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Fiber Monitoring using a Sub-Carrier Band in aSub-Carrier Multiplexed Radio-over-Fiber

Transmission System for applications in AnalogMobile Fronthaul

Patryk J. Urban, Senior Member, IEEE, Gustavo C. Amaral, and Jean Pierre von der Weid, Senior Member, IEEE

Abstract—In this paper, we describe an efficient method formonitoring fiber links utilizing sub-carrier multiplexing (SCM).By assigning a void sub-carrier frequency band for monitoringpurposes, the method re-uses the data transmitter without anyimpact on data transmission and provides capability of in-servicereflectometry measurements of fiber optic lines with 10m spatialresolution and 1.0dB fault detection sensitivity. Its promisingproperties and performance enable potential application inemerging networks such as relatively short distance analoguemobile fronthaul.

Index Terms—WDM/SCM-PON, Backscattering Techniques,OTDR

I. INTRODUCTION

RADIO Access Network (RAN) needs to evolve to meet5th Generation mobile communication (5G) demands.

This includes supporting 1000 times larger mobile data vol-ume, tens of billions of connected devices, 10 to 100 timeshigher achievable user data rates, 10 times longer batterylife as well as latency reduced by up to a factor of five[1]–[3]. Moreover, the envisioned dense deployment of lowpower mobile access points [4], [5], so called small-cells,leads research efforts towards efficient solutions to reduceoperational expenses of underlying fiber networks.

Therefore, the development of RAN has recently focusedvery much on mobile fronthaul (MFH) which is the linkconnecting a Digital Unit (DU) with a Radio Unit (RU) (Fig.1-a). Historically, this was just a short backplane interconnectwithin a Radio Base Station (RBS), but, recently, due to theCloud RAN (C-RAN) concept developments, extension ofMFH enables centralization of most of the network logics. Thisis translated into the simplification of the cell-site engineeringand reduced geographic distribution of maintenance sites [6].Fiber-optic MFH has two fundamental flavors: digital MFH(d-MFH) and analog MFH (a-MFH), Fig. 1-b and Fig. 1-c, respectively. The d-MFH is mostly supporting CommonPublic Radio Interface (CPRI) [7]. However, the 5G goalsput CPRI optics at a challenge of very high bit-rate operation

P. J. Urban is with Ericsson Research, Ericsson AB, Stockholm, Sweden(e-mail: patryk.urban@ericsson.com, patryk.j.urban@ieee.org).

G. C. Amaral and J. P. von der Weid are with the Center for Telecommuni-cations Studies, Pontifical Catholic University of Rio de Janeiro (PUC-Rio),22451-900, Brazil (e-mail: gustavo, vdweid@opto.cetuc.puc-rio.br).

and may bring the old-fashioned analog Radio-over-Fiber (a-RoF) transmission technology back on top. A-RoF keepsexpensive Digital-to-Analog and Analog-to-Digital Converters(DAC, ADC) inside a centralized DU-RU site, which feeds aremote Radio-head (Rh) equipped with an antenna [5], [8].

Fig. 1. RAN: (a) typical radio base station with a short CPRI interconnectbetween collocated DU and RU and a short copper/coax cable to the antenna;(b) extended CPRI link in a main-remote setup; and (c) extended analogfronthaul with short CPRI interconnect between collocated DU and RU.

Even though a-RoF reduces the cell-site complexity andalleviates the need for large optical bandwidth, it may suffermore from well-known transmission impairments to whichd-RoF is effectively more resistant [9]. As an example, a320MHz radio bandwidth in a-RoF link, together with guardbands, would correspond to a fraction of bandwidth occupiedby an equivalent CPRI signal (10Gbps) [7]. The transmissionimpairments, on the other hand, have been studied for yearsand respective efficient solutions have been addressed, e.g. in[10]. The different candidate technologies for 5G MFH havebeen impartially discussed in [11] leaving analog and digitalvariations of RoF stripped down to their essential pros andcons with a major conclusion being that a-RoF is less complex,less bandwidth demanding, more flexible and more transparentwhen contrasted with d-RoF [12].

