performance of a 60-ghz dcm-ofdm and bpsk-impulse ultra-wideband system with radio-over-fiber and...

IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 29, NO. 6, JUNE 2011 1295

Performance of a 60-GHz DCM-OFDM andBPSK-Impulse Ultra-Wideband System withRadio-Over-Fiber and Wireless TransmissionEmploying a Directly-Modulated VCSELMarta Beltran, Member, IEEE, Jesper Bevensee Jensen, Xianbin Yu, Member, IEEE,

Roberto Llorente, Member, IEEE, Roberto Rodes, Markus Ortsiefer, Christian Neumeyr,and Idelfonso Tafur Monroy, Member, IEEE

Abstract—The performance of radio-over-fiber optical trans-mission employing vertical-cavity surface-emitting lasers (VC-SELs), and further wireless transmission, of the two majorultra-wideband (UWB) implementations is reported when op-erating in the 60-GHz radio band. Performance is evaluated at1.44 Gbit/s bitrate. The two UWB implementations consideredemploy dual-carrier modulation orthogonal frequency-divisionmultiplexing (DCM-OFDM) and binary phase-shift keying im-pulse radio (BPSK-IR) modulation respectively. Optical trans-mission distances up to 40 km in standard single-mode fiberand up to 500 m in bend-insensitive single-mode fiber withwireless transmission up to 5 m in both cases is demonstratedwith no penalty. A simulation analysis has also been performedin order to investigate the operational limits. The analysisresults are in excellent agreement with the experimental workand indicate good tolerance to chromatic dispersion due tothe chirp characteristics of electro-optical conversion when adirectly-modulated VCSEL is employed.The performance comparison indicates that BPSK-IR UWB

exhibits better tolerance to optical transmission impairmentsrequiring lower received optical power than its DCM-OFDMUWB counterpart when operating in the 60-GHz band.

Index Terms—Dual carrier modulation, impulse radio, orthog-onal frequency division multiplexing (OFDM), radio over fiber,60 GHz radio, ultra wideband (UWB), vertical cavity surfaceemitting laser (VCSEL).

I. INTRODUCTION

ULTRA-WIDEBAND (UWB) is an attractive radio tech-nology which exhibits low interference, low latency

and potential high bitrate at low cost in short-range wireless

Manuscript received 1 June 2010; revised 1 December 2010. Thiswork was supported in part by the European Commission under theFP7-ICT-249142 FIVER, FP7-ICT-212352 ALPHA, and FP7-ICT-224402EURO-FOS Projects. The work of M. Beltran was supported by the Ministeriode Ciencia e Innovacion (MICINN), Spain, under FPI Grant BES-2006-12066.The work of X. Yu was supported by a Marie Curie International IncomingFellowship within the 7th European Community Framework Programme.M. Beltran and R. Llorente are with the Valencia Nanophotonics Tech-

nology Center, Universidad Politecnica de Valencia, 46022 Valencia, Spain(e-mail: [email protected]).J. B. Jensen, X. Yu, R. Rodes, and I. Tafur Monroy are with the DTU

Fotonik - Department of Photonics Engineering, Technical University ofDenmark, 2800 Kgs. Lyngby, Denmark (e-mail: [email protected]).M. Ortsiefer and C. Neumeyr are with Vertilas GmbH, 85748 Garching,

Germany (e-mail: [email protected]).Digital Object Identifier 10.1109/JSAC.2011.110616.

communications [1]. UWB is defined as any radio signalwith a fractional bandwidth of at least 0.20, or a 10-dBbandwidth of at least 500 MHz [2]. UWB uses regulatedspectrum from 3.1 to 10.6 GHz [3] supporting fourteenbands 528-MHz-wide-each as specified in the ECMA-368standard [4]. Maximum capacity in actual UWB equipmentis 480 Mbit/s per band (WiMedia specification v1.2 [3]). Thisgives an overall capacity of 6.72 Gbit/s per user when thefourteen bands are combined. This capacity is supported insingle-chip UWB implementations [5]. The maximum theo-retical UWB capacity would be achieved when the fourteenUWB bands are used bearing 1024 Mbit/s each (WiMediaspecification v1.5 [3]) giving 14.336 Gbit/s aggregated bitrateper user. Nevertheless, no commercial equipment to datesupports this configuration.This paper proposes the operation of UWB in the 60-GHz

band and evaluates the performance when vertical-cavitysurface-emitting lasers (VCSELs) are employed forelectro-optical conversion. Several technologies in the60-GHz band capable of multi-Gbit/s capacity have beenproposed in the last years. Some of them are WirelessHD(Jan. 2008), ECMA-387 (Dec. 2008), IEEE 802.15.3c(Oct. 2009), WiGig (Jul. 2010), and IEEE 802.11.ad. Someof them target wireless streaming consumer electronics, e.g.WirelessHD [6], whereas ECMA-387 and IEEE 802.15.3ctarget wireless personal area networks (WPAN). An interestingcharacteristic of ECMA-387 is that single-chip solutions areavailable [7], giving the advantage of reduced board spacerequirements and lower manufacturing cost.UWB in the 60-GHz band is interesting for several reasons:

1) The unlicensed frequency range regulated for generic60-GHz radio worldwide (57−66 GHz in Europeand Australia, 57−64 GHz in the U.S. and Canada,59−66 GHz in Japan) can allocate very well the UWBbandwidth in current regulation (up to 7.5 GHz).

