Terabit Transmission of High Capacity Multiband OFDM Superchannels on Field Deployed Single Mode Fiber

Roman Dischler1, Fred Buchali

1, Axel Klekamp

1, Wilfried Idler

1, Eugen Lach

1,

Achim Schippel2, Malte Schneiders

3, Sascha Vorbeck

3, Ralf-Peter Braun

4

(1) Alcatel-Lucent Bell Labs, Lorenzstr. 10, D-70435 Stuttgart, Germany, Roman.Dischler@Alcatel-Lucent.com

(2) Deutsche Telekom Netzproduktion GmbH, Technische Infrastruktur, Maybachstr. 57, 70469 Stuttgart

(3) Deutsche Telekom AG, Zentrum Technik Einführung, Heinrich-Hertz-Str. 3-7, 64295 Darmstadt, Germany

(4) Deutsche Telekom AG, Laboratories, Goslarer Ufer 35, 10589 Berlin, Germany

Abstract We demonstrate transmission of 5x253-Gb/s optical OFDM-superchannels in 100-GHz

spacing over 454 km field deployed fibers, achieving a capacity of 1.16 Tb/s. In addition transmission

of 398.5 Gb/s in one OFDM channel over the same distance is reported.

Introduction

Orthogonal Frequency Division Multiplexing

(OFDM) has emerged as a very promising

modulation format for high-speed and high-

capacity optical transmission due to its high

spectral efficiency and its resilience in the

presence of fiber dispersion and PMD. Several

transmission experiments [1-4] have shown that

optical OFDM is capable to transport Tb/s

capacities over several 100 km link length. The

almost rectangular shape of the optical OFDM

spectra allow to closely arrange multiple OFDM

signals in the frequency domain without guard

bands to form high-capacity superchannels [5],

leaving the classical system architecture with

fixed 50-GHz or 100-GHz channel grids.

In [6] we have demonstrated the transmission

of 253-Gb/s OFDM superchannels over field

deployed fibers provided by Deutsche Telekom,

reaching an encouraging length of 764 km. To

indicate a possible migration path towards grid

less networks, we choose an OFDM

configuration which represents a dense

multiband approach for signal generation, but is

compatible to a 100 GHz grid by leaving a

10 GHz guard band between the 253-Gb/s

OFDM channels.

This paper reports the results of further

transmission experiments, to study increased

transport capacities with the use of up to 5

OFDM channels in WDM configuration and

higher constellation modulation formats for

subcarriers.

All reported transmission experiments were

conducted with several 112-Gb/s NRZ-QPSK

channels in 100-GHz spacing in co-propagation

as additional system load.

Field Transmission Test Bed

The field trial link of Deutsche Telekom, shown

in Fig. 1, is located in Southern Germany. The

10 spans of G.652 fiber were divided into two

sections reaching a length of 454 km (east loop)

and 310 km (west loop). EDFA repeater

amplifiers were distributed in 6 locations, while

transmitter and receiver equipment were located

at the DTAG premises in Stuttgart. For the

experiments reported in this paper all dispersion

compensation modules in the amplifiers were

removed to prevent excess accumulation of non

linear distortions during transmission.

The transmitter setup to generate the

multiband OFDM signals is depicted in Fig. 2. It

is the same setup as used in [6]. The usage of 5

DFB laser sources allow the generation of

higher number of OFDM channels in a WDM like

configuration (Fig. 3, left). The multiband 253-

Gb/s OFDM signals consist of 15 subbands and

cover a bandwidth of 90 GHz each. For bit error

measurements, the respective DFB laser was

replaced by a tuneable ECL-source with lower

line width of about 100 kHz.

Öhringen

Feuchtwangen

Nürnberg

Stuttgart Tx, Rx

Ladenburg

Karlsruhe

78.9km

72.6km 75.4km

75.3km 79.7km

Repeater

west loop 310km

east loop 454km

Fig. 1: Map of field trial link with 2 sections over

6 repeater locations

Synth. ∆∆∆∆f, 2∆∆∆∆f, 5∆∆∆∆f

AWG

PBS

L1

L2

L3

5 x DFB Lasers

∆∆∆∆f = 5.9GHz

DLI Comb

gen. (15 lines)

OFDM 2 I/Q

OFDM 1 I/Q

100ns

54ns odd

even pol. mux.

