Distortion of OFDM Signals on Radio-Over-Fiber Links Integrated With an RF Amplifier and...

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 5, MARCH 1, 2008 467

Distortion of OFDM Signals on Radio-Over-FiberLinks Integrated With an RF Amplifier andActive/Passive Electroabsorption Modulators

Ju Bin Song, Member, IEEE, and A. H. M. Razibul Islam

Abstract—The integration of orthogonal frequency divisionmultiplexing (OFDM) and radio-over-fiber (RoF) techniques havemade cost-effective and high-data-rate mobile wireless Internetnetworks possible, such as wireless broadband networks. Thispaper describes the distortion effects of OFDM signals fed via anRF amplifier integrated with an RoF link employing active andpassive electroabsorption modulators (EAM) for chiefly broad-band in-building network applications. First, peak-to-averagepower ratio was investigated for RoF links. Second, the adjacentchannel power ratio, which estimates the degree of spectral re-growth due to the in-band and out-of-band interference resultingfrom distortion effects from nonlinear amplification, error vectormagnitude, and system distortion effects, was also observed forthe proposed system. In this study, we considered a combinedVolterra-series and impulse response-based analytical model forWiBro systems and compared it to the observed measurements.We analyzed the different nonlinear distortion effects for OFDMsignals driven via an RF amplifier that was integrated with anRoF link employing a distributed feedback laser as a transmitterand an active and passive InP EAM as a receiver. The results showsignificant agreement between the suggested analytical model andthe measurement case. This study is unique in that it examines thedistortion effects of RoF links with active and passive EAMs as anaccess point for OFDM-based wireless access networks.

Index Terms—Adjacent channel power ratio (ACPR), dis-tortion, error vector magnitude (EVM), orthogonal frequencydivision multiplexing (OFDM) signals, passive electroabsorptionmodulator, peak-to-average power ratio (PAPR), radio-over-fiber(RoF).

I. INTRODUCTION

WITH the natural progression of development in cel-lular communication technology, the need for multiuse

services and broadband convergence to combine voice, data,videos, and automated applications in a single device wasobvious. Orthogonal frequency division multiplexing (OFDM),a universally-adopted modulation scheme for broadband con-vergence networks, performs robustly against time-dispersiveimpairments such as multipath fading and impulsive interfer-ence and also has a spectral efficiency that rivals single-carrier

Manuscript received February 9, 2007; revised November 26, 2007. Thiswork was supported in part by the Korea Research Foundation (KRF) and inpart by Kyung Hee University.

J. B. Song and A. H. M. R. Islam are with the Telecommunications Labora-tory, Department of Electronic and Radio Engineering, Kyung Hee University,Yongin, 449-701, South Korea (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2007.915213

systems. However, since OFDM is a combination of a largenumber of subcarriers, the resulting OFDM signal has a largepeak-to-average power ratio (PAPR). The level of nonlineardistortions largely depends on the PAPR of an input signal ledto a power amplifier (PA). In a conventional class “A” linearamplifier, power efficiency is approximately an inverse linearfunction of the PAPR [1]. Since low power efficiency is unde-sirable for mobile terminals, nonlinear amplification becomesnecessary. As the in-band and out-of-band interference createdby the distortion effects of nonlinear amplification increasesthe bit-error-rate (BER) and spectral regrowth (SR), a trade-offbetween the degree of nonlinear distortion and total power con-sumption is necessary. The degree of SR can be characterizedby the adjacent channel power ratio (ACPR), and this ratio isthe limiting factor for achieving high efficiency amplification[2]. Generally, the ACPR is the ratio of power in the adjacentchannel to the power in the main channel. ACPR values arewidely used in the design of power amplifiers to quantify theeffects of intermodulation distortion and, hence, also serveas a measure of linearity. Error vector magnitude (EVM) isa measure of the amount of symbol error experienced by thereceiver and is a performance metric for signal distortionsexperienced by the system under observation. Spurious freedynamic range (SFDR) and third order intercept point (IP3)are also very important system parameters to assess nonlin-earity distortion effects that require particular attention, and totransmit subcarrier multiplexed OFDM wireless signals to aradio-over-fiber (RoF) link.