2) UWB is a mature technology with efficient softwareand single-chip solutions are also available [5], [8]. Thispermits UWB to be introduced in devices with specificspace and power requirements, like mobile phones.

3) UWB is, in origin, a coexistence technology. TranslatingUWB technology from the 3.1−10.6 GHz band to the

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1296 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 29, NO. 6, JUNE 2011

60-GHz band opens the opportunity of coexistence withother wireless transmissions in the band.

4) UWB operation in the 60-GHz band permits extend-ing the transmission reach, provided that the increasedatmospheric attenuation [9] is compensated increasingthe equivalent isotropic radiated power (EIRP) spectraldensity over −41.3 dBm/MHz, as in current UWBregulation worldwide, to 13 dBm/MHz, as permitted inregulation in force in the band [10].

Regarding the application scenario, UWB in the 60-GHzband has been indicated as a viable approach to providemulti-Gbit/s WPAN connectivity in scenarios where interfer-ence is a critical issue like aircrafts [11]. UWB WPAN in the60-GHz band can benefit from radio-over-fiber technology tointerconnect the large number of remote antenna units in UWBdistributed antenna systems. The high bandwidth offered bythe optical media enables a large number of UWB users ina single fiber. Furthermore, remote antenna units are signif-icantly simplified by centralizing modulation and frequencyup-conversion, and multi-standard wireless services are sup-ported on the same infrastructure. Finally, radio-over-fibertransmission of UWB signals (UWB-over-fiber) has beenindicated as an interesting solution for fiber-to-the-home(FTTH) access networks delivering high-definition (HD)audio/video [12]. Cost-effective standard single-mode fiber(SSMF) is widely used for FTTH with distances up to about40 km [13]. Recently-developed bend-insensitive single-modefiber (BI-SMF), which is backwards compatible with SSMFmaintaining the transmission properties of SSMF, open up aninteresting opportunity for UWB-over-fiber to be deployed atin-home environments. Several manufacturers offer BI-SMFby the time of writing [14]–[16]. Compared with SSMF,BI-SMF presents much lower bending loss at lower bendradius and eases indoor wiring in FTTH scenarios. BI-SMFfacilitates fiber installation where corners, twists and staplesare required, thus permitting easy installation at reducedcost [17]. BI-SMF is also expected to reduce the size ofthe fiber installation, optical cabinets and associated wiringinfrastructure.Fig. 1 depicts an example of the UWB radio-over-fiber

approach integrating the SSMF-based FTTH access networkand BI-SMF-based in building optical user distribution. Inthe example, UWB radio provides raw data connectivity, HDaudio/video and also connectivity to an UWB-enabled cellphone. The figure shows a head-end unit responsible of thecentral generation of the UWB signal. At the user premises,BI-SMF extends the optical access distribution to the remoteantenna units where the received UWB signals are photode-tected, filtered, amplified, and radiated to a UWB-enabledtelevision set [18] or computer [19].The approach in Fig. 1 is transparent to the specific UWB

implementation employed. Two main UWB implementationsare currently used: Pulse modulation (impulse-radio UWB)which is not channelized in force and multi-band orthogonalfrequency-division multiplexing (OFDM UWB) as specifiedin the ECMA-368 standard [4]. OFDM provides high spectralefficiency and substantial control over the use of the spectrum(e.g. by carrier nulling, time-frequency hopping, and powercontrol) with a simplified system design to facilitate coex-

core networkor MAN SSMF

SSMF

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Fig. 1. Radio-over-fiber system integrating FTTH optical access distributionand in-building optical-radio transmission of multi-standard multi-Gbit/sUWB signals for HD contents provision. MAN: Metropolitan area network.RAU: Remote antenna unit.

istence with other wireless technologies [20]. Furthermore,there is large market availability of low-cost OFDM-basedUWB solutions. On the other hand, impulse-radio UWB isflexible in terms of spectrum and bitrate and it is capable ofproviding simultaneous communications and high-resolutionranging [21].This work compares the experimental performance af-

ter radio-over-fiber and further wireless transmission in the60-GHz radio band of two major UWB implementations at1.44 Gbit/s: Standard OFDM based on dual-carrier mod-ulation (DCM-OFDM) and impulse radio based on binaryphase-shift keying modulation (BPSK-IR). This work tar-gets to give light on the best implementation for futureUWB systems in the 60-GHz band. Different fiber typesare evaluated including SSMF and BI-SMF. Direct modu-lation of a low-cost free-running uncooled VCSEL is em-ployed for electro-optical conversion of the UWB signals.Optical frequency up-conversion is performed employingrelaxed-frequency electro-optical components at the head-endunit in order to simplify the remote antenna units. Simula-tion analysis is performed to verify the experimental mea-surements. The system proposed could operate in a dual3.1−10.6-GHz/60-GHz configuration if desired, but dual op-eration is out of the scope of this analysis. 60-GHz bandoperation would re-use and extend UWB technology in termsof range and flexibility, and is the focus of this work.

