Design optimization of optical wireless communication owc focusing on light fidelity li fi using optical code division multiple access ocdma

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 10, October (2014), pp. 69-103 © IAEME
69
DESIGN OPTIMIZATION OF OPTICAL WIRELESS
COMMUNICATION (OWC) FOCUSING ON LIGHT
FIDELITY (LI-FI) USING OPTICAL CODE DIVISION
MULTIPLE ACCESS (OCDMA) BASED ON CARBON
NANOTUBES (CNTS)
Jafaar Fahad A. Rida1
, A. K. Bhardwaj2
, A. K. Jaiswal3
1
Dept. of Electronics and Communication Engineering, SHIATS -DU, Allahabad, India.
2
Dept. of Electrical and Electronics Engineering, SHIATS - DU, Allahabad, India,
3
Dept. of Electronics and Communication Engineering, SHIATS - DU, Allahabad, India
ABSTRACT
This research work focuses on the design and analysis ofOptical Wireless Communication
system (OWC) using Optical Code Division Multiple Access (OCDMA) based on Carbon
Nanotubes (CNTs) to bring in improvement in three parameters very important in any
communication system as data rate (R), bit error rate (BER), and signal to noise ratio (SNR).The
carbon nanotubes based OCDMA system supports ultrahigh speed network with data rate upto Tb/s
and exceptional BER performance in the system. As observed and presented in this paper, the carbon
nanotubes brought in the improved performance OCDMA system in OWC network with highest data
rate and lowest bit error rate. Future requirements of ultrahigh speed internet, video, multimedia, and
advanced digital services, would suitably be met with incorporating carbon nanotubes based devices
providing optimal performance. Considering the third order nonlinearity, carbon nanotubes are
observed to be highly efficient providing very fast response and are more suited to next generation
components required in communication system consuming much less power with time, extending the
life of batteries.
Keywords: OCDMA, CNTs, Optical Systems, OWC, Li-Fi, Effect Visibility with Bad Weather.
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING
AND TECHNOLOGY (IJARET)
ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 5, Issue 10, October (2014), pp. 69-103
© IAEME: www.iaeme.com/ IJARET.asp
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INTRODUCTION
The Optical Wireless Communications (OWC) is a type of communications system that uses
the atmosphere as a communications channel. The OWC systems are attractive to provide broadband
services due to their inherent wide bandwidth, easy deployment and no license requirement [1]. The
idea to employ the atmosphere as transmission media arises from the invention of the laser.
However, the early experiments on this field did not have any baggage of technological development
(like the present systems) derived from the fiber optical communications systems, because like this,
the interest on them decreased. At the beginning of the last century, the OWC systems have attracted
some interest due to the advantages mentioned above. However, the interaction of the
electromagnetic waves with the atmosphere at optical frequencies is stronger than that corresponding
at microwave [1].The traditional way to meet this requirement isto use wired physical connections.
But, wired physical connections have some inherent problems, in setting up and in its expansion.
Further, these need more space, time to setup, monetary investment in copper, maintenance etc.
Wireless systems offer an attractive alternative. Both, radio frequency (RF) and optical wireless
communication or free space optical application infrared (IR) and light fidelity (Li - Fi) are possible
options in implementing wireless systems. Unfortunately, the RF can support only limited bandwidth
because of restricted spectrum availability and interference; while this restriction does not apply to
IR. Thus, optical wireless (IR) technology [2-5] seems to be ideal for wireless communication
systems of the future. It can provide cable free communication at very high bit rates (a few Gbps as
compared to tens ofMbps supported by radio). In indoor optical wireless systems called light fidelity
(Li - Fi), laser diodes (LDs) or light emitting diodes (LEDs) are used as transmitter and photo-diodes
as the receivers for optical signals. These optoelectronic devices are cheaper as compared to RF
equipment as well as wire line systems. Further, optical wireless communication transmission does
not interfere with existing RF systems and is not governed by Federal Communications Commission
(FCC) regulations. The light fidelity (Li - Fi) signal does not penetrate walls, thus providing a degree
of privacy within the office area [11]. In addition to privacy, this feature of light fidelity (Li - Fi),
systems makes it easier to build a cell-based network.
• Applications of the OWC systems
Optical wireless communications systems have different applications areas:
a. Satellite networks: the optical wireless communications systems may be used in satellite
communication networks, satellite-to-satellite, satellite-to-earth [6].
b. Aircraft applications: satellite to aircraft or the opposite [7].
c. Deep Space: the deep space ,may be used for communications between spacecraft – to – earth or
spacecraft to satellite [6].
d. Terrestrial (or atmospheric) communications: terrestrial links are used to support fiber optic,
optical wireless networks "wireless optical networks (WON)" last mile link, emergency situations
temporary links among others. The number of personal computers and personal digital assistants for
indoor use are rapidly growing in offices, manufacturing floors, shopping areas and warehouses [8].
e. Light fidelity (Li - Fi) :Fi is a new way to establish wireless communication links using the LED
lighting networks. The Li-Fi protocols are defined by the international standard IEEE 802.15
established since 2011 by the IEEE comity. This is the same comity that has defined previously the
Ethernet 802.3 and Wi-Fi 802.11 standards [9-11].The carbon nanotube supports optical by three
main parameters very important to develop work with optical system application such as Electronic
structure of carbon nanotubes, Saturable absorption, and third order Nonlinearity. Depending on the
chiral vector, carbon nanotubes behave as semiconductor or metal. But here focuses on

71
semiconducting carbon nanotubes to improve optical integrated circuit. The optical absorption of
carbon nanotube determines their electronic energy gap and broadband operation is resulted of a
large distribution of (1 -1.5 nm) diameters. Third order susceptibility is responsible for processes
such as third harmonic generation (THG). Materials with a high nonlinearity combined with fast
response time are desired for roles such as photonic devices for communication and information
technology. The electronic proprieties are governed by a single parameter named the chiral vector,
and there are three parameters affecting the performance of carbon nanotubes diameter, chirality, and
number of walls. Carbon nanotubes are one of most commonly mentioned building blocks of
nanotechnology, with one hundred times the tensile strength of steel, thermal conductivity better than
all but the purest diamond and electrical conductivity similar to copper but with the ability to carry
much higher currents. They seem to be a wonder material, thin cylinders of graphite. Graphite (ܵ‫݌‬ଶ
)
is made up of layers of carbon atoms arranged in a hexagonal lattice like chicken wire, which itself is
very strong [12-16]. But let’s look at some of the different types of nanotubes and nanotube
pretenders such as One of major classification of carbon nanotubes is into Single – walled varieties
(SWNTs), which have a single cylindrical wall, and Multi-walled varieties (MWNTs), which have
cylinders within cylinders. There are two types for fabrication first, chemical (chemical vapor
deposition (CVD)) and second, other physical methods (Arc discharge, Laser ablation).The carbon
nanotubes with OCDMA system supports ultrahigh speed network with data rate upto Tb/s and
exceptional BER performance in the system. As observed and presented in this paper, the carbon
nanotubes brought in the improved performance OCDMA system network with highest data rate and
lowest bit error rate. Optical Code Division Multiple Access (OCDMA) can be seen that one of the
key issues to implement OCDMA networking and communication is how to encode and decode the
user’s data such that the optical channel can be shared, that is, we need to develop the practical
encoding and decoding techniques that can be exploited to generate and recognize appropriate code
sequences reliably [17]. Therefore, The OCDMA encoders and decoders are the key components to
implement OCDMA systems. In order to implement the data communications among multiple users
based on OCDMA communication technology, one unique codeword-waveform is assigned to each
subscriber in an OCDMA network, which is chosen from specific OCDMA address codes, and
therefore, different users employ different address codeword-waveforms. Optical code division
multiple access (OCDMA) technique is an attractive candidate for next generation broadband access
networks [18]. In an OCDMA network using on-off keying pattern, the user’s data is transmitted by
each information bit “1” which is encoded into desired address codeword. However, the transmitter
does not produce any optical pulses when the information bit “0” is sent. In terms of the difference of
signal modulation and detection pattern, OCDMA encoders/decoders are roughly classified into
coherent optical encoders/decoders and incoherent optical encoders/decoders [20]. The incoherent
optical encoders/decoders employ simple intensity-modulation/direct-detection technology and the
coherent optical en/decoders are based on the modulation and detection of optical signal phase. Here,
in this simulation about Data Rate (R) and Bit Error Rate (BER) with OOK formats and BPSK
formats in coherent system and OOK format and PPM formats in incoherent system. The efficient
utilization of bandwidth is a major design issues for ultra-high speed photonic networks, also it
increases data rate (R), and decreases bit error rate (BER) so as to perform with improved signal to
noise ratio (SNR). Silicon optical devices having band gap 1.12eV, called silicon photonics, has
attracted much attention recently because of its potential applications in the infrared spectral region
in optical system having refractive index ݊ଶ = 2 ∗ 10ିଶ଴
. Optical code division multiple access with
carbon nanotubes having band gap 2.9 eV and the refractive index ݊ଶ = 1.55 ∗ 10ି଼
, brought in the
improved performance. The two main techniques for multiplexing data signals are currently time
division multiplexing (TDM) and wavelength division multiplexing (WDM). Optical code division
multiple access (OCDMA) is an alternative method, which performs encoding and decoding through
an optical signature code in order to allow the selection of a desired signal so that different users can

