Journal of engineering and technology Performance of...

International Journal of Engineering and Technology Volume 5 No. 3,March, 2015

ISSN: 2049-3444 © 2015– IJET Publications UK. All rights reserved. 141

Performance of Coherent Optical OFDM in WDM System Based on QPSK

and 16-QAM Modulation through Super channels

Laith Ali Abdul-Rahaim1, Ibraheem Abdullah Murdas1 and Mayasah Razzaq1

Electrical Engineering Department, College of Engineering, Babylon University, Babylon, Iraq

ABSTRACT

In this paper, Polarization Division Multiplexing Coherent Optical Orthogonal Frequency-division Multiplexing (PDM CO-OFDM)

system is designed with two modulation formats (QPSK and 16-QAM) at 40 and 100 Gb/s bit rates to show their linear and

nonlinear behaver, and the maximum achievement reach at difference data rate. The system is first simulated with a single channel to

analyze the performance. Then the system of 8-WDM channels is simulated at 50 GHz (0.4 nm) channel spacing. In order to

evaluate the systems performance, spectrum result, electrical constellation diagrams, maximum reach versus the input power curves

and BER verses OSNR curves are presented. A reference BER to evaluate the performance of the systems is taken as 1×10-12. PDM

CO-OFDM QPSK is found to be the best performance for high capacity networks. In the case of single channel the optimal input

power is found to be -2 dBm and 1dBm, which corresponding to the maximum reach of 4080 km and 2480 km at 40 Gb/s and 100

Gb/s respectively. For 8-WDM system the optimum input power was found to be -5 dBm and -2dBm, which corresponding to the

maximum reach of 2320 km and 1440km at 40 Gb/s and 100 Gb/s respectively. The advantages of PDM CO-OFDM 16-QAM

(improve spectral efficiency, decrease electrical bandwidth) come at the expense of reduced maximum reach and optimum input

power. In the case of single channel the optimum input power was found to be -4 dBm and -1dBm, which corresponding to the

maximum reach of 1760 km and 960km at 40 Gb/s and 100 Gb/s respectively. For 8-WDM system the optimum input power was

determine to be -8 dBm and -4dBm, which give the maximum reach of 1040 km and 480km at 40 Gb/s and 100 Gb/s respectively.

Simulation results are obtained using Optisystem (version 11.0) software package.

KeyWords: Coherent Optical Ofdm, Qpsk And 16-Qam, Pdm, Wdm System.

1. INTRODUCTION

Nowadays, with the advent of emerged multimedia

applications, such as YouTube and Internet protocol TV

(IPTV), those have again continued to drive the bandwidth

demand. It does not that the growth of the internet traffic will

slow in the foreseeable future [1].

The Cisco’s projection of Internet traffic to 2015[2], which

shows bandwidth growth of a factor of 2 every 2 years. This

phenomenal growth places tremendous pressure on the

underlying information infrastructure at every level, from core

to metro and access networks [2]. In the following section,

several technologies are identified in optical communication

networks arising from the rapid increase in IP traffic and

merging applications.

To cope with the tremendous growth of internet traffic and

new services, the optical system that was dominated by a low-

speed, point-to-point transmission a decade ago needs to be

upgraded to support massive networking capabilities with a

transmission speed approaching 100 Gb/s[1]. The most

relevant technologies to increase the speed of optical

transmission system are identified as using Wavelength

Division Multiplexing (WDM), the transmission capacity can

be easily increased by multiplexing the output of several

transmitters for the existing fiber links without installation and

alternation of the fiber link as shown in fig. (1).The

multiplexed signal is fed into the optical fiber for transmission

to the second end, where a demultiplexer sends each channel

to it's belong receiver. When N channels at bit rates B1, B2 …

BN are transmitted simultaneously over a fiber of length L, the

total bit rate of the WDM link becomes as shown in the

following eq. [3, 4]:

𝐵𝑇 = 𝐵1 + 𝐵2 + ⋯ 𝐵𝑁 (1)

If bit rates in channel were equal, the system capacity is

improved by a factor of N. The most important parameters in

the design of a WDM system are the number of channels N,

the bit rate B at each channel operates, and the frequency

spacing Δvch between two Successive channels. The product

NB represents the WDM capacity and the product NΔvch

denotes the total bandwidth for a WDM system. Such an

approach can be considered as one of the most cost efficient

ways to increase the optical link throughput [4, 5].

A different approach to realize a high speed optical

transmission system is to use polarization division

Fig.(1) Configuration for WDM transmission

International Journal of Engineering and Technology (IJET) – Volume 5 No. 3, March, 2015


multiplexing (PDM). PDM is a way to increasing system

capacity or Spectral Efficiency (SE) by two. This done by

taking two independently channels with the same wavelength,

but orthogonal polarization states are transmitted

simultaneously in a same fiber. At the receiver end, the two

polarization channels are separated and detected independently

[5, 7]. The binary On/Off Keying (OOK) direct intensity

modulation format is the simplest modulation format and has

been the only modulation format used in the commercial

Dense WDM (DWDM) system until 10Gb/s per channel data

rate. For the 40Gb/s DWDM system, more advanced

modulation formats such as duobinary, Differential Binary

Phase Shift Keying (DBPSK), Quadrature Phase Shift Keying

(QPSK) as well as Differential Quadrature Phase Shift Keying

(DQPSK) have been introduced to enhance the Spectral

Efficiency SE [8]. An recent technology on the way to

100Gb/s line rates and to highest SE WDM networks are high-

order optical modulation formats, which encodes m = log2 M

data bits on M symbols. Transmission symbol rate, which is

reduced by M compared with the data rate. This method given

higher channel data rates by using existing equipment have

lower-speed and so we can increase the speed of present high-

speed electronics and Digital Signal Processing (DSP) [9].

By another words, when assuming a given data rate of a

channel, the transmission with lower symbol rates will reduce

the spectral width of a WDM channel and thus to improve the

SE. It can result in a reduced system reach through a higher

optical signal-to-noise ratio (OSNR) requirements coupled

with a reduced tolerance toward nonlinear impairments [10].

