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1 Ctaton: Ghaemooy, Zabh, opooa, Wau Oyewoe, Ahmad, Vahd and Letgeb, Erch 9 MMO free-pace optca communcaton empoyng ubcarrer ntenty moduaton n atmopherc turbuence channe Communcaton nfratructure: Sytem and Appcaton n Europe Frt nternatona CST Conference, EuropeComm 9, 6 pp 6-73 SS ubhed by: Sprnger URL: < Th veron wa downoaded from orthumbra Reearch Lnk: orthumbra Unverty ha deveoped orthumbra Reearch Lnk RL to enabe uer to acce the Unverty reearch output Copyrght and mora rght for tem on RL are retaned by the ndvdua author andor other copyrght owner Snge cope of fu tem can be reproduced, dpayed or performed, and gven to thrd parte n any format or medum for perona reearch or tudy, educatona, or not-for-proft purpoe wthout pror permon or charge, provded the author, tte and fu bbographc deta are gven, a we a a hypernk andor URL to the orgna metadata page The content mut not be changed n any way Fu tem mut not be od commercay n any format or medum wthout forma permon of the copyrght hoder The fu pocy avaabe onne: Th document may dffer from the fna, pubhed veron of the reearch and ha been made avaabe onne n accordance wth pubher poce To read andor cte from the pubhed veron of the reearch, peae vt the pubher webte a ubcrpton may be requred

2 MMO Free-Space Optca Communcaton Empoyng Subcarrer ntenty Moduaton n Atmopherc Turbuence Channe nvted paper Z Ghaemooy*, Senor Member, EEE, FET, W O opooa*, Student Member, EEE, ET, V Ahmad** and E Letgeb*** *orthumbra Communcaton Reearch Lab CRLab, orthumbra Unverty, ewcate upon Tyne, UK **Department of Eectrca Engneerng Tarbat Modare Unverty, Tehran, ran ***nttute of Broadband Communcaton, TU, Graz, Autra faryghaemooy@unnacuk Abtract n th paper, we anaye the error performance of tranmtterrecever array free-pace optca FSO communcaton ytem empoyng bnary phae hft keyng BSK ubcarrer ntenty moduaton SM n cear but turbuent atmopherc channe Subcarrer moduaton empoyed to emnate the need for adaptve threhod detector Drect detecton empoyed at the recever and each ubcarrer ubequenty demoduated coherenty The effect of rradance fadng mtgated wth an array of aer and photodetector The receved gna are neary combned ung the optma mamum rato combnng MRC, the equa gan combnng EGC and the eecton combnng SeC The bt error rate BER equaton are derved conderng addtve whte Gauan noe and og norma ntenty fuctuaton Th work part of the EU COST acton and EU project Keyword: Atmopherc turbuence, BSK, Free-pace optc, Laer communcaton, MMO ytem, Subcarrer moduaton, Spata dverty ntroducton ropagatng data-aden aer radaton over the atmophere termed FSO communcaton attractve for a number of reaon ncudng uncened pectrum and a narrow beamwdth [-3] There et a goba growng nteret n FSO and t wa etenvey reearched a part of the concuded EU framework 6 project and the COST acton Toward the end of 8 a new COST acton caed C8 on ropagaton too and data for ntegrated Teecommuncaton, avgaton and Earth Obervaton ytem wa tarted and wthn th acton group; our workng group contng of around partcpant nvoved n the optca wree communcaton An outdoor FSO nk eentay baed on ne-of ght LOS, thu, t pata oaton from potenta nterferer uffcenty mantaned by t narrow beamwdth profe But an obvou demert of the narrow aer beamwdth the