L4

L5 PBC

ECL- source

Fig. 2: Transmitter for 5x253-Gb/s multiband

OFDM signal

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

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

Tu.3.C.6

At the reception side we used a polarisation

diverse coherent receiver setup with tuneable

laser as optical local oscillator (LO) and offline

processing for frequency offset compensation,

synchronisation, MIMO-processing for

polarisation demultiplexing and channel

estimation, and bit error rate (BER)

measurement [4]. By tuning the LO to the

appropriate wavelengths, the tributaries of each

superchannel could be selected and processed

consecutively.

The total output power of the repeater

amplifiers into the transmission fibers was set to

16 dBm. This could not be varied by remote

control, so we adjusted the launched power of

the OFDM channels by variation of the power

ratio of OFDM- to QPSK-channels with tuneable

attenuators in front of a 2:1-coupler, which

superposed the signals of OFDM- and QPSK-

transmitter at the input of the first booster. A

typical spectrum of the combined signals is

shown in Fig. 3.

OFDM channels in WDM configuration

To study the transmission of high capacity

OFDM channels in WDM environment we

increased the number of generated OFDM

signals to 5x253 Gb/s-channels. Due to non-

linear interchannel distortions, we had to reduce

the launched power per channel. For this reason

the available OSNR at the receiver after the full

transmission length of 754 km was no longer

sufficient to achieve a BER below EFEC limit [7],

so we reduced the transmission length to

454 km by using the ‘east loop’ section only. For

time constraints, we measured every second

tributary of each 253 Gb/s-OFDM channel. All

measured BER depicted in Fig. 4 indicate

sufficient margin to the EFEC-limit with a

uniform performance for all channels and all

tributaries within the experimental stability of the

test bed.

In Fig. 5 we have summarised the optimum

launched power per channel versus the number

of transmitted channels. For a better comparison

we also depict the integrated power, which takes

the different number of spans into account. By

increasing the number of OFDM channels more

XPM products are generated by the non-linear

effects of the fiber. The simulation of a

comparable configuration with 112-Gb/s OFDM

channels in a 50 GHz grid in [8] showed a

reduction of 4 dB of integrated power going from

single channel to WDM transmission. While the

integrated power of 14.1 dBm for a single

channel found in the experiment is slightly lower

than the 16 dBm obtained by simulations, the

power reduction of 5.8 dB when transmitting

-55

-50

-45

-40

-35

-30

-25

-20

1547 1548 1549 1550 1551 1552

Wavelength (nm)

Power (dB)

100GHz 100GHz 100GHz 100GHz

-55

-50

-45

-40

-35

-30

-25

-20

1544 1549 1554 1559Wavelength (nm)

Power (dB)

OFDM QPSKQPSK

Fig. 3: Spectra of 5x253 Gb/s OFDM channels in 100 GHz grid (left), transmitter spectrum of 253 Gb/s-OFDM and 112 Gb/s-QPSK signals as additional system load (right)

1.E-04

1.E-03

1.E-02

1.E-01

1547.5 1548.5 1549.5 1550.5 1551.5LO Wavelength (nm)

BER

Transmission: 454km SMF (no DCF)OFDM: 5Ch 253Gb/s, QPSK subc. modulation

EFEC-limit

0.5

2.9

4.1

14.1

8.3

12.9

0

3

6

9

0 1 2 3 4 5 6

No. of Channels

Launched Pwr/Ch (dBm)

6

9

12

15

Integrated Power (dBm)

10 Spans

6 Spans

Fig. 4: BER of 5x253 Gb/s transmission over 454 km

(38 out of 75 tributaries measured)

Fig. 5: Optimum launched power per channel and

integrated power with variation of OFDM channels

Tu.3.C.6

5 channels indicates excess penalties for WDM

configuration in the experiment. We attribute this

observation to the use of identical modulation of

adjacent subbands on both sides of the

respective measured subband with a narrow

spacing of only 5.9 GHz, leading to strong non-

linear interaction on the first transmission spans,

before dispersive walk off leads to a

decorrelation of adjacent subbands [9].

