development of gpon upstream physical-media-dependent

2498 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 11, NOVEMBER 2004

Development of GPON UpstreamPhysical-Media-Dependent Prototypes

Xing-Zhi Qiu, Member, IEEE, Peter Ossieur, Student Member, IEEE, Johan Bauwelinck, Student Member, IEEE,Yanchun Yi, Dieter Verhulst, Student Member, IEEE, Jan Vandewege, Member, IEEE, Benoit De Vos, and Paolo Solina

Abstract—This paper presents three new gigabit-capable pas-sive optical network (GPON) physical-media-dependent (PMD)prototypes: a burst-mode optical transmitter, an avalanche pho-todiode/transimpedance amplifier (APD-TIA), and a burst-modeoptical receiver. With these, point-to-multipoint (P2MP) upstreamtransmission can be realized in a high-performance GPON at1.25 Gb/s. Performance measurements on the new burst-modeupstream PMD modules comply with GPON uplink simulations.The laser transmitter can quickly set and stabilize the launchedoptical power level over a wide temperature range with betterthan 1-dB accuracy. A burst-mode receiver sensitivity of 32.8dBm (BER = 10 10) is measured, combined with a dynamicrange of 23 dB at a fixed APD avalanche gain of 6. Full complianceis achieved with the recently approved ITU-T RecommendationG.984.2 supporting an innovative overall power-leveling mecha-nism.

Index Terms—Burst-mode, gigabit-capable passive optical net-work (GPON), optical access network, physical-media-dependentlayer, receiver, transmitter.

I. INTRODUCTION

THE passive optical network (PON) technology is basedon a passive star fiber network and offers a cost-effective

optical access solution with point-to-multipoint (P2MP) nature.With rapidly growing customer bandwidth requirements andproliferation of bandwidth in metro networks, broad-bandpassive optical networks (BPONs) [1]–[4] and the emerginggigabit-capable passive optical networks (GPONs) are expectedto prevail as the leading optical access technology eliminatingthe bandwidth bottleneck in the last mile. The full-servicesaccess networks (FSAN) GPON can provide high-band-width services to customers following different fiber-to-the-premises/cabinet/building/home/user (FTTx) scenarios.

A symmetric 1.25-Gb/s GPON system optimized for vari-able-length packet transmission is currently under developmentwithin the framework of the European IST project GIANT (gi-gapon access network) [5]. GIANT will demonstrate efficientgigabit transport for “triple play” suites of voice, video, and data

Manuscript received December 15, 2003; revised June 15, 2004. The burst-mode chip design work was supported in part by the Flemish Government underResearch Contract 010019 IWT Sympathi and in part by Alcatel Bell and STMi-croelectronics. The uplink building blocks simulations, development, and inte-gration were supported by in part by the European Commission under ResearchContract IST-2001-34523 GIANT and in part by Alcatel Bell.

X.-Z. Qiu, P. Ossieur, J. Bauwelinck, Y. Yi, D. Verhulst, and J. Vandewegeare with the Department of INTEC, Ghent University, B-9000 Gent, Belgium(e-mail: [email protected]).

B. De Vos is with Alcatel Bell, B-2018 Antwerp, Belgium.P. Solina is with Telecom Italia Laboratory, Turin 10148, Italy.Digital Object Identifier 10.1109/JLT.2004.836767

services with guaranteed quality of service (QoS) and with veryhigh bandwidth and transport efficiency [6], [7].

Fig. 1 illustrates the GPON access system. A continuousdownlink in the wavelength band of 1480–1500 nm carries1.25 Gb/s time-division-multiplexed (TDM) data from a singleoptical line terminator (OLT) toward multiple optical networkunits (ONUs) or optical network terminations (ONTs). Aburst-mode link in the 1310-nm window collects all ONU/ONTupstream traffic toward the OLT as variable-length packets at a1.25-Gb/s aggregate rate, in a P2MP time-division multiple-ac-cess (TDMA) scheme. The paper focuses on this uplink, whichis difficult to design due to the bursty nature of the multitalkertraffic.

Recently, the FSAN study group, a forum for the world’sleading telecommunications service providers and equipmentsuppliers to work towards a common goal of truly broad-bandaccess networks, initiated GPON network standardization viarecommendations for the GPON physical-media-dependent(PMD) layer and the transmission convergence (TC) layer.Both have now been approved by the International Telecommu-nication Union—Telecommunication Standardization Sector(ITU-T) and ratified as ITU-T Recommendation G.984.2 [8]and G.984.3 [9], respectively.

The paper presents an overview of the technology require-ments and specifications of the key GPON PMD building blocksin Section II. Section III illustrates the design of a generic burst-mode optical transmitter, followed by the design of a high per-formance dc-coupled burst-mode optical receiver in Section IV.The back-to-back GPON uplink modeling and its results are de-scribed in Section V. Finally, uplink burst-mode experiments arepresented in Section VI.

