Published in 2012 IEEE International Conference on Communications, Ottawa, Canada, June 2012
38 GHz and 60 GHz Angle-dependent Propagation for
Cellular & Peer-to-Peer Wireless Communications
Theodore S. Rappaport, Eshar Ben-Dor, James N. Murdock, Yijun Qiao
Wireless Networking and Communications Group (WNCG)
Electrical and Computer Engineering Department, The University of Texas at Austin, Austin, TX 78712
wireless@mail.utexas.edu
Abstract— As the cost of massively broadbandTM semiconductors
continue to be driven down at millimeter wave (mm-wave)
frequencies, there is great potential to use LMDS spectrum (in
the 28 – 38 GHz bands) and the 60 GHz band for cellular/mobile
and peer-to-peer wireless networks. This work presents urban
cellular and peer-to-peer RF wideband channel measurements
using a broadband sliding correlator channel sounder and
steerable antennas at carrier frequencies of 38 GHz and 60 GHz,
and presents measurements showing the propagation time delay
spread and path loss as a function of separation distance and
antenna pointing angles for many types of real-world
environments. The data presented here show that at 38 GHz,
unobstructed Line of Site (LOS) channels obey free space
propagation path loss while non-LOS (NLOS) channels have
large multipath delay spreads and can exploit many different
pointing angles to provide propagation links. At 60 GHz, there is
notably more path loss, smaller delay spreads, and fewer unique
antenna angles for creating a link. For both 38 GHz and 60 GHz,
we demonstrate empirical relationships between the RMS delay
spread and antenna pointing angles, and observe that excess path
loss (above free space) has an inverse relationship with
transmitter-to-receiver separation distance.
Keywords-60 GHz Propagation, 38 GHz Propagation; RF
channel measurement; Channel Models; Millimeter Wave
Communications; Cellular; Peer-to-peer; AOA; Channel Sounder;
Sliding Correlator; Broadband Wireless Access; mm-wave
I.
INTRODUCTION
There is great interest in using millimeter wave short range
communications for indoor use [1], [2], and more recently for
outdoor cellular mobile multi-Gbps systems [3], [4], [5], [6].
Recent analysis in [7] shows that the relative power
consumption of wireless devices improves as the RF
bandwidth increases, thus motivating the acceleration of multiGbps smart phone devices at mm-wave frequencies where
spectrum is widely available.
This research focuses on finding channel properties and
unique propagation paths using beam-steering. To date, little is
known about the comparative wideband propagation
characteristics of the different millimeter-wave bands using
steerable antennas for cellular use. In 1994, [8] introduced the
local multipoint distribution system (LMDS) concept for
carrier frequencies in the 20 to 40 GHz range. LMDS
promised outdoor use of millimeter wave systems for backhaul
and point-to-point or point-to-multipoint use, but the lack of
inexpensive electronics and steerable antennas prohibited the
use of such frequencies. A popular myth has been that nonLOS (NLOS) links are unable to offer sufficient strength for
mm-wave mobile radio, but as shown here, many NLOS paths
can be formed with steerable high directivity antennas such as
those that will be used in future mobile devices. As discussed
in [4], low-cost CMOS chips using steerable antenna arrays at
millimeter wave frequencies are now commercially viable [9],
and beam-forming algorithms have been well known for
several decades [10].
This paper presents 38 and 60 GHz propagation
measurements on The University of Texas at Austin campus
from March to August 2011. We first measured an outdoor
open pedestrian walkway with transmitter-receiver separation
distances between 19 and 129 meters, with the transmitter and
receiver antennas approximately 1.5 m high above ground.
This mimics peer-to-peer ad-hoc network applications and
simple two-way communications between terminals [5]. An
overhead view of the receiver locations around the campus
pedestrian courtyard is shown in Figure 1, with measurement
locations identical to those used in [5] (which only considered
60 GHz peer-to-peer measurements). The work reported here,
however, is the first to compare wideband peer-to-peer
propagation using steerable antennas at identical locations for
both 38 GHz and 60 GHz. In addition, here we present
cellular-type measurements at 38 GHz made using steerable
antennas with the transmitter (TX) antenna located on top of a
5 story building while the receiver (RX) antennas was set 1.5
m above ground and moved to various locations around
campus. Figure 2 shows an overhead view of the roof top
base station and receiver locations, which include both LOS
and NLOS links, for cellular-type measurements. Such cellular
deployments will likely provide multi-Gbps connectivity in
high user density areas [3], [4].
