short-range wireless communications for next generation networks.pdf

INTRODUCTIONWith the rapid evolution of wireless technolo-gies, ubiquitous and always-on wireless systemsin homes and enterprises are expected to emergein the near future. Facilitating these ubiquitouswireless systems is one of the ultimate goals ofthe fourth-generation (4G) wireless technologiesbeing discussed worldwide today. A wide rangeof heterogeneous systems, including wireless per-sonal area networks (WPAN), wireless local areanetworks (WLAN), cellular, WiMAX, and satel-lite systems will converge into 4G technologies.

As presented in Table 1, the following threetechnologies are expected to play a vital role inemerging WPAN for broadband or low through-put, low-power consumption applications: ultra-wideband (UWB), 60 GHz millimeter-wave- basedWPAN, and ZigBee technologies. UWB technolo-gy, proposed for high-speed short-range applica-tions, is one of the more active areas of focus inacademia, industry, and regulatory circles. UWBtechnology affects the basic policy of spectrumregulation by using both the occupied and theunoccupied spectrum across the 3.1–10.6 GHzband. Compared to UWB technology, 60 GHz

millimeter wave communications will operate incurrently unutilized spectrum and will providehigh data rates of up to several gigabits per secondfor indoor applications. This millimeter-wave envi-ronment provides desirable features such as readi-ly available pre-approved radio frequency (RF)spectrum allocation, inherent coexistence withdirectional antennas, security, and dense deploy-ment. Meanwhile, simple, low data rate WPANare finding numerous uses in the retail, medical,and logistics fields. In particular, the ZigBeeAlliance, an industry consortium promoting a low-power consumption WPAN wireless technology, isdeveloping specifications based on the low rateWPAN of the IEEE 802.15.4-2004 standard. Forthe WPAN technologies listed in Table 1, selec-tion of Medium Access Control (MAC) protocolsis also critical to the performance, cost, and usage.MAC protocols designed to support multigigabitWPAN demonstrate unique features due to appli-cations, data rates, and networking structures.

The physical layer (PHY) specifications andMAC protocols shown in Table 1 are standard-ized by the IEEE 802.15 WPAN Working Group.Figure 1 shows current standardization activitiesof the standard body. At the time of this writing,standard processes of 60 GHz millimeter-wave-based WPAN (TG3c) and WPAN low-rate alter-native PHY (TG4a) are being developed, andTG3a for UWB was disbanded from the stan-dardization. For current status, see the Web siteof the IEEE 802.15 WPAN [1].

In this article, we discuss technologies andissues for deploying next-generation short-rangewireless networks, focusing on three technolo-gies: UWB, 60 GHz millimeter-wave-basedWPAN, and ZigBee. We present an overview ofkey technical features, standardization, and regu-lation, as well as highlighting issues for success-ful deployment. The wireless MAC protocol ofmultigigabit 60 GHz WPAN also is addressed.

ULTRA-WIDEBAND WPANThe first report and order (RAO) of UWBissued by the Federal Communications Commis-sion (FCC) [2] allowed commercialized UWB

IEEE Wireless Communications • August 200770 1536-1284/07/$20.00 © 2007 IEEE

MAC protocols depend on:

Rela

High throughput

Energy efficiency

AC C E P T E D FROM OP E N CALL

CHEOLHEE PARK AND THEODORE S. RAPPAPORT, UNIVERSITY OF TEXAS AT AUSTIN

ABSTRACTThis article presents standardization, regulation,

and development issues associated with short-range wireless technologies for next-generationpersonal area networks (PAN). Ultra-wideband(UWB) and 60 GHz millimeter-wave communica-tion technologies promise unprecedented short-range broadband wireless communication and arethe harbingers of multigigabit wireless networks.Despite the huge potential for PAN, standardiza-tion and global spectrum regulations challenge thesuccess of UWB. On the other hand, ZigBee™ isexpected to be a crucial short-range technology forlow throughput and ultra low-power consumptionnetworks. The current status and direction offuture development of UWB, emerging 60 GHzmillimeter-wave PAN, and low data rate ZigBeeare described. This article also addresses wirelessMAC protocol issues of 60 GHz multigigabit PAN.

SHORT-RANGE WIRELESS COMMUNICATIONS FORNEXT-GENERATION NETWORKS: UWB, 60 GHZ

MILLIMETER-WAVE WPAN, AND ZIGBEE

The authors discussstandardization, regulation, anddevelopment issuesassociated withshort-range wirelesstechnologies fornext-generation personal area networks (PANs).

RAPPAPORT LAYOUT 8/2/07 3:23 PM Page 70

IEEE Wireless Communications • August 2007 71

devices in the United States and providedmomentum to the worldwide regulation andstandardization of UWB. The United States,Europe, and Asia are struggling to define a uni-fied spectrum regulation and standard. Globally,this short-range, high-throughput WPAN tech-nology with data rates of several hundredmegabits per second creates opportunities andchallenges. In this section, we present anoverview of UWB technologies, issues, and sta-tus of standardization and regulation.

UWB TECHNOLOGIESUWB technologies using short-pulse signals havebeen applied to radar systems since the 1960s,and their communication applications haveattracted industry and academia since the 1990s.In addition to communication applications, theUWB devices can be used for imaging, measure-ment, and vehicular radar. According to therequirement of the first RAO of the FCC, thefractional bandwidth or the transmission band-width of UWB signals should be greater than 0.2or 500 MHz, respectively; this open definitiondoes not specify any air interface or modulationfor UWB. In the early stages, time-domainimpulse radio (IR) dominated UWB technolo-gies and still plays a crucial role. However, driv-en by the first RAO and standardizationactivities, conventional modulation schemes,such as multiband orthogonal frequency domain

multiplexing (MB-OFDM) have appeared. Fig-ure 2 shows fundamental PHY features of twoUWB proposals, direct sequence (DS)-UWBand multiband MB-OFDM.