72
share the same bandwidth. In such as a system, data signals overlap both time and wavelength [18],
[19]. The performance of any communication system is fundamentally limited by the available
bandwidth, the signal to noise ratio of received signal, and the codes used to relate the original
information to the transmitted signal. These limits inevitably lead to increased errors and
corresponding loss of information. Next generation of optical communication system may preferably
incorporate carbon nanotubes based devices so as to achieve much higher data rate up to Tb/s in
comparison to present systems using silicon optical devices giving data rate upto Gb/s. Besides, such
systems with advanced energy source power realize in much longer life. Nevertheless, future
requirements of ultrahigh speed internet, video, multimedia, and advanced digital services, would
suitably be met with incorporating carbon nanotubes based devices providing optimal performance
[21].For ground space and or terrestrial communication systems, these links suffer from
atmospheric loss mainly due to fog, scintillation and precipitation.
Optical Wireless link provides high bandwidth solution to the last mile access bottleneck.
However, an appreciable availability of the link is always a concern. Wireless Optics (WOs) links
are highly weather dependent and fog is the major attenuating factor reducing the link availability.
Optical wireless links offer gigabit per second and data rates and low system complexity Terabit per
second with carbon nanotubes. The optical wireless communication (OWC) system has attracted
significant interest because it can solve the last mile problem in urban environments. The last mile
problem is when Internet providers cannot connect the fiber optic cables to every household user
because of the high installation costs. The only disadvantage of the OWC system is that its
performance depends strongly on weather conditions. Fog and clouds scatter and absorb the optical
signal, which causes transmission errors. Most previous studies consider only single-scattering
effects and assume that the received signal has no inter symbol interference (ISI), which is true
only for light-fog conditions [22]. Maintaining a clear line of sight (LOS) between transmit and
receive terminals is the biggest challenge to establish optical wireless links in the free space
especially in the troposphere [23]. The LOS is diminished due to many atmospheric influences like
fog, rain, snow, dust, sleet, clouds and temporary physical obstructions like e.g., birds and
airplanes [24]. Moreover, the electromagnetic interaction of the transmitted optical signal with
different atmospheric effects results in complex processes like scattering, absorption and extinction
that are a function of particle physical parameters. Hence the local atmospheric weather conditions
mainly determine the availability and reliability of such optical wireless links since there is always a
threat of downtime of optical wireless link caused by adverse weather conditions [25]. Optical
wireless links are also influenced by atmospheric temperature that varies both in spatial and
temporal domains. The variation of temperature in the optical wireless channel is a function of
atmospheric pressure and the atmospheric wind speed. This effect is commonly known as optical
turbulence or scintillation effect and causes received signal irradiance or power fades in conjunction
with the variation of temperature along the propagation path as shown in figure 1. As a result of
this scintillation phenomenon, the optical wireless channel distance and the capacity are reduced
[26].
Figure 1: General block diagram of optical wireless communication system.

73
Thereby restricting the regions and times where optical wireless links can be used potentially.
In order to take full advantage of the tremendous usefulness of optical wireless technology, a proper
characterization of different atmospheric effects and a meaningful interpretation of the filed
measurements in such adverse conditionsare required. Optical Wireless communication, also known
as free space optical (FSO), has emerged as a commercially viable alternative to radio frequency
(RF) and millimeter wave wireless for reliable and rapid deployment of data and voice networks.
RF and millimeter wave technologies allow rapid deployment of wireless networks with data rates
from tens of Mbit/sec (point-to-multipoint) up to several hundred Mbit/sec (point-to-point). Though
emerging license free bands appear promising, they still have certain bandwidth and range
limitations [27]. Optical wireless can augment RF and millimeter wave links with very high (>1
Gbit/sec) bandwidth. In fact, it is widely believed that optical wireless is best suited for multi
Gbit/sec communication. The general acceptance of free space laser communication (lasercom) or
optical wireless as the preferred wireless carrier of high bandwidth data has been hampered by
the potential downtime of these lasercom systems in heavy, visibility limiting, weather. There seems
to be much confusion and many preconceived notions about the true ability of lasercom systems in
such weather. There still is some confusion over how different laser wavelengths and LED for
wavelength 1550nm are attenuated by different types of weather [28]. Optical wireless
communication is now a well-established access technology, better known for its robustness in
transmitting large data volumes in an energy efficient manner. However the bit error rate (BER)
performance of a wireless optical communication ground link is adversely affected by cloud
coverage, harsh weather conditions, and atmospheric turbulence. Fog, clouds and snow play a
detrimental role by attenuating optical energy transmitted in terrestrial free space and thus decrease
the link availability and reliability.
This paper presents optimized design performance of incoherent OCDMA as well as coherent
OCDMA using carbon nanotubes (CNTs) based devices with reference to increased Data Rate (R)
and reduced Bit Error Rate (BER) which is far enhanced in comparison to Silicon based Optical
Devices. The carbon nanotubes (CNTs) based devices are having optical properties as well as brings
in miniatured dimension. Besides, it has been observed that a CNT – based FET switches reliably
use less power than silicon based optical devices, specifically in traditional t – gate multiplexer,
which is a fundamental logic block. Carbon nanotubes based optical devices can have a wide range
of applications in a wide variety of miniaturized circuits.
SYSTEM ASSUMPTION AND SIMULATIONS
In the present study, OCDMA scheme is of increasing interest for optical wireless system
because it allows multiple users to access the system asynchronously and simultaneously. OCDMA
is expected to provide further ultrahigh speed and real time computer communications where there is
strong demand for the systems to support several kinds of data with different traffic requirements
[21]. We have analyzed the improved performance in OOK and BPSK format with coherent
technique and OOK and PPM formats with incoherent (noncoherent) technique through some of
parameters as bit error rate (BER), data rate (R) and the effect some parameters on the optical
wireless communication or light fidelity as fog, rain, scattering, snow, dust, sleet, clouds, wind, and
temperarly physical obstruction. For ground space and or terrestrial communication scenarios, these
links suffer from atmospheric loss mainly due to fog, scintillation and precipitation signals and
then to upgrade the transmission bit rate distance product for ultra long transmission links. This
paper has also presented the bad weather effects such asrain, fog, snow, and scattering losses on the
transmission performance of wireless optical communication systems. We have focused on taken the
study of bit error rate, maximum signalto noise ratio, maximum transmission optical path lengths
and maximum transmission bit rates under these bad operating conditions.

74
A single wall carbon nanotube (SWTN) can be described as a single layer of graphite crystal
that is rolled up into a seamless cylinder, one atom thick usually with a small number (perhaps
20 - 40) of atoms along the circumference and along length (micron) along the cylinder axis [30].
This nanotube is specified by the chiral vector (‫ܥ‬௛ ).
‫ܥ‬௛ = ݊ ∗ ܽଵ + ݉ ∗ ܽଶ(1)
Where n and m are two integers indices called Hamada integers, described by the pair of
indices (n, m) that denote the number of unit vectors n* ܽଵ and m*ܽଶ in the hexagonal honeycomb
lattice contained in this vector ܽଵ and ܽଶ where |ܽ| =|ܽଵ| = |ܽଶ| =√3 * ܽ௖ି௖ = 0.246nm,
where ܽ௖ି௖= 0.142nm the c-c bond length ܽଵ and ܽଶ are graphite lattice vector ,which two vectors
real space vectors [13], [14], [31], [30]. The chiral vector makes an angle (ߠ) called the chiral angle
with the zigzag or direction.as figure 2. The vector connects two crystallographic ally equivalent
sites O and A on a two – dimensional (2D) graphene sheet where a carbon atom is located at each
vertex of the honeycomb structure [31]. The axis of the zigzag nanotube corresponds to ߠ = 0, while
the armchair nanotube axis corresponds to ߠ = 30, and the chiral nanotube axis corresponds to
0≤ ߠ ≤ 30. The seamless cylinder joint of the nanotube is made by joining the line AB to the
parallel line OB in figure 2, in terms of the integer (n, m), the nanotube diameter (݀௧) is given by
equation (2).
݀௧ =
|‫ ∗|܉‬ඥ‫ܖ‬ ૛ା‫ ܕ∗ ܖ‬ା‫ܕ‬૛
ૈ
(2)
The nearest – neighbor C-C distance 1.421 or 0.142 in graphite, ‫ܥ‬௛ is the length of the chiral
vector and the chiral angle (ߠ) is given by equation (3)
ࣂ = ‫ܖ܉ܜ‬ି૚
(
√૜ ∗࢓
૛࢔ା࢓
) (3)
Thus, a nanotube can be specified by either its (n, m) indices or equivalent by ݀௧ and ߠ [16].
Figure 2: explanation of synthesis of carbon nanotubes from graphite sheet
The information capacity C is defined as the maximum possible data bit rate R for error-free
transmission in the presence of noise, and depends on the parameters of the ‘‘communication
channel’’ (e.g., optical silicon and carbon nanotubes) devices and on the particular encoding
algorithm. While the use of more advanced codes may improve the system performance, the
bandwidth and the signal-to-noise ratio (SNR) in the communication channel put a fundamental limit
on information capacity [18], [20]. . Since the optical transmission lines or devices must satisfy very
strict requirements for bit error rate (BER) (10ିଽ
ܶ‫01݋‬ିଵହ
), to use optical fiber for longest distance
incorporating silicon based integrated circuit does not support its work enough to transmission rate

