Research ArticlePerformance of OFDM FSO Communication System withHybrid Channel Codes during Weak Turbulence
Ritu Gupta 1 Tara Singh Kamal2 and Preeti Singh3
1ECE IKGPTU Research Scholar Jalandhar 144601 India2ECE IEEE Life Sr Member RIET Abohar 152116 USA3ECE UIET-PU Chandigarh 160016 India
Correspondence should be addressed to Ritu Gupta erritugupta02gmailcom
Received 31 August 2018 Accepted 21 January 2019 Published 7 February 2019
Academic Editor Peter Mueller
Copyright copy 2019 Ritu Gupta et al is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
e performance of orthogonal frequency division multiplexing- (OFDM-) based free-space optics (FSO) depends on variousparameters such as number of subcarriers base band modulation nature of laser beam turbulence modelling and much moreVarious diversity techniques have been studied by researchers for the improvement of signal strength due to fading caused byatmospheric turbulence In this paper a novel channel coding scheme formed by serially concatenation of irregular low-densityparity check (LDPC) and trellis code modulation (TCM) codes linked by interleaver is proposed e proposed unified codingscheme is simulated and analyzed using the lognormal scintillation model which is suitable for weak turbulent conditions eobtained results are the comparative study of various channel coding schemes in terms of bit error rate (BER) vs signal-to-noiseratio (SNR) Simulation results confirm that newly designed hybrid code outperforms the independently coded and uncodedsystems under weak turbulence conditions by reducing the number of errors in the transmitted information that occurs due tofading It is found that the presented hybrid coded OFDM-FSO system with 16-level quadrature amplitude modulation (QAM)provides significant improvement with less decoding complexity and reasonable delay
1 Introduction
In the past free-space optics (FSO) has received the re-searcherrsquos attention due to its numerous advantages overradio-frequency (RF) communication such as less cost easydeployment and free of license and subsequently used inhigh-bandwidth applications [1 2] FSO has various knock-on effects in military applications disaster recovery back-haul connectivity and line-of-sight (LOS) link for com-munication between planes ships etc [1] due to varyingatmospheric turbulent conditions Because of atmosphericturbulence conditions link performance droops due towater particles and aerosols which causes changes in therefractive index affecting the propagation of the LASERbeam through atmosphere [3 4] Atmospheric turbulenceprovides high hindrance as LOS requirement is not con-tended Various diversity techniques have been analyzed inthe earlier research for reducing the impact of turbulence on
the performance of FSO link [5 6] Among various atmo-spheric aspects such as rain haze snow hail and fog themajor deterrent that affects the consistency and reliability ofthe FSO link is fog [7]
While data transmission through free space it is foreseenthat channel encoding plays the utmost important role andcontributes for better performance of data carriage evarious channel coding schemes also known as forward errorcorrection (FEC) schemes have been used in FSO com-munication due to their pros such as containing numerousvalid code words transmitting quickly and detecting orcorrecting block of errors [8ndash11] Convolutional coding(CC) has been considered as powerful error-correctingcodes as proved by Fang et al in [12] In most cases itmakes a good compromise between performance anddecoding complexity so it becomes a suitable choice fortransmission of data through wireless means whereas in thepresence of intrachannel nonlinear effects low-density
HindawiJournal of Computer Networks and CommunicationsVolume 2019 Article ID 1306491 6 pageshttpsdoiorg10115520191306491
parity-check (LDPC) codes outperform the turbo-productcodes in burst error-prone channels such as the optical fiberchannel [13 14] Simple trellis-coded modulation (TCM)codes itself are a hybrid scheme having a combination ofcoder and modulator and are appropriate for signal trans-mission in FSO communication because of its worthierperformance or rise in throughput (in bits per second) withsophisticated coding gain TCM improves the noise im-munity of digital communication systems without data-ratereduction or bandwidth expansion [15 16]
Wavelength division multiplexing (WDM) multiplexesthe multiple optical carriers into single fiber [17 18]whereas orthogonal frequency division multiplexing(OFDM) is chosen formulticarrier modulation because of itsinherent resilience to frequency selectiveness of the opticalcommunication channel [19] OFDM is computationallyeffectual by using inverse fast Fourier transform (IFFT) andfast Fourier transform (FFT) techniques to carry out themodulation and demodulation functions respectively Formitigation of short-term loss of signal strength due to fadingother diversity techniques such as time diversity and spatialdiversity have also been studied by Lee and Chan in [20] Inthis paper a novel unified coding scheme which is a serialconcatenation of LDPC and TCM codes has been proposede performance of the hybrid LDPC-TCM-coded FSO-OFDM system with 16-QAM has been analyzed using thelognormal turbulence model during weak turbulence ekey idea is to fully exploit the advantages of both OFDM andchannel coding for the FSO communication system
e remainder of the paper is organized as follows InSection 2 the system model and general assumptions arepresented Next simulation results are discussed in Section 3and Section 4 has shown the comparative analysis of variouschannel coding schemes Finally Section 5 concludes the paper
2 System Model
While it can be difficult to realize perfect knowledge of theturbulence distribution it is feasible to find and make use ofthe statistical moments of the turbulencee commonly usedstatistical models for depicting atmospheric turbulencechannels are the gamma-gammamodel negative exponentiallognormal distribution model K-distribution model and I-Kdistribution e lognormal distribution is suitable for weakturbulence and characterizes wireless optical communicationslinks over few hundred meters during the clear weatherconditions [15] e K-distribution model is fit for charac-terizing strong turbulence e negative exponential distri-bution defines the limiting instance of saturated scintillatione gamma-gamma (ΓΓ) distribution is a general model thatcan be functional under varied range of turbulence conditionsduring weak to strong [10] In this work weak turbulencecondition is considered and so the lognormal distributionmodel is highlighted because of its proficiency under suchconditions According to the lognormal distribution channelmodel the optical irradiance I is specified by
I exp(X) (1)
where X Gaussian RV with mean μ and variance σ2
Probability density function (pdf) of the receiving ir-radiance fluctuation is given as [21 22]
p(I) 12π
radic middot1σI
middot expminus(ln I + μ)2
2σ21113896 1113897 (2)
where I signal intensity assuming mean of I is 1 due tonormalized channel effect σ2 is a Rytov variance or varianceof light intensity
σ2 123C2Nk
76L116
(3)
where C2N refractive-index structure parameter k (wave
number) (2πλ) and L the distance between transmitterand receiver
e system model discussed in this paper is the single-input-single-output (SISO) OFDM-FSO system with hybridcoding scheme to endure the consequence of weak turbu-lence conditions such as maritime and light fog on FSO linke systemrsquos efficiency is estimated from the bit error rate(BER) and comparative analysis of the proposed hybridchannel coded scheme is done with the preexisting channelcoding schemes under weak turbulence conditions such asmaritime and light fog e simulations have been doneusing MATLAB 2016Ra is section of the paper confersthe design deliberations of the systems
21 System I OFDM-FSOSystemwith Single Channel Codinge system setup side is the same as it is done in wirelessnetworks except it uses optical transmitters such as LASERand optical receivers such as photodetectors for commu-nication e SISO-based OFDM-FSO system which hasbeen considered in MATLAB is shown in Figure 1
It consists of a transmitter a receiver and a free-spacechannel between them e transmitter uses a random bitgenerator FEC or channel encoder interleaver QAMmodulator IFFT OFDM multiplexer and an optical source(LASER) for transmission e receiver section has OFDMdemultiplexer FFT demodulator deinterleaver and de-coder Use of interleaver upsurges total delay because beforethe packets can be decoded the entire interleaved blockmustbe received Also the errorrsquos structure has not been exposedby interleavers e bit error rate (BER) has been analyzed atthe receiver end For channel modelling the lognormalturbulence model is obliged
e wavelength used is 1550 nm due to its low attenu-ation characteristics e system is designed with the use ofOFDM network e number of subcarriers used in thesystem is 4 because as the number of subcarriers increasesthe SNR needed to achieve the required BER also increases[22]
