applied sciences

Article An Effective Wide- Channel Access in Next-Generation WLAN-Based V2X Communications

Hanseul Hong 1 , Young Yong Kim 1, Ronny Yongho Kim 2 and Woojin Ahn 3,*

1 School of Electrical & Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea; [email protected] (H.H.); [email protected] (Y.Y.K.) 2 Department of Railroad Electrical & Electronic Engineering, Korea National University of Transportation, 157 Cheoldobakmulgwan-ro, Uiwang-si, Gyeonggi-do 16106, Korea; [email protected] 3 Korea Railroad Research Institute, 176, Cheoldobakmulgwan-ro, Uiwang-si, Gyeonggi-do 16106, Korea * Correspondence: [email protected]; Tel.: +82-31-460-5784

 Received: 18 October 2019; Accepted: 23 December 2019; Published: 27 December 2019 

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Abstract: As Intelligent Transport System (ITS) applications are diversified and amount of ITS data increases, high throughput and reliability are required in next-generation V2X communications. In order to meet such increased throughput and reliability requirements, IEEE 802.11bd, the next-generation V2X communication standard, has commenced standard development. One of the main features of IEEE 802.11bd is a 20-MHz bandwidth transmission. In this paper, a novel wide-bandwidth channel access scheme in next-generation (WLAN)-based vehicular communications is proposed. The proposed scheme is designed to provide fairness with other ITS devices and channel efficiency considering adjacent channel interference. By using the proposed scheme, through extensive simulations, it is verified that, while satisfying the fairness requirement with other ITS devices, the channel access delay of wide-bandwidth packet transmission can be optimized.

Keywords: WLAN; next generation V2X; channel access; wide-bandwidth transmission

1. Introduction Intelligent Transport Systems (ITSs) have been developed and widely deployed with Vehicle-to-Everything (V2X) communication. Vehicles equipped with various sensors (e.g., radar, Lidar, and cameras) combined with Vehicle-to-Vehicle (V2V) communication to exchange sensed data enable automatic driving and platooning. Vehicle-to-Pedestrian (V2P) communication prevents traffic accidents by providing warning signals. In addition, various V2X applications enable driving convenience, road safety, traffic efficiency, road management, and infotainment. As various applications using V2X communication are developed, the required throughput and reliability are diversified. Moreover, as more onboard sensors are equipped in each vehicle, the communication requirements of throughput and reliability have increased. At the time of this writing, the most widely deployed V2X communication system is Dedicated Short Range Communication (DSRC) using a set of Wireless Access in Vehicular Environments (WAVE) standards (Institute of Electrical and Electronics Engineers (IEEE) 1609 [1–3] and IEEE 802.11p [4]). Worldwide research projects and tests have proven that DSRC is a stable and efficient V2X communication system. Whereas DSRC is sufficient to provide basic and some advanced ITS services, more advanced future ITS services such as automated driving require more enhanced wireless

Appl. Sci. 2020, 10, 222; doi:10.3390/app10010222 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 222 2 of 18 communication performance. Two important requirements in next-generation V2X communication systems are throughput and delay reduction. In order to meet the requirements of next-generation V2X communication systems, the IEEE 802.11bd Task Group (TG) commenced V2X communication standardization in January 2019. IEEE 802.11bd’s objective is to improve the throughput, transmission range, and positioning performance, preserving backward compatibility and fairness with conventional Outside Context of a Basic Service Set (OCB) devices [5–7]. One of the proposed features to improve throughput is wideband transmission using a 20-MHz channel for large data transmission. A 20-MHz channel can double the throughput performance compared to legacy DSRC systems using 10-MHz channels. When the channel extension approach of IEEE 802.11n/ac is applied to 20-MHz transmission in V2X communication, the adjacent channel interference problem should be carefully considered. However, even though the channel access procedure is able to avoid adjacent channel interference problem in 20-MHz transmission, the channel extension approach of IEEE 802.11n/ac still cannot be applied in the V2X OCB environment owing to fairness concerns with conventional OCB devices and 10-MHz transmissions conducted by IEEE 802.11bd devices. When the channel load levels of two 10-MHz channels are not equal, the conventional channel extension scheme has higher channel access priority in the extended channel than the conventional OCB devices’ 10MHz channel access because the conventional channel extension scheme has no back-off procedure on the extended channel. One of the most important requirements that IEEE 802.11bd must meet is fairness with legacy devices deployed with IEEE 802.11p devices. Since the IEEE 802.11p OCB configuration does not require association with Access Points (APs), there are no primary or secondary channels as in the Basic Service Set (BSS) of other IEEE 802.11 networks such as IEEE 802.11n/ac. In addition, each channel is used independently for its own purpose with equal importance. Therefore, the fairness issue in V2X environments is more severe than in other IEEE 802.11 networks. There are prior researches to provide enhanced channel access mechanisms considering throughput and fairness under various IEEE 802.11 environments. Authors in reference [8] addresses unfairness caused by the wide-bandwidth transmission of IEEE 802.11ac in case of overlapping channels between neighboring APs. It provides channel allocation and methods to minimize the interference and enhance the fairness and network throughput considering different channel bonding levels. On the other hand, wireless mesh network of IEEE 802.11s is assumed in reference [9], where the authors propose an implementation method to provide proportional fairness without non-linear and non-concave optimization. However, these works assume static environments where APs or stations rarely move. Since there is no concept of AP in WLAN V2X scenario and all vehicles may change their positions dynamically, channel access methods considering static graphs are not suitable. The fair channel access scheme in WLAN V2X environment is considered in reference [10]. It compromises throughput and fairness considering multi-rate and multi-channel operation defined in IEEE 802.11p and IEEE 1609.4, respectively. It includes the grouping method of service channels (SCHs) regarding the data rate and the distance of transmission. However, in a real V2X environment, since WLAN V2X channels are pre-allocated and cannot be dynamically changed over time, throughput and fairness should be controlled with channel access schemes. Furthermore, 20-MHz transmission operation in the next generation V2X is not considered. In contrast to the related works, in this paper, the proposed channel access scheme jointly optimizes fairness and throughput performance for 20-MHz transmission under realistic WLAN V2X environment. In this paper, a novel and effective wide-bandwidth channel access scheme in next-generation V2X communication systems is proposed. The proposed scheme is carefully designed to provide both enhanced fairness with other stations and reduced channel access delays. The proposed scheme is designed to alleviate interchannel interference problems that exist in OCB environments. The proposed architecture is able to effectively prevent a hidden problem in the extended channel with the detection capability of exact channel usage time. The performance of the proposed scheme is analyzed in order to show the enhanced fairness. In addition to the proposed scheme, alternative channel-access Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 18 scheme, the proposed scheme and other variant wideband channel access schemes are compared by computer simulations. Through extensive simulations, the best performing scheme is selected. The rest of the paper is composed as follows. Section 2 presents background on conventional WLAN V2X communications and channel access schemes, Section 3 describes the proposed channel accessAppl. Sci. scheme2020, 10, 222 and alternative channel access schemes for the wide bandwidth of 20-MHz3 of 18 transmission in next-generation V2X communication systems, Section 4 provides extensive analysis of the fairness and delay performance of the proposed scheme, Section 5 evaluates the performance methods are given in the case of hardware limitations. In order to design the best scheme, the proposed of the proposed scheme, Section 6 discusses the simulation results, and Section 7 concludes the paper. scheme and other variant wideband channel access schemes are compared by computer simulations. Through extensive simulations, the best performing scheme is selected. 2. Background The rest of the paper is composed as follows. Section2 presents background on conventional 2.1.WLAN Conventional V2X communications V2X Communication and channel Scheme access in 5.9 schemes,GHz Band Section 3 describes the proposed channel access scheme and alternative channel access schemes for the wide bandwidth of 20-MHz transmission in next-generationThe most widely V2X deployed communication V2X communication systems, Section scheme4 provides is DSRC, extensive which employs analysis a of set the of fairness WAVE staandndards delay performanceincluding the of IEEE the proposed1609 series scheme, and IEEE Section 802.11p,5 evaluates as described the performance in [11]. The of themost proposed widely usedscheme, V2X Section frequency6 discusses band used the simulationfor V2X communication results, and Section is the 5.97 concludes GHz band, the which paper. is a licensed band. As described in reference [12], the Federal Communications Commission (FCC) in the U.S. initially allocated2. Background seven channels in the 5.9 GHz band in 1999 and defined each channel’s usage and rules in 2003 and 2006, as in Figure 1. 2.1. ConventionalAs shown in V2Xthe figure, Scheme178 (5.885 in 5.9 GHz GHz–5.895 Band GHz band) is called the Control Channel (CCH),The which most is widely used only deployed for safety V2X communicationmessages and control scheme messages. is DSRC, Channel which employs 172 (5.855 a set GHz of WAVE–5.865 GHzstandards band) including is dedicated the to IEEE safety 1609 messages series andbetween IEEE vehicles, 802.11p, and as describedChannel 184 in [(5.91511]. The GHz most–5.925 widely GHz band)used V2X is used frequency to transmit band frames used for to V2Xvehicles communication in a long commu is thenication 5.9 GHz range. band, whichThe remaining is a licensed channels band. canAs describedbe used to in send reference data frames [12], the regardless Federal Communications of message or data Commission type. Two (FCC)sets of in 20 the-MHz U.S. channels initially (Channelallocated seven174 and channels Channel in 176, the 5.9and GHz Channel band 180 in 1999 andand Channel defined 182) each can channel’s be used for usage 20- andMHz rules frame in transmissions.2003 and 2006, as in Figure1.

