Coexistence of Ultra-Wideband Systems with IEEE a Wireless LANs

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1 Coexisence of Ulra-Wideband Sysems wih IEEE-8.11a Wireless LANs J. Bellorado 1, S.S. Ghassemzadeh, L. J. Greensein 3, T. Sveinsson 1, V. Tarokh 1 Absrac In his sudy we provide a physical layer based analysis of he coexisence issues of Ulra-Wideband (UWB wih oher devices in he same specrum. Specifically, we have focused on he UWB inerference o and from devices using he Wireless Local Area Nework (WLAN sandard IEEE 8.11a. Our resuls indicae ha a UWB inerferer operaing a he peak allowable power densiy induces minimal inerference ino such WLAN devices in line-of sigh (LOS scenarios, even a close range. However, in he non-los (NLOS case, a UWB inerferer can severely affec he daa-rae susainable by 8.11a sysems. Moreover, 8.11a inerference ino UWB sysems is shown o reduce he signal-o-inerference raio (SIR by as much as 36 db when he inerferer is wihin LOS of he UWB receiver. Index Terms Inerference, UWB Signals, IEEE 8.11a, Impulse Radio, Co-exisence I. INTRODUCTION W ih he FCC s recen approval of unlicensed Ulra- Wideband (UWB communicaion devices in he GHz frequency range [1], he wireless indusry has aken an ineres in he inerference issues relaed o UWB ransmission. Specifically, we consider he ransmission of UWB sysems based on he Impulse Radio archiecure. Impulse radio uilizes low duy-cycle pulses wih duraion on he order of nanoseconds o spread radiaed power over large frequency bands. The UWB signal is formally defined as any signal ha occupies more han 5 MHz of specrum or has a fracional bandwidh in excess of % [1]. Because a UWB device spreads is signal energy over a large frequency range, i only radiaes a small percenage of is oal power in he operaing specrum of narrowband receivers. Hence, UWB signals should appear as low-power whie noise and have lile impac on underlying operaing devices. I has hus been assered ha UWB devices, wih appropriae resricions on effecive radiaed power (ERP, can coexis wih oher devices in he same frequency band wihou causing inerference, hereby making efficien use of already scarce specrum. To ensure his, he FCC has defined This maerial is based upon research suppored in par by he Naional Science Foundaion under he gran No. CCR and Alan T. Waerman award, gran No. CCR Any opinions, findings and conclusions or recommendaion expressed in his publicaion are hose of he auhors and do no necessary reflec he views of he Naional Science Foundaion. 1 The auhors are wih Division of Engineering and Applied Sciences, Harvard Universiy, Cambridge, MA, USA. S.S. Ghassemzadeh (saeedg@research.a.com, corresponding auhor is wih AT&T Labs-Research, Florham Park, NJ, USA. 3 L.J. Greensein is wih WINLAB-Rugers Universiy, Piscaaway, NJ, USA. he UWB power mask o allow a maximum ransmied specral densiy (PSD of dbm/mhz. There exis a myriad of poenial inerferers (licensed and unlicensed operaing in he specrum alloed for UWB use. In his sudy we focus on UWB inerference o and from WLAN devices ha use he IEEE sandard 8.1la []. WLANs employing IEEE 8.11a can suppor daa-raes as high as 54 Mbps and, herefore, use of hese devices has seen a significan increase in recen years. These sysems operae in he 5 GHz Unlicensed Naional Informaion Infrasrucure (U-NII frequency bands using orhogonal frequency division muliplexing (OFDM based ransmission [3]. In his specrum here exiss hree 1 MHz wide frequency bands ( , and GHz, wih he maximum ERP per MHz se o.5mw, 1.5mW and 5mW, respecively. These frequency bands are each divided ino five MHz wide channels, in which 8.11a employs 5 sub-carriers (48 for daa ransmission and 4 dedicaed for channel esimaion. A heoreical analysis of he inerference suppression capabiliy (in erms of received signal-o-inerference raio of a UWB sysem in he presence of narrowband and wideband inerference is given in [4]. There, he inerference induced on a UWB sysem is made general by modeling i as a wide-sense saionary random process. However, because UWB an WLANs may be used in close proximiy for similar applicaions, i is our