OPTIMIZATION OF OPTICAL SSB-OFDM SYSTEM WITH

DIRECT DETECTION FOR APPLICATION IN

METROPOLITAN/REGIONAL NETWORKS

Bruno M. M. Meireles and João L. Rebola

Department of Information Science and Technology, Instituto Universitário de Lisboa (ISCTE-IUL), Lisboa, Portugal

Group of Research on Optical Fiber Telecommunication Systems, Instituto de Telecomunicações, Lisboa, Portugal

Keywords: Direct-detection, Noise accumulation, Optical add-drop multiplexers, OPTICAL communications, optical

fiber, Orthogonal frequency division multiplexing (OFDM).

Abstract: Due to its spectral efficiency, dispersion robustness, simplicity and transparency to electrical modulation

format, orthogonal frequency division multiplexing (OFDM) is a promising technique that allows to

increase the capacity and to make the upgrade of optical telecommunications systems already installed. In

this work, the study and optimization of OFDM transmission in an optical communication system using

direct detection with a short and medium range (metropolitan and regional networks) for a target bit rate of

10 Gbps will be performed exhaustively. The optimization is carried out taking into account the impact of

noise accumulation and of signal distortion resulting from bandwidth narrowing introduced by the cascade

of add-drop multiplexing nodes on the network performance. Optical transmission of single sideband

OFDM signals is considered and the carrier-to-signal power ratio of the OFDM signal is optimized in order

to achieve the best network performance.

1 INTRODUCTION

Optical frequency division multiplexing (OFDM) is

a digital modulation technique that allows

transmitting multiple signals simultaneously in a

high-speed data channel. The data are separated into

narrow parallel subcarriers, where frequency

overlapping is possible without intercarrier

interference since all subcarriers are orthogonal

(Shieh, 2010). Hence, OFDM makes a more

efficient use of the spectrum than conventional

frequency division multiplexing. Different

modulation schemes, such as Quadrature Amplitude

Modulation (QAM) or phase shift-keying, can be

chosen for the different subcarriers independently

(Shieh, 2010). Moreover, since OFDM has a long

symbol period allows practically eliminating the

intersymbol interference due to fiber channel

dispersion (Shieh, 2010).

Another advantage of using OFDM is that

electrical channel equalization is done by a single

tap equalizer, while single carrier systems require

more complex electrical equalization techniques

such as adaptive equalization (Schmidt, 2008),

(Lowery, 2007).

An important drawback of OFDM is the signal

large peak-to-average power ratio (PAPR). OFDM

being a superposition of a higher number of

modulated subchannels signals may exhibit a high

instantaneous signal peak power (Hanzo, 2004). In

optical communications, high PAPR can impose

higher signal distortion in the presence of fiber

nonlinear effects (Shieh, 2008).

Currently there are two types of optical detection

used in OFDM transmission: coherent optical

OFDM (CO-OFDM) detection and direct-detection

OFDM (DD-OFDM). The use of direct detection is

widespread in existing optical communication

systems. Although, it has lower sensitivity than CO-

OFDM systems (Shieh, 2008, 2010), its simplicity

and lower cost, makes direct detection suitable for

metropolitan/regional networks, which do not

require state-of-the-art components as long haul

networks do.

Furthermore, the upgrade of installed links to

transmit OFDM signals is carried out mainly on the

electronic part of the communication link (through

133

M. M. Meireles B. and L. Rebola J..

OPTIMIZATION OF OPTICAL SSB-OFDM SYSTEM WITH DIRECT DETECTION FOR APPLICATION IN METROPOLITAN/REGIONAL NETWORKS.

DOI: 10.5220/0003488701330138

In Proceedings of the International Conference on Data Communication Networking and Optical Communication System (OPTICS-2011), pages

133-138

ISBN: 978-989-8425-69-0

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

the use of digital signal processing to perform the

Fast Fourier Transform (FFT) and its inverse) and

not on the existing optical components (Shieh,

2010).

