IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 10, MAY 15, 2012 803

Low-Complexity Optical Distribution ofGb/s BPSK UWB Signals

Mehrdad Mirshafiei, Student Member, IEEE, and Leslie A. Rusch, Fellow, IEEE

Abstract— Optical transport of ultra-wideband (UWB) signalsextends the reach of these power-limited signals to severalkilometers. We propose and experimentally demonstrate a simpleand low-cost optical method to generate UWB pulses. Weuse few optical components (source/modulator/photodetector), incontrast to most other optical generation techniques. UWB pulsescomplying with the U.S. Federal Communications Commissionspectral mask are generated by modulating the intensity of acontinuous-wave laser with a combined signal consisting of thedata signal and a sinusoidal signal. For impulse radio UWB,binary phase-shift-keying is accomplished simply by adjustingthe amplitudes of the data and the sinusoidal signal. Wirelesspropagation of the UWB impulses is investigated experimentallyat a very high bit rate. The receiver is implemented using a real-time oscilloscope to capture the received waveforms followed byoffline signal processing. In particular, we consider a minimum-mean-square error equalizer to counter the multipath-inducedintersymbol interference encountered at 1.75 Gb/s. Equalizationbrings the bit-error-rate within the forward-error correction limitof 10−3 after 2.5 m of wireless propagation at 1.75 Gb/s.

Index Terms— Microwave photonics, minimum-mean-squareerror (MMSE) algorithm, radio-over-fiber, ultra-wideband(UWB).

I. INTRODUCTION

ULTRA-WIDEBAND (UWB) radio transmission hasattracted much attention since the allocation of an unli-

censed frequency band by the US Federal CommunicationsCommission (FCC) in 2002 [1]. The wireless transmissionrange of UWB systems is limited to a few meters due topower restrictions and high intersymbol interference at thedata rates above 100 Mb/s [2]. Optical fiber distribution ofUWB signals extends the reach of such systems to severalkilometers. By generating UWB pulses in the optical domain,prior to fiber transmission, we avoid extra electrical to opticalsignal conversion.

Several approaches have been proposed for optical genera-tion of UWB waveforms. Generation of on-off-keying (OOK)UWB signals is convenient in the optical domain by using aMach–Zehnder modulator (MZM) [3], [4]. However, binaryphase-shift-keying (BPSK) has better error performance. Bi-phase modulation of Gaussian monocycle pulses was shown

Manuscript received September 9, 2011; revised January 18, 2012; acceptedFebruary 7, 2012. Date of publication February 20, 2012; date of currentversion April 13, 2012. This work was supported in part by TELUS Corpora-tion and in part by the Canadian Natural Science and Engineering ResearchCouncil and has been presented in part at the IEEE Microwave PhotonicsConference.

The authors are with the Electrical and Computer Engineering Department,Center for Optics, Photonics, and Lasers, Université Laval, Québec, QC G1V0A6, Canada (e-mail: mehrdad.mirshafiei.1@ulaval.ca; rusch@gel.ulaval.ca).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2012.2188502

in [5] based on a phase modulator and an asymmetric Mach–Zehnder interferometer. In [6], a 781.25-Mb/s binary BPSKUWB transmission system exploited relaxation oscillations ofa distributed feedback (DFB) laser. The bit-error rate (BER)results showed that 30 km of fiber transmission introducedno penalty, however, the pulses in [5], [6] were not FCCcomplaint. An arbitrary waveform generator was used in [7]to transmit BPSK UWB signals over a one meter wirelesschannel. A digital coherent receiver was employed, involvinga local oscillator laser beating with the signal in a 90° opticalhybrid. Two pairs of balanced photodiodes converted the signalto the electrical domain. Digital signal processing was per-formed afterwards to compensate fiber chromatic dispersion.

In this letter, we show a very cost-effective method ofUWB pulse generation by only using an optical source, anexternal modulator and a photodetector. This method is easilyapplicable to passive optical networks (PON). The data isadded to a sinusoidal waveform and the summation modulatesthe light intensity using a MZM. We will show that withproper choice of data amplitude, sinusoidal signal amplitudeand MZM bias point, BPSK UWB signals can be generatedat 1.75 Gb/s and lying within the UWB frequency band.Optical signals are converted to RF and transmitted wirelessly.We experimentally investigate this method, using a realtimeoscilloscope to capture the RF signal at the receive antennafollowing wireless propagation via two UWB antennas. Weinvestigate the equivalent isotropic radiated power (EIRP) oftransmitted wireless signals. The received signal is equalizedin MATLAB using a minimum-mean-square-error (MMSE)algorithm [8] to reduce the intersymbol interference (ISI). BERperformance of the system is investigated.

