Hans-Florian Geerdes

Performance analysis of dynamic OFDMA systems with inband signaling

Within the last decade, the orthogonal frequency- division multiplexing (OFDM) transmission scheme has become part of several standards for wireless systems. Today, OFDM is even a candidate for fourth-generation wireless systems. It is well known that dynamic OFDMA systems potentially increase the spectral efficiency. They exploit diversity effects in time, space, and frequency by assigning system resources periodically to terminals. Informing the terminals about new assignments creates a signaling overhead. Up to now, this overhead has not been taken into account in studies on dynamic orthogonal frequency-division multiplexing access (OFDMA) systems. Yet this is crucial for a realistic notion of the performance achieved by dynamic approaches. In this paper, we close this gap. We introduce two forms of representing the signaling information and discuss how these affect system performance. The study of the signaling impact on the performance is conducted for an exemplary dynamic approach. We find that the throughput behavior of dynamic OFDMA systems is significantly influenced by the signaling overhead. In many situations, neglecting the overhead leads to wrong performance conclusions. Also, the performance difference between dynamic and static schemes is now much more sensible to the specific parameter set of the transmission scenario (e.g., frame length, subcarrier number, etc.). This leads to the proposal of access points which should adapt certain system parameters in order to provide optimal performance.

Integrated Access Point Placement and Channel Assignment for Wireless LANs in an Indoor Office Environment

Wireless Local Area Network (WLAN) is currently among the most important technologies for wireless broadband access. The IEEE 802.11 technology is attractive for its maturity and low equipment costs. The overall performance of a specific WLAN installation is largely determined by the network layout and the radio channels used. Optimizing these design parameters can greatly improve performance. In this paper, access point (AP) placement and channel assignment are optimized using mathematical programming. Traditionally, these decisions are taken sequentially; AP placement is often modeled as a facility location problem, channel assignment as an (extended) graph coloring problem. Treating these key decisions separately may lead to suboptimal designs. We propose an integrated model that addresses both aspects simultaneously and thereby balances the two optimization objectives. Computational results show that indeed the integrated approach is superior to the sequential one.

UMTS radio network evaluation and optimization beyond snapshots

A new evaluation scheme for universal mobile telecommunications system (UMTS) radio networks is introduced. The approach takes the complex coupling of coverage and capacity through interference into account. Cell load estimates, otherwise obtained through Monte-Carlo simulation, can now be approximated without time-consuming iterative simulations on user snapshots. The two cornerstones are the generalization of interference coupling matrices from user snapshots to average load and the emulation of load control by an analytical scaling scheme. Building on the new evaluation scheme, two novel radio network optimization algorithms are presented: an efficient local search procedure and a mixed integer program that aims at designing the coupling matrix. Computational experiments for optimizing antenna tilts show that our new approaches outperform traditional snapshot models

Wireless network design: solution-oriented modeling and mathematical optimization

As the telecommunication industry matures, there is vital interest in good network designs. Many planning tasks in wireless networks are particularly complex and challenging. A solution-oriented modeling approach can help to devise efficient automatic planning and optimization schemes. This article discusses how good mathematical models can be obtained for real-world problems, which model structures facilitate good computational behavior, and which advanced tools are available for solving such models. The trade-off between capturing the fine characteristics of technologies and the simplicity of the model is illustrated for several technologies and example problems

Optimization methods for UMTS radio network planning

The UMTS radio network planning problem poses the challenge of designing a cost-effective network that provides users with sufficient coverage and capacity. We describe an optimisation model for this problem that is based on comprehensive planning data of the EU project Momentum. We present heuristic mathematical methods for this realistic model, including computational results.

Capacity optimization for UMTS: Bounds and benchmarks for interference reduction

UMTS has been designed as the next platform for mobile mass market communication. After a slow start, the usage of UMTS in terms of subscribers and volume is picking up momentum. Following the initial coverage-driven phase, network capacity is moving into focus. During network planning and optimization, cell capacity can be improved by explicitly designing for minimal interference. This article introduces planning methodologies that allow to minimize interference overhead while maintaining the established network coverage. Our main contribution is the first practicable approach for comparing interference to lower bounds and benchmarks. This comparison on realistic datasets suggests that our optimization methods produce first-class results.

Intelligent Distribution of Intrusion Prevention Services on Programmable Routers

The recent surge of new viruses and host attacks in the Internet and the tremendous propagation speed of self- distributing attacks has made network security a pressing issue. To protect an end-system, it must continuously be patched and additional security tools like an Intrusion Prevention System (IPS) are needed. The main problems are the effort for the users and network performance: The last years have shown that many users neither maintain their computers nor run an IPS. Accordingly, their systems remain vulnerable. On the other hand, the operation of an IPS inevitably decreases network performance as all packets are analyzed for malicious content before being forwarded. We recently proposed the operation of a flexible overlay network of intrusion prevention systems running on top of programmable routers to mitigate these issues. With this archi- tecture, security services can be dynamically distributed in the network. In this paper, we leverage this flexibility for minimizing the impact of intrusion prevention on network performance. Based on measurement from experiments with a prototype im- plementation of a programmable router, we derive an analytical description of the impact of a single active router on the network. This is then used to formulate two mixed integer optimization models for finding placement strategies for security services. Using detailed simulations on two example networks we show that the optimized strategies perform significantly better than reference placement strategies, including centralized intrusion prevention.

Mathematical Optimization Models for WLAN Planning

Wireless Local Area Networks (WLANs) based on the Ieee 802.11 standard family are used widely for wireless broadband Internet access. The performance aspects of WLANs range from deployment cost, coverage, capacity, interference, and data throughput to efficiency of radio resource utilization. In this chapter, we summarize some recent advances in applying mathematical optimization models for solving planning problems arising in placing access points (APs) and assigning channels in WLANs. For AP location, we present an optimization model aimed at maximizing the average user throughput. For channel assignment, we present two modeling approaches that use different performance metrics. We also discuss integrated models for joint optimization of AP location and channel assignment. We report computational experiments with real-life data, and show the advantages of mathematical optimization in WLAN planning.

Wie viel Kapazität hat ein Mobilfunknetz

Wer hat das noch nicht erlebt? Der Akku ist aufgeladen und reichlich Guthaben vorhanden, doch das Telefonieren mit dem Handy klappt trotzdem nicht. Meist liegt das daran, dass man sich in einem Funkloch befindet. Aber manchmal ist auch das Netz uberlastet. Anders gesagt: das Funknetz des Mobilfunkanbieters, das viele Antennen auf Hausdachern oder Masten umfasst, bietet entweder nicht genugend Abdeckung oder nicht genugend Kapazitat. Genau das will der Anbieter naturlich vermeiden. Mathematik hilft, diese Herausforderungen mit einer guten Planung des Mobilfunknetzes zu bewaltigen.