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122 J. OPT. COMMUN. NETW./VOL. 2, NO. 3 /MARCH 2010 Shams et al.

Electro-Optical Generation andDistribution of Ultrawideband

Signals Based on the GainSwitching Technique

Haymen Shams, Aleksandra Kaszubowska-Anandarajah, Philip Perry,P. Anandarajah, and Liam P. Barry

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Abstract—We demonstrate and compare the gen-eration and distribution of pulse position modulation(PPM) ultrawideband (UWB) signals, based on twodifferent techniques using a gain-switched laser(GSL). One uses a GSL followed by two externalmodulators, while the second technique employs twolaser diodes gain switched (GS) using a combined sig-nal from a pattern generator and an RF signal gen-erator. Bit-error-rate (BER) measurements and eyediagrams for UWB signals have been measured ex-perimentally by using the different GS transmitterconfigurations and various fiber transmission dis-tances. The simulation of both systems also has beencarried out to verify our obtained results, which showthe suitability of employing gain switching in a UWBover fiber (UWBoF) system to develop a reliable,simple, and low-cost technique for distributing theimpulse-radio UWB (IR-UWB) pulses to the receiverdestination.

Index Terms—Fiber optics communications; Radiofrequency photonics.

I. INTRODUCTION

D emands in wireless data transfer required fromthe end users have been increasing dramatically.

Nowadays, users need high-speed wireless access net-works to share photos, music, videos, and even high-definition video streams. Ultrawideband (UWB) tech-nology is an excellent and a reliable solution for futurewideband wireless personal access networks (WPANs)and high-bit-rate communication [1,2]. This is due to

Manuscript received September 18, 2009; revised January 13,2010; accepted January 13, 2010; published February 18, 2010 �Doc.ID 116917�.

The authors are with the Research Institute for Networks andCommunication Engineering (RINCE), Department of ElectronicEngineering, Dublin City University, Dublin 9, Ireland (e-mail:[email protected]).

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ts benefits of lower power consumption, low cost, highata capacity, and immunity to multipath fading.WB is a short-range ��10 m� wireless communica-

ion due to its power limitation from the Federal Com-unications Commission (FCC) regulation [3]. TheCC allocated in 2002 the unlicensed frequency bandetween 3.1 to 10.6 GHz for indoor UWB wirelessommunication systems with a power spectral densityPSD) lower than −41.3 dBm/MHz. One of the mainodulation schemes in UWB is impulse radio (IR-WB), which uses a carrier-free transmission with

ery narrow pulses [4]. IR-UWB is an attractiveethod due to its carrier-free modulation, lack of com-

licated frequency mixers, lack of intermediate fre-uency, and by consequence low power consumptionnd low cost. However, to increase its coverage area,WB signals can be easily integrated with optical

inks that provide low cost, low loss, and extremelyarge bandwidth [5,6]. This technique is known asWB over fiber (UWBoF) and can be used for distrib-ting UWB signals from the central station to differ-nt remote base transceiver stations. Hence, UWBoFan be used to extend the coverage of UWB to hun-reds of meters using optical fiber inside a building ormedium-range network in the home or office.

Several schemes for UWBoF have been proposed re-ently that depend on the optical generation of the im-ulse radio [7–11]. The generation of Gaussian mono-ycles and doublets as an impulse signal have beeneported in many research schemes for UWB systems.t has been demonstrated that these pulses have aider 3 dB bandwidth, and better performance in

erms of bit error rate (BER) and multipath immunityn comparison with other signals [12]. These pulsesan be created by using bandpass filtering of Gaussianulses to take the first- and second-order derivativesf the signal. UWB monocycle pulses have been gen-rated by using a gain-switched laser (GSL) and mi-rowave bandpass filter after the photodetector in [7],hereas other techniques have shown the optical gen-

