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Performance Analysis of V2V and V2I LiFi Communication Systems in Traffic Lights

Vehicular networks is a key technology for efficiently communicating both user’s devices and cars for timely information regarding safe driving conditions and entertaining applications like social media, video streaming, and gaming services, among others. In view of this, mobile communications making use of cellular resources may not be an efficient and cost-effective alternative. In this context, the implementation of light-fidelity (LiFi) in vehicular communications could be a low-cost, high-data-rate, and efficient-bandwidth usage solution. In this work, we propose a mathematical analysis to study the average throughput in a road intersection equipped with a traffic light that operates as a server, which is assumed to have LiFi communication links with the front lights of the vehicles waiting for the green light. We further assume that the front vehicle (the car next to the traffic light) is able to communicate to the car immediately behind it by using its own tail lights and the front lights of such vehicle, and so on and so forth. The behavior of the road junction is modeled by a Markov chain, applying the Queueing theory with an M/M/1 system in order to obtain the average queue length. Then, Little’s theorem is applied to calculate the average waiting delay when the red light is present in the traffic light. Finally, the mathematical expression of the data throughput is derived.

Nowadays, a major open issue in most cities is to improve traffic conditions by means of continuous surveillance of drivers. Manual traffic monitoring is a classical approach that is costly and inefficient since a high number of human resources are needed and only some vehicular crossings can be covered at partial times. To tackle this issue, the authors in [1] proposed a fog-based model for driving rule monitoring services. Such systems can easily be installed in traffic lights by means of a LiFi communication system to convey information in a smart city environment, where the traffic light communicates to all vehicles in the queue informing them about their individual average speed or driving infractions that happened in the previous streets, for example. However, the LiFi system is not limited to traffic information, since, as considered in Smart Cities applications [2], different types of information regarding the quality of life and city management can be conveyed to the passing vehicles. Building on this, we propose to use the already implemented infrastructure of light in traffic light and cars to convey this information in an efficient manner (since lights have to be used in vehicular systems), that does not use already crowded radio frequencies and provides a fast and reliable data link between cars using LiFi technology.

In this work, we develop a mathematical analysis to study the performance of a LiFi system installed in traffic lights that disseminates information regarding the city using the lights of the vehicles. Building on this, the traffic light acts as the server where information is first disseminated to vehicles in the city. The first car waiting in the red light receives the packet, and it conveys this information to the car right behind it using its tail lights and the front light of the next car. This procedure is repeated until it reaches the last car in the queue or cars start moving forward. At this point, we assume that vehicles move away from each other and the communication link is lost. This may not be the case in a practical scenario where communication links may still be functional even if vehicles are moving. However, we study the worst case scenario where vehicles can only communicate among them when they are not moving. The relevance of the proposed system is in the implementation of the Internet of Vehicles as studied in [3], where autonomous vehicles cooperate to maintain a smooth traffic flow on roads.

To this end, we assume that vehicles arrive and remain in the vehicle crossing where the traffic light is installed, a random, exponentially distributed time. Indeed, even if the time of the red and green lights is constant, the time that vehicles remain waiting in the intersection is, in many cases, random. This is due to the fact that drivers do not react instantaneously to the red-to-green switching, or they are at times distracted or in a hurry or even because pedestrians are crossing, among other reasons. Hence, we believe that modeling the dwelling time in the system (vehicular crossing) represents a good approximation to a practical system. Furthermore, the use of the exponential distribution corresponds to a first attempt at studying these type of systems. With this in mind, we also consider a simple crossing where no left/right turns are allowed and no U-turns are possible. As such, vehicles can only continue their original direction.

LiFi technology has been studied in the context of vehicular communication systems [4]. For instance, in [5], a cost-effective and inexpensive mechanism for the vehicle-to-vehicle (V2V) communication system using light is proposed. However, only two scenarios were considered. Namely, when a moving car is breaking and it alerts the vehicle behind it to be aware of such speed variation and when a high-speed vehicle approaches a junction it alerts other vehicles that may not detect it. Additionally, in [6], a vehicular communication system based on LiFi is proposed to communicate cars using the front and tail lights to improve road safety and traffic management by storing any infractions from cars, such as maximum speed violation, and sending this information to a central management system and taking legal actions in the future [7]. Conversely, we focus on a more general communication system, where security is an important objective, but other types of information can be conveyed, such as parking spaces, cultural events, weather conditions (rain, fog, or ice) that can affect driving conditions, and others. Another difference is that we focus our study on static or semistatic conditions where cars are waiting for the green light in a traffic light. As such, we study both V2V and V2I (vehicle-to-infrastructure) architectures since cars can communicate with the traffic light. Also, in [2], the authors propose a communication system using LiFi to communicate traffic lights and vehicles. The aim is to optimize traffic flow in the city and avoid car accidents by obtaining the most suitable routes and send alert signals when sudden speed changes are detected. However, the data transmission capacity is not evaluated nor the data throughput of the system.