75
between these devices. Further, the optical fiber has largest bandwidth to transport most information
but it needs the optical devices to support based on the new device called carbon nanotubes (CNTs)
preferably of the type Single Walled Carbon nanotubes (SWNTs) to get improved performance of
the system. Such devices may also be used as biological device and can be used one the human
bodies, in small length maximum 1 centimeter so they cannot be used like fiber or optical fiber cable
but they can also be used in manufacturing integrated optical circuits like encoder and decoder for
optical signal, also mode locked lasers which have highest efficiency for energy and transferring
optical signal with optical code division multiple access (OCDMA) to develop communication
system from increasing data bit rate and to improve the SNR in the system. The last mile problem is
when Internet providers cannot connect the fiber optic cables to every household user because of the
high installation costs. The only disadvantage of the OWC system is that its performance depends
strongly on weather conditions. Fog and clouds scatter and absorb the optical signal, which causes
transmission errors. the band gap energy for silicon optical fiber ‫ܧ‬௢ 1.12ܸ݁ and the energy band gap
for carbon nanotubes ‫ܧ‬௢ = 2.9ܸ݁ [10- 11], also the refractive index for them like for silicon optical
fiber (݊ଶ = 2 ∗ 10ିଶ଴
݉ଶ
/‫)ݓ‬ and the refractive index for carbon nanotubes (݊ଶ = 1.55 ∗ 10ି଼
݉ଶ
/
‫)ݓ‬ [18], [19],. Since the chip – level receiver are dependent on the number of photons (optical
energy) per chip in the received frame when it uses silicon optical devices the optical source power is
35.99 ∗ 10ିଷ
‫,ݐݐܽݓ‬ whereas when it uses carbon nanotubes the optical source power is 214.8 ∗
10ିଷ
‫.ݐݐܽݓ‬ That means when we use the carbon nanotubes devices the consumed power is very low.
Here, the time duration of time slot ܶ௕ = 3.33 ∗ 10ିଵ଴
sec or nanosecond in silicon optical devices,
but the time duration ܶ௕ = 2.58 ∗ 10ିଵଷ
‫ܿ݁ݏ‬ or femtosecond in carbon nanotubes, therefore, the
carbon nanotubes based ultrafast switching system, are formulated to attain optimized performance
of OCDMA technology.
• Optical and optoelectrónic components
Devices such as the laser diodes, high-speed photo-receivers, optical amplifiers, optical
modulators among others are derived of about thirty years of investigation and development of the
fiber optics telecommunications systems. These technological advances have made possible the
present OWC systems. Additionally, OWC systems have been benefited by the advances in the
telescopes generated by the astronomy [1],.The optical wireless communication network with carbon
nanotubes are better than silicon optical fiber (light source made from silicon), high power output
and very less power consumption to serve the applications of same energy. We can be use LED
source for light fidelity because of wide beam width for expended area and short distances, while the
laser diode (LD) for other application of optical wireless systems as connection between earth
satellite station and satellite, between buildings. There are three key function elements of optical
wireless communication system as shown in Figure. 1. The transmitter, the atmospheric channel and
the receiver. The transmitter converts the electrical signal into light signal. The light propagates
through the atmosphere to the receiver, which converts the light back into an electrical signal. The
transmitter includes a modulator, a laser driver, a light emitting diode (LED) or a laser, and a
telescope [34]. The modulator converts bits of information into signals in accordance with the chosen
modulation method. The driver provides the power for the laser and stabilizes its performance, it also
neutralizes such effects as temperature and aging of the laser or LED [32, 33]. The light sources
convert the electrical signal into optic radiation. The telescope aligns the laser LED radiation to a
collimated beam and directs it to the receiver. In the atmospheric channel, the signal is attenuated
and blurred as a result of absorption, scattering and turbulence. This channel maybe the traversed
distance between a ground station and a satellite or a path of a few kilometers through the
atmosphere between two terrestrial transceivers [35]. The receiver includes a telescope, filter, photo
detector, an amplifier, a decision device, and a clock recovery unit. The telescope collects the
incoming radiation and focuses it onto filter. The filter removes background radiation and allows

76
only the wavelength of the signal to pass through the electronic signal. The decision unit determines
the nature of the bits of information based on the time of arrival and the amplitude of the pulse. The
clock recovery unit synchronizes the data sampling to the decision making process.
• Light Emitting Diodes
In modern optical wireless communications, there are a variety of light sources for use in the
transmitter. One of the most used one is the semiconductor laser which is also widely used in fiber
optic systems. For indoor environment applications, where the safety is imperative, the Light Emitter
Diode (LED) is preferred due to its limited optical power. Light emitting diodes are semiconductor
structures that emit light. Because of its relatively low power emission, the LED's are typically used
in applications over short distances and for low bit rate (up to 155Mbps). Depending on the material
that they are constructed, the LED's can operate in different wavelength intervals. When compared to
the narrow spectral width of a laser source, LEDs have a much larger spectral width (Full Width at
Half Maximun or FWHM). Table 1 the semiconductor materials and its emission wavelength used in
the LED's. Such a device is a basic photonic building block and paves the way for application of
CNTs in nano-optics and photonics. A light emitting p -i -n diode from a highly aligned film of
semiconducting carbon nanotubes has been realized that emits light in the near-infrared spectral
range. A split gate design similar to the single-tube CNT diode allows for tuning both the rectifying
electrical behavior of the diode and its light generation efficiency. The CNT film diode produces
light that is polarized along the device channel, a direct consequence of the high degree of CNT
alignment in the film that reflects the polarization property of the 1D nature of individual tubes [1],
[32], [33].
Table 1: Material, wavelength and energy band gap for typical LED
Material Wavelength Range (nm)
AlGaAs 800 – 900
InGaAs 1000 – 1300
InGaAsP 900 – 1700
CNTs 700 - 2000
• Laser Diodes
The laser is an oscillator generating optical frequencies which is composed of an optical
resonant cavity and a gain mechanism to compensate the optical losses. Semiconductor lasers are of
interest for the OWC industry, because of their relatively small size, high power and cost efficiency.
Many of these lasers are used in optical fiber systems. Table 2 summarizes the materials commonly
used in semiconductor lasers. Laser diodes (LDs) are a more recent technology which has grown
from underlying LED fabrication carbon nanotube or silicon optical devices techniques. LDs still
depend on the transition of carriers over the band gap to produce radiant photons, however,
modifications to the device structure allow such devices to efficiently produce coherent light over a
narrow optical bandwidth.
Table 2: Materials used in semiconductor laser with wavelengths that are relevant for FSO
Material Wavelength Range (nm)
AlGaAs 620 - 895
GaAs 904
InGaAsP 1100 – 1650
1550
CNTs 700 - 2000

77
• Photodetectors
At the receiver, the optical signals must be converted to the electrical domain for further
processing, this conversion is made by the photo detectors. There are two main types of
photodetectors, PIN diode (Positive-Intrinsic-Negative) and avalanche photodiode" (APD). The main
parameters that characterize the photodetectors in communications are: spectral response,
photosensitivity, quantum efficiency, dark current, noise equivalent power, response time and
bandwidth. The photodetection is achieved by the response of a photosensitive material to the
incident light to produce free electrons. These electrons can be directed to form an electric current
when an external potential is applied to the device.
• Pin photodiode
This type of photodiodes has an advantage in response time and operates with reverse bias.
This type of diode has an intrinsic region between the PN materials, this union is known as
homojunction. PIN diodes are widely used in telecommunications because of their fast response. Its
responsivity, i.e. the ability to convert optical power to electrical current is function of the material
and is different for each wavelength. This is defined
ܴ =
ఎ௘
௛௩
(4)
Where η is the quantum efficiency, e is the electron charge (1.6 ∗ 10ିଵଽ
C), h is Planck's
constant (6.62 ∗ 10ିଷସ
J) and νis the frequency corresponding to the photon wavelength. InGaAs PIN
diodes show good response to wavelengths corresponding to the low attenuation window of optical
fiber close to 1500nm. The atmosphere also has low attenuation into regions close to this
wavelength. In this system, silicon optical devices and carbon nanotubes are used. The responsivity
in the carbon nanotubes based devices has the best sensitivity incorporating to other devices.
• Avalanche photodiode
This type of device is ideal for detecting extremely low light level. This effect is reflected in
the gain M:
‫ܯ‬ =
ூಸ
ூು
(5)
‫ܫ‬ீ is the value of the amplified output current due to avalanche effect and Ipis the current
without amplification. The avalanche photo diode has a higher output current than PIN diode for a
given value of optical input power, but the noise also increases by the same factor and additionally
has a slower response than the PIN diode.
Table 3: Characteristics of photo detectors used in OWC systems
Material Wavelength (nm) Responsivity
(A/W)
Gain Rise time
PIN. Silicon 300 – 1100 0.5 1 0.1-5 ns
PIN InGaAs 1000 – 1700 0.9 1 0.01-5 ns
PIN CNTs 700 - 2000 0.95 1 1-5 ps
APD
Germanium
800 – 1300 6 10 0.3-1 ns
APD InGaAs 1000 – 1700 75 10 0.3 ns
APD CNTs 700 - 2000 95 10 1.8 ps – 1fs

78
Due to the non-linear dependence of avalanche gain on the supply voltage and temperature,
APDs exhibit non-linear behavior throughout their operating regime. The addition of extra circuitry
to improve this situation increases cost and lowers system reliability. Additional circuitry is also
necessary to generate the high bias voltages necessary for high field APDs. As mentioned earlier,
most commercial indoor wireless optical links employ inexpensive Si photodetectors and LEDs in
the 850-950 nm range. However, some long-range, outdoor free-space optical links employ
compound photodiodes operating at longer wavelength to increase the amount of optical power
transmitted while satisfying eye-safety limits. Additionally, these long-range links also employ APD
receivers to increase the sensitivity of the receiver [36].Care must be taken in the selection of
photodiode receivers to ensure that cost, performance and safety requirements are satisfied.
• Optical amplifiers
Basically there are two types of optical amplifiers that can be used in wireless optical
communication systems: semiconductor optical amplifier (SOA) and amplifier Erbiumdoped fiber
(EDFA). Semiconductor optical amplifiers (SOA) have a structure similar to a semiconductor laser,
but without the resonant cavity. The SOA can be designed for specific frequencies. Erbium-doped
fiber amplifiers are widely used in fiber optics communications systems operating at wavelengths
close to 1550 nm. Because they are built with optical fiber, provides easy connection to other
sections of optical fiber, they are not sensitive to the polarization of the optical signal, and they are
relatively stable under environment changes with a requirement of higher saturation power than the
SOA.
• Optical antennas
The optical antenna or telescope is one of the main components of optical wireless
communication systems. Some systems may have a telescope in the transmitter and one in the
receiver, but the same device can be used to perform both functions. The transmitted laser beam
characteristics depend on the parameters and quality of the optics of the telescope. The various types
of existing telescopes can be used for optical communications applications in free space. The optical
gain of the antennas depends on the wavelength used and its diameter. The Incoherent optical
wireless communication systems typically expands the beam so that any change in alignment
between the transmitter and receiver do not cause the beam passes out of the receiver aperture. The
beam footprint on the receiver can be determined approximately by
‫ܦ‬௙ = ߠ‫ܮ‬ (6)
‫ܦ‬௙ is the foot print diameter on the receiver plane in meters, θis the divergence angle in
radians and L is the separation distance between transmitter and receiver (meters). The above
approximation is valid considering that the angle of divergence is the order of milliradians and the
distances of the links are typically over 500 meters. Li – Fi technology has the possibility to change
how we access the internet, stream video, receive emails and much more, the Li – Fi used optical
signal broadcast in free space by two ways. First, line of sight (LOS) or point to point link. Second,
non-line of sight (NLOS) or point to multi-point link (diffuse). The technology truly began during
the 1990’s in countries like Germany, Korea, and Japan where discovered LED’s could be retrofitted
to send information. This type of light would come in familiar forms such as infrared, ultraviolet, and
visible light, using infrared light at wavelength 1550nm. Also we can use visible light technique. Its
idea was very simple that if the LED’s is on then the logic 1 can be transmitted and if the LED’s is
off then the logic 0 can be transmitted. The LEDs can be switched on and off very quickly whereas
the carbon nanotubes switched in ultrafast speed with ultrafast response.

