A Novel WLAN Vehicle-To-Anything (V2X) Channel Access Scheme for IEEE 802.11P-Based Next-Generation Connected Car Networks

applied sciences

Article A Novel WLAN Vehicle-To-Anything (V2X) Channel Access Scheme for IEEE 802.11p-Based Next-Generation Connected Car Networks

Jinsoo Ahn 1 , Young Yong Kim 1 and Ronny Yongho Kim 2,* 1 School of Electrical & Electronics Engineering, Yonsei University, Seoul 03722, Korea; [email protected] (J.A.); [email protected] (Y.Y.K.) 2 Department of Railroad Electrical and Electronic Engineering, Korea National University of Transportation, Gyeonggi 16106, Korea * Correspondence: [email protected]; Tel.: +82-31-460-0573

Received: 15 September 2018; Accepted: 29 October 2018; Published: 1 November 2018

Featured Application: V2X system for next-generation connected car networks.

Abstract: To support a massive number of connected cars, a novel channel access scheme for next-generation vehicle-to-anything (V2X) systems is proposed in this paper. In the design of the proposed scheme, two essential aspects are carefully considered: backward compatibility and massive V2X support. Since IEEE 802.11p-based V2X networks are already being deployed and used for intelligent transport systems, next-generation V2X shall be designed considering IEEE 802.11p-based V2X networks to provide backward compatibility. Since all future cars are expected to be equipped with a V2X communication device, a dense V2X communication scenario will be common and massive V2X communication support will be required. In the proposed scheme, IEEE 802.11-based extension is employed to provide backward compatibility and the emerging IEEE 802.11ax standard-based orthogonal frequency-division multiple access is adopted and extended to provide massive V2X support. The proposed scheme is further extended with a dedicated V2X channel and a scheduled V2X channel access to ensure high capacity and low latency to meet the requirements of the future V2X communication systems. To demonstrate the performance of the proposed scheme thoroughly and rigorously, the proposed scheme is mathematically analyzed using a Markov model and extensive simulations are performed. In the dense V2X communication networks of the future, the proposed V2X communication scheme will provide high performance and reliability.

Keywords: V2X; OFDMA; random access; UORA; wireless LAN; resource unit; trigger frame; channel access

1. Introduction Connected cars are attracting attention increasingly and several commercial products for the connected car are expected in the near future. After a dedicated spectrum at 5.9 GHz was allocated for intelligent transport systems (ITSs), both in the US and in Europe in 1999 and 2008, respectively, different families of standards have been developed: the IEEE/SAE dedicated short-range communication (DSRC) in 2010 in the US, Release 1 of the ETSI/CEN Cooperative-ITS (C-ITS) in 2013 in Europe, and 3GPP Cellular-V2X (C-V2X) in early 2017 as a feature of Release 14 of the LTE standard [1–7]. DSRC and C-ITS both use the IEEE 802.11p standard [4,6,7], which is a short-range technology for connected car communications, for the physical and data link layers [4]. Based on the IEEE 802.11p standard, the SARTRE project, which ran from 2009 to 2012, deployed a platoon of

Appl. Sci. 2018, 8, 2112; doi:10.3390/app8112112 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2112 2 of 23 multiple connected cars driven autonomously in close formation [8]. In Japan, the fully automated truck platooning with a connected car system was tested on an express way in the Energy ITS project in 2012 [9]. The European Truck Platooning Challenge demonstrated the automated trucks of six major truck vendors that were driven in platoons on public roads [10]. Since currently wireless communication networks for connected cars are based on the IEEE 802.11p and wireless access in vehicular environments (WAVE) standards, the future wireless communication network for connected cars should provide a backward compatibility with IEEE 802.11p- and WAVE-based communication networks [11–13]. To enhance the safety and user experience of next-generation autonomous vehicles, not only enhancements in signal processing but also wireless network-based ITSs have been developed [14,15]. To provide a wireless network-based ITS, vehicle-to-anything, denoted as V2X, communication systems are expected to be used in autonomous driving vehicles [14–18]. V2X is a wireless communication technology used to exchange ITS information among cars, between a car and infrastructure network access points (APs), or among user devices in the car. Therefore, in the future V2X communication, from the V2X communication point of view, connected cars will include autonomous cars. To effectively provide users with a comfortable and safe driving environment, the ITS AP shall be considered as an ITS sensor or an ITS beacon for every navigating connected car. In other words, the ITS AP shall accommodate ITS information from connected cars, ITS pedestrians, and some local sensors. Then, it shall transmit the information gathered and processed by the ITS AP or crucial ITS information received from a remote node, such as ITS service provider servers, remote vehicles to numerous ITS nodes including connected cars and ITS pedestrians by using its wired and wireless connections. Therefore, the numerous communication links between ITS nodes and an AP must be reliable and need to have low delay properties for the AP to perform the role of a reliable ITS beacon. Unfortunately, if the number of ITS nodes that an AP needs to support is too large, the ITS AP cannot establish those wireless connections rapidly in a random-access-based V2X network. Because connected cars have large mobility, the connection establishment delay caused by many ITS nodes in next-generation V2X networks is much more important than in other wireless network scenarios. In this paper, the next-generation V2X (NG-V2X) scenario described in Figure1 is considered. ITS sensors are sensors collecting information related to ITS system. They can collect weather information, pavement condition information (road surface temperature, water film thickness, residual salt, etc.), and traffic information including vehicle classification [19,20]. They are anticipated to be used more widely for safety in various ITS scenarios in the future. The NG-V2X system must provide communications between a vehicle and anything including pedestrians, ITS sensors, vehicles, and ITS APs [21,22]. In another word, the NG-V2X must provide V2X communications. Thus, the NG-V2X system needs to consider communications among various and numerous stations (STAs) than ever before. Usually, 1000 m is considered as the ideal coverage range of an IEEE 802.11p-based system under the assumption of Line of Sight (LOS) communications environment like highway [23–27] without considering obstacles and severe fading and shadowing, which is not a realistic scenario [28]. It means that the IEEE 802.11p STAs can deliver data frames without any problem if there are not so many vehicles, as in a rural area or an uncrowded road unless there are shadowing and fading which can make the IEEE 802.11p coverage range smaller [29,30]. However, even though the actual coverage range is smaller than ideal coverage range of IEEE 802.11p-based system, the coverage range is sufficient to accommodate a large number of connected cars that could cause channel access problems in traffic jam scenarios because every car will be required to equip a V2X system in the near future [31–33]. Furthermore, the NG-V2X network we consider may include various STAs including pedestrian STAs and ITS sensor STAs as shown in Figure1; this means that the NG-V2X network may suffer severe channel access problems without novel and efficient channel access schemes. In other words, supporting numerous V2X STAs including all cars required to be equipped with the V2X system, ITS pedestrians, and ITS sensors will be difficult without novel and efficient NG-V2X channel access schemes. Appl. Sci. 2018, 8, 2112 3 of 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 24

ITS Sensor Appl. Sci. 2018, 8, UORAx FOR PEERITS Sensor REVIEW 1 ITS AP 3 of 24 Trigger UL DATA Frame Connected Car 1 M-BA By UL DATA ITS AP Pedestrian 1 ITS Connected UL DATA Sensor Car UORA ITS Sensor 1 ITS AP Trigger UL DATA Frame Connected Car 1 M-BA By UL DATA ITS AP Pedestrian 1 Connected UL DATA Car

Connected Pedestrian Car Pedestrian Connected Pedestrian Car ITS Pedestrian Sensor

Figure 1. A sample scenarioITS of the next-generation vehicle-to-anything (V2X) system components. Figure 1. A sample scenario ofSensor the next-generation vehicle-to-anything (V2X) system components. ITS, ITS, intelligent transport system; AP, access point. intelligent transport system; AP, access point.

TheseTheseFigure kinds kinds 1. A of sample of scalability scalability scenario problems problemsof the next-generation areare the most vehicle-to-anything well-known well-known problems problems(V2X) system of wireless of components. wireless networks. networks. ITS, intelligent transport system; AP, access point. However,However, different different from from other other cases, cases, downlink downlink ITSITS information has has a broadcast a broadcast property property and anduplink uplink ITS information may not require a large size of frames. Furthermore, the presence of a large number ITS informationThese kinds may notof scalability require a problems large size are of the frames. most Furthermore,well-known problems the presence of wireless of a large networks. number of of connected cars in the area of an AP means that their mobility might be reduced than in usual cases, connectedHowever, cars different in the areafrom of other an APcases, means downlink that IT theirS information mobility mighthas a broadcast be reduced property than and in usualuplink cases, and hence the data frames become more delay tolerable. In other words, if there is no traffic jam, a and henceITS information the data may frames not require become a large more size delay of frames. tolerable. Furthermore, In other the words, presence if there of a large is no number trafﬁc jam, connected car will pass through a coverage range of an ITS AP installed on the roadside in a short of connected cars in the area of an AP means that their mobility might be reduced than in usual cases, a connectedtime. However, car will if there pass throughis a heavy a traffic coverage jam, rangethe car of cannot an ITS leave AP the installed coverage on range the roadside soon. Figure in a 2 short and hence the data frames become more delay tolerable. In other words, if there is no traffic jam, a time.shows However, the concept if there of these is a heavy two scenarios. trafﬁc jam, An ITS the AP car can cannot be installed leave the on the coverage roadside range and ITS soon. sensors Figure 2 connected car will pass through a coverage range of an ITS AP installed on the roadside in a short showscan thebe installed concept on of thesethe roadside two scenarios. or buried An under ITS the AP road can befor installedthese two on scenarios. the roadside Figure and 2a shows ITS sensors a time. However, if there is a heavy traffic jam, the car cannot leave the coverage range soon. Figure 2 cansparse be installed case, and on Figure the roadside 2b shows or burieda dense undercase. In the the road Figure for 2a these scenario, two scenarios.the initial channel Figure 2accessa shows shows the concept of these two scenarios. An ITS AP can be installed on the roadside and ITS sensors a sparsedelay case,is important, and Figure not 2a bcollision. shows aOn dense the other case. hand In the, in Figurethe Figure2a scenario, 2b scenario, the a initialcollision channel may occur access can be installed on the roadside or buried under the road for these two scenarios. Figure 2a shows a delayassparse isshown important, case, in andFigure not Figure a3; collision.after 2b shows collision, On a dense the an other additionalcase. hand, In the channel inFigure the Figure 2aaccess scenario,2 bdelay scenario, the needs initial a to collision channel be considered mayaccess occur as shownbecausedelay inis of Figureimportant, the enhanced3; after not collision,a distributedcollision. an On channel additional the other acce hand channelss (EDCA), in the access Figure procedure delay 2b scenario, needsof the to IEEEa collision be considered802.11 may standard. occur because of theItas is enhanced shownworth notingin distributedFigure that 3; the after channeldelay collision, after access channel an (EDCA)additional collision procedure channel could beaccess of much the delay IEEE longer 802.11needs than to standard.the be data considered collision It is worth notingtime.because that This the of means the delay enhanced that after thechannel distributedtime loss collision due channel to collision, could accessbe which(EDCA) much is inevitableprocedure longer than becauseof the the IEEE data random 802.11 collision access standard. time. is the This only solution for a newcomer STA unless there were prior procedures such as an association or a meansIt is that worth the timenoting loss that due the todelay collision, after channel which iscollision inevitable could because be much random longer than access the is data the onlycollision solution dedicated resource scheduling, is not so long relatively, but the delay generated by random back-off for atime. newcomer This means STA unlessthat the there time loss were due prior to collision, procedures which such is inevitable as an association because random or a dedicated access is resourcethe after collision has a large value relatively. Hence, the random back-off delay needs to be minimized scheduling,only solution is not for so a long newcomer relatively, STA but unless the there delay were generated prior procedures by random such back-off as an association after collision or a has a ordedicated eliminated resource for an scheduling,effective channel is not accessso long procedure. relatively, but the delay generated by random back-off large value relatively. Hence, the random back-off delay needs to be minimized or eliminated for an after collision has a large value relatively. Hence, the random back-off delay needs to be minimized effectiveor eliminated channel accessfor an effect procedure.ive channel access procedure.