Several earlier mm-wave propagation studies have hinted at
the viability of NLOS links for mm-wave communications in
urban outdoor environments. Violette studied several outdoor
suburban environments using narrowband measurements at 9.6,
28.8, and 57.6 GHz [11], but conducted very few
measurements to find the availability of multiple paths formed
with different antenna pointing angles at TX and RX. In [11],
trees were the most common obstruction for NLOS
measurements, and exhibited such high attenuation that a clear
LOS link was required to receive a signal beyond 1 km
distance. In [12], an airport field, an urban street, and a tunnel
were studied for path loss using a narrowband 59 GHz system,
and a wideband chirp technique was used to measure multipath
delay spread. The work in [12] showed that the propagation
path loss exponent is between 2 and 2.5 for outdoor channels,
and RMS delay spread was less than 20 ns. Weather will be an
issue for mm-wave systems, as shown in [1] where wideband
Published in 2012 IEEE International Conference on Communications, Ottawa, Canada, June 2012
38 GHz measurements between buildings were conducted in
LOS, partially obstructed LOS, and NLOS links in clear
weather, rain, and in hail storms. Worst-case attenuation of 26
dB above free space was found during a severe hail storm [1].
Narrowband millimeter wave propagation during weather
events was studied in [13] where the Laws-Parsons model was
shown to give reasonable statistical results of measured
attenuation versus rain rate at 60 GHz, and showed the
importance of the rain drop diameter.
Figure 1– Overhead image of 38 and 60 GHz peer-to-peer measurement area
with transmitter location marked as TX and receiver locations as RX#.
In [14], a 230 m link at 35 GHz was studied and found excess
attenuation of at least 2.5 dB for all rain events, and an excess
attenuation of 10 dB or more occurring for 30% of the time.
II.
MEASUREMENT HARDWARE
The channel sounder used here employs a variable rate
PN sequence generator, adjusted to 400 Mcps for 38 GHz
measurements and 750 Mcps for 60 GHz. The system is a
superheterodyne with IF frequency of 5.4 GHz, which is fed to
the millimeter wave up- and down-converters. The 38 and 60
GHz switchable up and down converters were built by Hughes
Research Laboratories (HRL), and contain a mixer and LO
frequency multipliers to yield carrier frequencies of 37.625
and 59.4 GHz. The 37.625 GHz carrier is sent at a power of 22
dBm and the 59.4 GHz carrier is sent at a power of 5 dBm.
Since a sliding correlator requires a slower rate identical PN
sequence, chip rates of 749.9625 MHz (slide factor of 20,000)
and 399.95 MHz (slide factor of 8,000) were used for the 60
and 38 GHz receivers, respectively, to provide good
processing gain and minimal pulse distortion. The system is
able to measure at least 150dB of path loss in each band.
For 38 GHz, identical Ka-band vertically polarized horn
antennas with gains of 25 dBi and half-power beamwidth of
7.0o were used at the transmitter and receiver. The 60 GHz
peer-to-peer measurements used identical U-band vertically
polarized horn antennas with gains of 25 dBi and beamwidth of
7.3o at the transmitter and receiver. All antennas were rotated
on 3-D tripods.
III.
EXPERIMENTAL DESIGN
A. Peer-to-Peer ChannelMeaurements
For the peer-to-peer study, a single transmitter and ten
random receiver locations were chosen around a pedestrian
walkway area surrounded by buildings of 1 to 12 stories. The
transmitter was placed 20 meters away from a 7 story
building. The receiver was moved to locations with distances
of 19 to 129 meters from the transmitter. The locations in
Figure 1 offered typical urban reflectors and scatterers such as
automobiles, foliage, brick and aluminum-sided buildings,
lampposts, signs, and handrails.
In order to characterize the various LOS and NLOS links
present at each receiver location, the narrowbeam horn
antennas were systematically steered in the azimuth direction
Figure 2– Overhead image of the outdoor cellular measurement area with the
transmitter on a 5-story rooftop and receiver located on the ground.
