29 w.rosenkranz

8/2/2019 29 W.Rosenkranz

1/4

High Capacity Optical Communication Networks Approaches for Efficient Utilization of Fiber Bandwidth

Werner RosenkranzChair for Communications, University of Kiel, Germany

Kaiserstr. 2, D-24143 Kiel, [email protected]

Introduction

The rapidly worldwide growing data and internet traffic in telecommunication networks resultsin a sharp increase in the demand for transmission capacity. Thus a much more efficientutilization of the existing optical fiber network is an answer that meets economicalconstraints. An important step is the introduction of optical networks, where transmission,switching, and amplification will be carried out in the optical domain. These networks will bebased on DWDM technology, i.e. a great number of wavelength channels densely packedwith channel spacing of 100 GHz or less and transmission rates of 10 Gb/s per channel or

more are travelling in a single fiber over considerable distances resulting in an improvedutilization of the available bandwidth of the fiber channel. Thus for the first time, bandwidthefficiency an important feature in wireless communications becomes an important issuein fiber optics.

There are several approaches to achieve overall bandwidth efficiency. Amongst them areimproved modulation formats, channel coding, sophisticated management of chromatic andpolarization mode dispersion including equalization, wideband optical amplifiers, improvedWDM-multiplexers and demultiplexers. Sophisticated modeling of the WDM-optical channelincluding nonlinear effects is necessary in order to investigate the impact of the variousmethods. For practical reasons it is important to end up with structures that are realizable atthese high frequencies i.e. we have to avoid high complexity algorithms.

In this contribution we focus mainly on bandwidth efficient modulation formats at channeldata rates of 10 Gb/s and 40 Gb/s. By saving bandwidth both the dispersion problems andthe channel density are improved. A promising approach is partial response modulation andcoding as e.g. duobinary coding, which is a pseudoternary format [1],[4].

Optical Duobinary Signalling

Fig.1: Block diagram of duobinary coder with precoder (left). Power spectrum density ofduobinary and binary (NRZ) modulation formats (right)

A partial response encoded sequence c(k) is related to the binary data sequence d(k) by thefollowing encoding rule (Fig. 1)

1

0

( ) ( )n

c k d k

=

= . (1)

0 5 10 15 20

-30

-25

-20

-15

-10

-5

0

f [GHz]

[dB]

PRECODINGDUOBINARY-CODING(FIR- FILTER)

DELAYT

+

mod 2

DELAYT

+

+1+1

d(k)b(k)

c(k)

{0,1} {0,1}

{0,1,2}

b(k-1)

MZM

binary

duobinary


2/4

The encoding rule for a special code is fully described by n coefficients {}. The duobinary

code [2] has n=2 coefficients 0 = 1 = 1. That means a binary signal is encoded to a three-level duobinary signal by adding the current bit to the previous bit:

( ) ( ) ( 1)c k d k d k = + . (2)

The output symbols are 0, 1 and 2 or using an offset of 1 the symbols are 1, 0 and 1. Thiscode may be regarded as an FIR-filter which follows a power spectral density of the signal

multiplied with ( )2cos / 2T that is responsible for a zero at half of the data rate 1/T , where

T is the bit duration. So the spectral occupancy can be reduced by a factor of 2approximately. This results in less dispersion sensitivity.

A differential encoder is used as a precoder in order to avoid recursive decoding in thereceiver thus avoiding error propagation and reducing hardware complexity. The precodingrule for duobinary coding is

( ) ( ) ( 1)b k d k b k = , (3)

where d(k) is the transmitted binary data sequence, b(k) is the precoded binary sequence,and is the logic instruction XOR. Due to using a precoder and an encoder in thetransmitter, decoding in the receiver is very simple. The data bit d(k) is the absolute value of

the encoded symbol ( )c k {-1,0,1} i.e. d(k)=|c(k)|. Therefore at the receiver side direct

detection (square-law) photo-detector receivers for binary signaling may be used without anymodification.

Duobinary encoding devices can be realised either by a delay-and-add filter using a delay-line as in Fig. 1 or by a duobinary filter [3] as in fig. 2, i.e. a low-pass filter with a 3-dB cut-off

frequency of about bit rate. The transfer function approximates the main lobe of the cosinetransfer function eq.(1) of the duobinary filter. A Mach-Zehnder modulator is used to generatean optical duobinary signal. Assuming an ideal MZ modulator the complex envelope of theoutput optical field can be expressed by

pulsepatterngenerator

differentialencoder

T +LPF

duobinaryencoder

5 GHz

dual-driveMZ-mod.

10Gb/s

T +LPF

duobinaryencoder

5 GHz

bias

Q

Q

ternary

ternary

LPF

duobinaryfilter

2.5 GHz

bias

LPF

duobinaryfilter

2.5 GHz

dual-driveMZ-mod.

pulsepatterngenerator

differentialencoder

10Gb/s

Q

Q

ternary

ternary

Fig.2: Duobinary transmitter with Mach-Zehnder modulator and delay-and-add coder (left)and duobinary filter (right)

1 2 1 20

( ) ( ) ( ) ( )( ) cos exp

2 2

U t U t U t U t E t E j

U U

+=

(4)

where U is the switching voltage, and 1U and 2U are the voltages applied to each arm of

the device. Using the MZ modulator in push-pull configuration ( 1 2U U= ) with the tree-level

driving voltage, results in an optical duobinary signal. The off state corresponds to the 0.

The symbols -1 and 1 are represented by the on state with phase 0 and , respectively.

