A Review On FSO By Using Different Modulation Techniques

Abstract

Now a days as we are seeing that the free space optics is one of the emerging technology used for the replacement of the radio frequency wireless communication, due to their advantages over it like cost, speed, bandwidth, minimum error as well as efficient communication. The quality of services provided by the FSO is far better than the RF communication network. The free space optics is unlicensed band which make it less costly than the licensed band technology. As the atmospheric channel is used for transmission the effects of atmospheric condition effects the communication. In this paper we try to discuss all the modulation schemes which help to improve the performances of the optic system.

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International Journal of Engineering & Technology, 7 (3.8) (2018) 136-140
International Journal of Engineering & Technology
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Research paper
A Review On FSO By Using Different Modulation Techniques
Rajan Miglani1, Sachal Charak2, Sandeep Kumar Arora3*, Mahedi Masud4
1Lovely Professional University
2Lovely Professional University
3Lovely Professional University
4Taif University, Saudi Arabia
*Corresponding author E-mail:sandeep.16930@lpu.co.in
Abstract
Now a days as we are seeing that the free space optics is one of the emerging technology used for the replacement of the radio frequency
wireless communication, due to their advantages over it like cost, speed, bandwidth, minimum error as well as efficient communication.
The quality of services provided by the FSO is far better than the RF communication network. The free space optics is unlicensed band
which make it less costly than the licensed band technology. As the atmospheric channel is used for transmission the effects of atmos-
pheric condition effects the communication. In this paper we try to discuss all the modulation schemes which help to improve the per-
formances of the optic system.
Keywords:FSO;modulation;BER;OOK;PPM;PAM;PSK;OFDM;DPSK;QPSK;power efficiency; spectral efficiency
1. Introduction
The rapidly increases in the wireless communication is one of the
important change in the history of the communication technology.
The first wireless technology take place is telegram which is in-
vented in 1885. As the time move there is also the changes hap-
pened in the technology. Now that time is take place in which
every user want the high speed network which cannot provided by
the RF network so we have to see towards other technology and
the optical fiber fulfill our demands. Recent year we are more
focusing on the optical transmission [1]. The optical transmission
in which we are transmitting the information wirelessly called as
optical wireless communication or Free space optics (FSO). The
FSO is the technology in which we transmitted the signal which is
take places in the form of light through atmospheric channel. The
light signal which is generated by the laser or LED is transmitted
through the atmosphere and received by the PD (Photodiode) at
receiver end. The FSO typically used the infrared spectrum for the
transmission of the information signal. As the effects of the at-
mospheric conditions are less on IR wavelength but there are some
ranges which experience distortion due to the molecules take plac-
es in the atmosphere[2]. The history of the free space optics is one
of the ancient techniques which take in the eighth century. At that
time the Roman and Greek solider use sunlight for the communi-
cation [3]. After that the fire, smoke, semaphore etc is used for
point to point communication [4]. The photo phone is one of the
first wireless phone which is invented but not commercially used
in the market [5]. As the time changes the new advancement take
place and the invention of laser and Light Emitting Diode (LED)
take places which change the transmission of the optical wireless
communication. Till date the filament lamps are used for the
transmission of the information and the market of the FSO is also
strict at one point. The inventions of the laser help to move for-
ward in the direction of the FSO. The transmission should be in
line of sight with the receiver section for the transmission of the
voice signal, video etc in the optical wireless communication. The
free spaces optics have many advantages over any conventional
system like huge bandwidth which help us to transmit large num-
ber of data, narrow beam divergences which help us to transmit
the signal to long distances with less misalignment. It also help to
obtain high security as it is difficult to penetrate the narrow beam,
the less power and mass requirement, easy to install, high speed,
low cost as the cost of fiber and digging is removed so it is less
costly than the convention technology.
The FSO is categories on the transmission bases like ultra shot
range in which the communication is take places between the
chips to chip or ultra shot range communication,[6] short range
communication is define as the communication take place in wire-
less personal area network. This type of communication is take
place in conference hall, in a room etc, medium range is a com-
munication which take place in local area network. It is take place
in society, in campus or in a corporate etc.[6] The range of this
communication is up to kilometer, long range is a communication
take place in wide area network. In this we have inter building
connections, video surveillance network, back haul system etc.
