Ofdm oqam performances for high speed optical communication in long haul fiber over 1600 km

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Volume 5, Issue 8, August (2014), pp. 10-20
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IJECET
© I A E M E
OFDM/OQAM PERFORMANCES FOR HIGH SPEED OPTICAL
COMMUNICATION IN LONG HAUL FIBER OVER 1600 Km
Serge Roland Sanou1, 2, François Zougmoré1
1Laboratoire de Matériaux et Environnement (LAME), UFR-SEA, University of Ouagadougou,
Burkina Faso. 03 BP 7021 Ouagadougou 03
2Autorité de Régulation des Communications Electroniques et des Postes (ARCEP), Ouagadougou,
Burkina Faso. 01 BP 6437 Ouagadougou 01
ABSTRACT
Orthogonal Frequency Division Multiplex (OFDM) is a high-speed digital communication
technique which presents a big potential as an ideal solution for high-speed transmission in optical
fiber networks. This study shows the OFDM modulation associated with Offset Quadrature
Amplitude Modulation (OQAM) that is filtered using an analysis and synthesis filter banks in an
optical communication platform at the rate of 10 GB/s over 1600 Km using a single mode fiber
(SMF). The simulations are performed in the VPI Photonics software environment. The results show
that the filtered OFDM/OQAM provides better transmission performance than the classical
OFDM/QAM firstly because it does not require equalization to certain distances; secondly distances
are greater than those achieved with the conventional OFDM in similar studies. Making some
changes in the filter banks parameters improves significantly the performance of the system. The
bandwidth is maximized because we do not use the cyclic prefix (CP) in OFDM/OQAM. Moreover
the complexity of transmitters and receivers can be reduced, which shows OFDM/OQAM as an
effective solution to combat the effects of the chromatic dispersion (CD), the polarization mode
dispersion (PMD), the inter-symbol interference (ISI) and nonlinearities.
Keywords: OFDM, OQAM, Optical Fiber, Filter Banks, Bit Error Rate.
I. INTRODUCTION
OFDM multicarrier modulation techniques have been used to transmit information using
various channel transmission networks such as WiFi (IEEE802.11) or new mobile networks [1], [2].
Application to optical fiber networks is new and raises new issues as the transmission channel has
different characteristics [3], [4]. Techniques related to the conventional OFDM like the

2. International Journal of Electronics and Communication Engineering Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 8, August (2014), pp. 10-20 © IAEME
implementation of an appropriate channel coding (COFDM) is used to improve the performance of
OFDM on an optical medium. COFDM has been studied in our previous works [5], [6].
/ (1)
= (4)
11
New solutions that can save the cyclic prefix, OFDM/OQAM, are based on a prototype
function which is better localized in time and frequency domain. Another approach is related to the
use of OQAM modulation with a filter banks to perform a good signal processing which can achieve
a better performance than the classical OFDM with cyclic prefix. In fact, the idea of using filtered
OFDM/OQAM by a filter banks is based on the fact that OFDM is a common choice that can now be
replaced by Filter Bank-based MultiCarrier (FBMC) techniques which have some very interesting
characteristics, like the results showed by M. Bellanger [7], [8]. Then, it seemed to us to be a good
idea to investigate the combination of the two techniques where an OFDM/OQAM signal is filtered
by a filter banks. Filter Banks Multicarrier approach can be seen as an evolution and an extension of
the FFT approach of the OFDM. In order to keep the same size as the FFT used in classical OFDM
we implemented a polyphase structure. In this context, we used successively two modulations
4QAM and 16QAM for the generation of the OFDM baseband signal before applying the OQAM
and filter banks processes. Performance tests of the transmission chain were carried out on the basis
of the Error Vector Magnitude (EVM), the Q factor (Qeff) and the Bit Error Rate (BER). All these
tests were performed according to the Optical Signal to Noise Ratio (OSNR).
II. MATERIAL AND METHODOLOGY
1. OFDM/OQAM data structure
The principle of the OFDM is based on the division of the transmitted signal into many sub-carriers,
which makes it less sensitive to frequency selectivity, and by the extension of the OFDM
symbol duration using a Cyclic Prefix (CP) of sufficient length to avoid ISI. The OFDM/OQAM
signal is in baseband time domain [3]:
k N
( ) ki k ( S )
= S S −
OFDM OQAM S t C S t iT
i k N
SC
SC
=
=− +
+¥
=−¥
/ 2
/ 2 1
( ) ( ) ( ) j f k t iT S
k S S S t iT t iT e − − = P − 2p (2)
S
k
k t
f
−1
= ( )
( −D t £
t
)
G S
( t t t
) G S
t
f
p
1,
0, ,
£ −D
P = (3)
where SOFDM/OQAM(t) is the OFDM/OQAM signal, G is the guard interval characterizing the
cyclic prefix and (t) the rectangular function taking into account the guard interval. Cki is the i-th
information symbol of the k-th subcarrier, Sk(t) is the waveform of the k-th subcarrier, NSC is the
number of carriers, fk is the frequency of the k-th subcarrier, TS is the symbol period, ts is the
observation period of the OFDM/OQAM symbol.
In the context of OFDM/OQAM, we don’t use the cyclic prefix, so G = 0. The signal at the
output of the optical receiver is:
j w off t + Df
( )
( ) . ( ) 0
r t e r t
( ) ( )* ( ) 0 / r t S t h t OFDM OQAM = (5)

