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Rayleigh Back-scattering reduction by means of QuantizedFeedback Equalization in WDM-PONs

J osep Prat

Universitat Politecnica de Catalunya (UPC), J ordi Girona 1, TSC-D5, Barcelona; jprat@tsc.upc.edu

Abst ractQuantized-Feedback Equalization at the digital decision of the OLT-RX allows recovering thelow frequency components after filtering out the associated corrupting Rayleigh back-scattering noise

added in the bidirectional WDM-PON transmission, obtaining a substantial tolerance increase.

Introduction Wavelength multiplexing is considered as a

next key technological step in the progresstowards unlimited FTTH access [1-13].Wavelength reuse in reflective colourless ONUsfor efficient WDM-PONs is often limited by theintrinsic Rayleigh scattering diffused in theoptical fibre. Downstream and upstreamcommunications interfere between them in theform of random optical noise with an opticalspectrum corresponding to the courter-propagating signal; after the receiver, this noisemixes with the photo-detected received signal.

To minimize its effect, several techniqueshave been proposed, like optical spectrumbroadening, reducing their overlap by means ofwavelength dithering [2], phase dithering [3],wavelength shifting [4,5], spectrum slicing [6],

PSK [7-9] and FSK [10] modulations. At theelectrical side, it has been reduced by precoding[11] or SCM modulation [12]. Also it can belimited by the proper allocation of the distributionnode and the ONU gain adjustment [13].

In the basic scenario, the up-streamwavelength is generated at the OLT andtransmitted as a continuous wave carrier (CW)to the ONU, that modulates it imprinting the up-stream data. At the OLT receiver, the detectedup-stream signal mixes with the Rayleigh Back-scattering (RB) mainly caused by the down CW,

generating electrical noise; this noise is notspectrally flat but has a main component at lowfrequencies, resulting from the beating of thetwo carriers coming from the same laser but withuncorrelated phase noises. This produces astrong noise band centred at DC with a 3 dBbandwidth, which is generally the double of thelaser linewidth, in the range of MHz for typicalDFB lasers. Recently, this has been combatedby precoding the data with a code thatminimizes the low frequency components sothat they can be eliminated at the RX, togetherwith the RB noise [11].

Here, we propose to eliminate the lowfrequencies with an alternative approach, by

filtering the low frequency components out andrestoring them at the receiver by means ofelectronic Quantized Feedback Equalization(QFE) [14-16]. QFE is a form of DecisionFeedback Equalization (DFE) with loop pathlow-pass filtering. This is a simple technique thatallows using standard NRZ signalling with longidentical bit sequences without need forprecoding. This method is also helpful to simplifythe transceivers design as to alleviate the needfor DC coupling or very low frequency cut-offs. Itoffers interesting possibilities for burst-modehybrid WDM/TDM-PONs.

QFE and system scheme

The system configuration corresponds to aWDM-PON with colourless reflective ONU andcentralized wavelength generation at OLT.Figure 1 shows the experimental set-upassembled for the proof of this concept, and theQFE scheme at the receiver end.The OLT generates the downstream

transmitted optical signal, at 1551 nm, by a DFBlaser with a linewidth as low as 1.2 MHz.The ONU is based on an RSOA, equalized to

operate at 2.5 Gbit/s. The input power to theRSOA was adjusted to -12 dBm using thevariable attenuators; its gain is about 15 dB. A

L

RSOA

PRBSBERT

eq

HPF

LPF

+-D

D

OLT ONU

QFE

Fig.1: System and QFE scheme

ECOC 2010, 19-23 September, 2010, Torino, Italy

978-1-4244-8535-2/10/$26.00 2010 IEEE

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20:80 2x2 coupler is used at the ONU input. Totest the low frequency components in packetoperation, the data frame shown in Fig. 2 hasbeen generated following the G.984 GPONstandard requirement [17]: 128 bits with a 27-1PRBS plus 72 consecutive ones, plus again a

27-1 PRBS, plus 72 consecutive zeros. Theoutput extinction ratio is 5 dB, at 0 dBm outputpower.

