Institute for Systems Research Technical Reports

The current plan in the modernization of the A/G communications is to migrate from analog voice only system to integrated digital voice and data system. The next generation satellite systems can be an alternative to the terrestrial A/G systems by their low propagation and transmission delays, global coverage, high capacity, and free flight suitable characteristics. In this paper, we give an overview of the current and the future ATC architectures, describe the systems and the communications issues in these systems, and develop a framework in which LEO/MEO next generation satellite systems can be integrated to the future ATC systems.

Stochastic optimization is a direct application of the above root finding problem if the SA is driven by a gradient estimate of steady state performance. We study the convergence properties of an SA driven by agradient estimator which observes an increasing number of samples from the Markov chain at each step of the SA's recursion. To show almost sure convergence to the optimizer, a framework of verifiable conditions is introduced which builds on the general SA conditions proposed for the root finding problem.

We also consider a difficulty sometimes encountered in applicationswhen selecting the set used in the projection operator of the SA algorithm.Suppose there exists a well-behaved positive recurrent region of the state process parameter space where the convergence conditions are satisfied; this being the ideal set to project on. Unfortunately, the boundaries of this projection set are not known a priori when implementing the SA. Therefore, we consider the convergence properties when the projection set is chosen to include regions outside the well-behaved region. Specifically, we consider an SA applied to an M/M/1 which adjusts the service rate parameter when the projection set includes parameters that cause the queue to be transient.

Finally, we consider an alternative SA where the recursion is driven by a sample average of observations. We develop conditions implying convergence for this algorithm which are based on a uniform large deviation upper bound and we present specialized conditions implyingthis property for finite state Markov chains.

This paper is organized as follows: Section I introduces the development of methods for wireless communication over multipath fading channel. Section II gives Turin's evaluation of the effects of multipath and fading on the performance of DS-CDMA systems. Section III gives Pursley's evaluation of this issue. Section IV explains the structure and performance of conventional RAKE receiver. Section V shows Sourour's work on Multicarrier DS-CDMA. Section VI is a proposal for a multicarrier DS-CDMA system and some numerical results I have gotten. Section VII is devoted to the conclusions and future work.

Recognizing the challenge to produce multiple prototypes, this thesisresearch aims to integrate three state-of-the-art technologies: 3D solid modeling, rapid prototyping, and vacuum casting. A system architecture combining hardwareand computer software is designed and implemented.

The system utilizes computergraphics to construct a 3D model of an object through visualitzion. A softwaresystem, Maestro, processes a CAD file, generates support structures, and creates slice data to build prototypes by a stereolithography process. Thebuilt part serves as a master pattern for creation of a silicone rubber mold in a vacuum environment. This vacuum environment creates a material flow ratethat ensures replicas with superior quality in regards to surface finish anddimensional accuracy. This mold is then used to cast multiple replicas ofthe master pattern.

The unique contribution of this research is the application of thedeveloped system to meet a specific need in medical research - an effort torestore sight in blind individuals by implanting electordes in the visualcortex. Six replcas of a monkey skull are produced for surgeions to practicesurgical procedures. Image data obtained from CT scans of a mondkey head are successfully used to contruct a 3D solid model to fabricate a batch of six functional prototypes. The superior quality of these replicas hasoffered a unique opportunity for exploratory surgery in efforts to restoresight.

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.

By means of computer search, several such families are designed so that each family member is at least as good as any comparable code in the literature. (ﲇood here is defined in terms of minimum time diversity and minimum squared product distance, the most important parameters for performance over Rayleigh fading channels.) A protocol to implement these rate-compatible trellis codes in a type-II hybrid ARQ format with only a low-rate feedback channel is described. Upper bounds on the resulting bit error rate are developed and the results are used to select the best adaptive code from several possibilities. Simulation results comparing the proposed scheme with fixed-rate codes of the same throughput show substantial coding gain. Finally, a protocol modification limiting the variability of the code rate over a frame is described; this modification eliminates the need for excessive buffering, with a very small effect on performance