Arnon Shlomi

Free-space optical communication: detector array aperture for optical communication through thin clouds

Optical communication must contain clouds as parts of communication channels. Propagation of optical pulses through clouds causes widening in the spatial domain and attenuation of the pulse radiant power. These effects decrease the received signal and increase bit error rate (BER). One way to improve the BER of the communication system is by using adaptive methods to obtain more signal relative to noise power. Based on mathematical models of spatial widening of optical radiation derived by Monte Carlo simulation, a mathematical model for optimum performance of digital optical communication through clouds is developed. The purpose of the optimum adaptive communication system suggested here is to improve the BER by optimizing according to meterological conditions the spatial distribution of the detected radiation beam using a detector array where the external amplification of each detector is adaptable. Comparison and analysis of three models of communication systems in fog cloud channels are presented: (1) the optimum adaptive detector array aperture, (2) an ordinary single detector aperture of the same size, and (3) a small detector aperture. Improvement of more than four orders of magnitude in BER under certain conditions is possible with the new adaptive system model.

Vibration noise control in laser satellite communication

Laser satellite communication has become especially attractive in recent years. Because the laser beam width is narrow than in the RF or microwave range, the transmitted optical power may be significantly reduced. This leads to development of miniature communication systems with extremely low power consumption. On the other hand, the laser communication channel is very sensitive to vibrations of the optical platform. These vibrations cause angular noise in laser beam pointing, comparable to the laser beam width. As result, as significant portion of the optical power between transmitter and receiver is lost and the bit error rate is increased. Consequently, vibration noise control is a critical problem in laser satellite communication. The direction of the laser beam is corrected with a fast steering mirror (FSM). In this paper are presented two approaches for the FSM control. One is the feedback control that uses an LQG algorithm. The second is the direct feed- forward control when vibration noise is measured by three orthogonal accelerometers and drives directly the F SM. The performances of each approach are evaluated using MATLAB simulations.

Acquisition time calculation and influence of vibrations for microsatellite laser communication in space

The first step in creating an optical link between two LEO satellites is acquisition. In this process one of the satellites finds the maximal power of a received beam and locks on to it. This starts the tracking. In this paper we examine the time needed to finish the acquisition process and start tracking. The parameters included are the distribution function of satellite position, the size of the uncertainty area, the number of possible satellite positions, and the detection ability of a CCD. A model for the distribution function of position is given for two types of distribution: Gaussian and Uniform. Also considered the vibrations that come from internal systems of satellite, and from external sources. The characteristics of vibrations are considered and their influence on the scanning pattern that can deviate from the original path. A method of filtering the vibrations and compensating for them is suggested. The pointing system must be updated continuously from the star tracker with internal calculations of position, speed, velocity and vibrations characteristics. Examined also are several scanning methods: raster, spiral, Lissajo, Rose. Each method has its own possibilities and advantages, which are compared.

Optimum transmitter optics aperture for free space satellite optical communication as a function of tracking system performance

The basic configuration of free space satellite optical communication includes a transmitter and a receiver. The transmitter satellite must point the optical information beam to the receiver satellite in order to establish communication. An important aspect in satellite optical communication is to obtain minimum bit error rate using minimum power. This aim can be achieved with very small transmitter beam divergence angles. The disadvantages of too narrow a divergence angle are that the transmitter beam may sometimes miss the receiver satellite due to pointing vibrations, and that the transmitter optics aperture required is large and expensive. The optimum value of the received power as a function of the pointing vibration displacement determines the optimum bean divergence angle. The performance of the tracking system determines the amplitude limits of the vibrations of the transmitter beam in the spatial domain. A mathematical model including the performance of the communication system as a function of the performance of the tracking system is derived. From this model we derive the optimum transmitter telescope gain. An example for a practical communication system between a low earth orbit satellite (LEO) and a geostationary earth orbit satellite (GEO) is presented. Using this model makes it possible to choose appropriate optics for the transmitter with reduced size and weight.

Free-space optical communication: analysis of spatial widening of optical pulses for propagation through clouds

As part of a communication channel, clouds cause spatial widening and attenuation of optical pulse power. Free-space optical communication from satellite to earth (ground or airplane) occasionally involves clouds over part of the optical channel. Most of the energy of optical pulses propagating through thin clouds passes through the clouds. The propagating energy is concentrated around the center of the beam. The distribution of the energy relative to the center of the beam is not uniform. Using the received energy in an efficient way reduces the transmitter power needed for given bit error rate. The advantages of low transmitter power are less radiation exposure and greater immunity to eavesdropping. To use the received energy efficiently, a mathematical model of spatial widening of the optical beam is derived using Monte Carlo simulation. The simulation is carried out at three different wavelengths in the visible and the near IR. Important aspects of this work include the fact that (1) using shorter wavelengths such as 0.532 μm results in least spatial widening and maximal received power, and is thus preferable for optical communication, and (2) the mathematical model derived is a basis for adaptive communication with less transmitted energy consumption.

