System Level Modeling and Evaluation of Heterogeneous Cellular Networks

The cumulative impact of co-channel interferers, commonly referred to as aggregate network interference, is one of the main performance limiting factors in today?s mobile cellular networks. Thus, its careful statistical description is decisive for system analysis and design. A system model for interference analysis is required to capture essential network variation effects, such as base station deployment- and signal propagation characteristics. Furthermore it should be simple and tractable so as to enable first-order insights on design fundamentals and rapid exchange of new ideas. Interference modeling has posed a challenge ever since the establishment of traditional macro cellular deployments. The recent emergence of heterogeneous network topologies complicates matters by contesting many established aspects of time-honored approaches. This thesis presents user-centric system models that enable to investigate scenarios with an asymmetric interference impact. The first approach simplifies the interference analysis in a hexagonal grid setup by distributing the power of the interfering base stations uniformly along a circle. Aggregate interference is modeled by a single Gamma random variable. Its shape- and scale parameter are determined by the network geometry and the fading. The second model extends the circular concept by non-uniform power profiles along the circles. It enables to map substantially large heterogeneous out of-cell interferer deployments on a well-defined circular structure of nodes. Thereby it considerably reduces complexity while preserving the original interference statistics. The model is complemented by a new finite sum representation for the sum of Gamma random variables with integer-valued shape parameter that allows to identify candidate base stations for user-centric base station collaboration schemes as well as to predict the corresponding rate performance. The third approach applies stochastic geometry to model two-tier heterogeneous cellular networks with respect to the topology of an urban environment. It tackles the asymmetric interference impact by a virtual building approximation and introduces a new signal propagation model that directly relates to the topology characteristics such as building density and -size, which can straightforwardly be extracted from real world data. In the last part of the thesis, the applicability of the introduced models is validated against simulations with the Vienna LTE-Advanced Downlink System Level Simulator. For this purpose, the analytical models are calibrated against results from LTE Advanced link level simulations. This part also complements the hitherto user-centric investigations with a system-wide performance evaluation, addressing the impact of user clustering as well as small cell density- and isolation. Particular focus is laid on a systematic and reproducible simulation methodology as well as appropriate performance metrics, since conventional figures of merit tend to conceal performance imbalances among users.