TELKOM
NIKA
, Vol.13, No
.3, Septembe
r 2015, pp. 8
80~888
ISSN: 1693-6
930,
accredited
A
by DIKTI, De
cree No: 58/DIK
T
I/Kep/2013
DOI
:
10.12928/TELKOMNIKA.v13i3.1976
880
Re
cei
v
ed Ma
rch 6, 2
015;
Re
vised June
19, 2015; Accepte
d
Jul
y
4
,
2015
Performance Analysis of Different Modulation
Techniques for Free-Space Optical Communication
System
Huiy
ing Zha
ng
1,2
, Hongzuo Li
*1
,Xiao Dongy
a
1
, Cai Chao
1
1
Institute of sp
ace optic
al co
mmunicati
on,
Cha
ngch
un U
n
iversit
y
of Scie
nce an
d T
e
chnolo
g
y
,
Cha
ngch
un W
e
i
x
in
g Ro
ad N
o
. 7089,
130
02
1,Cha
ngc
hun,
Chin
a, T
e
l:185
043
44
559
2
Colle
ge of Info
rmation a
nd C
ontrol En
gi
neer
ing ,Jili
n Institu
t
e Of Chemical
T
e
chnolo
g
y
,
Jilin C
h
e
nde R
oad N
o
.45, 13
202
2,
Jili
n, Chi
na, T
e
l:18504
3
445
59
*Corres
p
o
ndi
n
g
author, e-ma
i
l
: CCLGY19
57
26@
163.com
A
b
st
r
a
ct
Free space
opt
ical syste
m
is
a hot
topic, w
h
ich has
gai
ni
ng
mor
e
an
d mor
e
attentio
n. Bu
t,
w
hen
the si
gn
al tra
n
s
mitted
i
n
th
e
chan
nel,
the
p
e
rformanc
e co
uld
be
sev
e
rely
de
grad
ed
d
u
e
to the
at
mos
p
her
e
turbul
ent. The purp
o
se of this
paper is to fin
d
a
most su
ita
b
le
mo
dul
atio
n
meth
od for FSO system un
der
F
S
O chann
el.
T
he p
e
rfor
man
c
e of
pow
er
efficienci
e
s,
b
a
n
d
w
i
dth effici
en
cy, BER a
nd
SNR for t
he f
o
u
r
m
o
dulation schemes have st
udied and com
p
ared in this paper incl
ude On-Off keying (OOK), Binary Phase
Shift Keying (B
PSK), Differential Ph
ase Shift
Keyi
ng (DPSK
) and Quadr
ature Phas
e Shif
t Keying (QPSK)
w
i
thout atmos
pher
ic turbu
l
e
n
c
e. Nu
meric
a
l
exper
iments s
how
that BPSK and QPSK s
c
he
mes
are b
e
tter
compar
ed to other sche
m
es in
BER performa
nce an
d pow
er
requir
e
ments. W
hen take
inte
nsity scintill
atio
n
und
er Gamma-
G
amma tur
bul
ence ch
an
nel
i
n
to consi
der
ati
on a
nd the
ave
r
age BER
is de
rived w
i
th Meij
er-
G function, BER perfor
m
a
n
ce
s of BPSK and
QPSK sc
heme appr
oxi
m
at
e the sa
me. Co
mpare
d
w
i
th BPSK,
BER perfor
m
ance for QPSK
is 3dB
lower.
From
the
simulation res
u
lts,
m
o
dula
tion for BPSK is robust
resist turbulenc
e. As a result, BPSK schem
e
is suit
able for free-space optic
a
l comm
unication system
.
Ke
y
w
ords
:
free-spac
e opt
ical co
mmunic
a
tion, b
i
t erro
r rate, mo
dul
ation sc
he
me,
signa
l–
nois
e
rate,
ga
mma-
ga
mma turbul
enc
e chan
nel
Copy
right
©
2015 Un
ive
r
sita
s Ah
mad
Dah
l
an
. All rig
h
t
s r
ese
rved
.
1. Introduc
tion
In recent yea
r
s, FSO com
m
unication h
a
s a
c
hi
eve
d
con
s
id
era
b
le
attentions d
u
e
to its
outstan
ding band
width,
capa
city
and high data
rat
e
. However,
whe
n
the laser be
am ca
rried
informatio
n transmitting
in
the spa
c
e, it i
s
g
r
eat
ly influ
enced
by atm
o
sp
heri
c
tu
rb
ulen
ce
which i
s
called scintillation. The behavior of
turbulence is non-predi
ctive,
produces random fluctuation
to
the received
optical irradi
a
n
ce t
hat ca
n cau
s
e
seri
ou
s disto
r
tion o
f
the receive
d
front of wa
ve
,
thus g
r
eatly redu
cing th
e receive
r
sen
s
i
t
ivity and
detection efficie
n
cy
re
sult
in perfo
rman
ce of
the FSO syst
em seve
rely
decrea
s
e
d
[1]. In order
to i
m
prove thi
s
probl
em,a va
riety of methods
have be
en u
s
ed to combat
the effect for
turbule
n
ce, such
as
red
u
ci
ng the receiving ap
ertu
re,
an
adaptive o
p
tics
syste
m
fo
r wavefront
corre
c
tion,
th
e multiple i
n
put multiple
output (MIM
O)
techn
o
logy i
s
adapte
d
and
an effective
modulatio
n schem
e is
co
n
s
ide
r
ation. Previous
studi
e
s
have sho
w
n t
hatmodul
atio
n sche
me i
s
an effectiv
e
mean
s to
re
sist atmo
sph
e
ric turbule
n
ce.