This is the network having range up to hundred kilometer and
Ultra long range communication is take place between the inter
satellite communication.[7] The range of this communication is
very high. This type of communication is used for broadcasting,
satellite to satellite communication etc. As we know that the at-
mosphere is used as the channel for this type of technology, so the
atmospheric effect is the drawback of this technology, the one
more drawback is the coverage area. The coverage area of the
FSO is very less as the line of sight is the requirement for the free
space optics.
When we talk about the atmospheric effects three hurdles come
across us are scattering, absorption and turbulence. When we talk
about the atmospheric absorption the different types of molecules
present in the atmosphere help to produces this effect.[8] As we

International Journal of Engineering & Technology
137
know that the water is easily absorbed the light, which means the
water vapors take places in the atmosphere help in the cause of the
atmospheric absorption. Scattering is the effect which is also take
places in the guided optical communication. The scattering is the
effect in which the light travelling in the medium is scattered by
the particles which take place in the atmosphere. What amount of
the scattering take place is dependent on the radius of the particle
through which the scattering takes place. The deflection in the
angle of the light take places which causes the scattering. When
the radius of the particle is less than the wavelength the types of
scattering take place is called as the Rayleigh scattering and when
it is almost same is called as the Mie scattering. Till now we are
talk about the gases or the solid particles take places in the atmos-
phere causes the effect. Now we talk about the atmosphere weath-
er conditions like rain, fog, snow etc which also make our com-
munication system performances poor.[1] From all these weather
condition the effect of the Fog is more as compare to the all other
effects because the wavelength we used for the communication is
almost same at which the effect of fog take places. It can change
the characteristics of the optical signal or can completely hinder
the passage of light because of absorption, scattering, and reflec-
tion. The effect of the snow is depend on the size of the snow
there are some cases where the size of snow particles is very large
which completely block the light which have to transmit. Till here
we are considering the losses which are caused by hazel atmos-
phere but there is some other effect take places in the clear atmos-
phere called as the turbulences. It is the effect which caused by the
temperature, solar wind and atmospheric pressure which help in
changing the reflective index of the light.[9] These type of effect
produces the fading or sparking in the signal due to which we have
to face the loss of information. The fluctuation in the signal causes
the change in the amplitude and the phase of the signal, so at the
receiver end the proper information or signal is not received which
reduces the performances of the network. The different types of
theory are introduced to reduce the effect of the turbulences. Beam
steering is defined as the dimensions of the swirl are larger than
the transmitter beam dimensions, it will deflect the beam as a
whole in random manner from its original path. The effect of the
beam wandering is commonly take places in the long range com-
munication like satellite communication. If the eddy dimension is
of the order of beam dimension, then the swirl will act like lens
that will focus and de-focus the incoming beam leading to irradi-
ance fluctuations at the receiver and the process is called scintilla-
tion. There are different types of model take places which is used
to reduces the effect of the scintillation. These models are named
as log normal, K distribution, I-k distribution which is used com-
monly at the low level of turbulences. As the level of turbulences
increased we have to shifted towards new models named as Gam-
ma-Gamma(GG) model, Double Weibull distribution model,
Double Gamma - Gamma model(DGG), quasi-static model etc.
when the dimension of the distortion is less than the beam dimen-
sion is called as the beam Spreading. The effect of the beam
spreading is take places at the receiver section at which the power
of the signal at the receiver end is reduced as well as the receiver
aperture angle is also effected[10]. The one more effect which
causes the deduction in the signal quality is called as background
effect. This effect is take places when due to the reflection, sun-
light scattering or when we convert the optical signal into the elec-
trical signal. This effect is commonly take place as a background
radiation. When the level of background noise is increases it effect
the receiver sensitivity. The passion model is used to reduce the
effect of the background noise.
To overcome these all above drawbacks we use the different types
of modulation schemes at the transmitter end. This modulation
schemes help us to modulate the intensity of the transmitted signal
which carry the information. The different types of modulation
take places in the optical wireless communication are Amplitude
modulation, phase modulation, frequency modulation and polari-
zation modulation. These different types of modulation is used as
per the requirement of the scenario which is disguised by the en-
ergy efficiency, power requirement, spectral efficiency etc[11]. In
this paper we present the review of the FSO communication by
primary focusing towards the modulation schemes used to im-
prove the performances of the link.
