AMERICAN UNIVERSITY OF BEIRUT IMPROVING DATA COMMUNICATIONS IN VEHICULAR AD HOC NETWORKS VIA COGNITIVE NETWORKS TECHNIQUES ALI JAWAD GHANDOUR

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2 AMERICAN UNIVERSITY OF BEIRUT IMPROVING DATA COMMUNICATIONS IN VEHICULAR AD HOC NETWORKS VIA COGNITIVE NETWORKS TECHNIQUES by ALI JAWAD GHANDOUR A thesis submitted in patial fulfillment of the equiements fo the degee of Maste of Engineeing to the Depatment of Electical and Compute Engineeing of the Faculty of Engineeing and Achitectue at the Ameican Univesity of Beiut Beiut, Lebanon Febuay 2010

3 AMERICAN UNIVERSITY OF BEIRUT IMPROVING DATA COMMUNICATIONS IN VEHICULAR AD HOC NETWORKS VIA COGNITIVE NETWORKS TECHNIQUES by ALI JAWAD GHANDOUR Appoved by: D. Hassan Atail, Associate Pofesso Electical and Compute Engineeing Adviso D. Zahe Dawy, Assistant Pofesso Electical and Compute Engineeing Membe of Committee D. Haida Safa, Assistant Pofesso Depatment of Compute Science Membe of Committee Date of thesis defense: Febuay 5, 2010

4 AMERICAN UNIVERISTY OF BEIRUT THESIS RELEASE FORM I, Ali Jawad Ghandou, authoize the Ameican Univesity of Beiut to supply copies of my thesis to libaies o individuals upon equest. do not authoize the Ameican Univesity of Beiut to supply copies of my thesis to libaies o individuals fo a peiod of two yeas stating with the date of the thesis defense. Signatue Date

5 ACKNOWLEDGMENTS Fist and foemost, I would like to thank Pofesso Hassan Atail fo being my adviso fo this maste thesis and fo poviding me with his invaluable feedback and guidance. Pof. Atail was always thee to suppot my wok though oganizing weekly meetings to discuss my pogess and plan what to do next. I would like also to thank all of those who helped me to complete this wok, especially Pof. Zahe Dawy and Pof. Haida Safa, membes of the thesis committee, fo thei valuable and insightful comments. Finally, a special thank to my paents, my sistes Nadine and Maya, my bothe Walid, and my bothes-in-law Rabih and Radwan fo thei pesence and suppot. Thank you fo eviving in me the spiit of challenge and inteest when I cacked unde pessue. v

6 AN ABSTRACT OF THE PROJECT OF Ali Jawad Ghandou fo Maste of Engineeing Majo: Electical and Compute Engineeing Title: Impoving Data Communications in Vehicula Ad Hoc Netwoks via Cognitive Netwoks Techniques Reseaches have suggested Vehicula Ad hoc Netwoks as a way to enable ca to ca communications and to allow fo the exchange of safety and othe types of infomation among cas. Seveal effots have attempted to standadize the communication and netwoking schemes fo vehicula settings. Of most impotance is the Wieless Access in Vehicula Envionments (WAVE) potocol stack standadized by the IEEE that allocates spectum fo vehicula communication. In this epot, we pove that WAVE does not necessaily povide sufficient spectum fo eliable exchange of safety infomation. We pesent a system that uses cognitive netwoking pinciples in vehicula netwoks to dynamically incease the spectum allocated to the contol channel (CCH) by the IEEE p amendment, whee all safety infomation is tansmitted. To do this, the poposed system elies on sensed data sent by the cas to oad side units that in tun fowad the aggegated data to a pocessing unit. The pocessing unit infes data contention locations and geneates spectum schedules to dispatch to the passing cas in an attempt to alleviate contention. Analysis and simulation esults indicate the effectiveness of the system in easing contention and allowing a geate numbe of cas in a paticula location to eliably communicate safety infomation. vi

7 CONTENTS ACKNOWLEDGEMENTS v ABSTRACT vi LIST OF ILLUSTRATIONS.. LIST OF TABLES... LIST OF ABBREVIATIONS... x xii xiii Chapte 1. INTRODUCTION Safety of tanspotation Systems Vehicula Ad Hoc Netwok VANETs Chaacteistics WAVE Stack Cognitive Netwoks Motivation Contibution RELATED WORK Physical Laye Amendment DSRC Spectum Allocation IEEE p Impovements MAC Laye Amendment vii

8 Oveview of BSS mechanisms in IEEE Basic Sevice Set Identification IEEE p WAVE Mode WAVE BSS MAC Amendment Summay The IEEE Standad Channel Routing Use Pioity Channel Selecto o Medium Contention Channel Coodination The IEEE Standad WBSS Establishment WBSS Temination Cognitive Systems Poposed Solutions in the Liteatue PROPOSED SYSTEM Contol Channel Extension Scheme Netwok Contention Metic System Design Entities Desciption and Layout Ca Opeation RSU Opeation LAPU Opeations Detemining the weight and theshold values Oveall Inteaction EXPERIMENTAL RESULTS Simulation Platfom Simulations Results. 67 viii

9 5. SCALABILITY ANALYSIS CONCLUSION AND FURTHER RESEARCH Conclusion Futue Wok BIBLIOGRAPHY 80 ix

10 ILLUSTRATIONS Figue Page 1.1. DSRC standads and communication stack Cognitive Cycle Spectum occupancy in each band in Dublin, Ieland Fequency channel layout of 5.9 GHz WAVE system Fame Contol Field in IEEE Illustation of MSDUs passed between the LLC and the MAC Pioitized access fo data tansmission on one channel Sync inteval, guad inteval, CCH inteval, and SCH inteval WAVE potocol stack and scope of WAVE Sevice Advetisement fomat Main sensing methods in tems of thei sensing accuacies and complexities Contention Window with and without netwok contention Poposed system topology Netwok cognition cycle Fomat of the ecod sent by the ca to the RSU Contention metic and coesponding contention levels and eactions Low Theshold linea model Medium contention theshold High contention theshold.. 61 x

11 3.9. Low Theshold linea model with Mobility High level view of the system s inteactions Sequence diagam detailing the inteactions between the system s components Congested scenaio elative to the low and medium thesholds Uncongested scenaio elative to the theshold Maximum delay vesus numbe of cas when the payload size is bytes 4.4. Access delay vesus numbe of cas and payload size Numbe of untansmitted packets befoe and afte extending the contol channel 74 xi

12 TABLES Table Page 1.1. Summay of spectum occupancy in each band in New Yok City and Chicago Channels assignment in DSRC Key paametes of DSRC/IEEE p PHY and IEEE a PHY Usage of diffeent addesses accoding to the ToDS and FomDS UP-to-AC mappings Ca gatheed data at inteval i Fomat of the RSU table fowaded to cas Paamete values fo deiving the weights of the contention expession Paamete values fo the mixed scenaio Chaacteistics of simulation envionments fo vehicula ad hoc netwoks Compaison of NS2, GooveNet and NCTUns Simulation paametes and thei values. 67 xii

13 ABBREVIATIONS NHTSA National Highway Taffic Safety Administation, 1 DSRC Dedicated Shot-Range Communication, 1 VANETs Vehicula Ad Hoc Netwoks, 2 RSU Road Side Unit, 2 MANETs Mobile Ad Hoc Netwoks, 2 ITS Intelligent Tanspotation Systems, 2 WAVE Wieless Access in Vehicula Envionments, 4 MAC Media Access Contol, 5 PHY Physical, 5 CN Cognitive Netwok, 6 QoS Quality of Sevice, 7 FCC Fedeal Communication Commission, 9 SCH Sevice Channel, 9 OFDM Othogonal Fequency-Division Multiplexing, 15 ISI Inte-Symbol-Intefeence, 16 AP Access Point, 19 IBSS Infastuctue Basic Sevice Set, 19 SSID Sevice Set Identifie, 19 WLAN Wieless Local Aea Netwok, 19 BSSID Basic Sevice Set Identification, 19 DA Destination Addess, 20 RA Recipient Addess, 20 SA Souce Addess, 20 BSS Basic Sevice Set, 22 WBSS WAVE BSS, 22 WSA WAVE Sevice Advetisement, 22 WA WAVE Announcement, 22 OBU On Boad Unit, 24 MSDU MAC sevice data unit, 25 LLC Logical Link Contol, 25 WSM WAVE Shot Message, 25 WSMP WSM Potocol, 25 MLME MAC Laye Management Entity, 25 EDCA Enhanced Distibuted Channel Access, 26 UP Use Pioity, 27 xiii

14 ACI Access Categoy Index, 27 AC Access Categoy, 27 AIFS Abitation Inte-Fame Space, 27 CW Contention Window, 27 TXOP Tansmit Oppotunity, 27 UTC Coodinated Univesal Time, 28 CCH Contol Channel, 28 WME WAVE Management Entity, 31 PSID Povide Sevice Identifie, 31 DARPA Defense Advanced Reseach Pojects Agency, 33 WRAN Wieless Regional Aea Netwok, 33 TV Television, 33 P-MP Point-to-Multipoint, 33 BS Base Station, 33 CPE Consume Pemise Equipment, 34 CRAHN Cognitive Radio Ad Hoc Netwok, 34 SNR Signal to Noise Ratio, 38 BER Bit Eo Rate, 38 NC-OFDM Non-Contiguous OFDM, 38 FFT Fast Fouie Tansfom, 39 CW Contention Window, 39 DCF Distibuted Coodination Function, 39 DIFS DCF Inte-Fame Space, 40 RTT Round Tip Time, 42 LAPU Local Acquisition and Pocessing Unit, 43 xiv

15 TO THE MAN WHO HAS CONTRIBUTED THROUGH HIS COMPASSION TO THIS STATE OF SUCCESS. MR. OUSSAMA JABER, YOU WILL ALWAYS BE REMEMBERED. xv