Ch 172 Ch 174 Ch 176 Ch 178 Ch 180 Ch 182 Ch 184

5.850 5.855 5.865 5.875 5.885 5.895 5.905 5.915 5.925 (GHz) SCH: SCH: may used for SCH: may used for SCH: CCH V2V Safety 20 MHz band 20 MHz band long range Figure 1. TheThe 5.9 5.9 GHz GHz channel channel allocation in the US.

IEEEAs shown 802.11p in defines the figure, the M Channeledium Access 178 (5.885 Control GHz–5.895 (MAC) and GHz Physical band) is (PHY) called layers the Control of V2X communicationChannel (CCH), schemes which in is the used 5.9 only GHz for band. safety The messageschannel access and controlschemes messages. in IEEE 802.11p Channel employ 172 the(5.855 Enhanced GHz–5.865 Distributed GHz band) Channel is dedicated Access (EDCA) to safety mechanism messages ofbetween IEEE 802.11e vehicles, with and modified Channel parameter184 (5.915 GHz–5.925 values. EDCA GHz isband) a contention is used to-based transmit channel frames access to vehicles scheme in a with long fourcommunication different Access range. CategoriesThe remaining (ACs) channels depending can be on used traffic to sendtype: data Video, frames Voice, regardless Best Effort, of message and Background. or data type. Except Two setsfor EDCAof 20-MHz parameters channels for (Channel differentiated 174 and channel Channel access 176, and per Channeltraffic type, 180 andEDCA Channel is basic 182)ally can a Distributed be used for Coordination20-MHz frame Function transmissions. (DCF) scheme. In a DCF scheme, when a station has data to transmit, it first sensesIEEE for a 802.11p DCF Interframe defines the Space Medium (DIFS) Accessperiod Controlto check (MAC)if the medium and Physical is idle before (PHY) it layers transmits of V2X the datacommunication frame. schemes in the 5.9 GHz band. The channel access schemes in IEEE 802.11p employ the EnhancedThere Distributed are two kinds Channel of channel Access (EDCA) sensing mechanism methods— ofPreamble IEEE 802.11e Detection with modified (PD) and parameter Energy Detectionvalues. EDCA (ED). is PD a contention-based senses the channel channel status access by decoding scheme with the signal four di durationfferent Access included Categories in the (ACs)IEEE 802.11depending preamble on tra andffic type:the frame Video, duration Voice, Best value Eff indicatedort, and Background. in the Duration Except field for of EDCAframe. parametersED senses t forhe channeldifferentiated status channel by detecting access perif there traffi isc type, any receivedEDCA is basicallysignal power a Distributed over a threshold Coordination value Function that is regarded(DCF) scheme. as decoding In a DCF a scheme,failure or when an error. a station When has the data channel to transmit, has ita firstbusy senses status, for there a DCF are Interframe different proceduresSpace (DIFS) depending period to checkon the if used the mediumchannel issensing idle before methods it transmits (PD or ED). the data If the frame. channel is sensed as busy Thereby PD are during two kinds a DIFS of period, channel then sensing a back methods—Preamble-off procedure is triggered Detection after (PD) the and DIFS Energy period Detection from the(ED). end PD of sensesthe sensed the channelbusy period. status The by back decoding-off procedure the signal waits duration for the included back-off induration, the IEEE which 802.11 is determinedpreamble and by the a randomly frame duration selected value back indicated-off counter. in the Duration field of frame. ED senses the channel status by detecting if there is any received signal power over a threshold value that is regarded as decoding a failure or an error. When the channel has a busy status, there are different procedures depending on the used channel sensing methods (PD or ED). If the channel is sensed as busy by PD during a DIFS period, then a back-off procedure is triggered after the DIFS period from the end of the sensed busy period. The back-off procedure waits for the back-off duration, which is determined by a randomly selected back-off counter. Appl. Sci. 2020 10 Appl. Sci. 2020,, 10,, x 222 FOR PEER REVIEW 4 4of of 18 18

On the other hand, if the channel is sensed as busy by ED during a DIFS period, then the stations On the other hand, if the channel is sensed as busy by ED during a DIFS period, then the stations must wait for an Extended Interframe Space (EIFS) period, which is a longer time period than DIFS must wait for an Extended Interframe Space (EIFS) period, which is a longer time period than DIFS after the end of the channel’s busy status sensed by ED. An EIFS period is sufficiently long to protect after the end of the channel’s busy status sensed by ED. An EIFS period is sufficiently long to protect the ongoing frame transaction where the transmitter of the frame that caused the channel to be busy the ongoing frame transaction where the transmitter of the frame that caused the channel to be busy needs to receive an acknowledge (ACK) frame to successfully complete the ongoing frame needs to receive an acknowledge (ACK) frame to successfully complete the ongoing frame transaction. transaction. Therefore, an EIFS period includes the time for ACK frame transmission, i.e., EIFS = Therefore, an EIFS period includes the time for ACK frame transmission, i.e., EIFS = Transmission time Transmission time of Ack frame at lowest PHY mandatory rate + SIFS + DIFS. By providing enough of Ack frame at lowest PHY mandatory rate + SIFS + DIFS. By providing enough time in the EIFS time in the EIFS period for a transmitter to receive an ACK frame, the hidden node problem caused period for a transmitter to receive an ACK frame, the hidden node problem caused by the ACK frame by the ACK frame transmitter can be avoided. transmitter can be avoided. In EDCA, in order to provide differentiated channel-access priorities depending on the traffic In EDCA, in order to provide differentiated channel-access priorities depending on the traffic type, as shown in Figure 2, an Arbitration Interframe Space (AIFS) is used instead of the DIFS of the type, as shown in Figure2, an Arbitration Interframe Space (AIFS) is used instead of the DIFS of the previously described normal DCF operation. AIFS values are different for the AC. The EIFS value is previously described normal DCF operation. AIFS values are different for the AC. The EIFS value also adjusted with the AIFS period, i.e., EIFS = Transmission time of Ack frame at lowest PHY is also adjusted with the AIFS period, i.e., EIFS = Transmission time of Ack frame at lowest PHY mandatory rate + SIFS + AIFS. mandatory rate + SIFS + AIFS.

Data to send Data to send Data to send

AIFS AIFS EIFS Busy Busy Data Data Data 802.11 (by PD) (with decoding fail) station AIFS: Arbitrary InterFrame Space PD: Preamble Detection ED: Energy detection FigureFigure 2. 2. ChannelChannel access access scheme scheme in in IEEE IEEE 802.11p 802.11p..

InIn IEEE IEEE 802.11p, 802.11p, the the default default transmission transmission bandwidth bandwidth for for frame frame transmission transmission is 10 MHz MHz.. Doubled Doubled timingtiming parameters parameters are are adopted adopted compared compared to to other other IEEE IEEE 802.11 802.11 systems systems [13] [13] because because of of a a larger larger delay delay spreadspread in in the the vehicular vehicular communication environment.

