Ruoyu Sun

Air–Ground Channel Characterization for Unmanned Aircraft Systems—Part I: Methods, Measurements, and Models for Over-Water Settings

The use of unmanned aerial systems (UASs), which are also known as unmanned aerial vehicles, and by the term “drones” in the popular press, is growing rapidly. To ensure safety, UAS control and nonpayload communication (CNPC) links must operate very reliably in a variety of conditions. This requires an accurate quantitative characterization of the air–ground (AG) channel, and this channel characterization is the focus of this paper. After providing motivation and background, we describe our methods and modeling approach, followed by a description of our simultaneous dual-band (L-band ∼970 MHz, C-band ∼5 GHz) measurement campaign and the over-water (OW) measurement sites. Example results for path loss and root-mean-square delay spread are provided, as well as the results for channel stationarity distance (SD), used in calculating small-scale Rician

Air-ground channels for UAS: Summary of measurements and models for L- and C-bands

K

Measurement Challenges for 5G and Beyond: An Update from the National Institute of Standards and Technology

- factor and correlations between the two receiver antennas that we employed in each frequency band. Two distinct SD measures—the power delay profile (PDP) correlation coefficient and the spatial autocorrelation matrix collinearity—were used and found to be of the same order. Path-loss exponents are near that of free space, but significant two-ray cancelation effects for these OW settings warrant more accurate models, which we provide. Delay spreads in the OW channels are also dominated by the two-ray components and are hence typically very small (∼10 ns) but can exceed 350 ns. A third intermittent multipath component (MPC) is also present a nonnegligible fraction of time; hence, we provide statistical wideband AG channel models to represent this. Future papers in this series will report results for the AG channel with ground sites in other types of environments.

Relationships among statistical channel parameters for an air-ground channel: Stationarity distance, ricean K-factor, and RMS delay spread

Unmanned aircraft systems (UAS) are being used in an ever-expanding variety of applications. For their safe integration into the worldwide airspace, reliable control and non-payload communication (CNPC) systems must be developed. The performance of any CNPC system depends critically upon the characteristics of the air-ground (AG) channel. Hence the authors, supported by NASA Glenn Research Center, gathered data from an air-ground measurement campaign explicitly aimed at characterizing this channel in two bands recently allocated for UAS, the L- and C-bands. The measurements were conducted in 2013, and produced the largest set of AG channel data ever gathered to date. This data, for seven ground site (GS) environments, was subsequently processed to develop models for the AG channels characteristics. In this paper we summarize the measurement campaign, measurement results, and completed models for propagation path loss, small-scale fading, airframe shadowing, wideband multipath characteristics, and correlations among the channel effects on two antennas and in the two bands. Path loss in the line-of-sight (LOS) AG channels generally follows that of free-space, but with significant two-ray attenuation peaks (exceeding 18 dB) recorded for relatively smooth earth surfaces. Small-scale fading was found to be well-modeled as Ricean, with K-factors on the order of 12 dB for L-band, 28 dB for C-band. Airframe shadowing was also found to be potentially severe, yielding attenuations of larger than 30 dB for both bands. As expected in the LOS conditions, channels were highly correlated for signals in the same band, but essentially uncorrelated between the two bands. Wideband models contain from one to seven intermittent multipath components, with power levels ~ 20 dB down from that of the LOS component; these yield root-mean square delay spreads up to ~ 2 microseconds. These results have been incorporated into the Radio Technical Commission for Aeronautics (RCTA)s Special Committee (SC)-228 work on UAS, and should be of use to future designers and developers of UAS communication systems.

Spatial and frequency correlations in two-ray air-ground SIMO channels

In less than a decade since the mainstreaming of cellular wireless technology, spectrum has become saturated by data-intensive smartphones, driving the so-called spectrum crunch. As a solution, the wireless community is pursuing the use of alternatives to current wireless technologies, including multiple-input/multipleoutput (MIMO) antenna arrays that allow increased simultaneous transmission capacity; the millimeter-wave (mmW) spectrum (30-300 GHz) to alleviate the spectrum crunch in current frequency bands; and ultradense networks transmitting wide-band modulated signals to allow short-range, high-speed data transfer.

Calibration of millimeter-wave channel sounders for super-resolution multipath component extraction

Based upon recent results from an air-ground channel measurement campaign, we investigate relationships between three common channel parameters: Ricean K-factor, stationarity distance (SD), and root-mean square delay spread (RMS-DS). Results show that SD and K are positively correlated, SD and dK/dt are negatively correlated, and SD and the temporal derivative of RMS-DS are also negatively correlated.

L- and C-band airframe shadowing measurements and statistics for a medium-sized aircraft

The correlation between signals received on multiple antennas is a function of the channel(s) between them, and analysis of this is useful for spatial diversity and spatial multiplexing. In this paper we compute the correlations in both the spatial and frequency domains for a classical two-ray channel in a SIMO setting for an air-ground setting. We analyze this for the “pure” (deterministic) two-ray case, and also include results for the case where the two-ray channel incurs Ricean fading. After describing the setting and basic analysis, we show corroborating NASA measurement results for an air-ground over-sea channel. We show that both the spatial and frequency domain correlations are numerically computable, oscillatory functions, dependent upon link geometric parameters (antenna heights and link distance), surface electrical parameters, and frequency. With knowledge of these parameters, one can in principle select antenna separation and/or frequency separation to achieve diversity in settings where this classical channel model applies. This can be beneficial when narrowbeam (tracking) antennas that suppress the surface reflection are impractical to deploy.