(similar to a beam-steering antenna array). For LOS links, the
transmitter and receiver were first pointed directly at each
other, corresponding to azimuth scanning angles of 0o for both
the transmitter and receiver. Next, the transmitter antenna was
pointed at the direction of a large scatterer. Then, the receiver
antenna was steered to point towards that same scatterer. If a
link was successfully established, a measurement was
recorded. Next, the transmitter orientation was left fixed on
the scatterer and the receiver antenna was then steered a full
360o to find and measure any additional links due to doublescattering or other propagation events. These additional links
were found at most receiver locations, with one or two
additional receiver angles at 60 GHz, and one to three
different receiver angles at 38 GHz, although at substantially
lower (-20 dB typ.) signal strength. All peer-to-peer receiver
locations had a LOS path to the transmitter and, as a
consequence, no outages were found for the peer-to-peer
measurements.
Measurements made at each receiver location for a
particular transmitter-receiver angle combination consisted of
the average of eight local area point PDP measurements,
where each point in the local-area was spaced equally on a
circular measurement track with 10λ separation between each
point. Each point PDP measurement consisted of a time
average of 20 power delay profiles (PDPs) acquired in rapid
succession over a fraction of second. As the receiver antenna
was moved around the local area circular track, it was oriented
to always point at the cause of the multipath link, as illustrated
in Figure 3. The eight local area PDPs were averaged together
to form a local average PDP at each location. Figure 4 shows a
scatter plot of the receiver and transmitter azimuth angles that
resulted in successful links. The plot shows a concentration in
the second and fourth quadrants. On the right side of Figure 4,
both antennas are pointed at or near the same reflector. The
Published in 2012 IEEE International Conference on Communications, Ottawa, Canada, June 2012
Figure 3 – As the receiver was moved around
the track at each location, it was re-oriented
to point at the presumed multipath source.
Figure 4– This scatter plot shows the azimuth angle combinations that resulted in links for the outdoor peer-topeer channel. 51% of the 38 GHz and 31% of the 60 GHz links were made with one of the antennas at an
extreme (>50o or <-50o) angle. Note more links are made at 38 GHz than at 60 GHz for identical locations.
results show that the single bounce scattering are more likely
to produce a link than double bounce scattering. While few
links are made with both antennas at extreme angles (>50o or
<-50o), 51% of the 38 GHz and 31% of the 60 GHz links were
made with one of the antennas at such extreme angles. The
lower number of available 60 GHz links and the lower
percentage of large azimuth angle links in the same
environment confirm larger absorption and path loss as
compared to 38 GHz, and also may be due to less transmit
power used at 60 GHz as compared to that used at 38 GHz.
B. Cellular Channel Measurements
Cellular (or rooftop-to-ground) measurements were
conducted using the same basic measurement methodology as
used in the peer-to-peer measurements, but also included the
ability to adjust the elevation angles for both transmitter and
receiver antennas. The increased height of the transmitter
relative to the receiver (due to the position of the transmitter
on the roof of a building) resulted in slightly more unique
links (typ. 2 to 4) for a given receiver-transmitter location and
angle-pointing combination than found for the peer-to-peer
channel, as fewer reflectors were obstructed from the
transmitter’s rooftop view. Moreover, whereas the peer-topeer channel had thin foliage scatterers and tree trunks, the
cellular channel was characterized by greater variability of
foliage density, and had many links where thin to thick density
foliage/vegetation caused scattering. The transmitter was
placed on the roof of Woolrich Laboratories (WRW), a 5 story
building (~60 feet), and was oriented to beam to the north of
WRW. Figure 2 shows the location of the transmitter and the
various receiver locations, including the surrounding
buildings. Figure 5 is a scatter plot of the transmitter and
receiver azimuth angles at which unique links were found.
There is a clear concentration of links between transmitter
azimuth angles of -30o and +20o. This property of the channel
characterizes an “urban canyon,” where multistory buildings
funnel energy along narrow pointing angles on both sides of
the receiver antenna boresight, and in which buildings block
any possible links at more extreme transmitter azimuth
pointing angles. In contrast, the peer-to-peer channel has a
larger number of links that were formed with extreme
transmitter azimuth pointing angles. The cellular receiver
azimuth angles show the impact of site-specific properties.
