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Unlicensed Millimeter Wave CommunicationsA New Opportunity for MM IC Technology at 60 GHz

Rory L. Van TuylHewlett-Packard Laboratories

Solid-state Technology LaboratoryPalo Alto, CA

ABSTRACTA recent FC C decision has opened the largest contiguous band-width in history, 59-64 GHz, for non-government unlicensedwireless commu nications. Emerging comm ercial applications inthis band will require inexpe nsive MMICs and low-c ost packag-ing. Will technology be up to the challenge?

INTRODUCTIONThe U.S. Federal Communications Commission on 15

Dec., 1995 set aside the entire 59-64 GHz frequency band forunlicensed applications. For the first time in theU.S., t willbe legal to transmit millimeter waves above 40GHz for non-government uses. This 5 GH z of unlicensed contiguousbandwidth allocated by the FCC offers an unprecedentedopportunity for very high bandwidth wireless applications,such as short-haul point-to-point data links, high data rateWireless LANs, wireless connections to (and within) homes,digital video links, and position sensors.

For these wireless applications to become pervasive, how-ever, they must become cost competitive. MMIC fabricationcosts of a few dollars per square millimeter should be ade-quate, provided that an inexpensive chip packaging and inter-connection system can be developed.

THEWORLD OF REGULATIONSIncreasingly, the livelihoodsof those who design and man-

ufacture GaAs ICs are moving away from reliance on Depart-ment of Defense dollars and toward dependence on FederalCommunications Commission decisions. The Wireless Rev-olution our industry is presently experiencing had its roots indecisions made years ago by the FCC and its counterpartagencies in other countries. Cellular phones and PCS are thetwo highest-profile examples of FCC spectrum allocations,and in the latter case, the communications industry wascharged billions of dollars at Federal auction for the privilegeof obtaining licenses to operate.

Normally, the communications industry takes the lead inrequesting spectrum policy. But in the case of millimeterwaves, it was requests by manufacturers- the auto equipmentindustry in the case of vehicular radar, and the computer

industry in the case of broadband communications- that haveshaped US. pectrum policy over the last year. Auto radar isnow approved for operation at 46 and 76 GHz, and broadbandcommunications can look forward to a home between 59 and64 GHz, in the Oxygen Absorption Band.2 This band is thesubject of worldwide regulatory attention:

Oxygen Band

Oxygen Band 0 ;sTransmFigure 1. International Regulato ry Status of Oxygen Absorption Band

The U.S. Federal Communications Commission is takingthe lead in two areas: auctioning spectrum for licensed use tobidding consortia, and giving spectrum away free to the gen-eral public. In the U.S., this general unlicensed spectrumcomes with few rules to hamper innovation (or to protectagainst interference). In the U.S., unlike in Europe and Japan,general unlicensed spectrum such as the 59-64 GHz band isnot earmarked for officially-approved applications only.

1.ftp.fcc.gov./publsurauslEngineerin~Technology/Orders selectfcc95499.txt (text) or fcc 95 499 .w ~Wordperfect)].See: FCC Report & Order 95-4 99. Available on-line from: ftp:// 2. Between 56 and 64 GHz, atmosph eric oxygen attenuates electromagneticwaves by as little as7 dB and as much as 15 dB per kilom eter at sea level.

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The 59-64 GHz band was regarded as Desert Propertyby the FCC, and was therefore released for unfenced grazingin 1995. However, the FCC has set aside a year (1996) for anindustry committee - the Millimeter Wave CommunicationsWorking Group - to propose additional rules for taming thisWild West radio spectrum. After that, its up to technology.

LIMITED ANGE, NLIMITEDPPLICATIONSWhat good is all this spectrum, anyway? One thing its no tgood for is long-range communication. Oxygen absorptionconfines propagation distances to the order of a kilometerwith the 10 Watt Equivalent Isotropic Radiated Power [EIRP]limit set by the FCC3 The good side of this is that interfer-ence range is also limited, so that frequencies can be reusedover short ranges.