Since a-RoF carries radio signals, which can feed the Rh,the architecture of the respective Optical Distribution Network(ODN), spanning from the DU-RU towards the Rh, is calledFiber To The Radio head (FTTRh). Longer reach of opticalfiber links, with respect to legacy copper cabling, enablesextension of FTTRh and further centralization of DU-RUin a cost-efficient way provided efficient multiplexing tech-niques are applied. Sub-Carrier Multiplexing (SCM) together

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with Wavelength Division Multiplexing (WDM) for mobilefronthaul applications has been studied in details in [13].Moreover, ODN should be kept simple, passive and low-cost, similar to ODN in Passive Optical Network (PON) wellknown from fixed broadband access area. Reliability becomeseven more critical for FTTRh rather than in, e.g., Fiber-To-The-Home (FTTH), as each Rh may connect several mobileusers. One of the major concerns in this context is signaldisruptions due to fiber failures resulting from extensive fiberbending, fiber cuts, etc. Therefore, a preventive and in-serviceremote fiber monitoring solution is needed in order to timelydetect and locate faults to shorten the service downtime. SinceSCM allows for providing several radio signals, which may berelated to different operators, antenna sectors and/or differentradio technologies, it also brings up an opportunity to assignone of the sub-carriers to link monitoring purposes either byusing one of the data sub-carriers (out-of-service monitoring)or by using an idle sub-carrier (in-service monitoring). Bothapproaches use the same optical transmitter for link monitoringand for data transmission, which we hereby refer to as anembedded monitoring technique.

Over decades now, reflectometry-based solutions for linkmonitoring, i.e. Optical Time Domain Reflectometry (OTDR)and Optical Frequency Domain Reflectometry (OFDR) haveproven to be the most adequate. A systematic overview offiber monitoring requirements, different approaches to satisfythose needs and proposed solutions for monitoring have beengiven in [14]. Embedded monitoring techniques [15], [16] havebeen considered and some solutions have been commercialized[17] in parallel to standardization supporting activities [18].Here, we focus on those, which touch upon embedded andSCM-related solutions as background to the proposed schemediscussed further in this paper.

In [15] a basic supervision solution integrated into datatransmitter at the Optical Line Terminal (OLT) and OpticalNetwork Unit (ONU) is described. It uses a single test pulsesent during data transmission pauses to perform the linkmonitoring. The method, therefore, is not centralized as itrequires end-to-end measure, as well as prohibitive of in-service tests. In [16] the authors devise a technique, whichrelies on a data pattern designed in such a way that most ofits energy is concentrated in a particular electronic frequency.The received echo is processed using a heterodyne electronicdetector circuit, which uses synthetic sinusoids generated bydata pattern shaping device as its local oscillator, so in-servicemonitoring is not a possibility.

The OFDR methodology is utilized further in [19] whichdedicates electronic frequency band for test signal outside thebaseband data signal bandwidth. Incoherent OFDR was usedin [20] and [21] and could, in principle, be compatible witha sub-carrier multiplexed RoF system. However, the requiredbandwidth for reasonable fault location with this technique istoo high to be compatible with simultaneous data transmissionand sub-carrier multiplexing.

A link monitoring method, which is applicable to SCMtransmission has been examined in [22]. There, the OTDRpulses were transmitted in the baseband of an optical carrier,whereas the data signals were transmitted in sub-carrier bands.

The technique showed that, although accompanied by limiteddynamic range for OTDR test, with proper distribution of themodulation index between a bank of SCM data signals andthe monitoring signal in baseband, the latter one has negligibleimpact on data transmission.

In this work we present a monitoring method using adedicated electronic frequency band, i.e. an idle sub-carrier,for link testing in an SCM data transmission system for a-MFH applications and discuss potential implications to theoverall system performance. The paper is organized as follows.In Section II, the SCM network is presented and discussedin details. In Section III, the monitoring setup is presentedand the mathematical formulation of the monitoring signalis developed. Section IV includes the results of real linkmonitoring employing the proposed technique. Finally, inSection V, we draw conclusions over the results.