However, improved RoF systems have recently emerged toprovide enhanced capacity, superior coverage and system per-formance, low radio-frequency dissipation, low signal attenua-tion in the fiber, and, ultimately, low system costs for the de-ployment of future broadband wireless data networks. Gener-ally, RoF is an analog optical link that transmits and distributesmodulated RF signals between a central site and remote sites,i.e., back and forth between where a base station is situatedand a number of remote sites where the antennas are placed.In some recently-proposed wireless networks, RoF technologyis shown to be appropriate for interconnecting control stationsand base stations as well as in building distributed wireless net-works, thereby allowing the concentration of complex equip-ment while simplifying the control system [3]. The nonlinearityeffects in the laser diode (LD), fiber channel, and photodetector(PD) can significantly affect the system performance of an RoFlink, as well.

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468 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 5, MARCH 1, 2008

Fig. 1. RoF downlink model for in-building access networks or extension of coverage.

In this regard, wireless broadband (WiBro), the name of mo-bile Internet systems in Korea, is a standard of IEEE 802.16e,which itself has been folded into WiMax. Next generation wire-less systems such as WiBro networks are deployed using orthog-onal frequency division multiple access (OFDMA). Because ofhigher frequency and bandwidth applications, extensive use ofmicrocells and picocells for these systems is inevitably requiredfor RoF links integrated with RF amplifiers. Previous studieshave shown the nonlinear distortion effects of OFDM signals onclipping and RF amplifiers [4]–[7], and others have assessed op-tical and wireless link integration. ACLR analysis of wide bandcode division multiple access (W-CDMA) signal for the RoFlink is represented in [8].

Until now, the distortion effects of OFDM signals over RoFlinks integrated with an RF amplifier and electroabsorptionmodulators (EAM) have not been reported with considerationtoward the parameters as mentioned above. We devised acomplete analytical model of the OFDM-RoF system and usedan electroabsorption modulator (EAM) as a receiver in orderto analyze the distortion effects. In this paper, we combined acomplete analytical model for the downlink system aforemen-tioned for WiBro. We assessed the performance of the modelby comparing the simulated model with suitable experimentaldesign considerations based on the distortion metrics for ourproposed system.

II. ANALYSIS OF THE PROPOSED LINK

In this paper, the RoF downlink model for in-building accessnetworks or extension of coverage is shown in Fig. 1 [8]. The RFamplifier is usually employed to access RoF links for wirelessnetworks. The directly modulated laser diode (LD) is usuallyused for transmitters of RoF links for in-building applications.The active EAM will be used for larger RF coverage. In theactive EAM case, the RF coverage can be increased using DCpower to increase responsivity for the active EAM, and an RF

amplifier to increase RF power for the downlink, as well as a lownoise amplifier for the uplink at the access point. The passiveEAM, which was designed for RF applications operating at 0bias voltages, can be used for smaller RF coverage without anypower supply at the access point.

Current analytical techniques have assessed the impactof power spectral densities (PSD) of OFDM signals usingclipping devices and high power amplifiers (HPA) that ex-hibit frequency-independent nonlinear behavior [5]. The samePSDs cannot be directly carried over to determine the impactin fiber-radio system elements because fiber-radio systemelements exhibit frequency-dependent nonlinear behavior.Therefore, to develop a complete analytical model that rep-resents the frequency-dependent and frequency-independentbehaviors of an RoF system integrated with an OFDM signalfed via an RF amplifier, a combination of Volterra series-basedanalytical techniques and impulse response techniques can beused. An OFDM signal can be represented as [9]

(1)

where is the central frequency, is the frequencyof the subcarrier , describes complex QPSK or QAMsymbols, and is the number of subcarriers. It was consideredthat an OFDM signal is cyclostationary, which implies that themean value and the auto-correlation of the process are periodicin time with some period . It follows according to the centrallimit theorem that for larger , possesses a Gaussian distri-bution with mean zero. Next, using the modulated narrowbandGaussian input signal shown in (1), we can write

(2)

SONG AND ISLAM: DISTORTION OF OFDM SIGNALS ON RADIO-OVER-FIBER LINKS 469

Here, is the voltage envelope of the modulated signal,is the phase, and is the carrier frequency. The modu-

lated signal is first amplified and then corrupted by additivewhite Gaussian noise (AWGN) with two-sided power spectraldensity . The frequency-dependent nonlinearity is shownin Fig. 2, where and are frequency responses oflinear systems and represents a memoryless device whosecharacteristics can be expanded in a power series

(3)

where represents RF amplifier nonlinearity, so that the input-output relationship can be written as