Orthogonal Frequency Division Multiplexing (OFDM) has

been preferably for physical-layer interface in wireless

communications recent decade. It has been widely studied in

mobile wireless communications to solve frequency-selective

fading problem and has been development to wireless network

standards and Digital Audio and Video Broadcastings (DAB

and DVB-T) in Europe, Asia, Australia, and other parts of the

world [11, 12]. The using of OFDM in optical

communications occurred recently, therefore a huge number of

papers on the theoretical and practical performance of OFDM

in many optical systems have published. Coherent Optical

Orthogonal Frequency Division Multiplexing (CO-OFDM)

combines the Characteristics of ‘coherent detection’ and

‘OFDM modulation’ for future high-data rate fiber

transmission systems [1, 13, and 14].

a. The Chromatic Dispersion (CD) and Polarization Mode

Dispersion (PMD) of the transmission system can be

effectively estimated and mitigated.

b. The spectra of OFDM subcarriers are partially overlapped,

resulting in high optical SE which is very suitable for the

future high speed optical transmission system [1].

c. OFDM takes advantage of digital signal processing of Fast

Fourier Transforms (FFTs) which suggests that the OFDM has

superior scalability over the channel dispersion and data rate

[1].

d. By using homodyne architecture, the electrical bandwidth

requirement can be greatly reduced for the CO-OFDM

transceiver, which is extremely attractive for the high-speed

circuit design, where electrical signal bandwidth dictates the

cost [1].

e. Migration towards higher order modulation in CO-OFDM

systems is enabled simply.

In contrast, this requires more complicated optical modulator

configuration in single-carrier systems [15]. Due to the

superior advantages offered by CO-OFDM, it has been

considered to be a promising technology for the high speed

optical transport system. All the mentioned technologies to

increase the speed of optical transmission system are used in

this paper and concentrates on the most promising version of

these technologies for future optical networks, the CO-OFDM

system. The Literature Survey that interest in the field of this

paper began with Shieh et al, [8] in 2007, which reported the

first experimental demonstration of CO-OFDM systems. 128

OFDM subcarriers with a nominal data-rate of 8 Gb/s were

successfully processed and recovered after 1000 km

transmission through standard-single-mode-fiber (SSMF)

without optical dispersion compensation. Bao et al, [9] in

2007, demonstrated the transmission performance through

simulation for WDM systems with CO-OFDM including the

fiber nonlinearity effect. The results showed that the system Q

of 8 channels at 10 Gb/s is over 13.0 dB for a transmission up

to 4800 km of SSMF. Also, they presented a novel technique

of Partial Carrier Filling (PCF) for improving the nonlinearity

performance of the transmission. The system Q of these

channels with a filling factor of 50 % at is improved from 15.1

dB to 16.8 dB for a transmission up to 3200 km of SSMF.

Although PCF technique broadens the bandwidth of an OFDM

signal, the bandwidth of OFDM signal with this filling is still

comparable with conventional intensity modulation and optical

duobinary signal.

Yao-Jun, [11] in 2011, employed a new method of fiber

nonlinearity post-compensation (FNPC) in a 40-Gb/s CO-

OFDM system. The FNPC located before the CO-OFDM

receiver includes an Optical Phase Conjugation (OPC) unit

and a subsequent 80 km High Nonlinear Fiber (HNLF) as a

fiber nonlinearity compensator. The OPC unit is based on a

four wave mixing effect in a semiconductor optical amplifier.

The fiber nonlinearity impairments in the transmission link are

post-compensated for after OPC by transmission through the

HNLF with a large nonlinearity coefficient. Simulation results

showed that the Nonlinear Threshold (NLT) (for Q> 10 dB)

can be increased by about 2.5 dB and the maximum Q factor is

increased by about 1.2 dB for the single-channel with periodic

dispersion maps. In the 7 WDM channels system with 50-GHz

channel spacing, the NLT increases by 1.1 dB, equating to a

0.7 dB improvement for the maximum Q factor. Wang, [12] in

2012, presented a comparison between 16-Amplitude and

Phase Shift Keying (APSK) modulated optical OFDM signal

and conventional 16-QAM based on 30.2-Gb/s single-channel

and 5×30.2-Gb/s WDM single-polarization CO-OFDM

systems. Simulation results showed that the 16-APSK

modulated optical OFDM signal has a higher tolerance toward

fiber nonlinearity, such as self-phase modulation (SPM) and

cross phase modulation (XPM). Silva, [12] in 2013,

demonstrated a transmission of three 450 Gb/s (9x50-Gb/s)

superchannels with SE of 3.73 b/s/Hz, using PDM-CO-OFDM

with QPSK modulation format, over Pure Silica Core Fiber

(PSCF) and amplification by Erbium Doped Fiber Amplifiers

(EDFAs). A maximum reach of 3842 km was obtained. The

results show that this approach can be used to perform 400

Gb/s long haul transmission with SE near to 4 b/s/Hz. The

references mentioned in this survey do not take into account

the performance of PDM CO-OFDM system with QPSK and



16-QAM modulation formats for high speed application at

different bit rate. Also, the linear and nonlinear limits, and the

maximum reach for these systems are addressed in this paper

carefully. The aims of this paper are to investigate the

performance PDM CO-OFDM system with QPSK and 16-

QAM modulation formats for high speed applications. The

studied data rate at 40 and 100 Gb/s. The system is first

simulated with a single channel and then 8-WDM channels are

connected at 50 GHz channel spacing. And to investigate the

linear and nonlinear limits and the maximum reach for these

systems. The performance of the following systems is tested

using Optisystem (version 11.0) software package. This paper

is organized as follows: In Section 1, an introduction to the

subjects of the paper, literature survey, and the objective to the

paper are given. In Section 2, a description of channel

impairments, an OFDM principles and Optical-OFDM are

considered. In Section 3, a design of PDM CO-OFDM systems

with QPSK and 16 QAM modulation formats are given. In

Sections 4, the results and discussions of numerical results

derived from analysis of the simulated system are given. In

Section 8, conclusions for the work are summarized.