II. MOBILE FRONTHAUL

Similarly to fixed fiber access, automatic remote fiber-fault detection and localization are fundamental requirementsfor an efficient proactive and reactive monitoring solution inMFH as it allows shorter service delivery downtime. Com-pleteness (ability to detect different types of fiber faults),comprehensiveness (ability to determine various parameters offiber faults), high sensitivity (∼1dB) and accuracy (<< 1dB)of measurements as well as neutrality to traffic flow are allequally important. These parameters stand for the capabilityto localize and identify minor and major fiber faults in everyODN section with distance and loss magnitude measurementprecision sufficient to determine the severity of network mal-function and the required troubleshooting steps. The solutionshould be single ended with the least impact to the existingODN [23].

We present an example resilient ODN architecture for a-MFH in Fig. 2. The RBS is situated in a Central Office (CO),and connected to both North (N) and South (S) branchesof the trunk fiber ring. The ring connects several opticalnodes, which are equipped with two in/through optical ports,and an internal add-drop path which connects to an antennaunit. Transmission in downstream direction is provided viaSub-Carrier Multiplexing (SCM). However, depending on theactual application scenario one or more sub-carriers can bedropped at every Optical Frontend Node (OFN), which donot share sub-carriers. Transmission in the upstream directionis provided via Wavelength Division Multiplexing (WDM), soevery OFN will add its unique wavelength channel to the ring.For multi-operator scenario one could also provide upstreamSCM on top of each WDM carrier.

Fig. 2. Example resilient ODN architecture for a-MFH.

More specifically in the RBS, the Radio Frequency (RF)interfaces are connected to so called RoF Master units or-ganized in linecards. The optical section of a Master RoF

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unit is composed of transmitter and receiver blocks. Thetransmitter is based on a directly or externally modulatedlaser whose output signal is sent through a Red Blue Filter(RBF) towards the switch. The other port of the RBF sendsthe upstream signal, via a wavelength demultiplexer, to a setof optical receivers dedicated for each wavelength channel. Inthe electrical section, laser diodes (LD) and photodiodes (PD)are placed next to sub-carrier multiplexers and demultiplexers,respectively as depicted in Fig. 3-a. The generic wavelengthpanel for the RBF is depicted in Fig. 3-b. Each linecardconnected to a switch is further referred to as an OpticalBackend Termination (OBT). These OBTs are connected toa ring ODN as shown in Fig. 2.

Fig. 3. a) Optical Backend Termination. The RBF works as a dichroicmirror and combines the up-stream and down-stream signals which occupydifferent wavelengths. LD: Laser Diode; PD: Photodiode; R/B: RBF. b)Generic wavelength panel for the proposed a-MFH architecture.

The preferred solution for a wavelength independent switchis a 1:2 coupler with adjustable split-ratio varying continuouslyfrom 0/100 to 100/0. The coupler adjusted to 0/100 or 100/0enables transmission/reception from one arm of the ring only(North or South), which would be the case for normal oper-ation of the network. From a backscattering link monitoringpoint of view, a probe signal launched from the OBT (furtherdetailed in Section III) would contain the information of thefull ring as seen from one of the terminations, and no forwardtransmitted signal would reach the monitoring unit placed atthe OBT. In case of a complete fiber break in the inter-nodelink, the split-ratio would have to be adjusted to optimizepower budgets for the different OFNs accessible by the Nor S branches and the monitoring unit would receive thesuperimposed signals from the two branches exactly as in a 2-branches star PON with the reservation that no forward signalwould reach the monitoring unit as the break prevents fullcirculation of the transmitted signals. In case of a fiber breakwhich introduces just partial attenuation, the split-ratio needsto be adjusted to 0/100 or 100/0 to transmit/receive to/from onearm of the ring. The choice of the arm would be made basedon the least number of affected nodes, i.e. nodes positionedafter the fiber fault from the viewpoint of chosen termination.

The transmitter block in the OFN (RoF Slave), Fig. 4, issimilarly built as its equivalent in the OBT. The signal from thetransmitter is coupled to one port of the RBF while the otherone is connected to the PD followed by an SCM demultiplexer.

The common port of the RBF is connected to a 1:2 50/50coupler, which divides the upstream signal equally in powerand sends it, via a 2:2 asymmetric coupler, to the N and Sbranches of the ring depicted in Fig. 2.

Fig. 4. Optical Frontend Node. LD: Laser Diode; PD: Photodiode.

