OFDMBackToWirelessFuture.pdf

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I N D U S T R Y T R E N D S

A

key limitation for wirelesstechnologies has been slow

transmission rates that didntcome close to providing thecapabilities of wireline ser-

vices. This left the technologies unableto offer fast downloads or handle mul-timedia and other desirable applica-tions.

Now, however, wireless approachesthat use orthogonal frequency divisionmultiplexing (OFDM)a modulationtechnique for transmitting largeamounts of digital data over a radiowaveare boasting high speeds. Forexample, IEEE 802.11a and 802.11g

wireless LAN (WLAN) standards offertheoretical maximum speeds of 54Mbits per second, with real-world datarates of up to 22 Mbps. This is higherthan the rates produced by previousWLAN technologies such as IEEE802.11b.

The European TelecommunicationsStandards Institutes proposed Hiper-LAN2 (high-performance radio LAN2) and Japans Mobile MultimediaAccess Communications broadbandWLAN technologies also use OFDM.

In addition to WLANs, vendors likeFlarion Technologies and NextNetWireless are using OFDM to bringhigher speeds to fixed-wireless metro-politan area networks (MANs).

Service providers are also looking atOFDM for their broadband mobile ser-vices, including those used in cellularphones and PDAs. In fact, even as code-division multiple access (CDMA)-basedthird-generation (3G) mobile wirelessdata and voice services finally enter the

US market, some companies are look-ing to OFDM to drive faster 4G wire-less systems.

However, OFDM probably wont

experience completely smooth sailing.For example, Allen Nogee, senior ana-lyst for wireless component technologyat In-Stat/MDR, a market researchfirm, said that with cellular 4G, even ifOFDM is proven to be superior tech-nology, market-leading CDMA-basedvendors and carriers are likely to fightto maintain their large customer base.

Also, numerous incompatible OFDMstandards may limit the technologysbroad usefulness and widespread adop-tion.

A CLOSER LOOK AT OFDMWhile new to wireless technology,

OFDM dates back to the late 1960s,as the subject of research into ways tominimize interference among trans-mission channels close to one anotherin frequency. OFDM has been used insuch disparate ways as asymmetric-DSL broadband Internet access, as wellas European digital audio and videobroadcast services.

The principal driving forces behindOFDMs increased popularity in wire-

less technology are the general demandfor faster approaches and the specificdesire to run multimedia applications,which are data intensive and thusrequire higher speeds.

OFDM has become practical formore widespread use because it relieson high-speed digital signal processors,and DSPs have only recently becomeavailable at a price that makes OFDMa competitive technology in the mar-ketplace.

The technologyOFDM splits a data-bearing radiosignal into multiple smaller signal setsand modulates each onto a differentsubcarrier, transmitting them simulta-neously at different frequencies.

The technology achieves higher band-width than other forms of multiplexingby using a number of parallel subcarri-ers spaced orthogonally as closely aspossible in frequency without overlap-ping or interfering, explained Douglas

Jones, professor of electrical and com-puter engineering at the University of

Illinois, Urbana-Champaign. Becauseadjacent subchannels are orthogonal toone another, they have no overlap andthus usually create little interference.

Generally, Jones said, OFDM usesthe fast Fourier transform (FFT) algo-rithm on both transmitter and receiverto mathematically transform signalsand thereby efficiently space the fre-quencies so that they are as close to-gether as possible, yet still orthogonal.

Wireless systems use various fre-quency-modulation methods to trans-

mit high volumes of material viaOFDM.

For example, IEEE 802.11a uses thebinary phase-shift keying approach toachieve 6- to 9-Mbps data rates; quad-rature phase shift keying for 12- to 18-Mbps rates; and quadrature amplitudemodulation for 24- to 54-Mbps rates.The faster approaches are more suscep-tible to interference and require moredigital signal processing power, and thusare not suitable for all situations.

OFDM: Back to theWireless FutureSteven J. Vaughan-Nichols


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OFDM also transmits 6-bit data seg-ments, instead of the typical binarysegments. This approach crams a lotof data into relatively little bandwidth.