16 CHAPTER 1 INTRODUCTION 1.1 Safety of Tanspotation System As economies ae developing and standads of living ae impoving, the numbe of cas on the oad is inceasing exponentially aound the globe. Evey yea, ca manufactues poduce hundeds of thousands of cas that ae deployed in the intenational maket. This fact is diectly elated to ca accidents that injue o kill thousands of people. In thei annual epot of 2007, the National Highway Taffic Safety Administation (NHTSA) in the United States showed that moe than two million people have been eithe killed o injued in moto vehicle accidents in the United States [1]. The most blatant eason that makes cas the least safe tanspotation system esides in thei coe natue: they shae common oads and ae not contolled by a cental entity as it is the case of tains and aiplanes. Thus, the safety of a given ca s occupants is not only a function of how this ca is contolled, but also highly dependent on the othe cas that ae being diven in its vicinity. One way to impove safety is to alet dives of dangeous situations befoe they ae able to obseve them. The most effective scheme fo addessing this issue is to implement distibutive safety potocols fo inhibiting conditions that ae known to incease the likelihood of ca accidents. The apid advances in wieless technologies povide oppotunities to implement advanced distibuted vehicle safety applications. In paticula, the new Dedicated Shot- Range Communication (DSRC) offes the potential to effectively suppot vehicle-tovehicle and vehicle-to-oadside safety communications. DSRC enables a new class of communication applications that will enhance the oveall safety and efficiency of the 1

17 tanspotation system while offeing new infotainment applications. Vehicles ae expected to be able to communicate to one anothe as well as to thei envionment. Also, ca computing sevices have taken a majo leap into the futue with the pesence of on-boad computes that contol navigation, lighting, beaking, and othe functions. The addition of GPS systems enabled the ca to be self-awae of its location and its speed. The intoduction of these computing capabilities and location awaeness is becoming a basic featue of evey new ca. Reseaches have poposed and investigated Vehicula Ad Hoc Netwok (VANET) as the mean towads standadizing vehicle-to-vehicle and vehicle-toinfastuctue communications. VANETs ae expected to become the most elevant ealization of mobile ad hoc netwoks due to the benefits of vehicula communications and the huge numbe of vehicles. 1.2 Vehicula Ad Hoc Netwok A Vehicula Ad Hoc Netwok is a distibuted netwok that does not ely on a cental administation fo communication among vehicles and between vehicles and fixed oad side equipment known as Road Side Unit (RSU). VANETs have unique chaacteistics in compaison to othe Mobile Ad Hoc Netwoks (MANETs). VANETs ae a conestone of the envisioned Intelligent Tanspotation Systems (ITS). ITS efes to pojects that aim to integate moden communication and infomation technology into existing tanspotation management systems and vehicles in ode to optimize vehicle life, fuel efficiency, safety, and taffic. VANETs will contibute to safe and moe efficient oads by poviding timely infomation to dives 2

18 and concened authoities, though vehicle-to-vehicle and vehicle-to-infastuctue communication VANETs Chaacteistics Vehicula Ad hoc Netwoks have gained much inteest in the last few yeas, as being a special case of the Mobile Ad hoc Netwoks. VANETs chaacteistics, which ae summaized into the following fou categoies, make VANETs moe distinguishing yet moe challenging than MANETs [2]: The vaying topology of VANETs epesents a unique challenge caused by the high mobility of the cas, which can each speeds up to 200 km/h. This causes the communication links to be shot lived as they may not exceed one minute. The situation is wosened when consideing multi-lanes situations. Moeove, inceasing the tansmission powe will limit fequency eusability and thus decease the thoughput of the netwok. Nevetheless, since cas move on oads, thei motion is constained and can be pedicted. VANETs will suffe fom fagmentation due to high mobility and opeation in spase aeas. This will cause some pats of the netwok to be uneachable. The effective diamete of the netwok is small due to poo connectivity. This geatly affects outing in VANETs. The two appoaches that ae cuently studied and employed in othe netwoks do not wok hee. Poactive appoaches fail since it is unealistic to hold the topology of the entie netwok as it changes quickly, and updates will consume a high amount of bandwidth. Reactive appoaches ae not appopiate since they equie path discoveies; these paths can become stale even befoe data 3

19 tansmission. Howeve, outing won t play that ole in VANETs since boadcast tansmission constitute a key pat of a VANET usage [3]. Pivacy and secuity challenges in VANET ae unique as it is the only type of netwoks that have to deal with eal life situations. Any malicious attempts could seveely endange taffic flow o even endange human lives. As a esult of VANETs chaacteistics, the intoduction and implementation of sevices in VANETs have been limited by two main factos. Fist, the high mobility of the cas with espect to a fixed sevice povide makes all existing potocols inconvenient to be used. Second, ca manufactues that ae woking on poviding VANETs sevices lack a common gound between them WAVE Stack Consideing the aguments pesented in the pevious section, the US Depatment of Tanspotation has asked fo the standadization of Intelligent Tanspot Systems. In esponse to this equest, the IEEE has eleased the Wieless Access in Vehicula Envionments (WAVE) Family of Standads between 2006 and This family addesses the above poblems by poviding a complete and coheent gound fo all automobile manufactues to wok on, and mechanisms to cope with the poblems incued by ca mobility. These standads define the achitectue to enable two modes of communications, vehicle-to-vehicle, and vehicle-to-infastuctue. By such, they enable the povision of seveal sevices in VANETs of which the most impotant ae safety elated ones, nevetheless othe non-safety sevices ae highlighted and suppoted. This family of standads can be seen as a new netwok stack simila to the taditional intenet stack as shown in Figue 1.1. The IEEE p Wieless Access in Vehicula Envionments (WAVE) standad is stictly limited to the lowe Media Access 4

20 Contol (MAC) and Physical (PHY) layes [4], while the oveall DSRC communication stack between the link laye and the applications is standadized by the IEEE 1609 woking goup. In paticula, the IEEE WAVE Netwoking Sevices standad coves the WAVE connection setup and management [5], wheeas the IEEE WAVE Multi-Channel Opeations standad sits ight on top of the IEEE p and enables opeation of uppe layes acoss multiple channels, without equiing knowledge of PHY paametes [6]. The IEEE and standads ae of less elevance to ou wok. The IEEE WAVE Resouce Manage standad specifies application laye issues of the potocol stack, as it defines the sevices and intefaces of the WAVE Resouce Manage application. Finally, the IEEE WAVE Secuity Sevices fo Applications and Management standad defines secue message fomats and pocessing. It is a laye independent standad that povides secuity sevices to othe layes. Fig DSRC standads and communication stack [7]. 1.3 Cognitive Netwoks Cuently, netwoks ae sometimes not capable of efficiently handling some of the exchanged data. This often esults in sub-optimal pefomance. By allowing 5

21 netwoks to obseve, act, and lean, cognitive netwoks attempt to povide an optimized pefomance. Fig Cognitive Cycle [10]. The tem Cognitive Netwok (CN) was coined by Clak et al. in [8]. Clak poposed a netwok that can assemble itself given high level instuctions, eassemble itself as equiements change, automatically discove a malfunction, and automatically fix a detected poblem o explain why it cannot do so. Moeove, Thomas et al. in [9] descibed cognitive netwoks as "a netwok with a cognitive pocess that can peceive cuent netwok conditions, and then plan, decide and act on those conditions. The netwok can lean fom these adaptations and use them to make futue decisions, all while taking into account end-to-end goals". CN is an emeging eseach field. The idea of cognition was fist intoduced by Mitola who suggested cognitive adio in [10]. His suggestion of embedding intelligence into adio opeation caught the imagination and attention of the eseach community. 6

22 The concept of cognition has evolved fom adios into the lage netwok. Cognitive netwoks diffe fom cognitive adios by the fact that the action space of the fome extends beyond the MAC and PHY layes and the netwok may consist of moe than just wieless devices. Mahonen discusses in [11] how cognitive adios may be just a logical subset of cognitive netwoks. The adaptive chaacteistic of the cognitive netwoks efes pimaily to its ability in employing gatheed infomation in ode to modify its chaacteistics. The cognition cycle as shown in Figue 1.2 includes an obsevation stage that meges the pevious expeience with the cuent sensed state, and a decision state that acts to modify the netwok chaacteistics accodingly. Cognitive netwoks take obsevations of netwok pefomance as input to a decision making pocess and then povide output as a set of actions that can be implemented in the modifiable elements of the netwoks. An ideal CN should be fowad-looking, athe than eactive, and attempt to adjust to poblems befoe they occu. A cognitive netwok is expected to delive, ove an extended peiod of time, bette end-to-end pefomance than a non-cognitive netwok. The intention is to use cognition to impove end-to-end objectives such as esouce management, Quality of Sevice (QoS), secuity, access contol, o thoughput. 7

23 Fig Spectum occupancy in each band in Dublin, Ieland [12]. The use of cognitive netwoks to addess spectum scacity is mainly motivated by the fact that the spectum usage of licensed bands is vey inefficient. In [12], it is mentioned that ou occupancy studies demonstate that less than a thid of the allocated adio spectum is being used at any given time. Figue 1.3 shows spectum occupancy in Dublin, Ieland whee aveage spectum use is found to be 13.6%. Moeove, a summay of spectum occupancy in each band in New Yok City and Chicago is shown in Table 1.1. Aveage spectum use tuns to be equal to 13.1% in New Yok City and 17.4% in Chicago. The uses of those bands eseve the sevice but do not use it most of the time. Cognitive netwok pinciples can be applied to incease the spectum utilization by dynamically modifying the opeational band. Theefoe, it enables the 8

24 coexistence of spectum between pimay and seconday uses in licensed and unlicensed bands. In such a case, pimay uses ae the licensed ones who eseved the spectum, while the seconday uses ae those who ae using the same spectum on a non intefeing basis. In this egad, it helps to note that the U.S. Fedeal Communication Commission (FCC) has ecommended adopting CNs as a method fo additional spectum access [13]. 1.4 Motivation In the yea of 1999, the FCC allocated 75MHz of Dedicated Shot Range Communications spectum at the 5.9 GHz band to be used exclusively fo vehicle-tovehicle and infastuctue-to-vehicle communications. The DSRC spectum is divided into 7 channels with a 10 MHz bandwidth allocated to each one. Six out of these channels ae sevice channels (SCH) while the cente one is the contol channel. A detailed desciption of the channel assignment in DSRC is povided in Chapte 2. 9

25 Table 1.1. Summay of spectum occupancy in each band in New Yok City and Chicago [12]. 10

26 In the DSRC band, all common safety messages ae supposed to be exchanged in the contol channel. Non-safety usage in the contol channel is limited to occasional advetisements of pivate applications that utilize a sevice channel, and is insignificant to the oveall channel load [14]. Simulations in the liteatue have indicated that the contol channel can suffe fom lage contention, implying that the 10 MHz channel allocated fo safety usage may not be enough [15] [16]. To assess the pefomance of safety applications ove the existing p potocol, and specifically, the 10 MHz contol channel, two paametes ae usually used: safety message delay and packet eception ate. Safety message delay is a cucial facto, since the dive eaction time to taffic waning signals can be in the ode of 700 milliseconds o longe [51]. Thus the safety message delay should be less than 200 milliseconds [17]. Othewise, the whole system will not be able to impove oad safety, since the dive will have no time to eact to emegency situations. Safety message delay is defined as the aveage delay a packet expeiences between the time at which the packet is geneated and the time it is successfully eceived at the eceive. This delay is the accumulation of many delays that ae the queuing delay in the highe MAC laye, the contention delay due to ca contention with othe cas fo channel access, the tansmission delay, and the popagation delay. The last two delays ae negligible and ae not affected with intefeence situations. Accodingly, we focus on the fist two delays and combine them into a single delay that we efe to in this epot as the Access Delay. The second and the most impotant paamete is the packet eception ate. A safety application must have a high eliability in deliveing safety packets especially 11

