Browsing by Author "Wang, H."
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Item Coherent Signal Processing Using Arrays of Arbitrary Geometry(1994) Wang, H.; Liu, K.J. Ray; ISRThe existing spatial smoothing (SS) technique, although it is effective in decorrelating coherent signals, can only be applied to uniformly spaced linear arrays which are very sensitive to the directions-of-arrival (DOA's) and can be used to estimate arimuth angles only. To significantly improve the robustness of DOA estimation and of beamforming and to estimate both arimuth and elevation angles, we developed techniques for applying SS to arrays of arbitrary geometry. We found that an array must have an orientational invariance structure with an ambiguity free center array for applying SS. We also study the cause of ambiguities in a multiple signal environment and find the necessary and sufficient conditions for an array manifold to be ambiguity free. If an array is also central symmetric, the forward/backward spatial smoothing can be used to improve the resolution. Finally, we expand the application of our technique not only to MUSIC and adaptive beamforming algorithms but also to ESPRIT algorithm. All the predicted results are verified by simulations.Item Control of Bifurcations and Routes to Chaos in Dynamical Systems(1993) Wang, H.; Abed, E.H.; ISRThis dissertation addresses issues in the control of nonlinear instabilities in high performance engineering systems. Specifically, we consider nonlinear control systems near the limits of their operating envelope. These are highly nonlinear situations occurring in the high-performance operation of a wide variety of systems. Such systems tend to exhibit nonlinear stabilities in terms of a jump to a new low-performance operating point, oscillatory behavior, chaotic behavior or system collapse in the absence of appropriate control action. Such situations necessitate the study of controlling nonlinear phenomena such as bifurcations and chaos.A new approach to the control of chaotic dynamical systems is introduced, namely control of routes to chaos. The theme is to design feedback control laws which ensure a sufficient degree of stability for a primary bifurcation in the routes to chaos. A thermal convection loop is used as a vehicle to illustrate the idea. Moreover, as the period doubling cascade is one of the most famous routes to chaos, the stabilization of period doubling bifurcations for general n- dimensional discrete-time nonlinear systems is investigated. The technique presented here affords considerable flexibility in terms of achievable behavior of the nonlinear system over a range of parameter values.
One contribution is the modeling, analysis and control of voltage collapse in electric power systems. a new mechanism of voltage collapse is suggested based on the framework of catastrophic bifurcations. Bifurcation control laws are designed to control these nonlinear phenomena at the inception of voltage collapse. The control laws are shown to result in improved performance of the system for a greater range of parameter values.
Another important application considered is the stall phenomenon in axial flow compressors. A combination of bifurcation analysis and nonlinear control is used to study the dynamics and active control of rotating stall in an axial flow compressor model. Both smooth and nonsmooth feedbacks are considered. Successful experimental verifications have been reported on these results.
Item The Design and the Testing of a 64-Processor Array(1988) Wang, H.; JaJa, J.; ISRArray architectures based on the VLSI technology allow the processing speed to increase by several orders of magnitude. While VLSI holds the promise of high parallelism by offering almost unlimited hardware at very low cost, there are several inherent constraints with respect to communication, design complexity, testability, etc. In this paper, we are concerned with design and testing of such an architecture. An array- processor chip consisting of 8x8 processing elements (PEs) each with 512 bits of memory was fully designed and fabricated using 2m CMOS technology. One of the novel features of this design is the capability to load data fast into all the PEs simultaneously. Extensive simulations were carried out on this design. This general purpose parallel processor chip was tested using the test workstation IMS-VS2000. An application board was also built by using the Macintosh II as the host controller.Item Dynamic Bifurcations in a Power System Model Exhibiting Voltage Collapse(1992) Abed, Eyad H.; Alexander, James C.; Wang, H.; Hamdan, Anan M. A.; Lee, Hsien-Chiarn; ISRDynamic bifurcations, including Hopf and period-doubling bifurcations, are found to occur in a power system dynamic model recently employed in voltage collapse studies. The occurrence of dynamic bifurcations is ascertained in a region of state and parameter space linked with the onset of voltage collapse. The work focuses on a power system model studied by Dobson and Chiang (1989). The presence of the dynamic bifurcations, and the resulting implications for dynamic behavior, necessitate a re- examination of the role of saddle node bifurcations in the voltage collapse phenomenon. The bifurcation analysis is performed using the reactive power demand at a load bus as the bifurcation parameter. Due to numerical ill-conditioning, a reduced-order model is employed in some of the computations. It is determined that the power system model under consideration exhibits two Hopf bifurcations in the vicinity of the saddle node bifurcation. Between the Hopf bifurcations, i.e., in the "Hopf window," period-doubling bifurcations are found to occur. Simulations are given to illustrate the various types of dynamic behaviors associated with voltage collapse for the model. In particular, it is seen that an oscillatory transient may play a role in the collapse.Item Signal Processing Techniques for Increasing Channel Capacity in Wireless Communications(1996) Wang, H.; Liu, K.J.R.; ISRAs the digital signal processing technology advances, the use of adaptive arrays to combat multipath fading and to reduce interference becomes increasingly valuable as a means of adding capacity to mobile communications. This dissertation address the major obstacles encountered in applying the two most applicable adaptive array algorithms to time division multiple access (TDMA) wireless communication systems.We first investigated the reference signal based adaptive diversity combining algorithm, which conventionally relies on feedback symbols in the absence of reference signals. Our computer simulation revealed that on a fast time varying fading channel, error propagation in the decision directed tracking mode severely degrades the performance. We developed a simultaneous diversity combining and decoding technique which incorporated QR decomposition-based recursive least-square parallel weights tracking and M-D decoding algorithms. In contrast to the conventional system where only one set of array weights is kept and updated, in our system, we update M sets of candidate weights. Thus we are able to make a more reliable symbol decision based on D symbols without compromising weights tracking speed. The M-D algorithm was first developed for the binary convolutional codes and then extended to Trellis-coded modulation. This technique significantly reduces error propagation. Simulation results showed that about 8 to 10dB improvement in the total interference suppression at low ISR and about 5dB improvement at high ISR can be achieved with a moderate increase in complexity.
