ULTRA WIDEBAND COMMUNICATIONS FOR SENSOR NETWORKS

Kent D. CollingInnovative Wireless Technologies

Forest, VA 24551

ABSTRACT

Application of Ultra Wideband (UWB) technology to anUnattended Ground Sensor (UGS) network providesunique capabilities for capacity, covert operation, and lo-cation awareness. These features make this RF technologyattractive for demanding military usage scenarios. Cou-pled with UWB's unique capabilities comes unique chal-lenges when applied to autonomous, self-forming, and bat-tery powered networks. In this paper we discuss essentialdesign tradeoffs necessary to incorporate a UWB wave-form into an ad hoc network ofsensors. Background UWBinformation is provided followed by brief analysis of keyrange, data rate, LPIILPD, interference rejection, syn-chronization, and location performance requirements. Thepaper concludes with system recommendations for appli-cation ofthe technology.

1. INTRODUCTION

The rapid growth of mobile ad hoc wireless technologieshas enabled novel and exciting new applications for mili-tary, homeland security, Ist responder, and industrial appli-cations. Unattended Ground Sensors (UGS) are a specialclass of wireless ad hoc networks that autonomously formmulti-hop networks and operate on battery power for ex-tended periods. Unlike wired networks, multi-hop wire-less networks introduce difficult technical challenges dueto the unreliable nature of the wireless medium, energyconstrained devices, and the compounding effect of linkdegradation over multiple hops. The combination of thesechallenges as well as many others requires unique trade-offs to be taken into account while designing the network.

The choice of appropriate RF physical layer technology isa key enabler in UGS networks. Ultra Wideband (UWB)is an emerging RF technology that has unique performancecharacteristics that support covert LPI/LPD operation, ac-curate ranging capabilities, good multipath performance,and variable data capacity depending on sensor bandwidthrequirements. These characteristics make UWB basedcommunications an attractive candidate for UGS networks.

andPhilip Ciorciari

U.S. Army CERDEC STCDFort Monmouth, NJ 07703

In this paper we present tradeoffs of key capabilities andchallenges posed by application ofUWB technology to adhoc wireless networks. Specifically we address

+ Propagation Environment+ Range versus Data Rate+ LPI/LPD+ Interference Rejection+ Energy Efficient Synchronization+ Localization

The end goal of our analysis is to determine feasibility ofapplying a UWB based waveform to UGS with a migra-tion path to JTRS Cluster 5 devices. Core requirementsfor the analysis are based on the Soldier Radio Waveform(SRW) [1] and Blue Radio [2].

2. UWB BACKGROUND AND OPTIONS

UWB is a pulsed based technology which leverages theextreme spectral spreading that is associated with shortbursts of energy in the time domain. The result of thespectral spreading is covert operation and capacity to sup-port high data rate applications while consuming minimalamounts of power. Relative to traditional narrowband RFtechnologies UWB provides

+ Great LPD performance at equivalent performancecapacity

+ Higher capacity at equivalent LPD and transmitpower levels

FCC regulations limit the average and peak transmit powerofUWB waveforms. The range and data rates obtainableby pulsed UWB transceivers is determined by the averageand peak power limits, the pulse rate, and the coding gainof the system. Implementation of the technology varies,but primarily has centered on traditional pulsed approachesand more recently multi-carrier approaches.

Pulsed Modulations

Pulsed modulation schemes use very short electrical pulsesto transmit data. The width of pulses may be as narrow as

The work presented in this paper was supported by the U.S. Army Communications Electronics Research and Development Command, Space andTerrestrial Communications Directorate at Ft. Monmouth, NJ under contract number DAAB07-03-C-J003.

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a few hundred picoseconds, producing bandwidths of sev-eral GHz. See Figure 1. Pulse shape determines the spec-tral shape of the waveform.

TX Spectrum at Filter Output wth Regulatory Limit for Indoor UWNB Radiation1 MHz RBW

i -lak-20-30-4050k-6C0-70-8C0-9a

-1 Oa}

-1 2C}-1 3C}-1 4()

1E9Frequency (Hz)

lE1O 2E1 C

Figure 1: Tx spectrum for indoor UWB at FCC limits.

