On the Impact of Multi-channel Technology on Safety-Message Delivery in IEEE p/ Vehicular Networks

Transcription

1 On the Impact of Multi-channel Technology on Safety-Message Delivery in IEEE 82.p/69.4 Vehicular Networks Marco Di Felice, Ali J. Ghandour, Hassan Artail, Luciano Bononi Department of Computer Science, University of Bologna, Italy Department of Electrical and Computer Engineering, American University of Beirut, Lebanon Abstract The IEEE 69.4-Multi-Channel Operation protocol has been proposed to support the co-existence of safety and non-safety (infotainments) applications over the Dedicated Short Range Communication (DSRC) channels at the 5.9 GHz band. However, the multi-channel approach over a single-radio transceiver might result in several performance degradations that have not been thoroughly investigated yet. In this paper, we analyze the performance of safety-related applications over multi-channel vehicular networks. We demonstrate through a simulation study that the synchronous channel switching enforced by the IEEE 69.4 protocol might easily compromise the performance of safety applications that rely on the periodic exchange of short lived broadcasts. Thus, we propose in this work the WAVE-enhanced Adaptive Broadcast (WAB) scheme. WAB provides a novel MAC contention control mechanism intended to reduce the impact of packet collisions caused by the synchronous channel switching, and to increase the packet delivery rate of broadcast messages in congested vehicular scenarios. The WAB scheme dynamically adapts to the channel conditions through a distributed load estimator metric, and implements priority mechanisms to ensure fairness among vehicles. Simulation results reveal that the proposed WAB scheme can significantly increase the packet delivery rate of broadcast messages when compared to the existing IEEE 82.p/69.4 scheme. I. INTRODUCTION Research advances in vehicular communication make concrete the possibility to deploy large-scale Vehicular Ad Hoc Networks (VANETs) by the next few years. Two main factors are giving impetus to the deployments of VANETs. On the one side, several research projects have focused on developing the methodological and architectural requirements to enable Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) communications through heterogeneous wireless technologies (e.g. projects [], [2] and [3] financed by European Union (EU) programs). In these projects, large variety of VANETs applications have been identified and implemented on small scale vehicular test-beds. The scope of the developed applications include enhancing drivers safety and reducing casualties [2], monitoring traffic conditions, and increasing the drivers comfort and entertainment. On the other side, the standardization efforts put by several IEEE Working Groups (WG) are nowadays converging towards the definition of a complete protocol stack for Wireless Access in Vehicular Environment (WAVE). At the lower layers, the IEEE 82.p [5] protocol defines the PHYsical (PHY) and lower Medium Access Control (MAC) operations on a single radio transceiver. However, the need to support different classes of applications with different Quality-of- Service (QoS) requirements is also striding the research toward multi-channel solutions that can favor the co-existence of safety and non-safety applications. To this aim, the FCC has allocated a Dedicated Short Range Communication (DSRC) spectrum for vehicular usage at 5.9 GHz, divided into 6 Service CHannels (SCH) and Control CHannel (CCH) of MHz each. Moreover, the IEEE 69.4 [4] standard has been defined to enable multi-channel communications on single-radio transceivers (expected to be the dominant case at the early stages of VANETs deployment) and multi-radio transceivers. Based on this standard, vehicles must perform synchronous switching between CCH Interval (CCHI) and SCH Interval (SCHI) of default length equal to 5 ms. During the CCH intervals, vehicles are tuned to the control channel (CCH) and can transmit routine beacon messages, event-driven safety messages and WAVE Service Advertisements (WSA). During SCH intervals, vehicles can tune their radios to one of the six service channels for non-safety communication (e.g. file-sharing, infotainment applications and oters). As a result, the problem of contention between safety and nonsafety applications is completely avoided in 69.4-based multi-channel VANETs. However, several research papers (e.g. [6] [7] [] [3]) revealed that halving the available bandwidth might seriously compromise the performance of safety and non-safety vehicular applications over realistic scenarios. In this paper, we investigate the performance of multi-channel IEEE 82.p/69.4-based VANETs, and we show that leveraging channel diversity does not always lead to an increase in the performance of wireless systems. Although our results are general, and can be applied to any kind of data exchanged among vehicles, we focused our analysis on the routine short-live broadcast messages transmitted during CCH intervals (referred to as HELLO messages in the following). HELLO messages are generated by each vehicle with a fixed frequency (typical values are or 2 Hz), and are crucial for the implementation of a large-class of safety systems (e.g.