2. Modulation Techniques Used in FSO
There are many modulation techniques used in the FSO. The most
commonly used modulation technique called as OOK (On Off
Keying). Due to its simplicity and BW (Bandwidth) efficiency
made the technique common. The OOK is the modulation tech-
nique which is based on the binary number in which the meaning
of ‘on’ is 1 or true and the meaning of ‘off’ is 0 or false. The 1 and
o is used to represent the presence of the light in which the 1 rep-
resent the presence of light and the 0 represent the absence of the
light.[ct]. The on off keying modulation technique required adap-
tive threshold for the better results as compare to the IM/DD mod-
ulation technique[12].The OOK modulation technique is com-
bined with the line coding technique to improve the results. As we
can study from that the OOK NRZ have moderate signal to noise
ratio (SNR), low cost where as the RZ is highly sensitive.[11].
The OOK is commonly suffering from the poor spectral and ener-
gy efficiency. The amplitude distortion is also the drawback of
this type of modulation technique. The power and the energy are
very important parameter whenever we choose the modulation
technique for the FSO system. The energy efficiency is the param-
eter which is related with the data rate and the spectral efficiency
is the parameter which is related with the information rate.
Too overcome the drawbacks of the OOK modulation techniques
we use other modulation techniques like PPM (Pulse Position
modulation), MPPM (Multi Pulse or Multicarrier PPM). The sin-
gle pulse PPM is helping us to obtain the transmission which is
energy efficient [14] whereas the spectral efficiency is not im-
proved as per the requirement. To improve the spectral efficiency
of the system we need to multiply pulse PPM. It helps us to max-
imize the energy efficiency as well as the spectral efficiency by
using the M number of laser source with N number of receiver.
The PPM is a modulation technique which eliminates the require-
ment of the dynamic threshold which is the prime parameter of the
OOK [15]. In [16] we obtain high spectral and energy efficiency
when the laser source is the half of the time slots used in the net-
work. As the PPM is the energy efficient we can use it where the
high amount of energy is required like deep sea or space commu-
nication. The MPPM modulation helps us to improve the peak to
average power ratio (PAPR)[17]. When we use the MPPM modu-
lation the performance of the peak power is better than PPM while
the average power have the better performances at PPM modula-
tion than the MPPM [19]. The drawback of this modulation tech-
nique is the switching speed. As we are working in optical to elec-
trical network the requirement of the switching speed is high be-
cause the bandwidth provided by the optical is very large than the
electrical. When we try to improve the switching speed parallel
the cost is also increased. The one more drawback of this modula-
tion technique is at the receiver end where it is difficult to demod-
ulate the modulated signal because of using binary convolution
codes. To overcome this disadvantage we use the decoding meth-
od called as SISO (Soft Input and Soft Output)[20].by using the
additional module we increases the complexity of the network.
The version of PPM is the differential pulse position modulation
technique (DPPM). In this modulation the transmission of the data
stream is take place, in which every pulse is follow by the empty
slot which helps us to improve the bandwidth efficiency of the
system [21].
Now we move to other modulation techniques used in the FSO
named as PWM (Pulse width modulation) and DPIM(digital pulse
interval modulation). The PWM have higher average width as
compare to the PPM for the same bit rate which help to reduces
the inter symbol interferences(ISI) but when we talk about the
power efficiency the PPM is much better than the PWM[22]. The

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International Journal of Engineering & Technology
PPM and the PWM are come under the synchronous modulation
technique where as the DPIM is the asynchronous modulation
technique. When we used the PPM and PWM we have to take care
about the synchronization between the modulator and demodulator
but the requirement of the synchronization is not take places in the
DPIM. In the digital pulse interval modulation the transmission of
the information is represented by the empty slots which take plac-
es in the data stream [11]. The spectral efficiency of the DPIM is
very much larger than the PPM and PWM due to the reason that it
does not have the fix length of data stream. In this the completion
of the symbol period is not required [23]. The problem in the
DPIM is at receiver end. At the receiver end when the data stream
is received in which the empty slots that does not contain any
information decoded as information slots which causes the error at
the receiver end. Pulse is followed by the empty slot which helps
us to improve the bandwidth efficiency of the system.
