Selected Papers

43-Gbit/s-channel-based DWDM System
Technologies for the Photonic Transport Network
Yutaka Miyamoto†, Masahito Tomizawa, and Yasuhiko Tada
Abstract
This paper describes recent technical progress and the evolution of dense wavelength division multiplexing (DWDM) system technologies based on 43-Gbit/s channels for the optical transport network(OTN). We describe the new digital frame format “OTU3: Optical Channel Transport Unit 3”, which wasrecently standardized in ITU-T as a novel network node interface of the OTN using 43-Gbit/s channels.
The new standard enables the transparent accommodation of multi-tributary service signals such as SDH(synchronous digital hierarchy), ATM (asynchronous transfer mode), Ethernet, and Fiber Channel in large
payloads, and enhances the WDM (wavelength division multiplexing) network management capability of
the OTN with its overhead byte. It also introduces the forward error correction (FEC) code byte in its
frame to ensure high-quality transmission at over 1 Tbit/s based on 43-Gbit/s channels. New transmission
technologies have established stable 43-Gbit/s-channel long-haul WDM transmission across the OTN.
They include a novel bandwidth-efficient RZ (return-to-zero) modulation format and a novel error-control scheme using optical spectral redundancy as well as several hybrid optical amplification technologies.

1. Introduction
The recent growth of Internet traffic has accelerated the demand for even higher capacities. 40-Gbit/schannel systems based on full electrical time divisionmultiplexing (ETDM) [1]-[3] have matured rapidly.
ETDM provides economical optical transport interfaces through the use of advanced LSI technology,
which is accelerating the research and developmentof high-capacity DWDM (dense wavelength divisionmultiplexing) transport systems based on 40-Gbit/schannels, which are nowadays considered to be
viable as next-generation systems throughout theworld [1]-[14]. They offer high capacity with fewer
channels, attain high spectral efficiency based on
today’s mature WDM multiplexers with 50–200-GHz
spacing, and could provide economic trunk transport
networks with a small footprint [15]. The 43-Gbit/schannel has been recently been accepted by ITU-T(International Telecommunication Union – Telecommunication Sector) as a prime candidate for the future† NTT Network Innovation Laboratories
Yokosuka-shi, 239-0847, Japan
E-mail: miyamoto.yutaka@lab.ntt.co.jp

36

optical transport network (OTN) based on WDM
technologies that will support the next-generation
data-centric network [16]. We conducted the first
field transmission experiment in NTT’s network to
confirm the feasibility of a 40-Gbit/s-channel-basedtransport system in 1996 [4]. This field experimentaccelerated the research and development of 40Gbit/s system technologies in NTT, leading to thefirst feasibility demonstration of a full-ETDM regenerative repeater system [1], [2], the first WDM laboratory experiment [5], the first WDM field trial [8],the first terabit-per-second laboratory experimentusing 43-Gbit/s ETDM channels with additional
overhead with forward error correction (FEC) [10]corresponding to the proposal of OTU3 digital framein ITU-T [19], and the first 1-Tbit/s WDM field trialbased on OTU3 [12].
This paper describes recent technical challenges
and the future evolution of DWDM transport systemtechnologies using 43-Gbit/s channels for the OTN.
In section 2, we describe the concept and impact of
the new digital frame format of the 43-Gbit/s channel: the optical channel transport unit 3 (OTU3),
which has been standardized in ITU-T to enhance the
network management capability of the OTN. In secNTT Technical Review

Selected Papers

tion 3, we introduce the advantages and issues of an
OTN based on 43-Gbit/s channels. Section 4
describes the concept and several attractive featuresof novel bandwidth-efficient return-to-zero (RZ) formats including carrier-suppressed RZ (CS-RZ) toenhance the transmission performance of long-haul
43-Gbit/s/channels. Section 5 introduces a new error
control scheme using optical spectral redundancy toenhance the signal-to-noise ratio for further evolutionof the OTN. We conclude with a short summary andmention the future evolution of the OTN based on 43Gbit/s channels.
2. Network requirements for the OTN and its
international standardization
Network traffic is now dominated by IP-based data
traffic instead of telephone-based traffic. The growthof Internet traffic demands that the backbone network
offers high quality and cost effectiveness, as well as
high capacity. The OTN based on high-capacity
WDM (Fig. 1) is very attractive for such a network.
The tributary client signals of the OTN are not only
conventional SDH/SONET signals but also several
types of IP-based client signal, such as GbE, 10GbE,
and IP over SDH/SONET. Therefore network operators require a new digital frame format to accommodate such new client signals. They also need several
operation, administration, and maintenance (OAM)
functions of the OTN to be embedded in the transported signal. For this purpose, the optical path layer
for the OTN was proposed [17], [18] and the optical-

channel transport unit (OTU) was standardized in
ITU-T Recommendation G.709 [16]. The bit-rates
for OTU are categorized into OTU1, OTU2, andOTU3. The OTU3 bit-rate class was proposed byNTT. The OTU3 frame enhances the network management capability for an OTN based on 43-Gbit/schannels with high capacity (over 1 Tbit/s) [19], [20].
Table 1 and Fig. 2 show the hierarchy of OTU and thedigital frame structure of OTU3, respectively, as
described in ITU-T Recommendation G.709. OTU3
is defined as a 43-Gbit/s channel with the following
features:• Client overhead transparency for multiple services• Digital supervision for enhancing WDM network
management• Enhanced transmission quality using FEC
The OTU payload was designed to accommodate
many types of client data signal without terminating
the overhead byte of the client signal. This ensurestransparent transmission of the client information signal together with the overhead signal used for client’s
network control across the backbone network, which
is desirable for providing a client’s virtual private network (VPN) across the OTN. The introduction of astructure for multiplexing ODUk-1 into ODUk is also
described in G. 709 amendment (ODU: optical dataunit). OTN multiplexing efficiently uses the wavelength resources, especially when the client has alower bit-rate interface/equipment. The large capacity of the OTU3 payload is attractive not only as a bulk
container but also as a multiplexed payload container
because of its improvement in multiplexing gain.

