Development of GPON Upstream Physical-Media-Dependent Prototypes

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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

AbstractThis 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 32.8

dBm(

BER= 1 0

1 0

)

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 TermsBurst-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 based

on 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 and

proliferation of bandwidth in metro networks, broad-band

passive optical networks (BPONs) [1][4] and the emerging

gigabit-capable passive optical networks (GPONs) are expected

to prevail as the leading optical access technology eliminating

the bandwidth bottleneck in the last mile. The full-services

access 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 development

within the framework of the European IST project GIANT (gi-

gapon access network) [5]. GIANT will demonstrate efficient

gigabit 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 supportedin part by the Flemish Government underResearch Contract 010019 IWTSympathi andin part byAlcatel Bell andSTMi-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 very

high bandwidth and transport efficiency [6], [7].

Fig. 1 illustrates the GPON access system. A continuous

downlink in the wavelength band of 14801500 nm carries

1.25 Gb/s time-division-multiplexed (TDM) data from a single

optical line terminator (OLT) toward multiple optical network

units (ONUs) or optical network terminations (ONTs). A

burst-mode link in the 1310-nm window collects all ONU/ONT

upstream traffic toward the OLT as variable-length packets at a

1.25-Gb/s aggregate rate, in a P2MP time-division multiple-ac-cess (TDMA) scheme. The paper focuses on this uplink, which

is difficult to design due to the bursty nature of the multitalker

traffic.

Recently, the FSAN study group, a forum for the worlds

leading telecommunications service providers and equipment

suppliers to work towards a common goal of truly broad-band

access networks, initiated GPON network standardization via

recommendations for the GPON physical-media-dependent

(PMD) layer and the transmission convergence (TC) layer.

Both have now been approved by the International Telecommu-

nication UnionTelecommunication 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 blocks

in 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 are

presented in Section VI.

II. FSAN GPON PMD PROTOTYPES

A. Burst-Mode Upstream PMD Building BlocksFig. 2 depicts the GPON physical layer as a set of PMD

building 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 avalanche

photodiode/transimpedance amplifier (APD-TIA) and the burst-

mode receiver (BM-RX). Table I lists the GPON PMD layer

Class B key specifications in the upstream direction as defined in

ITU-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 ODN

attenuation range of 520, 1025, 1530 dB, respectively. No

0733-8724/04$20.00 2004 IEEE


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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 customerspremises 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 PMDfunctional building blocksconsist of a downstream transmitter (DS-TX) andan upstream receiver(US-RX) at theOLT; 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 is

available 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-mode

receiver (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) Department

of Ghent University, Gent, Belgium, within the Flemish IWT

project SYMPATHI (symmetrical PON at high bit rate) [10].

Currently 1244-Mb/s burst-mode US-TXs and the US-RXs are

integrated into the GIANT GPON laboratory demonstrator [11].

This research contributed to the FSAN efforts toward ITU-T

standardization via Alcatel Bell and Telecom Italia Lab, and was

perfectly 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 dBm

combined 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 combined

tolerances 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 innovative

overall power-leveling mechanism (PLM) was proposed for

standardization [1], [10]. The PLM was adopted in the ITU-T

G984.2 as an optional PMD-layer implementation. Today, a

p-i-n photodiode-based RX at the OLT can obtain a minimum

RX sensitivity of 24 dBm, which is suitable for Class A

operation only, with 520-dB ODN loss. Therefore, the OLT

RX requirements in Table I dictate the use of an APD at 1244

Mb/s for Class B operation (1025-dB ODN loss), to reach a

minimum RX sensitivity of 28 dBm.

Although an increase of the avalanche gain or multiplica-

tion factor of the APD by proper biasing can improve the

RX sensitivity, overload figures may deteriorate as strong sig-

nals result in duty-cycle distortion or saturation of the APD-TIA

output. Optimizing the APD factor for achieving both high

sensitivity and wide dynamic range is not straightforward. Most

commercial APD-TIA modules are designed for point-to-point(P2P) transmission in continuous-wave (CW) operation. High-


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

sensitivity figures are obtained at a higher , while

overload is mostly defined with a smaller , or improved by a

slow automatic gain control (AGC) loop. The fast succession of

upstream bursts, however, separated by a short guard time of 4

B (25.6 ns at 1244 Mb/s), do not allow for a change of APD gain

in 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 APD

gain is set and a nearby ONU (strong packet P1) starts talking.

