OPTICS AND OPTOELECTRONICS VOLUME: 21 |NUMBER: 1 |2023 |MARCH
4×20 Gbps OFDM Based RoFSO T ansmission
Using Hyb id SCM-MDM Unde Di e en
Tu bulence Condi ions
Anku SOOD , Rahul KAUSHIK
Depa men o Elec onics and Communica ion Enginee ing, Jaypee Ins i u e o In o ma ion Technology,
A-10, Sec o -62, 201 309 Noida, U a P adesh, India
[email protected], ahul.k[email p o ec ed]
DOI: 10.15598/aeee. 21i1.4656
A icle his o y: Recei ed Aug 04, 2022; Re ised No 30, 2022; Accep ed Dec 21, 2022; Published Ma 31, 2023.
This is an open access a icle unde he BY-CC license.
Abs ac . A bandwid h e icien O hogonal
F equency-Di ision Mul iplexing - Radio o e F ee
Space Op ics (OFDM-RoFSO) sys em based on
2-le el mul iplexing wi h SCM and MDM domain is
implemen ed. A o al o 4-inpu da a s eams a e
mul iplexed be o e being ansmi ed h ough RoFSO
link. A 4 ×20 Gbps da a a e is achie ed by pe -
o ming mul iplexing a SCM le el wi h 45 GHz,
60 GHz subca ie equencies and hen a MDM le el
wi h HG01 and LG10 op ical modes. Wi h ce ain
design enhancemen s, channel capaci y can be u he
inc eased o mul i- old le els. The pe o mance o
ansmi ed inpu da a s eams 1–4 is in es iga ed
o Bi E o Ra e (BER), Q- ac o , ecei ed signal
powe , SNR and u bulence condi ions unde clea
ai , haze, ain, og and dus condi ions. Eye diag am
and elec ic cons ella ion pa e ns a e analyzed o
eliable da a ansmission. Thus, by using p oposed
design wi h 2-le el mul iplexing, a highe ansmission
capaci y and low c oss alk le els a e easily achie ed.
Keywo ds
OFDM, RoFSO, He mi e Gaussian (HG)
mode, Lague e Gaussian (LG) mode,
Subca ie Mul iplexing (SCM), Mode Di ision
Mul iplexing (MDM)
1. In oduc ion
In ecen imes, due o echnology enhancemen
a apid a e and implemen a ion o 5G ne wo k
applica ions, he demand o mul imedia se ices such
as ideo, li e s eaming and high-speed in e ne con-
nec i i y is con inuously g owing. Since he e is limi ed
spec um a ailabili y pe use , an e ec i e bandwid h
u iliza ion is highly equi ed. The e o e, RoFSO ech-
nology is p oposed o ackle hese challenges due o i s
ce ain ad an ages such as license ee spec um, low
ins alla ion cos , la ge bandwid h a ailabili y and high
channel capaci y. RoFSO is also p e e ed due o i s
lexibili y o link es ablishmen in ough geog aphical
e ains and o e c owded a eas whe e ibe ins alla ion
is no easible. So, wi h p ope equency planning,
a ious access poin s can be deployed using Wi eless
Local A ea Ne wo k (WLAN) echnology and i s p ac-
ical implemen a ion can be done wi h RoFSO sys em
[1] and [2].
Beside hese ad an ages, ansmission h ough
RoFSO link has ce ain limi a ions as well. The op i-
cal signal may expe ience a mosphe ic a enua ion due
o ain, haze, og, dus and snow condi ions [3] and
[4]. Poin ing e o and beam wande ing also a ec he
signal pe o mance and limi s he o e all ansmission
ange [5] and [6]. Expe iencing hese en i onmen al
challenges, exis ing RoFSO sys em equi es ce ain de-
sign enhancemen s. Thus, by implemen ing O hog-
onal F equency Di ision Mul iplexing (OFDM) wi h
RoFSO, signal a enua ion le els a e educed d as i-
cally. In OFDM, la ge numbe s o o hogonal subca -
ie s a lowe equencies a e u ilized o in o ma ion
ansmission, he eby educing ISI and p o iding low
signal a enua ion in FSO en i onmen [7].
In o ma ion ca ying capaci y o OFDM based
RoFSO sys em can be enhanced by implemen ing mul-
iplexing a subca ie , wa eleng h, op ical modes and
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OPTICS AND OPTOELECTRONICS VOLUME: 21 |NUMBER: 1 |2023 |MARCH
pola iza ion le els. Lim e al. has implemen ed Sub-
ca ie Mul iplexing (SCM) echnique o da a ans-
mission h ough FSO link. Wi h SCM, sys em BER
pe o mance is analyzed o numbe o use s in oduced
in he sys em. As he numbe o use s inc eases, he e
is a dec ease in ecei ed signal powe le els [8]. Sim-
ila ly, Mode Di ision Mul iplexing (MDM) using O -
bi al Angula Momen um (OAM) is ano he mul iplex-
ing echnique which o e s high channel capaci y and
has low design complexi y. MDM can be used o FSO
da a ansmission o MIMO based sys ems. Zhang
e al. has explained he ansmission using MDM o
ou di e en op ical modes a l=−3,−2,+2 and
+3 which shows minimum signal a enua ion and high
spec al e iciency [9].