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Shams et al. VOL. 2, NO. 3 /MARCH 2010/J. OPT. COMMUN. NETW. 123

eration of UWB signals based on optical phase modu-lators with dispersive devices or an optical frequencydiscriminator, a photonic microwave delay-line filter,or optical spectral shaping [8]. The pulse modulationscheme is also an important aspect of UWB distribu-tion systems. Several modulation schemes have beenemployed in order to satisfy the application and thedesign parameters such as pulse amplitude modula-tion (PAM), on–off keying (OOK), pulse positionmodulation (PPM), and binary phase-shift keying(BPSK) [13]. The implementation of an experimentalUWBoF system for distributing IR-UWB signalsbased on OOK pulses has been carried out in [9,10]and the optical BPSK modulation scheme to generatebiphase Gaussian monocycle pulses has been carriedout in [11]. In this paper, we demonstrate two differ-ent approaches for the generation and distribution ofelectro-optic IR-UWB signals based on a GSL andPPM modulation scheme at a bit rate of 1.625 Gbps.The first technique is based on using one GSL and twoMach–Zehnder modulators (MZMs) for external datamodulation [14], whereas the second approach isbased on a directly modulated (DM) GSL. The secondapproach uses a simpler direct modulation scheme forconversion of the electrical non-return-to-zero (NRZ)data signal to optical return-to-zero (RZ) data [15,16].To explore the trade-off between cost, performance,and reach, the performance of the two system setupshas been evaluated using experimental implementa-tions and simulations over different fiber links by us-ing three different configurations of a GSL diodetransmitter.

The structure of this paper is organized as follows.Section II gives an overview of our optical distributionsystem. Section III outlines the experimental andsimulation results using the first transmitter setupfor the electro-optic generation of IR-UWB based on aGSL and two external modulators. Section IV thengoes on to present the experimental and simulationresults of the second approach using two directlymodulated GSLs, and finally Section V presents asummary and conclusion.

II. SYSTEM OVERVIEW

The major building block of our system is the opticaldistribution system, shown in Fig. 1. It consists of acentral node or an optical distribution center (ODC)that generates a PPM optical signal using a GSL anda pulse pattern generator (PPG) at 1.625 Gbps. Theseoptical signals are then distributed through fiber linksto a number of remote antenna units (RAUs). In theRAU, the signals are photodetected, shaped to UWBsignals using an electrical bandpass filter (BPF), andradiated through air to the radio terminal (RT). At theuser end (the RT), the UWB signals are amplified and


emodulated. The system performance has been veri-ed by using a bit-error-rate tester (BERT) and aigh-speed digital sampling scope. This system allows

or the transmission of broadband data over hundredsf meters, with possible application areas of the pro-osed system involving the distribution of high-uality video stream content from DVDs and personalideo recorders to high-definition television (HDTV)isplays. In the next two sections, we will present twoifferent setups for generating PPM optical signals.he first one is achieved by using a pair of externalodulators driven by a pattern generator, while the

econd one is achieved by combining the signal from aattern generator and an RF signal generator and us-ng the composite signal to gain switch two lasers.

II. GENERATION OF IR-UWB BY USING TWO EXTERNAL

MODULATORS

. Experimental Setup

The schematic diagram of the first proposed systems shown in Fig. 2. In the ODC section, the opticalulses are generated using the technique of gainwitching. Three different laser configurations, oper-ting at 1550 nm, were used for the gain switching: aabry–Perot laser diode (FP-LD), a distributed-

eedback laser diode (DFB-LD), and an externally in-ected DFB-LD (EI DFB-LD). The FP-LD used was aommercially available InGaAsP device with a thresh-ld current of 9 mA. The DFB-LD was a commercialEL laser within a 14 pin butterfly package. The EIFB-LD is composed of a modulated DFB laser and a

ontinuous-wave (CW) DFB-LD in a master–slaveonfiguration [17]. The optical spectra for the threeSLs are shown as insets (i), (ii), and (iii) in Fig. 2,

espectively.