79
• Channel Topologies(The atmospheric channel)
The characteristics of the wireless optical channel can vary significantly depending on the
topology of the link considered. Various system configurations for optical wireless local area
networks have been investigated since then. They differ in the degree of directionality of the
transmitter and receiver and the orientation of the units. The latter factor underlies the development
of two major classes of link topology: line-of-sight (LOS) links, in which an.
LOS path between receiver and transmitter exists, and nonLOS or diffuse links, which rely
on diffuse signal reflections off the room surfaces.
• Point-to-point wireless optical links (Line-of-sight)
Point-to-point wireless optical links operate when there is a direct‚ unobstructed path
between a transmitter and a receiver. Figure 3 presents a diagram of a typical point-to-point wireless
optical link. A link is established when the transmitter is oriented toward the receiver. In narrow
field-of-view applications‚ this oriented configuration allows the receiver to reject ambient light and
achieve high data rates and low path loss. The main disadvantage of this link topology is that it
requires pointing and is sensitive to blocking and shadowing [36], [37].
Figure 3: A point-to-point wireless optical communications system
LOS links exhibit low power requirements when transmitted optical power is concentrated in
a narrow beam thus creating a high power flux density at the receiver. Furthermore, such links do not
suffer from multipath signal distortion. If additionally a narrow field-of-view (FOV) receiver is used,
an efficient optical noise rejection and a high optical signal gain are achievable [38]. Generally
speaking, narrow LOS links (NLOS, narrow transmit beam and small receiver FOV) are applicable
to point-to-point communications only. NLOS links cannot support mobile users because alignment
of receiver and transmitter becomes necessary. However, elements that are meant for point-to-point
links are being incorporated into different link configurations in search for better power efficiency
and higher data rates. For example, the so-called tracked system [39] utilizes a narrow beam
transmitter and a small FOV receiver with the addition of steering and tracking capabilities. In LOS
optical wireless LANs, the base station is typically located on the room ceiling. In order to serve
multiple mobile users within a relatively large coverage area, then arrow transmit beam is now
replaced by a wide light cone, which defines a communication cell. This configuration has been
called “cellular” [40] A large area communication cell is achieved at the cost of reducing the power
efficiency since more launch power is needed to ensure the required power flux density at the
receiver. In cellular configuration, optical signal is delivered to all the terminals within the light
cone. Communication between portables is accomplished through a base station, that is, in a star
network topology. An important development in LOS-LANs may be described as a merger of
cellular and NLOS tracked systems. The essence is in the utilization of two-dimensional arrays of
emitters and detectors. Base station is placed above the coverage area. The sources in the transmitter
array emit normally to the plane of the array. Then, an optical system performs spatial-angular
mapping, that is, a light beam is deflected into a particular angle depending on the spatial position of
the source in the array. As a result, the communication cell is split into microcells, each illuminated

80
by a single light source of the array. Power savings can be realized by switching off the sources that
do not illuminate a user terminal. Transmitter can be designed so that sources inthe emitter array
transmit different data streams, thus significantly increasing the overall capacity of the
communication system. The pixels in the detector array exhibit low capacitance and small FOV
because of their small size. The small detector capacitance allows for an increase in the transmission
bandwidth and the small FOV reduces the ambient light reception.
• Point-to-multipoint wireless optical links(Diffuse Links)
Diffuse transmitters radiate optical power over a wide solid angle in order to ease the
pointing and shadowing problems of point-to-point links. Figure 4 presents a block diagram of a
diffuse wireless optical system. The transmitter does not need to be aimed at the receiver since the
radiant optical power is assumed to reflect from the surfaces of the room. This affords user terminals
a wide degree of mobility at the expense of a high path loss. These channels‚ however‚ suffer not
only from optoelectronic bandwidth constraints but also from low-pass multipath distortion [2‚ 41‚
42]. Unlike radio frequency wireless channels‚ diffuse channels do not exhibit fading. This is due to
the fact that the receive photodiode integrates the optical intensity field over an area of millions of
square wavelengths‚ and hence no change in the channel response is noted ifthe photodiode is moved
a distance on the order of a wavelength [2‚ 43]. Thus‚ the large size of the photodiode relative to the
wavelength of light provides a degree of spatial diversity which eliminates multipath fading.
Figure 4: A diffuse wireless optical communications system
In classical diffuse links [42], base station is located at a desktop level and transmitter emits
upwards. Usually, transmitter radiation pattern is Lambertian, therefore the entire room ceiling and
large portions of the walls are illuminated. Since infrared is diffusely scattered by most room
surfaces, signals reach receiver after multiple reflections off the room walls and furniture. The
immense number of signal paths leads to signal distortion and, as a consequence, may cause inter
symbol interference. Another issue of concern is power efficiency. As a rule, diffuse configurations
are characterized by high signal path loss. Therefore, a receiver having a large effective collection
area and a wide FOV must be used. Nevertheless, diffuse links cannot compete with LOS links in
terms of power efficiency. The high optical signal path loss and the multipath distortion limit the
achievable transmission speed to a few tens of Mbps. On the other hand, while LOS links can easily
be blocked, diffuse links have the advantage of being very robust to shadowing and blockage.
Diffuse system is very well suited for point-to-multipoint connectivity and with it star, as well as
mesh networks can be established this architecture is referred to as multi spot diffusing (MSD).
Transmitter projects the light power in form of multiple narrow beams of equal intensity, over a
regular grid of small areas (spots) on a diffusely reflecting surface such as a ceiling. This way, the

81
signal power is uniformly distributed within the office and the link quality does not depend on the
receiver-transmitter distance. Each diffusing spot, in this arrangement, may be considered a
secondary light source having a Lambertian radiation pattern. Receiver consists of several narrow
FOV receiving elements aimed at different directions. A good portion of optical signal power is
received by each receiver branch via a finite number of distinct signal paths; a number equal to the
number of spots seen by the branch Like in LOS links, the latest development in quasi diffuse links
is the use of emitter [10] and detector arrays [44, 45, 46]. Utilization of a compact two-dimensional
array of semiconductor light sources allows for a reconfigurable transmitter output. Each light source
in the array is responsible for creating a single diffusing spot on the room ceiling, that is, the number
of sources equals the number of diffusing spots needed to cover the communication cell. If there is
no need for optical signal within certain parts of the communication cell, the corresponding light
sources are switched off. Thus, the system provides only the active users with signal and saves some
power by not distributing optical signal where it is not needed. With such a transmitter design,
independent communication channels (different information streams are launched through different
diffusing spots) are feasible, thus providing a means for spatial diversity the fundamental difference
in signal propagation environments in LOS and diffuse links determines the advantages and the
drawbacks of these link configurations. Despite all the efforts of a number of research groups over
the years, LOS links still have benefits that none of the proposed non-LOS. Thus, a receiver FOV
value of 30 would satisfy the requirements of both communication topologies channel. Then, an
optical encoder encodes the optical pulse and there would be an optical pulse code sequence within
the corresponding slot. The temporal sequence corresponding to each symbol is called one frame
whose length is represented byܶ௕. Each frame is divided into M slots and the length of each slot is
denoted by߬ = ܶ௕. Furthermore, each slot is composed of n chips and the time width of chip is
indicated by ܶ௖ = 5.181 ∗ 10ିଵ଻
‫,݀݊݋ܿ݁ݏ‬ where n corresponds to the code length of the optical
orthogonal code. Thus, there exists ܶ௕ = 3.33 ∗ 10ିଵ଴
sec, and power source is 35.99 ∗ 10ିଷ
‫ݐݐܽݓ‬
in silicon optical devices, and alsoܶ௕ = 2.58 ∗ 10ିଵଷ
‫,ܿ݁ݏ‬ and power source 214.8 ∗ 10ିଷ
‫ݐݐܽݓ‬ in
carbon nanotubes devices. For OOK modulation format, the slot length is equal to the length of a
frame. Assuming that both the chip time ܶ௖ = 5.181 ∗ 10ିଵ଻
‫ܿ݁ݏ‬ and throughput are held fixed.
Silicon optical devices the optical source power is 35.99 ∗ 10ିଷ
‫,ݐݐܽݓ‬ whereas when it uses carbon
nanotubes the optical source power is 214.8 ∗ 10ିଷ
‫.ݐݐܽݓ‬ That means when we use the carbon
nanotubes devices the consumed power is very low. Here, the time duration of time slot ܶ௕ = 3.33 ∗
10ିଵ଴
sec or nanosecond in silicon optical devices, but the time duration ܶ௕ = 2.58 ∗ 10ିଵଷ
‫ܿ݁ݏ‬ or
femtosecond in carbon nanotubes, therefore, the carbon nanotubes ultrafast switching based system,
are formulated to attain optimized performance of OCDMA technology. Parameters as indicated in
table 4 are assumed for achieving enhanced performance of carbon nanotubes based OCDMA in
comparison to silicon optical devices based devices which would in turn consume lesser power,
miniaturized in dimension and withstand higher temperature. The band gap energy for silicon optical
fiber ‫ܧ‬௢ 1.12ܸ݁ and the energy band gap for carbon nanotubes ‫ܧ‬௢ = 2.9ܸ݁ [10- 11], also the
refractive index for them like for silicon optical fiber (݊ଶ = 2 ∗ 10ିଶ଴
݉ଶ
/‫ݓ‬ ) and the refractive
index for carbon nanotubes (݊ଶ = 1.55 ∗ 10ି଼
݉ଶ
/‫.)ݓ‬Our simulation for coherent OCDMA used
OOK and BPSK formats with silicon optical devices and carbon nanotubes devices and also for
incoherent (noncoherent) OCDMA used OOK and PPM formats, as well as in a terrestrial optical
wireless system, the communication transceivers are typically located in the troposphere.
Troposphere is home to all kinds of weather phenomena and plays a very detrimental role for
FSO communications in low, medium, and high visibility range conditions mainly due to rain,
snow, fog and clouds. The estimated of fog, snow and rain attenuation effects using empirical
models.