3. International Journal of Electronics and Communication Engineering Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 8, August (2014), pp. 10-20 © IAEME
with Off = LD1 - LD2 and = LD1 - LD2; LD1 and LD1 are respectively frequency and
phase angular of the transmitter laser. LD2 and LD2 are respectively frequency and phase angular of
the receiver laser. The symbol * represents the convolution product and h(t) is the impulse response
of the optical fiber channel (SMF fiber).
OFDM has many variants and especially the one where the Cyclic Prefix is suppressed and
adding an extension of the FFT approach, like Filter Banks MultiCarrier (FBMC). There are mainly
three FBMC techniques that have been studied in the literature: Offset Quadrature Amplitude
Modulation (OQAM), Cosine Modulated multiTone (CMT), and Filtered MultiTone (FMT). The
term ‘offset’ refers to the time shift of half the inverse of the sub-channel spacing between the real
part and the imaginary part of a complex symbol. Our goal is to address OQAM. Opposite to OFDM,
which transmits complex-valued symbols at a given symbol rate, OQAM transmits real-valued
symbols by introducing a half symbol space delay between the in-phase and quadrature components
of QAM symbols, it is possible to achieve a baud-rate spacing between adjacent subcarrier channels
and recover the information symbol, free of ISI and Inter-Carrier Interference (ICI). The OQAM
transmitter structure used is the one presented in Fig. 1 a). In the Receiver in Fig. 1 b), the inverted
process is achieved using an analysis filter bank.
a)
b)
Fig. 1: Architecture of a) OQAM transmitter and b) OQAM receiver
H0, H1,… HM-1 are the prototype filter coefficients. The prototype filter design is based on the
Nyquist criterion where the global Nyquist filter is generally split into two parts, a half-Nyquist filter
in the transmitter and a half-Nyquist filter in the receiver.
The OQAM transmitter is divided into two main parts where a QAM signal, after serial to
parallel conversion is processed by a OQAM preprocessing and then filtered by a synthesis filter
bank. Similary, the OQAM receiver is also divided into two main parts where a received OQAM
signal, after serial to parallel conversion is filtered by an analysis filter bank and then processed by
an OQAM postprocessing to recover the received QAM signal
The analysis and synthesis filter banks can be expressed as functions of the prototype filter
P[m].The symmetry condition is satisfied by the squares of the frequency coefficients of the filter
[9]. The prototype filter is given by:
12
− −
2
1
k
K
= + − +
=
−
S ( 1)
[ ] [0] 2 ( 1) [ ]cos
1
m
KM
P m P P k k
k
p
(6)
with m=0,1,…, KM-2, the prototype filter length is L= KM±1 with M the number of subchannels
and K the overlapping factor.

4. International Journal of Electronics and Communication Engineering Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 8, August (2014), pp. 10-20 © IAEME
The frequency coefficients of the half-Nyquist filter used for K=2, 3 and 4 are given in
13
Table 1.
Table 1: Frequency domain prototype filter coefficients
K H0 H1 H2 H3
2 1 2 / 2 - -
3 1 0.911438 0.411438 -
4 1 0.971960 2 / 2 0.235147
The Prototype filter coefficients depend on different values of the overlapping factor K. The
purpose is to compare the associated three designed filters in the simulation
The k-th synthesis filter is defined by [10]:
−
L
k
= − )
2
1
(
2
[ ] [ ]exp m
p
k
M
g m P m j
p
(7)
The k-th analysis filter is simply a time-reversed and complex-conjugated version of the
corresponding synthesis filter. So it is as follows:
[ ] [ 1 ] * f m g L m k k p = − − (8)
−
L
k
= − )
2
1
(
2
[ ] [ ]exp m
p
k
M
f m P m j
p
(9)
f [m] g [m] k k = (10)
2. Optical Transmission Chain
The digital optical transmission channel architecture used is illustrated in Fig. 2:
Fig. 2: OFDM/OQAM optical transmission channel: a- data generation; b-OFDM/OQAM
transmitter; c-RF/Optical converter, d- Optical SMF fiber; e- Optical/RF Converter; f-
OFDM/OQAM Receiver g-data recovering
OFDM optical transmission chain is simulated in VPITransmissionMaker 9.1, [11] and
Matlab cosimulation environment. Because OQAM modulations are not available in
VPITransmissionMaker, cosimulation with Matlab has been used to add specific processing.
The developed processing platform is a universe of interconnected modules where some new
galaxies were created. The processing chain used is shown in Fig. 3. The simulation model OFDM