The gain switching of the lasers is achieved by ap-lying a bias below threshold in conjunction with anmplified sinewave at a repetition rate of 1.625 GHz.he gain-switched (GS) pulses, which are of the orderf 20 ps width, are shown in inset (iv) in Fig. 2. Theseptical pulses are divided into two equal paths by us-ng a 50:50 coupler. A pseudorandom bit sequence

ig. 1. Block diagram of our proposed optical distribution system.


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(PRBS) of length 27−1 at a bit rate of 1.625 Gbps froma PPG is applied to the MZMs. The data output �D� ofthe PPG is used to drive the upper MZM (passingpulses for all logical 1’s), while the complementaryoutput �D̄� of the PPG is used to drive the lower MZM(passing pulses for all logical 0’s). The output pulsesfrom the lower MZM are then delayed using an opticaldelay line (ODL) before being combined with the out-put of the upper MZM. This composite waveform rep-resents a PPM signal as shown in inset (v) of Fig. 2 forthe DFB-LD case for a sequence of 0010110100, whichis then transmitted along different lengths of stan-dard single-mode fiber (SSMF) to the RAU. At theRAU, an optically preamplified receiver consisting ofan erbium-doped fiber amplifier (EDFA), an opticalbandpass filter (OBPF), and a p–i–n photodetector(PIN-PD) is used to convert the optical pulses intoelectrical pulses.

These electrical pulses are amplified and shaped toa UWB signal by using an electrical UWB filter (band-pass 3.1–10.6 GHz), which acts as a differentiatorand converts these Gaussian pulses into doubletGaussian pulses. These pulses are shown as inset (vi)in Fig. 2 for the DFB-LD case and are broadly similarfor the other lasers tested. The electrical RF spectrumat point A is adjusted to meet the stringent FCC’smask and is shown in Fig. 3, and the dotted line rep-resents the S21 of the UWB filter. Here the spectrumshaping is achieved by the filter and the amplitude isadjusted to meet the mask by setting the optical re-ceived power using a variable optical attenuator. Theoutput of the filter is then connected directly to pointB in the RT, since we focus only on the UWB signaldegradation over the fiber transmission link. At the

Fig. 2. (Color online) Experimental setup for generating PPM pulsegain-switched FP, DFB, and EI DFB, respectively; (iv) gain switche


T, the signal is amplified and mixed with a 6.5 GHzignal (the fourth harmonic of 1.625 GHz) from the lo-al oscillator (LO) to downconvert the UWB signal toaseband. The output from the mixer is then ampli-ed and filtered with a low-pass filter (LPF) to removehe unwanted frequency components yielding a de-odulated data signal. BER measurements are then

erformed for a back-to-back (BTB) case as well as forifferent fiber lengths placed between the ODC andAU. A variable optical attenuator (VOA) is used withn inline powermeter to enable the monitoring of theptical received power �Prec� before the preamplifiedeceiver during the measurements of the BER. Eyeiagrams are also recorded at the output of the RT bysing a high-speed digital sampling oscilloscope.

It should also be noted that the simple demodulatorsed in this experiment requires at least three main

y using two external modulators: (i), (ii), and (iii) optical spectra forulses; (v) PPM pulses; (vi) doublet Gaussian pulses.

ig. 3. (Color online) Electrical spectrum for the generated PPMlectrical UWB pulses.

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tones within the electrical spectrum and is thereforeconstrained to bit rates of less than 3–4 Gbps. For theparticular UWB bandpass filter used in this experi-ment, the maximum bit rate is approximately1.7 Gbps. It is expected that the use of a different elec-trical filter and advanced signal processing at the de-modulator would increase this maximum rate.

