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A key component of establishing free-space optical communication between ground stations, satellites, and lunar nodes consists of accurate pointing, tracking, and beam acquisition. This investigation explores the use of sensor arrays external to the receiver aperture for sampling the incoming transmitted beam for the purpose of estimating the pointing error. Different arrays of sensors and different combinations of sensors processed with the traditional quadrant detector algorithm were evaluated based on the ability to accurately predict the direction of pointing error under a variety of beam widths and turbulence conditions. MATLAB simulations demonstrate that different configurations perform best under different conditions, and thus a combination of configurations with flexible processing capabilities provides the most promising approach. Beam width proved to be the most important factor in prediction accuracy.

Multi-point free-space optical communication (FSOC) has recently received extensive interest as a valuable solution for providing increased capacity in the upcoming communication networks. Diverse optical multiple access control (O-MAC) techniques have proved to successfully support multi-user FSOC. However, to adaptively and automatically select the most fitting O-MAC technique or combination thereof for signal demodulation, the number of transmitting users should be known a priori at the receiver side. In this letter, a technique based on histogram peak detection and unsupervised machine learning (e.g., k-mean, k-medoid, hierarchical, and fuzzy clustering), is proposed and experimentally demonstrated for number of users detection. The proposed methodology relies on amplitude information of received signals. Results show that when multiple users’ signals are received with equal amplitude, under-estimation of user number was observed; hence, a modified approach—a weighted clustering—was employed and experimentally validated. The work reported herein demonstrated the ability of the proposed methodology to simultaneously and accurately detect multiple transmitting users in real time and under severe atmospheric turbulence.

Optical wireless communication (OWC) has emerged as an attractive and promising solution for supporting high-capacity and high-density networks required by the fifth-generation (5G) communication standard and the Internet of Thing (IoT) system. As a wireless technology, OWC must ensure that multiple users can communicate through the network. To facilitate this, the ability of diverse optical multiple access control (O-MAC) techniques are analyzed in this paper. Combinations of different O-MAC methods that were individually tested in previous work are studied and experimentally validated. Results from testing four methods— Independent Component Analysis (ICA) and non-orthogonal multiple access (NOMA); ICA and signal subtraction; NOMA on one received mixed signal; and NOMA on two received mixed signals—proved successful for free space optical communication (FSOC) in a setup comprised of three users and a dual-path fiber bundle receiver. Users with same data rate and with various data rate were tested. Obtained results showed that received signals were demodulated with high accuracy.

We present several analytical models to calculate the intersection surface of a spherical cap with a solid cone where either one of them is experiencing perturbations in its attitude. These models are developed to quantitatively analyze the mutual alignment, and thus performance, of mobile free-space optical (FSO) communication nodes used in air-to-ground and ground-to-air FSO links. Same models can be also applied to various geometrical alignment problems in the presence of instability. Examples include coverage area of imaging, radar, and optical communication satellites while orbiting unstable orbits. These models provide a closed-form direct mapping between platform dynamics and FSO link performance facilitating a priori performance prediction and easing feedback requirements in real-time tracking systems.

FSO (Free-Space Optical)-based communication systems experience difficulty with receiving and separating signals arising from multiple transmitters, a capability that would facilitate implementation of MIMO (Multiple-In, Multiple-Out) systems. Current implementations require multiple, distinct optical antennas, each tracking a single user, which proves bulky and costly, especially if the transmitters are moving and must be tracked. A fiber-bundle receiver has the potential to use multiple pathways, corresponding to the individual fibers within the receiver, to capture different combinations of the incoming optical signals. If the bundle provides linear combining of the optical signals from both the individual fibers in the bundle and amongst the incoming optical signals, signal processing could extract the individual signals from the combinations. In this paper, we experimentally investigate whether the fiber-bundle receiver possesses sufficient linearity of operation to allow the separation of two signals by simple processing algorithms, for both turbulent and non-turbulent conditions. Data from two distinct sources enters a single-bundle, single field of view receiver, and a basis signal from one transmitter provides the reference for performing simple subtraction-based extraction of the unknown signal from the other transmitter. The experimental results show that the receiver does operate linearly, and that the linear operation remains sufficiently intact in the presence of turbulence to extract a recognizable copy of one signal from the other. The ability of the fiber bundle receiver to mitigate turbulence effects appears to assist in maintaining this sufficient level of linearity.

