A New Duty Cycle Based Digital Multiplexing Technique

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1 Proceedigs of the 7 IEEE Iteratioal Coferece o Telecommuicatios ad Malaysia Iteratioal Coferece o Commuicatios, May 7, Peag, Malaysia New Duty Cycle Based Digital Multiplexig Techique Mohamad Khazai bdullah 1, Ghafour mouzad Mahdiraji 1, Mohamed Faisal Elhag 1, hmed Fauzi bas 1, Edmod Zahedi ad Makhfudzah Mokhtar 1 1 Photoics Laboratory, Departmet of Computer ad Commuicatio Systems Egieerig, Uiversiti Putra Malaysia, 434 Serdag, Selagor, Malaysia. Departmet of Electrical, Electroic & System Egieerig, Faculty of Egieerig, Uiversiti Kebagsaa Malaysia, 436 UKM Bagi, Selagor, Malaysia. ghafour153@yahoo.com bstract ew multiplexig techique which is called duty cycle divisio multiplexig (DCDM) is preseted i this paper. Theoretical ad simulatio studies have bee carried out to evaluate the performace of this techique based o the sigal eergy ad symbol error rate (). wireless chael based o free space propagatio model is cosidered for the simulatio study. Two modulatio schemes of PSK ad QM are used to evaluate the techique, agaist the data rates. lso, the performace of the multiplexig techique is compared with the covetioal Time Divisio Multiplexig (TDM) techique as well as with the multilevel M-ary sigalig. The study shows that the eergy per bit i the DCDM techique, ulike that of the TDM techique icreases with the umber of users. The simulatio results correspod with the theoretical study i which the DCDM techique has better tha that of TDM. Keywords Duty cycle, multiplexig techique. I. INTRODUCTION Multiplexig is a essetial part i a commuicatio system where multiple data streams are trasmitted simultaeously through a sigle lik, whether the lik is based o coaxial cable, fiber, radio, or satellite [1, 3]. Multiplexig is widely employed due to its capability to icrease trasmissio capacity ad to reduce system costs []. There are three basic types of multiplexig techique i commuicatio systems; time divisio multiplexig (TDM) [4-6], frequecy or wavelegth divisio multiplexig (FDM) [3, 7] or (WDM) [8-1], ad code divisio multiplexig (CDM) [11-15]. TDM is the most widely used multiplexig techique i commuicatio systems today [16-17]. However, for multiplexig high umber of users with high data rates, highspeed multiplexer ad de-multiplexer are required, resultig i very high cost for TDM systems [18-19, 6]. t higher speeds clock recovery is aother essetial issue that may reder the system highly complicated ad costly for TDM systems [- 3]. Therefore, may ivestigatios have bee doe to desig ad develop reliable ad cost-effective clock recovery modules for TDM i both the electrical ad optical (thus, OTDM) versios [-5]. Realizig these problems, duty cycle divisio multiplexig (DCDM) is proposed i this paper as a alterative multiplexig techique. I this techique, differet users ca share commuicatio media ad trasmit data simultaeously by usig the same frequecy bad, but with a differet duty cycle. The proposed techique also has a iheret property which allows for better clock recovery (however, this is ot withi the scope of this paper). The purpose of this paper is to itroduce the ew multiplexig techique ad compares this techique with TDM. I sectio II, the basic fuctios ad characteristics of DCDM are explaied based o the theories. The results of a simulatio study are discussed i the subsequet sectio, which is the followed by a coclusio. II. DUTY CYCLE MULTIPLEXING TECHNIQUE. Basic Fuctio The DCDM techique is based the uipolar retur to zero (RZ) lie code. I this techique, each user trasmits a bit of with zero volts withi T s secod where T s is symbol duratio, ad bit 1 is trasmitted with + volts with a duratio less tha T s secods. The techique defies the duratio for i th user as i T s T (1) i + 1 where represets umber of multiplexig users. For example, for multiplexig 3 