2 Cooperative Adaptive Cruise Control, Cooperative Intersection Collision Avoidance and many others) that rely on periodic information exchange (vehicle velocity, acceleration, position and others) among vehicles. We show through simulations that the synchronous channel switching operations enforced by the IEEE 69.4 protocol might cause additional packet losses at the beginning and end of each interval due to synchronous collisions (also analyzed in [] and discussed in Annex C of [4]), and might yield to an increase number of untransmitted packets that fail to be generated and transmitted during the CCH interval. Based on these results, we propose a novel MAC scheme, called WAVE-enhanced Adaptive Broadcast (WAB), that introduces minimal changes to the contention control of the IEEE MAC 82.p in order to adapt its behavior to the channel switching operations performed at the upper-layer, and thus improve the delivery ratio of HELLO messages. The WAB scheme is designed to be adaptive to the channel conditions, through the implementation of a distributed channel load estimator and a novel contention control mechanism based on our previous work [4] for single-channel wireless ad-hoc networks. In case of low traffic conditions, the WAB scheme behaves like the legacy IEEE 82.p MAC protocol. In case of high channel contention, the WAB scheme introduces time differentiation among concurrent transmission attempts, in order to mitigate the problem of synchronous collisions. At the same time, it also introduces priority mechanisms to ensure fairness among vehicles, and to reduce the risk of un-transmitted packets. Contrary to other existing solutions [7] [], the WAB scheme does not assume any specific knowledge of the application layer, does not introduce any changes to the strict synchronization operations enforced by the IEEE 69.4 protocol, and is self-adaptive to channel load conditions, thus making it suitable for a fast varying and dynamic environment like the vehicular environment. The rest of this paper is organized as follows. Section II reviews related work. Section III presents the problem motivation through a simulation study. Section IV describes the suggested WBA scheme. Simulation results are presented in Section V. Finally, Section VI concludes the paper and presents future work. II. RELATED WORKS The IEEE 69.4 protocol is a component of the WAVE protocol stack for vehicular communication, depicted in Figure. The WAVE architecture foresees the utilization of a single-radio transceiver, that can switch among the DSRC channels at the 5.9 GHz band. This band is constituted of 6 Service CHannels (SCH) and Control CHannel (CCH) of MHz each, in addition to a 5 MHz guard band. The IEEE 82.p [5] standard defines the PHY and MAC operations on a single-channel. Moreover, the MAC 82.p scheme implements Quality-of-Service (QoS) differentiation among different Traffic Class (TC) through the Enhanced Distributed Channel Access (EDCA) scheme, derived from the IEEE 82.e standard. On the upper layer, the IEEE 69.4 SECURITY IEEE 69.2 IEEE 69. WSMP IEEE 69.3 IEEE 69.4 IEEE 82.p IEEE 82.p Fig.. APP layer TCP LLC layer WAVE upper MAC WAVE lower MAC WAVE PHY layer IP The WAVE protocol stack. safety non safety UDP scheme [4] enables multi-channel communication by single radio vehicles. The standard employs both time and frequency division access schemes. Time synchronization is assumed to be achieved through a Coordinated Universal Time (UTC), commonly obtained from Global Positioning System (GPS). Time is divided into two alternating intervals: (i) Control CHannel (CCH) interval where all vehicles should be tuned to the control channel (also known as channel 78) to exchange safety and control information and (ii) Service CHannel (SCH) interval where vehicles can switch to a SCH to be involved in a service of interest. Default length of the CCH and SCH intervals is 5ms. A complete CCH/SCH cycle defines a SYNCH interval. At the start of each CCH/SCH interval, a Guard Interval (GI) is introduced to account for channel switching delays. During GI, all transmissions are aborted and the Medium Access Control (MAC) layer is flagged as busy. As a result, un-transmitted packets at the end of an interval must be discarded, or delayed till the next SYNC interval. Several recent papers have addressed the problem of data congestion in the control channel of the IEEE 82.p/WAVE vehicular network. In [5], we showed that message delivery ratio in VANET does not exceed 4% under under the Nakagami radio propagation model, and thus we suggested the utilization of cognitive network concepts to extend the control channel bandwidth. In [9], the authors propose the utilization of different radios for control and service applications. More recent works have investigated the problems induced by the channel switching operations on the performance of vehicular applications. To this aim, the authors of [3] propose to adjust the duration of CCH/SCH intervals, based on the traffic load of each channel. The impact of synchronized collisions at the start-of-interval is demonstrated in [] where the authors propose to synchronize the application layer with the underlying time switching intervals so that no safety messages are generated during the SCH interval or during the GI. However, the suggested solution imposes additional complication for the cross-layer design since the application layer needs to be aware about the upper MAC layer status. Authors in [7] attempt to solve the problem of synchronous collisions