Nonlinearity would cause a tail effect and have an impact on

the receiver sensitivity, in case the strong packet is followed by

a weak packet (P2) emitted from a far-end ONU (Fig. 3). There-

fore, the dynamic range specification of 21 dB must be achieved

at a fixed APD gain and without a slow AGC loop. Symmetric

gain clamping, however, can be used to extend the RX dynamic

range.

To further relax the 21-dB dynamic range specification of

the OLT receiver, the transmit power level of the ONUs ex-

periencing a low ODN loss should be reduced. A PLM was

conceived operating each ONU in three discrete output power

modes, with the following mean launched power: 1) normal-

mode minimum and maximum 2 3 dBm, as stated in

Table I; 2) mode 1 normal 3 dB; and 3) mode 2

normal 6 dB. The PLM mode can be set locally via a serial

peripheral interface (SPI).

The PLM implementation also requires functionalities

belonging to the TC layer, such as the ONU capability to

increase/reduce the transmit power on the basis of downstream

messages sent by the OLT. The US-RX at the OLT measures

the received average power and compares it with two threshold

voltages ( 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 =

36) , and the input level ofthe weak packet P2 can be increased by

Y

dB( Y =

36) . Overall, PLMalleviates overload of the US-RX caused by the strong packet P1 and reduces

the burst-mode sensitivity penalty for the weak packet P2 caused by the tailof 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 one

mode to another, it sets its emitted power within the range of

the new mode and then resumes sending upstream data. (For

the 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 56 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 of

ONUs 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, over

a temperature range of 40 to 85 C, as required for outdoor

operation. The currents must be digitally set to avoid trimming

elements on the printed circuit board (PCB), reducing calibra-

tion costs. A dc-coupled interface between the driver and the

laser diode is needed. As the maximum TX enable and disable

time is limited to 16 b, or 12.8 ns at 1244 Mb/s, (specified in

ITU-T Recommendation G.984.2), whereas the launched data

after scrambling may have 72 consecutive identical digits, no

single time constant for an ac-coupled circuit can meet both

specifications. Moreover, ac-coupling would require heavy line

coding of the data and a much longer guard time in between

bursts. In contrast to GPON, Ethernet PON (EPON) systems douse ac coupling at the expense of more than a 20% throughput


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

decrease due to 8b10b line coding and of considerable loss in

transport efficiency and interactivity due to an extended inter-

packet gap (IPG).

The multiple upstream accessrequires 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 traffic

of operational services. The time needed to level a US-TX must

be tightly restricted, and fast initialization of ONUs must be per-formed after power-on or first connection to the network with a

minimum of control signals [13]. This is an important require-

ment when developing high-split-ratio GPON systems, as fast

network recovery after, for example, power failure is needed. On

the other hand, accurate tracking of slow laser-power-level drift

is also needed during the data transmission to keep the launched

power 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 element

for 1244-Mb/s upstream transmission, but the device choice

has a strong impact on the ONU cost. The BM-LDD consists of

a 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- m

SiGe BiCMOS process. CMOS processes with even shorter

submicrometer gates require lower supply voltages and cannot

provide the dc coupling, because the laser diode voltage drop

can be 1.6 V in the worst case, whereas the transient voltage

drop caused by parasitic inductances also has to be taken into

account. During transmission, the laser diode must be biased

above 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 the

nonlinear relationship between laser current and optical power,

both the laser modulation current and the bias current must be

regulated according to the optical power and extinction-ratio

requirements specified in Table I.

The laser driver has two differential pairs, each powered by

a current source and a current digitalanalog 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-made

10-b IDACs designed for BM-LDD outdoor operation over a

4085 C temperature range [14].

In contrast to commercially available CW laser drivers, a

BM-LDD cannot regulate the emitted power by means of a slow

averaging 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 short

time slot is allocated to a specific subscriber. Fast but accu-

rate optical level monitoring circuitry and an APC algorithm

were designed based on current-mode circuits [15], to overcomethe drawbacks of voltage-mode implementations at higher bit

rates. The optical-level monitoring circuit contains current mir-

rors and comparators. An active-input current mirror reduces

the impact of the parasitic capacitance of the laser back

facet monitor photodiode (PD), which is the main speed-lim-

iting factor of the level monitoring circuitry. The active-input

current 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.