Direct Sequence (DS)-UWB — The DS-UWB adoptsvariable-length spreading codes for binary phaseshift keying (BPSK) or (optional) quadrature bi-orthogonal keying (4BOK) modulations [3]. A datasymbol of BPSK (one bit) and 4BOK (two bits)modulation is mapped into a spreading sequenceand bi-orthogonal code with length rangingbetween 1 and 24, respectively. It can provide(optionally) a maximum data rate of 1.32 Gb/s.Chip rate can be changed from 1313 to 2730Mchips/s according to data rates and piconet chan-nels. There is a difference between each chip rateof piconet channels of about 13 MHz, and carrierfrequency is exactly a multiple of three times thechip rate. As a receiver structure, the Rake receiv-er is generally assumed to mitigate an adverseeffect of multi-path. DS-UWB supports two spec-trum bands of operation, the lower band (3.1–4.85GHz) and the optional upper band (6.2–9.7 GHz).Each operation band has six piconet channels,where distinguished spreading codes, chip rates,and center frequencies are specified.

For DS-UWB, efficient synchronization anddetection are critical to realize the technology.Although Rake reception is generally assumedto capture the spread energy over multi-path

n Table 1. Summary of characteristics of UWB, 60 GHZ millimeter-wave-based WPAN, and ZigBee.

Systems UWB 60 GHz WPAN ZigBee

Standard status Dissolved in IEEE In progress Approved

Frequency allocation 3.1–10.6 GHz57–64 GHz (U.S.)59–66 GHz (Japan)57–66 GHz (Europe)3

2.4–2.4835 GHz1

901–928 MHz2

868–868.6 MHz4

Channel bandwidth ≥ 500 MHz Not yet available 2,1 0.6,2 0.3 MHz4

Number of RF chan-nels

25

1464 (IEEE 802.15.3c) 16,1 10,2 14

Maximum data rate100 Mb/s (10 m)200 Mb/s (4 m)480 Mb/s (optional)

2 Gb/s (at least)≥ 3 Gb/s (optional)

250 kb/s1

40 kb/s2

20 kb/s4

Modulation DSSS,5 OFDM6 Not yet available BPSK,2,4 OQPSK1

Maximum coverage ~10 m ~20 m ~20 m

Channel access Hybrid multiple access(Random and guaranteed access)

CSMA/CA(Optional) guaranteed time slot

1 2.4 GHz band ZigBee2 915 MHz bands ZigBee3 This frequency band is under consideration.4 868 MHz band ZigBee5 DS-UWB6 MB-OFDM

The United States,Europe, and Asia arestruggling to definea unified spectrum

regulation and standard. Globally,

this short-range,high-throughput

WPAN technologywith data rates of

several hundredmegabits per secondcreates opportunities

and challenges.


IEEE Wireless Communications • August 200772

propagation, the complexity of the ideal Rakereceiver creates a realization problem. In addi-tion, long acquisition times must be addressed.Regarding piconets, there is a question as towhether the given spreading gains and structuresare sufficient for coexistence of piconets.

Multiband OFDM — Finding a realistic solution toavoid pulse-related issues of impulse radio (IR)UWB drives the MB-OFDM scheme that com-bines OFDM modulation and multiband transmis-sion [4]. The MB-OFDM uses a 128-point inversefast Fourier transform (IFFT) and FFT with asubcarrier spacing of 4.125 MHz (528 MHz /128).Each data subcarrier is modulated by a quadra-ture phase-shift keying (QPSK) symbol. Zero-padded suffix of length 32 samples (60.61 ns) anda guard interval of five samples (9.47 ns) are usedto prevent multi-path or inter-block interferenceand guarantee band switching time, respectively.Modulation can use time- and frequency-domainspreading of an order of two by repeating identicaldata in time- or frequency domain, respectively.Maximum data rate of 480 Mb/s can be provided.The multiband mechanism of MB-OFDM pro-vides five band groups over 3.1–10.6 GHz andeach band group consists of three different bandsexcept the last group that is composed of twobands. In addition, a time-frequency code (TFC),that specifies a sequence of frequency bands forOFDM symbol transmission, is used to interleaveinformation data over a band group.

Although this proposal avoids the disadvan-tages of IR schemes, the inherently high peak-to-average power ratio (PAPR) of the OFDMand the sensitivity to frequency offset, timingerror, and phase noise creates challenges for cir-cuit design and a power-efficient implementa-tion. For example, in an extreme case, the PAPRof the MB-OFDM can be approximately 20 dB,and this large PAPR may restrict the achievablepower efficiency and power amplifier design.

UWB REGULATION AND STANDARDIZATION

After the Notice of Inquiry (1998) and Noticeof Proposed Rule Making (2000), the FCCissued the first RAO related to UWB in Febru-ary 2002 [2]. The RAO approved unlicensedspectrum use in the 3.1–10.6 GHz band forcommunication and imaging devices with thepower spectrum density (PSD) limited to –41.3dBm/MHz or 75 nW/MHz to restrict the inter-ference from UWB to other services. The sec-ond RAO, which became effective in March2005, reaffirmed the first RAO and modifiedtest methods for UWB devices. In Europe, theElectronic Communications Committee (ECC)TG3 of the European Conference of Postal andTelecommunications Administrations (CEPT)published ECC Report 64 for UWB in Febru-ary 2005, wherein it asserted that Europeshould prepare its own spectrum mask. In addi-tion, the European Telecommunications Stan-dards Institute (ETSI) proposed its own UWBspectrum mask with more stringent PSD limitsthan the FCC mask at both band edges [5].European spectrum regulation was scheduledto be defined in 2006. In Japan, the Ministry ofInternal Affairs and Communications (MIC)has approved a preliminary emission policyover the 3.4–4.8 GHz and 7.25–10.25 GHzbands, where the emission limits are similar tothe FCC rule. Over the 3.4–4.8 GHz band, adetection and avoidance (DAA) mechanism isrequired for the emission level of –41.3dBm/MHz; otherwise, –70 dBm/MHz isrequired. In Korea, the Ministry of Informationand Communication (MIC) plans to allow testspectrum for UWB over the 3.1–5 GHz band.Above all, Korea wants to protect the WiBro,satellite digital multimedia broadcasting(DMB), and 3G cellular services from UWB-related interference. With the expected globalintroduction of UWB technologies, Task Group

n Figure 1. IEEE 802.15 Working Group for WPANs.