27 that boadcast scenaios constitute a key pat of a VANET s usage [3]. Theefoe, the sende can't expect acknowledgments, and thus can't tell if a packet was successfully eceived. As a esult, a deployed system with high packet eception ate can povide eliability and insue that all packets sent ae eceived without equiing feedback fom the eceive to the sende. Accoding to [18], the pobability of message delivey failue in a vehicula netwok should be less than Packet eception ate is defined as the atio of the numbe of packets successfully eceived to the total numbe of packets tansmitted. These two paametes wee analyzed widely in the liteatue, whee a consensus appeas to exist that the safety message delay in a vehicula netwok does not exceed equied value. The delay was found to be aound 1.2 ms in [19]. On the othe hand, the situation is not that good consideing the packet eception ate. Values deived fom liteatue show that the packet eception ate fell well below the expected value of In [3] and [19], the pobability of eception was found to be less than 0.6 at zeo distance unde the Nakagami adio popagation model. Given these findings, it is infeed that a deployed system with the existing 10 MHz contol channel cannot povide pefomance guaantees unde ealistic oad conditions, and theefoe ende unacceptable system pefomance. 1.5 Contibution The contibution of this epot is a system aimed to mitigate the spectum scacity issue in the DSRC contol channel by applying cognitive netwoks pinciples. The system is able to quantify netwok contention at the contol channel though a contention metic. Ou system is chaacteized by the following popeties. Fist, ou 12

28 system is dynamic as opposed to static appoaches in aiming at inceasing the spectum allocated to VANETs in the standad. A vehicle is allocated additional spectum on speculated need. The system elies on a leaning algoithm to identify fee pats of the spectum and allocate them to vehicles based on a linea model that aggegates data gatheed by cas to infe contention conditions in egions of the oad. Second, ou poposed system is totally distibuted and decentalized, and thus it avoids a single point of failue. Thid, the employed infastuctue is only equied to possess modest pocessing powe, which makes the deployment of the system pactical and moe feasible. Finally, the pocessing is local and the pocessing units need not to be inteconnected diectly, thus potecting the pivacy of dives behavios since no diving pattens ae being collected and no global pofiles of cas and dives ae developed. One vey impotant aspect in ou appoach is the implementation of a cognitive netwok model. Ou system obseves the netwok conditions though multiple souces (cas) that measue the contention paametes and sense the TV spectum. The system leans about contention egions and white spectum by aggegating data elayed by the cas. The decision making phase incopoates extending the DSRC contol channel to the sensed white spectum at contention egions enabling cas to communicate safety infomation eliably. The emaining of the epot is oganized as follows. In chapte 2, we povide a liteatue eview about the VANETs and DSRC and discuss some elated woks pefomed to solve the poblem of spectum scacity in VANETs. In chapte 3, we intoduce ou new system, its diffeent components and how the cognitive cycle takes place. We pesent in chapte 4 some simulations esults that veify the opeations of the system. We discuss in chapte 5 the scalability of the system and look fo any possible pefomance 13

29 bottleneck. Finally we conclude in chapte 6 and popose futue items of wok to enhance the poposed system. 14

30 CHAPTER 2 RELATED WORK 2.1 Physical Laye Amendment The physical laye of the netwok potocol stack is esponsible fo tansmitting aw bits ove a communication channel. The IEEE p standad scope coves both the PHY and MAC layes. We will stat in this section by intoducing the physical laye of DSRC DRSC Spectum Allocation As we mentioned ealie, The FCC allocated 75 MHz of the adio spectum fo DSRC. The 5.9 GHz DSRC spectum is composed of six sevice channels that ae 10 MHz each and one 10 MHz contol channel (see Table 2.1). Table 2.1. Channels assignment in DSRC GHz CH. 175 CH GHz Reseved CH. 172 CH. 174 CH. 176 CH. 178 CH. 180 CH. 182 CH. 184 Sevice Sevice Sevice Contol Sevice Sevice Sevice 5 MHz 10 MHz 10 MHz 10 MHz 10 MHz 10 MHz 10 MHz 10 MHz The modulation scheme adopted fo a single channel is Othogonal Fequency- Division Multiplexing (OFDM), whee each channel is divided into 64 sub-channels (48 ae data + 4 unifomly distibuted pilot sub-channel + 12 guad sub-channels) with a vaiety of modulation and coding ates. The sub-channels ae ovelapped so that the inte-caie spacing is MHz and it is equal to the bandwidth of a single sub- 15

31 channel. Actually, this bandwidth is half than that used in a and it is appopiate to combat fequency selective fading and Dopple shifts. Given this, Bit Rates can vay between 3, 4.5, 6, 9, 12, 18, 24, and 27 Mbps. The typical communication distance in WAVE system is fom 300m to 1000m. Futhemoe, the educed channel bandwidth 10MHz is used to incease the toleance fo multi-path popagation effects of signal in vehicula envionment. All paametes in time domain fo IEEE p is doubled when compaed to the IEEE a (see Table 2.2). This leads to a eduction of the effect of Dopple Spead by having naowe bandwidth. Also doubling the guad band inteval will yield to a eduction in the Inte-Symbol-Intefeence (ISI) due to multi-path popagation [20]. The following list contains the channels of DSRC and the type of applications that ae suppoted by each channel (see Figue 2.1) [21]: Channel 172 is eseved fo medium powe safety applications. Channel 174 is eseved fo medium powe applications. Channel 175 is a combination of channels 174 and 176. Channel 176 is eseved fo medium powe applications. Fig Fequency channel layout of 5.9 GHz WAVE system. 16

32 Channel 178 is the contol channel. It suppots all powe levels, safety application boadcasts, sevice announcements, and vehicle-to-vehicle boadcasts messages. Channel 180 is eseved fo low powe configuations and povides little intefeence when units ae sepaated by 15 mete o moe. Channel 181 is a combination of channels 180 and 182. Channel 182 is eseved fo low powe configuations and povides little intefeence when units ae sepaated by 15 mete o moe. Channel 184 is eseved fo a high powe sevice channel that is used to coodinate intesection applications. Table 2.2. Key paametes of DSRC/IEEE p PHY and IEEE a PHY. 17

33 2.1.2 IEEE p Impovements MAC level amendments ae softwae updates that ae elatively easy to make, but on the othe hand PHY level amendments should be limited in ode to avoid designing an entiely new wieless ai-link technology. Thus, the philosophy of IEEE p design is to make the minimum necessay changes to IEEE PHY. Accodingly, thee changes ae made. Fist, IEEE a aleady defines 10 MHz wide channels, and it is easy to be implemented since it only involves doubling of all OFDM timing paametes used in the egula 20 MHz a tansmissions, and thus, channels used in p ae 10 MHz wide channels as descibed in the pevious section. Also, the IEEE p intoduces some impoved eceive pefomance equiements in adjacent channel ejections. Finally, the spectum masks equied by the IEEE p ae moe stingent than the ones demanded of the cuent IEEE adios. 2.2 MAC Laye Amendment The IEEE MAC defines a pope aangement that allows fo a set of adios to establish and maintain a coopeating goup. Within the goup, adios can feely communicate among themselves but all tansmissions coming fom outside the goup ae not toleated and filteed out. This type of goup is called a Basic Sevice Set (BSS), and thee ae ongoing eseach effots to povide secue and obust communications within a BSS. The IEEE p amendment at the MAC laye aims at enabling a vey efficient communication goup setup without incuing significant ovehead as it is the case in the cuent IEEE MAC. We will stat by intoducing BSS mechanism in the IEEE standad. Afte this, we will pinpoint the enhancement intoduced by the IEEE p amendment. 18

34 2.2.1 Oveview of BSS mechanism in IEEE The Basic Sevice Set is a tem used to descibe the collection of stations which may communicate togethe ove the ai-link. The BSS may o may not include Access Point (AP). A BSS including an AP is called Infastuctue Basic Sevice Set (IBSS). All communications taking place within an IBSS ae sent fist to an AP which in tun fowads it to the destination station. This mechanism allows a station to filte out the tansmissions fom othe unelated stations neaby. Afte eceiving the AP beacon, the station will join a BSS by setting its local hopping time and channel sequence accoding to the infomation contained in the AP beacon in ode to maintain synchonization with the AP. The emaining steps constitute the authentication phase and association phase. The Sevice Set Identifie (SSID) acts as a passwod when a mobile device ties to connect to the BSS. SSID o netwok name is a name that identifies a paticula Wieless Local Aea Netwok (WLAN). The SSID can be up to 32 chaactes long. A BSS not including an AP is called an Independent Basic Sevice Set (Independent BSS) o Ad Hoc netwok. Independent BSS is the simplest of all IEEE netwoks since no netwok infastuctue is equied Basic Sevice Set Identification Evey BSS has a unique identification shaed by all membes called the Basic Sevice Set Identification (BSSID), which is the MAC addess of the AP sevicing the BSS. The BSSID should not be confused with the SSID. BSSID is the name of a BSS known to the stations at the MAC level and is a 48-bit long field. The majo advantage of the BSSID is filteing. Seveal distinct IEEE netwoks may ovelap physically, but thee is no eason fo one netwok to eceive link-laye boadcasts fom a 19

35 physically ovelapping netwok. Fig Fame Contol Field in IEEE An IEEE fame may contain up to 4 addesses depending on the ToDS and FomDS bits defined in the Contol Field (see Figue 2.2). Addess 1 always indicates the immediate ecipient addess which can be the Destination Addess (DA), BSSID o the Recipient Addess (RA). If ToDS is set, Addess 1 contains the AP addess. If ToDS is not set, Addess 1 is the addess of the end-station. Addess 2 is always the physical tansmitte addess. If FomDS is set, this is the addess of the AP. If FomDS is not set then it is the addess of the station. Moving on to the Addess 3 field, you will note fom Table 2.3 that it also depends upon the ToDS and FomDS bit settings. When the FomDS bit is set to a value of 1, the Addess 3 field contains the Souce Addess (SA). If the fame has the ToDS bit set, then the Addess 3 field contains the Destination Addess (DA). The fouth and last addess field, which is Addess 4, is used fo the special situation whee a wieless distibution system is employed and a fame is being tansmitted fom one access point to anothe. In this situation both the ToDS and FomDS bits ae set. Thus, neithe the oiginal destination no the oiginal souce addess is applicable and Addess 4 is then limited to identifying the souce of the wieless fame. Table 2.3 summaizes the usage of the diffeent Addesses accoding to the ToDS and FomDS bits setting. 20