Transmitters in pulse based modulation schemes use sim-ple switching circuits. Linear power amplifiers, which canbe a significant cost in carrier based modulation schemes,are not required. The pulsed based transmitters can also bedesigned to consume much less power than transmitterswith linear power amplifiers.

Pulsed UWB systems use pseudo random spreading codesto obtain coding gain and add security to their networks.In systems in which a large amount of coding gain is ap-plied, the amplitude of individual UWB pulses at the re-ceiver can be less than the root-mean-square amplitude ofthe receiver noise. By correlating the receive signal withthe spreading code, receive data can be extracted from thenoise floor. Probability of detection of the signal by a re-ceiver without the spreading code is low.

The timing requirements of pulsed UWB systems arestringent. To obtain coding gain, the receiver correlatormust maintain synchronization with the received signalwithin tens of picoseconds. Obtaining and maintainingsynchronization when signal levels are too low to detectindividual pulses is a challenge of pulsed UWB systemdesign. Development of a low power correlator requires ahigh specification integrated circuit implementation whichcan be costly for low volume production.

To improve performance in multipath environments,pulsed UWB systems use rake receivers. A rake receiveruses multiple receivers in parallel to capture and construc-tively combine signal arrivals from multiple paths. Rakereceivers improve the performance of UWB receivers inmulti-path environments at the cost of added complexityand power consumption.

Multi-Band OFDM

MB-OFDM is a modulation technique that subdivides eachband into many lower data rate sub-channels. Proposed

standards divide the allowable UWB spectrum into 528MHz channels comprised of 122 sub-carriers. The sub-carriers are spaced such that each is orthogonal to others,eliminating inter-channel interference. On the transmitside, input data is mapped to the phases of sub-channels byperforming an Inverse Fast Fourier Transform (IFFT). Onthe receive side, a Fast Fourier Transform (FFT) deter-mines the bit values from the phases of the sub-channels.The two main advantages of the MB-OFDM approach are

+ Selective sub-carrier removal provides jammingresistance not available in pulsed approaches

+ Implementation similar to traditional radio archi-tectures with a migration path to a CMOS solution

The OFDM technique uses a simple approach for dealingwith multipath signal arrival. It simply adds a cyclic pre-fix to each transmitted symbol which basically extends thelength of the symbol so most the multi-path energy canarrive at the receiver before it extracts the symbol informa-tion. This requires only one transmit and one receivechain at all times, even in the presence of multipath.

The design of the MB-OFDM transceiver chain parallelsthat of traditional radio architectures with DAC, ADC,gain, filtering, and I/Q mixers contained within the RF ar-chitecture. This common radio architecture allows for im-plementations to be done in CMOS and off-the-shelf dis-crete RF components, thus lower cost and earlycommercial market adoption. The FFT and IFFT calcula-tions are intensive digital operations. Rapid advances inthe speed and power consumption of digital technologyhave made this approach possible for UWB applications,but consume significant power.

Because the OFDM technique provides very good spectralcontrol, interference with narrow band receivers can beavoided as long as they do not saturate the receiver frontend. Interference from other intentional or unintentionalsources is avoided by selectively omitting specific sub-carriers from the transmit signal. Adaptive algorithms candynamically sense and notch interference from unwantedsources, see Figure 2. Band selectable filtering may beemployed at the interface to the antenna to provide addi-tional jamming resistance due to saturation of widebandreceivers from high power narrowband signals.