2.2.2.2. Conventional Conventional Channel Channel Extension Extension Scheme Scheme (Conventional (Conventional AIFS) AIFS) WhenWhen a a 20 20-MHz-MHz channel channel is is composed composed by by two two contiguous contiguous 10- 10-MHzMHz channels, channels, the themost most simple simple 20- MHz20-MHz channel channel access access scheme scheme reuses reuses the the existing existing wide wide-bandwidth-bandwidth channel channel access access scheme scheme defined defined in in IEEEIEEE 802.11n 802.11n and and IEEE IEEE 802.11ac 802.11ac [14], [14], which which is is used used to to access access wider wider channels channels of of 40 40 MHz, MHz, 80 80 MHz, MHz, and and 160 MHz channels. In In IEEE IEEE 802.11n 802.11n and and IEEE IEEE 802.11ac, 802.11ac, a a primary primary channel channel is is defined defined as as a a common accessaccess channel channel used used by by all all stations stations including including AP AP in in a a BSS, BSS, and and secondary secondary channels channels are are defined defined as as the the remainingremaining channels channels of of 40 40 MHz, MHz, 80 80 MHz, and 160 MHz with the primary ch channel.annel. TheThe wide wide channel channel access access in in IEEE IEEE 802.11n 802.11n and and IEEE 802.11ac follows follows the the EDCA EDCA channel channel access access procedureprocedure on on the primary channel channel and and ED ED sensing sensing on on the the secondary channel channel for for a a certain certain period period beforebefore the the end end of of the the channel channel access access procedure procedure in the primary cha channel.nnel. The The ED ED sensing sensing period period of of the the secondarysecondary channel channel depends on the operating band: PIFS in the 5 GHz band and DIFS in the 2.4 GHz GHz bandband.. When When a a busy busy channel channel is is indicated indicated in in the the secondary secondary channels channels by by ED ED sensing, sensing, the the station station may may transmittransmit a a frame frame using using only only t thehe primary primary channel, channel, or or a a back back-o-offff procedureprocedure is is restarted restarted with with a a new new backback-o-offff counter, preserving the contention window value and retransmissionretransmission counter.counter. TheThe previously describeddescribed channel-access channel-access rule rule for for 40-MHz 40-MHz channel channel access access can can be applied be applied to 20-MHz to 20- MHzchannel channel access access in IEEE in IEEE 802.11bd. 802.11bd. In reference In reference [14], the[14], primary the primary channel channel was definedwas defined as the as 10-MHz the 10- MHzchannel channel where where an EDCA an EDCA channel channel access access procedure procedure is performed, is performed, and theand secondarythe secondary channel channel was wasdefined defined as the as extended the extended 10-MHz 10- channelMHz channel composing composing the 20 MHz the 20 channel. MHz channel. In order toIn provideorder to enhanced provide enhancedfairness with fairness stations with using stations the using secondary the secondary channel, channel, using AIFS using instead AIFS instead of PIFS of is PIFS currently is currently under underdiscussion discussion [14], as [14], depicted as depicted in Figure in 3Figure. However, 3. However, despite despite using AIFS, using which AIFS, has which a longer has a waiting longer waitingperiod thanperiod PIFS, than there PIFS, still there exists still a fairnessexists a fairness problem problem with stations with stations on the secondary on the secondary channel channel because becausethere is nothere back-o is noff backof the-off secondary of the secondary channel. channel. The fairness The fairness problem problem becomes becomes worse when worse the when channel the channelload in theload secondary in the secondary channel channel is higher is than higher the than channel the channel load of theload primary of the primary channel channel because becau stationsse stations on the secondary channel perform the back-off procedure as a result of EDCA procedure. Appl. Sci. 2020, 10, 222 5 of 18

Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 18 on the secondary channel perform the back-off procedure as a result of EDCA procedure. The fairness The fairness problem should be carefully considered because fairness is one of the crucial problem should be carefully considered because fairness is one of the crucial requirements of IEEE requirements of IEEE 802.11bd [5,6]. 802.11bd [5,6].

Data to send AIFS xIFS

Primary Ch 174 Busy 10 MHz Data Ch 176 Busy 10 MHz

: ED (Energy Detection)+ PD (Preamble Detection) sensing : ED sensing

FigureFigure 3.3. Application of conventional channel-extensionchannel-extension schemescheme toto 20-MHz20-MHz channelchannel access.access.

2.3.2.3. Adjacent Channel InterferenceInterference ProblemProblem inin V2XV2X CommunicationsCommunications ForFor the the vehicular vehicular environment, environment, there there have have been been a number a number of prior of studies prior [15 studies,16] on [15 adjacent,16] on- adjacent-channelchannel interference interference problems. problems. Adjacent Adjacent channel channelinterference interference is caused is by caused a power by a leakage power leakage of signals of signalstransmitted transmitted in neighboring in neighboring channels, channels, which which is nonnegligible. is nonnegligible. Because Because of of the the adjacent-channel adjacent-channel interferenceinterference problem,problem, transmissiontransmission inin nearbynearby channelschannels causescauses aa degradationdegradation ofof thethe signalsignal qualityquality andand anan increasedincreased channelchannel accessaccess delay.delay. Therefore,Therefore, toto reducereduce negativenegative impactsimpacts onon thethe signalsignal qualityquality ofof thethe frameframe transmissiontransmission onon adjacentadjacent 10-MHz10-MHz channels,channels, authorsauthors inin referencereference [[17]17] suggestedsuggested schemesschemes toto alleviatealleviate suchsuch channelchannel interferenceinterference causedcaused byby transmissiontransmission inin adjacentadjacent channels.channels. TheThe previouslypreviously describeddescribed adjacent-channeladjacent-channel interferenceinterference problemproblem shouldshould bebe takentaken intointo accountaccount forfor thethe channelchannel accessaccess rulerule ofof wide-bandwidthwide-bandwidth transmission.transmission. InIn thethe wide-bandwidthwide-bandwidth channelchannel accessaccess procedureprocedure in in IEEE IEEE 802.11n 802.11n or IEEEor IEEE 802.11ac, 802.11ac, when when a busy a busychannel channel is detected is detected in the secondary in the secondary channel bychannel ED sensing, by ED sensing, the station the station may transmit may transmit frames frames using using only theonly primary the primary channel. channel. This This is calledis called a fallbacka fallback operation. operation. However, However, in V2X in V2X communication, communication, a fallback a fallback operation operation causes ancauses adjacent-channel an adjacent- interferencechannel interference problem because problem it transmitsbecause it frames transmits using frames adjacent using channels adjacent even channels though a even station though senses a transmissionstation senses on transmission the neighboring on the channel. neighboring The fallback channel. operation The fallback causes operation more severe causes adjacent-channel more severe interferenceadjacent-channel by ignoring interference the transmissions by ignoring in nearby the transmissions channels. Therefore, in nearby in the channels. proposed Therefore, channel-access in the methodproposed for channel 20-MHz-access transmission, method it foris suggested 20-MHz not transmission, to allow the it transmission is suggested of a not 10-MHz to allow fallback the operationtransmission in case of a of 10 a- busyMHz channel fallback indicated operation in in a secondary case of a channel,busy channel as in reference indicated [18 in]. a secondary channel, as in reference [18]. 3. Proposed Scheme and Its Variants

3.1.3. Proposed Proposed Scheme Channel Accessand Its Scheme Variants for 20-MHz Transmission

3.1. ProposedA channel Channel access Access scheme Scheme for 20-MHz for 20-MHz frame Transmission transmission should ensure fairness with stations using an extended 10-MHz secondary channel, with the definitions of the primary channel and A channel access scheme for 20-MHz frame transmission should ensure fairness with stations secondary channel as in [14]. In a BSS environment, the primary channel is a main channel used using an extended 10-MHz secondary channel, with the definitions of the primary channel and by all stations, and the secondary channels can be other BSSs’ primary channels, which may be secondary channel as in [14]. In a BSS environment, the primary channel is a main channel used by geographically separated where adjacent channel interference and channel extension fairness problems all stations, and the secondary channels can be other BSSs’ primary channels, which may be are not very serious. geographically separated where adjacent channel interference and channel extension fairness However, in the OCB environment, each channel is equally important, with its own purpose. problems are not very serious. Stations using other channels cannot be geographically separated owing to the moving nature of However, in the OCB environment, each channel is equally important, with its own purpose. vehicles. Therefore, adjacent channel interference and channel extension fairness problems are taken Stations using other channels cannot be geographically separated owing to the moving nature of into account more seriously and carefully in an OCB environment (e.g., IEEE 802.11p and IEEE vehicles. Therefore, adjacent channel interference and channel extension fairness problems are taken 802.11bd) than a BSS environment (e.g., IEEE 802.11n, IEEE 802.11ac, and IEEE 802.11ax). In the design into account more seriously and carefully in an OCB environment (e.g., IEEE 802.11p and IEEE 802.11bd) than a BSS environment (e.g., IEEE 802.11n, IEEE 802.11ac, and IEEE 802.11ax). In the design of the 20-MHz channel access scheme, fairness and adjacent interference avoidance are Appl. Sci. 2020, 10, 222 6 of 18

Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 18 of the 20-MHz channel access scheme, fairness and adjacent interference avoidance are considered, considered,and a minimized and a minimized channel-access channel delay-access must delay be provided. must be provided. In the proposed In the proposed scheme, whilescheme, fairness while withfairness stations with operating stations operating in an extended in an 10-MHz extended channel 10-MHz is ensured, channel minimized is ensured, channel-access minimized channel delay is- providedaccess delay by is preventing provided overprotectionby preventing overprotection caused by dynamic caused sensing by dynamic methods. sensing methods.