Given the increased interest in aerial communications, free-space optics (FSO) has gained considerable attention for its ability to deliver wider bandwidth, license-free, and secure communication. FSO was initially developed for fixed platforms. As such, the current push to introduce mobility is one of the greatest challenges for FSO developers. This paper presents the first open-loop alignment/stability analysis of hovering multirotors proven to maintain an FSO link despite inherent instability. Communication distance, wavelength, and platform deviation are among the many parameters evaluated in our model. We characterize fiber-bundle transceivers, as an example of optical arrays and their applicability to aerial FSO communication. Our simulation is based on static lab measurements combined with theoretical analysis and optical geometrical intersection models. Analyses indicated that rotational deviation has a much higher impact on performance than translational deviation. Current commercial multirotor platforms proved adequate for future FSO communication, with 16% to 30% expected throughput when employing suitable optical arrays. We conducted a parametric sweep test to determine optimal and marginal receiver parameters based on platform characteristics and performance metrics. This analysis was conducted and reported as one example of the many applications that our developed mathematical models support.

Free Space Optical (FSO) communication technology has a data rate higher than other wireless communication techniques. Although FSO has enjoyed widespread notoriety for its point-to-point communication, the increasing demand of mobile platforms and multi-user communications requires considerable improvement over current FSO systems. Developing multi-point mobile FSO systems is highly desirable. To aid in this effort, the work presented in this paper details a solution for overcoming the Blind Source Separation (BSS) problem in a multi-user communication link. A multi-user FSO link with various atmospheric conditions that leverages the FastICA algorithm is detailed below. Experimental results have been obtained with a system composed of two transmitters operating at different wavelengths and two receivers. Results demonstrate that using this method, the original optically transmitted signals can be separated from their received mixtures, even during high turbulence conditions.

Increasing usage and higher demand for wireless traffic are causing a critical need for increased bandwidth and capacity communication networks. Accordingly, multipoint free-space optical communication (FSOC) links are beginning to draw significant attention as a valuable and promising solution for satisfying new network requirements. Extensive interest has focused on investigating and developing hardware design solutions to perform multiuser FSOC. However, to meet the low size, weight, and power needs of this technology, we propose using a robust and solid signal-processing method, namely independent component analysis (ICA), to implement an optical multiple access technique. A high-speed, multipoint free space optical system composed of two independent users and two receiving sensors is introduced. FastICA and joint approximate diagonalization of eigenmatrices algorithms are exploited for multiuser detection, and extensive analysis of power ratio, data rate difference, computational complexity, achievable system capacity, turbulence generation accuracy, and turbulence severity effects on signal reconstruction is performed. Experimental results demonstrate that ICA provides suitable source separation for all tested configurations. Additionally, transmission parameters that maximize signal reconstruction performances were identified.

A fiber-bundle approach for beam steering at the transmitter was recently proposed and investigated that allowed tracking of the receiver without the use of mechanical devices for mobile free-spaced optical (FSO) platforms experiencing misalignment due to movement and atmospheric turbulence. This paper investigates different transmitter configurations along with a general pointing, acquisition and tracking (PAT) algorithm on key parameters including power availability at the receiver, switching time, link recapturing time, and coverage area at the receiver plane. The alignment control algorithm with different ways to track receiver is also evaluated on these key parameters. The algorithm performance versus transmitted power, switching time and the threshold power for beam steering was examined. The investigation finds that a combination of switching combined with splitting of the light between fibers is a suitable solution to provide the best trade-off between link up time and the coverage area at the receiver, and thus is a good general purpose design choice, and the optical tracking system with target location information demands fewer efforts to recover and maintain the dynamic link than the one without the awareness of the receiver location information.

