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Performance Limits of All-Optical OFDM Systems

Julian Hoxha, Gabriella Cincotti

Engineering Department, via della Vasca Navale 84, I-00146 Rome, Italy

e-mail: gabriella.cincotti@uniroma3.it

ABSTRACTLimits of the bit error rate (BER), information spectral efficiency and transmission reach are analyzed for an all-optical orthogonal frequency division multiplexing (AO-OFDM) with 200Gbaud/s symbol rate and quadrature

phase-shift keying (QPSK) or quadrature amplitude modulation (QAM).

Keywords: All optical OFDM, coherent receivers, ASE noise, phase noise.

1.INTRODUCTION

The throughput of a single-wavelength, single-polarization high-speed optical transmission can be increased byenhancing the spectral efficiency and using higher-order modulation formats. Orthogonal frequency divisionmultiplexing (OFDM) can provide the maximum 1 bit/s/Hz spectral efficiency, by distributing data over a set oforthogonal subchannels, that partially overlap in the frequency domain. Higher-order modulation formats, suchQAM, pulse amplitude modulation (PAM) and phase shift keying (PSK) can further increase the system

throughput by transmitting multiple bits over a single symbol. In this case, coherent optical detection is required,that presents higher receiver sensitivity, but is severely affected by phase noise.Novel approaches to implement OFDM directly in the optical domain have been proposed and experimentally

demonstrated [1-3]. An AO-OFDM system can scale Tb/s-rate signals into lower speed optical subcarriers toreduce the receiver complexity; in addition, these systems do not require expensive and power-consuming high-speed digital signal processing (DSP), and they are more spectral efficient, compared to coherent-OFDM (CO-OFDM), because no guard interval or training symbols are needed. The AO-OFDM system allows the sameflexible bandwidth allocation as other CO-OFDM approaches, but with lower sampling rate and complexityequalization. In addition, AO-OFDM performs the optical discrete Fourier transform (ODFT) by using anarrayed waveguide grating (AWG)-based passive device, so that the subchannels are generated directly in theoptical domain, with a larger power efficiency with respect to CO-OFDM.

Although AO-OFDM schemes have been largely investigated during the past five years, a comprehensiveanalysis of the system performances is still lacking in literature, in particular considering the phase noise effects.

In the present paper, we present a detailed investigation, through numerical simulations, of coherently modulatedAO-OFDM systems over a multispan transmission, in terms error vector magnitude (EVM) and bit error rate(BER). The maximum spectral efficiency has been evaluated, as well as the influence of phase noise, amplifiedspontaneous emission (ASE) noise from inline amplifiers and fiber nonlinear effects.

2.SYSTEM SETUP

Figure 1 shows the AO-OFDM system architecture: the source is a mode-locked laser (MLL) that generatesa train of optical Gaussian pulses with 200 GHz rep-rate and 2 ps full width half maximum (FWHM); the opticalspectrum is wide enough to cover the bandwidth of all the N= 8 subcarriers. A 1:8 power splitter sends theoptical pulses into eight independent IQ Mach-Zehnder modulators (MZM), that are used to encode informationwith arbitrary modulation formats. An AWG-based device implements the optical inverse discrete Fouriertransform (OIDFT), and it is used to simultaneously filter and multiplex the optical subcarriers. The passivedevice has eight input/output ports, eight grating waveguides and 200 GHz free spectral range (FSR); the

orthogonal AO-OFDM subcarriers are 25 GHz frequency spaced [4, 5]. Eight independent pseudo randombinary sequences (PRBS) of 65536 symbols are either quadrature phase skit keying (QPSK) or 16-QAMmodulated and digital-to-analog converted; the RF signals, with 2Vpeak- to-peak voltage, drive the IQ-MZMs,that are biased at the minimum transmission point -V. The optical signal generated at one output of the AWG-

based device is transmitted over a fiber link composed of 10 spans; each span is a 80 Km standard single modefiber (SSMF), with = 0.2 dB/km attenuation coefficient, n2 = 2.7 10

-20nonlinear index,Aeff = 80 m

2fiber core

effective area, and D = 17 ps/nm/km dispersion coefficient at 1550 nm. In each span, an Erbium Doped FiberAmplifier (EDFA) with 4 dB noise figure (NF) compensates the in-span losses, and a 13.6 km dispersioncompensating fiber (DCF) is used to mitigate chromatic dispersion. A band pass filter (BPF) at the receivereliminates the out-of-band noise. The transmission of the optical signal through the fiber is simulated by usingthe Optilux software [6].At the receiver side, an identical AWG-based device performs the ODFT to demultiplex the AO-OFDMsubchannels at its outputs. In a conventional OFDM scheme, coherent demodulation is performed using eight 90hybrids, a CW local oscillator (LO) and time gating, to sample the received symbols. Therefore, eight additionalelectro-absorption modulators (EAM) are required in the coherent receiver [3], increasing the power