2. OPTICAL-OFDM

The radio frequency (RF) domain OFDM has been studied in

last forty years. OFDM has only recently been applied to

optical communications. In 2006, three groups independently

proposed optical OFDM for long-haul application. Two major

research directions appeared Direct Detection Optical-OFDM

(DDO-OFDM) and Coherent Optical-OFDM (CO-OFDM)

[26]. Both techniques have advantages. CO-OFDM has the

highest performance in power and spectral efficiency and

robustness against polarization dispersion. DDO-OFDM has

simpler optical receiver architecture, but a frequency guard

band is needed to prevent the interference from mixing

products which reduces the electrical and optical spectral

efficiency. Besides, as some power is allocated to the

transmitted carrier, DDO-OFDM also requires more

transmitted optical power. Currently, there is extensive

research into the performance of both systems and on

techniques to mitigate the disadvantages [27]. A generic O-

OFDM system can be divided into five functional as shown in

fig. (2) (i) RF OFDM transmitter, (ii) RF-To-optical (RTO)

up-converter, (iii) optical fiber link, (iv) Optical-To-RF (OTR)

down-converter, and (v) RF OFDM receiver[1].

The common feature for Direct Detection Optical-

OFDM (DDO-OFDM) is, of course, the use of direct detection

at the receiver. Here there is no need for an LO laser at the

receiver like in the case of Coherent-Optical OFDM (CO-

OFDM) systems. This is the key advantage of DDO-OFDM,

and this is one important reason why there have been varieties

of DDO-OFDM system versions proposed and designed for

different applications [3]. DDO-OFDM can be classified into

two categories according to how the optical OFDM signal is

generated: linearly mapped DDO-OFDM (LM-DDO-OFDM),

where the optical OFDM spectrum is a replica of baseband

OFDM and nonlinearly mapped DDO-OFDM (NLM-DDO-

OFDM), where the optical OFDM spectrum does not display a

replica of baseband OFDM [3].

NLM-DDO-OFDM aims to obtain the linear mapping between

the baseband OFDM and optical intensity even though there is

no linear mapping between the baseband OFDM and the

optical field. Since the OFDM signal is encoded in the

amplitude not the field, and subsequently nonlinearly mapped

DDO-OFDM do not have the same capability of the dispersion

resilience as linearly mapped DDO-OFDM therefore it is not

fit for long-haul transmission. However, because of its simple

implementation, it has become a very attractive option for the

short-reach SMF application, multimode fiber and optical

wireless systems. In this paper we focus our work on long-haul

SSMF transmission, thus only linearly mapped DDO-OFDM

will be discussed [1].

The baseband OFDM can be made positive either by adding a

DC bias, as in DC-biased optical OFDM (DCO-OFDM), or by

clipping the bipolar OFDM signal at zero level and removing

all the negative going signals, as in asymmetrically clipped

optical OFDM (ACO-OFDM). If only the odd subcarriers are

loaded with data symbols at the input of IFFT, all of the

clipping noise falls on the even subcarriers, and the data

carrying odd subcarriers are not impaired [8].

As shown in fig.(3), the optical spectrum of an LM-DDO-

OFDM signal at the output of the O-OFDM transmitter is a

linear copy of the baseband OFDM spectrum plus an optical

carrier. The position of the main carrier can be one OFDM

spectrum bandwidth away, or at the end of OFDM spectrum.

Detailed signal representations and modeling for DDO-OFDM

systems can be found in [3]. In the receiver, a single photo-

detector is used where the optical carrier mixes with the

optical subcarriers to regenerate the electrical OFDM signal.

The output of the optical receiver consists of three terms: the

first term is a Direct Current (DC) component that can be

easily filtered out. The second term is the fundamental term

consisting linear OFDM subcarriers that are to be retrieved.

The third term is the second-order nonlinearity product of the

optical carrier and the OFDM subcarriers as shown in fig.(3)

that needs to be removed.

The following approaches can be used to minimize the pena

Fig.(2) Schematic of a generic Optical-OFDM

communication system

Fig.(3) Spectrum of DDO-OFDM signal



due to the second-order nonlinearity term; the effect of the

unwanted nonlinearity term can be avoided by allocating a

sufficient guard band. This guard band is set by RF I/Q

modulator at the transmitter, as shown in fig. (4). Another way

to reduce the distortion of the nonlinearity term to an

acceptable level is by reducing the scaling coefficient that

describes the OFDM band strength related to the main carrier

as much as possible [1,3].

2.1 Coherent Optical-OFDM

In CO-OFDM the electric field spectrum of the transmitted

optical signal is a replica of the baseband RF OFDM signal,

with no need for any optical carrier component to be

transmitted. Instead, the carrier component needed for OTR

conversion is locally generated at the receiver. Fig.(5) shows

the conceptual diagrams of CO-OFDM system with homodyne

architecture.

The RTO converter is simply an optical I/Q modulator

comprises two Mach-Zehnder modulators (MZMs) to

transform the I and Q components of the baseband OFDM

signal to the optical domain directly, as in fig(5).

2.2 Polarization Division Multiplexed of CO-OFDM It is well known that SMF supports two modes in polarization

domain. As shown in fig.(6), a Two-Input-Two-Output (TITO)

scheme of CO-OFDM is usually applied to support

polarization division multiplexed (PDM) transmission. It

consists of two set of CO-OFDM transmitters and receivers,

each transmitter and receiver pair for a single polarization. In

such a scheme, because the transmitted OFDM information

symbol can be considered as polarization modulation or

polarization multiplexing, the capacity will be doubled

compared with Single-Input-Single-Output (SISO) scheme

[27].

2.3 Bit Error Rate & Optical Signal to Noise Ratio

Bit Error Rate (BER) estimation can be made by treating the

noise sources in terms of Gaussian noise statistics. BER for

optimum setting of decision threshold for choosing whether bit

is a 1 or 0 is given by the Gaussian error function [27]

𝐵𝐸𝑅 =1

2𝑒𝑟𝑓𝑐 [

𝐼1−𝐼0

√2(𝜕1−𝜕0)] (2)

Where 𝐼1 and 𝐼0 are the average photocurrent generated by a 1

bit and 0 bit, respectively. 𝜕1 and 𝜕0 σ are the noise variances

for 1 bit and 0 bit, respectively. Optical Signal to Noise Ratio

(OSNR) the ASE added by the optical amplifiers in a long-

haul transmission link ultimately limits the feasible

transmission distance. When we consider a long-haul

transmission link with 𝑁𝑠𝑝𝑎𝑛𝑠, the total ASE power 𝑃𝐴𝑆𝐸𝑡𝑜𝑡 in

[W] added by all optical amplifiers along the link equals 𝑃𝐴𝑆𝐸𝑡𝑜𝑡 =

𝑃𝐴𝑆𝐸𝑁𝑠𝑝𝑎𝑛𝑠. This is valid under the assumption that a single

amplifier adds a noise power 𝑃𝐴𝑆𝐸 and that all spans have the

same insertion loss. After 𝑁𝑠𝑝𝑎𝑛𝑠, the ratio between signal

power and ASE power is known as the OSNR and is defined

as [13],

𝑂𝑆𝑁𝑅 =𝑃𝑜𝑢𝑡

𝑃𝐴𝑆𝐸𝑁𝑠𝑝𝑎𝑛𝑠

(3)

where 𝑃𝑜𝑢𝑡 defines the signal power at the output of the last

amplifier of the transmission link and has unit watt.

