Performance and Reliability of DSRC Vehicular Safety Communication: A Formal Analysis
© Xiaomin Ma et al. 2009
Received: 31 March 2008
Accepted: 26 November 2008
Published: 18 January 2009
IEEE- and ASTM-adopted dedicated short range communications (DSRC) standard toward 802.11p is a key enabling technology for the next generation of vehicular safety communication. Broadcasting of safety messages is one of the fundamental services in DSRC. There have been numerous publications addressing design and analysis of such broadcast ad hoc system based on the simulations. For the first time, an analytical model is proposed in this paper to evaluate performance and reliability of IEEE 802.11a-based vehicle-to-vehicle (V2V) safety-related broadcast services in DSRC system on highway. The proposed model takes two safety services with different priorities, nonsaturated message arrival, hidden terminal problem, fading transmission channel, transmission range, IEEE 802.11 backoff counter process, and highly mobile vehicles on highway into account. Based on the solutions to the proposed analytic model, closed-form expressions of channel throughput, transmission delay, and packet reception rates are derived. From the obtained numerical results under various offered traffic and network parameters, new insights and enhancement suggestions are given.
Transportation safety is one of the most important intelligent transportation system (ITS) applications. Active safety applications, that use autonomous vehicle sensors such as radar, lidar, and camera are being developed and deployed in vehicles by automakers to address the vehicle accident problem. Communications systems are expected to play a pivotal role in the ITS safety applications. Message communication in the ITS is normally achieved by installing a radio transceiver in each vehicle allowing wireless communications. Vehicle-to-vehicle (V2V) communication is of critical importance to many ITS safety-related services. The DSRC standard with 75 MHz at 5.9 GHz band was projected and licensed to support low-latency wireless data communications among vehicles and from vehicles to roadside units in USA [1–3]. Essentially, the DSRC radio technology is IEEE 802.11a adjusted for low-overhead operations in the DSRC spectrum. It is being standardized as IEEE 802.11p [2–5].
Safety applications usually demand direct V2V ad hoc communication networks due to highly dynamic topology of the networks and the stringent delay requirements . They will likely work in a broadcast fashion since safety information can be beneficial to all vehicles around a sender and safety message senders might not expect a response at the application level. For the purpose of high reliability and simple implementation, some direct (or single-hop) broadcast solutions to safety-related message transmission have been suggested and investigated. Xu et al.  propose several single-hop broadcast protocols to improve reception reliability and channel throughput. Torrent-Moreno et al.  propose a priority access scheme for IEEE 802.11-based vehicular ad hoc networks and show that the broadcast reception probability can become very low under saturation conditions. Jiang et al.  raise channel congestion control issues for vehicular safety communication, and introduce feedback information to enhance system performance and reliability. ElBatt et al.  discuss the suitability of DSRC periodic broadcast message for cooperative collision warning application. To date, all analyses and observations are mainly based on simulations [8, 11]. Although the connectivity of unicast wireless networks is studied theoretically , the factors that affect DSRC system performance and reliability such as IEEE 802.11 backoff counter process, hidden terminals, channel access priority, message generation interval, and high mobility on the road have not been theoretically addressed yet for the analysis of the DSRC safety broadcast communications. As a matter of fact, the exact analysis of such broadcast ad hoc networks with hidden terminal is still an open problem .
In this paper, we first introduce and justify an effective solution to the design of the control channel in DSRC with two levels of safety services covering most of the possible safety applications. Then, we construct an analytical model based on Markov chain method in  to evaluate performance and reliability indices such as channel throughput, transmission delay, and packet reception rates of a typical network solution for DSRC-based safety-related communication under highway wireless communication environment. We apply our proposed model to evaluate the impact of message arrival interval, channel access priority scheme, hidden terminal problem, fading transmission channel, and highly mobile vehicles on the performance and reliability. Based on the observations of numerical results under typical DSRC environment, some enhancement schemes are suggested or validated accordingly.
The remainder of this paper is organized as follows. Section 2 briefly reviews the DSRC communication system and environment, specifies requirements for the safety-related communication, and presents DSRC control channel design to cover most of the possible safety applications. Section 3 demonstrates an analytic model for broadcasting two levels of safety-related messages using the control channel in the highway scenario. Consequently, closed-form expressions of performance and reliability indices are derived. In Section 4, the proposed analytical model is validated by extensive simulations. In terms of the numerical results, some important observations and constructive enhancement suggestions are given. This paper is concluded in Section 5.