ineres o examine he coexisence of hese wo sysems in proximiy o each oher. Therefore, i is he inen of his sudy o provide an examinaion of he muual inerference beween 8.11a and UWB. We firs provide an analyical formulaion of he problem. We hen provide resuls in he form of an inerferer s impac on he SIR of each sysem as a funcion of device separaion. We also show simulaion resuls of he achievable daa-raes of 8.11a WLANs in he presence of UWB inerference. While many mehods of UWB inerference excision have been proposed (such as receiver design [5], direcional anennas, noch filers, pulse shaping [6], ec. we have no included he use of such echniques in our sudy. In Secion II, we presen a background on UWB and WLAN sysems, as implemened for his sudy. Secion III gives resuls of he inerference beween hese wo sysems and a brief conclusion is given in Secion IV. GLOBECOM /3/$17. 3 IEEE

2 A. The UWB Transceiver II. BACKGROUND A common mehod for implemening a UWB signal is widely referred o as Impulse Radio [7]. Impulse radio uses a rain of shor duraion monocycle pulses o produce a waveform wih very low duy cycle (ofen less han 1%. Pulse posiion modulaion is used o modulae daa ono he waveform. In his sudy we consider he use of a Gaussian monocycle pulse shape [8] given by { τ } ( exp ( v = A e τ, (1 where he pulse ampliude (A is se so ha he ransmi PSD requiremen esablished in [1] is adhered o. The pulse ime consan (τ conrols he cener frequency of he UWB signal, which is given by fc = 1 πτ. The Fourier Transform of he pulse waveform is given as { } π e V( f = ja π fτ exp ( π fτ. ( The 3-dB bandwidh of ( is abou 116% of he cener frequency. Since he receiver noise bandwidh, ypically, is close o he 3-dB value, we use i o calculae he receiver s noise power. The ransmie signal is given in [7] as Ns 1 j= l= ( b r jl, c j d p( = v jt lt z T d T, (3 where N s is he number of consecuive pulses modulaed by each daa bi, T r is he pulse repeiion period, and T b = N s T r is he bi period. The ime-hopping (TH sequence {z j,l } ensures ha muliple-access inerference will be suppressed by addiion of an exra (pseudorandom shif T c in he posiioning of he pulses. We consider he daa sequence {d j } and ime-hopping sequence {z j,l } o consis of elemens from he se {-1,1}. Table I gives he UWB ransmier parameers considered in our inerference sudy. A monocycle Gaussian pulse of abou ps widh (99.999% of he power of he pulse is conained in his inerval is ransmied wih a pulse repeiion period of 5ns. Our choice of parameers yields a signal wih a duy cycle (raio of pulse widh o pulse repeiion inerval of abou 1%, which is comparable o some propose devices (e.g., see [9]. The pulse ampliude (A is adjused so ha he ransmi power reflecs he maximum effecive radiaed power specral densiy limi of 41.3 dbm/mhz, as required by FCC [1]. The value of he pulse ampliude (A was deermined hrough a simulaion of he cyclo-saionary UWB waveform given in (3 for an arbirary ampliude o deermine he PSD of he signal over he enire frequency band ( GHz. Because scaling a random waveform by a known consan will scale is PSD by he square of ha consan, he appropriae value of A is easily calculaed o reflec he PSD limi. TABLE I UWB TRANSMITTER PARAMETERS Symbol Quaniy Values P PSD Limi -41.3dBm/MHz τ Pulse Time Parameer 35ps f c Cener frequency 6.45GHz T v Pulse widh.ns BW uwb Noise Bandwidh 7.5 GHz T r Pulse repeiion period 5ns N s # Pulses pr. Daa bi T c Pseudo random ime ns shif NF UWB Noise Figure 6dB N,UWB Noise Power -7dBm TABLE II THE IEEE 8.11a WLAN SYSTEM PARAMETERS Symbol Quaniy Values P Transmi Power 4 mw/mhz f c Cener frequency 5. GHz BW 8.11a Bandwidh MHz NF 8.11a Noise Figure 6 db N,8.11a Noise Power -95 dbm The oupu noise power of he UWB receiver is N,UWB = 174dBm/Hz + NFUWB + 1 log1 BWUWB = 174dBm/Hz + 6 db + 98 db-hz = 7 dbm where NF UWB and BW UWB are he noise figure and receiver noise bandwidh of he UWB receiver, respecively (Table I. B. The 8.11a WLAN A WLAN PHY simulaor based on ha of [] operaing in he U-NII frequency band cenered a 5. GHz was buil in Malab o deermine is achievable daa-rae wih and wihou UWB