Since the transmitter is nearly in line with the urban canyon
“wall,” paths formed by directional antennas in a dense urban
canyon environment can be easily predicted from the location
of the base-station relative to the canyon center (i.e. the midpoint between the two rows of buildings that form the urban
canyon).
Figure 5– Scatter plot of the transmitter and receiver azimuth angles for
measured links in the cellular environment at 37.625 GHz. The distribution of
TX azimuth angles for successful links is shown on the bottom. The
distribution of receiver azimuth angles is shown on the left.
Thus, most NLOS links will be formed with the transmitter
pointing toward the canyon center line, and the receiver
pointing to the side of the urban canyon away from the
transmitter.
IV.
MEASUREMENT RESULTS AND ANALYSIS
A. Peer-to-Peer Channel in Pedestrian Walkway
Both LOS and NLOS links were studied for the 38 and 60
GHz peer-to-peer channel. NLOS links resulted in much
higher path loss, typically ranging from 15 to 40 dB greater
than free space LOS paths (See Figs. 6 and 7). NLOS links
were also characterized by greater RMS delay spreads as
compared to LOS links, ranging from 1 ns (the nominal spread
of the probing pulse) to 122 ns for 38 GHz, and from 1 ns to
Published in 2012 IEEE International Conference on Communications, Ottawa, Canada, June 2012
36.6 ns for 60 GHz. Figure 6 shows a path loss scatter plot for
the 60 GHz peer-to-peer channel, and gives path loss
Figure 6– Scatter plot of the measured path loss values relative to 3 m free
space path loss for the 60 GHz outdoor urban peer-to-peer channel.
scattering off rough surfaces at 38 GHz, and with less free
space loss than at 60 GHz. Therefore, at 38 GHz, many more
links (some very weak) are made as compared to 60 GHz.
When the best path is selected, we see the path loss for
identical beamwidth antennas in 38 GHz and 60 GHz NLOS
channels are similar (n=3.71 @ 38GHz vs. 3.76 @ 60GHz).
Figure 8 shows the cumulative distribution function (CDF)
of the RMS delay spreads measured for the peer-to-peer
channel at both 38 and 60 GHz. From the plot, it is apparent
that LOS links do not show any resolvable multipath, with
differences between 60 GHz and 38 GHz links attributable to
the difference in the chip rates. Notably, the 38 GHz channel
was characterized by higher RMS delay spreads compared to
the 60 GHz channel, with mean RMS delay spreads of 23.6 ns
and 7.4 ns for the 38 and 60 GHz channels, respectively. This
is likely due to lower free space path loss and more objects in
the environment serving as scatterers at 38 GHz than at 60
GHz. The plot shows the expected and maximum values for
the RMS delay spread.
Figure 7– Scatter plot of the measured path loss values relative to 3 m free
space path loss for the 38 GHz outdoor urban peer-to-peer channel.
exponents for all LOS links, all NLOS links, and for the
strongest (best) NLOS link selected from all unique antenna
pointing combinations for each receiver location. All path loss
measurements were based on a common free space close-in
anchor/reference distance of 3 meters at which the free space
reference path loss is 77.5 and 73.5 dB for 60 and 38 GHz,
respectively. The 60 GHz LOS links have a path loss exponent
of 2.25, slightly greater than free space and a standard
deviation of 2 dB. The LOS path loss exponent being slightly
greater than 2 is due to atmospheric absorption of 60 GHz
[15]. For millimeter wave systems using beam-steering
antenna arrays, once the LOS path is completely blocked, the
beam will be steered until the strongest NLOS link is
identified. Thus, we considered the strongest NLOS link that
can be made at each location, as this link is the one which
future systems will need to select. Figures 6 and 7 indicate
advantages for systems that can find the strongest NLOS link
over those which receive a random NLOS link. Figure 6
shows the strongest 60 GHz NLOS links have a path loss
exponent of 3.76, as compared to 4.22 when all possible
NLOS links are considered. The stronger NLOS links were
also characterized by lower RMS delay spreads.