Ethernet network

SDH network

OTN

Fig. 1. Optical transport network (OTN).

Vol. 1 No. 8 Nov. 2003

37

Selected Papers

1

16 17

3824

Optical channel data unit (ODU)

13824 3825

16 17

4080
RS(255, 239)1
2

RS(255, 239)
Optical channel transport unit (OTU)

3

RS(255, 239)

4

RS(255, 239)
Och overhead

Och payload

FEC redundancy

Fig. 2. Digital frame of optical channel transport unit.

Table 1. Hierarchy of OTU.
Line rate (Gbit/s) Frame period (µs)

Line rate increase

OTU 1

2.666057143

48.971

STM-16 × (255/238)

OTU 2

10.709225316

12.191

STM-64 × (255/237)

OTU 3

43.018413559

3.035

STM-256 × (255/236)

The OTU frame offers us overhead bytes of WDM
network management for the network node interface,
for the first time. Namely this format will be used allover the world as the Inter Domain Interface (IrDI) tolink the networks of different telecom-carrier companies or sub-networks within a carrier’s network. The
overhead provides functions of frame synchronization, digital performance monitoring, data communication channel, automatic protection switching, andtrail tracing. These overheads are also attractive for
future OTN evolution into the multiprotocol lambda
switching (MPλS) network, which will offer rapid,
cost-effective provisioning.
Reed Solomon (RS) (255, 239) code is introduced
as an out-of-band FEC code in the OTU frame at theexpense of a 7% increase in line rate. The RS(255, 239) algorithm is widely used in today’s submarine systems. The FEC code can improve the bit
error rate (BER) from 10–4 to 10–15, resulting in acoding gain of about 6 dB at BER of 10–12, if theerrors obey random (Poisson) statistics. The strong
FEC capability increases the system margin to ensure
38

several network evolution scenarios. The margin is
effective for increasing the regenerative repeater span
across several optical network elements such as optical cross connects (OXCs) and optical add/drop multiplexers (OADMs). It also allows more WDM channels for a given regenerative span and transmission
performance. FEC is also helpful for digital performance monitoring. When bit errors occur in an optical channel, they are detected and corrected at thereceiver side in the FEC decoder, without clients seeing any degradation. This ensures error-free transmission of the client signal, even if some bit errors
occur in the OTN section. This feature lets network
operators improve network operations based on early
warning of forthcoming failures.
3. Advantages and enabling technologies of an
OTN based on 40-Gbit/s channels
3.1 Advantages of 40-Gbit/s channels
An OTN based on 40-Gbit/s channels has several
advantages for a data-centric high-capacity backbone
network [15], [19], [20].
• Multi terabit-per-second capacity with high signal spectral efficiency• Small footprint and low cost
• Large bulk payload for data-centric client signal
There are two standards for the 40-Gbit/s digitalframe structure: OTU3 and STM-256 (STM: synNTT Technical Review

Selected Papers

chronous transport module). OTU3 is basically standardized for multi-channel systems, while STM-256
is for single-channel systems. It should be noted thatOTU3 can be used in single-channel systems, while
STM-256 may be used in multi-channel systems
using the pre-OTN regime. Considering that recent
systems require WDM upgrading, not only in pointto-point topology but also toward optical ring or
mesh topology, OTU3 is suitable for today’s network
requirements in terms of transmission quality (by theuse of out-of-band FEC*1) and management capability. Figure 3 shows the recent relationship between
the signal spectrum efficiency and channel spacingwith channel rate as a parameter. When the channel
rate is increased from 2.4 to 40 Gbit/s, the modulation
bandwidth of a 40-Gbit/s signal completely fills theavailable channel bandwidth of today’s mature WDM
filter with 50–200-GHz channel spacing. Therefore,
the combination of today’s mature DWDM system
and 40-Gbit/s channels simply offers us high signal
spectral efficiency up to 1 bit/s/Hz. Efficiencies ofover 0.4 bit/s/Hz yield capacities beyond 1 Tbit/s
within the typical bandwidth of 4 THz (30 nm) for

optical fiber amplifier repeaters available in the C, L,
and S bands. Thus multi-Tbit/s transmission is possible with a multi-band inline-repeater system, while 1
Tbit/s can be easily achieved with a simple singleband repeater system.
The small number of channels of the WDM system
is also attractive because the network operation system has fewer objects (network elements) to supervise. This simplification also allows the network
management capability to be enhanced. Such features
also reduce the footprint of equipment in offices andnetwork maintenance requirements.
3.2 Issues and enabling technologies
The most important issue in achieving a cost-effective OTN based on 40-Gbit/s channels is to overcome
the transmission distance limitation due to imperfections in the optical fiber such as fiber nonlinear
effects, group velocity dispersion (GVD), and polarization mode dispersion (PMD). The decrease in signal-to-noise ratio due to the increase in signal bandwidth requires an increase in the optical signal
launched power. This will further increase the impactof fiber nonlinear effects. It is also very important inachieving uniform transmission performance over theentire fiber bandwidth (C, L, and S bands) given that

*1 out-of-band FEC: Out-of-band FEC is the FEC whose redundancy byte is defined independantly of the encoded signals mapped in
the payload.

