Networking at 60 GHz: the Emergence ofMultiGigabit Wireless

Upamanyu MadhowDepartment of Electrical and Computer Engineering

University of CaliforniaSanta Barbara, CA 93106, USA

Email: madhow@ece.ucsb.edu

Abstract—The large swathes of unlicensed spectrum avail-able worldwide at 60 GHz offer the potential for an orderof magnitude increase in wireless link speeds, to multiGigabitrates, relative to current technology. Oxygen absorption andrain attenuation bound the attainable range outdoors using thisband, while blockage by walls and furniture limits the rangeindoors. However, these are the same features that make 60GHz ideally suited for short-range networks with aggressivespatial reuse, both indoors (with link ranges of the order of10 meters) and outdoors (with link ranges of the order of100 meters). With the rapid scaling of silicon processes, low-cost CMOS implementations for 60 GHz radios are becomingavailable. However, the design of commercially viable multi-Gigabit networks based on such radios must account for theunique physical attributes of “millimeter wave” communication,and often requires a drastic rethinking of design guidelinesestablished for wireless networking at lower carrier frequencies.In this paper, we give a flavor of the technical issues involvedthrough an overview of a subset of our recent research in thisarea, including modeling and design of indoor 60 GHz networks,interference analysis for outdoor 60 GHz mesh networks andits implication for medium access control design, and analysisand prototyping of spatial multiplexing in line-of-sight indoorchannels.

I. INTRODUCTION

Millimeter wave communication has the potential for trans-forming wireless communication as we know it. While cellularand wireless local area networks at lower frequencies (1-5GHz) constantly struggle with the scarcity of spectrum, themm wave band has huge swathes of spectrum available at nocost. In the US, there is 7 GHz of unlicensed spectrum inthe 60 GHz ”oxygen absorption” band that is well suited toshort-range indoor and outdoor links. Much of this overlapswith unlicensed 60 GHz spectrum in Europe and Japan, whichopens the path for worldwide standardization and commercialproducts. Effective use of this spectrum potentially enableswireless to “catch up” with wires, leading to systems suchas Gigabit wireless Ethernet, wireless USB, and wireless un-compressed high definition video, as well as short-range fiberalternatives. While our focus in this paper is on short-rangemultiGigabit 60 GHz networking, we note that the US FederalCommunications Commission (FCC) has also made available13 GHz of spectrum in the 70-95 GHz (away from the oxygenabsorption band) for longer range, semi-unlicensed, highlydirectional, point-to-point ”last mile” links.

Fig. 1. Some examples of indoor 60 GHz applications.

Fig. 2. 60 GHz outdoor mesh networks can provide a quickly deployablebroadband infrastructure.

Industry interest in indoor 60 GHz networking (e.g., seeFigure 1) has been steadily increasing over the past few years.The European standards body, Ecma, has issued a 60 GHzstandard for short-range indoor links [3]. The IEEE 802.15.3cgroup has just completed a 60 GHz Wireless Personal AreaNetwork (WPAN) standard [4]. The Wireless HD consortium[1] has released specifications for uncompressed streamingof high definition (HD) video (which requires about 3 Gbpsof goodput). Finally, major companies in the PC, consumerelectronics, and semiconductor space have recently cometogether to form the WiGig (Wireless Gigabit) alliance [2],with the stated goal of promoting “a global ecosystem” ofmultiGigabit wireless products.

In addition to indoor applications of 60 GHz that industryis focusing on, we believe that 60 GHz outdoor mesh net-

works with short-range (e.g., 100 meter) links have significantpotential. Applications include quickly deployable backhaulfor dense picocellular networks, as well as point-to-multipointalternatives to fiber-to-the-home. For emerging economieswithout an extensive wired infrastructure, mm-wave commu-nication offers the potential for leapfrogging existing wirelineinfrastructure solutions.

