Wimax Femtocell Architectural Solution Computer Science Essay

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WiMAX femtocell solution is a system based on WFAP and some other network functions like backhaul network and core network. Backhaul part is provided by customer's DSL or cable broadband, FTTx connection and some ISP, whereas the core network functionalities and services are provided by core network.

Multiple profiles have been defined by WiMAX forum for WiMAX deployment these namely called as profile A, B and C. Each of these profiles differ in the capability set with respect to ASN (Access Service Network) GW (GateWay) that each network elements implement.

So an equivalent profile for femtocell has not been defined yet by the WiMAX forum, but the profile is expected to be defined and will provide feature distribution between a WFAP and ASN gateway.

The new profile of femtocell deployment will be derived by the different requirement that femtocell has in interference management, mobility , security and QoS and manageability as opposed to conventional WiMAX access point.

Fig: Visualizes network architecture for a Femtocell solution assuming profile-C based network [1]

In the above scenario the WiMAX FAP is assumed to be the profile C based station which suits in building deployments. A WFAP is the source of air interface functionality for small coverage areas and has capacity to support 5 to 6 customers and connectivity to the core network is provided by DSL modem or an external DSL modem as a backhaul interface. Profile C-ASN gateway resides in the core network and terminates multiple WFAPs and establishes connectivity to the CSN network elements and the network management (NMS) domain.

And the remote management of WFAP devices is supported by NMS domain. The typical functions performed by NMS include provision, troubleshoot, and create and update an accurate neighbor list. The entire NMS operations can be done by minimal user involvement and operator input. End-to-end QoS is an other important consideration while deploying WiMAX femtocells since a different operator provides backhaul connectivity so attempts should be made to provide expected QoS

4.2 What are the technical challenges for deployment of WiMAX femtocells?

WiMAX femtocells will open new avenues for services and business models for the indoor users and since WiMAX is the current high data rate supportive technology for wireless broadband communication as compared to cellular or other 3G networks and hence our analysis will focus on WiMAX femtocells its capacity, solutions and challenges. Though there are certain issues that are to be considered before the services can be deployed. These issues mainly covers the issues like access methods, interference between macrocell and femtocell coverage and the methods to avoid such interference.

WiMAX is the most demanding technology today because of its comparatively good service quality and data rate that it supports as compared to 3G. On the other hand femtocells or femto access points are low power small base stations with the size like WiFi access points which provide indoor coverage to mobile stations using a fixed internet broadband connection like DSL or cable or fixed wireless backhaul.

There are certain challenges like resource allocation, frequency planning, power level that need to be addressed are still under investigation before these systems are widely deployed.

When deploying a femtocell network, the access method, which refers to the rights of the users when making use of the femtocells, needs to be defined. Two main different strategies for femtocell access have been proposed so far:

4.2.1 Access Methods[2]

• Public Access where all the users can access all the femtocells of a given operator.

• Private Access where only the subscriber of the femtocell and a list of invited users can access a given femtocell. So public access femtocells will provide a better network performance in terms of QoS and throughput. However, this will also increase the number of HO (Handovers) and therefore, the signaling between the existing femtocells. Furthermore, when considering public access the operator must take into account the property and security concerns of the customers, since those who pay for a femtocell are normally not keen to share their resources with any other users. That is why in the first instance operators will use femtocells with private access.[2]

There are some challenges with the deployment of WiMAX femtocells that need to be considered before the services are deployed. In the WiMAX Forum, the Service Provider Working Group (SPWG) has developed stage 1 requirements for WiMAX femtocell systems. Mentioned below are some of the key high level requirements for WiMAX femtocells [2]

4.2.2 Coverage Range: WiMAX femtocells access points are expected to give coverage range to from tens of centimeters to tens of meters

4.2.3 Femtocells network operation independency: femtocells may be deployed to support the macrocells and be independent managed and operated. This means that femtocells sould be able to provide services within WiMAX femtocell access point's coverage without the presence of any macro network.[3]