Mature WDM filter
10

4G

10
G

20

G

40

G

10

0G

1 bit/s/Hz

1

10

0.5

0.1

1
NRZ

Total capacity in C+L band (Tbit/s)

Signal spectral efficiency (bit/s/Hz)

Channel rate2.

Duobinary
RZ
CS-RZ
0.01
1

10

50

100

200

0.1
1000

Channel spacing (GHz)

Fig. 3. Enhancement of spectral efficiency using 43-Gbit/s channel.

Vol. 1 No. 8 Nov. 2003

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various types of fiber will exist; a variety of OTN
upgrade scenarios should be supported. Nowadays,
several digital and analog technologies have beentested to overcome these problems for 40-Gbit/schannel transmission, as shown by Table 2. For digital technologies, the RS(255, 239) FEC code in
OTU3 is important for enhancing the performance
regardless of the limiting factors. Another attractive
digital error correction that has recently been proposed [21] offers further BER improvement without
increasing the line rate, as described in Section 5. For
analog technologies, three key technologies havebeen tested: the modulation and detection format[22], hybrid optical amplification using an erbiumdoped fiber amplifier (EDFA) and a distributed
Raman amplifier (DRA) [23], and dispersion compensation in terms of GVD and PMD including dispersion slope management [6]. Hybrid optical amplification is very attractive for reducing the impairment
due to fiber nonlinearities, as well as enhancing thesignal-to-noise ratio. However, it does not increase
the robustness against the linear impairment caused
by GVD and PMD. Guidelines and shut-down mechanisms to ensure safety for human eyes and network
facilities are the key to actually deploying high-power
distributed Raman amplifier systems [24]. In contrast
to dispersion compensation and hybrid optical amplification, selecting the most appropriate modulation
format in combination with FEC is very effective forincreasing the robustness against all types of impairments. Here we focus on the impact of the modulation format and its combined effect with error correcting technologies on 40-Gbit/s WDM transmis-

sion performance.
4. Bandwidth-efficient RZ modulation format
4.1 Requirements
The requirements that the modulation format mustsatisfy are listed below for both single-channel and
WDM applications.
• Compact modulation spectrum• Simple configuration of transmitter and receiver• High nonlinearity tolerance• Simple dispersion characteristics in linear andnonlinear transmission
A compact modulation spectrum is desirable
because this offers high signal spectrum efficiency in
WDM systems and increases the GVD tolerance.
High signal spectral efficiency of over 0.4 bit/s/Hz (ora spacing of less than 100 GHz) should be achievedin 40-Gbit/s-channel DWDM systems based on
today’s mature WDM filters [3], [15], which offer
higher signal spectral efficiency than 50-GHz-spaced
10-Gbit/s-channel DWDM systems. High nonlinearity tolerance is also necessary for single- and multichannel long-haul applications. Simple transmitterand receiver configurations are desirable for costeffective transmitter and receiver blocks. Since thedispersion tolerance of a 40-Gbit/s channel is only
100 ps/nm, automatic dispersion compensation isstrongly required. The simplest dispersion compensation approach is to set the total dispersion to zero.
Therefore, the modulation format should be robust
against fiber nonlinearity to offer us an optimized
system parameter for operation in the zero total dis-

Table 2. Enabling technologies for 43-Gbit/s channel transmission.
Enabling
technology

AnalogDigitalModulation
formatDistributed
amplification

Dispersion
compensation

FEC

Signal-to-noise ratio

Yes

Yes

No

Yes

Linear
• GVD
• PMD

Yes
Yes

No
No

Yes
Yes

Yes
Yes

Nonlinear
• SPM
• XPM
• FWM

Yes
Yes
Yes

Yes
Yes
Yes

Yes
Yes
Yes

Yes
Yes
Yes

Limiting
factors

GVD: group velocity dispersion, PMD: polarization mode dispersion,
SPM: self phase modulation, XPM: cross phase modulation, FWM: four-wave mixing,
FEC: forward error correction.

40

NTT Technical Review

Selected Papers

persion region.
4.2 Proposed bandwidth-efficient RZ format
Our approach to such a modulation format is to
reduce the bandwidth of the RZ format using phasereversal in the pulse generation and data encoding
processes [22]. The bandwidth-efficient RZ format isvery attractive for achieving stable long-haul DWDM
transmission at channel rates of over 40 Gbit/s. It
offers high-spectral efficiency while suppressing theeffect of fiber nonlinearity, providing robustness
against changes in GVD caused by fiber temperaturefluctuation and network reconstruction, which areimportant benefits in terrestrial systems. Since the RZ
format is robust against fiber nonlinearity, several RZ

formats were proposed for transoceanic systems.
Conventional RZ formats, however, require a modulation bandwidth of over 4B, as shown in Fig. 4(c)(where B is the line rate). This induces a large
crosstalk penalty if we target spectral efficiencies ofover 0.4 bit/s/Hz. Several approaches to reducing themodulation bandwidth of the RZ format apply periodic phase modulation in the pulse generation
process. The carrier-suppressed RZ (CS-RZ) format[7], [25] reduces it to less than 3B by using alternatebit-phase reversal in the pulse generation process, as
shown in Fig. 4(b). This also enhances the tolerances
to fiber nonlinearity.
The idea of the CS-RZ format is shown in Fig. 5. In
the conventional RZ format, the electrical field of RZ(a) DCS-RZ(b) CS-RZ(c) conventional RZ