In this paper, we provide an overview of a subset ofresearch activities in mm wave communication at UCSB,with the goal of highlighting the observation that designapproaches for 60 GHz networks must differ considerablyfrom those that we are familiar with from our experience withWiFi and cellular networks at lower carrier frequencies. Someof the most important design considerations are summarizedbelow:• Millimeter wave links are inherently directional. Foromnidirectional transmission, free space propagation lossscales as λ2, where λ = c/fc is the carrier wavelength, with cdenoting the speed of light and fc the carrier wavelength. Thewavelength at 60 GHz is 5 mm, while the wavelength at 2.4GHz (WiFi band) is 12.5 cm, so that the propagation loss foromnidirectional transmission and reception is 625 times, or28 decibels (dB), worse at 60 GHz than at 2.4 GHz. However,for a given antenna aperture, directivity scales as 1

λ2 . Thus,fixing the antenna aperture at each end, we gain by a factorof 1

λ4 , making the overall propagation loss scale as 1λ2 ; this

corresponds to a net gain of 28 dB in going from 2.4 GHz to60 GHz. Figure 3 illustrates how much more directive a mmwave node is compared to a WiFi node with similar formfactor. Indeed, employing such directivity at both transmitterand receiver is required for the range/rate combinationswe wish to achieve, given the difficulty of producing largeamounts of RF power at mm-wave frequencies using low-cost,low-power silicon implementations. Automating directivityusing electronic beamsteering, therefore, is an importantenabler for flexible network deployments. Another reasonthat we need beamsteering is to steer around obstacles(exploiting multihop relay or multipath reflection or acombination thereof): the ability of electromagnetic wavesto diffract around obstacles is severely impaired for smallerwavelengths.• Millimeter wave links are susceptible to blockage: obstacleslook much larger when the carrier wavelength is small.For indoor networking, blockage by a human can produceas much as 30 dB loss. Given the difficulty of producingpower at mm wave frequencies, we must steer around, ratherthan burn through, obstacles, using relays, reflections, or acombination thereof.• Millimeter wave spatiotemporal channels are differentfrom those at lower radio frequencies that WiFi and cellulardesigners are familiar with. For example, spatial multiplexingis possible even for line-of-sight (LoS) mm wave links,whereas it requires a rich scattering environment for lowercarrier frequencies. As another example, mm wave channelsare typically characterized by a few dominant paths (e.g.,the LoS and one bounce reflections), so that propagation is

accurately predicted by tracing a few rays.

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Fig. 3. Mm wave nodes yields far more directivity than WiFi nodes withsimilar form factors.

II. INDOOR 60 GHZ NETWORKS: THE BLOCKAGE

PROBLEM

As we have noted, directionality is a must for 60 GHzlinks in order to maintain their link budget. In this case, thedominant component of the received energy comes from theLoS path between the transmitter and receiver. However, LoSblockage by furniture, walls, and humans will be a routineoccurrence on indoor 60 GHz networks. Can we build robustnetworks in spite of this? In this section, we summarizeresults from [5], [6] regarding this issue. One possibilityfor handling blockage is to use electronic beamsteering toutilize reflections off surfaces such as wall, ceiling or floor.Since the availability of strong enough reflected paths is afunction of the environment, we wish to determine whetherit is possible to obtain robust performance even if we do notrely on reflections. The logical alternative, then, is the useof multihop relay. The questions then are: can we make dowith a small number of relays, and how should we design ournetwork protocol to provide relaying capability in the presenceof time-varying blockage, taking into account the directionalnature of the links.

We begin by building a simulation model that models block-age. We analyze the effect of obstacles on the received signalstrength via a site-specific mm wave propagation model basedon Huygens’ principle, as pictured in Figure 4. A primarywavefront generates secondary wavelets, some of which areblocked by obstacles. The loss due to multiple obstacles (seeFigure 4) can be calculated by a spatial convolution (efficientlyperformed using FFT in the transform domain).

We can now build Matlab models such as the office settingshown in Figure 5, add in humans endowed with a simple

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mobility model (we employ random waypoint), and computethe link gains between network nodes.

The next step was to design and evaluate a MAC protocolthat enables relaying. The proposed MAC protocol is designedfor a network in which every link is constrained to bedirectional, without fallback to an omnidirectional mode forcoordination as in most prior work on directional networkingat lower carrier frequencies. The protocol, in which an accesspoint controls the remaining nodes (even though it is not al-ways connected directly to all the nodes), includes proceduresfor topology discovery and updates, and recovery from LOSlink outages via multihop relay to the blocked nodes.