3 Packet Delivery Ratio (PDR) WAVE MC (Mode I), CW=3 WAVE MC (Mode I), CW=7 WAVE MC (Mode I), CW=5 WAVE SC (Mode I), CW=3 WAVE SC (Mode I), CW=7 WAVE SC (Mode I), CW=5 Packet Delivery Ratio (PDR) WAVE MC (Mode II), CW=3 WAVE MC (Mode II), CW=7 WAVE MC (Mode II), CW=5 WAVE SC (Mode II), CW=3 WAVE SC (Mode II), CW=7 WAVE SC (Mode II), CW=5 Packet Un-transmitted Risk Index (PURI) Wave MC (Mode II), CW=3 Wave MC (Mode II), CW=7 Wave MC (Mode II), CW= (a) (b) (c) Fig. 2. Figure 2(a) and 2(b) show the PDR for the Mode I and Mode II scenarios, respectively. Figure 2(c) shows the PURI index for the Mode II scenario. TABLE I SIMULATION PARAMETERS Number of runs Scenario type 4-lanes highway [4-] Vehicle Speed 25 m/s SCH interval 5 ms CCH interval 5 ms HELLO size 4 bytes Bit Rate 6 Mbps of safety messages by uniformly distributing the packets to be transmitted during the CCH interval time. However, this additional delay might likely increase the risk that packets do not get transmitted due to the expiration of the CCH interval. III. MOTIVATIONS In this section, we investigate through a simulation study the impact of the multi-channel operations enforced by the IEEE 69.4 protocol on the performance of vehicular applications that require the delivery of periodic HELLO messages. These results motivate the need of enhanced MAC contention control mechanisms, that dynamically adapt to the channel decisions performed by the upper WAVE stack. We model a multi-channel VANETs through the ns-2 simulation tool, with the additional extensions for the WAVE protocol stack described in [6]. These extensions provide a realistic modeling of the IEEE 69.4 multi-channel operations performed by vehicles with a single radio transceiver. We consider a 4-lanes highway scenario, with a varying number of vehicles. Unless specified otherwise, we use the parameters of Table I. Each SYNC interval, simulated vehicles generate a HELLO message, and attempt to transmit it on the control channel. As described in Section II, HELLO messages can only be transmitted during the CCH Interval. If the HELLO message is generated during the SCH interval, then the vehicle must buffer it till the start of the next CCH interval. As a result, the delivery ratio and delay of these messages strongly depend on its generation time by the application layer. For this reason, we distinguish two different scenarios modes of packet transmission in our analysis: Mode I. Each vehicle generates one HELLO message, at a random time during the SCH Interval. Mode II. Each vehicle generates one HELLO message, at a random time during the CCH Interval. We do not make specific assumptions on the content of these messages. Instead, we evaluated different network configurations, in which the HELLO messages is mapped to different TC, characterized by different values of the initial Contention Window (CW) size. For each initial CW value, we considered two different WAVE protocol configurations: WAVE Multi-Channel (MC) Configuration. WAVE MC implements the legacy behavior of the IEEE 69.4 protocol, in which the vehicles periodically switch between CCH and SCH intervals, as described in Section II. WAVE Single-Channel (SC) Configuration. WAVE SC implements the configuration in which the vehicles are always tuned on the control channel, and thus do not perform channel switching operations. This configuration is introduced to evaluate the impact of the periodic CCH/SCH switching operations introduced by the IEEE 69.4 protocol. Figure 2(a) shows the Packet Delivery Ratio (PDR) as a function of the number of vehicles, for the Mode I scenario. It is easy to verify that the WAVE MC configuration suffers from higher packet losses than the WAVE SC configuration. The gap between the two configurations in terms of PDR enlarges for increasing number of vehicles (i.e. higher channel contention), and for higher-priority TCs (i.e. for smaller values of the initial CW ). This is due to the fact that in WAVE MC configuration, each vehicle has to buffer the HELLO message till the end of the SCH interval, and then initiates the backoff procedure at the start of the next CCH interval. As a result, synchronous collisions are expected to happen at the start of the CCH interval among different contending vehicles. This behavior is referred to as start-of-interval collisions in Annex C of the standard [4].

