Schwarz waveform relaxation with adaptive pipelining

Schwarz waveform relaxation (SWR) methods have been developed to solve a wide range of diffusion-dominated and reaction-dominated equations. The appeal of these methods stems primarily from their ability to use nonconforming space-time discretizations; SWR methods are consequently well-adapted for coupling models with highly varying spatial and time scales. The efficacy of SWR methods is questionable, however, since in each iteration, one propagates an error across the entire time interval. In this manuscript, we introduce an adaptive pipeline approach wherein one subdivides the computational domain into space-time blocks, and adaptively selects the waveform iterates which should be updated given a fixed number of computational workers. Our method is complementary to existing space and time parallel methods, and can be used to obtain additional speedup when the saturation point is reached for other types of parallelism. We analyze these waveform relaxation with adaptive pipelining (WRAP) methods to show convergence and the theoretical speedup that can be expected. Numerical experiments on solutions to the linear heat equation, the advection-diffusion equation, and a reaction-diffusion equation illustrate features and efficacy of WRAP methods for various transmission conditions

Discrete multitone modulation for short-range optical communications

As the need for higher information throughput increases, standard solutions such as copper lines and radio links seem to approach their limits. Therefore, optical solutions, after having conquered the long and medium-range networks, are nowadays also migrating into short-range data communication scenarios, offering the possibility of high capacity information transfer for both professional as well as consumer applications. The challenge is to offer cost-effective and robust optical solutions at relatively short (¿ 1 km) transmission distances, where traditional single-mode fiber for long-haul transmission systems are unsuitable. Solutions such as multimode glass fibers (MMF), plastic optical fibers (POF), using light-emitting diodes (LED) or low-cost vertical cavity surface emitting laser diodes (VCSEL), and optical wireless links (based on LEDs) are therefore being proposed and seem to be promising candidates. These solutions feature low costs, easy handling and installation, flexibility, and robustness, which are all very suitable characteristics for consumer needs. However, this comes at the expense of less bandwidth when compared to single-mode fiber systems. This thesis investigates the use of digital signal processing in order to overcome the bandwidth limitations in short-range optical communication systems, ensuring that such solutions are future-proof. In particular, discrete multitone (DMT) modulation is proposed and investigated in order to increase the capacity of such systems. Derived from the more general orthogonal frequency division multiplexing (OFDM), DMT is a baseband multicarrier modulation technique that is already widely employed in copper-based digital subscriber lines (DSL) systems such as asymmetrical DSL (ADSL) and very high data rate DSL (VDSL). By dividing a high-speed serial data stream into multiple parallel low-speed sub-streams and transmitting them simultaneously using different frequencies, DMT can be used to efficiently combat various signal impairments such as dispersion and narrowband interference. Due to the use of intensity-modulation and direct-detection (IM/DD) in low-cost optical systems, where only the intensity of light is modulated and not the phase, the application of DMT is different from standard electrical systems. Characteristics such as high crest factor, which is the ratio of the peak to root-mean-square amplitude value of the DMT signal, and clipping have different consequences and are studied in this thesis. After an introduction to the principles of DMT and rate-adaptive bit-loading, an analytical model of the optical IM/DD channel for short-range optical communications is presented. Making use of this model, the theoretical capacity of such a channel is derived for both a Gaussian and a first-order low-pass electrical-to-electrical channel response by means of the water-filling method. It is found that the crest factor of the modulation signal plays a dominant role in defining the capacity of the optical IM/DD channel. Furthermore, by including characteristics of DMT modulation such as clipping and quantization, it is shown that the calculated capacity values can be refined and optimum parameters for DMT transmission over an optical IM/DD channel exist. Following this, the optimum clipping values and number of subcarriers for maximizing DMT transmission performance over an optical IM/DD channel are investigated. It is shown that the optimum clipping value, which depends on various system parameters such as receiver noise power and modulation order, can be determined by using an analytical expression. In the case of the number of subcarriers, larger values generally lead to better performance when DMT with bit-loading is used. Additionally, various experiments to explore the system limits of DMT techniques have been performed and the results for POF, MMF, and optical wireless are presented. It is shown that record bit-rates of up to 47 Gbit/s can be achieved using DMT. Finally, an efficient way to implement DMT is presented, together with results regarding the implementation of a real-time DMT transmission system operating at 1.25 Gbit/s. System complexity issues of real-time hardware implementation are also discussed, showing that pipelining and parallelization are essential in high-speed designs, adding to the need of extra hardware resources. Moreover, it is verified that for DMT, the Fast Fourier Transform (FFT) operations require most hardware resources. After the presentation of some alternative modulation techniques such as pulse-amplitude-modulated DMT (PAM-DMT), which also were investigated by the author, this thesis ends with the conclusions and some recommendations for further research work

[Research activities in applied mathematics, fluid mechanics, and computer science]

This report summarizes research conducted at the Institute for Computer Applications in Science and Engineering in applied mathematics, fluid mechanics, and computer science during the period April 1, 1995 through September 30, 1995

ICASE

This report summarizes research conducted at the Institute for Computer Applications in Science and Engineering in the areas of (1) applied and numerical mathematics, including numerical analysis and algorithm development; (2) theoretical and computational research in fluid mechanics in selected areas of interest, including acoustics and combustion; (3) experimental research in transition and turbulence and aerodynamics involving Langley facilities and scientists; and (4) computer science

Towards Tactile Internet in Beyond 5G Era: Recent Advances, Current Issues and Future Directions

Tactile Internet (TI) is envisioned to create a paradigm shift from the content-oriented communications to steer/control-based communications by enabling real-time transmission of haptic information (i.e., touch, actuation, motion, vibration, surface texture) over Internet in addition to the conventional audiovisual and data traffics. This emerging TI technology, also considered as the next evolution phase of Internet of Things (IoT), is expected to create numerous opportunities for technology markets in a wide variety of applications ranging from teleoperation systems and Augmented/Virtual Reality (AR/VR) to automotive safety and eHealthcare towards addressing the complex problems of human society. However, the realization of TI over wireless media in the upcoming Fifth Generation (5G) and beyond networks creates various non-conventional communication challenges and stringent requirements in terms of ultra-low latency, ultra-high reliability, high data-rate connectivity, resource allocation, multiple access and quality-latency-rate tradeoff. To this end, this paper aims to provide a holistic view on wireless TI along with a thorough review of the existing state-of-the-art, to identify and analyze the involved technical issues, to highlight potential solutions and to propose future research directions. First, starting with the vision of TI and recent advances and a review of related survey/overview articles, we present a generalized framework for wireless TI in the Beyond 5G Era including a TI architecture, the main technical requirements, the key application areas and potential enabling technologies. Subsequently, we provide a comprehensive review of the existing TI works by broadly categorizing them into three main paradigms; namely, haptic communications, wireless AR/VR, and autonomous, intelligent and cooperative mobility systems. Next, potential enabling technologies across physical/Medium Access Control (MAC) and network layers are identified and discussed in detail. Also, security and privacy issues of TI applications are discussed along with some promising enablers. Finally, we present some open research challenges and recommend promising future research directions