2013 IEEE Wireless Commuicatios ad Networkig Coferece (WCNC): MAC Broadcastig i Multichael Cogitive Radio Ad Hoc Networks Zaw Htike Departmet of Computer Egieerig Kyug Hee Uiversity 1 Seocheo,Giheug, Yogi, Gyeoggi 449-1 Korea Email: htike@etworkig.khu.ac.kr, Choog Seo Hog Departmet of Computer Egieerig Kyug Hee Uiversity 1 Seocheo,Giheug, Yogi, Gyeoggi 449-1 Korea Email: cshog@khu.ac.kr Abstract Cogitive radio etwork techology is regarded as a ew way to improve the spectral efficiecy of wireless etworks. It has bee well studied for more tha a decade ad umerous precious works have bee proposed. However, very few existig works cosider how to broadcast messages i cogitive radio etworks that operate i multichael eviromets ad oe of these provides a full broadcast mechaism. Therefore, i this paper, we propose a broadcastig mechaism for multichael cogitive radio ad hoc etworks. The, we aalyze the mechaism regardig the speed of message dissemiatio, umber of trasmissios, portio of the users that receive the broadcast message ad so forth. Idex Terms Cogitive radio ad hoc etworks, multichael, message broadcastig I. INTRODUCTION A cogitive radio (CR) etwork is ormally costructed with primary users (PUs) that are licesed to use the specific chaels ad secodary users (SUs) or cogitive users that are typically ot licesed to utilize the chaels. The beauty of CR techology is i its allowig the SUs to access the licesed chaels without ay harmful iterferece with the PUs operatios [1]. I geeral, SUs detect the free or idle portios of a chael ad access the chael. Whe the PU appears o the chael that is curretly used by SUs, all SUs must defer their trasmissios ad migrate to other available chaels. Chael availability is determied by PU activities, which chage dyamically i frequecy, space ad time; therefore, the set of available chaels for each SU might also chage dyamically [2]. Thus, at a give time, SUs may operate o differet chaels idepedetly as show i Figure 1 [3]. A. Problem Statemet As we all kow, broadcastig is essetial i may wireless etwork applicatios, such as deliverig multimedia messages, route discovery i gossip based routig protocols ad so o [4]. The problem is how to deliver a message to all users that are This research was supported by the MKE(The Miistry of Kowledge Ecoomy), Korea, uder the ITRC(Iformatio Techology Research Ceter) support program supervised by the NIPA(Natioal IT Idustry Promotio Agecy) (NIPA-2012-(H01-12-4). Dr. CS Hog is the correspodig author. Chaels CH-1 CH-2 CH-3 CH-4 x y a Occupied by PUs b c d w z a b Secodary Users Fig. 1. At ay give time, SUs may operate o differet chaels idepedetly. curretly dwellig o differet chaels. Sice it is a broadcast message, it should be received by all or most of the SUs i the etwork. Moreover, the message should be delivered to all users withi a reasoable time iterval. However, o existig protocol for the CR etwork addresses this problem. This was the mai ispiratio i our proposig a mechaism that eables broadcastig i multichael CR ad hoc etworks. B. Related Works Oe possible solutio to eable broadcastig i a multichael eviromet is usig a dedicated cotrol chael or broadcast chael [5][6]. I this approach, all users have to tue ito a specific chael to receive or trasmit the broadcast messages. This approach is applied i may Medium Access Cotrol (MAC) protocols for CR etworks because of its simplicity. This dedicated chael is ormally called the Commo Cotrol Chael (CCC) [7]. This solutio is also used i IEEE 19.4 (stadard for wireless access i vehicular eviromet (WAVE)) [8]. I IEEE stadard 19.4, oe out of seve chaels is assiged as a cotrol chael (CCH) ad the rest are called service chaels (SCH). All users switch to the specific chael, the cotrol chael, at a particular time iterval, the CCH iterval, to trasmit ad receive the broadcast messages [9]. However, this approach is ot applicable i CR etworks due to the lack of availability of costatly idle chaels. If we assig a chael for broadcastig i a CR etwork, it will be susceptible to PU activities, because, whe PUs appear o the dedicated cotrol or broadcast chael, all SUs must vacate the chael 978-1-4673-5939-9/13/$31.00 2013 IEEE 739