4 Figure 2(b) shows the Packet Delivery Ratio (PDR) as a function of the number of vehicles, for the Mode II scenario. In this case, the synchronous collisions problem is mitigated since vehicles are not forced to buffer the message before accessing the channel. As a result, the performance difference between the WAVE SC and the WAVE MC configurations is lower than Figure 2(a). However, the WAVE MC configuration still suffers of higher packet losses under saturated conditions compared to WAVE SC. This is mainly caused by the un-transmitted packets, that correspond to the events in which vehicles are unable to transmit the HELLO message on the control channel because of the expiration of the CCH interval, and thus discard the message itself. Figure 2(c) shows the probability of these events, denoted as Packet Un-transmitted Risk Index (PURI) for Mode II scenario. It is easy to verify that PURI increases with the number of vehicles, as a direct consequence of the increase in the access delay required to win the contention for the control channel medium. IV. WAB SCHEME Simulation results in the previous section clearly demonstrate that the multi-channel switching operations enforced by the IEEE 82.p/69.4 scheme can significantly reduce the delivery ratio of HELLO messages over congested scenario. As discussed earlier, the successful delivery of these messages is fundamental for the proper functioning of large class of safety-related vehicular applications which rely on the periodic dissemination of information (e.g. speed, acceleration, current position and others) from neighbor vehicles. For this purpose, we propose here a WAVEenhanced Adaptive Broadcast scheme (WAB) that is designed to improve the delivery rate of HELLO messages over based vehicular networks. The WAB scheme dynamically adapts to the contention level of the control channel, through a cooperative channel load estimator. In presence of high contention, it utilizes random delay to induce time diversity among concurrent transmissions, thus avoiding the problem of synchronous collisions at the start of an interval shown in Figure 2(a). Contrary the proposed solution in [7], the additional delay is introduced only in case of presence of high channel contention, and without compromising the performance of the vehicular applications. Moreover, the WAB scheme mitigates the problem of un-transmitted packets depicted in Figure 2(c), through the adoption of dynamic priority mechanisms for channel access. The block architecture of the WAB scheme is shown in Figure 3. The Channel Load Estimator Block provides an indicator of the contention level of the CCH, through the Slot Utilization (SU) metric defined in [4]. To this purpose, the Channel Load Estimator Block relies on local information gathered by the vehicle during each transmission attempt, as well as on information measurements shared by other vehicles. The Load Dissemination Block is responsible for broadcasting the SU measured information, by piggybacking them on the virtual collision/re transmission attempt NO Channel Access BLOCK backoff counter reaches zero BLOCK send to MAC layer MAC 82.p Channel Trasmission Fig. 3. transmit YES transmit on the CCH HELLO MESSAGE provide SU signal channel busy Channel Load Estimator update SU BLOCK Slot Utilization (SU) Channel Load Dissemination BLOCK Block architecture of the WAB scheme. broadcast SU periodic HELLO messages. The Channel Access Block is invoked each time the vehicle needs to transmit a HELLO message, and implements the backoff procedure foreseen by the IEEE MAC 82.p scheme. The only difference with the legacy IEEE 82.p MAC scheme is represented by the Channel Transmission Block, that is invoked once the backoff counter reaches the value of zero. Based on the current SU value, and on the priority of each transmission, the Channel Transmission Block decides to transmit the HELLO message on the channel, or to defer the transmission attempt by re-starting the backoff procedure. The complete operation of the WAB scheme is shown in Algorithm. In the following, we provide detailed descriptions of each block: Algorithm WAB scheme Operations Step. Channel Access Block Choose a random Backoff Counter between [:CW ] and start Exponential Random Backoff Mechanism if CCH interval expires then Num unt=num unt + Defer till start of the next CCH, and go back to Step end if Step 2. Channel Load Estimator and Transmission Blocks if Backoff Counter == then Compute SU new through Equation Compute SU = SU new α + SU old ( α) Compute P T = f(su, φ) through Equation 4 if (Random[,[ < P T ) then Transmit HELLO message on the control channel Reset Num att=, Num unt= else Increase Num att=num att + Double CW and go back to Step. end if end if Step 3. Channel Load Dissemination Block When vehicle γ receives a HELLO message from a neighbor: a. Compute correlation factor through Equation 7 b. Update SUγ coop through Equation 6