inerference. The WLAN ransmi power was se o have he maximum allowable power for devices operaing in he lower U-NII frequency band of 4 mw in MHz of bandwidh. The oupu noise power of he 8.11a receiver is N,8.11a = 174dBm/Hz + NF8.11a + 1 log1 BW8.11a = 174dBm/Hz + 6 db + 73 db-hz (4 = 95 dbm where NF 8.11a and BW 8.11a are he noise figure and he receiver bandwidh of he 8.11a device, respecively. Table II summarizes he WLAN sysem parameers. C. The Propagaion Medium UWB Channel Model: The UWB propagaion pah loss has been sudied exensively. In [1], for example, he pah loss model was given for LOS and NLOS in a residenial environmen as GLOBECOM /3/$17. 3 IEEE

3 8.11a Transmier UWB Transmier TABLE III SIR 8.11a (in db in he presence of UWB Inerference LOS Pah UWB inerfering ransmier 8.11a Receiver Scenario 1: UWB Inerference o 8.11a ( 1 γ log ( PL d = PL + d + S (5 1 LOS Pah UWB Receiver 8.11a inerfering ransmier Scenario : 8.11a Inerference o UWB Here he consans PL and γ were deermined empirically o be 47 db and 1.7 for LOS pahs; and 51 db and 3.5 for NLOS pahs, respecively. S is he well-known shadow fading represened by a zero-mean Gaussian random variable. In our simulaions we have fixed he sandard deviaion of S o.8 db for LOS pahs and 3.8 db for NLOS pahs. The effec of a mulipah channel is no considered in his sudy, as we assume an ideal RAKE-receiver ha collecs he power conained in all received pahs [11]. WLAN Channel Model: Due o he narrow bandwidh of he 8.11a signal, we assumed a fla-fading channel model for his sysem. A Rician channel wih a K-facor of 6 db was assumed for LOS pahs and a Rayleigh channel was assumed for NLOS [1]. Since he median pah loss of a signal propagaing hrough space is independen of is bandwidh, we use he same pah loss model (5 for he WLAN signal. III. INTERFERENCE SIMULATION AND RESULTS In his paper we consider an inerference scenario where a single receiver, 8.11a or UWB, is posiioned wihin LOS of an inerfering ransmier, as depiced in Fig. 1. We sudied he inerference effecs on he vicim receiver when i is in he LOS and he NLOS pah of is desired ransmier. The disance from he UWB ransmier o he receiver ( and he disance from he 8.11a ransmier o he receiver ( are boh varied from 1 o 1 meers. In he Fig. 1: Illusraion of he wo scenarios for he inerference sudy. SIR 8.11a (db (m Fig. : Signal-o-inerference raio of an 8.11a sysem (SIR 8.11a for an NLOS environmen in presence of a UWB inerferer. duwb (m in LOS (m in NLOS m m m TABLE IV Achievable daa-raes (Mbps of 8.11a in he presence of UWB. (m LOS (m NLOS duwb m m m following, we refer o he disance beween he receiver and he desired ransmier as he ransmier-receiver (T-R separaion, whereas he disance beween he inerferer and he receiver is referred o as he inerferer-receiver (I-R separaion. The ransmi power of each sysem, denoed by P UWB and P 8.11a, is wihin FCC guidelines as deailed in he previous secion. In boh cases, he db-received power from each ransmier is calculaed as PR,UWB ( duwb = PUWB PL( duwb (6 PR,8.11a ( d8.11a = P8.11a PL( d8.11a where PL, he propagaion pah loss, is given by (5. In he following sub-secions we perform a deailed analysis of he muual inerference of he aforemenioned sysems. In doing so, we view he UWB power wihin he 8.11a receiver bandwidh as whie noise and no aemp is made o miigae is effecs. The processing gain offered by our UWB sysem is considered by assuming an ideal Rake combining a he UWB receiver [4]. A. UWB Inerference ino 8.11a Devices To compue he inerference from he UWB signal o an 8.11a receiver, i is necessary o calculae he PSD of he UWB signal over he appropriae frequency band. This calculaion was done by simulaing a UWB sysem using he parameers specified in Table I. We assume ha he daa sequence {d j } and he TH sequence {z j,l } are independen, whie random processes aking on values in he se {-1,1} wih equal probabiliy. Denoing he oal power of he ransmie signal in he frequency range [f 1, f ] by P UWB (f 1, f, we calculae he UWB inerference on he 8.11a sysem as GLOBECOM /3/$17. 3 IEEE