The technology prioritizes the reduc-tion of interference among channelsand places less emphasis on perfectingthe quality of individual channels. This

is because OFDM transmits signals inparallel, so a problem with one chan-nel will cause minimal overall difficul-ties, which often can be fixed via error-correction technology.

As Figure 1 shows, OFDM is almostalways implemented in embeddedchipsets made up of radio transceivers,FFT processors, system I/O, serial-to-parallel and parallel-to-serial transla-tors, and OFDM logic.

Reducing interference

OFDM also improves performanceby minimizing multipath distortion, aproblem experienced by single-carrierwireless technologies such as CDMA.Multipath distortion occurs when atransmitted wireless signal bounces offa building, tree, or other surface, cre-ating copies that arrive at the destina-tion at different times, thereby de-grading overall signal quality.

OFDM deployments typically pro-tect against interference via forward

error correction. OFDM also combatsthe problem by transmitting each bitrelatively slowly, the University ofIllinois Jones explained. For example,to transmit 1 Mbit per second, a sys-tem could send one bit per microsec-ond. However, if a transmission weredelayed by slightly more than a micro-

second, it would overlap the next sig-nal, causing interference.

However, if a system transmits 1,000bits in parallel over 1,000 OFDM sub-channels, one bit could be transmittedper millisecond and still yield an aggre-gate 1 Mbit per second data rate. In thiscase, a signal delay of slightly morethan a microsecond would overlap onlyone-thousandth of the next bits trans-mission period, thereby causing almostno noise.

OFDMs reduction in interference,

distortion, and multipath delay elimi-nates the need, experienced by CDMA,for additional system elements such asequalizers to cope with this problem.This, in turn, reduces system cost,complexity, and power consumption.

ChallengesOFDMs radio components are built

to run at a systems peak, not average,power usage. This consumes a lot ofpower, which is a problem for battery-

powered devices such as laptops andPDAs.

Other challenges include phase dis-tortion, noise, and sensitivity to fre-quency instability caused when areceivers voltage-controlled oscillatordoesnt run at exactly the same carrierfrequency as the transmitters VCO.

OFDM VARIATIONSA key obstacle to OFDM adoption is

that there are numerous incompatibleversions and no single standard. (Seethe sidebar The Many Flavors ofOFDM.) For example, IEEE 802.11aand HiperLAN2 implement differentOFDM versions.

Like other multicarrier systems,OFDM has vast variations between itssignal-power peaks and valleys. Thereare many ways to deal with this prob-

lem. For example, IEEE 802.11a imple-mentations of OFDM limit power out-put, to reduce energy consumption, andretransmit packets if data is missing.

However, other OFDM implemen-tations use digital signal processors indifferent ways to eliminate peak-and-valley problems. They are thus incom-patible because the DSPs cant talk toone another.

Numerous companies use theirown proprietary OFDM approaches.

Informationsource

Errorcorrectionencoder

InterleaverSymbol

map

Serial toparallel

translator

Inversefast

Fouriertransform

Parallelto serial

translator

(a)

Symboldemap

Errorcorrectiondecoder

DeinterleaverReceivesample

data

Serial toparallel

translator

FastFourier

transform

Parallelto serial

translator

(b)

Figure 1. When OFDM transceivers communicate, the transmitter (a) and receiver (b) must align perfectly and, because of potential inter-

ference, must use error correction for the technology to work optimally. They also require symbol mapping and demapping to translate

data between the 6-bit segments that OFDM uses and standard binary segments. Although the data will start and end in serial format,

translators are necessary because OFDM sends and receives in parallel. And fast Fourier algorithms transform signals, spacing the

frequencies so that they are close together yet still orthogonal.


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there are many operators proceedingwith CDMA.

On the other hand, Rajiv Laroia,Flarions founder and chief technicalofficer, predicts OFDM will play amajor role in 4G mobile networks.