(4)

in which and are the impulse responses of the linearsystems. The bandpass frequency-dependent nonlinear ampli-fier model proposed for our system will exhibit the th-ordertransfer function as follows:

(5)

Here, is the frequency response of the bandpass filters. Fi-nally, the th-order kernel of the frequency-dependent nonlin-earity is obtained by taking the -dimensional inverse Fouriertransform of its th-order transfer function. Thus

(6)

In Fig. 2, is the undistorted OFDM signal, is thenonlinear distorted OFDM signal and the nonlinearity is rep-resented by its kernels , , orby its transfer functions , ,which are assumed to be symmetric. The frequency-dependentnonlinearity is modeled as in Fig. 2, where the characteristic ofthe memoryless device is also given by

(7)

with , and , and andare frequency responses of two bandpass first-order Butterworthfilters

(8)

Where is the center frequency and is the 3-dB bandwidthof the bandpass filters. The center frequency and the 3-dB band-width of the bandpass filters were set to be equal to the centerfrequency and the 3-dB bandwidth of the OFDM signals, re-spectively. The transfer functions for this nonlinearity can beobtained from (5) by noting that , , and

and for , and and are givenin (5).

In the model, distributed feedback (DFB) LD output light-wave intensity is directly modulated about a bias point by thetime-varying OFDM signal. Therefore, the transfer function ofthe DFB LD is modeled by the time varying components of theLD rate equations [10], expressed as

(9)

(10)

where and are the steady state components andand are the time varying components of the carrier density

and photon density . The input current to the DFB LD iscomposed of the steady-state component plus a time-varyingcomponent . is the optical confinement factor, is thegain slope constant, is the carrier density at which the netgain is zero, is the photon lifetime, is the electron life-time, is the fraction of spontaneous emission coupled into thelasing mode, is the volume of the active region, and is theelectronic charge. The parameter specifies the gain compres-sion characteristics of the active region. Applying Volterra seriesanalysis as mentioned in [11], the th order DFB LD transferfunction can be expressed as (11), shown at the bottom of thepage, where

(12)

(11)


TABLE ICALCULATION OF � FOR DIFFERENT VALUES OF �

Fig. 2. Frequency-dependent nonlinearity model.

(13)

(14)

A number of terms for and are given in Table I. Typicalparameters of the DFB LD [11] were used with some particularparameters used for Ortel DFB laser, which was employed inthe experiment in this paper.

Formula (11) represents frequency-dependent nonlinear be-havior for the RoF link. The DFB LD output optical power isgiven by

(15)

where is Planck’s constant, is the frequency of operation(optical frequency), is the internal quantum efficiency of thelaser, and is the differential quantum efficiency of the laser[12]. The optical signal from the LD propagates to the receiverover a single mode fiber (SMF) designed for low loss at thewavelength of the optical source (1540 nm in our proposedmodel). The main source of distortion in an optical fiber is chro-matic dispersion. Intermodal distortion effects are not presentfor single mode fibers. The transfer function of the fiber [13]can be written as

(16)

where represents length of the fiber, is the dispersion factor,and is the carrier frequency for which the adiabatic and tran-sient chirp of the laser. Optical losses due to the coupling be-tween laser and fiber and between fiber and the EAM are ne-glected. In an analytical model, an EAM is used as a receiverin the downlink access. The objective of using the EAM in this

case is to obtain the RF output signal from the optical input inthe EAM. Therefore, the internal optical gain of the EAM[14] can be expressed as

(17)

where denotes the internal optical gain with theFranz–Keldysh effect, is the incident optical power, isthe detected photocurrent, is the dark current, and is thepower of the absorbed light, defined as

(18)

Here, is the reflectivity, is the coupling co-efficient of theincident light to the waveguide, is the absorption coefficient,and is the device length.

Based on the transfer functions obtained, the com-plete OFDM-RoF system analytical model canbe expressed as: total OFDM-RoF system output

. Hence

(19)

where is obtained in (5), isas shown in (11) and , , and

are taken from (15)–(17), respectively.For the analytical model, the PSDs at the output of the RF am-

plifier are shown in Fig. 3(a) and (b) for the frequency-indepen-dent and dependent cases, respectively. In both cases, the -axisis normalized to the maximum value in the PSD of the distortedsignal. In the frequency-dependent case as shown in Fig. 3(b),the adjacent channel spectrum is getting wider at dB com-pared to the frequency-independent case in Fig. 3(a). Fig. 4shows the analytical modeling of the frequency-dependent casefor different Butterworth filter coefficients. One can see that theadjacent spectrum gets wider as the coefficient value increasesfrom 1/15 to 1/10.