I
n
this pap
er, p
e
rform
a
n
c
e f
o
r four m
o
d
u
lation sch
e
m
es
comp
ared in order
to find the most
suitabl
e sche
mes for FSO
system.
There are m
any modul
ation metho
d
s
in
FSO syst
em,su
c
h a
s
a
m
plitude mo
dulation,
freque
ncy m
odulatio
n an
d phase mod
u
lation. In t
he former research,the int
ensity modul
ation
techni
que
th
at is
On
-Off-keying
(OOK
) mo
dul
atio
n
sche
me
i
s
widely used becau
se of
i
t
s
band
width eff
i
cien
cy an
d it is ea
sy to mo
dulati
on
and
demod
ulation
.
However,th
e ability to re
sist
atmosp
he
ric
turbule
n
ce fo
r O
O
K mo
du
lation i
s
p
a
rti
c
ula
r
ly wea
k
. Co
mpa
r
ed
with tradition
al
intensity mo
d
u
lation/direct
detectio
n
(IM/
DD), ph
ase
modulatio
ndu
e to its
high
e
r
sen
s
itivity and
excelle
nt p
r
o
pertie
s
th
at
better
suite
d
for
FS
O sy
stem, wavefront
comp
en
sation
te
chn
o
l
o
g
y
doe
s not
re
q
u
ired
but th
e
atmosphe
ri
c turbul
en
ce
can b
e
bette
r
sup
p
re
ssed
[2]. In this p
a
per,
s
e
veral modulation inc
l
ude On-Off k
e
ying(OOK),
Binary Phas
e Shif
t Keying (BPSK), Different
ial
Phase S
h
ift Keying (DPSK), and
Quadrature
Phase S
h
ift Keying (QPSK) descri
bed
Evaluation Warning : The document was created with Spire.PDF for Python.
TELKOM
NIKA
ISSN:
1693-6
930
Perform
a
n
c
e
Analysis of Di
fferent Modul
ati
on Techniq
ues for F
r
e
e
-Space
…
(Hui
ying Zh
ang
)
881
respe
c
tively.
In addition, p
e
rform
a
n
c
e such a
s
power efficien
cy, band
width eff
i
cien
cy,bit error
rate
(BER) a
nd
sign
al to
n
o
ise
rate
(S
NR) fo
r th
e fou
r
mo
dulatio
n
scheme
s
are
comp
ared
un
der
the FSO cha
nnel
with an
d witho
u
t the
atmosphe
ri
c
fluctuation. Und
e
r
the si
mulation re
sults,
perfo
rman
ce
for the four modulation
scheme
s
a
r
e
compa
r
e, combine
d
wit
h
the theore
t
ical
analysi
s
re
su
lts,
a suitabl
e
modul
ation scheme
t
hat
best fo
r free
spa
c
e
opti
c
al
co
mmuni
cati
on
system i
s
ca
rried out.
The
re
st of the pa
pe
r is
orga
nized
as fo
llows. Se
ction 2 the
FS
O sy
stem m
odel a
nd
cha
nnel m
o
d
e
l has i
n
tro
duced in thi
s
pa
per. In
Section 3, t
he pe
rform
a
nce
of the four
modulatio
n schem
es a
r
e d
i
scusse
d with
out rega
rd
to the atmosp
he
re turbul
ent. In Section 4,the
BER pe
rformance fo
r t
he fou
r
mo
dulation
sch
e
mes u
nde
r Gamm
a-Ga
mma
cha
n
n
e
l is
comp
ared
an
d the
Meije
r-G fun
c
tion i
s
used to
si
m
p
lify the an
al
ysis. Fi
nally,
the pe
rforma
nce
are si
mulate
d
and com
p
a
r
e
d
according t
o
the theory
model.
2.
Modelof FS
O Sy
stemand Chann
e
l
2.1. Sy
stem
Model
A typical FSO system i
s
consi
s
ted of la
ser
so
urce, transmitte
r,
re
ceiver an
d
informatio
n
sin
k
. Th
e re
ceiving
en
d i
s
m
a
inly in
cl
udeo
ptic
al
a
n
tenna
s, p
h
o
t
o-dete
c
tor,
d
e
modul
ator a
n
d
pro
c
e
s
sor [3]. The block di
agra
m
of FS
O system i
s
shown in Figu
re 1.
Figure 1.
Blo
ck di
agram of
FSO system
Tran
smitting
sign
al modul
ated by pha
se m
odulato
r
and then tra
n
s
mitted by light beam
into the FSO
cha
nnel. Ge
n
e
rally, the ch
annel
refe
rs t
o
the atmo
sp
heri
c
chan
nel
and the
spa
c
e
cha
nnel. As
the sign
al travel throug
h
the spa
c
e chann
el, prop
erties of FS
O system
wil
l
be
deteriorated because
of at
mosphere turbulent, such
as ab
sorption, scattering
and scintillations.
Assu
ming tha
t
the chan
nel
without mem
o
ryand
with
a
dditive white
Gau
ssi
an noi
se (A
WG
N).