II. ULTRA-WIDEBAND RADIO-OVER-FIBER IN THE

60-GHZ BAND

A. Experimental Setup

The experimental setup of a UWB radio-over-fiber systemin the 60-GHz band employing a directly-modulated VCSELis shown in Fig. 2. At the head-end unit, a 1550-nm 10-Gbit/sVCSEL (VERTILAS GmbH) [22] is directly modulated by theUWB signal. Frequency up-conversion to the 60-GHz band isperformed by driving a Mach-Zehnder electro-optical intensitymodulator (Vπ of 3.7 V, 3-dB bandwidth of 35 GHz, chirpof −0.7) by a local oscillator signal multiplied by 2. Themodulator is biased to perform optical carrier suppressionwhich relaxes the RF frequency requirement whereas mitigatesthe RF power fading caused by fiber chromatic dispersion [23].The peak-to-peak amplitude of the signal driving the modu-lator is 3.5 V. Subsequently, the optical signal is distributedover optical fiber to the remote antenna units where the UWB

BELTRAN et al.: PERFORMANCE OF 60-GHZ UWB RADIO-OVER-FIBER AND WIRELESS TRANSMISSION 1297

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Fig. 2. Experimental setup and principle of operation of a UWB radio-over-fiber system in the 60-GHz band employing direct modulation of a VCSEL.Amp: Electrical amplifier. BT: Bias tee. PC: Polarization controller. MZM: Mach-Zehnder modulator. LO: Local oscillator. OBPF: Optical band-pass filter.VOA: Variable optical attenuator. PD: Photodetector. PS: Phase shifter. DSO: Digital storage oscilloscope.

signal is up-converted to the 60-GHz band after photodetection(u2t Photonics, XPDV3120R). FTTH distribution over 25 kmand 40 km of SSMF and integrated FTTH and in-buildingdistribution over 25 km of SSMF extended by 500 m ofBI-SMF (OFS, EZ-bend) is considered. The UWB signalin the 60-GHz band is amplified and filtered before beingapplied to an antenna for 5 m of wireless transmission. Theantennas are rectangular horn antennas with frequency rangeof 50−75 GHz, 20-dBi gain and 3-dB beamwidth of 12°.The Erbium-doped fiber amplifier (EDFA) EDFA #1 in Fig. 2(average output power of 15 dBm) is used to compensatefor the insertion loss of the modulator. The second EDFA,EDFA #2 in Fig. 2, sets a maximum average power at thephotodetector input (henceforth referred to as received opticalpower) of 10 dBm. A variable optical attenuator decreasesthe power level to analyze the performance as a functionof the received optical power. An optical band-pass filter(3-dB bandwidth of 0.8 nm) is used to suppress amplifiedspontaneous emission (ASE) noise.At the receiver, the received UWB signal in the 60-GHz

band is amplified and filtered, and down-converted by elec-trical mixing (RF bandwidth of 55−65 GHz, IF bandwidthof DC−10 GHz, conversion loss of 4.6 dB). The same localoscillator signal used to drive the modulator at the transmittermultiplied by 4 (output frequency of 58.6−62.2 GHz) isemployed for frequency down-conversion. A phase shifter isemployed for assuring proper phase matching for accuratedown-conversion. The down-converted signal is captured bya real-time digital storage oscilloscope (13-GHz bandwidth,40-GS/s sampling rate) and analyzed by digital signal pro-cessing (DSP).VCSEL driving, i.e. bias current and UWB peak-to-peak

voltage, local oscillator frequency, and amplification and fil-tering stages in the 60-GHz band (TX and RX Amp+Filteringin Fig. 2) are configured differently for DCM-OFDM UWBand BPSK-IR UWB. The different configurations employedgive the best performance in the optical back-to-back (B2B)configuration at the maximum received optical power of10 dBm.

B. DCM-OFDM Performance

The DCM-OFDM UWB signal at point (1) in Fig. 2, shownin Fig. 3, is generated by combining the outputs of three

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Fig. 3. RMS average spectrum of the generated DCM-OFDM UWB signalat point (1) in Fig. 2 (resolution bandwidth: 1 MHz).

standard UWB transmitter modules (Wisair). The modulessupport the UWB Band Group #1 [4]. The time-frequencycodes TFC5, TFC6 and TFC7 are selected for each mod-ule respectively. In this way, each module transmit in theBand #1 (3.168−3.696 GHz, 3.432-GHz central frequency),Band #2 (3.696−4.224 GHz, 3.96-GHz central frequency) andBand #3 (4.224−4.752 GHz, 4.488-GHz central frequency) ofthe UWB Band Group #1, respectively. The time-frequencycodes employed perform Fixed Frequency Interleaving (FFI),i.e. the information is transmitted in each band all the time.The FFI configuration maximizes bitrate compared with theTime Frequency Interleaving (TFI) (or frequency hopping)configuration which minimizes interference. The maximumbitrate of 480 Mbit/s is configured for each band, whichis achieved employing DCM data modulation, providing anaggregated bitrate of 1.44 Gbit/s and a spectral efficiency of0.91 bit/s/Hz.The bias current and UWB peak-to-peak voltage applied

to the VCSEL are set to 750 mVpp and 9.8 mA, respec-tively. The local oscillator frequency is set to 16.125 GHzso that the DCM-OFDM UWB signal is up-converted to64.5 GHz. The configuration of the RF amplification andfiltering block at the remote antenna units is a band-pass filter(58.125−61.875 GHz) and two low-noise amplifiers with again of 18.7 dB and 16.2 dB, respectively. The configurationat the receiver is a high-power amplifier (28.7 dB gain) and aband-pass filter (57.5−62.5 GHz).The performance of the demodulated DCM-OFDM UWB

signal at point (3) in Fig. 2 is evaluated by the errorvector magnitude (EVM) parameter. The EVM is mea-sured on the constellation diagram for each frequency band,Band #1, Band #2 and Band #3 in Fig. 3, employing