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The measurements are only valid after the transmission of a

sufficient number of successive 1s and 0s. 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 for

consecutive strings of1s or 0s 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 is

the case in most published burst-mode laser drivers [16], [17].

Successive 1s and 0s can be programmed in a power lev-

eling sequence upstream (PLSu) field with a maximum of 120

B in length, and in a physical layer operation administration

and maintenance upstream (PLOAMu) field. Both are specified

in G984.3 for power control measurements by an ONU. The

PLOAMu, part of GPON overheads, is up to a length of 13 B,

where 10 B ofdata field can be used as a laser control field for

accurately 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 initialization

phase or the PLOAMu during data transmission for regulating

its launched power. This supports a fast APC algorithm based on

a binary-like search with maximum power level protection [13].

The I-bias and I-mod can start from loosely specified but safe

preset values, and there is no need for the storage of a number

of calibration values in lookup tables as some burst-mode laser

drivers require. The digital APC algorithm [18] quickly and ac-

curately adjusts the IDAC settings of I-bias and I-mod until the

level errors are small enough, after which a level-OK signal is

generated 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 25

to 1400 A/mW) and laser slope efficiency in almost any cir-

cumstances. No laborious calibration of the laser characteristic

curve is needed, and no off-chip component needs to be trimmed

for different laser types. There are no time-critical external con-

trol signals to initiate a regulation cycle [19]. The BM-LDD chip

is 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 the

US-TX is adjustable in a 6-dB range. During initialization, an

ONU first sets its power from low power mode 2. The minimum

mean power required for guaranteeing a 10-dB extinction ratio

is specified at 5.5 dBm (mode 2). After level measurements

performed 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 an

APD-TIA module and a BM-RX chip as shown in Fig. 5. It

receives optical packets from all active subscribers in very fast

succession, with varying signal level and phase from packet to

packet. 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 case

from packet to packet due to the following contributions:

1) up to 15 dB of differential attenuation in the ODN

(1025 dB for Class B);

2) up to 5-dB tolerance on the mean launched power of an

ONU (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 a

packet 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 in

severe 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, and

slow AGC can hardly be employed in the GPON US-RX. How-

ever, dc coupling implies the presence of dc offsets, which may

drift due to temperature variations. As dc offset can become a

limiting factor in obtaining high sensitivity, a BM-RX should

be 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 detection

and amplitude recovery. The BM-RX must quickly extract the

decision 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 are

illustrated in Fig. 5. High sensitivity, wide dynamic range, and

fast response are important figures of merit. A 3-dB improve-

ment on the sensitivity can increase the splitting ratio by a factor

of 2, which almost doubles the number of subscribers that can

be connected to the network at little extra cost. A large dynamic

range guarantees a long logical reach and increases the network

flexibility. A high RX sensitivity can be obtained by applying

a 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-TIA

when 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 detected

signal. A tradeoff must be made to set an appropriate APD gain

so that the combined requirement of RX sensitivity and dynamic

range can be met. This optimum gain further depends upon the

transimpedance gain of the preamplifier and the input voltage

sensitivity 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 that

this 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


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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 experimental

results show that an APD gain of around 5 to 6 is a good

compromise to meet the requirements as given in Table I, [ 24].

It is clear that other optimum values will result from different

values of the transimpedance gain. Hence, one cannot decouple

optimization of the avalanche gain from the transimpedance

gain due to the limitation of a minimum input voltage sensi-

tivity of the BM-RX itself. Note that this value is quite low

when compared with conventional optical receivers. The reason

for this is that at a high avalanche gain, the dynamic range of the

receiver is limited by the tail following a strong packet, which

needs 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 differential

input signal for the BM-RX. The BM-RX mainly contains auto-

matic threshold extraction circuitry and limiting amplifiers with

symmetric gain clamping and dc-offset compensation as shown

in Fig. 5. The level measure PLM block was designed to mea-

sure the level of the incoming signal by comparing it with two

threshold voltages that are programmable. Fig. 5 gives a simpli-

fied view of the BM-RX chip and its interfaces toward the LT

digital ASIC and the clock-phase alignment (CPA). However,

the design of the CPA chip is not included in this paper. The

BM-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 to

generate an offset bias for the limiting amplifier stages. Due to

the short guard time, the BM-RX needs an active reset to erase

the threshold of a preceding packet.