Mobile free space optical (FSO) nodes have been characterized as a way to resolve movement and misalignment incompatibilities that are inherent in optical nodes. By introducing multibranch angular diversity mobility to FSO nodes, connection reliability is enhanced. In this paper, receiving branches are assumed to be circularly placed on a single plane, and transmitting branches are positioned on a different platform. A coverage of 360 in this unbalanced branch configuration is achieved with the deployment of diversity combining, wherein the transmitter follows a transmit selection diversity rule. Channel turbulence is modeled as weak lognormal with spatially correlated fading samples. On the receiver side, selection combining (SC), equal gain combining (EGC), maximum ratio combining (MRC), and threshold generalized SC (T-GSC) schemes serve as candidates for combining purposes. Statistical discussion relative to branch signals and combiner output is presented, and bit error performance and outage probability are numerically evaluated. When compared with MRC, T-GSC was found to have lower estimation error. The T-GSC scheme additionally demonstrated overall superior performance over SC and EGC for mobile FSO.

In spite of exceptional advantages demonstrated by optical wireless signals, alignment distortion from building sway or transceiver displacement, make the development of corresponding high-speed links less attractive. This paper investigates a fiber-bundled design providing limited transceiver mobility. By placing three fibers horizontally behind a convex lens, angular diversity is exploited. A modified scheme of switched diversity is utilized to combine signals from various receiving fibers. Coverage area, combiner processing load, bit error rate, and outage probability served as performance criteria throughout the analytical and simulation assessment. High bit error performance was achieved while maintaining low implementation complexity over a turbulent fading channel.

Wireless optical nodes with mobility capabilities (mobile free space optics [FSO]) have emerged as a way to overcome limitations inherent in FSO links, for example, transmitter–receiver restrictive alignment requirement. This paper proposes the use of an angular diversity model for characterizing the performance of mobile FSO over atmospheric turbulence, and transmitter–receiver misalignment switch-and-examine combining technique is suggested for a diversity combining under unbalanced multireceiving configuration. A laser beam is modeled as spatial Gaussian profile, and channel fading is modeled as a lognormal distribution with spatially uncorrelated samples. Analytical and statistical discussion on the resultant output is presented, and bit error performance and processing load are numerically evaluated and compared with those obtained using selection combining diversity schemes. Copyright © 2011 John Wiley & Sons, Ltd.

Pointing, acquisition and tracking of a free-space optical node in a mobile network experiencing misalignment due to adverse factors including vibration, motion and atmospheric turbulence requires a different approach than traditional free-space optical transceivers. A recent fiber-bundle approach for beam steering at the transmitter was investigated to provide continuous beam coverage at the receiver without the application of mechanical devices. Utilizing multiple fibers-lenses sets at the receiver was also proposed to enhance the tolerance of optical link misalignment. This paper investigates the feasibility of applying multiple fibers with a single lens at the transmitter and multiple fiber-lenses sets at the receiver to improve the transmission performance on tolerating link misalignment. Two series of laboratorial experiments were implemented to compare the optical link robustness for different transceiver setups as the fiber-lens distance (zfl) was varied to modify the beam divergence: multi-fiber transmitter working with single-fiber-lens receiver and multi-fiber transmitter working with multi-fiber-lens receiver. The results showed that using multiple fibers in both the transmitter and the receiver reveals significant potential to maximize the up time of the link.

An Electrical Engineering Summer Academy for Pre-College Students was held at the University of Tulsa, Tulsa, OK, during the summers of 2007 and 2008. The Academy participants included students having just completed 7th to 11th grade and teachers from middle school through high school. The students and teachers participated in team-building, professional development, and technical activities designed to teach them about the engineering profession and the field of electrical engineering. Activities included laboratories in electrical circuits, designing an electric car, soldering, a field trip, and discussion about ethics. Students worked in two- and four-person teams and made presentations on their experiences. The Academy was evaluated using formal assessment instruments and faculty observations. Both individual activities and the overall program were evaluated. The evaluation of the 2007 Academy revealed ratings of 4.5 or greater out of 5 on most aspects of the Academy, though some areas indicated a need for improvement, such as clarity of written materials and the availability of additional material for advanced students who finished early. Improvement in knowledge was demonstrated on over 50% of the questions on a survey administered at the beginning and again at the end of the Academy. Changes were proposed and implemented for the 2008 Academy in response to the assessment data. The impact of these changes and the lessons learned in the process are also presented.