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M (d

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(c) (d)

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Fig. 4. Performance measured at point (3) in Fig. 2 of the DCM-OFDMUWB radio-over-fiber system in the 60-GHz band combining optical and 5-mwireless transmission. (a), (b) UWB Band #1 in Fig. 3. (c), (d) UWB Band #2in Fig. 3. (e), (f) UWB Band #3 in Fig. 3.

commercially-available software (Agilent, 89600-Series Vec-tor Signal Analyzer). The EVM of the DCM-OFDM UWBsignal in the 60-GHz band combining optical and 5-m wirelesstransmission is shown in Fig. 4 as a function of the receivedoptical power. Six optical transmission cases are shown: 500 mof BI-SMF, 25 km of SSMF, 40 km of SSMF, 25 km ofSSMF compensated by 25 km of inverse dispersion fiber(IDF), 40 km of SSMF readjusting the VCSEL bias currentfrom 9.8 mA to 13.85 mA, and 40 km of SSMF compen-sated by dispersion compensating fiber (DCF) equivalent tocompensation of 40-km SSMF. The EVM in Fig. 4 is limitedby electrical noise at low received optical power. Decreasingthe received optical power further increases the EVM due tothe reduction in signal-to-noise ratio (SNR). In addition, theEVM improves at low received optical power with respect tothe optical B2B configuration after optical transmission over25 km of SSMF, as shown in Fig. 4(a), (c) and (e), and40 km of SSMF, as shown in Fig. 4(b), (d) and (f). Thisis ascribed to gain in the fiber transfer function induced bythe interaction of the chirp of the directly-modulated VCSELwith fiber chromatic dispersion [24], as verified in Section III.The gain is dependent on the fiber length. The increase ofpower level after 25-km and 40-km SSMF transmission withrespect to B2B as well as its dependence on fiber length can beverified in Fig. 5(a), (c) and (d). The minimum EVM obtainedat high received optical power is degraded after 25-km and40-km SSMF transmission with respect to the optical B2B

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Fig. 5. RMS average spectrum of the demodulated DCM-OFDM UWBsignal at point (3) in Fig. 2 combining optical and 5-m wireless transmission(resolution bandwidth: 5 MHz). (a) B2B at −2.5-dBm received optical power.(b) 500-m BI-SMF at −3 dBm. (c) 25-km SSMF at −2.7 dBm. (d) 40-kmSSMF at 2.3 dBm.

configuration. This is due to signal distortion by the fibertransfer function induced by chromatic dispersion and furthermodified by the chirp of the VCSEL [24]. Signal distortion andits dependence on frequency and fiber length can be observedin Fig. 5(c) and (d).

Signal distortion prevents recovering the completeDCM-OFDM UWB signal in the 60-GHz band after 25 kmof SSMF, as shown in Fig. 4(e), and also after 25 kmof SSMF extended by 500 m of BI-SMF, not shown inFig. 4 for simplicity. Successful recovery of the completeDCM-OFDM UWB signal in the 60-GHz band is achievedemploying dispersion management by matched IDF, asshown in Fig. 4(a), (c) and (e). The 25-km SSMF linkis compensated by 25 km of IDF. Compared with DCF,IDF is more suitable for being used as transmission fiber,thus extending the FTTH network reach [25]. Similar EVMperformance is obtained in the three DCM-OFDM UWBbands. The EVM improves by approximately 2 dB dependingon the received optical power with respect to the optical B2Bconfiguration. Equivalently, the optical receiver sensitivityimproves by 1.5 dB depending on the EVM threshold.The optical receiver sensitivity at EVM below −17 dB[4] is −2.1 dBm. The EVM improvement with respect tothe optical B2B configuration is ascribed to gain in thefiber transfer function induced by the interaction of theVCSEL chirp with a residual fiber chromatic dispersion, asa similar EVM improvement is obtained after transmissionover 500 m of BI-SMF, as also shown in Fig. 4(a), (c)and (e). The power level increases after 500-m BI-SMFtransmission with respect to the optical B2B configuration, ascan be verified in Fig. 5(a) and (b). The DCM-OFDM UWBsignal in the 60-GHz band is successfully distributed over500 m of BI-SMF, which is sufficient for most in-buildingnetworks, with an optical receiver sensitivity of −2.1 dBm atEVM<−17 dB. The signal is not distorted as the minimumEVM obtained at high received optical power is not degradedwith respect to the optical B2B configuration. This can alsobe verified in Fig. 5(a) and (b).


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Fig. 6. Peak spectrum of the DCM-OFDM UWB signal in the 60-GHzband at point (2) in Fig. 2 (video bandwidth: 10 MHz). RF carrier frequency:64.5 GHz. (a) B2B at 5-dBm received optical power. (b) 500-m BI-SMF at4.5 dBm. (c) 25-km SSMF at 4.8 dBm. (d) 40-km SSMF at 2.3 dBm.