In OFDM the required FFTIFFT is calculated using 16points e simulation parameters considered in the pro-posed SISOmodel as depicted in Figures 1 and 2 are given inTable 1
22 Design of the OFDM-FSO System with LDPC EncoderLow-density parity-check (LDPC) codes are powerfulchannel (FEC) codes that enhance the constant size
2 Journal of Computer Networks and Communications
redundancy to correct random and burst errors [13] Due toits various pros such as high code rate and low hardwarecomplexity [11] LDPC codes are currently used in FSOcommunication and perform decent during strong turbu-lence e LDPC code is a special linear group of codes withsparse parity-check matrix with N-K by N Many methodshave already been settled for the construction of parity-check matrix [13 14] e LDPC decoder halts when apermissible code word is found and signicantly has apotential to reduce the eort ese codes are advantageouswhen considering long codes due to parallel implementationof decoders In presented LDPC encoder the matrix di-mensions are 32400 64800 and the BER vs SNR is shown inFigure 3 In the said system an irregular LDPC code is usedwith code rate 12 From the results it can be observed thatat 25 dB the error rate of 10minus6 is achieved
23 Design of the OFDM-FSO System with TCM EncoderTCM has lesser computational complexity because ofcombination of encoder and modulator in a single block andreasonable BER performance without dropping bandwidthutilization rate e 12 rate trellis structure and 16-QAMmodulation have been considered for the analysis e BERvs SNR plot is shown in Figure 4 and it is found that at27 dB the error rate is 10minus6
FECcoder
IP datastream (b bits)
OFDMmodulator
OP datastream
FECdecoder PhotodetectorOFDM
demodulator
Optical Tx
FSOlink
Figure 1 Block diagram of OFDM-based FSO communication system [10]
Tx (N = 1)
Information source (b bits)
Rx (M = 1)
Output information
LDPCencoder
TCMencoder
OFDMmodulator
OFDMdemodulator
TCMdecoder
LDPCdecoder
FSOlink
Figure 2 Block diagram of SISO OFDM-based FSO hybrid LDPC- and TCM-coded system
Table 1 System parameters used in the proposed model
SNR vs BER for LDPC coding with lognormal distribution
Figure 3 BER vs electrical SNR of OFDM-FSO system with LDPCencoder
Journal of Computer Networks and Communications 3
24 System II OFDM-FSO System with Hybrid ChannelCodes e system designed with the combination of twochannel codes is shown in Figure 2 As shown in gure theother system arrangements are the same as given in Figure 1except two channel codes are cascaded to make a hybridsystem as the other codes are concatenated in [23 24]
e concatenation of error correcting codes is constructedto achieve copacetic performance with reasonable complexityLDPC is opted because of better data compression capabilityand TCM increases the bit rate by doubling the constellationpoints of the signal e other system specications are thesame as the OFDM-FSO system discussed in Section 21 eSNR vs BER plot is shown in Figure 5 As predicted from thegure at 30 dB the error rate is 10minus8
3 Results and Discussion
e results attained from various channel codes for weakturbulence conditions have been validated using MATLABand the scintillation has been modelled using the lognormalturbulence model In the simulation the Monte Carlomethod has been adopted for the verication of results byrandom attempts
In the proposed hybrid coded system the pair of singleinner-outer encoders and decoders has been used for mini-mum component cost computational complexity and syn-chronization issues Using proposed hybrid coding schemethe systemrsquos reliability is increased having acceptable BERwith reasonable computational complexity or delay latencye various channel coding schemes under weak turbulentconditions are analyzed in the following sectione results ofsystem model 1 and 2 are compared in Figure 6
As expected the system with proposed hybrid codesperforms well as compared to individually coded system In
Figure 6 the hybrid coded 16-QAM modulated systemrequires an SNR of 20 dB to attain a BER of 10minus6 while thesystem with LDPC and TCM coded individually requiresSNR of 25 dB and 27 dB respectively To oset the dipoundcultyof applying systems the performance dierence can comeout with the ensuing hybrid coded FSO system design
4 Comparative Analysis under DifferentTurbulence Conditions
In this section the performance estimation of variouschannel codes for OFDM-based FSO systems over weakturbulence conditions has been tabulated in Table 2 in acompressed format e table is representing the required
0 5 10 15 20 25 30SNR (dB)
10ndash7
10ndash6
10ndash5
10ndash4
10ndash3
10ndash2
10ndash1
100
BER
TCM coding for lognormal distribution
Figure 4 BER vs electrical SNR of OFDM-FSO system with TCMencoder
0 5 10 15 20 25 30 35SNR (dB)
SNR vs BER for hybrid coding with lognormal distribution
10ndash10
10ndash8
10ndash6
10ndash4
10ndash2
100
BER
Figure 5 BER vs electrical SNR of OFDM-FSO systemwith hybridLDPC-TCM encoder
0 5 10 15 20 25 30 35SNR (dB)
Lognormal distribution turbulence model
10ndash10
10ndash8
10ndash6
10ndash4
10ndash2
102
100
BER
HybridUncoded
TCMLDPC
Figure 6 BER vs electrical SNR of OFDM-FSO system withvarious channel codes using lognormal distribution
4 Journal of Computer Networks and Communications
SNR for achieving the BER of 10minus6 with various codingschemes
Form Table 2 it has been revealed that the proposedhybrid coding scheme gives the best performance amongother considered coding schemes during weak turbulencee different turbulence conditions affect the visibility andhence signal is attenuated which further causes the BERrough simulations significant improvement in perfor-mance has been obtained by concatenating LDPC codes withinterleaved schemes of TCM codes over fading channelseproposed hybrid coding has been compared with the LDPCand TCM coding with same trellis complexity and theuncoded systems with single antenna fading channels Asshown in Figure 6 with the proposed hybrid scheme theBER of 10minus9 is achieved at 30 dB of the SNR value duringweak turbulence conditions whereas BER of 10minus8 and 10minus7has been achieved at 30 dB by using LDPC and TCM codesrespectively e system has been simulated with MATLABcodes under various channel conditions using the lognormaldistribution turbulence model
5 Conclusion
Atmospheric turbulence comes out to be the major bot-tleneck in free-space optical communication systems andlimiting its use in extended applications Consistent researchis being carried out for reducing these effects and channelcoding plays the most important role is paper presenteda unified channel coding scheme constituted by concate-nation of irregular LDPC and TCM codes of 12 rate eyapplied the advantages of LDPC and TCM codes to formless complex reliable and high-performance concatenatedcodes For OFDM-based FSO system different conventionalchannel coding schemes have been examined and comparedwith a new hybrid channel coding scheme e OFDM-FSOsystem using the proposed scheme has been compared withthe conventionally used coding schemes under weak tur-bulent conditions e presented analysis of results indicatesthat for attaining BER of 10minus6 the required SNR for LDPCTCM and hybrid LDPC-TCM scheme is 25 dB 27 dBand 20 dB respectively e OFDM-FSO system designedwith novel hybrid channel coding shows an improvementof 5 dB as compared to solo LDPC and 7 dB as comparedto TCM alone e comparison of results is done by sim-ulating the system model using MATLAB codes
Data Availability
Input data have been randomly generated and the otherparameters have been stated in the manuscript
Conflicts of Interest
e authors declare that they have no conflicts of interest
References
[1] A Malik and P Singh ldquoFree space optics current applicationsand future challengesrdquo International Journal of Opticsvol 2015 Article ID 945483 7 pages 2015
[2] S P Joseph and M Mathew ldquoA review on performanceimprovement techniques in wireless optical communicationrdquoInternational Journal of Science and Research vol 4 no 8pp 745ndash749 2015
[3] M N O Sadiku S M Musa and S R Nelatury ldquoFree spaceoptical communications an overviewrdquo European ScientificJournal ESJ vol 12 no 9 pp 55ndash68 2016
[4] R Gupta and P Singh ldquoHybrid FSO-RF system a solution toatmospheric turbulences in long haul communicationrdquo In-ternational Journal of Scientific and Engineering Researchvol 5 no 11 pp 602ndash605 2014
[5] T A Tsiftsis H G Sandalidis G K Karagiannidis andM Uysal ldquoOptical wireless links with spatial diversity overstrong atmospheric turbulence channelsrdquo IEEE Transactionson Wireless Communications vol 8 no 2 pp 951ndash957 2009
[6] D Shah B Nayak and D Jethawani ldquoStudy of differentatmospheric channel modelsrdquo International Journal of Elec-tronics and Communication Engineering amp Technology vol 5no 1 pp 105ndash112 2014
