Experimental Demonstration of SCM-PON Monitoring with Baseband

Embedded OTDR

Author Names: Diego Rodrigo Villafani Caballero1, Luis Ernesto Ynoquio

Herrera1, Gustavo Castro do Amaral

1, Patryk Urban

2 and Jean Pierre von

der Weid1.

1Center for TelecommunicationsStudies, Pontifical CatholicUniversityof Rio de Janeiro,

Rio de Janeiro, Brazil.

2Ericsson Research, Ericsson AB, Stockholm, Sweden.

Corresponding author: Diego Rodrigo Villafani Caballero

Email: diego@opto.cetuc.puc-rio.br

Telephone Number: +55 (21) 98010 3737

Present address: CETUC/PUC-Rio. R. Marquês de São Vicente, 225 - Gávea, Rio de

Janeiro - Brazil, 22430-060.

Diego R. Villafani C.was born in Sucre, Bolivia, in 1989. He received the M.Sc. degree in

electrical engineering from the Pontifical Catholic University of Rio de Janeiro, Brazil,in 2013.

He is currentlyworking towards his Ph.D.at the department ofelectrical engineering in the same

institution.His research interests include optical fiber monitoring, optical fiber communications

andfuture optical access network technologies.

Luis E. Y. Herrera was born in Cajamarca, Peru, in 1977. He received the Ph.D. degree in

electrical engineering from the Pontifical Catholic University, Rio de Janeiro, Brazil, in 2016.

He is currently working as a Posdoctorate Researcher in the same institution. His present areas

of interest include optical fiber networks and fiber optic reflectometry.

Gustavo C. Amaralwasborn in Rio de Janeiro, Brazil, in 1989. He received the Ph.D. degree in

electrical engineering in 2016 from the Pontifical Catholic University, Rio de Janeiro. He is

currently working as a Postdoctorate Research Assistant at the Optoelectronics Laboratory with

Prof. J. P. von der Weid. His research interests include PON monitoring, polarization-encoded

quantum communication, and quantum optics.

Patryk J. Urban received the M.Sc. degree in optical telecommunications from the Szczecin

University of Technology, Szczecin, Poland, 2004, and the Ph.D. degree in optical access

networks from the Eindhoven University of Technology, Eindhoven, The Netherlands, 2009.

During 2003– 2005, he worked on nonlinear optics in the Optical Telecommunications and

Photonics Group, Szczecin University of Technology. During 2005–2009, he was a Ph.D.

Researcher on wavelength-reconfigurable WDM/TDM access networks at COBRA Research

Institute, The Netherlands, and during 2009–2010 as a Research Fellow at CNIT, Pisa, Italy,

where he performed further research on interferometric crosstalk mitigation methods in WDM-

PONs. In 2010, he joined Ericsson Research, Ericsson AB, Sweden, and currently holds a

position of a Senior Research Engineer and focuses on future optical access network

technologies including fiber-based mobile fronthaul/backhaul, fixed access and PON

supervision. He acts as a Reviewer for IEEE, OSA, IET, and Elsevier. During 2006–2008, he

was a Member of the IEEE/LEOS Benelux Student Chapter Board and since 2013 has been a

Member of IEEE Sweden Section Board. He is also a founder and first chairman of IEEE

Photonics Sweden Chapter. He is an author or coauthor of more than 70 publications, patent

applications, granted patents, and contributions to FSAN ODN Monitoring SG where he acted

as Ericsson’s representative.

Jean Pierre von der Weid was born in Rio de Janeiro, Brazil, in 1948. He received the Ph.D.

degree in physics in 1976 and has worked with fiber optics since 1983. Since 1990, he has been

working as a Professor at the Pontifical Catholic University, Rio de Janeiro, and led the

Optoelectronics and Instrumentation Group at the Center for Telecommunications Studies. He

also developed the interferometric technique for polarization mode dispersion measurements in

optical fibers as well as methods for evaluating and mitigating PMD impairments in high-bit-

rate transmissions. His research interests include PON monitoring and quantum cryptography.

