Xiaomin Ma

Performance and reliability of DSRC vehicular safety communication: a formal analysis

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.

Unsaturated Performance of IEEE 802.11 Broadcast Service in Vehicle-to-Vehicle Networks

In this paper, we propose an analytic model for performance evaluation of IEEE 802.11 ad hoc broadcast networks without the assumption of saturation condition that is not true in safety related vehicle-to-vehicle (v2v) communication. We use a discrete time M/G/l queue to model occasional occurrences of safety related message in each vehicle. By means of probability generating function (PGF) and a recursive algorithm, we obtain solution to the analytic model. Based on the solution, we derive several important performance indices for IEEE 802.11 broadcast service, such as packet delay, throughput, packet delivery ratio, and service time distribution. The proposed analytic model is validated by extensive simulations. Numerical results also reveal characteristics of the broadcast service in a typical v2v wireless network.

A Quantitative Approach to Evaluate DSRC Highway Inter-Vehicle Safety Communication

Inter-vehicle safety applications on highways rely considerably on the real-time and reliable communication in the vehicular ad hoc network. For engineers and researchers, it is greatly necessary and important to evaluate the provisions of the communication systems before they start designing them for such applications. This paper presents a quantitative approach to evaluate the quality of service that the inter-vehicle communication (using IEEE- and ASTM-adopted dedicated short range communication (DSRC)) can provide to the inter-vehicle safety application in the context of highway scenarios. Based on the proposed model, we analyze performance indices such as delay and packet delivery ratio (PDR). Outputs of the model are validated by computer simulations. On one hand, our quantitative model provides not only a convenient framework to conduct fast evaluation for the communication of the inter-vehicle safety applications but also a means to analyze the suitability of 802.11a- based DSRC for highway safety applications. On the other hand, it reveals the requirements for further improvement of the performance.

Saturation Performance of IEEE 802.11 Broadcast Scheme in Ad Hoc Wireless LANs

Broadcast communication is important in inter- vehicle communication. Vehicles broadcast safety related messages to pre-warn the vicinity vehicles. The new IEEE 802.11p is being developed based on IEEE 802.11a to address the need for vehicle-to-vehicle and vehicle-to-roadside-unit communication. In this paper, we study the saturation performance of IEEE 802.11 broadcast by developing an analytic model as well as by conducting simulation. Having investigated the literature and studied the features of the broadcast, we point out that the analytic models for IEEE 802.11 unicast cannot be simply applied for broadcast. We construct our model to characterize the operation of the backoff counter in broadcast. Based on solutions to the model, closed form expressions of performance indices including throughput, packet delay, and packet deliver ratio are derived. Numerical results reveal several characteristics of the broadcast service. Our research work provides a new means to approach analysis and enhancement of IEEE 802.11 broadcast performance and reliability in wireless ad hoc networks.

On the Broadcast Packet Reception Rates in One-Dimensional MANETs

In this paper, we investigate saturation performance of broadcast service in IEEE 802.11 based one-dimensional mobile ad hoc networks (MANETs) under multi-hop environment. Having analyzed the features of the multi-hop broadcast MANETs, for the first time we derive a closed form expression of broadcast packet reception rates, which is one of the important reliability metrics for broadcast communications. The analytic model takes IEEE 802.11 backoff counter process and hidden terminal problem into account. The proposed analytic model is validated by extensive simulations. From the obtained numerical results under various network parameters, new observations and potential enhancement suggestions are given.

