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Femtocell's access methods

Femtocells are low-power and low-cost base stations that provide residential cellular services, providing a coverage of roughly 10 m [1]. They can integrate with mobile operator through a broadband connection, typically ADSL. In General, femtocell makes that the traffic from the home cellular system, deviates through the broadband connection, releasing the resource consumption of the macrocell.

The 3GPP has introduced the concept of Closed Subscriber Group (CSG), which essentially identifies a group of subscribers who have access permission to one or more cells. Femtocells basically have three different operating modes: open access, closed access and a combination of both named hybrid access [2] and [3], the access methods are:

  • Open ccess: In this case, the UE can access the femtocell without any restrictions, this is seen in the Figure # 1 wherein the EU2 has unrestricted access to the femtocell in the building.
  • Closed Access: The administrator defines the only femtocell users (CSG) that can access the network. For this mode, emergency calls are exempt.
  • Hybrid Access: In this type of access, a limited amount of resources is allocated for access to users who are not part of CSG.

 

Figure # 1. Example of different femto-cell's access methods [2]: Open access, closed access and hybrid access.

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Quality of Service end-to-end in femto-cells environments

The telecommunications network of an ISP (Internet Service Provider) is actually an interaction of various networks types, within LTE (4G) can be highlighted as an access network to the end user. The information flow reaches to the user through several infrastructures, on which the ISP carries data traffic, from this perspective, some necessary points will be discussed in order to ensure the Quality of Service (QoS) in such environments.

From the point of view of LTE management, it is permitted to define profiles and classes of services, which are key points when negotiating QoS mobile requirements during the communication establishment, the transit of packets, even in handovers. However, it is necessary to consider the access and transport network if we want to ensure end-to-end QoS.

 

Transport Network

A typical scenario of an Internet service provider could be described as shown in Figure # 1, it can be seen that LTE is only a part of the provider's access network. The ISP has different types of access networks depending on the provided services. All the necessary infrastructure to transport information from the access networks, must transit through the core network. The core is also connected with other different network types, where the end service is usually located (Internet, PSTN, ftp services, video streaming, voice or other), which the user wants to access.

 

Figure # 1. ISP's simplified topology [2].

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The spectrum and multiplexing in LTE

The spectrum

Regarding the spectrum, LTE becomes quite flexible, allowing bandwidths of 1.25MHz, 1.6MHz, 2.5MHz, 5MHz, 10MHz, 15MHz and 20MHz in the downlink and uplink [4]. Furthermore, it supports broadcast transmission in downlink and uplink-downlink modes, on the other hand, radio resources for broadcast transmissions can be modified according to the operator needs.

The various scenarios which may present between the interaction of different service providers and other networks that they have, are not largely affected because manufacturers have planned coexistence, within the same geographic area, of the EUTRAN with other networks such as 3G and coexistence between adjacent operators, so too, is the case of the overlapping in countries boundaries [4].

The LTE spectral efficiency exceeds largely to HSPA +, these are the results of Telefónica [9], the study provides a scenario of urban centers with high density of buildings, 2x2 MIMO antenna configuration for both cases and using 64QAM as modulation scheme, with this, the study says that LTE spectral efficiency exceeds to HSPA + by 20% at full load. It is apparent from Figure # 1 than for rural or suburban centers LTE benefits will outweigh, furthermore along the chart, the LTE superiority is denoted.

 

Figure # 1: Spectral efficiency in terms of resource use [9].

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Macrocells and femtocells Interaction

Macro-cells are not very efficient [1] in the residential area due to penetration problems that have been mentioned in a previous article, also in the macro-cells there are many users and is more difficult to provide QoS to all of them. For these reasons Internet Service Providers (ISP) have opted for femto-cell implementation at residential level, in order to increase the quality of services provided.

Femto-cells are low-power base stations that provide low cost and high quality wireless services to residential, in this area offer a coverage of approximately 10 m [2], which are designed to integrate automatically with macro-cell networks. Femto-cells integrate with mobile operator through a broadband connection, typically DSL [2] and [3], as can be seen in Figure # 1. However it can also be performed by a wireless link as mentioned in [4]. In General, the femto-cell redirects the traffic from the home cellular system, through the broadband connection, releasing the resource consumption of the macro-cell.

 

Figure # 1. Typical Femto-cell [3].

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