Figure 7 shows a path loss scatter plot for the 38 GHz peerto-peer channel, which exhibited free space LOS propagation
(n=2). Interestingly, NLOS links for the 38 GHz channel had
a greater path loss exponent of 4.57 than 60 GHz peer-to-peer
NLOS links when all NLOS links were considered, but a
slightly better path loss exponent of 3.71 when considering
only the strongest NLOS links at each location. Comparing
Figs. 4, 6 and 7, we can infer differences in specular scattering
at 38 versus 60 GHz. The longer wavelength of 38 GHz results
in rough surfaces appearing more smooth, so that the same
environment will provide more links created through
Figure 8– CDF of the RMS delay spread for the 38 and 60 GHz urban outdoor
peer-to-peer channels.
Figure 9– Larger absolute azimuth pointing angles at the receiver, transmitter,
or both are associated with probabilistically higher RMS delay spreads. Fit
lines are added to illustrate the increased mean RMS delay spread.
Figure 9 shows a scatter plot of measured RMS delay
spread vs. the sum of the absolute values for the azimuth
pointing angles of the transmitter and receiver antennas for
each link over all locations. The plot reveals that large
scanning angles at the transmitter or receiver (or both) exhibit
high RMS delay spread paths more often than narrow
scanning angle paths. This is likely due to the greater travel
distance of NLOS links formed with large or extreme azimuth
pointing angles (greater than 500), and hence the larger
number of objects illuminated by the transmitter for these
links. This is an important trend for beam-steering algorithm
Published in 2012 IEEE International Conference on Communications, Ottawa, Canada, June 2012
development, as wide scan-angle links will require greater
channel equalization as well as longer propagation time than
shorter LOS links.
always prohibit formation of a link. As shown in Figure 11,
partially obstructed LOS links had approximately 5 – 15 dB
greater attenuation than unobstructed LOS links. All NLOS
links considered together results in a path loss exponent of
3.88, whereas when only the NLOS link with the lowest path
loss at each location is considered, the path loss exponent
decreased to 3.26, again showing the value of beam-forming
for mm-wave cellular. As expected, the cellular measurements
have larger standard deviations due to greater distances and a
more complex scattering environment.
Figure 10– Higher excess path loss (i.e. path loss in excess of the measured
average distant-dependent LOS path loss) is associated with higher RMS
delay spreads.
Figure 10 shows a scatter plot of RMS delay spread versus
excess path loss (i.e. path loss above the measured average
LOS path loss at all receiver locations). The figure reveals that
a relationship exists between the RMS delay spread of a link
and its excess path loss above free space. Links with low
excess loss were commonly found to be caused by a single
reflection off a metallic object in the environment. Since
diffuse scattering is minimal for such links, strong received
signals are present with typically low signal spread. Our
measurements suggest that the highest RMS delay spreads are
expected when both mobile devices are relatively close to each
other, but when the transmitter and receiver must steer to steep
azimuth angles to establish a link (for example, when the LOS
path is blocked by a wide object) Additionally, the results
suggest that higher transmit powers may increase the number
of NLOS links, yet these links often also possess higher RMS
delay spreads and are more lossy. Therefore, transmit power
should be rigorously selected with beamwidth to give the best
tradeoff of range versus required system complexity, data rate,
and processing power.
B. Cellular Channel in an Urban Campus
The 38 GHz millimeter-wave cellular channel is
characterized by greater link distances than peer-to-peer
channels due to the fact the transmitter is substantially
elevated relative to the receiver. Figure 11 shows a scatter plot
of the measured path loss for the different locations and links
measured in the 38 GHz cellular channel with a
anchor/reference free space reference distance of 5 m (free
space reference path loss of 77.9 dB). Separation distances
ranged from 61 m to 265 m. As with the 38 GHz peer-to-peer
measurements, LOS links (marked by black circles in Figure
11) showed a path loss exponent very close, and slightly
lower, than free space. The measured path loss exponent of
n= 1.95 is likely due to the urban canyon effect. Unlike the
peer-to-peer channel, several of the cellular LOS paths were
partially obstructed by tree branches, foliage, and building
corners. Notably, building corners/edges often did not
completely attenuate LOS links, even when the building edges
resulted in near complete obstruction of the LOS “optical”
path, demonstrating that shadowing due to buildings will not
Figure 11– Path Loss scatter plot for cellular channel at 37.625 GHz. A 5 m
free space reference distance is used.
