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Improving Nonlinear Precompensation in Direct-Detection Optical OFDMCommunications Systems

Liang Du, Arthur Lowery

Monash University, Clayton, Victoria 3800, Australia; arthur.lowery@eng.monash.edu.au

Abstract

Carrier boosting at the receiver enables direct detection optical OFDM (DDO-OFDM) to outperform coherent

OOFDM in the nonlinear limit. Boosting also improves the effectiveness of nonlinearity precompensation

substantially.

Introduction

Optical OFDM uses multiple subcarriers and

electronic equalisation to compensate for almost

unlimited dispersion in fibre links: transmission rates

of 121Gbps over 1009 km have been achieved per

WDM channel [1]. OFDM is resilient to both noiseand dispersion but the close packing of the

subcarriers causes strong nonlinear mixing between

them. This means that Optical OFDM operates

optimally at channel powers less than -5 dBm [2] at

10 Gbit/s.

Nonlinearity precompensation can mitigate

nonlinear effects in coherent optical OFDM (CO-

OFDM) systems [3],[4]; an improvement in received

electrical signal quality of over 10 dB is possible for

low dispersion fibres [4]. Postcompensation has

also been demonstrated to mitigate for fibre

nonlinearity for CO-OFDM systems [5]. CO-OFDM

requires a sophisticated optical receiver with anarrow-linewidth local oscillator: Direct-Detection

OOFDM (DDO-OFDM) transmits a carrier along

with the subcarriers and so only needs a photodiode

for reception. However, because the carrier

propagates along with the signal, there is far more

nonlinear mixing, so nonlinearity precompensation

is less effective [6]. Previously we showed that

boosting the strength of the carrier relative to the

sidebands just before the photodiode can improve

the noise performance of DDO-OFDM systems [7].

Boosting also allows the carrier to be transmitted at

a lower level, thus potentially reduces the strength

of nonlinear mixing in the fibre.This paper shows using simulations that carrier

boosting can also greatly improve the nonlinear

performance of DDO-OFDM systems, to exceed

coherent systems. Additionally, nonlinearity

precompensation becomes much more effective

with carrier boosting.

Nonlinear compensation

The transmitter is very similar to [6], where the 5-

GHz upper sideband is positioned 5-GHz above the

optical carrier as shown in Fig. 1. 10 Gbit/s is

transmitted using 4-QAM and 512 subcarriers. For

precompensation, phase modulation is applied to

the subcarriers and carrier, in proportion to the

instantaneous optical power in the sideband [6].

Figure 1: Spectrum of DDO-OFDM after

precompensation phase modulation.

Phase modulation in the time domain can

completely compensate for the intermixing of the

OFDM subcarriers in dispersionless fibre; however,

in dispersive fibre the waveform evolves duringpropagation, so that the precompensation is less

effective. The amount of precompensation is

generally reduced to find an optimum signal quality

at the receiver. We are interested in low-dispersion

fibres as these have a much poorer nonlinear

performance and so greatly benefit from nonlinearity

compensation [3]. In the following examples, a

4000-km system with of 2 ps/nm/km fibre was

simulated using VPItransmissionMaker. Amplifier

noise was turned off to isolate the nonlinear effects

as we are interested in the nonlinear limit. Span

lengths of 80-km were used with an attenuation of

0.2 dB/km. At the input of every span, the optical

power was re-amplified to -3 dBm.

The optical signal was also amplified just before the

photodiode receiver, and the carrier boosted with a

1-GHz band-pass optical filter with a variable stop-

band attenuation. The optimum transmitted carrier

level without boosting is when the carrier power

equals the sideband power: we also investigated

suppressing (cutting) the carrier by a further 5 dB

at the transmitter.

Results

Fig. 2 plots received signal quality, Q, averaged

over all subcarriers [2] versus the precompensation

P.4.08ECOC 2008, 21-25 September 2008, Brussels, Belgium

Vol. 5 - 1471978-1-4244-2228-9/08/$25.00 (c) 2008 IEEE

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