2. DSRC System Descriptions and Solution to Safety Message Broadcast
2.1. DSRC System for Safety Applications
The 5.9 GHz DSRC is a wireless interface expected to support high speed, short-range wireless interface between vehicles, and surface transportation infrastructure, as well as to enable the rapid communication of vehicle data and other content between on board equipment (OBE) and roadside equipment (RSE).
The DSRC spectrum consists of seven ten-megahertz channels that include one control channel and six service channels. Channel 178 is the control channel, which is generally restricted to safety communications only [2, 3]. The DSRC physical layer uses an orthogonal frequency division multiplex (OFDM) modulation scheme to multiplex data. The DSRC physical layer follows exactly the same frame structure, modulation scheme, and training sequences specified by IEEE 802.11a physical layer. However, DSRC applications require reliable communication between OBEs and from OBE to RSUs when vehicles are moving up to 120 miles/hour and having communication ranges up to 1000 meters. According to the updated version of the DSRC standard, the MAC layer of the DSRC adopts 802.11a layer specification with minor modifications. This is a random access scheme for all associated devices in a cluster based on carrier sense multiple access with collision avoidance (CSMA/CA). In the 802.11 MAC protocols, the fundamental mechanism for medium access is the distributed coordination function (DCF). DCF is meant to support an ad hoc network without the need of any infrastructure element such as an access point, but DCF is not able to provide predictable quality of service (QoS). The development of a robust and efficient MAC protocol will be central to the new generation DSRC devices.
Broadcast procedure of 802.11 MAC follows the basic medium access protocol of DCF except that no positive acknowledgement and retransmission exist. Broadcast of DSRC MAC occurs when a broadcast packet arrives at DSRC MAC layer from the upper layer and the MAC senses the channel status first and stores the status. Next, once an idle period equal to DIFS/EIFS is observed, MAC takes the next operation according to the stored channel status and the current value of its backoff time. If the current value of the backoff counter is not zero, MAC begins the backoff countdown process. If the current value of the backoff counter is zero and the status of the medium is busy, MAC generates random backoff time and begins the backoff countdown process. If the backoff counter counts down to zero, MAC begins data packet transmission immediately. During the backoff countdown process, carrier sense persists. If the medium becomes busy again, MAC goes back to the DIFS/EIFS observation process. During or after a broadcast transmission, MAC does not monitor the success or failure of the transmission. Once transmission completes, MAC simply releases the medium and competes for it when a new packet is ready to be sent.
There are two types of safety-related life messages that will likely be transmitted over the control channel [7, 15]: event-driven (or emergency) safety messages and periodic (or routine) safety messages. Event-driven messages will contain information about environment hazards. They will be transmitted when an emergency or a nonsafe situation is detected to make all the vehicles in the area aware or activates an actuator of an active safety system. Event-driven communications happen only occasionally, but must meet a requirement of fast and guaranteed delivery. Routine messages will contain state of vehicles (e.g., position, speed, and direction) and will be broadcasted by all vehicles at a frequency 10–20 times per second.
2.2. DSRC Environment
Under V2V communication environment, the vehicles are highly mobile and the network topology changes very frequently. These changes are due to the high relative speed of vehicles, even when they are moving in the same direction. Two vehicles can directly communicate only when they are within their radio range. For safety communication in DSRC, the high mobility of vehicles on the road may cause two adverse effects on performance of message sending and receiving. On one hand, during the transmission of safety-related message, some of the receivers may move out of the transmission range of the sender, resulting in failure of receiving the message. On the other hand, high mobility makes worse Doppler spread on OFDM, which leads to higher packet error rates and consequently lower channel capacity.
V2V communications present scenarios with unfavorable characteristics to deploy wireless communications, for example, multiple reflecting objects that are able to degrade the strength and quality of the received signal. While there are many factors that can affect the bit error rate (BER) on a multihop communication environment, mobility of nodes is one of the most important factors that can cause packet errors.
The problem of hidden terminals is a critical issue affecting the performance and the reliability of ad hoc networks. Hidden terminals are two terminals that although they are outside the interference range of one another, they share a set of terminals that are within the transmission range of both. Broadcast in IEEE 802.11 does not use virtual carrier sensing and thus only relies on physical carrier sensing to reduce collision . In the case of broadcast communication, the potential hidden terminal area needs to include the receiving range of all the terminals within transmission range of the sender. Thus, the potential hidden terminal area in broadcast can be dramatically larger than that of unicast. In other words, the broadcast fashion of V2V safety communications makes them more sensitive to hidden terminal problems.
2.3. Requirements of Safety-Related Communication
Parameters for road traffic.
