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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 4, APRIL 2013
Broadband Millimeter-Wave Propagation
Measurements and Models Using Adaptive-Beam
Antennas for Outdoor Urban Cellular
Communications
Theodore S. Rappaport, Fellow, IEEE, Felix Gutierrez, Jr., Student Member, IEEE,
Eshar Ben-Dor, Student Member, IEEE, James N. Murdock, Student Member, IEEE,
Yijun Qiao, Student Member, IEEE, and Jonathan I. Tamir, Student Member, IEEE
Abstract—The spectrum crunch currently experienced by mobile cellular carriers makes the underutilized millimeter-wave
frequency spectrum a sensible choice for next-generation cellular
communications, particularly when considering the recent advances in low cost sub-terahertz/millimeter-wave complementary
metal–oxide semiconductor circuitry. To date, however, little is
known on how to design or deploy practical millimeter-wave
cellular systems. In this paper, measurements for outdoor cellular
channels at 38 GHz were made in an urban environment with
a broadband (800-MHz RF passband bandwidth) sliding correlator channel sounder. Extensive angle of arrival, path loss, and
multipath time delay spread measurements were conducted for
steerable beam antennas of differing gains and beamwidths for
a wide variety of transmitter and receiver locations. Coverage
outages and the likelihood of outage with steerable antennas were
also measured to determine how random receiver locations with
differing antenna gains and link budgets could perform in future
cellular systems. This paper provides measurements and models
that may be used to design future fifth-generation millimeter-wave
cellular networks and gives insight into antenna beam steering
algorithms for these systems.
Index Terms—Angle of arrival (AOA), beamforming antennas,
cellular, fifth generation (5G), millimeter-wave propagation measurements, mobile communications, 38 GHz.
Manuscript received February, 2012; revised August, 2012; accepted
November 25, 2012. Date of publication December 20, 2012; date of current
version April 03, 2013. This work was supported in part by the U.S. Army
Research Laboratory and in part by Samsung DMC R&D Communications
Research Team (CRT) and Samsung Telecommunications America, LLC.
Portions of this paper were published in IEEE’s Global Communications
Conference [15], Radio Wireless Symposium [17], Wireless Networking &
Communication Conference [18], and International Conference on Communications [19].
T. S. Rappaport is with NYU WIRELESS, New York University and also
with Polytechnic Institute of New York University (NYU-Poly), New York, NY
USA 10003 (e-mail: tsr@nyu.edu).
F. Gutierrez, Jr. is with the Electrical and Computer Engineering Department, University of Texas at Austin, Austin, TX 78712 USA and also with NYU
WIRELESS, Brooklyn, NY 11201 USA (e-mail: felixgutierrez@poly.edu).
E. Ben-Dor is with Javelin Semiconductor, Inc., Austin, TX 78704 USA
(e-mail: esharbd@gmail.com).
J. N. Murdock was with the University of Texas at Austin, Austin, TX 78712
USA. He is now with Texas Instruments, Inc., Dallas, TX 75266 USA (e-mail:
james.murdock741@gmail.com).
Y. Qiao is with the Electrical and Computer Engineering Department, Rice
University, Houston, TX 77251 USA (e-mail: markqiao@gmail.com)
J. I. Tamir is with the Electrical Engineering and Computer Sciences Department, University of California, Berkeley, CA 94720 USA (e-mail: jtamir@eecs.
berkeley.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TAP.2012.2235056
I. INTRODUCTION
U
NDERSTANDING radio propagation is vital for the
successful design and implementation of new wireless
communication systems operating at higher frequencies and
bandwidths. The advancement of sub-terahertz (THz) semiconductor technology has now made millimeter-wave cellular
systems feasible [1]. While outdoor channel measurements at
millimeter-wave frequencies (e.g., frequencies above 30 GHz,
or wavelengths 10 mm or less) have been conducted by many
(for example, rain attenuation [2]–[4]; foliage attenuation [5],
[6]; multipath delay spread [7], [8]; angle of arrival (AOA)
[9]; reflection coefficients of materials [10], [11]; and coverage
outage probability [12], [13]), past work has been done for
either ground level or fixed point (i.e., 28 GHz LMDS) wireless
communications. Previous researchers have not considered the
propagation of millimeter waves using steerable antennas for
cellular/mobile applications.
This paper provides a comprehensive propagation study
for outdoor urban millimeter wave (e.g., sub-THz) cellular
networks with beam steering. Our work considers a variety of
elevated transmitters that represent typical fifth-generation (5G)
base-station locations at heights of two or more stories above
ground level, and dozens of ground-level receiver locations.