(a) (b)

Figure 2. Two different scenarios of V2X system: (a) Sparse scenario of V2X; (b) Dense scenario of V2X. (a) (b)

FigureFigure 2. Two 2. Two different different scenarios scenarios of of V2X V2X system: system: ( aa)) Sparse Sparse scenario scenario of ofV2X; V2X; (b) (Denseb) Dense scenario scenario of V2X. of V2X. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 24

In the design of the proposed scheme, two essential aspects, namely, backward compatibility and massive V2X support, are carefully considered. Since the IEEE 802.11p-based V2X network is already being deployed and used for ITSs, NG-V2X networks shall be designed under the consideration of the IEEE 802.11p-based V2X network to provide backward compatibility. Since all future cars are expected to be equipped with V2X communication devices, dense V2X communication scenarios will be common and massive V2X communication support will be required. As part of the efforts to solve two challenging issues of V2X network, the IEEE 802.11 working group has started to study the NG-V2X system. A study group for NG-V2X (NGV-SG) has been formed to consider various scenarios for the V2X communication system. It is expected that the study group will produce an NG-V2X standard based on the conventional IEEE 802.11p standard to ensure backward compatibility. It is considering a V2X system that will be more reliable, have a higherAppl. Sci. capacity,2018, 8, 2112 and support higher mobility, though specific details of the requirements are being4 of 23 discussed.

BUSY BUSY USER1 UL DATA BA AP

BUSY BUSY STA1 UL DATA BA STA1

Collison STA2 BUSY BUSY UL DATA STA2 Collison STA3 BUSY BUSY UL DATA STA3

BUSY BUSY BUSY STA4

STA5 BUSY BUSY UL DATA STA5

BUSY BUSY BUSY STA6

BUSY BUSY BUSY STA7

STA8 BUSY BUSY UL DATA STA8

FigureFigure 3. 3. ChannelChannel collision collision in in dense dense scenario scenario of of V2X. V2X.

InIn the designproposed of the scheme, proposed an scheme,IEEE 802.11-based two essential extension aspects, namely,is employed backward to provide compatibility backward and compatibility,massive V2X support, ability to are operate carefully in a considered. mode in wh Sinceich the the devices IEEE 802.11p-based can interoperate V2X with network conventional is already IEEEbeing deployed802.11p devices and used [34], for ITSs,and NG-V2Xthe emerging networks IEEE shall be802.11ax designed standard-based under the consideration orthogonal of frequency-divisionthe IEEE 802.11p-based multiple V2X access network (OFDMA) to provide is adopted backward and compatibility. extended to Since provide all future massive cars V2X are support.expected This to be paper equipped proposes with a V2X novel communication NG-V2X channel devices, access dense scheme V2X based communication on uplink OFDMA. scenarios Our will mainbe common contributions and massive in this V2X paper communication consist of two support parts: will (1) bea required.scalable and efficient channel access methodAs partusing of modified the efforts IEEE to solve 802.11ax two challenging uplink OF issuesDMA ofrandom V2X network, access (UORA) the IEEE appropriate 802.11 working for massivegroup has V2X started support to studyand (2) the an NG-V2Xaccurate mathemat system. Aical study modeling group and for NG-V2Xanalysis on (NGV-SG) the adopted has IEEE been 802.11axformed to uplink consider OFDMA various scenariosrandom access for the (UOR V2X communicationA). The adopted system. IEEE It 802.11ax-based is expected that theOFDMA study schemesgroup will including produce UORA an NG-V2X for the standard proposed based NG-V2X on the channel conventional access IEEEis modified 802.11p to standard work into 10 ensure MHz channelbackward bandwidth compatibility. by doubling It is considering the symbol a V2X leng systemth of thatIEEE will 802.11ax be more symbols. reliable, The have operation a higher frequencycapacity, and band support of the higherproposed mobility, NG-V2X though channel specific access details scheme of the is the requirements 5.9 GHz ITS are band being which discussed. is the sameIn operation the proposed band as scheme, the conventional an IEEE 802.11-based IEEE 802.11p extension based ITS is employedsystems. The to provideproposed backward scheme includescompatibility, a scalable ability channel to operate access in a mode method in which appropriate the devices for canV2X interoperate applications, with and conventional it also enables IEEE effective802.11p devicescommunication [34], and between the emerging ITS sensors IEEE and 802.11ax ITS APs standard-based without impacting orthogonal V2X system frequency-division performance. Furthermore,multiple access a scheduled (OFDMA) NG-V2X is adopted channel and access extended scheme to provide is also prop massiveosed for V2X the support. extra optimization. This paper Toproposes show ahow novel the NG-V2X proposed channel NG-V2X access channel scheme ac basedcess onscheme uplink can OFDMA. enhance Our network main contributions scalability, in this paper consist of two parts: (1) a scalable and efficient channel access method using modified IEEE 802.11ax uplink OFDMA random access (UORA) appropriate for massive V2X support and (2) an accurate mathematical modeling and analysis on the adopted IEEE 802.11ax uplink OFDMA random access (UORA). The adopted IEEE 802.11ax-based OFDMA schemes including UORA for the proposed NG-V2X channel access is modified to work in 10 MHz channel bandwidth by doubling the symbol length of IEEE 802.11ax symbols. The operation frequency band of the proposed NG-V2X channel access scheme is the 5.9 GHz ITS band which is the same operation band as the conventional IEEE 802.11p based ITS systems. The proposed scheme includes a scalable channel access method appropriate for V2X applications, and it also enables effective communication between ITS sensors and ITS APs without impacting V2X system performance. Furthermore, a scheduled NG-V2X channel access scheme is also proposed for the extra optimization. To show how the proposed NG-V2X channel Appl.Appl. Sci. Sci. 20182018, ,88, ,x 2112 FOR PEER REVIEW 5 5of of 24 23 mathematical modeling, and analysis, more accurate and closer to real standards than ever to the bestaccess of our scheme knowledge can enhance on uplink network OFDMA scalability, random mathematical access (UORA) modeling, are provided. and analysis, In the mathematical more accurate modelingand closer of to UORA, real standards some errors than of ever the to previous the best ofwo ourrks knowledgeare fixed and on the uplink actual OFDMA UORA random procedure access of the(UORA) IEEE are802.11ax provided. standard In the is mathematical considered. modelingThen, the of delay UORA, and some user errors capacity of the performance previous works of the are proposedfixed and theNG-V2X actual UORAchannel procedure access scheme, of the IEEE including 802.11ax some standard further is considered. optimization Then, methods, the delay andare evaluateduser capacity in various performance simulation of the scenarios proposed to NG-V2X demonstrate channel how access the proposed scheme, includingschemes enhance some further user capacityoptimization performance methods, of are the evaluated NG-V2X in network. various simulation scenarios to demonstrate how the proposed schemesThe enhancerest of paper user capacity is organized performance as follows. of the NG-V2XIn Section network. 2, we introduce some background technologiesThe rest including of paper is the organized conventional as follows. IEEE In Section802.11p2 ,standard we introduce and someUORA, background which are technologies used as a basicincluding channel the access conventional concept IEEE in this 802.11p work. standard In Section and 3, UORA, the proposed which areNG-V2X used as channel a basic access channel scheme access usingconcept the inUORA this work. procedure In Section is explained.3, the proposed In Section NG-V2X 4, mathematical channel accessmodels scheme of UORA using and the novel UORA and efficientprocedure V2X is enhancing explained. methods In Section including4, mathematical schedule modelsd NG-V2X of UORA channel and access novel are and explained. efficient Since V2X theenhancing proposed methods mathematical including modeling scheduled of the NG-V2X UORA channelprocedure access considers are explained. the latest Sinceupdates the on proposed UORA proceduremathematical of IEEE modeling 802.11ax, of the the UORA proposed procedure modeling considers is the most thelatest up-to-date updates and on accurate UORA proceduremodeling to of theIEEE best 802.11ax, of our knowledge. the proposed The modeling proposed is NG-V2X the most sc up-to-dateheme is specifically and accurate designed modeling to transport to the best ITS of informationour knowledge. in dense The proposed scenarios NG-V2X for the schemenext-gener is specificallyation V2X designednetworks. to In transport Section ITS 4, informationthe system performancein dense scenarios of the proposed for the next-generation uplink V2X channel V2X networks.access is investigated In Section 4through, the system extensive performance simulation. of Inthe Section proposed 5, delay uplink and V2X packet channel loss accessrate performances is investigated are throughexamined extensive in various simulation. simulation In scenarios Section5, includingdelay and conventional packet loss rateIEEE performances 802.11p and high are prio examinedrity channel in various access simulation scheme proposed scenarios by includingprevious studiesconventional to compare IEEE 802.11pthe channel and highaccess priority schemes. channel The conclusion access scheme with proposed a remark by of previous relationship studies with to NG-V2Xcompare is the presented channel in access Section schemes. 6. The conclusion with a remark of relationship with NG-V2X is presented in Section6. 2. Background Technology 2. Background Technology In the IEEE 802.11p standard, different from other IEEE 802.11 standards, authentication and associationIn the processes, IEEE 802.11p which standard, cause a differentlarge delay from before other real IEEE data 802.11 transmission, standards, are authenticationnot required. Each and IEEEassociation 802.11p processes, STA can whichtransmit cause its data a large to delayan AP before after receiving real data transmission,the beacon frame are not with required. the channel Each accessIEEE 802.11pprocedure. STA Figure can transmit 4 shows its the data conventional to an AP after IEEE receiving 802.11 transmission the beacon frame procedure with thewith channel large delayaccess link procedure. establishment Figure 4 andshows the the IEEE conventional 802.11p pr IEEEocedure 802.11 with transmission simplified procedure link establishment. with large Althoughdelay link IEEE establishment 802.11p has and a thesimpli IEEEfied 802.11p link establishment procedure with procedure, simplified the link channel establishment. access procedure Although isIEEE still 802.11ptime-based has random a simplified access; link hence, establishment if multiple procedure,STAs transmit the their channel data access simultaneously, procedure all is stillthe datatime-based frames randomcollide in access; the air. hence, Because if multiple the random-access-based STAs transmit their channel data simultaneously, access procedure all theof IEEE data 802.11frames is collide distributed in the air.and Because involves the a random-access-basedlarge retransmission channeldelay, it accessis not proceduresuitable for of IEEEhighly 802.11 dense is connecteddistributed car and scenarios involves like a large the inte retransmissionrchange and delay, urban it traffic is not jam suitable cases. for highly dense connected car scenarios like the interchange and urban traffic jam cases.