A transmitter and receiver design based on the use of fiber optic bundles have been proposed and studied both experimentally in the laboratory and theoretically through simulation, and have shown promise for providing enhanced functionality in mitigating the effects of turbulence and weather on the pointing, acquisition and tracking problem. In this paper, the operation of an FSO link constructed from these designs is analyzed under varying environmental conditions. A transmitter with a linear fiber array transmitted at 1550 nm to a receiver constructed using a hexagonal array of 19 fibers located 4.6 meters away. Transmissions occurred for different weather conditions over grass, concrete, and brick surfaces at different locations on campus. Data at 100 kb/s was transmitted across the link and the collected signal was recorded for off-line statistical analysis, including achieved bit-error rate. The investigation showed that the system performance was only slightly sensitive to temperature and terrain. The largest contributing factor to high BER time periods was when the receiver optics were blown out of the path of the transmitted beam due to instabilities in the construction of the mounting unit of the receiver. Improvements in the mounting structure are expected to eliminate this effect and allow a more accurate assessment of the receiver's capabilities.

A transmitter and receiver design based on the use of fiber optic bundles have been proposed and studied both experimentally in the laboratory and theoretically through simulation, and have shown promise for providing enhanced functionality in mitigating the effects of turbulence and weather on the pointing, acquisition and tracking problem. In this paper, the operation of an FSO link constructed from these designs is analyzed under similar environmental conditions but transmitting at two different wavelengths. A transmitter with a linear fiber array transmitted over a brick surface at either 1310 nm or 1550 nm wavelength to a receiver constructed using a hexagonal array of 19 fibers located 15 feet away. Data at 100 kb/s was transmitted across the link and the collected signal was recorded for off-line statistical analysis, including achieved bit-error rate. The investigation finds that, when eliminating effects due to artifacts in source operation, dependence of link operation on wavelength is minimal, even though the optical alignment was optimized for only 1550 nm.

We investigate differential measurement of strain using fiber Bragg gratings and spatial demultiplexing methods. Differential measurement is shown to improve strain resolution in the presence of noise by an average of 34% compared to absolute measurements of the Bragg wavelengths. Spatial demultiplexing of the gratings provides potentially high-resolution sensing without calculating the sensor wavelengths. The differential method is still temperature dependent. We apply the differential technique to the problem of oil-water boundary detection in production columns, with promising preliminary results.

We use a whole-animal model and wavelength-selective optical stimulation to investigate relationships between optical stimulus characteristics and neural signaling. Light-emitting diodes are used to selectively stimulate rod and cone pathways in Rana pipiens. A suction electrode is used to make in vivo measurements of the compound action potential from the optic nerve as the wavelength and intensity of the stimulus is varied. Our results demonstrate that the cone and rod pathways can be separately stimulated and analyzed with our method and, thus, provide a means to model the response of such pathways to more complex stimuli.

We have constructed a fiber-bundle based FSO receiver and investigated its performance as a function of transmission misalignment, turbulence, and weather. We also investigate a wavelength diversity scheme, which consists of switching between multiple transmission wavelengths, for reducing the impact of turbulence. Three wavelengths, 850nm, 1310nm, and 1550nm, are emitted by one or more transmitting fibers, and the effects of turbulence and misalignment experimentally evaluated in an indoor environment. The receiver retained the link for a reduced range of misalignment at all wavelengths without adjustments. Initial outdoor experiments were conducted using 1550nm under various lighting, wind, and turbulence conditions.

Increased RF noise floor, crowded and scarce wireless spectrum, and security are issues challenging present-day aerial wireless communication. Optical wireless (or free-space optical (FSO)) is an alternative communication system applicable to air-to-air (A2A) or air-to-ground (A2G) applications. An FSO link is able to provide high bandwidth, low-power, and secure communication for various unmanned aerial systems (UASs) and ground nodes, including mobile and fixed stations. However, unlike RF, optical communication is highly sensitive to misalignment and platform instability that causes unavoidable communication interruptions. Developing aerial optical wireless communication systems requires modeling the effects of UAS instability on the FSO system performance, including FSO link throughput. This work aims to develop a rigid and simple modeling framework for A2A FSO communication links. The framework is abstract, general, and platform-independent. The model relates instability to averaged percentage alignment (APA), which can be used to predict throughput and other performance-related parameters. A planer arrays of optical transmitters and receivers is used and proposed as a promising technology for FSO communication with large field-of-view and diversity-ready performance. Also, a novel alignment model using geometric intersection is developed and analyzed against simulated and real multi-rotor platforms.