In this work, the performance of an optical DD-

OFDM system is studied and optimized, with the

objective of using this signalling scheme in

metropolitan/regional networks (with about 600 km

of total length), where the accumulation of noise and

bandwidth narrowing introduced by a cascade of

optical nodes can have a significant impact on the

network performance.

2 SYSTEM DESCRIPTION

2.1 OFDM Transmitter

Figure 1: OFDM transmitter architecture.

Figure 1 shows the OFDM transmitter architecture

scheme used in this work. The input serial data is

first mapped to symbols using QAM. Then, the data

symbols are converted into N parallel blocks of data

(N subcarriers). N blocks of zeros are inserted in the

middle of the data subcarriers, making a total of 2N

parallel subcarriers within one OFDM symbol. The

introduction of these zeros (oversampling), as well

as the low pass filter (LPF) before the IQ modulator,

is used to reduce the aliasing (Alves, 2009). Then an

inverse-FFT is applied to the subcarriers resulting in

a time domain waveform that contains a

superposition of all 2N subcarriers. After, cyclic

prefix and guard interval are inserted. The resulting

waveform is then modulated by an IQ modulator to a

radio frequency (RF) carrier with frequency f

(Lowery, 2006). Then, the two signal I and Q

components are added and the OFDM electrical

signal is obtained.

Before optical transmission, the OFDM electrical

signal needs to be modulated onto the optical

domain using an external modulator (Shieh, 2010).

In this work, a Mach-Zehnder modulator (MZM)

operating in linear and nonlinear regime will be

considered. In the linear regime, no signal distortion

is introduced by the MZM. In the nonlinear regime,

the bias voltage of the MZM should be chosen to

improve the MZM linearity and minimize signal

distortion. Hence, the MZM is biased at the

quadrature point (Leibrich, 2009). Then, the optical

carrier power (in relation to the OFDM signal

power) is controlled by the single-side band (SSB)

filter, in order to improve the sensitivity of the

OFDM reception (Lowery, 2007). After SSB

filtering, the OFDM signal is transmitted through a

singlemode fiber (SMF).

2.2 OFDM Receiver

Figure 2: OFDM receiver architecture.

Figure 2 shows the DD-OFDM receiver architecture.

The optical receiver includes an optical amplifier, an

optical filter followed by a PIN photodetector and by

an OFDM electrical receiver. At the OFDM

electrical receiver, the baseband signal is recovered

using an IQ demodulator with a LPF in each

demodulation arm. The resulting baseband signal is

sampled using an A/D converter, then cyclic prefix

and guard interval are removed and a FFT is applied

to the resulting signal.

After FFT, the zero subcarriers are removed and

each subcarrier is equalized in order to compensate

for phase and amplitude distortion due to

transmission (Agrawal, 2004). Equalization is

performed by applying the inverse of the estimated

channel response using training sequences (Lowery,

2006). After equalization, each sub-channel is

demapped and the original bits are recovered. At this

point, the DD-OFDM system performance is

evaluated by estimating the bit error rate (BER).

2.3 System Performance Evaluation

In order to perform the system performance

evaluation, two distinct methods are used: BER

estimated using Monte Carlo simulation and direct

error count (DEC), which is named BER

DEC

and is

defined per subcarrier k by (Alves, 2010).

cos(2f

sin(2f

sin(2f

cos(2f

2+1

OPTICS 2011 - International Conference on Optical Communication Systems

134



number of bit errors

total number of transmitted bits

DEC

BER k 

(1)

and the BER obtained from the error vector

magnitude (EVM), named BER

EVM

and given by



11 3 1

log 1

EVM

RMS

BER k Q

MMEVMk





  







(2)

where M-QAM mapping per subcarrier is assumed

and EVM

RMS

is the root mean square of the EVM

defined by



 



() ()

()

RMS

sksk

EVM k









(3)

In equation (3), N

is the number of OFDM symbols

transmitted per OFDM frame and



()l

and



()l

are the M-QAM symbols corresponding to

the k

subcarrier of the l

OFDM symbol of the ideal

constellation and the constellation obtained at the

equalizer output, respectively (Alves, 2010).