The one simplest modulation technique which not uses the adap-
tive threshold called as subcarriers intensity modulation (SIM). In
the SIM the information is first modulated by using RF signal and
then converted into the optical to modulate the intensity of the
light [24]. When we compare the SIM with OOK we can see that
the throughput of the SIM is much better than the OOK. The fluc-
tuation in the phase is less in the SIM as compare to the on off
keying modulation where as when we talk about the power effi-
ciency the OOK is much better than the SIM but the SIM provide
us the high amount of capacity [25]. The OFDM (orthogonal fre-
quency division multiplexing) one of the important multiplexing
technique which provided us the resistance against the inter sym-
bol interferences (ISI). In OFDM the multiplexing of the two sub-
carriers take place which is orthogonal to each other. At one time
slot the one subcarrier at the 90 and the other subcarrier at 0 so the
chance of ISI is reduced very much. As the OFDM is very emerg-
ing technology used in the wireless communication which provid-
ed us very good result, so the researcher try to use it in the optical
wireless communication as the demand of the users increased day
by day. To use the OFDM in the optical communication is one of
the great challenges as the merging of these both techniques is
very difficult. Both of these techniques come from different do-
main as the OFDM in come under RF communication and the
FSO is come under the optical wireless communication so one
uses the electrical signal and other use the light signal for data
transmission. As both have different constraint that is OFDM is
bipolar where as the optical is unipolar. In OFDM we used coher-
ent reception where as the FSO used direct reception [26]. The
optical orthogonal frequency division multiplexing (OOFDM)
provided us the advantage of long distance data transmission as
compare to the other modulation schemes but we have to make a
tradeoff between the distance and the power loss. The capacity of
the system which uses the OOFDM is also improved at minimum
cost [28]. The below is the table I in which we provided the calcu-
lation formulas of the BER for different types of modulation
schemes.
Till now in this paper we only focus on the binary modulation
techniques. Now the time comes we have to focus on the multi
level modulation techniques which used in the FSO to improve the
capacity of the system. The M –ary ASK have the capability to
provided us the less sensitivity against the distortion as compare to
the OOK modulation [30]. The different types of multi level mod-
ulation help us to obtain the higher level of spectral efficiency
which we cannot obtain by using the binary types of modulation
schemes.[29]. The multi level modulation techniques are receiver
sensitive. As the dispersion parameters effect star increasing in the
FSO link the sensitivity provided by the modulation start decreas-
ing. The first multi level modulation technique we are going to
study is Pulse Amplitude Modulation (PAM). The PAM is a mod-
ulation technique in which the amplitude of the pulse is used for
the modulation of the signal. It is analog modulation technique
which is also used with light devices to improve the energy effi-
ciency of the devices. The LED is one of the examples of this
which use the PAM for controlling purpose. The PAM is the BW
efficient scheme as compare to other modulation schemes like
PPM,OOK etc. the complexity provided by the PAM is at higher
side. In [31] see that increase in the number of bits for a particular
value of the signal to noise ratio there is change in the BER to-
wards the negative side when we are using the LPPM where as
when we are using the MPPM when the number of bits increases
for the particular SNR the enlargement show in the BER perfor-
mances. There is the declined in the average power of the LPPM
with increases in the no. of bits of the signal where as the is bal-
loon in the bandwidth of the signal but when we talk about the M-
PAM there is totally vice a versa situation take places with in-
creases in the bits help us to improve the average power of the
signal but the decline shows in the BW of the signal[31]. So as per
our requirements we use these techniques. As per real scenario we
analyze that the requirement of the power is more or bandwidth. If
the power efficiency requirement is more we use the LPPM and
when the demand of the bandwidth increased we use the M-PAM
in the FSO system. Quadrature Amplitude Modulation (QAM) is
the modulation technique in which we use the two carrier signal
which is modulated by the amplitude modulation. When we talk
about the wireless communication we use the QAM with PAM
called as Q-PAM. The Q PAM is a modulation technique which
helps us to improve the intensity of the light which is generated by
the laser. The improved intensity means more carriers to modulate
so the power and spectral efficiency rise. The variable emission is
required to grab the advantages of the Q-PAM modulation tech-
nique. The variable emission is provided by the variable lasers and
the cost of the laser is too much which means that this technique is
costly which the drawback of it is.