CS-RZ pulse format+
Duobinary data format

CS-RZ pulse format+
NRZ data format

RZ pulse format+
NRZ data format

Complete
Carrier suppression

Carrier suppression

40 GHz0.0–10.0

80 GHz

–20.0
–30.0
–40.0
1551.51552.5–10.0

Power (dBm)

0.0Power (dBm)

Power (dBm)

Carrier

40 GHz0.0

120 GHz

–20.0
–30.0

Wavelength (nm)160 GHz

–20.0
–30.0
–40.0
1551.5–40.0
1551.51552.51553.5
Wavelength (nm)1553.5–10.0

1552.51553.5

Wavelength (nm)

CS-RZ: carrier-suppressed RZ

Fig. 4. Bandwidth reduction of RZ format.
B

2B

Frequency

Frequencyf0f0(a) The spectrum of CS-RZ(c) The spectrum of RZ

0

0

0

0

0

0

0

0

0

Time
π

π

Time

π

(b) The envelop of CS-RZ(d) The envelop of RZ

Fig. 5. Bandwidth reduction using two-mode beat pulse.

Vol. 1 No. 8 Nov. 2003

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Selected Papers

CS-RZ
pulse gererator

NRZ
data modulator

MZ#1

MZ#2

CS-RZ
Optical transmission fiber

LD

CDR

MZ mod.

ad20-GHzCLK

40-Gbit/s
NRZ data

c
b

b

c

d

0 p 0 p40-GHz
CS-RZ pulsea20-GHz CLK

CDR: clock data recovery
LD: laser diode
CLK: clock signal

40-GHz CS-RZ pulse
generation(duty cycle: 2/3)

Fig. 6. Configuration of CS-RZ transmitter.

pulse train is expressed as formula (1), where theoptical-pulse-repetition frequency B is equal to theline rate, E0/2 is the amplitude, and f0 is the opticalcarrier frequency.
E(t) =

E0· {1 + cos(2πBt)} · exp(–j2π f0 t).
2(1)

The envelop of the signal is sinusoidal with the frequency of B as shown in Fig. 5(d), and the Fourier
spectrum consists of three mode-locked modes withthe bandwidth over 2B as shown in Fig. 5(c).
In the CS-RZ format, on the other hand, the electrical field of CS-RZ pulse train is expressed as formula (2).
E(t) = E0 · cos 2πBt · exp(–j2π f0 t).
2

(

)

(2)

The envelop is sinusoidal with the half frequency(B/2) of conventional RZ as shown in Fig. 5(b). Twobeating modes are basically used in the Fourier spectrum and the mode spacing is exactly equal to the line
rate as shown in Fig. 5(a). It should be noted therequired bandwidth for CS-RZ pulse train is reduced
to the half of conventional RZ thanks to the phasereversal in CS-RZ format. This two-mode-beat pulse
train is modulated with the non-return-to-zero (NRZ)
format to generate the proposed format. We called
this format the “CS-RZ” format because of the spectral feature of carrier suppression in the modulation
spectra.
Figure 6 shows the basic CS-RZ transmitter configuration and its operation. Two Mach-Zehnder (MZ)
42

modulators are connected in series. One is used as an
NRZ data modulator and the other as a two-mode
beat pulse generator. We were the first to propose anddemonstrate the field-deployable chirp-free CS-RZ
transmitter based on a two-stage MZ modulator
design that offers the several advances needed toachieve high-bit-rate data modulation at over 40
Gbit/s: stable polarization-maintaining interconnection between the pulse generator and the data modulator, compact integration, precise phase adjustment
and stable synchronization between the pulse generation with an adjustable duty cycle and data modulation. As shown in Fig. 6, the MZ modulator is biased
at the transmission null point and driven at a frequency of half the line rate. In the 40-Gbit/s CS-RZ transmitter, the required clock is 20 GHz. The frequencydoubling feature is very attractive because it reduces
the operating frequency of the driving signal, whichsimplifies signal handling and relaxes the clock driver
specification compared with the conventional RZ format. Due to the phase encoding characteristics of the
MZ modulator, the generated clock pulse train isaccompanied by alternate bit phase reversal, as
shown in Fig. 6.
Figure 7 shows the advantage of CS-RZ format
over the conventional NRZ and RZ formats. The
transmission performance of the CS-RZ format surpassed those of the NRZ and RZ formats in a 360-km
dispersion-managed fiber with 200-GHz-spacing
WDM. In the case of the NRZ format, the self-phasemodulation and signal-to-noise ratio strongly
degrade the transmission performance. On the other
NTT Technical Review

Receiver sensitivity at BER=10–9 (dBm)

Selected Papers

to 2B, can be achieved with the duobinary CS-RZ(DCS-RZ) format [22], as shown in Fig. 4(a), whichuses phase-reversal modulation in both the pulse generation and the data encoding processes based on the
optical duobinary format; namely, the DCS-RZ format has alternate-mark-bit-phase reversal. Improvement in dispersion tolerance and DWDM spectral
efficiency (up to 0.4 bit/s/Hz), and high fiber-nonlinearity tolerance have been reported for DCS-RZ.

