vdsl transmission over a fiber extended-access network

VDSL transmission over a fiber extended-accessnetwork

Jason J. Lepley, Manoj P. Thakur, and Ioannis Tsalamanis

Department of Electronic Systems Engineering, University of Essex, Wivenhoe Park, Colchester,Essex, CO4 3SQ, United Kingdom

Carlos Bock, Cristina Arellano, and Josep Prat

Department of Signal Theory and Communications, Universitat Politècnica de Catalunya,Barcelona, Spain

[email protected]

Stuart D. Walker

Department of Electronic Systems Engineering, University of Essex, Wivenhoe Park, Colchester,Essex, CO4 3SQ, United Kingdom

RECEIVED 27 MAY 2005;REVISED 1 JULY 2005;ACCEPTED5 JULY 2005;PUBLISHED 28 JULY 2005

We describe two schemes developed to carry very-high-data-rate DSL (VDSL)signals over a fiber to the cabinet architecture for the upgrade of legacy coppernetworks. The first technique exploits the subcarrier bandwidth of uncooledsemiconductor lasers for the transmission of multiple VDSL signals by usinginexpensive interfacing hardware in the optical network unit (ONU). With ahybrid fiber–copper link comprising> 100 m of twisted-pair copper cable andas much as 45 km of single-mode fiber, data transmission comparable with thatfor fiber to the home is achieved. In an extension to this design we demonstrate asystem using a reflective semiconductor optical amplifier in a carrierless remoteONU configuration. Such a design might provide wavelength agility in thefiber access network for signal routing or active bandwidth allocation. © 2005Optical Society of America

OCIS codes:060.4250, 060.2330.

1. Introduction

Fiber to the home (FTTH) provides the ultimate wireline access medium because of itseffectively unlimited bandwidth. However, without significant opportunity for new revenuegeneration, for example, through high uptake rates of triple-play services, the economicmodel for wide-scale FTTH deployment remains weak [1, 2]. The business case for fiber-to-the-curb or -cabinet (FTTC) deployments, particularly with regard to capital expenditure,however, is much stronger. Here the fiber cable replaces much of the existing copper linkbut leaves the final copper drop link untouched. The cost advantages are therefore drawnfrom a potential reduction in the fiber plant (depending on the network topology deployed),greater reuse of existing (copper-based) infrastructure, and lower installation and purchasecosts of the end-user customer-premises equipment (CPE). Moreover, data rates of very-high-rate digital subscriber line (VDSL) and the spectrally enhanced VDSL2 technologiescan now exceed 100 Mbytes/s symmetric transmission over relatively short distances (aslong as∼ 300 m), making it an ideal transmission format for FTTC (cabinet) deployments.

One considerable expense in deploying DSL over an FTTC architecture is the require-ment for the installation of a remote digital subscriber loop access multiplexer (DSLAM),

© 2005 Optical Society of AmericaJON 7627 August 2005 / Vol. 4, No. 8 / JOURNAL OF OPTICAL NETWORKING 517

deployed in the optical network unit (ONU). The heavy power requirement and increasedfootprint of such systems places a significant burden on both the capital expenditure andoperating expenditure of this architecture, and alternatives are sought [3]. In this paper,we present results of experiments on a scheme to provide fiber optic extended DSL sig-nals over an FTTC architecture with inexpensive low-power (< 600 mW) ONU interfacingequipment while retaining the DSLAMs at the central office (CO). Furthermore, provi-sion for multidwellings or multiple sets of CPE is afforded through the use of subcarriermultiplexing in the ONU–optical line terminal (OLT) interfacing equipment. A noteworthyadvantage of this system is that it provides a readily deployable upgrade path for fiber pene-tration into legacy copper-based access networks. This could form part of a staged upgradesolution that, with sufficient uptake of triple-play services, could culminate in an FTTHnetwork.

We study two ONU hardware architectures; the first uses a conventional optoelectronicinterface comprising a laser and photodiode. This could, for example, consist of a bidirec-tional optical transceiver or of a photodiode and laser pair. A particularly suitable choice oflaser for this application would be the VCSEL, as it is inherently inexpensive and suited tointegrated circuit design. For the purpose of demonstration in this work we use a distributedfeedback laser operating at∼ 1544 nm. In a further implementation of the ONU hardware,we describe an optical interface consisting of a photodiode and a reflective semiconductoroptical amplifier (RSOA) [4]. The RSOA consists of a conventional SOA in combinationwith a rear facet mirror such that the amplified lightwave is retroreflected. This character-istic provides an increased gain from the device due to the double pass of the light throughthe gain region; an additional characteristic is its ability to modulate the incoming signal,removing the need for a local light source. Although the present cost of RSOAs wouldmake this particular solution economically inpractical at present, there is now some re-search interest in vertical-cavity SOAs, with demonstrated optical gains of> 10 dB [5].Such devices, being closely related to VCSELs, offer the potentially low manufacture andconstruction costs required for consumer products. Furthermore, their uncolored naturemay lead to significant cost savings in the ONU devices because of the economies of scaleand logistical benefits of using a generic system design.

