Editor's Note: Wireless sensor networks lie at the heart of emerging applications in nearly every industry segment. In building these networks, designers contend with issues that encompass real-time communications, efficient high-bandwidth data exchange, multiple network topologies, selection of optimal routing strategies, and more. The book, Building Wireless Sensor Networks, offers detailed treatments on critical requirements and promising solutions in each of these areas and more.
This excerpt focuses on design challenges and methods associated with creating a vehicular ad hoc network (VANET). To share data as vehicles pass on roads or rest in parking areas, a VANET must contend with issues as varied as the physics of signal propagation, the fluid nature of data routing, and the security vulnerabilities associated with participation in an ad hoc network. Because of the changing nature of a VANET, designers need a broad understanding of these issues.
In this excerpt from the book, the authors offer an in-depth discussion that defines the nature of VANET challenges and discusses alternatives for their solution. Continuing the description of VANETs begun in part 1, this installment of this series provides an in-depth discussion of the four main effects responsible for signal disturbance.
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Adapted from Building Wireless Sensor Networks , by Smain Femmam, Editor.
Chapter 3. Routing and data diffusion in vehicular ad hoc networks
By Frédéric Drouhin and Sébastien Bindel
22.214.171.124. Path loss
In a perfect environment, electromagnetic waves are only affected by the frequency and the distance between the transmitter and the receiver. In such an environment, no obstacles are present between stations, and it is denoted as free space . Let G t be the transmission gain, P t the power of the transmitted signal, and W the power density at a distance d , which is computed as follows according to [PAR 00]:
From the relationship between W and P r the received signal power, equation [3.3] can be formulated as follows:
with A r the effective aperture of the received antenna, λ the wavelength and G r the reception gain. From equations [3.2] and [3.3], the [FRI 46] equation determines the signal attenuation in a free space environment:
The Friis equation describes a vanilla environment and can be considered suitable for describing a signal propagation in far field environments. Regarding the ground environment and the position of the antennas, the propagation loss model described by Friis can be improved by taking into account the signal reflection on the ground. This model is called a two ray ground and describes the line of sight component and the multi-path component (ground reflection) of the received signal. According to [RAP 01], for a very large distance d and a perfect polarization and reflection, the calculation of the received signal power P r can be formulated according to the equation [3.5].
with h t and h r the height of the transmitter and receiver antenna and L the system loss and fixed at 1 according to [3.5]. A common strategy adopted by network simulators is to use the two-ray ground model, equation [3.5], when the distance d is larger than a cross-over distance d c and a free-space model, equation [3.4], in the other case:
Friis and the two-ray ground model are not suitable for describing dense environments, such as urban or building environments. A common model adopted for describing such environments is the log-distance model, where the path loss L at a distance d is expressed as follows:
with L 0 the reference path loss value based on measurement made at distance d 0 . A close version of this model applies three consecutive log-distance models with different α coefficients according to the distance between the emitter and the receiver.
126.96.36.199. Large-scale shadowing
Large-scale shadowing, also called local mean attenuation, occurs due to the obstruction of the signal when it meets an obstacle such as a building, a truck or an hill. Confirmed by empirical studies in both indoor and outdoor environments, the most common model used is log-normal shadowing. It takes into account the path loss based on a reference distance d 0 and applies a log-normal shadowing in order to compute the power reception ¬P r (d ) as follows:
with a typical value for β ∈ [2.7, 5] and χσ is a log-normal distribution with a standard deviation σdB ∈ [4, 12]dB in an outdoor environment.
188.8.131.52. Small-scale fading
Small-scale fading refers to the effect of the multi-path of the wave. It occurs when the interaction between the signal and the obstacle produces a split of the current signal with a different speed and strength. Such effects can be modeled by some statistical laws. The Rayleigh model describes an environment with several multi-paths that have the same strength. Such models are used to describe environments where the signal is highly disturbed. The amplitude of the received signal z can be described by the Rayleigh distribution defined by equation [3.9].
with 2σ2 the root mean square of the received signal and the assumption that each path is uniformly distributed [−π,π]. An environment wherein the strength of a path is higher than others can be described with a Rice distribution. The amplitude of the received signal, z , can be described with the Rice distribution defined in equation [3.10].
where I 0 is the Bessel function (zero order) and A the amplitude of the predominant path. The strength of the predominant path and others ratio is determined by K = A2 /2σ2 describing the fading degree. Regarding K , the Rice distribution can model a Rayleigh if K = 0 and a Gaussian distribution when K → ∞.
The motion of an emitter and a receiver produces a frequency shift of the incoming electromagnetic waves. It results in an offset in the carrier frequency as depicted in Figure 3.4, where two observers A and B are looking for a vehicle going to B direction. A typical scenario is the sound emitted by a moving car with an observer behind it and an observer in front of the car. For A observer, the sound frequency is higher than the frequency measured at the car, whereas the frequency at B is lower than that measured at the car.
Figure 3.4. Illustration of the Doppler effect
The resulting frequency fr observed when a source emits at a frequency of f is computed as follows:
where c is the celerity of the wave, v s the velocity of the source and v r the velocity of the observer. Regarding the motion of the source, v s and v r are positive if the receiver is moving towards the source and the receiver is moving away from the receiver, respectively. Under three assumptions, the antenna of the receiver is omnidirectional, the radio wave is propagated horizontally and the angle of receiving radio waves is uniformly distributed, the Doppler effect can be described by a Jakes model, wherein the normalized Jakes Doppler power spectrum is given by:
where f d is the maximum Doppler frequency.
184.108.40.206. A note on praxis measurements
In a theoretical manner, a link between two entities is considered as enabled or lost. In praxis, such a vision is too simplistic. As depicted in Figure 3.6, the signal affected by a path loss, shadowing and a fast-fading effect cannot be easily considered as enabled or lost. [ZAM 07] have demonstrated with their channel model that a link is characterized by three phases (Figure 3.5). First, the connected region where the link has a high packet reception ratio. Second, the transitional region, wherein the link is considered as unreliable and the packet ratio is fluctuated. Third, the disconnected region, where the link has a low packet reception ratio.
Figure 3.5. The three regions of the link quality
In Figure 3.6, the signal affected only by a Friss model shows a disconnection at 2000 m. With realistic propagation models, the good region of the link extends to 700 m, at which point the transitional region is then extended to around 1500 m, and consequently at the end, the list is then considered to be disconnected.
click for larger image
Figure 3.6. Attenuation, shadowing, fast fading effect: power reception over distance with two ray ground, shadowing, Rice (K = 2) and Doppler model
It is important for performance evaluations to test data delivery solutions under realistic propagation conditions. That is why [ABB 15] have designed a realistic propagation model from real measurements. Its effectiveness was investigated and confirmed by [HIL 17].
The next installment of this series looks at VANET routing protocols and their related mechanisms.
Reprinted with permission from Elsevier/ISTE Press, Copyright © 2017
Frédéric Drouhin is an Assistant Professor in the Laboratoire Modélisation Intelligence Processus Systèmes (MIPS) at the Université de Haute Alsace.
Sébastien Bindel is an Associate Professor in the Département Réseaux et Télécommunications at Université de Haute-Alsace.