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1. Introduction

Wireless communication technologies play an increasingly important role in recent years. The GSM telephone network has experienced a rapid deployment within 15 years: starting with the first GSM call in 1991, there are more than 3.5 billion subscribers as of the 4th quarter 2008 [122]. Furthermore, triggered by the rapid deployment for small portable computers like laptops and PDAs, or home applications such as video streaming between a file server and the TV, there is now also a growing demand for wireless data network access. The development of Wireless Local Area Networks (WLANs) started in the middle of the 1990s [120]. The first solutions were proprietary, such as RadioLAN WIN or Lucent WaveLAN computer plug-in cards, i. e. there was no common standard available for the radio interface which ensured that WLAN devices could communicate with devices from other manufacturers. This lack of interoperability prevented WLANs from being sold in large numbers.
Andreas Könsgen

2. The IEEE 802.11 Standard Series

In communication systems, it is crucial that the interface which controls the information exchange between networked nodes is standardised between different manufacturers. The most important institutions for standardisation in this area are the European Telecommunication Standardisation Institute (ETSI), the IEEE (Institute of Electrical and Electronics Engineers) and the ITU (International Telecommunications Union) with the subdivision ITU-T focused on wired communication systems and ITU-R for radio communication systems. The ETSI introduced, for example, the GSM standard for wireless telephones (Global System for Mobile Communication) and initiated the mobile broadband communication standard UMTS (Universal Mobile Telecommunication System). In the field of local area networks, the ETSI specified Hiperlan (High Performance LAN) providing data rates up to 2 Mbit/s and Hiperlan/2 (an enhanced version with data rates up to 54 Mbit/s). The IEEE introduced the 802 series of standards which define various kinds of communication networks.
Andreas Könsgen

3. Theoretical Aspects of Wireless LAN Performance

In the beginning of this chapter, a survey is given about existing literature concerning the performance ofWireless LAN and the enhancement by spectrum management. After this introduction, theoretical considerations about the capacity and the delay of IEEE 802.11 are discussed in detail starting with a model used in the literature. This model however applies to the case of an ideal, lossless channel, which means that the stations have infinite range, there is no attenuation of the signals on the path between the transmitter and the receiver. In this work, the model which has been introduced for the ideal channel is then extended for a more realistic channel by assuming an exponential attenuation of the signal and requirements for minimum signal levels and signal-to-noise-ratios (SNR) at the receivers. It is discussed how the results of these models apply for Transmit Power Control. Furthermore, novel considerations about the convergence speed of Dynamic Frequency Selection are given. The chapter concludes with calculations to compare the performance of a parallelised transmission for multiple users with a sequential transmission.
Andreas Könsgen

4. Spectrum Management Algorithms

The aim of this work is the investigation of spectrum management methods for IEEE 802.11a based networks, as they are introduced by the 802.11 h standard. As shown in the previous chapter, the standard defines Dynamic Frequency Selection and Transmit Power Control. The standard, however, only defines the signalling which is needed to maintain these spectrum management extensions. It does not specify the decision algorithms which select the frequency channel and the transmit power to be used. The standard only covers the topics which are important for the interoperability between hardware of different manufacturers. In this chapter, different methods to maintain Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) are introduced. The Transmit Power Control is extended by Link Adaptation (LA), which is not specified in the standard as well. Existing WLAN hardware rarely supports spectrum management by DFS and TPC according to the 802.11h standard. Link Adaptation is usually provided as a proprietary extension.
Andreas Könsgen

5. Cross-Layer Architecture

In the previous chapters, spectrum management methods were discussed which enhance the performance of wireless LANs based on the dynamic selection of transmit frequencies, transmit power and physical bit rate. These methods are applicable to optimise the coexistence between neighbouring networks. However, also in case of a single network with an access point which serves a number of stations, optimisations are possible.
Andreas Könsgen

6. Simulation Environment

The previous chapters discussed the theory and principles of enhancing the performance of wireless networks. For the validation of the proposed methods, simulations are required which implement a model of the network scenario under investigation.
Andreas Könsgen

7. Simulation Results

In this chapter, the performance of the spectrum management introduced in chapter 4 is evaluated, which also includes a validation of the theoretical results about wireless networks with limited range which was given in section 3.2. Results for cross-layer scheduling described in chapter 5 are given in this chapter as well.
Andreas Könsgen

8. Conclusions and Outlook

In this work, performance enhancements of IEEE 802.11a/h wireless LANs have been investigated. Two approaches have been discussed to achieve this goal: the first one is spectrum management including dynamic change of frequency channel, transmit power and physical bit rate, the other one is using a cross-layer scheduler at the access point to maintain the communication inside a radio cell to serve multiple users, by sequential or parallelised transmission of packets.
Andreas Könsgen


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