3 pontng and trackng requrement n the event of magnment Th can however be corrected ung actve trackng but at the cot of ncreaed compety and cot [3-5] Fog, aeroo, ran and gae and other partce upended n the atmophere reut n aer rradancententy attenuaton The attenuaton coeffcent typcay range from a few dbkm n a cear atmophere to ~7 dbkm n a dene fog regme [4] The huge attenuaton uffered durng dene fog retrct the carrer ca FSO nk to ~5 m [5, 6]; etendng the nk range n uch condton requre aternatve cheme uch a hybrd RFFSO [7, 8] Another factor that account for the FSO performance degradaton n a cear atmophere the rradance fuctuaton cntaton and the phae fuctuaton, whch reut from random nde of refracton varaton aong the propagaton path due to the atmopherc turbuence [3, 9, ] The On-Off keyng OOK gnang format ha been wdey ued n the commercay avaabe FSO ytem But n channe wth the atmopherc turbuence nduced fadng, the OOK cheme requre adaptve threhod to perform optmay [, ] Th fact among other ha ed to the ncreaed nteret n the tudy of SM n FSO ytem [-4] t ha ao been hown that ung a fed threhod OOK cheme reut n uboptma ytem, whch not ony nferor to a SM moduated FSO nk but ao ha a BER foor [5] n [6] the ow denty party check LDC codng ha been epored to ameorate the effect of cntaton on a SM-FSO nk t reported that the LDC coded SM n atmopherc turbuence acheved a codng gan of > db compared to the mary coded OOK n [7] the ue of pace-tme bock code wth coherent and dfferenta detecton technque ha been reported to acheve a mar performance However, nvokng error contro codng ntroduce huge proceng deay and effcency degradaton n vew of the number of redundant bt that w be requred a outned n [8] n th paper the BER anay of the SM-FSO baed mutpe-nputmutpe-output MMO confguraton n the preence of the og norma atmopherc turbuence preented conderng the foowng near recever combnng technque: EGC, MRC and SeC Both cae of patay correated and uncorreated optca fed at the recever are condered n the preence of the addtve whte Gauan noe n a reated work reported n [9], the pata dverty condered at the recever de for both OOK and M moduated FSO ytem Apart from mtgatng cntaton, the MMO ytem advantageou n combatng temporary nk bockage by brd and magnment when combned wth a wde aer beamwdth, thereby emnatng the need for an actve trackng t ao much eaer to provde ndependent aperture averagng wth mutpe eparate aperture ytem The terretra FSO bacay a LOS nk wth a neggbe deay pread; hence nter-ymbo nterference S not an ue The ret of the paper arranged a foow: Secton dcue ubcarrer ntenty moduaton; the error performance dcued n Secton 3 whe the concudng remark are gven n Secton 4 Subcarrer ntenty Moduaton n optca communcaton ytem SM acheved by moduatng the data onto a rado frequency gna, whch then ued to vary the rradancententy of an

4 optca ource a contnuou aer ource n th cae n th work, the ubcarrer aumed to be pre-moduated ung the BSK Other moduaton method can ao be ued and th one of the major advantage of SM Conderng weak turbuence and aumng that the og ntenty of aer radaton traverng the atmophere obey the norma dtrbuton e ~ -,, then the probabty denty functon pdf of the ntenty o ep gven by []: p n 5, ep where the average receved ntenty 5 ma, ma the peak receved aer ntenty and the receved average ntenty wthout turbuence The a meaure of the trength of aer ntenty fuctuaton and vad for < [] The ntantaneou photocurrent can be modeed a []: r t R ξm t n t, where ζ the moduaton nde, R the photodetector reponvty and nt the addtve whte Gauan noe AWG Conderng M ubcarrer, the compote ubcarrer gna durng the kth ymbo duraton gven by: M m t A g t kt d co w t ϕ, 3 j j where, for data bt and, gt - kt the rectanguar pue hapng functon, and T the ymbo duraton The ubcarrer anguar frequency and the phae are repreented by and, repectvey, whe the peak ubcarrer amptude For the contnuou wave aer tranmtter to operate wthn t dynamc range and to avod gna dtorton due to cppng, the condton ζmt mut away hod Conderng BSK moduated ubcarrer wth non-varyng amptude, the peak amptude A j A And f ζ normazed to unty, then A M Durng the kth ymbo, the photocurrent a gven by: k cj j M r t R ξa g t kt d k co wcjt ϕ j j n t 4 The band pa fter BF n Fg hep eparate the ubcarrer and uppre any ow varyng R component n 4 The ubcarrer frequence are ubequenty demoduated ndependenty by frequency down converon ung a reference carrer gna coω cj t φ j A ow pa fter LF empoyed to recover d k wth mnma dtorton The reut per branch an antpoda gna gven by:

5 d k RξAg t kt t nd t, 5 D where n D t the pot demoduaton AWG wth a varance The pot demoduaton eectrca gna-to-noe rato SR e, per ubcarrer then derved from 5 a: ξra 6 For a fed vaue of ζ therefore, ung mutpe ubcarrer ncreae throughputcapacty but at SR e penaty gven by ogm db Th ugget that mutpe ubcarrer houd ony be condered when the need for ncreaed capacty outwegh the accompanyng power penaty a b Fg : SM-FSO bock dagram; a tranmtter, and b recever TA Tranmpedance ampfer

6 3 Error erformance 3 Wthout Dverty For a coherenty demoduated BSK SM operated n a channe wth the atmopherc turbuence, the uncondtona probabty of bt error obtaned by averagng the condtona error rate over rradance fuctuaton tattc derved a []: e m m w Q Ke K w ep ep, n θ dθ Rξ where A K, and repreent the zero of the mth order Hermte poynoma He m and the correpondng weght factor, repectvey 7 3 Wth Recever Dverty Spata dverty empoyed both at the tranmtter and recever a hown n Fg n the anay and the ubequent ecton, the nge SM-FSO condered Frt, the ue of an array of -photodetector anayed whe tranmtter array treated n the ubequent ecton t t d ˆ t t Fg : H -MMO FSO ytem bock dagram

7 We aume the pata eparaton of the photodetector to be greater than the rradance pata coherence dtance ρ o e > ρ o reutng n uncorreated receved rradance Th aumpton reatc becaue ρ o on the order of few centmetre [] We ao aumed that the beamwdth at the recever uffcenty broad to cover the entre fed of vew FOV of the detector The photocurrent are neary combned before beng apped to the coherent demoduator to etract the tranmtted data from the receved ubcarrer gna To factate a far comparon between the nge-tranmtter-nge-recever ytem and the pata dverty ytem, each aperture area n the photodetector array aumed to be A D, where A D the recever aperture area for nge-tranmtternge-recever ytem The SR e can thu be derved a: ξ ξ a a RA a a RA 8 MRC Under the MRC, the gan reutng n the optmum SR e gven beow:, ξ MRC RA v 9 where ξ RA the SR for each nk The uncondtona BER wth the MRC near combnng gven by []: [ ] θ θ d S d Q MRC MRC e v v v v,

8 Where θ ep ep and et, we conder the ub-optmum but more practca near combnng technque EGC For the EGC, gan n 8 are a made contant, thu reutng n the foowng SR e : ξ < ξ EGC RA RA v Thu, MRC EGC v v < and the uncondtona BER can be obtaned a []: µ θ θ m Z EGC e u u e K Q w dz d Z Z K n ep, where A R K ξ, { } n ep u u u z Z Z Z µ and µ e u n n whe e u n SeC Here, the combner ampe a and eect the nk wth the hghet SR e, t BER derved a []: [ ] m SeC e K Q erf w ep, 3 33 Effect of Sgna Correaton on the BER For -photodetector eparated by a pata dtance e than the aer radaton pata coherence dtance e < ρ, the receved radaton are correated To how the effect of th on the ytem error performance, we conder equay paced photodetector empoyng the optmum MRC near combnng The uncondtona