–20.0
mit

–22.0

li
ss

Lo

CS-RZ
RZ
NRZ

–24.0
–26.0
–28.0

1.4 dB
–30.0
3.0

5.0

7.0
9.0
11.0
Repeater output power (dBm)

13.0

Fig. 7. Enhancement of SPM tolerance using CS-RZ.

hand, RZ and CS-RZ achieve stable transmission performance. The transmission performance of the CSRZ format is more than 1 dB better than that of the
RZ format. This enhancement of CS-RZ is further
enhanced when the signal spectral efficiency is over
100 GHz.
Further RZ modulation bandwidth reduction, down

43-Gbit/s
NRZ
1560.60 nm
l1

4.3 Terabit-per-second capacity upgrade basedon 43-Gbit/s channels using FEC
Figure 8 shows the experimental setup that first
demonstrated the impact of the CS-RZ format and RS(255, 239) FEC code on an OTN that uses 43-Gbit/schannels in a 1.2-Tbit/s (30 × 43 Gbit/s) WDM experiment [10]. Two types of 43-Gbit/s test signal withand without FEC were used to check the FEC performance.
30-channel 43-Gbit/s CS-RZ signal transmission
was established over a 376-km dispersion-flattened
dispersion-managed line with total repeater output
power of over +20 dBm (+5 dBm/channel average).

43-Gbit/s
CS-RZ

Vb

AWG

l2

200-GHzspacing

MZ#1

l30

MZ#2
SMF

1609.19 nm

InP-HEMT
IC based 4: 1 MUX#1: 127 km
RDF
21.3 GHz
CS-RZ signal
generator

10.7-Gbit/s
PRBS

SMF
#2: 127 km
RDF

10.7 GHz
9.95 Gbit/s
STM-64FEC
encoder

CDR

SMF#3: 122 km

UTC-PD

Ch. power
monitor

InP-HEMT IC
based 1: 4 DEMUX

CLK
recovery

RDF

BPF

A

Error
detector
STM-64analyzer

FEC
decoderFig. 8. Experimental setup for 1.2-Tbit/s (30 × 43 Gbit/s) WDM transmission.

Vol. 1 No. 8 Nov. 2003

43

17

–20
–30
–40
–50

Measured BER of
all STM-64 tributary channels < 10–12

15

24

13

8

16

–24

1

50 nm

11

1580

9
1550

–15
–9

30

–12
–9
–7
–6
–5
–4

Uncorrected log10 BER

19Corrected log10 BER

0–10
Q-factor (dB)

Relative power (dB)Selected Papers

–3
–60
1520

1540

1560

1600

1620

1640

1570

1590

1610

1630

Wavelength (nm)Signal wavelength(nm)
(a)

(b)

Fig. 9. Terabit/s WDM upgrade using FEC based on 43-Gbit/s optical channel.
NRZ spectrum

Consensus→ no error(signal=“0”)Vth+group

EXOR

CS-RZ spectum
CS-RZ
transmitter

1

ModesplitterVth–groupA“low”
Selector2
B“high”EXOR
Disagreement→ error(signal=“0”)NRZ spectrum

Proposed receiverFig. 10. Receiver configuration for proposed error-control.

Figure 9 shows the 1.2-Tbit/s WDM transmission
performance. The WDM spectra after transmission
are shown in Fig. 9(a). There is no four wave mixing
crosstalk component in the WDM spectra. Figure9(b) shows the BER improvement achieved using
FEC. All the 43-Gbit/s channels were successfullytransmitted with error free operation by employing
FEC for all channels.
The use of 43-Gbit/s channels with FEC in a terabit-per-second WDM system allows the span loss tobe as high as 30 dB, even if the channel power is 6 dB
less than the level used in the 8 × 40 Gbit/s channel
WDM experiment without FEC, as described in ref.
[7]. The STM-64 client channels were multiplexed
and transmitted with bit error rates better than 10–15
while keeping client overhead transparency.

44

5. Novel high-gain error control using optical
spectrum redundancy
5.1 Spectrum redundancy in CS-RZ format
The CS-RZ format is characterized by its dualmode spectrum redundancy. The CS-RZ pulses are
generated as the beat between two different longitudinal modes, and each mode is intensity-modulatedwith NRZ data [7]. This view of the CS-RZ spectrum
suggests that the frequency components can be separated by optical filtering; each separated signal
should have the NRZ format, as experimentally verified [27]. Figure 10 shows a general diagram of ournovel error control technique, where two opticalelectrical (O/E) conversion blocks are employed [21].
After dual O/E conversion, we employ special digitalprocessing to achieve the error control (details are
NTT Technical Review

Selected Papers

described in the following sub-sections). It should be
noted that the two frequency components of the CSRZ signal experience different noise environments
created within the optical amplifiers in the transmission line because the components have different central frequencies and spectra. We assume that amplified spontaneous emission (ASE) from optical amplifiers is the dominant noise source in the systems. The
ASE noise on each component is generated by a different energy state transition of the Er3+ electrons in
the optical fiber amplifiers. Therefore, mode-splitting
yields two NRZ signal encoded by the same logic,
each of which is accompanied by independent noise.