users, the 1 st, d ad 3 rd user uses duratio of T s, T s ad 3T s respectively to trasmit bit 1s Therefore, differet users ca share commuicatio media to trasmit simultaeously by usig the same frequecy bad but with differet duty cycles. Fig. 1(a) shows a example of a rectagular sigal that carries bit sequeces of 111. Fig. 1(b), (c), ad (d), represets the same sigal with T s, T s ad 4 4 3T s duty cycle respectively, represets as differet users. Fig. 4 1(e) shows the patters whe the 3 users are multiplexed with the same period of T s. The multiplexed sigal ca the be modulated by usig a sigle modulator. t the receiver side, after demodulatig the received sigal, the origial sigal ca be extracted from the demodulated sigal based o the sigal voltage amplitude ad the duty cycle s time duratio /7/$5. 7 IEEE. 56

2 mplitude (V) T s T s Fig. 1. (a) Origial rectagular sigal carryig bits 111 with duty cycle of 1% T s, (b), (c) ad (d) 3 multiplexig users with duty cycles of 5, 5 ad 75% T s for user 1, user ad user 3 respectively, (e) multiplexed sigal of the 3 users. Based o the umber of multiplexig users, there are possible combiatios of bits for users. I the multiplexed sigal, each of these combiatios produces a uique symbol. Fig. (a) shows the 8 possible bits combiatios for the case of 3 multiplexig users, ad the multiplexed symbols for the 8 cases are preseted i Fig. (b). By havig the kowledge about this uiqueess at the receiver side, the origial data for each user ca be easily distiguished ad recovered by takig 1 sample per slot for +1 slots per T s secods. This techique allows for automatic bit error detectio ad correctio, based o the sequece of sampled amplitudes per symbol duratio. For the case of multiplexig users, if oly oe sample per slot is take, the, the 1 st sample (take from the 1 st slot), has +1 possible levels, the d sample (take from the d slot), has possible levels, the th sample (take from the th slot), has possible levels ( or volts), ad the last sample ( +1) th, (take from the last slot), has 1 possible level which is volt. B. Sigal Eergy The calculatio of the average eergy per bit of the multiplexig sigal ca be obtaied by cosiderig all possible combiatios of bits for a specific umber of users. For example, Fig. (b) shows the method for calculatig the average eergy per bit for the case of 3 multiplexig users. The average eergy per bit of the DCDM techique ca be defied as: + 1+ k k 1 E () avg / bit Ts 6( + 1) where is sigal amplitude i volt, T s is sigal symbol duratio i secod ad is umber of users. The eergy per bit will be icreased liearly as the umber of multiplexed user is icreased. This is importat as it helps to improve the sigal trasmissio quality at a larger umber of users. Cases User User User (a) Case 1 Case Case 3 Case 6 Case 7 Case 8 3 E 1 E ( T s E 3 s E 3 Case 4 5 s Case 5 E 5 4 ( Ts E avg bit Ts /4 4 Ts E 6 6 s E 9 7 s E 14 8 s ( Ts / T 3 3 Fig.. (a) The 8 possible bits combiatio for multiplexig 3 users, (b) multiplexed symbols for the 8 cases ad the average eergy per bit for 3 users. C. Compariso of DCDM with M-ary ad TDM techique M-ary sigalig is ormally used to trasmit multiple amplitudes, each oe represetig bits or more. The mai purpose of M-ary is to icrease the badwidth efficiecy whereby may bits ca be trasmitted by a sigle symbol. If each bit is assumed to represet a differet user, the M-ary ca also be used for multiplexig, thus worthy of compariso with the ew scheme. Two comparisos are doe which are based o the umber of sigal voltage levels ad o the average eergy per bit versus umber of users. Fig. 3 shows the theoretical results for the sigal voltage level versus umber of users for the DCDM, TDM ad M-ary techiques. Compariso study shows that the umber of voltage levels i the TDM remaied fixed to levels, for the DCDM techique it icreased as ( +1), ad for the M-ary techique it icreased as ( ). Thus, i order to multiplex 15 users, the TDM, DCDM ad M-ary techiques require, 16 ad 3768 sigal voltage levels respectively. This result shows the disadvatage of the M-ary techique which makes it impossible to use as a multiplexig techique for high umber of users. Therefore, the ext compariso is cotiued betwee the TDM ad DCDM oly. (b) s 57