B. Results and Discussions

In Fig. 4, the BER is plotted versus the received op-tical power at the RAU for the three different opticaltransmitters. The eye diagrams corresponding to eachof the transmitter configurations are shown as insetsin each of the figures. Figure 4(a) shows that the GSFP-LD can be used to achieve error-free performance�BER�10−9� while transmitting over fiber lengths upto 450 m. The degraded performance over 650 m ismainly due to mode partition noise (MPN). Dispersionin the fiber causes the fluctuations of the energy be-tween the longitudinal modes of the FP-LD to betranslated into intensity fluctuations on the transmit-ted optical signal, thus reducing its signal-to-noise ra-tio [18]. Another contributory factor to the perfor-mance degradation is the amplified spontaneousemission (ASE) from the EDFA (because no ASE re-moval filter is used). Optical filtering cannot be em-ployed with this multimode transmitter scenario be-cause it would worsen the effect of MPN [18]. In thecase of the DFB laser [Fig. 4(b)], near-error-free trans-mission, while transmitting over 1 km of SSMF, canbe achieved. In this case the system transmissionreach is limited by noise resulting from the degrada-tion in the side mode suppression ratio (SMSR) of thelaser (�10 dB for gain-switched DFB), which causesMPN problems as outlined above for the FP laser, aswell as a relatively large temporal jitter �5 ps� in thegenerated optical pulses. By employing external lightinjection [19], the performance of the DFB-LD can begreatly improved as shown in Fig. 4(c). This improve-ment is achieved as a result of the enhancement of thepulse SMSR ��30 dB�, which eliminates MPN, and areduction of the temporal jitter ��1 ps�. Hence, withthis transmitter configuration, error-free transmis-sion over 37 km of SSMF has been achieved.

C. Simulation Results

Simulations have also been carried out by using theVPItransmissionMaker simulation platform to modelthe proposed setup and verify our experimental re-sults. The key parameters of the devices have beenchosen to meet the real experimental parameters. TheGSL was simulated by using a transmission linemodel (TLM) of a semiconductor laser module at awavelength of 1550 nm, and the output pulses were


ptimized by adjusting the bias and amplitude of theF drive. In the case of the EI DFB-LD, a CW module

aser was used to realize the external injection. Theavelength and power of the CW module laser wereptimized to obtain pulses exhibiting low jitter and aarrow optical spectrum. We also used a SSMF with aroup velocity dispersion (GVD) of 16 ps/nm/km andloss of 0.2 dB/km. The optical receiver has a ther-

(a)

(c)

(b)

ig. 4. (Color online) Measured BER versus received optical poweror different SSMF links and the eye diagram for the lowest BERsing three different transmitter configurations: (a) FP-LD, (b)FB-LD, and (c) EI DFB-LD.


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mal noise with 10−11 A/ �Hz�1/2 and a responsivity of1 A/W. In the downconversion at RT, the local oscilla-tor phase was adjusted to give the lowest BER and anopen eye diagram for each transmission distance. Thesimulation results for the BER versus the received op-tical power for the three configurations are shown to-gether in Fig. 5, and the simulated eye diagrams arealso shown in Fig. 6. These results show the systemperformance for each of the three transmitter configu-rations for BTB and maximum transmission reachscenarios. For the FP-LD, the simulation illustratesthat ASE noise causes the FP-LD to portray the worstreceiver sensitivity in the BTB case, and an error floorcould be seen at a BER of 10−11. The maximum reach�650 m� is caused primarily by the multimode spec-trum, which, when transmitted, results in MPN asdiscussed in Subsection III.B. The simulated eye dia-gram for BTB and after transmission over 650 m areshown in Figs. 6(a) and 6(b). In the case of theDFB-LD an improvement in the BTB receiver sensi-tivity is achieved mainly due to the removal of ASEwith the use of an OBPF. However, the maximumtransmission reach can only be doubled due to the de-graded SMSR of the GS-DFB, which results in MPN.Figures 6(c) and 6(d) show the simulated eye diagramfor the BTB and after transmission over 1500 m. TheEI DFB-LD exhibits the best BTB receiver sensitivityas a result of the reduced pulse-to-pulse jitter. More-over, it also achieves the maximum reach, which canbe attributed to the enhanced SMSR, reduced jitter,and reduced chirp. This can also be seen in the simu-lated eye diagram for the BTB case in Fig. 6(e). FromFig. 6(f), after transmission over a fiber link of 47 km,the eye diagram is degraded due to the combined ef-fect from the fiber’s attenuation and dispersion; how-ever, it is clear that we are able to achieve a signifi-cant increase in performance and reach with the EI-DFB. The experimental and simulated results are alsosummarized in Table I to show the performance forthe three transmitter configurations. The table dem-

Fig. 5. (Color online) Simulated BER versus received optical power
for the three different GSLs.

nstrates that there are small differences between theimulated and experimental measurements in the re-eiver sensitivities for BTB and the power penaltyeeded for maximum reach at BER�10−9.