5. International Journal of Electronics and Communication Engineering Technology (IJECET), ISSN 0976
6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue
8, August (2014), pp. 10-20 © IAEME
–
for Long-Haul Transmission available in VPITransmissionMaker was used as a model of
inspiration [12].
Fig. 3: Global s
scheme of the simulation
New galaxies OFDM_OQAM_Coder for OFDM/OQAM co
for OFDM/OQAM decoding, RF_Up_Converter for frequency
for frequency down shifts have been implemented in the transmitter
designed using Matlab in cosimulation with VPITransmissionMaker which provide an interface for
that. Fig. 4 shows the details galaxies.
a)
c)
receiver. They have been
Fig. 4: Simulation galaxies: a) Coder galaxy, b) Decoder galaxy, c) RF_Up_Converter galaxy and
d) RF_Down_Converter galaxy
We monitor the OSNR so as to fix its successive values at the transmitter side which can
influence the calculation of BER, modeling the variable effect of imperfections in the optical
transmission channel. For this the galaxy Set_OSNR is used. The performa
the OSNR measured at the receiver side before the entrance of the signal in the photodiode, by using
an OSNR meter. In order to use the successive values of OSNR in the Decoder galaxy, the OSNR
meter uses a variable called OSNR tha
global transmission chain.
14
coding, OFDM_OQAM_Decoder
up shifts and RF_Down_Converter
and the receiver
s b)
d)
performances nces are evaluated using
that t is also used as the parameter of the Const module in the

6. International Journal of Electronics and Communication Engineering Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 8, August (2014), pp. 10-20 © IAEME
Some global and OFDM parameters used for the simulation are given in the Table 2:
Table 2: Parameters used in the simulation
PARAMETERS VALUES UNITS
Global Parameters
TimeWindow 8*1024/ BitRateDefault second
BitRateDefault 10e9 bit/s
SampleRateDefault 4* BitRateDefault Hz
CarrierFrequency 7.5 GHz
ReferenceFrequency 193.1 THz+7.5GHz THz
OFDM Parameters
BitPerSymbolQAM 2 (or 4) -
CyclicPrefix 0 -
NumberOfCarriers 64 -
NumberFFTPoints 128 -
15
3. Equalization
An equalization process has been added to the global transmission chain to perform a better
identification of symbols before the calculation of EVM, BER and Qeff factor.
For the simulation, we used a Decision Feedback Equalizer (DFE) module which implements
a nonlinear equalization through Volterra Filter available in VPITransmissionMaker 9.1. The
adaptive filter uses the least-mean square (LMS) algorithm to minimize the mean-squared error
between the ideal training signal and the filtered distorted received signal at its output [13].
One of the benefits of OFDM/OQAM in optical fiber is that it doesn’t require equalization
process to certain distance, in order to be able to recover the data. However, the equalization is
mandatory for modulations level higher than 4QAM. The Fig. 5 presents the curve of 4QAM and
16QAM equalization error function.
a)
b)
Fig. 5: Equalization process: a) Volterra equalization error function for 4QAM and b) Volterra
equalization error function for 16QAM
4. Estimation of The EVM, BER, QEFF Factor and OSNR
The EVM is a measure of the quality of the transmission through the quality of the
demodulation.
EVMRMS is the value of the root square (Root Mean Square) of the difference between the
received symbols and ideals symbols, normalized. It is given by [14]:
− +
I I Q Q
( )
1 / 2
1
2 2
1
2 2
1
1

7. +
−
=
=
=
− −
N
r r r
N
r r r r r
RMS
I Q
N
N
EVM
(11)