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0

I~~~~

Band 1 Band 2 Band 33.432 GHz :3.960GHz 4.488 GHz

N,Iotch due tLv 55nulledsub-carries

This equation can be written to separate the frequency de-pendent term as

L (dB) A0+A1 logio(f)+ ]OfYogio(dldo) + 0x a

logiO(dldo) +sfor d > do 1 meter where A1 log1o(f) is the frequency de-pendent loss which is the integrated value over the selectedfrequency band and

A = Ao+Alloglo(f).-90 33 38 43 The values of Ao , A1, y, cy and s are dependent on the

freq, GHz geometry of the transmitter and receiver, and the channel

Figure 2. MB-OFDM response of three lower bands with environment around them. We analyzed the channel losssub-carrier nulling in Band 3. as a median path loss based on the first three terms, and a

multipath loss based on the last two terms. [3-4, 8-10]

3. PROPOGATION MODEL

The channel impulse model for pulse based UWB is sig-nificantly different than modulated waveforms. First thereis not Rayleigh type fading since there is not modulation.Second, since the pulse waveform is very narrow, the mul-tipath returns tend to form clusters of rays where the rays

within a cluster are caused by reflections from objects near

the transmitter and receiver, and the clusters are caused bybuilding structure and other large reflectors.

The UWB channel model representa-tion can be divided into threefunctions, the path loss at 1 meterwhich is frequency dependent, thepath loss at distance d, and thevariation about the median caused bymultipath. The transmit antenna andreceive antenna are shown becausethe location of each and thecorresponding channel path betweenthem is a critical part of the analysis.

The channel loss equation can bewritten as

Tx Antenna

Pathloss @ 1lmeter

Pathloss @ dmeters

Multipath S-Vlognormal

Rx Antenna

L (dB) = A + I Oogi 0(dldo) + I0 x a. log1 o(dldo) +s

for d> do = 1 meter and

* L(dB) is the channel loss in dB,+ A is the channel path loss at 1 meter,

d is the distance between the transmitter and re-

ceiver

yy is the power of d loss based on geometry and en-

vironment,x and cy represent the fading characteristics of y

which is represented as a zero mean Gaussian dis-tribution with standard deviation cy, ands is the lognormal fading.

Though the generalizations made in the derivation of thismodel, particularly the path loss model, were not 100%accurate, the theoretical results were beneficial for predict-ing performance trends during our feasibility study. Thismodel served as the basis for our theoretical predictionsand expectations were validated via simulation and UWBhardware measurements both indoor and outdoor.

4. RANGE VERSUS DATA RATE

Data rate is the measure of bits successfully transferredbetween sender and receiver in a given time period. De-sired range is achieved by integrating multiple pulses per

bit to achieve the necessary signal gain. Thus the systemtradeoff of data rate versus range is directly proportional.

Using the path loss equations derived above we calculateda range versus data rate estimate for an unattended groundsensor network with nodes at 1.8 meter height. Plots ofrange versus pulse rate are provided in Figure 5 for an

ideal correlator with FCC in-band radiation limits. Rangeis a function of EIRP. FCC Part 15.517 and Part 15.519limit the average EIRP to -41.3 dBm/MHz and the peakEIRP to 0 dBm/50 MHz [7]. The average EIRP is equal tothe peak EIRP times the pulsewidth/interpulse period. Thechip rate is 1/interpulse period. The roll off at higher pulserates is due the FCC average EIRP limit. The lower end ofthe curve is determined by the FCC peak EIRP limit andwithout correlator effects would be flat. However, oscilla-tor stability limits the maximum correlator gain. The plotsshow that the correlator gain rolls off for lower data ratedue to oscillator instability. A 5 PPM oscillator was usedin this calculation. A more stable reference would supporta longer integration time. As the correlator size is in-creased, the maximum distance also increases for a givendata rate.

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-6J _

-40

-45 1-50-

-55

-60 - jl-65i

-70

-75

-80-,~ ,

-85

Free Space Range vs Pulse RateFCC Radiation Limit (3.1-10.6 GHz)

1400

1200

f 1000

? 800

° 600

w 400

200

0

_ No Corr8-bit Corr32 bit Corr

-64 bit Corr128 bit Corr256 bit Corr512 bit Corr1024 bit Corr

N1 CO LOu- (O r- 00 0)O O O O O O O O

U] ] U] ] U] ] U] U] U

Pulse Rate (pulses/sec)

Figure 5: Free space range vs. pulse rate.

Figure 6 illustrates the anticipated results for the UrbanLOS model. The expected range decreases due to pathlossand multipath effects of an urban environment.