Highly directional steerable horn antennas at the transmitter
and receiver were used to measure the propagation channel
for angle of arrival (AOA), multipath time delay spread, and
propagation path loss. An 800-MHz null-to-null passband
bandwidth spread-spectrum sliding correlator channel sounder
[14], [15], similar to that used in [4] and [16], was built to perform extensive outdoor cellular millimeter-wave propagation
measurements at 37.625-GHz center frequency. An RF signal
power of 22 dBm was delivered to the transmit base-station
antenna, which was a 7.8 half-power beamwidth Ka-band
vertically polarized 25-dBi horn antenna to produce 47-dBm
EIRP [15], [17]–[19]. The receiver uses another Ka-band
vertically polarized horn antenna of either 13.3-dBi gain (49.4
beamwidth), or 25-dBi gain (7.8 beamwidth).
The idea of using beamsteering to form links within cellular
networks is not new [20], [21], but past work has not considered
the use of millimeter-wave spectrum and the additional capabilities of small form factor steerable antennas at the handset
0018-926X/$31.00 © 2012 IEEE
RAPPAPORT et al.: BROADBAND MILLIMETER-WAVE PROPAGATION MEASUREMENTS AND MODELS
Fig. 1. Use of highly directional receiver antennas in cellular millimeter-wave
systems would significantly reduce interference since co-channel interference
from off-boresight directions would be rejected. Figure adapted from [20].
and base station for mobile/cellular use. Moving to the millimeter-wave spectrum would provide orders of magnitude of
available spectrum to cellular carriers when compared to today’s
global 4G allocation, while simultaneously supporting inband
backhaul. Spatial-division multiple access (SDMA) and beam/
path combining could be utilized along with temporal or frequency-based multiple-access techniques to greatly increase capacity and spectrum reuse. For example, in [22], a cell tower
coverage region is broken into six sectors of 60 each, which
yields at least a 5 increase in the number of users that a cell
site can handle. The benefits provided by spatial-division multiple-access (SDMA) systems [23] include an extended range
due to high gain antennas, reduced interference by intelligently
controlling the beam direction, and increased cell capacity. The
problem of intercell interference, which currently plagues dense
heterogeneous network deployments, would be significantly reduced with the use of highly directional steerable beam antenna
arrays at the mobile and/or base station. As illustrated in Fig. 1,
interference becomes less likely due to the narrow beamwidth
at the base station. Thus, millimeter-wave cellular systems are
most likely to be noise limited at heavily shadowed locations
rather than limited by interference. The use of small directional
antennas leads to a new research field of antenna pointing protocols. Early work on millimeter-wave antenna pointing protocols
appears in [24], and is based on iterative antenna training using
pseudonoise sequences. Additional millimeter-wave protocols
for pointing antennas at both base station and mobile handsets
were recently presented in [25] and [26], and through the use of
narrowband pilot signals, antenna pointing directions and multipath angular spreads can be rapidly determined [27], [28].
Recently, Samsung Electronics proposed a multibeam cellular system operating at millimeter-wave frequencies (see
Fig. 2). Steerable directional beams are used at the base station
and mobile handset with the base stations also capable of
communicating with each other for coordination and backhaul
infrastructure [22]. As seen in Fig. 2, inner cells use wireless
backhaul to send data to outer cells which have fiber-optic links
to the packet data server gateways [29], [33]. The cell size must
be sufficiently small to provide for substantial spectral reuse
gains and sufficient capacity within the cell, yet large enough
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Fig. 2. Cell system may use steerable antenna arrays to communicate with mobile devices using multiple beams. Wireless backhaul is used from the inner cells
to the outer cells, where fiber-optic connections move the data to the packet data
server gateway. (Figure reproduced with permission from Jerry Z. Pi of Samsung Telecommunications America [29].)
to minimize the number of base stations and capital equipment
costs. This work gives early insights into cell coverage density
for typical urban millimeter-wave cellular deployments, and the
potential for combining energy from multiple pointing angles
using spatial multiple input multiple output (MIMO). Other
studies of millimeter-wave cellular systems may be found in
[30].
In this paper, we intentionally did not consider Doppler effects in our measurements, since Doppler is well understood
to induce time-selective fading that can be mitigated by packet
sizing and appropriate coding over the coherence time of the
channel [14].
This paper is organized as follows. The experimental design
and measurement methodology are explained in Section II. Results and analysis of path loss, multipath delay spread, and AOA
measurements in urban outdoor environments are discussed in
Section III. Section IV summarizes the results of this work and
concludes this paper.
II. EXPERIMENTAL DESIGN FOR THE CELLULAR URBAN
MEASUREMENTS
Channel sounding requires spatial averaging over a local area
in order to increase the signal-to-noise ratio (SNR) and understand small-scale fading [14]. At each receiver location and for
each TX-RX antenna orientation, we used a circular track to
collect power delay profiles (PDPs) at eight local-area measurement points spaced in 45 increments along the track. The radius
of the track yielded a
( 8 cm) separation distance between
adjacent measurement points along the circular track. The PDPs
at each of the eight local-area measurement points were collected over a 2-s period and averaged together to form a single
PDP. This process was repeated for each unique combination
of TX and RX antenna pointing angles and for each receiver
location [15], [19]. The assumption that received power does
not change significantly between local-area points was tested
by calculating the standard deviation of the path loss in each set
of eight local-area track points. The standard deviation of path
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loss over the local area was 1.20 dB, on average, with a maximum standard deviation of 1.74 dB at one location. The precise
small-scale fading behavior of individual multipath components
at millimeter-wave frequencies is the subject of current measurements in New York City.
