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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 WIRELESSCOMMUNICATION (OWC) FOCUSING ON LIGHT

FIDELITY (LI-FI) USING OPTICAL CODE DIVISIONMULTIPLE ACCESS (OCDMA) BASED ON CARBON

NANOTUBES (CNTS)

Jafaar Fahad A. Rida1, A. K. Bhardwaj

2, A. K. Jaiswal

3

1Dept. of Electronics and Communication Engineering, SHIATS -DU, Allahabad, India.2Dept. of Electrical and Electronics Engineering, SHIATS - DU, Allahabad, India,

3Dept. 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 datarate 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 ENGINEERINGAND 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

Journal Impact Factor (2014): 7.8273 (Calculated by GISI)

www.jifactor.com

IJARET

I A E M E


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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]. Theidea 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 possibleoptions 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 satellitecommunication 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 orspacecraft 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 thechiral vector, carbon nanotubes behave as semiconductor or metal. But here focuses on


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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 informationtechnology. 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 ()is made up of layers of carbon atoms arranged in a hexagonal lattice like chicken wire, which itself is

very strong [12-16]. But lets 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 vapordeposition (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

users 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 users 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 = 2 10 . Optical code division multiple access withcarbon nanotubes having band gap 2.9 eV and the refractive index = 1.55 10 , brought in theimproved 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 throughan optical signature code in order to allow the selection of a desired signal so that different users can


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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 preferablyincorporate 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 persecond 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 ofthis scintillation phenomenon, the optical wireless channel distance and the capacity are reduced

[26].

Figure 1: General block diagram of optical wireless communication system.


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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 forwavelength 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 isstrong 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 lengthsand maximum transmission bit rates under these bad operating conditions.


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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 (

).

= + (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* and m*in the hexagonal honeycomblattice contained in this vector and where || =|| = || =3 * = 0.246nm,where = 0.142nm the c-c bond length and are graphite lattice vector ,which two vectorsreal space vectors [13], [14], [31], [30]. The chiral vector makes an angle () called the chiral anglewith 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

= 0, while

the armchair nanotube axis corresponds to = 30, and the chiral nanotube axis corresponds to0 30. The seamless cylinder joint of the nanotube is made by joining the line AB to theparallel line OB in figure 2, in terms of the integer (n, m), the nanotube diameter ( ) is given byequation (2).

= | | (2)The nearest neighbor C-C distance 1.421 or 0.142 in graphite, is the length of the chiral

vector and the chiral angle (

) is given by equation (3)

= ( ) (3)Thus, a nanotube can be specified by either its (n, m) indices or equivalent by and [16].

Figure 2: explanation of synthesis of carbon nanotubes from graphite sheet

The information capacity Cis 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) (1010), to use optical fiber for longest distanceincorporating silicon based integrated circuit does not support its work enough to transmission rate


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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 cablebut 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 1.12 and the energy band gapfor carbon nanotubes

= 2.9[10- 11], also the refractive index for them like for silicon optical

fiber

( = 2 10 /) and the refractive index for carbon nanotubes

( = 1.55 10 /) [18], [19],. Since the chip level receiver are dependent on the number of photons (opticalenergy) per chip in the received frame when it uses silicon optical devices the optical source power is35.9910, whereas when it uses carbon nanotubes the optical source power is 214.810. That means when we use the carbon nanotubes devices the consumed power is very low.Here, the time duration of time slot = 3.33 10 sec or nanosecond in silicon optical devices,but the time duration = 2.58 10 or femtosecond in carbon nanotubes, therefore, thecarbon nanotubes based ultrafast switching system, are formulated to attain optimized performance

of OCDMA technology.

Optical and optoelectrnic components

Devices such as the laser diodes, high-speed photo-receivers, optical amplifiers, opticalmodulators 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 propagatesthrough 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 theincoming radiation and focuses it onto filter. The filter removes background radiation and allows


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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 DiodesIn 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 ofsemiconducting 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 900InGaAs 1000 1300

InGaAsP 900 1700

CNTs 700 - 2000

Laser DiodesThe 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 stilldepend 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


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PhotodetectorsAt 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 photodiodeThis 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

= (4)Where is the quantum efficiency, e is the electron charge (1.610C), h is Planck's

constant (6.6210J) and is the frequency corresponding to the photon wavelength. InGaAs PINdiodes 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 photodiodeThis type of device is ideal for detecting extremely low light level. This effect is reflected in

the gain M:

= (5) is the value of the amplified output current due to avalanche effect and Ipis the currentwithout 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


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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 inthe 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 amplifiersBasically 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-dopedfiber 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 antennasThe 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 beamcharacteristics 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

= (6)

is the foot print diameter on the receiver plane in meters, is the divergence angle in

radians and L is the separation distance between transmitter and receiver (meters). The aboveapproximation 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 1990s in countries like Germany, Korea, and Japan where discovered LEDs 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 LEDs is on then the logic 1 can be transmitted and if the LEDs 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.