2. DSL over FTTC Architecture

Figure1 presents the link structure used to carry multiple DSL signals over a FTTC net-work. The network contains some key features, among which are the compatibility withpassive optical network (PON) architectures and retention of the CO DSLAM equipment.The OLT–ONU hardware has been designed to operate at low power with a small foot-print, offering compatibility with the legacy cabinet and distribution point infrastructures.An ONU multiplexes each of the DSL signals from the CPE with a maximum of 24 VDSLbandwidth signals expected to be contained within a 1 GHz modulation bandwidth opticalcarrier in the current implementation. Future designs may encompass single- or vestigial-sideband techniques to improve the spectral efficiency of the subcarrier signals.

3. Local Carrier ONU Design

A set of hardware has been developed, based on the architecture of Fig.1; Fig. 2 showsa schematic of the experimental setup. The system comprises a DSL modem at the CPEcarrying 100BASE-T fast Ethernet traffic over a VDSL band 998 compliant channel. Thisis connected to the ONU equipment by 106 m of 24 AWG CAT-3 UTP (unshielded twisted-pair) cable with a loss of∼ 8 dB at 10 MHz. Such a distance would be representative of afinal drop UTP cable from a cabinet or distribution point. Clearly final drop cables can varyin quality quite substantially and may suffer from cross-talk issues as well as impairments


from the electrical characteristics of, for example, bridged taps and splices. The bit-loadingcharacteristics of the modem would attempt to optimize the performance of the data linkeven over such poor-quality links; however, this might ultimately lead to a slight variationin the data-carrying capabilities of certain legacy copper cables.

Central office

OLTDSLAM ONU

CPE

Distributionpoint

Customerpremises

Fibreaccessnetwork

Central office

OLTDSLAM ONU

CPE

Distributionpoint

Customerpremises

Fibreaccessnetwork

Fig. 1. Network architecture used for the DSL over optics solution.

100BASE-T

HPF

LO

Photodiode

UTP

DirectionalCoupler

HPFDFB-LD

LO

Photodiode

DirectionalCoupler

Internet

ONU

OLT/CO

Optical fibre

Upstream

Downstream

DSLAM

CPE modem

DFB-LD

Fig. 2. VDSL over FTTC experimental setup.

The UTP from each CPE terminates in the hardware represented by the ONU in Fig.2.The line initially terminates into an electronic directional coupler to split the upstream anddownstream signals. The design of the directional coupler consists of a lossless differen-tial balanced op–amp pair with a measured isolation of 23 dB in the forward direction and> 80 dB in the return direction. The forward direction is defined here as being from the op-tical receiver toward the modem or DSLAM termination (i.e., the direction of the arrows inFig. 2), and the converse is true for the reverse direction. The directional coupler has beenfurther designed to be frequency independent, thereby allowing for a band-plan agnosticONU solution capable of carrying both quadrature-amplitude-moduation- and discrete-multitone-modulation– (DMT) based signals. This design characteristic of both the OLT


and ONU equipment makes the scheme capable of carrying true interoperable universalDSL signals. Other designs for the directional coupler were considered, among which arethe transformer-based hybrids and directional couplers; however, these suffer from limitedupstream–downstream isolation and significantly higher losses.

The differential output of the directional coupler is converted to a common-mode signalbefore being upconverted to a channel within the subcarrier multiplexed (SCM) spectrum.A passive high-pass filter is used after the mixer to remove the residual base-band sig-nal that results from imperfect conversion. In the forward path, the upconverted signal iscombined with the other SCM channels before being used to directly modulate a semicon-ductor laser, although to avoid duplicity the combined circuits are not shown in Fig.2 andbecause multiple channels have not been transmitted in this experiment. Frequency upcon-version of the DSL signals enables multiple signals to be combined within the∼ 1 GHzmodulation bandwidth of the optical components. In the current scheme, the VDSL sig-nals have a base-band spectral width of 12 MHz (24 MHz including both sidebands). Withthe 998-138-1200 spectral band plan (as defined in the ETSI specification [6]) this offersa maximum 67 Mbytes/s upstream and 40 Mbytes/s downstream data transmission rate.Given the spectral width of the upconverted signal, a 1 GHz modulation spectrum in theoptical carrier would permit 40 VDSL channels, however. with the inclusion of 16 MHzguard bands the channel count would be reduced to∼ 25.

