A primary challenge in SDI systems is to transfer information reliably from sensors and within the battle management system. The volatile, hostile environment associated with an SDI system may include multiple-hop communications through channels with anomalous propagation conditions in the face of jamming and multiple-access interference. This paper addresses multiple-hop communications system performance in the face of such disturbances. A systems viewpoint is taken by postulating an SDI configuration involving multiple nodes, which may consist of sensors, weapons platforms, and battle management stations as well as hostile platforms such as jammers. A computer model is used to set up the geometry as well as to evaluate link ,performance parameters such as carrier-to-noise ratio (CNR), jamming-to-signal ratio (JSR), and bit error rate (BER). These can then be used to evaluate derived parameters such as packet error probabilities, packet delay, and throughput. Thus, conditions under which the system is potentially stressed can be considered in a direct manner, and alternative means to alleviate the sytem degradation can be considered.

Time-frequency hop pulse train signals used in applications such as coherent multi-user radar or asynchronous spread spectrum communications must be selected on the basis of both auto- and cross-ambiguity characteristics. Neither codes based upon Costas arrays, which have nearly ideal auto- but not so good cross-ambiguity properties, nor codes based on linear congruences, which have nearly ideal cross- but unattractive auto-ambiguity properties, are entirely satisfactory. As a compromise, a new class of time-frequency hop codes is developed, based upon an extension of the theory of quadratic congruences. An uniform upper bound is placed on the entire cross-ambiguity function surface, and bounds are placed on the position and amplitude of spurious peaks in the auto-ambiguity function. These bounds depend on time-bandwidth product and code length exclusively, which leads to a discussion of the trade-offs involved between these two parameters.

The objective of our research is to develop innovative techniques for routing packets in a satellite network using point-to-point laser communications. In this paper we summarize our findings on topology analysis, introduce a link assignment algorithm used to study the effects of link retargeting, and briefly present a 2-level hierarchal model which offers four node-disjoint paths between all source destination pairs and attempts to minimize network congestion while offering low propagation delays. Network survivability is a key consideration, and impulse changes to the network are assumed. Satellite losses, link losses, and traffic flow are expected in bursts.

Optical waves have been established as a powerful means for information transmission. Because of the inherent device non-linearities and dispersions of the transmission media, we are far short of achieving its full potentials. Nearly all high speed optical communications in use today have been in digital form. It is still difficult to transmit high speed digital and broadband video simultaneously. Therefore to make the most efficient and flexible use of the optical medium, or to achieve a very broad bandwidth or both, an alternative approach is thus necessary. We propose a novel modulation method which achieves the equivalent of the conventional FDM system but uses the optical wave as the main carrier. The information is modulated directly on the laser and the (sub carriers are generated by the interference of two coherent optical waves, or a coherent pair. We shall call this system DBM (Double-Beam-Modulation) which is equivalent to DSBM (Double-Side-Band-Modulation) with the carrier suppressed. The modulator can be realized in many different ways [1-3]. Currently we are investigating the modulator using a SWAOM (Standing-Wave Acousto-Optical-Modulator) and SWSAW (Standing-Wave-Surface-Acoustic-Wave) for carrier frequencies to 3 GHz. We are also studying the feasibility of using SWOMSW (Standing-Wave-Opto-Magneto-Static-Wave) [4] to extend the frequency to tens of GHz range. Frequency locking of multiple stripe lasers may soon be possible for frequencies to 100 GHz. The characteristics, advantages, and technologies related to the DBM's are discussed. The recent experimental progress is presented.

In this paper we examine the utility of holographic optics for their use as collection, pointing and filtering elements in a space-based optical communications system architecture. In the first section, we examine the possible applications and requirements for satellite lasercom pointing, acquisition and tracking subsystems. These requirements reflect the multifarious and dynamic aspects of a network architecture such as SDI. Next we examine some of the unique performance aspects of holographic elements and how they enhance network performance. We then analyze the element performance based on the features of the hologram. We evaluate the technologies and materials currently employed in the fabrication of holograms and discuss the capabilities and limitations of this technique to meet space-based requirements. We outline in the conclusions the potential enhancements to laser communications offered through the use of holograms, and recommend further work to extend their feasibility.

The networking problem in distributed fusion system is considered. At a given time instant, the decisions from some sensors may not be available at the fusion center due to the transmission delays. Assuming that the fusion center has to make a decision based on the data from the rest of the sensors, provided that at least one peripheral decision has been received, it is shown that the optimal decision rule that minimizes the probability of detection for fixed probability of false alarm at the fusion center is the Neyman-Pearson test at the fusion center and the sensors as well. Whenever the Lagrange multipliers method holds valid, the optimal set of thresholds is given via a set of nonlinear, coupled equations that depend on the decision policy and cannot be solved in general. A suboptimal, computational efficient algorithm is developed to solve for the sensor and fusion thresholds sequentially. Numerical results are provided to demonstrate the closeness of the solutions obtained by the suboptimal algorithm to the optimal solutions.

This paper discusses link-assignment and adaptive routing algorithms that have been developed for networks consisting of a large number of mobile nodes having directive links. We focus on the special case where data rates are large (tens of Mbits/sec) and the propagation delay between nodes may be large compared to packet transmission delay. In such circumstances, queue sizes at store-and-forward nodes can become extravagant if retransmissions are required because of lost or damaged packets. The algorithms employ techniques that provide a high probability of successful message delivery on the first transmission attempt in the unreliable network. The link-assignment algorithm builds and maintains a link topology that provides multiple node-disjoint paths between each source and destination. Decisions regarding topology changes are also based on link permanency and ability to carry the anticipated traffic load. The adaptive routing algorithm establishes routing tables for multiple independent paths that are maximally node-disjoint between all sources and destinations. Load splitting techniques are used to achieve better load balancing and also to provide a mechanism for rapid adaptation around failed links. All algorithms are distributed in the sense that each node makes independent decisions. Heuristic optimization techniques are used to reduce computation time to within affordable limits. The routing algorithm also responds well to transients in traffic loading.

