International Civil Aviation Organization
ACP WG-F/31
IP02
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
31st MEETING OF WORKING GROUP F
Seattle, Washington, USA
6-10 October, 2014
Agenda Item 10:
Any Other Business
L-Band and C-Band Air-Ground Channel Measurement & Modeling
Update
(Prepared by David Matolak, Kurt Shalkhauser and Robert Kerczewski)
(Presented by Robert Kerczewski)
SUMMARY
This information paper is intended to provide an update to ICAO WG-F of the
progress of L-Band and C-Band air-ground channel model development based
on data gathered through a propagation flight measurement campaign
performed by NASA. The L-Band and C-Band frequencies being modeled will
support the line-of-sight Control and Non-Payload Communications (CNPC)
for unmanned aircraft systems. An update on the development of over-water
channel models and initial results for hilly terrain are presented.
1.
INTRODUCTION
1.1
The National Aeronautics and Space Administration (NASA) has been conducting airground (AG) channel propagation measurement and modelling for two frequency bands proposed for use
by Unmanned Aircraft Systems (UAS) Control and Non-Payload Communications (CNPC), under
NASA’s UAS Integration in the National Airspace System Project. Information papers presented at
previous meetings of Working Group F have provided: a description of plans to conduct a series of flight
tests to measure the propagation characteristics of the air-ground (AG) channel at 960-977 MHz and
5030-5091 MHz and use the resulting data to enable the development of detailed AG channel models
(ACP-WG-F/25 WP 13, ACP-WG-F/26 IP8); the propagation measurement system and channel
modelling approach (ACP-WG-F/28 IP3); a description of the first four sets of flight tests (ACP-WGF/29 IP 5); and initial analysis of channel measurement data and channel model elements for over-sea
terrain condition (ACP-WG-F/30 IP 8).
ACP WG-F31 IP02
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1.2
Recent work on the air-ground (AG) channel characterization process has focused on
several tasks including data processing for extraction of channel parameters for remaining test
environments (hilly terrain, mountainous terrain, suburban, and urban) and statistical analysis for
development of accurate AG channel models for the various environments. In this update we provide
some example results for these tasks, but first we provide an update to the over-water path loss modeling
which has been extended to include over-fresh-water in addition to over-sea (salt water) conditions. We
also comment on dispersion.
2.
UPDATE OF AIR-GROUND CHANNEL MODELING FOR
OVER-WATER CONDITIONS
2.1
NASA performed AG channel characterization flight tests for over-sea (salt water)
conditions over the Pacific Ocean near Oxnard, California, USA on 11 June 2013. Flight tests for over
fresh water conditions took place over Lake Erie near Cleveland, Ohio, USA on 22 October 2013.
Based upon data gathered from these over-water flight tests, we have results for both path loss and
dispersion. These flight tests measured channel impulse responses for both straight and oval-shaped
flight paths. Impulse responses were measured in both L-band and C-band simultaneously: the two
transmitters (one L-band, one C-band) were located at the ground site (GS), and four receivers—two for
each band—were on the aircraft. Two separate antennas connect to the two receivers in each band.
2.2
Path loss generally follows the curved-earth two-ray (CE2R) model, with deviation due
to a number of factors, including water surface roughness, non-ideal antenna patterns, intermittent
multipath components, and some slow variation of received power due to channel measurement
equipment imperfections. Path loss models for the over-water settings are as follows, with path loss in
units of dB:
L-band
(1)
where threshold elevation angle t = 5 degrees, = -1 for travel toward the GS and +1 for travel away
from the GS, distance ranges from 1  d 28 km for fresh water and 2.2  d 24 km for sea water. The
variable X is a zero-mean Gaussian random variable, with standard deviation X dB, and CE2R denotes
the curved-earth 2-ray model.
C-band
(2)
The same parameter definitions in the L-band model apply to the C-band model as well. Model
parameters are provided in Tables 1 and 2.
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ACP-WGF31 IP02
TABLE 1. L-BAND PATH LOSS PARAMETERS
Setting
A0,L,s
(dB)
nL,s
Fresh
57.7
Sea
50.6
X,L,s
X,L,l
(dB)
LL,s
(dB)
L0
(dB)
(dB)
LL,l
(dB)
dt
(km)
1.4
2.8
1.8
1.1
3.2
1.8
6.6
1.5
2.8
1.2
1.0
4.8
1.1
9.1
TABLE 2. C-BAND PATH LOSS PARAMETERS
Setting
A0,C,s
(dB)
nC,s
Fresh
53.6
Sea
60.0
X,C,s
LC,s
(dB)
A0,l
(dB)
nC,l
(dB)
1.8
2.6
2.2
53.7
1.6
2.3
1.7
63.9
X,C,l
LC,l
(dB)
dt (km)
(dB)
1.8
2.8
1.5
6.6
1.5
2.5
0.5
9.1
2.3
Dispersion was found to be fairly small, as expected in these open over-water
environments, with root-mean square delay spreads (RMS-DS) typically less than 50 ns, with the
occasional value up to nearly 250 ns. Delay spread statistics have been reported in a prior NASA report
[1]. Detailed models (tapped delay line) for the over-water wideband channels are being developed, and
should be complete in the coming month.
