Free Space Optical (FSO) Communication uses light propagation in free space (air, outer space, and vacuum) to wirelessly transmit data for telecommunications and communication networking. FSO Communication is a key wireless and high-bandwidth technology for high speed large-capacity terrestrial and aerospace communications, which is often chosen as a complement or alternative to radio frequency communication. The propagating optical wave can be influenced negatively by random atmospheric changes such as wind speed, temperature, relative humidity, and pressure, thermal expansion, earthquakes, and high-rise buildings. This edited book covers the principles, challenges, methodologies, techniques, and applications of Free Space Optical Communication for an audience of engineers, researchers, scientists, designers, and advanced students.
Inspec keywords: free-space optical communication
Other keywords: spatial diversity; distributed sensing; quantum-based satellite communication; local weather data; phase turbulence; terrestrial link; optical spatial filtering; aperture averaging; wavelet-based signal processing; relay-assisted systems; signal encryption strategies; space time block codes performance; beam width; partially coherent Gaussian beam; FSO Communication; atmospheric attenuation; microwave photonics; spectral analysis; maximum likelihood thresholds; refractive index; combined channels; free space optical communications; atmospheric turbulence channels; Kalman filter; on-off keyed laser communications; acousto-optic chaos; encrypted chaos propagation; light propagation; beam wandering
Subjects: Free-space optical links; General electrical engineering topics
The demand for high bandwidth and secure communication is increasing. Free space optical (FSO) wireless communications technology could be one possible alternative option to the RF technologies that can be adopted in certain applications to unlock the bandwidth bottleneck issue, specifically in the last mile access networks, between mobile base stations in RF cellular wireless networks, and for radio over fiber; and over the last decade, we have seen growing research and development activities in FSO communications in the field of high data rate wireless technology applications as well as the emergence of commercial systems.
Optical communication can be deployed over a fiber-optic link or a free-space optical (FSO) link. Fiber-optic links are based on a closed two waveguide layers (core and cladding), protected by the jacket, for signal transmission, which are characterized by low-loss, extremely large bandwidth, small distortions, and represent relatively stable channels. FSO links are known for the “free” communication media, and have been widely used in shot-reach optical transmission systems. Despite the advantages of FSO channels, the performance of FSO transmission systems may be degraded by the atmospheric effects such as absorption, scattering effects, and atmospheric turbulence. Atmospheric turbulence is the main reason for channel fluctuation, as a result, the received beam will be affected by the intensity scintillation and spatial phase distortion. When spatial-mode multiplexing is used for FSO communication, mode crosstalk may become the limiting factor for the system capacity. In this chapter, we first analyze the atmospheric turbulence, and build a model to emulate the atmospheric turbulence channel. Then orbital angular momentum (OAM) multiplexing/demultiplexing will be introduced, followed by the description of approaches to mitigate the turbulence effects.
In this chapter, the average bit error rate (BER) and outage performance of a polarization shift keying (PolSK)-based FSO system is analyzed over moderate and strong atmospheric turbulences. By using wavelength and time diversity techniques, the performance of the system is enhanced. Also, the PolSK-based system is extended with spatial diversity and MIMO techniques using OC, EGC, and MRC combining schemes. The performance of SISO and MIMO FSO systems are analyzed in moderate and strong turbulence regimes, the results are compared and plotted. The better BER performance of 10-7 at SNR 1/4 30dB is achieved by using MIMO with MRC combining in weak turbulence without pointing errors. The performance degradation effect of atmospheric turbulence and pointing errors can be reduced by using MIMO scheme, that is by increasing the number of transmit and/or receive apertures. Through the analytical and numerical studies, it is observed that MIMO systems provide the better BER of 10-13 and 10-22 for moderate and strong turbulences at SNR at 60 dB, respectively.
Free space optic (FSO) communication systems are ideal for setting up high-speed short distance (a few kilometers) communication links. They have been in existence for about two decades but have not gained widespread acceptance and popularity. This is primarily due to a major drawback of the FSO systems. Changing the environmental conditions (atmospheric turbulence and fog) can drastically affect their throughput and reliability. Atmospheric turbulence can affect the FSO link in a manner similar to the impairments introduced by multi-path propagation (signal fading) in a wireless channel. It is well known that the random variation of signal strength due to fading in a communication channel can result in severe bit-error-rate (BER) performance degradation and an increase in the outage probability. A number of receiver combining schemes and space-time block codes (STBCs) have been designed to improve the performance of wireless communication systems over fading channels. FSO systems have the potential to provide high data-rate communication with the advantages of quick deployment times, high security, and no frequency regulations. These features have resulted in FSO becoming a unique technology within the domain of wireless communication.