The asymmetric splitter usually provides a 10/90 split ratio,but it could be different depending on the available powerbudget (LD and PD parameters). The through-path of theOFN is provided by the two high power branches of theasymmetric splitter, which introduces minor attenuation tothe output signals. If deeper fiber penetration is required, theRBF-to-coupler connection can be extended with a fiber span,enabling the O/E part of the OFN to be moved further awayfrom the ring. The downstream and upstream optical signalpair travels over the same optical link in counter directionsand, therefore, experiences the same link loss and symmetricsignal delay.

It is important to point out that potential leakage of theupstream signals feeding back the upstream LDs should beprevented by an LD isolator. For high capacity demandingscenarios, multi-wavelength transmission in downstream couldbe provided. Besides, proper set of OBT transmitters and addi-tional filtering capability would be needed at the OFN receiverto provide downstream wavelength selectivity. Alternatively,to avoid colored OFNs, arrays of Vertical-Cavity Surface-Emitting Lasers (VCSELs) and PDs could be considered.

III. FIBER MONITORING IN ANALOGUE MOBILEFRONTHAUL

A. Proposed Architecture

Fig. 5 presents the basic monitoring architecture installedat the Optical Backend Termination according to Fig. 3. Amonitoring unit is connected to the SCM MUX using one ofthe downstream sub-carrier channels to monitor the fiber.

Fig. 5. Monitoring system architecture. Only one output fiber is depicted forsimplicity.

An optical circulator is placed between the downstream laserand the RBF to bring the backscattered light signal back to

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the monitoring unit. The frequency is varied stepwise and thephase and amplitude of the modulated backscattered signal ismeasured. This scheme is similar to the step frequency methodproposed in [24] and differs from the conventional I-OFDR,in which the frequency is swept linearly and the heterodynebeat of the backscattered signal and the reference is detected[25]. The intensity of the modulated Rayleigh scattered signaldecreases as the monitoring frequency increases, so that thelower the channel frequency the better. Nevertheless, withproper amplification the monitoring signal could be connectedto any of the sub-carrier channels so that a dedicated sub-carrier would not be necessary.

B. Analytical ModelConsider an optical fiber link with length L and also

consider that the input power is modulated at an angularfrequency Ω as P =P0 cos (Ωt) generating a modulated opticalintensity along the fiber. The detected backscattered signalcurrent S(k) from the monitored fiber link is given by thesum of reflections that may occur, for example at the end ofthe link, and the Rayleigh Backscattered Signal (Rayleigh BS)along the fiber.

S (k)=∑i

DP0Rie−2αziej2kzi

+

L∫0

C (z′)m (z)F 2 (z′)DP0e−2αziej2kzidz′

(1)

The factor D is the detectivity of the photodetector, α is thefiber attenuation and Ri is the reflectivity at the reflection pointzi. The factor C(z) represents the Rayleigh BS coefficient ofthe fiber along the link and is a piecewise constant function,eventually changing its value when two different fibers areconnected or spliced. The fault loss function F (z) is also apiecewise constant function describing the losses at the faultsalong the fiber. Its value is 1 up to the first fault, decreasingto δ1 after the first fault with loss δ1 and is sequentiallymultiplied by δi after each fault position zi. The loss functionis squared because the light passes twice along the lossy pointin its round trip from the optical transmitter to any locationafter the fault in the fiber link and back again. The factork = nΩc is the modulation wave vector, where n is thegroup index of refraction of the fiber. The bounded function0 < m(z) < 1 describes the wavelength-dependent randomamplitude fluctuations of the Rayleigh intensity, also calledCoherent Rayleigh Noise (CRN) [25].

Considering that the scattering coefficient is non-zero onlywithin the fiber length 0<z <L, the limits of integration inthe integral term in Eq. 1 can be extended to ±∞. Taking theInverse Fourier Transform (IFT) of the recorded data withinthe channel bandwidth will result in:

S(z)=∑i

DP0Rie−2αzi 1√

2π

+∞∫−∞

W (k) ej2kzie−jkzdk

+A√2π

+∞∫−∞

L∫0

m (z)F 2 (z′)e−2αz′ej2kz

′W (k)e−jkzdz′dk

(2)

where C(z) can be assumed to have a constant value C allalong the link if a single type of fiber is used, and we took theliberty of simplifying the constants into A = CDP0. In prac-tice, measurements on the received signal are performed on alimited set of frequencies within the sub-carrier band allocatedfor data transmission defining a limited set of correspondingwavenumbers, which will act as the window for the IFT andis represented by W (k).

