Onlin Publication Dat: 15 th Jun, 01 Publishr: Asian Economic and Social Socity Computr Simulation to Gnrat Gaussian Pulss for UWB Systms Ibrahim A. Murdas (Elctrical Dpartmnt, Univrsity of Babylon, Hilla,Iraq) Murad A. Al-Hllo (Elctrical Dpartmnt, Univrsity of Babylon, Hilla,Iraq) Citation: Ibrahim A. Murdas., Murad A. Al-Hllo (01) Computr Simulation to Gnrat Gaussian Pulss for UWB Systms, Journal of Asian Scintific Rsarch, Vol., No. 5, pp. 69-74.
Journal of Asian Scintific Rsarch, Vol., No. 5, pp. 69-74 Computr Simulation to Gnrat Gaussian Pulss for UWB Systms Abstract Author(s) Ibrahim A.Murdas Elctrical Dpartmnt, Univrsity of Babylon, Hilla,Iraq E-mail:ibr197800@yahoo.com Murad A. Al-Hllo Elctrical Dpartmnt, Univrsity of Babylon, Hilla,Iraq E-mail: murad_65@yahoo.com In this work w prsnt a computr simulation of a simpl tchniqu for th gnration of powr-fficint, FCCcompliant Ultra-Widband (UWB) (monocycl and doublt) pulss. A Simulatd systm consist of a lasr sourcs, smiconductor optical amplifir, optical modulator, fibr Bragg grating (FBG).This tchniqu basd on combination of optically rconfigurabl photonic microwav dlay-lin filtr and using th cross gain modulation (XGM) in Smiconductor Optical amplifir (SOA). In SOA th cross gain modulation (XGM) tchniqu was usd to produc a UWB monocycl puls with a full width at half-maximum of 35 ps, by ntring th output monocycl pulss from first stag to th scond stag that includd rconfigurabl photonic microwav dlaylin filtr th last act as first- or scond-ordr diffrnc, which can b approximatd as a first- or scond-ordr drivativ, aftr drivativ th monocycl puls th doublt pulss was obtaind. Kywords: Smiconductor Optical Amplifir, Cross Gain Modulation, Optical Signal Procssing, Microwav Photonics spcifid by th FCC. Diffrnt approachs Introduction hav bn rcntly proposd and dmonstratd [J. Yao, 007]. Th major limitation of th Ultra widband (UWB) is a fast mrging approachs in is that ach schm can only tchnology that has rcntly attractd gnrat on typ of UWB puls (Gaussian considrabl intrst for its applications in monocycl or doublt). For som applications, short-rang, high-capacity wirlss such as puls shap modulation (PSM), it is communication systms and snsor ntworks, thanks to advantags such as a vry high data rat, low powr consumption, and immunity to multipath fading. Among ths tchniqus, th implmntation of th first- or th scond-ordr drivativs of a Gaussian puls, to gnrat a Gaussian monocycl or a Gaussian doublt, is considrd as a simpl and fficint tchniqu for UWB puls gnration [R.J. Fontana 004]. UWB pulss can b gnratd in th lctrical domain using lctronic circuitry. Rcntly; th gnration of UWB pulss in th optical domain has bn a topic of intrst. Th gnration of UWB pulss in th optical domain provids a highr flxibility, which dsirabl that both Gaussian monocycl and doublt can b gnratd in a singl systm. In [L. Zuniga, 006], diffrnt wavforms can b obtaind, but th switching spd btwn th wavforms is limitd by th spd of th liquid crystal modulator. Vry rcntly, a dsign was proposd to gnrat UWB monocycl and doublt pulss in on systm [R.J. Fontana, E. A. Richly,007], in which a fibr Bragg grating (FBG) was usd to srv as a frquncy discriminator, to prform phas modulation to intnsity modulation (PM-IM) convrsion. By locating th optical carrir at th linar or th quadratur rgion of th FBG rflction spctrum, UWB monocycl or doublt pulss nabls th gnration of UWB pulss with wr gnratd [J.Li, 007]. Th main switchabl puls shaps and polaritis. In addition, th hug bandwidth offrd by photonics nabls th gnration of UWB drawback of this schm is th rquirmnt for a high-spd tunabl lasr sourc (TLS) to raliz th wavform switchability. In addition, pulss to fully occupy th spctrum rang th high snsitivity of th FBG to 69