3. SIMULATION OF PDM CO-OFDM In order to increase the transmission reach of the optical signal

and improve the system performance, the system must be

designed properly by the accurate selection of the various

components in the system. This Section introduces the

description of the single channel PDM CO-OFDM system

Fig.(6) PDM CO-OFDM conceptual diagram.

Fig.(5) Architecture of CO-OFDM system with homodyne

architecture

Fig.(4) DDO-OFDM with RF I/Q modulator.



with QPSK and 16-QAM modulation formats from the

transmitter to the receiver. Then, the 8-WDM channels system

is discussed considering all the system components.

a. Single Channel Description

In this section, a description of the single channel system is

presented. The simulation parameters of this system are shown

in table (1). A Pseudo Random Binary Sequence (PRBS)

generator is usually required to generate pseudo random binary

sequences. PRBS drives 20 and 50 Gb/s bit rate. Using

polarization division multiplexing, entire system bit rate will

be increased to 40 and 100 Gb/s. The simulated single

channel system is shown in fig. (7) with three main parts: PDM CO-OFDM Transmitter, Optical Fiber, and PDM CO-

OFDM Receiver. The description of each part of the system is

presented in the following subsections.

Table (1) Simulation Parameters

Parameter Value Units

Bit rate 20 and 50 Gb/s

Time window 0.8192e-06 for 20 Gb/s and

0.32768e-06 for 50 Gb/s s

Sequence length 16384 bit

Transmitter Structure

The transmitter section is shown in fig.(8):

In this case, polarization multiplexing is used, the laser output

is split into two orthogonal polarization components by

Polarization Beam Splitter (PBS), which are modulated

separately and then combined using a Polarization Beam

Combiner (PBC). The parameters of the simulated CW laser

are shown in table (2).

Table (2) Parameters of the CW laser


Frequency 193.1 THz

power variable dBm

Linewidth 0.1 MHz

Initial phase 0 degree

The transmitter section of the simulated system for each

polarization is built with two parts: RF OFDM transmitter and

RTO Convertor.

RF OFDM transmitter is built up using Sequence Generator,

OFDM modulator and two low pass Roll off electrical filters

as shown in fig.(9). It starts with the Sequence Generator to

generate two parallel M-ary symbol sequences from binary

signals using two forms of sequence generator that is QPSK

and 16-QAM. After that, it passes through OFDM modulator

component. In it, the complex symbol streams converted from

serial to parallel, and then the digital time domain signal is

obtained using IFFT, which is subsequently converted into a

real-time waveform through a DAC. After that, each OFDM

signal component (I and Q) passes through low pass Roll off

filter to reduce the influence of the aliasing components on the

system performance. The parameters of the simulated RF

OFDM transmitter are shown in table (3).

Fig.(7) Single Channel PDM CO-OFDM System.

Fig.(9) Block diagram of the RF OFDM transmitter

Fig.(8) The transmitter section of single

channel system.



Table (3) Parameters of the RF OFDM Transmitter

and Receiver.

OFDM Modulator


Sequence generator

type QPSK and 16-QAM

Bit/sysmbol 2 for QPSK and 4 for 16-QAM

Number of

Subcarriers 128

Position Array 64

Number of

IFFT/FFT points 256

FFT type Radix-2

Baud Rate for

50Gb/s

25 for QPSK and 12.5 for

16-QAM Gbaud/s

Baud Rate for

20Gb/s

10 for QPSK and 5 for 16-

QAM Gbaud/s

Sampling

Frequency for

50Gb/s


QAM GHz

Sampling

Frequency for

20Gb/s


QAM GHz

Spectral Efficiency 2 for QPSK and 4 for 16-

QAM b/s/Hz

Low Pass Cosine Role Off Filter

Cut off frequency 0.6 * Bit rate for QPSK and

0.3* Bit rate for 16-QAM GHz

Insertion Loss 0 dB

Depth 100 dB

Roll off factor 0.2

The RTO up-converter (the I/Q optical modulator) is built up

using an X-coupler, two Mach-Zehnder modulators, and an

optical combiner. The optical signal from the laser source is

applied to the first input port of the coupler to yield the I and Q

carrier components, at the output ports, which are fed to the

MZMs, as shown in fig.(10). Lithium Niubate MZM (LiNb-

MZM) with dual-drive type is used. Each MZM is driven by

the positive and negative signals of one of the components of

the baseband OFDM signal (I or Q) at the two inputs of

modulating signal of the MZM. The output signals from the

two MZMs are combined by the optical combiner to form the

CO-OFDM signal. The parameters of the simulated RTO

Converter are shown in table (4).

After that, an EDFA amplifier is used as a booster amplifier to

compensate the losses incurred in the transmitter and then

boosts the signal in the optical fiber channel. The EDFA is

operated in the power control mode with a gain of 10 dB and a

noise figure of 4 dB.

Table (4) Parameters of the RTO Converter.

