Broadband Millimeter Wave

1850 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 4, APRIL 2013

Broadband Millimeter-Wave PropagationMeasurements and Models Using Adaptive-Beam

Antennas for Outdoor Urban CellularCommunications

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 mo-bile cellular carriers makes the underutilized millimeter-wavefrequency spectrum a sensible choice for next-generation cellularcommunications, particularly when considering the recent ad-vances in low cost sub-terahertz/millimeter-wave complementarymetal–oxide semiconductor circuitry. To date, however, little isknown on how to design or deploy practical millimeter-wavecellular systems. In this paper, measurements for outdoor cellularchannels at 38 GHz were made in an urban environment witha broadband (800-MHz RF passband bandwidth) sliding corre-lator channel sounder. Extensive angle of arrival, path loss, andmultipath time delay spread measurements were conducted forsteerable beam antennas of differing gains and beamwidths fora wide variety of transmitter and receiver locations. Coverageoutages and the likelihood of outage with steerable antennas werealso measured to determine how random receiver locations withdiffering antenna gains and link budgets could perform in futurecellular systems. This paper provides measurements and modelsthat may be used to design future fifth-generation millimeter-wavecellular networks and gives insight into antenna beam steeringalgorithms for these systems.

Index Terms—Angle of arrival (AOA), beamforming antennas,cellular, fifth generation (5G), millimeter-wave propagation mea-surements, mobile communications, 38 GHz.

Manuscript received February, 2012; revised August, 2012; acceptedNovember 25, 2012. Date of publication December 20, 2012; date of currentversion April 03, 2013. This work was supported in part by the U.S. ArmyResearch Laboratory and in part by Samsung DMC R&D CommunicationsResearch Team (CRT) and Samsung Telecommunications America, LLC.Portions of this paper were published in IEEE’s Global CommunicationsConference [15], Radio Wireless Symposium [17], Wireless Networking &Communication Conference [18], and International Conference on Communi-cations [19].T. S. Rappaport is with NYU WIRELESS, New York University and also

with Polytechnic Institute of New York University (NYU-Poly), New York, NYUSA 10003 (e-mail: [email protected]).F. Gutierrez, Jr. is with the Electrical and Computer Engineering Depart-

ment, University of Texas at Austin, Austin, TX 78712 USA and also with NYUWIRELESS, Brooklyn, NY 11201 USA (e-mail: [email protected]).E. Ben-Dor is with Javelin Semiconductor, Inc., Austin, TX 78704 USA

(e-mail: [email protected]).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:[email protected]).Y. Qiao is with the Electrical and Computer Engineering Department, Rice

University, Houston, TX 77251 USA (e-mail: [email protected])J. I. Tamir is with the Electrical Engineering and Computer Sciences Depart-

ment, University of California, Berkeley, CA 94720 USA (e-mail: [email protected]).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 thesuccessful design and implementation of new wireless

communication systems operating at higher frequencies andbandwidths. The advancement of sub-terahertz (THz) semi-conductor technology has now made millimeter-wave cellularsystems feasible [1]. While outdoor channel measurements atmillimeter-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 coverageoutage probability [12], [13]), past work has been done foreither ground level or fixed point (i.e., 28 GHz LMDS) wirelesscommunications. Previous researchers have not considered thepropagation of millimeter waves using steerable antennas forcellular/mobile applications.This paper provides a comprehensive propagation study

for outdoor urban millimeter wave (e.g., sub-THz) cellularnetworks with beam steering. Our work considers a variety ofelevated transmitters that represent typical fifth-generation (5G)base-station locations at heights of two or more stories aboveground level, and dozens of ground-level receiver locations.Highly directional steerable horn antennas at the transmitterand receiver were used to measure the propagation channelfor angle of arrival (AOA), multipath time delay spread, andpropagation path loss. An 800-MHz null-to-null passbandbandwidth spread-spectrum sliding correlator channel sounder[14], [15], similar to that used in [4] and [16], was built to per-form extensive outdoor cellular millimeter-wave propagationmeasurements at 37.625-GHz center frequency. An RF signalpower of 22 dBm was delivered to the transmit base-stationantenna, which was a 7.8 half-power beamwidth Ka-bandvertically polarized 25-dBi horn antenna to produce 47-dBmEIRP [15], [17]–[19]. The receiver uses another Ka-bandvertically polarized horn antenna of either 13.3-dBi gain (49.4beamwidth), 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 consideredthe use of millimeter-wave spectrum and the additional capa-bilities 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 1851