5 Channel Access Block. When a vehicle η needs to transmit a HELLO message during the CCH interval, and if the medium is found to be busy, it starts the Exponential Random Backoff Mechanism foreseen by the MAC 82. protocol. To this aim, it randomly initializes a Backof f Counter variable between zero and Contention Window (CW ). Let Backoff Counter CW be the initial backoff counter value randomly chosen by vehicle η while running the exponential random backoff mechanism. Backof f Counter is decremented when η senses the channel to be idle for an empty slot duration, while it is paused (freezed) in case a transmission is detected and the CCH is perceived to be busy. When Backof f Counter reaches zero, and if the CCH interval has not been expired, then η broadcasts the HELLO message on the medium. At the first transmission attempt, CW = CW min = 6 according to IEEE 82.p. Channel Load Estimator Block. Let N um Busy Slots represent the number of transmission attempts that vehicle η observes on the channel while in the backoff mechanism. Also, let N um Available Slots represent the total number of slots available for transmission observed on the channel. Based on the values of Num Available Slots and Num Busy Slots, the vehicle η computes the current Slot Utilization (SU new ) metric defined as follows: Num Busy Slots SU new = () N um Available Slots It is easy to see that SU new provides a value between and, providing a lower bound on the contention level of the control channel. A value of indicates a full saturated channel. Since each vehicle η might have previous estimations of the SU new, we account for the channel history and we defined Slot Utilization (SU) as a weighted average of the current measured value and the old stored values of slot utilization. SU = SU new α + SU old ( α) (2) Here, SU new is the value computed through Equation during the current transmission attempt, whereas SU old is the previous stored value of the SU metric, and α is a parameter that decides the relevance of history in the current decision. SU old is initialized to zero at system reset. We propose a dynamic formulation of α, that accounts for the relevance of each SU new measurement. Intuitively, we can relate the relevance of SU new with the duration of the channel observation: the longer the backoff interval (represented by N um Available Slots), the more the resultant SU new can be considered significant. Thus, we express α in the following equation as the minimum between the significance of the collected SU new and an maximum allowed α max value (set to.7 on our experiment): Num Available Slots α = min(, α max ) (3) CW Probablity of Transmission P T Slot Utilization φ= φ=2 φ=4 φ=8 Fig. 4. The values of P T computed through Equation 4. Channel Transmission Block. Based on the SU value, the vehicle η computes a probability of transmission P T, and randomly chooses a value between [ : [. If such value is less than P T, then the HELLO message is transmitted on the CCH, following the normal behavior of the legacy MAC 82.p scheme. Otherwise, vehicle η defers the transmission and increases the number of transmission attempts (Num att). In this case, the vehicle behaves like a virtual collision occurred during its attempt, i.e. it doubles the size of CW and invokes again the Channel Access Block, by initiating a new backoff process. The P T value is a function of the channel load, expressed by the SU value. The intuition behind our idea is that under high values of SU, any transmission attempt might be easily translate into a packet collision. Thus, the synchronous collisions problem can be mitigated by enforcing the vehicles to defer its transmission and schedule a retransmission attempt, with an increased value of CW. However, simply expressing the P T as a linear function of SU might introduce two additional problems: (i) it might cause system fluctuations between high and low channel loads, as discussed in [4] and (ii) it might increase the number of un-transmitted packets as a direct consequence of the additional transmission deferral introduced by the WAB scheme. To cope with these problems, we formulate P T as a function of two parameters, the SU value and a priority value (denoted by φ), as follows: P T = ( SU φ ) (4) Figure 4 shows the value of the P T as a function of SU and for different values of the priority φ. It is easy to verify that the parameter φ introduces an effective channel prioritization mechanism, where different φ layers are mapped to different values of P T, for the same amount of contention on the CCH, expressed by the SU metric. In WAB, the parameter φ is dynamically computed by each vehicle η as follows: φ = Num att Num unt (5) Here, Num att is the number of transmission attempts for the same HELLO message. It is increased by one each