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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 developmentof 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 servemultiple 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 ofthe source in the array. As a result, the communication cell is split into microcells, each illuminated


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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 transmissionbandwidth 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 ofsquare 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 roomsurfaces, 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 aregular grid of small areas (spots) on a diffusely reflecting surface such as a ceiling. This way, the


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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 thenumber 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 thedrawbacks 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 by. Each frame is divided into M slots and the length of each slot isdenoted by = . Furthermore, each slot is composed of n chips and the time width of chip isindicated by = 5.181 10 , where n corresponds to the code length of the opticalorthogonal code. Thus, there exists

= 3.33 10sec, and power source is

35.9910

in silicon optical devices, and also = 2.58 10 , and power source 214.810 incarbon nanotubes devices. For OOK modulation format, the slot length is equal to the length of aframe. Assuming that both the chip time = 5.181 10 and throughput are held fixed.Silicon optical devices the optical source power is 35.9910, whereas when it uses carbonnanotubes the optical source power is 214.810. That means when we use the carbonnanotubes devices the consumed power is very low. Here, the time duration of time slot = 3.33 10sec or nanosecond in silicon optical devices, but the time duration = 2.58 10 orfemtosecond 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 opticalfiber 1.12 and the energy band gap for carbon nanotubes = 2.9 [10- 11], also therefractive index for them like for silicon optical fiber ( = 2 10 / ) and the refractiveindex for carbon nanotubes ( = 1.55 10 /).Our simulation for coherent OCDMA usedOOK 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.


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Table 4: parameters assumed in simulated OCDMA system

Parameters Silicon optical devices Carbon nanotubes

Time duration (

)

3.3310sec

2.5810sec

Data rate () 3.0010 / 7.751910/System marginal loss, m 3 dB 3 dBLoad resistance (RL) 5 0 1 0 12.910Temperature for material 300K 973 K

Refractive index () 2 1 0/ 1.5510/Source power laser () 35.9910 214.810Recharge electron (e) 1.610 1.610Light of speed 3 1 0 3 1 0Transmitter lens diameter, Dt 100 100 Boltzmanns constant

()

1.3810

/

1.3810

/

Area of devices 2.510 2.510High visibility, Vhig 50 Vhigh, km 80 50 Vhigh, km 80Medium visibility, Vmedium 6 Vmedium, km 50 6 Vmedium, km 50

Low visibility, Vlow 0 , 0.5 0 , 0.5Receiver aperture diameter

(antenna size) ,Dr50 50

Band gap energy () 1.12 2.9 Time chip () 5.18110 5.18110Wavelength center () 155010 155010System marginal loss, m 3 dB 3 dB

Receiver noise figure, NF 5 dB 5 dBFade 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).


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The Incoherent OCDMA in Optical Wireless Communication with NonlinearityIn 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 tonoise 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].

= (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) andtheoptical 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. is the carrier hopping incoherent OCDMA system with wavelength that is theoriginal OOK signal is passed through a filter (e.g, prism or grating based) that separates components differently by their central wavelength, the single channel spectral width is itscentral frequency,

is the frequency spacing between different carriers.

is the gain to the crosstalk

between channels equal

( = 5 ).

= [ +

( ()

] (7)

Figure 5: illustrated data rate (R) for OOK format incoherent OCDMA with silicon opticaldevices 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


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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].

= ( ( () ) (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 doesnt send any

optical signal [47].

Figure 6: illustrated bit error rate (BER) for OOK format incoherent OCDMA with siliconoptical 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 by. Each frame isdivided into M slots and the length of each slot is denoted by = . Furthermore, each slot iscomposed of n chips and the time width of chip is indicated by

= 5.181 10

, where n

corresponds to the code length of the optical orthogonal code. Thus, there exists

= 3.33 10

sec, and power source is 35.9910 in silicon optical devices, and also = 2.58 10 , and power source 214.810in carbon nanotubes devices. It is aimed to improvethe performance of the incoherent OCDMA systems by OOK formats and PPM formats using carbon

nanotubes (CNTs), and silicon optical devices.

= (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].


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= (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 opticaldevices and carbon nanotubes

It is indicated that in the simulated system with the existing coding technique forPPM/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 10 , source power of laser = 214.8 10 , and refractive index = 1.55 10 /from table 1 [1],[5],[6].Bit error rate in PPM/ OCDMA format is given by equation (11) = !!()! (11)

Let M is the number of simultaneous users and is the single pulse width used in siliconoptical devices and carbon nanotubes, is code with =8 different wavelength channel anddifferent values of in silicon optical devices is =1.12 e V, and carbon nanotubes is =2.9 eV. 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 incoherentOCDAM 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.