3.
STATISTICAL ANALYSIS: SMALL-SCALE FADING
AND DIVERSITY FOR THE OVER-WATER AG
CHANNEL
3.1
As is well known, the over-sea (or over-fresh-water) AG channel is predominantly a lineof-sight (LOS) channel with a surface reflection. This is well-approximated by the CE2R model, which
is largely deterministic. Other channel effects including intermittent multipath components and random
magnitude and phase of the sea-surface reflection can be quantified statistically. For path loss this is
embodied in the variation of loss about the (often linear) fit versus distance. For small scale fading this is
done by specifying the distribution of the amplitude of the channel gain, after large scale effects are
removed. In our LOS case, the most logical choice for modeling the amplitude distribution is the Ricean
distribution. This two-parameter distribution is characterized by the strength of the LOS (non-fading)
component and the strength of the remaining (assumed ~ Rayleigh distributed) components. Most often,
the channel amplitude is normalized to have a mean-square value of 1, and in this case, the Ricean
distribution is characterized by its so-called K-factor or Rice factor, where k=A2/(22), with A the
amplitude of the LOS component, and 22 the power in the remaining components. Usually K=10log(k)
dB is specified. The normalized Ricean probability density function is given by
(3)
with r the pdf variable (amplitude), and I0 the modified Bessel function of first kind, order zero.
3.2
In order to actually compute the parameters of this small scale fading distribution,
strictly, one must determine the spatial region over which the small scale fading statistics can be
assumed constant. The extent of this region is termed the stationarity distance. Our conservative estimate
(based on a procedure applied in other multipath channels) for this distance is 250C, where C is the Cband wavelength. In short, the channel statistics can be considered roughly constant within any interval
less than or equal to this distance.
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ACP WG-F31 IP02
OxnardCA***06-11-2013***FT1***L-band Rx1
22
KML
Rician K Factor (dB)
20
KMB
KML Linear Fit [n=0.07 , =1.0]
18
KMB Linear Fit [n=0.08 , =1.0]
16
14
12
10
5
10
15
20
Link Distance (km)
Figure 1. L-band K-factor vs. link distance for over-sea setting
With this stationarity distance, we were able to compute Ricean K-factors for the over-sea setting. In
Figure 1 we show L-band K-factor vs. link distance computed by two different methods: ML denotes a
maximum likelihood fit, and MB denotes a moment-based method for estimating K. Linear fits to the Kfactor are also provided, with n=slope and = standard deviation of the linear fit. It is clear that the Kfactor exhibits some two-ray-like behavior, with K generally larger than approximately 11 dB. Similar
results pertain to the C-band K-factor, with K generally above 25 dB. The larger value of K for C-band
results from the less-severe cancellation in the two-ray representation: the sea surface is smoother for Lband than it is for C-band since the L-band wavelength is approximately five times larger than the C-band
wavelength, and in addition, the reflection coefficient is smaller at C-band than at L-band. The linear fit to
C-band K-factor has an intercept (at dmin~ 2 km) approximately 29 dB, slope n=0.17 and standard
deviation  = 1.7 dB.
OxnardCA***06-11-2013***FT1***L-band max(Rx1, Rx2)
Total Received Power (dBm)
-35
PRx1
-40
PRx2
max(PRx1, PRx2)
-45
-50
-55
-60
-65
-70
-75
5
10
15
20
Link Distance (km)
Figure 2. L-band received power vs. link distance for over-sea setting.
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ACP-WGF31 IP02
3.3
Figure 2 shows a plot of the L-band received power for one straight over-sea flight path.
The curves for the two individual L-band receivers (with antennas separated by ~1.84 m), along with the
curve for the maximum power among the two, are shown vs. link distance. The two-ray “lobing” effect
is clearly evident, and we note that for our stationarity distance of 250C ~15 m, the separation of the
plots for the two receivers does show that when one antenna is receiving a minimum value of power, the
other is not—this is a direct manifestation of the two-ray effect, since the curves are offset by ~ d, and
this d is a function of the antenna separation and link distance. Thus if we had the ability to
instantaneously select the antenna with the larger received signal, we could claim that for this setting,
antenna diversity can increase received signal strength by up to approximately 10 dB in L-band. For
larger antenna separations, the two curves for received power on the two antennas should become even
more distinct in any such straight flight track. Since practical diversity schemes require estimation of
received power over some time duration, the “diversity” implied by Figure 2 may not be easily
obtainable in practice. In contrast, inter-band diversity between the C- and L-bands would be of great
value, since the signals in the two bands are essentially uncorrelated. Over the entire flight track of
Figure 2, the K factors for the two C-band receivers are 30.4 and 29.9 dB, and for the L-band receivers
they are 12.8 dB.
4.
