes a call in femtocell coverage the signal transmitted from the handset is sensed continuosly, if the transmitted signal strength exceeds the threshold value the handset is assigned to macrocell network hence avoiding the interference of femtocell and macrocell handsets.
Fixed Spectrum Allocation for femtocells
4. Handover: Handover is the process of transferring the control of mobile device when it moves from one cell coverage to other seamlessly. Here when the mobile device moves from femtocell to macrocell or between two femtocell, the provider needs to provide uninterrupted services to the mobile device. This mechanisim of successful transferring control of mobile device is called handover. There are three types of handover explained below.
Inbound Handover
Inbound Handover: This type of handover takes place when the mobile device is moving from the external macrocell network to the femtocell coverage area. The user equipment (mobile device) continuously measures the signal strength of all the neighboring cells. Whenever the signal strength received by the device exceeds the threshold level the device gets ready for handover. It will then get authenticated by the femtocell which transfers maximum signal power to the device. The femtocell then authenticates the mobile device. The hand over is same as that of handover between two macrocell except the signaling connection between the two cells is through internet. Each femtocells has its unique identifier number which helps the successful handover of device to the corresponding femtocell.
The figure explains the handover procedure of inbound handover.
Outbound Handover: When the mobile device moves from femtocell to macrocell then its called outbound handover. When the transmitted signal from the femtocell handset exceeds the threshold level it is handoverd to macrocell network.
Outbound Handover
Femto-Femto Handover: when mobile device moves from one femto network to other. The signaling is carried through backhauling. The whole of handover is carried out by femtocell themselves.
Conclusions
Femtocell is very effective low power and short range device that is deployed in home for increasing the coverage area for defined number of mobile devices indoors. It allows users to enjoy services similar to wi-fi under license spectrum. They also help in reducing the overall traffic on the macrocell hence increasing the reliability and efficiency of service provider network. The femtocells are user handy devices which can be deployed easily. Femtocells system has complex architecture that differ based on the type of services that will be provided by femtocells. The SIP/IMs model and Legacy network model are the two widely use architectures in femtocells system. Femtocell Network Gateway /Femtocell convergence server bridges between the femtocell aceess points and mobile operators core network. Exponential increase in use of smart phones and hence in mobile data traffic has resulted in need development of femtocell. Though there are some technical issues in femtocell implementation various strategies have also been developed to mitigate each one of them. The research is ongoing to completely overcome the present challenges of Interference and handover.
