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Monday, June 27, 2011

Femtocells in Fixed Mobile Convergence

The concept of fixed mobile convergence (FMC) consists of extending all or a part of the services provided by the wireless telecom service provider’s core network to domestic and small and medium enterprise subscribers through the public Internet Protocol (IP) network, taking advantage of the proliferation of wireless local and personal area networks deployments in these scenarios. This convergence will require subscribers to be able to switch an active voice or data session between fixed wireless and mobile networks, ensuring a seamless network transition this FMC context, the deployment of third-generation (3G) femtocells in residential and small enterprise scenarios has been adopted as one of the competing technologies to provide this convergence.

A base station router (BSR) femto node is a low-cost and low-power Universal Mobile Telecommunications System (UMTS) Node B, mainly conceived for domestic use, that admits a limited number of simultaneous communications, and which is connected with the mobile core network through the user’s digital subscriber line (DSL).
3G femtocells provide solutions to different problems that are facing 3G networks. First of all, indoor coverage will be easily increased. UMTS indoor coverage is much more difficult to achieve than second-generation (2G) coverage, because UMTS typically operates at higher frequencies, making it more difficult to penetrate building structures. In addition to this, UMTS networks are expected to provide services with high throughput requirements, which will need higher signal levels, and therefore indoor solutions will be compulsory. Secondly, femtocells have associated with them a better performance in terms of data rate. Unlike macrocells that support hundreds of users, femtocells will support fewer simultaneously active users, and therefore High Speed Packet Access (HSPA) connections will be able to deliver higher data rates per user than in the macro cellular environment. With higher data rates and fewer users, the quality of service requirements demanded in 3G networks within home and office scenarios are expected to be easily fulfilled, enhancing user satisfaction rates. This fact leads to small femtocells that are be able to deliver a better voice and multimedia quality of experience (QoE). In addition to the obvious voice quality gains attributed to better coverage, femtocells enable support for a new generation of higher rate voice codecs that leverage fewer user per access point and the proximity of the handset to the femtocell. Finally, operators will get other important business benefits due to the fact that femtocells will relieve the macro network from the indoor traffic that uses a substantial part of the mobile network resources, increasing the overall network capacity and reducing the cost of backhauling traffic to the operator’s core network.

The femtocell deployment scenarios can be classified in different ways, depending on an architecture-centered or a user-centered vision. Using an architecture-centered approach, femtocell deployment scenarios can be classified taking into account the relationship between femtocells and macrocells within the architecture of the 3G network. Initially, femtocells were considered as a means to offload traffic from the existing congested mobile network and convey it through the data network. There are two types of deployments in such scenarios, depending on spectrum availability. If sufficient spectrum is available, the preferred solution is to devote a separate channel to femtocell deployments to minimize the macrocell–femtocell interaction. Since spectrum is a scarce resource, in many cases femtocell deployments have to share spectrum with the macrocell network.

The other type of deployment is focused on the extension of the cellular network in areas where user data networks are available and where macrocell base station deployment presents limitations. The dimensioning process of deployments in typical scenarios has not been analyzed in depth, since under these conditions the dominant effect is probably the femtocell–femtocell interaction. The interaction of femtocells is different from that of micro base stations or macro base stations, since it is assumed there is little coordination among cells at the radio interface.

On the other hand, when selecting a user-centered approach, several types of deployments can be found. When a BSR femtocell is deployed in a small business or office to provide better coverage, the model is called open access, and a controlled network is created with respect to radio interference and handover between BSRs. Coexistence between this type of network and the macrocell network seems consistent.

In contrast, in residential scenarios, closed subscriber group (CSG) is the typical configuration used. BSR femtocell access is limited to the subscriber identity module (SIM) cards accepted by the node owner under the operator’s supervision while any external SIM card is rejected. This aspect introduces a particular interference scenario: a user can be located nearer to a neighbor’s node than to his own node, but there is no option to connect to the neighbor’s node. In such cases, the neighbor’s interference can prevent the user from connecting to his own node in certain areas of his house. This scenario is inconceivable in a macrocell environment since users are always able to connect to the node with the best power reception. CSG introduces a significant amount of uncertainty in such deployments because it is UMTS Node B for residential use that allows a limited number of simultaneous communications. It connects with the mobile core network through the DSL at the user’s home.

The CSG presents another limitation, which is macrocell coverage at the user’s home for SIMs not belonging to the CSG. In this case, the BSR femto node may cause interference with the macrocell signal and create a coverage gap for non-CSG users. All radio parameters must be adapted to ensure a grade of coverage robust enough for the user’s entire home. Another critical aspect of BSR femtocell nodes is that they are deployed by users—no configuration is performed by the operator, and the whole installation process is automatic. The network should also be adapted to support potentially frequent and random BSR femtocell powering on/off.

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