V. UPSTREAM LINK MODELING AND SPECIFICATION

Optical transmission system simulations are widely used

for high-capacity core networks. However, how to accurately

model fast burst-mode P2MP transmission is still a challenging

research 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 ( 0 11 dBm with 2-dBmargin in case of PLM) followed by a weak packet (

0

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 different

methods, each method having its own advantages.

First, transistor-level models were used to validate the eye

diagrams of the transmitter, and in particular the laser driver.

Indeed, the required speed and accuracy necessitates a detailed

modeling of the interface between laser diode and driver, for

example, with regard to bonding wires. On the receiver side,

careful modeling of any memory effects that could destroy a

succeeding packet is needed. For example, Fig. 6 shows the

simulated differential outputs of the BM-RX (lower traces) to-

gether with the input current of the TIA (upper trace) in the

worst case of a weak packet (almost not visible) preceded by

a strong packet.

In a second step, abstract models of the transistor-level cir-

cuits of both the BM-LDD and BM-RX were combined with

detailed descriptions of the laser diode, the fiber plant, and the

avalanche diode. In this way, eye diagrams are produced that

can be used to quickly evaluate the influence of several systemparameters, such as the extinction ratio at the US-TX and the


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Fig. 7. Simulated eye diagram at the BM-RX output. The launched outputpower of the US-TX was set to 0 5 dBm with an extinction ratio of 10.5 dB, a20-km G.652 fiber was included, and the BM-RX input level was 0 28.5 dBm,with an APD gain of 6 at 1244 Mb/s. The input referred noise current of the TIA

was 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 the

simulated optical uplink end-to-end eye diagram for a received

average optical power of 28.5 dBm with an APD gain 6

at 1244 Mb/s.

In a last refinement, a detailed description of the mechanics

giving rise to bit errors in the receiver is needed. The cal-

culation of the error probability for the US-RX needs to be

split into two partsthe first part corresponding to the am-

plitude extraction performed by the BM-RX, and the secondpart corresponding to the clock-phase extraction performed

by the CPA. The sensitivity penalty for burst-mode receivers

using APDs has been analyzed in depth [24]. The analysis

takes into account detailed APD statistics, additive Gaussian

noise, intersymbol interference, and dc offsets in the receiver

channel. The penalty was calculated via comparison of bit-error

rates (BERs), obtained using numerical integration, both in

continuous-mode and burst-mode operation. It shows that

dc offsets and finite extinction ratios can easily dominate

the penalty due to the noise-corrupted threshold. Important

guidelines were given to the design of high-sensitivity and

wide-dynamic-range BM-RXs. Fig. 8 shows a calculated BERcurve compared with a measured BER curve obtained from the

first-version 1244-Mb/s burst-mode prototypes (US-TX-ver-

sion1 and US-RX-version1), which were not yet optimized for

best performance. At a BER of , it can be observed that

the measured curve and the calculated curve differ by about 0.4

dB. This difference can be attributed to a tail within the US-RX

and to small optical power fluctuations (in the range of 0.2 dBm

with a time constant of several seconds) of the BM-TX (which

uses an uncooled laser diode).

As described in [25], the error probability of the signal

leaving the BM-RX can be combined with a model for the

clock-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 average

optical 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

0

3.0-dBm average, with

an optical extinction ratio of 10 dB.

packets launched by ONUs. The recommended allocations of

the physical-layer overhead specified by G984.2 are illustrated

in Fig. 9. The mandatory total length of overhead at 1244 Mb/s

is 12 B (promoted for standardization [1] and adopted in the

ITU-T G984.2) and consists of guard time (mandatory 32 b),

preamble time (44 b), and delimiter time (20 b). The length

of the guard time is determined by the laser turn-on/turn-off

time, time shifts caused by 1) slight variations of the fiber

delay, 2) the APD and transistor discharge time, and 3) the

fiber propagation delay equalization granularity determined

by a time ranging process. The preamble can be split into

two parts, a so-called threshold determination field (TDF) for

amplitude recovery and the CPA field for clock-phase recovery,

both of which are programmable under the OLTs control. As

explained in [24] and [25], quick extraction of the decision

threshold and clock phase from a short preamble at the start

of each packet results in a sensitivity penalty. Both penalties

depend in a complex way upon the length of the TDF and

CPA field, respectively. An optimum distribution between both

should be found to maximize the performance of the GPON.A combination of measurements and simulation will be used