Va ious op ical modes such as HG, LG and LP can
be used o signal ansmission. Fa eed e al. has anal-
ysed he pe o mance o a ious op ical modes o a link
ange o 9 km. Compa a i e esul s show ha while us-
ing LG modes o ansmission, minimum BER le els
a e achie ed which indica es minimum In e -Symbol
In e e ence (ISI) and low signal a enua ion [10]. By
in oducing 2-le el mul iplexing, channel capaci y o
RoFSO sys em can be u he imp o ed. A RoFSO
sys em wi h hyb id Wa eleng h-Di ision Mul iplexing
- Mode Di ision Mul iplexingWDM-MDM scheme is
p esen ed he e. In his con igu a ion, 4-inpu da a
s eams ha ing 4×10 Gbps da a a e, a e u ilised o
b oadband and communica ion se ices wi h adio e-
quency o 40 GHz [11].
Kaka i e al. has explained a 2-le el mul iplex-
ing scheme using PDM-MDM modes o ansmission.
A high da a a e o 2×320 Gbps is achie ed o
16–QAM based OFDM-RoFSO sys em. He e, Op i-
cal modes HG00 and HG01 a e used a MDM le el
and o hogonal pola iza ion is done a PDM le el [12].
Fu he mo e, Singh e al. explained he 16-QAM,
OFDM based RoFSO sys em using combined WDM-
PDM modes o ansmission. In his design 1.6 Tbps
ansmission a e is achie ed unde ad e se wea he
condi ions [13]. To u he enhance he channel ca-
paci y, a 3-le el mul iplexing using WDM, MDM and
PDM o 160 Gbps da a ansmission was p oposed by
Huang e al. The esul s showed a mul i- old inc ease
in channel capaci y wi hou a ec ing i s pe o mance
in e ms o ecei ed signal powe and show no channel
c oss alk le els a ecei e ou pu [14]. To u he im-
p o e he ecei ed signal powe and BER, he Op ical
Single Sideband (OSSB) modula ion is p e e ed o e
Op ical Dual Sideband (ODSB) modula ion as i min-
imizes he ading e ec o signal ansmission h ough
FSO link [15]. The e o e, BER is used o analyze
channel pe o mance by obse ing numbe o e o bi s
ecei ed a ou pu . Also, a e age BER is de i ed
o analyzing Op ical Wi eless Communica ion (OWC)
link pe o mance unde di e en u bulence condi ions
[16] and [17]. Simila ly, o 60 GHz mm wa e ans-
mission h ough FSO link, ecei ed signal powe le els
a e compa ed o op ical wa eleng h o 850 nm and
1550 nm. Compa a i e esul s show imp o ed pe o -
mance while using 1550 nm wa eleng h o ansmis-
sion [18].
In he wo k p esen ed he e, a 2-le el mul iplexing
is implemen ed o OFDM based RoFSO sys em using
Subca ie Mul iplexing (SCM) a le el 1 and Mode
Di ision Mul iplexing (MDM) a le el 2. In o ma ion
h ough ou da a s eams is ansmi ed simul ane-
ously a e mul iplexing ini ially a SCM le el and hen
a MDM le el.
Using 4–QAM encode design, each channel is
u ilized o da a ca ying capaci y o 20 Gbps.
So, a o al in o ma ion a e o 80 Gbps is achie ed
in his design. OSSB modula ion is p e e ed in his
design o minimize he ading e ec . A SCM le el,
subca ie equencies o 45 GHz and 60 GHz a e as-
signed o inpu signals. Simila ly, a MDM le el, HG01
and LG10 op ical modes a e used o ansmission.
While in eg a ing abo e design wi h OFDM, dual mul-
iplexing o SCM-MDM has no been epo ed in any
o he p e ious s udies. Fu he , i may be wo h
no ed ha by combining SCM wi h MDM will enhance
he o e all channel capaci y. The e o e, in his wo k,
an a emp has been made o in es iga e he esea ch
wo k wi h dual mul iplexing o SCM-MDM o a ious
pe o mance pa ame e s.
Sec ion 2. b oadly desc ibes OFDM–RoFSO sys-
em using 2-le el mul iplexing. I explains mul iplexed
RF subca ie signal a 45 GHz and 60 GHz equen-
cies using OSSB modula ion. I also desc ibes spa ial
HG01 and LG10 modes a MDM le el used as op ical
ca ie s o signal ansmission h ough RoFSO link.
Sec ion 3. desc ibes a compa a i e analysis o
a ious op ical modes used o ansmission and hei
pe o mance unde weak, mode a e and s ong u bu-
lence condi ions o a link ange o 500 m o 8,000 m.
Concluding ema ks a e gi en in Sec. 4.
