This paper describes spectrum occupancy measurements performed in Chicago, IL in November 2005 and proposes long-term studies in multiple locations. The Chicago project consisted of deploying a high dynamic range spectrum measurement system, a data collection and processing system and conducting spectrum occupancy measurements in all bands between 30 MHz and 3,000 MHz (see Figure 1). These measurements were taken over a two-day period and are added to an existing body of data compiled in other cities and regions including Washington, D.C., and New York City. While these studies are critical in determining what bands have low utilization, longer-term studies are crucial in developing new spectrum access technologies such as cognitive radio algorithms related to Dynamic Spectrum Sharing (DSS). The observed low spectrum occupancy in a business center like Chicago indicates that a DSS radio system could access a huge amount of "prime" spectrum. The unoccupied, large contiguous spectrum blocks show that DSS radios can use conventional contiguous waveforms and that high temporal agility may not be required to significantly expand the data capacity of an accessible section of spectrum. From both short-term and long-term spectrum occupancy studies, candidate bands for spectrum sharing can be readily identified along with unique signal characteristics within these bands. The most important use of the data will be to support senior U.S. (and non-U.S.) government officials in taking action to enhance the use of the currently under utilized RF spectrum resources and to make the R&D investments and policy changes needed to support the development of dynamic spectrum sharing radios. Figure 1: Antenna Array with Chicago Loop IntroductionAs the popularity of new wireless applications and devices continues to grow, the demand for spectral capacity is becoming insatiable. Over the past few years such applications as data centric smart phones, Bluetooth headsets, keyboards and mice, broadband WiFi internet connections, satellite radios, and GPS navigation systems (to name only a few) have moved from obscure gadgets for the wealthy to standard devices and capabilities used by the masses. Demand for spectral capacity is fundamentally experiencing a "Quadruple Whammy" composed of the dramatic increase in the number of discrete applications, the rapid rise in the deployment of these applications, the growth in the amount of time each application is utilized, and the radical growth in the data rates used when these applications are operating. These trends are placing enormous demands on the finite spectral capacity. At the same time, it is observed in this, and related studies, that most of the spectrum, in most of the places, most of the time is completely unused. These observations scream for an enhancement to the current block auction system to fundamentally include the time domain in the allocation and command and control regulatory systems to facilitate new dynamic spectrum sharing (DSS) technologies to be developed and deployed ...
Analysis of 20‐second resolution magnetometer data from an array of temporary stations operated around Søndre Strømfjord, Greenland during the summer of 1986 shows the signatures of localized ionospheric traveling convection vortices. An example of an isolated event of this kind observed near 08 local time is presented in detail. This event consists of a twin vortex pattern of convection consistent with the presence of two field‐aligned current filaments separated by about 600 km in the east‐west direction. This system of currents is observed to move westward (tailward) past the array of stations at about 4 km/sec. The event is associated with relative quiet time ionospheric convection and occurs during an interval of northward IMF. It is, however, associated with a large fluctuation in both the Z and Y components of the IMF and with a large sudden decrease in the solar wind number density. The propagation of the system is inconsistent with existing models of FTE current systems, but nevertheless appears to be related to a readjustment of the magnetopause boundary to a sudden change in the solar wind dynamic pressure and/or to a change in reconnection brought about by a sudden reorientation of the IMF.
The magnetic field on the ground due to a small (_• 200 km scale size) localized field-aligned current system interacting with the ionosphere is calculated in terms of an integral over the ionospheric distribution of field-aligned current. Two different candidate current systems for flux transfer events (FTEs) are examined. The first system has current flowing down the center of a cylindrical flux tube with a return current uniformly distributed along the outside edge. The second system has upward current on one half of the perimeter of a cylindrical flux tube with downward current on the opposite half. The peak magnetic field on the ground is found to differ by a factor of 2 between the two systems, and the magnetic perturbations are in different directions depending on the observer's position. Assuming the current system moves in a constant linear poleward direction, we predict the ground magnetic field versus time which would be measured by a ground observatory. Using FTE detection statistics at the magnetopause, we estimate the detection rate of FTEs at a ground station under the dayside convection reversal to be between 12 and 60 min per sighting depending on which FTE current system is considered.
SUMMARYSpectrum measurement studies have shown that substantial portions of the allocated wireless spectrum are highly underutilized. Frequency-agile radios (FARs) have the potential to make opportunistic use of such spectrum holes without causing harmful interference to users of the allocated spectrum. Toward this goal, we develop a framework for modeling the interference caused by FARs employing spectrum access mechanisms based on the simple ListenBefore-Talk (LBT) scheme. Two variations of LBT are considered: individual LBT, whereby the FARs act independently of each other; and collaborative LBT, whereby the FARs communicate with each other in order to more accurately identify the spectrum holes. Our analysis of the LBT scheme reveals the fundamental interdependencies among key system design metrics and provides a basis for analyzing more complex spectrum access methods. In particular, the analysis of LBT provides a lower bound on the capacity gain achievable by FARs employing spectrum-sharing schemes. Our numerical results show that the individual LBT scheme can provide substantial capacity gains, while even more gain can be achieved using the collaborative LBT schemes. Our analysis suggests that much greater gains should be achievable via spectrum access schemes that incorporate location information and/or more sophisticated group behaviors.
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