802.15.1WPAN-Bluetooth

802.15.1Bluetooth2.4 GHz

Alternative PHY(3 alternative PHYs)

802.15.1aBluetooth2.4 GHz

802.15.3High rate (HR)

2.4 GHz

802.15.3aUWB

3.1-10.6 GHz

802.15.3cmmW WPAN57-64 GHz

802.15.4Low rate (LR)

868/915MHz, 2.4GHz

802.15.4aalternative PHY

3-10 GHz, 2.4 GHz

*20/40/250 kb/s*~55 Mb/s*Bluetooth (SIG) v1.1

*Bluetooth (SIG) v1.2*Draft in review

*110/200/480 Mb/s*Disbanded Jan. 2006

*≥ 2 Gb/s*(optional ≥ 3 Gb/s

*UWB*(optional) CSS

802.15.3WPAN-HR

IEEE 802.15WPAN802.15.2

coexistence802.15.5

mesh networking

802.15.3bMAC maintenance

802.15.4benhancement

*ieee802.org/15/index.html

802.15.4WPAN-LR

MAC layer(3 MACs)

Completed

Physical layer(6 PHYs)

In process Disbanded UWB 60 GHz mmW WPAN ZigBee relatedThe failure of unifiedstandardization willcause confusion inthe market andimpair the potentialof UWB. The MB-OFDM proposalof the WiMediaAlliance is receivingwide support frommany companies andconsortiums.



1/8 under Study Group (SG) 1 of ITU-Rformed in July 2002 is preparing recommenda-tions for UWB regulations.

The standard process of the IEEE 802.15TG3a, defining an alternative PHY specificationcompatible with the MAC standard of IEEE802.15.3-2003, was dissolved in January 2006 dueto the heated competition and resulted deadlockbetween two proposals, DS-UWB and MB-OFDM. The failure of unified standardizationwill cause confusion in the market and impairthe potential of UWB. The MB-OFDM proposalof the WiMedia Alliance is receiving wide sup-port from many companies and consortiums,such as the Wireless USB Promoter Group(2004) and the Wireless 1394 Trade Association(2004). In addition, Ecma Internationalapproved two standards for UWB, based onMB-OFDM technology in December 2005. Onthe other hand, DS-UWB is about one yearahead of the MB-OFDM camp in silicon devicesand products with optional higher data rates.Although the UWB Forum of DS-UWB suggest-ed a common signaling mode (CSM) and a dual-PHY standard as a compromise in the failedstandardization process, the Multiband OFDMAlliance (MBOA, now merged with WiMediaAlliance) responded negatively. Besides theIEEE 802.15 TG3a, there are several projectsrelated to UWB standardization all over theworld, such as the pervasive ultra-wideband low-spectral energy radio systems (PULSERS) withinthe 6th Framework Programs (FP6) of the Euro-pean Information Society Technologies (IST).As of July 2007, Dell Computers and Lenovoannounced pending shipments of MB-OFDMUWB technology in some laptops using Wiquestsilicon, and UWB chip makers such as Alereonand Intel expect to be shipping MB-OFDM inthe next several weeks.

60 GHZ MILLIMETER-WAVE-BASEDWPAN

The 60 GHz millimeter-wave radio can providemedium- and short-range wireless communica-tions with a variety of advantages. Huge andreadily available spectrum allocation, densedeployment or high frequency reuse, and smallform factor can pave the way to multigigabitwireless networks [6]. This section begins bydescribing some distinguished features thatmake the millimeter-wave band attractive forbroadband short-range wireless systems. Anoverview of standardization, regulation, andrelated issues also is provided.

WHY 60 GHZ?Throughout the world approximately 5–7 GHzbandwidth is available over the 60 GHz millime-ter-wave band. This continuous and clean band-width facilitates multigigabit wirelesstransmission. Moreover, this readily available,pre-approved spectrum allocation obviates regu-lation conflicts. When considering the regulatorydelay of UWB, this prospect is very encouraging.

In addition to clear and large spectrum alloca-tion, the 60 GHz millimeter-wave channel showshigher path loss than the lower microwave bands.In addition, the atmospheric oxygen (O2) absorp-tion and rain interference are known to increaseattenuation by 10–15 dB/km beyond 2 km. Thepropagation properties of indoor 60 GHz chan-nel decrease interference to other systems or col-located 60 GHz networks and increase frequencyreuse factors and space efficiency [7]. Further-more, high signal attenuation can strictly confinethe physical range of a network, which enablesmore secure wireless communications.

The size of a product, as well as technicalperformance, can become a crucial factor for

n Figure 2. UWB technologies; DS-UWB and MB-OFDM [1].

