Daniel Jiang

IEEE 802.11p: Towards an International Standard for Wireless Access in Vehicular Environments

Vehicular environments impose a set of new requirements on todays wireless communication systems. Vehicular safety communications applications cannot tolerate long connection establishment delays before being enabled to communicate with other vehicles encountered on the road. Similarly, non-safety applications also demand efficient connection setup with roadside stations providing services (e.g. digital map update) because of the limited time it takes for a car to drive through the coverage area. Additionally, the rapidly moving vehicles and complex roadway environment present challenges at the PHY level. The IEEE 802.11 standard body is currently working on a new amendment, IEEE 802.1 lp, to address these concerns. This document is named wireless access in vehicular environment, also known as WAVE. As of writing, the draft document for IEEE 802.11p is making progress and moving closer towards acceptance by the general IEEE 802.11 working group. It is projected to pass letter ballot in the first half of 2008. This paper provides an overview of the latest draft proposed for IEEE 802.11p. It is intended to provide an insight into the reasoning and approaches behind the document.

Design of 5.9 ghz dsrc-based vehicular safety communication

The automotive industry is moving aggressively in the direction of advanced active safety. Dedicated short-range communication (DSRC) is a key enabling technology for the next generation of communication-based safety applications. One aspect of vehicular safety communication is the routine broadcast of messages among all equipped vehicles. Therefore, channel congestion control and broadcast performance improvement are of particular concern and need to be addressed in the overall protocol design. Furthermore, the explicit multichannel nature of DSRC necessitates a concurrent multichannel operational scheme for safety and non-safety applications. This article provides an overview of DSRC based vehicular safety communications and proposes a coherent set of protocols to address these requirements

Broadcast reception rates and effects of priority access in 802.11-based vehicular ad-hoc networks

One key usage of VANET is to support vehicle safety applications. This use case is characterized by the prominence of broadcasts in scaled settings. In this context, we try to answer the following questions: i) what is the probability of reception of a broadcast message by another car depending on its distance to the sender, ii) how to give priority access and an improved reception rate for important warnings, e.g., sent out in an emergency situation, and iii) how are the above two results affected by signal strength fluctuations caused by radio channel fading? We quantify via simulation the probability of reception for the two-ray-ground propagation model as well as for the Nakagami distribution in saturated environments. By making use of some IEEE 802.11e EDCA mechanisms for priority access, we do not only quantify how channel access times can be reduced but also demonstrate how improved reception rates can be achieved. Our results show that the mechanisms for priority access are successful under the two-way-ground model. However, with a non-deterministic radio propagation model like Nakagamis distribution the benefit is still obvious but the general level of probability of reception is much smaller compared to two-ray-ground model. The results indicate that -- particularly for safety-critical and sensor network type of applications -- the proper design of repetition or multi-hop retransmission strategies represents an important aspect of future work for robustness and network stability of vehicular ad hoc networks.

Overhaul of ieee 802.11 modeling and simulation in ns-2

NS-2, with its IEEE 802.11 support, is a widely utilized simulation tool for wireless communications researchers. However, the current NS-2 distribution code has some significant shortcomings both in the overall architecture and the modeling details of the IEEE 802.11 MAC and PHY modules. This paper presents a completely revised architecture and design for these two modules. The resulting PHY is a full featured generic module able to support any single channel frame-based communications (i.e. it is also able to support non-IEEE 802.11 based MAC). The key features include cumulative SINR computation, preamble and PLCP header processing and capture, and frame body capture. The MAC accurately models the basic IEEE 802.11 CSMA/CA mechanism, as required for credible simulation studies. The newly designed MAC models transmission and reception coordination, backoff management and channel state monitoring in a structured and modular manner. In turn, the contributions of this paper make extending the MAC for protocol researches much easier and provide for a significantly higher level of simulation accuracy.

Empirical determination of channel characteristics for DSRC vehicle-to-vehicle communication

Dedicated Short Range Communication (DSRC) wireless band, allocated by the FCC for vehicular communication, constitutes the basis for one of the first vehicular ad-hoc networks/systems that is likely to be deployed. Therefore, it is important to characterize the physical properties of the DSRC channel.In this work we propose that due to the complexity, unpredictability and wide variety of road environments a statistical parametric model should be used to describe the physical channel behavior, and its parameters should be inferred from empirical data.Based on this methodological approach we construct channel gain models for two different environments: an open space and a typical highway with moderate traffic. To model the distribution of channel gain amplitude we choose the well-known two-parameter Nakagami model and estimate the distance dependency of its parameters from empirical road data. Spatial correlation of the channel strength is also estimated for a few separation distances.The results obtained show that in both environments the Nakagami average power parameter O falls off as the inverse-square of the sender-receiver separation distance up to a crossover distance of about 160m and as the inverse-fourth of the distance thereafter. The Nakagami fading parameter m lies between 1 and 4 for the open area and between 0.5 and 1 for the highway. The spatial correlation coefficients lie between 0.4 and 0.75 for the open environment, but between 0.9 and 1 for the highway. These results provide valuable input to support the design of optimal modulation, coding, diversity and protocol schemes for vehicle-to-vehicle and vehicle-to-infrastructure communication.

