Elisabeth Uhlemann

Evaluation of the IEEE 802.11p MAC Method for Vehicle-to-Vehicle Communication

In this paper the medium access control (MAC) method of the upcoming vehicular communication standard IEEE 802.11p has been simulated in a highway scenario with periodic broadcast of time-critical packets (so-called heartbeat messages) in a vehicle-to-vehicle situation. The 802.11p MAC method is based on carrier sense multiple access (CSMA) where nodes listen to the wireless channel before sending. If the channel is busy, the node must defer its access and during high utilization periods this could lead to unbounded delays. This well-known property of CSMA is undesirable for time-critical communications. The simulation results reveal that a specific node/vehicle is forced to drop over 80% of its heartbeat messages because no channel access was possible before the next message was generated. To overcome this problem, we propose to use self-organizing time division multiple access (STDMA) for real-time data traffic between vehicles. This MAC method is already successfully applied in commercial surveillance applications for ships (AIS) and airplanes (VDL mode 4). Our initial results indicate that STDMA outperforms CSMA for time-critical traffic safety applications in ad hoc vehicular networks.

Guest Editorial Special Section on Wireless Technologies in Factory and Industrial Automation—Part II

The three papers in this special section focus on wireless technologies in factory and industrial automation. The papers which appear in this second part cover everything from protocol design and evaluation to the design and assessment of system-level solutions for wireless sensor networks in industrial automation.

Optimal Incremental-redundancy strategy for type-II hybrid ARQ

Given a type-II hybrid ARQ scheme, we propose a general solution to find the optimal partitioning of n-k parity bits over at most M transmissions. The solution is in terms of maximizing the average code rate for a given puncturing pattern.

Scalability Issues of the MAC Methods STDMA and CSMA of IEEE 802.11p When Used in VANETs

Position messages will be the foundation for many emerging traffic safety applications based on wireless communications. These messages contain information about the vehicles position, speed, direction, etc. and are broadcasted periodically by each vehicle. The upcoming IEEE 802.11p standard, intended for vehicle ad hoc networks (VANETs) has flaws caused by the unpredictable behavior of its medium access control (MAC) scheme, which imply that traffic safety applications cannot be supported satisfactorily when the network load increases. We study the MAC mechanism within IEEE 802.11p being a carrier sense multiple access (CSMA) algorithm and compare it with a self-organizing time division multiple access (STDMA) scheme when used for broadcasting periodic position messages in a real-istic highway scenario. We investigate their scalability in terms of the number of vehicles that the VANET can support using met-rics such as channel access delay, probability of concurrent transmissions and interference distance. The results show that STDMA outperforms CSMA of 802.11p even when the network is not saturated.

Introducing Connected Vehicles [Connected Vehicles]

The term connected vehicles refers to applications, services, and technologies that connect a vehicle to its surroundings. Adopting a definition similar to that of AUTO Connected Car News, a connected vehicle is basically the presence of devices in a vehicle that connect to other devices within the same vehicle and/or devices, networks, applications, and services outside the vehicle. Applications include everything from traffic safety and efficiency, infotainment, parking assistance, roadside assistance, remote diagnostics, and telematics to autonomous self-driving vehicles and global positioning systems (GPS). Typically, vehicles that include interactive advanced driver-assistance systems (ADASs) and cooperative intelligent transport systems (C-ITS) can be regarded as connected. Connected-vehicle safety applications are designed to increase situation awareness and mitigate traffic accidents through vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications. ADAS technology can be based on vision/camera systems, sensor technology, vehicle data networks, V2V, or V2I systems. Features may include adaptive cruise control, automate braking, incorporate GPS and traffic warnings, connect to smartphones, alert the driver to hazards, and keep the driver aware of what is in the blind spot. V2V communication technology could mitigate traffic collisions and improve traffic congestion by exchanging basic safety information such as location, speed, and direction between vehicles within range of each other. It can supplement active safety features, such as forward collision warning and blind-spot detection.