36 Table 2.3: Usage of diffeent addesses accoding to the ToDS and FomDS. ToDS FomDs Addess 1 Addess 2 Addess 3 Addess DA SA BSSID N/A 0 1 DA BSSID SA N/A 1 0 BSSID SA DA N/A 1 1 RA TA DA SA Legend: TA = Tansmitte Addess RA = Receive Addess BSSID = Basic Sevice Set All stations filte on the Addess 1 field as it always indicates the ecipient addess. If the Addess 1 field contains a goup addess (e.g., a boadcast addess), the BSSID is compaed to ensue that the boadcast o multicast oiginated fom a station in the same BSS. A special case of the BSSID is the wildcad BSSID, which is composed of all 1s. A wildcad BSSID shall not be used in the BSSID field expect fo management fames of subtype pobe equest IEEE p WAVE Mode Vehicula safety communications equie an instantaneous data exchange capabilities and cannot affod scanning channels fo the beacon of a BSS and executing multiple handshakes to establish the communications. The IEEE MAC opeations descibed in the pevious section ae too much time-consuming to be applied in IEEE p. Thus, it is necessay fo all IEEE p adios to be in the same channel and configued with the same Basic Sevice Set Identification (BSSID) to enable immediate safety communications. A key amendment intoduced by the IEEE p WAVE is the tem WAVE mode. In the WAVE mode, a station is allowed to tansmit and eceive data fames with the wildcad BSSID value and without the need to pioi belong to a Basic Sevice Set (BSS). Based on this appoach, two vehicles can 21

37 immediately communicate with each othe upon encounte without any additional ovehead as long as they opeate in the same channel using the wildcad BSSID WAVE BSS Even fo non-safety communications and sevices, the ovehead of taditional BSS setup may be too expensive. Based on this, IEEE p intoduces a new BSS type called WAVE BSS (WBSS). WBSS is fomed by fist tansmitting an on demand beacon which is used fo advetising a WAVE BSS. Such advetisement contains the needed infomation fo eceive stations uppe laye mechanisms above the IEEE to undestand the sevices offeed in the WBSS in ode to decide whethe to join, as well as the infomation needed to configue itself into a membe of the WBSS. In othe wods, a station can decide to join and complete the joining pocess of a WBSS by only eceiving a WAVE advetisement with no futhe inteactions. The delay in joining a WAVE BSS is thus educed compaed to an infastuctue BSS because MAC level authentication and association do not apply to a WAVE BSS. It is the duty of a sevice povide to peiodically boadcast the infomation of sevices that can be accessed by potential uses in ange. The sevice infomation, e.g. sevice pofile, channel numbe and outing infomation, ae composed into the WAVE Sevice Advetisement (WSA) infomation element, which is caied by the WAVE Announcement (WA) fame at p MAC laye and peiodically boadcasted on CCH. When a use eceives a WSA fame fom a sevice povide and the sevice matches the use s equiement, the sevice use will locally decide to join in the WBSS. In the next SCH inteval, the use switches to the SCH channel as specified in the WSA 22

38 to pefom the sevice. In case the sevice uses IP as the netwok potocol, in contay to pue boadcast sevices, an additional handshake is equied between the sevice povide and the sevice use to establish the data link MAC Amendment Summay We summaize the changes intoduced at the MAC fo WAVE opeations as follows: A station in WAVE mode can send and eceive packets with the wildcad BSSID, egadless of whethe it is a membe of a WAVE BSS o not. A set of coopeating stations in WAVE mode that communicate using a common BSSID is called WAVE BSS. WBSS can be initialized by having a adio in WAVE mode sends a WSA that includes all equied infomation fo a eceive to join. A adio is consideed to belong to a specific WBSS when it is configued to send and eceive data fames with the BSSID of that WBSS. It leaves a WBSS when its MAC stops sending and eceiving fames using the BSSID of that WBSS. A station cannot be a membe of moe than one WBSS at the same time. A station in WAVE mode should not join an infastuctue. Also, it should neithe use active o passive scanning, no MAC authentication o association pocedues. A WBSS ceases to exist when it has no membes. The initiating adio is no diffeent fom any othe membe afte the establishment of a WBSS. Theefoe, a WBSS can continue if the initiating adio ceases to be a membe. 23

39 2.3 The IEEE Standad The WAVE achitectue defines two types of nodes, the Road Side Unit (RSU) and the On Boad Unit (OBU). The OBUs ae assumed to eside on cas o any othe mobile nodes in the system. The RSU is a fixed device placed in stategic locations (oad cossings fo example) and it is used to povide fixed location sevices and updates. The standad constitutes an uppe MAC laye in the WAVE stack and povides an inteface with the physical and lowe MAC laye defined in IEEE p. This standad descibes multi-channel opeation in the WAVE system, as the WAVE system is composed of one contol channel and seveal sevice channels. All contol infomation and citical safety infomation ae assumed to be tansfeed on the contol channel. When DSRC is initially deployed, it is envisioned that WAVE devices have only a single adio fo accessing o sensing single channel and thus they can monito a single channel at a time. Consequently, RSUs and OBUs ae equied to access the contol channel at the contol channel intevals. This access has the highest pioity and a device must suspend all it communications on the sevice channels to listen to the contol channel. This makes channel coodination the most citical in the system s opeations. The achitectue with channel coodination specifies the following tansmit opeations sevices: channel outing, use pioity and channel selecto. These sevices ae explained in details in the following sub-sections. 24

40 2.3.1 Channel Routing When a MAC sevice data unit (MSDU) is passed fom the Logical Link Contol laye (LLC) to the MAC, the MAC shall examine the EtheType field of the IEEE heade to detemine whethe the MSDU is a WAVE Shot Message (WSM) o IP datagam. The Ethenet type field defined in RFC 1042 is used to identify the highe laye potocol above the LLC. The fomat of MSDUs passed fom the LLC to the MAC is illustated in Figue 2.3. Fig Illustation of MSDUs passed between the LLC and the MAC. If the EtheType field indicates WSM Potocol (WSMP), the WSMP heade and WSMP data follow. The WSMP datagam is fowaded to the appopiate queue by the channel oute accoding to the channel identified in the WSMP heade and to the packet pioity. A packet is discaded without tansmitting any eo to the sending application if an invalid channel numbe is found in the WSMP datagam. IP datagam tansmission is a little bit diffeent. Initially, the IP application egistes the tansmitte pofile with the MAC Laye Management Entity (MLME). It then initializes IP data exchanges. The tansmitte pofile contains the SCH numbe, powe level, data ate and the adaptable status of powe level. A channel oute outes an IP datagam passed to it fom LLC to a data buffe that coesponds to the cuent egisteed SCH. A WAVE device can have only one active tansmitte pofile at any given time. An IP packet is dopped without geneating any eo message if a no valid SCH is specified in the tansmitte pofile. It is woth pointing out that IP datagams ae 25

41 not allowed in the contol channel since IP heade length is 40 bytes while contol channel allows messages less than 40 bytes to ease access to this busy channel. Fig Pioitized access fo data tansmission on one channel [6] Use Pioity The WAVE system defines 8 levels of pioity fo non-safety applications along with safety applications. Pioity is taken into consideation while pefoming MAC contention by using mechanisms of the Enhanced Distibuted Channel Access (EDCA) of e. Upon the aival of a datagam to the channel oute, it fowads the datagam to the appopiate channel and data queue (see Figue 2.4). The appopiate pioity queue 26

42 is selected by mapping the Use Pioity (UP as defined in tansmitte pofile) to an Access Categoy Index (ACI). The UP-to-AC mapping is defined in Table 2.4. Each Access Categoy (AC) has an independent channel access function. The diffeentiation in pioity between AC fo channel access paametes is implemented using the appopiate EDCA paamete set values. The EDCA paametes ae Abitation Inte- Fame Space (AIFS), Contention Window (CW) and Tansmit Oppotunity (TXOP) limit. The intenal contention algoithm accoding to IEEE standad calculates the back-off independently fo each AC based on access paametes. The AC with the smallest back-off wins the intenal contention, and the winning AC then contends extenally fo the wieless medium. The channel selecto schedules data fo extenal contention by de-queuing pioity queues based on thei ACI. The channel selecto also configues and confims the media use of desied channel infomation. Table 2.4. UP-to-AC mappings. 27

43 2.3.3 Channel Selecto o Medium Contention The channel selecto is in chage of the following impotant tasks: Deciding when to monito a specific channel. Specifying the set of legal channels at a paticula point in time. Indicating how long the WAVE device should monito and utilize a specific channel. Dopping data if it is supposed to be tansmitted ove an invalid channel, fo example, the channel no longe exists Channel Coodination Channel coodination is the most citical in the system s opeations. It is mainly elated with coodinating channel intevals, to assue that highe laye packets ae outed to the pope channel. These channel intevals ae unique time intevals fo the sevice and contol channel with espect to an extenal time efeence, called the Coodinated Univesal Time (UTC). In the potocol, it is specified that all WAVE devices should be able to monito the Contol Channel (CCH) duing a common time inteval called the CCH inteval. Single-channel WAVE devices ae not capable of simultaneously monitoing the CCH and exchange data on the SCH and thus synchonization is needed. Synchonization can insue that all devices switch togethe to the contol channel at the same time inteval. This guaantees that all devices can listen to safety infomation and othe high pioity infomation. Duing the contol channel inteval all WSMs with high pioity ae to be tansmitted. WSAs and less pioity messages can be tansmitted on the CCH o the SCH intevals. As descibed in the standad, synchonization is 28