3.1.1. Device Architecture In order to provide maximum performance, the dev deviceice architecture of the proposed 20-MHz20-MHz channel access scheme is designed to include two preamble detectors to perform PD sensing in both 10-MHz10-MHz channels. The The proposed proposed station station architecture architecture to to support support the the proposed scheme is shown in Figure 44.. TheThe proposedproposed stationstation architecturearchitecture hashas anan additionaladditional simplifiedsimplified receiverreceiver modulemodule withwith PHYPHY layer receiver and and simple simple MAC MAC layer. layer. In In the the proposed proposed station station structure, structure, one one preamble preamble detector detector is isa normala normal part part of ofIEEE IEEE 802.11 802.11 MAC MAC and and PHY PHY architecture architecture [19], [ 19and], andthe other the other preamble preamble detector detector is part is partof the of additionalthe additional simplified simplified receiver receiver module. module. The The additional additional simplified simplified receiver receiver module module has has a simplifiedsimplified MAC MAC layer layer and and PHY PHY layer layer receiver receiver for for a secondary a secondary channel channel to perform to perform PD sensing PD sensing on that on secondarythat secondary channel. channel. The added The added PHY receiver PHY receiver only in onlycludes includes the receiver the receiver part of part a normal of a normal PHY layer PHY [19].layer The [19]. added The added simplified simplified PHY and PHY MAC and MAC layers layers for a forsecondary a secondary 10-MHz 10-MHz channel channel only only includes includes the functionalitythe functionality of PD of sensing: PD sensing: the decoding the decoding of the LENGTH of the LENGTH field in the field preamble in the preamble’s’s L-SIG and L-SIG Duration and fieldDuration in the field MAC in theheader MAC of headerthe received of the receivedframes. The frames. simplified The simplified receiver module receiver detects module the detects channel the statechannel of the state secondary of the secondary channel according channel according to the decoded to the LENGTH decoded LENGTHfield and Duration field and field. Duration Since field. the simpleSince the MAC simple layer MAC is only layer for is only the detectionfor the detection of channel of channel usage usage time in time the in secondary the secondary channel, channel, the decodedthe decoded frames frames are arenot nottransferred transferred to the to theupper upper layer. layer.

PHY Layer (IEEE 802.11p) MAC Layer (IEEE 802.11p/1609.4) Upper Layer Channel access Frame Analog Constellation operation Insert GI IFFT Interleaver FEC Coder Decoder and RF Mapper (EDCA, PCF, ) Application Layer PHY MAC WSMP SAP Frame Generation SAP IPv6 TCP Analog Remove Constellation FFT Deinterleaver FEC Decoder LLC Layer and RF GI Demapper Ack/ Channel Recovery process Coordination

Channel state indication Analog Remove Constellation PHY FFT Deinterleaver FEC Decoder Frame and RF GI Demapper SAP Decoder No Data transfer to PHY receiver Simple MAC Layer upper layer

Figure 4. ProposedProposed station architecture for 20 20-MHz-MHz channel access access..

3.1.2. Proposed Proposed Channel Access Scheme (All Back Back-O-Offff AIFS) Based on the proposed d deviceevice architecture as in Figure 4 4,, eacheach 10-MHz10-MHz channelchannel isis ableable toto performperform carrier sensing (e.g. (e.g.,, PD and ED) independently. Such independent carrier sensing capability enables the back back-o-offff of the entire 20-MHz20-MHz channel, as depicted in Figure5 5,, whichwhich guaranteesguarantees fairfair contentioncontention with other 10-MHz10-MHz frame transmissions in each 10-MHz10-MHz channel.channel. In the proposed 20-MHz20-MHz frame transmission scheme, the 20 20-MHz-MHz channel is determined to be idle when both 10 10-MHz-MHz channels are sensed as idle because of PD and ED sensing in eacheach 10-MHz10-MHz band. Channel sensing of the primary channel is performed by an IEEE 802.11 transceiver, and channel sensing of the secondary channel is performed by the previously described newly added receiver with simplified simplified MAC layer and PHY layer. When When the the b back-oack-offff duration expires, the station is able to transmit a 20-MHz20-MHz frame without secondarysecondary-channel-channel idle status checking for the AIFS duration as inin thethe conventionalconventional AIFSAIFS scheme.scheme. Appl. Sci. 2020, 10, 222 7 of 18 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 18

Data to send AIFS AIFS Resume backoff process

Primary Ch 174 Busy 10 MHz Data Ch 176 Busy 10 MHz

: ED (Energy Detection)+ PD (Preamble Detection) sensing Figure 5. ProposedProposed channel access scheme for 20 20-MHz-MHz transmission transmission..

3.2. Alternative Alternative Channel Access Schemes for 20 20-MHz-MHz Transmission Since there might be hardware restrictions in adding the proposed simplified simplified receiver, several variant 20-MHz20-MHz frame transmission schemes with a single transceiver are also pro proposed.posed. Without the proposed simplified simplified receiver, receiver, both both detection detection and and decoding, decoding, i.e., ED i.e., and ED PD, and of PD, received of received 10-MHz 10 frames-MHz framesin the secondary in the secondary channel channel are di ffiarecult difficult to implement. to implement. Therefore, Therefore, with with a single a single transceiver, transceiver, only only ED EDsensing sensing is performed is performed in the in secondary the secondar channel.y channel. To the To best the of best our of knowledge, our knowledge, today’s today’s common common WLAN WLANimplementation implementation already employs already such employs ED sensing such in ED secondary sensing channels in secondary for wide-bandwidth channels for channel wide- bandwidthoperation. Since channel fairness operation. must be considered Since fairness more seriously must be in OCB considered communication more seriously environments in OCB than communicationBSS communication environments environments, than BSS new communication back-off procedures environments, with ED sensing new back in- secondaryoff procedures channels with EDneed sensing to be designed.in secondary Since channels fairness need and to medium be designed. access Since delays fairness have and a trade-o mediumff relationship, access delays in have the adesign trade- ofoff the relationship, proposed variantin the design schemes, of the the proposed joint optimization variant schemes, of both aspectsthe joint must optimization be considered. of both aspects must be considered. 3.2.1. Modified Conventional Wideband Channel Access Scheme with AIFS Period (Start AIFS + End AIFS) 3.2.1. Modified Conventional Wideband Channel Access Scheme with AIFS Period (Start AIFS + End AIFS)Since the conventional AIFS-based wideband channel access scheme does not perform back-off operations in secondary channels, in order to consider fairness carefully in secondary channels under Since the conventional AIFS-based wideband channel access scheme does not perform back-off an OCB environment, the conventional AIFS-based wideband channel access scheme must be modified. operations in secondary channels, in order to consider fairness carefully in secondary channels under The modification of the conventional AIFS method includes the sensing of both 10-MHz channels an OCB environment, the conventional AIFS-based wideband channel access scheme must be Appl.before Sci. performing2020, 10, x FOR the PEER back-o REVIEWff procedure, as illustrated in Figure6. 8 of 18 modified. The modification of the conventional AIFS method includes the sensing of both 10-MHz channels before performing the back-off procedure, as illustrated in Figure 6. Data to send As in the conventional AIFS method, EIFSchannel sensingAIFS in the primary channel can be performed with PD and ED sensing. In the proposed modification, since the modified conventional wideband channel access scheme only requires one transceiver, ED-based channel sensing can only be used in thePrimary secondaryCh 174 channel Busybecause PD sensing in the secondary channel is not general. When10 MHz a busy channel is detected with ED sensing in the secondary channel, the busyData-channel duration cannot be known. Therefore, in order to minimize the impact of 20-MHz frame transmission using the Ch 176 Busy 10 MHz secondary channel and protect the ongoing frame transaction (i.e., data frame + ACK) of the secondary channel, a relatively long waiting period of the EIFS duration must be applied. The rest of For secondary busy, EIFS the channel access procedure after the back-off operation is conducted: ED + PD sensing as in the conventional: ED sensing AIFS- sensing because of pkt error based wideband channel access method. Figure 6. Access variant scheme for 20-MHz channel with modification of conventional wideband Figure 6. Access variant scheme for 20-MHz channel with modification of conventional wideband channel access scheme. channel access scheme. As in the conventional AIFS method, channel sensing in the primary channel can be performed 3.2.2. Sensing of 20-MHz Channel for all Durations of Back-Off Procedure (All Back-Off EIFS) with PD and ED sensing. In the proposed modification, since the modified conventional wideband channelAnother access 20 scheme-MHz channel only requires access onevariant transceiver, scheme ED-basedis the modification channel sensing of the canproposed only be channel used in accessthe secondary method described channel because in Section PD 3.1.2 sensing to allow in the for secondary one-transceiver channel design. is not general.In order Whento guarantee a busy fairness,channel is20 detected-MHz channel with EDsensing sensing for inthe the duration secondary of the channel, back-off the p busy-channelrocedure is performed, duration cannotas in the be proposedknown. Therefore, method. Since in order this to scheme minimize also the requires impact one of 20-MHz transceiver, frame ED transmission sensing can usingonly be the performed secondary on the secondary channel during the back-off procedure. In this scheme, the 20-MHz channel is determined to be idle when each 10-MHz channel is sensed to be idle as a result of PD and ED sensing in the primary channel and ED sensing in the secondary channel. When a station has data to transmit using a 20-MHz channel, AIFS sensing and a back-off procedure are performed in the 20-MHz band, which can ensure a channel idle state for each decrement of the back-off counter. Since only ED sensing can be applied to the secondary channel, any detected busy state invokes EIFS duration waiting, as shown in Figure 7.

