Steven W. Ellingson

Cognitive Radio and Networking Research at Virginia Tech

More than a dozen Wireless @ Virginia Tech faculty are working to address the broad research agenda of cognitive radio and cognitive networks. Our core research team spans the protocol stack from radio and reconfigurable hardware to communications theory to the networking layer. Our work includes new analysis methods and the development of new software architectures and applications, in addition to work on the core concepts and architectures underlying cognitive radios and cognitive networks. This paper describes these contributions and points towards critical future work that remains to fulfill the promise of cognitive radio. We briefly describe the history of work on cognitive radios and networks at Virginia Tech and then discuss our contributions to the core cognitive processing underlying these systems, focusing on our cognitive engine. We also describe developments that support the cognitive engine and advances in radio technology that provide the flexibility desired in a cognitive radio node. We consider securing and verifying cognitive systems and examine the challenges of expanding the cognitive paradigm up the protocol stack to optimize end-to-end network performance. Lastly, we consider the analysis of cognitive systems using game theory and the application of cognitive techniques to problems in dynamic spectrum sharing and control of multiple-input multiple-output radios.

Spectral occupancy at VHF: implications for frequency-agile cognitive radios

Frequency-agile cognitive radio is a potential solu- tion to the problem of inefficient use of radio spectrum in the 30- 300 MHz (VHF) range. This is especially attractive if networks based on this technology can operate in spectrum left unused by existing users, as opposed to being allocated new spectrum through refarming. This paper presents a preliminary survey of this band in a large U.S. city, with the goal of quantifying spectral occupancy and thereby gaining some insight into the feasibility of this approach. A comparable measurement of the 25-90 MHz band in a rural environment is also presented, for comparison. In the urban measurement, it is found that sufficient spectrum is probably available: for example, it is estimated that approximately 80% of the 30-60 MHz band is essentially free of signals to within about 7 dB of the environmental noise limit at 30 kHz spectral resolution. However, these measurements are limited in duration, spatial sampling, temporal density, and spectral resolution. Requirements for a more comprehensive measurement campaign are proposed.

A polarimetric survey of radio-frequency interference in C- and X-bands in the continental united states using WindSat radiometry

Transmissions from ground-based systems in C- and X-bands present a significant challenge to the use of these bands for passive microwave remote sensing from aircraft and satellites. Because future missions plan to continue to use these frequencies, it is important to characterize and understand the nature of interference in as much of the candidate spectrum as possible. This paper presents a statistical analysis of interference observed in the continental U.S. using six months of data collected from the C- and X-band channels of the WindSat microwave radiometer. Our findings are consistent with those of previous studies by Li et al. and Njoku et al., which are based on data obtained from the Advanced Microwave Scanning Radiometer-EOS using somewhat similar center frequencies and bandwidths. Results show significant radio-frequency interference (RFI) at C-band, including brightnesses in horizontal and vertical polarizations in excess of 330 K, while X-band RFI is less obvious through direct examination of measured linearly polarized brightnesses. Evidence of lower levels of RFI is provided through use of the spectral and polarization indexes of Li et al., which reveal likely RFI contributions at X-band as well. Further confirmation of X-band RFI is obtained through analysis of the polarimetric channels, which are shown to provide direct evidence of RFI in contrast to the linearly polarized channels. A temporal analysis of the largest C-band RFI sources is also provided in an attempt to further understand their properties.

The LWA1 Radio Telescope

LWA1 is a new radio telescope operating in the frequency range 10-88 MHz, located in central New Mexico. The telescope consists of 258 pairs of dipole-type antennas whose outputs are individually digitized and formed into beams. Simultaneously, signals from all dipoles can be recorded using one of the instruments “all dipoles” modes, facilitating all-sky imaging. Notable features of the instrument include high intrinsic sensitivity (≈ 6 kJy zenith system equivalent flux density), large instantaneous bandwidth (up to 78 MHz), and four independently steerable beams utilizing digital “true time delay” beamforming. This paper summarizes the design of LWA1 and its performance as determined in commissioning experiments. We describe the method currently in use for array calibration, and report on measurements of sensitivity and beamwidth.

Antennas for the next generation of low-frequency radio telescopes

The next generation of large telescopes for radio astronomy at frequencies below 100 MHz will consist of tens of thousands of wide-band dipole-like antennas, each individually instrumented with a receiver and combined using digital signal processing. At these frequencies, the sensitivity of a telescope is limited by Galactic noise, with the result that even simple dipoles can deliver extraordinary useable bandwidth. In this paper the necessary characteristics for these antennas are explained, some bounds on performance are developed, and a few candidate designs are analyzed. It is shown that antenna systems consisting of simple wire dipoles, a 360 K active balun, and a long coaxial feedline can achieve Galactic noise-limited performance over large portions of the range 10-100 MHz. It is further shown that when these antennas are used as elements in a compact array, their Galactic noise-limited characteristics are not significantly affected.

