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Visible Light Communications

LED lamps have recently become the technology of choice for lighting in many environments, from indoor office spaces to outdoor vehicular use. These LED lamps can be modulated at relatively high speeds, opening up the possibility to use them to provide simultaneously illumination and data transmission, so-called visible light communications (VLC). One of the most compelling applications for VLC is in large unobstructed indoor spaces where many users desire concurrent high-speed connectivity, such as conference centers and classrooms; Wi-Fi fails here because of its limited spectrum re-use. Other promising applications include indoor geo-location, vehicle-to-anything (V2X) communications, indoor positioning, and transmission inside airplane cabins. In our research we consider modulation, signal processing, user tracking, and resource allocation for this new optical communications modality

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Advanced Modulation and Coding for Optical Communication Systems

Optical channels offer a tremendous opportunity for high-throughput data transfer. Yet with this opportunity come some fundamental challenges. Novel modulation and coding schemes are needed to access and exploit the optical bandwidth resources, and to survive the deleterious effects of the optical medium. In our work we develop higher-order modulation techniques suitable to optical (visible, IR, and UV) systems. We explore the potential benefits of using error control coding on the system performance, with particular emphasis on LDPC codes. We also consider the use of multiple transceivers (single technology and hybrid RF/optical) to establish more robust and higher performance MIMO communication systems.

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Cross-Layer Design and Optimization of Optical Networks

The dramatic increase in throughput demands on transport systems has propelled the development of all-optical networks. These networks can provide tremendous capacity when they are designed with their own limitations in mind, such as coarse wavelength granularity and physical impairments. In this research we consider the holistic design of optical networks that include the interdependence of three network layers: the traffic grooming layer, the lightpath management layer, and the physical fiber layer.

The network is first viewed from the top-down, where sub-wavelength circuit requests arrive with specific quality of service requirements. Current traffic grooming approaches are altered to incorporate their dependence on the lightpath management and physical layer constraints. The system is then examined from the bottom-up, so that the quality of transmission and efficiency of resource utilization can be optimized as the higher layer protocols evolve. Total capacity is measured from an information theoretic view point and system optimization uses ideas from game theory.

The results of the research are practical algorithms for improved capacity and survivability of future optical networks as well as providing a quantitative proof of their superiority. The enhancement of network capability will help satisfy our society’s ever-increasing need for information. It encourages the development of applications that require significant bandwidth. It also stimulates cross-fertilization of ideas from the two fields of networking and communications. When fully tested, the algorithms and software will be made publicly available via this web-site.

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Ultraviolet Communications

The objective of the research is to improve the distance-throughput product of ultraviolet communication systems by several orders of magnitude so that they may be used as inexpensive short-distance non-directed links, such as for voice and data. The approach taken combines a mix of communication-theoretic methods (modulation, coding, multi-beam transmission) and improved UV technologies (fine-tuned optics, powerful electro-optic devices) to obtain this gain. The research culminates in the development of an experimental testbed demonstrating and validating the concepts.

The novel contribution of this work is the blending of advanced communication techniques with clever optics to improve the link quality and make it suitable for a variety of applications. Previous research efforts have been one-dimensional, focusing on the devices, the physics, or the modulation, and exclusively for military applications. Commercial applications require significantly higher capacity and reliability, and multiple simultaneous users. This research provides a systematic plan for accomplishing this goal, starting with a mathematical description of the channel and ending with a prototype system.

A substantial improvement of the throughput and reliability of ultraviolet communication systems can lead to increased economic and scientific interest in this relatively immature technology. One can envision an explosion of applications currently unserved by inexpensive technology, such as the last-mile broadband connectivity in urban areas, densely-packed wireless sensor networks, and military applications.