The BER of the OFDM symbol is obtained by

averaging the BERs obtained for all subcarriers.

Although the effect of signal distortion on the

system performance is not accurately taken into

account using the BER

EVM

method, when noise

accumulation is dominant over signal distortion, the

BER

EVM

provides very precise estimates of the

system performance (Alves, 2010).

3 RESULTS

All results have been obtained considering an

OFDM signal with the parameters shown in Table 1

for a target bit rate of 10 Gbit/s and are based on the

work done by Lowery (2007).

Table 1: Simulation parameters.

Modulation

Nº of

transmitted bits

per OFDM

symbol

Nº of subcarriers

Bit rate per

OFDM stream

4 QAM 1024 512 10 Gbps

OFDM symbol

duration - T

Guard interval

Cyclic prefix

OFDM signal

bandwidth

102.4 ns 7.4206 ns 6.4 ns



5 GHz

Figure 3 a) shows the power spectral density

(lowpass equivalent representation) of the double-

sideband (DSB) OFDM signal at the MZM output

centered at the RF carrier of 7.5 GHz, with a

bandwidth of 5 GHz.

In a preliminary study, the bandwidths of the

electrical filters of the OFDM transmitter and

receiver have been optimized and 6

order Bessel

filters with a bandwidth of 4 GHz and 2.9 GHz,

respectively, have been found as optimum and used

throughout this work.

3.1 Optical SSB Filter Optimization

In this section, the bandwidth of the optical SSB

(OSSB) filter is optimized in order to suppress the

optical carrier power and improve the DD-OFDM

system performance (Lowery, 2007). Furthermore,

the use of SSB signalling (in comparison with DSB),

allows to decrease the distortion introduced by the

fiber chromatic dispersion during optical

transmission. The carrier-to-signal power ratio

(CSPR) is defined as CSPR = P

OFDM

(Alves,

2010), where P

is optical carrier power and P

OFDM

is the SSB OFDM signal power. For illustrative

purposes, Figure 3 b) shows the OFDM signal

spectrum after the OSSB filter for a CSPR of 0 dB.

a) b)

Figure 3: Power spectral density of the OFDM signal. a)

DSB signal at the modulator output. b) SSB signal after

filtering for a CSPR of 0 dB.

The optimization is performed by estimating the

performance of the DD-OFDM system (using the

BER

DEC

and BER

EVM

) as a function of the CSPR (by

varying the OSSB filter bandwidth) for the optical

signal-to-noise ratios (OSNRs) of 11 dB, 15 dB and

20 dB. A 2

order supergaussian optical filter is

considered at the optical receiver. The optimization

is performed for an external modulator working in

the linear (Figure 4) and nonlinear (Figure 5)

regimes.

Figure 4 shows that for an OSNR of 11 and 15

dB, the optimum CSPR is 0 dB as predicted in the

works by Lowery (2006, 2007) and Jansen (2007).

This corresponds to an OSSB filter bandwidth of 5

GHz. For an OSNR of 20 dB, it is possible to

observe that the optimum value is shifted to higher

CSPR values and therefore higher OSSB filter

bandwidths. This shift of the CSPR for higher

OSNRs has been also observed in the work by

OPTIMIZATION OF OPTICAL SSB-OFDM SYSTEM WITH DIRECT DETECTION FOR APPLICATION IN

METROPOLITAN/REGIONAL NETWORKS

135

Jansen (2007) and is attributed to the higher

influence of the intermixing of OFDM subcarriers

after the photodetection [the photodetection mixing

products are described in detail by Lowery (2008)]

on the performance, when the amplified spontaneous

emission (ASE) noise power is lower.

Figure 4: BER

EVM

and BER

DEC

(diamonds) as a function of

the CSPR for the OSNRs of 11 dB (blue), 15 dB (red) and

20 dB (green), for a linear external modulator and for the

receiver optical filter bandwidths of 100 GHz (dashed

lines) and 30 GHz (continuous lines).