Fig. 1. Use of highly directional receiver antennas in cellular millimeter-wavesystems would significantly reduce interference since co-channel interferencefrom off-boresight directions would be rejected. Figure adapted from [20].

and base station for mobile/cellular use. Moving to the mil-limeter-wave spectrum would provide orders of magnitude ofavailable spectrum to cellular carriers when compared to today’sglobal 4G allocation, while simultaneously supporting inbandbackhaul. Spatial-division multiple access (SDMA) and beam/path combining could be utilized along with temporal or fre-quency-based multiple-access techniques to greatly increase ca-pacity and spectrum reuse. For example, in [22], a cell towercoverage region is broken into six sectors of 60 each, whichyields at least a 5 increase in the number of users that a cellsite can handle. The benefits provided by spatial-division mul-tiple-access (SDMA) systems [23] include an extended rangedue to high gain antennas, reduced interference by intelligentlycontrolling the beam direction, and increased cell capacity. Theproblem of intercell interference, which currently plagues denseheterogeneous network deployments, would be significantly re-duced with the use of highly directional steerable beam antennaarrays at the mobile and/or base station. As illustrated in Fig. 1,interference becomes less likely due to the narrow beamwidthat the base station. Thus, millimeter-wave cellular systems aremost likely to be noise limited at heavily shadowed locationsrather than limited by interference. The use of small directionalantennas leads to a new research field of antenna pointing proto-cols. Early work on millimeter-wave antenna pointing protocolsappears in [24], and is based on iterative antenna training usingpseudonoise sequences. Additional millimeter-wave protocolsfor pointing antennas at both base station and mobile handsetswere recently presented in [25] and [26], and through the use ofnarrowband pilot signals, antenna pointing directions and mul-tipath angular spreads can be rapidly determined [27], [28].Recently, Samsung Electronics proposed a multibeam cel-

lular system operating at millimeter-wave frequencies (seeFig. 2). Steerable directional beams are used at the base stationand mobile handset with the base stations also capable ofcommunicating with each other for coordination and backhaulinfrastructure [22]. As seen in Fig. 2, inner cells use wirelessbackhaul to send data to outer cells which have fiber-optic linksto the packet data server gateways [29], [33]. The cell size mustbe sufficiently small to provide for substantial spectral reusegains and sufficient capacity within the cell, yet large enough

Fig. 2. Cell system may use steerable antenna arrays to communicate with mo-bile devices usingmultiple beams.Wireless backhaul is used from the inner cellsto the outer cells, where fiber-optic connections move the data to the packet dataserver gateway. (Figure reproduced with permission from Jerry Z. Pi of Sam-sung Telecommunications America [29].)

to minimize the number of base stations and capital equipmentcosts. This work gives early insights into cell coverage densityfor typical urban millimeter-wave cellular deployments, and thepotential for combining energy from multiple pointing anglesusing spatial multiple input multiple output (MIMO). Otherstudies of millimeter-wave cellular systems may be found in[30].In this paper, we intentionally did not consider Doppler ef-

fects in our measurements, since Doppler is well understoodto induce time-selective fading that can be mitigated by packetsizing and appropriate coding over the coherence time of thechannel [14].This paper is organized as follows. The experimental design

and measurement methodology are explained in Section II. Re-sults and analysis of path loss, multipath delay spread, and AOAmeasurements in urban outdoor environments are discussed inSection III. Section IV summarizes the results of this work andconcludes this paper.

II. EXPERIMENTAL DESIGN FOR THE CELLULAR URBANMEASUREMENTS

Channel sounding requires spatial averaging over a local areain order to increase the signal-to-noise ratio (SNR) and under-stand small-scale fading [14]. At each receiver location and foreach TX-RX antenna orientation, we used a circular track tocollect power delay profiles (PDPs) at eight local-area measure-ment points spaced in 45 increments along the track. The radiusof the track yielded a ( 8 cm) separation distance betweenadjacent measurement points along the circular track. The PDPsat each of the eight local-area measurement points were col-lected over a 2-s period and averaged together to form a singlePDP. This process was repeated for each unique combinationof TX and RX antenna pointing angles and for each receiverlocation [15], [19]. The assumption that received power doesnot change significantly between local-area points was testedby calculating the standard deviation of the path loss in each setof eight local-area track points. The standard deviation of path