Now let talk about the modulation schemes which use their phases
for the modulation of the signal. In these categories the first modu-
lation scheme come under our mind is Binary Phase shifting Key-
ing modulation (BPSK). The PSK is the digital modulation
scheme in which we use the phase of the signal for modulation. In
this change in the phase take place with respect to the reference
signal. The BPSK is a binary PSK in which we work with two
phase that are 0o and 180o. There we have only to phases which we
use for modulation of the signal that is the reason we called it as
binary PSK. The BPSK is commonly used in the wireless devices
like Bluetooth, Radio frequency identification where the range of
the source and the destination is less. From [15] we can see that
the demand of power used by the BPSK is very less as compare to
the QPSK (Quadrature Phase Shift Keying). The QPSK is modu-
lation technique in which we have four phases around that the
modulation takes place. The QPSK use two bits at a time which
increases the capacity of the network. For particular value of the
signal to noise ratio is less for the BPSK as compare to the QPSK
to obtain the BER. As we discuss above that we can modulate two
bit at a time by using the QPSK, so the bandwidth efficiency pro-
vided by the QPSK is much more than the PSK. For modulating
two bits the requirement of the power is also high so at the criteria
of the power efficiency the QPSK fails when we compare it with
the BPSK. When we are move ahead in the phase modulation we
come across at a modulation technique called as DPSK (Differen-
tial Phase Shifting Key).
DPSK is a modulation technique in which we change the phase of
the signal without considering the reference signal. DPSK is used
to overcome the drawback of the QPSK and PSK. The different
types of modulation effects which effects the signal reduces when
we use the signal which is modulated by the DPSK[32] the per-
formance of the differential encoding QPSK(DQPSK) is almost
same with as compare with the DPSK. The data rate provided by
the DQPSK is twice higher than DPSK as the two bits are trans-
mitted at the same time. The DQPSK have more spectral efficien-
cy as compare with the DPSK but when we talk about the power
the DPSK is better. The carrier less amplitude and phase modula-
tion (CAP) is a modulating technique which is almost same like
the QAM but the only difference is in the phase of the signal. The

International Journal of Engineering & Technology
139
QAM modulated two carrier signal where as the CAP combine
them and filter it. When we use this modulation scheme in the
FSO we find that the energy efficiency of the signal improved
when we compare it with PAM. The cost of using CAP is less
than the PAM. The all above discussion we have in which we try
to touch all the modulation schemes used in the FSO. The above is
the Table II in which we try to summarize all the above discussion.
The table shows us the different types of qualities take places in
the different modulation schemes.
Table 2: Survey of different modulation schemes.
Modulation
schemes
References
Qualities in FSO
OOK
11,12
Adaptive threshold, low power
efficiency, low cost, ease to imple-
ment, binary technique, moderate
SNR, highly sensitive.
PPM
14,15,11
Synchronous, energy efficient,
average power is more than MPPM,
dynamic threshold not required.
MPPM
17,11
Improved PAPR, high spectral
efficiency, low peak power.
DPPM
21,11
Higher spectral efficiency compare
to the MPPM, improved power
efficiency than PPM
PWM
22
High average width and spectral
efficiency as compare to PPM,
better response towards ISI, low
power efficiency than PPM, syn-
chronous.
DPIM
11,22,23
No requirement of synchronization,
large bandwidth as compare to PPM
and PWM, difficult to demodulate.
SIM
24,11,25
Adaptive threshold, high throughput
than OOK, less phase fluctuation,
more capacity, low power efficien-
cy, less costly.
OOFDM
26
Highly resist toward ISI, long range,
difficult to combine.
M-ASK
30
Less sensitive compare to OOK.
PAM
29
Higher spectral efficiency, more
complex to implement, moderate
power efficiency.
MPAM
31
Higher spectral efficiency compare
to PAM, required dynamic thresh-
old at destination.
QAM
23,29
Higher intensity, more power and
spectral efficiency as compare to
PAM.
BPSK
11,14
Power efficient as compare to
QPSK
QPSK
32,11
High capacity, low power efficien-
cy, high spectral efficiency.
DPSK
32,31
More capacity, less data rate as
compare to the DQPSK, less spec-
tral efficiency.
DQPSK
32
Higher data rate, high bandwidth
efficiency, low power efficiency.
CAP
33
More energy efficient as compare to
the PAM, less costly, simple im-
plementation.
3. Conclusion
In this paper we survey on the different modulation techniques
which we are using in the free space optical communication to
improve the performance level of the system. From here we con-
clude that all the modulation techniques have there on perspective
of advantages and disadvantages. Some techniques are good in
energy efficiency and some are good in power efficiency. It is
dependent on the situation which modulation technique is required
by user.
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