In the return path, the output of the circulator is connected to a photodiode before beingpassively split, downconverted, and reapplied as the return signal to the directional coupler.The output of the laser is connected by a circulator to the fiber link, which consists of anunamplified section of non-zero-dispersion shifted fiber (NZDSF) with various lengths. Thefiber carries the signal to the OLT terminal equipment, which includes a circuit identicalto the ONU, the output of which connects directly to the CO modem (or DSLAM). Forexperimental purposes both OLT and ONU transmitters operate within the 1550 nm band.

Results obtained demonstrate two key experimental observations. First, the transmis-sion performance of each VDSL signal over a range of subcarrier multiplexed channelscan be assessed. The assessment was performed across the approximate frequency range50–1000 MHz, principally governed by the mixer bandwidth. Second, performance acrossincreasing optical distance can be observed. Figure3 shows baseline performance of themodem transmission rate through the ONU–OLT equipment for a range of subcarrier fre-quencies. The results indicate a mean downstream rate of 46.4 Mbytes/s and an upstreamrate of 24.1 Mbytes/s; these compare with the 67 and 40 Mbytes/s respectively available tothe fast-998 band plan used, corresponding to transmission efficiencies of 69% and 60%,respectively. The decreased efficiency results almost exclusively from the upconversionand downconversion processes, namely, the conversion loss of the mixers. It is expectedthat this efficiency could be improved with better linearization in the mixers. For compari-son, the worst-case measurements from Fig.3 were 40.7 and 20.4 Mbytes/s for upstreamand downstream, respectively (corresponding to 61% and 51% transmission efficiencies).The variability of the results was partly due to phase mismatching between the local os-cillator and the received signal in the downconversion mixers, which could be removed byincorporating a phase-locked loop into the circuit design.

As a measure of the unconverted efficiency, transmission of base-band signals throughthe same circuit (i.e., bypassing the mixers) produced data transmission efficiencies of 91%and 97% for the downstream and upstream signals, respectively. This resulted in transmis-sion rates of 96 and 48.5 Mbytes/s over a 105/50 extended-998 band plan.

To measure transmission performance over the fiber optic extended link, the setup ofFig. 2 was used with increasing lengths of NZDSF, and again performance was measuredas an average across the full subcarrier spectrum. The results for the baseline optical (i.e.,a patch lead), 20 km, and 45 km of the fiber are plotted in Fig.4. As expected, these show a


gradual decrease in the data rate with distance, reducing the mean downstream rates to 37and 28 Mbytes/s for 20 and 45 km, respectively, while the upstream rates are reduced to24 and 12 Mbytes/s for the same respective distances. In keeping with the expected accesstopologies, no optical amplification was used. The results therefore follow a predictabledegradation due to the increased losses and their consequent reduced signal-to-noise ratio,as received by the modems.

0

10

20

30

40

50

60

0 200 400 600 800 1000Subcarrier channel frequency (MHz)

TX ra

te (M

bps)

DS rate

US rate

Fig. 3. Baseline data rate versus subcarrier frequency though the OLT + ONU interface.

0

10

20

30

40

50

60

70

ElectricalBaseline

(baseband)

ElectricalBaseline

(upconverted)

OpticalBaseline

20km NZDSF 45km NZDSF

Dat

a tra

nsm

issi

on ra

te (M

bps)

UpstreamDownstream

Fig. 4. Mean data rate across subcarrier spectrum over increasing transmission distance.

Chromatic dispersion is not thought to be an issue with this scheme because of thesmall (12 MHz single-sideband) bandwidth of the data signals. A further test with 25 kmG.652 single-mode fiber (with its higher dispersion) incurred a similar penalty to the 20 kmsection of the NZDSF, and a further experiment, replacing the fiber link with a dispersion-compensating fiber module (with a dispersion of−1300 ps/nm at∼ 1550 nm, designed tocompensate for∼ 75 km of G.652 fiber) with a loss similar to the 20 km NZDSF, again


yielded transmission similar results to the 20 km NZDSF measurement.

4. Carrierless ONU Design

For the second part of this work, the design of the ONU equipment has been altered toinclude the RSOA device (as shown in Fig.5). The OLT equipment remains the same asthat shown in Fig.2.

HPF RSOA

LO

Photodiode

UTP

DirectionalCoupler

ONU

Fibre

to CPE to PON/CO

HPF RSOA

LO

Photodiode

UTP

DirectionalCoupler

ONU

Fibre

to CPE to PON/CO

Fig. 5. RSOA-based ONU hardware.

In this ONU configuration the RSOA is used as a carrierless transmitter, in that it am-plifies and modulates the reflected light beam rather than generating a new light beam. Thisextends the functionality of the RSOA-based ONU to wavelength assignment or routing ar-chitectures [7], and the colorless nature of the ONU designs may lead to positive reductionsin the cost of the ONUs as the design becomes more generic.