Fig. 5 shows the experimental setup for the measurementof the distortion effects of OFDM signals. We used AgilentSignal Studio to generate WiBro OFDM signals at a frequency


Fig. 3. PSD of OFDM signal at the output of the RF amplifier for: (a) Fre-quency-independent case. (b) Frequency-dependent case.

of 2.39 GHz, which is a WiBro system frequency band. Thesignals were then passed to a vector signal generator (AgilentE4438C) to observe and generate the OFDM signals. The sig-nals were fed into the RF amplifier (HP 8349B), and to the DFBlaser for direct modulation, and then passed through the opticalfiber links. We analyzed the OFDM signals using a vector spec-trum analyzer (Agilent E4403B) and a mobile WiMax test set(Agilent E6651A) at the active and passive EAM after transmit-ting them through various lengths of SMFs. We used an RoF de-velopment kit for analog applications by Microwave Photonicsfor this purpose. The DFB laser was designed for analog modu-lation at 1540 nm [11], [12]. The bias voltage for an active EAM,which was developed at the BT laboratory, was optimized forRF modulation [8]. The passive EAM used in this experimentconsisted of a multiple quantum well absorption layer in a lowcapacitance ridged structure. InGaAsP/InP material system wasused for operation around a wavelength of 1540 nm. The chipwas mounted in a high frequency package with fiber pigtails and

Fig. 4. PSD of OFDM signal at the output of the RF amplifier for different filtercoefficients.

had a total fiber to fiber loss of only 5 dB at zero bias. The 3-dBmodulation bandwidth of this unbiased device is 8.2 GHz, whichcompares to a value of over 14 GHz under normal bias condi-tions. The typical variation of transmission and responsivity ofthis device with applied DC bias for TM polarized light at awavelength of 1540 nm are shown in Fig. 6 [15]. The passiveEAM can act as a photodetector for the downlink path, and asa modulator of the remaining downstream light for the uplinkpath. The passive EAM can be operated without DC bias for afull duplex link and a short radio range. A balance optical linkloss can be achieved if the EAM is biased appropriately, but forpassive operation, the downstream link loss is higher than theupstream loss, since the passive EAM is less efficient as a pho-todetector than as a modulator at zero bias. However, the overalllink was balanced more closely by using different values of an-tenna gain for each path at the access point. The radio systemcan be connected to the optical access link. Full rate transmis-sion was possible with the passive radio access point antennalocated in an office and the access point antenna free to accesswithin the radio cell.

In order to assess the distortion effects of OFDM signals forthe proposed RoF link connected with an RF amplifier, we ob-served the distortion effects of OFDM signals with respect todifferent fiber lengths, number of subcarriers of OFDM signals,and different arrangements of RoF systems at the output of ac-tive and passive EAMs.

At an input power of dBm, we observed simulated PSDat the output of the active EAM of the RoF link with 3-km fiberand compared it with the experimental PSD as shown in Fig. 7.The parameters for simulation for the InP-based DFB laser wereinferred from [11], [12]. The filter coefficient was 1/10. Thesimulated results include frequency-dependent and frequency-independent nonlinear effects using the proposed model. Sincesome other system effects such as loss of the connectors were


Fig. 5. Experimental arrangement for OFDM signal transmission and measured distortion effects.

Fig. 6. Typical transmission and responsivity of packaged EAM as a functionof applied DC bias [13].

included in the experimental arrangement, the simulated PSDshowed less distortion by an amount of around 6 dB at the peakof the spectrum compared to the measurement case for the activeEAM output PSDs as shown in Fig. 7. To be consistent withtypical lengths of fiber used for coverage extension of the basestation for in-building applications, the length of the fiber in theexperiment was less than 3 km.

Fig. 8 shows the measured output PSDs of the RoF links forthe active EAM with the RF amplifier and the passive EAMwithout RF amplifier. The WiBro OFDM signal at 2.39 GHz isa 16-QAM modulated signal with full carrier modulation for the1024 subcarrier in Fig. 8. Fig. 8(a) shows the measured outputPSD of the passive EAM without RF amplifier. The fiber lengthwas 0.5 km for the passive EAM. It was observed that the SRwas significantly increased with a wider adjacent channel spec-trum when the active EAM with the 30-dB gain RF amplifier,

Fig. 7. PSDs of the OFDM signal at the output of the EAM of the RoF link.

which is operating in nonlinear region, and a 3-km fiber wasused as shown in Fig. 8(b).