While our method resembles setups for upconversion ofUWB signals using an MZM, our approach is quite different.In [9], a UWB monocycle pulse was upconverted using thesummation of the pulse and a local oscillator to modulate alaser diode via an MZM. Similarly, upconversion of orthogo-nal frequency-division multiplexing OFDM-UWB signals wasdemonstrated in [10]. In our approach we generate UWBpulses rather than upconverting previously generated UWBwaveforms.

II. EXPERIMENTAL SETUP

Fig. 1 shows the block diagram of the UWB signal generatorand the receiver structure. A continuous wave (CW) laserbiased above threshold is used as the source. A polarizationcontroller (PC) followed by a 10 GHz Mach–Zehnder modu-lator (JDSU OC-192) perform the data modulation. A powercombiner (Marki PD-0010) combines the data coming from

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804 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 10, MAY 15, 2012

Fig. 1. Schematic diagram of the experimental setup. PC: polarizationcontroller. MZM: Mach–Zehnder modulator. VOA: variable optical attenuator.SMF: single-mode fiber. PD: photodetector. LNA: low-noise amplifier. LPF:lowpass filter.

a bit error tester (BERT) with a 7 GHz sinusoidal signalgenerated from the signal generator. Note that the sinusoidalsignal also serves as the BERT clock input. A variable opticalattenuator is used to change the optical power of the pulses, inorder to measure the BER curves presented later in Section IV.Single-mode fiber (SMF) can be placed after the VOA to studythe effect of fiber transportation on the UWB pulses.

A photodetector (PD) performs the optical-to-electrical(O/E) conversion. A DC-block eliminates the DC value ofthe electrical signal. The resulting UWB pulses are amplifiedand transmitted using commercial SkyCross SMT-3TO10MAantennas. The received waveforms are amplified using a lownoise amplifier (LNA, Mini-Circuits ZVA-183-S) and capturedby a a PC-controlled 10 GHz realtime oscilloscope. Detectionand BER calculation is performed by offline digital signalprocessing (DSP) in MATLAB (Section IV).

III. THEORETICAL STUDY

The transfer function of the MZM, configured as shown inFig. 1, is expressed as

Pout = Pin cos2(

π

2Vπ(Vb + D (t) + Vm sin ωt)

)(1)

where Pin and Pout are the input and output optical powers,Vπ is the MZM halfwave voltage, Vb is the bias voltage, D(t)is the data signal, and Vm is the amplitude of the sinusoidalsignal. This transfer function can be expanded using the Besselfunctions

Pout = Pin

{1

2+ 1

2J0 (πVπ/Vm) cos

(π

Vπ(Vb + D (t))

)

−J1 (πVπ/Vm) sin

(π

Vπ(Vb + D (t))

)sin ωt

+ J2 (πVπ/Vm) cos

(π

Vπ(Vb + D (t))

)cos 2ωt + ...

}.

(2)

The second and higher order harmonics in (2) are outof the UWB band and are filtered out by the bandpassresponse of the UWB antennas. The DC term in (2) shouldbe data independent so that it can be removed using a DCblock. In particular we will find values of oscillator amplitudeVm , and data amplitude for logical zero and one (D0 andD1, respectively) such that the DC term equals 0.5Pin .

Clearly for cos( πVπ

(Vb + D)) = 0 we meet our requirement,yielding D0 = −Vπ/2 − Vb, and D1 = Vπ/2 − Vb. Thus, Thedata amplitude is Vπ while Vb and Vm are not constrained.With this choice for the data amplitude, (2) can be expressedas

Pout ={ −Pin J1 (πVπ/Vm) sin ωt D (t) = D1

Pin J1 (πVπ/Vm) sin ωt D (t) = D0 (3)

We can see that a PSK modulation format is achieved withan amplitude depending on Vm . Another solution to havinga data independent DC results in OOK modulation format aspresented in [11].

IV. EXPERIMENTAL RESULTS AND DISCUSSION

In this section, we use the setup shown in Fig. 1 alongwith the mathematical expressions developed in Section IIIto generate UWB pulses with PSK modulation format. Themodulator we used has Vπ = 3.2, requiring a data amplitudeof 3.2 V. We bias the MZM at its quadrature point. The datachanges the operating point of the modulator between thepositive and negative slopes of the modulator transfer function.The peak-to-peak of the sinusoidal signal was around 4 V. Thisvalue does not maximize the output power, but was chosenexperimentally by examining the output signal eyediagram.