3 Number of voltage level Number of voltage level versus umber of user DCDM TDM M-ary Number of user Fig. 3. Number of sigal voltage level versus umber of user for the M-ary ad DCDM techique. Based o sigal eergy, the average eergy per bit for the DCDM is calculated as preseted i Fig., based o Eq. (). For the TDM, the same method show i Fig. is used to calculate the average eergy per bit of the TDM sigals, which is defied as E avg / bit 1 T. (3) Fig. 4 shows the average eergy per bit of the TDM ad DCDM for multiplexig up to 15 users. The figure shows that the average eergy per bit of the DCDM icreased liearly with the umber of multiplexig user whereas the average eergy per bit for TDM reduced. This icremet of eergy i the DCDM is very importat because, it makes this techique able to support higher umber of multiplexig user compared with the TDM, cosiderig the eergy per bit oly. The other advatages of the DCDM techique iclude simple trasmitter desig, error detectio ad correctio capability ad better clock recovery. For example, as far as trasmitter desig is cocered, the OTDM techique requires oe modulator for each user [7, 8]. The use of multiple modulators is costly ad may lead to cross-talks [4]. O the other had, the DCDM techique requires oly a sigle modulator for all the users. This is much chipper ad at the same time avoidig the cross-talk problems. lso ote that there are various trasitios takig place withi the period T s. If the duty cycle is less tha 1%, there will always be a trasitio betwee ay two cosecutive symbols except for the case whe all users are trasmittig bits cosecutively which is highly ulikely (especially as the umber of users icreases). The trasitio is importat i that it provides a simple meas to recover the clock timig. verage eergy per bit (J) Normalized eergy per bit versus umber of user DCDM TDM Number of user Fig. 4. Normalized average eergy per bit versus umber of user for DCDM ad TDM. III. SIMULTION STUDY simulatio study has bee doe i order to compare the performace of the DCDM with the TDM techique. The simulatio has bee set for a wireless trasmissio with the additive white Gaussia oise (WGN). Two modulatio schemes of 16-PSK ad 16-QM have bee applied to compare the performace of the two multiplexig techiques.. Simulatio Set Up The parameters used i this simulatio study are preseted i Table I, applicable for both the DCDM ad TDM techiques. block diagram of the DCDM techique is depicted i Fig. 5. The performace simulatio is studied cosiderig two of the mai impairmet factors which are atteuatio ad oise. s the trasmitter ad receiver atea are assumed to be i lie-of-sight, therefore, the free space propagatio model is used to calculate the atteuatio i the commuicatio media. The atteuatio of the commuicatio media calculated based o the Friis free space equatio [9] is 4π d f Pr Pt + Gt + Gr log (4) c where P t ad P r are the trasmitted ad received power i watts, G t ad G r are the trasmitter ad receiver atea gai, d is the trasmitter ad receiver separatio distace i meters, f is the carrier frequecy ad c represets the speed of light for 8 space which is 3 1 m/s [9-3]. P t is assumed be the same as the basebad sigal power which is calculated from the eergy cotet of the multiplexed sigals show i Eq. () ad (3) for the DCDM ad TDM respectively. Fig. 5. Block diagram of the DCDM techique for multiplexig 7 users. TBLE I DESIGN ND PERFORMNCE PRMETERS SSUMED FOR THE SIMULTION Fixed desig parameters for the DCDM ad TDM techique Number of sample per DCDM 1 + umber of user symbol TDM umber of user Number of pulse per simulatio 1 Number of multiplexig user 15 Modulatio scheme 16-PSK, 16-QM Number of level (M) DCDM 16 TDM Propagatio model Free space propagatio loss (FSPL) Carrier frequecy 4 GHz Trasmitter power Basebad sigal power Trasmitter ad receiver atea Fixed, lie-of-sight Noise parameters k 1.38e -3 J/ºK, F 1 db T at 3 ºK, T o 3 ºK Sigal voltage amplitude per user 15 mv Trasmitter atea gai (Gt) 16 db Receiver atea gai (Gr) db Distace (d) 8 km 58