Laroia said, Wireless systems orig-inally architected for circuit-voice andthen adapted for data, such as 3G, can-not cost-effectively provide the wholeInternet experience to mobile users.On the other hand, he explained,OFDM can increase transmissionspeeds at more attractive prices.

According to Navin Sabharwal, direc-tor of residential and networking tech-

nologies for Allied Business Intelligence,a market research firm, this isnt a con-cern. He said OFDM doesnt need to bestandardized because it is an underly-ing signal-modulation technique thatvendors should be able to adapt for dif-ferent applications.

The OFDM Foruman associationof hardware manufacturers, softwarefirms, and other wireless OFDMusersdisagrees. As is the case withother technologies, OFDM needs stan-dardization to enable widespread use,

encourage adoption, and thereby growthe market, argued Shawn Taylor, chiefscientist at Wi-LAN Inc., an OFDMfounder and vendor of high-speedwireless communications products.

LOOKING AHEADOFDM is already playing an impor-

tant role in WLANs and is beginningto do so with MANs. During the nextfive years, some industry observersincluding Paul Kellett, senior directorof research for Pioneer Consultingpredict there will be a war between

OFDM and CDMA for dominance ofthe broadband wireless market.

However, noted In-Stat/MDRsNogee, there isnt much of a battle to-day. Despite OFDMs technical advan-tages in some areas, he said, the tech-nology is new to wireless, while CDMAalready has 150 million worldwide sub-scribers. And, he explained, OFDMoperates differently than CDMA, sousers cannot simply substitute it in theirexisting implementations.

Moreover, Nogee said, although

OFDM has proven itself with packet-based data, it is not clear whether thetechnology can either handle largenumbers of voice customers or workwith both voice and data as well asCDMA.

And thus, noted Edward Rerisi, asenior analyst at Allied BusinessIntelligence, As of today, we know ofno major wireless operator goingahead with an OFDM platform for itsmobile data-delivery solution. But

T

herefore, Laroia concluded,OFDM is especially well suited

for mobility applications in cel-lular networks. I

Steven J. Vaughan-Nichols is a free-lance writer based in Arden, North Car-olina. Contact him at [email protected].

The Many Flavors of OFDM

The various types of OFDM include:

Vector OFDMBroadband silicon vendor Broadcom and Cisco Systems developed VOFDM.

This system works with spatial diversity, which uses multipath signal reflections

to increase bandwidth and range via special antennas and signal processing.

It uses antennas to capture the signals and high-powered processing to nor-

malize the delays into a higher throughput data stream. VOFDM is most often

used in fixed-wireless metropolitan area networks (MANs).

Wideband OFDMThe OFDM Forum says Wi-LANs W-OFDM should be the standard ver-

sion. Rather than using tightly packed orthogonal carriers, W-OFDM intro-

duces additional frequency space between the orthogonal channels. This fur-ther reduces interference and permits higher tolerance for OFDM transmission

problems such as jitter. Businesses and wireless Internet service providers are

using W-OFDM in MANs, for which transceivers tend to be outdoors and

require a more tolerant approach.

F-OFDMFlarion created F-OFDM by incorporating fast-frequency-hopping spread-

spectrum technology, which repeatedly switches frequencies during a radio

transmission. This system transmits a signal across a much wider frequency

band than is required. This spreads the energy across a higher number of chan-

nels on a wider spectrum, thereby increasing signal capacity. Flarion designed

F-OFDM, which is currently undergoing field testing, to deliver broadband

services to cellular-phone and other mobile users.

MIMO-OFDMMultiple-input, multiple-output OFDM was developed by Iospan Wireless.

Basically, MIMO-OFDM uses OFDM to break up a signal and wirelessly trans-

mit the pieces simultaneously via multiple antennas. The receiver subsequently

reassembles the pieces. MIMO-OFDM lets providers offer fixed broadband

wireless access systems that dont require a line of sight between transmitter

and receiver.

Editor: Lee Garber, Computer, 10662 Los

Vaqueros Circle, PO Box 3014, Los Alamitos,

CA 90720-1314; [email protected]

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OFDMBackToWirelessFuture.pdf