Urban LOS Range vs Pulse RateFCC Radiation Limit (3.1-10.6 GHz)

250.00

200.00

a)aD 150.00

cm100.00

50.00

0.00

No Corr8-bit Corr32 bit Corr128 bit Corr256 bit Corr

t 512 bit Corr1024 bit Corr

o1 o1 o o o o

Pulse Rate (pulses/sec)

Figure 6: Urban LOS range vs. pulse rate.Another tradeoff is selection of the appropriate RF band-width. In Figure 7 we divided the 3.1-10.6 GHz band into15 sub bands of 500 MHz and calculated the power per-centage received within each sub band. For free space andLOS conditions, the frequency attenuation is proportionalto 20 log(frequency). The frequency attenuation for ruralcondition is based on the COST 231 Hata model. TheCOST 231 Hata urban and WIM frequency attenuationsare also shown.

Power vs Frequency* L(db) Free Space E L(dB) Rural

E L (dB) Hata Urban E L (dB) WIM Urban

40

35

30 I

g 25

20

115

10

50 $1 1KlIL 1 2 |dLE

Frequency (MHz)

Figure 7: Power attenuation vs. frequency.It is apparent from the plots that a small percentage ofpower is obtained from the higher portion of the band, par-ticularly for urban conditions. 85% of the cumulative

power is received in the lower two sub bands, 3.1-4.1GHz, based on the WIM urban model and the lower sixsub bands, 3.1-6.1 GHz, based on the COST 231 Hata ur-ban model. A potential range increase tradeoff, dependingon application, is to downband the UWB waveform to fre-quency more suitable for the propagation environment.FCC regulations become an issue with this tradeoff.

5. LPI/LPD OPERATION

LPI/LPD requirements are concerned with preventing de-tection and exploitation of radio transmissions. LPD ap-plies to techniques which minimize power spectral densityand hence detection. LPI is concerned with preventing de-tection and exploitation of the signal by decoding, spoof-ing, or position monitoring. LPI/LPD performance is usu-ally determined by measuring the signal level received bya threat receiver at a specified range versus a friendly re-ceiver at the same range. The threat receiver can have anumber of characteristics with respect to bandwidth, sensi-tivity and scanning frequency. The performance gain ofthe friendly receiver includes matched filter gain associ-ated with receiver and spread spectrum processing.

Transceivers using UWB waveforms have the potential toenable wireless sensor network communications at muchlower power levels with equivalent capacity than can beprovided by narrowband systems. Widely spread trans-missions coupled with low transmit power levels providesUWB with a lower probability of being detected by unin-tended persons than competing narrowband technologies.This LPD characteristic ofUWB waveforms is an obviousadvantage for UGS applications. The reduced signalpower also translates into reduced power consumption ofthe transceivers and thus longer battery life.

Detection of a UWB transmission by a threat receiver maybe conducted in either the frequency domain using anLNA and a spectrum analyzer or in the time domain usingan LNA and an oscilloscope. Both techniques were evalu-ated in our simulations. To be undetectable in the fre-quency domain, the received UWB signal must be lower inenergy than the equivalent input noise of the LNA andspectrum analyzer in any sub-bandwidth of the entire re-ceiver bandwidth. To achieve this, the transmit powermust be low and evenly spread across the frequency spec-trum. In simulations ofUWB in the frequency domain thespectra produced when a UWB signal is received is com-pared to that produced by the LNA and spectrum analyzernoise when no UWB signal is present.

Simulations were run to compare total transmit energy ofthe UWB pulse stream to the integrated noise power of athreat receiver. Figure 8 shows a plot of energy receivedin the 3 GHz to 6 GHz band as a function of distance from

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a 15 cm high transmit antenna. The transmit power is setto the FCC mask limit for indoor communications. Thethreat receiver antenna is assumed to have the same char-acteristics as the UWB receiver antenna. The noise floorof the receiver is -36.8 dB. The energy of the UWB signalis detectable above the noise floor of the LNA when thedetector is approximately 30 meters from the UWB source.