44 pefomed wheneve a device joins a WBSS. The standad is discussed in the following section. The synchonization pocedue is shown in Figue 2.5. Fig 2.5. Sync inteval, guad inteval, CCH inteval, and SCH inteval. Figue 2.5 shows how the CCH and SCH intevals ae divided and altenated. All devices ae equied to monito the CCH at CCH intevals as impotant safety infomation may be tansmitted. The Sync inteval is simply the sum of the CCH inteval and SCH inteval. Guad intevals ae intoduced to minimize the effect of timing inaccuacies. Sync Inteval is 100 ms length and default values fo contol and sevice channel intevals ae 50 ms [6]. At each channel inteval, pevious MAC activities ae suspended and the cuent ones ae stated o esumed, this ensues that each packet is tansmitted on the coect RF channel. Fo instance, a RSU may povide music tansfe sevice. The OBU of a vehicle inteested in this specific sevice switches to a sevice channel to begin eceiving the audio file. If the tansfe takes too long to complete, the vehicle must switch to the contol channel to eceive safety messages and then switch back to the sevice channel to esume the file tansfe. Thus, using the contol channel coodination, a vehicle can peiodically monito the contol channel fo safety messages while it continues to use available sevices in the netwok. 29

45 2.4 The IEEE Standad The IEEE constitutes the Netwoking Sevices laye in the WAVE system as descibed befoe. It coesponds to the tanspot and netwok layes in the egula TCP/IP stack and it defines the netwok sevices in VANETs. As shown in Figue 2.6, this standad incopoates two planes: the Management Plane and the Data Plane. The following sections will descibe selected aeas of this standad that ae elevant to ou wok. Fig. 2.6: WAVE potocol stack and scope of While the Data Plane suppots the use of UDP-TCP/IP stack especially fo wied communication between the connected RSUs o between them and a seve, the WSMP is of impotance to the wieless communication and it is optimized fo shot message tansmission ove the WAVE envionment. By default, The WSMP geneates WSMs to be sent only ove the CCH. The MAC addess is employed to specify the sende of the message, while the destination is initially the MAC boadcast addess. Since evey OBU and RSU checks the CCH egulaly, evey node will be able to eceive such a message. 30

46 In ode to make use of the othe available channels effectively, WSMs could be geneated ove a sevice channel when a WBSS is unning ove it. In the following sections, we define how a WBSS is maintained and teminated though the Management Plane WBSS Establishment When an application unning on a RSU o OBU needs to establish a connection with the netwok, it equests the initiation of a WBSS fom the WAVE Management Entity (WME). It specifies a set of paametes late used when the WBSS geneates a WSA ove the CCH enabling evey WAVE node in the tansmission ange of the souce node to hea it. One of these paametes is the pesistence that detemines if the WSA esponsible fo advetising the sevice will be epeated in each contol channel inteval o it will be just pomoted once at the time of the ceation of the WBSS. Fo the fomat of the WSA displayed (see Figue 2.7), we can pick some majo fields: Povide Sevice Identifie (PSID): It is a fou-byte numeic field used to identify a paticula application entity. PSID is used in announcement messages fom povide to identify that this application s sevice is povided by this device. Theefoe, the PSID must uniquely identify a paticula application. The devices eceiving the announcement can immediately detemine if they should espond o not. Using PSID, the intended application is identified without fist equiing a bi-diectional data exchange to establish a link. PSID ole is vey simila to the ole of the pot numbe in the egula intenet stack. 31

47 Channel Numbe: It specifies on which SCH the communication with this application will occu. The choice of this channel could be eithe specified by the application o left to the WME which will pick the least congested SCH and un the WBSS ove it. MAC Addess (optional): It is the 48-bit MAC addess of the device hosting the povide application. This is pesent if diffeent fom the MAC addess of the device tansmitting the WSA. Upon heaing the WSA, the WME of the eceiving WAVE node will check if the PSID contained in the advetisement is of any inteest to any egisteed application unning on the WAVE device, and then it could choose to join the WBSS o simply ignoe it. Afte joining a WBSS, an application is fee to geneate WSMs and IP messages ove the specified SCH in addition to only WSM ove the CCH. 32

48 Fig WAVE Sevice Advetisement fomat [5] WBSS Temination WAVE identifies a vey simple and efficient way of WBSS temination. Thee is no need of communication between devices to stop a WBSS. In fact, evey node locally teminates the WBSS (in othe wods disjoins the WBSS) fo one of the following easons: 1. All applications unning on this WBSS device indicated that they do not need any additional sevice fom this WBSS and that they teminated all thei activities on it. 2. The WBSS device needs to join a conflicting WBSS (unning on a diffeent channel) to satisfy the need of a highe pioity application. 33

49 3. The WBSS device senses that SCH is idle fo a cetain time meaning that no activity is being done ove this channel and though this WBSS. 2.5 Cognitive Systems Seveal cognitive systems wee poposed in the liteatue. The Defense Advanced Reseach Pojects Agency (DARPA) has launched the XG pogam, which aims to develop the technology fo dynamically using unoccupied spectum without intefeence with othe spectum uses. XG pomises achievement of 10 times incease in spectum access and is consideed to be the key to dynamic topology, bandwidth, and esouce management in futue wieless netwoks [22]. Anothe cognitive based system is cuently being developed by the IEEE woking goup, which is chateed with the development of a cognitive based Wieless Regional Aea Netwok (WRAN), Physical and MAC layes fo use by license-exempt devices in the spectum that is cuently allocated to the Television (TV) sevice [23]. The system is composed of a fixed point-to-multipoint (P-MP) wieless ai inteface in which a Base Station (BS) manages its own cell and all associated Consume Pemise Equipments (CPEs). The BS contols the medium access in its cell and tansmits in the downsteam diection to the vaious CPEs, which espond back to the BS in the upsteam diection. In addition to its taditional ole, a BS also manages a unique featue of distibuted sensing. This is needed to ensue pope incumbent potection and is managed by the BS, which instucts the vaious CPEs to pefom distibuted measuement of diffeent TV channels. Based on the feedback, the BS decides which steps, if any, ae to be taken, i.e. allowing tansmissions on channels o stopping it. Moeove, a cognitive adio fo shotange in office wieless communication is poposed in [24]. In this system, the 34

50 tansceive senses the fequency occupancy conditions using two kinds of spectum sensing and finds the available fequencies. The poposed system uses a wide fequency band fom 3-12 GHz and specifies a maximum communication distance of 30 metes. Recently, Cognitive Radio Ad Hoc Netwoks (CRAHNs) have been poposed as a fom of distibuted cognitive netwoks. A CRAHN diffes fom classical ad hoc netwoks in that it deals with a changing spectum envionment and it should potect the tansmission of the pimay uses of the spectum. In ode to adapt to dynamic spectum envionment, the CRAHN incopoates the following functions in the classical layeing potocol: spectum sensing, spectum decision, spectum shaing, and spectum mobility [26]. Spectum sensing is used to identify spectum holes and then employs a spectum decision function to decide on the appopiate band to use. Spectum shaing povides the capabilities to coodinate between the available nodes, and finally spectum mobility is used in case of a handove when a pimay use is detected and when the cognitive uses evacuate the spectum. Refeing to the spectum sensing poblem, it is consideed one of the most challenging issues in cognitive netwok systems. Spectum sensing technology is most vital to the implementation of a CN system using dynamic spectum esouce management [27]. The enegy detecto-based sensing appoach, also known as adiomety o peiodogam, is the most common way of spectum sensing because of its low computational and implementation complexities. The signal is detected by compaing the output of the enegy detecto with a theshold which depends on the noise floo [28] [29]. Anothe spectum sensing appoach is based on wavefom detection. This method is only valid in wieless envionments with known signals pattens whee the eceived signal is coelated with a copy of itself. Othe sensing 35

51 methods that ae found in the liteatue include the cyclostationaity and identificationbased sensing techniques [30]. In the fome, featues of the eceived signals ae used to detect pimay uses, whee such featues ae caused by the peiodicity in the signal and can be detected using an autocoelation function. In the latte, seveal chaacteistics (enegy, channel bandwidth and othes) ae extacted fom the eceived signal and used to identify the technology used by the pimay use. The identification of the technology is supposed to povide a complete knowledge of the spectum chaacteistics. The last spectum sensing technique we will descibe hee is matched-filteing. Matchedfilteing is known as the optimum method fo detection of pimay uses. The main advantage of matched filteing is the shot time it needs [25]. Howeve, it equies pefect knowledge of the pimay uses signaling featues such as bandwidth, opeating fequency, modulation type and ode, pulse shaping, and fame fomat. Thus, the implementation complexity of the sensing unit is impactically lage [29]. The above-descibed techniques basically diffe in how they tade off accuacy fo complexity, with enegy-based detection being the least complex and least accuate technique, and match-filteing being the most complex and most accuate technique [30]. 36

52 Fig Main sensing methods in tems of thei sensing accuacies and complexities [30]. 2.6 Poposed Solutions in the Liteatue Some eseaches suggested that non-safety usage of DSRC ought to be seveely esticted duing peak hous of taffic to insue that automotive safety is not compomised [15]. Howeve, such solutions fo spectum scacity could impact the commecial side of DSRC and may not be acceptable. Altenatively, while acknowledging the poblem of eliable safety communications ove the contol channel, eseaches have poposed enhancements to the existing safety applications. One of the most impotant poposed solutions is epetition [31][19][32]. In this appoach, the sende vehicle epeats the tansmission of the safety message seveal times to incease the eliability of safety communications. It is shown in [19] that such scheme would in fact incease the pobability of eception ate to above 99%, solving the low eception ate poblem, and consequently making safety communications eliable ove the contol channel. Howeve, in thei analysis the authos of [19] assumed a bandwidth of 20 MHz, and a low message size of 200 bytes. This scheme scales badly to the incease of safety message size due to secuity ovehead. It is stated in [33] that with the addition of 37

53 secuity featues, message size would each 800 bytes. Thus, epetition might incease the pobability of eception to acceptable values, but will incu taffic on the contol channel. As a matte of fact, taffic on the contol channel will be multiplied by the numbe of epetitions employed. This will cause significant delays. To assess this hypothesis, we pefomed a simulation that adopts the same topology and paametes of [19], but cas wee distibuted on fou lanes, and a message size of 800 bytes was used. We implemented epetition and esticted it to a maximum of 10 times. This estiction is below some epetition values poposed in [32] that eached 20 and 30 times. In ou simulations the delay eached 1.2 seconds in some cases, and the aveage was above 200ms. This clealy violates the delay estiction, and in that sense epetition schemes will also fail to povide eliable safety communications. Given the above analysis, we popose to apply Cognitive Netwoks pinciples to VANET envionments to alleviate the contention that could take place in the contol channel. 38

