In recent years, free space optical (FSO) communication has gained significant importance owing to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power, and low mass requirements. FSO communication uses optical carrier in the near infrared band to establish either terrestrial links within the Earth’s atmosphere or inter-satellite/deep space links or ground-to-satellite/satellite-to-ground links. It also finds its applications in remote sensing, radio astronomy, military, disaster recovery, last mile access, backhaul for wireless cellular networks, and many more. However, despite of great potential of FSO communication, its performance is limited by the adverse effects (viz., absorption, scattering, and turbulence) of the atmospheric channel. Out of these three effects, the atmospheric turbulence is a major challenge that may lead to serious degradation in the bit error rate performance of the system and make the communication link infeasible. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of this paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. The latter part of this paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers (link, network, or transport layer) to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high capacity and low cost backhaul solutions.

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Optical Communication in Space: Challenges and Mitigation Techniques

Underwater wireless information transfer is of great interest to the military, industry, and the scientific community, as it plays an important role in tactical surveillance, pollution monitoring, oil control and maintenance, offshore explorations, climate change monitoring, and oceanography research. In order to facilitate all these activities, there is an increase in the number of unmanned vehicles or devices deployed underwater, which require high bandwidth and high capacity for information transfer underwater. Although tremendous progress has been made in the field of acoustic communication underwater, however, it is limited by bandwidth. All this has led to the proliferation of underwater optical wireless communication (UOWC), as it provides higher data rates than the traditional acoustic communication systems with significantly lower power consumption and simpler computational complexities for short-range wireless links. UOWC has many potential applications ranging from deep oceans to coastal waters. However, the biggest challenge for underwater wireless communication originates from the fundamental characteristics of ocean or sea water; addressing these challenges requires a thorough understanding of complex physio-chemical biological systems. In this paper, the main focus is to understand the feasibility and the reliability of high data rate underwater optical links due to various propagation phenomena that impact the performance of the system. This paper provides an exhaustive overview of recent advances in UOWC. Channel characterization, modulation schemes, coding techniques, and various sources of noise which are specific to UOWC are discussed. This paper not only provides exhaustive research in underwater optical communication but also aims to provide the development of new ideas that would help in the growth of future underwater communication. A hybrid approach to an acousto-optic communication system is presented that complements the existing acoustic system, resulting in high data rates, low latency, and an energy-efficient system.

Underwater Optical Wireless Communication

In recent years, free space optical communication has gained significant importance owing to its unique features: large bandwidth, license-free spectrum, high data rate, easy and quick deployability, less power and low mass requirements. FSO communication uses the optical carrier in the near infrared band to establish either terrestrial links within the Earth's atmosphere or inter-satellite or deep space links or ground-to-satellite or satellite-to-ground links. However, despite the great potential of FSO communication, its performance is limited by the adverse effects viz., absorption, scattering, and turbulence of the atmospheric channel. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of the paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. The latter part of the paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers i.e., link, network or transport layer to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of the optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high-capacity and low-cost backhaul solutions.

/pdf/optical-communication-in-space-challenges-and-mitigation-4wng3q8rfl.pdf

Underwater wireless communications can be carried out through acoustic, radio frequency (RF), and optical waves. Compared to its bandwidth limited acoustic and RF counterparts, underwater optical wireless communications (UOWCs) can support higher data rates at low latency levels. However, severe aquatic channel conditions (e.g., absorption, scattering, turbulence, etc.) pose great challenges for UOWCs and significantly reduce the attainable communication ranges, which necessitates efficient networking and localization solutions. Therefore, we provide a comprehensive survey on the challenges, advances, and prospects of underwater optical wireless networks (UOWNs) from a layer by layer perspective which includes: 1) Potential network architectures; 2) Physical layer issues including propagation characteristics, channel modeling, and modulation techniques 3) Data link layer problems covering link configurations, link budgets, performance metrics, and multiple access schemes; 4) Network layer topics containing relaying techniques and potential routing algorithms; 5) Transport layer subjects such as connectivity, reliability, flow and congestion control; 6) Application layer goals and state-of-the-art UOWN applications, and 7) Localization and its impacts on UOWN layers. Finally, we outline the open research challenges and point out the future directions for underwater optical wireless communications, networking, and localization research.

Underwater Optical Wireless Communications, Networking, and Localization: A Survey

In this letter, we introduce a detection technique based on differential signaling scheme for outdoor free space optical communications. This method requires no channel state information (CSI) and does not suffer from the computational load compared with the conventional receivers, where adjusting dynamically the detection threshold level (either based on CSI knowledge, or by using pilots) leads to increased computational time and reduced link throughput. This letter shows that the performance of the proposed technique only depends on the correlation of propagating optical beams. Especially under highly correlated-channels condition the fluctuation of the detection threshold level in the receiver is significantly small. We also show experimentally that under weak turbulence regime the variance of detection threshold level reduces for the correlated channel.