Power Input to Detector Spectrum Analyzer in 3 GHz to6 GHz Band versus Distance from UWB Source

X a.a<-E a

n c

en X

-30

-31

-32

m -33

-34

-35

-360 20 40 60 80 1OC

Distance UWB Source to Detector (m)

- UWB Power 3 GHz to 6 GHz - - - Noise Floor

Figure 8: Spectral detection ofUWB by threat receiver.

The calculated range of detection is dependent on the reso-

lution bandwidth of the threat receiver. In our simulationsthe resolution bandwidth is 19.53 KHz. By reducing theresolution bandwidth further, the spectral peaks of theUWB signal is raised higher relative to the noise floor.Limitations on the sensitivity of detection by using narrow

resolution bandwidths are due to the time required to makea measurement. It is inversely proportional to the resolu-tion bandwidth. As resolution bandwidth is decreased, thetime required to make a measurement over a given span

will eventually exceed the transmission time of the UWBsignal. In addition, the finite duration of the UWB signaland variations in periodicity will produce finite widths inthe spectral lines of the signal. Detection sensitivity can

not be increased by reducing resolution bandwidth belowthe width of the spectral lines.

Simulations were conducted to determine the LPD per-

formance of a UWB signal in the time domain. This ap-

proach involves evaluating whether the UWB waveformcan be detected by observing an increase in the number ofpeaks in signal level received above a threshold level. Thesimulations create histograms of the time domain signalamplitudes exceeding a set threshold level at several dis-tances from the UWB transmitter. The transmit power isagain set to the limit of the FCC mask for indoor commu-

nications. The shaded area of the plot in Figure 9 showsthe distribution of signal amplitudes received when no

UWB signal is present. The UWB signal is detectablewhen the distance to the source is approximately 30 metersand the noise figure of the threat receiver is 4 dB.

Figure 9: Time detection ofUWB by threat receiver.

6. INTERFERENCE REJECTION

Interference rejection is concerned with countering denialof valid transmissions to receivers through jamming. In-terference rejection is usually determined by measuring theloss in UWB receiver sensitivity or change in BER result-ing from the jamming source.

In the jamming simulations, narrowband and widebandinterfering signals were added to the desired signal re-

ceived by the UWB receiver. The narrowband interferingsignal used was an IEEE 802.11a waveform. The band-width of this waveform is less than 20 MHz, which is nar-

row compared to the UWB waveform. The BER of theUWB receiver output was observed as the level of the in-terfering signal was varied relative to that of the receivedsignal. The waveform used for broadband interferencesimulations was another UWB signal. Again the BER ofthe UWB receiver was observed as a function of interfer-ing signal level.

For narrowband interference simulations the UWB re-

ceiver was positioned 3 meters from a UWB transmitterradiating the maximum FCC level for indoor UWB com-

munications. The IEEE 802.11a signal level at the UWBreceiver antenna was varied relative to the UWB signaland the BER of the UWB system was determined. Figure10 shows the BER of the UWB receiver versus the ratio ofthe IEEE 802.1 la and UWB signal levels. The BER of theUWB receiver is greater than 5% when the IEEE 802.11asource is just over 30 dB greater than the UWB signallevel. The UWB signal was spread with 32 chips per bitfor this data and the pulse rate was 10 Mpps.

The performance of a UWB receiver with UWB interfer-ence is also shown in Figure 10. Resistance to jammingfrom the interfering UWB transmitter results from the lowduty cycle of both transmitters. The interfering transmitteris not using the same spreading code as the desired UWBsignal, thus the interfering UWB pulses infrequently occur

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10 m range

0.10

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0.00o.6oo o.603 o.606 o.6og o.612 o.615 o.618 o.621 o.624 o.627 o.630

C.,,.I.t., O,tp,t A.plit,d. Th,..h.ld

0

in the time window of the signal pulses. The desired UWBwaveform in these simulations use 32 pulses per bit trans-mitted. The interfering signal produces bit errors when itssignal level become high enough that a single interferingpulse arriving with a desired pulse produces a greater re-sponse in the correlator output than the response of desiredsignal integrated over the entire 32 pulses of a bit.