immediately. If the PUs trasmissio period is sigificatly log o the chael, the presece of the PUs may block chael access for SUs. Moreover, the available chael sets i CR etworks chage dyamically, hiderig the establishmet of a ever-available chael for all SUs. As a alterative, the authors of [10] proposed eablig broadcastig by usig multiradio. Obviously, this is ot a cost effective ad simple solutio sice it requires multiple trasceivers. I our proposed mechaism, we use either a dedicated broadcast chael or multiple trasceivers for message broadcastig. I this mechaism, all SUs just eed to collaborate i message dissemiatio. We preset a detailed descriptio i Sectio II. We aalyze the proposed mechaism i Sectio III. The, Sectio IV cocludes the paper. Note that, i this paper, we solely focus o message broadcastig i a multichael etwork eviromet. II. BROADCASTING IN A MULTICHANNEL ENVIRONMENT A. System Model We assume the etwork type is ad hoc without a cetralized coordiator ad that there are m available chaels (CHs), M = {CH 1,CH 2,...,CH m }. We also assume that there are N umbers of SUs i the etwork ad SUs are evely distributed o available chaels. Every SU i the etwork is equipped with a sigle trasceiver. The chael coditio is ideal, which meas there are o hidde termials, ad odes (SUs) 1 are withi the trasmissio rage of each other. If a chael is curretly used by PUs, this chael is regarded as uavailable for SUs, ad SUs dwell oly o PU-free chaels. B. Message Broadcastig The basic message broadcastig scheme is as follows. Whe a SU receives a broadcast message from upper layer applicatios, it will broadcasts the message by embeddig a couter(r) with the message o the curret operatio chael, CH i. All message trasmissios follow the priciple of the Distributed Coordiatio Fuctio (DCF) of IEEE 2.11 [11]. Ay eighborig SU that receives the broadcast message checks the couter ad, if the couter is o-zero, it will retrasmit the message after decreasig the couter by oe. If a SU receives the same message with a differet couter value, the couter value will be updated with a lower value. The message will be retrasmitted util its couter reaches zero. Sice SUs are evely distributed o differet chaels idepedetly, if a ode trasmits the broadcast message o a chael, say CH i, oly a portio of the eighbor odes, which are curretly dwellig o CH i, will receive it. However, the message should be broadcast o every chael at least oce i order to be delivered to all eighbors. Therefore, istead of the source SU trasmittig the message o every chael, eighbor SUs eed to collaborate i message dissemiatio. Whe a SU broadcasts a message o CH i with couter, r, some eighborig SUs which are curretly dwellig o CH i will receive it. Whe a SU receives the broadcast message 1 I this paper, we use the terms, ode ad secodary user iterchageably. o CH i, first, it chooses oe of the available chaels, CH j M,i j, radomly. The, it switches to CH j ad detects the presece of the PUs. If it detects the chael is free, it will retrasmit the message after decreasig r by oe. If the selected chael is occupied by PUs, it will choose aother available chael. Neighbor odes o CH j perform the same way. They choose oe of the available chaels radomly ad trasmit the broadcast message ad so o. If a chael is chose by more tha oe ode, odes eed to coted to broadcast the message. If a ode successfully trasmits the message, the rest will switch to other available chaels to retrasmit the message. A SU ca participate more tha oce i message dissemiatio. For example, if a SU receives the same message after trasmittig, ad if the couter is ozero, it will participate agai by retrasmittig the message. For example, if a SU receives the same message after trasmittig, ad if the couter is o-zero, it will participate agai by retrasmittig the message. Figure 2 shows message broadcastig i a multichael eviromet. CH-1 CH-2 CH-m 1 st phase(r) 2 d phase(r-1) 3 rd phase(r-2) a Fig. 2. c b b c e d g f C. Selectig the Couter Values e d f g g f Secodaryusers Message broadcastig procedure. Time Chael switchig Message broadcastig A large couter value ca icrease the umber of users that receive the broadcast message or the probability that the broadcast message is received by all users. However, the large couter value may cause udesirable trasmissio overheads. If we choose a small couter value, message broadcastig will be termiated withi a short time ad it might ot be delivered to a sigificat portio of users. It is ot straight forward to set the couter for the messages. As we metioed above, if a ode trasmits a message successfully o CH i ad all eighbor odes o CH i receive it, the each ode chooses oe of the chaels radomly for retrasmissio. Cosider the worst case, suppose all odes o CH i choose the same chael (CH j M,i j) ad switch to CH j for the retrasmissio. The, there will be, at most, oe retrasmissio o CH j i the secod phase. Agai, all odes choose the same chael (CH k M,k j,i) ad there will also be oly oe retrasmissio o CH k i the third phase ad so o. I order to trasmit the message o 7
