Andreas Winterstein

A digital PLL based downmix and phase detection unit for retro-directive antenna systems

Retro-directive antennas (RDAs) are a promising technique for communication systems as they provide self-tracking of moving terminals. However, many classic implementations are only able to re-transmit a received message and/or suffer from small realizable frequency gaps between received and transmitted signals. To overcome these limitations, a system architecture based on phase-locked loops (PLLs) has been proposed which allows for arbitrary frequency gaps, different array geometries, and makes use of the array gain on reception. In this work, we show a digital implementation of this retro-directive receiver to be realized on a field-programmable gate array (FPGA). We demonstrate the phase detection and downmix performance by digital hardware simulation with noisy input signals. The dynamic system behavior for signal acquisition and re-acquisition is shown in time domain. Suitability for communication systems is analyzed by subjecting the system to binary phase shift keying (BPSK) modulated signals. We provide simulative proof-of-concept for the proposed RDA system and its general ability to simultaneously process a communication signal and estimate its direction-of-arrival (DOA).

Use of High Altitude Platform Systems to augment ground based APNT systems

An Alternative Position Navigation and Timing (APNT) service is needed to backup satellite navigation for civil aviation in case of a radio frequency interference. Current APNT services rely on legacy systems and range sources based on ground stations and therefore, present limitations in terms of geometric diversity due to poor visibility. Moreover, there is still no straight forward solution how to maintain the correct synchronization of the ranging sources in case of GNSS outage. In this work, we propose the potential use of High Altitude Platform Systems (HAPS) first as pseudolites to enhance a ground APNT system. We evaluate the coverage of the platform and the enhancement in terms of dilution of precision. This lead to a ground-stratospheric APNT system with increased navigation service area. Besides, we propose these platforms for time synchronization and for integrity monitoring of the ground stations.

Demonstrator concept for a satellite multiple-beam antenna for high-rate data relays

This paper describes concept and development of some key components for a demonstrator of a multiple-beam antenna used for high-rate data relays. The demonstrator shall validate the whole antenna concept, showing among others antenna multibeam capability and advantages of the deployed digital beamforming.

Demonstration of a hybrid self-tracking receiver with DoA-estimation for retro-directive antenna systems

Recent studies have proposed self-tracking receiver systems with direction-of-arrival (DOA) estimation based on phase-locked loops (PLLs). Up to now, such a system has not been implemented. In this work, we present a hybrid hardware setup combing analog and digital signal processing, which provides this functionality. Successful phase detection and DOA estimation are demonstrated. The observed pointing errors are below 4.0°. The presented results are the proof-of-concept for the self-tracking receiver architecture. This technique can be directly applied to build retro-directive antennas (RDAs).

Analysis of a PLL-based down converter and phase detection circuit for self-tracking arrays

In this paper we analyze a down conversion and phase detection circuit for a self-tracking receiving antenna array which uses a nested phase-locked loop (PLL) architecture. Analytic expressions for the system model are derived and stability is checked using the Hurwitz criterion. The results lead to design constraints for the employed loop filters, ensuring stability of the proposed circuit. The general influence of parameter choice on system bandwidth and adaption speed is evaluated.

An adaptive calibration and beamforming technique for a GEO satellite data relay

High throughput data links from low earth orbit (LEO) satellites through a geostationary orbit (GEO) satellite data relay have been proposed to increase the available contact times to ground stations. Exact antenna beam pointing and tracking of moving targets are thereby key requirements for the relay satellite. In this work we propose an adaptive calibration and beamforming methodology based on least mean squares (LMS) which is suitable for a GEO data relais. The target System consists of the combination of a high gain reflector fed by a digitally steerable patch antenna array. The proposed method is first presented by numerical co-simulation of the antenna and the calibration algorithm. The results are then validated in an outdoor experimental setup with all digital signal processing implemented in a field-programmable gate array (FPGA). We demonstrate the tracking ability and pointing performance of the digitally enhanced reflector antenna with gain fluctuations smaller than 3 dB over a field of view of 2.9°�. The demonstrated performance shows that the digitally enhanced reflector antenna is a suitable candidate for long distance space communications.

A low-cost modular transmit front-end with analog beamforming capability

In this paper, we present a radio frequency (RF) transmitter front-end that can be used in analog beamforming applications. The proposed system is modular and can be employed at different frequencies whereby C-band operation is shown. Using low-cost components and substrates, this design makes analog beamforming affordable also for commercial applications with unit costs below 130 € and expected overall gain of 11.1 dB. We demonstrate the system performance in terms of S-parameter measurements with frequency translation to 7.0 GHz. Reproducibility of the achieved results is shown by analyzing multiple front-end prototypes. The beamforming capability is verified by antenna pattern measurements with an eight-element array.

A Hybrid Self-Tracking Receiver Implementation With Direction-of-Arrival Estimation for Retro-Directive Antenna Systems

Self-tracking receivers have been suggested for satellite and mobile communication applications. In this paper, we propose and demonstrate an array receiver (Rx) architecture based on phase-locked loops. The system is capable of automatically tracking a moving source and providing direction-of-arrival (DOA) estimates calculated from array signal phase differences. After deriving the system transfer functions, we show a digital implementation of the receiver and experimentally prove its phase detection capability. In combination with an eight-element linear array and analog front ends, we build a hybrid receiver and demonstrate its DOA estimation performance in static and dynamic outdoor setups. This paper gives the details about theoretic principles, implementation concept, and realization of the proposed receiver. We demonstrate DOA errors below 4° and show the reproducibility of the results. Using dynamic experiments, we verify the tracking ability of the system, which makes it applicable to retrodirective antennas.

A simulation technique for RF amplifier circuits using ANSYS electronics desktop

This paper describes a simulation technique for RF amplifier circuits using an electromagnetic simulator. To validate the proposed approach two RF circuits were designed, manufactured and measured. Simulated and measured results were compared and good agreement was observed.

Hardware-implemented adaptive calibration algorithms for antenna arrays

An adaptive calibration technique based on the least mean squares (LMS) algorithm is presented which does not rely on integrated calibration networks and is therefore suitable for e.g. array-fed reflector (AFR) antenna systems. Using hardware-implemented algorithms, makes the calibration real-time capable and independent of external computation. Possibilities to enhance the LMS by step-size controlling techniques are introduced and compared by simulation, regarding their adaption speed. The work focuses on the transition of the presented algorithms towards digital hardware, especially field-programmable gate arrays (FPGAs). The implementation feasibility of the proposed techniques is assessed and timing considerations as well as resource-effective realization are discussed. Suitable algorithms are selected and their implementation on an FPGA is described. Differences to simulation are outlined and optimization through pipelining is explained. The performance of the realized algorithms is evaluated on a Ka-band receiver system. Figures of merit are the adaption speed and the resulting signal-to-noise ratio (SNR) improvement. Stepsize control offers slight performance improvements over basic LMS with fixed step-sizes. This comes at the expense of higher resource demand and implementation effort.