學生：王志嘉
指導教授：許子衡 老師

Vehicular networks are becoming a key issue in the development of on-board services. Fleet management applications opened the way several years ago with the usage of cellular networks (CN) to receive vehicle positions at a monitoring station.
According to current state of vehicle telematics, connectivity requirements of vehicular services can be divided in two communication paradigms: vehicle to vehicle (V2V) communications and vehicle to infrastructure (V2I) communications.
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V2V is the most popular network architectures last year.V2I are more and more important.
Electronic fee collection systems or fleet management applications, where the most extended technologies are Dedicated Short Range Communications (DSRC), infrared, and wireless LAN.
In V2V solutions, vehicular ad-hoc networks
(VANETs) using wireless LAN and DSRC cover the communication necessities of safety services overall.
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V2V solutions over VANET are currently the most studied systems, paying a special attention to safety applications
The cellular networks have been elected, and peer to peer networks (P2P) are used to create a group-based communication paradigm over the CN basis.
The P2P technology creates a virtual network over CN, which enables the transmission of information among vehicles and between vehicles and infrastructure, and bounds the propagation of messages.
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Cellular Networks ： A Suited Technology for Vehicular Communication (i)
VANET is stated the technology which deals with more communication requirements of telematic services inside intelligent transportation systems (ITS) are CN, defending our idea of using cellular networks in a general telematic framework.
In the field of VANET, the most extended technologies are the wireless LAN transceivers, such as 802.11a/b/g and the new 802.11p.
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Cellular Networks ： A Suited Technology for Vehicular Communication (ii)
The most used technologies in cellular networks in
Europe are GPRS (General Packet Radio Service) and
UMTS (Universal Mobile Telecommunications
System).
In vehicle domain is important to deal with the most controversial performance parameter: the latency.
The values usually obtained of several hundreds of milliseconds are too high to send a critical notification to an adjacent vehicle
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Cellular Networks ： A Suited Technology for Vehicular Communication (iii)
For distances from 50 to 100 meters, nevertheless, current UMTS technology is able to give propagation times even better than VANET approaches.
The High Speed Packet Access (HSPA) is being launched with the objective of improving the operation of data applications. HSPA includes a better throughput and, what is more important, latency times in the order of tens of milliseconds.
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Cellular Networks ： A Suited Technology for Vehicular Communication (iv)
A V2V vehicular network based on CN must deal with the message propagation method used.
A group-based P2P technology has been used to deal with this feature. Following this technique, messages are only propagated to vehicles whose travel can be affected. As a result, complex routing protocols and penetration rate restrictions are avoided
.
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Cellular Networks ： A Suited Technology for Vehicular Communication (v)
Regarding the cost of a CN-based system, and comparing it with the hardware used in the VANET, two issues must be taken into account: the hardware platform cost and the cellular network usage cost.
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The network infrastructure uses a P2P approach over the cellular network basis to enable vehicles to receive and send contextual information about its current environment.
Fig. 1 shows a general diagram of the proposed high level communication architecture.
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Fig. 1. Main elements and interactions of the CN-based network design
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Information about the geometry of each area is maintained in the Group Server (GS) entity.
Information about areas is received from the Group
Server using a TCP/IP link over UMTS. A local element called Environment Server (ES) manages special messages inside the area.
Messages are encapsulated in P2P packages, and two different techniques of emission have been developed.
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P2P messages can be broadcasted in the area or sent to a specific vehicle.
In order to divide the messages transmitted in the network according with its content, every coverage area can include several transmission services. Every transmission service has its own P2P group to share messages, and this group may change between coverage areas
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The main processes carried out in the operation of the network architecture.
Initially, users have to subscribe to transmission services to send or receive information.
GS provides the P2P parameters to maintain the communication in all the active transmission services over the new area. Because of GS sends the area geometry to the vehicle, the latter communicates with
GS only when it detects itself out of the service area.
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Messages which require a special attention are processed by the infrastructure through ES.
As Fig. 1 shows, Environment Servers can be connected to the rest of the road side infrastructure, which may be composed of speed radars, identification sensors, video cameras.
Therefore some events detected by the road side hardware can be notified to vehicles, and useful reports can be sent to a third party entity.
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A complete hardware/software prototype has been created and tested. The vehicle used for the system deployment is a car widely sensorized at the
University of Murcia.
Fig.2 shows this car and the main components for the network architecture. The vehicle, through an agreement with the manufacturer, has been prepared to be enhanced with several sensors, such as odometry, a gyroscope, an accelerometer or a GPS sensor.
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Regarding the vehicle communications, a cellular network PCMCIA transceiver has been used. The model is a Huawei E612, which allows GPRS and
UMTS data connections
The GPS receivers used in both the prototype vehicle and the common one also offer time synchronisation to the computers, in order to obtain meaningful latency results..
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Fig. 3 shows this utility, named Message Console.
This tool includes an event-based mechanism to notify safety problems.
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The system has been tested over an enclosed zone.
Several coverage areas have been defined around the
University of Murcia, and over a near motorway.
Fig. 4 shows the path of six kilometres followed in all the tests. This place is suitable for our purposes, due to the good UMTS deployment and the low traffic density, so it is possible to repeat the same circuit in every test.
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The message payload contains: the packet type, the location of the vehicle (latitude and longitude), the source vehicle identifier, the message type, and an extra information field.
Fig. 5 shows a graph of the obtained latency in the message propagation between the two vehicles over a real mobility scenario.
As can be seen, the latency times obtained for every message are around 800 ms, with several peaks due to the mobility effect.
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Fig. 6 plots the latency in a V2I transmission model, where the same circuit was considered and the equipment aboard the common vehicle was connected to the wired network and used to receive the messages transmitted by the mobile unit.
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The latency obtained in the downlink channel using a
V2I paradigm is showed in Fig. 7, where the mobile terminal is now in charge of receiving messages from the infrastructure (the wired node).
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Table I summarises the total amount of messages sent in every test, the latency means and the dropping rates obtained in each case.
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The paper proposes a network design which covers both V2V and V2I communication paradigms, and takes into account the requirements of most of vehicular services.
The network architecture has been developed in both software and hardware terms, so a vehicle prototype has been defined in order to test the implementation, and the infrastructure side has been developed and run over servers located in our laboratories.
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Current works are being carried out over the proposed communication infrastructure, in the line of providing the driver with context-aware information and safety notifications.
New tests are planned with these modifications, using moreover the HSPA technology and considering problematic scenarios.
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