II. FSAN GPON PMD PROTOTYPES

A. Burst-Mode Upstream PMD Building Blocks

Fig. 2 depicts the GPON physical layer as a set of PMDbuilding blocks. The 1.25-Gb/s upstream transmitter (US-TX)mainly contains the burst-mode laser diode driver (BM-LDD),while the upstream receiver (US-RX) comprises the avalanchephotodiode/transimpedance amplifier (APD-TIA) and the burst-mode receiver (BM-RX). Table I lists the GPON PMD layerClass B key specifications in the upstream direction as defined inITU-T Recommendation G984.2. The optical distribution net-work (ODN) consists of passive optical elements such as split-ters, fibers, connectors, and splices forming an optical path.Three classes (Class A, B, C) are specified with a different ODNattenuation range of 5–20, 10–25, 15–30 dB, respectively. No

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QIU et al.: DEVELOPMENT OF GPON UPSTREAM PMD PROTOTYPES 2499

Fig. 1. GPON network architecture for FTTx scenarios. From a single OLT at an access node, it connects a maximum of 32 ONUs/ONTs at the customer’spremises via shared media of the ODN, which mainly contains a maximum of 20-km fiber and one or more passive optical splitters.

Fig. 2. GPON PMD functional building blocks consist of a downstream transmitter (DS-TX) and an upstream receiver (US-RX) at the OLT; a downstream receiver(DS-RX) and an upstream transmitter (US-TX) at the ONT. The US-TX contains a laser diode and a burst-mode laser diode driver (BM-LDD), and the US-RXcontains an avalanche photodiode/transimpedance amplifer (APD-TIA) and a burst-mode receiver (BM-RX). A burst-mode clock-phase alignment (BM-CPA) isalso developed, whose detailed design is not included in this paper.

GPON 1.25-Gb/s upstream chip set supporting Class B ODN isavailable on the open market at the time of this writing.

After extensive research, three burst-mode chips were spec-ified to be designed in 0.35- m SiGe BiCMOS technology:a 1244.16-Mb/s BM-LDD with fast and accurate digital auto-matic power control (APC), a high sensitivity, and wide dy-namic range APD-TIA receiver front end, and a burst-modereceiver (BM-RX) chip for fast but accurate signal recovery.This GPON burst-mode chip set was designed and tested suc-cessfully at the INTEC (information technology) Departmentof Ghent University, Gent, Belgium, within the Flemish IWTproject SYMPATHI (symmetrical PON at high bit rate) [10].Currently 1244-Mb/s burst-mode US-TXs and the US-RXs areintegrated into the GIANT GPON laboratory demonstrator [11].This research contributed to the FSAN efforts toward ITU-Tstandardization via Alcatel Bell and Telecom Italia Lab, and wasperfectly in line with the ITU-T GPON standardization progress[12].

B. Overall Power Leveling Mechanism

Table I specifies a minimum OLT RX overload of 7 dBmcombined with a minimum RX sensitivity of 28 dBm. Aflexible network deployment requires high sensitivity (high

splitting factor and long reach) but also a wide dynamic range(long/short optical network paths and different splitting fac-tors). These requirements are aggravated by the combinedtolerances on all-optical and elctrooptic (EO) components.As the combined 7 28 dBm is a very demanding speci-fication for 1244-Mb/s burst-mode operation, an innovativeoverall power-leveling mechanism (PLM) was proposed forstandardization [1], [10]. The PLM was adopted in the ITU-TG984.2 as an optional PMD-layer implementation. Today, ap-i-n photodiode-based RX at the OLT can obtain a minimumRX sensitivity of 24 dBm, which is suitable for Class Aoperation only, with 5–20-dB ODN loss. Therefore, the OLTRX requirements in Table I dictate the use of an APD at 1244Mb/s for Class B operation (10–25-dB ODN loss), to reach aminimum RX sensitivity of 28 dBm.

Although an increase of the avalanche gain or multiplica-tion factor of the APD by proper biasing can improve theRX sensitivity, overload figures may deteriorate as strong sig-nals result in duty-cycle distortion or saturation of the APD-TIAoutput. Optimizing the APD factor for achieving both highsensitivity and wide dynamic range is not straightforward. Mostcommercial APD-TIA modules are designed for point-to-point(P2P) transmission in continuous-wave (CW) operation. High-


TABLE IKEY PMD PARAMETERS OF GPON CLASS B 1244-Mb/s UPSTREAM

sensitivity figures are obtained at a higher , whileoverload is mostly defined with a smaller , or improved by aslow automatic gain control (AGC) loop. The fast succession ofupstream bursts, however, separated by a short guard time of 4B (25.6 ns at 1244 Mb/s), do not allow for a change of APD gainin between bursts. It requires quite some time for stabilizing theAPD gain after adjustment. Moreover, no slow AGC is possible,as a sudden overload of the TIA could happen when a high APDgain is set and a nearby ONU (strong packet P1) starts talking.Nonlinearity would cause a “tail” effect and have an impact onthe receiver sensitivity, in case the strong packet is followed bya weak packet (P2) emitted from a far-end ONU (Fig. 3). There-fore, the dynamic range specification of 21 dB must be achievedat a fixed APD gain and without a slow AGC loop. Symmetricgain clamping, however, can be used to extend the RX dynamicrange.

