Outage Analysis of Cooperative Space-Time Codes
with Network Coding
Gordhan Das Menghwar and Christoph F. Mecklenbr
¨
auker
Institute of Communications and Radio Frequency Engineering
Vienna University of Technology, Austria
Email:
{gdas, cfm}@nt.tuwien.ac.at
Abstract—With the increasing demand of reliable communi-
cations on move, cooperative communications has proved to be
an efficient solution to reduce the effects of unreliable nature of
wireless channel. In this paper, we study the Space-Time (ST)
cooperative communications aided by a fixed relay, forwarding
network coded bits to the destination. We present the perfor-
mance analysis in terms of outage probability. With analytical
results, we show that the investigated scheme outperforms the
cooperative transmission based on ST codes only. To be specific,
the analytical results show that ST codes with the help of
dedicated relay forwarding network coding can achieve diversity
order of 3 while ST codes based cooperative communications,
without use of network coding, only offers diversity order of 2.
I. INTRODUCTION
One of the major challenges of wireless channel is to
deal with the fading caused by multipath propagation of the
signal transmitted from source to the destination. To reduce
the effects of fading, multiple independent copies of the
signal are transmitted to average out the channel variations
due to multipath propagation. This technique is known as
diversity [1]. The most basic ways to provide diversity are
by means of frequency, time, and space [2]. The last form of
diversity, formally known as spatial diversity, is a way where
signal is transmitted or received by multiple, geographically
separated, antennas to achieve multiple independently faded
copies of the transmitted signal.
Cooperative communications has vastly been investigated
to provide the spatial diversity and the capacity improvements
to the wireless users [3][4]. It is a communication technique
where participating users act as a relay for each other, to
forward some information on behalf of the other user, for
reliable recovery of the message at the destination. Further-
more, as we move on to high frequencies, coverage problem
becomes prominent. Use of relay node is proposed by the
Third Generation Partnership Program’s Long-Term Evolution
Advanced (3GPP-LTE-Advanced) at cell edges to extend the
area of reliable reception [5][6]. With the introduction of the
relay node, we not only get coverage extension, but improved
capacity limits can also be achieved for the edge users. To
realize the relay assisted benefits, various relaying protocols
have been proposed [7][8].
Decode-and-Forward (DF) relaying strategy or protocol is
one of the many investigated relaying protocols where relay
decodes and then re-encodes the message received from the
transmitting user and forwards it to the destination.
Recently, two ST cooperating users exploiting the availabil-
ity of free user acting as a fixed relay was proposed and inves-
tigated [9][10]. The protocol uses two ST cooperating users
transmitting to a common destination and includes a relay node
forwarding network coded bits, of the messages received from
cooperating mobile stations, to the destination. It was shown
that the scheme outperforms cooperative transmission using
ST codes only for a range of SNR, and then beyond that range,
the protocol starts to degrade its performance.
In this paper, we change the channel model from [9][10]
by exploiting the orthogonal channel assignment to the relay
node. With analytical results, we show that the proposed
scheme improves the performance over cooperative transmis-
sion using ST codes only.
The rest of the paper is structured as follows. In Section II,
we describe our system model. Section III gives the mutual
information and the resulting outage probability of the scheme.
We discuss our results in Section IV. Finally, Section V
concludes our paper.
II. S
YSTEM MODEL
As in [9][10], we consider a cellular region having two
mobile stations m
1
, m
2
, and a single base station at the center
of the cell. Both of the mobile stations have independent
information to be sent to a common destination. Furthermore,
these mobile stations are considered to be at the edge of the
cellular region with a fixed relay node between them and
the base station, as shown in Fig 1. Before explaining our
Fig. 1. Cooperative communications
scheme, i.e., cooperative ST codes with network coding, we
2010 International ITG Workshop on Smart Antennas (WSA 2010)
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