Free space optical (FSO) communication is an upcoming attractive alternative technology for transporting high-bandwidth data when the existing RF/fiber-optic communication is neither realistic nor viable. However, the presence of FSO channel turbulences such as fog, smoke, rain, dust, snow, and/or sand can critically degrade the quality of the FSO communication system. There is a great technical development in today's optical components such as LED, laser, optical detector, detector's sensitivity at high bandwidth, modulation techniques, power requirements, total weight, and total size. In spite of all these technological developments, the major limitation of FSO communication quality is the atmosphere. Optical absorption and scattering due to the FSO channel's components in the atmosphere drastically reduce the transmitted optical power. Further, the arbitrary atmospheric formation due to random fluctuation of optical turbulence can alter the wavefront quality of the traveling optical signal, develop the intensity fading and thus result in random signal losses and inter symbol interference (ISI) at the receiver plane. Weather conditions thus ultimately determine the FSO communication system quality not only in ground-to-ground FSO applications but also for deep space laser satellite optical communications because a portion of the optical beam always in the atmospheric turbulence medium that causes time-varying scattering effects.
In this chapter, the main contribution by proposing the OSF as the detection method in FSO is producing an optical method that capable to filtering noise and minimizing scintillation in real time where these advantages are accordance to the benefit of FSO as the low cost platform of telecommunication. The OSF-detection method capable to solve the weaknesses of DD method where spatial noise, beam wander and scintillation as the results of turbulence effects can be suppressed optimally before the signal detected by PD. The OSF detection method also can solve the major problem in optical amplification using Erbium-doped fiber amplifier (EDFA) where beam wandering can cause the loss signal that should be coupled into the stage of amplification process. Thus, implementing the OSF as a detection method in FSO brings significance for the improvement of performance.
In this chapter, the key theoretical aspects of dynamic atmospheric turbulence related effects such as scintillations and beam wandering are discussed. The theoretical aspects of the major turbulent atmospheric effects scintillations, beam wandering, beam pointing stability and their simulation and experimental verification are discussed in Section 7.2. Experimental measurement of the laser beam profile at the receiver plane is discussed in Section 7.3, which will be used as the basis for the experimental characterization of the atmospheric turbulence effects later in this chapter. The experimental work presented in this chapter is primarily focused on determination of the individual contribution of scintillations and beam wandering to the received signal variance. This chapter also deals with experimental validation of the theoretical calculations of scintillation index fluctuations and beam wandering effects on the received signal variances. Investigations on the different physical optic principles and advanced signal processing methods to improve the signal quality at the receivers for FSO links are also discussed.
In this chapter, an all-optical FSO relay-assisted system is proposed to mitigate the destructive effects of the distance-dependent AT-induced fading. Relays are inserted directly in the link in order to reduce the AT-induced link loss, thus extending the link span and ensuring higher link availability as well as improving the overall system performance. Two all-optical relaying schemes are proposed and investigated, namely all-optical amplify-and-forward (AOAF) and all-optical regenerate-and-forward (AORF) FSO relay-assisted approaches. For the AOAF approach, the performance analysis of triple-hop AOAF FSO communications is done under the impact of nonhomogeneous atmospheric turbulence. The AORF relaying approach is then proposed to overcome the limitation imposed by AOAF system, where the signal and noise are accumulated at each relay, thus limiting the number of relay nodes that can be used.
The performance of the terrestrial free-space optical (FSO) communication is adversely affected by atmospheric conditions such as fog, rain, dust particles, and smoke. Even in clear atmospheric conditions, the FSO link experiences severe fading due to the turbulence effect. Turbulence is induced by random variations in refractive index due to temperature and pressure fluctuations along the FSO beam propagation. Turbulence can introduce deep fading (20 to 30 dB); as a result, the outage probability increases significantly. In order to mitigate the channel fading, the receiver or transmitter diversities or a combined transmitter and receiver diversity (known as multiple-input multiple-output (MIMO) techniques) are often considered along with other techniques such as modulation, temporal and wavelength diversities. In this chapter, the FSO link performance in the presence of the turbulence fading using transmitter and receiver diversity is discussed.
