International Journal of Computation and Applied Sciences IJOCAAS, Volume2, Issue 2, April 2017, ISSN: 2399-4509
Optimal value of cyclic prefix and effective
area for Single polarization O-OFDM
Rend N. Alrays
Sinan M. Abdul Satar
Razi J. Al-azawi
II.
THEORY
The propagation of the fundamental mode in single one
polarization state is called the single polarization. The aim of
this paper is to discuss the main two subjects which are the
cyclic prefix and the effective area; these two parameters are
presented below.
Abstract— OFDM is a multi-carrier-modulation (MCM) which is
deliberated as a special case of FDM technology. Single
polarization fibers means that the fundamental mode propagate
in only one polarization state. Single polarization Optical OFDM
system is simulated with Opti-system V14.0 program. ''Coherent
optical orthogonal frequency division multiplexing (CO-OFDM)''
system with different effective area and cyclic prefix length was
investigated to reach high data rates with long distance by using
single mode fiber (SMF). The performance of the system has
been deliberated by calculating signal to noise ratio (SNR), bit
error rate (BER) and optical signal to noise ratio (OSNR), it was
found that as the effective area increase, the BER decreases. The
optimal value is at a BER of 75*10-6 where the cyclic prefix is at
1/16 (6.25%) and the used effective area is 150 µm2.
Index Terms— BER, Cyclic prefix, Effective Area, Launch
power, O-OFDM, Optisystem, Single Polarization.
A-
CYCLIC PREFIX (CP)
In media transmission, duplicating the finish of the symbol
and sending it toward the start is called Cyclic Prefix [8].
This additional part will be removed at the receiver; the main
objective of using cyclic prefix is to decrease the effect of
Inter-symbol-interference and to offer strength to the OFDM
signal, the cyclic prefix length (Tcp) must be picked more
noteworthy than the delay spread (Δt) to decreases the InterCarrier-Interference and Inter-Symbol-Interference properties
as displayed in equation (1):
I. INTRODUCTION
Tcp< DL
igh speed digital communications has broadly used
Orthogonal Frequency Division Multiplexing all through
last three decades. There are many obstacles of the
OFDM implementation are such as Digital Signal Processing
(DSP), massive complex computation and high speed memory
due to the Very Large Scale Integrated circuit (VLSI) that
marked as an nominal technologies [1]. For high capacity
transmission systems Optical Orthogonal Frequency division
Multiplexing (O-OFDM) is considered as a combination of
modulation and multiplexing arrangement. [2]
The revolutionary work of Robert W. Chang was published on
the formation of band-limited orthogonal signals and it's
deliberated as the first OFDM system [3]. A genuine
increment of consideration in optical OFDM from the optical
communication has lately perceived. Optical OFDM was
recommended as a remarkable modulation scheme for high
data systems either in Direct Detection (DD) [4, 5] or coherent
detection [6].
The ''Coherent optical orthogonal frequency-division
multiplexing (CO-OFDM)'' has been broadly examined due to
its favorable circumstances such as flexible bandwidth
scalability and allocation, low required sampling rates and
high spectral efficiencies [7].
H
𝒄 𝑹
𝒇𝟐 𝒎
(1+rcp)
(1)
Where tcp refer to the interval of the cyclic prefix, D is the
chromatic dispersion, L is the length of the fiber, f is
frequency of optical carrier, R is the data rate, m is the number
of bits, rcp is denoting as the ratio of cyclic prefix length [9].
Adding the cyclic prefix to the signal has a weak point which
is the data loss by means of the decrease of Signal to Noise
Ratio (SNR) as appeared in equation below.
SNRloss= −𝟏𝟎𝐥𝐨𝐠⁡(𝟏 −
𝑻𝒄𝒑
𝑻
)
(2)
Where T is the data symbol length and Tcp is the cyclic prefix
length, to minimize the SNR loss, the cyclic prefix ought not
be larger than required. The reduced Guard Interval in
Coherent-OFDM systems accomplished less ICI because
shorter symbol intervals are employed [10, 11], if the cyclic
prefix length is high, this takes higher vitality for transmission
and it won't convey any information and this lead up to a ruin
in the system performance, on the other hand the cyclic prefix
can't compensate dispersion effects if the cyclic prefix length
is undersized [9].
B-
Rend N. Alrays, is currently a MSc. Student in the University of Technology.
E. mail: [email protected],
Sinan M. Abdul Satar, is currently a MSc. Student in the University of
Technology, E. mail: [email protected] Razi J. Al-Azawi is currently an
assistant
prof.
in
the
University
of
Technology,
E.mail:
[email protected]
EFFECTIVE AREA
The nonlinear effects of the fiber are an effective parameter on
the power of the electromagnetic field in the medium. The
effective area (Aeff) of the fiber is deliberated as a standout
amongst the most vital optical property which shows the
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International Journal of Computation and Applied Sciences IJOCAAS, Volume2, Issue 2, April 2017, ISSN: 2399-4509
power density and influenced unequivocally on the nonlinear.
The effective area of the fiber is computed by using the next
equation [12].
OFDM (SP-O-OFDM) system block diagram is shown in
figure (2) which comprises of the Optical-OFDM transmitter,
which embraces the BER test set which contain the pseudorandom bit sequence generator to generate the data bits with
bit rate of 40 Gb/s and sequence length of 20480 bits, a 16
Quardrature amplitude modulation (16QAM) which encode a
binary signal to an electrical as it carries 4 bits per symbol, an
OFDM modulation block is used to generate an OFDM signal
with 128 maximum number of subcariers, 15 points of prefix
points, 6 is the number of pilot symbols and the average
OFDM power is 15 dBm.