Deadline-aware scheduling of cooperative relayers in TDMA-based wireless industrial networks

In this paper we consider a scenario in which a set of source nodes wishes to transmit real-time data packets periodically to a central controller over lossy wireless links, while using a TDMA-based medium access control protocol. Furthermore, a number of relay nodes are present which can help the source nodes with packet retransmissions. The key question we consider in this paper is how to schedule the TDMA slots for retransmissions while taking advantage of the relay nodes, so that the average number of packets missing their deadlines is minimized. We provide a problem formulation for the general deadline-aware TDMA relay scheduling problem. Since the design space of the general problem is large, we also present one particular class of restricted TDMA relay scheduling problems. We suggest and numerically investigate a range of algorithms and heuristics, both optimal and suboptimal, of the restricted scheduling problem, which represent different trade-offs between achievable performance and computational complexity. Specifically, we introduce two different Markov Decision Process (MDP) based formulations for schedule computation of the restricted TDMA relay scheduling problem. One MDP formulation gives an optimal schedule, another (approximate) formulation gives a sub-optimal schedule which, however, comes very close to the optimal performance at much more modest computational and memory costs.

Deadline dependent coding-a framework for wireless real-time communication

A framework for real-time communication over a wireless channel is proposed. The concept of deadline dependent coding (DDC), previously suggested by the authors, is further developed using soft decision decoding of block codes to maximize the probability of delivering information before a given deadline. The strategy of DDC is to combine different coding and decoding methods with automatic repeat request (ARQ) in order to fulfil the application requirements. These requirements are formulated as two quality of service (QoS) parameters: deadline (t/sub DL/) and probability of correct delivery before the deadline (Pd), leading to a probabilistic view of real-time communication. An application can negotiate these QoS parameters with the DDC protocol, thus creating a flexible and dependable scheme.

How Severe Is the Hidden Terminal Problem in VANETs When Using CSMA and STDMA

The hidden terminal problem is often said to be the major limiting performance factor in vehicular ad hoc networks. In this article we propose a definition of the hidden terminal problem suitable for broadcast transmissions and proceed with a case study to find how the packet reception probability is affected by the presence of hidden terminals. Two different medium access control methods; carrier sense multiple access (CSMA) from IEEE 802.11p and self-organizing time division multiple access (STDMA), are subject of investigation through computer simulations of a highway scenario with a Nakagami fading channel model. The results reveal that the presence of hidden terminals does not significantly affect the performance of the two MAC protocols. STDMA shows a higher packet reception probability for all settings due to the synchronized packet transmissions.

Packet combining and doping in concatenated hybrid ARQ schemes using iterative decoding

We consider serially concatenated block codes in a hybrid ARQ scheme using iterative decoding. The extrinsic information generated in the iterative decoding process is saved and used when a retransmission is decoded. Two different strategies are examined; one using the extrinsic information only in the very first iteration, whereas the other uses it in all subsequent iterations until another retransmission arrives. The latter can be seen as turbo or concatenated code combining whereas the former, where the extrinsic information is used only once may be seen as code doping, providing an alternative perspective. The strategy of saving the extrinsic information is also compared to traditional type-III, equal gain diversity combining. Using the extrinsic information from previous retransmission is shown to improve performance not only in terms of bit error rate but also in terms of throughput and convergence speed and requires only negligible additional decoder complexity. The performance of this strategy is however not as good as simple equal gain combining. As a consequence, the investigated schemes are not competitive alternatives, however, the code doping procedure can be used in conjunction with traditional diversity combining schemes, improving further on convergence speed.

Delay and interference comparison of CSMA and self-organizing TDMA when used in VANETs

IEEE 802.11p is the proposed wireless technology for communication between vehicles in a vehicular ad hoc network (VANET) aiming to increase road traffic safety. In a VANET, the network topology is constantly changing, which requires distributed self-organizing medium access control (MAC) algorithms, but more importantly the number of participating nodes cannot be restricted. This means that MAC algorithms with good scalability are needed, which can fulfill the concurrent requirements on delay and reliability from road traffic safety applications. The MAC method of IEEE 802.11p is a carrier sense multiple access (CSMA) scheme, which scales badly in terms of providing timely channel access for a high number of participating nodes. We therefore propose using another MAC method: self-organizing time division multiple access (STDMA) with which all nodes achieve timely channel access regardless of the number of participating nodes. We evaluate the performance of the two MAC methods in terms of the MAC-to-MAC delay, a measure which captures both the reliability and the delay of the delivered data traffic for a varying number of vehicles. The numerical results reveal that STDMA can support almost error-free transmission with a 100 ms deadline to all receivers within 100 m, while CSMA suffers from packet errors. Moreover, for all considered cases, STDMA offers better reliability than CSMA.