To capture the various line-of-sight (LOS) and non-LOS
(NLOS) links present at each receiver location along the track,
25-dBi narrowbeam (7.8 beamwidth) horn antennas at the
TX and RX were systematically and iteratively steered in the
azimuth and elevation directions, emulating a beam-steering
antenna-array architecture. For LOS links, the transmitter and
receiver were pointed directly at each other in both azimuth
and elevation directions, corresponding to 0 azimuth scanning
angles for the transmitter and receiver. Often, it was possible to
receive many NLOS links for different TX/RX pointing angle
combinations, with multipath signals often 10–20 dB weaker
from the strongest received signal [15], [17]. Measurements
were recorded for all observed LOS and NLOS links at all
receiver locations.
The transmitter was placed at four rooftop locations within
the University of Texas at Austin campus: WRW-A, ENS-A,
ENS-B, and ECJ. At each transmitter location, the antenna was
mounted on a tripod 1.5 m above the roof toward the building’s
edge. Fig. 3 shows a map of all transmitter and receiver locations. There were a total of 36 unique receiver locations in the
environment, of which seven locations were measured from two
different transmitter locations, yielding 43 unique TX–RX measurement locations. At each measurement location, there were
typically 8 to 12 unique antenna pointing combinations between
the TX and RX that provided viable communication links, resulting in a total of 732 unique measured links, where each
TX–RX pointing angle combination was locally averaged at the
receiver.
All RX measurement locations used steerable receiver antennas with a 25-dBi vertically polarized rectangular horn antenna. In addition, about five of the same receiver locations for
each base-station transmitter were also measured using a wider
beamwidth 13.3-dBi vertically polarized rectangular horn antenna. The 25-dBi receiver antenna locations ranged from 29
to 930 m from the transmitter. The 13.3-dBi antenna locations
were between 70 m and 728 m from the transmitter.
The first transmitter location was on the northern edge of
a five-story rooftop (23 m) labeled as WRW-A. An image
of the environment from the transmitter perspective is presented on the top left of Fig. 4. Eleven receiver locations were
examined from WRW-A using the 25-dBi antenna, with six
being partially obstructed and five having clear LOS. Using the
13.3-dBi antenna, six receiver locations were measured, two
of which were partially obstructed and four had clear LOS.
Transmitter-to-receiver (TR) separation distances at WRW-A
ranged from 61 to 265 m. The next two transmitter locations
were an eight-story building (36 m) along the northern (ENS-A)
and eastern (ENS-B) edges of the rooftop. Fig. 4 shows the
perspective of the transmitters with the top right for ENS-A
and bottom right for ENS-B. For ENS-A, five of the 11 receiver
locations were obstructed and six locations had clear LOS using
the 25-dBi antenna and two of the five selected receiver locations for the 13.3-dBi antenna measurements were obstructed.
Fig. 3. Map of the northeastern corner of the University of Texas at Austin
campus showing the transmitter and receiver locations. All receiver locations
were measured using a narrowbeam 25-dBi gain antenna. About half of the receiver locations were also measured using a wider beam 13.3-dBi antenna.
ENS-A locations had TR distances of 75 to 295 m. For ENS-B,
three of the 11 receiver locations were obstructed and the rest
had clear LOS. ENS-B had the longest TR distances of 132
to 930 m. The last transmitter location was at the northeastern
corner of the ECJ building and was approximately 8 m above
a parking lot and a busy four-lane street. The bottom left of
Fig. 4 shows the transmitter’s perspective from ECJ. Most
locations near the urban residential area were obstructed from
the transmitter by foliage and, thus, the TR separations were
shorter for ECJ, ranging from 29 to 225 m.
III. CELLULAR URBAN MEASUREMENT RESULTS
A. AOA Distributions
AOA distributions were generated for each transmitter location (e.g., base station). We defined a link as being any signal
with lower path loss than 160 dB, which is the maximum measured by the channel sounder system [17], [19] . A scatter plot
showing all of the receiver and transmitter azimuth angle combinations for all links made at the WRW-A transmitter location
is shown in Fig. 5. The right side of Fig. 5 contains a histogram
of the number of links for each receiver azimuth angle in 10
incremental bins. The bottom of Fig. 5 shows the distribution
of transmitter azimuth angles. Only measurements performed
with the 25-dBi RX antenna are included in the azimuth angle
histograms since the narrower beam is better suited for AOA information. The 13.3-dBi RX antenna measurements were conducted in the same locations and yielded similar results, although the number of observed links was slightly less due to
RAPPAPORT et al.: BROADBAND MILLIMETER-WAVE PROPAGATION MEASUREMENTS AND MODELS
Fig. 4. Images of the four transmitter (base station) locations looking toward
their environments: WRW-A (top left), ENS-A (top right), ECJ (bottom left),
and ENS-B (bottom right).