Figure II shows simulation results for the office scenariodepicted in Figure 5. Several features are worth noting: first,stable network throughput is achieved despite routing occur-rence of blockage; second, multihop relay is critical, withseveral of the wireless terminals connected to the access pointusing more than one hop most of the time; third, the controloverhead for implementing multihop relay is moderate.

While much further research is needed for refining networkprotocols and developing cross-layer performance evaluationmodels, the results reviewed here demonstrate that blockageis not a showstopper for 60 GHz indoor networks, as long aswe use appropriately designed relaying mechanisms.

III. OUTDOOR 60GHZ NETWORKS: DEAFNESS AND

SPATIAL REUSE

We now turn to the outdoor mesh network pictured in Figure2. Time-varying blockage is typically not a problem for suchrooftop or lamppost deployments, but centralized control bya single node (as in the in-room network of the previoussection, or as in cellular networks) is not scalable. The keybottleneck in designing a mesh network with highly directionallinks is deafness: we can no longer rely on neighbors usingcarrier sense for implicit coordination. On the other hand,directionality also leads to a reduction in interference, so thatwe can have more aggressive spatial reuse. Thus, mediumaccess control for highly directional networks must be quitedifferent from WiFi networks at lower frequencies, with thefocus shifting away from interference management to devisingmechanisms for coordination between transmitter and receiverdespite the deafness caused by directionality. Our approachto MAC design, therefore, is to consider pseudowired, half-duplex links (i.e., assume complete deafness), and to use

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a reactive rather than a proactive approach for combatinginterference.

Our first step in validating this approach is to understandthe validity of the pseudowired approximation. We summarizeresults from [7], where we analyze the spatial interference cor-responding to completely uncoordinated transmissions, for ascenario depicted in Figure 7. We consider a particular receiverof interest, beamformed towards the transmitter it is interestedin receiving from. Interfering transmitters are placed on theplane at random, pointing their antennas in random directionstowards their desired receivers. We compute the probability ofcollision at the desired receiver using the protocol and physicalmodels [8]: in the protocol model, a collision occurs if thereceived power due to any interfering transmitter exceeds athreshold; in the physical model, a collision occurs if the totalreceived power due to all interfering transmitters exceeds athreshold. We obtain a compact analytical characterization for

the protocol model, and employ a combination of analyticalbounding and Monte Carlo simulation for the physical model.

φ2

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Fig. 7. The geometry of interference with directional antennas.

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While we use simple models for directional transmissionand reception, as shown in Figure 8, our results are broadlyapplicable to antennas with narrow beamwidth.

Figs. 10 and 11 show the collision probabilities for ideal“flat-top” arrays and for linear arrays with ρR2

0 = 1 andα = 10 dB/km and beamwidth=10◦ and different values ofthe SINR threshold β. Figure III shows some typical results,plotting collision probabilikty versus the desired Signal-to-Interference-and-Noise-Ratio (SINR) β; the latter is inverselyrelated to the threshold that the interference needs to exceedto create a collision. Figs. 10 and 11 show the collisionprobabilities for ideal “flat-top” arrays and for linear arrayswith ρR2

0 = 1 and α = 10 dB/km and beamwidth=10◦. Ata desired SINR of 15 dB, the collision probability is lessthan 10%, even with completely uncoordinated transmissionswith a fairly dense deployment of interfering transmissions(3-4 neighbors per node, on average, within the nominal linkrange).

The preceding results show that a pseudowired approxima-tion is an excellent starting point for MAC design for highly

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directional mesh networks. We have recently designed MACprotocols based on this approach (i.e., emphasizing transmit-receive coordination rather than interference management),and have obtained promising results to be reported in [9]).The key idea is to use decentralized learning, in which eachnode persists with transmission schedules that are found towork, and adjusts implicitly to the schedules of other nodesbased on its history of success and failure, without needing tohear transmissions that are not destined for it. This enablesconvergence to TDM-like schedules with minimal controloverhead. Similar ideas had previously been applied to MACdesign for real-time QoS in omnidirectional networks [10],but the use of learning and memory is particularly well suitedfor handling the problem of deafness in highly directionalnetworks.