This chapter has examined the performance of partially coherent FSO communication links from the information theory perspective, taking into account the adverse effects of atmospheric loss, turbulence-induced scintillations and PEs. In particular, a spatially partially coherent Gaussian-beam wave and important link design criteria have been jointly considered, in which the latter consists of the receiver aperture dimension and its resulting aperture averaging effect, transmitter beam width, link range, knowledge of CSI and weather conditions. By using the combined optical slow-fading channel model to describe the optical channel characteristics, a comprehensive analysis of the error performance, average channel capacity and outage probability of the FSO system have been presented. Moreover, the lowest-order Gaussian-beam wave model has been introduced in the proposed study, to characterize the propagation properties of the optical signal through random turbulent medium in an accurate manner; taking into account the diverging and focusing of the PCB, and the scintillation and beam wander effects arising from the atmospheric turbulent eddies. Correspondingly, the proposed study have presented a holistic perspective for optimal planning and design of horizontal FSO links employing spatially partially coherent laser beams.
Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading. Relay-assisted systems have been introduced as an effective method to extend coverage and mitigate the effects of fading in FSO links. In this chapter, we have analyzed and investigated the outage performance of relay-assisted FSO links with AF and DF relays. For serial relaying, it has been demonstrated that the outage probability is minimized when the consecutive nodes are placed equidistant along the path from the source to the destination. For parallel relaying, it has been shown that all of the relays should be located at the same place (along the direct link between the source and the destination) closer to the source and the exact location of this place depends on the system and channel parameters. Multi-hop parallel relaying which is the combined use of serial (multi-hop) and parallel relaying for FSO mesh networks has been also studied. Our analysis yields that multi-hop parallel relaying smartly exploits the distance dependency of the fading variance in FSO systems and bring substantial improvements with respect to standalone uses of multi-hop and parallel relaying. As an alternative way of implementation, all-optical relaying has been also considered. Unlike the earlier relaying schemes, the signals are processed in optical domain and therefore the requirement of OE and EO conversions is avoided. Comparisons between conventional and all-optical relaying demonstrate that the latter presents a favorable trade-off between complexity and performance and can be used as a low-complexity solution.
A new approach for mitigating atmospheric turbulence effects on free-space laser communication performance is presented. The method is based on evaluating Maximum Likelihood Thresholds using Kalman filter estimates in on-off keyed laser communications in atmospheric turbulence. Experimental results presented in this chapter clearly demonstrate the usefulness of the method. Although the results are shown for low data rate of 6.3 Kbps, the concept is valid for much higher data rate taking advantage of the higher bandwidth capability of laser communications. When looking at the table of the comparison of various threshold approaches, it is shown clearly that the anticipatory a posteriori processing of 50 data points provides a very good bit error performance, if the extra computational burden of processing 50 prior data points for every likelihood ratio test at every sample point is acceptable. The next best approach is the use of the dual Kalman filters which yields a BER that is within a factor of two of a posteriori approach, but without having to reprocess 50 prior data points before each threshold decision. Clearly, this is an acceptable alternative to the a posteriori approach with its more extensive processing and its unavoidable delays. Because the required update rates of h is determined by the slowly varying atmospheric turbulence, high data rate links using the dual Kalman approach will allow many more times the number of data samples and bits to be tested against and validated by the most up-to-date h. The dual Kalman approach is a new approach that is expected to give better BER performance in low and high data rate atmospheric optical links and to simplify the necessary processing in the receiver. The central issue addressed in this chapter is whether Kalman Filtering is helpful in dealing with atmospheric turbulence, and this chapter shows that it is within the bounds of statistical approximations.