Average vehicle distance
10 m (jammed), 30 m (smooth)
Message generation interval
Lanes in each direction
70 km/h~120 km/h
Packet payload size, P
2.4. Solutions to Broadcasting of Safety Message
To support safety applications in the DSRC system with high reliability and low delay, the basic link-layer behavior and the environment of safety communications in the control channel can be defined as follows [8, 9, 15].
Vehicle safety communication networks are entirely distributed ad hoc wireless networks.
Two types of the safety messages are broadcasted in the control channel; event-driven safety messages consist of all real-time safety critical information, while routine safety messages consist of the state of vehicles, and some safety-related information with loose delay requirement.
Most of the identified safety applications are based on direct or single-hop broadcast communication among vehicles within the range of one another.
Transmission power of each vehicle for safety-related communication should be strong enough to reach all potentially affected vehicles that need to take actions immediately.
Each safety message is usually very short (100~300 bytes), and thus usually mapped to a single frame.
A real-time priority scheme similar to IEEE 802.11e is adopted to differentiate two safety services by using different distributed coordination function (DCF) backoff window sizes: the higher priority class uses the window and the lower priority class uses the window .
The above framework of direct (or single-hop) safety message broadcast is justified as follows. (1) As an emergency situation takes place, the potentially affected vehicles that need to be alerted immediately must be very close to where the safety message is sent out. So direct message broadcasting would be enough to reach all such vehicles. (2) Some safety-related services that desire multihops of message forwarding (e.g., road caution hazard notification, and post crash notification) can be transmitted as routine safety message because delay requirement for the services is relatively longer (0.5–2 seconds). (3) Compared with multihop broadcast, single-hop broadcasting communication has the characteristics of lower delay, higher reliability, and being easier to implement and analyze.
Considering that reliability of safety message transmission is the most critical among other performance indices, we introduce or suggest several potential mechanisms to enhance the packet reception rates. (1) Increase backoff window sizes to reduce chances of packet collisions; (2) increase carrier sensing range to withstand the effect of hidden terminal; (3) design proper repetitions of the emergency packet within the packet lifetime; (4) give the event-driven safety service preemptive priority over the routine safety service so that possible collisions between two types of service will be reduced. Normally, routine safety message transmissions dominate the channel. Once an emergency messaging takes place, routine services stop and emergent message delivery will occupy the channel. In this paper, an enhancement scheme that combines step (3) with (4) is modeled and analyzed and all repetitions are separated by SIFS. The reason is that SIFS is long enough for all receivers to be able to identify individual packet, but is not too long to be mistaken by other vehicles as the end of a transmission.
3. System Model and Performance Analysis
3.1. Assumptions for IEEE 802.11 Broadcast in DSRC
In this paper, we focus on reliability and performance analysis of the DSRC control channel with two levels of safety services. Real world radio networks are influenced by many factors. In our model, we assume that IEEE 802.11 broadcast DCF works under the following scenarios.
We consider a highway environment where vehicles are exponentially distributed and they travel in free-flow conditions. As seen in Figure 1, the vehicular V2V network built along a highway is simplified as a one-dimensional (1D) mobile ad hoc networks which consist of a collection of statistically identical mobile stations randomly located on a line.
Given the tagged vehicle (the vehicle sending message) placed in origin, all vehicles have the same carrier sensing range which is assumed to vary between the range . The average number of vehicles in carrier sensing range of the tagged vehicle on the road is .
As shown in Figure 1, when the vehicular V2V network considered is simplified as a one-dimensional network, the potential hidden terminal area of the tagged vehicle in broadcast communication drops in the range of and assuming that the carrier sensing range equals the range within which one node interferers with other node. The average number of the potential hidden vehicles of the tagged vehicle on the road is .
There are two queues in each vehicle. One is for routine messages and the other is for emergency messages. They sense and access the channel independently. If two services conflict with each other in a vehicle, the emergency packet will be served first. The queue length of packets each vehicle can store at the MAC layer is unlimited. So each vehicle can be modeled as two independent discrete time M/G/1 queues . Two broadcast services share the common control channel.
The relative velocity of vehicles in the network is assumed to be uniformly distributed in the interval , where is the maximum relative speed. The average relative velocity of two vehicles in the network is assumed to be a constant value .
V2V communications present scenarios with unfavorable characteristics of channel fading in DSRC. The channel fading is reflected by simply introducing packet error probability , where P is the length of the packet, is the length of packet header, and is the fixed bit error rate (BER) probability. can be numerically evaluated for a Rician fading channel . When data bits are transmitted over Nakagami-m fading links, can be easily obtained using the closed form expressions given in . Capture effect is not considered in this paper.