tion mechanism that dispenses with the use of frames,
codes, and all complex calculations. The “consensus”
test is achieved simply by using two Exclusive-OR(EXOR) circuits.
To recover data, the scheme employs bit-by-bit
selection of the decision threshold voltage, as shownin Fig. 11. For signal “1” (upper figure), the decision
with a low threshold voltage provides better performance than the conventional optimum threshold,while for signal “0” (lower figure), the decision witha high threshold voltage provides better performance.
As illustrated in Fig. 11, if the Vth+ group provides noerror and the Vth– group does provide errors, the signal is very likely to be binary encoded by “0”. In thiscase, Vth+ is selected in the bit-period. The selector
logic is as follows. Assume it has two main-signal
ports 1 and 2, and two control ports A and B, as
shown in Fig. 10. The main-signal port 1 is associated with control port A, while 2 is associated with B.
If A is “low” and B “high”, the selector selects port 1.
If A is “high” and B “low”, the selector selects port 2.
The selector holds the same state as the previous bit
when the two EXOR circuits generate double “low”(consensus in both groups) or double “high” (disagreement in both groups). As the bit-by-bit selector,
we can use a high-speed 2:1 multiplexing circuit.

Voltage

5.2 Novel error control principle
Error-control circuits are used to implement twobasic digital operations: error detection and data
recovery.
To detect errors, the proposed scheme subjects the
split signal to “consensus” logic. Chromatic dispersion compensation is assumed, so the delay difference between the two NRZ signals should be offset.
In the receiver, the transmitted signal is split into two
by an optical mode-splitting filter. CS-RZ alreadyoffers 100% redundancy, and we use this spectral
redundancy to achieve error control. After dual (ordiverse) optical reception, each split baseband signal
is divided into two morebranches. Each divided signal
enters a decision circuit, each of
Noise distribution
which has one of two different
threshold levels: a high decisionthreshold Vth+ or low decision
V1
threshold Vth–. Hence, there are
four decision circuits in total
Threshold for “1”and we form them into two×groups according to the threshV0
old: the V th + group and V th –group. The outputs of thegroups represent the decisionanswers of the same logic with
Bit-by-bit threshold selection
independent noise. Within eachgroup, the “consensus” or “disagreement” is tested. If the
V1
result is “consensus”, the group×will have the correct decision,
Threshold for “0”so the threshold is likely to provide no error. If the result isV0“disagreement”, the group contains a wrong decision, so thethreshold is erroneous. This
Fig. 11. Bit-by-bit threshold selection improves BER.
forms a bit-by-bit error detecVoltage

TimeVol. 1 No. 8 Nov. 2003

Time

45

Selected Papers

5.3 Performance estimation
The probability that errors will propagate to downstream circuits is determined by two factors: double
errors in each group, and double disagreement amonganswers in both groups. The former factor can be
expressed as the product of the probabilities of erroneous decisions regarding the same signal with independent noise. When considering the upper port ofthe selector in Fig. 10, the probability of double errors
is given by 1/2*P2(1,Vth+)+1/2*P2(0,Vth+), whereP(x,V th y ) means the error probability of signal“x”=(0,1) with threshold “y”=(+,–) decision. In theabove expression, the mark ratio is assumed to be 1/2.
Similarly, the probability of double errors in thelower port of the selector is given by
1/2*P2(0,Vth–)+1/2*P2(1,Vth–). When considering the
double disagreement factor of error propagation, it
should be noted that we use electronic “copies” of thesame signals with independent noise (the total number of signals is four). Therefore, the upper decision
circuit in the Vth+ group and the upper decision circuitin the Vth– group experience exactly the same noise.
We assume that the dominant errors are due to ASE
noise from optical amplifiers because the thermal
noise and shot noise in the receiver are negligible.
The probability of double disagreement is estimatedas the lower of two probabilities: min{P(0,V th+),P(0,V th–)}=P(0,V th+) for “0” and min{P(1,Vth+),P(1,Vth–)}=P(1,Vth–) for “1”. In the equilibrium condition, when errors exist, the probability of the selector selecting the upper or lower port can be assumed
to be approximately 1/2. Therefore, the probability ofan error propagating through the receiver is given by
Pe =

1 2
1
P (1,Vth+) + P2(0,Vth–)441
1+ P(0,Vth+) + P(1,Vth–).
2
2

(3)

In the above expression, P2(0,Vth+) and P2(1,Vth–)are neglected. Here, for simplicity, we assume thatthe noise in the system is additive white Gaussian
noise (AWGN), where “0” and “1” have the sameamount of standard deviation due to noise, aroundeach average. It is well known that AWGN is notaccurate in optical amplified systems, but this simplified model can provide a good understanding of thefundamental features of this new scheme. We use the
Q factor of the ordinary AWGN model. Although it
usually includes a threshold parameter, we place thethreshold parameter outside the Q factor in the following expression assuming Q=I/(2σ), where I is thesignal voltage for “1” and σ is the standard deviation.
46

The average voltage for signal “0” is assumed to be 0
for simplicity. The error probability is given by
Pe =2(1–x)Q
2xQ
1
1
Erfc2{} + Erfc{
},
8
2
√2
√2

(4)

where x is the ratio of decision threshold voltage
Vth+ and signal “1” voltage. Here, Erfc(u) is the wellknown complementary error function defined as
Erfc(u) =2 ∞exp{–u2}du.
π u

∫

(5)

