Multi-coded Variable PPM for High Data Rate Visible Light Communications

Transcription

1 Joural o the Optical Society o Korea Vol. 6 No. Jue 0 pp DOI: Multi-coded Variable PPM or High Data Rate Visible Light Commuicatios Hyu-Dog Moo ad Sug-Yoo Jug* Departmet o Electroic Egieerig Yeugam Uiversity 4- Dae-dog Gyeogsa Korea (Received Jauary 0 : revised April 8 0 : accepted May 3 0) I this paper we propose a ew modulatio scheme called multi-coded variable pulse positio modulatio (MC-VPPM) or visible light commuicatio systems. wo groups o sigals (Pulse Width Modulatio (PWM) ad Pulse Positio Modulatio (PPM) groups) are multi-coded by orthogoal codes or trasmittig data simultaeously. he each multi-level value o the multi-coded sigal is coverted to pulse width ad positio which results i ot oly a improved data rate but also a processig gai i receptio. I additio we itroduce average duty ratio ad cyclic shit cocepts i PWM through which dimmig cotrol or light illumiatio ca be supported without ay degradatio i commuicatio perormace. hrough simulatio we coirm that the proposed MC-VPPM shows a comparable BER curve ad much greater achievable data rate tha the covetioal VPPM scheme usig a visible light optical chael eviromet. Keywords : Visible Light Commuicatio (VLC) Multi-codes Variable PPM PPM PWM OCIS codes : ( ) Optical commuicatios; ( ) Free-space optical commuicatio; ( ) Light-emittig diodes I. INRODUCION Next-geeratio LED lightig is more advatageous tha existig luorescet ad icadescet lightig i terms o log lie expectacy high tolerace to humidity low power cosumptio ad miimal heat geeratio []. I additio LEDs are used ot oly or illumiatio but also or may products such as moitors cell phoes cars ad others. Recetly there have bee may attempts to coverge LED with I techology [ 3]. Amog them Visible Light Commuicatio (VLC) which is the covergece o illumiatio ad commuicatio has emerged [4-6]. A correspodig VLC stadardizatio was recetly published by the IEEE Stadards Associatio [7 8]. Geerally VLC uses the Itesity Modulatio with a Direct Detectio (IM/DD) scheme which uses the amplitude (or itesity) o light to trasmit data. Huma eyes perceive oly the average itesity whe light chages aster tha the Maximum Flickerig ime Period (MFP) which is deied as 5 ms [8]. hereore both lightig ad commuicatio ca be simultaeously implemeted. By cosiderig both terms together may modulatio methods have bee proposed such as iverted pulse positio modulatio (I-PPM) subcarrier iverted pulse positio modulatio (SC-I-PPM) [9] pulse width modulatio (PWM) [0] ad variable PPM (VPPM) [8]. Amog them VPPM is the modulatio scheme proposed by the IEEE 80.5 stadard group. o support illumiatio with dimmig cotrol ad commuicatio simultaeously it uses biary PPM or commuicatio ad PWM or dimmig cotrol. However the mai drawback o the VPPM scheme is that the data rate is limited to the badwidth o a LED sice it uses oly biary PPM modulatio. I this paper we propose a ew VLC modulatio scheme called multi-coded variable pulse positio modulatio (MC- VPPM). wo groups o sigals (PWM ad PPM group) are multi-coded by orthogoal codes or trasmittig data simultaeously. he each multi-level value o the multicoded sigal is coverted to pulse width ad positio which results i ot oly a improved data rate but improved processig gai i receptio as well. I additio we itroduce average duty ratio ad cyclic shit cocepts i PWM. o this eect it is possible to support dimmig cotrol or light illumiatio without ay degradatio i commuicatio perormace. hrough simulatios we compare BER ad *Correspodig author: syjug@yu.ac.kr Color versios o oe or more o the igures i this paper are available olie