The phenomenon of acousto-optic (A-O) diffraction, first studied extensively in the late 1920s and 1930s, is used in many areas of signal processing, although this behavior is complex, and despite extensive generalized analyses and applications, comprehension of the phenomenon in its entirety is still incomplete. A-O diffraction refers to the interaction of light and sound waves, and it is used to controllably diffract light beams. The behavior of an A-O cell depends on several system parameters, and, in particular, the thickness of the crystal L and the wave numbers of both sound (K) and light (k). These quantities are summarized as a figure-of-merit by the Klein-Cook parameter (Q) which is used to characterize the regimes of A-O operation. For strict Bragg operation, which finds the most applications for these devices in practice, Chatterjee and Chen showed that Q should be larger than 8π. In this regime, under perfect Bragg-matching, there is only one diffracted order. If Q is much smaller than one, the mode of operation is called the Raman-Nath regime, which is characterized by multiple diffracted orders with intensities given by Bessel functions. Weak interaction theory is used in the analyses of AO diffraction, and this theory rests upon the assumption of uniform plane waves of sound and light. These assumptions, though not physically realistic, allow for tractable analyses and lead to observable results. The transfer function approach utilizes a plane wave angular spectrum of the field distribution (valid for small deviations from the exact Bragg angle), which allows the scattered fields to be represented by Fourier integrals in the angular domain. This makes it possible to apply the FFT algorithm to numerically generate the scattered fields of arbitrary inputs. Transfer function expressions for both Bragg orders are developed and may be readily applied in the Fourier transform domain. These expressions are convenient for modeling the effects of various parameters (such as phase shift and Q), as well as arbitrary input profiles.
A distributed sensing system utilizes multiple geographically separated sensors to observe the world. The sensor systems can process data and then transmit it, receive data and then process it, or some combination of the two. For this chapter, we will examine a distributed radio frequency (RF) sensor system set on mobile platforms. The main challenges of any distributed system are localization, synchronization, and processing capabilities. This paper presents a novel solution that utilized a free space optical (FSO) link for communication between nodes. The FSO link will aid in localization and synchronization while also providing a high-speed communications path between sensor units to allow maximum flexibility in processing optimization. An example of FSO communication link between unmanned air vehicles (UAVs) is given to show the viability of establishing multi-Gbit/s optical communication links in presence of atmospheric turbulence. Optical communication offers the advantages of sensor information exchanges at high data rates as well as secure communications needed for a number of tactical applications. Before we discuss about the distributed system, a brief introduction into signals, systems, and signal processing is provided.
Spread-spectrum techniques are widely used in radio communication and telecommunication. Any signal like acoustic, electrical, and electromagnetic signal produced with a specific bandwidth is spread in frequency hence results in a wider bandwidth. Spread spectrum techniques are deployed in telecommunication because of many significant advantages, e.g., to achieve secure communications, to detect the eavesdropping, to resist natural interference, to bound power flux density for satellite down links and resistance to noise and jamming. In spread spectrum technique, frequency hopping (FH) is used as a basic modulation technique by which any telecommunication signal can be transmitted on a wider bandwidth (radio bandwidth) as compared to frequency value of the original signal. Spread spectrum techniques deploy FH, direct sequence (DS), or mix of both methods so that it can be used for multiple access and reduces the interference to other receivers to get the overall privacy. At the receiver side, the received signals are correlated to extract the original information being sent. The two main motivation behind spread spectrum are: to create anti-jamming for unauthenticated person and to provide low probability of interception. The classical data transmission can also be achieved by using quantum communication protocols like dense coding, quantum key distribution (QKD), and quantum teleportation. Most of these protocols use quantum states of light to transmit the information through optical fibers. The problem of channel access become apparent when number of users increases. In classical communication networks, when many users want to use the same channel at the same time, many multiple access methods are used. On the other side, quantum communication networks also deploy frequency and wavelength division multiple access techniques, hence each user make use of same channel at different frequencies and different time instance. In addition to this, many multiple access techniques employ orbital angular momentum of single photons and coherent states. Quantum spread spectrum multiple access scheme described here for optical fibers could be used for free-space data transmission. In this scheme, multiple users send their photons via same optical fiber, sharing their path, time window, and frequency band. A number of techniques have been proposed previously for data transmission for quantum optical communication which provides heavy losses while combining and separating the transmitted data from different users. The described scheme is developed to follow classical spread spectrum methods, and also follows add-drop architecture with simple extraction and combination points.