[7] N Gupta S D Prakash H Kaushal V K Jain and S KarldquoPerformance analysis of FSO communication using differentcoding schemesrdquo AIP Conference Proceedings vol 1391pp 387ndash391 2011
[8] H Hennigera E Bernhard D M Stuart andC D Christopher ldquoCoding techniques to mitigate fading onfree-space optical communication linksrdquo in Proceedings ofSPIE-9e International Society for Optical Engineering SanDiego CA USA August 2008
[9] J S Nandaniya N B Kalani and G R Kulkarni ldquoCom-parative analysis of different channel coding techniquesrdquoInternational Journal of Computer Networks amp Communi-cations vol 4 no 2 pp 84ndash89 2014
[10] I B Djordjevic B Vasic and M A Neifeld ldquoLDPC codedOFDM over the atmospheric turbulence channelrdquo OpticsExpress vol 15 no 10 pp 6336ndash6350 2007
[11] A Joseph ldquoDesign of the high-speed framing FEC and in-terleaving hardware used in a 54 km free-space opticalcommunication experimentrdquo in Proceedings of Free-SpaceLaser Communications vol 7464 San Diego CA USAAugust 2009
[12] X Fang A Khalighi C Patrice and B Salah ldquoChannelcoding and time-diversity for optical wireless linksrdquo OpticsExpress vol 17 no 2 pp 872ndash887 2009
[13] Y Kou S Lin and M P C Fossorier ldquoLow density paritycheck codes construction based on finite geometries GlobalTelecommunications Conferencerdquo in Proceedings of Globe-com 2000-IEEE Global Telecommunications Conference vol 2pp 825ndash829 San Francisco CA USA November 2000
[14] D J C MacKay ldquoGood error-correcting codes based on verysparse matricesrdquo IEEE Transactions on Information 9eoryvol 45 no 2 pp 399ndash431 1999
[15] M Lin K Wang X Xin et al ldquoRapid soft-decision trelliscoded 32-QAM for free space optical communicationrdquo inProceedings of Asia Communications and PhotonicsConference Guangzhou China November 2012
Table 2 Comparative analysis of various coding schemes (requiredSNR for achieving BER of 10minus6)
[16] N G Costas ldquoSome implications of TCM for optical direct-detection channelsrdquo IEEE Transactions on Communicationsvol 37 no 5 pp 481ndash487 1989
[17] M Grover P Singh P Kaur and C Madhu ldquoMultibeamWDM-FSO system an optimum solution for clear and hazyweather conditionsrdquo Wireless Personal Communicationsvol 97 no 4 pp 5783ndash5795 2017
[18] M Grover P Singh and P Kaur ldquoMitigation of scintillationeffects in WDM FSO system using multibeam techniquerdquoJournal of Telecommunications and Information Technologyvol 2 pp 69ndash74 2017
[19] Y Wang D Wang and M Jing ldquoOn the performance ofcoherent OFDM systems in free-space optical communica-tionsrdquo IEEE Photonics Journal vol 7 no 4 pp 1ndash10 2015
[20] E J Lee and V W S Chan ldquoPart 1 optical communicationover the clear turbulent atmospheric channel using diversityrdquoIEEE Journal on Selected Areas in Communications vol 22no 9 pp 1896ndash1906 2004
[21] L Yang X Song J Cheng and J F Holzman ldquoFree-spaceoptical communications over lognormal fading channelsusing OOK with finite extinction ratiosrdquo IEEE Access vol 4pp 574ndash584 2016
[22] Z Ghassemlooy W Popoola and S Rajbhandari OpticalWireless Communications System and Channel ModellingCRC Press Taylor amp Francis Group Boca Raton FL USA2013
[23] J Yuan Z Jiang Y Mao and W Ye ldquoForward error cor-rection concatenated code in DWDM systemsrdquo Frontiers ofOptoelectronics in China vol 1 no 1-2 pp 20ndash24 2008
[24] Yi Yi Gong and K Ben Letaief ldquoConcatenated space-timeblock coding with trellis coded modulation in fading chan-nelsrdquo IEEE Transactions on Wireless Communications vol 1no 4 pp 580ndash590 2002
Chemical EngineeringInternational Journal of Antennas and
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Navigation and Observation
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Advances in
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parity-check (LDPC) codes outperform the turbo-productcodes in burst error-prone channels such as the optical fiberchannel [13 14] Simple trellis-coded modulation (TCM)codes itself are a hybrid scheme having a combination ofcoder and modulator and are appropriate for signal trans-mission in FSO communication because of its worthierperformance or rise in throughput (in bits per second) withsophisticated coding gain TCM improves the noise im-munity of digital communication systems without data-ratereduction or bandwidth expansion [15 16]
Wavelength division multiplexing (WDM) multiplexesthe multiple optical carriers into single fiber [17 18]whereas orthogonal frequency division multiplexing(OFDM) is chosen formulticarrier modulation because of itsinherent resilience to frequency selectiveness of the opticalcommunication channel [19] OFDM is computationallyeffectual by using inverse fast Fourier transform (IFFT) andfast Fourier transform (FFT) techniques to carry out themodulation and demodulation functions respectively Formitigation of short-term loss of signal strength due to fadingother diversity techniques such as time diversity and spatialdiversity have also been studied by Lee and Chan in [20] Inthis paper a novel unified coding scheme which is a serialconcatenation of LDPC and TCM codes has been proposede performance of the hybrid LDPC-TCM-coded FSO-OFDM system with 16-QAM has been analyzed using thelognormal turbulence model during weak turbulence ekey idea is to fully exploit the advantages of both OFDM andchannel coding for the FSO communication system
e remainder of the paper is organized as follows InSection 2 the system model and general assumptions arepresented Next simulation results are discussed in Section 3and Section 4 has shown the comparative analysis of variouschannel coding schemes Finally Section 5 concludes the paper
2 System Model
While it can be difficult to realize perfect knowledge of theturbulence distribution it is feasible to find and make use ofthe statistical moments of the turbulencee commonly usedstatistical models for depicting atmospheric turbulencechannels are the gamma-gammamodel negative exponentiallognormal distribution model K-distribution model and I-Kdistribution e lognormal distribution is suitable for weakturbulence and characterizes wireless optical communicationslinks over few hundred meters during the clear weatherconditions [15] e K-distribution model is fit for charac-terizing strong turbulence e negative exponential distri-bution defines the limiting instance of saturated scintillatione gamma-gamma (ΓΓ) distribution is a general model thatcan be functional under varied range of turbulence conditionsduring weak to strong [10] In this work weak turbulencecondition is considered and so the lognormal distributionmodel is highlighted because of its proficiency under suchconditions According to the lognormal distribution channelmodel the optical irradiance I is specified by
I exp(X) (1)
where X Gaussian RV with mean μ and variance σ2
Probability density function (pdf) of the receiving ir-radiance fluctuation is given as [21 22]
p(I) 12π
radic middot1σI
middot expminus(ln I + μ)2
2σ21113896 1113897 (2)
where I signal intensity assuming mean of I is 1 due tonormalized channel effect σ2 is a Rytov variance or varianceof light intensity
σ2 123C2Nk
76L116
(3)
where C2N refractive-index structure parameter k (wave
number) (2πλ) and L the distance between transmitterand receiver
e system model discussed in this paper is the single-input-single-output (SISO) OFDM-FSO system with hybridcoding scheme to endure the consequence of weak turbu-lence conditions such as maritime and light fog on FSO linke systemrsquos efficiency is estimated from the bit error rate(BER) and comparative analysis of the proposed hybridchannel coded scheme is done with the preexisting channelcoding schemes under weak turbulence conditions such asmaritime and light fog e simulations have been doneusing MATLAB 2016Ra is section of the paper confersthe design deliberations of the systems
21 System I OFDM-FSOSystemwith Single Channel Codinge system setup side is the same as it is done in wirelessnetworks except it uses optical transmitters such as LASERand optical receivers such as photodetectors for commu-nication e SISO-based OFDM-FSO system which hasbeen considered in MATLAB is shown in Figure 1
It consists of a transmitter a receiver and a free-spacechannel between them e transmitter uses a random bitgenerator FEC or channel encoder interleaver QAMmodulator IFFT OFDM multiplexer and an optical source(LASER) for transmission e receiver section has OFDMdemultiplexer FFT demodulator deinterleaver and de-coder Use of interleaver upsurges total delay because beforethe packets can be decoded the entire interleaved blockmustbe received Also the errorrsquos structure has not been exposedby interleavers e bit error rate (BER) has been analyzed atthe receiver end For channel modelling the lognormalturbulence model is obliged