Fig. 5. Scatter plot of the RX and TX azimuth angles for the links made with
the WRW-A transmitter at 38 GHz. The distribution of links as a function of the
transmitter azimuth angle for steerable 25-dBi transmitter and receiver antennas
is shown below the scatter plot, and the distribution of the number of links as
a function of the receiver azimuth angle and TR distance is seen to the right of
the scatter plot.
a broader beamwidth that could not resolve individual links and
12 dB less link budget.
The transmitter azimuth angle distribution in Fig. 5 is very
narrow, with only 1.7% of the links having an off-boresight
transmitter azimuth angle larger than
. The concentration of transmitter azimuth angles near boresight could be explained by the low number of nearby scatterers in the rooftop
environment. Hence, potential scatterers are more predominant
at the ground-based mobile receiver antenna, which commonly
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has a wide array of objects surrounding it. The data show that future millimeter–wave base-station transmitter antennas need to
point in the general direction of the receiver, and further suggest
that base stations need not provide a beam that is steerable over
more than a 60 span in urban environments. Indeed, as seen in
Fig. 5, the angular path distribution for the receiver azimuth angles is more spread out, most notably for shorter TR distances,
yet the majority of usable receiver antenna angles are still concentrated near boresight since these links tend to travel shorter
distances (e.g., less path loss) and have less extreme reflection
angles. For the receiver, 70.7% of the links had azimuth angles
within
of boresight with 64.6% for links shorter than
150-m TR distance. A total of 75.9% of the links were made
within
(70.9% for links less than 150 m) and 84.5%
within
(81% for links less than 150 m). These numbers are summarized in Table I for all transmitter locations.
The site-specific environmental features dominate propagation for millimeter wave cellular. For example, at base station
WRW-A, many more links occur for the positive receiver azimuth angles due to the natural asymmetry of the transmitter
antenna with respect to the nearby ENS building. ENS is situated northeast of WRW as seen in Fig. 6, and the transmitter
location is almost exactly aligned with the western side of ENS.
For the majority of measured locations, the receiver was placed
in the courtyard seen in the upper left of Fig. 4. Thus, there
was a larger open area to the right of the receiver in the westward direction than to its left where ENS blocked most of the
eastern direction as seen in Fig. 6. The presence of ENS and
other nearby multistory buildings (e.g., ECJ, PAT, RLM) reduced the number of large receiver azimuth angles by narrowing
the view of the receiver antenna from both sides. This is especially true for receiver locations 4, 6, and 8. These types of behaviors were consistently observed throughout this study, suggesting that site-specific RF planning based on ray tracing or
other predictive methods will be useful for deployments [18],
[31] .
The scatter plots and histograms for ENS-A and ENS-B
transmitter locations are shown in Figs. 7 and 8, respectively.
An even tighter distribution of transmitter azimuth angles was
observed, since the increase in transmitter antenna height led
to longer TR separation distances and fewer scatterers in the
vicinity of the transmitter. However, the building spacing in
the environments of ENS-A and ENS-B was much greater than
for WRW-A as seen in Fig. 4. Therefore, a larger spread of
receiver azimuth angles was observed for the ENS locations,
with 53.3% and 66.1% of the links being less than
off-boresight for ENS-A and ENS-B, respectively, and similar
behavior when angles up to
were considered as seen
in Table I. The longer TR distances of ENS-B links and, thus,
greater propagation loss explain why there are fewer links with
large receiver angles.
The ECJ transmitter location was the lowest in height, only
8 m above ground. One would expect a lower base-station antenna height would yield results that closely resemble ground
communications links [15] rather than higher transmitter locations. Fig. 9 shows that only 11.5% of all links were made with
transmitter azimuth angles greater than
. And 64.8%
of the receiver azimuth angles were less than
for ECJ
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TABLE I
SUMMARY OF THE RECEIVER AZIMUTH AOA DISTRIBUTIONS FOR ALL ENVIRONMENTS
Fig. 6. Environment of the WRW-A transmitter location. The transmitter position was nearly in line with the western side of ENS, thus reducing the number
of reflections coming from the receiver’s eastern direction (e.g., right side of
image).
Fig. 8. Scatter plot and distribution of the RX and TX azimuth angles for links
at ENS-B at 38 GHz using steerable 25-dBi TX and RX antennas.
Fig. 7. Scatter plot and distribution of the RX and TX azimuth angles for links
at ENS-A at 38 GHz using steerable 25-dBi TX and RX antennas.
growing to 75.4% of angles within
of boresight. Similar results were measured when considering only links shorter
than 150-m TR separation (see Table I).