GHz

3

+1 +1 +1 . . .-1 +1 -1-1

Spreading

or

Modulation

00011011

f

Option Channel #5Channel #1

Band#1

MG-OFDM

DS-UWB

Frequency Time

Zero-padding

• Zero-padding

• QPSK • 128-point IFFT (528 MHz)• Time-domain spreading• Frequency-domain spreading

• High PAPR - Non-linear distortion - Low power efficiency

• TFC• 4 piconets (mandatory) - Channel 1 (band ~1-3)

• FFT• Equalization (frequency)

• BPSK• 4-BOKS (option)

• Variable spreading gain - 1,2,3,6,12,24

• 4 piconets (mandatory) - Lower band

• Rake receiver• Equalization (time)

Detection

or00011011

Σ

Frequency

Band#2

Lower band Upper band

Multi-path channel

Band#3

Band#13

Band#14

3432MHz

3960MHz

4488MHz

9768MHz

10296MHz

4 5 6 7 8 9 10 11

Option

Σ

PA



marketing. An antenna or other RF componentsof 60 GHz frequency can be implemented in aslimmer size than other microwave band systemsdue to a short wavelength of about 5 mm. Thus,we can shrink mobile or portable wireless devicesof 60 GHz millimeter-wave-based WPAN.

MILLIMETER-WAVE-BASEDWPAN REGULATION AND STANDARDIZATION

The United States and Japan have issued spec-trum regulations about the millimeter-waveband, and Europe is preparing its regulation. InAugust 2000, Japan issued a regulation for 60GHz millimeter-wave for high-speed data com-munication, and in the regulation, both thelicensed (54.25–59 GHz) and the unlicensedband (59–66 GHz) were specified. For thelicense-exempted band, maximum output poweris limited below 10 mW, and the maximum chan-nel bandwidth is restricted as 2.5 GHz. Regard-ing the licensed band, 100 mW output powerwith 20 dBi antenna gain is allowed. The FCCassigned 57–64 GHz frequency band as an unli-censed band by FCC 47 CFR 15.255. In short,the rules are similar to other regulations forunlicensed bands and mainly consider WLANapplications. Power density measured at 3 m isrestricted to an 18 µW/cm2 in peak with averagedensity of 9 µW/cm2. The maximum peak poweris limited to 500 mW. In Europe, standards arebeing developed for 57–66 GHz band by CEPT(ECC) and ETSI. At present, maximum transmitpower of 57 dBm effective isotropic radiatedpower (EIRP) with a minimum spectrumrequirement of 500 MHz is proposed.

After the FCC gave authorization for 57–64GHz use without a license, standardization for

short-range broadband wireless communicationsover the band was accelerated by the IEEE802.15 Task Group TG3c and was officiallyapproved in March 2005. TG3c is developing amillimeter-wave-based alternative PHY for theexisting high-rate WPAN standard IEEE802.15.3-2003. The proposals for physical layerspecification were scheduled to be presented inNovember 2006, and standardization activitiesare expected to be finished by 2007. Accordingto the system requirement, this standard willprovide a data rate of at least 2 Gb/s, and option-ally in excess of 3 Gb/s. For current status, see[1]. In Japan, after undertaking several researchprojects during the 1990s, the association ofradio industries and businesses (ARIB) defineda fixed wireless access (FWA) standard, STD-T74, for video distribution and gigabit wirelesslink in May 2001. Recently Japan has launchedthe Millimeter-Wave Personal Area Communi-cation Systems (MPACS) expert group in theAsia Pacific Telecommunity (APT). In Europe,the MEDIAN project (1994–1997) investigated ashort-range WLAN (4–6 m) that could provide amaximum data rate of 150 Mb/s, using OFDMmodulation. After MEDIAN, the BroadWayproject is developing a dual frequency hybridWLAN bridging the 5 GHz and 59–64 GHzbands. It can provide (optionally) a maximumdata rate of 720 Mb/s at short range, using a 64-QAM OFDM modulation. In addition, theMAGNET and WINNER projects of IST arefocusing on short-range WPAN. In addition tothese ETSI projects, the WIGWAM project,funded by Germany, also is developing a short-range communication system with a data rate of1 Gb/s, using OFDM modulation.

ISSUES FOR 60 GHZMILLIMETER-WAVE-BASED WPAN

The 60 GHz millimeter-wave band evidently pro-vides short-range wireless systems with unprece-dented advantages in terms of bandwidth, highspace efficiency, security, and form factor. How-ever, this millimeter-wave spectrum also poseschallenges that must be resolved for successfuldeployment as described in the following.

Channel Analysis and Modeling — Measured data ofindoor broadband 60 GHz channels show highdependence on the surrounding environments.According to the measurements being analyzed inthe TG3c, path loss exponent and RMS delayspread are approximately 1.2–4.4 and 0.25–45 ns,respectively [7, 8]. The angle of arrival (AoA) dis-tribution standard deviation modeled as a Lapla-cian distribution is about 14°. Thus, it is desirableto specify channel parameters, including path lossexponent, delay spread, and angle spread, in con-nection with specific application scenarios orusage models. The channel subcommittee of TG3cclassifies channel environments according to sur-rounding (e.g., residential or office) and propaga-tion line-of-sight or non-line-of-sight (LOS orNLOS) environments. One of the difficulties inanalyzing measured data is that measurements arecoupled with specific antennas and settings. Forgeneral channel modeling, how to decouple orcouple these specific settings is a difficult task.

n Figure 3. 60 GHz indoor channel model by using a modified Saleh-Venezuela model with angle of arrival (AoA): cluster arrival rate λ = 1/2.833ns, cluster decay factor Γ = 9.09 ns, ray arrival rate λ = 1/0.642 ns, ray decayfactor γ = 1.25 ns, and standard deviation of AoA σ = 10.47° with threemean angle arrivals, 0°, 96°, and 200°. Channel parameters are based on themeasured data from IEEE 802.15 Working Group TG3c.