0.35

0.3

025

0.2

m0.15

0.1

0.05

020 30 40 50 60 70

Inteference to Desired Signal Ratio (dB)

+-o-802.11a Interference UWB Interference

Figure 10: Narrowband jamming ofUWB receiver.

The previous results were based on pulse based UWB.Because the MB-OFDM technique provides very goodspectral control, interference with narrow band receiverscan be avoided. Interference from intentional or uninten-tional interferers are avoided by selectively omitting spe-cific sub-carriers within the 528 MHz sub-bands or remov-ing whole sub-bands from the frequency hoppingsequence. Band selectable front end filters are needed toavoid saturation of the analog to digital converters. Thisfrequency agility makes a MB-OFDM approach a goodwaveform candidate for UGS applications. See Figure 11.

3 lower MBOAbands

802.1 1a

time scheduled protocols. Rapid synchronization allowsslotted operation and reduces the need for contention in-tervals and subsequent collisions. Overhearing ofneighbor traffic is alleviated by channelization, eitherthrough orthogonal codes or frequency hopping. Acquisi-tion and tracking of the code or hop sequence are timeconsuming in the synchronization process. With anLPI/LPD UWB physical layer, effective carrier sensemechanisms are not readily available since integration ofmultiple pulses is necessary to acquire synchronizationwith the incoming signal. Thus, synchronization acquisi-tion with a UWB waveform is a major challenge for effec-tive ad hoc network operation.

Synchronization acquisition time is highly dependent onthe length of the spreading code which in turn has a directimpact on the LPI/LPD performance of the UWB wave-form. If the pulse energy is concentrated in repetitivepulses above the noise floor, detection is not difficult.However, to achieve LPI/LPD performance, the pulse en-ergy must be spread across multiple pulses positioned atrandom intervals. Searching the full length of a long codecan take many seconds using a single correlator, thus limit-ing the capabilities of the network communications. Thetime required for a UWB signal to acquire synchronizationis proportional to the length of the spreading sequence andis inversely proportional to the rate at which a correlatorscans the received signal for synchronization. The meantime to acquire synchronization is to a first order inverselyproportional to the probability the receiver will recognizequickly enough that the scanning correlator is synchro-nized with the incoming signal, and stop scanning. If itfails to recognize that synchronization has been achieved,it will continue to scan and must then scan through the en-tire spreading code before it has another chance to recog-nize that it is synchronized.

8. LOCATION AWARENESS

Figure 1 1: MB-OFDM 3-band spectrum vs. 802.1 la.7. ENERGY EFFICIENT NETWORKING

Energy efficiency is the driving requirement in the designof ad hoc network communication protocols. Medium Ac-cess Control (MAC) protocols have direct control of theenergy saving features of the radio. Major causes of en-ergy waste are collisions, idle listening, and overhearing ofneighbor traffic. Synchronization acquisition in manyways is the key enabler in the overall capability of the ad-hoc link. Collisions and idle listening are minimized with

Geolocation in indoor and urban areas is an essentialemerging technology for military, public safety, and com-mercial situational awareness. Maintaining situationalawareness in indoor and urban environments is difficultbecause buildings, walls and other obstacles obstruct vi-sion and RF propagation. Propagation channels are typi-cally characterized by direct line of sight (DLOS) and non-line of sight (NLOS) performance. As more obstruction isadded to the propagation channel, the DLOS path ampli-tude decreases relative to the NLOS path due to multi-pathand shadow fading. Thus, the DLOS path, which is thekey enabling parameter for geolocation, is harder to detect[5]. The technical challenge is to develop geolocationtechniques that can distinguish between DLOS and NLOSsignals and provide accurate location information.