Experimental Demonstration of SCM-PON Monitoring with Baseband

Embedded OTDR

The combination of subcarrier multiplexing and passive optical networks can

provide an efficient and cost-effective solution for the fibre and wireless

convergence in access networks. Moreover, in order to reduce operational

expenditures, a reliable monitoring technique should provide in-service

evaluation of the physical-layer. Here, we perform the experimental

demonstration of an SCM-PON system with baseband embedded optical time-

domain reflectometer monitoring. Different modulation formats were tested to

evaluate the penalty generated by the monitoring system. Based on the long-term

evolution downlink test model (E-TM3.1) our results show negligible power

penaltywhile achieving a ~12dB dynamic range with 10-meters spatial resolution.

Keywords: Physical Layer Monitoring; Embedded Optical Time Domain

Reflectometer; Passive Optical Network; Analogue Radio over Fibre.

Introduction

The evolution of Radio Access Networks (RAN) to a centralized approach is of great

interest to the fifth generation mobile communications (5G). The centralization of the

network equipment will reduce the Capital Expenditure (CAPEX) and Operational

Expenditure (OPEX) since the antenna site is simplified and the Central Office (CO)

can be shared among different operators and Radio Access Technologies (RAT).

Furthermore, the centralization of the RAN and the use of optical equipment will reduce

the energy consumption and bring the possibility of allocating large bandwidths to the

radio access segment of the network [1].

The use of Fibre to the Radio head (FTTR-h) architectures combined with

analogue Radio over Fibre transmission (a-RoF), enables cost-efficient centralization of

the network logic in a CO and further simplification of the Optical Frontend Node

(OFN). This is because the radio signals occupy an optical subcarrier which can directly

feed the Radio head (Rh) without any upper layer intervention. In this context,

Subcarrier Multiplexing Passive Optical Network (SCM-PON) is a natural candidate for

the wireline transport part of mobile networks.

In order to further reduce the costs associated to the OPEX of the network, link

monitoring is essential. Therefore, we study the possibility of embedding an Optical

Time Domain Reflectometer (OTDR) within the CO for optical fibre monitoring and

fault localization. Similar as in Wavelength Division Multiplexing Passive Optical

Network (WDM-PON) monitoring, where one takes advantage of a periodic Arrayed

Waveguide Grating (AWG) to transmitthe monitoring probe signal and the data signal

in the same drop port [2], we take advantage of the fact that a-RoF utilizes high

frequencies for data transmission to propose that the OTDR probe pulse should occupy

the base band of the same optical carrier [3]. Notice that, in this scheme, the Radio-

Frequency (RF) and OTDR signals should occupy different portions of the frequency

spectrum, i.e., this concept is better applied to the transportation of RF signals without

down-conversion to an Intermediate Frequency (IF) that may occupy the same

frequency band as the OTDR signal.

Viability of such an embedded structure is assured if both of the following

conditions are met: monitoring does not impact data transmission (such that in-service

monitoring is possible); and the achievable Dynamic Range (DR) suffices to guarantee

link monitoring of the Optical Distribution Network (ODN). Since ODNs in FTTR-h

don`t usually exceed the limit of 20 kilometres due to the bandwidth limitation imposed

by chromatic dispersion [4] and latency requirements [5], a DR higher than 10 dB may

be considered as viable for OTDR monitoring solutions.

In this paper, we perform an experimental demonstration of an embedded OTDR

in an SCM-PON. Two modulation formats are used in order to evaluate the impact of

the monitoring system over the transmitted data.We useda Bit Error Rate transmitter

(BER-t) with 51.84 Mbps to generate anAmplitude Shift Keying(ASK) modulation

format; anda Vector Signal Generator (VSG)producingthe Long Term Evolution (LTE)

downlink test model (E-TM 3.1)with 64-Quadrature Amplitude Modulation (64-QAM).

Our results show that monitoring can be conducted simultaneously with data

transmissiongenerating small or negligible power penalty under different modulation

formats. Furthermore, in-service monitoring achieves ~12 dB DR with 10 meters spatial

resolution.

Experimental Setup

Figure 1.Block diagram representation of the embedded-OTDR experimental setup.