6 time the WAB scheme defers the current transmission attempt, while it is reset to zero in case of message transmission. This is to avoid the risk of starvation, i.e. a vehicle that continuously defers its transmissions because of the contention control implemented by Equation 4. Num att is initially set to one. The Num unt counts the number of consecutive un-transmitted packet events, i.e. the number of consecutive CCH intervals during which vehicle η was unable to transmit the HELLO message because of CCH interval expiration. We introduce this parameter to ensure fairness among the vehicles, i.e. to guarantee the correct behavior of vehicular applications that need to receive periodic information (e.g. the actual position) from each neighbor vehicle. As before, Num unt is initialized to one and reset to zero after a message transmission. Channel Load Dissemination Block. The effectiveness of the WAB scheme depends on the ability to track correctly the contention level on the CCH, that however might change dynamically as a consequence of vehicular mobility, and network load fluctuations. To increase the accuracy of the slot utilziation estimation, the WAB protocol enhances the local measurements (performed by each vehicle while backoffing through Equation 2), with measurements performed by other vehicles. Let SU η be the slot utilization value computed by the vehicle η through Equation 2. We assume that the SU η value is piggybacked by vehicle η when transmitting the HELLO message. A vehicle γ receiving the HELLO message from η sets its cooperation slot utilization SUγ coop as follows using its own computed SU γ and the broadcasted SU η : SU coop γ = SU γ β η γ + SU η ( β η γ) (6) Here, βγ η is the correlation factor governing the impact of η estimation of slot utilization on the local estimation of slot utilization at γ. Again, we propose a dynamic formulation of βγ η that accounts for the spatial correlation between vehicles γ and η. Due to the spatiality of the wireless communication, it is likely that close vehicles might have a more similar vision of the CCH utilization than far vehicles. The βγ η value is an indication of how far the vehicle γ is from the sender vehicle η normalized between ]:[. A value close to zero indicates that γ and η are very close and thus are more likely to perceive the same channel conditions. In that case, γ can safely rely on the slot utilization value shared by η. Thus, vehicle γ computes βγ η as follows: β η γ = D(γ, η) R η (7) where R η is the transmission range of the sender vehicle η that broadcasts the HELLO message. We can fairly assume fixed transmission ranges and thus R η = R γ. D(γ, η) is the actual distance between the two vehicles, computed through GPS devices. It is worthy to note that vehicle γ will use SUγ coop (computed through Equation 6) to compute the probability of transmission P T, while it will use SU γ (computed through Equation 2) when transmitting an HELLO message. V. PERFORMANCE EVALUATION In this Section, we evaluate the performance of the proposed WAB scheme against the existing IEEE 82.p/69.4 standard. To this aim, we use ns2 with our extension described in [6] to model multi-channel VANETs. We consider a 4- lanes highway scenario while varying the number of vehicles. Unless specified otherwise, we use the simulation parameters reported in Table I. We mainly focus on two metrics in our analysis: Packet Delivery Ratio (PDR): PDR is defined as the probability of a vehicle to receive a HELLO message transmitted inside its reception zone. Per-Vehicle Throughput: Per-Vehicle Throughput is defined as the amount of bytes successfully received by a vehicle, per-second. In Section III, we demonstrated that the interval of generation of HELLO messages have a deterministic impact on the contention conditions on the CCH, and thus on the performance of the vehicular applications. For these reasons, we keep the distinction among the two scenario modes introduced in Section III. Section V-A and Section V-B present the results for the Mode I and Mode