loss over the local area was 1.20 dB, on average, with a max-imum standard deviation of 1.74 dB at one location. The precisesmall-scale fading behavior of individual multipath componentsat millimeter-wave frequencies is the subject of current mea-surements 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 theTX and RX were systematically and iteratively steered in theazimuth and elevation directions, emulating a beam-steeringantenna-array architecture. For LOS links, the transmitter andreceiver were pointed directly at each other in both azimuthand elevation directions, corresponding to 0 azimuth scanningangles for the transmitter and receiver. Often, it was possible toreceive many NLOS links for different TX/RX pointing anglecombinations, with multipath signals often 10–20 dB weakerfrom the strongest received signal [15], [17]. Measurementswere recorded for all observed LOS and NLOS links at allreceiver 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 wasmounted on a tripod 1.5 m above the roof toward the building’sedge. Fig. 3 shows a map of all transmitter and receiver loca-tions. There were a total of 36 unique receiver locations in theenvironment, of which seven locations were measured from twodifferent transmitter locations, yielding 43 unique TX–RXmea-surement locations. At each measurement location, there weretypically 8 to 12 unique antenna pointing combinations betweenthe TX and RX that provided viable communication links, re-sulting in a total of 732 unique measured links, where eachTX–RX pointing angle combination was locally averaged at thereceiver.All RX measurement locations used steerable receiver an-

tennas with a 25-dBi vertically polarized rectangular horn an-tenna. In addition, about five of the same receiver locations foreach base-station transmitter were also measured using a widerbeamwidth 13.3-dBi vertically polarized rectangular horn an-tenna. The 25-dBi receiver antenna locations ranged from 29to 930 m from the transmitter. The 13.3-dBi antenna locationswere 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 imageof the environment from the transmitter perspective is pre-sented on the top left of Fig. 4. Eleven receiver locations wereexamined from WRW-A using the 25-dBi antenna, with sixbeing partially obstructed and five having clear LOS. Using the13.3-dBi antenna, six receiver locations were measured, twoof which were partially obstructed and four had clear LOS.Transmitter-to-receiver (TR) separation distances at WRW-Aranged from 61 to 265 m. The next two transmitter locationswere an eight-story building (36 m) along the northern (ENS-A)and eastern (ENS-B) edges of the rooftop. Fig. 4 shows theperspective of the transmitters with the top right for ENS-Aand bottom right for ENS-B. For ENS-A, five of the 11 receiverlocations were obstructed and six locations had clear LOS usingthe 25-dBi antenna and two of the five selected receiver loca-tions for the 13.3-dBi antenna measurements were obstructed.

Fig. 3. Map of the northeastern corner of the University of Texas at Austincampus showing the transmitter and receiver locations. All receiver locationswere measured using a narrowbeam 25-dBi gain antenna. About half of the re-ceiver 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 resthad clear LOS. ENS-B had the longest TR distances of 132to 930 m. The last transmitter location was at the northeasterncorner of the ECJ building and was approximately 8 m abovea parking lot and a busy four-lane street. The bottom left ofFig. 4 shows the transmitter’s perspective from ECJ. Mostlocations near the urban residential area were obstructed fromthe transmitter by foliage and, thus, the TR separations wereshorter for ECJ, ranging from 29 to 225 m.

III. CELLULAR URBAN MEASUREMENT RESULTS

A. AOA Distributions

AOA distributions were generated for each transmitter loca-tion (e.g., base station). We defined a link as being any signalwith lower path loss than 160 dB, which is the maximum mea-sured by the channel sounder system [17], [19] . A scatter plotshowing all of the receiver and transmitter azimuth angle com-binations for all links made at the WRW-A transmitter locationis shown in Fig. 5. The right side of Fig. 5 contains a histogramof the number of links for each receiver azimuth angle in 10incremental bins. The bottom of Fig. 5 shows the distributionof transmitter azimuth angles. Only measurements performedwith the 25-dBi RX antenna are included in the azimuth anglehistograms since the narrower beam is better suited for AOA in-formation. The 13.3-dBi RX antenna measurements were con-ducted in the same locations and yielded similar results, al-though the number of observed links was slightly less due to


Fig. 4. Images of the four transmitter (base station) locations looking towardtheir 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 withthe WRW-A transmitter at 38 GHz. The distribution of links as a function of thetransmitter azimuth angle for steerable 25-dBi transmitter and receiver antennasis shown below the scatter plot, and the distribution of the number of links asa function of the receiver azimuth angle and TR distance is seen to the right ofthe scatter plot.