2. Sys em Model
Fo an OFDM based RoFSO sys em, a ansmission
a e o 80 Gbps is achie ed by using 2-le el mul iplex-
ing while conside ing ou inpu da a s eams o ans-
mission. A le el 1, Sub-ca ie Mul iplexing (SCM)
wi h 45 GHz, 60 GHz equency is ca ied ou . A
le el-2, Mode Di ision Mul iplexing (MDM) is ca ied
ou by conside ing HG01and LG10 op ical modes o
ansmission. Da a inpu s 1–2 and 3–4 a e combined
a MDM MUX and ansmi ed h ough FSO link as
shown in Fig. 1 and i s simula ion se -up is p o ided
in Appendix (Fig. 14). A gene alized ma hema ical
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CW Lase
Sou ce
HG 01
Mode
Sub-
Ca ie
MUX
(SCM)
Le el 1
RF
Combine
Op ical
Modula o
0°
Ѳ°
Phase Shi e
Op ical
Modula o
0°
Ѳ°
Phase Shi e
Mode
Di ision
MUX
(MDM)
Le el 2
Da a 1 + 4-QAM
Encode
OFDM + QM
Modula o
45 GHz
60 GHz
Da a 2 + 4-QAM
Encode
OFDM + QM
Modula o
Mixe
OSSB
Da a 3 + 4-QAM
Encode
OFDM + QM
Modula o
Da a 4 + 4-QAM
Encode
OFDM + QM
Modula o
Sub-
Ca ie
MUX
(SCM)
Le el 1
60 GHz
45 GHz
Mode
Spli e
(MDM
De-MUX)
HG 01
Mode
60 GHz
45 GHz
OFDM + QM
Demodula o
OFDM + QM
Demodula o
Da a 1
Da a 2
LG10
Mode
Gaussian
BPF (45 GHz)
Gaussian
BPF (60 GHz)
60 GHz
45 GHz
OFDM + QM
Demodula o
OFDM + QM
Demodula o
Da a 3
Da a 4
Gaussian
BPF (45 GHz)
Gaussian
BPF (60 GHz)
SCM
De-MUX
SCM
De-MUX
A mosphe ic
Tu bulence
Channel
Mixe
Mixe
Mixe
APD
(Da a 1-2)
LG 10
Mode
APD
(Da a 3-4)
Fig. 1: Schema ic o p oposed OFDM based RoFSO sys em using 2-le el mul iplexing a SCM, MDM le els.
exp ession o LG modes is gi en as [11]:
Ψm,n ( , φ) = 2 2
w2
o|n
2|Ln
m2 2
w2
oexp  2
w2
o·
·exp jπ 2
λRosin (|n|φ), n ≥0
cos (|n|φ), n ≥0,
(1)
whe e, mand na e mode dependencies on X-index and
Y-index ep esen ing he azimu h and adial angles,
espec i ely; wois he spo size, R ep esen s he adius
o cu a u e and L(n,m)is he Lague e polynomial.
Simila ly, a gene alized ma hema ical exp ession o
HG modes is gi en as [11]:
Ψm,n ( , φ) = Hm √2x
w2
ox !exp −x2
w2
ox exp jπx2
λRox 
·Hn √2y
w2
oy !exp −y2
w2
oy exp jπy2
λRoy ,
(2)
whe e, mand n ep esen he mode dependencies on X
pola iza ion and Ypola iza ion, espec i ely; Hmand
Hna e he He mi e polynomials.
In his sys em using Pseudo Random Bi Sequence
(PRBS) gene a o , a 20 Gbps da a a e is gen-
e a ed and applied a 4-QAM encode o ob ain
encoded in o ma ion bi s a i s ou pu . The signal is
hen applied a OFDM modula o which is con igu ed
o gene a e 512 subca ie s a lowe equency ha -
ing 1024 IFFT poin s. While using OFDM-RoFSO
ansmission, he signal expe iences minimum chan-
nel a enua ion a lowe equency le els and educed
In e -Symbol In e e ence (ISI). Using quad a u e
modula o , Iand Qphase componen s o signal a e
gene a ed and applied a he mixe s age. A mixe 1,
a subca ie equency o 45 GHz and a mixe 2, a sub-
ca ie equency o 60 GHz is mixed wi h inpu base-
band signal which co esponds o inpu da a s eams 1
and 2 espec i ely. The ou pu RF signal om mixe 1
and mixe 2 a e combined o gene a e SCM signal o
da a s eams 1–2. Simila ly, da a s eams 3–4 a e
gene a ed by using same SCM pa ame e s.
The SCM signal along wi h he op ical ca ie is ap-
plied a inpu po s o DD-MZM as shown in Fig. 1.
He e, DD-MZM is cus omized o gene a e OSSB mod-
ula ed signal a i s ou pu . In his design, HG01 and
LG10 op ical modes a e conside ed o ansmission.
The SCM da a s eams 1–2 and 3–4 a e mul iplexed
by using MDM-MUX be o e being ansmi ed h ough
RoFSO link.