CSMA based Inter-Vehicle Communication Using Distributed and Polling Coordination

Intelligent transportation systems rely heavily on inter-vehicle communication (IVC) systems to support, among other things, safe driving by relaying essential information in real time. It is very important to guarantee the delivery of an imminent collision warning in real time in order to make appropriate actions to avoid it. Several existing MAC layer protocols are modified to address real-time traffic in ad hoc vehicle environment; carrier sense multiple access (CSMA) is one such protocol. This paper implements a priority in CSMA using different backoff time spacing (BTS) to allow higher priority traffic to access the medium faster than those with medium-to-low priority. The backoff time spacing is inversely proportional to the priority; the higher the priority, the lower the backoff time. The paper also proposes a new media access control (MAC) protocol implementation that combines distributed and polling coordination functions to further enhance the BTS-priority-based CSMA and to ensure the absolute delivery of traffic with the highest priority. A computer simulation was carried out to evaluate the performance of the proposed protocol. When compared with the BTS-based CSMA implementation, the simulation results indicate significant improvements in the system throughput and successful packet rate for the traffic with highest priority.

Capture effect on R-ALOHA protocol for inter-vehicle communications

As a component of the intelligent transportation system (ITS) and one of the concrete applications of mobile ad-hoc networks, inter-vehicle communication (IVC) has attracted research attention from both the academia and industry. A key issue in the wireless network is the design of medium access control (MAC) layer protocols for distributed channel assignment. A promising class of protocols for IVC networks is R-ALOHA-based protocols. In this paper, we document our investigation of near-far effects and capture effects on R-ALOHA for a real-time distributed packet wireless network or IVC in the presence of multipath, and shadowing. The research recorded in this paper also studied the stability of R-ALOHA protocol for IVC analytically and by simulation. In order to analyze how fast R-ALOHA makes the network stable once communication among terminals is initiated, we construct a discrete Markov chain and derive new recursive formulas based on solutions to Markov chain to evaluate distribution and mean value of system stabilization time in the fading environment.

Analytical Model for Broadcast Packet Reception Rates in Two-Dimensional MANETs

In this paper, an analytic model is built to derive a closed form broadcast packet reception rates in IEEE 802.11 based two-dimensional (2-D) mobile ad-hoc networks (MANETs), which is the extension of 1-D MANET analytical model in [4]. The analytic model takes two straight parallel line geometry, IEEE 802.11 backoff counter process, hidden terminal problem, message arrival interval, and fading transmission channel into account. The proposed analytic model can be applied to vehicular ad-hoc networks (VANETs) built along a highway. From the obtained numerical results under various network parameters, the new model is validated and new observations are obtained.

Reconsider Broadcast Packet Reception Rates in One-Dimensional MANETs

In this paper, a new analytic model is built to derive a closed-form expression of broadcast packet reception rates (PRRs) in IEEE 802.11 based one-dimensional (1-D) mobile ad-hoc networks (MANETs), which is the extension of 1-D MANET analytical model in [7]. Comparing with the existing models, the proposed model accounts for the impact of more factors and network parameters on PRRs such as non-saturated packet arrival in each mobile node, varied transmission, carrier sensing, and interference ranges, distances of receivers to the sender, and collisions caused by concurrent transmissions. As an example, the proposed model is applied to evaluate a dedicated short range communication (DSRC) safety-related service on a highway. Based on the simulation results, some important observations to the design and analysis of such wireless communication networks are provided.

Analysis of sliding frame R-ALOHA protocol for real-time distributed wireless networks

In this article, performance of sliding frame (SF) R-ALOHA protocols for real-time distributed wireless networks is investigated analytically and by simulation. First, a discrete Markov chain and Monte Carlo modeling are constructed to evaluate dynamic behavior of the protocol in transient state including distribution of the system stabilization time (SST) and the average number of successful terminals. Furthermore, a hierarchical decomposition is conducted to simplify steady state analysis, thus generating two one-dimensional Markov chains for closed-form performance of SF R-ALOHA under local wireless environment. Terminals with Poisson message arrivals and Poisson message length are analyzed. Consequently, performance indices, such as throughput, the average message delay, and packet dropping probability, are derived from the proposed analytic models. Capture effects on the SF R-ALOHA system in the presence of multipath and shadowing are also obtained by the Markov chain analysis. The numerical results from the analytic models are compared with that from simulation and equilibrium point analysis, proving correctness, accuracy, and scalability of the decomposition. The results also reveal performance characteristics of the SF R-ALOHA system.