a broader beamwidth that could not resolve individual links and12 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-boresighttransmitter azimuth angle larger than . The concentra-tion of transmitter azimuth angles near boresight could be ex-plained by the low number of nearby scatterers in the rooftopenvironment. Hence, potential scatterers are more predominantat the ground-based mobile receiver antenna, which commonly

has a wide array of objects surrounding it. The data show that fu-ture millimeter–wave base-station transmitter antennas need topoint in the general direction of the receiver, and further suggestthat base stations need not provide a beam that is steerable overmore than a 60 span in urban environments. Indeed, as seen inFig. 5, the angular path distribution for the receiver azimuth an-gles is more spread out, most notably for shorter TR distances,yet the majority of usable receiver antenna angles are still con-centrated near boresight since these links tend to travel shorterdistances (e.g., less path loss) and have less extreme reflectionangles. For the receiver, 70.7% of the links had azimuth angleswithin of boresight with 64.6% for links shorter than150-m TR distance. A total of 75.9% of the links were madewithin (70.9% for links less than 150 m) and 84.5%within (81% for links less than 150 m). These num-bers are summarized in Table I for all transmitter locations.The site-specific environmental features dominate propaga-

tion for millimeter wave cellular. For example, at base stationWRW-A, many more links occur for the positive receiver az-imuth angles due to the natural asymmetry of the transmitterantenna with respect to the nearby ENS building. ENS is sit-uated northeast of WRW as seen in Fig. 6, and the transmitterlocation is almost exactly aligned with the western side of ENS.For the majority of measured locations, the receiver was placedin the courtyard seen in the upper left of Fig. 4. Thus, therewas a larger open area to the right of the receiver in the west-ward direction than to its left where ENS blocked most of theeastern direction as seen in Fig. 6. The presence of ENS andother nearby multistory buildings (e.g., ECJ, PAT, RLM) re-duced the number of large receiver azimuth angles by narrowingthe view of the receiver antenna from both sides. This is espe-cially true for receiver locations 4, 6, and 8. These types of be-haviors were consistently observed throughout this study, sug-gesting that site-specific RF planning based on ray tracing orother 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 wasobserved, since the increase in transmitter antenna height ledto longer TR separation distances and fewer scatterers in thevicinity of the transmitter. However, the building spacing inthe environments of ENS-A and ENS-B was much greater thanfor WRW-A as seen in Fig. 4. Therefore, a larger spread ofreceiver azimuth angles was observed for the ENS locations,with 53.3% and 66.1% of the links being less thanoff-boresight for ENS-A and ENS-B, respectively, and similarbehavior when angles up to were considered as seenin Table I. The longer TR distances of ENS-B links and, thus,greater propagation loss explain why there are fewer links withlarge receiver angles.The ECJ transmitter location was the lowest in height, only

8 m above ground. One would expect a lower base-station an-tenna height would yield results that closely resemble groundcommunications links [15] rather than higher transmitter loca-tions. Fig. 9 shows that only 11.5% of all links were made withtransmitter azimuth angles greater than . And 64.8%of the receiver azimuth angles were less than for ECJ


TABLE ISUMMARY OF THE RECEIVER AZIMUTH AOA DISTRIBUTIONS FOR ALL ENVIRONMENTS

Fig. 6. Environment of the WRW-A transmitter location. The transmitter posi-tion was nearly in line with the western side of ENS, thus reducing the numberof reflections coming from the receiver’s eastern direction (e.g., right side ofimage).

Fig. 7. Scatter plot and distribution of the RX and TX azimuth angles for linksat ENS-A at 38 GHz using steerable 25-dBi TX and RX antennas.

growing to 75.4% of angles within of boresight. Sim-ilar results were measured when considering only links shorterthan 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 re-ceiver locations near a residential area. At receiver locationsnear the transmitter, many scatterers were identified, includingthose at large azimuth angles off-boresight. On the other hand,fewer links with large receiver angles were found for the res-idential area receiver locations since these areas were heavilyshadowed by trees, thus incurring additional penetration loss.

Fig. 8. Scatter plot and distribution of the RX and TX azimuth angles for linksat ENS-B at 38 GHz using steerable 25-dBi TX and RX antennas.

Fig. 9. Scatter plot and distribution of RX and TX azimuth angles for links atECJ at 38 GHz using 25-dBi steerable antennas at TX and RX.

