Long Term Evolution (Advanced) - LTE(-A)
Femtocells

As several studies show, most of the user's traffic is generated from indoor. The 4G networks are supposed to use not only low frequency bands (e.g., 800/900 MHz) but also frequencies above 2 GHz. Higher frequencies are related to worse propagation of the signal. This problem is emphasized especially for indoor environment. Therefore, indoor users are unable to reach sufficient signal quality corresponding to their requirements. Those problems are addressed by new concept of so-called femtocells (in LTE-A, denoted as HeNB). The femtocell can increase throughput for indoor users and, also, to offload macrocell by serving users receiving signal of low quality from a macrocell.

The femtocell is low cost base station, which is supposed to be deployed in user's premises such as a house or an office. The femtocell is connected to the backbone network (Internet) through a wired connection such as DSL (Digital Subscriber Line) or optical fiber. This backbone connection delivers user's data from the femtocell to a destination (a server or target user) and vice versa. The transmitting power of the femtocell is typically set to cover just indoor area to provide sufficient signal quality to close users.

The maximum transmitting power of femtocell is roughly up to 21 dBm. Thus, the femtocell's coverage is only in order of tens of meters.

The femtocells can provide three types of access:

- Open access: All users under the coverage of the femtocell can connect to it. Radio resources as well as backbone connection capacity are shared with the same priority for all users attached to the femtocell. A benefit of the open access consists in an opportunity to offload a macrocell by serving several outdoor users in areas with heavy traffic load or by serving users far from the macrocell. On the other hand, dense deployment of open access femtocells can lead to significant rise in amount of initiated handovers.
- Closed access: The femtocell with the close access admits only users included in so-called Close Subscriber Group (CSG) list. The CSG list defines users that can exploit the femtocell for access to the Internet. It is managed by a subscriber, that is, by a user who is in charge of the femtocell. Typically, only few users are assumed to be included in the CSG (roughly four or eight users are assumed for home femtocells). In case of the closed access, only limited amount of users listed in CSG shares radio as well as backbone resources of the femtocell. Therefore, higher quality can be quarantined to these users comparing to open access. On the other hand, the interference incurred by the femtocell to non-CSG users should be carefully managed to avoid an impairment of performance of non-CSG user.
- Hybrid access: The hybrid access is a combination of both open and closed accesses. In this case, the CSG list is defined and a part of femtocell transmission capacity (radio as well as backhaul) is accessible only by CSG users. The rest of the radio resources and backbone can be shared by other non-CSG users.

A critical issue related to femtocells deployment consists in increase of interference. The interference of femtocells can be classified to interference from femtocell to macrocell users (cross-tier interference) and femtocells interfere to other femtocells (co-tier interference). Level of negative impact of the interference depends also on allocation of radio resources among femtocells and macrocells. Two ways of radio resources allocation can be identified:

- Co-channel deployment: The frequency bands for macro and femtocells are overlapping. It means the same carriers are shared by macrocells and femtocells. In this case, the interference can be mitigated by proper control of femtocell transmitting power or by allocation of radio resources in order to minimize interference.
- Orthogonal (or dedicated) channel deployment: Different set of carriers is assigned for macrocells and femtocells. This way, cross-tier interference is eliminated. On the other hand, reuse of frequencies is lowered and radio resources are not utilized efficiently.