2 08 Joural o the Optical Society o Korea Vol. 6 No. Jue 0 the achievable data rate o MC-VPPM ad covetioal VPPM schemes i a visible light optical chael eviromet. Whe comparig BER to VPPM much greater achievable data rate is show i BER. he rest o the paper is orgaized as ollows. Sectio II describes the proposed MC-VPPM scheme. Simulatio results ad coclusios are give i Sectios III ad IV respectively. II. MULI-CODED VARIABLE PPM SCHEME Fig. shows the block diagram o the proposed multi-coded VPPM scheme or VLC. As show i the Figure the trasmittig data block is divided ito two separate parts. b is the PWM data block that cotais the atipodal sigaled data. b is the PPM data block that cotais the atipodal sigaled data. he PWM ad PPM data blocks are each ecoded by usig ad biary orthogoal multi-codes o legth. he two multi-coded sigal vectors o legth are obtaied as ollows: [ d ] L d N s [ d ] d = C b = pp m = C b pp m = d pp m L N s pp m d. C C = [ c L c L ] = [ c L c L ] pp m. are ad biary orthogoal () () code matrices. ad deotes the -th orthogoal codes o the PWM ad PPM data group respectively. he each elemet o ad have ad symmetric possible multi-level values i.e.. I this paper we propose a ew cocept called average duty ratio o pulse (%) which determies the average itesity o the light source. Dimmig cotrol ca be supported by varyig the average duty ratio. I the proposed scheme the ollowig values are chose as iitial system parameters: the average duty ratio o pulse (%) or dimmig support; the pulse width variatio iterval (%) or PWM; the pulse width margi (%) or limitig maximum ad miimum pulse width; ad the umber o pulse positios N or PPM. Based o these basic system parameters the possible amout o PWM ad PPM data ( ) ad the orthogoal codes o legth are determied as ollows: ( X M ) ( M X ) Mi K K (4) log max. (5).. Sigalig Format o trasmit the sigal each elemet o a multi-coded sigal vector or PWM is coverted to a pulse width vector as show below: (3) FIG.. Block diagram o the proposed multi-coded VPPM scheme.

3 Multi-coded Variable PPM or High Data Rate Visible Light - Hyu-Dog Moo ad Sug-Yoo Jug 09 w = [ w w L w Ns ] (6) d + L w = (7) ad the correspodig pulse width is give as w L + K W = w L + K w L + + K ( d < 0) ( d > 0) L :eve (8) L : odd Here is the duratio o a rame ad deotes the pulse spacig or PPM. able shows the PWM mappig rule accordig to eq. (7) ad (8). I the case o PPM each elemet o a multi-coded sigal vector or PPM is coverted to a pulse positio vector as ollows: m = [ m L m Ns ] (9). ABLE. PPM mappig rule m d m L 0 L + L + 4 M L 4 L L L L L ( d + ) L =. (0) able shows the PPM mappig rule accordig to eq. (0). Based o the pulse width ad pulse positio vector the trasmitted multi-coded VPPM sigal has the orm N () i= = 0 s () = m w ( s ) s t P t i represets the trasmittig pulse o width positio ad eergy dt. M he trasmitted sigal that cotais data occupies ABLE. PWM mappig rule d L : eve w Pulse width L 0 L K d L : odd w Pulse width L 0 L + K M M M M M M L 4 + L L + L 4 + L 3 + L [ X K] 3 + L [ X K] X + L [ X + K] L [ X + K] + 3 [ X K] [ X K] - - X [ X + K] [ X + K] M M M M M M + L L L + K + L L L + + K