At
the re
ceive
r
, the re
ceive
d
signal is
assu
med to
be
distorted by the
noise
and
det
ected
by the PD
then the d
e
te
cted p
hoto
c
u
rre
nt is extra
c
ted
by the B
P
F and the
received
sign
al is Y=
hx+N.
Whe
r
e,
is the
efficien
cy of
photo
-
ele
c
tri
c
conve
r
si
on
, h re
presen
ts the
ch
ann
el state
that
distrib
u
ted b
y
Gamma
-G
amma mo
del
, x is the transmitted
si
gnal, N i
s
th
e Additive white
Gau
ssi
an noi
se that ze
ro
mean an
d variance is N
0
/2.
2.2.
Chan
nel Mo
del Distribu
ted b
y
Gamma-Gamma
When the
optical
signal
transmittedi
n t
he
channel it will be affected
by various
interferen
ce, l
i
ke
rain,
clou
ds, fog
and
a
t
mosph
e
ri
c tu
rbule
n
ce, which i
s
e
a
sily l
ead to i
n
ten
s
i
t
y
sci
ntillation, beam wand
e
r
, scatte
ring, refra
c
ting,
ph
ase vari
ation
s
, wavefro
n
t aberratio
n
s a
nd
turbule
n
ce. T
herefo
r
e, th
e
pha
se-co
h
e
r
e
n
t mod
u
lation
and
de
mod
u
lation te
chni
q
ues a
r
e
used
to
degrade
the
impact
of at
mosp
he
ric di
sturb
a
n
c
e
in
sp
ace
optical commu
nication. Ge
ne
rally,
there a
r
e fou
r
models to d
e
scrib
e
the in
tensity
distrib
u
tion. Becau
s
e, this pape
r
mainly focu
ses
on the p
a
ralle
l link in
the F
S
O system, t
herefo
r
e,
we
take G
a
mma
-gamma
mode
l that both fit for
stron
g
tu
rbul
ence a
nd
we
ak tu
rb
ulen
ce to
describe
atmo
sph
e
ri
c ch
ann
el, an
d its pa
ramet
e
rs
most suita
b
le
for the actu
al param
eters. The
beam
intensity fluctuation pro
b
a
b
ility density of
Gamma
-ga
m
ma model i
s
given by [4, 5].
Evaluation Warning : The document was created with Spire.PDF for Python.
ISSN: 16
93-6
930
TELKOM
NIKA
Vol. 13, No. 3, September 20
15 : 880 – 888
882
()
/
2
(1
)
2
2(
)
(I)
(
2
)
()
(
)
fI
K
I
(1)
Whe
r
e, I
>
0, i
t
is inte
nsity
of sig
nal li
ght
,
(·) is
the Gamma func
tion, K represents
the
solutio
n
of Bayesia
n
equ
ation,
and
a
r
e p
a
r
a
m
e
t
ers
th
a
t
r
e
p
r
es
en
t th
e
la
r
g
e
–
s
c
a
l
e
and
small
-
scal
e o
p
tical wave in
tensity fluctua
t
ion, which are given as:
2
1
12
/
5
7
/
6
0.
49
(e
x
p
[
]
1
)
(
1
1.
11
)
R
R
2
1
12
/
5
5
/
6
0.
51
(e
x
p
[
]
1
)
(
1
0.
69
)
R
R
(2)
Whe
r
e,
2
R
represents the scin
tillation index
whi
c
h is
con
s
ide
r
a
s
Ryto
v paramete
r
,
the
mathemati
c
al
model i
s
gi
ven by
22
7
/
6
1
1
/
6
1.
23
Rn
Ck
L
. Here,
2/
k
,
is wavel
ength,L i
s
th
e
comm
uni
cati
on distan
ce,
2
n
C
is the refra
c
tive indexstru
ctu
r
e par
a
m
eter t
hat is determi
ned by wind
spe
ed an
d al
titude acco
rdi
ng to the H-V turbulen
ce
model.When
2
1
R
, it means the light
intensity flu
c
tuation i
s
we
a
k
a
nd
whe
n
t
h
e
2
1
R
it mean
s th
e st
ron
g
inte
n
s
ity fluctuatio
n. Since
the Gamm
a-Gamma
mod
e
l cove
r the
all po
ssi
ble
of the turb
ulen
ce, so i
n
this
pape
r the
perfo
rman
ce
of BER under Gamma
-Ga
mma model i
s
used.
3. Basic Mod
u
lation Sche
mes
As we
all kn
o
w
, there
are
different ki
nd
s
of pha
se
m
odulatio
n sch
e
mes
are fit for FSO
comm
uni
cati
on sy
stem. In this
section,we will di
scuss the SNR,
the bandwidt
h
efficiency and
power
efficie
n
cy u
nde
r dif
f
erent m
odul
ation
schem
e
s
, but th
e at
mosp
he
ric tu
rbule
n
ce n
o
t
take
into c
o
ns
ideration [5].
3.1. OOK Mo
dulation
Becau
s
e
of
On-off
key (OOK)
schem
e is the
sim
p
lest m
odula
t
ion form of
bina
ry
amplitude shift k
e
ying
(2ASK), it is
widely
us
ed in
FSO c
o
mmunication sys
tem.