e wavelength used is 1550 nm due to its low attenu-ation characteristics e system is designed with the use ofOFDM network e number of subcarriers used in thesystem is 4 because as the number of subcarriers increasesthe SNR needed to achieve the required BER also increases[22]
In OFDM the required FFTIFFT is calculated using 16points e simulation parameters considered in the pro-posed SISOmodel as depicted in Figures 1 and 2 are given inTable 1
22 Design of the OFDM-FSO System with LDPC EncoderLow-density parity-check (LDPC) codes are powerfulchannel (FEC) codes that enhance the constant size
2 Journal of Computer Networks and Communications
redundancy to correct random and burst errors [13] Due toits various pros such as high code rate and low hardwarecomplexity [11] LDPC codes are currently used in FSOcommunication and perform decent during strong turbu-lence e LDPC code is a special linear group of codes withsparse parity-check matrix with N-K by N Many methodshave already been settled for the construction of parity-check matrix [13 14] e LDPC decoder halts when apermissible code word is found and signicantly has apotential to reduce the eort ese codes are advantageouswhen considering long codes due to parallel implementationof decoders In presented LDPC encoder the matrix di-mensions are 32400 64800 and the BER vs SNR is shown inFigure 3 In the said system an irregular LDPC code is usedwith code rate 12 From the results it can be observed thatat 25 dB the error rate of 10minus6 is achieved
23 Design of the OFDM-FSO System with TCM EncoderTCM has lesser computational complexity because ofcombination of encoder and modulator in a single block andreasonable BER performance without dropping bandwidthutilization rate e 12 rate trellis structure and 16-QAMmodulation have been considered for the analysis e BERvs SNR plot is shown in Figure 4 and it is found that at27 dB the error rate is 10minus6
FECcoder
IP datastream (b bits)
OFDMmodulator
OP datastream
FECdecoder PhotodetectorOFDM
demodulator
Optical Tx
FSOlink
Figure 1 Block diagram of OFDM-based FSO communication system [10]
Tx (N = 1)
Information source (b bits)
Rx (M = 1)
Output information
LDPCencoder
TCMencoder
OFDMmodulator
OFDMdemodulator
TCMdecoder
LDPCdecoder
FSOlink
Figure 2 Block diagram of SISO OFDM-based FSO hybrid LDPC- and TCM-coded system
Table 1 System parameters used in the proposed model
SNR vs BER for LDPC coding with lognormal distribution
Figure 3 BER vs electrical SNR of OFDM-FSO system with LDPCencoder
Journal of Computer Networks and Communications 3
24 System II OFDM-FSO System with Hybrid ChannelCodes e system designed with the combination of twochannel codes is shown in Figure 2 As shown in gure theother system arrangements are the same as given in Figure 1except two channel codes are cascaded to make a hybridsystem as the other codes are concatenated in [23 24]
e concatenation of error correcting codes is constructedto achieve copacetic performance with reasonable complexityLDPC is opted because of better data compression capabilityand TCM increases the bit rate by doubling the constellationpoints of the signal e other system specications are thesame as the OFDM-FSO system discussed in Section 21 eSNR vs BER plot is shown in Figure 5 As predicted from thegure at 30 dB the error rate is 10minus8
3 Results and Discussion
e results attained from various channel codes for weakturbulence conditions have been validated using MATLABand the scintillation has been modelled using the lognormalturbulence model In the simulation the Monte Carlomethod has been adopted for the verication of results byrandom attempts
In the proposed hybrid coded system the pair of singleinner-outer encoders and decoders has been used for mini-mum component cost computational complexity and syn-chronization issues Using proposed hybrid coding schemethe systemrsquos reliability is increased having acceptable BERwith reasonable computational complexity or delay latencye various channel coding schemes under weak turbulentconditions are analyzed in the following sectione results ofsystem model 1 and 2 are compared in Figure 6
As expected the system with proposed hybrid codesperforms well as compared to individually coded system In
Figure 6 the hybrid coded 16-QAM modulated systemrequires an SNR of 20 dB to attain a BER of 10minus6 while thesystem with LDPC and TCM coded individually requiresSNR of 25 dB and 27 dB respectively To oset the dipoundcultyof applying systems the performance dierence can comeout with the ensuing hybrid coded FSO system design
4 Comparative Analysis under DifferentTurbulence Conditions
In this section the performance estimation of variouschannel codes for OFDM-based FSO systems over weakturbulence conditions has been tabulated in Table 2 in acompressed format e table is representing the required
0 5 10 15 20 25 30SNR (dB)
10ndash7
10ndash6
10ndash5
10ndash4
10ndash3
10ndash2
10ndash1
100
BER
TCM coding for lognormal distribution
Figure 4 BER vs electrical SNR of OFDM-FSO system with TCMencoder
0 5 10 15 20 25 30 35SNR (dB)
SNR vs BER for hybrid coding with lognormal distribution
10ndash10
10ndash8
10ndash6
10ndash4
10ndash2
100
BER
Figure 5 BER vs electrical SNR of OFDM-FSO systemwith hybridLDPC-TCM encoder
0 5 10 15 20 25 30 35SNR (dB)
Lognormal distribution turbulence model
10ndash10
10ndash8
10ndash6
10ndash4
10ndash2
102
100
BER
HybridUncoded
TCMLDPC
Figure 6 BER vs electrical SNR of OFDM-FSO system withvarious channel codes using lognormal distribution
4 Journal of Computer Networks and Communications
SNR for achieving the BER of 10minus6 with various codingschemes
Form Table 2 it has been revealed that the proposedhybrid coding scheme gives the best performance amongother considered coding schemes during weak turbulencee different turbulence conditions affect the visibility andhence signal is attenuated which further causes the BERrough simulations significant improvement in perfor-mance has been obtained by concatenating LDPC codes withinterleaved schemes of TCM codes over fading channelseproposed hybrid coding has been compared with the LDPCand TCM coding with same trellis complexity and theuncoded systems with single antenna fading channels Asshown in Figure 6 with the proposed hybrid scheme theBER of 10minus9 is achieved at 30 dB of the SNR value duringweak turbulence conditions whereas BER of 10minus8 and 10minus7has been achieved at 30 dB by using LDPC and TCM codesrespectively e system has been simulated with MATLABcodes under various channel conditions using the lognormaldistribution turbulence model
5 Conclusion
Atmospheric turbulence comes out to be the major bot-tleneck in free-space optical communication systems andlimiting its use in extended applications Consistent researchis being carried out for reducing these effects and channelcoding plays the most important role is paper presenteda unified channel coding scheme constituted by concate-nation of irregular LDPC and TCM codes of 12 rate eyapplied the advantages of LDPC and TCM codes to formless complex reliable and high-performance concatenatedcodes For OFDM-based FSO system different conventionalchannel coding schemes have been examined and comparedwith a new hybrid channel coding scheme e OFDM-FSOsystem using the proposed scheme has been compared withthe conventionally used coding schemes under weak tur-bulent conditions e presented analysis of results indicatesthat for attaining BER of 10minus6 the required SNR for LDPCTCM and hybrid LDPC-TCM scheme is 25 dB 27 dBand 20 dB respectively e OFDM-FSO system designedwith novel hybrid channel coding shows an improvementof 5 dB as compared to solo LDPC and 7 dB as comparedto TCM alone e comparison of results is done by sim-ulating the system model using MATLAB codes
Data Availability
Input data have been randomly generated and the otherparameters have been stated in the manuscript
Conflicts of Interest
e authors declare that they have no conflicts of interest
References
[1] A Malik and P Singh ldquoFree space optics current applicationsand future challengesrdquo International Journal of Opticsvol 2015 Article ID 945483 7 pages 2015
[2] S P Joseph and M Mathew ldquoA review on performanceimprovement techniques in wireless optical communicationrdquoInternational Journal of Science and Research vol 4 no 8pp 745ndash749 2015
[3] M N O Sadiku S M Musa and S R Nelatury ldquoFree spaceoptical communications an overviewrdquo European ScientificJournal ESJ vol 12 no 9 pp 55ndash68 2016
[4] R Gupta and P Singh ldquoHybrid FSO-RF system a solution toatmospheric turbulences in long haul communicationrdquo In-ternational Journal of Scientific and Engineering Researchvol 5 no 11 pp 602ndash605 2014
[5] T A Tsiftsis H G Sandalidis G K Karagiannidis andM Uysal ldquoOptical wireless links with spatial diversity overstrong atmospheric turbulence channelsrdquo IEEE Transactionson Wireless Communications vol 8 no 2 pp 951ndash957 2009
[6] D Shah B Nayak and D Jethawani ldquoStudy of differentatmospheric channel modelsrdquo International Journal of Elec-tronics and Communication Engineering amp Technology vol 5no 1 pp 105ndash112 2014
