Transmitter Pair Research Articles

Genetic algorithms for designing multihop lightwave network topologies

Multihop lightwave networks are a means of utilizing the large bandwidth of optical fibers. In these networks, each node has a fixed number of transmitters and receivers connected to a common optical medium. A multihop topology is implemented logically by assigning different wavelengths to pairs of transmitters and receivers. By using tunable lasers or receivers, it is possible to modify the topology dynamically when node failures occur or traffic loads change. The reconfigurability of logical multihop lightwave networks requires that optimal topologies and flow assignments be found. In this article, optimization of these logical topologies by genetic algorithms is investigated. The genetic algorithm takes topologies as individuals of its population, and tries to find optimal ones by mating, mutating and eliminating them. During the evolution of solutions, minimum hop routing with flow deviation is used to assign flows, and evaluate the fitness of topologies. The algorithm is tested with different sets of parameters and types of traffic matrices and the solutions are compared against histograms of random samples from the solution space. These tests show that the solutions found by the genetic algorithm are comparable with and in some cases better than those found by existing heuristic algorithms.

Capacity of a mobile multiple-antenna communication link in Rayleigh flat fading

We analyze a mobile wireless link comprising M transmitter and N receiver antennas operating in a Rayleigh flat-fading environment. The propagation coefficients between pairs of transmitter and receiver antennas are statistically independent and unknown; they remain constant for a coherence interval of T symbol periods, after which they change to new independent values which they maintain for another T symbol periods, and so on. Computing the link capacity, associated with channel coding over multiple fading intervals, requires an optimization over the joint density of T/spl middot/M complex transmitted signals. We prove that there is no point in making the number of transmitter antennas greater than the length of the coherence interval: the capacity for M>T is equal to the capacity for M=T. Capacity is achieved when the T/spl times/M transmitted signal matrix is equal to the product of two statistically independent matrices: a T/spl times/T isotropically distributed unitary matrix times a certain T/spl times/M random matrix that is diagonal, real, and nonnegative. This result enables us to determine capacity for many interesting cases. We conclude that, for a fixed number of antennas, as the length of the coherence interval increases, the capacity approaches the capacity obtained as if the receiver knew the propagation coefficients.

Global workpiece positioning system (GWPS) Part 2: Development of an ultrasonics-based system

Abstract Datum establishment uncertainty is a major source of error affecting the geometric accuracy of machined features. In part one of this two-part paper, a novel concept, the Global Workpiece Positioning System (GWPS), was presented for datum establishment in a multiple-setup machining operation, where a laser triangulation based prototype was implemented and its concept feasibility validated. However, the approach based on a laser triangulation probe has several limitations. These include sensitivity to the optical quality and orientation of the artifacts, a short stand-off, and the necessity to operate in a scanning mode while held in the spindle of the machine tool. Ideally, it would be preferred if the sensors were fixed and outside of the workspace of the machine tool. This would eliminate the time required for a probe scanning cycle and potentially minimize the effect of machine tool positioning uncertainty on the locating accuracy of the GWPS. A solution to this problem lies in the development of a GWPS that is analogous to the current satellite-based Global Positioning System (GPS). In such a system, a set of receiver and transmitter pairs would operate in a manner similar to the satellite transmitters and ground-based receivers of the GPS, and a universal system to locate the workpiece in the work zone of the machine tool would be accomplished. To explore the potential of this approach, an ultrasonics-based GWPS was examined where, as an initial step, the laser triangulation probe was replaced with an ultrasonic receiver and a series of ultrasonic transmitters was affixed to the workpiece. This approach was found to permit a wider and longer detectable range of the artifacts (ultrasonic transmitters). Thus, the workpiece and artifacts can be deployed more freely. A 2D machining experiment demonstrates that the workpiece can be positioned by the ultrasonic GWPS without the use of physical contacts. An average positioning error of 0.185 mm (0.007 in.) was obtained in five test runs. Although the positioning accuracy is insufficient for machining applications, the ultrasonic GWPS may be applicable to less demanding situations where greater artifact sensor flexibility is needed. Furthermore, the use of time-of-flight ranging best embodies the general concept of GWPS. If advancements in electromagnetic GPS technology continue to occur at the current pace, it is anticipated that an electromagnetic time-of-flight GWPS will achieve the accuracy needed for machining applications in the future.

Multihop Lightwave Network Design

Multihop lightwave networks are proposed for exploiting the large bandwidth of optical fiber. In these networks, each node has a fixed number of transmitters and receivers connected to a common optical medium. A multihop topology is implemented logically by assigning different wavelengths to pairs of transmitters and receivers. The problem of designing a multihop lightwave network takes the form of selecting the connection diagram (wavelength assignment), and partitioning the flow to accomodate each element of the node-to-node traffic intensity matrix among the links created by the connectivity diagram. We devised a new algorithm for the design of multihop lightwave networks by using Vogel's approximation method and simulated annealing utilization on any link. We tested the performance of the new algorithm on various example problems and compared its results with those of the existing solution techniques.