With high channel data rates and relatively big backoff window size W 0 , the consecutive freeze effect  in IEEE 802.11 DCF on the broadcast performance is neglected.
All nodes within one-hop range of the transmitted node are assumed to have synchronized time scale. It has been proven that by extensive simulations, the impact of the asynchronous time scale on the performance is negligible; if the transmitted packet is short, the backoff window size is big enough, and the channel data rate is high .
3.2. Backoff Process in IEEE 802.11 Broadcast
3.3. Performance of Channel for Tagged Vehicle
We consider a vehicular wireless ad hoc broadcast network with dynamic topology where each vehicle can send out a packet if there is no transmission sensed within the carrier sensing range of the vehicle. So here a channel is defined with respect to any vehicle sending out packet (referred to as the tagged vehicle).
where may be either for emergent transmission or for routine transmission; is the hidden vulnerable period, p e is packet error probability defined in Section 3.1, and is link breaking probability for a communication pair, which will be defined and evaluated later in Section 3.5. Note that here "successful" means all nodes within transmission range of the tagged vehicle have received broadcast information from the tagged vehicle. From (7), we can see that the successful transmission takes place under the following conditions: (1) no nodes within transmission range of the tagged vehicle transmit at the time instant when the tagged vehicle starts to transmit; (2) no nodes in the two potential hidden terminal areas (see Figure 1) transmit during a vulnerable period (normalized to the number of time slots through dividing by length of a virtual slot); (3) no transmission errors occur during the packet transmission; (4) no vehicles receiving the packet move out of the transmission range of the tagged vehicle throughout the packet transmission. The reason for the vulnerable period calculation is that the collision caused by nodes in potential hidden area could happen during the period that begins a transmission period before the tagged node starts its transmission and ends after the tagged node completes its transmission. In the one-dimensional mobility model as shown in Figure 1, there are two potential hidden terminal areas with respect to the tagged node. In each potential hidden terminal area, a transmission from a hidden node will be sensed by other hidden nodes in the same area, which may cause silence of the other nodes. Since two potential hidden terminal areas in Figure 1 are away from each other, vehicles in one area cannot hear the transmission status of vehicles in the other area. Transmissions in two areas are mutually independent. Each hidden terminal has chances to fail the target vehicle transmission: either by the tagged vehicle starts sending while a hidden terminal is transmitting or by that one hidden terminal starts transmitting while the tagged vehicle is transmitting.
3.4. Service Time
The MAC layer service time is the time interval from the time instant when a packet becomes the head of the queue and starts to contend for transmission to the time instant when the packet transmission is over. This time is important when we examine the performance of higher protocol layers. Apparently, the distribution of the MAC layer service time is a discrete probability distribution when the smallest time unit of the backoff timer is a time slot σ. Here, we model the characteristics of each vehicle in the network as two M/G/1 queues and approach service time distributions through probability generating function (PGF).
In order to derive the average service time distributions, the probability must be determined, while calculation depends on the duration of service time. In this paper, we apply an iterative algorithm to calculate .
The iterative steps are outlined as follows.
Calculate service time distributions through PGF.
Packet transmission delay is the average delay a packet experiences between the time at which the packet is generated and the time at which the packet is successfully received. It includes the medium service time (due to backoff, busy channel waiting, interframe spaces, transmission delay, and propagation delay, etc.), and queuing delay.
3.6. Link Breaking Probability
Define X to be the distance from the position of any vehicle at instant when the tagged vehicle is requesting channel for packet transmission to the boundary of the tagged vehicle transmission range.
3.7. Normalized Channel Throughput
3.8. Packet Reception Rate
Packet reception rate (PRR) is defined as the ratio of the number of packets successfully received to the number of packets transmitted. So PRR can be interpreted as the probability that all vehicles within transmission range of the tagged vehicle receive the broadcast message successfully in a virtual slot.
PRR expression (37) is divided into five parts (1) all nodes will receive the transmitting packet from the tagged node if no nodes within the transmission range of the tagged node transmit at the time instant when the tagged node starts to transmit; (2) only part of nodes will receive the transmitting packet as there is at least one node in the transmission range of the tagged node transmitting in a virtual slot; (3) some nodes in may fail to receive the broadcast packet if any nodes in the two potential hidden terminal areas (see Figure 1) transmit during the vulnerable period ; (4) some nodes in may fail to receive the broadcast packet if any transmission error occurs during the packet transmission; (5) some nodes in may fail to receive the broadcast packet if the nodes move out of the transmission range during the transmission period.