For clarity purposes, the expression in Eq. 2 is split into twoterms: SF (z), corresponding to the localized reflections; andSR(z), corresponding to the continuous Rayleigh backscat-tered signal contribution. This way, S(z) = SF (z)+ SR(z).The first term, SF (z), describes a sum of reflection peaks,which, after integration in k, yields:

SF (z) =∑i

Bie−2αziW (z − 2zi) (3)

where W (z) is the transformed window function. If we assumethe window to be sufficiently broad in frequency to span thewhole fiber length, its transform becomes sharp in distance andcan be replaced by δ(z − 2zi), the offset Dirac delta function.Upon this simplification, we can relate the reflectivity of thelocalized reflections along the fiber link directly to the peaksin the transformed SR(z) data series. Neglecting the muchsmaller Rayleigh contribution to the signal intensity at positionzi, the reflectivity is given by:

Ri =SF (zi) e

2αzi

DP0(4)

Focusing on the mathematical development regarding thesecond term of S(z), we find that, upon integration on k,SR(z) becomes:

SR (z) = A

+∞∫−∞

m (z′)F 2 (z′) e−2αz′W (z − 2z′) dz′ (5)

which represents the convolution between the signal intensityand the window function. Using the same window approxima-tion employed previously, we write Eq. 5 as:

SR (z) =A

2m(z2

)F 2(z2

)e−2αz (6)

In practical terms, the measured function is S(z). Note,however, that SR(z) equals S(z) except for a discrete numberof reflection points which are usually discarded in loss calcu-lations [26]. Hence, we replace SR(z) by S(z), which is theexperimentally acquired data, and write the loss function F (z)by taking its square root and employing the normal OTDRtwofold scale factor for the z-axis. The loss function F (z)can, therefore, be calculated in dB as:

5 log(S (z)

)= A

∣∣dB

+F (z)∣∣dB−αz

∣∣dB

+ 12m (z)

∣∣dB

(7)

The information conveyed by Eqs. 7 and 4 is that theFourier transform, S(z), of the frequency dependent fibertransfer function S(k) obtained from the Network Analyzer(NA), describes the backscattering profile of the fiber includingfaults, losses and localized reflections along the link. Thereason why this kind of loss detection is scarcely observed can

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be attributed to the fact that single reflections are much moreintense than the incoherent Rayleigh backscattered signal,usually filtered out, as in [24], which focused in discretereflections.

The CRN function m(z) appears as a wavelength dependentadditive random noise, which also depends on RF windowused for raw data acquisition. Hence, if measurements of thereceived signal are performed at different optical wavelengthsor different sets of RF modulation frequencies all terms inEq. 7 will remain unchanged except for m (z). Since thisfunction is randomly varying between 0 and 1, the averageof a great number of different measurements will converge tothe constant 1

2 (−3dB in logarithmic scale), which means thatthe coherent Rayleigh noise can be averaged out by averagingthe IFT of different sets of measurements.

It should be pointed out that the measurement of thebackscattered signal transfer function is not an OFDR-likemeasurement, where the backscattered signal beats with areference signal and the time scale is given by the RF sweeprate. In our approach, the frequency value is swept from aninitial value (fi) up to a final value (ff ) which, from theNetwork Analyzer performance’s point of view, corresponds tosubdividing the full frequency span in frequency steps (1600steps at most). The device holds the output frequency valuefor a given time before setting it to the next step value. Settingthe hold time as the round trip time of the signal inside thefiber, we enforce a steady state measurement at each frequencystep.

C. Experimental Setup

Fig. 6 displays the experimental setup of the SCM-PONfiber monitoring system. For simplicity, a single fiber wasdirectly connected to the optical circulator. The sub-carrierfrequency, generated by a Network Analyzer (NA), was chosento be 205MHz with a monitoring signal swept inside a 10MHzband. Therefore, the total input signal span modulating the LDranged from 200 to 210 MHz or any 10 MHz range withinthe 200− 220 MHz sub-carrier band.

Fig. 6. Experimental setup for SCM-PON fiber monitoring.