54 CHAPTER 3 PROPOSED SYSTEM 3.1 Contol Channel Extension Scheme The application of cognitive netwok in VANETs especially in DSRC is enabled by the physical laye descibed in IEEE p.The modulation scheme adopted fo a single channel is OFDM, whee the contol channel is divided into 64 sub-channels (48 ae data) with a vaiety of modulation and coding ates. The available bit ates can vay between 3, 4.5, 6, 9, 12, 18, 24, and 27 Mbps. To achieve bette thoughput in the netwok it would be desiable to be able to achieve highe bitates. Howeve, this comes at a cost: inceasing the bit ate is done by inceasing the modulation ode and deceasing the coding ate. This will in tun incease the equied signal to noise atio (SNR) and thus incease the bit eo ate (BER) which will wosen the situation [34]. As a matte of fact, the widening of the contol channel spectum can be done by simply extending it to the new additional band as detemined by the cognitive netwok mechanism. The newly added spectum is divided into sub-channels having the same paametes as those defined in p (inte-caie spacing of MHz) while peseving the othogonality equiement. Hence, the numbe of available sub-caies inceases to suit the system s equiements. The sub-caie is the caie fequency of a sub-channel. This allows the bandwidth of the sub-channels to stay constant, and the bit ate to incease but by peseving the modulation ode and the coding ate. This fom of an OFDM, whee the implementation achieves the high data ates via collective usage of a lage numbe of non-contiguous sub-channels, is called non-contiguous OFDM (NC-OFDM) [35]. NC-OFDM povides the necessay agile spectum usage needed 39

55 when potions of the taget licensed spectum ae occupied by both pimay and seconday uses. When they detect a pimay use, the seconday uses deactivate subcaies that can potentially intefee with him. In this egad, we should note that thee ae techniques that have been poposed to implement NC-OFDM tansceives, like the one descibed in [51] which intoduces an algoithm fo quick puning of the fast Fouie tansfom (FFT) that epesents the coe component of an NC-OFDM tansceive. This fact, along with the othe ca communication technologies that ae being developed and mentioned above, makes ou poposed system moe feasible, and in fact moe attactive since it allows fo the ealization of a system that is moe in line with the ecommendations and tends fo solving spectum scacity issues. 3.2 Netwok Contention Metic As a matte of fact, a majo pat of the system opeation is to detemine the aeas along the oad that suffe fom contol channel contention. We define a metic that quantifies contention at each location of the oad. Contention o Netwok Contention in this context is used to descibe data tansmission contention at the contol channel and not vehicula taffic congestion. We say that the contol channel suffes fom data tansmission contention if and only if the needed bitate exceeds the offeed bitate (available bitate). This implies that the amount of infomation tansmitted in a given peiod of time ovewhelm achievable bitates. At the fist sight, the Contention Window (CW) of might seem an appopiate candidate as a contention metic. Howeve, the Distibuted Coodination Function (DCF) technique fo medium access mechanism of IEEE specifies that when a packet aives at the MAC fom the uppe laye, the status of the channel should 40

56 be checked. If the channel is sensed idle fo a peiod equal to DCF Intefame Space (DIFS), the mobile station andomly selects a backoff time (time slot) within a backoff window. The backoff time is deived fom a unifom distibution ove the inteval [0, CW - 1], whee CW is a value between [CWmin, CWmax]. The backoff time is deceased only when the medium is idle and is fozen (paused) when anothe station is tansmitting. Each time the medium becomes idle, the station waits fo a DIFS and continuously decements the backoff time. As soon as the backoff time expies, the station is authoized to access the medium and tansmit. Hence, wheneve the station senses the medium to be busy, it pauses its backoff time until the medium is found idle again. This will incu delays that ae not modeled by any sense in the contention window CW. These delays actually infe contention in the medium that ends up affecting the tansmission of safety infomation. We theefoe ague that the contention window is not a complete metic to measue netwok contention. We veified this by simulation unde two diffeent scenaios, one suffeing fom netwok contention and the othe not. The contention window used by each station just befoe its successful tansmission was collected while inceasing the numbe of tansmitting stations. The gaph in Figue 3.1 clealy fails to diffeentiate between the two scenaios as the plots coincided clealy yielding no clea cut detemination of the contention. 41

57 Fig Contention Window with and without netwok contention. In the liteatue, some metics ae defined to be used to measue load at the contol channel. The Intefee Numbe is defined in [32] and the Communication Density in [36]. These metics howeve lack the needed pacticality to be implemented in a eal system. Some of the vaiables used in these metics ae had to estimate in eal time by individual cas such as vehicula density and intefeence ange. Theefoe, in ode to detemine contention at a given location, we popose a netwok contention metic C (t) to be implemented within the system that epesents its assessment of the contention level in egion at time t. If this metic is above a cetain theshold C th, it will then be assumed that egion suffes fom netwok contention. contention We made the contention metic at time t ely on C (t-1) and on the newly sensed Ĉ so as to account fo changes in the contention level and to make the system obust to fallacious data. Fo a given location, the contention is elated to the aveage numbe of safety packets tansmitted plus thei aveage sizes, and the channel capacity as eflected by the achievable bitate accoding to the adaptive modulation 42

58 scheme employed in IEEE Thus, if the system accounts fo nea histoy (i.e., C (t-1)) while consideing the pesent contention that is sensed by the cas in, the new C (t) should theefoe model actual contention accuately. We model contention at egion and time t in equation (1) using a linea pediction model simila to the one used to calculate the Round Tip Time (RTT) in the TCP potocol [37]: C ( t) γ C ( t 1) + ( 1 γ ) Cˆ = (1) The paamete γ eflects the weight given to histoy while (1-γ) coesponds to the weight of the sensed contention. The sensed metic data elayed fom n cas. The tem Ĉ depends on the evaluation of Ĉ is a linea combination of two factos, the fist being the poduct of the access delay D of safety packets and the channel s offeed bitate B divided by the aveage payload size S, while the second facto is the aveage numbe of untansmitted safety packets U pe total attempted tansmissions. The fist facto is in effect the channel capacity divided by the thoughput, which is the invese of the effective channel utilization. The access delay D in the context of ou system (as defined in section 1.4) is the delay between the time when a node (ca) decides to send a packet and the time the packet is successfully eceived at the eceive. With highe contention, D inceases due to the caie sensing mechanism, whee each node pauses its backoff time duing the MAC backoff pocess wheneve it senses a busy channel. Those incued delays that ae actually affected by the channel available bitate B, povide a patial contention indication. As a matte of fact, IEEE employs an adaptive modulation scheme that attempts to incease data ates to make use of favoable channel conditions, and educes the data ate as the channel degades. Adaptive modulation and coding attempts to maximize aveage spectal efficiency while maintaining a minimum bit eo pobability [38]. Given that all cas in the same egion 43

59 ae subject to the same channel condition, it is valid to assume that all of them, while employing the same adaptation schemes, will tansmit at equal bit ates. It follows that the bit ate used by a cetain ca at an instance of time t in a egion will indeed epesent the achievable bitate at t in. The payload size S is used to get the aveage delay pe byte. Finally, the numbe of untansmitted packets U also inceases with contention since the contol channel inteval is limited to 50 ms and the collision avoidance mechanism imposes that cetain packets will neve be able to get tansmitted if contention pesists. The esult is a unitless sensed contention metic that is calculated as follows: Cˆ D B S () t = α + βu (2) D, B, S and U being D, B, S and U espectively at egion. The computations of the weights α and β ae tackled in Section System Design The poposed system is composed of seveal entities that enable its oveall opeations. The majo ones ae the vehicles, the Road Side Units (RSUs), and local pocessing unit efeed to as the Local Acquisition and Pocessing Units (LAPUs) Entities Desciption and Layout The system topology is depicted in Figue 3.2. It compises thee majo components. In the following we biefly define the chaacteistics of each component, but shotly aftewads we descibe in details the functionalities of these components. 44

60 Local Acquisition and Pocessing Unit (LAPU) Road Side Unit (RSU) Tansmission ange of RSU Link between RSU and LAPU Fig Poposed system topology. The fist component, the vehicle, is assumed to have an On-Boad computing Unit (OBU) that implements the WAVE standad family peviously descibed and whose adio is softwae defined. It also compises an on-boad GPS, a navigato with its associated maps, a spectum sensing unit, and an on-boad cental compute with I/O intefaces. In fact, all those assumptions ae easonable and seveal of these featues ae aleady deployed in pesent commecial cas, o ae expected to be deployed in the nea futue. Fo example, BMW is developing cas with onboad GPS and navigato that can pedict the diving pattens of the dive and give a coect guess of the next ca stop with 80% accuacy [39]. The second component of the system is the Road Side Unit, which efes to the definition indicated in the WAVE standad. It has the ability to communicate simultaneously with multiple OBUs. It is assumed to have a softwae-defined adio and memoy capabilities fo caching elevant infomation that we pesent late. In ou system, The RSUs do not pefom spectum sensing fo two main easons. Fist, the 45

61 RSUs ae stationed on stategic static locations, and thus cannot povide sensing infomation fo locations whee accidents o emegency situations occu. The second eason is that we aim to minimize the eliance on infastuctue, as to make the system moe scalable, and build the fist step towads an infastuctue fee system in the futue. The thid and last majo component is the Local Acquisition and Pocessing Unit, which is a compute with modest pocessing capabilities and a egula size associated database. While no entity fom the ones pesented so fa is cognitive by itself, the system as a whole is cognitive since leaning, adaptation, and eaction will be taking place at diffeent locations and though vaious entities, a fact that is depicted in Figue 3.3. The specific spectum that the cas use as seconday uses is the TV spectum since it is clealy undeused as we shoed in section 1.4. Each RSU is assumed to be connected to its next hop RSUs (along the oad diections which could be one o two ways) though the same LAPU. On the othe hand, each LAPU is independent by itself, and accoding to ou cuent design, it does not have to be connected to othe LAPUs along the oad. In the poposed system, vehicles ae egaded as consumes as they ae supposed to finally benefit fom the expanded spectum in ode to enhance the dissemination of safety elated data. They also act as infomation gathees to feed the decision-making pocess with timely and necessay infomation. By this, they pefom a dual ole in the cognitive netwok paadigm: on one hand, they ae the seconday uses in the expanded spectum, and on the othe hand, they ae esponsible fo the sensing step that constitutes the fist stage towads cognition. 46