Beacon Association ACK AP Frame Response

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Figure 4. Cont. Appl. Sci. 2018, 8, 2112 6 of 23 Appl.Appl. Sci. Sci. 2018 2018, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 66 ofof 2424

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AnAn uplink uplink delay delay of of IEEE IEEE 802.11p 802.11p frame frame delivery delivery can can be be described described as asas TTTT=+=+αα T T TV2X V=V2_2_ X XαTCCH CCH CCH_duration duration duration+ TXOPT TXOPTXOP M n MM o (1)(1)(1) + ∑ ++Tback{}{}TiT−o f f (i)() +++T++SCH_busy_time(i ()) iNiT + N( ()i)TCCH_duration  TiTbackback−− off off() SCH SCH____ busy busy time time () iNiT () CCHCCH__ duration duration i=0 i=i=00 wherewhere α α represents representsrepresents the thethe ratio ratioratio of ofof the the partial partial control control chan channelchannelnel (CCH) (CCH) duration duration if if the the frame frame arrives arrives during during thethe CCHCCH duration.duration. duration. MMM isisis thethe the numbernumber number ofof of retransmissionsretransmissions retransmissions untiluntil until transmissiontransmission transmission successsuccess success oror or failure.failure. failure. N N((iN)i) is(isi ) theisthe the numbernumber number ofof ofCCHsCCHs CCHs forfor for eacheach each retransmission.retransmission. retransmission. IfIf Ifi i isiis is 0,0, 0, thethe the valuevalue value camecame came fromfrom from initialinitial initial transmission.transmission. transmission. TTCCH_durationCCH_duration isis aisa valuevalue a value ofof of aa CCH aCCH CCH durationduration duration usuallyusually usually setset set toto to 5050 50 msms ms oror 0 0 ms. ms.ms. TTTXOPTXOPTXOP isisis aa a timetime time componentcomponent component forfor transmittingtransmitting a a data datadata frame frameframe and andand receiving receiving its its immediateimmediate response.response. TTback-offback-off representsrepresentsrepresents aa a randomrandom delay delay duringduring which which an an IEEE IEEE 802.11 802.11 STA STA must must wait wait before before transmitting transmitting its its frames frames to to avoid avoid wireless wireless channel channel collision.collision. The The back-off back-off value value is is chosen chosen randomly randomly between between 0 0 and and the the value value of of the the congestion congestion window window (CW)(CW) size, size, and and the the STAs STAs must mustmust reduce reducereduce the thethe back-off back-offback-off valu valuevaluee every every time time slot slot interval interval if if the the channel channel is is idle idle duringduring the the time time slot slot and and the the value value is is positive. positive. BecauseBecause IEEEIEEE 802.11p 802.11p couldcould switchswitch itsits logical logical channel channel repeatedly, repeatedly, an an uplink uplink channel channel access access procedureprocedure by by connected connected carscars cancan bebe performedperformed duringduring thethe the servicservic serviceee channelchannel channel (SCH)(SCH) (SCH) durationduration duration only.only. only. Basically,Basically, IEEE IEEE 802.11p 802.11p could could switch switch its its logical logical channel channel every every 50 50 ms. ms. In In IEEE IEEE 802.11p, 802.11p, there there are are two two typestypes of ofof logical logicallogical channel: channel: CCH CCH and and SCH;SCH; SCH; CCHCCH CCH isis is used used used for for for transmitting transmitting transmitting downlink downlink downlink ITS ITS ITS information information information by by by an anITSan ITS ITS AP, AP, AP, and and and SCH SCH SCH can can can be be usedbe used used for for for every every every ITS ITS ITS station statio statio ifnn theif if the the STAs STAs STAs have have have received received received a beacon a a beacon beacon frame frame frame from from from the theITSthe ITS ITS AP. AP. AP. If a If V2XIf a a V2X V2X system system system deploys deploys deploys a dedicated a a dedicated dedicated frequency frequency frequency channel channel channel for thefor for logicalthe the logical logical CCH CCH CCH and and SCH,and SCH, SCH, the V2Xthe the V2Xdelay,V2X delay, delay,TCCH_duration T TCCH_durationCCH_durationcan can can be be be set set set to to 0.to 0. In0. In thisIn this this case, case, case,TV T 2TVXV22XcanX can can be be be described described described as as as below. below. below.

MM TT=+=+++ M{}{}n T() iT () i o TTVXVX22 TXOP TXOP  Tbackback−− off off() iT SCH SCH____ busy busy time time () i (2)(2) TV2X = TTXOP +i∑=i=00 Tback−o f f (i) + TSCH_busy_time(i) (2) i=0 BecauseBecause recentrecent V2XV2X systemssystems tendtend toto useuse aa dedidedicatedcated channelchannel structurestructure ratherrather thanthan toto useuse aa channelchannelBecause switchingswitching recent structurestructure V2X systems inin aa tend singlesingle to channel usechannel a dedicated [35,36],[35,36], channel wewe determineddetermined structure toto rather useuse this thanthis dedicateddedicated to use a channel CCHCCH structureswitchingstructure inin structure thisthis study.study. in a Figure singleFigure channel 55 describesdescribes [35, 36 anan], example weexample determined ofof thethe to channelchannel use this structurestructure dedicated consideredconsidered CCH structure inin thisthis in study.thisstudy. study. Figure5 describes an example of the channel structure considered in this study.

SCHSCH SCHSCH SCHSCH SCHSCH CCHCCH SCHSCH SCHSCH SCHSCH SCHSCH

CH#CH# 1 1 CH#CH# 2 2 CH# CH# 3 3 CH# CH# 4 4 CH# CH# 5 5 CH# CH# 6 6 CH# CH# 7 7 CH# CH# 8 8 CH#CH# 9 9

Figure 5. Example of IEEE 802.11p Channel Structure. FigureFigure 5. 5. Example Example of of IEEE IEEE 802. 802.11p11p Channel Channel Structure. Structure.

Appl. Sci. 2018, 8, 2112 7 of 23

V2X systems consist of several components including vehicle-to-vehicle (V2V), vehicle-to- infrastructure (V2I), vehicle-to-device (V2D), etc., and V2V and V2I communications shall be operated in identical frequency bands if the V2X system is supported by a single communication protocol rather than using complex multiple vehicular communication systems. Only the channels in the bands can be diversified for each purpose. Some channels could be defined as CCHs, and others as SCHs. Some SCHs might be used for V2I, and others for V2V or a mixed purpose. As we mentioned earlier, the delay this paper focuses on is a V2I delay in an SCH. More specifically, this paper considers the V2I uplink delay in the SCH channel. Because each vehicle transmits its own frames to a V2X infrastructure STA without any centralized control, the delay and collision they cause could be a great threat against contention-based V2X systems. Although the IEEE 802.11p standard is not a very recent standard, it is verified and being deployed for novel V2X systems. Although the latest research groups, including 3GPP and IEEE, are considering NG-V2X network standards, they are still considering IEEE 802.11p as the baseline technology for NG-V2X systems. Therefore, we consider IEEE 802.11p-based V2X channel access schemes for the NG-V2X systems in this work. The NG-V2X system that we consider shall support backward compatibility to IEEE 802.11p. This means that if the SCH or CCH is used for both conventional IEEE 802.11p and NG-V2X, the existence of conventional IEEE 802.11p STAs needs to be considered and conventional IEEE 802.11p communication should not be adversely affected. This can be interpreted that the channel access scheme we propose needs to be examined under the scenarios of legacy IEEE 802.11 coexistence. In this study, we propose a specific uplink channel access mechanism based on IEEE 802.11p. We consider highly dense scenarios of V2X vehicles in a traffic jam to measure how reliable and scalable the V2X system is in extreme scenarios. To handle the dense scenario, we have chosen to use the trigger frame that is defined in the IEEE 802.11ax standard but not in IEEE 802.11p. Basically, the trigger frame is used for soliciting uplink multiuser transmission by an STA whose address is indicated in the trigger frame. Because the trigger frame needs the target uplink STAs’ addresses, using the trigger frame for soliciting uplink multiuser transmissions in the V2X system that does not have an association procedure for exchanging STA information is not appropriate. Therefore, performing uplink multiuser transmission in the V2X system requires a random-access-based channel access procedure and not dedicated scheduling using the basic trigger frame. Fortunately, a resource unit (RU)-based random-access procedure called UORA procedure is defined in the IEEE 802.11 ax standard as an optional feature. The UORA is a unique channel access scheme for allowing STAs to perform RU-based random access. Figure6 shows the basic concept and operation of the UORA procedure. In this procedure, the AP allows STAs to access some RUs indicated as UORA RUs, which are indicated by a special value of association ID (AID). If the AID of some RUs is written in the special AID value, any STA receiving the UORA trigger frame counts the number of UORA RUs. The number is used for decreasing the STAs’ back-off counter as in the conventional IEEE 802.11 distributed coordinating function back-off procedure. Unlike in the conventional procedure, the back-off counter decreases based on the number of RUs and not based on the number of time slots the STAs have waited. Although the UORA procedure is not defined for IEEE 802.11p and hence cannot be used in the existing IEEE 802.11p-based V2X system, we propose to use the UORA procedure for the NG-V2X system by demonstrating its performance gain by proposing novel enhancing algorithms in this work. Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 24

V2X systems consist of several components including vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), vehicle-to-device (V2D), etc., and V2V and V2I communications shall be operated in identical frequency bands if the V2X system is supported by a single communication protocol rather than using complex multiple vehicular communication systems. Only the channels in the bands can be diversified for each purpose. Some channels could be defined as CCHs, and others as SCHs. Some SCHs might be used for V2I, and others for V2V or a mixed purpose. As we mentioned earlier, the delay this paper focuses on is a V2I delay in an SCH. More specifically, this paper considers the V2I uplink delay in the SCH channel. Because each vehicle transmits its own frames to a V2X infrastructure STA without any centralized control, the delay and collision they cause could be a great threat against contention-based V2X systems. Although the IEEE 802.11p standard is not a very recent standard, it is verified and being deployed for novel V2X systems. Although the latest research groups, including 3GPP and IEEE, are considering NG-V2X network standards, they are still considering IEEE 802.11p as the baseline technology for NG-V2X systems. Therefore, we consider IEEE 802.11p-based V2X channel access schemes for the NG-V2X systems in this work. The NG-V2X system that we consider shall support backward compatibility to IEEE 802.11p. This means that if the SCH or CCH is used for both conventional IEEE 802.11p and NG-V2X, the existence of conventional IEEE 802.11p STAs needs to be considered and conventional IEEE 802.11p communication should not be adversely affected. This can be interpreted that the channel access scheme we propose needs to be examined under the scenarios of legacy IEEE 802.11 coexistence. In this study, we propose a specific uplink channel access mechanism based on IEEE 802.11p. We consider highly dense scenarios of V2X vehicles in a traffic jam to measure how reliable and scalable the V2X system is in extreme scenarios. To handle the dense scenario, we have chosen to use the trigger frame that is defined in the IEEE 802.11ax standard but not in IEEE 802.11p. Basically, the trigger frame is used for soliciting uplink multiuser transmission by an STA whose address is indicated in the trigger frame. Because the trigger frame needs the target uplink STAs’ addresses, using the trigger frame for soliciting uplink multiuser transmissions in the V2X system that does not have an association procedure for exchanging STA information is not appropriate. Therefore, performing uplink multiuser transmission in the V2X system requires a random-access-based channel access procedure and not dedicated scheduling using the basic trigger frame. Fortunately, a resource unit (RU)-based random-access procedure called UORA procedure is defined in the IEEE 802.11 ax standard as an optional feature. The UORA is a unique channel access scheme for allowing STAs to perform RU-based random access. Figure 6 shows the basic concept and operation of the UORA procedure. In this procedure, the AP allows STAs to access some RUs indicated as UORA RUs, which are indicated by a special value of association ID (AID). If the AID of some RUs is written in the special AID value, any STA receiving the UORA trigger frame counts the number of UORA RUs. The number is used for decreasing the STAs’ back-off counter as in the conventional IEEE 802.11 distributed coordinating functionAppl. Sci. 2018back-off, 8, 2112 procedure. Unlike in the conventional procedure, the back-off counter decreases8 of 23 based on the number of RUs and not based on the number of time slots the STAs have waited.

