22-05-2014, 02:47 PM
A Cooperative MAC protocol for Ad Hoc Wireless Networks
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Abstract
Cooperative communications fully leverages the
broadcast nature of the wireless channel and spatial diversity,
thereby achieving tremendous improvements in system capacity
and delay. By enabling additional collaboration from stations
that otherwise will not directly participate in the transmission,
cooperative communications ushers in a new design paradigm for
wireless communications. In this paper, we extend a cooperative
MAC protocol called CoopMAC [1] into the ad hoc network
environment1 . The new protocol is based on the idea of involving
in an ongoing communication an intermediate station that is
located between the transmitter and the receiver. The interme-
diate station acts as a helper and forwards to the destination
the traffic it receives from the source. Thus, a slow one-hop
transmission is transformed into a faster two-hop transmission,
thereby decreasing the transmission time for the traffic being
handled. Extensive simulations in a large scale wireless ad-
hoc network (150 stations) show that CoopMAC significantly
improves the ad hoc network performance in terms of throughput
and delay, and indicate how such cooperative schemes can boost
the performance of traditional solutions (e.g., IEEE 802.11).
INTRODUCTION
The notion of cooperation takes full advantage of the
broadcast nature of the wireless channel and creates spa-
tial diversity, thereby achieving tremendous improvement in
system robustness, capacity, delay, a significant reduction in
interference, and extension of coverage range. Moreover, by
enabling additional collaboration from stations that otherwise
will not directly participate in the transmission, cooperative
communication unveils a new protocol design paradigm for
wireless communications.
The initial attempts for developing cooperative communi-
cations focused on physical layer schemes [2]–[4]. These ap-
proaches refer to the collaborative processing and retransmis-
sion of the overheard information at those stations surrounding
the source and the destination. By combining different copies
of the same signal transmitted by source and different relay
stations, the destination can improve its ability to decode the
original packet
A Cooperative MAC Protocol for ad-hoc networks
The protocol that is described in this section is based on
a MAC protocol that is called CoopMAC and is proposed
in [1]. CoopMAC is a cooperative protocol for infrastructure
wireless LANs. Its main aim is to support and improve
the communication of wireless stations in a cell with the
corresponding AP. In this paper we extend the functionality
of this protocol, adding new features, in order to design a new
cooperative MAC protocol for ad-hoc wireless networks.
The set of new features of cooperative MAC spans both
the data plane and control plane of the protocol stack. For
ease of explanation, the term relay and helper will be used
interchangeably in the following discussion. As shown in
Figure 1(b), ST As , ST Ah and ST Ad represent the source,
helper and destination station, respectively. Rsd , Rsh and
Rhd denote the sustainable rates between ST As and ST Ad ,
between ST As and ST Ah , and between ST Ah and ST Ad ,
respectively.
Simulation Settings
To quantify the performance of our proposed MAC, and to
assure a fair comparison with IEEE 802.11, we have devel-
oped an event-driven simulator. Four possible rates, namely
1 Mbps, 2.2 Mbps, 5 Mbps and 11 Mbps, which constitute
the permissible set of rates defined in IEEE 802.11b, were
used in our simulations. For each simulation, stations were
randomly placed in a circle of radius R = 350 m. The number
of stations varied from 15 to 150. The coverage areas for
different transmission rates are concentric circles of radius 100
m, 82.3 m, 76.7 m and 58.6 m for 1 Mbps, 2 Mbps, 5.5 Mbps
and 11 Mbps, respectively.
The destination of each packet was chosen randomly from
all of the neighbors that could be reached directly by a source
station. For each scenario we collected two types of statistics:
the aggregate network throughput and the service delay. The
data presented hereafter was averaged over several runs, each
of which had a different random initial seed and ran for a
period of time that was long enough to get stabilized results.
Simulation Results
Figure 2(a) reveal the relation between the network through-
put and the number of stations deployed. The MSDU packet
size is 1024 bytes. To obtain the system capacity, the network
is saturated and each station is in a backlogged state. It is
apparent that the cooperative MAC significantly outperforms
IEEE 802.11b.
Indeed, the Cooperative MAC protocol is anticipated to
deliver more throughput than the legacy IEEE 802.11 DCF due
to several reasons: First, it accelerates the slow transmissions
by replacing them with faster two-hop transmissions. Second,
the proposed protocol not only improves the performance of
slow stations, but also makes it possible for fast stations to
access the channel earlier, as the data transmissions from slow
stations take significantly less time.
CONCLUSIONS AND FUTURE WORK
In this paper we study a cooperative MAC scheme for ad-
hoc wireless networks. We measure its performance using
simulation results from a large scale network of 150 stations.
The thorough study shows that the cooperative protocol out-
performs IEEE 802.11 in most of the cases and set up a base
for considering the use of cooperation at the MAC layer as
an answer to the constraints on traditional protocols in dense
network environment.
As for future work, cross layer approaches will be con-
sidered, combining cooperation in the MAC and the PHY
layer. In the proposed protocol, a data frame is transmitted
sequentially two times: Once from the transmitter and once
from the helper. Since the receiver is able to overhear both
transmissions, inherently the protocol can support cooperative
schemes in the PHY layer. We are planning to combine several
PHY layer schemes with the cooperative MAC to study the
further improvements we can gain by such a combination.