To further relax the 21-dB dynamic range specification ofthe OLT receiver, the transmit power level of the ONUs ex-periencing a low ODN loss should be reduced. A PLM wasconceived operating each ONU in three discrete output powermodes, with the following mean launched power: 1) normal-mode minimum and maximum 2 3 dBm, as stated inTable I; 2) mode 1 normal 3 dB; and 3) mode 2normal 6 dB. The PLM mode can be set locally via a serialperipheral interface (SPI).

The PLM implementation also requires functionalitiesbelonging to the TC layer, such as the ONU capability toincrease/reduce the transmit power on the basis of downstreammessages sent by the OLT. The US-RX at the OLT measuresthe received average power and compares it with two thresholdvoltages ( and ). The OLT then decides whether theincoming optical signal is too low or too high or within the

Fig. 3. In the worst case, the dynamic range requirement for the US-RX canbe as high as 21 dB. By implementing the PLM, the input level of the strongpacket P1 can be decreased by X dB (X = 3–6), and the input level ofthe weak packet P2 can be increased by Y dB (Y = 3–6). Overall, PLMalleviates overload of the US-RX caused by the strong packet P1 and reducesthe burst-mode sensitivity penalty for the weak packet P2 caused by the “tail”of the strong packet P1 as the guard time is limited to 4 B (see Section IVfor the details). The PLM can improve the uplink burst bit-error-rate (BBER)performance, especially in the worst case shown in this figure.

range. When an ONU receives the message to change from onemode to another, it sets its emitted power within the range ofthe new mode and then resumes sending upstream data. (Forthe detailed PLM procedure, one can refer to G984.2, App. II.)

A first benefit of the proposed PLM is a reduction of the dy-namic range requirement at the OLT receiver with 5–6 dB (from21 to 15 dB) as shown in Fig. 3. Another advantage is that it in-creases the laser lifetime and reduces the power consumption ofONUs working in mode 1 and/or mode 2. It also reduces pos-sibly strong optical reflections due to nearby ONUs.

III. DESIGN OF A GPON ONU US-TX

A. US-TX Requirements

Where performance requirements dominate for the OLT-RX,a single RX serving all ONUs, the US-TX located in each sub-scriber ONU is very important in terms of system cost and com-patibility. The US-TX must provide gated laser bias and drivecurrents, programmable between 1 and 160 mA in total, overa temperature range of 40 to 85 C, as required for outdooroperation. The currents must be digitally set to avoid trimmingelements on the printed circuit board (PCB), reducing calibra-tion costs. A dc-coupled interface between the driver and thelaser diode is needed. As the maximum TX enable and disabletime is limited to 16 b, or 12.8 ns at 1244 Mb/s, (specified inITU-T Recommendation G.984.2), whereas the launched dataafter scrambling may have 72 consecutive identical digits, nosingle time constant for an ac-coupled circuit can meet bothspecifications. Moreover, ac-coupling would require heavy linecoding of the data and a much longer guard time in betweenbursts. In contrast to GPON, Ethernet PON (EPON) systems douse ac coupling at the expense of more than a 20% throughput


Fig. 4. Simplified functional block diagram of the GPON ONU US-TX and its interfaces to the network termination (NT) digital application-specified integratedcircuit (ASIC).

decrease due to 8b10b line coding and of considerable loss intransport efficiency and interactivity due to an extended inter-packet gap (IPG).

The multiple upstream access requires laser power level stabi-lization within the short time slots allocated to a specific ONU:a US-TX must not send upstream light in timing windows allo-cated to other ONUs, as this would disturb the upstream trafficof operational services. The time needed to level a US-TX mustbe tightly restricted, and fast initialization of ONUs must be per-formed after power-on or first connection to the network with aminimum of control signals [13]. This is an important require-ment when developing high-split-ratio GPON systems, as fastnetwork recovery after, for example, power failure is needed. Onthe other hand, accurate tracking of slow laser-power-level driftis also needed during the data transmission to keep the launchedpower variation small, thus decreasing the dynamic range re-quirement at the OLT US-RX side.

B. BM-LDD Design Challenges

Fig. 4 shows the architecture of the US-TX. It mainly con-tains a high-speed laser diode and a generic and intelligentBM-LDD chip. Laser diodes are not really the limiting elementfor 1244-Mb/s upstream transmission, but the device choicehas a strong impact on the ONU cost. The BM-LDD consists ofa laser driver stage, optical level monitoring, pattern detection,a dual-mode (fast/slow) digital APC algorithm, and SPI inter-face logic. The BM-LDD chip was developed in a 0.35- mSiGe BiCMOS process. CMOS processes with even shortersubmicrometer gates require lower supply voltages and cannotprovide the dc coupling, because the laser diode voltage dropcan be 1.6 V in the worst case, whereas the transient voltagedrop caused by parasitic inductances also has to be taken intoaccount. During transmission, the laser diode must be biasedabove its threshold to reduce the turn-on delay and to limitthe duty-cycle distortion. Due to the wide spread of individual

laser characteristics, their temperature dependence, and thenonlinear relationship between laser current and optical power,both the laser modulation current and the bias current must beregulated according to the optical power and extinction-ratiorequirements specified in Table I.