Match Zehnder Modulator


Electrical gain (1&2) -1

MZMs extinction ratio 60 dB

MZMs insertion loss 1 dB

MZMs switching RF voltage 4 V

MZMs switching bias voltage 4 V

Booster Amplifier

EDFA amplifier gain 10 dB

Noise Figure 4 dB

b. Optical Fiber Channel

The CO-OFDM signal is then lunched in the optical fiber

channel. The optical fiber channel as shown in fig.(11) is

composed of spans of 80 km of SMF. The fiber has a loss of

0.2 dB/km, a dispersion of 17 ps/nm/km, a dispersion slope

coefficient of 0.075 ps/(km.nm2), a nonlinearity coefficient of

2.6×e(-20) m2/W and an effective cross-section of 70 μm2. The

value of the PMD parameter is 0.001 ps/√𝑘𝑚 and it is

expected that CO-OFDM has high PMD tolerance. Fiber

dispersion is compensated by the Dispersion Compensation

Fiber (DCF) of 16 km. Its attenuation constant is 0.5 dB/km,

the dispersion coefficient value is -85 ps/(km.nm), the

dispersion slope coefficient is -0.3 ps/(km.nm2) and effective

area is 22 μm2. The attenuation of SMF and DCF are

compensated by EDFA in each span. The simulation

parameters of the optical fibers channel are listed in table (5).

Fig.(10) Block diagram of the RTO up-converter

Fig.(11) Block diagram of the of the optical fiber channel



Table (5) Parameters of the optical fiber channel


SMF

length 80 km

Attenuation Constant

α 0.2 dB/km

Dispersion parameter

D 17 ps/(nm.km)

Dispersion slope S 0.075 ps/(km.nm2)

Effective area 70 μm2

PMD parameter 0.001 ps/√𝑘𝑚

Nonlinear refractive

index 𝑛2 2.6×e (-20) m2/W

to be continued…

SBS threshold 2.33 dBm

SRS threshold 31.7 dBm

EDFA_1

Gain 16 dB

Noise Figure 4 dB

DCF

length 16 km

Attenuation Constant

α 0.5 dB/km

Dispersion parameter

D -85 ps/(nm.km)

Dispersion slope S -0.03 ps/(km.nm2)

Effective area 22 μm2

PMD parameter 0.001 ps/√𝑘𝑚

Nonlinear refractive

index 𝑛2 2.6×e (-20) m2/W

EDFA_2

Gain 8 dB

Noise Figure 4 dB

c. Receiver Structure

The signal is applied to the receiver to detect it and convert it

into an electrical signal after the propagation in the optical

fiber channel. An optical band pass filter is used in front of the

detection process to decrease the noise and the crosstalk in the

received signal. The LO is then split into two branches using

PBS and the received signal is separately demodulated by each

LO component using two single polarization receivers. The

parameters of the simulated LO are shown in table (6).

Table (6) Parameters of the LO.


Frequency 193.1 THz

power 0 dBm

Linewidth 0.1 MHz

Initial phase 0 degree

As shown in fig(12), the PDM CO-OFDM receiver section for

each polarization consists from two parts as the transmitter

section : OTR Convertor and RF OFDM receiver.

The OTR down-converter (homodyne receiver) is built up

using four X-couplers, a 90 phase shifter, four PIN

photodetectors, and two electrical subtractors, as shown in fig.

(13). This OTR conversion network employs balanced

detectors for noise cancellation. The first coupler is used to

split the incoming complex CO-OFDM signal into two parts to

be used for extracting I and Q components of the OFDM

separately. Similarly, the second coupler is used to split the

LO signal into two parts to be mixed with the CO-OFDM

signal in I and Q branches. The LO signal in one of the

branches is phase-shifted with 900 by the optical phase shifter

to be coupled to the CO-OFDM signal at the third coupler. The

outputs from the third coupler are used to be fed to the PDs of

the balanced detector to generate I component of the baseband

OFDM signal after subtracting the photocurrents output from

the two PDs. The Q component of the baseband OFDM signal

is generated by the same way at the balanced detector

containing the fourth coupler. The simulation parameters of

the OTR convertor are listed in table (7).

Table (7) Parameters of the OTR diagram.

Photo detector


Photodetector type PIN

Responsivity 1 A/W

Dark current 10 nA

X-Coupler

Coupling Coefficient 0.5

Additional Losses 1 dB

Fig.(12) The Receiver section of the single channel system.

Fig.(13) Block diagram of the OTR down converter.



The received electrical signals (I and Q) are then amplified

with two electrical amplifiers having a gain of 20dB as shown

in fig.(14). After amplification, the signals are passed through

Low Pass Roll off filters to eliminate the frequencies above

required band, usually, the same characteristics as the low-pass

filters used in the RF-OFDM transmitter.

Then these signals are fed to an OFDM demodulator

component. In it, the received baseband OFDM signal is

sampled by ADC, and then the FFT of each OFDM subcarrier

is taken to find the original transmitted symbol. After that

Sequence Decoder decodes two parallel M-ary symbol

sequences to binary signals. The decoding process is done by

QPSK and 16-QAM Sequence decoder. Then, the binary

signal passes through NRZ pulse generator to generate a Non

Return to Zero (NRZ) electrical signal. The 3R generator is

used to generate the original bit sequence, and the modulated

electrical signal to be used for BER analysis. The simulation

parameters of the RF OFDM Receiver are the same parameter

in table (3).

3.5 WDM System Description The description of the PDM CO-OFDM system with 8-WDM

channel can be divided into two parts:

3.5.1 Transmitter Structure

The block diagram of the transmitter side of the 8-WDM PDM

CO-OFDM is shown in fig. (15). The PDM CO-OFDM

channels are fed into a multiplexer. The simulation parameters

of the multiplexers and the demultiplexers used in the WDM

system are shown in table (8).

Table (8) Parameters of the optical multiplexers and

demultiplexers for WDM System.

Mux/Demux


Frequency spacing 50 GHz GHz

Bandwidth 40 for QPSK and 20

for 16-QAM GHz

Insertion loss 2 dB

Filter type Gaussian

Filter order 4

As in the case of the single channel transmission, an optical

booster amplifier with the same parameters is used in the

WDM system to accommodate the losses incurred in the signal

at the transmitter.

3.5.2 Receiver Structure

Once the propagation of the signal passes through the optical

fiber channel, it is then received and detected in the receiver.

The receiver structure of the 8WDM channels PDM CO-

OFDM system is shown in fig.(16).The channels are

demultiplexed with same parameters used in table (8).

Fig.(14) Block diagram of the RF OFDM receiver

Fig.(15) The transmitter side of the simulated WDM system.



