Moblty Based Routng Protocol wth MAC Collson Improvement n Vehcular Ad Hoc Networks Zhhao Dng, Pny Ren, Qnghe Du Shaanx Smart Networks and Ubqutous Access Rearch Center School of Electronc and Informaton Engneerng, X an Jaotong Unversty dngzhhao@stu.xtu.edu.cn Abstract. Intellgent transportaton system attracts a great deal of research attenton because t helps enhance traffc safety, mprove drvng experences, and transportaton effcency. Vehcular Ad Hoc Network (VANET) supports wreless connectons among vehcles and offers nformaton exchange, thus sgnfcantly facltatng ntellgent transportaton systems. Snce the vehcles move fast and often change lanes unpredctably, the network topology evolves rapdly n a random fashon, whch mposes dverse challenges n routng protocol desgn over VANET. When t comes to the 5G era, the fulflment of ultra low end-to-end delay and ultra hgh relablty becomes more crucal than ever. In ths paper, we propose a novel routng protocol that ncorporates moblty status and MAC layer channel contenton nformaton. The proposed routng protocol determnes next hop by applyng moblty nformaton and MAC contenton nformaton whch dffers from exstng greedy permeter stateless routng (GPSR) protocol. Smulaton results of the proposed routng protocol show ts performance superorty over the exstng approach. Keywords: Intellgent transportaton system, VANET, routng protocol, GPSR. I Introducton The number of vehcles has gone through an explosve growth n the last few years. Correspondngly, the ncrease number of vehcles has also brought serous traffc problems, such as traffc am, gas polluton and the severe traffc safety problems. In the meanwhle, most of the modern vehcles are equpped wth electronc devces lke vehcular sensors, the global poston system (GPS) and dgtal control center, whch provde the possblty of buldng a communcaton network. Therefore, the emergng VANETs s amed at provdng effcent network servce among vehcles[1-3]. In VANETs, vehcles can communcate wth adacent vehcles, base statons, and roadsde facltes whle travelng on the roads, so vehcles are able to transmt emergency messages to others whch can ensure the safety of drvng when there s an emergency. In most cases, vehcles are not only requred to communcate wth the surroundng vehcles but also wth the far vehcles. Hence, the mportant ssue n VANETs s how to buld a relable and stable mult hop communcaton network when vehcles are not wthn ther communcaton range. There already have been many routng protocols for
mult hop communcatons n MANET whch can be dvded nto several categores. The exstng famous topology based routng protocols are AODV, DSR, OLSR, etc [4-6]. The exstng famous locaton based routng protocols are GPSR, GPCR, etc[7-9]. As a unque form of MANETs, VANETs has a lot of same characterstcs compared to MANETs. On the one hand, they are all autonomous networks so nodes can establsh wreless connectons wthout the control of base statons. In addton, consder the constant movng status of nodes n VANETs and MANETs, so there s no have fxed network topology. Besdes, VANETs dffers from MANETs for the followng aspects. Frstly, wreless channel n VANETs s unstable, and t s easly affected by roadsde buldngs, road condtons and other vehcles. Secondly, the network topology changes quckly and the lnk expraton tme s short because of the hgh velocty and unstable drecton of vehcles. Hence, t s a challenge to desgn a relable and effcent routng protocol over VANETs. There exsts many related works on the routng protocol n VANETs. In [10], authors desgn a movng-zone based routng protocol by establshng movng zones that vehcles wll collaborate wth each other to send nformaton n a dynamc movng zone. The authors n [11] use dstance factor to mnmze control overhead by selectng the least number of possble hops. Authors n [12] propose an mproved GPSR routng protocol by usng vehcle densty and desgn a new data structure n order to carry route nformaton whch can be consdered when search routng. In ths paper, we propose a novel routng protocol by usng the nformaton of moblty along wth a MAC collson mprovement. As s llustrated n [11], ncreased densty of nodes challenges the utlzaton of resources, whch motvates us to optmze MAC performance. We assume the network operates on the IEEE 802.11 DCF mechansm [12]. In the frst step, we calculate the lnk expraton tme usng the moblty. In the second step, we estmate the delay n channel contenton under 802.11 DCF mechansm and we also consder the greedy strategy. Fnally, we calculate the forward weght of node and combne wth GPSR routng protocol to determne the next hop. The rest of ths paper s organzed as follows. We n secton II ntroduce the proposed routng protocol wth moblty and MAC collson mprovement. We n secton