However an average BER of 5.9E-4 for all 5

channels and a performance of all measured

tributaries below the EFEC limit indicate a

successful transmission of 1.16 Tb/s usable

capacity over 454 km of field deployed SMF with

significant margin.

Transmission of 1x398.5Gb/s OFDM signal

To further increase the capacity of a single

OFDM channel we replaced the formerly used

QPSK data modulation of the subcarriers by

8QAM. In an OFDM configuration with 244

modulated subcarriers we achieve a data rate of

398.5 Gb/s for user data and FEC overhead,

reaching a spectral efficiency of 4.3 bit/s/Hz if

we take 7% FEC overhead and 90 GHz used

bandwidth into account.

Fig. 6 shows a comparison of the back-to-

back OSNR sensitivity of a 253-Gb/s signal with

QPSK modulation and a 398.5-Gb/s signal with

8QAM. Measurements of similar OFDM

configurations without multiband generation in

our setup typically show a difference of 4dB in

OSNR sensitivity for a BER of 10-3 when going

from QPSK to 8QAM, thus the current multiband

transmitter setup gives about 2 dB of excess

penalty to the 8QAM configuration carrying

398.5 Gb/s.

Finally we transmitted the 398.5 Gb/s OFDM

multiband signal together with 8x112 Gb/s

QPSK over the 454 km SMF of the ‘east loop’.

The BER measurement shown in Fig. 7

indicates a performance of all tributaries below

the EFEC limit [7], thus error free transmission

(BER<10-14) after the application of EFEC is

ensured [7]. However, with an average BER of

2.16E-3 there is only little margin to the EFEC

limit.

Summary

We have successfully demonstrated the

transmission of 5x253 Gb/s OFDM super-

channels over 454 km G.652 field installed fiber

of Deutsche Telekom in a 100 GHz grid,

achieving a capacity of 1.16 Tb/s for user data.

The transmission was performed in co-

propagation with 10x112 Gb/s QPSK signals.

Comparison of transmission using 1, 3 and 5

OFDM channels show a significant reduction of

optimum launched power due to a strong non-

linear interaction of identically modulated OFDM

subbands.

In a second experiment we demonstrate the

transmission of a capacity of 398.5 Gb/s in a

single OFDM superchannel over 454 km field

deployed fiber, achieving a spectral efficiency of

4.3 bit/s/Hz within the used 90 GHz bandwidth.

Acknowledgment

This work was supported by the European

CELTIC program and financially supported by

the German BMBF project 100GET.

References [1] S.L. Jansen et al., OFC 2008, PDP2

[2] Q. Yang et al., OFC 2008, PDP7

[3] Y. Ma et al., OFC 2009, PDPC1

[4] R. Dischler et al., OFC 2009, PDPC2

[5] W. Shieh, OFC 2009, OWW1

[6] R. Dischler et al, OFC 2010, PDPD2

[7] ITU -T Recommendation G.975.1, 2004,

Appendix. I.9

[8] X. Liu et al, ECOC 2008, Mo.3.E.2

[9] R. Dischler et al., Proc. OFC 2009, OWW2

15 20 25 30 35

OSNR / 0.1nm (dB)

BER

1E-2

1E-3

1E-4

1E-5

1E-6

398.5Gb/s8QAM

253Gb/sQPSK

1.E-04

1.E-03

1.E-02

1.E-01

1550.1 1550.3 1550.5 1550.7 1550.9

LO Wavelength (nm)

BER

Transmission: 454km SMF (no DCF)OFDM: 398.5Gb/s, 8QAM subc. modulation

EFEC-limit

Fig. 6: Back-to-back measurement of 253 Gb/s

and 398.5 Gb/s OFDM superchannel

Fig. 7: BER of 398.5 Gb/s-OFDM channel after

transmission over 454 km SMF

Tu.3.C.6