IV. SPATIAL MULTIPLEXING OVER LOS LINKS

An important aspect of mm wave communication is thatspatial multiplexing gains can be realized even in LoS envi-ronments, using nodes with compact form factor. Spatial mul-tiplexing corresponds to different transmit antennas sendingdifferent data streams, which are then separated out using anantenna array at the receiver; different transmit antennas mustsee different enough array responses at the receiver for this tobe possible. In other words, the Multiple Input Multiple Output(MIMO) channel matrix between transmit array and receivearray should have rank that is at least as large as the numberof independent data streams. At lower carrier frequencies, fortwo transmit antennas at moderate separation (compatible, say,with a laptop form factor), we must have a rich scatteringenvironment to satisfy the rank condition. However, becauseof the small wavelength at 60 GHz, it is possible to obtainspatial multiplexing even in LoS environments; for example,using multiple antennas on a laptop form factor over a rangeof 10 meters.

We have verified that spatial multiplexing is indeed pos-sible in a LoS environment via a series of prototypes, themost recent of which is described in [11]. Our prototypealso demonstrate a novel MIMO architecture that scales toGigabit speeds: rather than performing spatial processing anddemodulation together as in WiFi systems, we recognize theopportunity presented by the vastly different time scales ofspatial channel variation and data transmission. This permits usto adapt our spatial processing for separating out the multipledata streams on a slow time scale, and then performingoperations such as synchronization and demodulation on theseparated data streams. Figure 12 shows a recent 4×4 MIMOprototype, and Figure 13 shows that the MIMO “channelseparation” algorithm is indeed successful in separating outthe data streams. See [11] for details.

The antenna separations used to demonstrate indoor LoSspatial multiplexing are compatible with form factors fordevices such as laptops, set-top boxes, and plasma screentelevision sets. Thus, spatial multiplexing can be used toincrease the rate and robustness of applications such aswireless uncompressed HD [1]. However, we must determine

Fig. 12. A brassboarded 4 × 4 MIMO prototype tested indoors.

Fig. 13. Eye diagrams demonstrating successful separation of data streamsusing our dual time-scale architecture.

how sensitive the multiplexing gain is to the propagationenvironment. We provide a glimpse of some results regardingthis from [12].

We first examine the available degrees of freedom as afunction of node form factor by counting the number ofdominant eigenmodes for a half-wavelength linear array thatspans the node dimension. For form factors typical of a set-top box transmitter (length 1/3 m) and a large TV receiver(length 1m), we plot the spatial degrees of freedom in Figure14. Clearly, spatial multiplexing gains of the order of 10xare possible, in principle (the challenge is to design practicaltransceiver architectures, and to guarantee robustness in theface of multipath and blockage). To this end, we consider achannel model for the environment depicted in Figure 15.

While referring to [12] for detail, for a typical array config-uration in the setting of Figure 15, we show some capacity cal-culations as we vary the relative position of the transmitter andreceiver. Figure 16 shows that when the LoS path is available,the capacity does depend on the position, but is mostly alwayslarge. However, the dependence on position exhibits far moresevere fluctuations when the LoS path is blocked, as shown inFigure 17. This problem can be alleviated by increasing thenumber of antennas, but transceiver architectures that allow usto scale up in this fashion remain very much open for design.

It is worth noting that the preceding capacity calculations

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Fig. 14. Spatial degrees of freedom in a LOS environment given linear arraysof lengths LT = 1/3 m and LR = 1 m. and a carrier frequency of 60 GHz

LTX

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Fig. 15. Top-down view of indoor environment illustrating LOS and singlereflection paths.

are for antenna arrays in which each individual “antenna” isactually a subarray that can be electronically beamsteered (thedirection is optimized in our capacity calculations). For a 2×2MIMO LoS link, each subarray typically steers along the LoS,but there are certain positions when steering to utilize a wallreflection turns out to yield higher capacity.

V. CONCLUSIONS

The sampling of research results provide here indicate howdifferent the design considerations are for mm wave networkscompared to networks at lower carrier frequencies; see also therecent special issue [13] for a snapshot on current research inGigabit indoor WPANs. We end by summarizing our mainobservations regarding mm wave networking. Blockage is a

Fig. 16. Capacity as a function of transmitter position for the 2× 2 system.