To the best of our knowledge, this is the first paper that attempts to study the relevance and potential use of the LiFi technology in both V2V and V2I communication systems, clearly showing the benefits and limitations of data dissemination making use of bandwidth outside the already crowded radio frequencies. This is done by deriving the theoretical throughput in different environments. The main contributions of the paper are as follows:
(i) We study the benefits and limitations of an information dissemination system for V2I and V2V downlink communication systems where a LiFi server is installed in traffic lights and vehicles aligned waiting in the queue transmit relevant information
(ii) A mathematical model based on a continuous-time Markov chain (CTMC) is used to evaluate the performance of the LiFi system
(iii) Different scenarios are proposed to evaluate the system performance in terms of average throughput for different packet sizes, data rates, average times that the traffic light is in red/green, and traffic conditions
(iv) Based on the numerical evaluation of the proposed system, we give clear guidelines for the system parameter selection in order to offer adequate throughput in terms of the number of vehicles that can download the data in the junction.

The rest of the paper is organized as follows: Section 2 describes in detail the LiFi system’s characteristics that can be used in such communication system. Then, in Section 3, we provide the main assumptions and describe the system that is mathematically modeled in Section 4. We conclude the paper with relevant numerical results presented in Section 5 and Conclusions.

  1. System Model
    The proposed environment considers a vehicle-to-infrastructure (V2I) system where traffic lights transmit information regarding city driving conditions, to avoid traffic jams or roads in construction, for example. Also, diverse information can be downloaded to the vehicles, including weather, pollution, and general information like relevant news and even commercial sales in specific points of the city. As such, some data can be consumed by vehicles while other data can be passed to users and commuters. In the case of autonomous vehicles, this data exchange is of great importance to enhancing the driving conditions by changing routes and adapting speed, effectively reducing commute times in big cities. Such data are proposed to be transmitted using LiFi by taking advantage of light infrastructure already installed throughout cities and in all sort of vehicles, including public transportation buses. This V2I communication is intended for the data transmission from the traffic light to the first car in the queue waiting during the red light. Afterward, this first car conveys this information to the rear car using the tail lights and the front lights, respectively. And this procedure is repeated until the last car in the queue is reached or until the red light turns to the green light and vehicles begin to move, and the car lights are no longer aligned, hindering the communication process. As such, we assume an error-free communication due to an accurate alignment of the transmitters and receivers as well as the close distance among vehicles only when they are not moving during the red light. When cars begin to move, we assume that communication can no longer be established since the distance between vehicles is no longer constant and small, due to the natural movement of the cars moving in the roads, and a good alignment is no longer possible all the time. However, we leave the open issue of a LiFi communication system when vehicles are moving for future research works.

For simplicity, we assume that all processes involved in the crossing are Poisson processes. This is a major assumption in the work. However, as a first attempt to study the performance of the system, we believe that it provides an approximation to the throughput of LiFi communications in traffic lights. In future works, we will look into more accurate models in such systems.

Building from this, interarrival times of vehicles in both direction A and B are assumed to be random variables, exponentially distributed with rates and , respectively. Also, average waiting times of vehicles in the crossing are assumed to be random variables, exponentially distributed with mean for direction A and for direction B. Finally, traffic light remains in state , an exponentially distributed random time with rate γ.

Note that we model a crossing where vehicles in each direction do not interfere on the flowing of cars in the other direction. As such, we do not consider a crossing where wide turns nor U-turns are allowed. Finally, a single lane is considered. However, the results derived in this work can be easily extended to multiple lanes if we assume that only cars in the same lane can communicate among them, i.e., cars in a particular lane cannot communicate with cars in another lane.

  1. LiFi System
    Light-fidelity is a technology that uses electromagnetic waves in the visible range (380 nm to 750 nm approximately [8]). Therefore, LiFi presents, as its name implies, a broad fidelity in light communications, since it provides high transmission data rates (10 Gbps [9]), free bandwidth, high level of security, and high propagation capabilities with respect to other types of waves, because transmissions though water, terrestrial surface, and even in outer space are possible. LiFi has been cataloged as a VLC (visible light communication) technology, because it is a light transmission technology. The main differentiator of LiFi with respect to the other VLC technologies lies in the fact that the previous VLC technologies have been conceived mainly with PPP (point to point) communication, that is, as a substitute for a cable, while LiFi has the characteristic to be a complete network system with bidirectional and multiuser communication.

LiFi uses light emitting diodes (LED’s) to make full connections in wireless network systems. Each LED in a LiFi system acts as an access point, and due to the size (order of millimeters) of the AP’s, their networks are called attocell networks, unlike WiFi femtocell networks. These improvements in the attocell networks provide the necessary infrastructure for the IoT technologies and contribute to the fifth generation 5G [10] of cellular systems. LiFi arises from a worldwide need, the scarcity of spectrum bands available for wireless-fidelity (WiFi) technology.

The use of LiFi systems in indoor applications could represent the largest field of action of this technology in the coming decades since it could become a solution for massive deployment of nodes, making use of bandwidth-efficient [11] and secure connections [12] in the Internet of Things. As a result of many experimentation in modulation [13], color and power, the use of LiFi has been envisioned to be used in urban, marine, and in outer space environments. To this date, the use of LiFi is mainly focused for indoor environments where illumination conditions are rather stable, where nodes have low or no mobility and there are few obstacles between transmitters and receivers. However, current efforts on LiFi allows the use of this technology in outdoor environments like the one shown in Figure 3 where we depict efficient-bandwidth use scenarios in future communications systems.

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