[7] N Gupta S D Prakash H Kaushal V K Jain and S KarldquoPerformance analysis of FSO communication using differentcoding schemesrdquo AIP Conference Proceedings vol 1391pp 387ndash391 2011
[8] H Hennigera E Bernhard D M Stuart andC D Christopher ldquoCoding techniques to mitigate fading onfree-space optical communication linksrdquo in Proceedings ofSPIE-9e International Society for Optical Engineering SanDiego CA USA August 2008
[9] J S Nandaniya N B Kalani and G R Kulkarni ldquoCom-parative analysis of different channel coding techniquesrdquoInternational Journal of Computer Networks amp Communi-cations vol 4 no 2 pp 84ndash89 2014
[10] I B Djordjevic B Vasic and M A Neifeld ldquoLDPC codedOFDM over the atmospheric turbulence channelrdquo OpticsExpress vol 15 no 10 pp 6336ndash6350 2007
[11] A Joseph ldquoDesign of the high-speed framing FEC and in-terleaving hardware used in a 54 km free-space opticalcommunication experimentrdquo in Proceedings of Free-SpaceLaser Communications vol 7464 San Diego CA USAAugust 2009
[12] X Fang A Khalighi C Patrice and B Salah ldquoChannelcoding and time-diversity for optical wireless linksrdquo OpticsExpress vol 17 no 2 pp 872ndash887 2009
[13] Y Kou S Lin and M P C Fossorier ldquoLow density paritycheck codes construction based on finite geometries GlobalTelecommunications Conferencerdquo in Proceedings of Globe-com 2000-IEEE Global Telecommunications Conference vol 2pp 825ndash829 San Francisco CA USA November 2000
[14] D J C MacKay ldquoGood error-correcting codes based on verysparse matricesrdquo IEEE Transactions on Information 9eoryvol 45 no 2 pp 399ndash431 1999
[15] M Lin K Wang X Xin et al ldquoRapid soft-decision trelliscoded 32-QAM for free space optical communicationrdquo inProceedings of Asia Communications and PhotonicsConference Guangzhou China November 2012
Table 2 Comparative analysis of various coding schemes (requiredSNR for achieving BER of 10minus6)
[16] N G Costas ldquoSome implications of TCM for optical direct-detection channelsrdquo IEEE Transactions on Communicationsvol 37 no 5 pp 481ndash487 1989
[17] M Grover P Singh P Kaur and C Madhu ldquoMultibeamWDM-FSO system an optimum solution for clear and hazyweather conditionsrdquo Wireless Personal Communicationsvol 97 no 4 pp 5783ndash5795 2017
[18] M Grover P Singh and P Kaur ldquoMitigation of scintillationeffects in WDM FSO system using multibeam techniquerdquoJournal of Telecommunications and Information Technologyvol 2 pp 69ndash74 2017
[19] Y Wang D Wang and M Jing ldquoOn the performance ofcoherent OFDM systems in free-space optical communica-tionsrdquo IEEE Photonics Journal vol 7 no 4 pp 1ndash10 2015
[20] E J Lee and V W S Chan ldquoPart 1 optical communicationover the clear turbulent atmospheric channel using diversityrdquoIEEE Journal on Selected Areas in Communications vol 22no 9 pp 1896ndash1906 2004
[21] L Yang X Song J Cheng and J F Holzman ldquoFree-spaceoptical communications over lognormal fading channelsusing OOK with finite extinction ratiosrdquo IEEE Access vol 4pp 574ndash584 2016
[22] Z Ghassemlooy W Popoola and S Rajbhandari OpticalWireless Communications System and Channel ModellingCRC Press Taylor amp Francis Group Boca Raton FL USA2013
[23] J Yuan Z Jiang Y Mao and W Ye ldquoForward error cor-rection concatenated code in DWDM systemsrdquo Frontiers ofOptoelectronics in China vol 1 no 1-2 pp 20ndash24 2008
[24] Yi Yi Gong and K Ben Letaief ldquoConcatenated space-timeblock coding with trellis coded modulation in fading chan-nelsrdquo IEEE Transactions on Wireless Communications vol 1no 4 pp 580ndash590 2002
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawiwwwhindawicom Volume 2018
Hindawiwwwhindawicom Volume 2018
Navigation and Observation
International Journal of
Hindawi
wwwhindawicom Volume 2018
Advances in
Multimedia
Submit your manuscripts atwwwhindawicom
redundancy to correct random and burst errors [13] Due toits various pros such as high code rate and low hardwarecomplexity [11] LDPC codes are currently used in FSOcommunication and perform decent during strong turbu-lence e LDPC code is a special linear group of codes withsparse parity-check matrix with N-K by N Many methodshave already been settled for the construction of parity-check matrix [13 14] e LDPC decoder halts when apermissible code word is found and signicantly has apotential to reduce the eort ese codes are advantageouswhen considering long codes due to parallel implementationof decoders In presented LDPC encoder the matrix di-mensions are 32400 64800 and the BER vs SNR is shown inFigure 3 In the said system an irregular LDPC code is usedwith code rate 12 From the results it can be observed thatat 25 dB the error rate of 10minus6 is achieved
23 Design of the OFDM-FSO System with TCM EncoderTCM has lesser computational complexity because ofcombination of encoder and modulator in a single block andreasonable BER performance without dropping bandwidthutilization rate e 12 rate trellis structure and 16-QAMmodulation have been considered for the analysis e BERvs SNR plot is shown in Figure 4 and it is found that at27 dB the error rate is 10minus6
FECcoder
IP datastream (b bits)
OFDMmodulator
OP datastream
FECdecoder PhotodetectorOFDM
demodulator
Optical Tx
FSOlink
Figure 1 Block diagram of OFDM-based FSO communication system [10]
Tx (N = 1)
Information source (b bits)
Rx (M = 1)
Output information
LDPCencoder
TCMencoder
OFDMmodulator
OFDMdemodulator
TCMdecoder
LDPCdecoder
FSOlink
Figure 2 Block diagram of SISO OFDM-based FSO hybrid LDPC- and TCM-coded system
Table 1 System parameters used in the proposed model
SNR vs BER for LDPC coding with lognormal distribution
Figure 3 BER vs electrical SNR of OFDM-FSO system with LDPCencoder
Journal of Computer Networks and Communications 3
24 System II OFDM-FSO System with Hybrid ChannelCodes e system designed with the combination of twochannel codes is shown in Figure 2 As shown in gure theother system arrangements are the same as given in Figure 1except two channel codes are cascaded to make a hybridsystem as the other codes are concatenated in [23 24]
e concatenation of error correcting codes is constructedto achieve copacetic performance with reasonable complexityLDPC is opted because of better data compression capabilityand TCM increases the bit rate by doubling the constellationpoints of the signal e other system specications are thesame as the OFDM-FSO system discussed in Section 21 eSNR vs BER plot is shown in Figure 5 As predicted from thegure at 30 dB the error rate is 10minus8
3 Results and Discussion
e results attained from various channel codes for weakturbulence conditions have been validated using MATLABand the scintillation has been modelled using the lognormalturbulence model In the simulation the Monte Carlomethod has been adopted for the verication of results byrandom attempts
In the proposed hybrid coded system the pair of singleinner-outer encoders and decoders has been used for mini-mum component cost computational complexity and syn-chronization issues Using proposed hybrid coding schemethe systemrsquos reliability is increased having acceptable BERwith reasonable computational complexity or delay latencye various channel coding schemes under weak turbulentconditions are analyzed in the following sectione results ofsystem model 1 and 2 are compared in Figure 6
As expected the system with proposed hybrid codesperforms well as compared to individually coded system In
Figure 6 the hybrid coded 16-QAM modulated systemrequires an SNR of 20 dB to attain a BER of 10minus6 while thesystem with LDPC and TCM coded individually requiresSNR of 25 dB and 27 dB respectively To oset the dipoundcultyof applying systems the performance dierence can comeout with the ensuing hybrid coded FSO system design
4 Comparative Analysis under DifferentTurbulence Conditions
In this section the performance estimation of variouschannel codes for OFDM-based FSO systems over weakturbulence conditions has been tabulated in Table 2 in acompressed format e table is representing the required
0 5 10 15 20 25 30SNR (dB)
10ndash7
10ndash6
10ndash5
10ndash4
10ndash3
10ndash2
10ndash1
100
BER
TCM coding for lognormal distribution
Figure 4 BER vs electrical SNR of OFDM-FSO system with TCMencoder
0 5 10 15 20 25 30 35SNR (dB)
SNR vs BER for hybrid coding with lognormal distribution
10ndash10
10ndash8
10ndash6
10ndash4
10ndash2
100
BER
Figure 5 BER vs electrical SNR of OFDM-FSO systemwith hybridLDPC-TCM encoder
0 5 10 15 20 25 30 35SNR (dB)
Lognormal distribution turbulence model
10ndash10
10ndash8
10ndash6
10ndash4
10ndash2
102
100
BER
HybridUncoded
TCMLDPC
Figure 6 BER vs electrical SNR of OFDM-FSO system withvarious channel codes using lognormal distribution
4 Journal of Computer Networks and Communications
SNR for achieving the BER of 10minus6 with various codingschemes