For NRZ-
OOK,the si
gn
al is
defined
as
00
(t
)
A
(
t
n
T
)
c
o
s
kn
T
b
c
n
sa
g
w
t
. Here, the value of
n
a
is 0
or 1,
T
g
rep
r
e
s
ent
s n
o
-return
-
to-ze
r
o p
u
lse,Tb
is th
e sy
mbol inte
rva
l
. Take
no
turbul
en
ce
into
con
s
id
eratio
n
,
conditional
bit erro
r ratio
(p
e-OOK
) for the NRZ-OO
K coded o
p
tical data can
be
expre
s
sed
as a fun
c
tion of the Signal
-to-Noise Ratio (SNR) as follo
ws
11
1
()
22
22
e
OOK
p
er
f
c
S
N
R
Q
S
N
R
.
In NRZ
-OOK,
the requi
red
band
width is
equal to bit rate ,that is B
OOK
=R
b
. Ac
c
o
rding to
comm
uni
cati
on theo
ry, the band
width
efficien
cy is
defined a
s
th
e ratio b
e
twe
en the bit rat
e
R
b
can
be t
r
an
smitted and
re
quire
d b
and
width B. Th
e
r
efore, the b
and
width effi
cien
cy of O
O
K
is
1
b
ook
R
B
. Powe
r requi
reme
ntis
one
of the in
dicators
that n
eed
to be
con
s
id
ered. A
c
cordi
n
g
to BER exp
r
e
ssi
on, the
req
u
ired
po
we
r i
s
d
e
fined
a
s
2
1
N
PS
N
R
. Here,
is
t
he
re
spo
n
siv
i
t
y
of the PD,
2
N
is the total n
o
ise
power. T
o
g
e
t the pa
rt
icul
ar
BER whil
e transmitting OOK
throug
h
an ideal
cha
n
nel, the norm
a
lize
d
avera
g
e
power requi
reme
nt is
2
1
OO
K
N
PS
N
R
.
3.2. BPSK M
odulation
In pha
se
mo
dulation te
ch
nique,the i
n
formatio
n
is
expre
s
sed i
n
term
s of th
e carrier.
Whe
n
bina
ry digital sig
nal
to control the
phase
of a sinusoidal
ca
rrier that called
Binary Phase
Shift Keying (BPSK). It is a two
stage
phase
shift
ke
ying where the phase of the carrier i
s
set
to
0 or
a
c
cording the
val
ue of the
m
odulatin
g si
g
nal. If a sy
mbol '1
' is tran
smitted, t
h
e
modulate
d
si
gnal
i
s
exactl
y
as
th
e ca
rri
er with pha
se
0,
a
s
the
sa
me,
pha
se
standin
g
for '0'.
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TELKOM
NIKA
ISSN:
1693-6
930
Perform
a
n
c
e
Analysis of Di
fferent Modul
ati
on Techniq
ues for F
r
e
e
-Space
…
(Hui
ying Zh
ang
)
883
The BPSK modulation is the simpl
e
st f
o
rm of PSK
and it more robust to resi
st noise than OOK
[6-8].
As
for BPSK,eac
h
bit of the modulating s
i
gn
al caus
es
a trans
m
itting s
y
mbol with T
s
duratio
n that
equal
with t
h
e bit d
u
ratio
n
T
b
. That i
s
, t
he requi
red bandwidth for
BPSK is equal to
the bit rate. B
BPSK
=R
b.
This is con
s
ist
ent with the
OOK. Acco
rding to the
definition of the
band
width eff
i
cien
cy, the b
and
width effi
cien
cy for BP
SK can be
e
x
presse
d a
s
1
b
BP
Sk
R
B
,
that is the theoretical bandwidth effi
ciency fo
r BPSK i
s
unit. And the condi
tional
BER equation of
BPSK is
1
2
e
BPS
K
p
er
f
c
S
N
R
. The powe
r
re
qu
ireme
n
ts can
readily be d
e
rived from t
he BER
expre
ssi
on
s. In the ca
se
o
f
equal BER,
usin
g
the n
o
rmali
z
ed
average p
o
wer
re
quire
ment
s o
f
BPSK to NRZ-OOK, the po
wer requirement for BPSK c
a
n be
written as
1
22
BP
SK
NRZ
O
O
K
P
P
.
Theo
retically, the NRZ-O
O
K req
u
ire
as m
u
ch a
s
22
times
power
than BPSK to obtain
a
particular BER performance.
3.3. DPSK M
odulation
DPSK is a relative phase modul
ation model
, the information
whi
c
h is t
r
ansmitted
rep
r
e
s
ente
d
by the pha
se
differen
c
e
b
e
twee
n the a
d
jacent symb
ols. When
eq
uals 0, thi
s
mean
s two
adja
c
ent sy
mbol sig
nal
s that before
and after are the sam
e
. And so, the
phen
omen
on
of inverted
π
can be
avoided
with
DPSK modulat
ion scheme.
As synchronous
demodulation, compared to BPSK modulation, t
he phase and f
r
equency of
the carrier need
not
to k
n
ow,
but,
the loc
a
l
c
a
rrier is nec
e
ss
ary.
The BER for
DPSK
c
a
n be
calculated as
follows
1
2
2
e
D
PSK
SNR
pe
r
f
c
.