II, respectively. Section V-C analyzes the system performance in a dynamic environment with varying contention conditions on the CCH. A. Mode I Analysis In this scenario mode, each vehicle generates one HELLO message at a random instant within each SCH interval. Figure 5(a) shows the PDR plots for the WAB scheme and the legacy IEEE 82.p/69.4 MAC scheme (denoted by WAVE MC as in Section III). We vary the number of active vehicles in the scenario, and we consider different configurations of the Contention Window (CW ) size, that correspond to different Traffic Classes (T C) of the application producing the HELLO messages. Figure 5(a) shows that the adaptive contention control mechanism implemented by the WAB scheme is able to mitigate the impact of synchronous collisions, and thus to significantly increase the delivery rate over all the configurations considered with different number of vehicles. Interestingly, the performance of the WAB scheme is only marginally affected by the initial setting of the CW. Moreover, the gap among different CW values almost disappears under moderate and high channel contention conditions (i.e. number of vehicles > 6). This demonstrates the ability of the WAB scheme to self-adapt to the channel load conditions. Figure 5(b) confirms that the improvement in terms of PDR does not come at the expense of decrease in network throughput, i.e. the higher delivery ratio is not simply due to the fact that vehicles avoid transmitting HELLO messages on the CCH. Instead, Figure 5(b) shows that WAB scheme significantly improves the per-vehicle throughput over highlycongested scenarios. This is due to the fact that through the

7 Packet Delivery Ratio (PDR) WAVE MC (Mode I), CW=3 WAVE MC (Mode I), CW=7 WAVE MC (Mode I), CW=5 WAB (Mode I), CW=3 WAB (Mode I), CW=7 WAB (Mode I), CW=5 Per-vehicle Throughput (B/s) WAVE MC (Mode I), CW=3 WAVE MC (Mode I), CW=7 WAVE MC (Mode I), CW=5 WAB (Mode I), CW=3 WAB (Mode I), CW=7 WAB (Mode I), CW=5 Channel Utilization Channel Utilization, WAVE MC (Mode I) Channel Utilization, WAB (Mode I) Transmission Events, WAVE MC (Mode I) Transmission Events, WAB (Mode I) (a) (b) Simulation Time (c) Fig. 5. PDR and Per-Vehicle Throughput metrics for the Mode I analysis are shown in Figure 5(a) and 5(b), respectively. Channel utilization during the CCH interval is shown in Figure 5(c). Packet Delivery Ratio (PDR) WAVE MC (Mode II), CW=3 WAVE MC (Mode II), CW=7 WAVE MC (Mode II), CW=5 WAB (Mode II), CW=3 WAB (Mode II), CW=7 WAB (Mode II), CW= Per-vehicle Throughput (B/s) WAVE MC (Mode II), CW=3 WAVE MC (Mode II), CW=7 WAVE MC (Mode II), CW=5 2 WAB (Mode II), CW=3 WAB (Mode II), CW=7 WAB (Mode II), CW= Packet Delivery Ratio (PDR) WAVE MC (Mode II), size=2 B WAVE MC (Mode II), size=4 B WAVE MC (Mode II), size=6 B WAB (Mode II), size=2 B WAB (Mode II), size=4 B WAB (Mode II), size=6 B (a) (b) (c) Fig. 6. PDR and Per-Vehicle Throughput metrics for the Mode II analysis are shown in Figure 6(a) and 6(b), respectively. PDR as a function of the HELLO message payload size (in bytes) is shown in Figure 6(c). contention control algorithm governed by Equation 4, vehicles avoid synchronous collisions at the start of the interval, and thus maximize the number of HELLO messages successfully transmitted during the CCH interval, while preserving a high utilization of the channel. Figure 5(c) clarifies this concept, by showing the temporal schedule of message transmissions during a CCH interval (of length 46 ms), for a configuration with 4 active vehicles. It is easy to see that when using the legacy WAVE MC configuration (IEEE 82.p/69.4) most of the transmissions are concentrated on the first half of the interval, since all vehicles start the backoff process at the instant ms (i.e. at the start of the CCH). As a result, most of these transmissions result in synchronous collisions, while the channel turns to be empty in the second half of the interval (see Figure 5(c)). On the other hand, the WAB scheme introduces temporal diversity among vehicles transmissions, so that the CCH interval is mostly occupied by transmission attempts all over its duration. B. Mode II Analysis In this scenario mode, each vehicle generates one HELLO message at a random instant within each CCH interval. Figure 6(a) shows the PDR plots for the WAB and the legacy WAVE MC schemes, while varying the number of active vehicles using different configurations of CW size. Figure 6(a) shows that the WAB scheme significantly improve the delivery of HELLO messages over all different configurations, with a performance improvement of around % in the case with