# Institute for Systems Research Technical Reports

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This archive contains a collection of reports generated by the faculty and students of the Institute for Systems Research (ISR), a permanent, interdisciplinary research unit in the A. James Clark School of Engineering at the University of Maryland. ISR-based projects are conducted through partnerships with industry and government, bringing together faculty and students from multiple academic departments and colleges across the university.

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Item A 2D Nondestructive Inspection Method to Detect a Through Crack by, Electrostatic Boundary Measurements(1996) Berenstein, Carlos A.; Chang, Der-Chen; Wang, Emei; ISRA detailed procedure is proposed to find the position and length of a linear crack originating at the boundary of a 2-d object by means of electrostatic boundary measurements.Item 3D Wavelet-Based Video Codec with Human Perceptual Model(1999) Gu, Junfeng; Baras, John S.; ISR; CSHCNThis thesis explores the use of a human perceptual model in video compression, channel coding, error concealment and subjective image quality measurement.The perceptual distortion model just-noticeable-distortion (JND) is investigated. A video encoding/decoding scheme based on 3D wavelet decomposition and the human perceptual model is implemented. It provides a prior compression quality control which is distinct from the conventional video coding system. JND is applied in quantizer design to improve the subjective quality ofcompressed video.

The 3D wavelet decomposition helps to remove spatial and temporal redundancy and provides scalability of video quality. In order to conceal the errors that may occur under bad wireless channel conditions, a slicing method and a joint source channel coding scenario that combines RCPC with CRC and uses the distortion information toallocate convolutional coding rates are proposed. A new subjective quality index based on JND is proposed and used to evaluate the overall performance at different signal to noise ratios (SNR) and at different compression ratios.

Due to the wide use of arithmetic coding (AC) in data compression, we consider it as a readily available unit in the video codec system for broadcasting. A new scheme for conditional access (CA) sub-system is designed based on the cryptographic property of arithmetic coding. Itsperformance is analyzed along with its application in a multi-resolution video compression system. This scheme simplifies the conditional access sub-system and provides satisfactory system reliability.