III presents the smulaton results under our proposed routng protocol and GPSR routng protocol. Fnally, the conclusons are derved n Secton IV. II The Proposed Routng Protocol We n ths secton present our modfed routng scheme n detal. In the begnnng, our greatest wsh s to reduce the transmsson delay as much as possble snce a lot of vehcle applcatons are delay senstve. And t's also essental to enhance lnk connectvty due to the varous speed and drecton of dfferent vehcles. We frst assume the nformaton of vehcle speed and drecton can be collected by GPS and other vehcular sensors. There exsts a transmsson range and each vehcle can communcaton wth each other wthn the transmsson range. In common case, we naturally consder that routng wth the shortest path s the best whch wll reduce the
number of hops at utmost. The same dea has been used by GPSR routng protocol whch can be called the greedy forwardng strategy. However, the actual stuaton s not always the case and we take Fg.1 for example. The source vehcle 1 wants to transmt packets to destnaton vehcle 10. The path obtaned by GPSR routng protocol s [v1, v5, v6, v10] and t needs three hops. Recall the DCF mechansm n 802.11p, vehcles n the same cluster must experence a MAC layer contenton process to use the wreless channel and every tme ust one packet can be sent. We can see that vehcle 6 has fve neghbors n ts communcaton range so t mght experence a serous MAC delay than vehcle 8 whch only has two neghbors. Therefore, ths path may spend more tme than the path [v1, v5, v8, v9, v10] whch wll spend four hops to arrve the destnaton. Now we frst estmate the MAC delay by analyzng the contenton process of the MAC layer. Fgure 1 An example of dfferent routng selectons A. MAC Delay Analyss
25 Backoff Tmes 20 15 10 = 0.01 = 0.02 = 0.03 = 0.04 = 0.05 = 0.06 5 0 1 2 3 4 5 6 7 8 9 10 Number of vehcles n a cluster Fgure 2 Average backoff tmes n a cluster under dfferent packet arrval rate IEEE 802.11p s the standard protocol n VANETs whch llustrates the man techncal standards for the PHY layer and MAC layer [13]. The basc scheme s DCF whch known as carrer sense multple access wth collson avodance (CSMA/CA). In ths stuaton, vehcles must sense the channel state before transmttng packets. The contenton process n MAC layer can be smplfed as follows. Frstly, vehcle ntates a random backoff procedure untl the medum s dle for DIFS (DCF Inter-Frame Space) tme. If the medum s busy, t must wat and contnue the backoff procedure untl the medum s dle once agan. When the backoff tme equals a zero value, a vehcle should frst send a RTS (Request to Send) packet, then vehcle wat a CTS (clear to Send) packet n order to make sure the contenton s success. Once a vehcle receves the CTS packet n a slot tme, the contenton s success and t can transmt a packet mmedately. On the contrary, f no CTS packet receved or vehcle detects a collson wthn a slot tme, the vehcle turns back to the backoff state and start a new round of backoff. Usually, the common algorthm of backoff procedure n CSMA/CA s wellknown BEB (Bnary Exponental Backoff) algorthm [14]. In BEB, the backoff wndow doubles when the contenton faled and there exsts a mnmum contenton wndow and a maxmum contenton wndow. The backoff tme s generated randomly from the wndow. Through the ntroducton to the contenton process above, we can see that the number of backoff tmes need to be reduced as lttle as possble n order to reduce the delay. In the backoff state, we assume Pt () represents the probablty that vehcle senses channel dle for tme nterval t. Therefore, P ( slot) can represent the success
contenton probablty of vehcle after the backoff state when sensng channel dle for a slot tme. Correspondngly, the falure probablty s 1 ( ). Hence, the P slot probablty of k -th backoff before a successful transmsson s k 1 N( K) (1 P( slot)) P( slot) (1) Then, we can get the average backoff tmes of vehcle as follows: n k 1 lm (1 ( )) ( ) n k 1 N k P slot P slot (2) Now, we wll calculate the probablty. The arrval of a packet of a vehcle can be seen as a Posson process and the packet arrval process can be seen follow the Posson dstrbuton [15]. The probablty that n packets arrve at vehcle wthn a tme nterval t can be expressed as P ( slot) n ( t) t P ( t, n) e (3) n! where denotes the packet arrve rate of a vehcle. We notce that the overall packet arrval rate n cluster s ( ) C( ) (4) where denotes the whole number of vehcles n cluster. Consequently, for vehcle n cluster, the probablty that no vehcle else to transmt packets durng tme nterval s wrtten as follows: ( ) t P( t,0) (5) C( ) t Then we can solve ths lmt expresson. For the sake of smplcty, we frst gnore the lmt symbol and replace formula (5) to formula (2). The expresson can be