Randomly Randomly chosen chosen RUs by multiple STAs

UORA UORA Trigger null Trigger null Frame M-BA Frame M-BA with STA1 UL DATA with null 3 RUs 3 RUs STA3 UL DATA Collision

STA1 OBO 2 Ye s New OBO 3 Ye s New OBO STA2 OBO 5 No 2 2 Ye s New OBO STA3 OBO 1 Ye s New OBO 6 No 3

Figure 6. Basic concept and operation of uplink OFDMA random access (UORA) procedure. Figure 6. Basic concept and operation of uplink OFDMA random access (UORA) procedure. 3. Proposed Novel V2X Channel Access Schemes The basic model of UORA channel access we considered in previous sections can be applied to a novel V2X channel access scheme. Because UORA channel access can be applied to non-AID STAs, IEEE 802.11p-based V2X STAs that do not have an association procedure to obtain an AID are good targets for applying the UORA channel access procedure. In addition to conventional IEEE 802.11p and UORA channel access that we have considered, we consider a limited association procedure for enhancing the V2X system. Although the legacy V2X system considers communications between vehicles and other STAs, including V2X infrastructure STAs, other vehicles, etc., we can assume that some extra sensor STAs could be installed near a V2X infrastructure STA communicating with the V2X infrastructure STA in the NG-V2X scenario. Sensor STAs may have extremely low or no mobility in contrast to V2X vehicular STAs. This means that the association procedure, which the conventional V2X system does not support due to the high mobility and short link lifetime of V2X STAs, can be useful for these V2X sensor STAs of NG-V2X. The sensor STAs deployed for accommodating traffic information could transmit the information to a V2X infrastructure STA. In many cases, the sensor STAs intend to report the information to certain V2X infrastructure STAs periodically. In this case, using random-access channel access mechanisms including DCF and UORA is very inefficient. To minimize the inefficiency, an association procedure between the V2X infrastructure STA and the sensor STAs for scheduled transmission opportunity needs to be considered. If the V2X system allows association procedures for some special STAs including the sensor STAs, V2X infrastructure STAs can allocate AIDs to the STAs. In this case, the V2X infrastructure STAs can trigger OFDMA transmissions of the special STAs by using a trigger frame with dedicated AIDs the V2X infrastructure STAs have allocated. Figure7 shows the UORA channel access procedure with dedicated AIDs. RUs with dedicated AIDs cannot be accessed by V2X-UORA STAs, and only the special STA having a unique dedicated AID can access the matched RU and transmit its own information. Unless dedicated RUs are used, the special STAs also need to participate in the contention procedure, causing longer system delay and higher packet loss rate. Therefore, to optimize V2X-UORA channel access, dedicated RU-based channel access with the association procedure needs to be considered to be applied. For further optimization, we consider the scheduled NG-V2X system that we propose. The UORA trigger frame we use for NG-V2X is especially useful for soliciting uplink transmissions from multiple STAs. However, if a channel is determined to be used for UORA transmission and time sync is calibrated by other frames transmitted by an ITS AP, a beacon frame or a new indication frame is sufficient and the trigger frame is not required for the UORA channel access procedure. This means that the overhead caused by transmitting trigger frames can be eliminated so that the scheduled NG-V2X could achieve a higher user capacity threshold than the NG-V2X we considered. Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 24

Although the UORA procedure is not defined for IEEE 802.11p and hence cannot be used in the existing IEEE 802.11p-based V2X system, we propose to use the UORA procedure for the NG-V2X system by demonstrating its performance gain by proposing novel enhancing algorithms in this work.

3. Proposed Novel V2X Channel Access Schemes The basic model of UORA channel access we considered in previous sections can be applied to a novel V2X channel access scheme. Because UORA channel access can be applied to non-AID STAs, IEEE 802.11p-based V2X STAs that do not have an association procedure to obtain an AID are good targets for applying the UORA channel access procedure. In addition to conventional IEEE 802.11p and UORA channel access that we have considered, we consider a limited association procedure for enhancing the V2X system. Although the legacy V2X system considers communications between vehicles and other STAs, including V2X infrastructure STAs, other vehicles, etc., we can assume that some extra sensor STAs could be installed near a V2X infrastructure STA communicating with the V2X infrastructure STA in the NG-V2X scenario. Sensor STAs may have extremely low or no mobility in contrast to V2X vehicular STAs. This means that the association procedure, which the conventional V2X system does not support due to the high mobility and short link lifetime of V2X STAs, can be useful for these V2X sensor STAs of NG-V2X. The sensor STAs deployed for accommodating traffic information could transmit the information to a V2X infrastructure STA. In many cases, the sensor STAs intend to report the information to certain V2X infrastructure STAs periodically. In this case, using random-access channel access mechanisms including DCF and UORA is very inefficient. To minimize the inefficiency, an association procedure between the V2X infrastructure STA and the sensor STAs for scheduled transmission opportunity needs to be considered. If the V2X system allows association procedures for some special STAs including the sensor STAs, V2X infrastructure STAs can allocate AIDs to the STAs. In this case, the V2X infrastructure STAs can trigger OFDMA transmissions of the special STAs by using a trigger frame with dedicated AIDs the V2X infrastructure STAs have allocated. Figure 7 shows the UORA channel access procedure with dedicated AIDs. RUs with dedicated AIDs cannot be accessed by V2X-UORA STAs, and only the special STA having a unique dedicated AID can access the matched RU and transmit its own information. Unless dedicated RUs are used, the special STAs also need to participate in the contention procedure, causing longer system delay and higher packet loss rate. Therefore, to optimize V2X-UORA channel access, dedicated RU-based channel access with the association procedure needs to be considered to be Appl. Sci. 2018, 8, 2112 9 of 23 applied.

Randomly Randomly chosen RUs chosen RUs

UORA UORA Trigger Sensor 3 UL DATA Trigger Sensor 5 UL DATA Frame M-BA Frame M-BA with STA1 UL DATA with null 3 RUs 3 RUs STA3 UL DATA STA2 UL DATA

STA1 OBO 2 Ye s New OBO 3 No 1 STA2 OBO 4 No 2 2 Ye s New OBO STA3 OBO 1 Ye s New OBO 6 No 4 FigureFigure 7. 7. V2XV2X UORA UORA channel channel access access procedure procedure wi withth dedicated dedicated resource resource unit(s) unit(s) (RUs). (RUs). 4. System Model For further optimization, we consider the scheduled NG-V2X system that we propose. The UORASome trigger researchers frame we have use for tried NG-V2X to model is especially the UORA useful procedure for soliciting using a uplink Markov transmissions chain model from with multiplesome assumptions STAs. However, for simplified if a channel analysis is determin in the earlyed stageto be ofused IEEE for 802.11ax UORA standardization.transmission and As time the syncIEEE is 802.11ax calibrated standard by other has frames been transmitted developed, by the an specific ITS AP, procedure a beacon of frame UORA or hasa new been indication defined. frameTo modify is sufficient and correct and the the trigger Markov frame model is not and required the analysis for the method, UORA channel Lanante access et al. triedprocedure. to generate This a more generalized model and analysis method [37]. Many researchers have tried to evaluate its performance, especially its efficiency and throughput [38,39]. One of the purposes of this paper is similar to that of those studies on system modeling. Because the research is almost close to the actual IEEE 802.11ax UORA procedure, a small disagreement still exists between the Markov chain model and the actual UORA procedure. Therefore, we tried to enhance the Markov model and the analysis method.

4.1. Markov Chain Model As an example, assume the UORA trigger frame with nine UORA RUs. Because the 20-MHz IEEE 802.11ax trigger frame can have nine RUs at most, this is a reasonable situation. Figure8 shows the Markov chain model of the 9-RU UORA procedure. If the UORA counter of an STA is smaller than the number of UORA RUs in the received trigger frame, the STA can transmit its frame via the UL OFDMA transmission opportunity. If the initial random back-off number the STA obtained is 0 or smaller than the number of UORA RUs in the trigger frame, the STA can transmit its frame just after receiving the trigger frame with UORA RU(s). Based on Figure8, each probability branch can be described as below.

P{0, k|i, l} = (1 − p)/W0 k ∈ (0, W0 − 1) l ∈ (0, 9) i ∈ (0, m) P{i, k|i, k + 9} = 1 k ∈ (0, W − 10) i ∈ (0, m) i (3) P{i, k|i − 1, l) = p/Wi k ∈ (0, Wi − 1) l ∈ (0, 9) i ∈ (0, m) P{m, k|m, l) = p/Wm k ∈ (0, Wm − 1) l ∈ (0, 9)

where m is the required number of retransmissions to reach the maximum UORA CW size. The maximum UORA CW size is called OCWmax and the initial size of UORA CW is called OCWmin. Once the OCW reaches OCWmax for successive retransmission attempts, the OCW, Wi in (3), shall remain at the value of OCWmax, Wm in (3), until the OCW is reset. The probability p means the probability of transmission failure when an STA transmits its frame via the UORA procedure. If we assume that the transmission is always successful unless there are collisions, p can be interpreted as the probability of UORA collision. From the Markov chain model and probability branches, we can derive the stationary distribution of the Markov chain. From the state (0,0), the following can be derived:

(1 − p) m r b0,0 = ∑ ∑ bi,k (4) W0 i=0 k=0 Appl. Sci. 2018, 8, 2112 10 of 23

For each state (0,k) where k > 0, we can derive the following expression.

m r (1−p) b = b + + ∑ ∑ b 0,k 0,k r W0 i,k i=0 k=0 m r (1−p) = b + + 2 ∑ ∑ b 0,k 2r W0 i,k i=0 k=0 (5) j k m r W0−1−k (1−p) = ( r + 1){ W ∑ ∑ bi,k} 0 = j k i 0 k=0 W0−1−k = ( r + 1)b0,0

For 0 < i < m, k > 0, the expression for each bi,k can be derived as below.

r j k r bi−1,k Wi−1−k bi−1,k b = b + + p ∑ = ( + 1)p ∑ i,k i,k r Wi−1 r Wi−1 k=0 k=0 (6) r b r b b = p( ∑ m,k + ∑ m−1,k ) m,k Wm Wm−1 k=0 k=0

For bi,0 with i > 0, p r bi,0 = ∑ bi−1,k (7) Wi−1 k=0 From Equations (5) and (7), the following expression can be derived.

m r j W − −k k 1−p p b = ( 0 1 + 1){ ( ∑ ∑ b )} 0,k r p Wi i,k i=0 k=0 (8) j k − m−1 r = ( W0−1−k + 1)( 1 p )( b + p b ) r p ∑ i+1,0 Wm ∑ m,k i=0 k=0

If we deﬁne the virtual state term bm+1,0 for Equation (7), Equation (8) can be re-expressed as follows. m W0 − 1 − k 1 − p b0,k = ( + 1)( )(∑ bi+1,0) (9) r p i=0 In a similar way, the following equation can be derived from Equations (6) and (7).

