Sven G. Bilén

On a Truthful Mechanism for Expiring Spectrum Sharing in Cognitive Radio Networks

We study how truthfulness can be enforced as a dominant strategy when a number of secondary cognitive radios participate in an online expiring spectrum sharing auction, held by the primary to lease its spectrum holes. The types of secondary cognitive radios, announced to the primary, are composed of valuation and arrival-departure periods. We show how, by the suitable choice of channel allocation and pricing schemes, the collusion incentive among secondary cognitive radios can be reduced. The performance of the deployed dynamic auction is compared with that of the Vickrey-Clarke-Groves (VCG) offline auction as a benchmark.

Enhanced electrodynamic tether currents due to electron emission from a neutral gas discharge: Results from the TSS‐1R Mission

During the reflight of the first electrodynamic Tethered Satellite System (TSS-1R) mission, the unplanned separation of the tether at the Orbiter end resulted in the highest tether current during the mission. In the moments just prior to the tether separation with 19.7 km of tether deployed and a generated electromotive force (EMF) of 3482 V, currents reaching approximately 0.97 A were shunted through the tether to the Orbiter electrical ground, which was in contact with the ionosphere primarily through its main engine surfaces. This current level was nearly twice as large as observed during any nominal operating period. As the failure point of the tether entered into the ambient plasma, the current increased to 1.1 A and maintained this level even after the break for approximately 75 s. The principal surprise in these results was that the broken end of the tether, with only a few short strands of copper wire, could support higher currents than the much larger Orbiter conducting surface areas. Analysis of possible current enhancement mechanisms revealed that only a gas-enhanced electrical discharge, providing an electron emission source, was plausible. Ground plasma chamber tests confirmed this analysis. The TSS-1R results thus represent the highest electron current emission from a neutral plasma source yet demonstrated in a space plasma. This is of interest for current collection processes in general and plasma contactor development in particular.

Analysis of chamber simulations of long collecting probes in high-speed dense plasmas

Chamber tests of simulated electrodynamic tethers (EDTs) of different geometries operating in a dense high-speed plasma are described. The geometries tested and described here are cylindrical and flat-ribbon tape. By moving the probe samples relative to the plasma source it was possible to vary the density and therefore the effective width over a range of approximately 1 to 2 Debye lengths (/spl lambda//sub D/) for the cylinder sample and 6 to 19 /spl lambda//sub D/ for the tape samples. Several important conclusions can be drawn from the tests. 1) The current-voltage characteristics of the cylinder behave as predicted by orbital-motion-limited (OML) current collection theory in the saturation region. 2) The tape tether had comparable current levels to a theoretical equal area OML cylinder up to an effective width of at least /spl sim/11/spl lambda//sub D/ and possibly wider. 3) Orienting the tape samples parallel or perpendicular to the plasma flow yielded different current responses (perpendicular is larger) above a bias potential that is near the estimated energy of the incoming beam ions. The observed difference was generally more pronounced at larger effective widths (higher densities). 4) It was also necessary to be above this bias potential to have a V/sup 0.5/ current-voltage character appropriate for an ideal cylinder in the OML regime. It is concluded that wide ribbon-like tape tethers can be effective current collectors but that velocity effects will be a factor to consider, especially as relative width of the tape tether (with respect to /spl lambda//sub D/) grows.

Strategies to Achieve Truthful Spectrum Auctions for Cognitive Radio Networks Based on Mechanism Design

In this paper, we investigate the design of a truthful auction for the case when a primary spectrum owner is willing to lease its idle spectral resources in sequential time periods. The secondary cognitive radios participate in the spectrum sharing auction by declaring to the primary their types, which consist of their arrival and departure time instances and valuations. The adapted methodology aims at reducing the collusion incentive among secondary users through the proper choice of the pricing policy and replacing second-price policy, such as in Vickrey-Clarke-Grove (VCG) auctions, by the critical value auction. Furthermore, the proposed auction is dynamic and is performed on-line, in contrast to static off-line schemes such as VCG. Simulation results confirm the anti-cheating property of the proposed auction scheme.

Experimental investigation of electron collection to solid and slotted tape probes in a high-speed flowing plasma

This paper presents the analysis and comparison of measurements of electron current collection to round cylinder, solid tape, and slotted tape electrodynamic-tether samples in a mesosonic flowing plasma. A Hall thruster was used to simulate a flowing unmagnetized space plasma in a large 6-m /spl times/ 9-m vacuum chamber. Guarded tether samples were employed to mitigate end effects. Plasma parameters were determined based on the ion saturation and electron retardation regimes of a cylindrical Langmuir probes current characteristics. Solid tape samples with widths spanning from 4.9 to 41.9 Debye lengths, and slotted tapes with center-to-center line spacings spanning from 1.4 to 13.2 Debye lengths were tested. Several conclusions can be drawn from the analysis of the results: 1) the plasma flow leads to significant current enhancements over that predicted by the orbital-motion-limited theory; 2) the electron current collected per unit area on solid tapes decreases as the width of the tape is increased; 3) beyond a threshold bias close to the beam energy, solid and slotted tapes both collect more current when oriented transverse to the flow; 4) slotted tapes are more efficient electron collectors per unit area than solid tapes; and 5) our data suggests that the electron current collected on slotted tapes decreases with increasing line spacing until a possible minimum is attained, beyond which it is expected to start increasing again. The minimum was attained in the case of the samples oriented transverse to the flow, but not in the case of the samples aligned with the flow, for which the critical spacing is likely higher (due to an increased sheath interaction radius of each line caused by flow-induced sheath elongation).