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FSO and Hybrid RF/FSO Networks and Communications

Broadband wireless communication has many advantages over wired systems, such as ease in deployment, mobility, and lower installation cost. Channel diversity techniques relying on multiple parallel links have been used to improve the throughput and reliability of wireless channels. We are interested in systems consisting of independent parallel channels, possibly through the use of different technologies, creating a heterogeneous system. Each channel in these systems can have a different symbol rate and modulation scheme. In this work we propose an adaptive transmission technique to maximize the throughput of these noninterfering parallel channel systems. Our work is motivated by the potential advantages of hybrid free space optical (FSO)/radio frequency (RF) systems.

Due to the considerable demand for wireless throughput, radio frequency resources have become cluttered and thus expensive. Communicating over the optical domain, the so-called FSO channel, with its nearly boundless frequency spectrum, has been proposed as a viable alternative for high-speed “lastmile” connectivity. Weather conditions and scintillation can severely affect the FSO channel, and it is therefore wise to pair it with a parallel lower rate but reliable RF system, forming what is currently referred to as a FSO/RF hybrid communication system. Conventionally these systems only use the RF channel as a backup when the FSO channel is weak, having to switch between the two channels frequently depending on variations in the channel conditions. Though simple, this non-adaptive technique results in an ineffiCient use of resources, especially when significant fading occurs. Instead, we propose to adapt the symbol rates and modulations in a jointly-encoded FSO/RF hybrid system, where both the FSO and RF subsystems are simultaneously active.

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Fiber-optic Channel Characterization

This research pursues the characterization of multiuser optical communication systems to increase the performance, distance, and capacity of existing and future systems. The focus is on the physical layer - i.e., the actual single fiber links that connect the nodes of large. The intense demand for more data, voice, and video in and out of every home and office has led to multiuser systems that are typically broadband and high power, sometimes leading to severe linear and nonlinear degradation in the signal due to properties inherent to the fiber. Mastering these physical limitations can translate into a hundred to thousand fold increase in capacity of existing infrastructure, with a potential savings of billions of dollars.

The goal of this research project is to identify, characterize, and combat signal degradation in fibers, and to use this information to design the highest capacity multiuser systems. While the physical properties of fibers have been broadly measured and defined, the proper design of fiber systems to optimize signal properties for communications has not been fully considered. A set of tools has been developed that provides a method of obtaining an analytical closed-form solution of a nonlinear differential equation describing the propagation of optical signals through silica fiber. The result is a Volterra model for the fiber which can be used to both quantify the signal degradation due to the fiber and to design algorithms and devices to combat this effect. The model is used to analyze the performance of optical TDMA, CDMA, and WDMA networks including realistic distortions such as intermodulation products, intersymbol interference, and multiple user interference.

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Signal Processing for Optical Communication systems

The dramatic increase in throughput demands from backbone transport data networks has propelled the development of all-optical wavelength-division-multiplexed (WDM) networks. As the capacity demands on these systems increase, the physical layer degradation becomes so severe that sophisticated signal processing techniques are necessary to maintain the quality of service. Channel nonlinearity differentiates multichannel fiber-optic systems from conventional wireline and wireless systems. Consequently, novel signal processing and communication theoretic approaches are required to design and analyze the channel; this research project addresses this need. As a consequence, WDM system designs are improved and the data throughput available to society though these networks is substantially increased.

This research develops a discrete time-wavelength nonlinear model for the WDM system and uses this model to design powerful signal processing techniques. Up to now, the WDM channel has been considered as a set of parallel channels, ignoring the cross-channel effects or modeling them simply as noise. Motivated by techniques that have been so successful in wireless communications, such as multiuser detection, MIMO processing, multichannel precoding, etc., this research develops algorithms to apply to the WDM fiber channel that can produce substantial capacity gains. A two-dimensional discrete-time polynomial model for a WDM system is formulated to account for intra-channel and inter-channel linear and nonlinear effects. Multichannel processing algorithms across time and wavelength for interference mitigation are designed and evaluated. Since nonlinear interference limits the performance of networks, constrained coding to diminish this interference is used to trade capacity for performance. In all-optical networks, crosstalk emanating from other lightpaths can limit performance. Employing idle lightpaths judiciously provides multiuser coding and path diversity (redundancy and memory) to the entire network.

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