82
Table 4: parameters assumed in simulated OCDMA system
Parameters Silicon optical devices Carbon nanotubes
Time duration (ܶ௕) 3.33 ∗ 10ିଵ଴
sec 2.58 ∗ 10ିଵଷ
sec
Data rate (ܴ௕) 3.00 ∗ 10ଽ
ܾ݅‫ܿ݁ݏ/ݏݐ‬ 7.7519 ∗ 10ଵଵ
ܾ݅‫ܿ݁ݏ/ݏݐ‬
System marginal loss, αm 3 dB 3 dB
Load resistance (RL) 50 ∗ 10ଷ
Ω 12.9 ∗ 10ଷ
Ω
Temperature for material 300K 973 K
Refractive index (݊ଶ) 2 ∗ 10ିଶ଴
݉ଶ
/‫ݐݐܽݓ‬ 1.55 ∗ 10ି଼
݉ଶ
/‫ݐݐܽݓ‬
Source power laser (ܲ௢௢௞) 35.99 ∗ 10ିଷ
‫ݐݐܽݓ‬ 214.8 ∗ 10ିଷ
‫ݐݐܽݓ‬
Recharge electron (e) 1.6 ∗ 10ିଵଽ
1.6 ∗ 10ିଵଽ
Light of speed 3 ∗ 10଼
3 ∗ 10଼
Transmitter lens diameter, Dt 100 ܿ݉ 100 ܿ݉
Boltzmann’s constant (‫ܭ‬௕) 1.38 ∗ 10ିଶଷ
‫ܭ/ܬ‬ 1.38 ∗ 10ିଶଷ
‫ܭ/ܬ‬
Area of devices ݉2.5 ∗ 10ିହ
݉2.5 ∗ 10ିହ
High visibility, Vhig 50 ≤Vhigh, km ≤80 50 ≤Vhigh, km ≤80
Medium visibility, Vmedium 6 ≤Vmedium, km ≤50 6 ≤Vmedium, km ≤50
Low visibility, Vlow 0 ≤ ܸ݈‫,ݓ݋‬ ݇݉ ≤ 0.5 0 ≤ ܸ݈‫,ݓ݋‬ ݇݉ ≤ 0.5
Receiver aperture diameter
(antenna size) ,Dr
50 ܿ݉ 50 ܿ݉
Band gap energy (‫ܧ‬௧) 1.12 ݁ ܸ 2.9 ݁ ܸ
Time chip (ܶ௖) 5.181 ∗ 10ିଵ଻
‫ܿ݁ݏ‬ 5.181 ∗ 10ିଵ଻
‫ܿ݁ݏ‬
Wavelength center (ߣ௖) 1550 ∗ 10ିଽ
݉ 1550 ∗ 10ିଽ
݉
System marginal loss, αm 3 dB 3 dB
Receiver noise figure, NF 5 dB 5 dB
Fade margin, Fm 20 dB 20 dB
Snow rate, S 0.2 mm/h 0.2 mm/h
Rain rate, R 1 mm/h 1 mm/h
RESULT AND DISCUSSION
The optical wireless communication (OWC) is general term for explaining wireless
communication with optical technology. Usually, includes infrared (IR) and light fidelity (Li - Fi) or
optical wireless fidelity (Wi - Fi) communication for short range and free space optics (FSO)
communication for longer range. The model have been deeply investigated to present the modulation
and code in Incoherent OCDMA as OOK and PPM formats, also in coherent OCDMA as OOK and
BPSK formats to improve performance system with carbon nanotubes (CNTs) (nano technique) and
to compare with silicon optical devices (micro technique) integrated devices. Here, also to present
the bad weather effects on the transmission performance (channel topology) and system operating
characteristics of optical wireless communication (OWC) for different visibility ranges over wide
range effecting parameters. In this paper, we have investigated the transmission analysis of OCDMA
in optical wireless communication system using silicon optical devices and carbon nanotubes
(CNTs) under the set of the wide range of operating parameters as shown in table 4. There are three
parameters very important in any communication systems such as Signal to noise ratio (SNR), Data
bit rate (R), and Bit error rate (BER).

83
• The Incoherent OCDMA in Optical Wireless Communication with Nonlinearity
In the incoherent approach to the CDMA, the original OOK- modulated signal is divided into
several parts and each part is delayed by the amount determined by the code used. The way the OOK
signal is divided depends on the particular realization of the incoherent OCDMA, and poor signal to
noise power (SNR, dB) as shown in figure 5 which illustrates the variation between the signal to
noise ratio and the data rate. When the SNR increases in OOK format, the data bit rate (R) increases
also and it also explains the results with carbon nanotubes curv2 to get Tb/s better than silicon
optical devices curv1 to get Gb/s. This shows the efficiency performance of OCDMA system with
carbon nanotubes curve 2 and silicon optical devices curve 1, expressing results by equation (6) and
equation (7) [18], [20].
ࡿࡺࡾ =
ࡺࣅ∗ ࡼ࢖ࢋࢇ࢑
࣎
૛ࢀ
∗ࡺࣅ∗ࡹ∗ࡳࣅቆ
࣓૛
૛ቀ∆ࢃࡻ
૛శ ∆ࢃ૛ቁ
ቇࡼ
࢖ࢋࢇ࢑శ ඨ∆ࢃ૛శ∆ࢃࡻ
૛
૛∆ࢃ∗∆ࢃࡻ
∗ ࡼࡺ
(6)
Where the number of users (M) are increasing, the signal to noise ratio is decreasing because
the carbon nanotubes has high energy band gap and high refractive index third nonlinearity, that
means the enhanced the nonlinearity in optical code division multiple access (OCDMA) andܲ௣௘௔௞the
optical power input for coherent OCDMA system, M is number of users of the system, d is distance
silicon or carbon for area integrated and others parameters mention in previous section in
assumption.ܰఒ is the carrier – hopping incoherent OCDMA system with wavelength that is the
original OOK signal is passed through a filter (e.g, prism or grating – based) that separates ܰఒ
components differently by their central wavelength, ∆ܹ the single channel spectral width ܹைis its
central frequency,߱ is the frequency spacing between different carriers.‫ܩ‬ఒ is the gain to the crosstalk
between channels equal (‫ܩ‬ఒ = 5 ݀‫.)ܤ‬
ࡾ =
૚
ࢀ
[૚ − ࢒࢕ࢍ૛
‫ۏ‬
‫ێ‬
‫ێ‬
‫ێ‬
‫ۍ‬
૚ + ࢋ࢞࢖
ష૛ࢀ
ࡹ∗࣎∗ ࡳࣅ
൮
൬
࣓
૛∆ࢃࡻ
൰
૛
૚శ ࢽ૛∗ࢊ૛∗ࡺ૛∗ ࡼ࢖ࢋࢇ࢑
૛(
૛
૜∗√૜
శࡺࣅ(ࡹష૚)ࡲ
ࣅ∗
࣎
ࢀ
൲
‫ے‬
‫ۑ‬
‫ۑ‬
‫ۑ‬
‫ې‬
] (7)
Figure 5: illustrated data rate (R) for OOK format incoherent OCDMA with silicon optical
devices and carbon nanotubes
The figure 6 illustrates that the bit error rate (BER) is decreasing when the SNR is increasing
this is given by equation (8) .The bit error rate in the system with carbon nanotubes is better than

84
with silicon optical devices as shown in figure 6. Here, the curve 2 is for carbon nanotubes, while the
curve 1 is for silicon optical devices [20].
࡮ࡱࡾ =
૚
࢒࢔૛
ࢋ࢞࢖(−
૛ࢀ
ࡹ∗࣎∗ࡳࣅ∗൮
൬
࣓
૛∆ࢃࡻ
൰
૛
૚శ ࢽ૛∗ ࢊ૛∗ ࡺ૛∗ ࡼ࢖ࢋࢇ࢑
૛(
૛
૜√૜
శࡺࣅ(ࡹష૚)ࡲࣅ∗
࣎
ࢀ
൲
) (8)
As to the OOK modulation manner, there are only two binary symbols and each symbol
corresponds to one data bit. When data bit is “1”, the optical encoder sends an optical pulse code
sequence to the network. Otherwise, when data bit is “0”, the optical encoder doesn’t send any
optical signal [47].
Figure 6: illustrated bit error rate (BER) for OOK format incoherent OCDMA with silicon
optical devices and carbon nanotubes
In PPM modulation format, the different symbol is expected by the distinct position where
the pulse locates, for example, the pulse at the first slot represents the first symbol; the pulse at the
second slot represents the second symbol, etc. Then, an optical encoder encodes the optical pulse and
there would be an optical pulse code sequence within the corresponding slot. The temporal sequence
corresponding to each symbol is called one frame whose length is represented byܶ௕. Each frame is
divided into M slots and the length of each slot is denoted by߬ = ܶ௕. Furthermore, each slot is
composed of n chips and the time width of chip is indicated by ܶ௖ = 5.181 ∗ 10ିଵ଻
‫,݀݊݋ܿ݁ݏ‬ where n
corresponds to the code length of the optical orthogonal code. Thus, there exists ܶ௕ = 3.33 ∗ 10ିଵ଴
sec, and power source is 35.99 ∗ 10ିଷ
‫ݐݐܽݓ‬ in silicon optical devices, and alsoܶ௕ = 2.58 ∗
10ିଵଷ
‫,ܿ݁ݏ‬ and power source 214.8 ∗ 10ିଷ
‫ݐݐܽݓ‬ in carbon nanotubes devices. It is aimed to improve
the performance of the incoherent OCDMA systems by OOK formats and PPM formats using carbon
nanotubes (CNTs), and silicon optical devices.
ࡿࡺࡾ =
૚
࢑ష૚
૜ࡺ
ା
ࢎ࢜
૝ࣁࡿ૛ࢀ
(9)
Where K is the number of simultaneous users, ܶ௕ is the signaling period “symbol interval “, n
is the code of length, where each user is assigned a set of N codes (code length), each corresponding
to a particular “ digit”. In the M-ary system with M=8 [17].