Fig. 17. Capacity as a function of transmitter position when the LOS path isblocked for the 2× 2 link. To compensate for reflection loss, transmit powerhas been increased by 10 dB.

dominant consideration indoors, and it is crucial to address itthrough a combination of network design (e.g., relays) andnode design (e.g., multiple antennas, each of which couldbe an electronically steerable subarray). MAC design for mmwave mesh networks focuses on coordination in the face ofdeafness, and deemphasizes interference management. Spatialmultiplexing is easily available at such small wavelengths, butthe gains are sensitive to changes in the geometry. Of course,the issues we have raised, and the solutions we have examinedthus far, only scratch the surface. The engineering of mm wavenetworks will require a significant and sustained effort fromthe research community in the years to come.

ACKNOWLEDGEMENTS

This work was supported by the National Science Founda-tion under grants CNS-0520335, ECS-0636621, CCF-0729222

and CNS-0832154. The work surveyed here is in collaborationwith a number of students and colleagues. The work onindoor WPAN (Section II) involves Dr. Sumit Singh, FedericoZiliotto, Prof. Elizabeth Belding, and Prof. Mark Rodwell. Thework on outdoor mesh networks (Section III) involves Prof.Raghu Mudumbai and Dr. Sumit Singh. The work on spatialmultiplexing (Section IV) involves Eric Torkildson, Dr. ColinSheldon, Dr. Munkyo Seo and Prof. Mark Rodwell.

REFERENCES

[1] WirelessHD, http://www.wirelesshd.org[2] Wireless Gigabit (WiGig) Alliance, http://www.wirelessgigabitalliance.

org[3] ECMA 60 GHz Standard ECMA-387, http://www.ecma-international.org/

publications/standards/Ecma-387.htm[4] IEEE 802.15 WPAN Task Group 3c, Millimeter wave alternative PHY,

http://www.ieee802.org/15/pub/TG3c.html[5] S. Singh, F. Ziliotto, U. Madhow, E. M. Belding and M. J. W. Rodwell,

“Millimeter wave WPAN: cross-layer modeling and multihop architec-ture,” Proc. IEEE Infocom 2007 Minisymposium, Anchorage, Alaska,USA, May 2007.

[6] S. Singh, F. Zilliotto, U. Madhow, E. M. Belding, M. Rodwell, “Blockageand directivity in 60 GHz wireless personal area networks: from cross-layer model to multihop MAC design,” IEEE Journal on Selected Areasin Communications, special issue on Realizing Gbps Wireless PersonalArea Networks, vol. 27, no. 8, pp. 1400-1413, October 2009.

[7] R. Mudumbai, S. Singh, U. Madhow, “Medium access control for 60GHz outdoor mesh networks with highly directional links,” Proc. IEEEInfocom 2009 Mini-conference, Rio de Janiero, Brazil, April 2009.

[8] P. Gupta and P. R. Kumar, “The capacity of wireless networks,” IEEETransactions on Information Theory, vol. 46, no. 2, pp. 388-404, March2000.

[9] S. Singh, R. Mudumbai, U. Madhow, “Distributed coordination with deafneighbors: efficient medium access for 60 GHz mesh networks,” Proc.IEEE Infocom 2010, San Diego, CA, March 2010, to appear.

[10] S. Singh, P. A. K. Acharya, U. Madhow, E. M. Belding-Royer, “StickyCSMA/CA: implicit synchronization and real-time QoS in mesh net-works,” Ad Hoc Networks, 2007.

[11] C. Sheldon, M. Seo, E. Torkildson, M. Rodwell, U. Madhow, “Four-channel spatial multiplexing over a millimeter-wave line-of-sight link,”IEEE MTT-S International Microwave Symposium Digest (MTT’09), pp.389-392, Boston, MA, June 2009.

[12] E. G. Torkildson, C. Sheldon, U. Madhow, M. J. W. Rodwell,“Millimeter-wave spatial multiplexing in an indoor environment,” Proc.1st International Workshop on Multi-Gigabit MM-Wave and TeraHzWireless Systems (MTWS’09), to appear.

[13] IEEE Journal on Selected Areas in Communications, special issue onRealizing Gbps Wireless Personal Area Networks, vol. 27, no. 8, October2009.