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Proceedings of IC-NIDC2009

BASEBAND OFDM OPTICAL SINGLE-SIDEBAND

TRANSMISSION WITH PARALLEL OPTICAL SSB

MODULATION FOR DIRECT DETECTIONJing Ning, Yaojun Qiao, Yuefeng Ji

Key Laboratory of Information Photonics and Optical Communications(BUPT), Ministry of Education,

Beijing University of Posts and Telecommunications, Beijing 100876, [email protected], [email protected], [email protected]

Abstract

A novel baseband OFDM optical SSB

transmission scheme using parallel optical SSBmodulation is proposed for long-haul opticaltransmission. Since the proposed scheme adopts thetraditional SSB modulation theory and only uses

the even sub-carriers to transmit data, simulationsshow that it has higher tolerance to the signal-signal

beat interference and the nonlinearity effectscomparing to the known baseband direct detection

OFDM schemes.

Keywords: Orthogonal frequency-divisionmultiplexing (OFDM); Parallel optical single-sideband modulation (P-OSSB); Conventional

optical single-sideband modulation (C-OSSB);Signal-signal beat interference (SSBI); Fibernonlinearity

1 Introduction

Orthogonal frequency-division multiplexing(OFDM) is a multi-carrier modulation technique

where the data stream is carried by manyorthogonal and lower rate subcarrier tones; it hasbeen widely employed into wirelesscommunications and numerous digital standards [1].Recently, OFDM has aroused much attention in

optical fiber communications as its high toleranceto chromatic dispersion (CD) [2-4] and polarization

mode-dispersion (PMD) [5-7]. Besides powerfulbut complex coherent reception studies, directdetection optical OFDM (DD-OFDM) using only asimple direct detection receiver was also identified.Meanwhile, in order to overcome CD, opticalsingle-sideband (OSSB) modulation method shouldbeen applied into the DD-OFDM system.

Currently, some schemes [8-10] have beenproposed to realize the baseband-DD-OFDM OSSB

transmission, which do not require modulating the

electrical baseband OFDM signal into the radio-frequency (RF) field and can save half bandwidth

comparing with other DD-OFDM systems [2-4].However, these baseband-DD-OFDM OSSBsystems mainly contain two defects. The major

drawback is that they are failed to consider how toovercome the influence of the signal-signal beat

interference (SSBI) produced by the photodiodereception [11]. Additionally, the ability of

combating fiber nonlinearity is also poor. Theseschemes will be referred to as conventional opticalSSB baseband DD-OFDM (C-OSSB) in thefollowing.

In order to overcome these problems, we proposea novel scheme to generate a baseband-DD-OFDMoptical SSB signal by using a parallel optical

single-sideband (P-OSSB) modulator. The P-OSSB

modulator based on the traditional SSB modulationtheory is composed of two parallel dual-driveMZMs. Meanwhile, for overcoming the SSBI, onlythe even subcarriers are used to transmit the dataand the odd subcarriers are left unused, because theSSBI will only fall on the odd subcarriers [11].

Simulation results show that the proposed scheme

has higher tolerance to SSBI and nonlinearityeffects comparing to C-OSSB OFDM systems.

2 The P-OSSB OFDM Transmitter

Architecture

2.1 Parallel Single-sideband Modulation Theory

According to the traditional SSB modulation

theory [12], the following is also a SSB signal:

( ) 1 ( ) ( ) a t n t jn t (1)

where ( )n t is a real baseband signal, and ( )n t is the

Hilbert transform of ( )n t . Therefore, if modulated

the SSB signal on an optical carrier 0f , an optical

SSB signal will be obtained:

0( ) ( )exp( 2 )E t a t j f t (2)

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978-1-4244-4900-2/09/$25.00 2009 IEEE


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Basing on the traditional SSB modulation theory,we proposed a P-OSSB modulator which iscomposed of two parallel dual-drive MZMs eachwith different bias point, as shown in Figure1. Firstof all, in order to generate a chirp free optical signal,the two dual-drive MZMs are both driven by equal

amplitude and negative polarity signals. So inFigure 1, only one driving signal into each MZM isshown, and the concretely driving signals for thetwo MZMs are:

1 ( ) / 4 d xm t V (3)

2 ( ) / 2 / 2d xm t V

(4)

whereV

is the modulators switching voltage; x is

a modulation parameter; ( )m t and ( )m t are

respectively the real baseband OFDM signal and

the Hilbert transform of ( )m t ; / 4V and / 2Vrepresent different bias points of the two MZMs.

According to 1d , 2d and the modulation function of

MZM, we can obtain the output expression of theP-OSSB transmitter:

0

1exp( 2 ) [1 ( ) ( )]

2

output

x xE j f t m t j m t

V V (5)

From Eq. (2) and (5), it can be seen that theoptical SSB signal will be obtain by using the P-

OSSB modulator. In Eq. (5), the /

x V must be less

than one.

