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.

We consider resilient design for stressed optical networks. We compare existing commercial architectures with alternative architectures in both the low and high stress regimes.

Multiple failure restoration in mesh optical networks

Coherent optical communication systems using semiconductor lasers can provide improved performance over direct detection systems due to higher receiver sensitivities. However, excessive local oscillator intensity fluctuations of present state-of-the-art semiconductor lasers usually prevent conventional heterodyne receivers from operating at the quantum limit. The amount of degradation can be substantial (2-40 dB).1.2 For wide-bandwidth systems in particular intensity noise due to the re- laxation oscillation of the local oscillator laser can degrade receiver sensitivities at frequencies well below the relaxation resonance frequency.3 A dual-detector heterodyne receiver has been proposed for the purpose of local oscillator intensity noise cancellation.4 The concept has been demonstrated for small receiver bandwidths (<150 MHz).5 In this paper we describe techniques and results of noise cancellation over a 3-GHz bandwidth. Intensity noise at the relaxation oscillation resonance has been successfully suppressed by 20 dB. Analytical performance bounds based on measurable device and circuit parameters are found to give excellent predictions of experimental results

Cancellation of local oscillator intensity noise caused by the relaxation oscillation of GaAIAs lasers with a dual-detector heterodyne receiver

We propose an optical broadcast architecture for intra-data center communication to address the growth of traffic volume in data center networks. The bandwidth of current wavelength-division multiplexed (WDM) systems is limited by the low loss region of an optical fiber and the bandwidth of optical amplifiers. However, the bandwidth of an optical fiber greatly exceeds the bandwidth of optical amplifiers. We investigate the scalability and power requirements of an out of amplifier band optical broadcast architecture for intra-data center communication. We show that the broadcast architecture is limited by splitting loss and not fiber loss for short distance communication. We also analyze the delay performance of the broadcast architecture as a function of wavelength loading and the size of data flows. We show that the broadcast architecture requires fewer fiber lengths and optical switches than traditional switched network architectures.

Out of amplifier band optical broadcast intra-data center network architecture

Minimum shift keying for frequency modulation of semiconductor lasers

1TCP is a popular transport layer protocol. The performance of TCP is known to be unsatisfactory in wireless network, due to high bit error rates or drop outs. Employing error correction at the transport layer can alleviate the problem to some extent. We make three contributions in this paper. First, we optimize the efficiency of repetitive codes. In particular, we propose a good estimate on throughput and obtain a simple formula to estimate the optimal number of paths that maximizes efficiency. Second, we analyze the optimization of efficiency when using Reed-Solomon Erasure Code with TCP. We show that the improvement is dramatic for stable channels but not as significant for noisy channels. Third, we find that the bottleneck on the efficiency for multipath TCP transmission with coding is the protocol TCP. Therefore, to drastically increase efficiency in wireless network, we should use another protocol instead of TCP.

Performance analysis and optimization of multipath TCP

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