Optimal data rate selection for vehicle safety communications

This paper answers a simple but important question in VANET research: what is the optimal data rate to be used in DSRC-based vehicle safety communications? While it is generally accepted that the default choice is 6 Mbps, this assumption is not rooted in strong technical considerations. This paper provides a systematic evaluation of optimized data rates choices in a variety of scenarios. The answer found enables researchers to generally eliminate one dimension of complexity in relevant VANET studies. Additionally, the methodology used in this paper is possibly as interesting as the conclusion.

IEEE 1609.4 DSRC multi-channel operations and its implications on vehicle safety communications

This paper provides an overview of IEEE 1609.4, a work in progress standard for multi-channel operations over the 5.9GHz Dedicated Short Range Communications (DSRC) spectrum. In the U.S., the DSRC spectrum is organized into several channels. IEEE 1609.4 defines a time-division scheme for DSRC radios to alternately switch within these channels to support different applications concurrently. We describe the main features of IEEE 1609.4 in detail and discuss the main concerns with the original protocol design. In particular, we focus on those issues that can have a significant impact on vehicle safety communications. While IEEE 1609.4 is currently being updated and revised, this paper is intended to contribute to the technical discussions, and to bring attention to the most relevant and critical issues. This paper also contains results from software simulations conducted to study vehicle safety communications under stressful but realistic conditions. These results confirm concerns for the currently proposed scheme and provide a motivation for updating and revising the standard.

Design methodology and evaluation of rate adaptation based congestion control for Vehicle Safety Communications

Vehicle Safety Communications (VSC) is advancing rapidly towards product development and field testing. While a number of possible solutions have been proposed, the question remains open as how such a system will address the issue of scalability in its actual deployment. This paper presents a design methodology for congestion control in VSC as well as the description and evaluation of a resulting rate adaption oriented protocol named PULSAR. We start with a list of design principles reflecting the state of the art that define why and how vehicles should behave while responding to channel congestion in order to ensure fairness and support the needs of safety applications. From these principles, we derive protocol building blocks required to fulfill the defined objectives. Then, the actual protocol is described and assessed in detail, including a discussion on the intricate features of channel load assessment, rate adaptation and information sharing. A comparison with other state-of-the-art protocols shows that “details matter” with respect to the temporal and spatial dimensions of the protocol outcome.

IEEE 802.11 based vehicular communication simulation design for NS-2

DSRC is a promising IEEE 802.11 based wireless technology that enables advanced active vehicle safety, among other applications. NS-2 is a common and familiar communication simulation tool for researchers. The current NS-2 distribution package, however, is not completely ready to accommodate the requirements of IEEE 802.11 (i.e. DSRC) based vehicular communication research. In this paper, we present an overview on the pre-existing wireless simulation architecture design in the NS-2 distribution code. We then describe our improvements to the IEEE 802.11 PHY and MAC modules in NS-2. The resulting simulator is a more accurate simulation tool for DSRC in particular and IEEE 802.11 based wireless communication research in general.

Joint power/rate congestion control optimizing packet reception in vehicle safety communications

In Vehicle Safety Communications (VSC) based on IEEE 802.11p, vehicles establish a mutual awareness of their presence by periodically broadcasting status messages, aka beacons. If vehicle density is high and beaconing is not regulated, the channel can become congested, impairing reception performance and safety benefit. As a countermeasure, a number of congestion control approaches have been suggested, adapting transmit (Tx) power, beacon generation rate (Tx rate), or both. However, in general these approaches did not show what the optimal outcome for congestion control would be and how and why their solution would lead to the desired result. In this work, we analyze answers to the first question and provide a methodology for the second. We systematically derive a joint power/rate control strategy for VSC which optimizes reception performance for a targeted sender-receiver distance. We start by laying out why we consider average (or percentile of) packet Inter-Reception Time (IRT) at the targeted awareness distance to be a suitable metric for our purpose. Then, we analyze a wide range of Tx parameters to identify which combinations optimize reception in a homogeneous scenario. We show that for each sender-receiver distance, there is an optimal Tx power which, unlike the corresponding Tx rate, is independent of node density. In addition, we analyze the Pareto optimal Tx parameter combinations for two groups of vehicles with different target distances adapting at the same time. We show that the majority of these combinations use the same Tx power as identified in the homogeneous case. We conclude that a simple and efficient strategy to optimize reception performance is to select Tx power w.r.t. the targeted distance and to adapt Tx rate w.r.t. channel load.