The transmitter optical output was connected to port #1of an optical circulator while a photodetector followed byan electrical signal amplifier was connected to port #3. Thebackscattered sub-carrier signal is also fed to the NA in aself-homodyne detection scheme. We use the supposition of aself-homodyne detection scheme due to the already discussedfact that the measuring periods are far greater than the opticalpath delay and, thus, the steady state regime is guaranteed.The response of the NA can, therefore, be interpreted as thefiber’s transfer function.

In order to remove the effects of any residual reflectionsat the circulator and also to compensate the effects of thetransfer functions of each of the electrical equipment – RFamplifiers, PD, LD – we record a reference measurement witha fully attenuated signal at the circulator’s #2 port. Since themonitoring signal range is 10 MHz, the achieved round tripspatial resolution is 10 meters [27]. Test measurements wereperformed with 12- and 1-km fibers in order to verify thesensitivity of the technique.

D. Results

Fig. 7 shows the backscattered signal intensity measured inthe range between 200 and 210 MHz for the setup shown inFig. 6. Two spools of single mode fibers with 8.2 and 4.1km with a 2-dB loss induced with a bend at the splice wereused. The loss was calibrated with a standard OTDR priorto the measurement. Full RF modulation depth was used andthe sweep time and video bandwidth of the Network Analyserwere adjusted accordingly to ensure the steady state condition.Sweep averaging was also employed to increase SNR.

Fig. 7. Backscattered signal as function of the modulation frequency.

Although apparently no information is conveyed by the sig-nal in Fig. 7, the corresponding IFT trace, which is displayedin Fig. 8, clearly exhibits the fiber’s signature. Our monitoringsignal is then chosen as the IFT trace of the backscatteredmodulated signal with the horizontal scale properly dividedby a factor of two accounting for the two-way optical pathalong the fiber.

As expected from the model, the fiber characteristics andfault positions can be obtained from the fit of the trace to apiecewise linear function. Note that the red-dotted line thatappears in Figs. 8 and 9 is a linear fit inside the stretchesbefore and after the break and help visualizing the result. Thisfit was performed based on a priori knowledge of the fault’slocation and does not incur into signal processing methods.This result, however, would be automatically available in caseof employing an automatic fiber fault detecting technique suchas the one described in [28].

The measurement of a shorter fiber (∼ 1.1 km) was alsoconducted in order to assess the technique’s performance. Fig.9 presents the monitoring signal for a 1.1 km fiber with a 1-dB induced bend loss at ∼ 600 meters. Again, we see thatthe technique is able to detect small faults even in short-rangefiber links. Regard, however, that a diminished dynamic range

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Fig. 8. FFT of the measured backscattered signal. The end position of thefiber link is an open FC-APC connector. Time scale was converted to distancescale.

is achieved for shorter fibers when contrasted with that of alonger fiber since the integral of the Rayleigh backscatteredpower is lower in this case.

Fig. 9. Monitoring signal corresponding to a 1-dB loss induced at 600 m ina 1.1 km-long single mode fiber.

Our results show a Dynamic Range limited to a maximum of∼7 dB, which is affected mainly by our detector’s sensitivity(3pW/Hz1/2 NEP and 4 · 104V/W gain). Even though raisingthe probe signal power would also increase the dynamicrange, this would harshly affect data transmission and isnot considered as a possibility. Also, we could resort tolower frequency sweeps since the 1/f factor on the probingsignal is determinant for a lower dynamic range. Nevertheless,our goal is to allocate the data channel for monitoring solowering our frequency band does not figure as a possibilityfor increased dynamic range. Therefore, we believe that, inorder to achieve higher dynamic range without impairing thesystem’s architecture, higher sensitivity detectors should beemployed.

