can select Carrier-2 as next carrier and forward its packet to
Carrier-2 since Carrier-2 has a high probability that it can
forward Source’s packets to the access point via a commu-
nic ation path consisting of other vehicles. The second
challenging problem is how to combine the road traffic
statistics (e.g., density) information with the vehicle trajec-
tory information for better forwarding decision making.
In next sections, we will deal with the two challenges
raised in this section through the Link delay modeling (in
Section 3) and the Trajectory-based forwarding (in Section 4).
3THE LINK DELAY MODEL
This section analyzes the link delay for one road segment
with one-way vehicular traffic given the vehicle arrival rate
, the vehicle speed v, and the communication range R; note
that a constant vehicle speed v is used for the link delay
analysis and that the impact of the variable vehicle speed on
the link delay will be shown in comparison of simulation
results at the end of this section; the results indicate that our
link delay model is a good approximation to the simulation
result. We leave the link delay for a two-way road segment
as future work. Three terms for the link delay model are
defined as follows:
Definition 1 (Network Component). Let Network Compo-
nent be a group of vehicles that can communicate with each
other via either one-hop or multihop communication, that is, a
connected ad hoc network. Fig. 2 shows a network component
consisting of vehicles n
1
; ...;n
k
.
Definition 2 (Forwarding Distance). Let Forwarding Dis-
tance (denoted as l
f
) be the physical distance a packet travels
via wireless communication within a road segment starting
from the entrance. Fig. 2 shows the forwarding distance l
f
for
the network component.
Definition 3 (Carry Distance). Let Carry Distance (denoted
as l
c
) be the physical distance a packet is carried by a vehicle
within a road segment. Fig. 2 shows the carry distance l
c
of
vehicle n
1
.
Let v be the vehicle speed. B y ignoring the small
communication delay, the link delay d
ij
along a road with
the length of l is the corresponding carry delay. We have,
d
ij
¼
l
c
v
; where l
c
¼ l l
f
: ð1Þ
Therefore, the expected link delay E½d
ij
is
E½d
ij
¼ðl E½l
f
Þ=v: ð2Þ
In (2), in order to obtain the expected link delay E½d
ij
,we
need to derive the expected forwarding distance E½l
f
first.
Clearly, the forwarding distance l
f
equals the communication
length of the network component that is near the entrance as
shown in Fig. 2. To illustrate our modeling approach, we use
Fig. 3a to explain how the forwarding distance l
f
change over
time under different traffic arrival patterns.
JEONG ET AL.: TRAJECTORY-BASED DATA FORWARDING FOR LIGHT-TRAFFIC VEHICULAR AD HOC NETWORKS 745
Fig. 1. Packet delivery scenarios. (a) A light-traffic road network. (b) A
road network with unbalanced traffic density.
Fig. 2. Forwarding distance l
f
and Carry distance l
c
.
Fig. 3. Forwarding distance ( l
f
) for vehicle arrivals. (a) Forwarding
distance (l
f
) over time. (b) Vehicle arrival sequence on one-way road
segment.