Notice that PRR is a very important reliability measure, which evaluates how all vehicles within the transmission range of the tagged transmitting vehicle receive the broadcast safety-related message. Since two levels of safety services share a common control channel, their one-hop theoretical PRRs should be identical.
4. Model Validation and Numerical Results
Parameters for communications in DSRC.
BPSK, QPSK, 16-QAM, 64-QAM
1/2, 2/3, 3/4
OFDM symbol duration
8 μ s
Channel data rate
6, 9, 12, 24, 36, 54 Mbit/s
DIFS for 802.11a
64 μ s
Slot time, σ
16 μ s
SIFS for 802.11a
32 μ s
Propagation delay, δ
1 μ s
40 μ s
PLCP header length
8 μ s
4.1. Model Validation
In order to validate the proposed analytic model, we write our own event-driven simulation program in MATLAB. Our simulation is conducted under a highway DSRC environment within road length of 5000 miles. The simulation program includes main physical (except modulation, demodulation, and coding) and MAC behavior of IEEE 802.11 broadcast ad hoc communication with DSRC parameters. The program adopts assumptions that both vehicles on the road and packet generation interval are exponentially distributed. According to the results from , intervehicle spacing in a network that is disconnected due to low traffic volume can be characterized by exponential distribution. With distributed asynchronous channel access and limited transmission range and carrier sensing range, the consecutive freeze effect in IEEE 802.11 DCF, asynchronous time scale, and the hidden terminal problem are naturally reflected in the simulation process. The time resolution of the simulation program is exactly the minimum time unit (1 μ s) specified in IEEE 802.11 standard. Each simulation round lasts 1200 seconds.
Comparing the obtained reliability and performance under typical DSRC environment with requirements set for safety-related ad hoc communication network, we can see that it is no problem for packet delivery delay for emergency safety service (<1.2 milliseconds) to meet the requirement (500 milliseconds); it is even not a problem for routine safety service to reach its 5 hops away destination (2.5 kilometers) within 5 × 2 milliseconds = 10 milliseconds. However, the obtained packet reception rates (<0.8) fail to meet reliability requirement ( ) for DSRC safety critical messaging.
4.2. Observations and Discussions
From Figure 4–6, it is observed that increasing data rate (from 24 Mbps to 54 Mbps) helps significantly improve the delay. However, increasing data rate reduces the channel throughput under unsaturated channel condition. Data rate changes have minor effect on packet reception rate. As the road traffic is getting heavier (<0.2 vehicles/mile), transmission delays and packet reception rates are getting worse, but channel throughputs are increased accordingly because the channel is still unsaturated.
4.3. Impact of Enhancement
As shown in Figure 9, although the enhancement brings longer packet delivery delay, the maximum delay introduced ( milliseconds) is still much less than the required message lifetime 500 milliseconds. Increasing the number of message repetitions N r and carrier sensing range ( ) helps improve reliability of safety message transmission. But as emergency traffic is getting higher ( pck/s in Figure 10), excessive repetitions may make the PRRs worse instead (see the curve with ). The reason for the observation is that increasing N r actually increases time period on which a transmitting vehicle occupies the channel, thus bringing more chances of collisions and interference. It is noticed that optimal selection of N r depends on network environment, network parameters, and vehicle traffic on the road.
In this paper, we investigate reliability and performance of DSRC ad hoc V2V communication networks with two levels of safety-related services analytically and by simulation. Several important performance indices for broadcast such as channel throughput, packet reception rates, and packet delivery delay are derived from the proposed analytical model taking IEEE 802.11 backoff counter process, fading channel, hidden terminal, nonsaturation traffic, mobility, and so forth, into account. Numerical results reveal characteristics of the DSRC communication system for safety application. From the analysis of DSRC safety services on highway, we observe that (1) under typical DSRC environment, IEEE 802.11a is able to meet the safety message delay requirement, but is not able to guarantee high reliability because of possible transmission collision and harsh channel fading; (2) hidden terminal problem in broadcast is more severe than that in unicast; (3) high mobility of vehicles has minor impact on the reliability and performance of the direct single hop broadcast network with high data rate; (4) with direct broadcast and preemptive emergent message transmission, it is possible to meet both performance requirement and reliability requirement simultaneously through adjusting backoff window size, appropriate number of packet repetitions, and enough range of carrier sensing.
Our future research work will focus on development and analysis of new effective and robust MAC protocols toward 802.11p, which includes adaptively adjusted network parameters in terms of current traffic load and network situation for optimized performance and reliability.
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