The design of a free-space optical (FSO) transceiver suitable for the demands of establishing reliable communication between mobile nodes such as UAVs requires the solution of difficult problems related to pointing, acquisition and tracking. To accommodate the inevitably larger misalignment conditions encountered in mobile applications, the receiver must possess a large field of view (for angular misalignments), be tolerant of translations away from the optical axis of the receiver antenna, and maintain a large collecting area to realize a workable power budget for the overall link. The transmitter must be able to deliver the best quality beam, in terms of peak power and power distribution, in the direction of the receiver as possible. The link must also tolerate the effects of atmospheric turbulence and weather, which normally cause signal fade or complete signal loss. One approach to turbulence mitigation is to use a wavelength diversity scheme. The optimum wavelength for transmission varies with turbulence strength, absorption, and weather effects such as rain and fog. We have constructed an FSO transmitter and receiver based on optical fiber bundles and adapted the transmitter to be capable of wavelength diverse transmission. In this paper, we present an experimental investigation of the performance of the system as a function of transmission misalignment, turbulence, and weather for a wavelength diversity scheme, which consists of switching between transmission wavelengths, for reducing the impact of turbulence. Three wavelengths, 850nm, 1310nm, and 1550nm, are emitted by one or more transmitting fibers, and the effects of turbulence and misalignment experimentally evaluated in an indoor environment. A system is designed to detect changes in transmission and switch the transmitter to the appropriate wavelength. The receiver retained the link for a reduced range of misalignment at all wavelengths without adjustments, indicating that adjustment of the receiver immediately after a wavelength changes was not necessarily required.

This paper presents novel, modular optical detector arrays of various shapes and configurations. Recently developed Modular Optical Wireless Elements (MOWE) architecture serves as the basis for large and complex optical detector arrays that can be constructed as geometric shells and provide wide-area even omnidirectional field-of-view (FoV). Programmable optical modules synchronously sample the environment, and then route measurements to the user through a dedicated electrical backbone. The arrays are inexpensive, easy to construct, and can be made with homogeneous/inhomogeneous optical properties. Applications include remote sensing, motion detection, optical navigation, and medical imaging, among others. We present the MOWE detector array concept with a detailed optical analysis and a suggested design methodology, as well as a number of various demonstrations. We also utilize wavelength-diversity of MOWE arrays to demodulate two overlapping signals, showing that many diversity-based algorithms can be conveniently prototyped and implemented.

Experimental results of the transmission to total-internal reflection switching of a step-on CO/sub 2/ laser beam by a dielectric-clad, isotropic liquid crystal film is presented. The use of the isotropic phase allows for polarization independence of the processes, while maintaining a moderate thermal nonlinearity. Dependence of switching speed on incident angle and energy density is evaluated. Experimental observations of the nonlinear switching effect with microsecond CO/sub 2/ laser pulses are also presented. >