At the OLT-RX, after the circulator, an opticalpreamplifier is located before the photo-detection with a PIN+TIA, with an overallsensitivity of -33 dBm in back-to-back.

For this investigation, the analogue photo-received signal is high-pass filtered (HPF).Typically this is done naturally with a DC-blockbetween the amplification stages, with a low cut-off frequency in the range of tens of KHz. Here,this is substantially increased, to about 10 MHz,

with a serial capacitor of about 180 pF, in orderto remove the low frequencies components withthe associated RB main component.

Finally, the signal is digitally regenerated witha QFE. It is based on a conventional datarecovery (decision comparator and synchronoussample&hold) whose output is filtered and fed-back to the input, combining with the receivedsignal. The loop has a low-pass filter (LPF) witha similar cut-off frequency as the previous HPF,so that both are almost complementary. Thefeedback loop operates as restoring the slowenvelope before it is lost, in absence of errors.

Its propagation delay is required to be short toavoid error propagation.

Here, as regenerator, a commercial limitingamplifier with data&clock recovery has beenused, and the feedback loop has been externallyadded to the IC, with single pole low-passfiltering, at about 12 MHz cut-off; the feedbackdelay is about 700 ps. The decision threshold is

optimized in each case.

Experimental results

Figure 2 shows the measured received data atthe receiver, before and after the bit decision forthe three cases: conventional decision receiver,receiver with HPF and conventional decision,and receiver with HFP and QFE. We observethat in the second case the data is faulty in thelong consecutive bit sequences, while with QFEthese are partially restored, allowing an openeye diagram signal.

The launched power has been varied, from -3to +13 dBm, modifying the Optical Signal to RBRatio (oSRR), from 15 to 25 dB. In thisconfiguration, the RB received noise power waswell above the ASE and RX noises, in order toanalyze the system tolerance to the RB. TheoSRR could not be extended further due to thelimited gain of the ONU.

Figure 3 shows the obtained BER for theupstream transmission as a function of theoSRR for the conventional reception and for theQFE. Without HPF+QFE, a BER of 10

-4 isobtained with an oSRR of about 22 dB (this not

a general optimum that could be obtained withan ideal RSOA and ER, but is serves as thecomparison reference here). Adding the HPF,without QFE, the signal could not be wellrecovered due to the high signal distortion.When applying HPF+QFE, an improvement of

Fig. 2: Received data at the receiver, before andafter decision. First pair: conventional decision

receiver; second: receiver with HPF and

conventional decision; last pair: HFP and QFE.

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

oSRR (dB)

log

BER

DFB+convent.

DFB+HPF+QFE

ECL+convent.

ECL+HPF+QFE

DFB+HPF

ECL+HPF

Fig. 3: BER versus oSRR ratio for the conventionaland the QFE receiver.

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the sensitivity of about 2 dB has been obtained,to about 20 dB oSRR, for 10-4 - 10-5 BER. ForBER above 10-3, the QFE can produce errorpropagation, that can be handled by controllingthe input amplitude to the circuit.The same experiment was performed with an

external cavity laser (ECL) as OLT-TX laser,with a narrower linewidth of about 100 KHz. Inthis case, the improvement when applying QFEincreases to about 3 dB. This is explained by thehigher portion of RB noise filtered out at lowfrequencies by the HPF, as compared to theDFB TX, that has a wider noise spectrum overthe cut-off.

Conclusions

QFE can be a practical form of data recoveryin WDM-PON systems limited by Rayleigh

Backscattering noise. It can increase itstolerance in several dB and, moreover, canrecover the low frequency components of thedata burst, here below 10 MHz. More substantialincrease of tolerance against RB is expected tobe obtained when expanding the modulationbandwidth of the RSOA and when combiningthis technique with optical spectrum widening.This work was supported by the European FP7 SARDANA

project.

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