Item Absolute Stability Theory, Theory, and State-Space Verification of Frequency-Domain Conditions: Connections and Implications for Computation(1997) Chou, Y.S.; Tits, A.L.; Balakrishnan, V.; ISRThe main contribution of the paper is to show the equivalence between the following two approaches for obtaining sufficient conditions for the robust stability of systems with structured uncertainties: (i) apply the classical absolute stability theory with multipliers; (ii) use the modern theory, specifically, the upper bound obtained by Fan, Tits and Doyle [IEEE TAC, Vol. 36, 25-38]. In particular, the relationship between the stability multipliers used in absolute stability theory and the scaling matrices used in the cited reference is explicitly characterized. The development hinges on the derivation of certain properties of a parameterized family of complex LMIs (linear matrix inequalities), a result of independent interest. The derivation also suggests a general computational framework for checking the feasibility of a broad class of frequency- dependent conditions, and in particular, yields a sequence of computable ﲭixed- -norm upper bounds , defined with guaranteed convergence from above to the supremum over frequency of the aforementioned upper bound.Item An Accurate and Efficient Approach to Statistical Simulation for Large-Scale Analog Circuits(1992) Chao, C-Y.; Milor, Linda; ISRA systematic approach to statistical simulation for large scale analog circuits is presented. The statistical model takes into account mismatch between devices due to variations in the process and noise, as well as interdie variations. The number of statistical parameters is O (n), where n is the number of devices. Additionally, because large-scale analog circuits are very computationally intensive to simulate, a two level approach is used, combining a behavioral model relating block performances to circuit performances and regression models relating a set of primary statistical variables to block performances. The efficiency of this approach to statistical simulation is demonstrated with the examples of an A/D converter.Item Accurate Segmentation and Estimation of Parametric Motion Fields for Object-based Video Coding using Mean Field Theory(1997) Haridasan, Radhakrishan; Baras, John S.; ISR; CSHCNWe formulate the problem of decomposing a scene into its constituent objects as one of partitioning the current frame into objects comprising it. The motion parameter is modeled as a nonrandom but unknown quantity and the problem is posed as one of Maximum Likelihood (ML) estimation. The MRF potentials which characterize the underlying segmentation field are defined in a way that the spatio-temporal segmentation is constrained by the static image segmentation of the current frame. To compute the motion parameter vector and the segmentation simultaneously we use the Expectation Maximization (EM) algorithm. The E-step of the EM algorithm, which computes the conditional expectation of the segmentation field, now reflects interdependencies more accurately because of neighborhood interactions. We take recourse to Mean Field theory to compute the expected value of the conditional MRF. Robust M-estimation methods are used in the M- step. To allow for motions of large magnitudes image frames are represented at various scales and the EM procedure is embedded in a hierarchical coarse-to-fine framework. Our formulation results in a highly parallel algorithm that computes robust and accurate segmentations as well as motion vectors for use in low bit rate video coding.*This report has been submitted as a paper to the SPIE conference on Visual Communications and Image Processing - VCIP98 to be held in San Jose, California on Jan 24- 30, 1998.*Item The Acoustic Features of Speech Phonemes in a Model of Auditory Processing: Vowels and Unvoiced Fricatives.(1987) Shamma, S.; ISRThe acoustic features of three types of stimuli (a harmonic series, naturally spoken vowels, and unvoiced fricatives) are analyzed based on the response patterns they evoke in a model of auditory processing. The model consists of a peripheral cochlear stage, followed by two central neural networks. At the peripheral stage, the asymmetrical nature of the cochlear filters, combined with the preservation of the fine temporal structure of their outputs, provide for robust and level-tolerant spatiotemporal representation of the speech signals. At the subsequent central stages, the cochlear patterns are processed by two layers of lateral inhibitory networks (LIN) to extract the perceptually important parameters of the stimuli. For the harmonic series, an in-phase and an out-of-phase version (one harmonic inverted) are used to illustrate the role of the spatiotemporal cues in encoding the spectral and temporal features of the stimuli. With the more complex vowel sounds, the primary acoustic features encoded by the LIN outputs are the few largest harmonic components of the stimuli, i.e., those closest to the formant frequencics. The output patterns computed for different (male and female) speakers display moderate variability, especially in the locations of the output peaks. However, the results also suggest that the relative levels of the LIN peaks (or the weight distribution of the patterns) is a more stable and characteristic feature of the different vowel groups. The results for the unvoiced fricatives indicate that the most invariant and distinctive acoustic feature the auditory model extracts, is the location of the high frequency edge of each stimulus spectrum.Item An Active Set Method for Solving Linearly Constrained Nonsmooth Optimization Problems.(1987) Panier, E.R.; ISRAn algorithm for solving linearly constrained optimization problems is proposed. The search direction is computed by a bundle principle and the constraints are treated through an active set strategy. Difficulties that arise when the objective function is nonsmooth require a clever choice of a constraint to relax. A certain nondegeneracy assumption is necessary to obtain convergence.Item Active Stabilization of Rotating Stall: A Bifurcation-Theoretic Approach(1992) Liaw, Der-Cherng; Abed, Eyad H.; ISRActive control of the onset of stall instabilities in axial flow compressors is pursued using bifurcation analysis of a dynamical model proposed by Moore and Greitzer (1986). This model consists of three ordinary differential equations with state variables being the mass flow rate, pressure rise, and the amplitude of the first harmonic mode of the asymmetric component of the flow. The model is found to exhibit a stationary (pitchfork) bifurcation at the inception of stall, resulting in hysteresis. Using throttle opening as a control, analysis of the linearized system at stall shows that the critical mode (zero eigenvalue) is unaffected by linear feedback. Hence, nonlinear tools must be used to achieve stabilization. A quadratic feedback control law using the measurement of asymmetric dynamics is proposed which stabilizes the bifurcation and eliminates the undesirable hysteretic behavior.Item Active Vibration Damping by Parametric Control(1994) Shahidi, R.; Shayman, M.; ISRIn recent years, smart composites have been introduced in the market that open up new possibilities for controlling or sensing flexible structures. Internal properties of these composite materials which are either embedded into the structure or bonded to the surface of the structure can be manipulated, through application of magnetic field (in magneto-strictive material), voltage (in piezoelectric material), or heat (in Shape Memory Alloys).This dissertation is primarily concerned with those composites with variable stiffness or modulus of elasticity. The objective is to control the stiffness of the structure to achieve stability. A lumped parameter model of a non-shearable and inextensible beam is derived as an approximation to the continuum model with attached composites. Due to the ability to manipulate the modulus of elasticity of the composite, the spring constants of the finite dimensional model of the beam are identified as the control variables. We design nonlinear feedback control laws to damp vibrations in the resultant simple Hamiltonian control system while satisfying the input constraints. Then, the analysis is specialized to a class of bilinear system which is the state linearized version of the original system. Optimality of the proposed controller is studied and under special conditions a discontinuous control law is proposed which achieves a faster dissipation of the energy than the continuous one for some simulated cases.