III. PEAK-TO-AVERAGE POWER RATIO

OFDM’s high time-domain PAPR has been a major obstaclein the implementation of power-efficient transmitters. This highPAPR significantly reduces the average power at the output ofthe high-power amplifier used at the transmitter. The peak of thesignal can be up to times the average power, where is thenumber of carriers. These large peaks increase the amount of in-termodulation distortion among the subcarriers and out-of-bandradiation as the signal peaks get into its nonlinear region. So,an RF power amplifier over a large dynamic range is needed as


Fig. 8. Distortion of OFDM signals at the EAM outputs of the RoF links.(a) Measured PSD at the passive EAM output without RF amplifier and a 0.5-kmfiber. (b) Measured PSD at the active EAM output with the RF amplifier and a3-km fiber.

the high PAPR ratio reduces the power efficiency of the system.PAPR of the OFDM symbol is defined as

(20)

where denotes the expectation. In our analysis,

since is the useful time duration in and cyclic prefix(CP) changes neither the average nor the peak power of thesignal, CP is removed from the analysis. As the basebandOFDM signal is used in the PAPR analysis as the RF frequencyand is much higher than the subcarrier frequency, basebandOFDM PAPR will be equivalent to passband PAPR. For large

and uncorrelated input data, the central limit theorem ap-plies and the transmitted signal becomes complex Gaussiandistributed. Therefore, the amplitude of the OFDM signal has a

Rayleigh distribution with zero mean and a variance .The PAPR can be written as

(21)

and the PDF of the OFDM signal magnitude is given as

(22)

where . The probability that the magnitude of at leastone single sample does not exceed a certain threshold isthus

(23)

We assume the samples to be statistically independent. There-fore, the probability that the magnitude of the entire OFDMsymbol exceeds a certain threshold level can be approximatedas

(24)

or

(25)Fig. 9 depicts the measured complementary cumulative dis-

tortion function (CCDF) at the RF amplifier output versus theinput power level at the input port of the RF amplifier, whichhas about 30-dB gain. The CCDF of additive white Gaussiannoise (AWGN) was compared to the measured CCDF. The totalnumber of subcarriers used was 1024 for WiBro systems. Theinput OFDM signal was 10% subcarrier-modulated 16-QAMsignals. The OFDM signal with dBm input power level was21.6 dBm at the output of the RF amplifier, which is close to thenonlinear region of the amplifier resulting in increased CCDFas shown in Fig. 9. Fig. 10 shows the measured CCDF in termsof the percentage of subcarrier modulation for different modu-lation types. The CCDF was measured at the active EAM forthe 3-km RoF link without the RF amplifier. The input powerof the OFDM signals was dBm at the input port of theDFB LD of the central unit. The CCDF between QPSK and16-QAM was similar to the results in this experiment, but themeasured CCDF was changed approximately 1 dB under 1%of the between 100%, 50%, and 10%


Fig. 9. CCDF at the RF amplifier output for different input levels.

Fig. 10. CCDF at an active EAM of the RoF link without an RF amplifier fordifferent percentages of subcarrier modulation.

subcarrier modulation. The CCDF of the full subcarrier mod-ulated was less than the CCDF of the AWGN. Fig. 11 depictsthat the CCDF results at the output of the passive EAM of theRoF link with the RF amplifier. The input power level at theinput port of the RF amplifier was dBm, and the outputwas dBm at the output of the RF amplifier. The 10%subcarrier QPSK modulated was used for a 3-km fiber and the20% subcarrier QPSK modulated was used for 0.5- and 1-kmfiber. Fig. 11 shows that the simulated CCDF results for a 3-kmfiber were similar for the measured CCDF results. It was alsoobserved that the PAPR increased due to a higher number ofsubcarriers for the OFDM signal.

IV. ADJACENT CHANNEL POWER RATIO

The ACPR is defined differently among various wireless stan-dards. The main difference is the way in which adjacent channelpower affects the performance of another wireless receiver for

Fig. 11. CCDF at a passive EAM of the RoF link integrated with the RF am-plifier for different percentage of subcarrier modulation.

which the offending signal is co-channel interference. We cal-culated ACPR values using the following equation:

(26)

where is the desired in-band, and is the PSDof the amplified signal .

