Chenxi Qiu

A Review of Communication, Driver Characteristics, and Controls Aspects of Cooperative Adaptive Cruise Control (CACC)

Cooperative adaptive cruise control (CACC) systems have the potential to increase traffic throughput by allowing smaller headway between vehicles and moving vehicles safely in a platoon at a harmonized speed. CACC systems have been attracting significant attention from both academia and industry since connectivity between vehicles will become mandatory for new vehicles in the USA in the near future. In this paper, we review three basic and important aspects of CACC systems: communications, driver characteristics, and controls to identify the most challenging issues for their real-world deployment. Different routing protocols that support the data communication requirements between vehicles in the CACC platoon are reviewed. Promising and suitable protocols are identified. Driver characteristics related issues, such as how to keep drivers engaged in driving tasks during CACC operations, are discussed. To achieve mass acceptance, the control design needs to depict real-world traffic variability such as communication effects, driver behavior, and traffic composition. Thus, this paper also discusses the issues that existing CACC control modules face when considering close to ideal driving conditions.

A Delaunay-Based Coordinate-Free Mechanism for Full Coverage in Wireless Sensor Networks

Recently, many schemes have been proposed for detecting and healing coverage holes to achieve full coverage in wireless sensor networks (WSNs). However, none of these schemes aim to find the shortest node movement paths to heal the coverage holes, which could significantly reduce energy usage for node movement. Also, current hole healing schemes require accurate knowledge of sensor locations; obtaining this knowledge consumes high energy. In this paper, we propose a Delaunay-based coordinate-free mechanism (DECM) for full coverage. Based on rigorous mathematical analysis, DECM can detect coverage holes and find the locally shortest paths for healing holes in a distributed manner without requiring accurate node location information. Also, DECM incorporates a cooperative movement mechanism that can prevent generating new holes during node movements in healing holes. Simulation results and experimental results from the real-world GENI Orbit testbed show that DECM achieves superior performance in terms of the energy-efficiency, effectiveness of hole healing, energy consumption balance and lifetime compared to previous schemes.

Efficient Data Collection for Large-Scale Mobile Monitoring Applications

Radio frequency identification (RFID) and wireless sensor networks (WSNs) have been popular in the industrial field, and both have undergone dramatic development. RFID and WSNs are well known for their abilities in identity identification and data transmission, respectively, and hence widely used in applications for environmental and health monitoring. Though the integration of a sensor and an RFID tag was proposed to gather both RFID tag and sensed information, few previous research efforts explore the integration of data transmission modes in the RFID and WSN systems to enhance the performance of the applications. In this paper, we propose a hybrid RFID and WSN system (HRW) that synergistically integrates the traditional RFID system and WSN system for efficient data collection. HRW has hybrid smart nodes that combine the function of RFID tags, the reduced function of RFID readers, and wireless sensors. Therefore, nodes can read each others sensed data in tags, and all data can be quickly transmitted to an RFID reader through the node that first reaches it. The RFID readers transmit the collected data to the back-end servers for data processing and management. We also propose methods to improve data transmission efficiency and to protect data privacy and avoid malicious data selective forwarding in data transmission. Comprehensive simulation and trace-driven experimental results show the high performance of HRW in terms of the cost of deployment, transmission delay and capability, and tag capacity requirement.

An Efficient Wireless Power Transfer System to Balance the State of Charge of Electric Vehicles

As an alternate form in the road transportation system, electric vehicle (EV) can help reduce the fossil-fuel consumption. However, the usage of EVs is constrained by the limited capacity of battery. Wireless Power Transfer (WPT) can increase the driving range of EVs by charging EVs in motion when they drive through a wireless charging lane embedded in a road. The amount of power that can be supplied by a charging lane at a time is limited. A problem here is when a large number of EVs pass a charging lane, how to efficiently distribute the power among different penetrations levels of EVs? However, there has been no previous research devoted to tackling this challenge. To handle this challenge, we propose a system to balance the State of Charge (called BSoC) among the EVs. It consists of three components: i) fog-based power distribution architecture, ii) power scheduling model, and iii) efficient vehicle-to-fog communication protocol. The fog computing center collects information from EVs and schedules the power distribution. We use fog closer to vehicles rather than cloud in order to reduce the communication latency. The power scheduling model schedules the power allocated to each EV. In order to avoid network congestion between EVs and the fog, we let vehicles choose their own communication channel to communicate with local controllers. Finally, we evaluate our system using extensive simulation studies in Network Simulator-3, MatLab, and Simulation for Urban MObility tools, and the experimental results confirm the efficiency of our system.

A Distributed Three-Hop Routing Protocol to Increase the Capacity of Hybrid Wireless Networks

Hybrid wireless networks combining the advantages of both mobile ad-hoc networks and infrastructure wireless networks have been receiving increased attention due to their ultra-high performance. An efficient data routing protocol is important in such networks for high network capacity and scalability. However, most routing protocols for these networks simply combine the ad-hoc transmission mode with the cellular transmission mode, which inherits the drawbacks of ad-hoc transmission. This paper presents a Distributed Three-hop Routing protocol (DTR) for hybrid wireless networks. To take full advantage of the widespread base stations, DTR divides a message data stream into segments and transmits the segments in a distributed manner. It makes full spatial reuse of a system via its high speed ad-hoc interface and alleviates mobile gateway congestion via its cellular interface. Furthermore, sending segments to a number of base stations simultaneously increases throughput and makes full use of widespread base stations. In addition, DTR significantly reduces overhead due to short path lengths and the elimination of route discovery and maintenance. DTR also has a congestion control algorithm to avoid overloading base stations. Theoretical analysis and simulation results show the superiority of DTR in comparison with other routing protocols in terms of throughput capacity, scalability, and mobility resilience. The results also show the effectiveness of the congestion control algorithm in balancing the load between base stations.