Denial of Service Attacks in Cognitive Radio Networks through Channel Eviction Triggering

Security issues associated with successful operation of cognitive radio networks (CRNs) recently are gaining a lot of attention. In this paper we focus on a specific class of Denial of Service attack that is executed through Channel Eviction Triggering (CET), whereby the adversary nodes unduly invoke mechanisms inherent in CRN operation to protect the licensed users and thus disrupt secondary access to the otherwise idle licensed bands. Skewing the spectrum sensing decision of a CRN through sensing mis-reports is a manifestation of CET attacks. Whereas most studies in the literature focus on making the cooperative sensing more robust against such sensing misreports, we tackle the problem from an incentive perspective and develop strategies to minimize the utility difference of truthful and cheating cognitive radios so as to alleviate the incentive of mis-reporting. Our numerical results verify the effectiveness of the proposed CET defense scheme.

Resonance-probe measurements of plasma densities in electric-propulsion plumes

We are developing a resonance-probe plasma diagnostic that uses a microwave network analyzer for use in electric-propulsion research. To show the feasibility of our resonance-probe implementation, we have measured plasma densities in the plume of the 5-kW-class P5 Hall-effect thruster and compared them to measurements made with a Langmuir probe. Our preliminary work in this area indicates that the resonance-probe technique shows considerable promise. The resonance-probe technique should prove to be a useful tool to support electric-propulsion research since it able to provide high temporal-resolution electron density measurements. Nomenclature B 0 background magnetic-flux density, T c speed of light in a vacuum, 2.998 × 10 8 m/s k Boltzmanns constant, 1.38 × 10 −23 J/K l RP resonance-probe tip length, m m e electron mass, 9.109 × 10 −31 kg m i ion mass, kg n e electron plasma density, m −3 q charge magnitude, 1.602 × 10 −19 C T e electron temperature, K v p propagation (phase) velocity, m/s ε 0 free space permittivity, 8.85 × 10 −12 F/m λ wavelength of excitation frequency, m ω ce angular electron-cyclotron frequency, rad/s ω pe angular electron-plasma frequency, rad/s ω uh angular upper-hybrid frequency, rad/s

Software-defined radio: a new paradigm for integrated curriculum delivery

Software-defined radio is a rapidly developing field that is driving the development of and innovation in communications technology, and promises to significantly impact all communications sectors. Entities developing these SDR systems require a trained workforce that has been prepared with the mindset, knowledge, skills, and tools required to address both the system (breadth) and technical (depth) aspects of SDR systems. Developing SDRs necessarily involves a collection of disciplines including, but not limited to, electromagnetics, radio-frequency engineering, communications, digital signal processing, embedded systems, computer programming, and systems engineering. Whereas electrical engineering and computer science and engineering curricula at the university level may include courses in all of these areas, a students typical curriculum does not; nor does it usually involve the integration of all these topics. However, SDR can be employed as an integrative construct that facilitates systems thinking and cross-domain learning via peers. In this article, we present several significant educational efforts across six U.S. universities that have developed integrated curricula in SDR, most including a significant laboratory component.

JANUS: exploring the high redshift universe

Gamma-ray bursts (GRBs) provide extremely luminous background light sources that can be used to study the high redshift universe out to z ~ 12. Identification of high-z GRBs has been difficult to date because no good high-z indicators have been found in the prompt or afterglow emission of GRBs, so ground-based spectroscopic observations are required. JANUS is an Explorer mission that incorporates a GRB locator and a near-IR telescope with low resolution spectroscopic capability so that it can measure the redshifts of GRBs immediately after their discovery. It is expected to discover 50 GRBs with z > 5 as well as hundreds of high redshift quasars. JANUS will facilitate study of the reionization phase, star formation, and galaxy formation in the very early universe. Here we discuss the mission design and status.

Development and Chamber Testing of a Miniature Radio- Frequency Ion Thruster for Microspacecraft

*† ‡ Satellite attitude control, including spin rate, is an area of interest for the application of electric propulsion devices. One such device is the RF ion thruster, which can make fast changes in thrust level by changing the applied power level or acceleration potential. Application to micro- or nano-satellites requires the miniaturization of these devices. A Miniature RF-Ion Thruster (MRIT) currently is being designed and under going tests in a vacuum chamber at Penn State University. This article describes the design of this two-grid ion thruster using Xenon gas as propellant, the test set-up, as well as the initial test results for several different modes of operation. The MRIT is tested for power levels on the order of ten watts and uses an excitation frequency of 13.56 MHz (industrial unregulated frequency). With an input power of 15 W and an acceleration grid voltage of approximately 1 kV, the maximum exhaust velocity predicted for the MRIT is 38 km·s -1 , which gives a specific impulse of approximately 3800 s. The maximum calculated thrust level is about 0.6 mN. One possibility for in situ, on-orbit testing of the MRIT would be on board LionSat as a satellite spin control device. LionSat is the Penn State student nanosatellite participating in the Nanosat–III program competition.