4 0 Joural o the Optical Society o Korea Vol. 6 No. Jue 0 a total o rames o duratio ad requires a time duratio o. Because we set the average duty ratio o a pulse at (%) or dimmig support the average pulse eergy satisies the ollowig costrait: E X { E p w } = p = E p w = E L + w. () he sigalig example o the proposed multi-coded VPPM is show i Fig.. I order to produce the correspodig sigals b ad b ; ad the orthogoal code matrices; C ad C are geerated based o Walsh-Hardamard (WH) orthogoal codes o legth 4 as ollows: C = C = (3) From the Figure we ca see that there are pulses that exceed their ow rame duratios ( ) ad itroduce Iter- Frame Iterereces (IFI). his is because we use PWM ad PPM mappig simultaeously. Because IFI makes it diicult to detect ad demodulate the trasmitted MC- VPPM sigal o the receiver side we itroduce a Cyclic Shit scheme which ca remove IFI. By cosiderig the eergy costraits i Eq. () ad (3) ad providig Cyclic Shit the shape o the trasmittig pulse ca be desiged as ollows: ad P i w () t 0 t W = 0 otherwise m δ + W P m w > ( IFI case) () t = 0 t W ( m δ) 0 m δ t otherwise. (5) he graphical represetatio o the Cyclic Shit scheme by usig the example give i Fig. is show as below: Fially a LED is drive by the curret sigal cotrolled by the MC-VPPM sigal. he LED emits the light sigal which has the average optical power give as ollows: Pt = s s 0 X ()dt t (6).. Receiver Desig Ater passig through the VLC optical chael is received by a photodiode (PD). he received sigal is give as [4 5 ]: () t R X () t h() t () t r = +. (7) i P m w mδ + W ( No IFI case) () t = P ( t m δ) w (4) Here is the detector resposibility [A/W] is Additive White Gaussia Noise (AWGN) ad * idicates the covolutio operator. he average received optical power becomes FIG.. Sigalig example o the proposed multi-coded VPPM scheme. ( =70% =0% =0% =4 =3 =3 =4)

5 Multi-coded Variable PPM or High Data Rate Visible Light - Hyu-Dog Moo ad Sug-Yoo Jug FIG. 3. Cyclic Shitig example o the proposed multi-coded VPPM scheme. represets the chael DC gai. I we cosider a LOS case with o relectios the chael DC gai is: H ( ) = 0 ( m + ) A m cos ( φ) s( ψ) g( ψ) cos( ψ) 0 ψ Ψc π d (8) 0 ψ > Ψ is the order related to the trasmitter l semiagle (at hal power) give by l. cos For example = 60 (Lambertia trasmitter) correspods to. is the physical detectio area o the PD is the distace betwee the LED ad the PD is the agle o irradiace ad is the agle o icidece. is the sigal trasmittace o the optical ilter is the cocetrator gai ad c is the cocetrator ield o view (FOV). We assume that the Gaussia oise has a total variace that cosists o shot oise thermal oise ad iter rame itererece by a optical path dierece. N = σ + σ + R P (9) shot thermalt rifi I the duratio o the sigal is log eough the ISI is egligible. hereore the mai oise sources become shot ad thermal oises. A shot oise variace is give by [4] σ = qγ( P + P ) B+ qi I B (0) shot rsigal rifi bg is the electroic charge is the received sigal power is the equivalet oise badwidth ad c is the backgroud curret. We deie the oise badwidth actor =0.56. he thermal oise variace is also give as [4]: 8πk 6π k Γ σ = η + η () k k 3 thermal AIB AIB 3 G gm the irst two terms represet eedback-resistor oise ad RE chael oise respectively. is Boltzma s costat is absolute temperature is the ope-loop voltage gai is the ixed capacitace o a photo detector per uit area Γ is the FE chael oise actor is the FE trascoductace ad = Here we assume that the detector resposibility is ideal ( =) ad the receiver is exactly sychroized with the trasmitter. Upo receptio o the receiver perorms demodulatio at the ith sigal iterval ad pulse width ad produces the correlatio metric o legth as ollows: [ r r ] r = L r () 0 N s ( 00) ( 0 L ) ( ) ( L ) ( L L ) [ ] 0 0 = r L r L r L r L r r (3) ( ) i + i ( ) s + + a b = r() t qa b() t r s dt. (4) Here * deotes the template pulse which is the trasmitted pulse dispersed by the VLC optical chael.