Two-level schemes use two decision stages to ascertain coarse code acquisition of spread spectrum signals. The first stage employs passive correlators which are capable of real time search but with lim-ited reliability. The second stage employs active correlators which allow longer integration times and more reliable decisions but which cannot search in feal time. By using these stages in tandem the advantages of real time rapid search and reliable decisions can be obtained. We have developed several such schemes along with analytical models to characterize their performance in severe environments that include fading and noise jamming.

In this paper we give a solution to the problems of finding the best set of K completely unmerged paths through a trellis with Mi≥K states at depth i in the trellis, i = 0, 1, 2, ..., N. Here, "best set" means that the sum of the metrics of all K paths in the set is minimized and "completely unmerged" means that no two paths pass through a common state. The solution involves using the Viterbi algorithm on (an expanded trellis with {Mi/k} states at depth i. We then apply this result to optimally separate the tracks of K targets in a multi-target radar system with measurement errors and extraneous measurements.

In recent years, a number of techniques have been suggested that combine two or more repetitions of the same message to improve the efficiency and reliability of digital communications systems. This combining is performed so that the combined message can be decoded properly, even though the individual transmissions contain uncorrectable errors. Significant performance gains can be achieved when multiple repetitions of contaminated messages are combined. Although combining techniques previously have been analyzed for binary transmission schemes employing majority vote decision, calculation of the performance in the case of M-ary channels is more complex. This paper describes a methodology called plurality voting, and demonstrates the performance obtained when message combining is used on an M-ary channel with this decision rule.

A systematic approach is proposed to construct networks that can be controlled by destination tags. The networks can be designed so that they have a unique path between any of its inputs and any of its outputs and can also be designed so that multiple paths are provided. When multiple paths exist, the network is fault-tolerant in the sense that a fault in the path does not prevent communication through a redundant path. This approach is based on the fact that, for a unique-path network of size N with logN stages and 2x2 switches, a binary tree structure can be associated with each input switch. The tree is such that the root, nodes, edges and levels of the tree correspond to the input switch, switches connected to the input switch, links between these switches and stages of the network, respectively. Sets of tree structures can be devised and used as the underlying "skeletons" of fault-tolerant networks that are controllable by destination tags. There are several ways to organize binary trees and map them into networks in order to implement destination tag routing. This paper concentrates on a special case of the approach and experiments are conducted on constructing networks whose switches are 2x2, 3x3 or 4x4 switches (although nonuniform switches of various sizes are also possible.) Merits and advantages of these designs are discussed with respect to routing and rerouting schemes and fault tolerance. Several existing seemingly varied and independent designs of unique-path and fault-tolerant networks are found to be governed by this approach. In particular, Augmented Data Manipulator (ADM) networks and related designs are investigated and improved with respect to fault-tolerance and routing and rerouting schemes, and new designs with better fault-tolerance capabilities are suggested.

The limit of load balancing due to decreasingly available bandwidth is studied. In the computer simulation, the bandwidth of the communication link is translated into 1.) the transfer delay associated with relocating a job and 2.) the uncertainty of the remote queuing status, due to the combined effects of the system dynamics and the non-zero time required in probing the status of the remote stations. The computer simulation is based on a sender-initiated load balancing scheme and is tested for a number of decision thresholds. Among these, an adaptive threshold that is a function of the traffic pattern and the system capability, "balances" the load best; it may not always minimize the average delay, but it always keeps the service stations (or nodes, or processors) similarly loaded. Results are obtained for both stationary and bursty traffics, and for both fixed-length and exponential-length jobs. For all the loading ratios tested, the balancing scheme is capable of improving the system performance over that of the no-balancing cases, when the transfer delay is less than 10 ms. For greater transfer delays, the improvement may drop to negative values.

The development of a novel technique of forming optical quality surfaces in semiconductor materials is described. The technique exploits the precise sputter-etching afforded by a beam of 20 keV Ga+ ions focused to a 250 nm spot to micromachine features in the surfaces of semiconductor laser dice and wafers. Diode laser output mirrors of quality comparable to that of cleaved facets and two section coupled-cavity lasers which exhibit discrete and continuous tunability have been fabricated. Turning mirrors, angled at 45° to the epitaxial layers of the die to produce surface-emitting diode lasers, have also been micromachined with the focused ion beam (FIB). We have observed 330 mW of optical power from a surface-emitting 10 element phase-locked array. Development of FIB micromachining techniques needed to form the structures required for constructing a two-dimensionally coherent array of diode lasers are also described.

Stripe domain garnet light deflectors are being developed to have the increased deflection efficiency needed to perform the optical beam steering function in a laser crosslink, intersatellite communication system. Two approaches are being taken to develop a deflector with a goal of greater then 50% efficiency at 0.8 and 1.06 μm wavelengths. These are development of garnet materials with increased magneto-optic performance and operation of the garnet element in a Fabry-Perot etalon. Modeling of etalon behavior and anticipated garnet material properties indicates efficiencies up to 90% are possible at 1.06 μm and up to 60% at 0.8 μm. At our present stage of material development and without an etalon, efficiencies of 11% at 0.8 μm wavelength and 14% at 1.06 μm have been achieved.