The three DCM-OFDM UWB bands are recovered after40-km SSMF transmission, as shown in Fig. 4(b), (d) and(f), however with poor quality. The EVM after 40-km SSMFtransmission at high received optical power can be improvedby readjusting the VCSEL driving with respect to the opticalB2B configuration. The EVM is improved due to the read-justment of the VCSEL chirp, without employing dispersioncompensation or management of the VCSEL chirp [26]–[28].This is shown in Fig. 4(b), (d) and (f) for readjustment of thebias current applied to the VCSEL from 9.8 mA to 13.85 mA.Considering an EVM threshold of−17 dB, the optical receiversensitivity for successful recovering of the three DCM-OFDMUWB bands is 1 dBm, limited by the signal distortion in theUWB Band #1. This corresponds to a power penalty of 1.3 dBwith respect to the optical B2B configuration.The expected performance employing dispersion compen-

sation by DCF is also investigated. The 40-km SSMF linkis compensated by 6.5 km of DCF which has dispersionequivalent to compensation of 40 km SSMF. The EVM inthe three DCM-OFDM UWB bands improves at high receivedoptical power and is degraded at low received optical powerwith respect to the uncompensated 40-km link, as shownin Fig. 4(b), (d) and (f). The optical receiver sensitivity atEVM<−17 dB is 1 dBm limited by the UWB Band #3,corresponding to a power penalty of 1.6 dB with respectto the optical B2B configuration. It should be noted thatthe successful transmission after dispersion compensation byIDF or DCF with EVM<−17 dB is achieved increasing theamplification EDFA #2 in Fig. 2 by 5 dB to compensate forthe increased loss of the combined fiber link.Additionally, the impact of the gain of the receiving antenna

on performance is studied. Fig. 4(a) shows the EVM of theDCM-OFDM UWB Band #1 after 25-km SSMF transmissionwhen a Cassegrain antenna with 39.7-dBi gain is employedat receiver instead of the 20-dBi antenna. The optical receiversensitivity improves by 1.3 dB at low received optical powerwhereas the minimum EVM is maintained at high receivedoptical power.

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Fig. 7. Constellation diagrams of the demodulated DCM-OFDM UWBBand #1 at point (3) in Fig. 2 combining optical and 5-m wireless transmis-sion. (a) B2B, EVM= −19.5 dB at 5-dBm received optical power. (b) 500-mBI-SMF, EVM= −16.1 dB at −3 dBm.

Fig. 6 shows examples of DCM-OFDM UWB spectra inthe 60-GHz band measured at point (2) in Fig. 2. The powerlevel decreases by approximately 2 dB as the received opticalpower decreases by 1 dB. The signal spectrum meets currentregulation in the 60-GHz band [29] in all configurations.However, spectra of noise and distortion are observed due toimperfect filtering. These spectra could cause interference toother radio signals in the current 60-GHz band (57−66 GHz)or to future frequencies outside 57−66 GHz, particularlyaround 52 GHz. The undesired spectra could be reducedby adequate filtering in practice or by system design. Theundesired spectra do not impact on performance as this isevaluated over the bandwidth of each DCM-OFDM UWBband.Fig. 7 shows examples of DCM-like constellation diagrams

at point (3) in Fig. 2. Signal degradation translates into moredisperse constellation points thus degrading the EVM. Theconstellation diagrams in Fig. 7 confirm the good perfor-mance with EVM<−17 dB of the radio-over-fiber system forgeneration with combined fiber and wireless transmission ofhigh-quality DCM-OFDM UWB signals in the 60-GHz band.

C. BPSK-IR Performance

The BPSK-IR UWB signal at point (1) in Fig. 2 is generatedby an arbitrary waveform generator (Tektronix, AWG 7122B)at 23.04 GS/s. A pseudo random bit sequence (PRBS) of211−1 word length at 1.44 Gbit/s is employed. The BPSK-IRUWB pulse is a fifth-order derivative Gaussian shape with astandard deviation of 0.068 ns so as to be in good compliancewith the UWB EIRP spectral density mask [2] with the highestspectral efficiency of 0.28 bit/s/Hz [30]. In this way, the systemcould operate in dual band 3.1−10.6 GHz/60 GHz as thebaseband signal, which is also available after photodetection,could be radiated meeting current UWB regulation [31]. TheBPSK-IR UWB signal, shown in Fig. 8, comprises a singleband (3.26−8.45 GHz at 10 dB, 5.58-GHz peak frequency).BPSK data modulation is employed in order to avoid spectralpeaks at multiples of the bitrate, which limit UWB reach [31],as verified in Fig. 8(b).The bias current and UWB peak-to-peak voltage applied to

the VCSEL are set to 820 mVpp and 13 mA, respectively.The local oscillator frequency is set to 16.165 GHz so thatthe BPSK-IR UWB signal is up-converted to 64.66 GHz. Theconfiguration of the RF amplification and filtering block at theremote antenna units is a low-noise amplifier (18.7-dB gain),


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Fig. 8. Programmed BPSK-IR UWB signal applied to the AWG. (a) Partof the signal in the time domain corresponding to the bit sequence 10110.(b) Normalized power spectral density of the signal (solid line) and UWBEIRP spectral density indoor mask [2] (dashed line).