4.2.4 Self-organization/configuration: WFAP are deployed as plug and play without configuration and on site tuning so they are able to support the network managed self configuration.[3]

4.2.5 Operator controlled remote integration, activation, and deactivation: Since the WiMAX femtocell access points are considered to be end-user nomadic devices and they operate in licensed spectrum so their transmission should be fully managed by the femtocell service provider The femtocell network should be able to determine WFAP's locations before they are activated in order to make sure that the transmission frequency and power levels meet the local regulatory requirements.[3]

4.2.6 Synchronization and interference management: The inter femtocell interference and neighboring macrocell interference need to be considered, for this the femtocells need to be synchronized with each other and with neighboring macrocells.[3]

4.2.7 Handover: Seamless handover between macrocell BSs and WFAPs as well as between WFAPs should be supported, but at pedestrian speeds. In addition to the aforementioned system requirements, there is an expectation that the 802.16m air interface will provide further optimization for femtocell operation. For example:

• Signal measurement report to support advanced interference management, radio resource management, and WFAP location

• Optimization of MSs scanning, selection, network entry, and handover to desired BSs in multilayer networks consisting of macrocell BSs and large numbers of femto BSs these features would further optimize femtocell operations and facilitate their usage[6]

4.2.8 Interference to/from femtocell s and other BSs (macrocells): in order for femtocells it must be ensured that there should be no interference with other neighboring cells in surroundings. In typical BSs installation lot of efforts and method and manpower are used to avoid interference issues. For femtocells end users will simply take the device attach it to power and broadband and expect it to work. At the same time the network operator management system must be certain that femtocells will not interfere with each other and with surrounding macrocell networks.[3]

4.2.9 Regularoty issues: Femtocells operate in licensed spectrum so need a regulatory approval. Femtocells activate and transmit radio frequencies that are not allowed by local regulations so ways and means have to be ensured that all femtocells be activated remotely by the network and network must be aware of the femtocells exact location to avoid transmissions that have regulatory implications. Interference and customer health concerns must meet regulatory requirements.[3]

4.2.10 Intrasystem and intersystem interference: This the interference between femtocells and macrocells and this is one of the challenging problem in those areas where femtocells are densely deployed and in near this problem is not being supported by MSs but in future it MSs may be able to support network for more optimal interference management. For instance the problem may be addressed by using separate frequencies for indoor femtocells and /or using dynamic power control and distribution/centralized radio resource management techniques.[3]

Although the access method for deployed femtocells still remains an open question, customers surveys [15] show that private access is the customer's favorite option. However, this approach imposes some interference problems to macrocell and femtocell users . Some of these problems are summarized in the following: First, a DL (DownLink) user connected to a far macro-cell could be jammed due to the presence of a closer DL femtocell user who is using the same frequency/time. Second, a UL (UpLink) user connected to a femtocell could be jammed due to the presence of a close UL user connected to a macrocell using the same frequency/time. Therefore, interference avoidance techniques need to be applied to reduce the impact of femtocells on the macrocells and vice versa. In WiMAX networks, intra-cell interference may be neglected due to the sub-carrier orthogonality features of OFDMA (Orthogonal frequency-Division Multiple Access). Operators must therefore cope with inter-cell interference in order to enhance the network performance. To overcome inter-cell interference, OFDMA networks are flexible in terms of radio resource management techniques, supporting different frequency reuse schemes and sub-channel allocation techniques, which in turn may decrease the intercell interference and increase the network capacity. However, these fixed schemes and techniques are not the most suitable solution in mobile scenarios, where the behavior of the channel and the users are continuously changing. A new approach to the frequency assignment problem tailored to OFDMA networks called Dynamic Frequency Planning is presented . DFP can decrease the network interference and increase significantly the network capacity by dynamically adapting the radio frequency parameters to the environment. It operates on a regular basis to cope with the changing behavior of the traffic and the channel throughout the day. DFP can run from a few times a day down to on a second by second basis depending on the needs of the operator. DFP can also be applied on WiMAX femtocell scenarios to avoid macrocell to femtocell interference and also femtocell to femtocell interference, improving the network capacity and the user experience in outdoor and indoor scenarios. [3]

4.3 Rural & Urban Considerations:

There are two considerations that we have mentioned so far one is the urban area sand second is the rural area where there is no facility of cable of DSL broadband connection.

