Vehicle-to-Infrastructure (V2I) Architecture
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Vehicle-to-Infrastructure (V2I) architecture allows multiple vehicles to communicate with the road's infrastructure. This is then directed towards a central tower that allows multiple vehicles to operate on the same bandwidth. This helps all servers maintain the acceleration and position of all vehicles on streets and roads. This assists in determining everything such as fastest path and/or nearby accidents. Because of this, traffic safety enhancement is the largest factor when it comes to obtaining data from vehicles on and off the road. Also roadside infrastructures are included to provide warnings to vehicles about weather and accidents on its path. For this process to work, gathered speeds and locations within the proximity are reported to the server then to the other vehicles. Because of the large number of vehicles on the road, a central server is needed to relay all the data to each individual vehicle.
- Dedicated Short Range Communication (DSRC)
There are multiple infrastructures that a vehicle can communicate with such as other cars or a control building. Vehicle to Vehicle (V2V) lets multiple vehicles communicate with one another on a given network. DSRC works as a two-way short-range wireless connection. It works similar to WiFi since it allows vehicles to exchange data such as speed, distance, position, and mass of separate vehicles . The primary use of DSRC is for collision prevention. These are achieved by frequent data exchanges among vehicles within a certain range. Each vehicle that utilizes DSRC, casts data from the vehicle to neighboring vehicles multiple times per second within a range between 100-1000 meters in a radius based on the technology . Each vehicle also receives "safety messages" to warn other vehicles of collisions that the vehicle is driving towards to prepare the driver for what is ahead. Even though DSRC is mainly for collision prevention, it can also be used for assisted navigation such as GPS, electronic payments for tolls, improved fuel efficiency and present traffic updates.
According to the U.S. Federal Communications Commission, there is an allocated 75 MHz of licensed spectrum in the 5.9 GHz band for DSRC . This is what the "Dedicated" is DSRC refers to. This spectrum is divided into several channels. Safety messages are exchanged on Channel 172 and have been designated for safety among vehicles . The term "Short Range" in DSRC is meant to convey that the communication takes place over a few hundred meters which is shorter than cellular and WiMax services. DSRC communication relies on a manufacturer based standard among devices from different manufacturers for interoperability. The concept of proposed system architecture is shown in Fig. 1. In the United States, vehicles operate on a 5.9 GHz band to operate DSRC. This is then divided into seven 10 MHz channels . Because its running on a 10 MHz channel, all frames within a modulation takes only half as long to transmit than on a 20 MHz channel. This helps reduce collision probability for every frame transmission per second.
- Vehicular Ad-Hoc Network (VANET)
In VANETs, a vehicle moving along streets in an urban city establishes a network among themselves. Since its development, there have been an increase in commercial and research. In VANETs, mobile nodes are the travelling vehicles since it has high mobility and speed. The main disadvantage of VANET is that the network topology changes rapidly to other forms of vehicle communication. Since VANET equipped vehicles only move on predefined streets, they do not have problems for resource limitation.
It is possible for vehicles to obtain a geographic position by using GPS. This can provide good time synchronization through the network. Vehicles within a VANET infrastructure moves within the constraints of traffic flow. This is done while communicating with others. Ad hoc networks use less specialized hardware for infrastructure support. This allows all the stability of the network to be placed on individual nodes. Without dedicated communication hardware, there are other methods placed to attempt to optimize the network's communication to develop a hierarchical based system within the network to help with clustering. To support the VANET environment's dynamic nature, clustering must be updated every so often to reflect geographical changes with vehicle movements. The network's clustering must be extremely quick to minimize time lost within the network .
VANET has a set of unique characteristics to aid in traditional ad-hoc devices on a mobile network. VANET with high dynamic topology, enough energy and storage space, moving track predictable and diversified automotive network scenarios, has many significant applications in transportation and communication, such as vehicle safety, road traffic efficiency, and information and entertainment . VANET does not have a difficult time when it comes to vehicle shadowing. This happens when a smaller vehicle is shadowed by a much larger vehicle which complicates its communication with infrastructures on the road. In a VANET system, the synchronization between vehicles at a particular speed might be fast due to the network's topology modification . Keeping vehicles anonymous with its data such as the location of vehicles on highways are unidentified to each other. Periodic data exchanges from individual vehicles explain direct infrastructures/vehicles about its position. Yet, the address-position map (APM) will vary frequently because of the relative movements among neighboring vehicles.