4. Simulation Results:

The simulation results in this paper showed the behaver of

PDM CO-OFDM QPSK and 16-QAM system at 40and

100Gbit/s to understand the linear and nonlinear limits, and the

maximum achievable reach at each data rate. The data rate of

100Gbit/s was chosen to match the speed of the next

generation optical networks.

4.1 Single Channel PDM CO-OFDM System with

QPSK modulation:-

The main results in this system are:

4.1.1 Spectrum Result

Fig.(17) shows the RF spectrum analyzer PDM CO-OFDM

QPSK signal before and after 1200 km transmission distance

at 100 Gb/s.

Fig.(18) shows the optical spectrum of PDM CO-OFDM

QPSK signal before and after 1200 km transmission distance

at 100 Gb/s. The 20 dB bandwidth of the signal is around 25

GHz.

4.1.2 Received Electrical Constellation Diagram

Fig. (19) shows the received constellation diagram after

removing the chromatic dispersion by DCF for 1200 km with

different input power. The constellation of each OFDM

symbols corresponding QPSK data points is recovered with

four distinct clusters of. The residual noises spreading

constellation point are mainly from ASE in transmission

system, nonlinearity of optical fibers and PMD. According to

the SPM formula, there is no doubt that a higher input power

would lead to a larger nonlinearity distortion. As shown in

fig(20), when the fiber transmission distance increases, the

distortion increases. This simulation result is consistent with

the discussions in section 2.2 that the phase shift caused by

Fig.(16) The receiver side of the simulated WDM

system.

Fig.(17) RF Spectrum of PDM CO-OFDM QPSK signal (a)

before (b) after 1200 km transmission distance.

Fig.(18) Optical spectrum of PDM CO-OFDM QPSK signal

(a) before and (b) after 1200 km transmission distance.



fiber nonlinearity is positively related with fiber transmission

length.

4.1.3 Measurements of Maximum reach

The maximum reach was measured as a function of input

power at 40 and 100 Gbit/s (Fig.(21)), to characterize the

linear and nonlinear limits for QPSK PDM CO-OFDM

transmission. This done by assuming the BER = 10-12. The

results are summarized in table (9).

For 100 Gbit/s QPSK PDM CO-OFDM transmission the input

power had to be increased by ~4dBm to get the same distances

in that been get in transmission at 40 Gbit/s. This is because

the in-band noise power scales linearly with the symbol rate

due to the associated broadening spectral.

Fig.(19) Received constellation diagram after 1200 km with

different input power. a) -2 dBm, b) 0 dBm, c) 2 dBm

(C)

Fig(20) Received constellation single channel system with 0dBm

input power after different transmission distances (a) 1520 km,

(b) 1840 km (c) 2160 km.

(C)



Table (9) Maximum reach and Optimum Input

Power of single channel PDM CO-OFDM QPSK

system at 40 and 100Gb/s.

Bit rate (Gb/s) Max. Reach (km) Optimum Input

Power (dBm)

Single channel CO-OFDM PDM QPSK system

40 4080 -2

100 2480 1

4.1.4 OSNR Measurement

To characterize the PDM CO-OFDM QPSK signal at 40

Gbit/s and 100 Gbit/s, the BER was determined as a function

of OSNR, and the results are shown in fig.(22). The required

OSNR for the 100 Gb/s is 8.32 dB at BER=10-12, which is 2.19

dB higher than that for 40 Gb/s. This can be attributed to

increase in symbol rate by a factor of more than two resulted

in additional 2.19 dB OSNR retribution due to the bandwidth

increasing.

4.2 Single Channel PDM CO-OFDM System with

16-QAM modulation format

The bit-rate of 100Gbit/s can be achieved using a QPSK PDM

CO-OFDM system operating at 25Gbaud. Indeed, such a

system can give ultra-long distances, potentially sufficient to

use in trans-oceanic routes. The QPSK CO-OFDM PDM

signal needed lower OSNR than higher-order modulation

formats for the equivalent data rates. It also advantage from

the single amplitude (constant modulus), that improve the

tolerance to nonlinearity compared to modulation of multi-

level formats. The advantage of generating 100Gbit/s PDM

CO-OFDM signals using 16-QAM, however, is has lower

symbol rate of 12.5Gbaud requires less by a factor of two

compared to QPSK. A lower symbol rate dangles the

requirements on the bandwidth of the electronics at the

transmitter and the receiver, reducing the cost of electronic and

electro-optical components used. 16-QAM can also increase

the spectral efficiency in WDM transmission, so can be

reduced the spacing between the WDM channels. Another

advantage of 16-QAM over QPSK signals is that at higher bit-

rates (e.g. 200Gbit/s), it becomes impractical to use since a

QPSK signal would require a symbol rate of 100Gbaud for

which electronics at the transmitter and receiver is not easily

available. Therefore, a 25Gbaud PDM CO-OFDM 16-QAM

could be better to obtain a 200Gbit/s. Similarly to the QPSK

this section shows the comprehensive study of PDM CO-

OFDM 16-QAM at 40 and 100 Gb/s in terms of maximum

reach. Similar to the QPSK experiments simulate in section

(4.1), the main results measured in the 16-QAM PDM CO-

OFDM single channel are:


Fig.(23) shows the RF spectrum analyzer PDM CO-OFDM

16-QAM signal before and after 560 km transmission distance

at 100 Gb/s.

Fig.(24) shows the optical spectrum of PDM CO-OFDM 16-

QAM signal at 100Gb/s before and after 560 transmission

distance. The 20 dB bandwidth of the signal is around 12.5

GHz.

Fig.(21) Measured maximum reach of single channel PDM

CO-OFDM QPSK transmission at 40Gbit/s and 100Gbit/s.

Fig.(22) Measured OSNR for 40 Gbit/s and 100 Gbit/s

single channel PDM CO-OFDM QPSK. Fig.(23) RF Spectrum of 100 Gb/s PDM CO-OFDM 16-

QAM signal (a) before (b) after 560 km transmission

distance



4.2.2 Received Electrical Constellation Diagrams

Fig.(25) shows the received constellation diagram after 560

km at different input power. Fig.(26) shows the received

constellation diagram with 0 dBm input power at different

distances.