Data to send AIFS EIFS Resume backoff process

Primary Ch 174 Busy 10 MHz Data Ch 176 Busy 10 MHz

Stop backoff for busy : ED + PD sensing : ED sensing

Figure 7. Alternative 20-MHz channel access scheme with all back-off approach for 20-MHz transmission.

4. Delay Analysis of All Back-Off AIFS Channel Access Scheme The performance of 20-MHz transmission can be measured with two metrics: 1. fairness with other devices transmitting a 10-MHz frame in a secondary channel, and 2. the throughput of frames transmitted using a 20-MHz channel. The fairness metric can be measured by the mean access delay of frames carried in a 10-MHz frame in the secondary 10-MHz band. The throughput metric can be measured by the mean access delay and frame transmission latency carried in the 20-MHz channel. For simplicity, in the performance analysis, there are several assumptions: full buffer traffic and one AC with the same AIFS number (AIFSN) and contention window value. The mathematical modeling of this paper is based on the Markov chain modeling in [20]. In addition, the transmitted frame size in each 10-MHz band is assumed to be the same in every transmission. Since the total latency is the sum of the access delay, frame transmission delay, and additional delay for retransmission, the average value of the latency can be expressed as Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 18

Data to send EIFS AIFS

Primary Ch 174 Busy 10 MHz Data Ch 176 Busy 10 MHz

Appl. Sci. 2020, 10, 222 8 of 18 For secondary busy, EIFS : ED + PD sensing : ED sensing sensing because of pkt error channel and protect the ongoing frame transaction (i.e., data frame + ACK) of the secondary channel, a relativelyFigure long 6. Access waiting variant period scheme of the for EIFS20-MHz duration channel must with be modification applied. The of conventional rest of the channelwideband access channel access scheme. procedure after the back-off operation is conducted as in the conventional AIFS-based wideband channel access method. 3.2.2. Sensing of 20-MHz Channel for all Durations of Back-Off Procedure (All Back-Off EIFS) 3.2.2.Another Sensing 20 of- 20-MHzMHz channel Channel access for allvariant Durations scheme of Back-Ois the ffmodificationProcedure (Allof the Back-O proposedff EIFS) channel accessAnother method 20-MHz described channel in Section access 3.1.2 variant to schemeallow for is theone modification-transceiver ofdesign. the proposed In order channel to guarantee access fairness,method described20-MHz channel in Section sensing 3.1.2 tofor allow the duration for one-transceiver of the back- design.off procedure In order is toperformed, guarantee as fairness, in the proposed20-MHz channel method. sensing Since this for scheme the duration also requires of the back-o one transceiver,ff procedure ED is sensing performed, can only as in be the performed proposed onmethod. the secondary Since this channel scheme during also requires the back one-off transceiver, procedure. ED In sensing this scheme, can only the be 20 performed-MHz channel on the is determinedsecondary channel to be idle during when the eac back-oh 10-MHzff procedure. channel Inis sensed this scheme, to be idle the 20-MHzas a result channel of PD isand determined ED sensing to inbe the idle primary when each channel 10-MHz andchannel ED sensing is sensed in the to secondary be idle as channel. a resultof When PD and a station ED sensing has data in theto transmit primary usingchannel a 20 and-MHz ED sensingchannel, in AIFS the secondary sensing and channel. a back When-off procedure a station are has performed data to transmit in the using 20-MHz a 20-MHz band, whichchannel, can AIFS ensure sensing a channel and a back-o idle stateff procedure for each are decrement performed of in the the back 20-MHz-off counter. band, which Since can only ensure ED sensinga channel can idle be state applied for each to the decrement secondary of the channel, back-o ff anycounter. detected Since busy only state ED sensing invokes can EIFS be applied duration to waiting,the secondary as shown channel, in Figure any detected7. busy state invokes EIFS duration waiting, as shown in Figure7.

Data to send AIFS EIFS Resume backoff process

Primary Ch 174 Busy 10 MHz Data Ch 176 Busy 10 MHz

Stop backoff for busy : ED + PD sensing : ED sensing

Figure 7. 7. AlternativeAlternative 20-MHz 20-MHz channel channel access access scheme scheme with all with back all-off back-o approachff approach for 20-MHz for transmission20-MHz transmission.. 4. Delay Analysis of All Back-Off AIFS Channel Access Scheme 4. Delay Analysis of All Back-Off AIFS Channel Access Scheme The performance of 20-MHz transmission can be measured with two metrics: 1. fairness with The performance of 20-MHz transmission can be measured with two metrics: 1. fairness with other devices transmitting a 10-MHz frame in a secondary channel, and 2. the throughput of frames other devices transmitting a 10-MHz frame in a secondary channel, and 2. the throughput of frames transmitted using a 20-MHz channel. The fairness metric can be measured by the mean access delay transmitted using a 20-MHz channel. The fairness metric can be measured by the mean access delay of frames carried in a 10-MHz frame in the secondary 10-MHz band. The throughput metric can be of frames carried in a 10-MHz frame in the secondary 10-MHz band. The throughput metric can be measured by the mean access delay and frame transmission latency carried in the 20-MHz channel. measured by the mean access delay and frame transmission latency carried in the 20-MHz channel. For simplicity, in the performance analysis, there are several assumptions: full buffer traffic For simplicity, in the performance analysis, there are several assumptions: full buffer traffic and and one AC with the same AIFS number (AIFSN) and contention window value. The mathematical one AC with the same AIFS number (AIFSN) and contention window value. The mathematical modeling of this paper is based on the Markov chain modeling in [20]. In addition, the transmitted modeling of this paper is based on the Markov chain modeling in [20]. In addition, the transmitted frame size in each 10-MHz band is assumed to be the same in every transmission. frame size in each 10-MHz band is assumed to be the same in every transmission. Since the total latency is the sum of the access delay, frame transmission delay, and additional Since the total latency is the sum of the access delay, frame transmission delay, and additional delay for retransmission, the average value of the latency can be expressed as delay for retransmission, the average value of the latency can be expressed as

Lw = dw + Tw + (mean retransmission delay), (1) where Lw is the mean latency, dw is the mean access delay, and Tw is the mean transmission delay of the 20-MHz frame transmission. When the transmitted frame is a broadcast frame, the retransmission delay can be omitted. Similarly, the mean latency of frame transmission in the primary 10-MHz channel is the sum of the access delay and mean transmission delay, Tp, and the mean latency of the frame transmission in the secondary 10-MHz channel is the sum of the access delay and transmission delay in the secondary channel, Ts. Since the same transmission delay value is used, Tp = Ts in the analysis. Appl. Sci. 2020, 10, 222 9 of 18

To obtain the access delay of frames carried in 20-MHz channels in the all back-off AIFS scheme, the transmission probability of each station should be calculated. Since the channel access scheme of all back-off AIFS includes an EDCA back-off operation with the modification of the channel state decision, the Markov-chain state transition diagram is the same as the Markov-chain diagram depicted in [20], with the assumption of the same AC. Based on the approach in [20], the transmission probability can be calculated as the sum of state probabilities with back-off counter 0, which is expressed as