Computr Simulation to Gnrat Gaussian pulss.. nvironmntal changs would affct th stability of th systm. UWB signals ar producd by pulsd missions, whr a vry wid RF bandwidth is rlatd to a narrow puls width. Unlik many convntional radio transmittrs in which a modulatd signal is up convrtd & amplifid, in UWB systms information is ncodd in th sris of basband pulss and transmittd without a carrir. Hnc, th transmittrs rquir prcis puls shaping to produc th rquird spctrum and maximis th antnna s mission. Producing missions with flat & wid PSDs rquirs xtrmly accurat puls dsigns. Most of th approachs proposd for gnrating UWB signals with charactristic monocycl or doublt wavforms ar implmntd mainly by using lctronic circuits in th lctrical domain.[ Q. Wang and J. Yao, 006] To distribut UWB signals ovr a longr distanc, stat-of th- art optical fibr with xtrmly low loss is considrd an xcllnt candidat for a transmission mdium. Thrfor th gnration and distribution of UWB signals dirctly in th optical domain has bn a topic of intrst rcntly. Th gnrat UWB doublts in th optical domain by using a spcially dsignd frquncy-shift kying modulator that consists of four optical phas modulators with thr lctrods. In th sam way th hybrid systm for gnrating UWB monocycl signals in th gain-switchd Fabry Prot lasr diod was usd to gnrat an optical puls train; a UWB monocycl signal is thn producd in th lctrical domain by a microwav diffrntiator [ Q. Wang and J. Yao, 006], th gnratd UWB signals by using an optical phas modulator in combination with a lngth of singl-mod fibr (SMF) was prformd In Rf. [D. Wntzloff,006], instad of a long SMF, a fibr Bragg grating (FBG) is mployd as a frquncy discriminator to prform PM IM convrsion. UWB monocycl or doublt signals can b gnratd by altring th location of th optical carrir at th linar or th quadratur slops of th FBG spctral rspons.rcntly proposd to gnrat UWB signals by using photonic microwav dlay lin as diffrntiatd. Bcaus Gaussian monocycl or doublt pulss can b gnratd by implmnting th first- or th scond-ordr drivativ of a Gaussian puls this was achivd in [Q. Wang, J. Yao, 007]. Gaussian Puls Gnration Mathmatical analysis Gaussian monocycl or doublt puls can b gnratd by implmnting th first- or th scond-ordr drivativ of a Gaussian puls whr th zro-man Gauss function is dscribd by Equation (1), whr σ is th standard dviation [M. Ghavami t al 007]: G 1 x ( x) (1) Whr: G(x) ar calld Gaussian wavforms bcaus thir mathmatical dfinition is similar to th Gauss function. Th basis of ths Gaussian wavforms is a Gaussian puls rprsntd by th following Equation ( ) y 1 ( ) t g t K1 () Whr: y g1 is th basis of th Gaussian puls, < t <, τ is th tim-scaling factor, and K 1 is a constant. Mor wavforms can b cratd by a sort of high-pass filtring of this Gaussian puls. Filtring acts in a mannr similar to taking th drivativ of Equation (). For xampl, a Gaussian monocycl, th first drivativ of a Gaussian p uls, has th form: t ( t ) y ( ) g t K (3) Whr: y g is th first drivativ of a Gaussian puls, < t <, τ is th tim-scaling factor and K is a constant. A Gaussian monocycl has a singl zro crossing. Furthr drivativs yild additional zro crossings, on additional zro crossing for ach additional drivativ. If th valu of τ is fixd, by taking an additional drivativ, th fractional bandwidth dcrass, whil th cntr frquncy incrass. A Gaussian doublt is th scond drivativ of quation (3) and is dfind by [M. Ghavami t al 007] 70
Journal of Asian Scintific Rsarch, Vol., No. 5, pp. 69-74 y t ( t ) g3 ( t) K3 (1 ) (4) Whr: y g3 is th scond drivativ of a Gaussian puls, < t <, τ is th timscaling factor and K3 is a constant. Aftr gnrating th optical monocycl pulss by using th optical amplifir th doublt Gaussian pulss can b gnratd according to quation (4). Th scond -ordr drivativ of Gaussian pulss can b approximatd by th first- or th scond-ordr diffrnc. It is known that th a scond-ordr diffrnc can b ralizd using a thr-tap microwav dlaylin filtr with cofficints of (1, -, 1) [Q. Wang and J. Yao, 007 ]. Principl of photonic microwav dlay lin filtr Fig. (1) Illustrat th principl of photonic microwav dlay lin