05-09-2012, 03:49 PM
CDR-MAC: A Protocol for Full Exploitation of Directional Antennas in Ad Hoc Wireless Networks
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Abstract
In this paper, we propose a new Medium Access Control (MAC) protocol for full exploitation of directional antennas in
wireless networks. The protocol introduces a circular directional transmission of the Request To Send (RTS) control packet, spreading
around a station information about the intended communication. The stations that receive the directional RTS, using a simple scheme
of tracking the neighbors’ directions, defer their transmission toward the beams that could harm the ongoing communication. In this
way, the proposed protocol takes advantage of the benefits of directional transmissions as the increase of spatial reuse and of
coverage range. Additionally, it reduces the hidden-terminal problem, as well as the deafness problem, two main factors for the
decrease of the efficiency of directional transmissions in ad hoc networks. The performance evaluation of the protocol shows that it
offers a significant improvement in static, as well as mobile, scenarios, as compared to the performance of the proposed protocols that
use omnidirectional or directional transmissions.
INTRODUCTION
AD hoc wireless networks have attracted a growing
interest in many application domains, which include
commercial applications, military applications, and applications
in environments where the presence of an infrastructure
network is impossible or not affordable. Currently, the
technology of transmitting data over the air is based on
omnidirectional antennas; therefore, most of the research in
designing Medium Access Control (MAC) protocols has
assumed their deployment. The result is a number of
protocols, including the industry standard IEEE 802.11
[11], [17], which seems to effectively solve the challenges
projected in this environment.
RELATED WORK
The idea of using directional antennas in radio communications
has inspired many researchers. Zander [24] has
proposed to use directional antennas in slotted ALOHA
multihop packet radio networks. Some other approaches of
using directional antennas have focused on broadband and
cellular networks [1], [10], [23].
More recently, a few proposals have explored the
required modifications in the MAC protocol of 802.11
wireless networks in order to adapt this kind of antenna in
802.11 ad hoc networks. These proposals use combinations
of directional and omnidirectional transmissions for the
four-way handshake frames. Nasipuri et al. [16] propose a
variation of the Request To Send/Clear To Send (RTS/CTS)
mechanism of IEEE 802.11 adapted for use with directional
antennas. Their protocol sends the RTS and CTS packets
omnidirectionally in order to enable the transmitter and
receiver to locate each other and then sends the data packet
and acknowledgment (ACK) in direct mode. Ko et al. [13]
propose another MAC protocol that sends a directional RTS
that is followed by an omni-CTS. They assume that the
transmitter knows the receiver’s location, so it directionally
transmits the RTS to it. They propose an alternative scheme
in case there is a lack of information for the location of the
receiver. In this case, the RTS is transmitted in omnimode in
order to seek the receiver.
Directional Antennas
A directional antenna can transmit a signal in any direction,
using an array of antennas called array of elements.
Individual omnidirectional transmissions from these elements
interfere positively or negatively with each other,
resulting in an increase of signal strength in one or more
directions and elimination in others. The greater the number
of elements of a directional antenna, the better the increase
of the signal in the desired direction. There are directional
antennas with one (omnidirectional), two, four, eight, 16,
and so forth elements. The interested reader can find an
intensive study of directional antennas in [15] and [19].
As the number of antenna elements increase, the
beam width and the signal gain can be controlled more
effectively. An important element of our protocol is covering
the whole area around the transmitter with successive
sequential transmissions. In the rest of the paper, we will
assume that we can provide effective omnitransmission
with M sequential directional transmissions when we have
M antenna elements. In certain cases, this number may
need to be higher, but our conclusions, which are based on
the numerical experiments we performed, will not change.
Circular Directional RTS
Our protocol is based in a simple and innovative scheme of
RTS transmission. In this scheme, the RTS is transmitted
directionally consecutively in a circular way until it scans
all the area around the transmitter. The circular transmission
of the CTS is an alternative that may enhance the
performance of the protocol as well. Since this may result in
the unpredictable behavior of the communicating stations,
we will not study this factor in this work. We are going to
investigate this idea in our future studies.
As mentioned, we assume antennas with a predefined
number of beams, M in Fig. 2, which cover the area around
the transmitter. Assume that, with beam 1, the transmitter
starts transmitting its RTS in a predefined direction.
Shortly afterward, it turns its transmission beam on the
right, sending the same RTS with the next one (beam 2).
It continues this procedure again and again until the
transmission of RTS covers the entire area around the
transmitter (until it sends the RTS with beam M).
Random Scenarios with Mobility
It is very common for the nodes of a wireless LAN to be
mobile. Mobility is a typical characteristic of such an
environment. Thus, it is important to examine the behavior
of our protocol in scenarios where the nodes are mobile.
The efficient update of the Location Table and the accurate
transmission of the frames toward the right direction are
very important issues for the right behavior of the protocol.
To study the efficiency of the protocol in a mobility
environment, we consider some random scenarios. We
measure the throughput in every scenario for static, as well
as for mobile, nodes. We examine cases with both low and
high mobility for the nodes. With low mobility, nodes are
assumed to move with speeds of 2 m/s, whereas, with high
mobility, they move at 10 m/s.
CONCLUSIONS
In this work, we propose a MAC protocol suitable for
networks with directional antennas. Our protocol, utilizing
a new scheme for the broadcasting of RTS, employs only
directional transmissions, increasing in this way the coverage
area. Particularly, the proposed algorithm is based on a
CDR that scans the area around the transmitter, informing
the neighbors of the intended communication. Using a
simple and effective scheme, the neighbors decide for their
transmission differentiation in order not to destroy the
ongoing transmission. In this way, there is a strong decrease
in the hidden-terminal problem. The protocol does not
assume any knowledge of the neighbors’ locations. Due to
the dynamic nature of the functionality of the protocol, it
behaves efficiently in an environment with static, as well as
mobile, users. The previous features result in an efficient
integrated scheme that can be implemented easily.