0˚ mean AoA96˚ mean AoA200˚ mean AoA

0.25

Am

plit

ude

0

0.5

0.75

1

360

60 GHz indoor channel model: modifiedSaleh-Venezuela with angle of arrival

Angle spread (°) Delay spread (ns)0

30300

10

20240

180120

600



Regarding multiple antenna systems, multiple-input multiple-output (MIMO) channel measure-ments are very rare and correlation and rank of 60GHz MIMO channel[s] have not been analyzed suf-ficiently yet. In the literature, there is still confusionwhether an indoor 60 GHz channel can allow spa-tial multiplexing (SM) of general MIMO systems.

When considering documents of TG3c, tappeddelay line or modified Saleh-Venezuela (S-V)models can be applied for 60 GHz indoor chan-nel modeling [7]. The tapped delay line modelhas channel taps (or coefficients) of specific rela-tive power with Rayleigh or Rician small-scalevariations. The modified S-V model adds AoAinformation to a conventional model. S-V chan-nel modeling, which has been widely used forindoor, narrow band channel modeling, describesmulti-path propagation as a combination of clus-ters and rays per each cluster with Poisson arrivalrates. Figure 3 shows simulation result of 60 GHzindoor channel based on the modified S-V modelwith AoA and measurement parameters of [7, 8].

Modulations — Modulation issues of 60 GHz mil-limeter-wave WPAN can create similar circum-stances or controversies for UWB betweenDS-UWB and MB-OFDM. As described in theprevious section about standardization, severalEuropean projects have proposed OFDM-basedsystems, and Japan’s project has designed a sin-gle-carrier system using amplitude-shift keying(ASK) modulation. Directional or array gainsthat can be combined with modulations thoughspatial multiplexing of multiple antennas seemto require more consideration.

OFDM modulation of 60 GHz WPAN mustaddress high PAPR and sensitivity of phase andcarrier offset. High PAPR causes nonlinear distor-tion and low-power efficiency in the power amplifi-er. Due to the high carrier frequency, phase noiseand carrier offset will degrade the performance ofOFDM. General single carrier modulations over60 GHz multipath channels are unfeasible due tothe required high complexity of equalization. How-ever, block transmission using single-carrier modu-lation with frequency domain equalization(SC-FDE) can achieve complexity and perfor-mance comparable with OFDM modulation with-out high PAPR and phase noise sensitivity [9].Block transmission of SC-FDE appends cyclic pre-fix at the transmitter to prevent interblock interfer-ence and uses FFT, frequency domain equalization,and IFFT at the receiver. Data detection is per-formed in the time domain after IFFT operation.Performance of SC-FDE using frequency-domainequalizations under 60 GHz indoor channel envi-ronment are presented in Fig. 4.

Broadband Circuit Technologies — Conventionally, 60GHz RF circuits have been realized using III-Vcompound technologies such as gallium arsenide(GaAs) and indium phosphide (InP). GaAs-based monolithic microwave integrated circuits(MMIC) are available in the market today. How-ever, the price of the solution is still too expen-sive to be adopted in WLAN or consumerapplications, and mass production also is diffi-cult. While complementary metal oxide semicon-ductor (CMOS) technology is considered as anideal solution in the point of cost and circuit

integration, RF CMOS for 60 GHz frequencyrequires more performance improvement. Con-sidering performance and cost, currently, silicongermanium (SiGe) technology seems to be apromising alternative to the GaAs or InP tech-nology. The SiGe solution provides MMIC cir-cuits with a tolerable noise figure and powerdissipation. Besides circuit technologies, 60 GHzcircuits make possible or require new implemen-tation methodologies. For example, antenna-on-chip design or implementing antennas by usingwire bonding shrinks the size of the system. Ana-logue processing for equalization and synchro-nization may deal with speed and powerconsumption issues.

ZIGBEE TECHNOLOGY

Although wireless control and sensor networkingare at the other end of high-speed networks withquality of service (QoS) support, these low-powerconsumption and low-data rate WPAN are drivenby applications in the retail, medical, and logisticsfields. This low throughput and low-cost wirelesstechnology, named ZigBee, adopts the IEEE802.15 TG4 low-data rate WPAN standard. Itsspecification, which was defined by the ZigBeeAlliance, specifies network, security, and applica-tion profiles based on PHY and MAC sublayersof IEEE 802.15.4-2003 standard [10, 11].

IEEE 802.15.4-2003 STANDARD AND ZIGBEEThe IEEE 802.15.4-2003 PHY operates in threedifferent industrial, scientific, and medical (ISM)bands; 2450, 915, and 868 MHz. The 2450 MHz(global) band provides 16 channels; the 915MHz (United States) and 868 MHz (Europe)

n Figure 4. Performance of single-carrier block transmission with frequencydomain equalization over 60 GHz indoor channel: 16-QAM modulation with256 data symbols and 64 cyclic prefix per block. Channel model of a 60 GHzindoor (office) environment based on measured data from France TelecomR&D, Table 11, which is available at http://www.ieee802.org/15/pub/TG3ccontributions.html, is used.

EBN0 (dB)

Single-carrier block transmission with frequency domain equalization

20

10-3

10-4

Ave

rage

bit

err

or p

roba

bilit

y P b

10-2

10-1

100

4 6

*without error propagation

8 10 12

SC-FDE with MMSE DFE*SC-FDE with ZF DFE*SC-FDE with MMSE LESC-FDE with ZF LEOFDM without channel codingAWGN



bands are assigned 10 channels and a singlechannel, respectively. For the three ISM bands,two PHY options, 2450 MHz and 915/868 MHzPHY, are specified that support data rates of250 kb/s and 40/20 kb/s, respectively. 2450 MHzPHY maps a data symbol composed of fourinformation bits into one of 16 quasi-orthogonal32-chip PN sequences and modulates it usingoffset quadrature phase shift keying (OQPSK).On the other hand, 915/868 MHz PHY maps abinary data symbol into one of two 15-chip PNsequences and transmits the chip sequence usingBPSK. The basic channel access mechanism ofTG4 is a carrier sense multiple-access with colli-sion avoidance (CSMA-CA) and is suitable forvery low-duty cycle operation. For applicationsrequiring specific data bandwidth, one of twooptional superframe structures provides a guar-anteed time slot (GTS) for low latency. A deviceimplementing the IEEE 802.15.4-2003 protocolcan be classified as a full function device (FFD)or reduced function device (RFD). In brief, boththe PHY specification and the MAC protocolachieve the required long battery life and sim-plicity by using spread spectrum transmissionand very low duty cycle.