We depict, in Fig. 1, the block diagram of our proposed architecture; this takes into

account both the embedded-OTDR monitoring unit and the emulated SCM-PON. To

demonstrate the feasibility of the experiment we used one RF optical subcarrier channel

generated in the vicinity of 2.0 GHz, the modulation format of the RF subcarrier is

defined by the transmitter,which can be either ASK or 64-QAM. This subcarrier

channel has a 20 MHz bandwidth when it carries a 64-QAM Orthogonal Frequency

Division Multiplexing (OFDM) signal and a 110.4 MHz bandwidth, measured at the

first null, when it carries an ASK signal. At the CO, a commercial standard OTDR

device was used to simplify the data acquisition and signal processing whilst ensuring

the embedding of the monitoring unit. Combination of monitoring and data signals is

done as follows. The downstream tuneable laser source is divided by an asymmetric

optical beam splitter; the high power branch is directed to an electro-optical Amplitude

Modulator (AM)excited by the data signal subcarrier; the low power branch is directed

to a tandem configuration of two Semiconductor Optical Amplifiers (SOA); the SOAs

are synchronously triggered by the detection pulse of the OTDR device and generate a

100 ns wide high peak power probe pulse (15 dBm); the high and low power branches

are combined in a Polarizing Beam Splitter (PBS) with orthogonal polarizations such

that no coherent interference effects spoil the output optical signal. Backscattered light

from the Fibre under Test (FUT) is directed back to the OTDR device by two optical

circulators (OC) and the fibre profile is traced.

Figure 2.OTDR, LTE and ASK electrical frequency spectrum.

Figure 2 shows the electrical frequency spectrum occupied by the OTDR signal,

the LTE subcarrier generated in 2 GHz, and the ASK signal up-converted to the central

frequency of 2 GHz.Note that the OTDR and data signals are located in different

frequencies and do not superimpose each other. Ensuring these different band

frequencies, for monitoring and data transmission, enables the system tomultiplex

additional channels in the vicinity of 2 GHz. This is desired since the goal is often to

serve different antenna sectors or different operators with different frequency channels

and modulation formats.

In the case of an LTE signal, the OFN architecture is rather simple since the

detected electrical signal from the photodiode directly feeds the Rh. In order to assess

the impact on data transmission, however, we make use of a matched filter at the

subcarrier's frequency and measure the Error Vector Magnitude (EVM) in the case of

simultaneous data transmission and OTDR link monitoring. To measure the EVM and

the IQ constellation, a VSG and a Vector Signal Analyser (VSA) were placed at the CO

and OFN respectively. In the case of an ASK modulation signal, the data signal is also

down-converted to the base band by a matched filter.ABER test, using a 223

−1 pseudo-

random bit sequence (PRBS) at 51.84 Mbps,is performedby using a BER-Transmitter

and Receiver (BER-T/R) located at the CO and OFN, respectively.

Fibre Monitoring Results and Impact on Data Transmission

Figure 3 and 4 show the embedded OTDR monitoring results for a 15.5-km fibre link

where twofibre spools, with 3.38 and 12.12 km, respectively, were used. By inducing a

3.3 dB fault at 3.38 km, the fault localization capability of the proposed method is

tested.

Figure 3.In-service embedded OTDR fault localizationin presence of the CRN.

It is a well-known factthat tuneable OTDR measurements suffer from the noisy

contribution due to Coherent Rayleigh Noise (CRN) given thatthe employed light

sources are highly coherent [6]. This effect is also present in Fig. 3 and challenges the

quantification of the fibre fault loss. Sweeping the optical source central wavelength

within the 0.8 nm-wide Dense Wavelength Division Multiplexing (DWDM) channel

has been proposed in order to diminish the CRN of coherent OTDR measurements and

was employed throughout our embedded-OTDR measurements [7]. Figure 4 shows the

embedded OTDR monitoring result after diminishing the CRN, where we can notice the

connector’s insertion loss in case of a clean link. A spatial resolution of 10-meters and

an 11.5 dB dynamic rangeare achieved when in-service monitoring is performed.

Hence, viability of the monitoring technique is assured for SCM-PON applications.

Figure 4.In-service embedded OTDR fault localization with diminished CRN.