Free-space satellite optical communication: adaptive information bandwidth to maintain constant bit error rate during periods of high satellite vibration amplitudes

Satellites in free space suffer from periods of high displacement amplitude vibrations (for example: during the operation of the thruster, the antenna pointing mechanism, or the solar array drive mechanism). In order to utilize the advantages of optical communication in space, very narrow divergence transmitted beams are used. The high amplitude vibrations of the satellite cause decrease of received signal power in the receiver satellite due to mispointing of the transmitted beam. One way to overcome this problem is to develop very complicated stabilization systems. The disadvantages of this solution are: complexity, reliability, and cost. Most of the time, the amplitude of the vibrations is low and does not affect the communication performance. Considering these facts, we derive a model of a communication system that adapts the communication system parameters to changes in received signal caused by changes in vibration amplitude. The purpose of this model is to keep the bit error rate (BER) low and constant by adapting the system bandwidth and the receiver parameters to the vibration amplitude. The duration amplitude and occurrence of the high amplitude vibrations are assumed to be known so the adaptation of the communication system parameters is simple. The adaptive model is derived for the on off keying (OOK) modulation method. An example for practical optical space communication systems based on the adaptive model is given. Comparison and analysis of the performance of standard and adaptive models of communication systems for variable amplitudes of vibration amplitude are presented.

Adaptive optical transmitter and receiver for space communication through clouds

Optical space communication will use clouds as part of communication channels. Propagation of optical pulses through clouds causes widening and deformation in the time domain and attenuation of the pulse radiant power. These effects decrease the received signal and limit the information bandwidth of the communication system. This work defines typical characteristics of optical pulse propagation through clouds. Characteristics of the optical pulses are calculated using Monte-Carlo simulation. Based on these characteristics a model for optimum performance of digital optical communication through clouds is presented. Examples for practical communication systems are given. An adaptive method to improve and in some cases to make possible communication is suggested. Comparison and analysis of two models of communication systems in cloud channels are presented: (1) adaptive transmitter and standard receiver (semi-adaptive system) and (2) adaptive transmitter and receiver (adaptive system). An improvement of more than eight orders of magnitude in bit error rate under certain conditions is possible with the new adaptive system model.

New approach to detector array receiver performance analysis for laser satellite communication

Laser satellite communication is one of the most promising methods of communication outside the earths atmosphere. In the continuing quest to optimize atmospheric optical wireless communication, arrays of photodetectors are replacing solitary photodetectors in receivers, affording the advantages of the small fast photodiode while effectively increasing the receiver aperture. Thus, power dispersed by atmospheric turbulence and scattering may be collected by the enlarged receiver area, and high BER, caused by low received power, can be decreased. We propose a mathematical model, which can be used to improve the data processing from detector photocurrent by incorporating thoroughly researched concepts from optical imaging theory such as atmospheric turbulence and aerosol optical transfer functions. This model forms the basis of an analytical tool, which will help in the implementation of smart detector arrays for WDM communication systems.

Possible solutions to mitigate vibration effects in laser intersatellite links

Free space laser communication between satellites networked together can facilitate high-speed communication between different places on earth. The advantages of an optical communication system by comparison with a microwave communication system in free space are: a) smaller size and weight, b) less transmitter power, c) larger bandwidth, d) higher immunity to interference, and e) smaller transmitter beam divergence. The use of optical radiation as a carrier between the satellites engenders very narrow beam divergence angles. Due to the narrow beam divergence angle and the large distance between the satellites, the pointing from one satellite to another is complicated. The problem is further complicated due to vibrations of the pointing system caused by two fundamental mechanisms, stochastic in nature; 1) tracking nose created by the electro-optic tracker and 2) vibrations created by internal and external mechanical mechanisms. The vibrations displace the transmitted beam and the receiver field of view with respect to one another. Such movement decreases the average received signal, and increases the bit error rate (BER). In this paper we will review five methods to mitigate the effect of vibrations on laser satellite communication system. The methods are a) receiver with adaptive detector arrays, b) Bandwidth/data rate/coding rate adaptation, c) Power minimization using adaptive beam-width, d) Communication diversity within the satellite network, and e) Power control.

Adaptive suboptimum detection of optical PPM signal with detection matrix and centroid tracking

In some applications of optical communication systems, such as satellite optical communication and atmospheric optical communication, the optical beam wanders on the detector surface due to vibration and turbulence effects, respectively. The wandering of the beam degrades the communication system performance. In this research, we derive a mathematical model of an optical communication system with a detection matrix to improve the system performance for direct detection pulse position modulation (PPM) We include a centroid tracker in the communication system model. The centroid tracker tracks the center of the beam. Using the position of beam center and an apriori model of beam spreading we estimate the optical power on each pixel (element) in the detection matrix. Based on knowledge of the amplitudes of signal and noise in each pixel, we tune adaptively and separately the gain of each individual pixel in the detection matrix for communication signals. Tuning the gain is based on the mathematical model derived in this research. This model is defined as suboptimal due to some approximations in the development and is a suboptimum solution to the optimization problem of n multiplied by m free variables, where U,mare the dimensions of the detection matrix. Comparison is made between the adaptive suboptimum model and the standard model. From the mathematical analysis and the results of the comparison it is clear that this model improves significantly communication system performance.