filtr [Q. Wang and J. Yao, 007] Th schmatic diagram of a gnral N-tap photonic microwav dlay-lin filtr is shown in Fig. 1. It consists of an optical sourc, an optical modulator, a dlay tin dvic, and a photodtctor (PD). Th microwav signal to b filtrd is modulatd onto th lightwav gnratd from th optical sourc via th optical modulator. Th modulatd lightwav is thn snt to an N-tap dlay-lin dvic to introduc diffrnt tim dlays with an idntical tim dlay diffrnc btwn two adjacnt taps. Th tim-dlayd signals ar thn applid to th PD. Th tim dlay diffrnc dtrmins th fr spctral rang (FSR) and th cofficints dtrmin th shap of th filtr rspons. Mathmatically, th frquncy rspons of an N-tap microwav dlay-lin filtr is givn [Q. Wang and J. Yao, 007]. (5) Whr H N ( ) is th frquncy rspons of dlay lin filtr, τ is th tim dlay diffrnc and ak is th cofficint of th kth tap. For a two-tap filtr with cofficint of (1,-1), th frquncy rspons is givn [Q. Wang and J. Yao, 007] H( ) jsin (6) For a thr-tap filtr with cofficints of (1, -, 1), th frquncy rspons is givn (7) H ( ) 4sin 3 j/ j If ωτ / is small, Eq. and 3 can b approximatd as H H H ( ) ( 3 ) N ( ) N 1 k0 j a j j jk (8).3 Gnration monocycl Gaussian puls W simulat a simpl mthod for gnrating UWB monocycl pulss basd on cross-gain modulation (XGM) in a optical amplifir (SOA) as shown in Fig.(). In this systm an optical Gaussian puls (pump) and a continuous wav (CW)( prob) ar applid to thsoa. Th XGM in th SOA, a pair of polarity-rvrsd optical Gaussian pulss is gnratd at th output of th SOA [Q. Wang and J. Yao, 006]. Th two polarity-rvrsd optical pulss ar thn tim dlayd by two cascadd FBGs to introduc a tim-dlay diffrnc. Whn th physical spacing btwn th two FBGs and thir rflctivitis ar proprly dsignd, a monocycl puls with th rquird dsign paramtrs is gnratd. k 71
Computr Simulation to Gnrat Gaussian pulss.. Fig. () Monocycl puls gnration systm [Q. Wang and J. Yao, 006] Th basic ida of this approach is to gnrat a pair of polarity-rvrsd pulss at diffrnt wavlngths with an appropriat tim dlay diffrnc btwn th pulss. Th matrial gain spctrum of an SOA is homognously broadnd thrfor th cross gain modulation (XGM) ffct in an SOA is usd to gnrat th polarity-rvrsd pulss. In th proposd approach, whn a high-powr pulsd pump light is injctd into th SOA, th variation of th pump powr modulats th carrir dnsity of th SOA so that th gain of th SOA varis invrsly with th input lasr powr. If a CW prob light is injctd into th SOA with th pump, th powr of th prob light will vary invrsly with th pump powr, and a pair of polarity-rvrsd pulss is gnratd, with on puls at th pump wavlngth and th othr at th prob wavlngth, th non-linar mchanism is (XGM) illustratd in Fig. (3)Sinc th polaritis of th pump and th prob pulss ar rvrsd, a dirct dtction of ths two pulss would lad to a cancllation of th two pulss. Howvr, if a propr timdlay diffrnc is introducd btwn th two pulss, a nw puls that has a shap of a monocycl is gnratd. Th puls width of th monocycl can b controlld by altring th tim-dlay diffrnc. Pum p Pro b C ti SOA Filtr Modulat d prob Fig. (3) Illustration th XGM mchanism in SOA ti Doublt Gaussian pulss If th input signal to th thr-tap microwav dlay-lin filtr is a Gaussian monocycl, a Gaussian doublt can b gnratd. This will b illustratd in Fig. (4) whr th monocycl pulss was gnratd and ntring to th photonic dlay- lin filtr th output wav (doublt) takn at th BPD. (4) Th compltd proposd switchabl systm to produc th monocycls and doublt Gaussian puls Th Simulation Rsults A simulation was achivd by using OptiSystm7 and Matlab packags.th systm layout a shown In figur (4) was simulatd with two parts on for monocycl and for doublt. Th two lasr sourcs ar usd on for th pump signal and th othr for prob signal. Th pump sourc was modulatd by lctrical data using psudo random bit squnc gnrator and NRZ data format whil th othr lasr sourc usd as CW prob data, th pump and prob ar combind togthr into SOA using couplr. According to XGM tchniqu th intnsity of prob data was modulats according th larg powr of pump signal and th puls with shap shown in Fig. (6) ar producd at th output of SOA, using filtr lik FBGs filtrs tund at pump prob wavlngths to chancl th tim dlay 7
