Dieter Kreuer

Network management in short-range mobile radio networks: Performance tuning optimizes collision behaviour

The authors evaluate several network management algorithms to increase the performance of short-range mobile radio networks concerning collision avoidance and reduction of collision duration measured in frames. It is shown that suitable choice of transmission power with independent antenna systems fulfils requirements of applications while increasing the communication performance. The evaluations are made by simulations of a three-lane unidirectional highway. It is shown that earlier investigations of optimal transmission ranges are not satisfactory compared to the requirements of applications.<<ETX>>

Two time variant models for satellite channels

The authors consider different methods to design mathematical models for a satellite channel. These channel models, being based on homogeneous Markov chains, are used as a basis for evaluations of the open systems interconnection (OSI) layer 2 protocols. The parameter design of the models is based on measured block error probabilities, obtained by the German PTT on the Olympus 20/30 GHz channel. Since the block error rate is one of the most important issues for a satellite channel, the modeling is based on block oriented data transfers. The authors compare block error probabilities of Gilberts and Fritchmans models with two new generative time variant models. The first is a one-dimensional Markov chain called the time variant (TV) model and the second is a two-layer Markov model called the two-layer time variant model (TLTV). Analyses and simulations show that the TLTV model exhibits the best results.<<ETX>>

Time variant models for satellite channel

Different methods used to design mathematical models for a satellite channel are considered. These channel models, based on homogeneous Markov chains, are used as a basis for evaluation of OSI layer two protocols. The parameter design of the models is based on measured block error probabilities, obtained by German PTT on the Olympus 20/30-GHz channel. Since the block error rate is one of the most important issues for a satellite channel, the modeling will be based on block-oriented data transfers. The authors compare block error probabilities of Gilberts (1960) and Fritchmans (1967) models with two generative time variant models. The first is a one-dimensional Markov chain (called time variant model, TV) and the second is a two-layered Markov model (called two layer time variant model, TLTV). Analyses and simulations show that the TLTV model exhibits the best results.<<ETX>>

Verbesserte ARQ-Strategien fur Kanale mit hohen Fehlerraten

In den letzten Jahren wurde die Qualitat und Leistungsfahigkeit von Satellitenstrecken standig verbessert, wodurch die Einfuhrung von VSATs (Very Small Aperture Terminals) und anderen Satellitensystemen mit niedrigem Link-Budget und begrenzter Speicherkapazitat ermoglicht wurde. Durch Kopplung von paketorientierten Netzen (LANs, HSLANs und ATM/BISDN) uber VSAT-Stationen treten sowohl Paketverluste durch Pufferuberlaufe in Brucken und Routern im terrestischen Netz, als auch Bitfehler auf der Satellitenstrecke auf Die resultierenden hohe Fehlerraten erfordern Verfahren zur Fehlerkorrektur, die einfach und effizient in Bezug auf Durchsatz und benotigten Speicherplatz sind. Es wurde eine Reihe von ARQ-Fehlersicherungsverfahren (Automatic Repeat Request) zur paketorientierten Kommunikation entwickelt, die zwar mit beschranktem Speicherplatz auskommen, jedoch entweder nur einegeringe Sicherheit bei hohen Blockfehlerraten aufweisen, oder durch den geringen Durchsatz eine grose Bandbreite erfordern. In diesem Beitrag stellen wir eine einfache und effiziente Strategie (genannt Stutter-XOR-Strategie, SXOR) zur Erhohung des Durchsatzes bekannter Verfahren vor. Im Gegensatz zu herkommlichen hybriden Verfahren (Kombination von ARQ und Forward-Error-Correction) kann diese nicht nur Bitfehler sondern auch Blockverluste korrigieren. Wir haben mehrere Varianten dieser Strategie entwickelt. Zwei von ihnen sollen hier analytisch und simulativ bewertet werden.

Improved E/sub b//N/sub 0/ estimation with soft quantized detection

In satellite communications, meteor burst communications, as well as digital land mobile radio networks, adaptive power control, variable encoding, or transmission rate control are suited to keep the link at a constant bit error rate under varying channel quality. To control adaptive systems of this kind, an analog process quantity has to be monitored, which indicates the current link quality. This quantity has to be measureable fast and accurately enough to allow for a quick reaction of the adaptive link control system. We present a method to quickly estimate the quality of the link, applying soft quantization of received bits. We demonstrate how the quantizer must be implemented in order to achieve a near optimum performance.

Adaptive resource sharing strategies for TDMA satellite networks

The Ka band (20/30 GHz) is very attractive for future digital satellite communication systems with small earth stations. However, rain fading causes severe attenuations for a small percentage of the average year. To guarantee near continuous availability, adaptive fade countermeasures have to be applied. To provide a constant information rate, most of these countermeasures require additional bandwidth, so that some reserve in bandwidth has to be included into the link budget. This paper analyses several strategies, how to share a common resource of bandwidth among a number of stations in a TDMA system. It is shown, that the same quality approach, the aim of which is to level the link qualities of all stations to the same E/sub b//N/sub 0/ value, can improve the average bit error rate of the net by up to two powers of ten with only 30 percent of reserve in time and 10 percent in frequency.

A prototype adaptive fade countermeasure system for TDMA operation at Ka band

Ka band, the 20/30 GHz frequency band for satellite communications, has gained much interest in recent times. It is well suited for applications with small sized earth stations. The major disadvantage of Ka band, however, is its sensitivity to rain fading. Several satellite experiments are being carried out at the moment studying so called adaptive fade countermeasures, the purpose of which is to improve link quality in presence of rain. In this paper, the design and early laboratory experiments of a prototype fade countermeasure system implemented on a TDMA controller are described. This system is used for an experiment carried out by German PTT, Swiss PTT and DLR to investigate the significance of adaptive fade countermeasures for the availability of a Ka band satellite link. First results show, that an effective improvement of the link is possible with simple fade countermeasures.

Modelling satellite channels-a new approach

The authors consider different mathematical models for the error characteristics of satellite channels. They define a descriptive model and four generative models. Two of these are the well-known Gilbert and Fritchman error channel models, while the other two are new Markov models, termed the time-variant (TV) model and the two-layer time-variant (TLTV) model. The Gilbert and Fritchman models, as well as the TLTV model, are compared with the descriptive model and actual channel measurements, obtained by the German PTT on the Olympus 20/30-GHz channel. Block error probabilities and burst error probabilities are compared. For both error types, the TLTV models exhibit the best results.<<ETX>>