transformed as follows: If the left sde of formula (6) multpled by follows: e n ( ) t k 1 ( ) t lm (1 ) (6) n k 1 ( ) t 1 e, we can get a new formula as N k e e n ( ) t ( ) t k ( ) t n k 1 (1 e ) N lm k (1 e ) e (7) Formula (6) mnus Formula (7) and results can be smplfed as follows: n (1 ( ) t k 1 ) (1 ( ) t n ) (8) k 1 N k e n e The frst term of formula (8) s geometrc seres and the second term of formula (8) can be transformed nto the problem of solvng the lmt of contnuous functon. Obvously, the second term equals zero. Fnally, when $n$ tends to postve nfnty and we can get the average backoff tmes of vehcle n cluster as follows: N 1 C ( ) e (9) slot Fg.2 shows the average backoff tmes of a vehcle under dfferent and dfferent number of vehcles n a cluster. It's obvous that the less the number of backoff, the
lower the MAC layer delay. So we should try to choose the vehcle wth fewer neghbor nodes as next hop. B. Lnk Expraton Tme Fgure 3 An example of calculatng LET under dfferent vehcle speed and drecton; (a)same drecton wth v v. (b)opposte drecton. (c)same drecton wth v v. (d)opposte drecton. We assume that the moblty nformaton ncludng vehcle speed and drecton can be obtaned by GPS devce. We also assume each vehcle can collect moblty nformaton of others whch wthn the communcaton range. Then we can estmate the lfetme of lnk by calculatng LET. LET measures the relablty of a lnk. The communcaton range of each vehcle s set to R. Accordng to the movng stuatons of vehcles on the road, the LET can be expressed nto four dfferent condtons whch shown n Fg.3. We assume the speed of vehcle and vehcle are, respectvely. The coordnates of the two vehcles are ( x, y ) and ( x, y ). Therefore, the dstance between vehcle The angle s and vehcle s 2 2 d ( x x ) ( y y ) (10) y y arctan x x Then we can get the vertcal dstance of vehcle Consequently, when vehcle and vehcle between two vehcles s as follows: v and vehcle : v (11) w d sn (12) are movng n the same drecton, the LET
when vehcle and vehcle two vehcles s as follows: LET R w d w 2 2 2 2 v v (13) are movng n the opposte drecton, the LET between LET R w d w 2 2 2 2 v v Now, we have estmated the MAC backoff tmes and LET of each vehcle, then we modfy the next hop selecton strategy of GPSR routng protocol. We assume each vehcle has a node weght and for a source vehcle, the weght of neghbor s calculated as follows: Nmax N LET dmax d W N LET (15) d where LET max between vehcle neghbors; N max max s the maxmum LET of all neghbors; and source vehcle. N max s the backoff tmes of vehcle max LET (14) represents the LET s the maxmum backoff tmes of all ; d max denotes the maxmum dstance between neghbors and destnaton; s the dstance between vehcle and destnaton; 1. Obvously, we choose the vehcle as next hop whch has the maxmum value of. The procedure of the proposed routng protocol can be summarzed for followng process. Frstly, the source vehcle measure the dstance between tself and destnaton vehcle. When these two vehcles are wthn ther communcaton range R, source vehcle can tramsmt packet to the destnaton vehcle drectly. If the dstance s more than R, we calculate the LET, backoff tmes and dstance of each neghbor vehcle. Then we can get the value of each neghbor vehcle and choose the vehcle wth W W W d maxmum value of as forward vehcle. And we repeat ths process to fnd the whole path. The detaled procedure of route dscovery s presented n Algorthm 1. A route mantenance strategy s consdered n our proposed routng protocol as well n order to avod lnk dsconnecton. The lnk dsconnecton s more common n VANETs because of the nfluence of vehcle speeds and drectons. The dea of route mantenance s to perodcally detect the dstance between vehcles n order to determne whether or not t s nterrupted. The detaled procedure of route mantenance s presented n Algorthm 2. Algorthm 1 Route Dscovery 1. Get speed and drecton nformaton of all vehcles; 2. Calculate the dstance between current transmt vehcle and all other vehcles; 3. Get the neghbor vehcles to neghbor set M;
4. n =0; 5. whle n < M.sze do 6. f vehcle n s destnaton vehcle 7. Select destnaton vehcle; 8. Return; 9. else 10. Apply Eq.(5) to calculate 11. end f 12. n = n + 1; 13. endwhle 14. Select the vehcle wth maxmum value of weght W Algorthm 2 Route Mantance 1 Update poston of all vehcles; 2 Get all source vehcle n set S; 3 n =0; 4 whle current vehcle sn't destnaton vehcle and n < S.sze do 5 Calculate the dstance d between current vehcle and next hop vehcle; 6 f d < 250 7 Let current vehcle be next hop vehcle 8 else 9 Apply current vehcle to route dscovery procedure; 10 c 11 end f 12 f current vehcle s destnaton vehcle 13 n = n + 1; 14 end f 15 endwhle W III Performance Evaluatons In ths secton, our proposed routng protocol s evaluated and compared wth the famous GPSR routng protocol. We consder a hghway model envronment wth vehcles are dstrbuted randomly for smplcty. The hghway model envronment s 1000m length and has sx lanes n two drectons. The wreless communcaton range
