## 3. Freestanding Position-Based Routing Protocol
### Radio Channel  
It is well known that in a wireless communication system, the impairments introduced by the channel can adversely affect the transmitted signal. In this context, the path loss and the fading are the most important channel properties to consider when analyzing the performance of a V2V wireless communication system, [37,58,59]. In fact, as the path loss increases the SNR at the receiver decreases, resulting in a degradation of the system performance, [37,58,59]. Most of the research dealing with the design of routing protocols for VANETs assumes that the radio channel follows the behavior of traditional propagation models, e.g., freespace, two rays, Nakagami, _etc._ Nevertheless, as shown in [37], these models do not hold for V2V communications in highway scenarios, because of the antennas height and the vehicle direction have different effects over the path loss [37, 55, 58, 59]. Considering an adequate radio channel model for the development of a routing protocol aimed to V2V applications is very important, as can be inferred from the analysis and results reported in [55]. In fact, the results in [55] show that different radio channel models have different impacts on the routing strategy performance. This is a consequence of the VANETs PHY layer intrinsic features, specifically the low antenna heights and the highly dynamic topology, [55, 58, 59, 60]. Recently, several channel models for V2V communications have been proposed in the literature, e.g., see [37, 61, 62, 63, 64] and references within [58, 59]. Among these works, [37, 62, 63, 64] provide path loss models for highway scenarios. On the other hand, [61] provides two different small-scale fading models for two different cases of V2V communications in highways: oncoming vehicles, and vehicles traveling in the same direction at the same speed. The models introduced in [61] have been previously used for the evaluation of the IEEE 802.11p standard, [27]. However, these models are not straightforward applicable to the evaluation and design of routing protocols for VANETs in highway scenarios, since the path loss and the shadowing are not considered. Particularly, the path loss is a critical parameter that should be considered when evaluating VANETs routing protocols, because of the SNR of the received signal at different nodes is dependent on the path loss. Furthermore, [61] does not provide a model for vehicles moving away each other in highways, which is a case occurring in highway VANETs [37]. Among the works that provide path loss characterization for highway scenarios (e.g., [37, 62, 63, 64]), the model introduced in [37] stands out because it is derived from extensive V2V measurement campaigns. Moreover, this model agrees well with previous findings as those reported in [63]. The path loss model introduced in [37] follows a classic power law and includes a large-scale fading (shadowing) term. Furthermore, the path loss model in [37] allows the characterization of three different V2V communications cases with one single model, specifically: oncoming vehicles, vehicles traveling in convoy, and vehicles travelling away from each other. Therefore, this work considers the highway path loss model introduced in [37] for the development of the routing strategy, since this model was specifically developed for V2V highway scenarios. The path loss model introduced [37] is described by Equation (1):  
\[PL(d)=PL_{0}+10 nlog_{10}\left(\frac{d}{d_{0}}\right)+X_{\sigma}+\zeta PL_{c} \quad d>d_{0} \tag{1}\]  
where \(d\) is the propagation (\(T_{x}\ -\ R_{x}\)) distance; \(PL_{0}\) is the path loss at a reference distance \(d_{0}\); \(n\) is the path loss exponent; \(X_{\sigma}\) is a zero-mean normally distributed random variable with standard deviation \(\sigma\); \(PL_{c}\) is a correction term that accounts for the offset between forward and reverse path loss. An important characteristic of this path loss model is the introduction of the \(\zeta\) variable, which is defined according to the relative direction of vehicles.