Whe
n
u
s
in
g differential de
codi
ng,
the
in
formatio
n
bit
“1” will
be
tra
n
s
m
itted by
shift the
pha
se of
mod
u
lated
sign
al
180°
rel
a
tive
to the p
r
eviou
s
p
h
a
s
e of th
e mod
u
late
si
gnal. And
bit '
0'
will be t
r
ansmitted without shift the
phase of
m
o
dulated
signal rel
a
tive to the previous of
modulate
d
si
gnal. The
re
q
u
ired
ban
dwi
d
th for DPSK
is eq
ual to t
he bit rate B
DPSK
=R
b
[9]. The
spe
c
tru
m
effi
cien
cy of
DP
SK is
relative
ly highe
r,
di
spersion
tole
rance, no
nline
a
r tole
ra
nce
and
PMD tole
ran
c
e can
be im
proved. When
getting the
sa
me
bit er
ror rate, the receiver
sens
itivity
is
3dB hig
h
e
r
th
an O
O
K mo
d
u
lation. In te
rms
of re
si
st n
o
ise,it i
s
b
e
tter th
an
OOK.
The
ba
nd
wid
t
h
efficien
cy is
equal
to BPS
K and
OOK,
that is,
1
D
PSk
. For t
he
conve
n
ien
c
e
of an
alysi
s
, the
average power
requi
rem
e
nt of DPSK
nor
m
a
lized to OOK
can be expressed
as
1
2
D
PSK
NR
Z
O
O
K
P
P
.
Under the same BER condition, the averag
e power of NRZ
-OOK
is twice than
DPSK.
3.4. QPSK Modulation
Different
from
BPSK and
DPSK, the QP
SK schem
e using two bits
are grouped t
ogether
to form sign
als. Wh
en si
gnal
s tran
sm
itted, t
here are fou
r
part
i
cula
r pha
se
s. The sp
ect
r
al
efficiency can be further enhan
ced by QPSK modul
ation. Since
the QPSK can be regarded as
the com
p
osition of two
ort
hogonal
signals of BPSK. Theref
ore, each
bit occupies T
b
se
con
d
s,
the sign
als
correspon
ding
to the bits last for T
s
=2T
b
. This me
an
s that the requi
red ban
dwidth
for
QPSK is double to BPSK modulation, that is, B
QPSK
=0.
5
R
b
.And the theo
retical ba
nd
wi
dth
effic
i
enc
y
for QPSK is
2
0.
5
b
QPS
k
b
R
R
. But, the practically the band
width
efficien
cy is
1.4
～
1.6bps
/
H
z
.
Sinc
e the QPSK is a four-s
tate p
hase
s
h
ifting k
e
ying in
whic
h two bits are
grou
ped tog
e
t
her an
d the carrie
r is p
h
a
s
e mo
dulated
, each bit ha
s half of the origin
al re
cei
v
ed
intensity. Therefore, BER f
o
r QPSK
can considered
as two orthogonal of BPSK combi
ned. B
E
R
for QPSK c
a
n be described as
2
QPSK
BP
SK
BE
R
e
r
f
c
S
NR
BER
. Because of the average po
we
r
requi
rem
ent
of BPSK requires a factor of
22
more po
wer th
an NRZ-OOK, con
s
eque
ntly, the
average power require
ment by the QPSK normaliz
e
d to
NRZ-OOK c
an be express
as
:
Evaluation Warning : The document was created with Spire.PDF for Python.
ISSN: 16
93-6
930
TELKOM
NIKA
Vol. 13, No. 3, September 20
15 : 880 – 888
884
1
1
1(
B
E
R
)
(2
)
22
QP
S
K
NR
Z
O
O
K
P
er
f
c
P
e
r
f
c
BER
(3)
From
Equati
on
(10), we
assumed that inve
rse
error
of BER for QPSK m
o
dulation i
s
equal to 2 as BPSK modulation. Therefore, the
average power
requirement by
the QPSK is the
same as BPSK, or it only a few amount power
m
o
re than BPSK. From theoreti
c
al analysis
above, it
shows that the b
andwidth
efficiency for OOK,
BPSK and
DPSK modulation schemes
are the
s
a
me, only the QPSK twic
e than other
s
c
he
mes
.
Als
o
from the disc
uss
i
on above, it is
c
l
ear that the required bandwidth for QPSK is
the lowes
t
than other modulation sc
hemes
,
however, the
data
rate i
s
t
he do
uble
d
compa
r
ed to
o
t
hers.
With
re
spe
c
t to the
perfo
rman
ce
of
BER, BPSK and QPSK perform
better.
From the equat
ion of the
average power requi
rem
e
nt for
BPSK, DPSK and QPSK
schemes norma
lized to NRZ-OOK respective
ly, it is cl
ear that the
DPSK requires
1.5dB more
power than
BPSK to obtain the
partic
ular BE
R performance, and t
he
requi
re power for BPSK and QPSK
are the
sa
me.
So the BPSK
and
QPSK schem
es are t
h
e
less po
wer ef
ficient modul
ation.
4. BER unde
r Atmos
pher
i
c Turbulenc
e
Apparently, the BER perf
o
rma
n
ce of OOK is
sev
e
rely influen
ced be
ca
use
of the
intensity scintillation in the
FSO channel. Therefor
e, taking the detecto
r noi
se and channel
turbule
n
ce int
o
acco
unt, the bit error
rat
e
on t
he
re
cei
v
ing termin
al
of OOK can b
e
cal
c
ul
ated
by
(
0
)(
e
|
0
)
(
1
)(
e
|
1
)
e
Pp
p
p
p
[9, 10]. Where, p(0) and p(1)are t
he probability of transmitting 1 and 0
respe
c
tively. p(e|0
)
a
nd
p(e|1)a
r
e th
e
condition
al
p
r
o
bability in th
e
ca
se
of
sen
d
ing 0
an
d 1.