Differential regulation of peptide and catecholamine characters in cultured sympathetic neurons

Mechanisms regulating peptidergic, noradrenergic and cholinergic development were compared in dissociated cell cultures of neonatal rat sympathetic ganglia. The majority of cultured neurons contained at least two neurotransmitters and many neurons contained three or more. These studies were undertaken to determine whether co-existing transmitters were co-ordinately regulated by the environment. Co-culture of sympathetic neurons with ganglion non-neuronal cells increased substance P and choline acetyl- transferase activity but decreased somatostatin and tyrosine hydroxylase activity. Conversely, elimination of non-neuronal cells virtually abolished neuronal expression of substance P and choline acetyltransferase and increased somatostatin and tyrosine hydroxylase. Consequently, under these conditions, somatostatin and tyrosine hydroxylase were similarly regulated, whereas substance P was associated with choline acetyltransferase. By contrast, stimulation of adenylate cyclase or treatment with membrane-permeable adenosine 3′,5′-phosphate analogs increased tyrosine hydroxylase and decreased choline acetyltransferase, but had no effect on substance P or somatostatin levels. Moreover, potassium- or veratridine-induced membrane depolarization increased tyrosine hydroxylase but decreased substance P, somatostatin and norepinephrine levels. However, inhibition of neurotransmitter release with magnesium or calcium-free medium prevented the decrease in norepinephrine levels but not the decrease in substance P and somatostatin. Consequently, the effects of membrane depolorization on peptide levels cannot be ascribed to release and subsequent depletion of substance P and somatostatin and must result from decreased net synthesis (synthesis minus catabolism) of the transmitters. Nerve growth-factor treatment also differentially regulated transmitter metabolism; nerve growth factor increased protein-specific activities of tyrosine hydroxylase and choline acetyltransferase but did not increase the protein-specific content of substance P and somatostatin. Quantitative transmitter expression was also influenced by neuron density; increasing density elevated substance P and choline acetyltransferase activity but decreased somatostatin and tyrosine hydroxylase activity per neuron. Finally, culture of sympathetic neurons in a defined (serum-free) medium also altered some but not all traits, decreasing substance P. somatostatin and choline acetyltransferase without any change in tyrosine hydroxylase. Our observations suggest that there is no fixed relationship between any pair of the four phenotypes examined and that each phenotype appears to be independently regulated in cultured sympathetic neurons. Nevertheless, under many circumstances noradrenergic and somatostatin development were co-ordinately regulated, while substance P and choline acetyltransferase development were similar, possibly indicating some functional relationship between these pairs of transmitters.

RESULTS OF A DIFFERENTIAL OMEGA TEST IN THE MACKENZIE RIVER DELTA

In 1969 the Gravity Division of the Observatories Branch, Department of Energy, Mines and Resources, Ottawa, in cooperation with the Research, Development, and Programming Division of the Telecommunications and Electronics Branch of the Department of Transport, undertook an evaluation of the worldwide Omega Navigation System in the Arctic for the Polar Continental Shelf Project. Omega is a long‐range, very low frequency radio navigation system. It consists of 4 (Norway, Trinidad, Hawaii, and New York) of the planned 8 transmitters and provides navigational coverage for the North Atlantic area, North America, and parts of South America (Scull, 1969 and Dick‐Peddie, 1968). These stations presently transmit two frequencies (10.2 kHz and 13.6 kHz) in a sequential pattern synchronized in phase by means of atomic clocks (Tracor, 1968 and Findlay, 1968). The Omega receiver measures the difference of phase of received signals from a pair of transmitters. This measurement defines one line of position (LOP) in a family of hyperbolic lines. Lines of positions defined by the zero phase difference are the lines of position that are numbered on an Omega chart, and the distance between two such lines is known as a lane. A position is determined by the intersection of two lines of position within known lanes.

Some Further Investigations of Ionospheric Cross-Modulation

The work in this paper forms the continuation and development of the experiments described in a paper by Ratcliffe and Shaw published in 1948. Measurements of the phase and amplitude of the modulation transferred to an unmodulated wave by a modulated disturbing wave in the ionosphere are used to provide information about that part of the ionosphere in which the cross-modulation occurs. By measuring almost simultaneously the height of reflection of the wanted wave and the characteristics of the transferred modulation, a value of 1.4 × 106 per second for the collision frequency of electrons with molecules at a height of 92 kilometres has been deduced. This measurement has been made possible by the use of a very sensitive automatic apparatus which is briefly described. Special attention has been paid to the `gyro-resonance effect' in cross-modulation first pointed out in a theoretical paper by Bailey in 1937. The experiments appear to show that the predicted enhancement of the cross-modulation when the disturbing station radiates on the local gyro-magnetic frequency does not exist. A modification of the original theory is given which is of great use in the calculation of the cross-modulation to be expected for a given pair of transmitters. This modified theory is used in the discussion of the gyro-resonance effect and of the observed decrease of the cross-modulation at dawn. The agreement between the calculated and observed magnitudes of cross-modulation is stressed in view of the fact that the calculations involve a knowledge of the absorption of the wanted wave incident obliquely upon the ionosphere.