Antenna -vs- istancefrom the Transmitterfor

EIRP= 1OWFrequency = 60 GH z

10m lOOm l k m 10kmDistance

Figure 2. Propagation of 60 GH z Signals in the AtmospherePerhaps the most frequently-mentioned application for suchrange-limited waves is indoor Wireless LANs. However,short-range outdoor applications are possible as well. The factthat no license is required, coupled with the complete absenceof constraint on what the applications must be, suggests thatusers, rather than central planners, will determine the applica-tions for 60 GHz radios.A perennial problem for radio has been the huge differentialbetween gigabit-per-second optical fiber data rates and thekilobits- to megabits-per-second rates possible with wireless.This data rate gap might be breachable in the 5 GHz ofspectrum allocated at 59-64 GHz. Since bandwidth is notseverely restricted, simple 2-level modulations like BPSK orFSK could be employed initially, with relatively simple hard-ware. More complex modulations, such as Orthogonal Fre-

quency Division M ultiplexing - a type of modulation capableof overcoming multipath propagation - have already beendemonstrated in a 27 Mbit/sec millimeter wave wireless LANprototype [11.

The distinguishing attribute of the 59-64 GHz band isclearly its ability to handle a large number of broadband datastreams simultaneously. If technical and economic hurdlescan be overcome, we expect that broadband applications wilfind a home in this band, and that robust data links capable ocarrying hundreds of megabits per second [e.g. ATM] willbecome commonplace. Is there a set of technologies that canmake this all happen?

Is TECHNOLOGYP To THECHALLENGE?At lower RF frequencies, such as 900 MHz, miniature dis-crete components surface-mounted on PC boards do an ade-quate job and can be manufactured cheaply offshore. Hencethere is little incentive for monolithic integration except fosuch components as synthesizer prescalers which can not bebuilt in discrete form. For applications at higherRF frequen-cies, such as 12 GHz for DBS, monolithic parts combine withdiscrete LNAs to do an adequate job at low cost. However, inthe 59-64 GHz band the fully-monolithic approach is the onlyeconomically viable alternative for commercial products.Fortunately, GaAs FET technology has evolved over the pastwo decades to the point where GaAs MMICs for millimetewave frequencies are now practical:

InGaAs HEMT 111-V Fet Evolution100-200 InGaAdAlInAs 1970s - 1990st narly 1990s

I0-100fT[GHzI20-5010-20 -aAs pHEMT0:1 0.25 0:s 1 .bGate Length (ym)Figure 3 TodaysE T echnology IS 10times faster than in the 1970sMillimeter Wave MMIC s are now pracucal

Modern FETs achieve their high bandwidth by a combinationof short gatelength and InGaAs channels (more Indium givegreater bandwidth). Designers today can choose betweenlow-noise processes (those using 53% Indium InGaAchannels, grown on either InP or GaAs substrates), andpower processes (those with 22% Indium InGaAs channelsgrown on GaAs substrates). The low-noise processes feature FETs with fT >150 GHz, noise figures < I dB at 60 GHzdrain voltages of 1-1.5V, and limited (-20 mW) output powe~~ ~~ ~

3. EIRP is equivalent to [Transmit Power]x[Antenna Gain].

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[2]. The power processes yield FETs with fT =75 GHz,operate with drain biases of 3-5 V, and deliver >lo0 mW ofpower at 60 GHz [3].