for further study to find such an optimum. Finally, note that

many topics, such as reflections from transmitters close to the

receiver 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 the

BM-RX sensitivity, dynamic range, and BBER, a back-to-back

uplink containing one US-TX, an APD-TIA, and a BM-RX

was established and tested at INTEC [26], [27]. Fig. 10 shows

the measured eye diagram of the first-version burst-modetransmitter prototype (US-TX-version1) with a distributed


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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 = 0 2.5 dBm,bias power

= 0

13.5 dBm). The measured turn-on and turn-off times are shown on the right side.

feedback (DFB) laser diode (Mitsubishi FU-445SDF). The

mean and bias optical power was set to 2.5 dBm and 13.5

dBm, respectively. The test results show that the eye diagramof the US-TX-version1 falls well into the mask specified in the

ITU-T G.984.2. The measured burst turn-on time is 6 pre-bias

bits, and the turn-off takes about 13 b; both meet the PMD

specification (TX enable/disable time 16 b) as illustrated in

Fig. 9. The launched optical power tolerance of the US-TX was

tested over the full temperature range (from 40 to 85 C) [14].

In order to support PLM, three different mean optical powers

of 5.5, 2.5, and 0.5 dBm, respectively, were set together

with a fixed bias level of 13.5 dBm during the temperature

tests. A maximum total optical power variation of 0.8 dB has

been achieved including the tracking error of the photodiode,

the tolerance of the reference current, and the offset variations

of 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 time

of this writing, this is the first GPON US-TX prototype to be

published supporting the PLM.Two US-TXs (version 1 prototypes) were subsequently con-

figured to send a strong packet P1 followed by a weak packet P2

toward the US-RX (version 1 prototype), emulating a worst-case

condition, as shown in Figs. 3 and 6. The US-TX2 (P2) was

set to 3-dBm mean optical power with an extinction ratio of

10 dB. The test result is shown in Fig. 11, where the upper and

lower trace represent the output of the APD-TIA and output

of the BM-RX-version1, respectively; the avalanche gain

was set to 6; the strong and weak packets received in the APD

have 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 time

between the two packets was 4 B, or 25.6 ns, followed by a pre-amble of 12 b of 1 and 12 b of0 for the RX amplitude re-


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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 was

measured at 0 10 dBm, and the weak signal was 0 29.5 dBm; the APD gainwas set to 6.

covery; 20 successive 10 patterns were included for the CPA

and delimiter field, as illustrated in Fig. 9. From Fig. 11, one can

see that the weakpacket at an input optical power of 29.5 dBm

is well recovered, though the bandwidth was limited for the 10

pattern (in front of the data payload) due to capacitive loading of

the external components required for the first-version BM-RX

chip, which was not yet optimized for best performance.

BERs were measured using packets for which the preamble

is composed of 24 b followed by a pattern of 20 times 10 and

a PRBS sequence with a length of 128 000 b; the guardtime was 25.6 ns. Only bit errors within the PRBS sequences

were taken into account. The average optical power of the

BM-TX-version1 was set at 2.5 dBm with an extinction

ratio of 10 dB. Fig. 12 shows the measured BERs for different

avalanche gains versus the average input optical power.

The receiver sensitivity threshold was found at an optical

input power of 31.6 30.8 30.2, and 28.9 dBm, for an

avalanche gain of 8, 7, 6, and 5, respectively. In addition, the

maximum 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 (the

second optimized prototype), showing even better performance.