Xm τw = bi,0, (2) i=0 where bi,0 is the state probability of the ith back-off procedure with back-off counter 0 of the Markov chain as defined in reference [20]. From the transmission probability τw, the probability of frame transmission in a 20-MHz channel by k stations in each slot, Pw(k), can be defined as ! Nw k Nw k Pw(k) = τ (1 τw) − , (3) k w · − where Nw is the number of stations with buffered data to transmit using 20-MHz frames. Since a collision occurs when two or more stations transmit frames in one slot, the collision col probability of 20-MHz frame transmission in one slot Pw can be derived as

col P = 1 Pw(0) Pw(1). (4) w − −

Similarly, the transmission probabilities transmitted in primary and secondary 10-MHz bands τp and τs can be calculated using the same scheme of Equation (2) from the model in reference [20]. The transmission probabilities of 10-MHz frames by k stations in each 10-MHz band in one slot Pp(k) and P (k) can be derived as s ! Np k Np k Pp(k) = τ (1 τp) − , (5) k p · − ! Ns k Ns k Ps(k) = τ (1 τs) − , (6) k s · − where Np and Nw denote the number of stations having buffered frames to transmit using 10-MHz frames in the primary and secondary channels, respectively. As in Equation (4), the collision probabilities of 10-MHz frame transmission in the primary channel and secondary channel can be obtained as col col P = 1 Pp(0) Pp(1) and P = 1 Ps(0) Ps(1), respectively. p − − s − − The mean access delay of frames transmitted in the 20-MHz channel, dw, can be converted to

dw = AIFS + E [length of a slot time for a 20 MHz frame]/τw, (7) where AIFS = SIFS + AIFSN aSlotTime. · Appl. Sci. 2020, 10, 222 10 of 18

Assuming that Tp = Ts, E [length of a slot time for a 20 MHz frame] is calculated as

E[length of a slot time for a 20MHz frame] = Pp(0) Ps(0) Pw(0) aSlotTime · · · +Pp(0) Ps(0) Pw(1) (Tw + AIFS + aSlotTime) · · · +Pp(1) Ps(0) Pw(0) ms(Tp + AIFS + aSlotTime) · · · +Pp(0) Ps(1) Pw(0) mp(Ts + AIFS + aSlotTime) · · · +Pp(1) Ps(1) Pw(0) (Tp + AIFS + aSlotTime) col · · · +Pp Ps(0) Pw(0) ms(Tp + EIFS + aSlotTime) · col · · (8) +Pp(0) Ps Pw(0) mp(Tp + EIFS + aSlotTime) col· col· · +Pp Ps Pw(0) (Tp + EIFS + aSlotTime) col · · · +Pp(1) Ps Pw(0) (Tp + AIFS + aSlotTime + mp(EIFS AIFS)) col · · · − +Pp Ps(1) Pw(0) (Tp + AIFS + aSlotTime + ms(EIFS AIFS)) · · · col − +Pp(0) Ps(0) Pw (Tw + EIFS + aSlotTime) · · col· +(1 Pp(0) Ps(0)) (Pw(1) + P ) (max(Tw, Tp) + EIFS + aSlotTime) − · · w · where ms(x) is the mean holding time for the secondary channel for duration x, and mp(x) is the mean holding time for the primary channel for duration x. Since busy-channel states may occur in the primary channel and the secondary channel in a cascaded manner, the mean holding time for the secondary channel, ms(x), can be obtained by a recursive function:

( ) = + P x/aSlotTime ( ( ))i 1 ( ) ( + ) ms x x id=1 e Ps 0 − Ps 1 mp Tsch_AIFS–x i aSlotTime P x/aSlotTime i 1 col × (9) + d e (Ps(0)) − P mp(T –x + i aSlotTime) i=1 s sch_EIFS × for x T , and ≤ sch_AIFS

Pns(x) j 1 ms(x) = x + (Ps(0)) − Ps(1)(mp(x T j aSlotTime) x + T + j aSlotTime) j=1 − sch_AIFS − × − sch_AIFS × P (x ns(x) aSlotTime)/aSlotTime n (x) i 1 + d − × e ( ( )) s ( ( )) ( ) ( ( ( ) ) + ) (10) i=1 Ps 0 Ps 0 − Ps 1 mp Tsch_AIFS– x ns x aSlotTime i aSlotTime P x/aSlotTime i 1 col − j × × k + d e (Ps(0)) − P mp(T –x + i aSlotTime), ns(x) = (x T )/aSlotTime i=1 s sch_EIFS × − sch_AIFS where T < x T and T = Ts + AIFS + aSlotTime and T = Ts + EIFS + sch_AIFS ≤ sch_EIFS sch_AIFS sch_EIFS aSlotTime. The mean holding time for the primary channel, mp(x), can be calculated in the same way as (9) col col and (10), with the substitution of Ps(k), Ps , mp(x), Tsch_AIFS, and Tsch_EIFS for Pp(k), Pp , mp(x), Tpch_AIFS, and Tpch_EIFS when Tpch_AIFS = Tp + AIFS + aSlotTime and Tpch_EIFS = Tp + EIFS + aSlotTime. The mean access delay of frames carried in the secondary 10-MHz channel, dp, can be converted to

dp = AIFS + E [length of a slot time in primary 10 MHz]/τp, (11) where E [length of a slot time in primary 10 MHz] is calculated as

E[length of a slot time in primary 10 MHz] = Pp(0) Iwp(0) aSlotTime · · +Pp(1) Iwp(0) (Tp + AIFS + aSlotTime) · · +Pp(0) Iwp(1) (Tw + AIFS + aSlotTime) col · · (12) +Pp Iwp(0) (Tp + EIFS + aSlotTime) · col · +Pp(0) I (Tw + EIFS + aSlotTime) n · wp · o + (1 Pp(0)) (1 Iwp(0)) (max(Tp, Tw) + EIFS + aSlotTime) − · − · where Iwp(k) is the interference probability to the primary channel by the 20-MHz frame sent by k stations. Appl. Sci. 2020, 10, 222 11 of 18

When iwp denotes the interference probability to primary channel by a certain station transmitting a 20-MHz frame, Iwp(k) can be derived as ! Nw k Nw k Iwp(k) = i (1 iwp) − (13) k wp · −

col and I = 1 Iwp(0) Iwp(1). wp − − Since a backoff counter decreases only when both channels are in the idle state in the all backoff AIFS scheme, the interference probability by a certain station transmitting a 20-MHz frame, iwp, can be written as iwp = p (AIFS + aSlotTime) τw, (14) sidle · where psidle(AIFS + aSlotTime) is the channel idle probability of the secondary channel for AIFS + aSlotTime, and is approximated as

AIFS+aSlotTime Ns ds aSlotTime d aSlotTime e· psidle(AIFS + aSlotTime) ( − ) . (15) ≈ ds

Similarly to Equation (11), the mean access delay of frames carried in the primary 10-MHz channel, ds, can be converted to

ds = AIFS + E [length of a slot time in secondary 10 MHz]/τs, (16) where E [length of a slot time in secondary 10 MHz] is calculated as

E[length of a slot time in sec ondary 10 MHz] = Ps(0) Iws(0) aSlotTime · · +Ps(1) Iws(0) (Ts + AIFS + aSlotTime) · · +Ps(0) Iws(1) (Tw + AIFS + aSlotTime) col · · (17) +Ps Iws(0) (Ts + EIFS + aSlotTime) · col · +Ps(0) Iws (Tw + EIFS + aSlotTime)  · · + (1 Ps(0)) (1 Iws(0)) (max(Ts, Tw) + EIFS + aSlotTime) − · − · where Iws(k) is the interference probability to the secondary channel by the 20-MHz frame sent by k stations. From the interference probability to secondary channel by 20-MHz frame transmission, iws, I (k) can be derived as ws ! Nw k Nw k Iws(k) = i (1 iws) − (18) k ws · −

col and I = 1 Iws(0) Iws(1), where iws is derived as ws − −

iws = p (AIFS + aSlotTime) τw, (19) pidle · where ppidle(AIFS + aSlotTime) is the channel idle probability of the primary channel for AIFS + aSlotTime, and is approximated as

AIFS+aSlotTime Np dp aSlotTime d aSlotTime e· ppidle(AIFS + aSlotTime) ( − ) . (20) ≈ dp

In order to show that the analysis is accurate, a brief simulation is performed under full-buffered traffic and simulation result is compared with analysis values. As a main consideration point of the proposed method is the fairness with 10-MHz transmissions on the secondary channel, the analysis result of the mean access delay of 10-MHz transmissions in the secondary channel is compared with the Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 18

iws= p pidle() AIFS + aSlotTime  w , (19)

where ppidle () AIFS+ aSlotTime is the channel idle probability of the primary channel for AIFS + aSlotTime, and is approximated as

AIFS+ aSlotTime d− aSlotTime N p p aSlotTime ppidle ()() AIFS+ aSlotTime . (20) d p

In order to show that the analysis is accurate, a brief simulation is performed under full-buffered traffic and simulation result is compared with analysis values. As a main consideration point of the proposed method is the fairness with 10-MHz transmissions on the secondary channel, the analysis result of the mean access delay of 10-MHz transmissions in the secondary channel is compared with Appl. Sci. 2020, 10, 222 12 of 18 the simulation result. As shown in Figure 8, the analysis can be verified as accurate by comparing it with the simulation result in terms of the mean access delay of 10-MHz transmissions in the simulationsecondary result. channel. As shown in Figure8, the analysis can be verified as accurate by comparing it with the simulation result in terms of the mean access delay of 10-MHz transmissions in the secondary channel.

