Debbie Kedar

Urban optical wireless communication networks: the main challenges and possible solutions

Urban optical wireless communication (UOWC) is rapidly gaining popularity as an effective means of transferring data at high rates over short distances. The UOWC terminal includes an optical transmitter and a receiver positioned, for example, on high-rise buildings separated by several hundred meters. Light beams propagating through the atmosphere carry the information from the transmitter to the receiver. UOWC boasts many advantages over its rivals. Notably, UOWC facilitates rapidly deployable, lightweight, high-capacity communication without licensing fees and tariffs. However, UOWC still faces many challenges, including how to improve communication performance in adverse weather conditions or during building sway. We present and evaluate some of the exciting new research approaches that have been suggested to deal with these issues, including optimization of telescope gain, new technologies for pointing systems, and solutions at the network level.

Non-line-of-sight underwater optical wireless communication network

The growing need for ocean observation systems has stimulated considerable interest within the research community in advancing the enabling technologies of underwater wireless communication and underwater sensor networks. Sensors and ad hoc sensor networks are the emerging tools for performing extensive data-gathering operations on land, and solutions in the subsea setting are being sought. Efficient communication from the sensors and within the network is critical, but the underwater environment is extremely challenging. Addressing the special features of underwater wireless communication in sensor networks, we propose a novel non-line-of-sight network concept in which the link is implemented by means of back-reflection of the propagating optic signal at the ocean-air interface and derive a mathematical model of the channel. Point-to-multipoint links can be achieved in an energy efficient manner and broadcast broadband communications, such as video transmissions, can be executed. We show achievable bit error rates as a function of sensor node separation and demonstrate the feasibility of this concept using state-of-the-art silicon photomultiplier detectors.

Optical wireless communication through fog in the presence of pointing errors

Terrestrial optical wireless communication (OWC) is emerging as a promising technology, which makes connectivity possible between high-rise buildings and metropolitan and intercity communication infrastructures. A light beam carries the information, which facilitates extremely high data rates. However, strict alignment between the transmitter and the receiver must be maintained at all times, and a pointing error can result in a total severance of the communication link. In addition, the presence of fog and haze in the propagation channel hampers OWC as the small water droplets scatter the propagating light. This causes attenuation due to the resultant spatial, angular, and temporal spread of the light signal. Furthermore, the ensuing low visibility may impede the operation of the tracking and pointing system so that pointing errors occur. We develop a model of light transmission through fogs of different optical densities and types using Monte Carlo simulations. Based on this model, the performance of OWC in fogs is evaluated at different wavelengths. The handicap of a transceiver pointing error is added to the model, and the paradoxically advantageous aspects of the transmission medium are exposed. The concept of a variable field of view receiver for narrow-beam OWC is studied, and the possibility of thus enhancing communication system performance through fog in an inexpensive and simple way is indicated.

A comparative study of wireless communication network configurations for medical applications

A comparative study of the performance characteristics of five multiple access configurations for a wireless communication network is presented and evaluated within the context of the medical field of application. A universal variable has been defined for comparison between the alternative configurations, embodying the data rate, number of channels, power consumption, and bandwidth requirements. Special medical specifications have been accommodated, while the broader applications of this approach have been addressed. CDMA is indicated as the preferred method, over ALOHA, slotted ALOHA, CSMA, and polling.

Non-line-of-sight optical wireless sensor network operating in multiscattering channel.

Networks of sensors are envisaged to be major participants in future data-gathering systems for civilian and military applications, including medical and environmental monitoring and surveillance, home security, agriculture, and industry. Typically, a very large number of miniature sensing and communicating nodes are distributed ad hoc at the location of interest, where they establish a network and wirelessly communicate sensed data either to one another or to a base station using various network topologies. The optical modality is a potential solution for the links, due to the small and lightweight hardware and low power consumption, as well as other special features. Notably, the backscattering of light by molecules and aerosols in the atmosphere can function as a vehicle of communication in a way similar to the deployment of numerous tiny reflecting mirrors. The scattering of light at solar-blind ultraviolet wavelengths is of particular interest since scattering by atmospheric particles is significant and ambient solar interference is minimal. In this paper we derive a mathematical model of a simple and low-cost non-line-of-sight (NLOS) optical wireless sensor network operating in the solar-blind ultraviolet spectral range. The viability and limitations of the internode link are evaluated and found to facilitate miniature operational sensor networks.

Backscattering-induced crosstalk in WDM optical wireless communication

The crosstalk effect of aerosol backscatter on the performance of a wavelength-division-multiplexed (WDM) optical wireless communication (OWC) system is investigated, analyzed, and quantified. An OWC link could be a segment within a metropolitan area network (MAN) or a ground-station-to-space link of a satellite communication system. In these cases, a WDM transmitter and receiver are housed in one transceiver unit with parallel, or near-parallel, optic axes. The crosstalk at the receiver is caused by light from the transmitted signal of the same transceiver, which has been backscattered by molecules and aerosols in the atmosphere. This is exacerbated in the presence of fog and haze, in which case both the desired signal from another transceiver is attenuated by scattering and the backscatter-induced crosstalk increases. A bit-error-rate (BER) model is derived that takes into consideration the dominant noise sources, including backscatter-induced crosstalk and signal mixing with amplified stimulated emission (ASE) from an optical preamplifier at the receiver. The numerical calculations in this paper indicate that, in moderate fog, the BER may increase by an order of magnitude or more due to backscatter, depending upon the atmospheric extinction coefficient.