The laser driver has two differential pairs, each powered bya current source and a current digital–analog converter (IDAC),for independent setting of the laser bias current I-bias and mod-ulation current I-mod. Both of them can be set quickly up to80 mA with a resolution of 0.1 mA, providing a total drive cur-rent up to 160 mA. The currents are generated by custom-made10-b IDACs designed for BM-LDD outdoor operation over a

40–85 C temperature range [14].In contrast to commercially available CW laser drivers, a

BM-LDD cannot regulate the emitted power by means of a slowaveraging measurement. Due to the bursty nature of the data,there is no stable average power available. Optical power stabi-lization is only possible when transmitting, i.e., when a shorttime slot is allocated to a specific subscriber. Fast but accu-rate optical level monitoring circuitry and an APC algorithmwere designed based on current-mode circuits [15], to overcomethe drawbacks of voltage-mode implementations at higher bitrates. The optical-level monitoring circuit contains current mir-rors and comparators. An active-input current mirror reducesthe impact of the parasitic capacitance of the laser backfacet monitor photodiode (PD), which is the main speed-lim-iting factor of the level monitoring circuitry. The active-inputcurrent mirror produces two copies of the photocurrent for the“1” and “0” level measurement, respectively [14]. During trans-mission, two current comparators compare the monitor current

with two reference currents (Iref-mod and Iref-bias) cor-responding to the desired “1” and “0” launched optical power.Each reference current needs only one calibration at room tem-perature and is digitally set via an SPI interface to avoid trim-ming elements on the PCB.


The measurements are only valid after the transmission of asufficient number of successive “1”s and “0”s. A pattern detec-tion block scans the incoming data when the BM-LDD is en-abled by a burst envelope signal (TH in Fig. 4) and searches forconsecutive strings of “1”s or “0”s with a given (programmable)length. The detection of a suitable string in the data stream en-ables the level monitoring and the APC control. Due to thisself-detection technique, no time-critical signal such as a pre-amble envelope or an arming signal is required for APC, as isthe case in most published burst-mode laser drivers [16], [17].Successive “1”s and “0”s can be programmed in a power lev-eling sequence upstream (PLSu) field with a maximum of 120B in length, and in a physical layer operation administrationand maintenance upstream (PLOAMu) field. Both are specifiedin G984.3 for power control measurements by an ONU. ThePLOAMu, part of GPON overheads, is up to a length of 13 B,where 10 B of “data” field can be used as a laser control field foraccurately tracking laser power drift during data transmission,performing a slow APC.

During each allocation period, according to the OLT control,the ONU can transmit either the PLSu during an initializationphase or the PLOAMu during data transmission for regulatingits launched power. This supports a fast APC algorithm based ona binary-like search with maximum power level protection [13].The I-bias and I-mod can start from loosely specified but safepreset values, and there is no need for the storage of a numberof calibration values in lookup tables as some burst-mode laserdrivers require. The digital APC algorithm [18] quickly and ac-curately adjusts the IDAC settings of I-bias and I-mod until thelevel errors are small enough, after which a “level-OK” signal isgenerated and sent to the NT digital application-specified inte-grated circuit (ASIC). This US-TX can drive most commercialgigabit laser diodes with a wide range of laser back-facet capaci-tance (from 2 to 15 pF), photodiode responsivity (from 25to 1400 A/mW) and laser slope efficiency in almost any cir-cumstances. No laborious calibration of the laser characteristiccurve is needed, and no off-chip component needs to be trimmedfor different laser types. There are no time-critical external con-trol signals to initiate a regulation cycle [19]. The BM-LDD chipis intelligent and easily programmed via an SPI interface, in con-trast with many burst-mode laser drivers published so far [16],[17]. To support the PLM, the launched optical power of theUS-TX is adjustable in a 6-dB range. During initialization, anONU first sets its power from low power mode 2. The minimummean power required for guaranteeing a 10-dB extinction ratiois specified at 5.5 dBm (mode 2). After level measurementsperformed at the BM-RX, an increase of 3 dB (mode 1) or 6 dB(normal mode) can be requested by the line termination (LT).

IV. DESIGN OF A GPON OLT US-RX

A. US-RX Requirements

The US-RX located in the OLT mainly consists of anAPD-TIA module and a BM-RX chip as shown in Fig. 5. Itreceives optical packets from all active subscribers in very fastsuccession, with varying signal level and phase from packet topacket. The packets are interleaved with a guard time of 4 B at1244 Mb/s (or 25.6 ns) as specified in G984.2.

The average signal level may vary 21 dB in the worst casefrom packet to packet due to the following contributions:

1) up to 15 dB of differential attenuation in the ODN(10–25 dB for Class B);

2) up to 5-dB tolerance on the mean launched power of anONU (from 2 dBm to 3 dBm);

3) a 1-dB optical path penalty over the ODN, which is com-pensated by an increase of the minimum receiver sensi-tivity.