IV. GENERATION OF IR-UWB BY USING DIRECT

MODULATION FOR THE LASER

. Experimental Setup

The schematic diagram for the second approach forhe UWBoF distribution system is shown in Fig. 7.he ODC uses the simpler direct modulation of a GSL

or data transmission at a rate of 1.625 Gbps. TheDC setup consists of two DFB-LDs biased below the

hreshold current value via a bias tee. These twoFB-LDs are modulated using a combination of a.625 GHz sinusoidal signal generator and a synchro-ous 1.625 Gbps pattern generator (27−1 PRBS). Theutput from the signal generator is amplified and thenivided into two equal paths before it is combinedith the NRZ data pattern for both data and invertedata. The resultant output waveforms are applied tooth DFB-LDs to generate Gaussian pulses throughhe GS process only when the data signal applied tohe laser is a bit 1. The output pulses are adapted us-ng the inspection of the eye diagrams to the best ex-inction ratio and minimum pattern-dependent effecty adjusting the bias level and the amplitude of thelectrical data. DFB-LD 1 generates a GS Gaussianulse for each logical bit 1, while DFB-LD 2 generatesGS Gaussian pulse for each logical bit 0.

The output from both lasers are then delayed rela-ive to one another by using the optical delay lineODL) before the data and inverted data pulses areombined using a 50:50 coupler and then transmittedirectly as a PPM signal over different lengths ofSMF. After fiber transmission, the signal is receivedy the same RAU and RT (shown in Fig. 2), which am-lifies the received PPM signal and transforms it intolectrical UWB pulses. Then, the UWB signal is trans-itted directly to the RT at point B to be demodulated

ig. 6. (Color online) Simulated eye diagrams for FP lasers at (a)TB and (b) 650 m, DFB lasers at (c) BTB and (d) 1500 m, and EI-FB lasers at (e) BTB and (f) 47 km.


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and amplified. BER measurements and eye diagramsare recorded for the received signal over different fiberlengths.

B. Results and Discussion

The system performance is measured using two dif-ferent configurations of our DM GS laser transmit-ters. In the first configuration we simply used two GSDFB laser diodes (DFB-LD) operating at the samewavelength and their optical spectrum is shown inFig. 7 inset (i). While in the second configuration, weexternally injected light from a third laser in a CWmode into the two GS DFB lasers to give an externallyinjected GS DFB (EI DFB-LD) laser and have the op-tical spectrum shown as in Fig. 7 inset (ii). Due to thelimited practical components, the DFB-LDs and EIDFB-LDs were chosen with different operating wave-lengths. The oscilloscope traces in Fig. 8 show theNRZ data, the laser drive signal consisting of the datacombined with the RF sinusoid, and the resulting op-tical pulses. It is interesting to note that the dataneeds only to create a small offset to generate narrowpulses with a good extinction ratio. The two opticaloutputs, which have pulse durations of around 20 ps,

TABRESULTS SUMMARY FOR IR-UWB GENERA

TransmitterConfiguration

BTBThreshold at

10−9

(dBm)

PowePenaltyMax. Re

at 10−

(dB)

FP-LD meas. −25.3 3.2simul. −25.1 3.5

DFB-LD meas. −37.5 5.7simul. −38.2 4.7

EI DFB-LD

meas. −38.9 9.9simul. −39.9 10.2

Fig. 7. (Color online) Schematic diagram for generating PPM byusing two directly modulated GSLs: (i) and (ii) optical spectra fordirectly modulated gain-switched DFB and EI DFB.


re combined together after delaying the lower branchy 285 ps relative to the beginning of a bit period. TheER measurements for the different fiber spans ver-us the received optical power at the RAU �Prec� arehown in Fig. 9. Figure 9(a) shows an error floor at0−8 due to significant levels of amplitude noise andemporal jitter on the GS pulses. This limits the per-ormance but is acceptable for a radio-based systemhat includes a high level of forward error correction.he results also show a penalty of 0.7 dB betweenTB and 1 km transmission at a BER of 10−8.