W − 1 − k b = ( i + 1)b (10) i,k r i,0

From the steady-state probability rule and Equations (7)–(10), we obtain

m Wi−1 m Wi−1 W0−1 m ∑ ∑ bi,k = ∑ ∑ bi,k + ∑ b0,k + ∑ bi,0 i=0 k=0 i=1 k=1 k=1 i=0 m Wi−1 j k W0−1 j k m Wi−1−k W0−1−k (11) = ∑ ∑ ( r + 1)bi,0 + ∑ ( r + 1)b0,0 + ∑ bi,0 i=1 k=1 k=1 i=0 = 1

If we have the exact values of Wi, we can derive all the states as functions of p from equations derived above. Unlike in distributed coordinating function channel access analysis [40], not only STAs in state bi,0 but also STAs in state bi,k, where k is equal or smaller than r, transmit their frames. Therefore, we can derive τ, the probability of transmission, as follows.

m r τ = ∑ ∑ bi,k (12) i=0 k=0 Appl. Sci. 2018, 8, 2112 11 of 23 Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 24

FromFrom the the transmission transmission probability, probability, we we can can re-express re-express the UORAthe UORA collision collision probability probabilityp as follows.p as follows. n−1 p = 1 − (1 − τ)− (13) p =−1(1) −τ n 1 (13)

BecauseBecause every every steady-state steady-state probabilityprobability can can be be expressed expressed as as a afunction function of ofp, pwe, we can can obtain obtain exact exact steady-statesteady-state probabilities probabilities if if we we havehave speciﬁcspecific Wi,, rr,, and and nn values.values.

(1-p)/W0 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1111 11111 0,10 0,11 0,12 0,13 0,14 0,15 0,16 0,17 0,18

111... 1 0,W0-4 0,W0-3 0,W0-2 0,W0-1

1 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 p/W1 1111 11111 1,10 1,11 1,12 1,13 1,14 1,15 1,16 1,17 1,18

111... 1 1,W1-4 1,W1-3 1,W1-2 1,W1-1 ...

1 m,0 m,1 m,2 m,3 m,4 m,5 m,6 m,7 m,8 m,9 p/Wm 1111 11111 m,10 m,11 m,12 m,13 m,14 m,15 m,16 m,17 m,18

111... 1 m,Wm-4 m,Wm-3 m,Wm-2 m,Wm-1

FigureFigure 8. 8.Markov Markov chainchain model of UORA channel channel access access with with 9 9resource resource units units (RUs). (RUs).

4.2.4.2. Simplified Simplified Markov Markov Chain Chain ModelModel TheThe Markov Markov model model we we havehave analyzedanalyzed in the traditional traditional way way is is slightly slightly complicated, complicated, and and we we can can simplifysimplify the the Markov Markov model. model. AsAs can be observed from from the the Markov Markov chain chain model model shown shown in inFigure Figure 8 8 andand the the equations equations we we have have derived,derived, allall r states have have similar similar stochastic stochastic properties properties except except the the first first r + r1 + 1 statesstates and and some some last last states. states. We We call call the the set set of of states states with with similar similar properties properties as as a a stage. stage. We We try try to to use use this numberthis number of stages of stages as a representative as a representa residualtive residual UORA UORA back-off back-off value. value. i,n WeWe use useB i,nB as as a astate state probabilityprobability of each each stage stage where where i iisis the the number number of ofretransmissions retransmissions as earlier as earlier andandn isn is the the number number of of stages. stages. TheThe stage-basedstage-based Markov Markov model model is is described described in in Figure Figure 9,9 and, and each each state state indicates Bi,n we defined. For each i, the maximum number of stages Ni and Ri, number of states in Bi,n indicates Bi,n we defined. For each i, the maximum number of stages Ni and Ri, number of states in where n = Ni, can be described as follows. Bi,n where n = Ni, can be described as follows. W − 2 i N =+Wi − 2 1 (14) N =i + 1 (14) i r

R ==−W − r(N −− 1) −− 1 (15) iRWrNiii (1 ii 1) (15) m B0,0 = (1 − p)m∑ Bi,0 (16) BpB=−(1 ) i=0 0,0 i ,0 (16) = i i 0 Bi,0 = pBi−1,0 = p B0,0 for 0 < i < m (17) Appl. Sci. 2018, 8, x FOR PEER REVIEW 12 of 24

Appl. Sci. 2018, 8, 2112 12 of 23 = = i BpBpBii,0− 1,0 0,0 for 0 0 (19) Wi i p m =−−+p p B = {(N − n − 1BNnrRB)in,0,0r +{(R } i( 1)B + i }B − ) =where{(N 0− ≤ ni <− m1,) nr +> 0R } (p + 1)B (19)(20) m,n m m W m,0 m 1,0 m m W 0,0 m Wi m From the total probability rule, ppm =−−+m Ni−1 m−1 Ni−1 +Nm−1 =−−+ + Bmn,,01,00,0{( NnrR m 1) m }() B m B m− {( NnrR m 1) m } () pB 1 (20) ∑ ∑ Bi,n = ∑ WW∑ Bi,n + ∑ Bm,n + B0,0 i=0 n=0 i=0 nmm=1 n=1 m−1 i p 2 Ni (21) From the total probability= B0,0{ ∑ rule,W {( Ni − 1) r + (Ni − 1)Ri − 2 r} i=1 i m m pmmNNN−−−111−1 2 Nm p + (piim+ 1){(Nm − 1) r + (Nm − 1)Rm − r} + + 1} = 1 Wm BBBB=++2 1−p iim,n  ,n  ,n 0,0 in==00 in == 01 n = 1 The simpliﬁed model we derived also provides the steady-state distribution as a function of m−1 pi N probability if speciﬁc=−+−− UORABNNRr{{(1)(1)} CW sizes and2 RU size are given.i 0,0  iiir (21) = As in Section 4.1, the probabilityi 1 Wi of transmission can2 be derived as follows.

m m ppm N ++(1){(1)(pNrNR −+−−++=2 1)1 m r } 1}1 mmmτ = B = B (22) Wp∑ i,0 − p 210,0 − m i=0 1

TheFrom simplified Equations model (13), we (21), derived and (22), also we provides can obtain th specifice steady-state values ofdistribution the steady-state as a function distribution of probabilityand the transmission if specific UORA probability CW sizes of the and simplified RU size modelare given. if we have specific system parameters.

r(1-p)/W0 (r+1)(1-p)/W0 1 1 1 1 0,0 0,1 0,2 ... 0,N0-1

r(1-p)/W0

R0(1-p)/W0 ...

(r+1)p/Wi rp/Wi 1-p

1 1 1 i,0 i,1 i,2 ... i,Ni-1 p 1 rp/Wi

Rip/Wi

(r+1)p/Wm rp/Wm

1-p 1 1 1 m,0 m,1 m,2 ... m,Nm-1 1 p

rp/Wm

Rm*p/Wm

Figure 9. Simpliﬁed Markov chain model of UORA channel access. Figure 9. Simplified Markov chain model of UORA channel access. Appl. Sci. 2018, 8, 2112 13 of 23

4.3. Delay Analysis from Simpliﬁed Model Assume that each state in the simpliﬁed Markov chain can be expressed as a random variable of an initial UORA back-off value. This means that if an STA gets a random UORA back-off value, the value can be mapped to an appropriate value of the stage. Simply, let us set a random variable N as the number of UORA trigger intervals for transmission given Rth retransmission. Now, the average overall delay can be described as

E[Toverall] = E[TInitial] + E[Tcontention] (23)

Toverall is the total delay from generating a frame to delivering it or to discarding it after M retransmissions. Tinitial is the waiting time for receiving the first UORA trigger frame after generating a frame. Tcontention is the time delay from receiving the first UORA trigger frame to successive delivery or to Mth retransmission failure. Because the time includes UORA back-off delays and retransmission delays, it is called contention delay in this paper. From the simplified model, we can obtain the probability distribution of the random variable N easily. For a given number of retransmission j,

 r+1 N = 0  Wj j  j k  r Wj−2 W 0 < Nj < r P[Nj] = j (24) W −2  W −r j −1  j r j W −2 k  N = j Wj j r where j ∈ [0, M], r is the number of RUs in the UORA trigger frame, and Wj is the size of the UORA CW for the given retransmission index j. To determine the average delay, we must obtain the average number of stages for each STA.

Wj−2 b r c E[Nj] = ∑ NP[Nj] (25) N=0

The intervals of the UORA trigger frames can be described as a random variable as well. We use a random variable I to represent it and describe the delay as below.

M i E[Toverall] = E[TInitial] + [∑ ∑ E[Tj]P[R = i]] (26) i=0 j=0

M i E[Toverall] = E[TInitial] + [E[I]∑ ∑ E[Nj]P[R = i]] (27) i=0 j=0 where R is the number of retransmissions with the distribution:

P[R = j] = (1 − p)pj j ∈ (0, M − 1) M−1 (28) P[R = M] = 1 − ∑ (1 − p)pk = pM k=0

The probability p is an identical parameter we used in the Markov chain model. Not only can we control p value to model UORA channel contention directly, but we can also set the p value to the value we obtained from Equation (13). If we assume that Wj and r are constant values over every j, E[Nj] will have a constant value N from Equation (24). In this case, Equation (27) can be re-expressed as follows.

M E[Toverall] = E[TInitial] + [E[I]N∑ (i + 1)P[R = i]] (29) i=0 Appl. Sci. 2018, 8, 2112 14 of 23

1 − pM+1 E[T ] = E[T ] + (E[I]N ) (30) overall Initial 1 − p If we assume that the interval I is deterministic and the initial frame generation time is random, Equation (30) can be expressed as

I 1 − pM+1 E[T ] = c + I N (31) overall 2 c 1 − p where Ic is a constant value of the UORA trigger interval. If the interval follows an exponential distribution and the interval I has an average value Ip, then

1 − pM+1 E[T ] = I + I N (32) overall p p 1 − p

The delays shown in Equations (31) and (32) are special cases as we referred. To simplify the equation, we assumed a constant size of CWs along with a constant size of RUs. If we have more complex patterns of CW size, the distribution of Nj will change and hence we need to calculate Equation (27) with appropriate calculation methods. Basically, computational software packages can calculate it easily even though Nj has a complex distribution. As a result, the delay can be calculated by using Equation (27), where (24), (25), and (28) are for the general UORA delay case.