In addition to the standard IEEE 802.15.4-2003, low rate alternative PHY task group, TG4ais defining an enhanced PHY specification overTG4 in throughput, power consumption, andrange. The approved draft specification of TG4aconsiders two PHY, that is, IR-based UWB forboth communication and ranging over 3–10 GHzband and (optionally) narrowband chirp spreadspectrum (CSS) for communication only over2450 MHz. IR-based UWB adopts a combina-tion of pulse position modulation and BPSK andprovides data rate of 842 kb/s with severaloptional rates. CSS spreads differential QPSK ofquasi orthogonal PN sequences by using chirpspread spectrum and supports data rates of 1Mb/s or optional 250 kb/s.

The ZigBee specification, defined by an asso-ciation of companies called the ZigBee Alliance,provides upper layer stacks and application pro-files that are compatible with the IEEE 802.15TG4 PHY and MAC sublayers. Similar to RFDand FFD, the specification also describes nodesas network coordinators, routers, and enddevices. Network coordinators manage the net-work, and routers direct messages betweennodes. Data traffic of ZigBee networking can beclassified as three different types: periodic, inter-mittent, and repetitive low-latency data. Accord-ing to these traffic types, optimized networkconfiguration can be chosen. Generally, ZigBeenetworks are suitable for low duty cycle (≈ 1 per-cent) communications for long battery life.Besides the technical specifications, the ZigBeeAlliance also manages product branding of Zig-Bee-compliant platforms. Recently Freescaleand several other companies announced plat-forms including chip sets, development environ-ment, and protocol stacks.

ISSUES AND CHALLENGESWhile low throughput and low-power consump-tion networking hold great potential, severalopen issues must be addressed for a successfulrealization of the potential. The crucial issue is

how to realize a low-power consumption andlow-cost network of ZigBee devices. This chal-lenging task cannot be accomplished by an iso-lated methodology alone; a system-levelapproach ranging over wide layers is required.

Networking and Application Profiles — Frequentlychanging network environments caused by nodemovement and failure require networking andupper layers to offer very robust and self-config-urable mechanisms. These mechanisms andapplications should select an appropriate net-working topology because power consumptiondepends on networking structure and devicetype. A robust routing algorithm also is an issuefor multihopping or mesh ZigBee networks. Fur-thermore, due to limited resources, includingmemory, power, and computational capacity,network stacks must be realized in an efficientand compact form; for example, the ZigBeeAlliance suggests an eight-bit microprocessorand either 32 kilobytes (FFD) or six kilobytes(RFD) memory.

Device Implementation — For ZigBee device real-ization, a highly integrated single chip usingCMOS technology is preferred to combine ana-log and digital circuits with tolerable RF perfor-mance. The choice of transceiver architecture isa trade-off among cost, complexity, performance,and power consumption. Zero-IF or direct con-version is one of the promising candidatesbecause it enables digital preferred design, lowpower, and simplicity. In addition to hardwaredesign, software implementation also must con-sider characteristics of ZigBee devices. Forexample, the protocol stack, operating system(OS), and compiler must operate with limitedmemory and computing capacity.

Multi-month to Multi-year Battery Life — The goal ofmulti-month to multi-year battery life createschallenging tasks for ZigBee designers in circuitdesign, battery technology, PHY specification,and protocol stacks. Most of ZigBee applicationsare expected to operate with a very low-dutycycle, thus power consumption in sleep or power-down mode can significantly affect battery life.Avoiding leakage current also requires carefulcircuit design. Most of the currently availableZigBee devices consume several orders of micro-amps in power-down mode. Reception andwake-up operations of an RF circuit require aconsiderable portion of total energy consump-tion. Using current technology, approximately20–50 mA is consumed for RF circuit operationin a ZigBee transceiver. In addition to circuitimplementation, applications and network pro-files must also support low-power operations.

MAC PROTOCOLS FORMULTIGIGABIT WPAN

A wireless MAC protocol is a rule by whichnodes of a network can share a common spec-trum in an efficient and fair manner. A MACprotocol of a short-range ad hoc network target-ed by a multigigabit WPAN, especially 60 GHzmillimeter-wave WPAN, requires a distinctive

While low throughputand low-power consumption networking holdgreat potential, several open issuesmust be addressedfor a successful realization of thepotential. The crucialissue is how to realize a low-powerconsumption andlow-cost network ofZigBee devices.

RAPPAPORT LAYOUT 8/2/07 3:23 PM 6


MAC protocol differentiated from the generalad hoc MAC protocols surveyed in [12]. TheIEEE 802.15 TG3c decided to define a newMAC protocol compatible with 60 GHz millime-ter-wave WPAN. Channel and networking char-acteristics of multigigabit WPAN, as well asassumed applications, should be reflected in theMAC protocol. Figure 5 shows the main issuesof 60 GHz multigigabit WPAN: directionalantennas, high throughput, relay channels,streaming, energy efficiency, and interferenceand co-existence. In this section, high through-put, directional antennas, and implementationissues of millimeter-wave-based WPAN MACprotocol are addressed.