An experimental study of antenna array calibration

The coupling matrix concept for predicting the radiation patterns of elements of an antenna array is studied. Measured data, as well as some numerical data, are used in the study. It is demonstrated that for some practical antennas of interest whose radiation patterns are affected by structure scattering, the coupling matrix concept may not work very well. As expected, the stronger the structure scattering, the greater the discrepancy between the predicted patterns and the actual patterns.

Removal of the GLONASS C/A Signal from OH Spectral Line Observations Using a Parametric Modeling Technique

Astronomers use the 1612 MHz OH spectral line emission as a unique window on galactic dynamics and the properties of evolved stars. In recent years, experiments using this OH line have become more difficult because of interference from the GLONASS satellite system. In this paper we demonstrate that the C/A component of the GLONASS signal can be removed using a parametric estimation/subtraction technique that exploits known properties of the modulation and requires no additional antennas. Using actual OH line observations, we demonstrate cancellation greater than 20 dB. This technique can be implemented using present-day digital signal processing hardware, and it does not significantly affect the relatively weak astronomy signals or the sensitivity of the measurement.

Design and Evaluation of an Active Antenna for a 29–47 MHz Radio Telescope Array

The eight-meter-wavelength transient array (ETA) is a new radio telescope consisting of 12 dual-polarized, 38 MHz-resonant dipole elements which are individually instrumented, digitized, and analyzed in an attempt to detect rare and as-yet undetected single dispersed pulses believed to be associated with certain types of astronomical explosions. This paper presents the design and demonstrated performance of ETAs dipole antennas. An inverted V-shaped design combined with a simple and inexpensive active balun yields sensitivity which is limited only by the external noise generated by the ubiquitous Galactic synchrotron emission over a range greater than the 27-49 MHz design range. The results confirm findings from a recent theoretical analysis, and the techniques described here may have applications in other problems requiring in situ evaluation of large low-frequency antennas

Mitigation of Radar Interference in L-Band Radio Astronomy

The 1215-1400 MHz band is important for spectroscopy of H I at high redshift, pulsar work, and SETI. Observations at these frequencies are complicated by pulsed interference from ground-based aviation radars. In this paper, we characterize one such radar received at the Arecibo Observatory using coherently sampled data sets obtained during a recent observation. Using these data, we demonstrate some simple methods for detection and removal of the radar pulses. One of these uses a coherent subtraction technique that has not previously been applied to the radar problem. This new technique provides an alternative to blanking, which is undesirable in pulsar and SETI work. We demonstrate that the radar studied in this paper can be suppressed by at least 16 dB in integrated spectra using the coherent subtraction technique. The maximum single-pulse power observed at the output of the canceler is ~15 dB less. The primary limitation appears to be the detector performance; as a result, the performance using blanking is about the same. Also, we demonstrate that the matched detector for pulses from this radar is relatively insensitive to astronomical transients (e.g., giant pulses), and we quantify the risk of such transients being falsely identified as radar pulses. The techniques described in this paper can easily be adapted to radar waveforms other than the one examined here.

Airborne radio-frequency interference studies at C-band using a digital receiver

Corruption of C-band microwave brightness observations by radio-frequency interference (RFI) has been reported in recent data from orbiting radiometers; methods for mitigating these effects are of great importance for the design of future spaceborne microwave radiometers. One approach that has been suggested involves the use of multiple subchannels at C-band as opposed to a single channel; the use of multiple subchannels allows RFI to be detected and mitigated by analyzing relationships among subchannel brightnesses. While this approach has been utilized in previous airborne measurements, demonstrations of the RFI mitigation performance achieved have been difficult to obtain. To address this issue, an enhanced airborne system for observing radio-frequency interference effects on C-band microwave radiometers was developed, and is described in this paper. The system includes a traditional microwave radiometer with four C-band subchannels, so that RFI removal is possible using a subchannel mitigation algorithm. In addition, the system includes a digital receiver with the capability of providing high temporal and spectral resolution observations of interference. This high-resolution data allows improved understanding of RFI sources to be obtained, and also allows analysis of subchannel mitigation algorithm performance. Observations using the system in a test flight near Wallops Island, VA are described. Results show the four subchannel approach generally to be effective in mitigating the observed RFI sources, although examples are also illustrated using the digital receiver data to demonstrate failure of this approach. While studies of the digital receiver data alone could be performed to demonstrate further improvements in RFI mitigation, issues with this initial dataset limit the extent of such studies. Nevertheless, the results obtained still demonstrate qualitatively the improved RFI mitigation that can be achieved in brightness observations through the use of digital receivers