5. Performance Analysis To support a real-world V2X system, a reasonable data arrival rate of V2X vehicular STAs must be considered. In this work, a 10-Hz V2X uplink data frame arrival rate is assumed to analyze V2X channel access performance. In other words, each V2X vehicle generates its data frame to a V2X infrastructure STA every 100 ms. As the V2I uplink delay is the key delay factor of the NG-V2X system, we focus on the channel access delay and packet loss rate (PLR) performance of the proposed V2X-UORA channel access. To demonstrate specific performance enhancement of the proposed V2X-UORA channel access, legacy IEEE 802.11p-based channel access simulation is also performed. The simulation is a system-level simulation and communication range is determined depending on its received signal power instead of its actual distance. Wideband operation can also be performed with a various number of channels and the wideband operation procedure follows IEEE 802.11 wideband operation rules. Conventional IEEE 802.11p system supports 2-band wideband operation by utilizing 2 contiguous 10 MHz channels and we assumed at most 4-band wideband operation can be supported in the next-generation V2X system. Each simulation was performed in a limited traffic generation time and a limited simulation time. Because we set unlimited retransmission tries, packet loss that affects the value of PLR means that the data packet has not been delivered in simulation time after it is generated. Therefore, the PLR is not the main target of our performance comparison but it is a constraint parameter which needs to be 0 value if the number user value is under user capacity threshold. The simulation time was set to five times the traffic generation time. This means that each V2X STA generates uplink data traffic during traffic generation time and then stops generating data frames for four times the traffic generation time. Conventional channel access procedures for transmitting data frames were performed during the whole simulation time in a conventional IEEE 802.11p scenario, and a V2X infrastructure STA transmits an UORA trigger frame every trigger frame interval over the whole simulation time. Simulation parameters we used are listed in Table1. Transmission Opportunity (TXOP) duration and frame arrival rate parameters on the list are calculated based on the corresponding specifications and OCW value for each case are determined by experimental results. Appl. Sci. 2018, 8, x FOR PEER REVIEW 15 of 24 is generated. Therefore, the PLR is not the main target of our performance comparison but it is a constraint parameter which needs to be 0 value if the number user value is under user capacity threshold. The simulation time was set to five times the traffic generation time. This means that each V2X STA generates uplink data traffic during traffic generation time and then stops generating data frames for four times the traffic generation time. Conventional channel access procedures for transmitting data frames were performed during the whole simulation time in a conventional IEEE 802.11p scenario, and a V2X infrastructure STA transmits an UORA trigger frame every trigger frame interval over the whole simulation time. Simulation parameters we used are listed in Table 1. Transmission Opportunity (TXOP) duration and frame arrival rate parameters on the list are calculated based on the corresponding specifications and OCW value for each case are determined by experimental results.

Appl. Sci. 2018, 8, 2112 Table 1. Simulation parameters. 15 of 23 Description Value NV NumberTable 1.ofSimulation V2X Vehicles parameters. X-axis NV/10 (1-band) OCW Contention Descriptionwindow size of UORA NV/20 Value (2-band) NV Number of V2X VehiclesNV/40 X-axis (4-band) TV2X Transmission Opportunity (TXOP) duration of NG V2X transmission NV/10 2880us (1-band) OCW Contention window size of UORA NV/20 (2-band) TS-V2X Transmission Opportunity (TXOP) duration of S-NG V2X transmission 2700us NV/40 (4-band) 720us (1-band) TV2X Transmission Opportunity (TXOP) duration of NG V2X transmission 2880us

TLegacyTS-V2X TransmissionTransmission Opportunity Opportunity (TXOP) duration duration of of S-NG NG-V2X V2X transmission transmission 405us2700us (2-band) 720us225us (1-band) (4-band) TLegacy Transmission Opportunity (TXOP) duration of NG-V2X transmission 405us (2-band) MQueue Size of frame queue in number of frames 1000 frames 225us (4-band) λV2X Frame arrival rate of each V2X vehicle. 10 Hz MQueue Size of frame queue in number of frames 1000 frames NsensorλV2X Frame arrivalNumber rate of of ITS each sensor V2X vehicle. 10 0 or Hz 30 Nsensor Number of ITS sensor 0 or 30 In Figure 10, we can observe the delay and the PLR of the conventional IEEE 802.11p-based channelIn Figureaccess procedure10, we can with observe 80 STAs the delayusing anda single the 10-MHz PLR of thechannel. conventional In Figure IEEE 10a, 802.11p-basedthe delay and thechannel PLR of access a single procedure STA in witha scenario 80 STAs with using 60 V2X a single STAs 10-MHz are shown, channel. whereas In Figure in Figure 10a, 10b, the delaythe delay and andthe PLRthe PLR of a of single a single STA STA in a in scenario a scenario with with 60 V2X 80 V2X STAs STAs are shown,case are whereas shown. inIt can Figure be observed10b, the delay that conventionaland the PLR IEEE of a single 802.11p STA works in a scenariowell in the with 60 80STAs V2X case STAs but case not arein the shown. 80 STAs It can case. be Almost observed all thatthe packetsconventional are delivered IEEE 802.11p in 10 worksms and well no inpacket the 60 loss STAs is observed. case but not Unlike in the in 80 the STAs 60 case.STAs Almostcase, we all can the observepackets arethat delivered the delay in does 10 ms not and have no packeta converge lossnce is observed. value. This Unlike means in thethat 60 conventional STAs case, we IEEE can 802.11pobserve cannot that the support delay does 80 V2X not haveSTAs a with convergence a single value.V2X infrastructure This means that STA conventional in the environment IEEE 802.11p we assumed.cannot support In other 80 words, V2X STAs the user with capacity a single threshol V2X infrastructured is a value STAbetween in the 60 environment and 80 for the we case assumed. of the FigureIn other 10. words, the user capacity threshold is a value between 60 and 80 for the case of the Figure 10.

(a) (b)

FigureFigure 10. Delay andand packet packet loss loss rate rate (PLR) (PLR) of aof single a single V2X V2X STA forSTA conventional for conventional IEEE 802.11p IEEE 802.11p channel channelaccess in access a single in a band single transmission: band transmission: (a) 60 V2X (a) STAs60 V2X scenario; STAs scenario; (b) 80 V2X (b) STAs80 V2X scenario. STAs scenario.

In Figure 11a, the average delay and the PLR performance over the number of users in a single-channel conventional IEEE 802.11 scenario are shown. The X-axis represents the number of users and the delay values are log scaled. We can observe that the case of 80 users makes catastrophic delay and it is not recommended to accommodate over 80 users in the system. V2X-UORA is also simulated in a similar scenario. In a single channel, the UORA trigger frame can allocate nine RUs for uplink OFDMA transmission at most. Therefore, we use UORA trigger frames with nine RUs in the V2X-UORA channel access scenario. Similar to Figure 11a, Figure 11b shows the average delay and the PLR performance of STAs using a single-band transmission. The only difference between Figures 11a and 11b is that Figure 11b depicts the V2X-UORA channel access this paper proposes. Because OCW sizes can affect delay and PLR performance, we set the OCW size to one-tenth the number of users for adaptive OCW control. Appl. Sci. 2018, 8, x FOR PEER REVIEW 16 of 24

In Figure 11a, the average delay and the PLR performance over the number of users in a single-channel conventional IEEE 802.11 scenario are shown. The X-axis represents the number of users and the delay values are log scaled. We can observe that the case of 80 users makes catastrophic delay and it is not recommended to accommodate over 80 users in the system. V2X-UORA is also simulated in a similar scenario. In a single channel, the UORA trigger frame can allocate nine RUs for uplink OFDMA transmission at most. Therefore, we use UORA trigger frames with nine RUs in the V2X-UORA channel access scenario. Similar to Figure 11a, Figure 11b shows the average delay and the PLR performance of STAs using a single-band transmission. The only difference between Figure 11a and Figure 11b is that Figure 11b depicts the V2X-UORA channel access this paper proposes. Because OCW sizes can affect delay and PLR performance, we set the OCW size to one-tenthAppl. Sci. 2018 the, 8 ,number 2112 of users for adaptive OCW control. 16 of 23 Delay (ms)Delay Packet Loss Rate(PLR)

(a) (b)

FigureFigure 11. 11. AverageAverage delay delay and and PLR PLR in in a a sing singlele band band transmission transmission case case (dashed (dashed circle circle is is the the converged converged delaydelay range): range): (a (a) )Conventional Conventional IEEE IEEE 802.11p 802.11p channel channel access access scenario; scenario; (b (b) )Proposed Proposed V2X-UORA V2X-UORA channel channel accessaccess scenario. scenario.

OCWOCW control control can can be be performed performed by by IEEE IEEE 802.11 802.11 beacon beacon frames frames transmitted transmitted by by the the V2X V2X infrastructureinfrastructure STA. STA. Because Because an an IEEE IEEE 802.11p 802.11p V2X V2X STA STA needs needs to to receive receive the the beacon beacon frame frame before before transmittingtransmitting its its frame frame to to the the V2X V2X infrastructure infrastructure STA, STA, receiving receiving the the OCW OCW control control value value included included in in thethe beacon beacon frame frame is is reasonable reasonable and and easy easy to to impl implementement in in NG-V2X NG-V2X systems. systems. After After receiving receiving the the OCW OCW controlcontrol value, value, each each V2X V2X can can generate generate its its own own UORA UORA back-off back-off value value for for UORA UORA channel channel access access based based onon the the received received OCW OCW control control value. value. We We will will examine examine th thee effects effects of of this this OCW OCW size size in in the the last last part part of of thisthis section. section. InIn this this paper, paper, we we will will call call the the largest largest number number of users of users that thathave have convergence convergence value value of delay of delaywith 0with PLR 0as PLR the user as the capacity user capacity threshold. threshold. Therefore, Therefore, we will need wewill to examine need to the examine user capacity the user threshold capacity inthreshold various inscenarios. various As scenarios. can be observed As can be from observed Figure from 11, Figure the proposed 11, the proposed V2X UORA V2X enhances UORA enhances its user capacityits user capacitythreshold threshold performance performance slightly. slightly.However, However, the specific the specificvalue of value performance of performance gain could gain vary,could depending vary, depending on the onspecific the specific implementation implementation method. method. In some In somebad implementation bad implementation methods, methods, the overheadthe overhead of V2X-UORA of V2X-UORA may may degrade degrade the the performanc performancee gain. gain. In Insome some bad bad implementation implementation scenarios, scenarios, itit is is possible possible that that the the V2X-UORA V2X-UORA channel channel access access has has a a smaller smaller user user capacity capacity than than the the conventional conventional channelchannel access access case. case. An An important important thing thing we we can can observ observee from from Figure Figure 11 is11 that is that the delay the delay value value in the in convergedthe converged delay delay range range (dashed (dashed circ circles)les) has has a relatively a relatively averaged averaged delay delay value value rather rather than than having having fluctuatingfluctuating values values of of delay, delay, unlike unlike the the conventional conventional IEEE IEEE 802.11p 802.11p case. FigureFigure 12 12 shows shows the the average average delay delay and and the the PLR PLR in in a a two-band two-band transmission transmission scenario. scenario. Figure Figure 12a 12a showsshows a a conventional conventional IEEE IEEE 802.11p 802.11p channel channel access access scenario scenario simulated simulated and and the the proposed proposed V2X-UORA V2X-UORA channelchannel access access applied applied is is shown shown in in Figure Figure 12b.12b. Because Because a a larger larger band band allows allows small small transmission transmission opportunityopportunity (TXOP) (TXOP) duration duration in in the the conventional conventional IE IEEEEE 802.11 802.11 case, case, the the user user capacity capacity threshold threshold is is improvedimproved compared compared withwith thethe singlesingle band band case. case. Unfortunately, Unfortunately, V2X-UORA V2X-UORA channel channel access access cannot cannot take takeadvantage advantage of the of reducedthe reduced TXOP TXOP duration duration from from the expanded the expanded bandwidth. bandwidth. Instead Instead of the of reduced the reduced TXOP, TXOP,V2X-UORA V2X-UORA channel channel access access can allocate can allocate more RUs,more owingRUs, owing to wideband to wideband operation. operation. According According to the latest version of the IEEE 802.11ax standard, the trigger frame can allocate 18 UL OFDMA RUs at most. For a realistic simulation, 18 RUs are used in the two-band transmission case of the V2X-UORA channel access. From Figure 12a,b, we can observe that the V2X-UORA channel access is much more scalable although it requires a slightly larger delay in the case of a small number of STAs. This means that the V2X-UORA channel access shows a much better user capacity threshold property compared to the conventional channel access case in the two-band transmission case. If the V2X system requires a quality of service (QoS) delay threshold less than 10 ms, the V2X-UORA channel access could meet the QoS requirement with the averaged delay and it uses the extended bandwidth more efficiently than the conventional wideband channel access case. Appl. Sci. 2018, 8, x FOR PEER REVIEW 17 of 24 to the latest version of the IEEE 802.11ax standard, the trigger frame can allocate 18 UL OFDMA RUs at most. For a realistic simulation, 18 RUs are used in the two-band transmission case of the V2X-UORA channel access. From Figure 12a,b, we can observe that the V2X-UORA channel access is much more scalable although it requires a slightly larger delay in the case of a small number of STAs. This means that the V2X-UORA channel access shows a much better user capacity threshold property compared to the conventional channel access case in the two-band transmission case. If the V2X system requires a quality of service (QoS) delay threshold less than 10 ms, the V2X-UORA channel access could meet the QoS requirement with the averaged delay and it uses the extended bandwidthAppl. Sci. 2018 more, 8, 2112 efficiently than the conventional wideband channel access case. 17 of 23

(a) (b)

FigureFigure 12. 12. AverageAverage delay delay and and PLR PLR in in two two bands bands transmission transmission case: case: (a (a) )Conventional Conventional IEEE IEEE 802.11p 802.11p channelchannel access access scenario; scenario; (b (b) )Proposed Proposed V2X-UORA V2X-UORA channel channel access access scenario. scenario.

