Information Theory and Reliable Communication

We describe a method to estimate the capacity limit of fiber-optic communication systems (or ?fiber channels?) based on information theory. This paper is divided into two parts. Part 1 reviews fundamental concepts of digital communications and information theory. We treat digitization and modulation followed by information theory for channels both without and with memory. We provide explicit relationships between the commonly used signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the performance of modulation constellations such as quadrature-amplitude modulation, combinations of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted specifically to the "fiber channel.'' We review the physical phenomena present in transmission over optical fiber networks, including sources of noise, the need for optical filtering in optically-routed networks, and, most critically, the presence of fiber Kerr nonlinearity. We describe various transmission scenarios and impairment mitigation techniques, and define a fiber channel deemed to be the most relevant for communication over optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber channel using ring constellations. Several scenarios are considered, including uniform and optimized ring constellations, different fiber dispersion maps, and varying transmission distances. We further present evidences that point to the physical origin of the fiber capacity limitations and provide a comparison of recent record experiments with our capacity limit estimation.

Capacity Limits of Optical Fiber Networks

Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber back-up, backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/monitoring, and quantum key distribution among others. 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 and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

/pdf/survey-on-free-space-optical-communication-a-communication-239v68hfyw.pdf

Survey on Free Space Optical Communication: A Communication Theory Perspective

The properties of a unique set of quantum states of the electromagnetic field are reviewed. These ‘squeezed states’ have less uncertainty in one quadrature than a coherent state. Proposed schemes for the generation and detection of squeezed states as well as potential applications are discussed.

Squeezed states of light

Squeezed states of the electromagnetic field have been generated by nondegenerate four-wave mixing due to Na atoms in an optical cavity. The optical noise in the cavity, comprised of primarily vacuum fluctuations and a small component of spontaneous emission from the pumped Na atoms, is amplified in one quadrature of the optical field and deamplified in the other quadrature. These quadrature components are measured with a balanced homodyne detector. The total noise level in the deamplified quadrature drops below the vacuum noise level.

https://link.aps.org/pdf/10.1103/PhysRevLett.55.2409

Observation of squeezed states generated by four-wave mixing in an optical cavity.

In our previous work [1], [2], we presented Optical Flow Switching (OFS) as a key enabler of scalable future optical networks. In the present work, we propose a practical scheduling algorithm and conduct an approximate throughput-delay analysis for OFS networks.

/pdf/performance-analysis-of-optical-flow-switching-f2swmh99o1.pdf

Performance Analysis of Optical Flow Switching

A data satellite network would have to support a broad spectrum of bursty unscheduled users with different delay constraints over time-varying atmospheric satellite channels. Since onboard resources are expensive and a broadband satellite will have many spotbeam-coverage cells within its service area, it is desirable to use agile scanning beam systems, adaptive to service requirements and channel conditions, and time-share a small number of active downlink beams. In addition, a congestion control mechanism of incoming traffic into the satellite service queues is required to provide a reasonable queueing delay within delay deadlines. In this paper, a jointly optimized scheme of multibeam allocation and congestion control based on incoming traffic and channel conditions over fading satellite downlinks with beam-sharing and delay constraints is developed. With the assumptions of quasistatic channel conditions and predictable incoming traffic rates (at least in the short term), an analytical solution for joint beam allocation and congestion control is considered. Numerical examples will be given to show that the joint scheme outperforms uniform allocation by serving, with fairness, more accepted traffic (e.g., a factor of 2 in the case of linearly distributed traffic across cells) and/or a smaller queueing delay.

Satellite multibeam allocation and congestion control with delay constraints

Heterodyne LASERCOM systems using GaAs lasers for ISL applications

Optical Flow Switching (OFS) is a promising architecture to provide end users with large transactions with cost-effective direct access to core network bandwidth. For very dynamic sessions that are bursty and only last a short time (~1S), the network management and control effort can be substantial, even unimplementable, if fast service of the order of one round trip time is needed. In this paper, we propose a fast scheduling algorithm that enables OFS to set up end-to-end connections for users with urgent large transactions with a delay of slightly more than one round-trip time. This fast setup of connections is achieved by probing independent paths between source and destination, with information about network regions periodically updated in the form of entropy. We use a modified Bellman-Ford algorithm to select the route with the least blocking probability. By grouping details of network states into an average entropy, we can greatly reduce the amount of network state information gathered and disseminated, and thus reduce the network management and control burden to a manageable amount; we can also avoid having to make detailed assumptions about the statistical model of the traffic.

/pdf/fast-scheduling-of-optical-flow-switching-27o041k5mo.pdf

Fast Scheduling of Optical Flow Switching

Clear air atmospheric turbulence causes significant fading for terrestrial-terrestrial and terrestrial-satellite free space optical communication systems. Typically extra link margin is used to assure link availability and reliability, however this extra margin is an inefficient and expensive use of resources. In this paper, we analyze data collected by an experimental system with a single laser transmitter located 250 meters from two coherent receivers. We first use the data to validate the use of a two-state continuous time Markov process to model outage statistics of the diversity system. In the two-state channel model, symbols received during an outage are assumed to be lost, and symbols received during a non-outage are assumed to be received correctly. This channel model can be used to analyze the performance of the transport layer. Next, we use statistical and spectral analysis techniques to create a linear prediction model for signal attenuation for both the single-receiver and diversity systems. The prediction model is an optimal estimator that predicts signal attenuation 1 ms into the future to 1.5 dB accuracy for the single-receiver cases and to 1 dB accuracy for the diversity case. The maximum amount of time the estimator can predict into the future with some confidence is about 5–10 ms. This channel prediction and adaptation can be used to greatly improve the efficiency of free-space optical communication systems in the atmosphere.

Experimental analysis of the time dynamics of coherent communication through turbulence: Markovianity and channel prediction

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