4.3.1 Urban Areas: In urban areas high density of possible customers makes operators seek some efficient solutions to increase the coverage. The common approach to increasing the capacity of a cellular network is to add more cells, each of them covering smaller areas. Hence, in an urban environment, the operators had to install more base stations with lower power. They also had to face the same dilemma as in a rural environment where the coverage inside buildings is ensured only by improving the outdoor signal quality. In this kind of environment, where multiple reflections on walls and diffractions on roofs occurs, it was very often not sufficient to use free space or empirical radio propagation models. Deterministic radio propagation models, such as those based on a ray tracing approach, can be used to compute efficiently the diffractions and reflections of the signal to compute accurate coverage maps. Even if this kind of tool helped the operators, they mainly helped to optimize the street level coverage but not the coverage inside the building. This is due to the fact that operators can have access to the building data of cities thanks to aerial pictures like Google maps for example, but it is quite impossible to have the whole data related to the content of the buildings of the city. Moreover the complexity of such a tool would be drastically high. [4]

Fig 1. Depicts the urban scenario in which femtocell is installed and cable, fiber or DSL connection is being terminated onto the femtocell. Fetmocell serves to improve indoor coverage and provide capacity and hence improving user experience.

Femtocell diagram

Fig1.Urban scenario

4.3.2 Rural Areas: In rural areas high base stations are needed to be installed in order to provide the coverage to long distances, these are called macrocells. The power of the macrocell base stations is high to maximize the covered distance. In that perspective, due to the low number of customers and high prices of high power base station installations and their maintenance costs, the approach for operators has always been to try to ensure minimal coverage so that voice calls can be performed outside. The deployment of such a network was very often done by combining wireless network planning tools and real measurements. The disadvantage of a macrocell network is that the network is deployed by only taking into account the outdoor coverage, which is why in many rural areas it is still necessary to go outside of the building to be able to make a call. Then, in order to optimize indoor coverage, the only approach for operators was to add more macrocells. Since such equipment is expensive, not only in terms of buying cost, but also in terms of maintenance, the operators always had to deal with an economic compromise: add macrocells in areas where the number of customers is big enough to make the installation profitable, and leave the other areas either without or with a minimal coverage, thus femtocell is likely to play a significant role to reduce the operator costs and provide coverage. Below we have discussed a rural model that shows a cost effective way to deploy WiMAX femtocells to overcome coverage issues.[4]

Fig 2. Depicts the rural area scenario where there is no DSL connectivity and additional microwave antenna and supporting equipments have to be installed along with the femtocells to synchronize with the WiMAX BSS to give the last mile connectivity and this provides cost effective way to deliver the services to end-users. C:\Users\Saleem Raza\Desktop\rural scenario.png

Fig 2. Rural scenario

4.4 How can we increase the capacity to accommodate larger number of customers?

There are different platforms available to support different data rate and throughput requirements, one efficient way to have larger data rate is with WiMAX and LTE (Long Term Evolution). WiMAX is being deployed commercially while LTE is in its process of evolution and development. Currently WiMAX is on the possible mark , but with WiMAX there are certain coverage issues in congested areas as we have highlighted previously in our report the effective way to improve coverage is with WiMAX Femtocells, it not only does improve coverage but also enhances systems capacity by off loading macro layer traffic and improves macrocell capacity (in the case of using macrocells to provide indoor coverage, more power from the base station will be needed to compensate for high penetration loss, resulting in a decrease in macrocell capacity [123]) in an efficient manner. According to their capacity, FAP can be classified into two categories, namely home FAP, which can support 3-5 simultaneous users, and enterprise FAP, which can support 8-16 users. By offloading macrocell traffic femtocell is a good way to accommodate more number of customers who due to traffic are unable to avail the services properly.

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