When performing real time monitoring, the RF modulationdepth of the monitoring tone must be lowered to 1/M (inan M -channel configuration). In our particular case, M = 8so the monitoring RF power was decreased correspondingly,thus impacting the SNR. Here, we make use of the previouslymentioned randomness of the CRN term m (z) of Eq. 7 to in-crease the SNR by averaging traces obtained with different sets

of measuring frequencies. The 10 MHz sweep was repeated10 times inside the 20 MHz band of the downstream RoFsignal, from 200 to 210 MHz, from 201 to 211 MHz and soforth in 10 interleaved frequency combs. The IFFT traces werecorrespondingly averaged and the result is a clearer acquiredsignal, which is reflected in the narrowing of the distributionof intensities around the piecewise linear fit as shown in Fig.10-a. A note should be made that averaging over 10 sets offrequencies was enough to bring the noise level down to thelevel obtained at full modulation depth shown in Fig. 8 andFig. 9. Further confirming the white noise characteristics of theCRN, we plot, in Fig. 10-b, the CRN distribution as a functionof the number N of independent frequency sets used for CRNaveraging. The overall trend follows quite well the generalN−1/2 dependency expected for a fully random behavior ofthe CRN.

Fig. 10. a) Statistical distribution of the CRN contribution for a monitoringsignal trace measured with a single frequency set (left) and averaged over 10frequency sets. b) CRN noise vs number of frequency sets for CRN averaging.The line is the expected N−1/2 dependence.

In principle, the number of independent measurements couldbe increased almost indefinitely, as far as the CRN can beconsidered as a white noise. The trade-off is the time requiredto perform the full measurement. Of course there should be alimit on that assumption, as for very close frequency combsthe RF phase is practically the same along all the fiber so thatthe signal from neighboring frequencies would suffer the sameinterference and, thus, would not contribute to CRN averaging.

The spatial resolution of the reported technique is limitedby the bin of the Fourier Transform, given by the inverse ofthe frequency span c/2n∆f . If we keep the resolution fixedat 10 m, the number of acquired frequency steps requiredfor the 2-km range measurement in Fig. 9 is 400 points,whereas 4.000 are needed for a 20-km range as in Fig. 8.Taking into consideration that the measurement time of asingle frequency step must be at least equal to the round triptime of flight 2nL/c, a measurement over a single frequencyset is performed within one minute assuming a 20-km range.10 minutes would then be necessary to acquire the IFT tracesover 10 independent sets of frequencies, which is a reasonablemonitoring time when compared to usual OTDR measure-ments. A shorter piece of fiber would be measured faster, sothat, for the same measurement time, an even higher decreasein the CRN amplitude would be obtained by averaging the IFTtraces.

Finally, we must consider the interplay between data trans-mission and monitoring in order to put forth an estimate of the

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impact of in-service monitoring. Although we do not reportreal data transmission along with the monitoring signal in thisproof of concept experiment, we expect the interference tobe very small since the monitoring signal has been allocatedin a void sub-carrier band with corresponding RF power (aneighth of the full modulation depth). Therefore, the impactof in-service monitoring is no higher than the impact of datatransmission in a neighboring SCM channel. Hence, if laserand amplifiers non-linearity, chromatic dispersion and otherfactors which affect the intermodulation products of analogchannels transmitted along an optical link are properly dealtwith, no further penalty is expected from the introduction ofa monitoring signal in a void SCM channel. Of course, themonitoring signal requires interruption of data transmissionalong the particular channel used for monitoring. This can bedealt with either by dedicating one extra channel solely formonitoring or by distributing the entire monitoring measure-ment over all channels: a short interruption on each channelassociated to a short monitoring time within the channel band-width with averaging performed over all measured frequencies.

IV. CONCLUSION

We have experimentally demonstrated a method for fibermonitoring based on the backscattered signal of a radio-frequency sub-carrier in a sub-carrier multiplexed radio-over-fiber transmission system. Even though the technique achieveslow dynamic range, it is an interesting candidate for shortrange mobile fronthaul applications with low optical split-ting ratios. In this context, we achieved a 10 meters spatialresolution and ∼ 7dB dynamic range enabling cost-effective,in-service monitoring. The theoretical background used tomodel the received signal is highly accurate and predicts themajority of the effects tested experimentally. To overcomeexcessive noise contribution from CRN, off-band averagingcan be employed even inside a single sub-carrier band.

We strongly believe that the technique has a broad ap-plication range but the reduced dynamic range might be ahindrance for its deployment. Therefore, we set the improve-ment of achievable dynamic range as our main future point ofinvestigation.

ACKNOWLEDGMENT

Boris Dortschy from Ericsson Research, Ericsson AB,Maria Marquezini from Ericsson Telecomunicacoes, CNPqand FAPERJ are acknowledged for their support.

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