http://hdl.handle.net/1721.1/4059 Sun, 26 May 2013 08:12:02 GMT 2013-05-26T08:12:02Z http://dspace.mit.edu:80/bitstream/id/4141/rle.gif http://hdl.handle.net/1721.1/4059 http://hdl.handle.net/1721.1/68611 Absorption of Microwaves by Atmospheric Gases Rosenkranz, Philip The theory of absorption of microwave emission by common atmospheric gases is reviewed. Fri, 01 Jan 1993 00:00:00 GMT http://hdl.handle.net/1721.1/68611 1993-01-01T00:00:00Z http://hdl.handle.net/1721.1/62578 Optical Communication Through the Turbulent Atmosphere with Transmitter and Receiver Diversity, Wavefront Control, and Coherent Detection Puryear, Andrew Lee Free space optical communication through the atmosphere has the potential to provide secure, low-cost, rapidly deployable, dynamic, data transmission at very high rates. However, the deleterious e ects of turbulence can severely limit the utility of such a system, causing outages of up to 100 ms. For this thesis, we investigate an architecture that uses multiple transmitters and multiple coherent receivers to overcome these turbulence-induced outages. By controlling the amplitude and phase of the optical eld at each transmitter, based on turbulence state information fed back from the receiver, we show that the system performance is greatly increased by exploiting the instantaneous structure of the turbulence. This architecture provides a robust highcapacity free-space optical communication link over multiple spectral bands, from visible to infrared. We aim to answer questions germane to the design and implementation of the diversity optical communication architecture in a turbulent environment. We analyze several di erent optical eld spatial modulation techniques, each of which is based on a di erent assumption about the quality of turbulence state information at the transmitter. For example, we explore a diversity optical system with perfect turbulence state information at the transmitter and receiver that allocates transmit power into the spatial modes with the smallest propagation losses in order to decrease bit errors and mitigate turbulence-induced outages. Another example of a diversity optical system that we examine is a diversity optical system with only a subset of the turbulence state information: this system could allocate all power to the transmitter with the smallest attenuation. We characterize the system performance for the various spatial modulation techniques in terms of average bit error rate (BER), outage probability, and power gain due to diversity. We rst characterize the performance of these techniques in the idealized case, where the instantaneous channel state is perfectly known at both the receiver and transmitter. The time evolution of the atmosphere, as wind moves tur- 3 bules across the propagation path, can limit the ability to have perfect turbulence state knowledge at the transmitter and, thus can limit any improvement realized by optical eld spatial modulation techniques. The improvement is especially limited if the latency is large or the feedback rate is short compared to the time it takes for turbules to move across the link. As a result, we make successive generalizations, until we describe the optimal system design and communication techniques for sparse aperture systems for the most general realistic case, one with inhomogeneous turbulence and imperfect (delayed, noisy, and distorted) knowledge of the atmospheric state. Thesis Supervisor: Vincent W. S. Chan Title: Joan and Irwin M. Jacobs Professor of Electrical Engineering and Computer Science Mon, 02 May 2011 00:00:00 GMT http://hdl.handle.net/1721.1/62578 2011-05-02T00:00:00Z http://hdl.handle.net/1721.1/54732 Generating Pictures from Waves: Aspects of Image Formation Accardi, Anthony The research communities, technologies, and tools for image formation are diverse. On the one hand, computer vision and graphics researchers analyze incoherent light using coarse geometric approximations from optics. On the other hand, array signal processing and acoustics researchers analyze coherent sound waves using stochastic estimation theory and diffraction formulas from physics. The ability to inexpensively fabricate analog circuitry and digital logic for millimeter-wave radar and ultrasound creates opportunities in comparing diverse perspectives on image formation, and presents challenges in implementing imaging systems that scale in size. We present algorithms, architectures, and abstractions for image formation that relate the different communities, technologies, and tools. We address practical technical challenges in operating millimeter-wave radar and ultrasound systems in the presence of phase noise and scattering. We model a broad class of physical phenomena with isotropic point sources. We show that the optimal source location estimator for coherent waves reduces to processing an image produced by a conventional camera, provided the sources are wellseparated relative to the system resolution, and in the limit of small wavelength and globally incoherent light. We introduce quasi light fields to generalize the incoherent image formation process to coherent waves, offering resolution tradeoffs that surpass the traditional Fourier uncertainty principle by leveraging time-frequency distributions. We show that the number of sensors in a coherent imaging array defines a stable operating point relative to the phase noise. We introduce a digital phase tightening algorithm to reduce phase noise. We present a system identification framework for multiple-input multiple-output (MIMO) ultrasound imaging that generalizes existing approaches with time-varying filters. Our theoretical results enable the application of traditional techniques in incoherent imaging to coherent imaging, and vice versa. Our practical results suggest a methodology for designing millimeter-wave imaging systems. Our conclusions reinforce architectural principles governing transmitter and receiver design, the role of analog and digital circuity, and the tradeoff between data rate and data precision. Thesis Supervisor: Gregory W. Wornell Title: Professor of Electrical Engineering and Computer Science Fri, 07 May 2010 17:34:28 GMT http://hdl.handle.net/1721.1/54732 2010-05-07T17:34:28Z http://hdl.handle.net/1721.1/50654 Avoiding Interruptions - QoE Trade-offs in Block-coded Streaming Media Applications ParandehGheibi, Ali; Medard, Muriel; Shakkottai, Srinivas; Ozdaglar, Asu We take an analytical approach to study Quality of user Experience (QoE) for video streaming applications. First, we show that random linear network coding applied to blocks of video frames can significantly simplify the packet requests at the network layer and save resources by avoiding duplicate packet reception. Network coding allows us to model the receiver’s buffer as a queue with Poisson arrivals and deterministic departures. We consider the probability of interruption in video playback as well as the number of initially buffered packets (initial waiting time) as the QoE metrics. We characterize the optimal trade-off between these metrics by providing upper and lower bounds on the minimum initial buffer size, required to achieve certain level of interruption probability for different regimes of the system parameters. Our bounds are asymptotically tight as the file size goes to infinity. Tue, 12 Jan 2010 19:05:11 GMT http://hdl.handle.net/1721.1/50654 2010-01-12T19:05:11Z