ToTo show thethe tendency tendency of of wideband wideband operation operation more more obviously, obviously, the four-bands the four-bands case is also case examined. is also examined.Because four-band Because four-band transmission transmission is commonly is commo appliednly in applied recent IEEE in recent 802.11 IEEE systems 802.11 and systems eight-band and eight-bandtransmission transmission is usually implemented is usually implemented by 4 bands + 4by bands 4 bands transmission + 4 bands structure, transmission examining structure, up to 4 examiningbands wideband up to 4 operation bands wideband is reasonable. operation Figure is 13reasonable.a shows the Figure overall 13a average shows delaythe overall and the average PLR of delaythe conventional and the PLR IEEE of 802.11the conventional DCF procedure IEEE with 802.1 a single,1 DCF two,procedure and four with bands a single, cases. Thistwo, showsand four that bandsthe wideband cases. This procedure shows that of thethe conventionalwideband proced channelure of access the conventional procedure can channel result access in a shorter procedure delay canwhen result the in number a shorter of usersdelay is when small, the but number the user of capacityusers is small, threshold but doesthe user not capacity increase threshold linearly despite does notthe increase number linearly of bands despite increasing the number linearly. of Figure bands 13 increasingb shows the linearly. overall Figure average 13b delay shows and the the overall PLR of averagethe proposed delay and V2X-UORA the PLR channelof the proposed access procedure V2X-UORA with channel a single, access two, procedure and four bandswith a cases. single, Unlike two, andFigure four 13 a,bands Figure cases. 13b showsUnlike that Figure using 13a, wideband Figure transmission13b shows that enhances using the wideband user capacity transmission threshold enhancesperformance the user more capacity efficiently thre althoughshold performance it does not enhance more efficiently delay performance although it and does it only not meetsenhance the delayQoS delayperformance requirement. and it only meets the QoS delay requirement. In addition to the V2X-UORA performance analysis, we need to consider the extra optimization method we mentioned in the previous section. We have assumed that NG-V2X systems can have an association procedure for some special STAs including sensor STAs deployed to gather traffic information near a V2X infrastructure STA. In this case, dedicated RUs can be allocated in the UORA trigger frame for the STAs. Because of the dedicated RUs, the number of UORA RUs decreases but the dedicated RU is always occupied by the indicated STA if there are no frame errors. This means that using the dedicated RUs increases RU utilization and hence can enhance the overall delay performance as well. Figure 14a shows that the delay and the PLR performance of the proposed V2X with and without dedicated RUs. We assumed 30 sensor STAs and the X-axis represents the extra number of V2X STAs that excludes the 30 sensor STAs. Each STA generates its data packet including traffic information every 100 ms. Figure 14b shows how the dedicated RU with an association procedure enhances the user capacity threshold performance. Unless the dedicated RU is used, the 30 sensors are also considered as contention STAs. However, replacing only a single UORA RU in the UORA trigger frame to a dedicated RU makes the sensor STAs not participate in the UORA contention procedure. As the number of sensor STAs increases, the dedicated RU method becomes more effective.

To examine the performance of the scheduled(a) NG-V2X channel access, we compared the performance with that of legacy channel access, high-priority EDCA (HP-EDCA) and (nonscheduled) NG-V2X in Figure 15. HP-EDCA is the IEEE 802.11p channel access procedure with high priority channel access parameters (AC_VO). Some studies have shown that applying the highest priority access category (AC_VO) to IEEE 802.11p channel access procedure improves its channel access efﬁciency through their analysis and simulation results [41,42]. As can be observed, scheduled NG-V2X shows higher user capacity performance than legacy channel access, HP-EDCA and NG-V2X cases we have stated. Although scheduled NG-V2X not only requires a more preliminary procedure including timing Appl. Sci. 2018, 8, x FOR PEER REVIEW 17 of 24 to the latest version of the IEEE 802.11ax standard, the trigger frame can allocate 18 UL OFDMA RUs at most. For a realistic simulation, 18 RUs are used in the two-band transmission case of the V2X-UORA channel access. From Figure 12a,b, we can observe that the V2X-UORA channel access is much more scalable although it requires a slightly larger delay in the case of a small number of STAs. This means that the V2X-UORA channel access shows a much better user capacity threshold property compared to the conventional channel access case in the two-band transmission case. If the V2X system requires a quality of service (QoS) delay threshold less than 10 ms, the V2X-UORA channel access could meet the QoS requirement with the averaged delay and it uses the extended bandwidth more efficiently than the conventional wideband channel access case.

(a) (b)

Figure 12. Average delay and PLR in two bands transmission case: (a) Conventional IEEE 802.11p channel access scenario; (b) Proposed V2X-UORA channel access scenario.

To show the tendency of wideband operation more obviously, the four-bands case is also examined. Because four-band transmission is commonly applied in recent IEEE 802.11 systems and eight-band transmission is usually implemented by 4 bands + 4 bands transmission structure, examining up to 4 bands wideband operation is reasonable. Figure 13a shows the overall average delay and the PLR of the conventional IEEE 802.11 DCF procedure with a single, two, and four bands cases. This shows that the wideband procedure of the conventional channel access procedure can result in a shorter delay when the number of users is small, but the user capacity threshold does not increase linearly despite the number of bands increasing linearly. Figure 13b shows the overall averageAppl.Appl. Sci. Sci. 2018delay2018, ,88, ,xand 2112 FOR the PEER PLR REVIEW of the proposed V2X-UORA channel access procedure with a single,1818 two,of of 24 23 and four bands cases. Unlike Figure 13a, Figure 13b shows that using wideband transmission enhances the user capacity threshold performance more efficiently although it does not enhance delaysynchronization performance and and RU it only indication meets the but QoS is also delay less requirement. adaptive than nonscheduled NG-V2X, it shows attractive user capacity performance gains.

(b)

Appl. Sci.Figure 2018 ,13. 8, x Overall FOR PEER average REVIEW delay and PLR: (a) Conventional IEEE 802.11p channel access scenarios;18 of 24

(b) Proposed V2X-UORA channel access scenarios.(a)

In addition to the V2X-UORA performance analysis, we need to consider the extra optimization method we mentioned in the previous section. We have assumed that NG-V2X systems can have an association procedure for some special STAs including sensor STAs deployed to gather traffic information near a V2X infrastructure STA. In this case, dedicated RUs can be allocated in the UORA trigger frame for the STAs. Because of the dedicated RUs, the number of UORA RUs decreases but the dedicated RU is always occupied by the indicated STA if there are no frame errors. This means that using the dedicated RUs increases RU utilization and hence can enhance the overall delay performance as well. Figure 14a shows that the delay and the PLR performance of the proposed V2X with and without dedicated RUs. We assumed 30 sensor STAs and the X-axis represents the extra number of V2X STAs that excludes the 30 sensor STAs. Each STA generates its data packet including traffic information every 100 ms. Figure 14b shows how the dedicated RU with an association procedure enhances the user capacity threshold performance. Unless the dedicated RU is used, the 30 sensors are also considered as contention STAs. However, replacing only a single UORA RU in the UORA trigger frame to a dedicated RU makes the sensor STAs not participate in the UORA (b) contention procedure. As the number of sensor STAs increases, the dedicated RU method becomes moreFigure Figureeffective. 13. 13. Overall Overall average average delay delay and and PLR: PLR: (a (a) )Conventional Conventional IEEE IEEE 802.11p 802.11p channel channel access access scenarios; scenarios; (b(b) )Proposed Proposed V2X-UORA V2X-UORA channel channel access access scenarios. scenarios.

In addition to the V2X-UORA performance analysis, we need to consider the extra optimization method we mentioned in the previous section. We have assumed that NG-V2X systems can have an association procedure for some special STAs including sensor STAs deployed to gather traffic information near a V2X infrastructure STA. In this case, dedicated RUs can be allocated in the UORA trigger frame for the STAs. Because of the dedicated RUs, the number of UORA RUs decreases but the dedicated RU is always occupied by the indicated STA if there are no frame errors. This means that using the dedicated RUs increases RU utilization and hence can enhance the overall delay performance as well. Figure 14a shows that the delay and the PLR performance of the proposed V2X with and without dedicated RUs. We assumed 30 sensor STAs and the X-axis represents the extra number of V2X STAs that excludes the 30 sensor STAs. Each STA generates its data packet including traffic information every(a) 100 ms. Figure 14b shows how the dedicated( bRU) with an association procedure enhances the user capacity threshold performance. Unless the dedicated RU is used, the FigureFigure 14. 14. AverageAverage delay delay and and PLR PLR in in a a single single band band transmission transmission with with 30 30 sensor sensor STAs: STAs: (a (a) )Conventional Conventional 30 sensors are also considered as contention STAs. However, replacing only a single UORA RU in IEEEIEEE 802.11p 802.11p channel channel access access scenario; scenario; (b (b) )Proposed Proposed V2X-UORA V2X-UORA channel channel access access scenario scenario with with the the the UORA trigger frame to a dedicated RU makes the sensor STAs not participate in the UORA dedicateddedicated RU RU method. method. contention procedure. As the number of sensor STAs increases, the dedicated RU method becomes more effective.

(a) (b)

Figure 14. Average delay and PLR in a single band transmission with 30 sensor STAs: (a) Conventional IEEE 802.11p channel access scenario; (b) Proposed V2X-UORA channel access scenario with the dedicated RU method. Appl. Sci. 2018, 8, x FOR PEER REVIEW 19 of 24

To examine the performance of the scheduled NG-V2X channel access, we compared the performance with that of legacy channel access, high-priority EDCA (HP-EDCA) and (nonscheduled) NG-V2X in Figure 15. HP-EDCA is the IEEE 802.11p channel access procedure with high priority channel access parameters (AC_VO). Some studies have shown that applying the highest priority access category (AC_VO) to IEEE 802.11p channel access procedure improves its channel access efficiency through their analysis and simulation results [41,42]. As can be observed, scheduled NG-V2X shows higher user capacity performance than legacy channel access, HP-EDCA and NG-V2X cases we have stated. Although scheduled NG-V2X not only requires a more preliminary procedure including timing synchronization and RU indication but is also less adaptive than Appl. Sci. 2018, 8, 2112 19 of 23 nonscheduled NG-V2X, it shows attractive user capacity performance gains.