Overall, fewer large receiver azimuth angle links were madewith the wider beam 13.3-dBi receiver antenna, since thesmaller receiver antenna gain offers less link margin and is thusunable to detect paths traveling longer distances. The data inFigs. 5–9 show that lower base-station height yields a greaterrange of transmitter azimuth angles for creating links. Thesite-specific environment controls the distribution of receiverazimuth angles, since the majority of reflectors are closer to


TABLE IISUMMARY OF PATH-LOSS EXPONENTS AND STANDARD DEVIATION FORALL 38-GHZ CELLULAR MEASUREMENTS WITH STEERABLE TX AND RX

ANTENNAS

the receiver position. These are key results that directly impactcell- and base-station layout and antenna design [18].

B. Propagation Path Loss

Path loss was extracted for every measured multipath com-ponent (e.g., link) made with every unique pair of transmitterand receiver pointing angles across the 43 measurement loca-tions. For each measured link, the radio path was identified asclear LOS for an unobstructed path when the TX and RX an-tennas were pointing at each other, partially obstructed LOSwith some physical obstructions when the beams were pointedat each other, and NLOS links where antennas were pointed offboresight, thereby exploiting reflections. Path loss scatter plotswere modeled using the standard log-normal shadowing model[14], where the measured path loss data was fitted with aMMSEbest-fit path loss exponent. The log-normal shadowing model isgiven by (1), where the path loss is in dB and is a func-tion of distance and assumed to be a random value. Path loss isrelated to a close-in free-space reference distance, 5 m,and is modeled by the path loss exponent and a shadowingrandom variable which is represented as a Gaussian randomvariable in dB with zero mean and dB standard deviation [34].

(1)

The log-normal shadowingmodel has been used tomodel anyarbitrary link without consideration of antenna characteristics.However, the introduction of steerable and highly directionalantennas leads to a significant dependence of the path-loss ex-ponent and on antenna orientations. Since TX and RX an-tennas were pointed at a wide range of angles, we consideredradio propagation path loss for two cases: one case was whenTX and RX antennas had a visible line of sight between eachother, and were pointed at each other (LOS-directed), and theother case was when the RX and TX antennas did not have a vis-ible LOS due to obstructions, and were not pointed at each otherso that reflections or scattering could be used to make a NLOSlink (NLOS-directed). LOS-directed antennas consistently hadlower path loss than NLOS, even in cases with partially ob-structed LOS link due to foliage or edges of buildings. The prop-agation results were discussed previously in [17]. Table II sum-marizes those results.The measured data are further separated by transmitter loca-

tions with the model parameters summarized in Table III. Underthe assumption that a system with antenna steering capabilitywill first search for the best possible link, the NLOS parame-ters are specified for the best (lowest path loss) NLOS link at

each receiver location. Table III shows that the transmitter lo-cation with the fewest partially obstructed LOS links, ENS-B,experienced the lowest LOS path loss including both clear andpartially obstructed links with exponents of 2.01 and 2.03 forthe 25-dBi and 13.3-dBi RX antennas, respectively. In contrast,the transmitter location with the most partially obstructed LOSlinks was ECJ, which had the highest LOS path-loss exponentsof 2.99 and 2.74. It is worth noting from Table III that whena link could be made, the wider beam 13.3-dBi RX antennaprovided less path loss when compared with the narrowbeam25-dBi antenna at the exact same location, indicating that widerbeamwidths capture more signal energy at the mobile RX, albeitwith less overall link budget available.Another environment-dependent quantity is the variability of

the path loss of LOS links, as expressed by , the standarddeviation (e.g., shadowing) of the least-squares fit error. Bycomparing ENS-B and ECJ LOS measurements, it is clear thatthe more obstructed regions, such as those measured from theECJ base station, have a more variable LOS path loss than theless obstructed environment for the ENS-B transmitter. WhileENS-B had modest variation in LOS path loss from its best fit,resulting in shadowing standard deviation of 6.56 and 5.31 dBfor the 25- and 13.3-dBi RX antennas, respectively, it can beseen that ECJ had a much higher shadowing variation of 13.92and 12.46 dB. The strongest NLOS links also had greater vari-ance 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 locationare shown in Fig. 10 using a 25-dBi receiver antenna. Somelocation-specific variations were observed since ENS-B hadthe lowest mean RMS delay spread of 5.3 ns versus the highestmean RMS delay spread of 16.5 ns at ECJ. As previouslymentioned, differences in environment and TR separationsmake these two transmitter location links quite different fromeach other. Another noticeable trend is the 99-percentile values,which show that WRW-A and ENS-B had significantly lowermaximum RMS delay spreads (65.4 ns and 27.6 ns at 99%, re-spectively) than the other two environments. A possible reasonfor WRW-A having fewer very high RMS delay spread links isthat none of the WRW-A receiver locations were near a street,while the other transmitter environments contained receiverlocations adjacent to a wide street. The street was found to be agood environment for yielding high RMS delay spreads, sinceit has many reflective objects spaced in nearly regular intervalsfor long distances and very few obstructions that block reflectedor scattered waves. For example, a street is typically lined withparked vehicles for many tens of meters. In addition, a streethas many light poles, moving vehicles, surrounding buildings,street signs, and pedestrians, all of which have been found tobe reflectors at millimeter-wave frequencies [15]. ENS-B linkshad, on average, a longer separation distance. These longerlinks had significantly lower delay spreads due to the attenua-tion of longer traveling paths, as discussed in Section III.The dependence on receiver antenna gain can be seen by com-