The optimum thresholds for this receiver are calculated as x=0.6 and 0.4 for Vth+ and Vth–, respectively.
The error control performance should be the relationship between raw (input) and controlled (output)
BER calculated from (4), especially if the raw BER
curve is not as steep as in the theoretical error-function.
5.4 Experimental verification
The proposed error controlling receiver was built
and tested with the addition of 7% FEC code, at thedata rate of 10.7 Gbit/s. In this experiment, we usedthe 10.7-Gbit/s RS(255, 239) standard FEC frame fordata transmission and 42.7-GHz CS-RZ pulses fordual mode generation. There will no difficulty in supporting 43-Gbit/s data transmission because 43Gbit/s electronic devices are already available.
Experimental results are shown in Fig. 12. The open
circles plot the raw BER without any error control by
the conventional receiver and with FEC decoder inactive. The green circles represent the results with theproposed receiver without FEC decoding. The thickblack line represents a theoretical estimation calculated from the relationship between raw BER and output
BER obtained from equation (2). The raw BER of
10–6 was improved to 5 × 10–10, which is slightly better than the above estimation. This is due to the simple approximation that noises are AWGN. In thisexperiment, the gain was 3 dB at the BER of 10–12.
The leftmost plots represent the performance of the
proposed receiver together with RS(255, 239). We
achieved error free operation (<10–12) at the received
optical power of –38.5 dBm. Total gain with the proposed scheme and 7% FEC was 9 dB. The important
point to note is that the gain of our technique follows
the simple sum-rule, e.g, FEC gain (6 dB) plus pulse
error-control gain (3 dB). This is due to the characteristics of the receiver: it proceeds on a bit-by-bit
basis and is thus independent of error statistics. The
usual FEC concatenation approaches do not offer a
sum-rule for coding gain because their performance
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Selected Papers

Data: 10.7 Gbit/s
CS-RZ pulse: 42.7 GHz
10–4
Proposed receiver (basic)
accompanied with RS(255,239)–5Bit error ratio

10

Without
error control

10–6transmission in the OTN. A novel error correction
scheme using optical spectral redundancy has great
potential to increase the system margin by 9 dB in
combination with the standard RS(255, 239) without
any additional line rate increase. These technologies
ensure the practicality and evolution of the OTN
based on 43-Gbit/s channel in combination with several hybrid optical amplification technologies including distributed Raman and erbium-dopod fiber amplifiers.

Proposed receiver(basic)

10–7

References
[1]

10–8
10–9
Theory
10–10
–11

10

6 dB

10–12
–40

3 dB

–35
–30
Averaged optical power (dBm)

Fig. 12. BER performance of proposed error-control
scheme.

depends on error statistics (errors after the first
decoding show strong burst-error characteristics).
This independence of error statistics of the proposed
receiver enhances its tolerance against PMD, whose
error statistics have burst-error characteristics [28].
6. Conclusion
This paper described recent progress in the opticaltransport network based on 43-Gbit/s channels. A
novel digital frame format for the optical transport
unit (OTU) has been standardized to ensure a highly
reliable and cost-effective optical transport network(OTN) that can transport today’s data-oriented multiclient services transparently. The OTU frame offers
an overhead byte that is dedicated to WDM network
operation and the transmission performance improvement offered by RS(255, 239) forward error correction code at the expense of only a 7% increase in line
rate. Novel bandwidth-efficient formats, including
CS-RZ, are very effective for achieving long-haultransport systems with high spectral efficiency ofover 0.4 bit/s/Hz. The ITU-T standard FEC not only
enhances the transmission performance of 43-Gbit/schannels, but also allows bit-error-rate performancemonitoring while keeping transparent client signal
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K. Hagimoto, M. Yoneyama, A. Sano, A. Hirano, T. Kataoka, T.
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Y. Tada, K. Hagimoto, K. Sato, K. Wakita, K. Kato, and Y. Miyamoto, “Field experiments on 40 Gbit/s repeaterless transmission over 198
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[7] Y. Miyamoto, A. Hirano, K. Yonenaga, A. Sano, H. Toba, K. Murata, and O. Mitomi, “320-Gbit/s (8 × 40 Gbit/s) WDM transmission
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Kawakami, H. Toba, K. Murata, M Fukutoku, Y. Yamane, K.
Noguchi, T. Ishibashi, and K. Nakajima, “320 Gbit/s WDM field
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[9] T. N. Nielsen, A. J. Stentz, K. Rottwitt, D. S. Vengsarkar, Z. J. Chen,
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[10] Y. Miyamoto, K. Yonenaga, S. Kuwahara, M. Tomizawa, A. Hirano,
H. Toba, K. Murata, Y. Tada, Y. Umeda, and H Miyazawa, “1.2Tbit/s (30 × 42.7-Gbit/s ETDM optical channel) WDM transmission
over 376 km with 125-km spacing using forward error correction andcarrier-suppressed RZ format,” Proc. of OFC2000, Baltimore,
U.S.A., post-deadline paper PD26, Mar. 2000.

