Paul Wagner

Combinatorial pulse position modulation for power-efficient free-space laser communications

A new modulation technique called combinatorial pulse position modulation (CPPM) is presented as a power-efficient alternating to quaternary pulse position modulation (QPPM) for direct-detection, free-space laser communications. The special case of 16C4PPM is compared to QPPM in terms of data throughput and bit error rate (BER) performance for similar laser power and pulse duty cycle requirements. The increased throughput from CPPM enables the use of forward error corrective (FEC) encoding for a net decrease in the amount of laser power required for a given data throughput compared to uncoded QPPM. A specific, practical case of coded CPPM is shown to reduce the amount of power required to transmit and receive a given data sequence by at least 4.7 dB. Novel hardware techniques for maximum likelihood detection and symbol timing recovery are presented.

The C3PO project: a laser communication system concept for small satellites

The satellite market is shifting towards smaller (micro and nanosatellites), lowered mass and increased performance platforms. Nanosatellites and picosatellites have been used for a number of new, innovative and unique payloads and missions. This trend requires new concepts for a reduced size, a better performance/weight ratio and a reduction of onboard power consumption. In this context, disruptive technologies, such as laser-optical communication systems, are opening new possibilities. This paper presents the C3PO1 system, “advanced Concept for laser uplink/ downlink CommuniCation with sPace Objects”, and the first results of the development of its key technologies. This project targets the design of a communications system that uses a ground-based laser to illuminate a satellite, and a Modulating Retro-Reflector (MRR) to return a beam of light modulated by data to the ground. This enables a downlink, without a laser source on the satellite. This architecture suits well to small satellite applications so as high data rates are potentially provided with very low board mass. C3PO project aims to achieve data rates of 1Gbit/s between LEO satellites and Earth with a communication payload mass of less than 1kilogram. In this paper, results of the initial experiments and demonstration of the key technologies will be shown.

Least Reliable Bits Coding (LRBC) for high data rate satellite communications

An analysis and discussion of a bandwidth efficient multi-level/multi-stage block coded modulation technique called Least Reliable Bits Coding (LRBC) is presented. LRBC uses simple multi-level component codes that provide increased error protection on increasingly unreliable modulated bits in order to maintain an overall high code rate that increases spectral efficiency. Further, soft-decision multi-stage decoding is used to make decisions on unprotected bits through corrections made on more protected bits. Using analytical expressions and tight performance bounds it is shown that LRBC can achieve increased spectral efficiency and maintain equivalent or better power efficiency compared to that of Binary Phase Shift Keying (BPSK). Bit error rates (BER) vs. channel bit energy with Additive White Gaussian Noise (AWGN) are given for a set of LRB Reed-Solomon (RS) encoded 8PSK modulation formats with an ensemble rate of 8/9. All formats exhibit a spectral efficiency of 2.67 = (log2(8))(8/9) information bps/Hz. Bit by bit coded and uncoded error probabilities with soft-decision information are determined. These are traded with with code rate to determine parameters that achieve good performance. The relative simplicity of Galois field algebra vs. the Viterbi algorithm and the availability of high speed commercial Very Large Scale Integration (VLSI) for block codes indicates that LRBC using block codes is a desirable method for high data rate implementations.

Laser-induced contamination of space borne laser systems: impact of organic contamination and mitigation by oxygen

Laser-induced contamination (LIC) is still a major risk for space based laser systems. In this paper the mitigation of LIC by oxygen is investigated. Tests were performed with a pulsed laser at 355 nm. The partial pressure of the contamination material was in the range of 10-5 -10-4 mbar. The mitigation effect showed a threshold behavior concerning the ratio between contamination and oxygen pressure. Also a cleaning effect was successfully demonstrated: previously created depositions were completely removed by irradiation at several tens Pa oxygen pressure without any remaining degradation of the optical surface.

In-situ laser-induced contamination monitoring using long-distance microscopy

Operating high power space-based laser systems in the visible and UV range is problematic due to laser-induced contamination (LIC). In this paper LIC growth on high-reflective (HR) coated optics is investigated for UV irradiation of 355 nm with naphthalene as contamination material in the range of 10-5 mbar. The investigated HR optics were coated by different processes: electron beam deposition (EBD), magnetron sputtering (MS) or ion beam sputtering (IBS). In-situ observation of contamination induced damage was performed using a long distance microscope. Additionally the onset and evolution of deposit formation and contamination induced damage of optical samples was observed by in-situ laserinduced fluorescence and reflection monitoring. Ex-situ characterization of deposits and damage morphology was performed by differential interference contrast and fluorescence microscopy. It was found that contamination induced a drastic reduction of laser damage threshold compared to values obtained without contamination. Contamination deposit and damage formation was strongest on IBS followed by MS and smallest on EBD.

Low cost adaptable laser transmitter for ground-based orbital observations

Several theoretical laser transmitter concepts for low Earth orbit free space optical applications were investigated. A suitable, cost effective design including a beam steering unit as well as a fully automated laser divergence control was realized and characterized. For this only commercial off the shelf components were used.

Detection and laser ranging of orbital objects using optical methods

Laser ranging to satellites (SLR) in earth orbit is an established technology used for geodesy, fundamental science and precise orbit determination. A combined active and passive optical measurement system using a single telescope mount is presented which performs precise ranging measurements of retro reflector equipped objects in low earth orbit (LEO). The German Aerospace Center (DLR) runs an observatory in Stuttgart where a system has been assembled completely from commercial off-the-shelf (COTS) components. The visible light directed to the tracking camera is used to perform angular measurements of objects under investigation. This is done astrometrically by comparing the apparent target position with cataloged star positions. First successful satellite laser ranging was demonstrated recently using an optical fiber directing laser pulses onto the astronomical mount. The transmitter operates at a wavelength of 1064 nm with a repetition rate of 3 kHz and pulse energy of 25 μJ. A motorized tip/tilt mount allows beam steering of the collimated beam with μrad accuracy. The returning photons reflected from the object in space are captured with the tracking telescope. A special low aberration beam splitter unit was designed to separate the infrared from visible light. This allows passive optical closed loop tracking and operation of a single photon detector for time of flight measurements at a single telescope simultaneously. The presented innovative design yields to a compact and cost effective but very precise ranging system which allows orbit determination.

Combinatorial FSK modulation for power-efficient high-rate communications

Deep-space and satellite communications systems must be capable of conveying high-rate data accurately with low transmitter power, often through dispersive channels. A class of noncoherent Combinatorial Frequency Shift Keying (CFSK) modulation schemes is investigated which address these needs. The bit error rate performance of this class of modulation formats is analyzed and compared to the more traditional modulation types. Candidate modulator, demodulator, and digital signal processing (DSP) hardware structures are examined in detail. System-level issues are also discussed.