Figure 4 shows also a good agreement between

the error probabilities estimated using the BER

DEC

and the BER

EVM

for lower OSNRs, in accordance

with the results presented in the work by Alves

(2010). Furthermore, it shows that the performance

of the DD-OFDM system is improved by using the

receiver optical filter with a bandwidth of 30 GHz,

due to higher ASE noise power reduction.

Figure 5 shows the BER as a function of the

CSPR, for the nonlinear external modulator, for

different modulation indexes m. It is possible to

observe that the optimum CSPR value is always near

0 dB, although, for higher modulation indexes, a

slight shift of the optimum CSPR is observable. It is

also possible to observe that, as the bandwidth of the

SSB filter increases (higher CSPR) and for higher

modulation indexes, the distortion due to the

external modulation non-linear regime starts to

impose the system performance. For lower

modulation indexes, the external modulator is

operating near the linear regime and the noise is the

main performance impairment, once the distortion

induced by the external modulator on the OFDM

signal is negligible.

Moreover, it is possible to infer that for a DSB-

OFDM signal (which corresponds to the rightmost

CSPR depicted for each modulation index), there

exists an optimum value for the modulation index,

which leads to the best system performance. This

conclusion is in agreement with the results presented

in the work by Alves (2010).

Figure 5 shows again a good agreement between

the estimates of the BER obtained using the BER

DEC

and the BER

EVM

Figure 5: BER

EVM

(dashed lines) and BER

DEC

(diamonds)

as a function of the CSPR for the model of the nonlinear

external modulator for different modulation indexes (m)

and an OSNR of 11 dB.

3.2 OFDM Signal Transmission along a

Cascade of Optical Nodes

In this section, the transmission of OFDM signals

along an optical communication system with several

fiber spans is studied. The impact of noise

accumulation and of bandwidth narrowing along the

several spans on the performance of the DD-OFDM

system will be investigated. Figure 6 shows the

scheme considered for the OFDM transmission

system, with a special emphasis on the optical node

configuration. The optical fiber considered in this

investigation is a standard singlemode fiber with a

dispersion parameter D

= 17 ps/(nm.km) and an

attenuation of α =0.2 dB/km. Linear transmission

along the fiber is considered.

The optical node is simply modelled by an ideal

inline optical amplifier with gain G, which adds

ASE noise to the signal (operation performed in the

simulation by a noise generator) and an optical filter

to reduce the noise power. In this scheme, the noise

added by each erbium-doped fiber amplifier (EDFA)

accumulates along the link. Notice also that the

signal distortion can be enhanced at the

photodetector’s input due to the cascade of optical

filters. It is also assumed that the optical amplifier

exactly compensates the fiber attenuation. In a first

approach, we also considered that fiber dispersion is

completely compensated by the electrical equalizer.

OPTICS 2011 - International Conference on Optical Communication Systems

136

Figure 6: OFDM transmission system with several spans.

Each span is composed by a fiber followed by the optical

node constitute by an EDFA and an optical filter.

It is worth recalling that one of the main goals of

this work is to study OFDM transmission in a

metropolitan/regional optical network, which has

about 600 km of distance coverage. So, it is

important to estimate the DD-OFDM system

performance for the optical link depicted in Figure 6,

and to determine if the commitment for the network

coverage length is accomplished.

Figure 7: BER

EVM

as a function of the number of sections

used in the optical link obtained for an OSNR of 25 dB

(diamonds) and 20 dB (continuous lines) for different

receiver optical filter bandwidths. The error probability

corresponding to the FEC limit is also depicted.

Figure 7 shows the error probability estimated

using the BER

EVM

as a function of the number of

sections for different receiver optical filters. The

MZM is assumed to be working in the linear regime

at the optimum CSPR. Figure 7 shows that the

probability of error increases as the number of

optical sections increases, since the OSNR is

degrading with the number of sections. For the

optical filters with a bandwidth of 10 GHz (blue)

and 20 GHz (red), the distortion suffered by the

OFDM signal (due the smaller optical filter

bandwidth and severe bandwidth narrowing due to

the cascade of optical nodes) is dominant over the

noise and no distinction is found between the error

probabilities estimated for both OSNRs of 20 dB

and 25 dB (measured at the output of the first optical

node). For higher optical bandwidths (larger than

30 GHz), the signal distortion introduced by the

optical filtering is not so significant and noise

accumulation along the optical link plays an

increasing role on degrading the network

performance. Similar conclusions were found for the

nonlinear MZM model, as long as it is working in

lower modulation indexes (near the linear regime).