47

Selected Papers[11] W. Weiershausen, H. Scholl, F. Kuppers, R. Leppla, B. Hein, H.
Burkhard, E. Lach, and G. Veith, “40 Gb/s field test on an installedfiber link with high PMD and investigation of differential group delay
impact on the transmission performance,” Proc. OFC’99, San Diego,
U.S.A., paper ThI5, pp. 125-127, 1999.
[12] S. Kuwahara, K. Yonenaga, Y. Miyamoto, Y. Kisaka, K. Sato, A.
Hirano, T. Ono, A. Matsuura, M. Tomizawa, T. Kataoka, Y. Tada, H.
Toba, N. Hirayama, and H. Asai, “1 Tbit/s field trial with 100GHz
spacing over 91km SMF using 43 Gbit/s/channel OTN interface prototype,” Electron. Lett., Vol. 37, pp. 904-906, 2001.
[13] T. Otani, M. Hayashi, M. Daikoku, K. Ogaki, Y. Nagao, K. Nishijima, and M. Suzuki, “Field trial of 63 channels 40 Gbit/s dispersionmanaged soliton WDM signal transmission over 320 km NZ-DSFs,”
Proc. of OFC2002, Anaheim, U.S.A., PD-paper FC9, 2002.
[14] M. Birk, L. Raddatz, D. Fishman, S. Woodward, and P. Magill, “Field
trial of end-to-end OC-768 transmission using 9 WDM channels over1000 km of installed fiber,” Proc. of OFC2003, Baltimore, U.S.A.,
paper TuS4, 2003.
[15] Y. Miyamoto, M. Yoneyama, T. Otsuji, K. Yonenaga, and N.
Shimizu, “40-Gbit/s TDM Transmission Technologies Based on
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[19] M. Tomizawa, Y. Miyamoto, T. Kataoka, Y. Tada, “Recent progressand standardization activities on 40 Gbit/s channel technologies,”
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[20] Y. Miyamoto, T. Kataoka, M. Tomizawa, and Y. Yamane, “40-Gbit/s
ETDM Channel Technologies for Optical Transport Network,” Optical fiber technology, Vol. 7, pp. 289-311 2001.
[21] M. Tomizawa, Y. Kisaka, A. Hirano, and Y. Miyamoto, “Error correction without additional redundancy by novel optical receiver withdiverse detection,” Tech. Dig. of OFC2002, Barfimore, U.S.A., pp.
368-370, paper WX7, Feb. 2002.
[22] Y. Miyamoto, K. Yonenaga, A. Hirano, and M. Tomizawa, “N × 40Gbit/s DWDM transport system using novel return-to-zero formats
with modulation bandwidth reduction,” IEICE Trans. Commun., Vol.
E85-B, pp. 374-385, 2002.
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[24] M. Tomizawa, A. Hirano, and Y. Miyamoto, “Safety issues in highpower optical fiber communication systems, including distributed
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[25] A. Hirano, M. Asobe, K. Sato, K. Yonenaga, Y. Miyamoto, H.
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7-1-4, Sep. 2002.

48

Yutaka Miyamoto
Senior research engineer, supervisor, NTT
Network Innovation Laboratories.
He received the B.E. and M.E. degrees in electrical engineering from Waseda University,
Tokyo, Japan, in 1986 and 1988, respectively. In
1988, he joined the NTT Transmission Systems
Laboratories, Yokosuka, Japan, where heengaged in R&D on high-speed optical communications systems including the 10-Gbit/s terrestrial optical transmission system FA-10G. Since
2003, he has been the leader of the High-Speed
Lightwave Transport System Research Group,
NTT Network Innovation Laboratories, Yokosuka, Japan. His current research interests include
optical transport network based on 40-Gbit/schannel and beyond, and their related devices.
He is a member of the IEEE and the Institute ofElectronics, Information and Communication
Engineers (IEICE) of Japan. He received the best
paper award of the first optoelectronics and communication conference (OECC’96) in 1996, andthe best paper award from the IEICE Communication Society in 2003.

Masahito Tomizawa
Senior research engineer, NTT Network Innovation Laboratories.
He received the B.E. degree in applied physics
in 1990, M.Sc degree in physics in 1992, and Dr.
Eng. degree in applied physics in 2000 fromWaseda University, Tokyo, Japan. In 1992, hejoined NTT Transmission Systems Labs, where
he was engaged in R&D of a high-speed opticalring architecture, forward error correcting codesin fiber optic systems, and automatic dispersion
equalization. Then he joined NTT Network Service Systems Laboratoried, where he devoted
himself to the installation of 10-Gbit/s ring systems in the whole of NTT’s network. Since 1999,
he has been with NTT Network Innovation Laboratories, where his main job is international
standardization of fiber optic systems in ITU-T.
He is also engaged in R&D of 40-Gbit/s systems,
polarization mode dispersion (PMD), and newreceiver designs. In 2003, he is on leave to the
Massachusetts Institute of Technology (MIT).
He received the Young Engineer Award fromIEICE in 1998. He is a member of IEEE and
IEICE.

Yasuhiko Tada
Senior Manager of Maintenance and Service
Operation Department, NTT East Corporation.
He received the M.S. degree from Tohoku University, Sendai, Japan, in 1982. In 1982, hejoined NTT Yokosuka Electrical Communications Laboratories, Yokosuka, Kanagawa, Japan,
where he was engaged in R&D of digital speech
signal processing (bit rate reduction codecs) andoptical cross-connect switching technologies. In
1998-2000, he was active in the development of
a commercial upgradable submarine WDM
transmission system. From 2000, he headed
research on high-speed lightwave communication technologies as the leader of the High-Speed
Lightwave Transport System Research Group,
NTT Network Innovation Laboratories, Yokosuka, Japan. Since 2003, he has led the EMC Engineering technology group as Senior Manager ofMaintenance and Service Operation Department,
NTT East Corporation, Tokyo, Japan. He is amember of the IEICE.

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