62 Envionment - Radio spectum - Vehicles and othe types of uses Sense and measue Queue Distibuted Sensing by Cas Updates LAPU Cuent State: Contention locations and additional spectum Act Decide Contention locations and additional spectum RSU Lean Fowad Fig Netwok cognition cycle Ca Opeation Evey 20 metes, a ca initiates the pocess of infomation gatheing. The 20m distance is a easonable assumption of the distance unit step of the system. It is mainly used to quantize the continuous oad into points whee each point epesents a 20m length egion of the oad. The pocess of infomation gatheing fo a specific egion woks as follows. Evey time the ca entes the contol channel inteval i, it tempoaily stoes the data pesent in Table 3.1. Additionally, when the ca entes the sevice channel inteval h, it takes measuements of the powe levels of b TV channels, whee each channel is defined to be 6 MHz in width [23]. It is wothy to note that the ca must cease the infomation gatheing pocess at the end of eithe a CCH o SCH inteval if it exceeds the 20m while in the middle of one of the two intevals. It is wothy to note that 47

63 the choice of the quantization step to be equal to 20 metes is mainly motivated by the fact that we do not need to incu lage taffic on the contol channel. This is made clea in chapte 5 about scalability analysis. Moeove, since the data is being aveaged at the RSU, we need to have continuous pofiles of contention metic. Contention metic value should not oscillate fom egion to anothe, especially that taffic jams will extend on lage numbe of egions. If the esolution is high, we will have moe noisy data which leads to geat inconsistencies, theefoe 20 m is a good compomise. td,j,i tx,i s,j,i a,i pl,v,h b,j,i Table 3.1. Ca gatheed data at inteval i. access delay time of each successfully tansmitted safety packet j numbe of sent safety packets at the end of CCH payload size of all sent safety packets numbe of attempted but not tansmitted safety packets powe level measued fo TV channel v at SCH inteval h available bitate at egion fo each safety packet j At the end of the pocess of infomation gatheing that is un each 20 metes as we mentioned ealie, the ca must pefom data aggegation in ode to minimize the size of the data it sends to the RSU when it late entes its tansmission ange. It basically has to ceate a ecod that summaizes the collected data fo the last 20 metes. In this step the ca aveages the diffeent infomation gatheed duing the peceding N contol channel intevals (an estimation of N is pesented late in this section) and M sevice channel intevals. Given that the CCH and SCH intevals occu altenatively, the following elation holds: 1. Moe specifically, the ca c computes: c The total numbe of tansmitted packets: TX = = tx N i 1, i c The total numbe of non-tansmitted packets: A = = a N i 1, i 48

64 The aveage numbe of non-tansmitted packets pe total attempted: U c A = TX c c + A c N tx 1 1, j, i, i c i = j = td The aveage access delay: D = c TX N tx j i = 1 1,,, i s c i j The aveage payload of sent packets: S = = c TX The total numbe of sensed powe measuements: P c = M This value comes fom the fact that the powe level sensing of the TV channels occus once evey sevice channel inteval. This is because it educes the amount of infomation that should be gatheed duing the contol channel inteval, and also because the sensing should detect tansmissions of pimay uses. That is, if sensing is done duing CCH, othe cas (seconday uses) could be using those TV channels, and this will ende the measues inaccuate. c pl h The aveage powe fo each sensed TV channel v at : PL =, v c P N tx M v h = 1,,, i b c i j j i The aveage bitate at : B = = 1 = 1,, c TX Evey 20 metes, the ca goes though multiple CCH intevals. In fact, a ca moving at an aveage speed of 60 Km/h will altenate between CCH and SCH intevals at least 12 times (knowing that the sum of CCH and SCH intevals is 100 ms). In this case, the numbe of CCH inteval N visited duing the 20 metes distance is appoximated 12.The ca is aggegating the data gatheed duing the 20m and stoing it into 1 ecod. Thus the aveages oftx, A, U, D, S and P computed fom 12 samples all fall in the same c c c c c c egion and add edundancy to the esults which in tuns educes eos. Figue 3.4 shows a ecod stoed by ca c fo egion. 49

65 Fig Fomat of the ecod sent by the ca to the RSU. Given the size of each field in the ecod of Figue 3.4, and assuming that each ca is sensing the spectum of 6 TV channels, the size of a single ecod is theefoe 124 bytes. The distance between two diect neighbos RSUs can span seveal kilometes. Taking a distance of 10Km as an example, the size of data that the ca needs to tansmit to the RSU is 62 Kbytes. Given that this data exchange will take place on one of the sevice channels, no exta load will be incued on the contol channel. Moeove, this 62 Kbytes will pose no issue when compaed to the size of the usual data that is communicated ove the sevice channel, like multimedia files and steams [40]. A moe pofound analysis of the scalability of the link between the cas and RSU is povided in Chapte RSU Opeation The RSU s main opeation is to act as a middlewae between the cas and the LAPU. Each RSU povides the sevice of infoming the cas about contention locations along thei paths with the associated additional spectum that they can use. In ode to benefit fom this sevice, a ca that is in poximity of an RSU, will subscibe to a WAVE Basic Sevice Set that is geneated by the RSU and advetised duing the CCH 50

66 inteval and offeing cognitive sevices. Duing the next sevice channel inteval, the ca will povide the RSU with all the stoed ecods since the last time it passed by a RSU. In addition to this, the ca will infom the RSU about the expected next RSU on its path. This infomation will enable the RSU to povide the ca with a table that coesponds to the path that the ca will follow indicating the contention locations and the additional spectum associated with them. In fact, the RSU stoes locally a set of tables, each of which coesponds to a diect next hop RSU. These tables ae updated by the LAPU and maintained in the RSU s local memoy. Upon the eceipt of the next hop infomation fom the ca, the RSU fowads the coesponding table, thus enabling the ca to extend its spectum to additional white spaces at specific contention locations along the way. Howeve, if the ca was unable to pedict o does not want to shae its next hop RSU, it can put a special value in the next hop field of the message sent to the RSU. This will pompt the RSU to send it all the next hop RSU tables. Thus the ca will have all its possible paths coveed with the coesponding contention locations and available spectum. The fomat of the table is depicted in Table 3.2. The RSU will also povide the ca with a set of TV channels to be sensed duing its jouney to the next hop RSU. Each RSU will make sue to povide a numbe of TV channels to be sensed pe ca accoding to the taffic flow in its egion guaanteeing that the distibuted sensing of the whole TV spectum does not equie a time geate than T max. T max is defined as the maximum time needed to sense the cuent state of the whole TV spectum utilization. T max should not exceed double the time equied by a ca moving at an aveage speed to each the next RSU. 51

67 In addition to its inteactions with the cas, the RSU fowads all the data eceived fom them to the LAPU, which will in tun make use of this data as will be descibed in the next section. Table 3.2. Fomat of the RSU table fowaded to cas. Next RSU Contention Location 1 Additional Spectum 1 Contention Location 2 Additional Spectum 2... Contention Location h... Additional Spectum h LAPU Opeations The LAPU is the main pocessing and stoage unit of the system. It is esponsible fo the geneation and the update of the tables maintained at its connected RSUs. The LAPU attempts to accuately estimate the contention locations along the path between each pai of its connected RSUs, and then ties to find all the available white channels in the TV spectum. Futhemoe, it associates additional spectum to these contention locations using a quantization pocess descibed late in this section, whee the amount of this spectum is popotionate to the degee of expeienced contention. Afte eceiving input data fom the RSU fo n cas poviding measuements fo egion, the LAPU calculates the final values of: The access delay fo : D n c= 1 = n TX c= 1 c TX D c c 52

68 The aveage numbe of non-tansmitted packets: U n c= 1 U c = n c= 1 c c ( A + TX ) c c ( A + TX ) The aveage numbe of payload sizes: S n c= 1 S c = n c= 1 TX TX c c The aveage bitate: B n c= 1 = n TX c= 1 c TX B c c metic With the above data, the LAPU is now able to calculate the measued contention Ĉ and incopoate it into the linea pediction model of C (t) (defined in Eqs 2 and 1, espectively). The LAPU pefoms a contention quantization pocess, whee each contention metic coesponding to a distinct egion is mapped into one of fou pedefined contention levels. These levels wee deived expeimentally and ae: No Contention, Low Contention, Medium Contention, and High Contention. Now, each location along the path between two consecutive RSUs is associated with a contention level, whee a subset of the white TV channels is appopiately povided to elieve the contention. This is illustated in Figue 3.5. The No Contention level is assigned to egions whee the calculated contention metic is less than C th. Next, the Low Contention level is assigned to egions whee the load falls within the following egion [channel capacity, 1.5 channel capacity]. This is indicated though a calculated contention metic between C th and 1.2 C th. Futhemoe, the Medium Contention level is assigned to egions whee the load is between one and a half and twice the channel capacity, which is indicated though a contention metic between 1.2 C th and 1.3 C th. Finally, the High Contention level coesponds to egions whee the load is at least double the channel capacity, and is associated with a contention metic highe than 1.3 C th. It is woth 53

69 mentioning that C th is assigned a value of 100, and this will be explained in the next section. C (t) Contention Level Reaction No Contention Do nothing Low Contention Medium Contention High Contention Assign 1 TV channel Assign 2 TV channels Assign all the white spectum Fig Contention metic and coesponding contention levels and eactions. As seen in Figue 3.5, the additional TV spectum allocated to each contention level is in ode of channels. Each TV channel spans 6 MHz, and will be divided into sub-channels of width MHz. As a esult each TV channel will encompass 38 sub-caies. Additionally, the same sub-channel assignment done in the contol channel can be applied, whee 48 out of the 64 sub-channels ae allocated fo data tansmission. Thus, allowing us to assume that the same popotions of 10 out of the 38 sub-channels will be allocated to be guad and pilot sub-channels, while the othe 28 will be allocated fo data exchange. Hence, the added value of each TV channel is 28 sub-channels to the initial 48 sub-channels of the 10 MHz contol channel. Given this, one TV channel is allocated to the Low Contention level, whee the bitate will incease by (28+48)/48=1.58 times, which is an addition that will be sufficient to deal with contention at that level. The same methodology is applied in the Medium Contention level whee two TV channels ae allocated and the bitate incease will be ( )/48=2.16 times, which again, will elieve the contention at the specified egion. In the last level, 5 TV channels ae allocated to combat contention. The 54