The combination of 21-dB-level variation, a short guard time(4 B), and a maximum of 72 consecutive identical bits within apacket also requires the US-RX to be dc coupled. It is impos-sible to choose a right time constant(s) without spoiling the ini-tial conditions at the start of a new packet. This would result insevere signal distortion and burst-mode penalty [20]. If ac cou-pling would be used between the APD-TIA and the BM-RX,the high-pass filtering nature of the ac-coupled circuit would re-ject the low-frequency contents in the data payload [21], [22].Indeed any mechanism containing the memory of a precedingpacket, such as ac coupling, slow dc offset compensation, andslow AGC can hardly be employed in the GPON US-RX. How-ever, dc coupling implies the presence of dc offsets, which maydrift due to temperature variations. As dc offset can become alimiting factor in obtaining high sensitivity, a BM-RX shouldbe able to automatically correct such offsets [23]. Moreover,the BM-RX requires fast but accurate threshold setting on in-dividual incoming packets to perform dynamic-level detectionand amplitude recovery. The BM-RX must quickly extract thedecision threshold within a preamble length of a few bytes (e.g.,3 B in this case) at the beginning of each packet [8].

B. US-RX Design Challenges

The main functions and design challenges of the US-RX areillustrated in Fig. 5. High sensitivity, wide dynamic range, andfast response are important figures of merit. A 3-dB improve-ment on the sensitivity can increase the splitting ratio by a factorof 2, which almost doubles the number of subscribers that canbe connected to the network at little extra cost. A large dynamicrange guarantees a long logical reach and increases the networkflexibility. A high RX sensitivity can be obtained by applyinga higher APD factor, for example, . This, however,does not increase the dynamic range or power budget propor-tionally. A strong optical signal emitted from a nearby ONU ex-periencing a minimum ODN loss would saturate the APD-TIAwhen the APD gain is set to ten. This will result in an over-load of the TIA or a severe duty-cycle distortion of the detectedsignal. A tradeoff must be made to set an appropriate APD gainso that the combined requirement of RX sensitivity and dynamicrange can be met. This optimum gain further depends upon thetransimpedance gain of the preamplifier and the input voltagesensitivity of the BM-RX. Indeed, in a traditional optical re-ceiver, the input voltage sensitivity of the postamplifier is usu-ally much better than the sensitivity of the preamplifier, such thatthis preamplifier solely determines the complete receiver sensi-tivity. A BM-RX, however, has a minimum voltage sensitivitybelow which the fast dc-coupled threshold extraction circuitry


Fig. 5. Design challenges for a high sensitivity and a wide dynamic range US-RX.

will not operate properly anymore. For a fixed differential tran-simpedance gain of 2 k and a differential input voltage sen-sitivity of the BM-RX of 20 mV, simulation and experimentalresults show that an APD gain of around 5 to 6 is a goodcompromise to meet the requirements as given in Table I, [24].It is clear that other optimum values will result from differentvalues of the transimpedance gain. Hence, one cannot decoupleoptimization of the avalanche gain from the transimpedancegain due to the limitation of a minimum input voltage sensi-tivity of the BM-RX itself. Note that this value is quite lowwhen compared with conventional optical receivers. The reasonfor this is that at a high avalanche gain, the dynamic range of thereceiver is limited by the tail following a strong packet, whichneeds to be decayed sufficiently within the guard time of 25.6 nsbefore a following, possibly much weaker, packet can be han-dled.

The APD-TIA converts the photocurrent into a differentialinput signal for the BM-RX. The BM-RX mainly contains auto-matic threshold extraction circuitry and limiting amplifiers withsymmetric gain clamping and dc-offset compensation as shownin Fig. 5. The “level measure PLM” block was designed to mea-sure the level of the incoming signal by comparing it with twothreshold voltages that are programmable. Fig. 5 gives a simpli-fied view of the BM-RX chip and its interfaces toward the LTdigital ASIC and the clock-phase alignment (CPA). However,the design of the CPA chip is not included in this paper. TheBM-RX threshold extraction requires both a positive and nega-tive peak detection circuit to extract the amplitude information(a “1” and a “0” level) at the beginning of each packet and togenerate an offset bias for the limiting amplifier stages. Due tothe short guard time, the BM-RX needs an active reset to erasethe threshold of a preceding packet.

V. UPSTREAM LINK MODELING AND SPECIFICATION

Optical transmission system simulations are widely usedfor high-capacity core networks. However, how to accuratelymodel fast burst-mode P2MP transmission is still a challengingresearch topic. Intensive study, modeling, and simulations havebeen performed for GPON upstream burst-mode PMD building

Fig. 6. Simulation result in case of a strong packet (�11 dBm with 2-dBmargin in case of PLM) followed by a weak packet (�29.5 dBm, almost notvisible) as handled by the BM-RX. The upper trace was the input current of theTIA, and the lower traces were plotted at the differential outputs of the BM-RX.The level difference of 19.5 dB is 4.5 dB larger than the required 15 dB listedin Table I yielding ample system margin).

blocks. Modeling has been performed using three differentmethods, each method having its own advantages.

First, transistor-level models were used to validate the eyediagrams of the transmitter, and in particular the laser driver.Indeed, the required speed and accuracy necessitates a detailedmodeling of the interface between laser diode and driver, forexample, with regard to bonding wires. On the receiver side,careful modeling of any memory effects that could destroy asucceeding packet is needed. For example, Fig. 6 shows thesimulated differential outputs of the BM-RX (lower traces) to-gether with the input current of the TIA (upper trace) in theworst case of a weak packet (almost not visible) preceded bya strong packet.