vehicles. Figure 6(a) confirms that the WAB scheme is independent of the initial setting of the CW size, and thus of the T C of the application generating the HELLO messages. Figure 6(b) shows the per-vehicle throughput for the Mode II analysis. Again, this result demonstrates that the WAB scheme is able to introduce a distributed contention control inside the network, that probabilistically reduces the number of MAC collisions on the control channel. Finally, Figure 6(c) shows the PDR plots for the WAB and the WAVE MC schemes, for different payload size of the HELLO messages. In WAB, the Channel Load Estimator Block described in Section IV does not assume any knowledge of the HELLO payload size. Instead, it infers the amount of contention based on the Slot Utilization defined by Equation 6, that is only a function of the number of slots used/unused, without any relationship with the transmission time involved by each message transmission attempt. As a result, the WAB scheme is able to probabilistically

8 Packet Delivery Ratio (PDR) WAVE MC WAB, Cooperation OFF WAB, Cooperation ON Simulation Time Fig. 7. The PDR in case of delayed vehicle arrivals is shown in Figure 7. reduce the contention of the CCH, regardless of the payload size of the HELLO message, as shown by Figure 6(c). C. Dynamic Contention Analysis In the previous analysis, we assumed stationary systems in which the number of vehicles is fixed during the whole simulation run, and the MAC contention conditions do not vary over time. This might be realistic over specific VANET environments, e.g. highways, where platoon of vehicles move at the same speed. In this section, we also analyze the behavior of the WAB scheme in highly-dynamic environments, like the urban one. In Figure 7, we consider a scenario in which 4 vehicles are initially active, and each vehicle generate one HELLO message at a random instant within the SYNC interval. From simulation time instant 2, a new vehicle is added to the scenario every 2 time instants, till instant 8 (time unit used is seconds). From time instant till instant 6, one vehicle is removed from the scenario, until the original configuration with 4 vehicles is restored. This scenario can model realistic urban scenarios, where traffic jams can dynamically occur at intersections with traffic lights. In Figure 7 we show the PDR over time of the legacy WAVE MC configuration, in addition to two different configurations of the WAB scheme: (i) Cooperation ON, which is the complete WAB protocol used so far, and (ii) Cooperation OFF, which is a derivative of the WAB protocol, where the Channel Load Dissemination Block is disabled, i.e. each vehicle only relies on local estimation of the slot utilization. Results in Figure 7 demonstrate that both WAB configurations are able to dynamically adapt to the load conditions, so that the impact of MAC collisions is greatly mitigated compared to the WAVE MC protocol. Also, Figure 7 reveals that cooperation is fundamental to track the channel load in a correct way. We can see that the introduction of cooperation results in a % increase in PDR. This remarkable benefit introduced by cooperation can justify the minimal overhead cost to pay (few bytes) due to the incorporation of the SU value in the HELLO messages. VI. CONCLUSION AND FUTURE WORKS In this paper, we analyzed the performance of safety-related applications over the new emerging WAVE stack for multichannel vehicular communication. First, we demonstrated that the synchronous channel switching enforced by the IEEE 69.4 scheme might significantly affect the packet delivery rate of safety applications. Then, we proposed a novel MAC contention control mechanism, called WAB scheme, that adapts to the multi-channel operations of the IEEE 69.4 scheme, in a transparent way. The WAB scheme estimates the CCH traffic conditions through a cooperative load estimator, and schedules the transmission of safety message based on the actual channel conditions. Simulation results confirm that our proposed solution can significantly enhance the packet delivery rate and throughput of safety applications on multi-channel environments, and can self-adapt to dynamic contention conditions through a complete distributed approach. Future works will include further analysis of the WAB scheme over largescale urban scenarios, and its possible adaptation for eventdriven safety applications. ACKNOWLEDGMENT Marco Di Felice is supported by Italian MIUR funds under the project PRIN-29 STEM-Net: STEM devices for selforganizing wireless NETworks. 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