Performance of duobinary signalling


3/4

The simulation model consists of a data source generating a pseudorandom binary

sequence at 10=Gb/s, a precoder, a duobinary encoder followed by a MZ modulator, the fibreand a simple receiver consisting of a PIN diode. The MZ modulator is described by equation(6). The chromatic dispersion of the fibre is modelled by a low-pass equivalent fibre transfer

function ( )2 20( ) exp / H f j LD f c = , with the fibre dispersion coefficient D =17 ps/km/nm.L is the fibre length, 0 is the operation wavelength, and c is the speed of light in the free

space.

We have considered the MZ in push-pull configuration. The duobinary encoder is realised intwo different ways. On the one hand as a delay-and-add filter followed by a pulse shapinglow-pass filter realised as 2nd-order Butterworth filter with 5-GHz bandwidth. On the otherhand as a duobinary filter realised as 2nd-order Butterworth filter with 2,5-GHz bandwidth. Abinary signal with sinusoidally shaped edges is used for comparison. We have consideredthe eye diagram of the received signal for different fibre lengths. The eye of the binarytransmission is fully closed after 130 km fibre length. The duobinary signals still can bedetected and extend the transmission distance substantially.

(a)0 0.5 1 1.5 2

x 10-10

0

0.5

1

1.5

2 120 km

time [s]

normalizedamplitude

0 0.5 1 1.5 2

x 10-10

0

0.5

1

1.5

2

2.5 180 km

time [s]

normalizedamplitude

(d)

(b)0 0.5 1 1.5 2

x 10-10

0

0.5

1

1.5120 km

time [s]

normalizedamplitude

0 0.5 1 1.5 2

x 10-10

0

0.5

1

1.5

2180 km

time [s]

normalizedamplitude

(c)0 0.5 1 1.5 2

x 10-10

0

0.5

1

1.5120 km

time [s]

normalizedamplitude

0 0.5 1 1.5 2

x 10-10

0

0.5

1

1.5

2180 km

time [s]

normalizedamplitude

(e)

Fig. 3: Eye diagrams of (a) binary transmission, (b) duobinary transmission realised by delay-

and-add filter followed by a pulse shaping low-pass filter, (c) duobinary transmission realisedby a duobinary low-pass filter and (d),(e) BER over SNR, fiber length 120 km and 180 km,data rate 10 Gb/s.

Experimental results

In a lab experiment a 10 Gb/s duobinary transmitter with duobinary filter (Bessel-filter fifthorder, 3-dB-cutoff frequency 2.8GHz) has been realized. Fig. 4 shows the eye diagrams after0km, 75km and 150km of standard single mode fiber for conventional binary (NRZ)modulation and duobinary modulation. Clearly the improvement is demonstrated.

Dispersion tolerance in dispersion managed multispan long-distance trunk lines

High-speed long-distance WDM-trunk lines over standard single mode fiber are necessarilydispersion compensated. State of the art is to use dispersion compensating fiber (DCF).

10 15 20 25 3010

-20

10-15

10-10

10-5

100

SNR / [dB]

BER

120 km

binaryduobinary, delay&add, TPduobinary, TP

10 15 20 25 3010

-20

10-15

10-10

10-5

100

SNR / [dB]

BER

180 km

duobinary, delay&add, TPduobinary, TP


4/4

Fig.4: Eye-diagram for duobinary 10Gb/s transmission. back-to-back (left) and after 150kmstandard single mode fiber

Therefore dispersion tolerance of the duobinary format in such an environment wasinvestigated by simulation. Weconsidered a 10Gb/s optical line of 10identically dispersion compensatedand optically amplified spans of 100kmeach. Each span is driven by 11dBm

optical power, thus nonlinear selfphase modulation occurs. In fig. 5 thedispersion tolerance of the system forthe duobinary and binary formats isshown. At the abscissa the amount ofdispersion-compensation in each spanis given (i.e. full-compensation for-1700ps/nm). Dispersion tolerance isestimated by the amount ofcompensation deviation which istolerable in the last span in order tomeet a quality criterion (Q-factor). Weconclude that undercompensation ispreferable and that duobinary format is

less critical to a sophisticated dispersion management.

Conclusions

By duobinary encoding a bandwidth reduction by approximately a factor of two is achieved atthe cost of only slightly higher complexity at the transmitter site. The receiver remainsunchanged with respect to the common IM/DD concept for NRZ-format. We show severalnew implementations using a Mach-Zehnder modulator. It is shown that receiver sensitivitywith respect to dispersion sensitivity may be improved by approximately 10 dB for long

distance spans. Also first results from lab experiments are reported. Several structures areoptimized with respect to minimum bit error ratio and eye-opening.

References

[1] Yonega, Kuwano. "Dispersion-Tolerant Optical Transmission System Using DuobinaryTransmitter and Binary Receiver". Journal of Lightwave Technology, vol. 15, no. 8, 1997.[2] Lender. "The Duobinary Technique for High-Speed Data Transmission". IEEEtransactions on communication and electronics, 1963.[3] Walklin, Conradi. "On the Relationship Between Chromatic Dispersion and TransmitterFilter Response in Duobinary Optical Communication Systems". IEEE photonics technologyletters, vol. 9, no.7, 1997.

[4] Wichers, Rosenkranz: "Optical duobinary modulation schemes using a Mach-Zehndertransmitter for Lightwave Systems". International Conf. on Transparent Optical NetworksICTON'99, 1999, p. 15-18

Fig. 5: Dispersion tolerance of a10x100km dispersion managed SSMF-

line in the nonlinear regime.

29 w.rosenkranz

Documents