HIGH THROUGHPUTA wireless data bus and uncompressed HDTVvideo streaming, which are potential applicationsof multigigabit WPAN, require unprecedentedhigh throughput; for an 1080i HDMI connec-tion, data rate of 2 Gb/s is assumed at the appli-cation level. This high throughput of 60 GHzWPAN requires not only a multigigabit physicallayer but an efficient MAC protocol as well.

Bandwidth waste caused by frame and packetstructures restricts achievable throughput. In thehigh speed physical layer compatible with multi-gigabit transmission, overheads existing in con-ventional preamble and header fields of packetor frame aggravate the throughput more severelythan narrow-band transmission. Thus, for exam-ple, a superimposed training sequence embeddedin the information data can be adopted by 60GHz WPAN to reduce the overhead. Maximumframe and packet sizes also are factors to investi-gate for throughput improvement. Throughput isgreatly influenced by signaling and control mech-anisms, as well as frame structures. In particular,retransmission caused by collisions or errors sub-stantially consumes the data bandwidth. To mini-mize retransmission, 60 GHz MAC protocol mayrealize a hybrid automatic repeat request (ARQ)that combines ARQ and error correction coding.In addition, retransmission and ARQ can refer tochannel state information (CSI), such as signal-to-noise ratio (SNR) for adaptive control opera-tions. In addition to parameter and controloptimization, an efficient channel access schemealso is crucial to throughput improvement.Reserved-based channel access, generally adopt-ed to provide guaranteed bandwidth in WPAN,may require modification under given 60 GHzWPAN networking and application scenarios.

DIRECTIONAL PROTOCOLSSevere attenuation of 60 GHz radio propagationand the resulting low-link budget make high gainor directional antennas feasible for 60 GHzWPAN. There is an ongoing discussion to con-sider directional MAC protocols at the IEEE802.15 TG3c although omni-directional antennasare assumed for standardized WPAN andWLAN. In particular, 60 GHz WPAN can obtainthe benefit of directional antennas using multi-ple antennas due to small wavelength (≈ 5 mm).Directional antennas have the potential of high-er spatial reuse, longer coverage, less interfer-ence, and higher energy efficiency thanomni-directional antennas, as surveyed in [13].

However, directional antennas and MAC pro-tocols, especially in the short-range ad hoc net-works, require cautious consideration in terms ofthe trade-off between complexity and perfor-mance. Although most proposed directional pro-tocols deal with switched-beam antennas due tosimplicity, fully adaptive arrays or generalMIMO antennas deserve to be studied further.In technical aspects, estimating directions orpositions is a challenging task under limitedcomputing capacity and ad hoc networking envi-ronments. Special signaling or procedure forestimations should be provided. Because thebeam width of a 60 GHz radio propagation isnarrower compared with microwave radio, mis-aligned directional antennas may lead to degra-dation and aggravation of received SNR andinterference level, respectively. Regarding chan-nel access, most of the proposed directional pro-tocols are based on the request-to-send/clear-to-send (RTS/CTS) and network allocationvector (NAV) of the IEEE 802.11 distributedcoordination function (DCF) mechanism [12].However, 60 GHz WPAN may require reserva-tion-based directional protocols due to QoS sup-port and applications. In addition, there aredirectional antenna-inherent issues, such as newhidden node problems and deafness caused bymainly asymmetric gains of directional antennas.For the assumed frequently changing ad hoc net-working, these node problems should beaddressed efficiently.

IMPLEMENTATION AND SIMULATION PLATFORMSWhen considering practical aspects of MAC pro-tocol implementation, there are a variety oftrade-offs among architectural requirements,performance requirements, and complexity.Optimal partition between hardware and soft-ware based on these factors is crucial to a suc-cessful realization of the MAC protocol.Growing trends toward system-on-a-chip (SoC)

n Figure 5. MAC protocol issues of 60 GHz WPANs: high throughput, direc-tional antennas, relay channels, video streaming, and interference and coexis-tence.

MAC protocols depend on:

• Channel - Radio propagation - Interference - Coexistence...

• Applications - Traffic pattern - Throughput - QoS...

Relay

High throughput

Streaming

Directional antenna

Interferencecoexistence

Energy efficiency



design combine a processor (e.g., ARM™) withdedicated custom hardware for MAC opera-tions. Generally, special frame handling such asacknowledgment or control handshaking is pro-cessed by hardware for fast response. The timer,a cyclic redundancy check (CRC), and encryp-tion/decryption are also hardware functions. Onthe other hand, network management and con-trol operations are implemented in software.Regarding partition issues, the interface is aninteresting topic because wireless MAC has linkswith radio, baseband, state registers, and hostparts. In addition, partition, buffering, and directmemory access (DMA) operations require care-ful attention to support high throughput ofmultigigabit wireless MAC.

Along with implementation issues, severalsimulation platforms are discussed in the docu-mentation of an IST project related to WPAN[14]. Although performance depends on specificrequirements and environments, OpNet™ andNetwork Simulator 2 (NS-2) seem to be widelyaccepted for network protocol designs to date.OMNeT++, Ptolemy II, GloMoSim, and recent-ly, SystemC also show strength in particularareas. These platforms, however, seem to requiremore WPAN protocols and wireless channelenvironments. In addition to these simulationplatforms, site-specific tools such as LANPlan-ner™, which visualizes coverage, capacity, andphysical location for WLAN design and deploy-ment, also are available.