In a second step, abstract models of the transistor-level cir-cuits of both the BM-LDD and BM-RX were combined withdetailed descriptions of the laser diode, the fiber plant, and theavalanche diode. In this way, eye diagrams are produced thatcan be used to quickly evaluate the influence of several systemparameters, such as the extinction ratio at the US-TX and the


Fig. 7. Simulated eye diagram at the BM-RX output. The launched outputpower of the US-TX was set to �5 dBm with an extinction ratio of 10.5 dB, a20-km G.652 fiber was included, and the BM-RX input level was �28.5 dBm,with an APD gain of 6 at 1244 Mb/s. The input referred noise current of the TIAwas 230 nA , and its differential transimpedance was 2 k.

APD gain at the US-RX, on the GPON physical-layer perfor-mance quantified by parameters such as differential range, split-ting factor, and feeder length. As an example, Fig. 7 shows thesimulated optical uplink end-to-end eye diagram for a receivedaverage optical power of 28.5 dBm with an APD gain 6at 1244 Mb/s.

In a last refinement, a detailed description of the mechanicsgiving rise to bit errors in the receiver is needed. The cal-culation of the error probability for the US-RX needs to besplit into two parts—the first part corresponding to the am-plitude extraction performed by the BM-RX, and the secondpart corresponding to the clock-phase extraction performedby the CPA. The sensitivity penalty for burst-mode receiversusing APDs has been analyzed in depth [24]. The analysistakes into account detailed APD statistics, additive Gaussiannoise, intersymbol interference, and dc offsets in the receiverchannel. The penalty was calculated via comparison of bit-errorrates (BERs), obtained using numerical integration, both incontinuous-mode and burst-mode operation. It shows thatdc offsets and finite extinction ratios can easily dominatethe penalty due to the noise-corrupted threshold. Importantguidelines were given to the design of high-sensitivity andwide-dynamic-range BM-RXs. Fig. 8 shows a calculated BERcurve compared with a measured BER curve obtained from thefirst-version 1244-Mb/s burst-mode prototypes (US-TX-ver-sion1 and US-RX-version1), which were not yet optimized forbest performance. At a BER of , it can be observed thatthe measured curve and the calculated curve differ by about 0.4dB. This difference can be attributed to a tail within the US-RXand to small optical power fluctuations (in the range of 0.2 dBmwith a time constant of several seconds) of the BM-TX (whichuses an uncooled laser diode).

As described in [25], the error probability of the signalleaving the BM-RX can be combined with a model for theclock-phase extraction, giving rise to a final burst bit-error rate(BBER). Some physical-layer upstream burst-mode overheadis added, for the US-RX to correctly receive the upstream

Fig. 8. Measured BER on the first-version prototypes (US-TX-version1and US-RX-version1) and simulated BER curves versus the received averageoptical power at 1244 Mb/s with an APD gain of 6. The APD was a MitsubishiPD8933. The input-referred noise current of the TIA was 230 nA , andits differential transimpedance was 2 k. The decision threshold is halfwayfrom the peak eye opening, and the sample moment was optimized for bestsensitivity. The transmitter optical power was set to a �3.0-dBm average, withan optical extinction ratio of 10 dB.

packets launched by ONUs. The recommended allocations ofthe physical-layer overhead specified by G984.2 are illustratedin Fig. 9. The mandatory total length of overhead at 1244 Mb/sis 12 B (promoted for standardization [1] and adopted in theITU-T G984.2) and consists of guard time (mandatory 32 b),preamble time (44 b), and delimiter time (20 b). The lengthof the guard time is determined by the laser turn-on/turn-offtime, time shifts caused by 1) slight variations of the fiberdelay, 2) the APD and transistor discharge time, and 3) thefiber propagation delay equalization granularity determinedby a time ranging process. The preamble can be split intotwo parts, a so-called threshold determination field (TDF) foramplitude recovery and the CPA field for clock-phase recovery,both of which are programmable under the OLT’s control. Asexplained in [24] and [25], quick extraction of the decisionthreshold and clock phase from a short preamble at the startof each packet results in a sensitivity penalty. Both penaltiesdepend in a complex way upon the length of the TDF andCPA field, respectively. An optimum distribution between bothshould be found to maximize the performance of the GPON.A combination of measurements and simulation will be usedfor further study to find such an optimum. Finally, note thatmany topics, such as reflections from transmitters close to thereceiver and tails occurring after a strong burst in the receiver,need further investigation in detail.

VI. UPLINK BURST-MODE EXPERIMENTAL RESULTS

To evaluate physical-layer uplink performances such as theBM-RX sensitivity, dynamic range, and BBER, a back-to-backuplink containing one US-TX, an APD-TIA, and a BM-RXwas established and tested at INTEC [26], [27]. Fig. 10 showsthe measured eye diagram of the first-version burst-modetransmitter prototype (US-TX-version1) with a distributed


Fig. 9. Specification of upstream burst-mode overhead (96 b in total). It consists of guard time (mandatory 32 b), preamble time (44 b), and delimiter time (20 b).