Low pass filter with depth of 100 dB and cutoff frequency of
75*symbole rate is used to lower the radiation of the band.
The RF signal of the ofdm is converted to an optical signal
over the RF to optical converter which comprises of a CW
laser source and two mach-zehender modulators, with laser
power of 10 dBm and 1 dB insertion loss of the modulator .
The optical fiber is of 1000 Km length, single mode fiber, an
optical amplifier is located after every 50Km of gain 10dB ,
16.75 ps/nm.km is the dispersion parameter and
0.2 dB/km is the attenuation constant.
The optical-OFDM receiver contain a detection as the optical
signal is identified and transformed into an electrical signal
with 1 A/W responsivity and photo detector type of PIN, the
OFDM demodulation component where the signal is
demodulated by using the fast fourier transform (FFT), a
16QAM is used to decode and convert the signals into bits.
The proposed system with single polarization Optical-OFDM
scheme was used, the used optical fiber is a standard-SingleMode-Fiber consists of 20 loops of 50Km, and the system
with variable value of effective area is used to explore the best
value to use in the future communication applications which is
varied from 60-150 µm2 in steps of 10. The cyclic prefix
length is also changed from (1/4 to 1/256).
∞
Aeff =
𝟐𝝅[∫𝟎 |𝛙|(𝒓)|𝟐⁡ 𝐫𝐝𝐫]⁡𝟐
∞
∫𝟎 |𝛙|(𝒓)|𝟒⁡ 𝐫𝐝𝐫
(3)
Where r is the radius of the fiber and r is calculated from the
central longitudinal axis of the fiber and ψ (r) is refers to the
modal field distribution.
The ultra-extensive effective area of the fiber (150 µm2) is
used to decrease the effects of the nonlinearities, as it allow
the higher launch of signal power into the fiber and it requires
a low attenuation to decrease the loss of the optical signal
[13], effective area of the fiber is shown in figure (1).
Figure (1): The effective area of the optical fiber.
III.
I.
SINGLE POLARIZATION OFDM SYSTEM
DESCRIPTION
Optical-OFDM system design is performed by using
Optisystem 14.0 [14]. The optisystem software is used to
produce the fiber channel and its influence on the
communications. The suggested single Polarization Optical
Figure (2): proposed Single Polarization Optical OFDM (SP-O-OFDM) system block diagram.
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International Journal of Computation and Applied Sciences IJOCAAS, Volume2, Issue 2, April 2017, ISSN: 2399-4509
IV. RESULTS AND DISCUSSION
For a Single polarization optical-OFDM signal is
transmitted over an extended length of 1000Km of single
mode fiber with a 40 Gbp/s coherent detection 16-QAM
Optical-OFDM. The optimal value of effective area is
150 µm2 at the cyclic prefix length of 1/16 (6.25%),
where it has a BER of 75*10-6. The in-phase component
of the RF spectrum after the OFDM modulation and
demodulation are presented in Figure (3, 4), The OSNR
and SNR are measured at 24.79 and 22.81 Sequentially.
Figure (5): The Received Data Constellation Diagram for
OFDM 16QAM at 40 Gbps.
Single Polarization O-OFDM
0
10
CP
CP
CP
CP
CP
CP
CP
-1
BER
10
1/4
1/8
1/16
1/32
1/64
1/128
1/256
-2
10
-3
10
Figure (3): The in-phase component of RF spectrum for 1/32 CP
after OFDM modulation.
-4
10
-2
0
2
4
Luanch Power (dBm)
6
8
10
Figure (6): BER as a function of the fiber launch power per
channel after 1000 km transmission for single polarization
system.
The lengths of cyclic prefix (rcp) are varied from 1/4, 1/8,
1/16, 1/32, 1/64, 1/128 and 1/256. Estimated pulse
broadening using analytical expression equation (1) is
tabulated in table (1).
TABLE 1
DELAY SPREAD FOR VARYING LENGTHS OF CP FOR
SINGLE POLARIZATION O-OFDM.
Cyclic Prefix
length(rcp)
1/4 (25%)
1/8 (12.5%)
1/16 (6.25 %)
1/32 (3.1%)
1/64 (1.56%)
1/128 (0.78%)
1/256 (0.39%)
Figure (4): The in-phase component of the RF spectrum for 1/32
CP after OFDM demodulation.
The constellation diagram of the received signal at the
OFDM demodulation receiver after carrier phase
estimation is presented in Figure (5). The power of the RF
is measured at −10 dBm after the OFDM modulation and
at -37dBm after the OFDM demodulation For the
effective area of 150 µm2 the launch power is changed
from -2 to 10 dBm, increasing power lead to decrease bit
error rate value, Figure (6) shows the BER as a function
of the fiber launch power per channel after 1000 km
transmission for Single polarization O-OFDM system,
which gives the lower BER at input power of 10 dBm
means increasing input power leads to decrease the BER.
Delay spread (ns)
1.609
1.448
1.367
1.327
1.307
1.297
1.292
V. CONCLUSION
In this paper, we have suggested and established the
enhancement of the Single Polarization Optical
Orthogonal Frequency Division Multiplexing system and
performed the optimal effective area and cyclic prefix
length to use in terms of BER, The optimal value of the
cyclic prefix is at 1/16 (6.25%), where the used effective
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International Journal of Computation and Applied Sciences IJOCAAS, Volume2, Issue 2, April 2017, ISSN: 2399-4509
area is 150 µm2, as it has a BER of 75*10-6, as the
effective area increase the system achieves the lower bit
error rate. The OSNR and SNR are measured at 24.79 and
22.81 respectively and the measured the peak to average
power ratio at this CP is 8.2 dB.
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