every CH, the couter value should be m, sice m is the total umber of chaels. Cosider agai the best case sceario as eighbor odes choose differet chaels for retrasmissio. For example, ode b chooses CH k ad c choosesch j, where k j, as show i Figure 2. Therefore, i the secod phase, there will be retrasmissios o differet chaels, where is the average umber of eighbor odes o CH i ad it ca be estimated as = N/m. Agai, if the odes that receive the broadcast message choose totally differet chaels, there will be mi( 2,m) retrasmissios o mi( 2,m) differet chaels ad so o. Thus, the message will be trasmitted o every chael withi (log m) phases. The we ca choose the couter value for the broadcast message trasmissios as log (m) < r m, (1) where x is the smallest iteger ot less tha x. III. PERFORMANCE EVALUATION A. Message Dissemiatio Rate We ra simulatios to show the message dissemiatio rate based o the r value ad the umber of eighbor odes o each CH. We assiged the umber of chaels as m = 6. Figure 3 (a) shows the portio of chaels to which the odes choose to trasmit ad (b) shows the ratio of users curretly dwellig o the selected chaels to the total users of the etwork. I other words, it shows the umber of users that ca receive the broadcast message. Ratio of CHs (%) 2 4 6 Couter(r) (a) Ratio of Users(%) 20 2 3 4 5 6 Couter(r) (b) N=12 N=18 N=24 Fig. 3. (a) Portio of chaels chose for retrasmissio ad (b) ratio of users that receive the message based o the r value. Obviously, the message dissemiatio rate is totally depedet o the umber of odes i the etwork. If we evaluate i terms of phases, we ca say that the deser the etwork, the faster the message dissemiatio. However, o the other had, if there are may users i the etwork, each user has less chace to seize the chael for trasmissio. It ca cause log trasmissio delays which we discuss i detail later. B. Number of Message Trasmissios Accordig to the proposed mechaism, the source ode (the ode that geerates the broadcast message) trasmits the broadcast message with couter value r o its curret operatig chael,ch i. Ay ode och i that receives the message chooses oe of the available chaels for retrasmissio. Let Y be the umber of chaels that are chose for retrasmissio. The, the probability that all eighbor odes o CH i choose the same chael (oly oe chael) is P(Y = 1) = m m. (2) Here, m represets the umber of permutatios with repetitio of m thigs take at a time. The probability of all odes choosig two chaels for retrasmissio is P(Y = 2) = m.(m 1).2 2 m. (3) The we have a probability mass fuctio as, P(Y = y) = m(m 1)(m 2)...(m (y 1))y y ) m (m y)! = y y m,y = 1,2,3,...,. (4) Let T be the umber of trasmissios o differet chaels. Geerally, the expected umber of T ca be expressed as E[T] = 1 (m i)! i i mi(m, 1 m, < m (m i)! i i m ), m. Accordig to the proposed broadcastig mechaism, the umber of trasmissios is expoetially icreased based o r. Thus, we have the total expected umber of trasmissios based o r as follows. E[T tot ] = 1+mi(m, 0. 1 + mi(m,. 1 + mi(m, r 1. 1 (m i)! i i m ) (m i)1 i i m )+... r 1 = 1+ mi(m, k.( 1 k=0 (m i)! i i m ) C. Average Delay for Each Broadcast Message (5) (m i)! i i m ))(6) Here, we defie the average delay as the ecessary time for successful trasmissio of a broadcast message o every chael. We have assumed that all message trasmissios follow the priciple of DCF. Therefore, at a give time slot, a SU will trasmit the broadcast message with probability τ. If more tha oe ode trasmits at the same time, it causes collisio with probability p ad we have, p = (1 (1 τ) 1 ), (7) where 0 < p < 1 ad 0 < τ < 1. The variables τ ad p ca be solved by the umerical method as i [11]. 741
TABLE I SIMULATION PARAMETERS Parameter Value Number of chaels (m) 6 Data rate (B) 2 Mbps Broadcast packet legth 1024 bytes Time Slot 9µs SIFS 16µs DIFS SIFS +2.TimeSlot I every time slot, the packet will be successfully trasmitted with probability p sus, the packet collisio occurs with probability p col, or the chael is idle or busy with probabilities p idle ad p busy. Here, let us apply some results from [12], ad the we have p idle = (1 τ) p busy = 1 p idle = 1 (1 τ) p sus = τ(1 τ) 1 (8) p col = p busy p sus = 1 (1 τ) τ(1 τ) 1. Let T data deote the time for trasmissio of a broadcast message.t idle,t col adt sus represet the duratio of chael idle, the duratio of packets collisio ad the duratio for a successful packet trasmissio. The we have, T idle T sus = atimeslot = T col = T data +DIFS. T data = Data L B. (9) Data L represets the total packet legth of broadcast message ad B deotes the data rate. Suppose, X is the