85
ࡾ =
ࡷ ࢒࢕ࢍ૛ࡹ
ࡹ∗࢔∗ࢀ࢈
(10)
The data rate with carbon nanotubes represented by curve 2 is better than silicon optical
devices as represents curve 1 as shown figure 7 expressed by equation (9) to get SNR and equation
(10) to get data bit rate (R).
Figure 7: represented data rate (R) for PPM format incoherent OCDMA with silicon optical
It is indicated that in the simulated system with the existing coding technique for
PPM/OCDMA system, the bit error rate increases much more with silicon optical devices but the bit
error rate is very low with carbon nanotubes used ߬ = ܶ௕ = 2.58 ∗ 10ିଷ
‫,ܿ݁ݏ‬ source power of laser
ܲைை௄ = 214.8 ∗ 10ିଷ
‫,ݐݐܽݓ‬ and refractive index ݊ଶ = 1.55 ∗ 10ି଼
݉ଶ
/‫ݐݐܽݓ‬ from table 1 [1],[5],[6].
Bit error rate in PPM/ OCDMA format is given by equation (11)
࡮ࡱࡾ =
૚
૛
ቂ∑
࢔!
࢓!(࢔ି࢓)!
ቄ
ࡱ࢚ࡺࣅ࣎
૛∗ࢀࢉ
ቅ
࢓
ቄ૚ −
ࡱ࢚ࡺࣅ࣎
૛∗ࢀࢉ
ቅ
࢔ି࢓
ࡹ
࢓ୀ ࡺࣅ
ቃ
ࡱ࢚
(11)
Let M is the number of simultaneous users and ߬ is the single pulse width used in silicon
optical devices and carbon nanotubes, ܰఒ is code with ܰఒ =8 different wavelength channel and
different values of ‫ܧ‬௧ in silicon optical devices is ‫ܧ‬௧ =1.12 e V, and carbon nanotubes is ‫ܧ‬௧ =2.9 e
V. Then, as a function of number of users, the bit error rate (BER) performance codes C is affected
by the multiple access interference (MAI).The multiple access interference affects the incoherent
OCDAM system. The bit error rate (BER) increases marginally with carbon nanotube as represented
by curv2 compared with silicon optical devices represented curv1, as shown in figure 8. Therefore,
we can say that the Data Rate (R) in incoherent OCDMA with OOK/OCDMA format gives better
results than PPM/OCDMA with carbon nanotubes (CNTs). In the OCDMA system increasing the
signal to noise ratio increases the data rate, while decreasing the bit error rate enhances the system
performance. For the best performance of optical communication with highest data rate and lowest
bit error rate, we investigated the optimized OCDMA performance with carbon nanotubes in
comparison with silicon optical devices.

86
Figure 8: represented bit error rate (BER) for PPM format incoherent OCDMA with silicon
• The Coherent OCDMA in Optical Wireless Communication with Nonlinearity
In the coherent approach to optical CDMA, the information is first encoded in pulse train
using standard OOK, for both silicon optical devices and carbon nanotubes. Here, we get improved
results with parameter signal to noise ratio (SNR) using carbon nanotubes than silicon optical
devices as given by equation (12). When the signal to noise ratio are increasing, the data bit rate is
increasing because the carbon nanotubes has high energy band gap and high refractive index third
nonlinearity, that means the enhancement in the nonlinearity properties in optical code division
multiple access (OCDMA) bought these result by equation (12) and equation (13) as shown
figure 9.
ࡿࡺࡾ =
ࡼ࢖ࢋࢇ࢑
ࡼࡺା ࡼ࢖ࢋࢇ࢑(ࡹି૚)∗
࣎
૛ࢀ
∗
૚
૚ାࢽ૛∗ࢊ૛∗ࡺ૛∗ࡼ࢖ࢋࢇ࢑
૛
∗ (ࡹି૚)૛ቀ
࣎
૛ࢀ
ቁ
૛ (12)
Where ܲ௣௘௔௞the optical power input for coherent OCDMA system, M is number of users of the
system, d is distance silicon or carbon for area integrated and others parameters mention in previous
section in assumption.
ܴ =
ଵ
்
[1 − ݈‫݃݋‬ଶ
‫ۏ‬
‫ێ‬
‫ێ‬
‫ۍ‬
1 + ݁‫݌ݔ‬
ቌି
ೄಿೃೀ
భశ ംమ∗ ೏మ∗ಿమ∗ ು೛೐ೌೖ
మ(ಾషభ)మ∗ቀ
ഓ
మ೅
ቁ
మቍ
‫ے‬
‫ۑ‬
‫ۑ‬
‫ې‬
] (13)
First, we substitute the optical wireless system parameters in equation (12) and equation (13)
and get the result as in figure 9 curve 1, increasing data rate (R) along with the SNR in the system,
subsequently, we substitute the carbon nanotubes parameters in same equation to get result as shown
figure 9 curve 2. For improved system, we need to improve SNR values and it is observed that the
data rate values with carbon nanotubes are better than silicon optical devices.

87
Figure 9: illustrated data rate (R) for OOK format coherent OCDMA with silicon optical
Figure 10: illustrated bit error rate (BER) for OOK format coherent OCDMA with silicon
The figure 10 illustrates the variation in bit error rate (BER) in the same system and show
that for carbon nanotubes this decreasing from 10ିଵଶ
‫01 ݋ݐ‬ିଵଵ
while for the silicon optical devices
BER varies from 10ିଵଵ
‫01 ݋ݐ‬ିଵ଴
, these making the improvement in system performance governed
by equation 14. This indicates the effect of SNR to improved coherent system. The encoder
incorporating silicon optical devices makes the light spreads by lens but while using carbon
nanotubes ( single walled carbon nanotubes ) (SWNTs) light spreading is narrowed down, the light
focuses on one point on the filter nanotubes that is observed to be the most active in applications of
passive optical CDMA network.
࡮ࡱࡾ =
૚
࢒࢔૛
ࢋ࢞࢖
(ି
ࡿࡺࡾ࢕
૚శ ࢽ૛∗ࢊ૛∗ࡺ૛∗ ࡼ࢖ࢋࢇ࢑
૛∗(ࡹష૚)૛∗ ቀ
࣎
૛ࢀ
ቁ
૛)
(14)
Although an on-off keying (OOK) intensity modulated based FSO link is widely reported, its
major challenge lies in the fact that it requires adaptive threshold to perform optimally in

88
atmospheric turbulence condition. Subcarrier intensity modulation (SIM) based on a binary phase
shift keying (BPSK) scheme in a clear but turbulent atmosphère is presented. Here, we indicated
BPSK coherent OCDMA to obtain result as shown figure 11 the data rate is increasing when the
signal noise ratio is increasing as given by equation (15) and equation (16), the band width in BPSK
equal twice the bandwidth in PPM.The BPSK coherent OCDMA ranging data rate better than OOK
coherent OCDMA overcomes the turbulence atmosphere. The resulting data rate as shown in figure
11 indicating the data rate with carbon nanotubes represented by curv2, better than silicon optical
devices represented by curv1. The data bit rate (R) is increasing when the signal to noise ratio (SNR)
is increasing, so we observe improved performance for this system bringing improved signal to
noise ratio resulting is reduction of consumption of the optical power energy in these applications.
ࡿࡺࡾ =
૚
࢑ష૚
૜ࡺ
ା
ࢎ࢜
૝ࣁࡿ૛ࢀࢉ࢕࢙૝૙.૙ૡ࣊
(15)
ࡾ = ૛ ∗
ࡷ ࢒࢕ࢍ૛ࡹ
ࡹ∗࢔∗ࢀ࢈
(16)
࡮ࡱࡾ =
૚
૛
ࢋ࢞࢖(−
ࡼ࢖ࢋࢇ࢑
ࡼࡺ
) (17)
Where ܲ௣௘௔௞is the optical power input for coherent OCDMA system, and the average noise
powerܲே = 0.1 ∗ 10ିଷ
, the band gap energy for silicon optical fiber ‫ܧ‬௢ 1.12ܸ݁ and the energy band
gap for carbon nanotubes ‫ܧ‬௢ = 2.9ܸ݁ [18- 20], also the refractive index for silicon optical
devices(݊ଶ = 2 ∗ 10ିଶ଴
݉ଶ
/‫)ݓ‬ and the refractive index for carbon nanotubes (݊ଶ = 1.55 ∗
10ି଼
݉ଶ
/‫.)ݓ‬Fourier Transform from the frequency domain to time domain with silicon optical
devices equal to optical power output ( P(t)= 35.99 ∗ 10ିଷ
݉‫) ݓ‬ as the figure 7 , while the carbon
nanotubes optical power output ( P(t)= 214.8 ∗ 10ିଷ
݉‫.) ݓ‬ The figure 10 illustrates bit error rate
(BER) in the same system for carbon nanotubes increasing from 10ିଵସ
‫01 ݋ݐ‬ିଵଶ
while the silicon
optical devices BER from 10ିଵଶ
‫01 ݋ݐ‬ିଵ଴
, to make the improvement in system performance
governed by equation 17.
Figure 11: observed data rate (R) for BPSK format coherent OCDMA with silicon optical