FigureFigure 8. 8.Comparison Comparison between between analysis analysis and and simulation, simulation, in in terms terms of of the the mean mean access access delay delay of of 10 10 MHz MHz transmissionstransmissions in in the the secondary secondary channel. channel.

However,However, the the main main assumption assumption in in the the analysis analysis is is full-bu full-bufferedffered environment, environment, where where each each STA STA alwaysalways has has data data to to transmit. transmit. Since Since the the full full bu bufferffer assumption assumption causes causes too too heavy heavy load load in in each each channel, channel, thethe simulation simulation is is performed performed with with reduced reduced number number of of STAs. STAs. 5. Results 5. Results In order to investigate the performance of the proposed channel-access schemes in terms of the In order to investigate the performance of the proposed channel-access schemes in terms of the fairness and efficiency (latency) of the 20-MHz frame defined in IEEE 802.11bd, rigorous simulations fairness and efficiency (latency) of the 20-MHz frame defined in IEEE 802.11bd, rigorous simulations are performed. The time driven simulator implemented using MATLAB is utilized in the performance are performed. The time driven simulator implemented using MATLAB is utilized in the evaluation. In the simulation, 20-MHz channel is composed of two contiguous 10 MHz-channels performance evaluation. In the simulation, 20-MHz channel is composed of two contiguous 10 MHz- and each 10-MHz channel has two states: channel busy state and channel idle state. Multiple channels and each 10-MHz channel has two states: channel busy state and channel idle state. Multiple STAs independently perform channel access when frames are generated with their interarrival times STAs independently perform channel access when frames are generated with their interarrival times following an exponential distribution. Two kinds of STAs are implemented: STAs performing 10-MHz following an exponential distribution. Two kinds of STAs are implemented: STAs performing 10- channel access and STAs performing 20-MHz channel access following the schemes in this paper. MHz channel access and STAs performing 20-MHz channel access following the schemes in this The transmission of each STA changes the state of the corresponding channel(s) to channel busy paper. The transmission of each STA changes the state of the corresponding channel(s) to channel state. In case of 20-MHz channel access, upon successful channel access, both primary and secondary busy state. In case of 20-MHz channel access, upon successful channel access, both primary and channels become channel busy state. If 20-MHz frame transmissions are unfair, i.e., greedy with secondary channels become channel busy state. If 20-MHz frame transmissions are unfair, i.e., greedy the 10-MHz frame transmissions of the secondary channel, then the 10-MHz frame transmitted in the secondary channel has a longer medium access delay than 10-MHz frames with only channel contention. In order to clearly measure the fairness performance, the traffic of the secondary 10-MHz channel is set to be congested. To implement the different levels of channel load, different number of STAs generating background 10 MHz frames is employed. Under such environments, the mean access delays of frames in the secondary channel for the proposed schemes are compared. Since fairness and efficiency need to be jointly designed, the latency performance of the proposed schemes is also investigated. The best scheme is that which provides the minimum impact on the secondary 10-MHz frame transmissions and short channel access delays of 20-MHz frame transmissions. In order to investigate the efficiency of the proposed schemes, the mean access delays of 20-MHz frames of the proposed schemes are compared. The mean access delay is the main component that causes latency in a broadcast environment, which is the typical communication environment of V2X. In the simulation, stations generate a frame to Appl. Sci. 2020, 10, 222 13 of 18 transmit with their interarrival times following an exponential distribution with an average interarrival time of 100 ms. For simplicity, an ideal channel is assumed, and all traffic is assumed for one AC. In addition, for simplicity, only broadcast frames, which do not require ACK, are transmitted in the simulation because the main type of V2X message is a broadcast message among many V2X applications [21]. As the effect of fairness can be clearly measured under a highly congested environment in the secondary channel, the simulation is performed under high traffic in the secondary channel and low traffic in the primary channel. Therefore, in the simulation, 200 stations transmit 10-MHz frames in the secondary channel, and 10 stations transmit 10-MHz frames in the primary channel. The other simulation parameters are listed in Table1.

Table 1. Simulation Parameters.

Name Value Frame Length (10-MHz frame) 500 Bytes Frame Length (20-MHz frame) 2000 Bytes AC AC_BE MCS 2

Figure9 shows mean and standard deviation of the medium access delay of 10-MHz frames transmitted in the secondary channel and mean and standard deviation of the medium access delay of 20-MHz frames as the number of stations transmitting 20-MHz frames increases from 10 to 200. As in Figure9a,c, the proposed 20-MHz channel access scheme, the all back-o ff AIFS scheme, provides the minimum impact on the medium access delay of 10-MHz frames transmitted in the secondary channel in terms of mean and standard deviation. The standard deviation of all back-off AIFS scheme provides smaller standard deviation than the conventional method, providing the minimum impact on the number of 10-MHz transmissions with high contention delay caused by 20-MHz transmissions. Since all back-off AIFS has good performance in terms of the medium access delay of 10-MHz transmission, it will be beneficial when delay sensitive data are transmitted using 10-MHz bandwidth in V2X applications. Thus, the proposed scheme provides the maximum fairness with 10-MHz frame transmissions in the secondary channel. In addition, the all back-off AIFS scheme shows good delay performance, in terms of mean and standard deviation of the medium access delay of 20-MHz frames. If there is a design limitation with one receiving antenna and the decoder, the proposed scheme cannot be used, and the alternative channel access schemes described in Section 3.2 should be used. When the station includes only one receiving antenna and decoder, trade-off occurs between the medium access delay of 10-MHz frames in the secondary channel and the medium access delay of 20-MHz frames depending on the channel access scheme with ED sensing of the secondary channel. Specifically, when the conventional AIFS scheme is applied to the channel access of a 20-MHz frame, fair contention with 10-MHz frame transmissions including stations using IEEE 802.11p is not possible. On the other hand, the start AIFS + end AIFS scheme and all back-off EIFS scheme ensure fairness and low impact on 10-MHz frame transmissions in the secondary channel. However, the EIFS sensing procedure in such channel access schemes (start AIFS + end AIFS scheme and all back-off EIFS scheme) causes overprotection of 10-MHz frames transmitted in the secondary channel. This results in a large medium access delays in the transmission of 20-MHz frames. In order to measure the performance of the proposed channel access method clearly, channel load in each 10-MHz channel needs to be considered. Medium access delay is highly affected by channel load in each 10-MHz channel. Because two 10-MHz channels are considered and high channel load clearly shows the impact on the performance, two cases for the channel load are considered: high channel load in the primary channel and high channel load in the secondary channel. The simulation results of Figure9 show how high channel load in the secondary channel a ffects the performance of the channel access schemes. In order to show the impact on the channel access schemes in case of high Appl. Sci. 2020, 10, 222 14 of 18

Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 18 channel load in a primary channel, the mean access delay of 10-MHz frames in the secondary channel isfairness measured and as low the impact number on of stations10-MHz transmittingframe transmissions 20-MHz framesin the increasessecondary from channel. 10 to 200However, under thethe highEIFS load sensing condition procedure of the in primary such channe channel.l access As shownschemes in (start Figure AIFS 10, all+ end channel AIFS access scheme schemes and all show back- similaroff EIFS performance scheme) causes since overprotection 20-MHz channel of 10 access-MHz isframes not easily transmitted initiated in duethe secondary to a high channelchannel. load This conditionresults in ina large the primary medium channel. access delays in the transmission of 20-MHz frames.

