Elsevier

Computer Communications

Volume 117, February 2018, Pages 84-103
Computer Communications

Theoretical interference analysis of inter-vehicular communication at intersection with power control

https://doi.org/10.1016/j.comcom.2017.10.001Get rights and content

Abstract

Interference problems caused by congestion of vehicles at intersections or on highways may significantly degrade vehicle-to-vehicle (V2V) communications, especially for active-safety assistance systems due to the importance of emergency information. In this paper, we propose a theoretical interference model of V2V communications at an intersection that uses a transmission-power-control method. To evaluate and address the interference problem at an intersection, we derived an analytical expression of the outage probability of a typical vehicle at an intersection and provided guidelines for an optimal power-control method, which cannot be obtained through simulations. We modeled the location of vehicles in queueing and running segments separately and analyzed their interference on the basis of a stochastic geometry approach. In our model, a simple power-control method is used: the transmission power of each vehicle is determined by the status of the vehicle, i.e., stopped or running. By changing the transmission power of vehicles in queueing segments, we can mitigate the interference received at vehicles running closer to an intersection. By using the theoretical results, we obtain an optimal power-control method, which can balance the outage probabilities of vehicles in queueing and running segments. We validated our analytical results and the effect of the power-control method on V2V communications by numerical simulations.

Introduction

Intelligent transportation systems (ITSs) have been attracting much attention because they have various applications for not only vehicles but also passengers and pedestrians. The applications include traffic and congestion control, safety assistance, and auto-driving, all of which will drastically change and greatly benefit our lives [1], [2]. The key technologies for ITSs, called vehicle-to-infrastructure (V2I) or vehicle-to-vehicle (V2V) communications, involve the networking of vehicles and other communication devices e.g., road side units (RSUs). The wireless communications protocols commonly used in V2I and V2V communications are dedicated short-range communications (DSRC). For example, IEEE 802.11p is used in the U.S. and provides media access control (MAC) and physical-layer design in DSRC [2]. In addition, other protocols, such as device-to-device communication mode in LTE, may be used in future ITSs [3].

Since mobile devices are positioned on road networks, V2V communications in urban environments present new research challenges. Interference caused by congestion at intersections or on highways may significantly degrade V2V communications [4], [5], [6]. To reduce the interference caused by a large number of vehicles using the same channels, several adaptive control methods of transmission power or broadcasting rate have been proposed in the area of active-safety assistance systems [6], [7], [8], [9]. For example, the method proposed by Moreno et al. [6] adaptively controls the transmission power of vehicles so that their max-min fairness is satisfied, whereas the method proposed by Fallah et al. [7] uses channel occupancy as a feedback measure and a network performance indicator.

In spite of their impact, the interference problems of V2V communications in urban environments have not been sufficiently analyzed theoretically. Most of their performance evaluations are simulation-based [6], [10], [11], or specific protocol behaviors (e.g. IEEE 802.11p) under homogeneous environments are considered [12], [13], [14]. However, theoretical analysis is important for optimizing power or rate control without a heuristic because time-consuming simulations are needed. Theoretical understanding may be useful for not only optimization but also sensitivity analysis of system parameters, which will help in designing future ITSs. In addition, it is also important to consider interference caused by congestion of vehicles, such as at intersections. Indeed, there are queues of vehicles at an intersection, so vehicles closer to an intersection may suffer serious interference problems due to the congestion.