89
Figure 12: represented bit error rate (BER) for BPSK format coherent OCDMA with silicon
The three parameters are used for quality communication systems: transmission reliability,
bandwidth efficiency, and power efficiency. We define the power efficiency as the number of lumen
the light source produces per watt. Light sources need to be regulated in terms of eye safety.
Transmission reliability, bit error rate is critical to the performance of a communication. Optical
energy is in the transmitted optical power must be large enough to provide adequate amount of
received optical power at the receiver’s location, so as to sustain reliable operation of the
communication system that is operating under the optical channel impairments and ambient noise
Bandwidth efficiency. Although there is plenty of spectrum available at optical frequencies, several
constituents of the communication system (e.g. the capacitance introduced by the photocurrent
sensitive area, which increases with the size of the area, occurrence of multipath in the channel)
limit the usable bandwidth that can support distortion-free communication [1], [26]. Also, the
ensuing multipath propagation in diffuse link/non-directed LOS limits the available channel
bandwidth system and impacts the behaviour of overlaying protocols and applications. Similar to the
OOK optical pulses, BFSK optical pulses also suffer from channel loss when passing through the
multipath channel.
• Factors affecting the terrestrial optical wireless communications systems
Several problems arise in optical wireless communications because of the wavelengths used
in this type of system. The main processes affecting the propagation in the atmosphere of the optical
signals are absorption, dispersion and refractive index variations. The latter is known as atmospheric
turbulence. The absorption due to water vapor in addition with scattering caused by small particles or
droplets or water (fog) reduces the optical power of the information signal impinging on the receiver.
Because of the above mentioned degradation factors, this type of communications system is
susceptible to the weather conditions prevailing in its operating environment, the disturbances
affecting the optical signal propagation through the atmosphere. Fog is the weather phenomenon that
has the more destructive effect over OWC systems due to the size of the drops similar to the optical
wavelengths used for communications links. Dispersion is the dominant loss mechanism for the fog.
Taking into account to the effect overthe visibility parameter OWC communications in lower
visibility range conditions mainly due to rain, snow, fog and clouds. The estimated fog, snow
and rain attenuation effects using empirical OWC model for fog attenuation is given by equation
(18), [48].

90
ࢻࢌ࢕ࢍ(ࣅ) =
૜.ૢ૚૛
ࢂ
ቀ
ࣅ
૞૞∗࢞∗૚૙ష૝
ቁ
ିࢗ
(18)
Where V is visibility range in km, λ is transmission wavelength in nm. ߙ௙௢௚(λ) is the total
extinction coefficient and q is the size distribution coefficient of scattering related to size
distribution of the droplets. In case of clear or foggy weather with no rain or snow, approximations
of the q parameter to compute the fog attenuation, that are very accurate for the narrow wavelength
range between 1300–1650 nm.
ࢗ = ቐ
૚. ૟ (ࢂ ≥ ૞૙࢑࢓)
૚. ૜ (૟࢑࢓ ≤ ࢂ ≤ ૞૙࢑࢓)
૙(ࢂ ≤ ૙. ૞࢑࢓)
(19)
Transmitted optical pulses in free space are mainly influenced by two main mechanisms of
signal power loss, absorption and scattering. Absorption is mainly due to water vapours and carbon
dioxide, and depends on the water vapour content that is dependent on the altitude and humidity. By
appropriate selection of optical wavelengths for transmission the losses due to absorption can be
minimized. It was found that scattering (especially Mie scattering) is the main mechanism of optical
power loss as the optical beam looses intensity and distance due to scattering. The beam loss due to
scattering canbe calculated from the following empirical, visibility range dependent formula (20),
[49].
ࢻ࢙ࢉࢇ࢚(ࣅ) =
૚ૠ
ࢂ
ቀ
૞૞૙
ࣅ
ቁ
૙.૚ૢ૞࢜
dB/km (20)
Where V is visibility range in km, λ is transmission wavelength in nm. Then, the total
attenuation of wireless medium communication system can be estimated as
ࢻ = ࢻࢌ࢕ࢍ(ࣅ) + ࢻ࢙࢔࢕࢝ + ࢻ࢘ࢇ࢏࢔ + ࢻ࢙ࢉࢇ࢚(ࣅ) (21)
When the optical signal passes through the atmosphere, it is randomly attenuated by fog and
rain. Although fog is the main attenuation factor for optical wireless links, the rain attenuation effect
cannot be ignored, in particular in environments where rain is more frequent than fog. As the size of
water droplets of rain increases, they become large enough to cause reflection and refraction
processes. These droplets cause wavelength independent scattering [49]. It was found that the
resulting attenuation increases linearly with rainfall rate; furthermore the mean of the raindrops size
is in the order of a few millimeters and it increases with the rainfall rate [50]. Let R be the rain rate
in mm/h, the specific attenuation of wireless optical link is given by equation (22), [51].
ࢻ࢘ࢇ࢏࢔ = ૚. ૙ૠ૟ࡾ૙.૟ૠ
(22)
If S is the snow rate in mm/h then specific attenuation in dB/km is given by equation
(23), [52,53]
ࢻ࢙࢔࢕࢝ = ࢇࡿ࢈
(23)

91
If λ is the wavelength, the parameters a and b for dry snow are given as the following
ࢇ = ૞. ૝૛࢞૚૙ି૝
ૃ + ૞. ૝ૢ૞ૡૠ૟, ‫܊‬ = ૚. ૜ૡ
The parameters a and b for wet snow are as follows, [54, 55]
ࢇ = ૚. ૙૛૜࢞૚૙ି૝
ૃ + ૜. ૠૡ૞૞૝૟૟, ‫܊‬ = ૙. ૠ૛
In order to estimate the coverage at millimeter wavelengths under direct Line of Sight
(LOS) conditions, the free space propagation model is used. The SNR dB requirements fo
rmodulation scheme at a fixed data rate of one Gbit/sec is obtained by silicon optical devices
and by carbon nanotubes the data rate of few Tbit/sec from the following formula (24), [56].
ࡿࡺࡾ = ࡼࢀ − ૜૙ + ࡳࢀ + ࡳࡾ − ૛૙ ‫࢑(܏ܗܔ‬࡮࡮ࢃࢀ) − ࢻ − ࡺࡲ − ࡲ࢓ (24)
Where ்ܲ is transmitter power, ‫ܩ‬் is the transmitter antenna gain, ‫ܩ‬ோ is the receiver antenna
gain, ߣ௖is the carrier wavelength, ݇஻is the Boltzmann’s constant (1.38*10ିଶଷ
‫,)ܭ/ܬ‬ Receiver
bandwidth (B.W=1MHz), Tis the ambient temperature in K, , Receiver Noise Figure, ‫ܨ‬௠ is the
Fade margin, and αis the total attenuation in dB/km. The maximum propagation distance (L) for
meeting the SNR requirements to formula (25), [57].
ࡸ = ૚૙ࢻ/૛૙
(25)
The transmitter and receiver antenna gains can be expressed as the following as equation (26)
and (27)
ࡳࢀ =
૜૛
ࣂࢊ࢏࢜
૛ (26)
ࡳࡾ = ቀ
࣊ࡰ࢘
ࣅ
ቁ
૛
(27)
Where ߠௗ௜௩ is the transmitter divergence of the beam in radians can be expressed as follows
formula (28)
ࣂࢊ࢏࢜ =
૝ࣅ
࣊ࡰ࢚
(28)
The basic formula for a typical optical link is an exponential decaying function as function
of the path length L as the following expression formula (29), [58, 59]
ࡼࡾ = ࡼࢀ
ࡰ࢘
(ࡰ࢚ା(ࣂࢊ࢏࢜ାࡸ)૛ ∗ ࢋିࢻࡸ
(29)
Where ܲோ is the received power after traveling the path length L through the lossy medium,
்ܲis the initial transmitted power, and αis the total attenuation coefficient of the medium. The bit
error rate (BER) essentially specifies the average probability of incorrect bit identification. In
general. The higher the received SNR, the lower the BER probability will be for most PIN receivers,
the noise is generally thermally limited, which independent of signal current. The bit error rate
(BER) is related to the signal to noise ratio (SNR) as follows formula (30), [60,61]

92
࡮ࡱࡾ = ቀ
૛
࣊∗ࡿࡺࡾ
ቁ ∗ ࢋ࢞࢖ ቀ
ିࡿࡺࡾ
ૡ
ቁ (30)
The maximum transmission bit rate or data bit rate (Rmax).which is a losses limited one,
and is given by equation (31), [62]
ࡾ࢓ࢇ࢞ = ࡾ ∗ ࢋ࢞࢖(−(ࢻࡸ) + ࢻ࢓) (31)
Where ܴ the maximum available transmission bit rate without any limitations, and ߙ௠ is the
system marginal loss. The optical wireless communication (OWC) for the bad weather effects on the
transmission performance and system operation characteristics of wireless optical communication
systems for different visibility ranges over wider range of the affecting parameters. Here, we
computed the signal to noise ratio (SNR), the data bit rate (R), the bit error rate (BER),Maximum
propagation distance, and Received signal power with low, medium, and high visibility which affects
on the performance of optical wireless communication (OWC) with used carbon nanotubes devices
and silicon optical devices depended on parameters from table 4.
Figure 13 has indicated that signal to noise ratio (SNR) marginally increasing through used
low visibility because of the optical wireless communication systems effect by bad weather as dense
fog, rain, and snow as well as scattering. On the other hand, the result obtained for SNR with carbon
nanotubes represented by curv2 better than silicon optical devices represented curv1. Figure 14 has
represented that the moderate increase in SNR in resulting the improved performance of OWC
systems with medium visibility range. The moderate fog, rain, and snow affect the light signal
between transmitter and receiver. It is also observed that carbon nanotubes represented by curv2
gives higher increased SNR compared to silicon optical devices. The SNR is observed to be higher
with medium visibility than with low visibility. Figure 15 has illustrated that the SNR has
represented the highest increase with high visibility compared to both medium and low visibility to
improve performance OWC. The performance of system is better with carbon nanotubes devices
than silicon optical devices as given by equations 18,19,20,21,22,23,24,25,26,27, and 28.
Figure 13: observed the Signal to noise ratio in relation to low visibility for OWC with
silicon optical devices and carbon nanotubes