EXAMPLE RESULTS FOR HILLY TERRAIN
4.1
Flight tests were conducted over hilly terrain near Latrobe, Pennsylvania, USA on April
15 2013. Figure 3 shows the Latrobe terrain. Here we provide some example results from recent
processing of the flight test data. Figure 4 (left) shows path loss vs. link distance for one receiver in each
band, along with CE2R model fits and free-space path loss for comparison. We also show a plot (right)
of L-band path loss for the two receivers in the over-sea setting. The hilly terrain flight path was a
straight path, parallel to a nearby ridge. As in the over-sea case, in hilly terrain the two-ray effects are
more pronounced at L-band than at C-band. Linear fits to the hilly terrain measured path loss on the loglog scale show standard deviations ranging from 1.9-3 dB for C-band, and 3.2-3.6 dB for L-band. When
comparing to the over-sea results, it is also clear that the two ray effect is more pronounced in the oversea setting than in the hilly terrain setting. This is as expected since water surfaces will generally be
much more reflective than earth surfaces. Nonetheless, the surface reflection in the hilly terrain does
cause “path loss peaks” seen in such “two-ray” channels—these peaks are just not as regular
(~predictable) as in the over-water cases. Specific models for the path loss in the hilly terrain setting,
analogous to models of (1) and (2), are currently under development.
Figure 3. Terrain in the vicinity of Latrobe, Pennsylvania, USA.
Path Loss (dB)
LatrobePA***04-15-2013***FT1***C-band Rx1 & L-band Rx1
145
Free Space C-band
140
Free Space L-band
2-Ray C-band
135
2-Ray L-band
130
Measured C-band
Measured L-band
125
120
115
110
105
100
95
2.24
3
4
5
6
7
8 9 10
Link Distance (km)
12
15 17
Path Loss Recorded by Sounder (dB)
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ACP WG-F31 IP02
OxnardCA***06-11-2013***Track2***L-band Rx1&2
150
140
130
Measured L-band Rx1
Measured L-band Rx2
Free Space PL
2-Ray Curved Earth
2-Ray Flat Earth
120
110
100
90
80
10
4
Link Distance (m)
Figure 4. Left: path loss vs. distance for one C-band and one L-band receiver, hilly terrain; and
Right: path loss vs. distance for two L-band receivers, over-sea.
4.2
The hilly terrain environment also proved more dispersive than the over-water AG
channel. This is expected due to the presence of hills and the nearby suburban area of Latrobe, PA. In
Figure 5 we show a plot of RMS-DS vs. link distance for the same straight flight path as in Figure 4. We
have applied a moving average to the results to smooth them and reduce noise. Of particular interest are
the RMS-DS “bumps” at several values of link distance. These occur when the geometry is such that a
strong reflection (or several) is received from one of the large buildings near Latrobe.
Figure 5. RMS-DS vs. distance for straight flight path, hilly terrain.
4.3
Figure 6 shows a sequence of power delay profiles (PDPs) for this flight track,
illustrating the cause of the “bump” in the RMS-DS plot at link distance approximately 5.8 km. In
addition to the strong LOS component (power ~ - 40 dBm), we have the surface reflection (~ - 60 dBm)
at very small relative delay plus another “distant reflection” from another obstacle. Our analysis of maps
and the geometry points to a very large building as the source of this reflection. Table 3 provides a
summary of RMS-DS statistics for the hilly terrain environment.
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ACP-WGF31 IP02
Figure 6. Sequence of power delay profiles for RMS-DS “bump” in Figure 4.
TABLE 3. HILLY TERRAIN RMS-DS STATISTICS
RMS Delay Spread
FT1
FT6
5.
Original Data Sequence
Moving Averaged over 100 PDPs
Moving Averaged over 1000 PDPs
Original Data Sequence
Moving Averaged over 100 PDPs
Moving Averaged over 1000 PDPs
Mean (ns)
12.9
35.7
Median (ns)
Max (ns)
10.9
11.1
11.2
10.9
18.6
19.3
267.2
87.0
57.0
995.7
371.8
349.2
Standard
Deviation
(ns)
9.8
6.6
5.7
55.7
42.9
40.4
SUMMARY
5.1
Continuing progress is being made on developing models for the UAS AG channel in
multiple GS environments. Path loss in hilly terrain was found to exhibit two-ray behavior as in the overwater settings, but with less regular variation with distance, given the less regular and less reflective
earth surface in comparison to the water surfaces. Small scale fading has been modeled with Ricean
statistics, with Ricean K-factors on the order of 13 dB for L-band, and 30 dB for C-band. Our technique
for estimation of stationarity distance will enable computation of small scale fading statistics for
additional GS environments. We also found that the hilly terrain AG channel is more dispersive than the
over-water channels. This is as expected, since terrain features and large buildings present in this setting
can act as good reflectors.
6.
NEXT STEPS
6.1
Future work includes processing additional data for other settings (urban, mountainous,
desert, and suburban), and subsequent development of path loss and small scale fading models.
Additionally, wideband tapped delay line models that account for all multipath components will also be
developed for all settings.