The ECJ environment included a parking lot and streets
within 150 m from the transmitter location with several receiver locations near a residential area. At receiver locations
near the transmitter, many scatterers were identified, including
those at large azimuth angles off-boresight. On the other hand,
fewer links with large receiver angles were found for the residential area receiver locations since these areas were heavily
shadowed by trees, thus incurring additional penetration loss.
Fig. 9. Scatter plot and distribution of RX and TX azimuth angles for links at
ECJ at 38 GHz using 25-dBi steerable antennas at TX and RX.
Overall, fewer large receiver azimuth angle links were made
with the wider beam 13.3-dBi receiver antenna, since the
smaller receiver antenna gain offers less link margin and is thus
unable to detect paths traveling longer distances. The data in
Figs. 5–9 show that lower base-station height yields a greater
range of transmitter azimuth angles for creating links. The
site-specific environment controls the distribution of receiver
azimuth angles, since the majority of reflectors are closer to
RAPPAPORT et al.: BROADBAND MILLIMETER-WAVE PROPAGATION MEASUREMENTS AND MODELS
TABLE II
SUMMARY OF PATH-LOSS EXPONENTS AND STANDARD DEVIATION FOR
ALL 38-GHZ CELLULAR MEASUREMENTS WITH STEERABLE TX AND RX
ANTENNAS
the receiver position. These are key results that directly impact
cell- and base-station layout and antenna design [18].
B. Propagation Path Loss
Path loss was extracted for every measured multipath component (e.g., link) made with every unique pair of transmitter
and receiver pointing angles across the 43 measurement locations. For each measured link, the radio path was identified as
clear LOS for an unobstructed path when the TX and RX antennas were pointing at each other, partially obstructed LOS
with some physical obstructions when the beams were pointed
at each other, and NLOS links where antennas were pointed off
boresight, thereby exploiting reflections. Path loss scatter plots
were modeled using the standard log-normal shadowing model
[14], where the measured path loss data was fitted with a MMSE
best-fit path loss exponent. The log-normal shadowing model is
given by (1), where the path loss
is in dB and is a function of distance and assumed to be a random value. Path loss is
related to a close-in free-space reference distance,
5 m,
and is modeled by the path loss exponent and a shadowing
random variable
which is represented as a Gaussian random
variable in dB with zero mean and dB standard deviation [34].
(1)
The log-normal shadowing model has been used to model any
arbitrary link without consideration of antenna characteristics.
However, the introduction of steerable and highly directional
antennas leads to a significant dependence of the path-loss exponent and
on antenna orientations. Since TX and RX antennas were pointed at a wide range of angles, we considered
radio propagation path loss for two cases: one case was when
TX and RX antennas had a visible line of sight between each
other, and were pointed at each other (LOS-directed), and the
other case was when the RX and TX antennas did not have a visible LOS due to obstructions, and were not pointed at each other
so that reflections or scattering could be used to make a NLOS
link (NLOS-directed). LOS-directed antennas consistently had
lower path loss than NLOS, even in cases with partially obstructed LOS link due to foliage or edges of buildings. The propagation results were discussed previously in [17]. Table II summarizes those results.
The measured data are further separated by transmitter locations with the model parameters summarized in Table III. Under
the assumption that a system with antenna steering capability
will first search for the best possible link, the NLOS parameters are specified for the best (lowest path loss) NLOS link at
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each receiver location. Table III shows that the transmitter location with the fewest partially obstructed LOS links, ENS-B,
experienced the lowest LOS path loss including both clear and
partially obstructed links with exponents of 2.01 and 2.03 for
the 25-dBi and 13.3-dBi RX antennas, respectively. In contrast,
the transmitter location with the most partially obstructed LOS
links was ECJ, which had the highest LOS path-loss exponents
of 2.99 and 2.74. It is worth noting from Table III that when
a link could be made, the wider beam 13.3-dBi RX antenna
provided less path loss when compared with the narrowbeam
25-dBi antenna at the exact same location, indicating that wider
beamwidths capture more signal energy at the mobile RX, albeit
with less overall link budget available.
Another environment-dependent quantity is the variability of
the path loss of LOS links, as expressed by , the standard
deviation (e.g., shadowing) of the least-squares fit error. By
comparing ENS-B and ECJ LOS measurements, it is clear that
the more obstructed regions, such as those measured from the
ECJ base station, have a more variable LOS path loss than the
less obstructed environment for the ENS-B transmitter. While
ENS-B had modest variation in LOS path loss from its best fit,
resulting in shadowing standard deviation of 6.56 and 5.31 dB
for the 25- and 13.3-dBi RX antennas, respectively, it can be
seen that ECJ had a much higher shadowing variation of 13.92
and 12.46 dB. The strongest NLOS links also had greater variance at these more obstructed locations [32].