4 The amout of thermal oise that is cosidered i the commuicatio media is give by [9-3] P kt B (5) where the P is thermal oise power (W), k is the Boltzma s costat of J/ºK, T is kow as the equivalet oise temperature ad B is the equivalet oise badwidth. The oise badwidth is assumed to be equal with the multiplexed sigal badwidth. The oise temperature is calculated as [9-3] T Tat + T ( F 1) (6) where the T at is oise temperature of the atea, T o is temperature of source ad F is atea oise figure i db. Temperature of the atea ad source are assumed to be at the room temperature of 7 oc. I order to calculate the of the DCDM ad TDM with both the PSK ad QM modulatio scheme, the simulatio is modeled i the Matlab eviromet. I this software the calculates usig the direct method, by comparig the trasmitted ad received data, (i.e. Number of error / Number of trasmitted bit). B. Simulatio Results Fig. 6 ad 7 show the simulatio results of the versus bit rate of the TDM ad DCDM for the case of PSK ad QM respectively. I this simulatio the umber of multiplexig users is fixed to 15 users. The data rate of each user is varied from 1 to 15 Mbps per user cotributig to the maximum total data rate of 5 Mbps. The results show that for all bit rates, the DCDM has better performace tha the TDM i both modulatio schemes. This is because of the higher eergy per bit of the DCDM sigals as previously show. By comparig the two modulatio schemes, Fig. 8 shows that, QM has better tha PSK for modulatig the DCDM sigal. lthough, the symbol error rate is high, the mai objective of comparig the performace of the two techiques is evertheless achieved. versus data rate for PSK PSK-DCDM PSK-TDM Fig. 6. versus bit rate per user of DCDM ad TDM for the case of PSK. versus data rate for QM QM-DCDM QM-TDM Fig.7. versus bit rate of DCDM ad TDM for the case of QM. versus data rate for DCDM PSK-DCDM QM-DCDM Fig. 8. versus data rate of DCDM usig PSK ad QM. IV. CONCLUSION The priciple of the DCDM techique has bee discussed i this paper by comparig with M-ary ad TDM techiques. The theoretical results show that if the M-ary techique used as multiplexig techique, the umber of sigal voltage level is icreased by which is impractical. However, it is possible for the DCDM i which the umber of sigal level is oly (1+ ). lthough TDM has the advatage of smaller amplitude levels, its eergy cotet decreases with the umber of users. O the other had DCDM eergy cotet improves with the umber of users. The simulatio results clearly show the better performace of DCDM tha that of TDM whether the sigals are QM or PSK modulated. For DCDM, QM proves to be a better modulatio scheme. lthough the simulatio cosiders oly thermal oise, the geerality of the trasmissio performace of DCDM i compariso with TDM is maitaied. The other iheret advatages of the DCDM techique cosidered for future reports are simpler trasmitter, better error detectio ad correctio, ad better clock recovery. REFERENCES [1] V. Zarimpas ad B. Hoary, Multimedia multiplexig protocol for future wireless commuicatio chaels, 3G Mobile Commu. Tech., 15 th Iter. Co., pp , 4. [] S.. ljuid. Developmet ad implemetatio of a ovel optical code divisio multiplexig techique for metropolita area etwork. PhD thesis, Uiversiti Putra Malaysia (UPM), Feb. 5, Chapter. [3] H. Sari, F. Vahaverbeke, ad M. Moeeclaey, Extedig the capacity of multiple access chaels, IEEE Commu. Maga., Ja., pp

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