Networking Reference
In-Depth Information
The following algorithm is inspired by the approach
taken in [ART 07] for transmission power control, where the
vehicular density is estimated based on the time the car stops
in traffic. Therefore, the vehicle needs to measure the stop
time ( T stop ) in the last T update time window. If T stop =0 ,the
traffic is in a free-flow state and the CW is set to CW min .If
T stop = T update , the vehicle is considered to be a part of a traffic
jam and CW = CW max . For intermediate values, the following
formula is used:
CW =(T stop /T update )(CW max − CW min )+CW min
The mechanism could be implemented without any
additional hardware, as the stop time can already be
calculated using data from the speedometer. The problem
could lie in the fact that a vehicle might stop for several other
reasons than a traffic jam, especially in an urban scenario.
- Speed-based neighbor estimation: A more accurate
estimation of local density based on traffic-flow theory is
proposed by Shirani et al. [SHI 09], who use vehicle speed
and jerk (the derivative of acceleration with respect to
time) to adjust the transmission power. Therefore, this final
mechanism calculates the local density and the CW as follows:
CW = D l
D max (CW max − CW min )+CW min
where D l = |jerk|/speed , and D max is the predefined upper
threshold.
Although this approach uses more information than the
previous approach, it still lacks the ability to handle, without
any delay, some situations common to city traffic, which result
in a low speed without necessarily implying a high vehicular
density (e.g. left-turn and stop sign). Moreover, jerk is not
currently measured on a regular basis in vehicles.
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