A revolutionary cost-effective "Microchip" system capable of measuring temperature and pressure over the entire wellbore was developed and tested in the field. The Microchip system includes two major components: the surface devices and the "tracer." A tracer (~7.5 mm in diameter) consists of fiber optic sensors, a SoC IC (System-on-Chip, which includes microcontroller, memory, transmitter and receiver circuits) and a battery, all encapsulated in a protective chemical coating shell. The surface devices include an "initiator" to reset the circuit of the tracer and a "data collector" to retrieve data from the tracer using wireless communication. When used in the field, tracers will be injected together with the drilling fluid. As the tracer travels through the wellbore, it will measure the temperature and pressure through the entire wellbore and store the data in the on-chip memory. When the tracers are carried out of the borehole by the drilling fluid, a "data collector" will communicate with the tracer to download the data stored in the on-chip memory on the tracer. Thirteen tracers were injected in an 11,050ft well and circulated with a 13ppg mud at 400-500gpm. After travelling about 6.6km in the 8-3/8 inch wellbore to a depth of 11,050 ft, seven tracers returned to the surface successfully. The first tracer returned to the surface after 57 min. Data recorded on the tracer shows a bottom hole pressure of ~7500psi and flowing bottom hole temperature of ~190 degrees Fahrenheit. Results are consistent with the static bottom hole pressure and temperature. The Microchip system can be used for subsurface measurement or as a diagnostic tool for identifying problematic zones. As an open platform, the Microchip system could also be expanded to other measurements by using different types of sensors. This Microchip system can be used in drilling and completion, pipeline monitoring and many other applications.

A novel receiver and transmitter that utilize fiber bundles are under development to address the challenges associated with acquiring and maintaining a free-space optical link between mobile platforms in the presence of vibration and atmospheric turbulence. The fiber bundles allow for greater misalignment tolerance at the receiver and greater control over the spatial area covered by the transmitted beam upon reaching the receiver. This paper reports on a simulation-based investigation of design choices that optimize the performance of the system under several operating scenarios. The simulation incorporates prior experimental data into the theoretical calculations for optical propagation to better describe the performance of the physical designs. For a given link length and available power, the coverage area and collected power are controlled by the pattern of transmitters, beam divergence, and the receiver construction. The investigation finds that the coverage area of the receiver can be optimized for a given link length by proper choices of these parameters, and that similar parameters may provide near optimal performance at other wavelengths. Trade-offs between choices of the key parameters are explored as a function of link length. The results provide guidance on the further development of the overall system.

In order for mobile optical transceivers to communicate, the transceivers must be able to acquire and maintain a free-space optical link between them despite misalignments due to movement and the effects of atmospheric turbulence. Recently, novel transmitters and receivers, along with specifically designed control algorithms, were proposed that incorporated a fiber-bundle approach for improving the ease of acquiring and maintaining the link between two transceivers. Preliminary transmitter and receiver nodes have been constructed for testing the capabilities of this approach. This paper investigates the performance of a transmitter and receiver pair through experimental methods. The performance is evaluated on several key parameters, including initial acquisition time, up time of the link when perturbed by movement or simulated atmospheric impairment, and the link recapturing time once a connection is lost. The dependence of the key parameters is evaluated for different levels and types of perturbations, as well as design choices at the transmitter and receiver. The results show that the optical control system successfully recovered and maintained the link while the receiver was in motion, although the performance was impacted by the angular misalignment tolerance of the receiver. The strengths and limitations of the approach revealed by the experiments are also discussed, along with paths for further improvements.

A number of existing spatial diversity schemes have been shown to improve the performance of optical wireless communication systems in diversity-rich environments. Among all, switched diversity has low complexity and is simple to implement. In this paper, an innovative spatial diversity scheme based on switched diversity is proposed. The scheme, namely switch-to-dominant combining, contributes to a higher bit error rate (BER) performance when compared to conventional switched diversity schemes, including switch-and-stay and switch-and-examine diversity. The optical multireceiver wireless system operates in a spatially correlated and lognormally distributed fading channel. Analytical analyses are conducted to demonstrate BER and processing load performance offered by the new scheme and compare them to available schemes, i.e., conventional switched combining and selection combining.

The term mobile FSO refers to wireless optical nodes with mobility capabilities. Mobile FSO has emerged as a way to combat some of limitations facing FSO links, i.e. incompatibility of transmitters and receivers relative to movement and misalignment. In this paper we utilize an angular diversity model for characterizing the performance of mobile FSO over atmospheric turbulence. Switch-and-examine combining (SEC) technique is our suggestion for the diversity combining solution in such unbalanced multi-transceiving configuration. The channel fading is modeled as a lognormal distribution with spatially correlated samples. Analytical/ statistical discussion on the resultant output is presented and bit error performance and processing load are numerically evaluated and compared to a selection combining (SC) diversity.