C. Millimeter-Wave Cellular RMS Delay Spread
The RMS delay spreads cumulative distribution functions
(CDFs) for all LOS and NLOS links at each transmitter location
are shown in Fig. 10 using a 25-dBi receiver antenna. Some
location-specific variations were observed since ENS-B had
the lowest mean RMS delay spread of 5.3 ns versus the highest
mean RMS delay spread of 16.5 ns at ECJ. As previously
mentioned, differences in environment and TR separations
make these two transmitter location links quite different from
each other. Another noticeable trend is the 99-percentile values,
which show that WRW-A and ENS-B had significantly lower
maximum RMS delay spreads (65.4 ns and 27.6 ns at 99%, respectively) than the other two environments. A possible reason
for WRW-A having fewer very high RMS delay spread links is
that none of the WRW-A receiver locations were near a street,
while the other transmitter environments contained receiver
locations adjacent to a wide street. The street was found to be a
good environment for yielding high RMS delay spreads, since
it has many reflective objects spaced in nearly regular intervals
for long distances and very few obstructions that block reflected
or scattered waves. For example, a street is typically lined with
parked vehicles for many tens of meters. In addition, a street
has many light poles, moving vehicles, surrounding buildings,
street signs, and pedestrians, all of which have been found to
be reflectors at millimeter-wave frequencies [15]. ENS-B links
had, on average, a longer separation distance. These longer
links had significantly lower delay spreads due to the attenuation of longer traveling paths, as discussed in Section III.
The dependence on receiver antenna gain can be seen by comparing the results in Figs. 10 and 11, which plot the CDFs between RX antennas. While all of the plots in Fig. 10 look sim-
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TABLE III
SUMMARY OF PATH-LOSS EXPONENTS AND SHADOWING STANDARD DEVIATIONS FOR ALL TRANSMITTER AND RECEIVER LOCATIONS AND LINKS AT 38 GHZ
finite probing pulse width, and no multipath distortion from
propagation) with one partially obstructed LOS link resulting
in a maximum of 15.5 ns. The NLOS measurements exhibited
higher and more varied RMS delay spreads, with a mean of
14.8 ns for the 25-dBi receiver antenna and 13.7 ns for the
13.3-dBi receiver antenna. The maximum NLOS RMS delay
spreads were 185 and 166 ns for the 25- and 13.3-dBi receiver
antennas, respectively. Nonetheless, more than 80% of the
NLOS links had RMS delay spreads under 20 ns and 90% of
the NLOS links had RMS delay spreads under 40 ns.
D. Trends in RMS Delay Spread
Fig. 10. RMS delay spread CDFs for each transmitter location and a CDF for
all of the measured links using the steerable 25-dBi RX antenna at 38 GHz. The
expected and 99-percentile values for each CDF are displayed on the plot.
Fig. 11. RMS delay spread CDFs for each transmitter location and a CDF for
all of the measured links using the steerable 13.3-dBi RX antenna at 38 GHz.
The expected and 99-percentile values for each CDF are displayed on the plot.
ilar to each other (except for ENS-B that has long TR separation
distances), a much wider variety of CDFs was produced when
using the wider beam antenna. As discussed later, the system
sensitivity had a strong effect on the RMS delay spread, as the
lower gain 13.3-dBi RX antenna had higher RMS delay spreads
at smaller TR separations compared to the 25-dBi antenna, yet
the lower gain RX antenna had lower RMS delay spreads at locations with longer TR separations.
As discussed in [17], a considerable difference in RMS delay
spreads between LOS and NLOS links was observed. Most
LOS measurements had very minimal RMS delay spread, on
the order of 1 ns, due solely to the transmitted pulse shape (i.e.,
To build power-efficient, low-overhead millimeter-wave mobile communication systems, future systems will require the
ability to adjust antenna pointing angles, while jointly considering multipath delay spread and path loss needed to make a
suitable link. The characteristics of the RMS delay spread from
this measurement campaign were studied in [17], where it was
found that the mean and worst case RMS delay spread increase
as the antenna angle is pointed away from the LOS angle (e.g.,
boresight) at the TX and RX. It was also found that mean and
worst case RMS delay spread decrease with increasing TR separation distances. The main reason for these trends is that a
stronger received signal is caused by one or a few strong multipath components arriving at different specific angles. These
strong multipath components dominate the delay spread and
motivate the use of millimeter-wave cellular where directional
low path-loss links can carry very high data rates with small
RMS delay spread. However, when the TX and RX antennas
are steered away from each other at relatively close TR separation distances of a couple of hundreds of meters or less, strong
LOS and other strong multipath components are less likely, and
the RMS delay spread becomes much greater since multipath
components arrive from many different scattering and reflection
mechanisms. At greater TR separation distances beyond several hundred meters, the number of receivable multipath components decreases due to propagation loss, thus causing fewer detectable multipath components and smaller RMS delay spreads.