The F ee Space Op ical (FSO) link equa ion is gi en
as [15]:
P x =P x "D2
x
(D x +θZ)2#η xη x ·10−
´αz
10 ,(3)
whe e, P x and P x a e he op ical signal powe a
ansmi e and ecei e end, espec i ely; D x and
D x a e he an enna ape u e diame e s a ansmi -
e and ecei e , espec i ely; θ ep esen s he beam
di e gence; zis he link dis ance, he op ical e iciency
is designa ed by η x and η x and ´αis he a mosphe ic
a enua ion.
Conside ing clea ai a enua ion, he signal ans-
mi ed be ween FSO link is analyzed o weak, mod-
e a e and s ong u bulence condi ions. Using Con-
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inuous Wa e (CW) lase , spa ial p o iles o HG01
and LG10 modes a e gene a ed as shown in Fig. 2(a)
and Fig. 2(b), espec i ely.
(a)
(b)
Fig. 2: Spa ial p o iles o gene a ed op ical modes o
a) HG01 mode; b) LG10 mode.
The equency spec um o RF Subca ie Mul i-
plexed (SCM) signal a 45 GHz and 60 GHz equency
is shown in Fig. 3.
Fig. 3: Spec um o RF Subca ie Mul iplexed (SCM) signal
o da a inpu s 1–2.
Fig. 4 shows he op ical spec um obse ed a MDM-
MUX ou pu while using HG01 and LG10 as inpu
applied modes.
Fig. 4: Measu ed op ical spec um a MDM MUX ou pu o
inpu da a s eams 1–2, 3–4.
A e mul iplexing a MDM le el, op ically mod-
ula ed signal is ansmi ed h ough RoFSO link.
Since op ical signal expe iences deg ada ion due o a -
mosphe ic a enua ion and u bulence condi ions, i s
powe le el can be aised by using an op ical ampli-
ie . A ecei e , he op ical modes a e sepa a ed by
using MDM De-MUX o ob ain mode 0 and mode 1
co esponding o HG01 and LG10 modes, espec i ely.
A e mode spli ing, he op ical signal is made o
inciden on APD de ec o su ace and i s elec ical
equi alen is ob ained a he ou pu . The equency
spec um obse ed a APD de ec o ou pu o 45 GHz
and 60 GHz subca ie equencies is shown in Fig. 5.
Fig. 5: Spec um o 45 GHz, 60 GHz in o ma ion signal a
APD de ec o ou pu o link ange, L= 500 m and
C2
n= 1 ·10−15 m−2/3.
The de ec o ou pu is hen p ocessed by he Gaus-
sian band pass il e o sepa a e 45 GHz and 60 GHz
subca ie equencies. A e demodula ion, he da a
s eams 1, 2 and 3, 4 co esponding o he op i-
cal modes HG01 and LG10 modes, espec i ely, a e
ob ained a decode ou pu as shown in Fig. 1.
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The ecei e simula ion ci cui is p o ided in Appendix
(Fig. 15). Va ious sys em simula ion pa ame e s used
in his analysis a e gi en in Tab. 1.
Tab. 1: Lis o simula ion pa ame e s.
Pa ame e Value
Bi a e 20 ·109bi s·s−1
Radio equency 45 GHz, 60 GHz
No. o subca ie s 512
No. o FFT poin s 1024
Ope a ing equency 193.1 THz
Tx. Ape u e diame e 5 cm
Beam Di e gence 2–6 m ad
Recei e ape u e diame e 20 cm
Link leng h 500 m – 8,000 m
Index e ac ion s uc u e 1·10−15 m−2/3 o
30 ·10−15 m−2/3
Wa eleng h 1550 nm
3. Resul s and Discussion
The design implemen a ion o OFDM based RoFSO
sys em is ca ied ou wi h Op iSys em 19.0 e sion and
simula ion esul s a e depic ed in Fig. 2, Fig. 3, Fig. 4,
Fig. 5, Fig. 12 and Fig. 13. The sys em pe o mance
is analyzed o ecei ed powe , Bi E o Ra e (BER),
Q- ac o e c. while conside ing a ious channel pa am-
e e s, such as link ange, a mosphe ic u bulence and
op ical modes o FSO ansmission. Va ious ansmi -
e con igu a ions such as:
•Di ec ansmission wi hou mul iplexing i.e. back
o back sys em.
•Subca ie Mul iplexing (SCM-MUX).
•Mode Di ision Mul iplexing (MDM-MUX).
•Hyb id SCM-MDM mul iplexing a e conside ed
o compa a i e measu emen s.
Analysis o ecei ed signal powe is ca ied ou a
APD de ec o ou pu o a ious mul iplexed design
con igu a ions as shown in Fig. 6. In his analysis,
compa a i e esul s a e obse ed o a link ange o
up o 8,000 m. The ixed alues o sys em pa am-
e e s a e λ= 1550 nm, he ecei e ape u e diam-
e e D= 20 cm, C2
n= 1 ·10−15 m−2/3and he
beam di e gence =5 m ad. A APD de ec o ou -
pu , he obse ed signal powe is −28 dBm, −34 dBm,
−39.5dBm and −44 dBm o back o back sys em,
−31 dBm, −37 dBm, −42 dBm and −46 dBm o SCM-
MUX sys em; −35 dBm, −40.5dBm, −45.25 dBm and
−49.5dBm o MDM MUX sys em; and −38 dBm,
−43 dBm, −47 dBm and −51 dBm o hyb id SCM-
MDM MUX sys em a a dis ance o 2,000 m, 4,000 m,
6,000 m and 8,000 m, om ansmi e , espec i ely.
Fig. 6: Plo o ecei ed powe a APD de ec o o a link ange
o up o 8 km using 4-QAM-OFDM-RoFSO sys em ha -
ing C2
n= 1 ·10−15 m−2/3,λ= 1550 nm, D= 20 cm.
Compa a i e esul s in Fig. 6 show ha while using
dual mul iplexed SCM-MDM sys em, he ecei ed sig-
nal powe is well wi hin he de ec able ange. Since
4-inpu da a s eams a e mul iplexed in dual SCM-
MDM design, a o al 80 Gbps ansmission a e is
achie ed, and imp o ed capaci y le els a e obse ed.
Fu he , by modi ying ce ain design pa ame e s a
SCM and MDM le els, he channel capaci y can easily
be enhanced.
Simila ly, he Q- ac o is analysed o a ious design
con igu a ions as shown in Fig. 7. The ixed sys em
pa ame e alues a e λ= 1550 nm, C2
n= 10−15 m−2/3,
beam di e gence =5 m ad and D= 20 cm.
Fig. 7: Q- ac o analysis o 4-QAM OFDM-RoFSO sys em,
conside ing a ious mul iplexing designs o a link ange
o up o 8,000 m and a subca ie equency o 60 GHz.
The obse ed Q- ac o alues a e 22.97, 12.83, 7.98
and 5.85 o Back o Back (B2B) sys em; 20.57, 11.04,
7.28 and 4.98 o SCM-MUX sys em; 17.79, 10.06, 6.26
and 4.28 o SCM-MDM MUX sys em; and 11.78, 7.16,
4.12 and 2.8 o MDM-MUX sys em o a link ange o
2,000 m, 4,000 m, 6,000 m and 8,000 m, om ans-
mi e , espec i ely. The plo s obse ed in Fig. 7 show
ha he Back o Back (B2B) sys em has high Q- ac o
and low in e e ence le els o a link ange o 1,000 m
o 8,000 m. Since o dual SCM-MDM MUX design,
he obse ed Q- ac o alues a e well wi hin he speci-
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OPTICS AND OPTOELECTRONICS VOLUME: 21 |NUMBER: 1 |2023 |MARCH
ied limi s, he e o e i can be conside ed o p ac ical
implemen a ion.
Simila ly, he pe o mance o OFDM based RoFSO
sys em is analysed o channel c oss alk (XT) while
conside ing SCM, MDM and dual SCM-MDM based
con igu a ions. Fo a mul iplexed channel, c oss alk is
de ined as in e e ence obse ed om all o he channels
excep he desi ed channel a ecei e . Since he chan-
nel c oss alk is measu ed in e ms o BER o a spec-
i ied Link ange (L), a compa a i e analysis is ca ied
ou o a ious mul iplexed con igu a ions, as shown
in Fig. 8. The ixed sys em pa ame e s conside ed a e
λ= 1550 nm, D= 20 cm and C2
n= 10−15 m−2/3.
Fig. 8: BER pe o mance measu emen o a link ange consid-
e ing weak u bulence o ecei e channel wi h a ious
c oss alk le els.
The plo in Fig. 8 shows ha he maximum c oss alk
le els a e obse ed o dual SCM-MDM mul iplexed
sys em wi h a maximum BER o 1.48 ·10−4. Since he
BER le els a e well wi hin he limi s o accu a e da a
ansmission, i s design implemen a ion can easily be
ca ied ou .
I is clea om abo e indings ha channel capac-
i y is imp o ed by using dual mul iplexed SCM-MDM
sys em and he esul s show accep able ecei ed powe
and channel c oss alk le els. Al hough he design is ca-
pable o ansmi ing in o ma ion on ou mul iplexed
channels simul aneously, a u he analysis is equi ed
o measu e indi idual channel pe o mance. In his de-
sign, he channel con igu a ions HG01-45 GHz, LG10-
45 GHz, HG01-60 GHz and LG10-60 GHz a e used
o channels 1–4, espec i ely. The indi idual chan-
nel pe o mance can be analysed based on BER, SNR,
elec ical cons ella ion and eye diag am o decide i s
implemen a ion easibili y in p ac ical en i onmen .
The measu emen s we e ca ied ou by conside ing
a mosphe ic u bulence condi ions, link ange and
a mosphe ic a enua ion.
Fo a dual mul iplexed SCM-MDM sys em, he BER
pe o mance o inpu da a s eams 1–4 is analysed o
a link ange o up o 8,000 m as shown in Fig. 9. The
ixed sys em pa ame e s a e λ= 1550 nm, D= 20 cm,
beam di e gence =5 m ad and C2
n= 1 ·10−15 m−2/3.
F om eye diag am analysis, he obse ed alues o min.
log o BER a e −1.41,−1.12,−1.10 and −1.07 o
channel 1; −1.96,−1.22,−1.12 and −1.09 o chan-
nel 2; −24.32,−15.13,−9.25 and −6.44 o channel 3;
and −30.86,−19.45,−12.71 and −9.73 o channel 4
o a link dis ance o 2,000 m, 4,000 m, 6,000 m and
8,000 m om ansmi e , espec i ely, as shown in
Fig. 9.
Fig. 9: Es ima ion o log o BER o a link ange o up o
8 km using op ical modes HG01 and LG10 a subca ie
equency o 45 GHz and 60 GHz.
The g aphs obse ed in Fig. 9 show ha as he link
dis ance be ween ansmi e and ecei e inc eases,
sys em Bi E o Ra e (BER) pe o mance de e io-
a es. Simila ly, he compa a i e analysis shows ha
he BER pe o mance is imp o ed while using highe
subca ie equency o 60 GHz wi h LG10 and HG01
op ical modes o ansmission.
So, LG10 and HG01 modes a 60 GHz subca ie
equency a e u he analysed by conside ing a ious
a mosphe ic u bulence condi ions. Scin illa ion index
pa ame e , C2
n alues conside ed o weak, mode a e,
and s ong u bulence condi ions a e shown in Tab. 2.
Tab. 2: Scin illa ion index pa ame e C2
n o a ious u bulence
le els.
Tu bulence
Condi ions
Scin illa ion
Model
Scin illa ion index
pa ame e ,C2
n
Weak Log-no mal 1·10−15 m−2/3
Mode a e Log-no mal 9·10−15 m−2/3
S ong Gamma-Gamma 30 ·10−15 m−2/3
Fig. 10 shows he pe o mance o HG01 and LG10
modes a 60 GHz subca ie equency unde di e -
en u bulence condi ions. The ixed design pa ame e s
conside ed a e: λ= 1550 nm, D= 20 cm and beam di-
e gence =5 m ad. Fo a link dis ance, L= 2,000 m,
he obse ed alues o min. log o BER a e −50.29,
−4.02 and −2.423 o HG01 mode and −61.04,−6.20
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OPTICS AND OPTOELECTRONICS VOLUME: 21 |NUMBER: 1 |2023 |MARCH
and −3.44 o LG10 mode o weak, mode a e and
s ong u bulence condi ions, espec i ely.
Fig. 10: Es ima ion o log o BER o weak, mode a e and
s ong u bulence condi ions using op ical modes
HG01 and LG10 a subca ie equency o 60 GHz
o a link ange o up o 4,000 m.
The cu es ob ained in Fig. 10 show ha he ans-
mission using LG10 mode has lowe BER in compa -
ison o HG01 mode a 60 GHz subca ie equency.
Fu he , analysis shows ha unde weak u bulence
condi ions, he sys em’s BER is low due o minimum
signal a enua ion as compa ed o mode a e and s ong
u bulence condi ions. Since LG10 mode a 60 GHz
subca ie equency shows imp o ed channel pe o -
mance, i can be u he analysed by conside ing a i-
ous a mosphe ic a enua ion condi ions o FSO signal
ansmission. A enua ion coe icien alues conside ed
o he analysis a e gi en in Table. 3.
Tab. 3: Speci ic a mosphe ic a enua ion coe icien s unde di -
e en wea he condi ions [3], [4] and [17].
Wea he condi ion A enua ion (dB·km−1)
Clea ai a enua ion 0.0647
Low Haze 1.537
Hea y Haze 10.115
Ligh Rain 6.27
Hea y Rain 19.28
Thin Fog 9
Hea y Fog 22
Ligh Dus 25.11
Dense Dus 297.38
The signal pe o mance is mainly a ec ed by a mo-
sphe ic a enua ion due o haze, ain, og and dus con-
di ions. Fig. 11 shows he pe o mance o LG10 mode
a 60 GHz while conside ing di e en wea he condi-
ions o signal ansmission. The sys em design pa-
ame e s conside ed in his analysis a e λ= 1550 nm,
D= 20 cm, beam di e gence equal o 5 m ad and
C2
n= 1 ·10−15 m−2/3. The obse ed SNR le els a e:
42 dBm, 41 dBm, 40 dBm and 38 dBm o low haze;
40 dBm, 37 dBm, 34 dBm and 30 dBm o ligh ain;
39 dBm, 36 dBm, 31 dBm and 25 dBm o hin og; and
36 dBm, 25 dBm, 15 dBm and 0 dBm o ligh dus o
a link ange o 500 m, 1,000 m, 1,500 m and 2,000 m
om ansmi e , espec i ely, as shown in Fig. 11.
alues conside ed o weak, mode a e, and s ong u bulence
condi ions a e shown in Table 2.
Table 2. Scin illa ion index pa ame e Cn2 o a ious u bulence le els.
Tu bulence
Condi ions
Scin illa ion
Model
Scin illa ion index
pa ame e , Cn2
Weak
Log-no mal
1×10−15𝑚−2/3
Mode a e
Log-no mal
9×10−15𝑚−2/3
S ong
Gamma-Gamma
30 × 10−15𝑚−2/3
Fig. 10 shows he pe o mance o HG01 and LG10 modes a
60GHz subca ie equency unde di e en u bulence
condi ions. The ixed design pa ame e s conside ed a e: λ =
1550nm, D = 20cm and beam di e gence = 5m ad. Fo a link
dis ance, L = 2000m, he obse ed alues o min. log o BER
a e -50.29, -4.02 and -2.423 o HG01 mode and
-61.04, -6.20 and -3.44 o LG10 mode o weak, mode a e
and s ong u bulence condi ions, espec i ely.
Fig.10. Es ima ion o log o BER o weak, mode a e and s ong u bulence
condi ions using op ical modes HG01 and LG10 a subca ie equency o
60GHz o a link ange o up o 4000 m.
The cu es ob ained in Fig. 10 show ha he ansmission
using LG10 mode has lowe BER in compa ison o HG01
mode a 60GHz subca ie equency. Fu he , analysis shows
ha unde weak u bulence condi ions, he sys em’s BER is
low due o minimum signal a enua ion as compa ed o
mode a e and s ong u bulence condi ions.
Since LG10 mode a 60 GHz subca ie equency shows
imp o ed channel pe o mance, i can be u he analysed by
conside ing a ious a mosphe ic a enua ion condi ions o
FSO signal ansmission. A enua ion coe icien alues
conside ed o he analysis a e gi en in Table 3.
Table3. Speci ic a mosphe ic a enua ion coe icien s unde di e en
wea he condi ions [3], [4], [17].
Wea he condi ion
A enua ion(dB/km)
Clea ai a enua ion
0.0647
Low Haze
1.537
Hea y Haze
10.115
Ligh Rain
6.27
Hea y Rain
19.28
Thin Fog
9
Hea y Fog
22
Ligh Dus
25.11
Dense Dus
297.38
The signal pe o mance is mainly a ec ed by a mosphe ic
a enua ion due o haze, ain, og and dus condi ions. Fig. 11
shows he pe o mance o LG10 mode a 60 GHz while
conside ing di e en wea he condi ions o signal
ansmission. The sys em design pa ame e s conside ed in his
analysis a e λ=1550nm, D=20cm, beam di e gence equal o
5m ad and 𝐶𝑛
2=1×10−15𝑚−2/3. The obse ed SNR le els
a e: 42 dBm, 41 dBm, 40 dBm and 38 dBm o low haze; 40
dBm, 37 dBm, 34 dBm and 30 dBm o ligh ain; 39 dBm,
36 dBm, 31 dBm and 25 dBm o hin og; and 36 dBm, 25
dBm, 15 dBm and 0 dBm o ligh dus o a link ange o 500
m, 1000 m, 1500 m and 2000 m om ansmi e ,
espec i ely, as shown in Fig. 11.
Fig.11. Obse ed SNR a APD de ec o ou pu o haze, ain, og and dus
condi ions using 4-QAM-OFDM-RoFSO sys em o 5000m link ange, Cn2
= 1×10−15𝑚−2/3 , λ = 1550 nm, D = 20 cm.
Compa a i e plo s in Fig. 11 show qui e s eady SNR le el
obse ed unde haze condi ions due o low a mosphe ic
a enua ion. Since a highe signal a enua ion is obse ed due
o ain, og and dus condi ions, he SNR le els de e io a e
qui e sha ply wi h inc ease in link dis ance. Table 4 shows
a ious SNR le els obse ed unde haze, ain, og and dus
condi ions.
Table 4. Obse ed SNR le els o di e en wea he condi ions.
Range
(m)
A mosphe ic A enua ion (dB/km)
SNR (dBm)
unde
Haze
condi ions,
α=1.537
SNR (dBm)
unde
Rain
condi ions,
α=6.27
SNR (dBm)
unde
Fog
condi ions,
α=9
SNR (dBm)
unde
Dus
condi ions,
α=25.11
500
42
40
39
36
1000
41
37
36
25
1500
40
34
31
15
2000
38
30
25
0
2500
36
26
18
0
3000
34
21
10
0
3500
32.5
16
0
0
4000
31
10
0
0
4500
29.5
4
0
0
5000
28
0
0
0
Fig.12 shows he cons ella ion diag ams o ecei ed
in o ma ion s eams o channels 1 o 4 wi h
𝐶𝑛
2= 1×10−15𝑚−2/3, λ = 1550nm. The ixed cons ella ion
Fig. 11: Obse ed SNR a APD de ec o ou pu o haze, ain,
og and dus condi ions using 4-QAM-OFDM-RoFSO
sys em o 5,000 m link ange, C2
n= 1 ·10−15 m−2/3,
λ= 1550 nm, D= 20 cm.