Journal of Asian Scintific Rsarch, Vol., No. 5, pp. 69-74.Fig(5) shows th XGM whr a strong pump light will rduction th gain of SOA, and causs to modulation of a wak CW prob light. Fig. (5) Gain rduction du to Strong pump puls Th pump powr is usually highr than th prob powr, a crtain asymmtry will occur in th gnratd monocycl as shown in Fig. (6). Now aftr th monocycls was gnratd th systm in Fig.(4) can usd to gnrat th doublt Gaussian pulss if it s configurd as a thr-tap microwav dlay-lin filtr with cofficints of (1, -, 1). This configuration was achivd by connctd th two arm of th couplr to th input port of th BPD, with an additional tim-dlay diffrnc τ introducd btwn th two branchs by adjusting th intrnal optical dlay lin in th BPD th doublt pulss with FWHM =5 ps ar shown in figur (8).All paramtr usd in th simulation shown in tabl 1.. Fig. (6) Asymmtry monocycl puls Monocycl that has a vry good shap is obtaind, as shown in Fig.( 7). Th FWHM of th monocycl puls is about 35 ps, which is narrowr than lctronic gnratd Gaussian puls. This is bcaus th tim-dlay diffrnc btwn th pump and th prob puls is smallr than th width of th Gaussian puls. Fig.(8) Doublts Gaussian puls at th output of systm Tabl 1: Simulatd paramtr Paramtr Valu Pump wavlngth 1549. μm Prob wavlngth 155.63 μm Fibrs Bragg Grating lngth (1, ) mm Driving currnt to th SOA 300 ma output powr of lasr sourc (prob) - 7 dbm output powr of lasr sourc (pump) -3 dbm Spacing btwn FBG1 and FBG.5 mm Input Puls shap Gaussian Lngth of SOA 100 μm Carrir dnsity 10^4 m -3 t FWHM 30 ps Volum of SOA 90 μm 3 Width of SOA 100 μm Fig. (7) output monocycls Gaussian puls 73
Computr Simulation to Gnrat Gaussian pulss.. Conclusions W prsnt a simpl flxibl mthod for gnrating UWB pulss (monocycl and doublt) basd on dlay-lin filtr and th XGM in SOA. Th numrical xprimnt implmntd using Mat lab and OptSystm packag to gnratd monocycls hav 35 ps at FWHM.Whn th Gaussian monocycl pulss wr gnratd. th filtr was configurd as a thr-tap dlay-lin filtr with cofficints of (1, -, 1), to gnrat a Gaussian doublt pulss with 5 ps at FWHM. Rfrncs D. Wntzloff,, A. Chandrakasan (006) Gaussian Puls Gnrators for Subbandd Ultra-Widband Transmittrs IEEE Transaction on microwav thory and Tchniqus Vol,54, NO. 4, pp. 1647-1650. J. Yao, Q.Wang, (007) Photonic Microwav Bandpass Filtr With Ngativ Cofficints Using a Polarization Modulator IEEE Photonics Tchnology Lttrs Vol 19, NO. 9, pp. 644-646. J. Li, K.Xu,(007) Ultra-widband puls gnration with flxibl puls shap and polarity control using a sagnac intrfromtr basd intnsity modulator Optics Exprss, Vol 15, No.6,pp.18156-18161. L. Zuniga, P. Ptropoulos (006) Dsign of a Fibr Bragg Grating for Dcoding DPSK Signals Optolctronics Rsarch Cntr, Univrsity of Southampton, Unitd Kingdom. M. Ghavami, L. B. Michal, R. Kohno (007) Ultra Widband Signals and Systms in Communication Enginring Copyright @ 007 John Wily & Sons Ltd. Q. Wang, F. Zng,( 006) Optical ultra widband monocycl puls gnration basd on cross-gain modulation in a smiconductor optical amplifir Optics Lttrs,Vol 31, No.1, pp3083-3085. Q. Wang and J. Yao, (006) UWB doublt gnration using a nonlinarly-biasd lctrooptic intnsity modulator, IEE Elctron. Lttr, Vol. 4,pp 1304-1305. Q. Wang and J. Yao, (007) Switchabl optical UWB monocycl and doublt gnration using a rconfigurabl photonic microwav dlay-lin filtr, Optics Exprss Vol.15, No. pp 14667-1467. R. J. Fontana, (004) Rcnt Systm Applications of Short-Puls Ultra-Widband (UWB) Tchnology,IEEE Transaction Microwav Thory and Tchniqu. Vol. 5, pp. 087-104. R.J. Fontana, E. A. Richly,(007) Obsrvations on Low Data Rat, Short Puls UWB Systms IEEE Intrnational Confrnc on Ultra-Widband (ICUWB), Singapor... 74