The 7 GHz sinusoidal signals ensures generation of UWBsignals at the center of the FCC spectral mask. We generatethe data bit 1 with the pattern ‘1111’ and a data bit 0with the pattern ‘0000’, corresponding to a symbol rate of1.75 Gb/s. The choice for this data duration was made toachieve reasonable bit-rate and signal bandwidth for highspeed UWB applications. The patterns ‘1111’ and ‘0000’generate out-of-phase pulses with the same duration.

Fig. 2a shows the measured eyediagram of the generatedPSK UWB pulses for a pseudorandom bit sequence (PRBS) oflength 27 −1. Fig. 2b plots the measured received signals after50 cm of wireless transmission in the lab. Although the eyeis open before transmission, the ISI caused by the multipathreflections severely distorts the received signal at the very highspeed (1.75 Gb/s). The non-return-to-zero (NRZ) nature of thegenerated UWB pulses exacerbates the ISI.

Fig. 2c (gray) shows the power spectral density (PSD) of thegenerated PRBS UWB sequence measured using an electricalspectrum analyzer. The EIRP can be calculated by measuringthe antenna gain response using a vector network analyzer asexplained in [3]. The normalized EIRP is calculated and shownin Fig. 2c (blue/dark). Comparing the EIRP with the PSDof transmit waveform (Fig. 2c, gray), the second harmonic,centered at 14 GHz, has been eliminated by the antennaand the EIRP respects the FCC spectral mask. The fractionalbandwidth of the waveform is 30%.

MIRSHAFIEI AND RUSCH: LOW-COMPLEXITY OPTICAL DISTRIBUTION 805

Fig. 2. PSK signals. (a) Eye diagram of the UWB signal at the transmitter.(b) Eye diagram of the received signal after 50 cm of wireless propagation.(c) Power spectral density of the transmit pulses obtained from the electricalspectrum analyzer (gray), the normalized EIRP (blue), versus the FCC mask(red). (d) BER performance for back-to-back and 20-km SMF.

Fig. 3. BER performance of the receiver with no equalization (solid blue)and with MMSE equalizer (dashed red) are compared for several wirelessdistances.

We consider a linear receiver matched to the transmit pulseimplemented in MATLAB by processing the captured datafrom the scope. The scope takes a maximum of 220 samplesat 40 Gsamples/s. We match the captured bit sequence with thePRBS data and perform timing acquisition to find the samplingtime. This is achieved by finding the peak of correlationfunction between the PRBS and the received signal. Thethreshold is set to zero. Detection is based on hard decisionby comparing the samples with the threshold. Because of thelow number of bits, we calculate the BER from the Q-factorusing a Gaussian noise approximation.

Fig. 2d plots the BER versus the average optical power.The penalty from SMF propagation is less than 1 dB, makingthe system suitable for use in passive optical networks (PON).This penalty can be attributed to chromatic dispersion. In [7],it was shown that applying dispersion compensation reducesthe penalty to less than 0.5 dB, even for longer SMF. Nosignificant change on the PSD was observed after adding theSMF. Fig. 3 shows the BER performance of the receiver afterseveral wireless distances. With no equalization the system is

limited to 1.5 m when respecting a FEC limit of 10−3. Errorfloors are caused by the ISI. We employ an MMSE equalizerto combat the ISI. The equalizer is trained with the PRBSdata bits for an overhead of about 15% to compute the cross-correlation between received sequence and input sequence andthe autocorrelation of the received sequence. Adaptive MMSEcan be used to lower the overhead. Fig. 3 shows much betterBER performance after equalization, where the BER at 2.5 mis well within the FEC range.

V. CONCLUSION

We experimentally demonstrated a simple, low-cost opticalUWB pulse generation method. A CW laser, a Mach–Zehndermodulator, and a photodetector were the only optical compo-nents in this technique. A combination of data and a sinusoidalsignal was used to generate BPSK UWB pulses at a bit-rate of1.75 Gb/s. Fiber propagation caused no significant degradationto the pulses, validating use of such systems in PONs.

Antenna transmission measurements and EIRP calculationswere reported. Bit error rate performance of a linear receiverwas investigated by offline signal processing. We could reachthe FEC limit of 10−3 after 2.5 m of wireless propagation byemploying an MMSE equalizer.

ACKNOWLEDGMENT

The authors would like to thank R. Farhoudi for his helpwith the minimum-mean-square-error equalizer.

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