Fig. 10. US-TX eye diagram (left) was measured using a fourth-order Thompson filter at 1244 Mb/s (mean optical power = �2.5 dBm,bias power = �13.5 dBm). The measured turn-on and turn-off times are shown on the right side.

feedback (DFB) laser diode (Mitsubishi FU-445SDF). Themean and bias optical power was set to 2.5 dBm and 13.5dBm, respectively. The test results show that the eye diagramof the US-TX-version1 falls well into the mask specified in theITU-T G.984.2. The measured burst turn-on time is 6 pre-biasbits, and the turn-off takes about 13 b; both meet the PMDspecification (TX enable/disable time 16 b) as illustrated inFig. 9. The launched optical power tolerance of the US-TX wastested over the full temperature range (from 40 to 85 C) [14].In order to support PLM, three different mean optical powersof 5.5, 2.5, and 0.5 dBm, respectively, were set togetherwith a fixed bias level of 13.5 dBm during the temperaturetests. A maximum total optical power variation of 0.8 dB hasbeen achieved including the tracking error of the photodiode,the tolerance of the reference current, and the offset variationsof the high-speed monitoring circuits, which is much betterthan the 5-dB tolerance specified in G.984.2, and leaves ample

margin for the tracking error of the PON optics. At the timeof this writing, this is the first GPON US-TX prototype to bepublished supporting the PLM.

Two US-TXs (version 1 prototypes) were subsequently con-figured to send a strong packet P1 followed by a weak packet P2toward the US-RX (version 1 prototype), emulating a worst-casecondition, as shown in Figs. 3 and 6. The US-TX2 (P2) wasset to 3-dBm mean optical power with an extinction ratio of10 dB. The test result is shown in Fig. 11, where the upper andlower trace represent the output of the APD-TIA and outputof the BM-RX-version1, respectively; the avalanche gainwas set to 6; the strong and weak packets received in the APDhave an input average power of 10.0 and 29.5 dBm, respec-tively. The transmitted payload was pseudorandom bit sequence(PRBS) with a packet length of 19.52 kB; the guard timebetween the two packets was 4 B, or 25.6 ns, followed by a pre-amble of 12 b of “1” and 12 b of “0” for the RX amplitude re-


Fig. 11. Strong packet followed by a weak packet emulates the worst-casecondition. The upper trace was plotted at the output of the APD-TIA, and thelower trace was plotted at the output of the BM-RX1. The strong signal wasmeasured at �10 dBm, and the weak signal was �29.5 dBm; the APD gainwas set to 6.

covery; 20 successive “10” patterns were included for the CPAand delimiter field, as illustrated in Fig. 9. From Fig. 11, one cansee that the weak packet at an input optical power of 29.5 dBmis well recovered, though the bandwidth was limited for the “10”pattern (in front of the data payload) due to capacitive loading ofthe external components required for the first-version BM-RXchip, which was not yet optimized for best performance.

BERs were measured using packets for which the preambleis composed of 24 b followed by a pattern of 20 times “10” anda PRBS sequence with a length of 128 000 b; the guardtime was 25.6 ns. Only bit errors within the PRBS sequenceswere taken into account. The average optical power of theBM-TX-version1 was set at 2.5 dBm with an extinctionratio of 10 dB. Fig. 12 shows the measured BERs for differentavalanche gains versus the average input optical power.The receiver sensitivity threshold was found at an opticalinput power of 31.6 30.8 30.2, and 28.9 dBm, for anavalanche gain of 8, 7, 6, and 5, respectively. In addition, themaximum receiver overload was 11.0 10.6 10.1, and

9.2 dBm at 8, 7, 6, and 5, respectively.Furthermore, the burst-mode BER was measured for a

back-to-back 1.25 Gb/s uplink containing one US-TX-version2(the second optimized chip BM-LDD), an APD-TIA module,and a recently developed fully functional US-RX-version2 (thesecond optimized prototype), showing even better performance.The improved performance is mainly due to the fact that thisreceiver is fully integrated, avoiding any capacitive loadingon sensitive nodes that leads to eye closure. The measuredburst-mode BER at the BM-RX-version2 output is shown inFig. 13. The two different slopes in the curve can be explainedby the fact that, for optical powers beneath 34.0 dBm, thedifferential signal at the output of the TIA is too small to becorrectly recovered by the BM-RX. The optimized receiversensitivity is 32.8 dBm 6 , and its dynamic rangereaches 23 dB 6 at BER . The experimentcarried out so far shows that the US-RX is capable of handling

Fig. 12. BER performance at 1244 Mb/s. The input signal of the BM-RXconsisted of packets, separated with a guard time of 25.6 ns. Each packetcontained a preamble of 24 b, followed by a pattern of 20 times “10” and aPRBS sequence 2 � 1 with a length of 128 000 b.

Fig. 13. Back-to-back burst-mode BER performance at 1244 Mb/s. Theoptimized US-RX sensitivity of �32.8 dBm was measured (M = 6), and itsdynamic range reaches 23 dB (M = 6) at BER < 10 .

bursts up to 125 s long, spaced with a guard time of 25.6 ns,and compliant with the GPON standards.