As the nature of the DSL signal is such that downstream and upstream data are fre-quency division multiplexed, the residual downstream signal that is amplified and reflectedis effectively filtered by the DSLAM. A separate photodiode is used for detection of thedownstream carrier signal. The upstream and downstream paths are power split by usinga 3 dB coupler, with care taken to ensure that Fresnel reflections (which may lead to op-tical feedback in RSOA) are minimized. As with the previous setup, the DSL signal usedis the 998 compliant VDSL standard with a 12 MHz spectral bandwidth and a maximum67 Mbytes/s downstream and 40 Mbytes/s upstream data rate.

Both base-band and SCM-based transmission tests were conducted to validate the op-eration of the RSOA-ONU architecture. The device had a transparency current of 50 mAand was biased at 80 mA; the upstream data were modulated on it at 40 mA peak–peak.Under these conditions, the RSOA provided a gain of∼ 8 dB, which is sufficient to ensuretransparency of the ONU with∼ 1.5 dB of additional gain. The modulation bandwidth ofthe RSOA was∼ 1.5 GHz, although the end-to-end bandwidth of the optical system wasrestricted to∼ 1 GHz by the OLT components.

First, the ONU was operated in the subcarrier multiplexing mode by using an upcon-version frequency of 260 MHz and the 998 67/40 band plan. With an unamplified 20 kmsection of NZSDF separating the ONU and OLT, the data rates achieved were 38 Mbytes/sdownstream and 24 Mbytes/s upstream, giving data transmission efficiencies of 57% and60%, respectively. These measurements show performance almost identical to those of theprevious transmitter design, suggesting that the RSOA would be an equally capable trans-mitter. With the mixer circuit bypassed to transmit in base-band mode, the data transmis-sion rates were 48 Mbytes/s downstream and 25 Mbytes/s upstream, demonstrating datatransmission efficiencies of 72% and 63%, respectively.


5. Conclusions

We demonstrate practical results of a novel subcarrier multiplexing scheme for transmissionof multiple-band-plan agnostic universal-DSL signals over a fiber optic extended network.Good performance is shown over the full subcarrier spectral range and distances as longas 45 km of single-mode fiber, making this a possible solution for X-Large (or extended)PONs. By using these low-power ONU–OLT interfaces in a FTTC–FTTN (neighborhood)architecture, we propose an inexpensive upgrade solution for staged fiber penetration intolegacy access networks. Solutions such as this permit FTTH comparable data rates withmaximal infrastructure reuse.

We have further extended the scheme by developing a carrierless ONU transceiver usingan RSOA device. This provides a colorless ONU architecture with its potential flexibilityin wavelength assignment and routing. The transmission rates achieved for the carrierlessONU design were very similar to those achieved for the localized carrier ONU design.This would suggest that the main factors affecting performance are not related to the rela-tive characteristics of the two transmitters and that the carrierless ONU design is equallycapable of being used in this application. Both solutions demonstrate triple-play-capablebandwidth over extended PON distances and, through the incorporation of SCM, enablemultiuser scalability from the distribution point.

Acknowledgments

J. J. Lepley, I. Tsalamanis, and S. D. Walker are partners of the the Information SocietyTechnologies MUSE consortium and would like to acknowledge their support in this work.The collaboration for this work between the University of Essex and Universitat Politecnicade Catalunya has been partly supported by the ePhoton programme.

References and Links[1] N. Frigo, P. Iannone, and K. Reichmann, “A view of fiber to the home economics,” IEEE Com-

mun. Mag.42, S16–S23 (2004).[2] T. Monath, N. Elnegaard, P. Cadro, D. Katsianis, and D. Varoutas, “Economics of fixed broad-

band access networks,” IEEE Commun. Mag.41, 132–139 (2003).[3] P. Silverman, ed., “DSL anywhere,” DSL Forum Marketing Report MR-001 (2004).[4] J. Prat, C. Arellano, V. Polo, and C. Bock, “Optical network unit based on a bidirectional reflec-

tive semiconductor optical amplifier for fiber-to-the-home networks,” IEEE Photon. Technol.Lett. 17, 250–252 (2005).

[5] E. S. Björlin, B. Riou, P. Abraham, J. Piprek, Y.-J. Chiu, K. A. Black, A. Keating, and J. E.Bowers, “Long wavelength vertical-cavity semiconductor optical amplifiers,” IEEE J. QuantumElectron.37, 274–281 (2001).

[6] ETSI Technical Specification TS 101 270 part 2—VDSL Transceiver Specification.[7] I. Tsalamanis, E. Rochat, J. Lepley, and S. Walker, “Access network architecture featuring

cascaded arrayed-waveguide gratings and polarisation multiplexing transmission,” presentedat Ninth European Conference on Networks and Optical Communications, Eindhoven, TheNetherlands, 2004.


vdsl transmission over a fiber extended-access network

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