70 appoach used to calculate the aveage value of ( t) Contention cases is shown in the following section ˆ fo the Medium and High Fo each ca c, the LAPU updates the coesponding TV channel powe value that c sensed as follows: c () t δpl ( t 1) + (1 δ PL PL = (3),, v, v ) The TV channel is deemed occupied o white depending on whethe PL,v (t) exceeds PL th o not, espectively. It is woth noting hee that fo the TV tansmission, channels assignment is static and all the white channels ae stoed in a database as specified by the FCC eleased in Novembe 2008 [41]. The leaning pocess we pesented can automatically detemine these static white channels, but it is especially useful fo detecting othe seconday uses tying to cognitively use the TV spectum. The values of γ in (1) and δ in (3) should be chosen in ode to guaantee quick convegence of the system while keeping it obust against wong data. The value of γ should be smalle than that of δ since each v C Ĉ is geneated by measues fom n cas while PL, is geneated fom only one ca. A typical value of γ could be 0.5 with equal c v weight to the histoy and to the aggegated sensed data. The value of δ could be assigned a value of 0.15 to emphasize the histoy component. Afte the LAPU has detemined the intensity of contention along the path between the RSUs it is connected to, and has allocated the appopiate additional spectum, the next step is to update the RSU tables. The RSU table is deemed to be stale when a majo contention on the oad is obseved. Actually, the contol channel contention is coelated with oad taffic congestions, which may span distances of hundeds of metes. Hence, it is expected that multiple contiguous sections of the oad will 55

71 expeience contention in the contol channel. In such a condition, it is detemined that the tables at the coesponding RSUs ae stale and equie updating. The LAPU does not necessaily esend entie tables but athe, it sends the updated sections of the table indexed by the contention locations whee the affected RSUs incopoates this data into thei tables Detemining the weight and theshold values This section descibes the expeimental pocedue used to simulate the IEE p daft standad and to deive the values of α and β in Equation 2 plus the thesholds used in the system. To decide on the values of α and β plus the othe thesholds, multiple scenaios of netwok contention wee developed. As mentioned befoe, the bitate in p ove the 10 MHZ contol channel can vay between 3 Mbps and 27 Mbps, while the nominal bit ate is consideed to be 6Mbps [42]. When consideing a contol channel inteval of 50 ms and a bit ate of 6Mbps, the channel capacity in the contol channel inteval becomes (6 Mbps 50 ms)/8=37,500 bytes, which is in essence the theshold case. That is, any input to the netwok that is moe than bytes duing 50 ms will ende the netwok congested. To deive the netwok contention theshold, thee scenaios coesponding to thee bitates: 3 Mbps, 6Mbps, and 12 Mbps wee devised. To have a epesentative sample, the numbe of cas in each scenaio was set to thee values: 50, 100, and 200 cas. Table 3.3 summaizes the paametes fo each scenaio, including the payload sizes, all of which yield loads that ae equal to the channel capacity. 56

72 Table 3.3. Paamete values fo deiving the weights of the contention expession. Bitate 3 Mbps 6 Mbps 12 Mbps Numbe of cas Payload size (in bytes) Fo all the simulations, the values of the access delay divided by the payload and multiplied by the offeed bitate in the channel, ( ) D B / S, wee plotted against the aveage numbe of untansmitted packets pe attempted numbe of tansmitted packets U. In this espect, the dependent vaiable is in effect the channel capacity divided by the thoughput, which is the invese of the effective channel utilization. Next, a linea egession was pefomed to deive the theshold using the combined data fom all scenaios. The esults ae illustated in Figue 3.6. As shown, we call the deived theshold the Low Theshold since thee ae two highe thesholds that denote moe sevee channel conditions. The simulations wee based on the netwok simulation softwae NS2, which povides a compehensive suppot fo the IEEE set of technologies, and is a widely utilized simulation tool among wieless communications eseaches. The latest vesion, namely ns2.34, is an ovehaul of the pevious vesion. It intoduces a new achitectue and a moe up-to-date modeling of the IEEE MAC and PHY layes. Moe impotantly to ou study, NS2 now includes suppot fo the IEEE p Dedicated Shot Range Communication standad, which theefoe povides a ealistic and accuate simulation of the poposed system [43]. In section 4.1, we will povide a detailed explanation about ou choice of NS2 as the netwok simulation tool to employ. 57

73 0.25 Low Theshold Model Invese of channel utilization * y = 0.349x R 2 = Mbps offeed and tansmitted 3 Mbps offeed and tansmitted 12 Mbps offeed and tansmitted Aveage untansmitted packets pe attempted tansmission Fig Low Theshold linea model. Fo each scenaio, the tansmission ange was set to mw (250 metes), in accodance with the typical values that ae discussed in [33] [44]. The sepaation between each two cas was set to 5 metes, and the cas wee assumed to be in a taffic jam, so that they ae elatively stationay, and wee distibuted equally on fou lanes. Each ca boadcasts once pe contol channel inteval a message to the cas in its tansmission ange, whee the size of the message was detemined by the simulated scenaio. Futhemoe, evey ca began boadcasting andomly within the contol channel inteval. Each simulation was epeated 100 times fo each scenaio, and at the end of each un, awk scipts wee used to pocess the tace files and geneate the two measues we discussed above: the invese of the effective channel utilization and the aveage numbe of untansmitted packets pe attempted numbe of tansmitted packets. The data fom the simulation was then analyzed to compute the diffeent paametes of the system. As we mentioned ealie, a linea model elating the invese of the 58

74 effective channel utilization and the aveage numbe of untansmitted packets was D B / S ) geneated. In this model, the y-value is epesented by the fist tem (i.e., ( ) while the x-value is denoted by the second tem (i.e., U ). Hence, using the cuve fit that is displayed in Figue 3.6, we can elate the two tems with a coelation equal to D B 0.81 as follows: = 349 U Next, by ealizing that the value of C ˆ () t S (see Equation 2) must hold tue fo C th, which we nomalize to 100, we get: D B 100 = α + βu. Finally, using the values of the y-intecept and the slope in the S linea egession equation (i.e., 192 and -349, espectively), we can easily compute α as 100/ and β as With these values, Equation 2 now becomes: D B Cˆ () t = U (equation 2.1), and C th =100. S To futhe validate ou finding, we simulate a scenaio of 300 cas with diffeent offeed bitates which is a moe ealistic scenaio. Table 3.4 shows the distibution the cas among the diffeent bitates. Table 3.4. Paamete values fo the mixed scenaio. Total Numbe of cas 300 Bitae 3 Mbps 6 Mbps 12 Mbps Numbe of cas Payload size (in bytes) This mixed scenaio is a theshold scenaio since the offeed bitate is equal to the needed bitate. We expect the contention window to be 100 fo this scenaio. The simulated contention window tuns to be This validates ou finding. 59

75 Next, two scenaios coesponding to two highe contention levels (i.e., medium contention and high contention thesholds) wee simulated. The fist scenaio simulated coesponds to needed bitate being one and a half the offeed, which is egaded as the theshold between low contention and medium contention. Figue 3.7 shows the medium contention case plotted. The scenaio was epeated 500 times and the esulting D B S and U wee plugged into the model demonstated above, whee the esultant aveage value of ˆ () t was found to be 120. The second additional scenaio coesponds C to a needed bitate that is double the offeed. This gives ise to a theshold between medium contention and high contention. Figue 3.8 shows the high contention case plotted. The scenaio was epeated 500 times and again, the esulting D B S and U wee plugged into the model, whee the aveage value of ( t) ˆ was found to be 130. C 60

76 0.2 Medium Theshold Model Invese of channel utilization * R² = Aveage untansmitted packets pe attempted tansmission Fig Medium contention theshold. Invese of channel utilization * High Theshold Model R² = Aveage untansmitted packets pe attempted tansmission Fig High contention theshold. 61

77 Finally, we an a simulation to justify that mobility does not affect ou findings. A scenaio of 100 cas is simulated with all cas moving at the speed of 95km/hou. The offeed bit ate used is 6Mbps/hou and the payload size is 375 bytes. This scenaio coesponds to the low theshold case. We expect the contention window to be 100. The simulated contention window tuns to be This indicates that mobility does not affect ou findings. Futhemoe, we plot in Fig. 3.9 the mobility scenaio contention metic points against the low theshold line. We can easily notice that the points fall aound the theshold and have a high coelation with it. This is intuitive and can be agued due to the fact that the cas will not tavel a long distance duing the 50 ms contol channel inteval. And since all cas ae moving in a elatively close speed, they can be assumed to be stationay elative to each othe Low Theshold Model with Mobility Invese of channel utilization * Mobility Scenaio Aveage untansmitted packets pe attempted tansmission Fig Low Theshold Model with Mobility. 62

78 We conclude this section by noting that a moe consevative appoach towads setting the thesholds could be adopted. Thus, all the thesholds could be futhe pushed to the left since they wee calculated based on the limiting scenaios whee the offeed and the utilized bitates wee exactly the same. 3.4 Oveall Inteaction In this section, the components of the system that wee descibed ealie will be put togethe to fom the complete pictue. A ca will be assigned a set of TV channels. Evey 20m, the ca will stoe a ecod containing the powe levels of these channels, the location coodinates, the contention metics, and a timestamp as descibed in the pevious sections. When a ca and an RSU become in the tansmission ange of each othes, an exchange of infomation will occu. The ca will povide the RSU with its next hop RSU along its path with all the ecods stoed since the last RSU. The RSU on the othe hand will check the next hop RSU sent by the ca and will poactively fowad to the ca the coesponding table(s) maintained in its memoy. This is depicted schematically in Figue 3.9 and moe pecisely using the sequence diagam of Figue The ca will now possess the table indicating the pedicted contention locations along its path till the next hop RSU with the associated additional spectum at these locations. The ca will then extend the contol channel accodingly and will use it egulaly until it eaches the next hop RSU along its path. It should be kept in mind that the ca should not diectly tansmit on this additional spectum. In fact, it should fist pefom a fast sensing to guaantee that the additional spectum is indeed fee at the moment of tansmission. But on the othe hand, the ca should not woy about othe 63

79 cas not eceiving the message since all cas should have the same table and should be listening on the same extended contol channel at the specified locations. Fig High level view of the system s inteactions. Meanwhile, the RSU elays the infomation eceived fom the ca to the LAPU. The LAPU, having a copy of all the tables stoed at all its connected RSUs, geneates its estimates about the contention locations and the fee spectum. The LAPU does not diectly update the tables stoed at the RSUs, but instead, it waits until a majo change in contention levels along the path is obseved. The updates ae not continuously geneated since they lead to poblems in the communication between the cas on the oad. In fact, when a table update is eceived by an RSU, a tansition step take place between the old and the new tables. Since othewise, if the RSU sends the new table ight afte eceiving it, the eceiving cas may lack the same extension of the contol channel as compaed to the cas in font of them which eceived the old table. This condition will definitively lead to communication poblems between these two goups of cas. Fo this eason, afte eceiving the new table, the RSU will make sue to send the old and the new tables to a set c of tansitioning cas passing by it. In addition, those 64