9 BER n atmopherc turbuence obtaned by averagng 4 over the jont pdf of the ntenty fuctuaton θ θ d K Q MRC ec n ep v 4 Adoptng the Tatark approach [], the correaton coeffcent X X B B ρ of an optca wave n turbuent atmophere between two pont wth a pata eparaton gven by: ρ L k L k L k L k 5 where k λ; t can be nferred that the correaton coeffcent nverey proportona to the pata eparaton for < λl e for e than atmopherc channe coherence dtance from whch the covarance matr 6 obtaned ρ ρ ρ ρ X C, 6 where j the pata eparaton between photodetector and j, 4 the og amptude varance whe ρ the correaton coeffcent between two photodetector wth pata eparaton The jont pdf of receved aer ntenty gven by [9]: 8 ep T X X C C p β β v v, 7

10 where β n n n The epreon for the uncondtona BER then obtaned a: e v v v pv Q d 8 MRC 34 Tranmtter Dverty n th cae we conder a nge photodetector and mutpe aer tranmtter The aer ource are aumed to be uffcenty paced o that the photodetector receve uncorreated aer radaton To enure a far comparon and to mantan a contant power requrement, we aume that the power avaabe for a nge-tranmtter ytem equay hared amongt the H-aer tranmtter A uch, the rradance from each aer repreent an H factor reducton compared to a nge tranmtter ytem Another obvou approach to aume that a the tranmtter have the ame power; n th cae the power requrement w have to ncreae by a factor H [8] Baed on the former, the receved photocurrent gven by: Aξd g t ktco w t ϕ n t H R r t k c H 9 Snce aer array are ony eparated by a few centmetre, the phae hft eperenced by the receved rradance due to path dfference therefore neggbe The SR e on each ubcarrer can therefore be derved a: MSO H H RξA v There et a cear marty between and hence; the uncondtona BER for the mutpe tranmtter-nge photodetector ytem the ame a that for the nge tranmtter-mutpe photodetector confguraton wth EGC near combnng 35 Tranmtter Dverty-Recever Dverty MMO n th ecton we conder a mutpe-aer and mutpe-photodetector ytem n content wth the earer aumpton, the tota tranmt power made equa to the

11 tranmt power when ung a nge aer to acheve the ame bt rate and the combned aperture area of the -photodetector the ame a when a nge photodetector ued The H-aer and -photodetector are aumed paced enough o that the receved aer radaton are uncorreated Frt, we conder when the photocurrent are combned ung EGC A prevouy dcued, a mutpe tranmtter-nge photodetector ytem wth H-aer dentca to a nge tranmtter-mutpe photodetector confguraton havng H-photodetector wth EGC combnng; thu the foowng repreent the SR e aumng dentcay dtrbuted rradance: H H RξA RξA v j H j H Th epreon the ame a H-photodetector nge tranmtter-mutpe photodetector ytem empoyng EGC near combnng Hence, the uncondtona BER obtaned by repacng by H n By neary combnng the photocurrent ung MRC, the ndvdua SR e on each nk : H RξA j H j Conderng the um of ndependent ognorma dtrbuted random varabe a another og norma dtrbuton [3] then the uncondtona BER derved a: [ S θ ] e dθ, 3 Where θ ep ep µ,, u and µ u are a prevouy defned ecept that now repaced by H For an optca MMO-FSO nk under conderaton therefore, the BER performance governed by wth repaced by H for the EGC combnng technque and 3 for the MRC 36 Reut and Dcuon The pot of 7 agant the normaed eectrca SR RE[] hown n Fg 3 wth M [,, 5, ], 3 and ζ When M the BER curve the ame a that reported n [] Mutpe ubcarrer can therefore be ued to ncreae the capacty but at an eectrca SR penaty defned by ogm db To how the effect of gna correaton, 8 potted n Fg 4 for [, 3] and ρ [,, 3, 6] at a turbuence eve 3 To acheve a BER of -6, the ue of two photodetector