Item The Acts Experiments program at the Center for Satellite and Hybrid Communication Networks(1997) Friedman, Daniel E.; Gupta, Sonjai K.; Zhang, C.; Ephremides, Anthony; ISR; CSHCNThis paper describes experiments conducted over ACTS and the associated T1~VSAT terminal. The experiments were motivated by the commercial potential of low-cost receive-only satellite terminals that can operate in a hybrid network environment, and by the desire to demonstrate frame relay technology over satellite networks. The first experiment tested highly adaptive methods of satellite bandwidth allocation in an integrated voice- data service environment. The second involved comparison of FEC and ARQ methods of error control for satellite communication with emphasis on the advantage that a hybrid architecture provides, especially in the case of multicasts. Finally, the third experiment demonstrated hybrid access to databases through the use of Mosaic and compared the performance of internetworking protocols for interconnecting LANs via satellite. A custom unit termed Frame Relay Access Switch (FRACS) was developed by COMSAT Laboratories for these experiments; the preparation and conduct of these experiments involved a total of twenty people from the University of Maryland, the University of Colorado, and COMSAT Laboratories, from late 1992 through 1995.Item Adapting the Spreadsheet to Engineering Problems.(1985) Palmer, M. E., III; Pecht, M. G.; Horan, John V.; ISRThe popularity of financial spreadsheet packages takes advantage of many advances in microcomputer technology, especially growth in the processor's memory-address space. They have extended abilities to represent data graphically and some have programming languages included. Using operations common to numerical analysis, spreadsheets can also be used to solve engineering problems.Item Adapting Treemaps to Stock Portfolio Visualization(1992) Jungmeister, Walter-Alexander; Turo, D.; ISRTreemap visualization techniques are extended and applied to stock market portfolios via a prototype application. Designed to facilitate financial decision-making, the prototype provides an overview of large amounts of hierarchical financial data and allows users to alter aspects of the visual display dynamically. Treemap concepts are illustrated via examples which address common portfolio management needs.Item Adaptive Array Systems Using QR-Based RLS and CRLS Techniques with Systolic Array Architectures(1991) Tang, C.F.T.; Tretter, S.A.; ISRIn this dissertation the basic techniques for designing more sophisticated adaptive array systems are first developed. Then several systolic architectures based on numerically stable and computationally efficient algorithms are proposed for adaptive array systems. Compared to the existing architectures proposed elsewhere in the literature, our new systolic architectures are more efficient structures for real-time signal processing applications and VLSI hardware implementation. The reasons are (1) the proposed systolic architectures are based on numerically stable and computationally efficient systolic algorithms, (2) there is no bottleneck in the whole architecture since QR decomposition by the square root free fast Givens method is used, (3) the whole architecture has a fully pipelined design since backward substitution is avoided, (4) it is a single fully pipelined open-loop system without any feedback arrangement, and (5) the systolic architectures function recursively to update the result for each new snapshot. Therefore, the new VLSI systolic architectures proposed in this dissertation using QR-recursive least squares (QR-RLS) and QR-constrained recursive least squares (QR-CRLS) techniques archieve minimal memory and maximal parallelism for real-time signal processing applications and VLSI hardware implementation.Item Adaptive Blind Multi-Channel Equalization for Multiple Signals Separation(1995) Li, Ye; Liu, K.J. Ray; ISRThis paper investigates adaptive blind equalization for multiple- input and multiple-output (MIMO) channels and its application to blind separation of multiple signals received by antenna arrays in communication systems. The performance analysis is presented for the CMA equalizer used in MIMO channels. Our analysis results indicate that a double infinite-length MIMO-CMA equalizer can recover one of the input signals, remove the intersymbol interference (ISI), and suppress the rest signals. In particular, for the MIMO FIR channels satisfying certain conditions, the MIMO-CMA FIR equalizer is able to remove the ISI and co-channel interference regardless of the initial setting of the blind equalizer. To recover all input signals simultaneously, a novel MIMO channel blind equalization algorithm is developed in this paper. The global convergence of the new algorithm for MIMO channels is proved. Hence, the new blind equalization algorithm for MIMO channels can be applied to separate and equalize the signals received by antenna arrays in communication systems. Finally, computer simulations are presented to confirm our analysis and illustrate the performance of the new algorithm.Item Adaptive Block Transofmr Coding of Speech Based on LPC Vector Quantization.