8. International Journal of Electronics and Communication Engineering Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 8, August (2014), pp. 10-20 © IAEME
_ _ _ (12)
= (13)
BER erfc (14)
16
with
−
−
r I and r Q
the real and imaginary part of the r-th received symbol. Ir and Qr are the real
and imaginary part of the r-th ideal symbol corresponding to the r-th received one. The calculation of
EVMRMS is performed in the receiver decoding process.
The Bit Error Rate (BER) is the measuring parameter the best known of the quality of a
digital transmission, and represents the ratio between the number of erroneous bits and the total
number of bits transmitted. The determination of the BER is based on the following definition:
N
BER err = =
N
Number of errorneous Bits
_ _ _
Number of Transmittde Bits
For a better estimation of BER, we used a Monte Carlo approach [6], which consists in a
stochastic simulation with a large number of random symbols, to estimate the behavior of the system.
Therefore, we can estimate that:
N
BER err
( )
N
MC LiN
® +m¥
Q factor (Qeff) calculation is based on the above BER formulas [3]:
1 eff Q
=
2 2
Q erfcinv( BER) eff = 2. 2* (15)
2 2
t +¥
− =
erfc x e dt
x
( )
p
(16)
with erfcinv(x) the inverted function of the complementary error function erfc(x) .
The simulation is performed under the effect of the Chromatic Dispersion (CD) and the
Optical Signal to Noise Ratio (OSNR), the ratio of the optical signal power and the noise power:
S
Noise
P
P
OSNR = (17)
with PS the power of the optical signal, PNoise the total power of the noise which models the
accumulation of all the noises associated with the optical transmission chain.
III. THE RESULTS
The simulations helped us to plot the evolution curves of EVM as a function of OSNR.
Similarly, the estimations of the BER and Q factor curves were performed according to the OSNR.
1. Received constellations
Fig. 6 represents the received constellations for 4QAM without equalization, 4QAM with
equalization and 16QAM with equalization. These constellations show a proper design of the
transmission chain over a distance of 1600 Km.

9. International Journal of Electronics and Communication Engineering Technology (IJECET), ISSN 0976
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a)
8, August (2014), pp. 10-20 © IAEME
Fig. 6: Received constellations: a) 4QAM, without equalization, b) 4QAM, with Volterra
equalization and c) 16QAM, with Volterra equalization
2. EVM as a function of OSNR
The two curves below in Fig
filter, in the quality of the received signal. The overlapping factor raises better results for K=4.
a)
Fig. 7: EVM comparison of filters: a) 4QAM, without equalization and b) 4QAM, with
–
Fig. 8 shows a comparison of the EVM of the two modulations 4QAM and 16QAM with K=4
and with equalization. The modulations give good performance. The differen
comes logical because 16QAM is a higher level modulation.
17
b)
c)
Fig. 7 illustrate the impact of the configuration parameter K of the
b)
Volterra equalization
difference ce between their curves

10. International Journal of Electronics and Communication Engineering Technology (IJECET), ISSN 0976
6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue
Fig. 8: EVM comparison based on 4QAM and 16QAM, with Volterra equalization
3. BER as a function of OSNR
BER comparison using different value of the overlapping factor K is
appreciate the impact of the filtering process in the quality of the global transmission chain
a)
Fig. 9: BER comparison of filters using a) 4QAM, without equalization, b) 4QAM, with Volterra
equalization and c) 16QAM, with Volterra equalization
After several simulations for the 16QAM modulation, only overlapping factor K=4 gives
good performances in term of Bit error rate (BER) suggesting, regarding other results,
value of the overlapping factor is K=4.
4. Qeff factor as a function of OSNR
The Q factor curves give a confirmation of the better impact of K when the filters are
designed with a value of 4.
8, August (2014), pp. 10-20 © IAEME
18
also a good way to
b)
c)
–
chain.
that the best

11. International Journal of Electronics and Communication Engineering Technology (IJECET), ISSN 0976
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a)
8, August (2014), pp. 10-20 © IAEME
Fig. 10: Qeff comparison of filters using a) 4QAM, without equalization and b) 4QAM, with
IV. CONCLUSION
–
OFDM/OQAM brings a new way of investigation that is being studied in wireless and optical
networks. The idea of using variants of OFDM is influenced by the need to
transmission impairments, to strengthen the transmission capacity and
schemes like OQAM that can be implemented without the use of a cyclic prefix and equalization in
some cases.
The simulations showed the superiority of OFDM/OQAM than standard OFDM with cyclic
prefix for optical communications, in
equalization process for modulations like 4QAM. This can be useful for simple applications with the
use of less complex receivers. The equalization process is mandatory for
scheme.
Furthermore, the study of FBMC techniques for optical communication is beginning and it
opens new ways of research and applications that can be used to maximize the bandwidth with better
characteristics of the transmission for photonics networks.
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