paring the results in Figs. 10 and 11, which plot the CDFs be-tween RX antennas. While all of the plots in Fig. 10 look sim-


TABLE IIISUMMARY OF PATH-LOSS EXPONENTS AND SHADOWING STANDARD DEVIATIONS FOR ALL TRANSMITTER AND RECEIVER LOCATIONS AND LINKS AT 38 GHZ

Fig. 10. RMS delay spread CDFs for each transmitter location and a CDF forall of the measured links using the steerable 25-dBi RX antenna at 38 GHz. Theexpected 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 forall 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 separationdistances), a much wider variety of CDFs was produced whenusing the wider beam antenna. As discussed later, the systemsensitivity had a strong effect on the RMS delay spread, as thelower gain 13.3-dBi RX antenna had higher RMS delay spreadsat smaller TR separations compared to the 25-dBi antenna, yetthe lower gain RX antenna had lower RMS delay spreads at lo-cations with longer TR separations.As discussed in [17], a considerable difference in RMS delay

spreads between LOS and NLOS links was observed. MostLOS measurements had very minimal RMS delay spread, onthe order of 1 ns, due solely to the transmitted pulse shape (i.e.,

finite probing pulse width, and no multipath distortion frompropagation) with one partially obstructed LOS link resultingin a maximum of 15.5 ns. The NLOS measurements exhibitedhigher and more varied RMS delay spreads, with a mean of14.8 ns for the 25-dBi receiver antenna and 13.7 ns for the13.3-dBi receiver antenna. The maximum NLOS RMS delayspreads were 185 and 166 ns for the 25- and 13.3-dBi receiverantennas, respectively. Nonetheless, more than 80% of theNLOS links had RMS delay spreads under 20 ns and 90% ofthe NLOS links had RMS delay spreads under 40 ns.

D. Trends in RMS Delay Spread

To build power-efficient, low-overhead millimeter-wave mo-bile communication systems, future systems will require theability to adjust antenna pointing angles, while jointly consid-ering multipath delay spread and path loss needed to make asuitable link. The characteristics of the RMS delay spread fromthis measurement campaign were studied in [17], where it wasfound that the mean and worst case RMS delay spread increaseas the antenna angle is pointed away from the LOS angle (e.g.,boresight) at the TX and RX. It was also found that mean andworst case RMS delay spread decrease with increasing TR sep-aration distances. The main reason for these trends is that astronger received signal is caused by one or a few strong mul-tipath components arriving at different specific angles. Thesestrong multipath components dominate the delay spread andmotivate the use of millimeter-wave cellular where directionallow path-loss links can carry very high data rates with smallRMS delay spread. However, when the TX and RX antennasare steered away from each other at relatively close TR separa-tion distances of a couple of hundreds of meters or less, strongLOS and other strong multipath components are less likely, andthe RMS delay spread becomes much greater since multipathcomponents arrive frommany different scattering and reflectionmechanisms. At greater TR separation distances beyond sev-eral hundredmeters, the number of receivable multipath compo-nents decreases due to propagation loss, thus causing fewer de-tectable multipath components and smaller RMS delay spreads.Since our data show that RMS delay spread increases and be-comes more variable as TX and RX antennas are pointed awayfrom boresight, future mobile devices at a particular locationshould prefer a link using relatively small off-boresight antennapointing angles ( ) compared to a link of similar strengththat uses large pointing angles off boresight ( ) [17]. Fi-nally, when considering the cell edge where the TR separationmay be nearly a kilometer, the measurement results show that


TABLE IVA COMPARISON OF THE OUTAGE STATISTICS FOR THE TWO TX LOCATIONS

expected RMS delay spreads are very low. Thus, less equaliza-tion is required near the cell edge. The reduced power and la-tency for equalization of these cell-edge links can be put to usein other processing areas, such as additional error coding for thislower SNR case.