VII. CONCLUSION

This paper presented the development of a 1244-Mb/s GPONburst-mode chip set and its integration into upstream PMD op-tical transmitter and receiver prototypes. Based on the P2MPuplink system study and modeling, innovative design conceptswere proposed, implemented, and proven. Valuable contribu-tions were provided to FSAN/ITU-T for GPON PMD standard-ization.

In the GPON burst-mode receiver, an APD gain of 5 to 6proved to yield a good compromise between dynamic range andsensitivity. For the first time, to the authors’ knowledge, testresults are demonstrated and confirmed by uplink simulations,which comply with the new ITU-T Recommendation G.984.2supporting the overall PLM. The GPON PMD and system pro-totypes with high performance will be demonstrated within theframework of the IWT SYMPATHI and IST GIANT project,respectively, which are expected to be one of the first proof-of-concept demonstrations of the advanced GPON technology.


ACKNOWLEDGMENT

The authors would like to thank all SYMPATHI and GIANTproject partners for their cooperation, especially STMicroelec-tronics and Alcatel Bell, which were coordinators for the SYM-PATHI and GIANT projects, respectively.

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Xing-Zhi Qiu (M’98) received the Ph.D. degree inelectronics engineering from Ghent University, Gent,Belgium, in 1993.

She joined the INTEC design laboratory of GhentUniversity in 1986. She has been active in thedevelopment of optoelectronic systems, especially ofburst-mode transmitter and receiver front ends. Shehas managed the development of the gigabit-capablepassive optical network (GPON) burst-mode chipset design and GPON upstream PMD subsystemdevelopment within the INTEC design laboratory.

She is author/coauthor of 70 international publications in the field of advancedtelecommunication systems, optical access network demonstrations, andmixed-mode analog/digital chip design.

Peter Ossieur (S’03) received the Eng. degree in ap-plied electronics from Ghent University, Gent, Bel-gium, in 2000, where he is currently working towardthe Ph.D. degree.

He has been a Research Assistant in the INTECdesign laboratory of Ghent University since 2000.His research focuses on analog integrated circuitsfor burst-mode laser drivers and receivers in passiveoptical network telecommunication systems and inthe modeling of burst-mode communication.

Johan Bauwelinck (S’03) received the Eng. degreein applied electronics from Ghent University, Gent,Belgium, in 2000, where he is currently working to-ward the Ph.D. degree.

He has been a Research Assistant in the INTECdesign laboratory of Ghent University since 2000.His research focuses on analog integrated circuits forburst-mode laser drivers in passive optical networktelecommunication systems.


Yanchun Yi received the Bachelor’s degree in elec-tronics engineering from Tsinghua University, Bei-jing, China, in 1998 and the Master’s degree from theUniversity of York, York, U.K., in 2002. He is cur-rently working toward the Ph.D. degree at the Infor-mation Technology Department of Ghent University,Gent, Belgium.

His research interest focuses on the simulation ofburst-mode optical communication systems.

Dieter Verhulst (S’03) received the engineeringdegree in applied electronics from Ghent University,Gent, Belgium, in 2001, where he is currentlyworking toward the Ph.D. degree.

He has been working at the INTEC design labora-tory of Ghent University in a passive optical networkresearch project with a focus on the high-speed digitalfunctions in burst-mode laser drivers and clock-phasealignment circuits.

Jan Vandewege (M’96) was born in Gent, Belgium,in 1949. He received the Electronic Engineering andPh.D. degrees from Ghent University, Gent, Belgium,in 1972 and 1978), respectively.

He founded the INTEC design laboratory ofGhent University, in 1985, which provides trainingto Ph.D.-level electronic engineers in the designof telecom and radio-frequency hardware and fastembedded software. He is currently a Full Professorat Ghent University. He has authored or coauthored152 international publications and ten international

patents in the field of telecommunication systems, optical access networks, andmixed-mode analog/digital chip design.

Benoit De Vos received the M.Sc. degree in elec-trical engineering from the Faculté Polytechnique deMons, Mons, Belgium, in 2000.

He has been working in the Research and Innova-tion Department of Alcatel Bell, Antwerp, Belgium,since September 2000. His research focuses on PMD-layer-related system studies for passive optical net-work (PON) systems. Currently, he is leading, withinAlcatel, the SYMPATHI project, which investigatesa gigabit-speed PON network.

Paolo Solina received the electronics degree from theTechnical Institute G. Peano of Turin, Italy, in 1974.

He joined Telecom Italia Lab (TILab, formerlyCSELT), Turin, Italy, in 1974. He contributed tothe development of high-bit-rate optical communi-cations systems within several projects: Esprit 169LION, Esprit 2512 IACIS, RACE 2024 BAF, andACTS 050 PLANET. He was also responsible forthe service trials based on passive optical networkplatforms within the EURESCOM projects P917BOBAN and P1015 FREEHANDS.

Mr. Solina has been a Member of the Full Services Access Network OpticalAccess Network (FSAN OAN) group since 1996. He has been the Editor of thenew ITU-T Recommendation G.984.2, which specifies the physical layer of thegigabit-capable passive optical network systems.

development of gpon upstream physical-media-dependent

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