4 I = P ( f, f PL( d, (7 UWB UWB 1 UWB where f 1 and f are he lower and upper band-edges of he 8.11a receiver, respecively. The oal inerference power is hen calculaed as he sum of he noise power (N,8.11a and he receive inerference power a he 8.11a receiver (I UWB. In order o calculae he SIR of he 8.11a sysem in presence of he UWB inerference, we deermine he received power from he 8.11a receiver using (6. The SIR of he 8.11a receiver is hen calculaed by ( SIR8.11 a = PR,8.11 a 1 log1 I% UWB + N%,8.11a. (8 Here I % UWB and N % are he inerference and noise,8.11a power a he 8.11a receiver given in linear scales, respecively. 1 Effec of UWB Inerference on 8.11a SIR Fig. shows SIR 8.11a for an 8.11a sysem operaing a a rae of 54 Mbps in an NLOS environmen. The scenarios of = 1, 5, and 1m are shown, as well as he noinerference case (i.e., =. Noe ha he UWB inerferer a a disance of 1 meer causes a 1 db decrease in SIR 8.11a for all T-R separaions (i.e. he curve ploed for he case of no-inerference is ranslaed down by 1 db. As he UWB ransmier disance is increased o 5m and 1m, he SIR 8.11a is decreased by abou 1 db and 5 db, respecively. In he absence of UWB inerference, SIR 8.11a is decreased from 53 db o 1 db for ranging from 1 o 14m. However, wih a 1m I-R separaion, a SIR 8.11a of only db can be mainained for T-R separaions of less han 8m. Thus, he inerference from he UWB sysem severely affecs he performance of 8.11a for small I-R separaions. For he wors case (, = 14m an SIR 8.11a of around -1 db is observed and, herefore, he 8.11a sysem is jammed. Even when he I-R separaion is 1m, a 5 db decrease in SIR 8.11a can affec he performance of 8.11a WLANs operaing a high daa-raes due o heir high SIR requiremens. Daa Rae (Mbps (m Fig. 3: Plo of he achievable daa-raes of an 8.11a sysem for NLOS pahs in he presence of a UWB inerferer. Noe ha, as specified by he 8.11a sandard, only daa-raes of 54, 48, 36, 4, 1, 9 and 6 Mbps are suppored. Table III shows some values of SIR 8.11a for boh LOS and NLOS environmens. I can be seen ha, when 8.11a is working in an inerference-free LOS environmen, an SIR 8.11a of more han 4 db is mainained for up o 1m. The SIR 8.11a decay due o UWB inerference is he same as for he NLOS case (i.e. 1 db, 1 db and 5 db for = 1, 5, and 1m, respecively. The lowes observed SIR 8.11a in he LOS case is 1 db a a T-R separaion of 1 meers, which is sill sufficien for reliable 8.11a communicaion. Effec of UWB Inerference on 8.11a Daa-Raes Alhough we have already presened resuls of he SIR 8.11a in he presence of a UWB inerferer, a opic of grea ineres in WLAN sysems is he available daa-raes for a user in he presence (and absence of inerference. Figure 3 depics he resuls of simulaions for 8.11a operaing in an NLOS environmen, while Table IV shows he achievable daa-raes for 8.11a working in LOS and NLOS environmens. From Fig. 3 and Table IV i is seen ha, wihou inerference, 54 Mbps can be achieved a 4m and 1 Mbps can be achieved up o 11 meers. I should also be noed ha he minimum 8.11a daa-rae of 6 Mbps can be susained a all disances inside of 14m. However, in he presence of UWB inerference he achievable daa-rae drops drasically wih increasing. For example, wih an I-R separaion of 1m, he rae drops o 4 Mbps for and he sysem is jammed a 1m. Also noe ha a UWB ransmier a 1m is able o jam he 8.11a sysem wih a T- R separaion of 4m and, wih boh ransmiers placed 1m from he receiver, he WLAN sysem is jammed. For he case of 8.11a operaing in LOS, Table IV shows ha a daa-rae of 54 Mbps can be mainained a 1m, so long as he inerferer is over 5m away from he vicim receiver. This daa rae is reduced o 4 Mbps when he inerferer is wihin 1m of he receiver. Hence, we conclude ha he effec of UWB on he achievable daa-rae of an 8.11a WLAN is highly dependen on he working environmen of he sysem. When 8.11a devices operae in LOS environmens hey show beer resisance o he UWB inerference. Conversely, he 8.11a sysems can be easily jammed in NLOS environmens. B. 8.11a Inerference ino UWB Devices Inerference from 8.11a on a UWB sysem was simulaed wih he purpose of exploring SIR UWB. Coninuous ransmission of he 8.11a was assumed, overlapping he enire ransmission ime of he UWB sysem. A known mehod for UWB receiver performance analysis [7, 13] is o assume an undisored received waveform and build a receiver mached o he Gaussian monocycle for use in ideal Rake combining. SIR UWB is hen defined as SIR = P 1 log I% + N% (9 ( UWB R, UWB a, UWB GLOBECOM /3/$17. 3 IEEE