Form Table 2 it has been revealed that the proposedhybrid coding scheme gives the best performance amongother considered coding schemes during weak turbulencee different turbulence conditions affect the visibility andhence signal is attenuated which further causes the BERrough simulations significant improvement in perfor-mance has been obtained by concatenating LDPC codes withinterleaved schemes of TCM codes over fading channelseproposed hybrid coding has been compared with the LDPCand TCM coding with same trellis complexity and theuncoded systems with single antenna fading channels Asshown in Figure 6 with the proposed hybrid scheme theBER of 10minus9 is achieved at 30 dB of the SNR value duringweak turbulence conditions whereas BER of 10minus8 and 10minus7has been achieved at 30 dB by using LDPC and TCM codesrespectively e system has been simulated with MATLABcodes under various channel conditions using the lognormaldistribution turbulence model
5 Conclusion
Atmospheric turbulence comes out to be the major bot-tleneck in free-space optical communication systems andlimiting its use in extended applications Consistent researchis being carried out for reducing these effects and channelcoding plays the most important role is paper presenteda unified channel coding scheme constituted by concate-nation of irregular LDPC and TCM codes of 12 rate eyapplied the advantages of LDPC and TCM codes to formless complex reliable and high-performance concatenatedcodes For OFDM-based FSO system different conventionalchannel coding schemes have been examined and comparedwith a new hybrid channel coding scheme e OFDM-FSOsystem using the proposed scheme has been compared withthe conventionally used coding schemes under weak tur-bulent conditions e presented analysis of results indicatesthat for attaining BER of 10minus6 the required SNR for LDPCTCM and hybrid LDPC-TCM scheme is 25 dB 27 dBand 20 dB respectively e OFDM-FSO system designedwith novel hybrid channel coding shows an improvementof 5 dB as compared to solo LDPC and 7 dB as comparedto TCM alone e comparison of results is done by sim-ulating the system model using MATLAB codes
Data Availability
Input data have been randomly generated and the otherparameters have been stated in the manuscript
Conflicts of Interest
e authors declare that they have no conflicts of interest
References
[1] A Malik and P Singh ldquoFree space optics current applicationsand future challengesrdquo International Journal of Opticsvol 2015 Article ID 945483 7 pages 2015
[2] S P Joseph and M Mathew ldquoA review on performanceimprovement techniques in wireless optical communicationrdquoInternational Journal of Science and Research vol 4 no 8pp 745ndash749 2015
[3] M N O Sadiku S M Musa and S R Nelatury ldquoFree spaceoptical communications an overviewrdquo European ScientificJournal ESJ vol 12 no 9 pp 55ndash68 2016
[4] R Gupta and P Singh ldquoHybrid FSO-RF system a solution toatmospheric turbulences in long haul communicationrdquo In-ternational Journal of Scientific and Engineering Researchvol 5 no 11 pp 602ndash605 2014
[5] T A Tsiftsis H G Sandalidis G K Karagiannidis andM Uysal ldquoOptical wireless links with spatial diversity overstrong atmospheric turbulence channelsrdquo IEEE Transactionson Wireless Communications vol 8 no 2 pp 951ndash957 2009
[6] D Shah B Nayak and D Jethawani ldquoStudy of differentatmospheric channel modelsrdquo International Journal of Elec-tronics and Communication Engineering amp Technology vol 5no 1 pp 105ndash112 2014
[7] N Gupta S D Prakash H Kaushal V K Jain and S KarldquoPerformance analysis of FSO communication using differentcoding schemesrdquo AIP Conference Proceedings vol 1391pp 387ndash391 2011
[8] H Hennigera E Bernhard D M Stuart andC D Christopher ldquoCoding techniques to mitigate fading onfree-space optical communication linksrdquo in Proceedings ofSPIE-9e International Society for Optical Engineering SanDiego CA USA August 2008
[9] J S Nandaniya N B Kalani and G R Kulkarni ldquoCom-parative analysis of different channel coding techniquesrdquoInternational Journal of Computer Networks amp Communi-cations vol 4 no 2 pp 84ndash89 2014
[10] I B Djordjevic B Vasic and M A Neifeld ldquoLDPC codedOFDM over the atmospheric turbulence channelrdquo OpticsExpress vol 15 no 10 pp 6336ndash6350 2007
[11] A Joseph ldquoDesign of the high-speed framing FEC and in-terleaving hardware used in a 54 km free-space opticalcommunication experimentrdquo in Proceedings of Free-SpaceLaser Communications vol 7464 San Diego CA USAAugust 2009
[12] X Fang A Khalighi C Patrice and B Salah ldquoChannelcoding and time-diversity for optical wireless linksrdquo OpticsExpress vol 17 no 2 pp 872ndash887 2009
[13] Y Kou S Lin and M P C Fossorier ldquoLow density paritycheck codes construction based on finite geometries GlobalTelecommunications Conferencerdquo in Proceedings of Globe-com 2000-IEEE Global Telecommunications Conference vol 2pp 825ndash829 San Francisco CA USA November 2000
[14] D J C MacKay ldquoGood error-correcting codes based on verysparse matricesrdquo IEEE Transactions on Information 9eoryvol 45 no 2 pp 399ndash431 1999
[15] M Lin K Wang X Xin et al ldquoRapid soft-decision trelliscoded 32-QAM for free space optical communicationrdquo inProceedings of Asia Communications and PhotonicsConference Guangzhou China November 2012
Table 2 Comparative analysis of various coding schemes (requiredSNR for achieving BER of 10minus6)
[16] N G Costas ldquoSome implications of TCM for optical direct-detection channelsrdquo IEEE Transactions on Communicationsvol 37 no 5 pp 481ndash487 1989
[17] M Grover P Singh P Kaur and C Madhu ldquoMultibeamWDM-FSO system an optimum solution for clear and hazyweather conditionsrdquo Wireless Personal Communicationsvol 97 no 4 pp 5783ndash5795 2017
[18] M Grover P Singh and P Kaur ldquoMitigation of scintillationeffects in WDM FSO system using multibeam techniquerdquoJournal of Telecommunications and Information Technologyvol 2 pp 69ndash74 2017
[19] Y Wang D Wang and M Jing ldquoOn the performance ofcoherent OFDM systems in free-space optical communica-tionsrdquo IEEE Photonics Journal vol 7 no 4 pp 1ndash10 2015
[20] E J Lee and V W S Chan ldquoPart 1 optical communicationover the clear turbulent atmospheric channel using diversityrdquoIEEE Journal on Selected Areas in Communications vol 22no 9 pp 1896ndash1906 2004
[21] L Yang X Song J Cheng and J F Holzman ldquoFree-spaceoptical communications over lognormal fading channelsusing OOK with finite extinction ratiosrdquo IEEE Access vol 4pp 574ndash584 2016
[22] Z Ghassemlooy W Popoola and S Rajbhandari OpticalWireless Communications System and Channel ModellingCRC Press Taylor amp Francis Group Boca Raton FL USA2013
[23] J Yuan Z Jiang Y Mao and W Ye ldquoForward error cor-rection concatenated code in DWDM systemsrdquo Frontiers ofOptoelectronics in China vol 1 no 1-2 pp 20ndash24 2008
[24] Yi Yi Gong and K Ben Letaief ldquoConcatenated space-timeblock coding with trellis coded modulation in fading chan-nelsrdquo IEEE Transactions on Wireless Communications vol 1no 4 pp 580ndash590 2002
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawiwwwhindawicom Volume 2018
Hindawiwwwhindawicom Volume 2018
Navigation and Observation
International Journal of
Hindawi
wwwhindawicom Volume 2018
Advances in
Multimedia
Submit your manuscripts atwwwhindawicom
24 System II OFDM-FSO System with Hybrid ChannelCodes e system designed with the combination of twochannel codes is shown in Figure 2 As shown in gure theother system arrangements are the same as given in Figure 1except two channel codes are cascaded to make a hybridsystem as the other codes are concatenated in [23 24]
e concatenation of error correcting codes is constructedto achieve copacetic performance with reasonable complexityLDPC is opted because of better data compression capabilityand TCM increases the bit rate by doubling the constellationpoints of the signal e other system specications are thesame as the OFDM-FSO system discussed in Section 21 eSNR vs BER plot is shown in Figure 5 As predicted from thegure at 30 dB the error rate is 10minus8
3 Results and Discussion
e results attained from various channel codes for weakturbulence conditions have been validated using MATLABand the scintillation has been modelled using the lognormalturbulence model In the simulation the Monte Carlomethod has been adopted for the verication of results byrandom attempts
In the proposed hybrid coded system the pair of singleinner-outer encoders and decoders has been used for mini-mum component cost computational complexity and syn-chronization issues Using proposed hybrid coding schemethe systemrsquos reliability is increased having acceptable BERwith reasonable computational complexity or delay latencye various channel coding schemes under weak turbulentconditions are analyzed in the following sectione results ofsystem model 1 and 2 are compared in Figure 6
As expected the system with proposed hybrid codesperforms well as compared to individually coded system In
