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IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 55, NO. 9, SEPTEMBER 2007 1667 Narrowband Interference Avoidance in OFDM-Based UWB Communication Systems Dimitrie C. Popescu, Senior Member, IEEE, and Prasad Yaddanapudi, Student Member, IEEE Abstract—In this letter we present a new method for miti- gating narrowband interference in orthogonal frequency-division multiplexing-based ultra-wideband communication systems. The proposed narowband interference avoidance (NBIA) method per- forms spectral shaping of the transmitted signal using binary sig- nature sequences with minimum total squared correlation (TSC) to avoid the narrowband interfering signal. We illustrate the pro- posed method with plots that show the spectrum of the transmitted signal with and without NBIA, and also present numerical results obtained from simulations showing improvement in the bit error rate (BER) performance of the system when NBIA is employed. Index Terms—Interference avoidance, orthogonal frequency- division multiplexing (OFDM), spectral shaping, ultra-wideband (UWB). I. INTRODUCTION U LTRA-WIDEBAND (UWB) communication systems have generated increasing interest among researchers lately because of their potential for providing high data rates, and their robustness in multipath fading environments. UWB systems require large bandwidths for transmission of UWB sig- nals (bandwidth larger than 500 MHz, or 25% of the center frequency), and must be capable of operating in the presence of various interfering signals coming from the existing (nar- rowband) communication systems. Thus, one of the challenges in the design of UWB communication systems is mitigation of narrowband interference. In our paper, we consider the orthogonal frequency- division multiplexing (OFDM)-based UWB system proposed by Gerakoulis and Salmi [1], which was dubbed interference sup- pressing OFDM (IS-OFDM) system and which provides good performance in the presence of narrowband interference, and present a new method for mitigating narrowband interference in this system that further improves performance. The pro- posed NBIA method performs spectral shaping [2] in an OFDM framework, using binary spreading sequences with minimum TSC [3]–[5] to avoid the spectrum of the narrowband interfer- ing signal. We note that the concept of spectral shaping was introduced in the general context of UWB communication sys- Paper approved by G. Li, the Editor for Wireless Communication Theory of the IEEE Communications Society. Manuscript received August 8, 2005; re- vised April 4, 2006 and November 8, 2006. This work was supported in part by the Texas Higher Education Coordinating Board (THECB) Advanced Technol- ogy Program (ATP) under Grant 000512-0261-2003. This paper was presented in part at the GLOBECOM 2005 Conference. D. C. Popescu is with the Department of Electrical and Computer En- gineering, Old Dominion University, Norfolk, VA 23529 USA (e-mail: [email protected]). P. Yaddanapudi is with the Department of Electrical and Computer Engineer- ing, University of Texas, San Antonio, TX 78249-0669 USA. Digital Object Identifier 10.1109/TCOMM.2007.904370 tems in [2], and is based on a spread-time code-division multiple access (CDMA) scheme [6], in which a signal is multiplied by a spreading sequence in the frequency domain as opposed to con- ventional spread spectrum systems in which the signal is multi- plied by the spreading sequence in time domain. The idea behind spectral shaping is to use a spreading sequence with zeros in the frequency band corresponding to the narrowband interfering sig- nal, which shapes the spectrum of the transmitted UWB signal to avoid the narrowband interference. For the IS-OFDM UWB system considered in our paper, spectral shaping is achieved by notching out several carriers that are in the same band as the nar- rowband interfering signal. This is accomplished by using the binary sequences with minimum TSC proposed by Karystinos and Pados [3]–[5], instead of the Hadamard sequences used in [1]. The paper is organized as follows. In Section II, we give a brief description of the IS-OFDM UWB system and of binary sequences with minimum TSC. In Section III, we discuss how these sequences can be used to perform spectral shaping for the IS-OFDM UWB system and state the NBIA procedure. We continue with the presentation of numerical results obtained from simulations in Section IV, and present final conclusions in Section V. II. SYSTEM DESCRIPTION In the IS-OFDM UWB system, the wide transmission band- width is divided into ˜ N frequency bins (or carriers) that are grouped into L groups with ˜ M frequency bins in each group, such that the total number of carriers ˜ N = L ˜ M . The input data stream with rate R enters a serial-to-parallel (S/P) converter that provides L data streams each with rate R/L. Each parallel stream of rate R/L corresponding to a given group of frequency bins enters a second S/P converter, which provides ˜ M parallel streams each with rate R/ ˜ N . The ˜ M parallel streams in each group are spread by orthogonal Hadamard sequences {w q } ˜ M 1 0 of length ˜ M that returns the rate to R/L, and is followed by S/P conversion back to ˜ M parallel streams each with rate R/ ˜ N . These are then combined in an interference suppressing scheme by adding the orthogonally modulated symbols in a given group to form a new set of ˜ M parallel symbols, such that the power of each of the ˜ M symbols in the frame carried by the given group of frequency bins is distributed over all ˜ M bins in the group, while symbols are separated by orthogonal Hadamard sequences in order to be able to distinguish them at the receiver and enable easy demodulation. The transmitter diagram for the IS-OFDM UWB system is depicted schematically in Fig. 1, and we refer the reader to the paper by Gerakoulis and Salmi [1] for complete details on the IS-OFDM UWB system. 0090-6778/$25.00 © 2007 IEEE
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Page 1: Narrowband Interference Avoidance in OFDM-Based UWB ...

IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 55, NO. 9, SEPTEMBER 2007 1667

Narrowband Interference Avoidance in OFDM-BasedUWB Communication Systems

Dimitrie C. Popescu, Senior Member, IEEE, and Prasad Yaddanapudi, Student Member, IEEE

Abstract—In this letter we present a new method for miti-gating narrowband interference in orthogonal frequency-divisionmultiplexing-based ultra-wideband communication systems. Theproposed narowband interference avoidance (NBIA) method per-forms spectral shaping of the transmitted signal using binary sig-nature sequences with minimum total squared correlation (TSC)to avoid the narrowband interfering signal. We illustrate the pro-posed method with plots that show the spectrum of the transmittedsignal with and without NBIA, and also present numerical resultsobtained from simulations showing improvement in the bit errorrate (BER) performance of the system when NBIA is employed.

Index Terms—Interference avoidance, orthogonal frequency-division multiplexing (OFDM), spectral shaping, ultra-wideband(UWB).

I. INTRODUCTION

U LTRA-WIDEBAND (UWB) communication systemshave generated increasing interest among researchers

lately because of their potential for providing high data rates,and their robustness in multipath fading environments. UWBsystems require large bandwidths for transmission of UWB sig-nals (bandwidth larger than 500 MHz, or 25% of the centerfrequency), and must be capable of operating in the presenceof various interfering signals coming from the existing (nar-rowband) communication systems. Thus, one of the challengesin the design of UWB communication systems is mitigation ofnarrowband interference.

In our paper, we consider the orthogonal frequency-division multiplexing (OFDM)-based UWB system proposed byGerakoulis and Salmi [1], which was dubbed interference sup-pressing OFDM (IS-OFDM) system and which provides goodperformance in the presence of narrowband interference, andpresent a new method for mitigating narrowband interferencein this system that further improves performance. The pro-posed NBIA method performs spectral shaping [2] in an OFDMframework, using binary spreading sequences with minimumTSC [3]–[5] to avoid the spectrum of the narrowband interfer-ing signal. We note that the concept of spectral shaping wasintroduced in the general context of UWB communication sys-

Paper approved by G. Li, the Editor for Wireless Communication Theory ofthe IEEE Communications Society. Manuscript received August 8, 2005; re-vised April 4, 2006 and November 8, 2006. This work was supported in part bythe Texas Higher Education Coordinating Board (THECB) Advanced Technol-ogy Program (ATP) under Grant 000512-0261-2003. This paper was presentedin part at the GLOBECOM 2005 Conference.

D. C. Popescu is with the Department of Electrical and Computer En-gineering, Old Dominion University, Norfolk, VA 23529 USA (e-mail:[email protected]).

P. Yaddanapudi is with the Department of Electrical and Computer Engineer-ing, University of Texas, San Antonio, TX 78249-0669 USA.

Digital Object Identifier 10.1109/TCOMM.2007.904370

tems in [2], and is based on a spread-time code-division multipleaccess (CDMA) scheme [6], in which a signal is multiplied by aspreading sequence in the frequency domain as opposed to con-ventional spread spectrum systems in which the signal is multi-plied by the spreading sequence in time domain. The idea behindspectral shaping is to use a spreading sequence with zeros in thefrequency band corresponding to the narrowband interfering sig-nal, which shapes the spectrum of the transmitted UWB signalto avoid the narrowband interference. For the IS-OFDM UWBsystem considered in our paper, spectral shaping is achieved bynotching out several carriers that are in the same band as the nar-rowband interfering signal. This is accomplished by using thebinary sequences with minimum TSC proposed by Karystinosand Pados [3]–[5], instead of the Hadamard sequences usedin [1].

The paper is organized as follows. In Section II, we give abrief description of the IS-OFDM UWB system and of binarysequences with minimum TSC. In Section III, we discuss howthese sequences can be used to perform spectral shaping forthe IS-OFDM UWB system and state the NBIA procedure. Wecontinue with the presentation of numerical results obtainedfrom simulations in Section IV, and present final conclusions inSection V.

II. SYSTEM DESCRIPTION

In the IS-OFDM UWB system, the wide transmission band-width is divided into N frequency bins (or carriers) that aregrouped into L groups with M frequency bins in each group,such that the total number of carriers N = LM . The input datastream with rate R enters a serial-to-parallel (S/P) converterthat provides L data streams each with rate R/L. Each parallelstream of rate R/L corresponding to a given group of frequencybins enters a second S/P converter, which provides M parallelstreams each with rate R/N . The M parallel streams in each

group are spread by orthogonal Hadamard sequences {wq}M −10

of length M that returns the rate to R/L, and is followed byS/P conversion back to M parallel streams each with rate R/N .These are then combined in an interference suppressing schemeby adding the orthogonally modulated symbols in a given group� to form a new set of M parallel symbols, such that the powerof each of the M symbols in the frame carried by the givengroup of frequency bins is distributed over all M bins in thegroup, while symbols are separated by orthogonal Hadamardsequences in order to be able to distinguish them at the receiverand enable easy demodulation. The transmitter diagram for theIS-OFDM UWB system is depicted schematically in Fig. 1, andwe refer the reader to the paper by Gerakoulis and Salmi [1] forcomplete details on the IS-OFDM UWB system.

0090-6778/$25.00 © 2007 IEEE

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1668 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 55, NO. 9, SEPTEMBER 2007

Fig. 1. The transmitter diagram for the IS-OFDM UWB system.

The orthogonal Hadamard sequences used for spreading ofthe data streams in the IS-OFDM UWB system as describedearlier, are binary valued sequences that are also used in spreadspectrum systems for transmitting data over a larger bandwidththan is necessary, or to provide multiple access in orthogonalCDMA systems in which multiple users access the same fre-quency band at the same time. We note that spread spectrumtransmission of data can also be achieved by using nonorthogo-nal binary valued spreading sequences like for example pseudo-random noise (PN) sequences. However, when nonorthogonalbinary valued sequences are used to provide multiple accessin (nonorthogonal) CDMA systems, each user’s signal will actas interference for the signals of the other users, and the mu-tual interference between any two users depends on the crosscorrelation of their corresponding spreading sequences. Tradi-tionally, PN sequences were used in the CDMA systems due totheir special correlation properties: high correlation of a givenPN sequence with itself and low correlation of the given PNsequence with other PN sequences or with shifted versions ofitself. These properties imply relatively simple and efficient re-ceiver structures based on single-user-matched filtering that canbe used in a wide variety of scenarios. Recently, new perfor-mance metrics have been introduced in the CDMA literature toevaluate the overall performance of CDMA systems, like sumcapacity, TSC, maximum squared correlation (MSC), or total

asymptotic efficiency (TAE) [5]. Minimizing the TSC metricwas used to design new binary valued spreading sequences [3],[4] that were shown to be optimal or near optimal with respectto the other criteria also [5].

The algorithm proposed by Karystinos and Pados [3], [4]yields a matrix of dimensions K × L, whose K columns form aset of binary valued spreading sequences of length L with mini-mum TSC. The algorithm works for both underloaded case whenK ≤ L, and overloaded case when K > N , and is initializedwith a Hadamard matrix of size N = 4�(max{K,L} + 1)/4�,where �x� rounds real number x to the nearest integer smallerthan x.

III. NARROWBAND INTERFERENCE AVOIDANCE PROCEDURE

The NBIA procedure proposed in our paper for the IS-OFDM UWB system is based on spectral shaping that wasintroduced by Da Silva and Milstein in [2], as a method formitigating narrowband interference in UWB communicationsystems. The basic idea behind spectral shaping is to multiplythe transmitted signal in frequency domain by a spreading se-quence with zeros in the frequency band corresponding to thenarrowband interfering signal. This operation shapes the spec-trum of the transmitted UWB signal to avoid the narrowbandinterference.

For the IS-OFDM UWB system, spectral shaping can beachieved by avoiding carriers that are in the same frequencyinterval as the narrowband interfering signal. This can be ac-complished by replacing the Hadamard sequences {wq}M −1

0

in frequency bin � where the narrowband interfering signal islocated, by alternative sequences that have zeros in places cor-responding to those particular carriers where the spectral gapin the transmitted signal is required. As mentioned in [2], thisreplacement has a twofold effect: it shapes the spectrum of thetransmitted signal to avoid the narrowband interference, and atthe same time, suppresses the narrowband interference at thereceiver that uses the same spreading sequences in the demod-ulation process. However, replacing Hadamard sequences byalternative sequences spoils the orthogonality of symbols trans-mitted over the same carrier, and creates interference amongthem at the receiver that affects performance. The level of in-terference depends on the cross-correlation properties of theset of alternative sequences, and is usually quantified by theTSC. Among all binary sequences (including the various PNsequences like maximal length, Gold, etc.), those developed byKarystinos and Pados [3], [4] have minimum TSC and allowdecoding of a given symbol under minimum total interferencefrom the other transmitted symbols. In the NBIA procedure, weuse these binary sequences with minimum TSC developed byKarystinos and Pados [3], [4] to perform spectral shaping ofthe transmitted UWB signal. We note that the interference in-troduced by replacing the orthogonal Hadamard sequences withalternative nonorthogonal binary sequences introduces an errorfloor in the BER performance of the IS-OFDM system even inthe absence of the narrowband interfering signal, as can be seenin Fig. 2 that illustrates the BER performance of the IS-OFDMsystem when Hadamard sequences are used for spreading in all

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POPESCU AND YADDANAPUDI: NARROWBAND INTERFERENCE AVOIDANCE 1669

Fig. 2. BER performance of the IS-OFDM UWB system in the absence ofnarrowband interference for several types of spreading sequences.

the L groups of frequencies and when alternative sequences areused in one of the groups.1

The NBIA procedure for spectral shaping, using binaryspreading sequences with minimum TSC, requires knowledgeof the narrowband interfering signal and/or of its position in thefrequency domain. This knowledge can be obtained by usingvarious techniques, for example, comparing the frequency do-main of the signal at the receiver with an interference thresholdderived using a spectral template of the desired signal [7]–[9].In this case, the carriers that are above the interference thresholdare considered to be affected by the narrowband interference,and will be subject to the NBIA procedure.

In order to present the NBIA procedure, we assume that nar-rowband interference has been detected in the band of frequen-cies [f1, f2]. Then, the bandwidth of the narrowband interferingsignal is equal to

FNBI = f2 − f1 (1)

and the signal is located in the IS-OFDM frequency bin number

� =⌈

f1

∆FIS-OFDM

⌉(2)

where ∆FIS-OFDM = Ftotal/L, Ftotal is the total bandwidth ofthe UWB signal, and �x� rounds real number x to the nearestinteger larger than x. In terms of the subcarriers in bin �, thenarrowband interfering signal spectrum starts at carrier number

ns =⌊

f1 − (� − 1)∆FIS-OFDM

∆f

⌋. (3)

1The simulation setup used in obtaining Fig. 2 is described in detail inSection IV.

The total number of carriers that must be avoided is

kf =⌈

FNBI

∆f

⌉+ 1 (4)

where ∆f is the width of a single carrier. The narrowbandinterfering spectrum ends at carrier number

ne = ns + kf (5)

Using the parameters in (3)–(5), the NBIA procedure can beformally stated as follows.

1) Start with an M × M Hadamard matrix and use the algo-rithm of Karystinos and Pados in [3], [4] to construct anM × (M − kf ) matrix S of binary sequences with mini-mum TSC.2

2) Augment matrix S to an M × M matrix S by placing kf

columns with all elements equal to zero in those placescorresponding to the kf subcarriers to be avoided.

3) Use columns of matrix S to replace the Hadamard se-quences {wq}M −1

0 in frequency bin �.We note that orthogonal Hadamard sequences will be used in

the rest of the IS-OFDM frequency groups.

IV. SIMULATION RESULTS

We considered an IS-OFDM UWB system with bandwidthFtotal = 1.25 GHz identical to that in [1], divided into N = 512frequency bins grouped into L = 8 groups, each with M = 64subcarriers. This implies that each frequency bin is ∆f =2.4414 MHz wide, and that the bandwidth of a group of fre-quency bins is approximately ∆FIS-OFDM = 156.25 MHz.We assumed a narrowband interfering signal (jammer) withbandwidth FNBI = 5 MHz, located between frequencies f1 =500 MHz and f2 = 505 MHz, which was generated similar to [1]by using a linear bandpass FIR filter with passband equal to5 MHz driven by white Gaussian noise with unit variance at theinput. From (2), the group number where the jammer is locatedis obtained as � = 4. Using (3)–(5), the subcarrier number wherethe jammer starts is ns = 12 and the number of subcarriers itoverlays is kf = 4. Thus, the jammer spectrum overlays overfrequency bins 12 through 15 in the fourth IS-OFDM group.The total power of the jammer is selected relative to that of theUWB signal such that specific values of the jammer-to-signal-ratio (JSR) are obtained.

We apply the NBIA procedure described in the Section III andconstruct the M × (M − kf ) (that is, 64 × 60 for our numeri-cal example) matrix S of binary spreading sequences using thealgorithm in [3] and [4]. Next, we augment S by adding kf = 4column vectors with all elements zero after column 11 to ob-tain matrix S of dimension M × M (that is 64 × 64 for ournumerical example) whose columns will be used to replace theHadamard sequences in the IS-OFDM group number n = 4.

To illustrate the spectral shaping effect of the NBIA proce-dure, we have plotted the power spectral density of the transmit-ted signal before and after the procedure was applied in Fig. 3.

2This corresponds to the overloaded case with K > L in the notation ofKarystinos and Pados [3], [4].

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1670 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 55, NO. 9, SEPTEMBER 2007

Fig. 3. Power spectral density of the transmitted signal before and after theNBIA is performed.

The plot shows the spectral gap that occurs in the spectrum ofthe transmitted UWB signal when NBIA is employed, in thefrequency band corresponding to the jammer.

We have also performed a series of simulations to investigatethe BER performance of the considered IS-OFDM UWB systemwith and without application of the NBIA procedure. We usedthe IS-OFDM UWB receiver presented by Gerakoulis and Salmi[1] and looked at the raw BER after demodulation/detectionwithout considering any error correction techniques. We notethat the use of error correction techniques will only improve theraw BER observed after demodulation/detection.

In the first simulation experiment, we did not consider thejammer present and looked at the BER with AWGN only whenHadamard sequences are used in all groups of frequencies, aswell as when PN sequences and binary sequences with mini-mum TSC are used in the fourth group of frequencies whileHadamard sequences are used in all other groups. The results ofthis experiment are shown in Fig. 2, which shows the error floorintroduced by the use of nonorthogonal spreading sequences be-yond which the BER cannot be decreased by increasing Eb/N0.We note that this floor does not depend on the presence of thenarrowband interfering signal, and is minimum for binary se-quences with minimum TSC.

In the second experiment, we simulated the system in AWGNwith the jammer active, for JSR values of 5, 10, and 30 dB, andlooked at the BER before and after application of the NBIAprocedure. The results of this simulation are presented in Fig. 4.The dashed curves in Fig. 4 correspond to the BER beforeapplication of NBIA, and also show an error floor beyond whichthe BER will not be improved by increasing Eb/N0. In thiscase the error floor is due to the spectral leakage caused bythe discontinuity in the discrete Fourier transform (DFT) blocks

Fig. 4. BER performance of the IS-OFDM UWB system with AWGN andnarrowband interferer, before and after NBIA is applied.

that occurs as a result of the stochastic nature of the narrowbandjammer that does not have an integer number of cycles in theDFT block.

This spectral leakage affects all the subcarriers in the trans-mitted signal not only those subcarriers colocated with the nar-rowband interference in the frequency domain. As can be seenfrom Fig. 4, the error floor introduced by the spectral leakagein AWGN is higher than that introduced by the use of binarysequences with minimum TSC seen in Fig. 2, and is especiallysignificant at high JSRs. The solid curves in Fig. 4 correspondto the BER after application of NBIA, and show that the errorfloor displayed is smaller than when NBIA is not applied. Wenote that for low and average JSR (5 and 10 dB), applying NBIAleads to an approximately one order of magnitude decrease in theBER floor, while for large JSR, (30 dB), the BER improvementis lower. We also note that the BER floor after application ofNBIA for large JSR values can be further decreased by compen-sating the effect of spectral leakage on the recovered symbols.This can be achieved by removing extra subcarriers adjacentto the jammer as in [8], or by using a windowing technique atthe receiver [10]. Fig. 5 illustrates the reduction in error floorafter NBIA is applied for high JSR values (20 and 30 dB), whenadditional adjacent subcarriers in the UWB signal are nulled forspectral leakage compensation.

In the third experiment, we looked at the BER performancein the presence of multipath and simulated the system usinga multipath channel between transmitter and receiver as de-scribed in [11] for the same JSR values of 5, 10, and 30 dB.Results of this experiment are presented in Fig. 6, and similarobservations as in the previous case can be made that the use ofthe NBIA procedure improves the BER performance. To pro-vide a better view of the performance improvement implied

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POPESCU AND YADDANAPUDI: NARROWBAND INTERFERENCE AVOIDANCE 1671

Fig. 5. BER performance of the IS-OFDM system with AWGN and narrow-band interferer for high JSR, with NBIA and spectral leakage compensation.

Fig. 6. BER performance versus Eb /N0 for several JSR values for the IS-OFDM UWB system with multipath and narrowband interferer.

by NBIA for multipath channel and narrowband jammer, wesimulated the system also for varying values of JSR and fixedEb/N0 = 5, 10, 15, 20, 25, and 30 dB, and the results of thissimulation are shown in Fig. 7. We note that, for a given valueof Eb/N0, the improvement in BER decreases with the increasein JSR, which may be due to the fact that, at higher JSRs, thespectral gap created in the transmitted signal may not be deepenough to cancel entirely the spectral peak of the jammer.

Fig. 7. BER performance versus JSR for several Eb /N0 values for theIS-OFDM UWB system with multipath and narrowband interferer.

In the fourth experiment, we looked at the BER performanceof the system when more than one narrowband jammer is ac-tive. We considered two distinct cases: one in which we assumedthat there are two narrowband jammers in the same IS-OFDMfrequency bin and another one in which the two narrowbandjammers are located in distinct IS-OFDM frequency bins. Theresults of this experiment are shown in Fig. 8. We note that whenthere are two narrowband jammers in distinct IS-OFDM, fre-quency bins applying the NBIA procedure yields over one orderof magnitude decrease in the BER floor that becomes close tothat of the system with only one narrowband jammer and NBIA.When the two narrowband jammers are in the same IS-OFDMfrequency bin, the BER floor is still reduced, although the gain isless than in the previous case. This decrease in performance maybe overcome by increasing the number of IS-OFDM frequencybins L, so that the narrowband jammers lay over different fre-quency bins.