5 SIR UWB (db Fig. 4: d (m UWB TABLE V SIR UWB (in db in he presence of 8.11a Inerference. (m LOS Signal-o-inerference raio of a UWB sysem (SIR UWB in an NLOS environmen in he presence of an 8.11a inerferer. (m NLOS m m m where I% 8.11 and % a N, UWB are he inerference and noise powers, in linear scales, a he UWB receiver, respecively. The resuls of our simulaions of SIR UWB in he presence of 8.11a inerference in an NLOS environmen are shown in Fig. 4 and Table V. Clearly, an 8.11a ransmier induces a significan reducion of SIR UWB when in close proximiy of he UWB receiver. When an 8.11a ransmier is posiioned 1m from he UWB receiver, a 36 db reducion of SIR UWB is incurred. When he 8.11a ransmier is moved o 5m and 1m from he UWB receiver, he reducion in SIR UWB becomes 5 db and 18 db, respecively. We noe ha, in he NLOS scenario, even when he UWB T-R separaion is 3 m and he I-R separaion is 1m, a drop in SIR UWB from 8.7 db o 1 db is observed. Thus, i can be concluded ha UWB sysems are sill vulnerable o narrowband inerference in spie of he high processing gain ha hey offer. IV. CONCLUSION We have compleed a sudy of he inerference beween UWB an WLANs. We have shown ha a UWB signal, meeing FCC guidelines can grealy impac he performance of an 8.11a WLAN sysem working in NLOS environmens. Moreover, 8.11a WLAN inerference will also grealy impac UWB receiver performance. In ligh of hese resuls, i is clear ha he guidelines se forh by he FCC concerning UWB ransmission are no sufficien o preven UWB sysems from inerfering wih narrowband sysems under all circumsances. Furhermore, he opic of proecing a UWB sysem from narrowband inerference is also of grea imporance. Thus, he various proposed mehods o address his issue, which include noch filering, pulse shaping, anenna echniques, signaling forma, and rae/power adapaion, are opics for furher research. V. ACKNOWLEDGEMENTS The auhors hank Prof. Aleksandar Kavčić of Harvard Universiy for his echnical suppor and encouragemen. REFERENCES [1] FCC Documen -163:Revision of Par 15 of he Commission s Rules Regarding Ulra-Wideband Transmission Sysems, April,, ET Docke No [] IEEE Sandar: Par 11: Wireless LAN Medium Access Conrol (MAC and Physical Layer (PHY Specificaions High Speed Physical Layer in he 5 GHz Band, [3] R. Van Nee and R. Prasad, OFDM for Wireless Mulimedia Communicaions. Arech House,. [4] L. Zhao and A.M. Haimovich, Performance of Ulra- Wideband Communicaions in he Presence of Inerference. IEEE Journal on Seleced Areas Special Issue on Ulra Wideband Radio in Muli-Access Wireless Communicaions, Dec., vol., pp [5] I. Bergel, E. Fishler and H. Messer, Narrow-Band Inerference Suppression in Time-Hopping Impulse-Radio Sysems. IEEE Conference on Ulra Wideband Sysems and Technologies,. [6] A. Taha and K. M. Chugg, A Theoreical Sudy on he Effecs of Inerference on UWB Muliple-Access Impulse Radio. Asilomar Conference on Signals, Sysems and Compuers,. [7] M.Z. Win and R. Scholz, Impulse Radio: How i Works. IEEE Communicaion Leers, vol., No. 1, January [8] X. Chen and S. Kiaei, Monocycle Shapes for Ulra Wideband Sysems. IEEE Inernaional Symposium on Circuis and Sysems (ISCAS,. [9] A. Peroff and P. Wihingon, PulseON Technology Overview, hp:// [1] S. S. Ghassemzadeh e. al., A Saisical Pah Loss Model for In-Home UWB Channels, IEEE Conference on Ulra Wideband Sysems and Technologies (IEEE-UWBST,. [11] J. Foerser, The Effecs of Mulipah Inerference on he Performance of UWB Sysems in an Indoor Wireless Channel, Vehicular Technology Conference (VTC, 1. [1] T.S. Rappapor, Wireless Communicaions, Principles and Pracice. Prenice Hall, Inc., [13] M.Z. Win and R. Scholz, Energy Capure vs. Correlaor Resources in Ulra-Wide Bandwidh Indoor Wireless Communicaion Channels. Miliary Communicaions Conference (MILCOM, GLOBECOM /3/$17. 3 IEEE