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:
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: