4 × 20 Gbps OFDM Based RoFSO Transmission Using Hybrid SCM-MDM Under Different Turbulence Conditions

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 ©2023 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 48 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 ©2023 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 49 OPTICS AND OPTOELECTRONICS VOLUME: 21 |NUMBER: 1 |2023 |MARCH 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- ©2023 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 50 OPTICS AND OPTOELECTRONICS VOLUME: 21 |NUMBER: 1 |2023 |MARCH 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. ©2023 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 51 OPTICS AND OPTOELECTRONICS VOLUME: 21 |NUMBER: 1 |2023 |MARCH 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- ©2023 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 52 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 ©2023 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 53 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. ©2023 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 54 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. ©2023 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 55 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. Re e ences [1] MANSOUR, A., R. MASLEH and M. ABAZA. New Challenges in Wi eless and F ee Space Op ical Communica ions. 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