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consumption and the system complexity. In the present approach, we propose a simplified coherent receiver witha pulsed LO, to both coherent detect and sample the received signals; therefore time gating is no longerrequested at the receiver [7]. In a coherent optical time-domain sampling (COTDS) receiver, the LO signal isgenerated by a MLL, identical to the one used at the transmitter, that can be synchronized with the receivedsignal; we assume a zero frequency offset between the transmitter and receiver lasers. The optical signal fromeach AWG outputs is mixed with the pulsed LO in a 2 x 4 90 hybrid and detected with two balanced

photodetectors for the I and Q components. The photodiodes are assumed noiseless and the output signals aresent to DSP circuits.

Figure 1. AO-OFDM system architecture.

3.SIMULATION RESULTS

We have investigated the system performances by evaluating the BER as function of the overall optical powersent to the first fiber span, using Monte-Carlo simulations. From an inspection of Fig. 2, we observe that BERincreases with the optical power, due to the nonlinear effects, that become dominant with respect to simplifiedspontaneous emission (ASE) noise [8].

Figure 2. BER versus launched optical power.

Figure 3 reports the maximum transmission distance versus the launched power for BER=10-3. For optical power

larger than -1 dBm, the nonlinearity effects becomes dominant over the ASE noise limitations.The optical signal to noise ratio (OSNR), evaluated over a 0.1 nm bandwidth is [9]:

0.1 10log 58chOSNR P L NF M = + (1)

where Pch is the power launched in the first fiber span, L = 18.7 dB the overall fiber span loss, NF = 4 theamplifier noise figure and M = 8 the number of spans. In our case, we have Pch= -1 dBm, and the OSNRis 24.3 dB.

The maximum spectral efficiency, can be evaluated according to the Shannon-Hartley formula, when theOSNR is evaluated over the 200 GHz bandwidth

( )2 1.6log 1SE OSNR= + (2)

Considering only ASE noise, the maximum SE is 3.22 bit/s/Hz.

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Figure 3. Maximum transmission distance versus launched power.

4.PHASE NOISE EFFECTS

The influence of the phase noise has been evaluated in terms of the OSNR, as

( )121

1

sin1

sin

N

p

n

OSNR

n

n

N

=

= +

(3)

where N is the number of OFDM subchannels and the sum takes into account the crosstalk among the

subchannels [4]. The phase noise varianceisp = 2, where is LO linewidth and is the detected symbolduration.

We consider a 16-QAM transmission, analysing two different cases, when time gating is used to sample thesignals at the outputs of the 90 hybrids, and when a MLL is used as pulsed LO. In the first case, the EAMs are

driven by a pulse sequence of =2 ps duration and 40 ps time interval and the CW LO has a =100 kHz

linewidth. On the other hand, the pulsed LO is the signal from a MLL with a =2510-9

GHz bandwidth.

The EVM as a function of the OSNR and the BER versus the EVM are shown in Fig. 4(a) and 4(b), respectively[10, 11]. We observe that the phase noise creates a floor in the system performance and that the LO laserlinewidth is the main critical parameter in the coherent receivers.

0 10 20 30 400

5

10

15

20

25

30

35

40

45

OSNR[dB]

EVM[%]

w time gating

w/o time gating

0 10 20 30 40 50-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

EVM[%]

log(BER)

w time gating

w/o time gating

(a) (b)

Figure 4: (a) EVM versus OSNR;. (b) BER versus EVM.

5.CONCLUSIONS

The performance limits of 200 Gbaud QPSK AO-OFDM systems in long-haul transmission have beennumerically investigated. The optimum launched power has been evaluated, by observing a BER worsening dueto nonlinear effects. The maximum transmission distance, using forward error correction (FEC) is 1100 km andthe maximum SE is 3.22 bit/s/Hz.

In addition, phase noise effects have been evaluated for 16-QAM modulation, considering both time gating anda pulse LO; in the latter case, the BER presents a floor and the LO laser linewidth becomes the main critical

parameter.

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ACKNOWLEDGEMENTS

This work was supported by the European Commission through ICT-ASTRON project (Contract No. 318714)funded under the 7th Framework Programme and the Italian Ministry of University and Research throughROAD-NGN project (PRIN2010-2011).

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[3] S. Shimizu et al.: Demonstration and performance investigation of all-optical OFDM systems based onarrayed waveguide gratings, Opt. Exp.,vol. 20, pp. B525-B534, 2012.

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