a band-pass filter (58.125−61.875 GHz) and a high-poweramplifier (28.7-dB gain). The configuration at the receiver isa high-power amplifier (28.7-dB gain), a low-noise amplifier(16.2-dB gain) and a band-pass filter (57.5−62.5 GHz).The performance of the demodulated BPSK-IR UWB sig-

nal at point (3) in Fig. 2 is evaluated by the bit-error-rate(BER) parameter. BER is measured on the eye diagramsemploying off-line custom DSP. The DSP software consistsof re-sampling by a factor of 1.008, low-pass filtering at acut-off frequency optimum for each configuration (5.1 GHz forB2B, 6.5 GHz for 25-km SSMF and 25-km SSMF extendedby 500-m BI-SMF, and 5 GHz for 40-km SSMF), matchedfiltering with the original UWB pulse shape, bit synchro-nization and calculation of the optimum decision threshold.The BER is calculated by bit-by-bit comparison with theoriginal PRBS over 120,000 bits in all configurations. BERperformance of the BPSK-IR UWB signal in the 60-GHz bandcombining optical and 5-m wireless transmission is shownin Fig. 9 as a function of the received optical power. Threeoptical transmission cases are considered: 25 km of SSMF,25 km of SSMF extended by 500 m of BI-SMF, and 40 kmof SSMF. Successful recovery of the BPSK-IR UWB signalwith BER below the limit of 2.2⋅10−3 employing forwarderror correction (FEC) is achieved in all fiber configurations.Very low optical receiver sensitivities of −12.5 dBm and−15.6 dBm after 25 km and 40 km of SSMF, respectively,and −14.3 dBm after 25 km of SSMF extended by 500 mof BI-SMF are obtained. This corresponds to an improvementof 0.1 dB, 3.2 dB and 1.9 dB, respectively, compared withoptical B2B. The BER is limited by electrical noise in Fig. 9.Decreasing the received optical power further increases theBER due to reduction in SNR. In addition, the improvement inoptical receiver sensitivity after fiber transmission is ascribedto gain in the fiber transfer function induced by the interactionof the chirp of the directly-modulated VCSEL with the fiberchromatic dispersion [24], as verified in Section III.Note that the same VCSEL driving adjusted initially in the

optical B2B configuration has been employed in all fiber con-figurations. Nevertheless, optimization of the VCSEL drivingin a given fiber configuration could lead to different perfor-mance, as has been shown in Section II.B for DCM-OFDMUWB.Fig. 10 shows examples of BPSK-IR UWB spectra in the

60-GHz band at point (2) in Fig. 2. Residual spectral lines atfrequencies multiple of the bitrate are caused by asymmetryof the BPSK pulses. This is due to the nonlinear transfer

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-log(

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Fig. 9. Performance measured at point (3) in Fig. 2 of the BPSK-IRUWB radio-over-fiber system in the 60-GHz band combining optical and5-m wireless transmission. The FEC limit of 2.2⋅10−3 is shown via a dashedline.

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ower

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Fig. 10. Peak spectrum of the BPSK-IR UWB signal in the 60-GHz bandat point (2) in Fig. 2 (video bandwidth: 10 MHz). RF carrier frequency:64.66 GHz. (a) B2B at −9.8-dBm received optical power. (b) 25-km SSMFat −15 dBm. (c) 40-km SSMF at −17.5 dBm.

function of the VCSEL and distortion from fiber dispersion.The power level decreases by approximately 2 dB as thereceived optical power decreases by 1 dB. The increase ofpower level after fiber transmission with respect to the opticalB2B configuration as well as its dependence on fiber lengthcan be verified in Fig. 10(a) and (c). The signal spectrummeets current regulation in the 60-GHz band [29] in allconfigurations. However, residual RF carrier at 64.66 GHz andnoise spectrum around 52 GHz are observed due to imperfectfiltering. These spectra could cause interference to other radiosignals in the current 60-GHz band (57−66 GHz) or to futurefrequencies around 52 GHz. The undesired spectra could bereduced by adequate filtering in practice or by system design.The undesired spectra do not impact on performance becausefrequency down-conversion is done with the same RF carrier,and DSP low-pass filtering is included at the receiver.BPSK-like eye diagrams at point (3) in Fig. 2 are shown

in Fig. 11. The open eye diagrams in Fig. 11, especiallyafter 40 km of SSMF, confirm the excellent performanceshown in Fig. 9. Signal degradation closes the eye diagramresulting in increased BER. At −12.7 dBm received opticalpower, the gain in the fiber transfer function after transmissionover 25-km SSMF+500-m BI-SMF enables recovering theBPSK-IR UWB signal at BER<2.2⋅10−3 at received opticalpower lower than in the B2B configuration for the sameperformance, unlike the 25-km SSMF configuration, as shownthe eye diagrams in Fig. 11(b) and (c). In addition, the gain


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Fig. 11. Eye diagrams of the demodulated BPSK-IR UWB signal atpoint (3) in Fig. 2 combining optical and 5-m wireless transmission. (a) B2B,BER= 8.57⋅10−6 at −10-dBm received optical power. (b) 25-km SSMF,BER= 3.4⋅10−3 at −12.7 dBm. (c) 25-km SSMF extended by 500-m BI-SMF,BER= 3.43⋅10−5 at −12.7 dBm. (d) 40-km SSMF, BER= 1.01⋅10−4 at−14.7 dBm.

in the fiber transfer function after 40-km SSMF transmissiondoes not compensate for the received optical power reductionat −14.7 dBm with respect to the optical B2B configurationat −10 dBm, as shown the eye diagrams in Fig. 11(a) and (d).Furthermore, Fig. 11(c) and (d) exhibit similar maximum eyeopening resulting in similar BER performance.