4.2.3 Maximum reach measurements

The performance of transmission PDM CO-OFDM 16-QAM

signals are simulated in this section. The curves of maximum

reach were determined as a function of input power into the

fiber (at BER = 10-12) and are shown in fig.(27). In the case of

40Gbit/s the optimum input power was found to be -4dBm,

corresponding to the maximum reach of 1460km. At 111Gbit/s

PDM CO-OFDM 16-QAM transmission the optimum input

power was been -1dBm to given a maximum reach of 960km.

It is clear that 16-QAM modulation given (more spectral

efficiency and lower electrical bandwidth). This is at the

(C)

Fig.(25) Received constellation diagram after

560 km with different input power. a) -2 dBm,

b) 0 dBm, c) 2 dBm.

Fig.(27) Measured maximum reach of single channel PDM

CO-OFDM 16-QAM transmission at 40Gbit/s and

100Gbit/s

Fig(26) Received constellation single channel system with

0dBm input power after different transmission distances (a)

560 km, (b) 720 km(c) 900 km.

(C)

Fig.(24) RF Spectrum of 100 Gb/s PDM CO-OFDM 16-

QAM signal (a) before (b) after 560 transmission distance



expense of decreased maximum reach and optimum input

power. The results are summarized in table (10).


Power of single channel PDM CO-OFDM 16-QAM

system at 40 Gb/s and 100Gb/s.


Power (dBm)

Single Channel CO-OFDM PDM 16-QAM System

40 1760 -4

100 061 -1


To characterize the PDM CO-OFDM 16-QAM signal at 40

Gbit/s and 100 Gbit/s, the BER was determined as a function

of OSNR, and the results are shown in fig .(28). The required

OSNR for the 100 Gb/s is 15.83 dB at BER=10-12, which is 2.4

dB higher than that for 40 Gb/s.

4.3. 8-WDM Channels PDM CO-OFDM system

with QPSK modulation format

The single-channel systems described in section (4.1) were

used to establish the transmission performance of an upper-

bound for all practical optical fiber transport systems based on

WDM systems. WDM allows the optical infrastructure to be

shared amongst many channels, thus decreasing the cost of

transmitted data information in a fully loaded system. In these

operating systems, nonlinear effects become important sources

of weaknesses. Because of these additional weaknesses, it is

clear that single-channel transmission alone can underestimate

the performance of WDM systems with several tens of

wavelength channels [3]. The aim of this section is to

investigate the performance of PDM CO-OFDM QPSK and

16-QAM signals in the presence of adjacent WDM channels.

Similar to the single channel system carried out in section 4.1,

the main results measured in the 8-WDM channels PDM CO-

OFDM QPSK are:


Fig.(29) shows the RF spectrum analyzer for central channel

(channel 4) PDM CO-OFDM QPSK signal before and after

1200 km transmission distance at 100 Gb/s.

Fig.(30) shows the optical spectrum of 8-WDM channels

PDM CO-OFDM QPSK signal before and after 1200 km

transmission distance at 100 Gb/s.

4.3.2 Received Electrical Constellation Diagrams 4.3.3

Fig. (31) shows the received constellation diagrams for central

WDM channel after 1200km at different input power. The

constellation is worse with WDM system, which shows the

increasing of the nonlinear effects. Fig. (32) shows the

received constellation diagrams with -4 dBm input power after

different transmission distances.

Fig.(28) Measured OSNR for 40 Gbit/s and 100 Gbit/s single

channel PDM CO-OFDM 16-QAM

Fig.(29) RF Spectrum of PDM CO-OFDM QPSK signal for

the 4th channel (a) before (b) after 1200 km transmission

distance.

Fig.(30) Optical Spectrum of PDM CO-OFDM QPSK signal for

the 4th channel (a) before (b) after transmission.




To characterize the performance of transmission 8-WDM

PDM CO-OFDM QPSK signals at 40 Gb/s and 100Gbit/s, the

maximum transmission distance was measured (at BER = 10-

12) as a function of the input power per channel (Fig.(33)). The

transmission measurements were performed for a central

WDM channel (channel 4), because of the central channel

experiences the maximum amount of nonlinearity. This causes

the worst-case scenario and, hence, the lower bound in the

transmission performance. The optimal input power and

maximum reach is decreased in 8-WDM system compared

with the single channel system. The results are summarized in

table (11).

(C)


different input power. a) -4 dBm, b) -2 dBm, c) 0 dBm.

(C)

Fig(32) Received constellation single channel system with -4

dBm input power after different transmission distances (a)

1200 km, (b) 1360 km (c)1520 km.

Fig. (33) Measured maximum reach of 8-WDM channels

PDM CO-OFDM QPSK system, measured at 40 and 100

Gbit/s.




Power of 8-WDM channels PDM CO-OFDM QPSK

system at 40 Gb/s and 100Gb/s.

Bit rate (Gb/s) Max. Reach (km) Optimum Input Power

(dBm)

8 WDM channels PDM CO-OFDM QPSK System

40 2320 -5

100 1440 -2


Fig.(34) shows the OSNR measurements, measured for the

central WDM channel, for both 40 and 100 Gb/s.

The required OSNR for the 100 Gb/s is 10.06 dB at BER=10-

12, which is 2.8 dB higher than that for 40 Gb/s , this compares

to 2.19 dB in a single-channel PDM CO-OFDM QPSK

system.

4.4. 8-WDM Channels PDM CO-OFDM system

with 16-QAM modulation format

Similar to the single channel system carried out in section 4.1,

the main results measured in the 8-WDM channels PDM CO-

OFDM 16-QAM are:


Fig.(35) shows the RF spectrum analyzer for central channel

PDM CO-OFDM 16-QAM signal before and after 560 km

transmission distance at 100 Gb/s.

Fig.(36) shows the optical spectrum of 8- WDM channels

PDM CO-OFDM 16-QAM signal before and after 560 km

transmission distance at 100Gb/s.

4.4.2 Received Electrical Constellation Diagrams

Fig. (37) shows the received constellation diagrams for central

WDM channel after 560 km at different input power. Fig. (38)

shows the received constellation diagrams with -4dBm input

power after different transmission distances.

Fig.(34)Measured OSNR for 8-WDM PDM CO-OFDM

QPSK at 40 and 100 Gbit/s.