(1989) Hussain, Yunus; Farvardin, Nariman; ISRIn this paper we describe an adaptive block transform speech coding system based on vector quantization of LPC parameters. In order to account for the power fluctuations, the speech signal is normalized to have a unit-energy prediction residual The temporal variations in the short-term spectrum, on the other hand, are taken into accour by vector quantizing the UC parameters associated with the vector of speech samples and transmitting the codeword index. For each block based on the codevector associated with the input vector, an optimum bit assignment map is used to quantize the transform coefficients. We consider two types of zero memory quantizers for encoding the transform coefficients, namely the Llyod-Max quantizer and the entropy-coded quantizer. The performance of these schemes is compared with other adaptive transform coding schemes. We show by means of simulations that the system based on entropy-coded quantizer design leads to very high performance and in most cases as much as 5 dB performance improvement in terms of segmental signal-to-noise ratio is observed over the adaptive block transform coding scheme of Noll and Zelinski [1]. The effects of the bit-rate and the size of the codebook on the performance of the systems are also studied in detail.Item Adaptive Channel Allocation for OFDM-Based Smart Antenna Systems with Limited Transceiver Resources(2002) Koutsopoulos, Iordanis; Tassiulas, Leandros; Tassiulas, Leandros; ISR; CSHCNSmart antennas constitute perhaps the most promising means of increasing capacity in wireless systems by allowing intra-cell channel reuse by several users. The employment of smart antennas at the physical layer raises significant issues in medium access control (MAC) layer. In this paper, we study the impact of smart antennas on MAC layer channel allocation in the presence of limited transceiver resources, where a transceiver is a communication unit that is used to set up a distinct beam. The problem is addressed in the context of Orthogonal Frequency Division Multiplexing (OFDM), which is the predominantly proposed signaling scheme for wireless broadband access. Since a beam can only serve users in different subcarriers, the problems of subcarrier and transceiver assignment are coupled. We propose heuristic algorithms to allocate channels to users, adjust beamforming vectors and assign users and channels in beams, with the objective to increase system throughput and provide QoS to users in the form of minimum rate guarantees. Our criteria for resource assignment and beam formation are based on spatial separability properties of users, beam vector cross-correlations and induced interference to the system. This unified cross-layer approach is shown to yield significant throughput benefits.Item Adaptive Control of Nonlinear Systems via Approximate Linearization(1993) Ghanadan, Reza; Blankenehip, Gilmer L.; ISRWe present a direct adaptive tracking control scheme for nonlinear systems that do not have a well defined (vector) relative degree and hence are not feedback linearizable. This techniques uses feedback and coordinate changes to transform a nonlinear system with parameter uncertainty into an approximate input-output linearized one. Our result is also applicable to slightly non-minimum phase nonlinear systems with unknown parameters. We prove that the presented adaptive design scheme results in an asymptotically stable closed loop system and show that the controller can achieve adaptive tracking of reasonable trajectories with bounds on the tracking error. We also present a state regulation scheme based on state approximate linearization. We demonstrate the adaptive approximate tracking results using a simplified model of an aircraft which is slightly non-minimum phase. The usefulness of our approach is also illustrated on a "benchmark" example that is not feedback linearizable.Item Adaptive Control of Nonlinear Systems with Applications to Flight Control Systems and Suspension Dynamics(1993) Ghanadan, Reza; Blankenship, G.L.; ISRIn this dissertation, we employ recent theoretical advances in differential geometric formulation of nonlinear control theory and adaptive control to develop a practical adaptive nonlinear control strategy.We first present a new scheme for tracking and decoupling of multi-input/multi-output nonlinear systems with parametric uncertainty in their dynamics. We obtain an adaptive right-inverse that can be used as a decoupling prefilter for the original system and to generate the input necessary such that the outputs track a desired path. The procedures are systematic and have been implemented into a computer code. an integrated symbolic-numerical software system, written a Mathematica and C, has been developed that includes capabilities for automatic generation of model equations, for design of nonlinear tracking, regulation, stabilization, and adaptive control laws, and for generation of simulation codes (in C) for performance evaluations. This system is then used to design a nonlinear adaptive control algorithm for active suspensions for vehicles with the objective to effectively isolate the sprung body dynamics from the road disturbances. We also consider the design of a magnetic levitation control system.