The experimental results were obtained using different inputpowers of the RF amplifier and different fiber lengths forcomparison with the analytical model. Fig. 12 shows closeagreement between the measured and simulated ACPRs at theoutput of the RF amplifier, as well as at the output of the activeEAM of the RoF link without the RF amplifier. The length ofthe fiber in this case was 0.2 km. At dBm, simulated andmeasured ACPRs at the RF amplifier output were dBcand dBc, while simulated and measured ACPRs at theoutput of the EAM without the RF amplifier were dBcand dBc, respectively.

Measured ACPRs at the output of active and passive EAMfor different fiber lengths with the RF amplifier were plottedin Fig. 13. At an input power of dBm and fiber lengthof 0.2 km, the ACPR for the passive EAM was about 10-dBchigher than that of the active EAM case, which kept essentiallythe same difference throughout the range of observation. In theactive EAM case, Fig. 13 shows that the ACPR increased as thefiber length increased. The simulated results for the active EAMwith a 3-km fiber and for the passive EAM with a 0.5-km fiberwere also similar to the measured results, as shown in Fig. 13.We concluded that due to enhanced SR, distortion increased asfiber lengths increased.

V. ERROR VECTOR MAGNITUDE

EVM measurements were performed on a vector spectrumanalyzer (Agilent E4403B) and a mobile WiMax test set (Agi-lent E6651A) that captures a time record and internally performs


Fig. 12. Measured and simulated ACPRs at the output of the RF amplifier and the EAM.

Fig. 13. Measured ACPRs at different fiber lengths.

a fast Fourier transform (FFT) to enable frequency domain anal-ysis. Signals are down-converted before EVM calculations aremade. Since different modulation systems such as QPSK and16-QAM with full carrier modulation, half carrier modulation,and 10% carrier modulation were used, some normalization istypically carried out to effectively calculate and compare EVMmeasurements. The normalization is derived such that the meansquare amplitude of all possible symbols in the constellation ofany modulation scheme is one. Thus, EVM is defined as theroot-mean-square (RMS) value of the difference between a col-lection of measured symbols and ideal symbols. These differ-ences are averaged over a given, typically large, number of sym-bols and are often shown as a percent of the average power per

symbols of the constellation. As such, EVM can be mathemati-cally given as

(27)

where is the normalized th symbol in the measured sym-bols, is the ideal normalized constellation point of the

th symbol and is the number of unique symbols in theconstellation.

Fig. 14 depicts a measured distortion of modulated symbolswhich are QPSK and 16-QAM signals at the passive EAMoutput of the RoF link with an RF amplifier. The number ofsubcarriers of WiBro signals was 1024 and full subcarriers aremodulated. The measured EVM was 7.9% for the RoF linkwith a 3-km fiber. The input power of the OFDM signals was

dBm, which is a 21-dBm level at the input of the RoF linkdue to the RF amplifier utilizing about 30-dB gain.

The difference of the measured EVM between full subcar-rier modulated QPSK and 16-QAM was about 0.5% as shownin Fig. 15. The difference of the EVM between full-, half-, and10%-modulated QPSK and 16-QAM signals was within 0.5%in the measurement. The symbol distortion is largely increasedwhen the input power level moves up to the IP3 of the RF ampli-fier, as shown in Fig. 15. The symbol EVM at the active EAMoutput of the RoF link with the RF amplifier for a 3-km fiberis decreased at the -dBm input level of the RF amplifier,though is increased at the 0-dBm input level due to the non-linear effects of the RF amplifier.

Fig. 16 depicts the 16-QAM symbol EVM of the RoF linkwithout an RF amplifier in terms of different lengths of fibers.


Fig. 14. Measured constellation diagram of demodulated signals from OFDM subcarriers at a passive EAM output of the RoF link with an RF amplifier and 3-kmfiber. (a) QPSK, full subcarrier modulation, and 1024 subcarriers; (b) 16-QAM, full subcarrier modulation, and 1024 subcarriers.

Fig. 15. Measured symbol EVM at the output of the RF amplifier and the activeEAM.