The
averag
e bit e
rro
r rate (BE
R
)can
be
de
scrib
ed a
s
0
0
hI
(I
)
(
)
2
e
Pf
Q
d
I
N
. Where,
N
0
represe
n
ts
noise, owin
g
to
(x
)
2
Q
(
2
x
)
er
f
c
, Q()i
s the
Gau
ssi
an-Q functio
n
, su
bstituting (1)
an
d
()
er
f
c
the
averag
e BER is rep
r
e
s
ente
d
as:
()
/
2
(1
)
2
0
0
11
2
(
)
(I
)
(
)
(
2
)
22
(
)
(
)
e
hI
Pe
r
f
c
I
K
I
d
I
N
(4)
Ac
c
o
rding to the Meijer-G f
unc
tions
,
the
()
v
K
and
()
er
f
c
will be expressed as follows [12]:
2,
0
1,
2
1
1
()
|
1
0,
2
er
f
c
x
G
x
2
2,
0
0,
2
1
()
|
24
,
22
v
x
Kx
G
vv
(5)
For th
e
conv
enien
ce
of th
e an
alysis,
d
e
fine
0
h
N
makin
g
(5) into
(4) t
he ave
r
ag
e
bit erro
r rate
can b
e
simplif
ied as:
2,
2
3,
2
11
1,
1
,
1
|
1
2(
)
(
)
2
2
0,
1
,
22
e
ook
PG
(6)
If the FSO sy
stem using B
PSK modulat
ion sc
heme, under
the atmosp
here turbulence
and d
e
tecto
r
noise, and
a
s
sumin
g
that t
he bal
an
ced
detectio
n
is u
s
ed, th
e ave
r
age BE
R of t
h
e
system i
s
rep
r
esent as:
()
/
2
(1
)
2
00
0
11
2
(
)
(I
)
(
)
(
)
(
)
(
2
)
22
(
)
(
)
e
hI
P
erf
c
S
N
R
f
I
d
I
e
rf
c
I
K
I
d
I
N
(7)
All the sa
me,
define
0
h
N
, by u
s
ing th
e form
ula (5) to
sim
p
lified the
expre
ssi
on
(7
)
Evaluation Warning : The document was created with Spire.PDF for Python.
TELKOM
NIKA
ISSN:
1693-6
930
Perform
a
n
c
e
Analysis of Di
fferent Modul
ati
on Techniq
ues for F
r
e
e
-Space
…
(Hui
ying Zh
ang
)
885
is:
2
2,
2
2
2,
3
1
1,
()
22
|
2(
)
(
)
,,
22
2
e
BPSK
PG
(8)
Similarly, average BER for DPSK and QPSK can be derived:
2
2,
2
2
2,
3
1
1,
()
22
|
2(
)
(
)
2
,,
22
2
e
D
PSK
PG
2
2,
2
2
2,
3
1
1,
()
22
|
()
(
)
,,
22
2
eQ
P
S
K
PG
(9)
5. Simulation and Re
sults
Followi
ng th
e
com
p
a
r
ison
pre
s
ente
d
a
b
o
ve, in thi
s
section, fo
ur
modulatio
n m
odel
will
be sim
u
late
d to verify the analytica
l
result
s by
experim
ent
s. Co
nsi
deri
ng the inte
nsity
sci
ntillationan
d without intensity sci
ntillation wh
e
n
talk abo
ut the BER perfo
rman
ce. Whi
l
e
discu
ss th
e
band
width
efficien
cy and
the req
u
ir
em
ent po
wer fo
r the
signal
transmitted, the
cha
nnel
fadi
ng a
nd tu
rb
ulen
ce
have
n
’t take
into
co
nsi
deratio
n [13]. Assu
med the
ma
in
simulatio
n
pa
ramete
rs of F
S
O use
d
in the experim
ent as Tabl
e1.
Table1. Simul
a
tion paramet
ers
Parameter
value
Modulation mode
NRZ-OOK,BPSK
,DPSK ,QPSK
Wavelength
1550nm
Efficiency
0.8
Maximum optical po
w
e
r
10mW
Beam divergence
10
mra
d
Modulation band
w
i
dth
100MHz
Responsivity
of
PIN
1A/W
Dark current
10nAmp
Time duration pe
r slot
10ns
Spectral densit
y
10N
w
/
m
2
Attenuation of F
S
O
20dB/km
Figure 2. Co
mpari
s
o
n
of the four mo
dul
ation
scheme
s
for
BER
Figure 3. Fou
r
modul
ation
scheme
s
for
BER
with turbulenc
e
Simulation of
BER performance for
NRZ-OOK, BPSK, DPSK a
nd QPSK is
given in
Figure 2. In orde
r to make
a com
pari
s
o
n
,
the atmo
sp
heri
c
turb
ulen
ce is
not into
con
s
ide
r
atio
n.
With the SNR inc
r
eas
e
d
the BER for the mo
dulat
ion scheme
s
sh
ow
s
de
creased from t
he
0
5
10
15
20
25
10
-1
2
10
-1
0
10
-8
10
-6
10
-4
10
-2
10
0
SN
R
BE
R
OOK
B
PSK
D
PSK
QP
S
K
0
5
10
15
20
25
10
-12
10
-10
10
-8
10
-6
10
-4
10
-2
10
0
SN
R
BER
OOK
O
O
K
w
i
th
tu
r
b
u
l
e
n
c
e
BPSK
BPSK
w
i
th
tu
r
b
u
l
e
n
c
e
D
PSK
D
PSK
w
i
th
tu
r
b
u
l
e
n
c
e
QP
S
K
Q
P
S
K
w
i
th
tu
r
b
u
l
e
n
c
e
Evaluation Warning : The document was created with Spire.PDF for Python.
ISSN: 16
93-6
930
TELKOM
NIKA
Vol. 13, No. 3, September 20
15 : 880 – 888
886
simulatio
n
re
sults. Co
ncl
u
sion
that
th
e BER
pe
rform
ance fo
r BPS
K is l
o
wer th
an oth
e
rs
ca
n be
obtaine
d
co
mpared with other mod
u
la
tion
sch
e
me
s. Whe
n
SNR is
10dB
ach
i
eved, the B
E
R
performance
of BPSK modulation i
s
better than QP
S
K
modulation
by 0.4 dB, and 2.56
dB bet
ter
than
DPSK, 4.46 dB
better than OOK. Hence,
when under the
same SNR,
BPSK modul
ation
displ
a
y a
better performance. On the ot
her
hand,
it i
s
clear that
BPSK
requi
res the less S
NR
than other
schemes at the particul
a
r BE
R from Fi
gure 2. That is, the
BPSK requires lest power.
The BER=10
-1
0
for instance, SNR for B
PSK is 13.10 dB
and for QPSK is 13.24 dB, for DPS
K
is
16.33, for
OOK is 22.5
dB, it illu
strates that the BPSK is 0.14
dB,
3.23 dB and 9.
4 dB better than
QPSK, DPSK and OOK m
odulation respectively. From
the sim
u
lat
i
on result
s,
it can
be inferred
that when th
e value of BER is 10
-15
achieved, difference of SNR between BPSK and QPSK is
small. Wh
en
the value for BER<1
0
-15
, it can co
ncl
u
d
e
that the performa
n
ce of BER for BPSK
and QPSK is approximatel
y the same.
Experimental
re
sults and theoretical
analysis results are
consi
s
tent. But, the structure of BPSK modulation
is simpler than QPSK.
Therefore, the BP
SK
modulatio
n schem
es i
s
b
e
tter for FSO
.
Taking the
atmosp
he
ric
fluctuation int
o
con
s
id
erati
on,
the simulatio
n
results a
s
shown in Figu
re 3 [14].
Und
e
r mod
e
rate turbule
n
ce, it is clear that
the performan
ce of
the four mod
u
lation
scheme
s
di
splay a di
stin
ct de
crease i
n
BER. In
this pa
per, o
n
l
y
the atmosp
heri
c
fluctu
ation
taken
into
co
nsid
eratio
n.
Comp
ared
Fi
gure
2
with
Fi
gure
3,
we
ca
n seen
that th
e pe
rforman
c
e of
BER lead to 8.2dB, 1.4dB, 5.8dB, 1.8dB det
erioration for OOK, BPSK, D
PSK and QPSK
respe
c
tively. Con
c
lu
sio
n
that the p
e
rform
a
n
c
e
s
of the fou
r
modul
ation
are
dramat
ically
aggravated b
y
the fluctuation. Although
, under t
he a
t
mosph
e
ri
c tu
rbule
n
ce ca
n
be dra
w
n, the
performanc
e
of BPSK s
t
ill bette
r than
other sc
hemes
and QP
S
K
sc
heme is approximatel
y
c
o
ns
is
tent
with BPSK [15]. Therefore,
the
performance for BPSK and QPSK modulation sc
heme
are m
o
st outstandi
ng than OOK and
DPSK scheme
in against the fluctuati
on. Furtherm
ore,
compared
with OOK,BER f
o
r DPSK i
s
about 3dB be
tter. The
relati
onship between transmitting
power an
d BER can b
e
sh
own fro
m
the Figure 4.
Figure 4. Rel
a
tionship bet
wee
n
BER an
d
transmitting p
o
we
r
Figure 5. BER performance of BPSK in
different scint
illation situati
o
n
Based on
the
compari
s
on among OOK,
QPSK
and
BPSK schem
es, it is obvious that
the trans
m
itting power for BPSK is
the lowes
t. When the trans
m
itting power is 4mW, OOK is
18dB hi
gher
and
QPSK is 3dB
hi
gher than
BPSK.
Hence, only co
nsidering t
he transmitting
power, modulation of BPSK is
s
u
itable for FSO s
y
s
t
em. In this
part, BPSK
s
c
h
eme for average
BER unde
r di
fferent levels
of turbule
n
ce
is simul
a
ted.
Assumi
ng th
e tran
smitting
rate is fixed
to
1Gbit/s, the
deviation o
f
the re
ceiv
ed si
gnal f
r
eque
ncy i
s
about 1
00M
Hz.
Und
e
r
wea
k
turbule
n
ce, st
rong
turbule
n
c
e
and
mod
e
rate turbulen
ce,simulatio
n
result
s a
r
e
sh
own
in
Figu
re
5.
From
Figu
re
5 we
can
see
n
that i
n
the
wea
k
atmo
sp
here
turbule
n
t, the sy
stem
has lo
w BER
and
whe
n
the atmosp
here turb
ulent is hi
gh,
the BER is hi
gh also. Whe
n
the
2
R
is 0.2, 0.5, 1.0, 5.0,
if keep the BER at 10
-10
,
the SNR that
we nee
d ab
out
are 17.5
d
B, 18.7dB, 21.8dB, 28.6
d
B
respe
c
tively.
