Adaptives hierarchisches Ray Tracing Verfahren zur parallelen Berechnung der Wellenausbreitung in Funknetzen

The continuous demand for mobile communications leads to the development of new cellular networks for transmission of data, voice and video sequences. Already established GSM mobile cellular networks, a great economic success of the last decade, will be slowly upgraded to and eventually replaced by the new, third generation, cellular networks according to the UMTS standard. On the other hand, local wireless data networks according to the IEEE 802.11 standard will extend the classic LAN computer networks beyond building boundaries and lead to the creation of local communication islands. Both cases imply high requirements in respect to the communication bandwidth and quality of service. Very efficient use of the frequency spectrum and a very accurate knowledge of the communication channel properties are needed to satisfy these requirements. Statistical communication channel models allow a general classification of the propagation environment properties. In addition to the statistical modeling, deterministic channel models provide very accurate predictions of the radiowave propagation in a scenario of a particular geometry. Deterministic channel modeling is computationally very intensive, contrary to virtually any statistical channel model, and thus applications has been limited to a set of simplified geometries of rather small extents. Together with measurements, deterministic channel models can be used for parameter extraction in new statistical channel models in the process of evaluation and optimization of future communication systems. A new parallel and adaptive method for the deterministic communication channel modelling has been developed in the context of this work. The method is based on the construction of propagation paths between transmitters and receivers by means of a ray tracing technique together with additional sources taking edge diffraction and rough surface scattering into account. A new fast algorithm based on modified binary space partitioning trees has been deployed for the ray/object intersection test. The construction of nearly optimal binary space partitioning trees for arbitrary geometries has been realized by heuristics, the simulated annealing method and genetic algorithms. A novel ray density normalization algorithm based on tagged rays has been developed to quickly identify new propagation paths. The ray density is adapted locally to the scene geometry to minimize the total number of rays required for the spatial sampling of the geometry. The combination of the classic ray launching method with the source imaging technique allows the precise computation of the shortest propagation paths while maintaining constant local ray density and short simulation runtimes. It allows the accurate computation of the frequency selective properties of the communication channel. Source imaging over multiple arbitrary oriented diffraction edges according to the extension of the Fermat's principle has been solved with the Newton-Raphson iteration and a Simulated Annealing technique. Scattering on rough fractal surfaces has been modelled by the method of moments. Further substantial acceleration of the ray tracing solution of complex electromagnetic wave propagation problems has been achieved through the parallelization of the algorithm. Parallel implementations on symmetric multiprocessors with shared memory and on distributed memory parallel computers are presented together with a new hybrid method for distributed/shared memory architectures. Theoretical limits and achieved speedup of the computation are discussed. The physical propagation model is validated with analytical models and channel sounder measurements. Finally some applications of the deterministic model are presented, among these the radio network planning of the Transrapid track in Shanghai and an EMC computation of a radar facility

A Prototype Adaptive Fade Countermeasure System for TDMA Operation at K_a Band

: K a 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 K a 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 availabilty of a K a band satellite link. First results show, that an effective improvement of the link is possible with simple fade countermeasures. 1. INTRODUCTION Operational satellites using the 20 GHz downlink / 30 GHz uplink frequency range..

Advances in digital SiPMs and their application in biomedical imaging

Similar to analog silicon photomultipliers (SiPMs), digital SiPMs (dSiPMs) essentially consist of an array of single-photon avalanche photodiodes (SPADs). Instead of a passive quench resistor, however, an active quenching circuit is locally integrated with each SPAD, making the sensor response faster and less sensitive to the gains of the individual SPADs. Moreover, additional circuits for the fully digital acquisition, processing, and readout of optical signals are integrated within the sensor. As a result, dSiPMs offer high photo-detection efficiency, high single-photon time resolution (SPTR), and high uniformity, as well as many practical advantages, such as a very compact form factor, low voltage operation, magnetic field compatibility, high stability of operation, low gain drift, and a high degree of scalability. At the same time, dSiPMs represent a new paradigm in low-level light sensing technology. That is, their fully digital operation makes them true photon counting devices, preserving at least partly the discrete spatio-temporal structure of the information embedded in the optical signal. This means that the operation of dSiPMs can be fully understood only in statistical terms, but also opens up novel possibilities for extracting information from the measured data. So far, the main driver behind the development of dSiPMs has been the detection of scintillation pulses in detectors for time-of-flight (TOF) positron emission tomography (PET). Several types of dSiPM have been developed in recent years. Moreover, first imaging devices based on dSiPMs have been realized by various groups. This review summarizes the main dSiPM concepts and technologies currently under development, provides an overview of the results obtained recently with dSiPMs-based PET and SPECT devices, and presents a critical outlook on the challenges and chances for dSiPMs in future radiomolecular imaging systems

A gated single-photon avalanche diode array fabricated in a conventional CMOS process for triggered systems

A bidimensional array based on single-photon avalanche diodes for triggered imaging systems is presented. The diodes are operated in the gated mode of acquisition to reduce the probability to detect noise counts interfering with photon arrival events. In addition, low reverse bias overvoltages are used to lessen the dark count rate. Experimental results demonstrate that the prototype fabricated with a standard HV-CMOS process gets rid of afterpulses and offers a reduced dark count probability by applying the proposed modes of operation. The detector exhibits a dynamic range of 15 bits with short gated"on" periods of 10ns and a reverse bias overvoltage of 1.0V

Readout schemes for low noise single-photon avalanche diodes fabricated in conventional HV-CMOS technologies

Three different pixels based on single-photon avalanche diodes for triggered applications, such as fluorescence lifetime measurements and high energy physics experiments, are presented. Each pixel consists of a 20µm x 100µm (width x length) single photon avalanche diode and a monolithically integrated readout circuit. The sensors are operated in the gated mode of acquisition to reduce the probability to detect noise counts interferring with real radiation events. Each pixel includes a different readout circuit that allows to use low reverse bias overvoltages. Experimental results demonstrate that the three pixels present a similar behaviour. The pixels get rid of afterpulses and present a reduced dark count probability by applying the gated operation. Noise figures are further improved by using low reverse bias overvoltages. The detectors exhibit an input dynamic range of 13.35 bits with short gated"on" periods of 10ns and a reverse bias overvoltage of 0.5V. The three pixels have been fabricated in a standard HV-CMOS process

Coating technology for locally varying optical function on 2d and 3d elements

In this paper, a coating technology will be presented that allows achieving a locally varying optical function on 2d and 3d elements. This can be used, for example, to compensate layer thickness errors occurring during the deposition on tilted surfaces of larger 3d optical components. Other applications can be coatings for lateral varying light extraction of large-area waveguides in displays, wave front correction or variable optical filters for hyper spectral cameras. In the coating plant PreSensLine at FEP (by Von Ardenne GmbH) deposition rates in the range of 20 to 50 nm∙m/min allow efficient fabrication of optical layer systems for use in IR,VIS and UV