III. SIMULATION ANALYSIS

Performance improvement after dispersive fiber transmis-sion compared with the optical B2B configuration has beenachieved in the experiments for both DCM-OFDM andBPSK-IR UWB signals. This is likely assisted by the chirpcharacteristics of the directly-modulated VCSEL [24], as hasbeen pointed out in Section II.B and II.C. Similarly, per-formance improvement after 20-km SSMF transmission ofon-off keying (OOK) baseband signals is observed in [28].Chirp management of a directly-modulated distributed feed-back (DFB) laser is performed by a tunable optical filterin [28]. In order to verify the experimental measurements, asimulation model for the UWB radio-over-fiber system in the60-GHz band has been developed employing the commercialsimulation tool VPITransmissionMaker (version 8.3).A single-mode rate-equation model of a VCSEL is em-

ployed. The characterized power- and voltage-bias currentcurves and thermal frequency shift of the VCSEL are in-cluded in the model. Furthermore, we previously verifiedthat the model for the VCSEL can reproduce the behaviorof the real VCSEL by measuring the output of the VCSELin the time and frequency domains under modulation witha non-return-to-zero (NRZ) PRBS of 27−1 word length at7.5 Gbit/s. 7.5 Gbit/s was chosen in order to have similarspectral width in the simulations and the experiments. Dueto the fifth derivative Gaussian pulse shape, the spectralproperties, and therefore also the chirp- and dispersion-related

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B2B 25 km SSMF 25 km SSMF+500 m BI-SMF 40 km SSMF

Received optical power (dBm)

-log(

BER

)

Fig. 12. Performance simulated at point (3) in Fig. 2 of the BPSK-IRUWB radio-over-fiber system in the 60-GHz band combining optical and5-m wireless transmission, to be compared with Fig. 9. The FEC limit of2.2⋅10−3 is shown via a dashed line.

characteristics are more similar to 7.5-Gbit/s OOK leadingto more accurate results. Parameters of the VCSEL modelgiving a simulated output of the VCSEL very similar to thatmeasured are found by multi-parameter sweep at a given drivevoltage and bias current. Simulation results are obtained at thesame drive voltage and bias current as in the experiment andsetting the corresponding VCSEL parameters obtained fromthe sweep.Simulated BER of the BPSK-IR UWB signal in the 60-GHz

band is shown in Fig. 12 combining optical and 5-m wirelesstransmission which is modeled as free-space loss. The sameoptical transmission cases as in Fig. 9 are considered: 25 kmof SSMF, 25 km of SSMF extended by 500 m of BI-SMF, and40 km of SSMF. Simulated BER exhibits the same behavioras experimental BER shown in Fig. 9. Furthermore, it isverified that no performance improvement is obtained afterfiber transmission with respect to optical B2B when the chirpof the VCSEL is disabled in simulation.

IV. CONCLUSION

UWB radio-over-fiber in the 60-GHz band for providingmulti-Gbit/s integrated long-reach FTTH and WPAN connec-tivity has been proposed and experimentally demonstrated.Radio-over-fiber transmission over 40-km SSMF without anydispersion compensation and further 5-m wireless trans-mission has been demonstrated for both DCM-OFDM andBPSK-IR UWB signals at 1.44 Gbit/s in the 60-GHz band.The BPSK-IR UWB implementation is more tolerant to fiberimpairments requiring lower received optical power than theDCM-OFDM UWB counterpart. Experimental results verifiedby simulation show that the UWB radio-over-fiber systemin the 60-GHz band can benefit from the chirp of low-costdirectly-modulated VCSELs to increase optical receiver sen-sitivity.We believe that these results underpin the flexibility of

UWB signaling not only in the 3.1−10.6 GHz band, but alsoin the 60-GHz band.

ACKNOWLEDGMENT

The authors would like to thank Nortelco and Tektronix forsupplying the arbitrary waveform generator.


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[10] 60 GHz Multiple-Gigabit WAS/RLAN Systems, ETSI Std. EN 302 567,Rev. 1.1.1, Mar. 2009.

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[12] R. Llorente et al., “Ultra-wideband radio signals distribution in FTTHnetworks,” IEEE Photon. Technol. Lett., vol. 20, no. 11, pp. 945–947,Jun. 2008.

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International Conference on Transparent Optical Networks (ICTON’10),Munich, Germany, Jun. 27/Jul. 1, 2010, paper Tu.A3.1.

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[17] M.-J. Li et al., “Ultra-low bending loss single-mode fiber for FTTH,”in Optical Fiber Communication Conference and Exposition (OFC’10),San Diego, CA, USA, Mar. 21–25, 2010, paper PDP10.

[18] Ultra Thin LCD TV - The UT Series. Hitachi Ltd. [Online]. Available:http://www.hitachi.com/New/cnews/071023.html

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in Optical Fiber Communication Conference and Exposition (OFC’09),San Diego, CA, USA, Mar. 22–26, 2008, paper JThA32.

[28] J. Yu et al., “42.8 Gbit/s chirp-managed signal transmission over 20kmstandard SMF at 1550nm without DCF,” Electron. Lett., vol. 43, no. 23,Nov. 2007.

[29] Operation within the band 57-64 GHz, FCC Rep. FCC 15.255, Oct.2002.

[30] H. Sheng, P. Orlik, A. M. Haimovich, L. J. Cimini, and J. J. Zhang, “Onthe spectral and power requirements for ultra-wideband transmission,”in 2003 IEEE International Conference on Communications (ICC’03),Anchorage, Alaska, USA, May 11–15, 2003, pp. 738–742.

[31] J. B. Jensen, R. Rodes, M. Beltran, and I. T. Monroy, “Shared medium2 Gbps baseband & 2 Gbps UWB in-building converged optical/wirelessnetwork with multimode fiber and wireless transmission,” in 36th Euro-pean Conference and Exhibition on Optical Communication (ECOC’10),Torino, Italy, Sep. 19–23, 2010, pp. 827–829.