Figure 12– CDF of the 37.625 GHz Cellular channel RMS delay spread.
Figure 13– RMS delay spread for the 38 GHz cellular channel as a function of
transmitter and receiver pointing angle combinations.
For 38 GHz cellular measurements, both LOS and partially
obstructed LOS links had minimal RMS delay spread on the
order of 1 ns (partial obstruction refers to e.g. a path through
tree branches that do not totally block the path) whereas
NLOS links exhibited RMS delay spreads greater than 15 ns
for 20% of the acquired links. These trends can be seen in
Figure 12 which presents a CDF of the RMS delay spread for
the 38 GHz cellular channel. The average RMS delay spread
for the NLOS links was 12.2 ns. The lower mean RMS delay
spread for the cellular channel compared to the 38 GHz peer-
Published in 2012 IEEE International Conference on Communications, Ottawa, Canada, June 2012
to-peer channel may be attributed to larger T-R distances
(causing greater delays and attenuation of late arriving
multipath) and propagation due to the urban canyon effect.
require beam steering over a small range of angles at the
transmitter, while the receiver may require more exhaustive
beam steering to find a link. We also found that the position of
the transmitter relative to the center-line of the urban canyon
(i.e. the line mid-way between the two columns of buildings
that form the urban canyon) will result in minimum path loss
when the transmitter antenna is steered most often to point
toward this center line. Rain and hail, however, will attenuate
link margin and reduce cell sizes while lowering data rates.
Future work is investigating cellular shadowing and link outage
over larger propagation distances at mm-wave frequencies.
ACKNOWLEDGMENTS
Figure 14– Increased excess path loss is associated with higher average RMS
delay spreads for the 38 GHz cellular channel.
Figure 13 shows a scatter plot of the measured RMS delay
spreads of the cellular channel vs. transmitter and receiver
azimuth scanning angle combinations. While large buildings
can cause high delay spreads with near-boresight antenna
pointing, the probability of a high RMS delay spread link is
generally lower for smaller azimuth pointing angles than for
greater angles, as illustrated by the linear fit line. Figure 14
shows a scatter plot of the RMS delay spread versus the excess
path loss over the reference free space path loss. The figure
shows how, like the peer-to-peer channel, RMS delay spread
increases with excess path loss. Indeed systems that can
identify the strongest NLOS link will more often avoid high
RMS delay spread links.
V.
CONCLUSION
Millimeter wave (38 and 60 GHz) peer-to-peer outdoor
urban channels, and a 38 GHz outdoor cellular channel, were
measured using a broadband sliding correlator channel
sounder with rotatable 7 degree beamwidth antennas. The key
trends include near free-space path loss and virtually no RMS
delay spread for all LOS links, while NLOS links have higher
RMS delay spread, as much as 122 ns (for the 38 GHz peer-topeer channel) and 107 ns for the 38 GHz cellular channel. In
general, NLOS links offer increasing RMS delay spread as the
azimuth pointing angles are increased away from boresight at
either or both the transmitter and receiver. An interesting
result is that many unique paths can be formed in NLOS and
LOS channels using narrow beam antennas. By picking the
best combination of transmitter and receiver antenna pointing
angles at any location, path loss and RMS delay spread can be
reduced substantially. A key observation is the increase in
RMS delay spread for increasing excess path loss (above free
space) for both the peer-to-peer and cellular channels.
The data presented here suggest that while NLOS paths
may be formed for millimeter-wave channels, they will require
equalization and will have greater propagation latency, higher
power consumption, and lower data rates than LOS channels.
However, a system capable of determining the best
combination of antenna pointing angles for NLOS conditions
will benefit from higher SNR and lower expected RMS delay
spread. The angles over which links could be formed for the 38
GHz cellular channel suggest that outdoor urban canyons,
where tall buildings surround the receiver on two sides, will
This project is sponsored by Samsung Telecommunications
America, LLC. The authors wish to thank Samuel J.
Lauffenburger and Jonathan Tamir for their contributions to
this project. The measurements were taken under FCC
Experimental License 0548-EX-PL-2010.
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