For systems that do not satisfy the restrictive regularity assumptions of the current adaptive nonlinear control methodologies, commonly based on exact feedback linearization technique, we develop a technique of adaptive approximate tracking and regulation. This technique achieves reasonable stable tracking performance under parameter uncertainty in nonlinear dynamics for a large class of nonlinear systems with guaranteed bounds on the tracking error and parameter estimates. While the controller structure is designed using the approximate system, the adaptive loop is constructed around the true system in order to avoid any parameter drift typically caused by dynamic uncertainty in the system. Furthermore, for adaptive regulation, our scheme removes the linear parameter dependence assumption on the location of the unknown parameters. It also replaces the involutivity condition for exact feedback linearization with an order n involutivity assumption for approximate feedback linearization. For nonlinear systems that are linearly controllable, we give a simple systematic design procedure using a dynamic state feedback that achieves adaptive quadratic linearization.

We then investigate the use of this technique in the design of flight control systems using a simplified planar VTOL aircraft model. While due to the non-minimum phase property of the VTOL system the previous results in adaptive nonlinear control theory are not applicable, a comparison between the performance of our adaptive controller to the non-adaptive case reveals that the adaptive controller performs about 90% better in signal tracking.

Item Adaptive Data Broadcast in Hybrid Networks(1997) Stathatos, K.; Roussopoulos, N.; Baras, John S.; ISR; CSHCNHybrid networks combine multiple communication modes and are fast, emerging as the most viable solution for the ever increasing demand for bandwidth and data services. Taking advantage of this new technology, we are proposing a hybrid scheme which effectively combines broadcast for massive data dissemination and unicast for individual data delivery. The goal is to build highly scalable systems with small response time. In this paper, we describe a technique that continuously adapts the broadcast content to match the hot-spot of the workload. We show that the hot-spot can be accurately obtained by, monitoring the ``broadcast misses'' observed through direct requests.This is a major departure from all other broadcast optimization schemes which are handicapped by their total reliance on complete knowledge of both ``hits'' and ``misses''. We also show that the proposed adaptive scheme performs effectively even under very dynamic and rapidly changing workloads. Extensive simulation results demonstrate both the scalability and versatility of, the technique. Another basic result obtained in this paper is that the overall, system's throughput depends only on the size of the hot-spot and not on the volume of the workload. This has far reaching implications for very large scale and high volume wide area information systems.

Item Adaptive Data Broadcasting Using Air-Cache(1996) Stathatos, K.; Roussopoulos, N.; Baras, John S.; ISR; CSHCNIn the Data AirWaves Project at University of Maryland, we are integrating Direct Broadcast Satellite (DBS) systems with terrestrial networks to provide a hybrid and effective communication substrate lying between data resources and remote/mobile user applications. Smooth integration of these two media balances the need for rapid data dissemination to very large numbers of clients and on-demand interactive data services. This paper describes the air-cache, a method for effective data broadcasting and an algorithm which rapidly adapts the content of the cache based on the ﲭisses which result in explicit (on- demand) data requests. Simulation results show that the hypothesis of adapting based only on the misses performs quite reasonably and has very little deviation from a system that has complete information - both hits and misses.