Energy-efficient cooperative broadcast in fading wireless networks

Cooperative broadcast, in which receivers are allowed to combine received packet from different senders to combat transmission errors, has gained increasing attention. Previous studies showed that broadcast optimization solutions are sufficient in non-fading environments but may suffer a low delivery ratio under wireless channel fading. Though previous work analyzed the tradeoff between energy and delay in cooperative broadcast, no works investigated the tradeoff in a fading environment. Thus, in this paper, we study this tradeoff with the consideration of fading. We formulate this problem as a Fading-resistant Delay-constrained Minimum Energy Cooperative Broadcast (FDMECB) problem, and prove that it is NP-complete. We then propose an approximation algorithm for theoretical interests. We further propose a heuristic algorithm that makes approximately optimal local decision to achieve global optimization. Our experimental results show that our algorithms outperform a previous non-fading resistant algorithm.

A Decentralized Network with Fast and Lightweight Autonomous Channel Selection in Vehicle Platoons for Collision Avoidance

Always keeping a certain distance between vehicles in a platoon is important for collision avoidance. Centralized platoon systems let the leader vehicle determine and notify the velocities of all the vehicles in the platoon. Unfortunately, such a centralized method generates high packet drop rate and communication delay due to the leader vehicles limited communication capability. Therefore, we propose a decentralized platoon network, in which each vehicle determines its own velocity by only communicating with the vehicles in a short range. However, the multiple simultaneous transmissions between different pairs of vehicles may interfere with each other. Directly applying current channel allocation methods for interference avoidance leads to high communication cost and delay in vehicle joins and departures (i.e., vehicle dynamics). As a result, a challenge is how to reduce the communication delay and cost for channel allocation in decentralized platoon networks? To handle this challenge, by leveraging a typical feature of a platoon, we devise a channel allocation algorithm, called the Fast and Lightweight Autonomous channel selection algorithm (FLA), in which each vehicle determines its own channel simply based on its distance to the leader vehicle. We conduct experiments on NS-3 and Matlab to evaluate the performance of our proposed methods. The experimental results demonstrate the superior performance of our decentralized platoon network over the previous centralized platoon networks and of FLA over previous channel allocation methods in platoons.

Quick and Autonomous Platoon Maintenance in Vehicle Dynamics For Distributed Vehicle Platoon Networks

Platoon systems, as a type of adaptive cruise control systems,will play a significant role to improve travel experience androadway safety. The stability of a platoon system is crucialso that each vehicle maintains a safety distance from itsproceeding vehicle and can take necessary actions to avoidcollisions. However, current centralized platoon maintenancemethod cannot meet this requirement. We suggest to use adecentralized platoon maintenance method, in which eachvehicle communicates with its neighbor vehicles and self-determines its own velocity. However, a vehicle needs toknow its distance from its preceding vehicle to determineits velocity, which is unavailable in vehicle communicationdisconnection caused by vehicle dynamics (i.e., node joinsand departures). Thus, a formidable challenge is: how torecover the platoon quickly in vehicle dynamics even when thedistance information is unavailable? To handle this challenge,we first profile a succeeding vehicle’s velocity to minimizethe time to recover the connectivity hole with its precedingvehicle and find that the profiles are almost the same at thebeginning regardless of its current velocity and distance to itspreceding vehicle. Accordingly, we devise a strategy, in whicha succeeding vehicle uses its stored common velocity profilewhen it is disconnected from its preceding vehicle and thenadjusts its velocity once the connection is built. Experimentalresults from simulation show the efficiency and effectivenessof our decentralized platoon maintenance method.

Fading-Resistant Link Scheduling in Wireless Networks

In this paper, we study the link scheduling problem considering the fluctuating fading effect in transmissions. We extend the previous deterministic physical interference model to the Rayleigh-fading model that uses the stochastic propagation to address fading effects. Based on this model, we formulate a problem called Fading-Resistant Link Scheduling (Fading-R-LS) problem, which aims to maximize the throughput of all links in a single time slot. We prove that this problem is NP-hard. Based on the geometric structure of Fading-R-LS, we then propose two centralized schemes with O(g(L)) and O(1) performance guarantee, respectively, where g(L) is the number of magnitudes of transmission link lengths. Our experimental results show that the superior performance of our proposed schemes compared to previous schemes.

Energy-Efficient and Delay-Constrained Broadcast in Time-Varying Energy-Demand Graphs

In this paper, we study the minimum energy broadcast problem in time-varying graphs (TVGs), which are a very useful high level abstraction for studying highly dynamic wireless networks. To this end, we first incorporate a channel model, called energy-demand functions, to the current TVGs, namely time-varying energy-demand graphs (TVEGs). Based on this model, we formulate the problem: given a TVEG, what is the optimal schedule (i.e., Which nodes should forward a packet in what times and at what power levels) to minimize the energy consumption of the broadcast? We prove the problem to be NP-hard and o(log N) in approximable. It is a challenge to find a solution for this problem on continuous time. Fortunately, we prove that the problem on continuous time is equivalent to the problem on certain discrete time points, called discrete time set (DTS). Based on this property, we propose polynomial time solutions for this problem with different channel models, and evaluate the performance of these methods from real-life contact traces.