(a) (b)

(c) (d)

FigureFigure 9. 9.Comparison Comparison between between channel channel access access schemes schemes in terms in of terms effects of on effects transmissions on transmissions in secondary in channelsecondary and channel mean access and mean delay access of 20-MHz delay frame:of 20-MHz (a) mean frame: access (a) mean delay access of 10-MHz delay frameof 10-MHz sent inframe the secondarysent in the channel; secondary (b) channel; mean access (b) mean delay access of 20-MHz delay frame; of 20-MHz (c) standard frame; ( deviationc) standard of deviation 10-MHz frame of 10- sentMHz in frame the secondary sent in the channel; secondary (d) standard channel; deviation(d) standard of 20-MHz deviation frame. of 20-MHz frame.

InIn orderorder toto clearlymeasure show the howperformance well the of proposed the proposed scheme channel performs access in termsmethod of clearly, channel channel access delay,load in the each mean 10 access-MHz delaychannel of 20-MHzneeds to framesbe considered. is compared Medium with theaccess mean delay access is highly delay ofaffected 10-MHz by frameschannel under load the in same each 10channel-MHz load channel. level. Because For 20-MHz two 10 frame-MHz transmission, channels are the considered number of and stations high transmittingchannel load 20-MHz clearly frames shows increases the impact from on 10 the to 200 performance, and for 10-MHz two frame cases transmission, for the channel the numberload are ofconsidered: stations transmitting high channel 20-MHz load in framesthe primary increases channel from and 10 high to 200 channel to provide load in the the same secondary channel channel. load levelThe forsimulation both cases. results The of result Figure in Figure9 show 11 how shows high the channel outstanding load in delay the secondary performance channel of the affects proposed the schemeperformance in terms of of the fair channel contention access with schemes. 10 MHz In transmissions. order to show the impact on the channel access schemes in case of high channel load in a primary channel, the mean access delay of 10-MHz frames in the secondary channel is measured as the number of stations transmitting 20-MHz frames increases from 10 to 200 under the high load condition of the primary channel. As shown in Figure 10, all channel access schemes show similar performance since 20-MHz channel access is not easily initiated due to a high channel load condition in the primary channel. Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 18

Appl. Sci. 2020, 10, 222 15 of 18 Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 18

Figure 10. Comparison between channel access schemes under high load in a primary channel.

In order to clearly show how well the proposed scheme performs in terms of channel access delay, the mean access delay of 20-MHz frames is compared with the mean access delay of 10-MHz frames under the same channel load level. For 20-MHz frame transmission, the number of stations transmitting 20-MHz frames increases from 10 to 200 and for 10-MHz frame transmission, the number of stations transmitting 20-MHz frames increases from 10 to 200 to provide the same channel load level for both cases. The result in Figure 11 shows the outstanding delay performance of the Figure 10. ComparisonComparison between between channel access schemes under high load in a primary channel channel.. proposed scheme in terms of fair contention with 10 MHz transmissions. In order to clearly show how well the proposed scheme performs in terms of channel access delay, the mean access delay of 20-MHz frames is compared with the mean access delay of 10-MHz frames under the same channel load level. For 20-MHz frame transmission, the number of stations transmitting 20-MHz frames increases from 10 to 200 and for 10-MHz frame transmission, the number of stations transmitting 20-MHz frames increases from 10 to 200 to provide the same channel load level for both cases. The result in Figure 11 shows the outstanding delay performance of the proposed scheme in terms of fair contention with 10 MHz transmissions.

Figure 11. Performance comparison between 10 MHz transmission and 20 MHz transmission under the same channelchannel load.load.

6. Discussion As described in the previous section, the conventional AIFS scheme, which is a similar approach to the wideband channel access scheme adopted in IEEE 802.11n and IEEE 802.11ac, does not perform fair contentioncontention with 10-MHz10-MHz transmissionstransmissions in thethe secondarysecondary channel.channel. This is because the channel accessFigure procedure 11. Performance of a 20-MHz20-MHz comparison frame in between the conventional 10 MHz transmission AIFS scheme and 20performs MHz transmission a back back-o-offff underprocedure only the in thesame primary channel channel, load. while channel access for 10-MHz10-MHz frame transmissionstransmissions in the secondary channel senses the channel forfor thethe entireentire back-oback-offff period.period. 6. DiscussionThe proposed 20-MHz channel access scheme, called the all back-off AIFS scheme, enables fair contention of both channels despite channel congestion on each channel because of the back-off As described in the previous section, the conventional AIFS scheme, which is a similar approach procedure that considers the channel states of the primary and secondary channels. In addition, as all to the wideband channel access scheme adopted in IEEE 802.11n and IEEE 802.11ac, does not perform back-off AIFS includes two receivers to enable PD and ED sensing in both channels, overprotection fair contention with 10-MHz transmissions in the secondary channel. This is because the channel leading to the performance degradation of 20-MHz frame transmission can be effectively prevented. access procedure of a 20-MHz frame in the conventional AIFS scheme performs a back-off procedure On the other hand, if there is one receiver in the station, only ED sensing can be applied to the only in the primary channel, while channel access for 10-MHz frame transmissions in the secondary secondary channel when PD sensing (decoder) is applied to the primary channel. In such an ED channel senses the channel for the entire back-off period. Appl. Sci. 2020, 10, 222 16 of 18 sensing case, since the decoding of frames is usually infeasible, a back-off operation in the secondary channel should be performed based on the EIFS period to provide enough time for the transmitter of frames in the secondary channel to receive an ACK frame. This EIFS sensing operation is a basic procedure in the IEEE 802.11 specification whenever frame decoding to acquire the frame transmission duration fails. Although the EIFS sensing ensures fairness and ACK frame protection, the EIFS period leads to the overprotection of 10-MHz frame transmissions in the secondary channel because the channel access scheme in a 10-MHz frame waits for the AIFS period before resuming the back-off procedure when it decodes frames and acquires the frame transmission duration. In other words, during the EIFS and AIFS time difference, the back-off counter is decreased and the transmission of the 10-MHz frame may be performed in case of expiration of the back-off procedure. Meanwhile, the channel access procedure for the 20-MHz frame is still waiting for the EIFS period. As a result, the medium access delay of the 10-MHz frame in the start AIFS + end AIFS scheme and all back-off EIFS period is further decreased, while the delay of the 20-MHz frame transmission is further increased.

7. Conclusions In this paper, a fair and efficient channel access scheme for a 20-MHz frame in next-generation WLAN-based V2X communication was proposed. As applications of V2X communication have proliferated recently, more throughput is required for WLAN V2X communication than the throughput supported by IEEE 802.11p. As a result, IEEE 802.11bd, the so-called Next-Generation V2X (NGV), was established. Standardization is being developed to enhance throughput and communication range performance while maintaining fairness with existing deployed IEEE 802.11p-compliant vehicular devices. In IEEE 802.11bd, the 20-MHz frame format was proposed to increase channel utilization and to improve throughput performance. The channel access scheme for a 20-MHz frame must ensure fair contention with the channel access procedure of 10-MHz frame transmissions in each 10-MHz channel while providing the acceptable transmission latency of a 20-MHz frame. The proposed channel access scheme was designed to jointly optimize fairness and throughput performance. The proposed scheme performs a back-off procedure for the entire 20-MHz bandwidth with PD and ED sensing of each 10-MHz channel using two receivers, leading to fair contention with 10-MHz frame transmission and preventing overprotection. Simulation results showed that the proposed scheme met fairness requirements and provided the best 20-MHz transmission performance by avoiding overprotection in a secondary channel. In this paper, alternative 20-MHz channel access schemes were also proposed in order to overcome hardware limitations. However, if only one receiver can be implemented in the station, since ED sensing can only be applied to the secondary channel, then a trade-off occurs between the fairness and the delay of the 20-MHz frame. In alternative 20-MHz channel access schemes, a joint design between fairness and delay performance was applied in order to provide an acceptable trade-off. In next-generation WLAN-based V2X communication, with the proposed scheme, the requirements of future V2X applications can be met by providing the maximum throughput performance and minimum impact on the extending channel.

Author Contributions: Conceptualization, H.H.; Data curation, H.H.; Formal analysis, H.H. and W.A.; Funding acquisition, R.Y.K.; Investigation, H.H. and Y.Y.K.; Methodology, H.H.; Project administration, W.A.; Software, H.H.; Supervision, R.Y.K. and W.A.; Validation, Y.Y.K., R.Y.K., and W.A.; Writing—original draft, H.H.; Writing—review and editing, Y.Y.K., R.Y.K., and W.A. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT& Future Planning (NRF-2017R1A2B4003987). Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2020, 10, 222 17 of 18

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