Figure 1. Parallel Optical Single-Sideband(P-OSSB) modulation configuration

2.2 The input signals

To obtain a real baseband OFDM signal and onlyuse the even subcarriers to transmit useful data, the

input vector to the IFFT must be constrained tohave Hermitian symmetry. If using Nsubcarriers to

transmit the useful data, 4N subcarriers will be

needed as the input of the IFFT. The correspondingmode is shown below:

* * * *

1 2 N-1 N N N-1 2 1

2N subcarriers 2N subcarriers

0 b 0 b 0 ....0 b 0 b 0 b 0 b 0.... b 0 bI (6)

where *K

b denotes the complex conjugate ofK

b .

Figure 2. Simulation setup for the P-OSSBbaseband OFDM system

3 Simulation Setup

The performance of the proposed system is

investigated using numerical simulations byVPIsystems VPItransmissionMaker WDM V7.6.

Figure 2 shows the simulation setup for the P-OSSB DD-OFDM system. The data rate is 10Gbps,the number of subcarriers is 256 and 16-QAMencoding is used, so the bandwidth of the system is

5GHz. After IFFT conversion, the cyclic prefix (CP)is added into each OFDM symbol, which accountsfor 1/16 of the all subcarriers. Therefore, theduration of an OFDM symbol is 25.6ns and the CPis 1.6ns. For simplicity, the nonlinearity of the twoparallel MZMs has been precompensatedcompletely. The output of the P-OSSB modulator issent into four spans of SSMF with a total distance

of 320km without any optical inline dispersionmanagement. After detection using a simple

photodiode, the signal is demodulated in the OFDMreceiver which includes the simple electricaldispersion equalization (EDC) behind the FFTconvert. In EDC, 8 symbols were used as thetraining sequence to estimate the phase rotation ofdifferent carrier frequencies caused by the CD infiber.

For comparison a C-OSSB DD-OFDM [8-9] isalso simulated. The data rate is also 10Gbps with256 4QAM modulated subcarriers. If using 4QAM

modulation for each subcarrier, 5GHz signalbandwidth will be obtained which equals to the

propose P-OSSB DD-OFDM scheme. At the sametime, CP and EDC are also included in the system.

4 Simulation Result

A Monte Carlo evaluation is conducted toidentify the transmission performance.

Figure 3 shows the baseband OSSB spectrumafter the P-OSSB modulator. The lower sidebandsuppression is almost ideal.

The frequency phase shift of 64 subcarriers after320km transmission is shown in Figure 4. As is


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known to that, as long as the length of the CP ismore than CD, CD simply causes a frequency phaserotation of each OFDM subcarrier. Therefore, byusing the training sequence, EDC can correctlyrecover the phase of each subcarrier. The insets inFigure 4 show the constellations before a) and after

b) the EDC respectively.

Figure 3. Optical spectrum after P-OSSB modulator

Figure 4. Frequency phase shift of 64 subcarriers

For comparing to C-OSSB baseband DD-OFDM,Figure 5 displays the BER performance for the bothschemes after 320km transmission. In thesesimulations, the nonlinearity of fiber is notconsidered. The results show 4.5dB benefits ofrequired OSNR for P-OSSB scheme comparingwith the C-OSSB scheme. This decrease of

required OSNR for P-OSSB can be explained by

the improved capability of overcoming the SSBI.

Figure 5. BER performance for the both schemes

Figure 6 depicts the system Q factor of thereceived data versus the optical launch power forthe both schemes after 320km transmission, withOSNR 23dB. It can be seen that the P-OSSBscheme has higher nonlinearity tolerance probablydue to the larger subcarriers spacing which could

suppress the nonlinear effects like four-wave-mixing (FWM) and cross-phase modulation (XPM)etc [11].

Figure 6. Q factor vs. the optical launch power

5 Conclusions

We have demonstrated a novel method oftransmitting baseband OFDM over fiber with P-

OSSB modulation and direct detection. Simulationsin 320km SMF link show that P-OSSB OFDM has4.5 dB OSNR benefits comparing with C-OSSBOFDM system in linear case, and strongercapability of combating the nonlinearity than C-

OSSB OFDM system. Therefore, the simplicity andpotential of the technique make P-OSSB OFDM a

promising alternative scheme for long-haul opticaltransmission.

Acknowledgements

This research was supported in part by National

863 Program (No.2009AA01Z253), National 973

Program (No. 2007CB310705), the NSFC

(No.60772024, No.60711140087), SRFDP

(200800130001), ISTCP (No. 2006DFA11040),

PCSIRT (No. IRT0609), P.R.China and FujitsuR&D Center Co., Ltd.

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