CONCLUSIONS

This article describes potential technology forthe next-generation WPAN, namely UWB, 60GHz millimeter-wave-based WPAN, and ZigBeeand gives an overview of related standardizationand regulation issues. We highlighted UWB and60 GHz millimeter-wave-based WPAN and Zig-Bee as harbingers of next generation WPAN formultigigabit and low-rate networks, respectively.UWB technology has a huge potential for short-range, broadband wireless communications andis under worldwide development. We empha-sized that the standardization breakdown ofIEEE 802.15 TG3a and different views regardingspectrum regulations are threatening globaldeployment of UWB. We described key featuresof two UWB technologies, DS-UWB and MB-OFDM, and discussed global status of standard-ization and regulation. In addition to UWBtechnology, 60 GHz millimeter-wave-basedshort-range wireless communications are expect-ed to play a crucial role for multigigabit WPANin the near future. We highlighted the character-istics and development status of 60 GHz short-range communications. To realize multigigabit60 GHz communications, accurate channel mod-eling, efficient modulations, and broadband cir-cuits must be addressed. In addition to high datarate WPAN, we also discussed a low data rate,low cost, and ultra low-power consumption tech-nology named ZigBee. Although it is on theother end of UWB and 60 GHz millimeter-waveWPAN in data rate, ZigBee has potential in sen-sor networks and the wireless inventory trackingcontrol of next generation wireless networks.

Together with PHY technology, upper layer

protocols also are crucial for next-generationbroadband WPAN. We focused on the MACsublayer of multigigabit WPAN and describedthe issues of high throughput and directionalantennas. Additionally, we highlighted an opti-mized partition of MAC functions between hard-ware and software.

In conclusion, UWB, 60 GHz WPAN, andZigBee are potential technologies to realizeshort-range wireless communications of 4G tech-nology. To successfully deploy these WPAN inthe near future, globally unified standardizationand regulations, as well as technical issues shouldbe addressed.

REFERENCES[1] IEEE 802.15 WG for WPAN Web site; available at

http://grouper.ieee.org/groups/802/15/[2] FCC, “First Report and Order: Revision of Part 15 of the

Commissions Rules Regarding Ultra-Wideband Trans-mission Systems,” ET Docket 98-153, Apr. 2002.

[3] R. Fisher et al., “DS-UWB Physical Layer Submission to802.15 Task Group 3a,” IEEE 802.15 3G doc., IEEE802.15- 04/137r3, July 2004.

[4] A. Batra et al., “Multiband OFDM Physical Layer Proposalfor IEEE 802.15 Task Group 3a,” MBOA-SIG, Sept. 2004.

[5] D. Porcino, “Standardization and Regulations for UWB,”ULTRAWAVES IST-2001-35189, D9.4, Jan. 2005.

[6] P. Smulders, “60 GHz Radio: Prospects and FutureDirections,” Proc. 10th IEEE Symp. Commun. and Vehic.Tech., Benelux, Nov. 2003, pp. 1–8.

[7] IEEE 802.15.3, “Channel Model Literature Summary andCapacity Calculations”; http://www.ieee802.org/15/pub/TG3c contributions.html

[8] H. Hao, V. Kukshya, and T. S. Rappaport, “Spatial Char-acteristics of 60-GHz Indoor Channels,” IEEE JSAC, vol.20, no. 3, Apr. 2002, pp. 620–30.

[9] M. Huemer et al., “A Review of Cyclically Extended SingleCarrier Transmission with Frequency Domain Equalizationfor Broadband Wireless Transmission,” Euro. Trans. Com-mun., vol. 14, no. 4, Jul.–Aug. 2003, pp. 329–41.

[10] ZigBee All iance, “ZigBee Specification,” doc.053474r06, v. 1.0, Dec. 2004.

[11] IEEE Std. 802.15.4, “Part 15.4: Wireless MediumAccess Control (MAC) and Physical Layer (PHY) Specifi-cations for Low-Rate Wireless Personal Area Networks(LR-WPANs),” Oct. 2003.

[12] S. Kumar, V. S. Raghavan, and J. Deng, “MediumAccess Control Protocols for Ad Hoc Wireless Networks:A Survey,” Elsevier Ad Hoc Networks, vol. 4, no. 3, May2006, pp. 326–58.

[13] A. Arora and M. Krunz, “Power-Controlled MediumAccess for Ad Hoc Networks with Directional Anten-nas,” Elsevier Ad Hoc Networks, 2006.

[14] IST PULSERS, “Definition of New Concepts/Architecture forUWB MAC and Networking,” tech. rep. D51.1a, July 2004.

BIOGRAPHIESCHEOLHEE PARK received his B.S. and M.S. degrees in elec-tronic engineering from Chung-Ang University, Seoul,Korea, in 1995 and 1997, respectively. Since 2004 he hasbeen working toward his Ph.D. degree in electrical andcomputer engineering at the University of Texas at Austin.His research interests include multicarrier communications,MIMO technologies, ad hoc MAC protocols, and 60 GHz adhoc networks. From 1997 to 1999 he was a research engi-neer at the Satellite and Mobile Terminal Divisions, HyundaiElectronics Industry Co., Korea. From 1999 to 2004 he waswith the Wireless Network Research Center, Korea Electron-ics Technology Institute, as a senior researcher.

THEODORE S. RAPPAPORT () received his B.S., M.S., and Ph.D.degrees from Purdue University. He teaches at the Univer-sity of Texas at Austin, where he is the William and Bet-tye Nowlin Chair in Engineering and the founding directorof the Wireless Networking and Communications Group(WNCG). He has been working in the wireless communi-cations field since the 1980s. He is an active researcherand entrepreneur, and has founded and serves on theboards of several pre-IPO wireless companies. He alsoguides students in the field of millimeter-wave wirelessdevices, indoor and RF propagation, and wireless networkdesign.

UWB, 60 GHzWPAN, and ZigBeeare potential tech-nologies to realizeshort-range wirelesscommunications of4G technology. To deploy theseWPAN in the nearfuture, globally unifiedstandardization andregulations, as wellas technical issuesshould be addressed.


short-range wireless communications for next generation networks.pdf

Documents