Only BER

EVM

results are presented, since when

noise is the dominant factor that impairs the system

performance (over signal distortion), BER

EVM

provides very accurate BER results (Alves, 2010).

To achieve the objective of covering a

metropolitan/regional network with a length of about

600 km and by assuming that each fiber section is

about 80 km long, approximately 8 sections are

needed to cover that distance [by considering that

the error probability is bounded to the value of 10

3

which corresponds to the limit for forward error

correction (FEC) (Shieh, 2010)]. From Figure 7, one

can conclude that, for an OSNR of 25 dB, the goal

of covering the typical maximum distance of

metropolitan/regional network is accomplished.

Then, the impact of optical fiber transmission on

the system performance is evaluated to confirm if

the distortion imposed by the optical fiber dispersion

is indeed compensated by the equalizer and can be

considered negligible or not. The results obtained

are shown in Figure 8.

Figure 8: BER

EVM

as a function of the optical fiber length

in the optical link. The error probability corresponding to

the FEC limit is also depicted.

Figure 8 shows that for an OSNR of 20 dB and

for a fiber length up to 600 km, the OFDM system

error probability is always below 10

10

. Therefore, it

is possible to state that the distortion imposed by the

SMF is sufficiently compensated by the equalizer.

The small oscillations observed in the BER are

15

OPTIMIZATION OF OPTICAL SSB-OFDM SYSTEM WITH DIRECT DETECTION FOR APPLICATION IN

METROPOLITAN/REGIONAL NETWORKS

137

attributed to the imperfect estimation of the

equalizer coefficients for each fiber length. A more

accurate coefficients estimation would lead to a less

oscillatory BER variation with the fiber length. For

an OSNR of 15 dB, as the noise power is higher, the

influence of the imperfect estimation of the equalizer

coefficients is not visible, and the BER variation

with the fiber length shows practically no

oscillations for fiber lengths below 1400 km. After

1400 km, the effect of fiber dispersion on the signal

can no longer be compensated by the equalizer, due

to the insufficient guard interval duration of the

OFDM signal (Shieh, 2010) and a peak of the error

probability occurs [as shown in Figure 8]. This

means that, it is possible to reach a fiber length of

approximately 1400 km without significant

distortion added by the SMF on the OFDM

transmission system. In conjunction with the results

presented in Figure 7, it can be stated that the

distortion introduced by the optical fiber along the

target distance of about 600 km is negligible, and

that noise accumulation and signal distortion due to

bandwidth narrowing along a chain of optical

multiplexing nodes are the dominant factors causing

the performance degradation for typical distances of

metropolitan/regional networks.

4 CONCLUSIONS

In this paper, it has been shown that it is possible to

cover a metropolitan/regional optical network using

an optical OFDM system with direct detection at the

bit rate of 10 Gbit/s. The optical OFDM system has

been optimized in order to attain the best network

performance and it has been shown that the error

probability is still below the FEC limit after 8

sections of optical nodes (for a typical distance of

about 600 km). This conclusion was obtained for an

OSNR of 25 dB and for optical filters (inside the

optical node) with a bandwidth above 30 GHz. ASE

noise accumulation and signal distortion resulting

from bandwidth narrowing play a significant role on

achieving this limit. We have also found that the

distortion introduced by the optical fiber along the

network link length is negligible.

The consideration of a more realistic model for

the optical multiplexing node based on, for example,

reconfigurable optical add-drop multiplexers and the

study of the impact of the detuning of the optical

filters (inside the optical nodes) on the network

performance are left for future work.

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