6 Joural o the Optical Society o Korea Vol. 6 No. Jue 0 Based o the maximum-likelihood (ML) decisio rule we detect the positio ad pulse width ( ) o the trasmitted pulses ad regeerate the multi-coded sigal vector as show below: [ mˆ ˆ ] m ˆ = (5) L m N s [ wˆ ˆ ] w ˆ = (6) ( mˆ ˆ ) w L w N s ( a b ) = arg max. (7) a L b L r = 0 K = 0 K he the two multi-coded sigal vectors received are give as ollows: [ d ˆ ] L dˆ N d ˆ = (8) [ d ˆ ] pw m L dˆ N s d ˆ = (9) dˆ dˆ ˆ m s = L (30) ˆ w = L. (3) Fially ad data cotaied i are decoded by orthogoal codes used at the trasmitter with hard decisio as ollows: z ˆ = Cd (3) bˆ = sig{ z } For perormace compariso we iclude the covetioal VPPM scheme [8] which is coded with a orthogoal code o legth 4 to obtai the same processig gai as the proposed MC-VPPM scheme. Note that we are ocusig o evaluatig the BER perormace o the proposed MC-VPPM modulatio scheme. hereore we cosider a ucoded system. Fig. 4 shows the compariso o BERs or the proposed MC-VPPM ad VPPM scheme accordig to the received rx by chagig the dimmig levels. BER plots were widely used to express the data trasmissio perormace o the proposed scheme i regard to commuicatio icludig optical commuicatio. We perorm a Mote-Carlo simulatio that repeats data trasmissio 0 9 times or obtaiig a 0-8 BER curve. I order to ivestigate the perormace i the operatig rx rage o iterest we chage ) the dimmig ratio ad ) the distace betwee the LED ad PD. Fig. 5 illustrates the relatioship betwee MC-VPPM s rx perormace accordig to the dimmig ratio uder the VLC sceario i [] ad the distace betwee LED ad PD. Because VLC perormace based o MC-VPPM aects the desig o LED illumiatio irastructure it will be meaigul to express commui- ABLE 3. he iitial system parameters o MC-VPPM SYSEM PARAMEERS Average duty ratio (X) 30% 50% 70% Pulse width margi (M) 0% 40% 0% Pulse width variatio (K) 0% Number o pulse positio (N) 4 Number o PWM data ( ) Number o PPM data ( ) 3 Orthogoal code legth ( ) 4 z ˆ = Cd. (33) bˆ = sig{ z } III. SIMULAION RESULS Util ow we have discussed the overall descriptio o the proposed MC-VPPM scheme. Now we preset the simulated result to validate the proposed scheme. he iitial system parameters o MC-VPPM or simulatio are show i able 3. We used the VLC chael eviromet [] ad parameters or determiig oise variaces i [4] ad []. FIG. 4. he compariso o BERs betwee the proposed MC-VPPM ad VPPM.

7 Multi-coded Variable PPM or High Data Rate Visible Light - Hyu-Dog Moo ad Sug-Yoo Jug 3 FIG. 5. he rx perormace o MC-VPPM accordig to the dimmig ratio uder the VLC sceario i [] ad the distace betwee the LED ad PD. ( LED power is uw optical rate : 400 khz ) catio perormace based o distace ad dimmig ratio uder the give VLC sceario. From the above result it is observed that MC-VPPM shows better BER perormace as the dimmig level decreases. his is because the itererece caused by icreased dimmig levels i determiig correlatio metrics results i perormace degradatio eve though more eergy is allocated to the trasmitted sigal thaks to higher dimmig level icreases i rx. Compared with the covetioal VPPM scheme the BER o MC-VPPM i 30% dimmig levels always shows better BER perormace tha VPPM. I regard to BERs o the proposed MC-VPPM with 50% ad 70% dimmig the proposed MC-VPPM still shows better perormace i the low rx regio eve though the BERs o the proposed scheme with 50% ad 70% dimmig i higher rx regios are a little bit less tha that o VPPM. However as oe ca see i Fig. 6 the achievable data rate o the proposed MC-VPPM scheme is much higher tha that o the VPPM scheme (by a actor o i our simulatio). his meas that it is possible to compesate or the degraded BER perormace o the proposed MC-VPPM scheme i high rx coditios by utilizig more power chael codig schemes with the beeit o a higher data rate. For example MC- VPPM ca use / chael codig schemes uder the same data rate coditio while VPPM caot use ay kid o chael codig scheme. hereore the proposed MC-VPPM scheme shows reasoably robust BER perormace compared with the covetioal VPPM scheme. As metioed beore we compared the achievable data rate (ADR) betwee the proposed MC-VPPM ad VPPM schemes. ADR represets how much data ca be trasmitted without error per uit time. hereore ADR ca be expressed as a uctio o the maximum data rate ad the bit error rate (BER): FIG. 6. A compariso o achievable data rate betwee the proposed MC-VPPM ad VPPM schemes. (optical rate o LED : 400 khz) ADR = ( P ) R (34) b max ad max deote BER ad the maximum trasmissio rate per uit time (bits/sec) respectively. Here the maximum trasmissio rate max is calculated as show below: R max total umber o data bits per block to be trasmitted ( bits) = time to trasmit oe data block usig give modulatio scheme (sec) L+ L L+ L = = ( ) ( bits /sec). s Ns L + δ (35) Fially we ca obtai the achievable data rate (ADR) o the proposed MC-VPPM give as ADR = ( Pb ) Rmax L + L = ( Pb ) ( bits /sec) Ns( L + ) δ (36) Fig. 6 shows the compariso o achievable data rate betwee two schemes whe the optical rate o the LED is 400kHz. As expected the achievable data rate o MC- VPPM is much greater tha that o the VPPM scheme. IV. CONCLUSION We have proposed a ew VLC scheme based o multi-coded variable PPM (MC-VPPM) or VLC. By covertig multi-level values o the multi-coded sigals to pulse positios ad pulse width the proposed MC-VPPM gives both improved data rate ad robust BER. For light illumiatio dimmig ca be also supported without ay degradatio i commuicatio perormace by itroducig average duty ratio ad cyclic shit cocepts i to the

