Constrained capacity density optimization by fractional frequency partitioning

Downlink performance of cellular networks is mainly limited by Inter-cell Interference (ICI). A promising concept for ICI mitigation is Fractional Frequency Reuse (FFR), which effectively allows to trade off overall performance against enhanced cell-edge performance. In this work, a novel FFR scheme is proposed. It assigns a given user to a frequency sub band depending on the achievable capacity density (bit/s/m²). An optimization problem is formulated, which aims at maximizing average per-user throughput while maintaining a minimum performance at cell-edge. Simulations are carried out for omnidirectional and sectorized cellular scenarios, using both Two-dimensional (2-D) and Three dimensional (3-D) antenna radiation patterns. The simulation results show that the proposed scheme outperforms conventional reuse-1- and reuse-3 schemes in terms of average and cell-edge performance.6

System level modeling and evaluation of heterogeneous cellular networks

Zsfassung in dt. SpracheThe 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-A 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.14

The Vienna LTE-advanced simulators: up and downlink, link and system level simulation

This book introduces the Vienna Simulator Suite for 3rd-Generation Partnership Project (3GPP)-compatible Long Term Evolution-Advanced (LTE-A) simulators and presents applications to demonstrate their uses for describing, designing, and optimizing wireless cellular LTE-A networks. Part One addresses LTE and LTE-A link level techniques. As there has been high demand for the downlink (DL) simulator, it constitutes the central focus of the majority of the chapters. This part of the book reports on relevant highlights, including single-user (SU), multi-user (MU) and single-input-single-output (SISO) as well as multiple-input-multiple-output (MIMO) transmissions. Furthermore, it summarizes the optimal pilot pattern for high-speed communications as well as different synchronization issues. One chapter is devoted to experiments that show how the link level simulator can provide input to a testbed. This section also uses measurements to present and validate fundamental results on orthogonal frequency division multiplexing (OFDM) transmissions that are not limited to LTE-A. One chapter exclusively deals with the newest tool, the uplink (UL) link level simulator, and presents cutting-edge results. In turn, Part Two focuses on system-level simulations. From early on, system-level simulations have been in high demand, as people are naturally seeking answers when scenarios with numerous base stations and hundreds of users are investigated. This part not only explains how mathematical abstraction can be employed to speed up simulations by several hundred times without sacrificing precision, but also illustrates new theories on how to abstract large urban heterogeneous networks with indoor small cells. It also reports on advanced applications such as train and car transmissions to demonstrate the tools’ capabilities.

Runtime Precoding: Enabling Multipoint Transmission in LTE-Advanced System-Level Simulations

System-level simulations have become an indispensable tool for predicting the behavior of wireless cellular systems. As exact link-level modeling is unfeasible due to its huge complexity, mathematical abstraction is required to obtain equivalent results by less complexity. A particular problem in such approaches is the modeling of multiple coherent transmissions. Those arise in multiple-input-multiple-output transmissions at every base station but nowadays so-called coordinated multipoint (CoMP) techniques have become very popular, allowing to allocate two or more spatially separated transmission points. Also, multimedia broadcast single frequency networks (MBSFNs) have been introduced recently in long-term evolution (LTE), which enables efficient broadcasting transmission suitable for spreading information that has a high user demand as well as simultaneously sending updates to a large number of devices. This paper introduces the concept of runtime-precoding, which allows to accurately abstract many coherent transmission schemes while keeping additional complexity at a minimum. We explain its implementation and advantages. For validation, we incorporate the runtime-precoding functionality into the Vienna LTE-A downlink system-level simulator, which is an open source tool, freely available under an academic noncommercial use license. We measure simulation run times and compare them against the legacy approach as well as link-level simulations. Furthermore, we present multiple application examples in the context of intrasite and intersite CoMP for train communications and MBSFN