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The extreme bandwidth ofUWB implies very high resolu-tion in the time domain which is advantageous for time-of-arrival (TOA), or time-difference-of-arrival (TDOA) basedranging because it makes it possible to resolve individualmultipath arrivals and to accurately identify the (time ofthe) first arrival corresponding to the direct path from thetransmitter. In urban environments buildings, vehicles andother reflective obstacles create a very high multipath RFenvironment. The indoor environment is even worse. Theinherent frequency diversity of UWB waveforms makesthem more tolerant to multipath fading when compared tonarrowband technologies. UWB waveforms commonlyuse bandwidths of 3 GHz and more. 3 GHz bandwidthimplies 0.33 ns resolution in the time domain which corre-sponds to 0.1 m in distance. Once the correct arrival isidentified ranging accuracy depends on SNR and perform-ance of the timing reference. Our experience in the labsuggests that accuracies in the centimeter range can be ob-tained with good SNR. This is much better accuracy thanis obtainable in urban/indoor environments using othernarrowband waveforms.

9. CONCLUSIONS

Due to the wide variety ofUGS applications and propaga-tion environments, the reality is no one RF waveform willmeet every need. UWB is no exception. UWB performswell in multipath environments where traditional narrow-band technologies tend to falter. The flexibility of rangeversus data rate tradeoffs make a UWB based waveformapplicable to both high and low data rate applications. Forcovert operations the LPI/LPD characteristics of the tech-nology are tremendous and when operated at power levelsand frequency bands comparable to competing technolo-gies, the UWB capacity potential is unmatched. UWBoffers unique ranging capabilities for geolocation applica-tion, though research remains to overcome resolution ofthe obstructed DLOS path.

Interference rejection and signal acquisition are the mainweaknesses. UWB does provide -15-20 dB co-channelinterference rejection, but the extremely low transmitpower levels and wideband receiver front end of UWBdesigns, requires additional jamming rejection techniquessuch as frequency agile waveforms, band selectable filter-ing, and directional antennas.

No effective carrier sense mechanisms with LPD UWBsignals, presents challenges for energy efficient ad hocnetworks. Measured synchronization times for an ex-tended range MB-OFDM UWB transceiver developed byInnovative Wireless Technologies are less than 100 us.Synchronization times of pulsed techniques at extendedrange have been measured on the order of milliseconds.

has an acquisition advantage over pulse based UWB andcan be effectively applied to an ad hoc sensor network so-lution.

10. REFERENCES

1. P. Hugo, "Soldier-Level Integrated Communications Envi-ronment (SLICE) Project - Soldier Radio Waveform (SRW)Overview" JTRS Cluster 5 Industry Day presentation, July2003.

2. "Networked Sensors for the Objective Force Advanced Tech-nology Communications", U.S. Army presentation and meet-ing notes, January 2003.

3. J. Foerster, "Channel Modeling Sub-committee Report (Fi-nal)," IEEE P802.15-02/490rO-SG3a, Dec. 3, 2002. Documentincludes MatLab program.

4. Saleh and R. Valenzuela, "A Statistical Model for IndoorMultipath Propagation," IEEE JSAC, Vol. SAC-5, No. 2, Feb.1987, pp. 128-137.

5. D. Ramsburg, R. Passmore, J. Silverstrim, "Final Report: In-novative Methods for Geolocation and Communication withUltra-Wideband Mobile Radio", Innovative Wireless Tech-nologies, May 2004.

6. K. Colling, "Final Report: Ultra-Wideband Communicationsfor Sensor Network Communications", Innovative WirelessTechnologies, June 2003.

7. First Report & Order FCC 02-48, adopted February 14, 2002,released April 22, 2002.

8. V. Erceg, L. Greenstein, et al, "An Empirically Based PathLoss Model for Wireless Channels in Suburban Environ-ments," IEEE JSAC, Vol. 17, No. 7, July 1999, pp. 1205-1211.

9. J.S. Lee, L.E. Miller, "CDMA Systems Engineering Hand-book", Artech House 1998, ISBN 0-89006-990-5, Chapter 2Mobile Radio Propagation Considerations.

10. Ghassemzadeh and Tarokh, "The Ultra-Wideband IndoorPath Loss Model", IEEE P802.15-02/277rl-SG3a, July 8,2002.

Thus for energy efficient networking, MB-OFDM UWB

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