FigureFigure 15. AverageAverage delay and and PLR PLR comparison comparison for for lega legacy,cy, High Priority EDCA, EDCA, proposed proposed NG-V2X, NG-V2X, proposedproposed Scheduled NG-V2X channel access.

SoSo far, performance comparison results are fo focusedcused on their performance comparison under under thethe error-free error-free situation situation for for fair fair an andd easy comparison. However, However, in in real real world, there there exist fading and shadowingshadowing in in wireless wireless channel channel and and system system performa performancence can can be be degraded degraded by by fading and shadowing leadingleading to to a a smaller smaller coverage coverage range range than than 1000 1000 m. m. In In order order to to examine examine the the effect effect of of fading fading and and shadowing shadowing inin wireless channel,channel, additionaladditional simulations simulations with with fading fading and and shadowing shadowing for for 250 250 m coveragem coverage range range and and1000 1000 m coverage m coverage range range are performed.are performed. In the In simulation,the simulation, more more realistic realistic simulation simulation environment environment are areconsidered. considered. Unlike Unlike the previousthe previous results, results, which which are with are with an ideal an ideal channel channel environment environment scenario scenario using usingthe ITU-R the ITU-R channel channel model inmodel Urban in Macro Urban (UMa) Macro scenario, (UMa) whichscenario, only which considers only LOS considers path loss LOS without path lossany considerationwithout any onconsideration shadowing and on non-LOSshadowing (NLOS) and path non-LOS loss, additional (NLOS) simulationspath loss, employadditional the simulationsUMa channel employ model the with UMa shadowing channel andmodel NLOS with transmission shadowing and probabilities NLOS transmission per distance probabilities considering peractual distance environment considering [43]. Channel actual modelenvironment parameters [43]. used Channel in the simulationsmodel parameters are listed inused Table in2 andthe simulationsthe result is are shown listed in Figurein Table 16 2. Theand speciﬁcthe result channel is shown model in Figure parameters 16. The in Tablespecific2 are channel based onmodel the parametersrecent channel in Table model 2 document are based of on IEEE the 802.11recent working channel groupmodel [ 44document]. of IEEE 802.11 working group [44]. Table 2. Channel model parameters for simulation.

Index Channel ModelTable 2. Channel Scenario model parameters Vehicles for (STAs) simulation. Distribution Radius Shadowing ITU-R IndexIdeal Channel Model Line Scenarioof sight (LOS) only Vehicles 1000(STAs) m (Uniformly)Distribution Radius Shadowing No (UMa) ITU-R ITU-R IdealCase 1 LineLOS of and sight non-LOS (LOS) (NLOS)only 10001000 m (Uniformly)(Uniformly) 3dB No (UMa)(UMa) ITU-RITU-R CaseCase 1 2 LOS andLOS non-LOS and NLOS (NLOS) 1000 250 mm (Uniformly)(Uniformly) 3dB3dB (UMa)(UMa) ITU-R Case 2 LOS and NLOS 250 m (Uniformly) 3dB As shown(UMa) inFigure 16, the performance of the legacy channel access procedure is degraded dramatically for case 1. It means that 1000 m, which is an ideal coverage range of IEEE 802.11p-based system, may not be achievable in real world. Although the proposed NG-V2X shows much better performance thanks to its narrow signal bandwidth, the performance also drops signiﬁcantly for case 1. For case 2, 250 m coverage range case, legacy channel access procedure shows the user capacity threshold performance that is relatively close to the ideal case performance, even though the delay increases. The proposed NG-V2X shows almost the same performance as the ideal case for case 2. It means that not only the proposed NG-V2X shows better user capacity as mentioned in the previous simulation results, but also it provides better coverage range performance for the next-generation V2X systems. In addition to performance comparisons among channel access schemes we have proposed, we also performed the delay and PLR performance simulation under various OCW scenarios. Basically, Appl. Sci. 2018, 8, 2112 20 of 23

it is easily expected that a too small OCW may cause frequent collisions and a too large OCW may cause unnecessary channel access delay. To check the exact influence of the OCW, we performed V2X-UORA channel access simulation multiple times with different OCW values and a fixed number of users. Figure 17 shows the simulation results in the case of two-band transmission. In Figure 17a,b, we cannot observe an exact tendency unlike in the case with various number of STAs that we have analyzed. However, it is obvious that there are some sweet spot ranges for a given number of STAs, as in Figure 17a. Therefore, if the OCW value makes the delay converged without packet loss for a given number of STAs, obtaining a specific optimal value of the OCW is not a critical issue because the sweet spot range that meets QoS requirements is wide enough. Thus, it can be interpreted as the controlling OCW not requiring extremely sensitive and immediately adaptive algorithms. Based on sensing results of sensor nodes and incoming data packets, a V2X infrastructure STA can control the Appl. Sci. 2018, 8, x FOR PEER REVIEW 20 of 24 OCW value with a relatively long-term decision-making algorithm.

Appl. Sci.Figure 2018, 16.8, x FORAverage PEER delay REVIEW and PLR comparison between legacy and proposed NG-V2X for various21 of 24 channelFigure cases 16. (idealAverage channel delay case, and casePLR 1comparison and case 2). between legacy and proposed NG-V2X for various channel cases (ideal channel case, case 1 and case 2).

As shown in Figure 16, the performance of the legacy channel access procedure is degraded dramatically for case 1. It means that 1000 m, which is an ideal coverage range of IEEE 802.11p-based system, may not be achievable in real world. Although the proposed NG-V2X shows much better performance thanks to its narrow signal bandwidth, the performance also drops significantly for case 1. For case 2, 250 m coverage range case, legacy channel access procedure shows the user capacity threshold performance that is relatively close to the ideal case performance, even though the delay increases. The proposed NG-V2X shows almost the same performance as the ideal case for case 2. It means that not only the proposed NG-V2X shows better user capacity as mentioned in the previous simulation results, but also it provides better coverage range performance for the next-generation(a) V2X systems. (b) In addition to performance comparisons among channel access schemes we have proposed, we FigureFigure 17. 17. AverageAverage delay delay and and PLR PLR in in two two bands bands transmis transmissionsion over over UORA UORA congestion congestion window window (CW) (CW) also performed the delay and PLR performance simulation under various OCW scenarios. Basically, value:value: (a (a) )Below Below user user capacity capacity threshold threshold (190 (190 STAs); STAs); ( (bb)) Below Below user user capacity capacity threshold threshold (200 (200 STAs). STAs). it is easily expected that a too small OCW may cause frequent collisions and a too large OCW may 6. Conclusionscause unnecessary channel access delay. To check the exact influence of the OCW, we performed V2X-UORA channel access simulation multiple times with different OCW values and a fixed numberWe presented of users. the Figure analysis 17 showsof UORA the channelsimulation access results and inthe the performance case of two-band enhancement transmission. of the In proposedFigure V2X-UORA17a,b, we cannot channel observe access an procedure exact tendency to support unlike IEEE in the 802.11p-ba case withsed various NG-V2X number systems. of STAs Unlikethat conventional we have analyzed. IEEE 802.11p However, channel it is access,obvious the th proposedat there are V2X-UORA some sweet channel spot accessranges does for a not given reducenumber the averageof STAs, channel as in Figure access 17a. delay Therefore, when mo if there channelOCW value bandwidth makes theis used. delay This converged means withoutthat usingpacket conventional loss for aIEEE given 802.11p number channel of STAs, access obtaining is still a gooda specific option optimal if a V2X value application of the OCW requires is not a a muchcritical lower issue delay because in a sparse the sweet STAs spscenario.ot range Howe that meetsver, the QoS proposed requirements V2X-UORA is wide exhibits enough. better Thus, user it can capacitybe interpreted performance as the in termscontrolling of meeting OCW the not QoS requir delaying requirementextremely sensitive in wideband and immediately transmission adaptive and densealgorithms. STAs scenarios. Based on This sensing means results that ofthe sensor proposed node V2Xs and UORA incoming is a datamore packets, suitable a V2Xchannel infrastructure access methodSTA canfor densecontrol V2X the OCWSTAs scenarios.value with Furtherm a relativelyore, long-term we examined decision-making how dedicated algorithm. RU-based V2X UORA channel access considerably enhances V2X user capacity performance. If the IEEE 802.11p-based NG-V2X system considers sensor STAs gathering traffic information, the proposed V2X UORA with dedicated RUs need to be considered. As we observed in the analysis of the UORA procedure, the UORA contention window can affect UORA channel access performance. Fortunately, we could observe that controlling OCW can be performed with a long-term decision-making algorithm because the performance does not change dramatically if the UORA CW value lies in the range of the UORA CW sweet spot. Because all proposed methods in this paper are designed under the consideration of backward compatibility and real implementation, they can be expected to be considered as an option in the NG-V2X channel access procedure.

Funding: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning, grant number NRF-2017R1A2B4003987.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Kenney, J.B. Dedicated Short-Range Communications (DSRC) Standards in the United States. Proc. IEEE 2011, 99, 1162–1182, doi:10.1109/JPROC.2011.2132790. 2. ETSI. ETSI TR 101 607 (v1.1.1), Intelligent Transport Systems (ITS), Cooperative ITS (C-ITS), Release 1; ETSI: Sophia Antipolis, Valbonne, France, 2013. 3. 3GPP. 3GPP TS 36.300: Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN), Overall Description Stage 2; 3GPP: Sophia Antipolis, Valbonne, France, 2018. Appl. Sci. 2018, 8, 2112 21 of 23

6. Conclusions We presented the analysis of UORA channel access and the performance enhancement of the proposed V2X-UORA channel access procedure to support IEEE 802.11p-based NG-V2X systems. Unlike conventional IEEE 802.11p channel access, the proposed V2X-UORA channel access does not reduce the average channel access delay when more channel bandwidth is used. This means that using conventional IEEE 802.11p channel access is still a good option if a V2X application requires a much lower delay in a sparse STAs scenario. However, the proposed V2X-UORA exhibits better user capacity performance in terms of meeting the QoS delay requirement in wideband transmission and dense STAs scenarios. This means that the proposed V2X UORA is a more suitable channel access method for dense V2X STAs scenarios. Furthermore, we examined how dedicated RU-based V2X UORA channel access considerably enhances V2X user capacity performance. If the IEEE 802.11p-based NG-V2X system considers sensor STAs gathering trafﬁc information, the proposed V2X UORA with dedicated RUs need to be considered. As we observed in the analysis of the UORA procedure, the UORA contention window can affect UORA channel access performance. Fortunately, we could observe that controlling OCW can be performed with a long-term decision-making algorithm because the performance does not change dramatically if the UORA CW value lies in the range of the UORA CW sweet spot. Because all proposed methods in this paper are designed under the consideration of backward compatibility and real implementation, they can be expected to be considered as an option in the NG-V2X channel access procedure.

Author Contributions: Conceptualization, J.A.; Data curation, J.A.; Formal analysis, J.A.; Funding acquisition, R.Y.K.; Investigation, J.A. and R.Y.K.; Methodology, J.A. and R.Y.K.; Project administration, R.Y.K.; Resources, R.Y.K.; Software, J.A.; Supervision, Y.Y.K. and R.Y.K.; Validation, Y.Y.K. and R.Y.K.; Writing—original draft, J.A.; Writing—review & editing, Y.Y.K. and R.Y.K. Funding: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning, grant number NRF-2017R1A2B4003987. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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