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Figure 8: represented bit error rate (BER) for PPM format incoherent OCDMA with siliconoptical devices and carbon nanotubes

The Coherent OCDMA in Optical Wireless Communication with NonlinearityIn 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.

= () () (12)Where the optical power input for coherent OCDMA system, M is number of users of thesystem, d is distance silicon or carbon for area integrated and others parameters mention in previous

section in assumption.

= [1 1 + () ] (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.


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Figure 9: illustrated data rate (R) for OOK format coherent OCDMA with silicon opticaldevices and carbon nanotubes

Figure 10: illustrated bit error rate (BER) for OOK format coherent OCDMA with silicon

optical devices and carbon nanotubes

The figure 10 illustrates the variation in bit error rate (BER) in the same system and show

that for carbon nanotubes this decreasing from 10 10 while for the silicon optical devicesBER varies from10 10 , these making the improvement in system performance governedby equation 14. This indicates the effect of SNR to improved coherent system. The encoderincorporating 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.

=

( () ) (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


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atmospheric turbulence condition. Subcarrier intensity modulation (SIM) based on a binary phase

shift keying (BPSK) scheme in a clear but turbulent atmosphre 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 OOKcoherent 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.

= . (15)

= (16) = ( ) (17)Where is the optical power input for coherent OCDMA system, and the average noise

power = 0.1 10 , the band gap energy for silicon optical fiber 1.12 and the energy bandgap for carbon nanotubes = 2.9 [18- 20], also the refractive index for silicon opticaldevices( = 2 10 /) and the refractive index for carbon nanotubes ( = 1.55

10

/).Fourier Transform from the frequency domain to time domain with silicon optical

devices equal to optical power output ( P(t)=

35.9910 )as the figure 7 , while the carbon

nanotubes optical power output ( P(t)= 214.810 ). The figure 10 illustrates bit error rate(BER) in the same system for carbon nanotubes increasing from 10 10 while the siliconoptical devices BER from 10 10 , to make the improvement in system performancegoverned by equation 17.

Figure 11: observed data rate (R) for BPSK format coherent OCDMA with silicon opticaldevices and carbon nanotubes


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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 receivers 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 systemsSeveral 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 atmosphericturbulence. 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].


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() = . (18)Where V is visibility range in km, is transmission wavelength in nm.

() is the total

extinction coefficient and q is the size distribution coefficient of scattering related to sizedistribution 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 13001650 nm.

= . ( ). ( )( . ) (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].

()= .dB/km (20)Where V is visibility range in km, is transmission wavelength in nm. Then, the total

attenuation of wireless medium communication system can be estimated as

= () + + + () (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 ratein mm/h, the specific attenuation of wireless optical link is given by equation (22), [51].

= . . (22)If S is the snow rate in mm/h then specific attenuation in dB/km is given by equation

(23), [52,53]

= (23)


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If is the wavelength, the parameters a and b for dry snow are given as the following

= . + ., = . The parameters a and b for wet snow are as follows, [54, 55] = . + . , = .

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].

=

+

+

(

)

(24)

Where is transmitter power, is the transmitter antenna gain, is the receiver antennagain, is the carrier wavelength, is the Boltzmanns constant (1.38*10 /), Receiverbandwidth (B.W=1MHz), Tis the ambient temperature in K, , Receiver Noise Figure, is theFade margin, and is the total attenuation in dB/km. The maximum propagation distance (L) for

meeting the SNR requirements to formula (25), [57].

= / (25)The transmitter and receiver antenna gains can be expressed as the following as equation (26)

and (27)

= (26) = (27)

Where is the transmitter divergence of the beam in radians can be expressed as followsformula (28)

=

(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]

= (() (29)Where is the received power after traveling the path length L through the lossy medium,is the initial transmitted power, and is 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]


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= (30)The maximum transmission bit rate or data bit rate (Rmax).which is a losses limited one,

and is given by equation (31), [62] = (() + ) (31)Where the maximum available transmission bit rate without any limitations, and 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 devicesthan 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 withsilicon optical devices and carbon nanotubes


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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 siliconoptical 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.


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Figure 16: explained the data rate in relation to low visibility for OWC with silicon opticaldevices and carbon nanotubes

Figure 17: observed the data rate in relation to medium visibility for OWC with silicon opticaldevices and carbon nanotubes

Figure 18: observed the data rate in relation to high visibility for OWC with silicon optical

devices and carbon nanotubes


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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 OWCsystems 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 opticaldevices and carbon nanotubes

Figure 20: represented the bit error rate in relation to medium visibility for OWC with silicon



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Figure 21: explained the bit error rate in relation to high visibility for OWC with siliconoptical devices and carbon nanotubes

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 distancecompared 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



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Figure 23: represented the Maximum propagation distance in relation to medium visibility forOWC 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 usedlow 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.


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Figure 25: explained the R

design optimization of optical wireless communication owc focusing on light fidelity li fi using...

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