It is the receiver's responsibility to determine the relevance of important messages and decide on appropriate actions . For a VANET system, location based broadcast to other vehicles is the most suitable communication technique when it comes to collision avoidance. Location-based information is an extremely vital measurement when it comes to distance and speed within a VANET system. Geographical routing protocol is important for VANETs since all nodes can determine their own position. All nodes know the position of their direct neighbor. The source node knows the location of the destination. Geographical routing protocol for VANET is more suitable for routing because it doesn't necessarily need route maintenance and does not occupy more bandwidth.
- Global Positioning System (GPS)
The Global Positioning System (GPS) is frequently used in road navigation. Global Position System (GPS) based vehicle tracking is an important application when dealing with mobile Geographic Information system (GIS) in V2V communication. Using GPS for V2V communication has many benefits. One of the main benefits of using GPS is that it is based off of geographic location with a satellite. The main downside of using this technology is that the connection can be lost when driving through a tunnel or a parking garage. There is a plethora of applications to be utilized for GPS vehicle communication. This includes shortest path algorithms based on distance or traffic in a busy city. One of the downsides to GPS is not getting an accurate position for neighboring vehicles to use for data communication with other vehicles. GPS, however, can be used in conjunction with other forms of V2V communication to achieve accurate data that is necessary to relay information to and from one another. One form of this is GPS used in conjunction with DSRC. DSRC gives local data amongst vehicles within a short distance , GPS is then used to relay the data that was gathered kilometers away based on the information given from another district that the vehicle is heading towards.
In general, GPS devices are used more to navigate rather than to be used for Vehicular communication. It allows needed information such as speed, location, and distance to be communicated from the satellite to the vehicle. Differential Global Positioning Systems (DGPS) can limit the amount of errors from GPS by minimizing or removing them. These include ionospheric effects which affect the propagation of radio waves to and from the vehicle and the tropospheric delay which receives and processes an algorithm to attempt to model or predict the impact of the signal travel time. DGPS is accurate compared to GPS since it gets the information of the vehicle up to a miniscule accuracy. DGPS assists autonomous vehicles with other peripherals to help vehicles accomplish driving tasks such as staying in lane, collision prevention, and checking for speed limits. Even though DGPS is only off by a few centimeters, there are ways to improve its performance. A common solution is integrating with an Inertial Navigation System (INS) .
The most common configurations integrate DGPS with high performance 6 degree of freedom INS units. Existing methods for this includes separated INS and GPS units and embedded GPS with INS hardware . GPS/INS integration is typically some form of a Kalman filter (KF), which uses a series of measurements over time. KF based GPS/INS integration can be classified into two categories. GPS-aiding INS where each state in the EKF are INS sensor errors; and the inputs to the EKF are measured between INS and GPS. And INS-aiding GPS where the extended KF states are the INS integration states and the extended KF inputs are GPS measurements .
- Medium Access Control (MAC)
The default MAC layer protocol in V2V Communications uses CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) to avoid collision. A node within the network's infrastructure can sense the communication channel and begin to send out messages once the channel becomes free. However, if two nodes cannot sense each other attempt to send messages to a similar destination concurrently, then both transmissions will fail and retransmissions will be needed. This problem is called the "hidden terminal problem". Another type of delay comes from redundant transmission. A vehicle may receive the same message multiple times from different senders. These redundancies will postpone the transmission of other emergency messages.
Many solutions have been proposed to reduce the V2V Communications delay. For the interference delay, the key is to let nodes in the interference range transmit at different time, i.e., assigning different transmission slots to these nodes. Decentralized MAC protocols are suitable for vehicular networks due to the dynamically changing set of vehicular nodes , and the MAC protocol combining the aspects of centralized and decentralized protocols is proposed in . Each cycle begins with a beacon message from an access point (AP), where the message contains information on the AP and the number of backoff slots. A cycle consists of reassociation slots, data contention slots, and data transmission slots. Based on the slot occurrence information on the previous cycle, the estimation of the number of active nodes and the decision of contention slot size are performed. The MAC protocol is designed for single-rate wireless networks.
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