Figure 6 the hybrid coded 16-QAM modulated systemrequires an SNR of 20 dB to attain a BER of 10minus6 while thesystem with LDPC and TCM coded individually requiresSNR of 25 dB and 27 dB respectively To oset the dipoundcultyof applying systems the performance dierence can comeout with the ensuing hybrid coded FSO system design
4 Comparative Analysis under DifferentTurbulence Conditions
In this section the performance estimation of variouschannel codes for OFDM-based FSO systems over weakturbulence conditions has been tabulated in Table 2 in acompressed format e table is representing the required
0 5 10 15 20 25 30SNR (dB)
10ndash7
10ndash6
10ndash5
10ndash4
10ndash3
10ndash2
10ndash1
100
BER
TCM coding for lognormal distribution
Figure 4 BER vs electrical SNR of OFDM-FSO system with TCMencoder
0 5 10 15 20 25 30 35SNR (dB)
SNR vs BER for hybrid coding with lognormal distribution
10ndash10
10ndash8
10ndash6
10ndash4
10ndash2
100
BER
Figure 5 BER vs electrical SNR of OFDM-FSO systemwith hybridLDPC-TCM encoder
0 5 10 15 20 25 30 35SNR (dB)
Lognormal distribution turbulence model
10ndash10
10ndash8
10ndash6
10ndash4
10ndash2
102
100
BER
HybridUncoded
TCMLDPC
Figure 6 BER vs electrical SNR of OFDM-FSO system withvarious channel codes using lognormal distribution
4 Journal of Computer Networks and Communications
SNR for achieving the BER of 10minus6 with various codingschemes
Form Table 2 it has been revealed that the proposedhybrid coding scheme gives the best performance amongother considered coding schemes during weak turbulencee different turbulence conditions affect the visibility andhence signal is attenuated which further causes the BERrough simulations significant improvement in perfor-mance has been obtained by concatenating LDPC codes withinterleaved schemes of TCM codes over fading channelseproposed hybrid coding has been compared with the LDPCand TCM coding with same trellis complexity and theuncoded systems with single antenna fading channels Asshown in Figure 6 with the proposed hybrid scheme theBER of 10minus9 is achieved at 30 dB of the SNR value duringweak turbulence conditions whereas BER of 10minus8 and 10minus7has been achieved at 30 dB by using LDPC and TCM codesrespectively e system has been simulated with MATLABcodes under various channel conditions using the lognormaldistribution turbulence model
5 Conclusion
Atmospheric turbulence comes out to be the major bot-tleneck in free-space optical communication systems andlimiting its use in extended applications Consistent researchis being carried out for reducing these effects and channelcoding plays the most important role is paper presenteda unified channel coding scheme constituted by concate-nation of irregular LDPC and TCM codes of 12 rate eyapplied the advantages of LDPC and TCM codes to formless complex reliable and high-performance concatenatedcodes For OFDM-based FSO system different conventionalchannel coding schemes have been examined and comparedwith a new hybrid channel coding scheme e OFDM-FSOsystem using the proposed scheme has been compared withthe conventionally used coding schemes under weak tur-bulent conditions e presented analysis of results indicatesthat for attaining BER of 10minus6 the required SNR for LDPCTCM and hybrid LDPC-TCM scheme is 25 dB 27 dBand 20 dB respectively e OFDM-FSO system designedwith novel hybrid channel coding shows an improvementof 5 dB as compared to solo LDPC and 7 dB as comparedto TCM alone e comparison of results is done by sim-ulating the system model using MATLAB codes
Data Availability
Input data have been randomly generated and the otherparameters have been stated in the manuscript
Conflicts of Interest
e authors declare that they have no conflicts of interest
References
[1] A Malik and P Singh ldquoFree space optics current applicationsand future challengesrdquo International Journal of Opticsvol 2015 Article ID 945483 7 pages 2015
[2] S P Joseph and M Mathew ldquoA review on performanceimprovement techniques in wireless optical communicationrdquoInternational Journal of Science and Research vol 4 no 8pp 745ndash749 2015
[3] M N O Sadiku S M Musa and S R Nelatury ldquoFree spaceoptical communications an overviewrdquo European ScientificJournal ESJ vol 12 no 9 pp 55ndash68 2016
[4] R Gupta and P Singh ldquoHybrid FSO-RF system a solution toatmospheric turbulences in long haul communicationrdquo In-ternational Journal of Scientific and Engineering Researchvol 5 no 11 pp 602ndash605 2014
[5] T A Tsiftsis H G Sandalidis G K Karagiannidis andM Uysal ldquoOptical wireless links with spatial diversity overstrong atmospheric turbulence channelsrdquo IEEE Transactionson Wireless Communications vol 8 no 2 pp 951ndash957 2009
[6] D Shah B Nayak and D Jethawani ldquoStudy of differentatmospheric channel modelsrdquo International Journal of Elec-tronics and Communication Engineering amp Technology vol 5no 1 pp 105ndash112 2014
[7] N Gupta S D Prakash H Kaushal V K Jain and S KarldquoPerformance analysis of FSO communication using differentcoding schemesrdquo AIP Conference Proceedings vol 1391pp 387ndash391 2011
[8] H Hennigera E Bernhard D M Stuart andC D Christopher ldquoCoding techniques to mitigate fading onfree-space optical communication linksrdquo in Proceedings ofSPIE-9e International Society for Optical Engineering SanDiego CA USA August 2008
[9] J S Nandaniya N B Kalani and G R Kulkarni ldquoCom-parative analysis of different channel coding techniquesrdquoInternational Journal of Computer Networks amp Communi-cations vol 4 no 2 pp 84ndash89 2014
[10] I B Djordjevic B Vasic and M A Neifeld ldquoLDPC codedOFDM over the atmospheric turbulence channelrdquo OpticsExpress vol 15 no 10 pp 6336ndash6350 2007
[11] A Joseph ldquoDesign of the high-speed framing FEC and in-terleaving hardware used in a 54 km free-space opticalcommunication experimentrdquo in Proceedings of Free-SpaceLaser Communications vol 7464 San Diego CA USAAugust 2009
[12] X Fang A Khalighi C Patrice and B Salah ldquoChannelcoding and time-diversity for optical wireless linksrdquo OpticsExpress vol 17 no 2 pp 872ndash887 2009
[13] Y Kou S Lin and M P C Fossorier ldquoLow density paritycheck codes construction based on finite geometries GlobalTelecommunications Conferencerdquo in Proceedings of Globe-com 2000-IEEE Global Telecommunications Conference vol 2pp 825ndash829 San Francisco CA USA November 2000
[14] D J C MacKay ldquoGood error-correcting codes based on verysparse matricesrdquo IEEE Transactions on Information 9eoryvol 45 no 2 pp 399ndash431 1999
[15] M Lin K Wang X Xin et al ldquoRapid soft-decision trelliscoded 32-QAM for free space optical communicationrdquo inProceedings of Asia Communications and PhotonicsConference Guangzhou China November 2012
Table 2 Comparative analysis of various coding schemes (requiredSNR for achieving BER of 10minus6)
[16] N G Costas ldquoSome implications of TCM for optical direct-detection channelsrdquo IEEE Transactions on Communicationsvol 37 no 5 pp 481ndash487 1989
[17] M Grover P Singh P Kaur and C Madhu ldquoMultibeamWDM-FSO system an optimum solution for clear and hazyweather conditionsrdquo Wireless Personal Communicationsvol 97 no 4 pp 5783ndash5795 2017
[18] M Grover P Singh and P Kaur ldquoMitigation of scintillationeffects in WDM FSO system using multibeam techniquerdquoJournal of Telecommunications and Information Technologyvol 2 pp 69ndash74 2017
[19] Y Wang D Wang and M Jing ldquoOn the performance ofcoherent OFDM systems in free-space optical communica-tionsrdquo IEEE Photonics Journal vol 7 no 4 pp 1ndash10 2015
[20] E J Lee and V W S Chan ldquoPart 1 optical communicationover the clear turbulent atmospheric channel using diversityrdquoIEEE Journal on Selected Areas in Communications vol 22no 9 pp 1896ndash1906 2004
[21] L Yang X Song J Cheng and J F Holzman ldquoFree-spaceoptical communications over lognormal fading channelsusing OOK with finite extinction ratiosrdquo IEEE Access vol 4pp 574ndash584 2016
[22] Z Ghassemlooy W Popoola and S Rajbhandari OpticalWireless Communications System and Channel ModellingCRC Press Taylor amp Francis Group Boca Raton FL USA2013
[23] J Yuan Z Jiang Y Mao and W Ye ldquoForward error cor-rection concatenated code in DWDM systemsrdquo Frontiers ofOptoelectronics in China vol 1 no 1-2 pp 20ndash24 2008
[24] Yi Yi Gong and K Ben Letaief ldquoConcatenated space-timeblock coding with trellis coded modulation in fading chan-nelsrdquo IEEE Transactions on Wireless Communications vol 1no 4 pp 580ndash590 2002
Chemical EngineeringInternational Journal of Antennas and
Propagation
International Journal of
Hindawiwwwhindawicom Volume 2018
Hindawiwwwhindawicom Volume 2018
Navigation and Observation
International Journal of
Hindawi
wwwhindawicom Volume 2018
Advances in
Multimedia
Submit your manuscripts atwwwhindawicom
SNR for achieving the BER of 10minus6 with various codingschemes