The improved performance is mainly due to the fact that this

receiver is fully integrated, avoiding any capacitive loading

on sensitive nodes that leads to eye closure. The measured

burst-mode BER at the BM-RX-version2 output is shown in

Fig. 13. The two different slopes in the curve can be explained

by the fact that, for optical powers beneath 34.0 dBm, the

differential signal at the output of the TIA is too small to be

correctly recovered by the BM-RX. The optimized receiver

sensitivity is 32.8 dBm 6 , and its dynamic range

reaches 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 0 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

0

32.8 dBm was measured( M =

6)

, and its

dynamic range reaches 23 dB ( M = 6) at BER


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ACKNOWLEDGMENT

The authors would like to thank all SYMPATHI and GIANT

project 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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127132.[11] T. Van Caenegem, E. Gilon, X.-Z. Qiu, P. Solina, and K. Cobb, Pro-totyping ITU-T GPON, the new efficient and flexible FTTP PON solu-tion, in Proc. Int. Symp. Subscriber Loops Services, Edinburgh, U.K.,Mar. 2124 2004.

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[14] J. Bauwelinck, D. Verhulst, P. Ossieur, X. Z. Qiu, J. Vandewege, and B.De Vos, DC-coupled burst-mode transmitter for a 1.25 Gbit/s upstreamPON, IEE Electron. Lett., vol. 40, no. 8, pp. 501502, Apr. 2004.

[15] J. Bauwelinck, D. Verhulst, P. Ossieur, X. Z. Qiu, J. Vandewege, andB. De Vos, Current mode circuits for fast and accurate optical level

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[22] , Burst-mode penalty of AC-coupled optical receivers optimizedfor 8B/10B line code,IEEE Photon. Technol. Lett., pp. 17241726, July

2004.[23] P. Ossieur, Y. C. Yi, J. Bauwelinck, X. Z. Qiu, J. Vandewege, and E.Gilon, DC-coupled 1.25 Gbit/s burst-mode receiver with automaticoffset compensation, IEE Electron. Lett., vol. 40, no. 7, pp. 447448,Apr. 2004.

[24] P. Ossieur, X.-Z. Qiu, J. Bauwelinck, and J. Vandewege, Sensitivitypenalty calculation for burst-mode receivers using avalanche photodi-odes, J. Lightwave Technol., vol. 21, pp. 25652575, Nov. 2003.

[25] B. Meerschman, Y. C. Yi, P. Ossieur, D. Verhulst, J. Bauwelinck, X.Z. Qiu, and J. Vandewege, Burst bit-error rate calculation for GPONsystems, in Proc. IEEE/LEOS Benelux Chapter, 2003, pp. 165168.

[26] Z. Lou, S. Verschuere, Y. Yi, D. Verhulst, X. Z. Qiu, and J. Vandewege,

Lab. test bed development for evaluation of the GigaPON uplink per-formance, in Proc. IEEE/LEOS Benelux Chapter, 2003, pp. 137140.

[27] X. Z. Qiu, P. Ossieur,J. Bauwelinck, Y. C. Yi, D. Verhulst, S. Verschuere,Z. Lou, W. Chen, Y. Martens, X. Yin, J. Vandewege, B. De Vos, and E.Gilon, FSAN GPON upstream burst-mode transmission experiments,

presented at the 30th Eur. Conf. Optical Communication (ECOC04),Stockholm, Sweden, Sept. 59, 2004.

Xing-Zhi Qiu (M98) 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 chip

set 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 (S03) 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 circuits

for burst-mode laser drivers and receivers in passiveoptical network telecommunication systems and inthe modeling of burst-mode communication.

Johan Bauwelinck (S03) 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 network

telecommunication systems.


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Yanchun Yi received the Bachelors degree in elec-tronics engineering from Tsinghua University, Bei-

jing, China, in 1998 and the Masters 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 of

burst-mode optical communication systems.

Dieter Verhulst (S03) 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 projectwitha focus on thehigh-speed digitalfunctions in burst-mode laser drivers and clock-phasealignment circuits.

Jan Vandewege (M96) 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 Facult Polytechnique deMons, Mons, Belgium, in 2000.

He has been working in the Research and Innova-tion Department of Alcatel Bell, Antwerp, Belgium,since September2000. Hisresearchfocuses on PMD-layer-related system studies for passive optical net-work (PON) systems. Currently, he is leading, within

Alcatel, the SYMPATHI project, which investigatesa gigabit-speed PON network.

Paolo Solina received theelectronicsdegree fromthe

Technical Institute G. Peano of Turin, Italy, in 1974.He joined Telecom Italia Lab (TILab, formerly

CSELT), Turin, Italy, in 1974. He contributed tothe development of high-bit-rate optical communi-cations systems within several projects: Esprit 169

LION, 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 Prototypes

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