In this paper, we propose a theoretical interference model for V2V communications at an intersection that uses a simple transmission-power-control method. By deriving the mathematical expression of performance metrics, we reveal the effect of power control on the interference problem and obtain a guideline for optimizing the power-control method, which cannot be obtained through simulations. To model the locations of vehicles at an intersection, we consider queueing (i.e., stopped) and running segments separately. Vehicles in running segments are assumed to be distributed in accordance with a Poisson point process (PPP). By taking the stochastic geometry approach, we derive a theoretical expression of the outage probability of a typical vehicle in queueing or running segments. The outage probability is defined as the probability that the signal-to-interference ratio (SIR) of a vehicle is smaller than a certain threshold. Although our PPP-based modeling is simpler than modeling vehicles’ locations in the real world, it is mathematically tractable and gives us a baseline to evaluate the performance of general V2V communications. In addition, to address the interference problem at intersections, we consider the power-control method in our model: vehicles in queueing segments are assigned less transmission power than those in running segments, i.e., the transmission power is determined on the basis of the status of vehicles: stopped or running. By changing the transmission power of vehicles in queueing segments, vehicles in running segments can reduce the interference from those in queueing segments, which can mitigate the interference problem at intersections. By computing the mathematical expression of the outage probability, we theoretically reveal not only the interference problem at an intersection but also the effect of the power control. Furthermore, we obtain a guideline for optimizing the power-control method, which balances the outage probabilities of vehicles in the queueing and running segments. Since adaptive power-control-methods (e.g., [6]) may take a long time to converge in a diverse environment, such optimal transmission power based on theoretical analysis can be useful as an initial value of the adaptive control methods. We validated our model through numerical simulations, and numerical results showed that the power-control method can be optimized through theoretical analysis. We found that if a vehicle is running closer to an intersection, its outage probability becomes higher, whereas vehicles in a queue have similar outage probabilities regardless of their positions. Furthermore, we found that our power control method can reduce the outage probability of a vehicle running closer to the intersection by roughly 10%.

The remainder of this paper is organized as follows. Section 2 summarizes related work. In Section 3, we explain the proposed model. We provide the theoretical expression of the outage probability in vehicular communications in Section 4. On the basis of these results, we discuss our transmission-power-control method and its optimization in Section 6. We discuss several numerical results in Section 7 and conclude the paper in Section 8.

Section snippets

Related work

In this section, we briefly summarize previous studies related to our paper. Since there are various important applications of V2V communications, such as safety assistance systems, several methods have been proposed for congestion-control in V2V communications. Moreno et al. [6] proposed an adaptive power-control method focusing on the max-min fairness among vehicles. Fallah et al. [7] also proposed an adaptive power-control method using channel occupancy ratio as a feedback measure and a

System model

In this section, we explain the proposed model. Fig. 1 shows a conceptual image of the model. First, there is an intersection crossing two streets: one along the x-axis, and the other along the y-axis. Each street is classified into two segments: one occupied by vehicles queueing (i.e., stopped) at the intersection, and the other occupied by vehicles running. We call the former a queueing segment SQ and the latter a running segment SR. Running segments are sub-classified into approaching and

Interference distribution

We first consider the total interference received at a tagged vehicle in an SR and SQ and derive the Laplace transforms of the interference distributions. By combining these results, we derive the outage probability of the tagged vehicle in the next section.

Outage probability of V2V communications at intersection

We next derive the mathematical expression of the outage probability of V2V communications at an intersection. We consider two transmission scenarios: downlink and uplink transmission of the tagged vehicle. In both cases, vehicles are assumed to communicate with the nearest vehicles in both SRs and SQs. In other words, in the downlink transmission scenario, the tagged vehicle attempts to receive information from its nearest vehicle while in the uplink one, the tagged vehicle transmits to the

Transmission power control

In this section, we discuss the transmission-power-control method in our model. On the basis of the results in the previous section, we give a theoretical explanation to the relationship between the outage probabilities of the tagged vehicle in an SR and SQ. By mathematically representing the relationship as a cost function and deriving its explicit expression as a function of θ, i.e., the power-control ratio (see Table 1), we optimize θ so that the outage probabilities can be well-balanced.

We

Numerical results

In this section, we discuss several numerical results. First, our analytical results were validated through numerical simulation. Furthermore, we investigated the performance of V2V communications at an intersection under various parameter settings. We then evaluated the effect of our power-control method on the interference problem at an intersection. In addition, we evaluated the impact of the system parameters on the power-control method and its optimization. Finally, we discuss the validity

Conclusions

We proposed a mathematical interference model with a simple power-control method for intersections. Using a stochastic geometry approach, we derived the theoretical values of the outage probability of a typical vehicle in an SQ an SR. By mathematically expressing the relationship between the outage probabilities of the tagged vehicle in an SQs and SR as a cost function, we obtain a guideline for an optimal power-control method, which cannot be obtained through simulations. We found that our

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