Since our data show that RMS delay spread increases and becomes more variable as TX and RX antennas are pointed away
from boresight, future mobile devices at a particular location
should prefer a link using relatively small off-boresight antenna
pointing angles (
) compared to a link of similar strength
that uses large pointing angles off boresight (
) [17]. Finally, when considering the cell edge where the TR separation
may be nearly a kilometer, the measurement results show that
RAPPAPORT et al.: BROADBAND MILLIMETER-WAVE PROPAGATION MEASUREMENTS AND MODELS
TABLE IV
A COMPARISON OF THE OUTAGE STATISTICS FOR THE TWO TX LOCATIONS
expected RMS delay spreads are very low. Thus, less equalization is required near the cell edge. The reduced power and latency for equalization of these cell-edge links can be put to use
in other processing areas, such as additional error coding for this
lower SNR case.
E. Cellular Urban Outage Study
An important open question for a cellular millimeter-wave
system in dense outdoor urban settings is the extent of cell coverage for a given transmitter height. The AOA studies discussed
previously showed that NLOS paths exist and can be used to increase coverage. The extent of the coverage was examined in
[18]. The cellular outage study was performed at the University
of Texas at Austin campus with measurements made within approximately 400 m around the transmitter locations. Measurements from two transmitter locations provided outage probability for base stations of different heights. The probability was
also broken into outages present for a system with sensitivity
of up to 160-dB path loss and a less sensitive system with up
to 150-dB path loss for the smaller gain RX antenna case. The
outage probability is summarized in Table IV.
As expected for the lower elevation transmitter (TX2-WRW),
links over 200 m were made less frequently than at the higher
transmitter location TX1-ENS, resulting in an outage rate of
39.6% based on a system sensitivity of 160-dB path loss. It is
important to note that for both the high and low base-station
transmitter locations, no outages were observed for all random
measurement locations within a 200-m radius. In addition, the
lower transmitter position benefited from a larger number of
suitable reflectors in the environment for links less than 200 m
away, since the vertical angle of incidence from the low transmitter location to a given reflector was reduced compared to incidence from the high transmitter location. This led to a higher
number of links with TR separation under 200 m that had less
than 150-dB path loss at the lower TX location than at the higher
TX location (10% outage compared to 27.3%). Further work in
other urban environments is needed to determine whether 200
m offers complete coverage for millimeter wave cellular.
IV. CONCLUSION
While several past studies have characterized the outdoor millimeter-wave channel for wireless backhaul and
ground-level communications, there has been a significant lack
of information about the millimeter wave cellular (base-station
to mobile) channel. Our work has provided 38-GHz radio
propagation channel data for outdoor systems capable of implementing antenna beam steering. The work here considered
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typical future-generation cellular base-station locations using
steerable antennas at both receiver and transmitter locations,
in both azimuth and elevation directions. We measured AOA
statistics of viable RF links. It was shown that the elevated
transmitters at heights of two to eight stories above ground
require 60 of scanning (up to
off-boresight) in the
azimuth direction to cover nearly all possible NLOS links. The
receiver antenna, however, would benefit from larger scanning
freedom. A NLOS link is rarely preferred over an LOS or
partially obstructed LOS link, since NLOS links tend to have
10 to 50 dB more path loss and higher expected RMS delay
spread. When the LOS direction is completely blocked by a
building or other shadowing objects, the work here shows that a
reflection, scattered, or diffraction path may still have sufficient
signal strength to be received, albeit at a lower signal level.
Distant-dependent propagation path-loss models were provided
to account for LOS, NLOS, as well as the best possible path
provided in NLOS conditions when using steerable antennas at
the TX and RX.
Our outage study indicates that increasing the base-station
transmitter height in a dense urban environment provides coverage to a greater percentage of locations past 300 m from the
transmitter, but the improvement is “spotty” in that large regions
past 300 m still lack coverage [18]. Our outage study indicates
that a lower base station is able to use many reflectors in the environment to cover all locations within a 200-m radius from the
base station. This work suggests that millimeter-wave cellular
systems may work best in dense urban environments with microcell deployments with cell radii less than 200 m.
This study answered important questions regarding path loss,
RMS delay spread, and signal coverage for millimeter-wave
urban outdoor cellular channels for steerable antenna architectures. However, many additional measurements are needed to
cover all environments of interest and to develop full channel
models for standards development. These are likely to include
antenna angle-dependent channel models due to the directionality and steerability of these novel communication systems.
Moreover, the spatial and temporal small-scale variations are of
great interest at these short wavelengths; thus, extensive work
remains to develop a complete millimeter-wave cellular channel
model. Finally, large environment dependency of receiver and
transmitter AOA distributions suggests the usefulness of sitespecific cell design using ray-tracing models.
ACKNOWLEDGMENT
The authors would like to thank students T. Forbes, S. J. Lauffenburger, and A. Duran for their contributions to the project,
Samsung researchers S. Rajagopal, S. Abu-Surra, and J. Z. Pi
for their ongoing interest and support of this work, Hughes Research Laboratory and National Instruments for donating equipment, and the reviewers and editor for their helpful comments.
This work was sponsored by Samsung DMC R&D Communications Research Team (CRT) through Samsung Telecommunications America, LLC.