However, the system reaches an error floor atround 10−5 on transmission over 10 km fiber due tohe large frequency chirp on the GSL that broadenshe spectrum to approximately 1 nm. The fiber disper-ion results in the chirped pulses being significantlyroadened such that the 1 and 0 data pulses overlapausing intersymbol interference (ISI). The eye dia-rams are also shown in Fig. 9(a) as insets for the low-st bit error rate at 1 km and after transmission over0 km. When we employ the EI GS-DFB laser diodese greatly improve the performance of the system

ransmission. The result shown in Fig. 9(b) revealshat the transmission length is extended up to 10 kmith error-free �BER=10−9� performance. It should beoted that the receiver sensitivity in the BTB case has

IN BY USING TWO EXTERNAL MODULATORS

MaximumReach

Error FloorValue

LongerTransmission

for ErrorFloor

450 m 10−5 650 m450 m 10−6 650 m1 km 10−6 1.5 km1 km 10−6 1.5 km37 km 10−4 47 km37 km 10−4 47 km

ig. 8. Data, combined data, and an RF sinusoid and output pulsesrom lasers for (a) data and (b) inverted data at 1.625 Gbps.

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been improved by 3 dB compared with the noninjectedcase due to the reduction of timing jitter (from�4 to �1 ps) and the enhancement in SMSR (from�10 to �30 dB) of the GS-DFB lasers, which reducesMPN caused by optical filtering in the experiment[18]. In the EI GS-DFB case, the system displays anerror floor around 10−7 for a distance of 25 km. As forthe noninjected case, this error floor is a result of ISIbetween the 1 and 0 data pulses. The significantly ex-tended reach in the EI GS-DFB case is due to thechirp reduction caused by the external injection (spec-tral width reduced from �1 to �0.3 nm) [20].

C. Simulation Results

The simulated model for the generation and distri-bution of hybrid UWB signal using directly modulated

(a)

(b)

Fig. 9. (Color online) Measured BER versus received optical powerfor different SSMF spans and the eye diagrams for two differenttransmitter configurations: (a) two GS-DFBs, (b) two EI GS-DFBs.


asers has been carried out for the noninjected and in-ected case. We used here two TLM semiconductor la-er modules with the same biased and RF drive powerombined with both data and inverted data to achievehe DM GS DFB-LD. The BER versus received opticalower is shown in Fig. 10 and the simulated eye dia-rams are also shown in Fig. 11 for the direct modu-ated GS-DFB and EI GS-DFB. Figures 11(a) and1(b) illustrate the simulated eye diagrams forFB-LD and demonstrate the timing jitter and MPNffect in the downconverted pulse between BTB and0 km transmission over fiber. These detrimental ef-ects are eliminated in the injected case as shown byhe eye diagrams in Figs. 11(c) and 11(d) for BTB andransmission over 25 km. However, this transmitteronfiguration shows a maximum reach only to 25 kmn EI GS-DFB, which is significantly different fromhe first configuration, which could achieve 37 km er-or free. This is mainly due to the degradation in thextinction ratio in the direct modulation case com-ared with that in the external modulators.

The important results from the directly modulatedS laser are summarized in Table II. The table clearly

ig. 10. (Color online) Simulated BER versus received opticalower for two transmitter configurations of the two directly modu-ated GSLs.

ig. 11. (Color online) Simulated eye diagrams for DM DFB-LD ata) BTB and (b) 10 km and DM EI DFB-LD at (c) BTB and (d)5 km.