In the next part of the dissertation, we proposed and studied the use of the constrained adaptive array algorithm for extracting signals from interferences at separable directions. This algorithm requires direction-of-arrival (DOA) information and does not need reference signals. However, most of the high resolution DOAs estimation methods are only effective for noncoherent signals, while in mobile radio channels, coherent signals are inevitable. We developed a general spatial smoothing (SS) technique and a forward backward spatial smoothing technique for two dimensional arrays to decorrelate coherent signals from arbitrary directions. We found and proved the necessary and sufficient conditions on an array configuration for applying SS. This array must have an orientational invariance structure with an ambiguity free center array, and the number of subarrays must be larger than or equal to the size of the largest group of coherent signals. We also studies the causes of ambiguities and found some ambiguity free array manifolds. We expanded the application of our SS to several high resolution DOA estimation and constrained adaptive beamforming algorithms. All the predicted results were verified by simulations. In the last part of the dissertation, we investigated the applications of adaptive array technique in DS/CDMA systems. We applied reference-signal- based simultaneous diversity combining and decoding to reduce fading and suppress interference caused by poor synchronization and power control.
Item Simultaneous Diversity Combining and Decoding for Fast Time- Varying Mobile Radio Channels(1996) Wang, H.; Liu, K.J. Ray; ISRIn slowly time-varying mobile radio channels, adaptive diversity combining can reduce multipath fading of desired signal and suppress interfering signals. However, for fast time-varying fading channels, there exist no effective techniques to achieve the same results. The continued use of decision directed adaptive array algorithms will cause error propagation. This paper presents a novel adaptive diversity combining technique with QRD- RLS based parallel weights tracking and a proposed M-D decoder. With moderate increase in complexity, this system significantly reduces error propagation in the decision directed array systems while maintaining the same tracking speed. Its effectiveness and much better performance then that of the conventional technique has been confirmed by computer simulation.Item Trie Hashing with Controlled Load .(1989) Litwin, Witold; Roussopoulos, N.; Levy, G.; Wang, H.; ISRTrie hashing is an access methods to primary key ordered dynamic files. The key address is computed through a trie. Key search needs usually one disk access since the trie may be in core and needs two accesses for very large files, when the trie has to be on the disk. We present a new variant of the method that allows to set up an arbitrary load factor for ordered insertions. In particular, one may create compact files, loaded up to 100%. We show that the capabilities of trie hashing make the method preferable to a B-tree by most of criteria that motivated the latter method supremacy over the database world.Item Two-Dimensional Spatial Smoothing for Multipath Coherent Signal Identification and Separation(1995) Wang, H.; Liu, K.J. Ray; ISRThe existing spatial smoothing (SS) technique, although it is effective in decorrelating coherent signals, is considered applicable only to uniformly spaced linear arrays which are very sensitive to the directions-of-arrival (DOAs) and can be used to estimate azimuth angles only. To significantly improve the robustness of DOA estimation and of beamforming and to estimate both azimuth and elevation angles in a 3D multipath mobile radio environment, we developed techniques for applying SS to arrays of nonlinear geometry. We found and proved the necessary and sufficient conditions on an array configuration for applying SS. This array must have an orientational invariance structure with an ambiguity free center array, and the number of subarrays must be larger than or equal to the size of the largest group of coherent signals. We also studied the cause of ambiguities in a multipath environment. We found the necessary and sufficient conditions for a three-sensor array manifold to be ambiguity free and identified several higher order ambiguity situations. If an array is also central symmetric, the forward/backward spatial smoothing can be used to improve the resolution. Finally, we expanded the application of our technique not only to MUSIC and adaptive beamforming algorithms but also to ESPRIT algorithms. All the predicted results are verified by simulations.