E. Cellular Urban Outage Study

An important open question for a cellular millimeter-wavesystem in dense outdoor urban settings is the extent of cell cov-erage for a given transmitter height. The AOA studies discussedpreviously showed that NLOS paths exist and can be used to in-crease coverage. The extent of the coverage was examined in[18]. The cellular outage study was performed at the Universityof Texas at Austin campus with measurements made within ap-proximately 400 m around the transmitter locations. Measure-ments from two transmitter locations provided outage proba-bility for base stations of different heights. The probability wasalso broken into outages present for a system with sensitivityof up to 160-dB path loss and a less sensitive system with upto 150-dB path loss for the smaller gain RX antenna case. Theoutage 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 highertransmitter location TX1-ENS, resulting in an outage rate of39.6% based on a system sensitivity of 160-dB path loss. It isimportant to note that for both the high and low base-stationtransmitter locations, no outages were observed for all randommeasurement locations within a 200-m radius. In addition, thelower transmitter position benefited from a larger number ofsuitable reflectors in the environment for links less than 200 maway, since the vertical angle of incidence from the low trans-mitter location to a given reflector was reduced compared to in-cidence from the high transmitter location. This led to a highernumber of links with TR separation under 200 m that had lessthan 150-dB path loss at the lower TX location than at the higherTX location (10% outage compared to 27.3%). Further work inother urban environments is needed to determine whether 200m offers complete coverage for millimeter wave cellular.

IV. CONCLUSION

While several past studies have characterized the out-door millimeter-wave channel for wireless backhaul andground-level communications, there has been a significant lackof information about the millimeter wave cellular (base-stationto mobile) channel. Our work has provided 38-GHz radiopropagation channel data for outdoor systems capable of im-plementing antenna beam steering. The work here considered

typical future-generation cellular base-station locations usingsteerable antennas at both receiver and transmitter locations,in both azimuth and elevation directions. We measured AOAstatistics of viable RF links. It was shown that the elevatedtransmitters at heights of two to eight stories above groundrequire 60 of scanning (up to off-boresight) in theazimuth direction to cover nearly all possible NLOS links. Thereceiver antenna, however, would benefit from larger scanningfreedom. A NLOS link is rarely preferred over an LOS orpartially obstructed LOS link, since NLOS links tend to have10 to 50 dB more path loss and higher expected RMS delayspread. When the LOS direction is completely blocked by abuilding or other shadowing objects, the work here shows that areflection, scattered, or diffraction path may still have sufficientsignal strength to be received, albeit at a lower signal level.Distant-dependent propagation path-loss models were providedto account for LOS, NLOS, as well as the best possible pathprovided in NLOS conditions when using steerable antennas atthe TX and RX.Our outage study indicates that increasing the base-station

transmitter height in a dense urban environment provides cov-erage to a greater percentage of locations past 300 m from thetransmitter, but the improvement is “spotty” in that large regionspast 300 m still lack coverage [18]. Our outage study indicatesthat a lower base station is able to use many reflectors in the en-vironment to cover all locations within a 200-m radius from thebase station. This work suggests that millimeter-wave cellularsystems may work best in dense urban environments with mi-crocell deployments with cell radii less than 200 m.This study answered important questions regarding path loss,

RMS delay spread, and signal coverage for millimeter-waveurban outdoor cellular channels for steerable antenna architec-tures. However, many additional measurements are needed tocover all environments of interest and to develop full channelmodels for standards development. These are likely to includeantenna angle-dependent channel models due to the direction-ality and steerability of these novel communication systems.Moreover, the spatial and temporal small-scale variations are ofgreat interest at these short wavelengths; thus, extensive workremains to develop a complete millimeter-wave cellular channelmodel. Finally, large environment dependency of receiver andtransmitter AOA distributions suggests the usefulness of site-specific cell design using ray-tracing models.