12 wth ρ 3 and 6 requre addtona ~8 db and ~35 db of SR, repectvey, compared to when > ρ o e when ρ ; wth three photodetector, the addtona SR requred to acheve the ame BER of -6 ~7 db and ~5 db for ρ 3 and 6, repectvey Th apparenty how the effect of correated ntenty and ao buttree the empha paced on the need for to be greater than ρ o n a pata dverty ytem a t reut n the mamum gan - umber of ubcarrer 5 BER SR db Fg 3: BER agant the eectrca SR for M [,,5,] ubcarrer and turbuence trength 3 To compare the MMO ytem wth the nge SM benchmark reported n [], the BER gven by 3 potted agant the normaed eectrca SR a depcted n Fg 5 at a turbuence eve 3 for varou vaue of and H The nk margn dverty gan reutng from the ue of tranmtterrecever array hown n Tabe A -MMO ytem requre an addtona ~4 db of SR compared to a 4-MMO ytem n the atter however, 4 photodetector w have to be uffcenty paced to avod any correaton n the receved gna a agant photodetector n the former Ao, at the tated BER, a 4 4-MMO baed SM-FSO nk requre ~4 db and db ower SR compared to the nge tranmtter wth 4 and 8 photodetector, repectvey

13 -3-4 Correaton coeffcent Recever -Recever -5 BER SR db Fg4: BER agant the eectrca SR at dfferent vaue of correaton coeffcent for number of photodetector [, 3] and turbuence trength X4 X X4 X8 X5 X -5 BER ormaed SR db Fg 5: MMO BER agant the eectrca SR for dfferent number of aer and photodetector array at turbuence trength 3 and M

14 Tabe : Lnk margn dverty gan at BER of -6 and 3 MMO confguraton Lnk margn db Concuon We have preented an epreon for the uncondtona BER of H -MMO FSO ytem that ue BSK ubcarrer ntenty moduaton under the og norma dtrbuted weak atmopherc turbuence envronment The error performance and the nk margn theoretca ung three dfferent near combnng technque have ao been preented Reut howng ceary the effect of correated receved rradance on the ytem error performance have ao been gven n a -detector array and at a turbuence trength of 3, a correaton coeffcent of 6 reut n addtona ~35 db of SR to acheve a BER of -6 ; th ncurred power penaty ncreae a the receved gna become more correated At a BER of -6, -MMO ytem requre ~4 db addtona SR when compared wth 4-MMO ytem But pacng of an array of 4 photodetector to enure uncorreated rradance recepton far more demandng than pacng photodetector Ao the ue of 4 4-MMO to dever the ame BER of -6 requre ~4 db and db ower SR than ung one aer ource wth 4 and 8 photodetector, repectvey A 4 4-MMO ytem thu the preferred confguraton to mtgate cntaton wthout ncreaed ytem compety Reference M Uya, L Jng, and Y Meng, "Error rate performance anay of coded free-pace optca nk over gamma-gamma atmopherc turbuence channe," EEE Tranacton on Wree Communcaton, vo 5, pp 9-33, 6 E Letgeb, M Geghart, and U Brnbacher, "Optca network, at me acce and appcaton," Journa of optca and fbre communcaton report, vo, pp 56-85, 5 3 D Kedar and S Arnon, "Urban Optca Wree Communcaton etwork: The man chaenge and pobe outon," EEE Optca communcaton, pp -7, 4 4 H Webrand and B S Ghuman, Free Space Optc: Enabng optca Connectvty n today' network ndanapo, : SAMS pubhng, 5 D Knger, "Free pace optc for aer communcaton through the ar," Optc & hotonc ew, pp 36-4, October 6 aac Km, Bruce McArthur, and E Korevaar, "Comparon of aer beam propagaton at 785 nm and 55 nm n fog and haze for optca wree communcaton," SE roceedng: Optca Wree Communcaton, vo 44, pp 6-37, 7 S Boom and W S Hartey, "The at-me outon: hybrd FSO rado," n ArFber, nc Whte paper, Vted: Juy 7 8 Km and E Korevaar, "Avaabty of free pace optc and hybrd FSORF ytem," roceedng of SE: Optca wree communcaton V, vo 453, pp 84-95, 9 X Zhu and J M Kahn, "Free-pace optca communcaton through atmopherc turbuence channe," EEE Tranacton on Communcaton, vo 5, pp 93-3, Augut

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