Hence, there exists a need for accurate and efficient V2V
channel models. To provide such a model, we incorporate
vehicles as obstacles and present a method to analyze the
existence of the LOS component of the signal for each
communicating pair. The focus on the existence of the LOS
component was motivated by the recent experimental V2V
studies reported in [11] and [12]. These studies showed that,
when existent, the LOS component of the signal carries orders
of magnitude more power than the remaining components
(e.g., due to reflection or diffraction). This was shown to be
especially true for highway environments. We therefore analyze
the data collected on Portuguese highways to show that,
as physical obstacles, vehicles have a significant impact on
signal propagation, by frequently obstructing the LOS between
the communicating vehicles. Based on the (non-)existence
of LOS, we implemented an efficient model for vehicles as
obstacles and showed that for the proposed VANET communication
standard, the Dedicated Short Range Communication
(DSRC) [13], the signal attenuation due to the obstructing
vehicles is significant. To further verify the predictions of the
proposed model, we conducted empirical measurements which
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16 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 29, NO. 1, JANUARY 2011
corroborated the results regarding the signal attenuation due
to vehicles. Not modeling the vehicles as obstacles thus leads
to unrealistic assumptions about the physical layer, and this
was shown to have significant implications on the behavior
of the upper layers of the protocol stack (e.g., [8], [14], and
[15]).