Form Table 2 it has been revealed that the proposedhybrid coding scheme gives the best performance amongother considered coding schemes during weak turbulencee different turbulence conditions affect the visibility andhence signal is attenuated which further causes the BERrough simulations significant improvement in perfor-mance has been obtained by concatenating LDPC codes withinterleaved schemes of TCM codes over fading channelseproposed hybrid coding has been compared with the LDPCand TCM coding with same trellis complexity and theuncoded systems with single antenna fading channels Asshown in Figure 6 with the proposed hybrid scheme theBER of 10minus9 is achieved at 30 dB of the SNR value duringweak turbulence conditions whereas BER of 10minus8 and 10minus7has been achieved at 30 dB by using LDPC and TCM codesrespectively e system has been simulated with MATLABcodes under various channel conditions using the lognormaldistribution turbulence model
5 Conclusion
Atmospheric turbulence comes out to be the major bot-tleneck in free-space optical communication systems andlimiting its use in extended applications Consistent researchis being carried out for reducing these effects and channelcoding plays the most important role is paper presenteda unified channel coding scheme constituted by concate-nation of irregular LDPC and TCM codes of 12 rate eyapplied the advantages of LDPC and TCM codes to formless complex reliable and high-performance concatenatedcodes For OFDM-based FSO system different conventionalchannel coding schemes have been examined and comparedwith a new hybrid channel coding scheme e OFDM-FSOsystem using the proposed scheme has been compared withthe conventionally used coding schemes under weak tur-bulent conditions e presented analysis of results indicatesthat for attaining BER of 10minus6 the required SNR for LDPCTCM and hybrid LDPC-TCM scheme is 25 dB 27 dBand 20 dB respectively e OFDM-FSO system designedwith novel hybrid channel coding shows an improvementof 5 dB as compared to solo LDPC and 7 dB as comparedto TCM alone e comparison of results is done by sim-ulating the system model using MATLAB codes
Data Availability
Input data have been randomly generated and the otherparameters have been stated in the manuscript
Conflicts of Interest
e authors declare that they have no conflicts of interest
References
[1] A Malik and P Singh ldquoFree space optics current applicationsand future challengesrdquo International Journal of Opticsvol 2015 Article ID 945483 7 pages 2015
[2] S P Joseph and M Mathew ldquoA review on performanceimprovement techniques in wireless optical communicationrdquoInternational Journal of Science and Research vol 4 no 8pp 745ndash749 2015
[3] M N O Sadiku S M Musa and S R Nelatury ldquoFree spaceoptical communications an overviewrdquo European ScientificJournal ESJ vol 12 no 9 pp 55ndash68 2016
[4] R Gupta and P Singh ldquoHybrid FSO-RF system a solution toatmospheric turbulences in long haul communicationrdquo In-ternational Journal of Scientific and Engineering Researchvol 5 no 11 pp 602ndash605 2014
[5] T A Tsiftsis H G Sandalidis G K Karagiannidis andM Uysal ldquoOptical wireless links with spatial diversity overstrong atmospheric turbulence channelsrdquo IEEE Transactionson Wireless Communications vol 8 no 2 pp 951ndash957 2009
[6] D Shah B Nayak and D Jethawani ldquoStudy of differentatmospheric channel modelsrdquo International Journal of Elec-tronics and Communication Engineering amp Technology vol 5no 1 pp 105ndash112 2014
[7] N Gupta S D Prakash H Kaushal V K Jain and S KarldquoPerformance analysis of FSO communication using differentcoding schemesrdquo AIP Conference Proceedings vol 1391pp 387ndash391 2011
[8] H Hennigera E Bernhard D M Stuart andC D Christopher ldquoCoding techniques to mitigate fading onfree-space optical communication linksrdquo in Proceedings ofSPIE-9e International Society for Optical Engineering SanDiego CA USA August 2008
[9] J S Nandaniya N B Kalani and G R Kulkarni ldquoCom-parative analysis of different channel coding techniquesrdquoInternational Journal of Computer Networks amp Communi-cations vol 4 no 2 pp 84ndash89 2014
[10] I B Djordjevic B Vasic and M A Neifeld ldquoLDPC codedOFDM over the atmospheric turbulence channelrdquo OpticsExpress vol 15 no 10 pp 6336ndash6350 2007
[11] A Joseph ldquoDesign of the high-speed framing FEC and in-terleaving hardware used in a 54 km free-space opticalcommunication experimentrdquo in Proceedings of Free-SpaceLaser Communications vol 7464 San Diego CA USAAugust 2009
[12] X Fang A Khalighi C Patrice and B Salah ldquoChannelcoding and time-diversity for optical wireless linksrdquo OpticsExpress vol 17 no 2 pp 872ndash887 2009
[13] Y Kou S Lin and M P C Fossorier ldquoLow density paritycheck codes construction based on finite geometries GlobalTelecommunications Conferencerdquo in Proceedings of Globe-com 2000-IEEE Global Telecommunications Conference vol 2pp 825ndash829 San Francisco CA USA November 2000
[14] D J C MacKay ldquoGood error-correcting codes based on verysparse matricesrdquo IEEE Transactions on Information 9eoryvol 45 no 2 pp 399ndash431 1999
[15] M Lin K Wang X Xin et al ldquoRapid soft-decision trelliscoded 32-QAM for free space optical communicationrdquo inProceedings of Asia Communications and PhotonicsConference Guangzhou China November 2012
Table 2 Comparative analysis of various coding schemes (requiredSNR for achieving BER of 10minus6)
[16] N G Costas ldquoSome implications of TCM for optical direct-detection channelsrdquo IEEE Transactions on Communicationsvol 37 no 5 pp 481ndash487 1989
[17] M Grover P Singh P Kaur and C Madhu ldquoMultibeamWDM-FSO system an optimum solution for clear and hazyweather conditionsrdquo Wireless Personal Communicationsvol 97 no 4 pp 5783ndash5795 2017
[18] M Grover P Singh and P Kaur ldquoMitigation of scintillationeffects in WDM FSO system using multibeam techniquerdquoJournal of Telecommunications and Information Technologyvol 2 pp 69ndash74 2017
[19] Y Wang D Wang and M Jing ldquoOn the performance ofcoherent OFDM systems in free-space optical communica-tionsrdquo IEEE Photonics Journal vol 7 no 4 pp 1ndash10 2015
[20] E J Lee and V W S Chan ldquoPart 1 optical communicationover the clear turbulent atmospheric channel using diversityrdquoIEEE Journal on Selected Areas in Communications vol 22no 9 pp 1896ndash1906 2004
[21] L Yang X Song J Cheng and J F Holzman ldquoFree-spaceoptical communications over lognormal fading channelsusing OOK with finite extinction ratiosrdquo IEEE Access vol 4pp 574ndash584 2016
[22] Z Ghassemlooy W Popoola and S Rajbhandari OpticalWireless Communications System and Channel ModellingCRC Press Taylor amp Francis Group Boca Raton FL USA2013
[23] J Yuan Z Jiang Y Mao and W Ye ldquoForward error cor-rection concatenated code in DWDM systemsrdquo Frontiers ofOptoelectronics in China vol 1 no 1-2 pp 20ndash24 2008
[24] Yi Yi Gong and K Ben Letaief ldquoConcatenated space-timeblock coding with trellis coded modulation in fading chan-nelsrdquo IEEE Transactions on Wireless Communications vol 1no 4 pp 580ndash590 2002
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[16] N G Costas ldquoSome implications of TCM for optical direct-detection channelsrdquo IEEE Transactions on Communicationsvol 37 no 5 pp 481ndash487 1989
[17] M Grover P Singh P Kaur and C Madhu ldquoMultibeamWDM-FSO system an optimum solution for clear and hazyweather conditionsrdquo Wireless Personal Communicationsvol 97 no 4 pp 5783ndash5795 2017
[18] M Grover P Singh and P Kaur ldquoMitigation of scintillationeffects in WDM FSO system using multibeam techniquerdquoJournal of Telecommunications and Information Technologyvol 2 pp 69ndash74 2017
[19] Y Wang D Wang and M Jing ldquoOn the performance ofcoherent OFDM systems in free-space optical communica-tionsrdquo IEEE Photonics Journal vol 7 no 4 pp 1ndash10 2015
[20] E J Lee and V W S Chan ldquoPart 1 optical communicationover the clear turbulent atmospheric channel using diversityrdquoIEEE Journal on Selected Areas in Communications vol 22no 9 pp 1896ndash1906 2004
[21] L Yang X Song J Cheng and J F Holzman ldquoFree-spaceoptical communications over lognormal fading channelsusing OOK with finite extinction ratiosrdquo IEEE Access vol 4pp 574ndash584 2016
[22] Z Ghassemlooy W Popoola and S Rajbhandari OpticalWireless Communications System and Channel ModellingCRC Press Taylor amp Francis Group Boca Raton FL USA2013
[23] J Yuan Z Jiang Y Mao and W Ye ldquoForward error cor-rection concatenated code in DWDM systemsrdquo Frontiers ofOptoelectronics in China vol 1 no 1-2 pp 20ndash24 2008
[24] Yi Yi Gong and K Ben Letaief ldquoConcatenated space-timeblock coding with trellis coded modulation in fading chan-nelsrdquo IEEE Transactions on Wireless Communications vol 1no 4 pp 580ndash590 2002
![EnergyEfficiencyAdaptationforMultihopRoutingin ...downloads.hindawi.com/journals/jcnc/2012/767920.pdfnetwork,wetakeintoconsiderationthehardware(orcircuit) energy consumption [8].](https://static.documents.pub/doc/80x56/5ac842097f8b9a40728c98d9/energyefciencyadaptationformultihoproutingin-wetakeintoconsiderationthehardwareorcircuit.jpg)