Compa a i e plo s in Fig. 11 show qui e s eady SNR
le el obse ed unde haze condi ions due o low a -
mosphe ic a enua ion. Since a highe signal a enua-
ion is obse ed due o ain, og and dus condi ions,
he SNR le els de e io a e qui e sha ply wi h inc ease
in link dis ance. Table 4 shows a ious SNR le els
obse ed unde haze, ain, og and dus condi ions.
Tab. 4: Obse ed SNR le els o di e en wea he condi ions.
Range
(m)
A mosphe ic A enua ion (dB·km−1)
SNR
(dBm)
unde
Haze
condi ions,
α= 1.537
SNR
(dBm)
unde
Rain
condi ions,
α= 6.27
SNR
(dBm)
unde
Fog
condi ions,
α= 9
SNR
(dBm)
unde
Dus
condi ions,
α= 25.11
500 42 40 39 36
1,000 41 37 36 25
1,500 40 34 31 15
2,000 38 30 25 0
2,500 36 26 18 0
3,000 34 21 10 0
3,500 32.5 16 0 0
4,000 31 10 0 0
4,500 29.5 4 0 0
5,000 28 0 0 0
Figu e 12 shows he cons ella ion diag ams o
ecei ed in o ma ion s eams o channels 1 o 4 wi h
C2
n= 1 ·10−15 m−2/3,λ= 1550 nm. The ixed
cons ella ion poin s show a eliable da a ansmission
o 4·20 Gbi s·s−1 o a link ange o 2,000 m.
The cons ella ion diag am o LG10 mode a 60 GHz
shows mo e accu a e poin s o eliable ansmission as
shown in Fig. 12(d). Figu e 13 shows he eye diag am
pa e ns obse ed o channel 1 o 4 while conside ing
a link ange o 2,000 m.
Figu e 13(d) shows ha he ansmission using LG10
mode a 60 GHz has clea eye opening which indica es
eliable da a ansmission h ough RoFSO link and no
channel c oss alk le els a e obse ed.
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OPTICS AND OPTOELECTRONICS VOLUME: 21 |NUMBER: 1 |2023 |MARCH
(a)
(b)
(c)
(d)
Fig. 12: Cons ella ion diag am o ecei ed signal a L= 2000 m
and α= 0.0647 dB·km−1 o a) HG01 mode a
45 GHz, b) LG10 mode a 45 GHz, c) HG01 mode
a 60 GHz and d) LG10 mode a 60 GHz.
(a)
(b)
(c)
(d)
Fig. 13: Eye diag am o ecei ed signal o L= 2000 m,
α= 0.0647 dB·km−1 o a) HG01 mode a 45 GHz,
b) LG10 mode a 45 GHz, c) HG01 mode a 60 GHz,
d) LG10 mode a 60 GHz.
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OPTICS AND OPTOELECTRONICS VOLUME: 21 |NUMBER: 1 |2023 |MARCH
4. Conclusion
In his design, ansmission h ough OFDM-RoFSO
sys em using dual SCM-MDM con igu a ion is
implemen ed. Inpu da a s eams 1,2 and 3,4 a e mul-
iplexed ini ially a SCM le el wi h 45 GHz, 60 GHz
subca ie equencies and hen a MDM le el (1-2,
3-4) wi h HG01, LG10 modes be o e being ansmi ed
h ough RoFSO link. Since each channel is ansmi -
ed wi h an in o ma ion ca ying capaci y o 20 Gbps,
a o al ansmission a e o 80 Gbps
(2×2×20 Gbi ·s−1) is achie ed while using 4-inpu
da a s eams simul aneously.
The FSO channel pe o mance is analysed o a max-
imum link ange o up o 8,000 m unde di e en
wea he condi ions. A compa a i e analysis based on
BER, Q- ac o and channel c oss alk is pe o med o
single and dual mul iplexed designs wi h SCM, MDM
domains. Sa is ac o y esul s a e obse ed o dual
SCM-MDM based design. Fu he , dual SCM-MDM
sys em is analysed o BER and a mosphe ic u bu-
lence condi ions while conside ing HG01, LG10 modes
a 45 GHz and 60 GHz subca ie equencies.
Resul s show signi ican imp o emen s in ecei ed
signal powe and BER o LG10 mode a highe subca -
ie equency o 60 GHz. Unde clea ai condi ions,
FSO channel shows low a mosphe ic a enua ion and
imp o ed SNR in compa ison o haze, ain, og and
dus y wea he condi ions. Due o he possibili y o
accommoda e mo e numbe o addi ional in o ma ion
s eams wi h ew design change, his app oach o e s
a highe ansmission a e, be e spec um e iciency
and imp o ed channel capaci y le els which can be
easily achie ed.
Acknowledgmen
We exp ess ou since e hanks o Jaypee Ins i u e o
In o ma ion Technology, Noida, India o p o iding
esea ch acili ies and a ious ools ela ed o he wo k
p esen ed in his design.
Au ho Con ibu ions
A.S. has de eloped he design concep , o mula ed
and pe o med simula ion using Op isys em ool and
measu emen s a e e i ied o design easibili y. Bo h
A.S. and R.K. con ibu ed o he inal e sion o he
manusc ip . R.K. supe ised he wo k done.
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