Marta Beltran received the M.Sc. degree intelecommunication engineering and PostgraduateMasters degree in communications technologies,systems and networks from the UniversidadPolitecnica de Valencia (UPV), Valencia, Spain,in 2006 and 2007, respectively, and is currentlyworking toward the Ph.D. degree at UPV.Since 2004, she has been a Research Techni-

cian with the Valencia Nanophotonics TechnologyCenter, UPV. She has participated in several na-tional and European research projects including

FP7-IST-UCELLS and FP7-ICT-FIVER. She has collaborated in teachingat the UPV in optical communications and electronics. Her research interestsinclude hybrid wireless-optical access networks, radio-over-fiber technologies,ultra-wideband radio technologies, optical generation and processing of RFsignals, and pulsed laser sources.

Jesper Bevensee Jensen received his PhD in 2008from the Technical University of Denmark, DTUFotonik.Since 2008 he has been employed as a post-

doc at DTU Fotonik, involved in the Europeanproject ICT-ALPHA. His research interests includeadvanced modulation formats, access and in-homenetwork technologies, wireless-wireline integration,ultrawideband-over-fiber, polymer optical fibers, andcoherent access technologies.

Xianbin Yu received the M.S. degree from TianjinUniversity, Tianjin, China, in 2002, and the Ph.D.degree from Zhejiang University, Hangzhou, China,in 2005. From October 2005 to October 2007,he was a Postdoctoral Researcher with TsinghuaUniversity, Beijing, China. In November 2007, hebecame a Postdoctoral Research Fellow with DTUFotonik, Technical University of Denmark, Lyngby,Denmark, where he is currently an Assistant Profes-sor.He co-authored one book chapter and over 60

peer-reviewed international journal and conference papers. His research inter-ests are in the areas of microwave photonics, optical fiber communications,wireless-over-fiber, ultrafast photonic wireless signal processing, and ultrahighfrequency short-range access technologies.Dr. Yu currently holds a Marie Curie international incoming fellowship

within the 7th European Community Framework Program.


Roberto Llorente received the M.Sc. degree inTelecommunication Engineering and the Ph.D. de-gree from the Universidad Politecnica de Valen-cia (UPV), Spain, in 1998 and 2006, respectively.Since then, he has been in research positions withinthe university, and in 2002 he joined the ValenciaNanophotonics Technology Center (VNTC).Currently, he is an Associate Professor of the

UPV, teaching radio-communications-related sub-jects, and he is Head of the Optical Systemsand Networks Unit at the VNTC. He has been

leading VNTC activities in the European projects FP5-IST-TOPRATEand FP6-IST-UROOF. Currently he is the Coordinator of the projectsFP7-IST-UCELLS and FP7-ICT-FIVER from January 2008 and January2010 respectively. He has authored or co-authored more than 40 papers inleading international journals and conferences and has authored three patents.His current research interest includes optical and electro-optical processingtechniques applied to optical transmission links and hybrid wireless-opticalaccess networks.

Roberto Rodes received the M.Sc. degree in Elec-trical Engineering in 2010 from the Technical Uni-versity of Denmark (DTU).He is currently working toward the Ph.D. degree

on advanced modulation of Vertical Cavity SurfaceEmitting Lasers (VCSELs) at DTU Fotonik.

Markus Ortsiefer was born in Cham, Germany, onDecember 20, 1970. He received the diploma degreein physics in 1997 and the doctoral degree in 2001,both from the Technical University of Munich.During his PhD work, he studied novel concepts

for long-wavelength VCSELs and developed theInP-based buried tunnel junction (BTJ)-VCSEL. Heis co-founder of VERTILAS where he was man-aging director from 2001 to 2003. Since 2003, heholds the position of the CTO and is responsiblefor the companies production and research activities.

Markus Ortsiefer has authored or co-authored more than 120 publications inscientific journals, conference proceedings and books and filed several patentson optoelectronic devices.Dr. Ortsiefer is a member of the German Physical Society (DPG).

Christian Neumeyr, photograph and biography not available.

Idelfonso Tafur Monroy received the M.Sc. degree in multichannel telecom-munications from the Bonch-Bruevitch Institute of Communications, St.Petersburg, Russia, in 1992, the Technology Licenciate degree in telecommu-nications theory from the Royal Institute of Technology, Stockholm, Sweden,and the Ph.D. degree from the Electrical Engineering Department, EindhovenUniversity of Technology, The Netherlands, in 1999.He is currently the Head of the metro-access and short range communica-

tions group of the Department of Photonics Engineering, Technical Universityof Denmark. He was an Assistant Professor until 2006 at the EindhovenUniversity of Technology and was an Associate Professor until Nov. 2009.Currently, he is a Professor at the Technical University of Denmark. Hehas participated in several European research projects, including the ACTS,FP6, and FP7 frameworks (APEX, STOLAS, LSAGNE, MUFINS). At themoment, he is in involved the ICT European projects Gi-GaWaM, ALPHA,BONE, and EURO-FOS. His research interests are in hybrid optical-wirelesscommunication systems, coherent detection technologies and digital signalprocessing receivers for baseband and radio-over-fiber links, optical switching,nanophotonic technologies, and systems for integrated metro and accessnetworks, short range optical links, and communication theory.

performance of a 60-ghz dcm-ofdm and bpsk-impulse ultra-wideband system with radio-over-fiber and...

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