FCC rules allow 10 Watts of EIRP in the 59-64 GHzband. This means that a 10mW transmitter (which is possiblewith a low noise process) working with a 30 dB gainantenna, would operate at the legal power limit. Likewise, a100 mW transmitter (which would require a power pro-cess), would transmit at the legal power limit with a 20 dBgain antenna.Though, it seems that the basic device technology is at handfor building practical applications in the 59-64 GHz band, thequestion remains: can sufficient circuit complexity beachieved at a low enough cost? GaAs FETs can now bemonolithically integrated using coplanar waveguide intercon-nects to produce 59-64 GHz MMICs of medium complexity,such as the divide-by-four prescaler shown below [4]:

Figure 4. This HEMT M MIC, designed by H P and fabricated by HughesResearch is reported elsewher e in this digest. The 1.7 x 1.1 mm chip istypical of state-of-the-art MMICs with coplanar interconnects. This circuitis a divide-by-4 prescaler usable in the 59-64 GH z band.[Photo courtesy of Chris J. Madden, HP L aboratories].This MMIC, realized with a low noise process, contains

10 FETs, with total gate periphery in excess of 750pm, thin-film capacitors and epi resistors. The absence of backside viaspromises lower manufacturing cost for this type of MMIC,but e-beam gate lithography used for gate fabrication willsomewhat offset this cost savings. Nonetheless, we should beable to expect MMICs for 59-64 GHz applications which costless than $10 per square millimeter. Therefore, the challengefor designers has now become: How do we get a 60 GHzradio onto a single chip? Time will tell.But MMICs are only part of the story. Low-cost assembliesare needed. How can these circuits be packaged cheaplyenough to make wireless applications affordable? Coplanarwirebond interconnects between chips are surprisingly low-loss at 60 GHz [ 5 ] ,and multichip module technologies could

well accommodate millimeter wave components along withI.F. and baseband circuits [6]. That leaves the problem ofcoupling signals to and from the millimeter-wave modules.Fortunately, the outlook is bright here as well: miniatureantennas - either on-chip or off-chip - will allow for directcoupling to free space, thereby making expensive millimeterwave connectors unnecessary. Simple antennas are shortenough to become part of a MMIC chip, and radiation to andfrom MMPC chips can be handled inexpensively and com-pactly. Furthermore, directional antennas for this band arepractical in size: radiating apertures of approximately 1 inchdiameter can accomodate a 20 dB gain antenna at 60 GHz [7].Combining MMIC chips and radiating structures into a lowcost package seems a real possibility - one which promises toopen the door to commercial applications in the 59-64 GHzband.

CONCLUSIONRecent FCC decisions allow commercial use of the 59-64GHz band today, without license, and with no restrictionsplaced on the type of applications. Propagation distances inthis band are short, so long-range communication is not pos-sible, but neither is long-range interference a problem.Todays MMIC technology seems well suited to the challengeof producing low-cost RF modules for the 59-64 GHz band,and miniature antennas offer low-cost connections to freespace. After years of exile in the world of gold-plated hard-ware, millimeter waves at last seem poised to flourish in com-mercial and consumer applications.

REFERENCES[ I ] Skellem , D., et al., A -Wave High Speed Wireless LAN for MobileComputing - Architecture and Prototype ModemlCodec Implementation,IEEE Hot Interconnects 96 Symposiu m Digest, August, 1996.[2] Nguyen, L. et al., Ultra-Hig h-speed Modulation-Doped Field-EffectTransistors: A Tutorial Review, Proc. IEEE, April, 1992, pp. 494-518.[3] Sharma, A., et al., Millimeter-Wave High Power Amplifiers UsingPseudomorphic HEMTs, 1994 IEEE MTT-S Conference Digest, pp. 813-816.[4] Madden, C., et al., A Novel 75 GH z InP HEM T Dynamic Divider,1996 GaAs IC Sym posium Digest [in this volume].[SI Krems, T., et al., Millimeter-Wave Performance of Chip Interconnec-tions Using Wire Bonding and Flip Chip, International Microwave Sympo-sium Digest, 1996, vol. 1, pp. 247-250.[6] Cooper, P., et al., Multichip -on-Flex Plastic Encapsulated M HDI-L owCost Substrateless M anufacturing For Microwave and Millimeterwave Mod-ules, International Microwave Symposium Digest, 1996, vol. 1, pp. 219-222.[7] Rutledge, D., et al., Chapter 1: Integrated-Circuit Antennas, in InfraredandMillimeter Waves, vol. 10, Academic Press, 1983.

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