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Theodore S. Rappaport (F’98) received the B.S.,
M.S., and Ph.D. degrees in electrical engineering
from Purdue University, West Lafayette, IN, USA,
in 1982, 1984, and 1987, respectively.
He is an Outstanding Electrical and Computer Engineering Alumnus and Distinguished Engineering
Alumnus from his alma mater. He holds the David
Lee/Ernst Weber Chair in Electrical and Computer
Engineering at Polytechnic Institute of New York
University (NYU-Poly), Brooklyn, NY, USA, and
is Professor of Computer Science and Professor of
Radiology at NYU. In 2012, he founded NYU WIRELESS, a multidisciplinary
research center involving NYU’s engineering, computer science, and medical schools. Earlier in his career, he founded the Wireless Networking and
Communications Group (WNCG) at the University of Texas at Austin (UT),
USA. Prior to UT, he was on the electrical and computer engineering faculty
of Virginia Polytechnic Institute and State University, Blacksburg, VA, USA,
where he founded the Mobile and Portable Radio Research Group (MPRG),
one of the world’s first university research and teaching centers dedicated to the
wireless communications field. In 1989, he founded TSR Technologies, Inc.,
Blacksburg, a cellular-radio/personal-communications-services software radio
manufacturer that pioneered cellular E-911 and test equipment that he sold in
1993 to what is now CommScope, Inc. In 1995, he founded Wireless Valley
Communications Inc., Austin, TX, USA, a site-specific wireless network
design and management firm that was sold in 2005 to Motorola, Inc. He has
testified before the U.S. Congress, has served as an international consultant for
the ITU, has consulted for more than 30 major telecommunications firms, and
works on many national committees pertaining to communications research
and technology policy. He is a highly sought-after consultant and technical
expert, and serves on the Board of Directors of the Marconi Society. He has
authored or coauthored more than 200 technical papers, over 100 U.S. and
international patents, and several best-selling technical books.
Dr. Rappaport was elected to the Board of Governors of the IEEE Communications Society (ComSoc) in 2006, and was elected to the Board of Governors
of the IEEE Vehicular Technology Society (VTS) in 2008 and 2011.
RAPPAPORT et al.: BROADBAND MILLIMETER-WAVE PROPAGATION MEASUREMENTS AND MODELS
Felix Gutierrez, Jr. (S’08) received the B.S. degree
in electrical engineering from the University of Texas
at Austin (UT), USA, in 2006, the M.S. degree in
electrical engineering from Texas A&M University,
College Station, TX, USA, in 2008, and is currently
pursuing the Ph.D. degree in electrical engineering at
UT.
He completed an internship with ETS-Lindgren,
Cedar Park, TX, USA, in 2010. He is currently a
Visiting Research Scholar at the Polytechnic Institute
of New York University (NYU-Poly), Brooklyn,
NY, USA. He has worked on millimeter-wave and sub-terahertz semiconductor
circuits and antennas for next-generation wireless communications.
Eshar Ben-Dor (S’08) received the B.S. degree in
electrical engineering from the Ohio State University
(OSU), Columbus, OH, USA, in 2009 and the M.S.E.
degree with a focus on integrated microwave circuits
and millimeter-wave communications from the University of Texas at Austin (UT) in 2011
During his time at OSU, he worked part-time in the
Nanoscale Patterning Laboratory on electron-beam
lithography resist technology. Currently, he is an IC
Design Engineer at Javelin Semiconductor, Austin,
TX, focusing on cellular-phone complementary
metal–oxide semiconductor power amplifiers.
James N. Murdock (S’10) received the B.S.E.E. and
M.S.E. degrees in electrical engineering at The University of Texas at Austin (UT) in 2008 and 2011,
respectively.
He has co-authored two journal publications and
11 conference or magazine publications. In 2011,
he completed an internship at Texas Instruments,
Dallas, TX, USA, in sub-THz antenna design. Currently, he is an Analog Design Engineer with Texas
Instruments, where he focuses on low-power radio
frequency design. His research interests include
sub-THz/THz design, low-power design, and scientific data archiving.
Mr. Murdock volunteers with the United Way, FIRST Robotics, and Communities in Schools Dallas Region.
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Yijun Qiao (S’08) received the B.S. degree in
electrical engineering from the University of Texas
at Austin, Austin, TX, USA, in 2012 and is currently
a graduate ECE student at Rice University, Houston,
TX.
As an Engineering Honors student, he joined
Prof. Theodore S. Rappaport’s research team in Fall
2010 and worked on the millimeter-wave channel
sounding project for one year. He developed a
measurement track now being used for propagation
research at Polytechnic Institute of New York
University (NYU-Poly), Brooklyn, NY, USA.
Jonathan I. Tamir (S’11) received the B.S. degree
in electrical engineering from the University of
Texas at Austin, Austin, TX, USA, in 2011 and is
currently pursuing the Ph.D. degree in electrical
engineering and computer sciences at the University
of California, Berkeley, CA, USA, with a focus on
signal processing for medical imaging and communication systems.