s set to 250m. The velocty of vehcles s varable from 30km/h to 80km/h. Table 1 presents the detaled parameters n our smulaton. Parameters Smualton scenaro Value Hghway Number of lanes 6 Smalaton area Data packet sze 1000m*25m 512bytes Cwmnmun 31 CWmaxmun 1023 1/3 1/3 1/3 Arrval rate( ) 10packets/ms Speed of vehcles 30-80km/h Number of vehcles 12-60 Communcaton range MAC protocol Routng protocol Smulaton tme 250m IEEE802.11p The proposed,gpsr 500TTI Fgure 4 Average end-to-end delay vs number of vehcles
Fg.4 llustrates the average end-to-end delay of our proposed routng protocol and GPSR routng protocol versus the number of vehcles n dfferent average vehcle speed. End-to-end delay refers to the tme that packets transmtted among two vehcles plus the delay n MAC layer. From the fgure, we frstly see that the delay ncreases as the vehcle densty ncreases both n GPSR routng protocol and our proposed routng protocol. It's reasonable because the contenton of MAC layer wll become more compettve whle the number of vehcles ncreases and the tme cost n contenton becomes bgger. Secondly, when the speed of vehcle ncreases, the delay wll ncrease accordngly as hgh speed leads to lnk nstablty. Thrdly, end-to-end delay under our proposed routng protocol s smaller than GPSR routng protocol. It's expected for the followng reason. As the moblty nformaton and MAC delay estmaton are consdered n our proposed routng protocol, the choce of next hop wll become more reasonable than GPSR whch only consders the dstance. Thus, our proposed routng protocol performs better than GPSR on end-to-end delay. Fg.5 plots the packet delvery rate of our proposed routng protocol and GPSR routng protocol versus the number of vehcles n dfferent average vehcle speed. From ths fgure, we can see packet delvery rate of both two routng protocols ncreases when the densty of vehcles ncreases. Ths s obvously because we can choose a more approprate vehcle node for packet forwardng when there are more vehcles. Besdes, when the number of vehcles s less, there are few effectve paths can be selected so the packet delvery rate s smlar to that of the two protocols. We can also see that packet delvery rate gets worse wth the contnuous ncrease of vehcle speed whch s expected. When the number of vehcles s large, the path n our proposed routng protocol s more relable, so the packet delvery rate s hgher than GPSR routng protocol. Fgure 5 Packet delvery rate vs number of vehcles Fg.6 llustrates the broken lnks aganst the number of vehcles n dfferent average vehcle speed. The metrcs of broken lnks measures the lnk relablty. If the number of broken lnks s less, the route can be consdered stable and relable. Otherwse, f wreless lnks nterrupt frequently, the overheads of exchangng control packets wll become larger and t wll spend some delay n reestablshng lnks.
Addtonally, ths fgure demonstrates that the number of broken lnks decreases as the number of vehcles ncreases and the lnk nterrupton s more serous when the speed s 80km/h. Fnally, the number of broken lnks of our proposed routng protocol s small compared to GPSR routng protocol whch llustrates the proposed routng protocol s more stable once agan. Fgure 6 Broken lnks vs number of vehcles IV Conclusons Ths paper descrbed an mproved routng protocol n VANETs by modfyng the forward procedure of exstng GPSR routng protocol. The hghlght of our proposed routng protocol s that we reduce the delay and strengthen the relablty of routes. We optmze the next hop selecton strategy of the well-known GPSR routng protocol by usng moblty and MAC delay estmaton. The method n our routng protocol s based on the vehcular electronc devces whch can provde a large amount of vehcular nformaton. We add moblty nformaton(speed and drecton) and MAC delay estmaton n next hop selecton strategy to make the route more relable and decrease delay. We also dscuss the route mantenance strategy n detal. Fnally, Abundant and convncng smulatons prove that our proposed routng protocol performs much better n forms of average end-to-end delay, packet delvery rate and broken lnks than GPSR. References 1. Y. H. Zhu, X. Z. Tan and J. Zheng, ``Routng n vehcular ad hoc networks: A survey,'' IEEE Vehcular Technology Magazne, vol. 2, no. 2, pp. 12--22, Jun. 2007. 2. M. Altayeb, I. Mahgoub, ``A survey of vehcular ad hoc networks routng protocols, Internatonal Journal on Innovaton and Appled Studes, vol. 3, no. 3, pp. 829--846, Jun. 2013.
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