Fig.(35) RF Spectrum of PDM CO-OFDM 16-QAM signal

for 4th channel (a) before (b) after 560 km transmission

distance.

Fig.(36) Optical Spectrum of PDM CO-OFDM 16-QAM

signal for 4th channel (a) before (b) after 560 km

transmission distance.




To characterize the linear and nonlinear transmission

performance of 8- WDM channels PDM CO-OFDM 16-QAM

system, the maximum reach was measured as a function of the

input power per channel (Fig. (39)). The results are

summarized in table (12).


Power of 8-WDM channels CO-OFDM PDM 16-

QAM system at 40 Gb/s and 100Gb/s.


Power (dBm)

8-WDM channels CO-OFDM PDM 16-QAM System

40 1040 -8

100 480 -4


Fig.(40) shows the OSNR measurements, measured for the

central WDM channel, for both 40 and 100 Gb/s.

The required OSNR for the 100 Gb/s is 17.5 dB at BER=10-12,

which is 3.4 dB higher than that for 40 Gb/s, this compares to

2.4 dB in a single-channel PDM CO-OFDM 16-QAM system.

The results in this Section represent the first investigation to

the performance of single channel and 8-WDM channels PDM

CO-OFDM QPSK and 16-QAM. Overall, it was clear that

maximum transmission distance in the case of WDM system

was lower than in single-channel system because of nonlinear

effects from the neighboring channels. The results for single-

channel and 8-WDM channels PDM CO-OFDM QPSK

system at 40 and 100Gbit/s are summarized in Table (13).

(C)


different input power. a) -4 dBm, b) -2 dBm, c) 0 dBm.

(C)

Fig(38) Received constellation single channel system

with -4 dBm input power after different transmission

distances (a) 560 km, (b) 640 km(c) 720 km.

Fig. (39) Maximum reach of 8-WDM channels PDM CO-

OFDM 16-QAM systems at 40 Gb/s and 100Gb/s.

Fig. (40) Measured OSNR for 40 Gbit/s and 100 Gbit/s 8-

WDM channels PDM CO-OFDM 16-QAM.



Table (13) Comparison of maximum reach and

optimum input power between single-channel and 8-

WDM channels PDM CO-OFDM QPSK system at 40

and 100Gbit/s.


Power (dBm)

Single channel CO-OFDM PDM QPSK System

40 4080 -2

100 2480 1

8-WDM channels PDM CO-OFDM QPSK System

40 2320 -5

100 1440 -2

At 111Gbit/s the existence of the additional 7-WDM channels

in the case of QPSK reduced the maximum achievable reach

from 1440 km to 2480 km, compared to the single-channel

experiment. Nevertheless, the maximum achievable

transmission distance of 1440 km in the case of WDM means

that 100Gbit/s QPSK solution can be potentially used for long-

haul transmission. Finally, the 8-WDM channel PDM CO-

OFDM 16-QAM transmission at 100Gbit/s was studied. The

presence of additional channels reduced the maximum reach

from 061km to 480 km compared to the single-channel

experiment. Still, 480km is long transmission distance for

100Gbit/s WDM PDM CO-OFDM 16-QAM. The results for

single-channel and 8-WDM channels PDM CO-OFDM 16-

QAM system at 40 and 100Gbit/s are summarized in Table

(14).

Table (14) Comparison of maximum reach and

Optimum input power between single-channel and 8-

WDM channels PDM CO-OFDM 16-QAM system at

40 and 100Gbit/s.


Power (dBm)

Single channel CO-OFDM PDM 16-QAM System

40 1760 -4

100 061 -1

8-WDM channels PDM CO-OFDM 16-QAM System

40 1040 -8

100 480 -4

5. Conclusions

The design of PDM CO-OFDM system operating with QPSK

and 16-QAM modulation formats has been investigated for

high speed optical transmission system. From the simulation

results, the following points can be concluded. For the PDM

CO-OFDM system operating with QPSK at 100 Gb/s, the

bandwidth of the optical signal is 25GHz. Whereas in the

system operating with 16-QAM the bandwidth is 12.5 GHz for

the same bit rate. This means that the optical signal of the

QPSK system occupies double the bandwidth of the 16-QAM

system for the same bit rate. The PDM CO-OFDM QPSK

signal also has a higher tolerance to nonlinearity compared to

CO-OFDM PDM 16-QAM. The advantages of 16-QAM

modulation (increased spectral efficiency, lower electrical

bandwidth) come at the expense of reduced maximum reach

and optimum input power. For example, in the case of

100Gbit/s PDM CO-OFDM 16-QAM the optimum input

power was found to be -1dBm, corresponding to the maximum

reach of 960km while for PDM CO-OFDM QPSK

transmission the optimum input power was found to be 1dBm,

corresponding to a maximum reach of 2480km. Increasing

system bit rate from 40 Gb/s to 100 Gb/s leads to increase

OSNR penalty. For example, the required OSNR for the 100

Gb/s PDM CO-OFDM QPSK is 8.32 dB at BER=10-12, which

is 2.19 dB higher than that for 40 Gb/s. Also, the required

OSNR for the 100 Gb/s PDM CO-OFDM 16-QAM is 15.83

dB at BER=10-12, which is 2.4 dB higher than that for 40 Gb/s.

This can be attributed to increase in symbol rate by a factor of

more than two resulted in an additional OSNR penalty due to

the doubling of the bandwidth. The PDM CO-OFDM QPSK

signal has a lower required OSNR than 16-QAM for the

equivalent bit rates. For example, the required OSNR for the

100 Gb/s PDM CO-OFDM QPSK is 8.32 dB at BER=10-12,

while for PDM CO-OFDM 16-QAM the required OSNR is

15.83 dB. Increasing the number of channels (increasing the

system capacity) leads to worsen the system performance due

to additional nonlinear effects from the neighboring channels.

For example, at 100Gbit/s the presence of the additional 7-

WDM channels in the case of PDM CO-OFDM QPSK

decreased the maximum achievable reach from 2480 km to

1440 km, compared to the single-channel experiment. Also,

the 8-WDM channel PDM CO-OFDM 16-QAM transmission

at 100Gbit/s was studied. The presence of additional channels

decreased the maximum reach from 960km to 480 km

compared to the single-channel experiment.

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