Interference Management in LTE-Advanced Heterogeneous Networks Using Almost Blank Subframes

Abstract: Long term evolution (LTE) is the standard that the Third-generation Partnership Project (3GPP) developed to be an evolution of UMTS. LTE offers higher throughput and lower latency than UMTS and this is mainly due to the larger spectrum used in LTE but in terms of spectrum efficiency LTE does not offer a lot of improvements compared to UMTS. The reason for that is that current technologies such as UMTS and LTE are approaching the theoretical boundaries in terms of spectral efficiency. Since spectrum has become a scarce resource nowadays, new ways have to be found to improve the network performance and one of the studied approaches to do that is to enhance the network topology. The concept of heterogeneous networks has attracted a lot of interest recently as a way to improve the performance of the network. The heterogeneous networks approach consists of complementing the Macro layer with low power nodes such as Micro or Pico base stations. This approach has been considered a way to improve the capacity and data rate in the areas covered by these low power nodes; they are mostly distributed depending on the areas that generate higher traffic. Since cell selection for the users is based on the downlink power level and due to the transmitting power differences between Macro and Pico nodes, Pico nodes might be under-utilized, meaning that a low number of users are attached to the Pico nodes. As a solution to this problem an offset to the received power measurements used in cell selection is applied allowing more users to be attached to the Pico nodes, this solution is called ‘Range Extension’ which refers to the extended coverage area of the Pico nodes. The problem with Range Extension is that it drastically increases the interference that the Macro nodes impose on the Pico nodes users in the Range Extension area in terms of data and control channels. Enhanced Inter-Cell Interference Coordination (eICIC) schemes have been proposed to combat the heavy interference in the Range Extension case ranging from frequency domain schemes like carrier aggregation to time domain schemes like Almost Blank Subframes (ABS). The focus of this thesis will be on the ABS solution which consists of reserving a group of subframes during which the Macro nodes are partially muted allowing the users in the range extension area to be served with lower interference. The objective of this thesis work is to introduce a closed form expression to calculate the Almost Blank Subframes allocation in order to maximize the normalized cell-edge users throughput. The derivations are carried out for a simplified model of a telecommunications network. The expression will be validated with simulations involving different users and Pico nodes distributions and different channel models (ITU channel models and Spatial Channel Models). Another goal is to try to have a deeper understanding and concrete conclusions about the different heterogeneous deployments.