This paper investigates the performance of a recently proposed control algorithm for mobile FSO node alignment through experimental methods. The performance is evaluated on coverage area at the receiver plane, the link recapturing time and the connection up-time during tracking. The dependence of performance on the transmitted power, switching time and the threshold power for beam steering was examined. The results show that the optical tracking system successfully recovered and maintained the link while the receiver was in motion and the signal coverage at the target area was enhanced significantly.

This paper reports on a simulation-based investigation of an optical wireless system using fiber-bundle based transmitters and receivers. The simulation incorporates both beam propagation theory and experimental data from evaluations of the individual components. The coverage area at the receiver is evaluated as a function of transmitter power, link length, misalignment, and the number of transmitter fibers employed. An optimum coverage area is achievable with proper selection of transmitter parameters for a given link length. Trade-offs between the number of fibers employed and the transmitter power are explored.

Optical wireless networks often require more than one transmitter to maintain consistent connection with a single receiver. Accordingly, an efficient control mechanism is required to address challenges associated with multiple access to the receiver. This paper proposes a receiver access control (RAC) mechanism for optical wireless mesh networks that focuses on the optimization of classical RTS/CTS technique and is applicable for indoor communications, as well. The novel algorithm was established for terminals and is optimized to enhance throughput of high speed optical links with a small number of terminals. It is based on a multi-slot channel reservation (MSCR) concept, and founded on terminal buffering in, which delays collided terminals for a limited period of time. The protocol performance is analytically characterized. Analytical simulation is shown to exhibit channel throughput.

The principal optical nonlinearities of isotropic liquid crystals are reviewed, and their potentials for all-optical limiting and switching applications are assessed. Our dynamic grating studies of a class of isotropic liquid crystals show that, in the nanosecond and longer time scales, the principal mechanisms for nonlinearities are laser induced ordering (molecular orientations), thermal and density fluctuations. We report optical limiting results obtained with bulk thin film and optical fiber fabricated from these isotropic crystals.

The availability of Channel State Information (CSI) at the receiver side of a Free Space Optical (FSO) link is required for optimal detection and diversity combining. Consequently, providing instantaneous CSI knowledge introduces challenges to receiver design and an increase in estimation error and bandwidth waste. In this work we investigate two selection-combining diversity schemes and their achieved gains when perfect instantaneous CSI is not available. An average SNR estimation value at the receiver is used for branch selection, and signal combining is performed with branch balance and unbalance. Channel fading is modeled as a lognormal distribution with spatially correlated samples. BER performance and outage probabilities are analytically characterized. Further, analytical results are verified using computer simulation.

Mobile Ad hoc networks (MANETs) are composed of a collection of independent nodes usually connected by low bandwidth Radio Frequency (RF) links which are challenging for transmitting video and high bandwidth applications for rescue workers in emergency situations. Earlier we proposed a routing protocol, called AODVH to use for high bandwidth communication for search and rescue operations in a disaster area which consists of hybrid nodes using Free Space Optical (FSO) and RF links, with FSO being the primary link and RF as backup in the case of failure of “FSO only” paths. The objective of this paper is to evaluate the performance of AODVH using ns-2 simulations with varying network size and compare with three other Ad hoc routing protocols. We also implemented and tested AODVH in the laboratory using a testbed consisting of Linux based Ad hoc nodes. Results show that AODVH performs better in terms of packet loss, end-to-end delay, overhead, packet delivery ratio, route discovery frequency and throughput when compared to with the other three protocols.

Hybrid Free Space Optics (FSO)/Radio Frequency (RF) communication systems have emerged as a way to improve network performance by providing enhanced availability and reliability. In an effort to mitigate individual drawbacks in the optical link during adverse weather conditions, network traffic flows simultaneously between channels. Based on the Shannon-Hartley theorem, channel capacity is dependent, among others, on both the signal to noise ratio (SNR) and channel bandwidth. As such, combined link throughput may be affected by channel state conditions. Because atmosphere turbulence can be modeled as a time-varying fading channel, capacity analysis can be investigated. In this paper, authors apply various availability scenarios of channel state information (CSI) on the optical link to derive closed-form expressions for combined link capacity. Numerical simulation provides a comparison to the results.