The EVM was measured and simulated at the output of the ac-tive EAM as well as the passive EAM. When the input levelof the OFDM signals at the LD input port of the RoF centralunit was 10 dBm, the measured EVM at the passive EAM was6.48% for a 3-km fiber. The simulated EVM using the proposedanalytical link model for active EAM in the case of the 10-dBminput level at the RoF central unit is close to the measured EVMas shown in Fig. 16. The measured EVMs show a decrease insymbol distortion by increasing RF input level at the LD due tooperating in the linear region and a low loss of the RoF link.

Fig. 17 depicts the 16-QAM symbol EVM for the RoF linkwith the RF amplifier in terms of the length of fiber. As theinput power level is increased due to the RF amplifier, the mea-sured EVM was increased to approximately 8% because of theincreased nonlinear effects of the amplifier with an RoF link.The input level of the OFDM signals was increased near to theIP3 of the RF amplifier, which was at a dBm output. TheEVM increased for the 0.5-km fiber, as well as the 3-km fiberas shown in Fig. 17. The measured EVM for the passive EAM

Fig. 16. OFDM symbol EVM at the output of the RoF link without RFamplifier.

was highly related to nonlinear effects due to the RF amplifierwith the RF link for shorter fiber lengths, as well.

VI. SYSTEM DISTORTIONS

The 1-dB compression point of the RF amplifier wasdBm and the IP3 was approximately 30 dB. The laser

relative intensity noise (RIN) dominates the noise sources inthe downlink. The measured RIN was dBm/Hz. TheSFDR was measured by summing the two signal generatorswith the splitter/combiner and connecting them to the input ofthe RoF central unit. The spectrum analyzer was connected tothe RF downlink connector of the EAMs. The measured SFDRwas approximately dB for the down link. The measuredIP 3 was dB and the downlink harmonics was less than

dBm. The critical factor for transmitting subcarrier-mod-ulated OFDM signals is to balance the downlink gain so as notto overdrive the laser, which would cause not only degradationin the EVM, but also difficulty in maintaining the stability ofthe spectral mask in the output of the EAMs.


Fig. 17. OFDM symbol EVM at the output of the ROF link integrated with anRF amplifier.

VII. CONCLUSION

This study analyzed the distortion effects of OFDM signalsdue to nonlinearity in the RF amplifier lined to the RoF link,and showed close agreement between the analytical model andmeasured cases. Further investigations into the proposed systemwill be of paramount importance for microcell and picocell ap-plications in future broadband convergence, such as in WiBroaccess networks. Determining downlink specifications in termsof the maximum allowed value for the PAPR, ACPR, EVM, andsystem distortions will eventually help to define the input powerand fiber length limitations in the design of suitable RoF links,which is integrated with an RF amplifier and EAMs. A passiveEAM was used as an RF access point for picocell applicationsto investigate distortion effects of OFDM signals. The symbolEVM of subcarrier-modulated OFDM signals for the active orpassive EAM output of the RoF link was with a 0.5-kmfiber, and was 7% for a 3-km fiber. To our knowledge, this is thefirst reported comparison of distortion effects using the trans-mission of OFDM signals over RoF links with active and pas-sive EAMs for the WiBro system.

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Ju Bin Song (S’99–A’00–M’02) was born in SouthKorea in 1964. He received the B.Sc. and M.Sc.degrees in 1987 and 1989, respectively, and thePh.D. degree from the Department of Electronic ndElectrical Engineering, University College London(UCL), University of London, U.K. in 2001.

From 1992 to 1997, he was a Senior Researcherin the Electronics and Telecommunications ResearchInstitute (ETRI), South Korea. He was a ResearchFellow in the Department of Electronic and ElectricalEngineering, UCL, U.K., in 2001. During 2002 and

2003, he was an Assistant Professor in the School of Information and Com-puter Engineering, Hanbat National University, Korea. He has been an AssistantProfessor in the Department of Electronic and Radio Engineering, Kyung HeeUniversity, since 2003. His current research interests include broadband accessnetworks, high-speed telecommunications, mobile multihop networks, and re-source sharing technologies.

A. H. M. Razibul Islam was born in Bangladesh in 1979. He received the B.S.degree in electrical and electronic engineering from the Islamic University ofTechnology (IUT), Bangladesh, in 2002, and the M.S. degree from the Depart-ment of Electronic and Radio Engineering, Kyung Hee University, South Korea,in 2007.

He has been a Lecturer in the Department of Electrical and Electronic Engi-neering, IUT, since 2003. His research interests includes nonlinearity of OFDMsystems, radio-over-fiber links, and their performance analysis.

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