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shows that there is a close match between experimen-tal and simulation results. The simulated and experi-mental BER shows a 1 dB difference in BTB. Thepower penalty is about 2 dB for the maximum reachfor both cases at 10−9, while the error floor is nearlythe same for the longest transmission distance forDFB and EI DFB at 10 and 25 km, respectively.

The advantage of the direct modulation scheme overthe previous setup is its simplicity as it does not re-quire external modulators, and hence there is no ad-ditional insertion loss, and a reduction in cost, whichis vital for the development of a low-cost solution fordistribution of UWB signals. In Table III, the compari-son between the two distribution setups is presentedin terms of the limiting factors that affect the systemdistribution and the required costs. In systems thatneed the distribution for a small residential networkor office building, the FP-LD or DM DFB-LD can bethe best choice for a low-cost and easily integratedsystem. However, for a large distribution system, theEI DFB-LD scheme with two external modulators al-lows a more reliable and stable system for many endusers with maximum reach.

V. CONCLUSIONS

UWB over fiber is an attractive and cost-effectivesolution for increasing the reach of UWB systems. Inthis work, we have successfully demonstrated the gen-

TABRESULTS SUMMARY FOR IR-UWB GENERATIO

TransmitterConfiguration

BTBThreshold at

10−9

(dBm)

Power Pefor Max. R

at 10−

(dB)

DMDFB-LD

meas. −29.5 2.4simul. −31.2 3.1

DM EIDFB-LD

meas. −33.4 2simul. −34.1 2.3

TABCOMPARISON BETWEEN THE TWO APPROACH

ApproachSetup

LaserConfiguration Costs

Externalmodulators

FP-LD $

DFB-LD $$$

EI DFB-LD $$$$Direct

modulationDFB-LD $$

EI DFB-LD $$$


ration and distribution of 1.625 Gbps UWB signalsodulated in PPM format by using two optical distri-

ution setups. Both setups have been studied over dif-erent fiber links using different GS laser configura-ions. The first approach uses one GS laser and twoxternal modulators. The results show that error-freeransmission can be achieved for the GS FP, DFB, andI DFB laser transmitters over 450 m, 1 km, and7 km, respectively.

We have also demonstrated an alternative approachor the generation and distribution of UWB signalsased on direct modulation of a GSL. The method useswo lasers driven with a signal composed of NRZ datand an RF sinusoid. The generated PPM pulses wereransmitted over different fiber links by using GS-FBs and EI GS-DFBs. The obtained results show

hat the system can provide error-free performancever 10 km with an EI GS-DFB. A degraded perfor-ance is also achievable with an uninjected GS-DFB

t 10 km. Since these radio systems typically operateith a BER worse than 10−3, this performance maylso be acceptable and offers a simpler transmitter de-ign. There is thus a trade-off between the cost/omplexity of the transmitter configuration in a UWB-ver-fiber system and the required reach anderformance of the distribution network. The noise ef-ects caused by pulse propagation from the differentources means that one must choose the transmitterased on the required reach of the network.

IIY USING TWO DIRECTLY MODULATED GSLS

yh

MaximumReach

Error FloorValue

LongerTransmission

for ErrorFloor

1.5 km 10−6 10 km1.5 km 10−5 10 km10 km 10−7 25 km10 km 10−7 25 km

IIIUPS IN TERMS OF THEIR LIMITING FACTORS

Limiting Factors

• MPN resulting from the energy fluctuations in thelongitudinal modes due to dispersive transmission

• SMSR degradation causes MPN problems• Large inherent timing jitter in the generated gain-

switching pulses• Attenuation and dispersion effect• Timing jitter in GS pulses• SMSR degradation causes MPN problems• Degraded extinction ratio

LEN B

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ACKNOWLEDGMENTS

This work was supported in part by the Science Foun-dation Ireland Research Frontiers Programme andthe Enterprise Ireland Technology Development Pro-gramme.

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