Finally, in the last experiment of this series, we compared theperformance of the IS-OFDM system with and without the pro-posed NBIA procedure with that of a conventional OFDM-basedUWB system with frequency domain interleaving followed byconvolutional coding. For the conventional OFDM-based UWBsystem, we consider a system with the same total bandwidthof 1.25 GHz and the same number N = 512 of subcarriers.We employ frequency domain interleaving with a depth of twofollowed by a convolutional coding with rate R = 1/2. The re-sults of this experiment are shown in Fig. 9. We note that whenthe systems operate only in AWGN, at low JSR (5 dB), theIS-OFDM system with NBIA has the best performance followedby the IS-OFDM system without NBIA, and both IS-OFDMsystems outperform the conventional OFDM-based UWB sys-tem. At higher JSR (10 dB and above), the conventional OFDM

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1672 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 55, NO. 9, SEPTEMBER 2007

Fig. 8. BER performance of the IS-OFDM system with more than one nar-rowband interferer.

Fig. 9. BER performance for OFDM UWB system with frequency interleavingand convolutional coding, IS-OFDM UWB system, and IS-OFDM UWB systemwith NBIA.

system has performance very similar to that of the IS-OFDMsystem without NBIA, while the IS-OFDM UWB system em-ploying NBIA still continues to have the best performance. Wealso note from Fig. 9 that even in the case of a multipath channel,the IS-OFDM UWB system employing the NBIA procedurecontinues to display the best performance, although in this case,the differences were not so significant.

We conclude this section by noting that we have also lookedat the sensitivity of the NBIA procedure, when the position ofthe jammer bandwidth is not identical to the bandwidth that theNBIA procedure avoids. For this, we simulated the system todetermine the variation in BER performance improvement im-plied by the proposed NBIA procedure with various offsets ofthe actual position of the center frequency of the narrowbandinterfering signal from the value (f1 + f2)/2 corresponding tothe center of the frequency band avoided by the NBIA proce-dure. Our simulations have shown that when the bandwidth ofthe narrowband jammer overlaps over about 70% or more withthe frequency band avoided by the NBIA procedure, the im-provement in BER implied by NBIA is essentially not affectedby the offset.

V. CONCLUSIONS

In this paper, we presented a new procedure for avoidingnarrowband interfering signals in OFDM-based UWB commu-nication systems. The NBIA procedure performs spectral shap-ing [2] of the transmitted signal using the newly developed bi-nary spreading sequences with minimum TSC [3]–[5] to avoidthose subcarriers located in the same frequency band as the nar-rowband interfering signal. The novelty of the proposed NBIAprocedure consists in application of the spectral shaping con-cept in an OFDM framework that is different from [2], as wellas in the new way the binary spreading sequences with mini-mum TSC are used, that is different from their original intendeduse of providing multiple access in CDMA systems. We notethat application of the NBIA procedure in the IS-OFDM sys-tem trades off orthogonality of transmitted symbols in group �of frequency bins where the narrowband interference is presentfor improved performance in the presence of the narrowbandinterference.

We illustrated the proposed NBIA procedure with an exam-ple that displays the spectrum of the transmitted signal withand without NBIA and shows the gap in the spectrum of thetransmitted signal when NBIA is performed. This occurs in thefrequency band of the narrowband interfering signal and impliesthat the transmitter avoids sending information in this band. Wealso presented numerical results obtained from simulations thatshow that the BER performance of the considered OFDM-basedUWB system is improved when NBIA is employed, and that it issuperior to that of conventional OFDM-based UWB system thatuses frequency domain interleaving and convolutional coding.

ACKNOWLEDGMENT

The authors are grateful to the anonymous reviewers for theirconstructive comments on the paper.

REFERENCES

[1] D. Gerakoulis and P. Salmi, “An interference suppressing OFDM systemfor ultra wide bandwidth radio channels,” in Proc. 2002 IEEE Conf. UltraWideband Syst. Technol., Baltimore, MD, pp. 259–264.

[2] C. Da Silva and L. B. Milstein, “Spectral-encoded UWB communica-tion systems,” in Proc. 2003 IEEE Conf. Ultra Wideband Syst. Technol.,Reston, VA, pp. 96–100.

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POPESCU AND YADDANAPUDI: NARROWBAND INTERFERENCE AVOIDANCE 1673

[3] G. N. Karystinos and D. A. Pados, “Minimum total squared correlationdesign of DS-CDMA binary signature sets,” in Proc. 2001 IEEE GlobalTelecommun. Conf., San Antonio, TX, vol. 2, pp. 801–805.

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