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FSO Detection Using Differential Signaling in Outdoor Correlated-Channels Condition

12 In this paper, we present a study of dual header-pulse interval modulation (DH-PIM) scheme for optical wireless communications. System theory and code properties of DH-PIM are discussed and expressions for the power spectral density, slow and packet error rates and optical power requirements are presented. The problem of baseline wander is also studied. The performance of DH-PIM is compared with other modulation schemes such as on-off keying (OOK), pulse position modulation (PPM), differential pulse position modulation and digital pulse interval modulation. We show that, DH-PIM offers higher bit rate and has a built-in frame synchronization capability. For a simple threshold detector receiver, it offers improved error performance compared with OOK, but marginally inferior performance compared with PPM. The optimum system performance in terms of optical power and bandwidth requirements is achieved at bit resolution of 5.

Performance of dual header-pulse interval modulation (DH-PIM) for optical wireless communication systems

SUMMARY This paper assesses the performance of dual header pulse interval modulation (DH-PIM) over indoor optical wireless systems. DH-PIM being anisochronous scheme offers a built-in symbol synchronization capability. Theoretical and simulation results demonstrate that DH-PIM offers shorter symbol length, improved transmission rate and bandwidth requirement and a comparable power spectral density profile compared with digital pulse interval modulation (DPIM) and pulse position modulation (PPM) schemes. It is shown that DH-PIM2; with wider pulse duration is the preferred option when the available channel bandwidth is limited and higher optical power is tolerable. Whereas DH-PIM1; with narrower pulse width, exhibits comparable power requirements but a marginally higher bandwidth compared with DPIM, and is also more bandwidth efficient than PPM at the cost of increased power requirement. However, at higher bit resolutions, i.e. M57; DH-PIM1 is both bandwidth and power efficient compared with PPM. Error rate analysis show that DH-PIM offers improved packet error rate compared with on-off-keying (OOK) and DPIM, but marginally inferior as compared with PPM. The power requirement and penalty due to intersymbol interference for non-dispersive and dispersive channels is analysed and the results show that for given parameters, DH-PIM requires marginally higher optical power compared with PPM and DPIM, but it supports the same bit rate at much less bandwidth requirement. Copyright # 2005 John Wiley & Sons, Ltd.

Indoor optical wireless systems employing dual header pulse interval modulation (DH-PIM)

In this paper a new pulse time modulation technique known as Multilevel-Digital Pulse Interval Modulation (MDPIM) is introduced and its properties are presented. Expressions for the pulse train, bandwidth, transmission rate and capacity are given. The performance of MDPIM is compared with other pulse modulation techniques. Also system block diagram and signal simulation are presented.

Multilevel Digital Pulse Interval Modulation Scheme for Optical Wireless Communications

The paper presents analysis and simulation for the convolutional coded DH-PIM using 1⁄2 convolutional code with the constraint length of 3. Decoding is implemented using the Viterbi algorithm. The proposed scheme is simulated for a constraint length of 7 and the results are compared with a number of different modulation techniques. DH-PIM with convolutional coding requires about 4-5 dB lower SNR for a given slot error rate compared with the un-coded DH-PIM.

Performance of convolutional coded dual header pulse interval modulation in infrared links

In indoor infrared wireless environment the system performance is affected by the presence of interference from artificial light sources, which can be reduced by employing an electrical high pass filter (HPF). But filtering will result in an interference known as baseline wander. The severity of baseline wander highly depends on the type of modulation technique. This paper discusses the effect of artificial light sources on indoor infrared systems employing dual-header pulse interval modulation (DH-PIM). The paper examines the effect of changing the cut-on frequency of the HPF on the optical power requirement and penalty of diffuse and non-diffuse infrared wireless links for different bit rates. Results presented are compared with other modulation techniques. It is shown that DH-PIM is more susceptible to the effects of baseline wander than pulse position modulation (PPM) and digital pulse interval modulation (DPIM). This is because DH-PIM power spectral density is higher at DC and low frequency region. However, compared with on-and off keying (OOK), DH PIM offers improved performance.

Nawras Aldibbiat

Papers

Performance of dual header-pulse interval modulation (DH-PIM) for optical wireless communication systems

Indoor optical wireless systems employing dual header pulse interval modulation (DH-PIM)

Multilevel Digital Pulse Interval Modulation Scheme for Optical Wireless Communications

Performance of convolutional coded dual header pulse interval modulation in infrared links

Baseline wander effect on indoor wireless infrared links operated by dual header pulse interval modulation