ACKNOWLEDGMENT

The authors would like to thank students T. Forbes, S. J. Lauf-fenburger, and A. Duran for their contributions to the project,Samsung researchers S. Rajagopal, S. Abu-Surra, and J. Z. Pifor their ongoing interest and support of this work, Hughes Re-search Laboratory and National Instruments for donating equip-ment, and the reviewers and editor for their helpful comments.This work was sponsored by Samsung DMC R&D Communi-cations Research Team (CRT) through Samsung Telecommuni-cations America, LLC.

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Theodore S. Rappaport (F’98) received the B.S.,M.S., and Ph.D. degrees in electrical engineeringfrom Purdue University, West Lafayette, IN, USA,in 1982, 1984, and 1987, respectively.He is an Outstanding Electrical and Computer En-

gineering Alumnus and Distinguished EngineeringAlumnus from his alma mater. He holds the DavidLee/Ernst Weber Chair in Electrical and ComputerEngineering at Polytechnic Institute of New YorkUniversity (NYU-Poly), Brooklyn, NY, USA, andis Professor of Computer Science and Professor of

Radiology at NYU. In 2012, he founded NYUWIRELESS, a multidisciplinaryresearch center involving NYU’s engineering, computer science, and med-ical schools. Earlier in his career, he founded the Wireless Networking andCommunications Group (WNCG) at the University of Texas at Austin (UT),USA. Prior to UT, he was on the electrical and computer engineering facultyof 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 thewireless communications field. In 1989, he founded TSR Technologies, Inc.,Blacksburg, a cellular-radio/personal-communications-services software radiomanufacturer that pioneered cellular E-911 and test equipment that he sold in1993 to what is now CommScope, Inc. In 1995, he founded Wireless ValleyCommunications Inc., Austin, TX, USA, a site-specific wireless networkdesign and management firm that was sold in 2005 to Motorola, Inc. He hastestified before the U.S. Congress, has served as an international consultant forthe ITU, has consulted for more than 30 major telecommunications firms, andworks on many national committees pertaining to communications researchand technology policy. He is a highly sought-after consultant and technicalexpert, and serves on the Board of Directors of the Marconi Society. He hasauthored or coauthored more than 200 technical papers, over 100 U.S. andinternational patents, and several best-selling technical books.Dr. Rappaport was elected to the Board of Governors of the IEEE Communi-

cations Society (ComSoc) in 2006, and was elected to the Board of Governorsof the IEEE Vehicular Technology Society (VTS) in 2008 and 2011.


Felix Gutierrez, Jr. (S’08) received the B.S. degreein electrical engineering from the University of Texasat Austin (UT), USA, in 2006, the M.S. degree inelectrical engineering from Texas A&M University,College Station, TX, USA, in 2008, and is currentlypursuing the Ph.D. degree in electrical engineering atUT.He completed an internship with ETS-Lindgren,

Cedar Park, TX, USA, in 2010. He is currently aVisiting Research Scholar at the Polytechnic Instituteof New York University (NYU-Poly), Brooklyn,

NY, USA. He has worked on millimeter-wave and sub-terahertz semiconductorcircuits and antennas for next-generation wireless communications.

Eshar Ben-Dor (S’08) received the B.S. degree inelectrical engineering from the Ohio State University(OSU), Columbus, OH, USA, in 2009 and the M.S.E.degree with a focus on integrated microwave circuitsand millimeter-wave communications from the Uni-versity of Texas at Austin (UT) in 2011During his time at OSU, he worked part-time in the

Nanoscale Patterning Laboratory on electron-beamlithography resist technology. Currently, he is an ICDesign 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. andM.S.E. degrees in electrical engineering at The Uni-versity 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. Cur-rently, he is an Analog Design Engineer with TexasInstruments, where he focuses on low-power radiofrequency 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 Com-

munities in Schools Dallas Region.

Yijun Qiao (S’08) received the B.S. degree inelectrical engineering from the University of Texasat Austin, Austin, TX, USA, in 2012 and is currentlya graduate ECE student at Rice University, Houston,TX.As an Engineering Honors student, he joined

Prof. Theodore S. Rappaport’s research team in Fall2010 and worked on the millimeter-wave channelsounding project for one year. He developed ameasurement track now being used for propagationresearch at Polytechnic Institute of New York

University (NYU-Poly), Brooklyn, NY, USA.

Jonathan I. Tamir (S’11) received the B.S. degreein electrical engineering from the University ofTexas at Austin, Austin, TX, USA, in 2011 and iscurrently pursuing the Ph.D. degree in electricalengineering and computer sciences at the Universityof California, Berkeley, CA, USA, with a focus onsignal processing for medical imaging and commu-nication systems.

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