93
Figure 14: illustrated the Signal to noise ratio in relation to medium visibility for OWC
with silicon optical devices and carbon nanotubes
Figure 15: represented the Signal to noise ratio in relation to high visibility for OWC
Figure 16 has presented that transmission bit rate or data bit rate (R) slowly increases through
low visibility because of the optical wireless communication systems get effected by bad weather, as
dense fog, rain, and snow as well as scattering. On the other hand, the result obtained for data bit rate
(R) with carbon nanotubes represented by curv2 provides better performance results than silicon
optical devices represented curv1. Figure 17 represent the data bit rate (R) moderate increase bring in
the improved performance OWC of systems with medium visibility range. The moderate fog, rain,
and snow affect the light signal between transmitter and receiver. It is also observed that carbon
nanotubes represented by curv2 provides higher increased data bit rate (R) compared to silicon
optical devices. The data bit rate (R) is higher with medium visibility than low visibility. Figure 18
has illustrated that the SNR represents the highest increase with high visibility compared to both
medium and low visibility to improve of performance OWC. The performance system is better with
carbon nanotubes devices data rate to get Tbit/sec than silicon optical devices with Gbit/sec is given
by equations 18, 19, 20, 21, 22, 23, 24, 25, 26, 27 and 31.

94
Figure 16: explained the data rate in relation to low visibility for OWC with silicon optical
Figure 17: observed the data rate in relation to medium visibility for OWC with silicon optical
Figure 18: observed the data rate in relation to high visibility for OWC with silicon optical

95
Figure 19 has indicated that bit error rate (BER) shows the highest increase through used low
visibility because of the optical wireless communication systems get effected by bad weather such as
dense fog, rain, and snow as well as scattering. On the other hand, the result obtained for BER with
carbon nanotubes represented by curv2 provides better performance than silicon optical devices
represented curv1. Figure 20 shows that the BER moderate increasing in the performance OWC
systems with medium visibility range. The moderate fog, rain, and snow affect the light signal
between transmitter and receiver. It is also observed that carbon nanotubes represented curv2 higher
increased BER compared to silicon optical devices. The BER is lower with medium visibility than
with low visibility. Figure 21 has illustrated that the BER has the lowest increase with high visibility
compared to both medium and low visibility, improving the performance of OWC. The performance
of system is better with carbon nanotubes devices than silicon optical devices as given by equations
18, 19, 20, 21, 22, 23, 24, 25, 26, 27 and 30.
Figure 19: illustrated the bit error rate in relation to low visibility for OWC with silicon optical
Figure 20: represented the bit error rate in relation to medium visibility for OWC with silicon

96
Figure 21: explained the bit error rate in relation to high visibility for OWC with silicon
Figure 22 has indicated that maximum propagation distance highest decreasing when we used
low visibility ranges because of the optical wireless communication systems effect by bad weather as
dense fog, rain, and snow as well as scattering. On the other hand, the result obtained for maximum
propagation distance with carbon nanotubes represented by curv2 provides better results than silicon
optical devices represented curv1. Figure 23 shows that with the maximum propagation distance
there is moderate decreasing in the performance of OWC systems with medium visibility range. The
moderate fog, rain, and snow affect the light signal between transmitter and receiver. It is also
observed that carbon nanotubes represented curv2 higher increased maximum propagation distance
compared to silicon optical devices. The maximum propagation distance is with medium visibility
higher than low visibility. Figure 24 shows the maximum propagation distance has represented the
lowest increased with high visibility compared to both medium and low visibility to improve
performance OWC. The performance of system is better with carbon nanotubes devices than silicon
optical devices as expressed by equations 18,19,20,21,22,23,24, and 25.
Figure 22: observed the Maximum propagation distance in relation to low visibility for OWC

97
Figure 23: represented the Maximum propagation distance in relation to medium visibility for
OWC with silicon optical devices and carbon nanotubes
Figure 24: illustrated the Maximum propagation distance in relation to high visibility for
OWC with silicon optical devices and carbon nanotubes
Figure 25 has indicated that Received signal power is marginally increasing with the used
low visibility because of the optical wireless communication systems gets effected by bad weather as
dense fog, rain, and snow as well as scattering. On the other hand, the result obtained for Received
signal power with carbon nanotubes represented by curv2 is better than silicon optical devices
represented curv1. Figure 26 has represented that the Received signal power is marginally increasing
in OWC systems with medium visibility range. The moderate fog, rain, and snow affect the light
signal between transmitter and receiver. It is also observed that carbon nanotubes represented by
curv2 provides higher Received signal power compared to silicon optical devices. The Received
signal power with medium visibility is higher than low visibility. Figure 27 has illustrated the
Received signal power has represented the highest increased with high visibility compared to both
medium and low visibility resulting to improved performing OWC. The performance of system is
better with carbon nanotubes devices than silicon optical devices as given by equations 18, 19, 20,
21, 22, 23, 24, 25, 26, 27 and 29.

98
Figure 25: explained the Received signal power in relation to low visibility for OWC with
Figure 26: represented the Received signal power in relation to medium visibility for OWC
Figure 27: observed the Received signal power in relation to high visibility for OWC with

99
CONCLUSION
Optimized design performance of OCDMA with carbon nanotubes (CNTs) based devices
have been observed providing highest data rate (R) and lowest bit error rate (BER) incorporating
techniques like OOK/OCDMA and PPM/OCDMA in Coherent as well as Incoherent OCDMA
system. Firstly, the incorporation of carbon nanotube, based devices result in improved system
performance in comparison to silicon optical devices, with increased data rate upto Tb/s and much
reduced bit error rate between 10ିଵଷ
‫01 ݋ݐ‬ିଵ଴
bits/sec. This has brought in considerable saving in
energy of sources in transmitter side, besides bring in better sensitivity of photodetectors in receiver
circuit due to reduced effect of total noise on the system. Secondly, it has been observed that number
of users would be increased by increase in code length and decrease in code weight. Next generation
of optical communication system may preferably incorporate carbon nanotubes based devices so as
to achieve much higher data rate up to Tb/s in comparison to present systems using silicon optical
devices giving data rate upto Gb/s. Besides, such systems with reduced energy source power realize
in much longer life Nevertheless, future requirements of ultrahigh speed internet, video, multimedia,
and advanced digital services, would suitably be met with incorporation of carbon nanotubes based
devices providing optimal performance. Three important parameters are very important for any
communication system, named signal to noise ratio SNR, Data rate R, and Bit error rate BER. In
optical code division multiple access (OCDMA) utilizing the nonlinear properties of materials used
in silicon optical devices and carbon nanotubes, bring in improvement in the system performance.
Maximum propagation distance, received signal power, signal to noise ratio, bit error rate, and
transmission rates for different visibility ranges are the major interesting design parameters as a
measurement of the system performance under different optical transmission systems As well as
optical wireless communication systems have presented the highest received signal power,
signal to noise ratio, transmission bit rates, and the lowest propagation distance and bit error rate for
different visibility ranges at carbon nanotubes compared to silicon optical devices.The Optical
Wireless Communications (OWC) is a type of communications system that uses the atmosphere as a
communications channel. The OWC systems are attractive to provide broadband services due to their
inherent wide bandwidth, easy deployment and no license requirement. The idea to employ the
atmosphere as transmission media arises from the invention of the laser. The visible light
communication (VLC) based on Li-Fi (Light Fidelity)-The future technology in optical wireless
communication refers to the communication technology which utilizes the visible light source as a
signal transmitter, the air as a transmission medium, and the appropriate photodiode as a signal
receiving component.
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AUTHOR’S BIBLIOGRAPHY
Jafaar Fahad A.Rida Received his bachelor of Electronic and Communication
Engineering Technical Najaf Collage Iraq in 2003. He obtained M.Tech.
Communication System Engineering from SHIATS Allahabad India in 2012. He
is Pursing Ph.D in Communication System Engineering in Depart ment of
Electronics and Communication Engineering in SHIATS, Allahabad. He has
experience for five years with CDMA technical company and MW System. He
has published several research papers in the field of Optical Systems
Communication and Carbon Nanotubes Engineering.
Dr. A.K. Bhardwaj Allahabad, 16.01.1965, Received his Bachelor of
Engineering degree from JMI New Delhi in 1998; He obtained his M.Tech.
degree in Energy and Env. Mgt. from IITNew Delhi in 2005. He completed his
Ph.D in Electrical Engg. From SHIATS (Formerly Allahabad Agriculture
Institute, Allahabad- India) in 2010. He has published several research paper in
the field of Electrical Engineering. Presently heis working as Associate Professor
and HOD in Electrical Engg. Department, SSET, SHIATS Allahabad- India.
Prof. A. K. Jaiswal, Is working as Professor and HOD of the department of
Electronic and Communication in Shepherd school Engineering and Technology
of SHIATS, Allahabad, India. His area of working is optical fiber communication
system and visited Germany, Finland for exploration of the system designing. He
has more than 35 years experience in related fields. He was recipient of national
award also developing electronics instruments.

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