Analyzing the performance of a nanosatellite cluster-detector array receiver for laser communication

This work analyzes laser communication between a cluster of nanosatellites, which is a concentrated formation of small lightweight satellites and a ground station. The scenario under consideration is a cluster of nanosatellites communicating by means of a laser beam with a detector array receiver that is located on the earths surface and equipped with a common optical system for all incoming beams. The beams are concentrated to spots over the detector plane by the receivers optics. The detector array enables the ground station to communicate with a tight concentration of the nanosatellites, which reduces system complexity and cost. A critical parameter that determines the successful receipt and subsequent decoding of a transmitted signal for a given configuration is the angular separation between the satellites within the cluster. This separation must be retained to prevent critical overlapping of the spots on the detectors surface. The maximum allowable overlapping is calculated in terms of given bit-error rate. The spatial spreading of the beams, caused by scattering from aerosols in different layers of the atmosphere, is calculated for the case of single scattering. A stratified model of the atmosphere is used. Turbulence influences the beam width, especially for the case of short exposure, and is primarily caused by temperature changes, which result in fluctuations in the refractive index. In this research, a new approach is adopted for analyzing communication network performance through the atmosphere by applying optical-transfer function (OTF) concepts used in imaging and remote sensing. We evaluate the effectiveness of this new approach in applications where spatial spread between the users is very important.

Subsea ultraviolet solar-blind broadband free-space optics communication

We examine the potential of subsea free-space optics (FSO) for sensor network applications leveraging the emerging technologies of highly sensitive photon-counting detectors and semi-conductor LED and laser light sources in the UV solar blind. Monitoring oil and gas production installations is the niche application discussed. The merits of FSO include the capacity for broadband communication that would enable the transmission of video data in real time, which is not possible with other technologies at present. However, subsea FSO is challenged by high extinction and the immense variability of background illumination in shallow waters. This has stimulated us to investigate the potential of underwater FSO in the UV solar-blind spectral range, where background illumination is nearly nonexistent and considerable scattering occurs. The achievable performance is compared to transmission at 520 nm, where, in Clear Ocean, data rates of 100 Mbps can be transmitted over distances of ~170 m, falling to under 15 m in harbor waters. It is anticipated that ranges of 12 m can also be obtained with UV solar-blind wavelengths, although experimental corroboration is not yet available.

Performance limitation of laser satellite communication due to vibrations and atmospheric turbulence: down-link scenario

SUMMARY In this paper, we analyse the effects of vibrations and the atmosphere on the performance of a broadband laser inter-satellite link (BLISL) which was studied within the framework of the BLISL joint Israeli– German applied research project. The use of optical radiation as a carrier between satellites and in satelliteto-ground links enables transmission using very narrow beam divergence angles. Due to the narrow beam divergence angle and the large distance between the satellite and the ground station or airplane the pointing is a complicated process. Further complication results from vibration of the pointing system caused by two fundamental mechanisms of a stochastic nature: (1) tracking noise created by the electro-optic tracker and (2) vibrations caused by internal satellite mechanical mechanisms. Additionally an inhomogeneity in the temperature and pressure of the atmosphere leads to variations of the refractive index along the transmission path. These variations of refractive index as well as pointing vibrations can cause fluctuations in the intensity and the phase of the received signal leading to an increase in link error probability. In this paper, we develop a bit error probability (BEP) model that takes into account both pointing vibrations and turbulence-induced log amplitude fluctuations (i.e. signal intensity fading) in a regime in which the receiver aperture D0 is smaller than the turbulence coherence diameter d0: Our results indicate that BLISL can achieve a BEP of 10 � 9 and data rate of 1Gbps with normalized pointing vibration of GT *s 2 ¼ 0:05 and turbulence of sX ¼ 0:3: Copyright # 2003 John Wiley & Sons, Ltd.

Analyzing performance of nano-satellite cluster - detector array receiver laser communication

This work presents research in the field of laser satellite communication between a cluster of nano-satellites and a ground station. The scenario under consideration is a cluster of nano-satellites, communicating by means of a laser beam with a detector array receiver, which is located on the Earths surface and equipped with a common optical system for all incoming beams. A critical parameter, determining the successful receipt of a transmitted signal for a given configuration, is the angular separation between the satellites within the cluster. This separation must be retained to prevent critical overlapping of the spots on the detector. The maximum allowable overlapping is calculated in terms of given BER. The spatial spreading of the beams, caused by scattering from aerosols in different layers of the atmosphere, is calculated for the case of single scattering, as appropriate for the stratified model used. Turbulence influences the beam width especially for the case of short exposure. In this research a new approach is adopted to characterize the atmospheric channel using OTF (Optical Transfer Function) concepts from the field of imaging and remote sensing. We evaluate the effectiveness of this new approach inapplications where spatial spread is very important, and detector array feasibility is currently under investigation.