The limitation on the achievable dynamic range, when contrasted with standard

OTDR measurements, originatesfrom the Rayleigh backscattering contribution from the

data channel, which elevates the noise floor level as depicted in Fig. 5, where both in-

service and offline monitoring have been conducted.The two different noise floor levels

at the end of the fibre link are due to the contribution from the data signal optical power

to the noise floor of the OTDR trace. This noise comes from different parts of the fibre,

so the OTDR noise floor signal is reduced after the 3.3 dB fault in the fibre link. Note

that the excessive noise is not a contribution of the data signal subcarrier, but of the

optical carrier that traverses the high-power branch as discussed in Fig. 1. Hence,

offline monitoring, in the context of Fig. 5, involves disconnecting the high-power

branch thus eliminating the data optical carrier rather than simply turning off the data

stream which excites the electro-optic amplitude modulator.

Figure 5.Noise Floor Comparison with in-service and offline monitoring.

Despite the higher noise floor level experimented by the proposed embedded-

OTDR system, both its fault localization and dynamic range are in accordance with

short reach SCM/WDM-PON architectures. Hence, the system is viable for deployment

in such environments, presenting a simple, reliable, and straightforward means of link

supervision.

Figure 6 shows the measured impact of the in-service monitoring over the

transmitted ASK modulation format.The BER variation in case of simultaneous data

transmission plus link monitoring, and when monitoring is turned off, can be observed.

In case of in-service monitoring, the slope of the curve changes when the Received

Optical Power (ROP) is increased, generating a 2.8 power penalty at a BER of 10-9

.

This noise is identified as a typical interferometric effect since it increases

proportionally with the ROP. Furthermore, it is the leading source of noise after the

ROP reaches -19 dBm. This interferometric effect can be considered to becaused by the

leakage in the Polarization Beam Splitter (PBS), where the data and monitoring signals

are combined in orthogonal polarization modes.

Figure 6. Bit Error Rate for an ASK modulation signal at different Received Data

Optical Powers with and without link monitoring.

As for the impact over the transmitted LTE signal, the results of the measured

EVM show that negligible penalty is introduced due to in-service monitoring. Fig. 7

depictsthe EVM variation with the received data optical power at the Rh measured

using the conjunction of VSG and VSA; this measurement shows both simultaneous

data transmission plus link monitoring, and when the monitoring is turned off. Figure 7

also shows the required EVM of 8%, which is the minimum requirement of the 3rd

Generation Partnership Project (3GPP) for 64-QAM modulation format [8]. The

minimum ROP should be higher than -22 dBm in order to achieve at least 8% EVM.

Figure 7.Error Vector Magnitude rms for an LTE signal at different Received Data

Optical Powers with and without link monitoring.

The orthogonal polarization mode combination at the output of the embedded-

OTDR system is, as previously commented, mandatoryto eliminate coherent effects

which can render the data transmission unviable. It is interesting, therefore, to

investigate the actual outcomes of the system when both monitoring and data optical

signals are combined in the same polarization mode. In view of that, we measure the

LTE signal while keeping the ROP at the OFN at -16 dBm, for which the EVM isclose

to 2% as presented in Fig. 7. Figure 8 depicts the measured constellation in both cases,

with the signals combined at the PBS into orthogonal polarization modes and with the

signals combined at a regular BS into the same polarization mode. Under different

conditions, when the orthogonal polarization mode combination is not assured, the data

signal quality is severely affected by the monitoring signal and the measured EVM will

increase dramatically (up to 12% for the presented results of Fig. 8), which is associated

with the overspreading of the constellation.

Fig. 6: Effect of different optical combination schemes on the 64

signal constellation.a) Orthogonal polarization modes; b) Same polarization mode.

Conclusions

We experimentally demonstrated an

embedded OTDR. The results show that the proposed technique achieves ~12dB

dynamic range and 10 meters spatial resolution

performed,small or negligible

dynamic rangesuffers an additional penalty due to the constant Rayleigh backscattered

signal from the data optical carrier and, therefore, is lower than would be expected for

conventional tuneable OTDR measurements. CRN mitigation is performed by sweeping

the optical carrier central wavelength within the available bandwidth for DWDM

channels. The proposition o

polarization modes is imperative for the syste

environment.

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