That mean
s if we want de
cre
a
se
the SNR, effective
measu
r
e sh
ould be take
n to
comp
en
sate
the p
o
wer of t
he
sign
al
wh
en tran
smit
u
nder the
turb
ulen
ce
ch
ann
el [16]. F
r
om
the
simulation results, the conclusion that BPSK has
a better perform
ance unde
r the weak turbulent
c
an be derived. That is
to s
a
y the BPSK s
c
h
eme
eff
e
c
t
ively res
i
s
t
weak
turbulent. Theoretic
a
l
analysi
s
for the band
width
efficiency of the four
mod
u
lation sche
mes ab
ove, we ca
n se
en
that
the band
widt
h efficien
cy for the fou
r
modulatio
n a
ll are
c
on
stan
t. The simul
a
tion re
sult
s are
0
0.
5
1
1.
5
2
2.
5
3
3.
5
4
10
-10
10
-8
10
-6
10
-4
10
-2
10
0
T
r
a
n
s
m
i
t
ti
n
g
P
o
w
e
r
BER
OO
K
QP
S
K
B
PSK
0
5
10
15
20
25
30
10
-2
0
10
-1
5
10
-1
0
10
-5
10
0
10
5
S
NR(
dB
)
BER
R
2
=5
.
0
R
2
=1
.
0
R
2
=0
.
5
R
2
=0
.
2
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TELKOM
NIKA
ISSN:
1693-6
930
Perform
a
n
c
e
Analysis of Di
fferent Modul
ati
on Techniq
ues for F
r
e
e
-Space
…
(Hui
ying Zh
ang
)
887
dra
w
n in
Fig
u
re 6 th
at a
s
the
sam
e
as
we
di
scu
s
sed. Simula
tion re
sult re
veals that th
e
bandwidth
efficiency for
OOK, BPSK and DPSK schem
e are about
1 respec
tivel
y
, but for QP
SK
is equal to 2. This suggest
that the QPSK modulation scheme
ca
n make full use of the syst
em
band
width util
ization a
nd th
e transmissio
n rate can be
improve
d
.
Figure 6.Simulation re
sult
s of band
widt
h
ef
f
i
cien
cy
Figure 7. Simulation for tra
n
smi
ssi
on po
wer
requi
rem
ent
Simulation of
the power
requirement
for
OOK, BP
SK, DPSK and QPSK
m
odulation
schemes are shown in Fi
gure
7. Just as
we di
scussed above, the BPSK and QPSK schem
e
requi
re
s
app
roximately the
sa
me
po
wer. But wh
en t
he
sam
e
BE
R p
e
rfo
r
man
c
e fo
r th
e
DPSK
and OOK scheme achiev
ed, the DPSK requires
l
e
ss power than OOK,
about 1.5dB lower.
From Figure
7 it is clear t
hat the BPSK
and the QPSK need the lowest
power, and about
4.52
dB lower than the OOK and
1.5dB lower than
DPSK. Hence, the BPSK and QPSK are m
o
re
suitabl
e for free sp
ace co
mmuni
cation
base
d
on re
q
u
irem
ent po
wer.
6. Conclu
sion
The pu
rp
ose
of this pap
er is to
pre
s
ent
the
co
mpari
s
o
n
for the four m
odulatio
n
scheme
s
, fro
m
the theore
t
ical analy
s
is and exper
i
m
ental re
sults
we can seen
that compa
r
ed
with OOK and DPSK modulation, the B
PSK and
QP
SK shows a
much be
tter performance in
terms of th
e BER without con
s
ide
r
ing the
atmosp
heri
c
di
sturba
nces. When take the
atmosp
he
ric fluctuation
into the Ga
mma-G
amma
distri
buted
cha
nnel, the
avera
ge B
E
R
perfo
rman
ce
of the fo
ur m
odulatio
ns sh
ows di
re
ct
d
e
clin
e
re
spe
c
tively. From t
he
simul
a
tion
it
can be seen that in the
weak and strong turbulent
, the BPSK
and QPSK format
s still have
almos
t
the
s
a
me BER performance. Simulation
under different turbulence conditions
for BPSK
modulatio
n, the expe
rime
ntal sh
ow
s th
at it has go
o
d
ch
ara
c
te
ristics to resi
sta
n
ce tu
rbul
en
ce.
For the
bandwidth effici
ency of OOK, B
PSK and QP
SK are
closed to uint, only
the QPSK is
th
e
twice times than others. Also t
he require
power for
QP
SK is the same as BPSK
format, and t
hey
need the lowest powe
r compared others.
Therefore,
BPSK and QPSK show outstanding
performanc
e
.
Compared with QPSK, there
is
3dB
lower for BPSK of BER
performance.
Furthermore,
the modulat
i
on and dem
odulation for BPSK is
si
mpler than
QPSK, and
the
transmissio
n band
width i
s
halved comp
ared
with BP
SK under th
e
same BE
R. Con
s
e
quently
,
BPSK schem
e is thought t
o
be th
e most suitable
modulation
and demodul
ation
scheme for F
S
O
comm
uni
cat
i
ons sy
st
em.
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Tr
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93-6
930
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