time iterval from chael access cotetio to the time whe a message is successfully broadcast o a chael, the the mea of time iterval X is give as E[X] 2 = T idle p sus +p col T col p sus +T sus. (10) The, the average delay for each broadcast message is goig to be E[Delay] = r.e[x]. (11) We ra simulatios to illustrate the average delay for each broadcast message while PU activities are take ito cosideratio. Table 1 represets the simulatio parameters we used. Broadcast packet arrivals follow the Poisso distributio ad, agai, all message trasmissios follow the priciple of IEEE 2.11 DCF [11]. I the simulatios, PU appearace is radom ad ca occur o ay chael at ay time. Whe a PU appears o a chael, we assume that this chael is o loger available for SUs ad all SUs vacate to other available chaels. As show i Figure 4, the average delay is slightly icreased while the umber of PUs i the etwork icreases. This is because high PU activities ca decrease the umber of available chaels for SUs. SUs eed to share less available 2 Detailed proof for E[X] ca be see i [12]. chaels. Thus, SUs require more time o cotetio to seize the chaels for message broadcastig. It icreases the value of E[X]. O the other had, if the etwork has fewer available chaels, it will require a small couter value for broadcastig, as discussed i Sectio II. Noetheless, for simplicity, we used r = 4 i our simulatios. Average delay (ms) 55 45 35 Number of PUs=1 Number of PUs=2 Number of PUs=3 20 25 35 45 55 65 Number of SUs Fig. 4. Average delay for message broadcastig. We also evaluated our mechaism regardig the message delivery ratio, the proportio of the successfully trasmitted messages to the umber of total messages which arrive. The message delivery ratio is also slightly affected by PU activities ad the umerical result ca be see i Figure 5. Similarly to the previous results, high PU activities cause less chael access time for SUs ad degrade the packet delivery ratio of the secodary etwork. Delivery ratio of broadcast messages (%) 20 10 0 Fig. 5. Number of PUs=3 Number of PUs=2 Number of PUs=1 20 Number of SUs Successful packet delivery ratio IV. CONCLUSION We have preseted a broadcastig mechaism for multichael CR ad hoc etworks. This mechaism eables broadcastig i multichael eviromets without usig a dedicated broadcast chael or multiple trasceivers. The, we showed the average time ecessary for broadcastig a message over a multichael eviromet. I future works, we will 742
aalyze our proposal i more depth, cosiderig the PU actives o each chael. REFERENCES [1] I. F. Akyildiz, W.-Y. Lee, ad K. R. Chowdhury, CRAHNs: Cogitive radio ad hoc etworks, Ad Hoc Networks, vol. 7, o. 5, pp. 810 836, July 2009. [2] I. F. Akyildiz, W.-Y. Lee, M. C. Vura, ad S. Mohaty, NeXtgeeratio/dyamicspectrumaccess/cogitiveradiowireless etworks: A survey, Computer Networks, vol., o. 13, p. 21272159, September 2006. [3] A. D. Domeico, E. C. Striati, ad M.-G. D. Beedetto, A survey o mac strategies for cogitive radio etworks, IEEE Commuicatios Surveys ad Tutorials, vol. 14, o. 1, pp. 21 44, FIRST QUARTER 2012. [4] L. Juhai ad X. Liu, A survey of multicast routig protocols for mobile ad-hoc etworks, IEEE Commuicatios Surveys ad Tutorials, vol. 11, o. 13, pp. 78 91, First quarter 2009. [5] J. Che, AMNP: ad hoc multichael egotiatio protocol with broadcast solutios for multi-hop mobile wireless etworks, IET Commuicatios, vol. 4, o. 5, pp. 521 531, March 2010. [6] C. Campolo, A. Moliaro, A. Viel, ad Y. Zhag, Modelig prioritized broadcastig i multichael vehicular etworks, IEEE Trasactios o Vehicular Techology, vol. 61, o. 2, pp. 687 1, March 2012. [7] B. F. Lo, A survey of commo cotrol chael desig i cogitive radio etworks, Physical Commuicatio, vol. 4, o. 1, pp. 26 39, March 2011. [8] IEEE stadard for wireless access i vehicular eviromets (WAVE)multi-chael operatio, IEEE Std. 19.4, September 2010. [9] Q. Che, D. Jiag, ad L. Delgrossi;, IEEE 19.4 DSRC multi-chael operatios ad its implicatios o vehicle safety commuicatios. IEEE Vehicular Networkig Coferece (VNC), 2009, pp. 1 8. [10] J. Qadir, C. T. Chou, A. Misra, ad J. G. Lim, Miimum latecy broadcastig i multiradio, multichael, multirate wireless meshes, IEEE Trasactios o Mobile Computig, vol. 8, o. 11, pp. 1510 1523, November 2009. [11] G. Biachi, Performace aalysis of the IEEE 2.11 distributed coordiatio fuctio, IEEE Joural o Selected Areas i Commuicatios, vol. 18, o. 3, pp. 535 547, March 2000. [12] Q. Wag, S. Leg, H. Fu, ad Y. Zhag, A IEEE 2.11p-based multichael MAC scheme with chael coordiatio for vehicular ad hoc etworks, IEEE Trasactios o Itelliget Trasportatio Systems, 2012. 743