8 4 Joural o the Optical Society o Korea Vol. 6 No. Jue 0 PWM. hrough simulatios based o VLC scearios we coirmed the robust BER curves ad much-greater achievable data rates o the proposed MC-VPPM i compariso with covetioal VPPM schemes. ACKNOWLEDGMEN his research was supported by the Basic Sciece Research Program through the Natioal Research Foudatio o Korea (NRF) uded by the Miistry o Educatio Sciece ad echology (No ). REFERENCES. R. D. Dupuis ad M. R. Krames History developmet ad applicatios o high-brightess visible light-emittig diodes J. Lightwave echol. 6 MAY (008).. D.-K. So E.-B. Cho I.-K. Moo Y.-S. Park ad C.-G. Lee Developmet o a illumiatio measuremet device or color distributio based o a CIE 93 XYZ sesor J. Opt. Soc. Korea (0). 3. C. G. So J. H. Yi J. S. Gwag J. H. Kwo ad G. Park Improvemet o color ad lumiace uiormity o the edge-lit backlight usig the RGB LEDs J. Opt. Soc. Korea (0). 4.. Komie ad M. Nakagawa Fudametal aalysis or visible-light commuicatio system usig LED lights IEEE ras. Cosumer Electro (004). 5.. Komie J. H. Lee S. Haruyama ad M. Nakagawa Adaptive equalizatio system or visible light wireless commuicatio utilizig multiple white LED lightig equipmet IEEE ras. Wireless Commu (009). 6. J. Vucic C. Kottke S. Nerreter K. Lager ad J. W. Walewski 53 Mbit/s visible light commuicatios lik based o DM-modulatio o a white LED J. Lightwave echol (00) IEEE Std IEEE Stadard or Local ad Metropolita Area Networks-part 5.7: Short-rage Wireless Optical Commuicatio Usig Visible Light IEEE Piscataway N.J. (0). 9. H. Sugiyama S. Haruyama ad M. Nakagawa Experimetal ivestigatio o modulatio method or visible-light commuicatio IEICE ras. Commu. E89-B (006). 0. S. Hidemitsu H. Shiichiro ad M. Nakagawa Brightess cotrol methods or illumiatio ad visible-light commuicatio system i Proc. 3rd Iteratioal Coerece o Wireless ad Mobile Commuicatios (Guadeloupe Frech Caribbea Mar. 007).. J. M. Kah ad J. R. Barry Wireless irared commuicatios Proc. IEEE (997).. I. E. Lee M. L. Sim ad F. W. L. Kug Perormace ehacemet o outdoor visible-light commuicatio system usig selective combiig receiver IE Optoelectro (009).