A DMAC Protocol to Improve Spatial Reuse by
Managing the NAV Table of the Nodes in VANET
授課老師：許子衡 老師
學生：羅英辰
Introduction

Vehicular communication has begun to attract more
attention in industry and academia.
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An infrastructure for wireless communication from
vehicle to road and from vehicle to vehicle is an important
technology in providing telematics services.

To improve the efficiency of VANET (Vehicle Ad-hoc
Network), the MAC (Medium Access Control) protocol
based on MANET (Mobile Adhoc Network) has been
carefully studied.

The use of a directional antenna allows for spatial reuse
and high communication throughput in the vehicular
environment which is characterized by high-mobility on a
restricted road.
Directional Antenna
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In using only an omni-antenna, the MAC protocol has an
exposed terminal problem due to the CSMA/CA (Carrier
Sense Multiple Access with Collision Avoidance)
mechanism.
A node in the idle state awaits a signal from the omnidirection of gain Go of the antenna, if the node receives a
transmitted signal, it transmits a message of gain Gd (>
Go), which is the maximum transmission gain.
As shown in Figure 1(a), DBTMA/DA broadcasts a BTr
(Receive busy tone) before transmitting a CTS and BTt
(Transmit busy tone) before the DATA in the control
channel.
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Figure 1(b) shows the operation of RIDMAC.
When Node A first sends a packet, the packet includes
information from the table.
Node B updates the table according to the packet received
from Node A and transmits RTR (Ready to Receive) to
receive data from the next node.
DMAC/NT
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In VANET, the location and mobility of a vehicle can be
predicted because the road environment has a consistent
Node.
We propose the DMAC/NT protocol to improve efficiency
in VANET by solving these problems without using
additional channels or extra frames.
The DMAC protocol prevents packet collision by setting
up the NAV (Network Allocation Vector) through an
exchange of RTS and CTS.
Performing collision-avoidance by setting up the NAV for
spatial reuse in DMAC can cause trouble when topology
variation exists, as shown in Figure 2.

As shown in Figure 3, we propose a DMAC protocol
suitable for VANET which is based on the multi-NAV
table of each node.

The table includes a node ID and the NAV of each node,
and is updated by transmitted ORTS and OCTS.

This protocol solves the deafness problem and improves
spatial reuse.

Figure 4 shows the status flows of the sender and receiver
nodes in DMAC/NT.
Evaluation And Analysis

The network layer used the IP and AODV protocols which
have the best efficiency in VANET.

The source node continually transmitted a CBR (Constant
Bit Rate) to create a packet collision.

The first scenario was designed to evaluate efficiency
according to the transmission interval of the packets in the
mobility by compulsion.
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The second scenario evaluated random mobility.
Conclusions
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This letter proposes a DMAC protocol suitable for
VANET.
This new protocol was designed to solve VANET’s
deafness problem and to enable efficient spatial reuse,
which has a similar mobility pattern, by creating a multiNAV table managing the Node ID and NAV of each node
according to its direction.
Through simulation, we confirmed that the DMAC/NT
protocol had an increased efficiency in total throughput
and average jitter because of improved spatial reuse.