IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 6, NO. 1, JANUARY 2007 41
A Cooperative Diversity Scheme Based on Quadrature Signaling
Veluppillai Mahinthan, Student Member, IEEE, Jon W. Mark, Life Fellow, IEEE,
and Xuemin (Sherman) Shen, Senior Member, IEEE
Abstract— A bandwidth and energy efficient cooperative di-
versity scheme based on quadrature signaling is proposed. The
quadrature signaling is achieved by transmitting in the in-phase
and quadrature components of QPSK modulation. The diversity
gain and the bit error rate of the proposed cooperative diversity
scheme improve with the inter-user channel quality. It is shown
that the proposed scheme can achieve a diversity order of two for
high inter-user signal-to-noise ratios, if the cooperating users are
located within a region that permits successful cooperation. The
cooperative region corresponding to a specified bit error rate is
defined and derived.
Index Terms— Cooperative diversity, MIMO, maximum ratio
combining, user collaboration, cooperative region.
I. INTRODUCTION
T
RANSMIT diversity is a powerful technique for combat-
ing multipath fading in wireless comm unications. It has
been used in 3
rd
generation mobile communication systems to
increase the downlink data rate. However, employing m ultiple
antennas in a mobile terminal to achieve the transmit diversity
in the uplink is not feasible due to the limited size o f the
mobile unit. In order to overcome this problem, a new mode
of transmit diversity, called cooperative diversity (CD), based
on user cooperation has been proposed in [1]-[6]. By user
cooperation, it is meant that when the sender transmits to
the destination, it also transmits co pies to other users, called
partners, for relaying to the destination. The antennas o f the
sender and the partners togeth er form a multiple transmit
antenna situation. CD schemes are immune not only against
small scale channel fading but also against large scale channel
fading.
An adaptive regenerate and forward scheme for CD net-
works is proposed in [3] and [4]. The partner forwards the
inform ation if it is decoded correctly
1
. The influence of the
data rate, path loss and network geometry on the cooperative
scheme is studied for an error free inter-user channel. In
this sch eme, the channel is divided into two sub-channels in
the time domain, which requires a b andwidth twice as large
compared to a non-cooperative diversity (NCD) scheme.
In this paper, a bandwidth and energy efficient CD scheme
based on quadrature signaling is proposed for a two-user
Manuscript received February 15, 2005; revised August 5, 2005, October
24, 2005, and October 27, 2005; accepted October 29, 2005. The associate
editor coordinating the re view of this paper and approving it for publication
was H. B¨olcskei. This work has been supported by the Natural Sciences
and Engineering Research Council (NSERC) of Canada under Grant No.
RGPIN7779. This paper was presented in part at the IEEE WCNC ’2005,
Ne w Orleans, LA, March 2005.
The authors are with the Department of Electrical and Computer Engi-
neering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 (email:
{mveluppi, jwmark, xshen}@bbcr.uwaterloo.ca).
Digital Object Identifier 10.1109/TWC.2007.05092.
1
The correctness can be verified using cyclic redundancy check (CRC)
system. Instead of expanding the bandwidth or reducing the
effective data rate of the cooperative system, we expand
the signal constellations of the modulation scheme to ac-
commodate the partner. Quadrature signaling is achieved by
transmitting in the in-phase and quadrature components of a
phase shift keying modulation scheme. Specifically, this paper
is concerned with fixed regenerate and forward employing
symbol by symbol detection and no cyclic redundancy check
(CRC) in the partner’s device, which provides implementation
simplicity to the scheme. On the other hand, an adaptive
regenerate and forward scheme provides more diversity gain
at the expense of device complexity, which is not presented
here due to space limitations.
The power and bandwidth of the proposed cooperative
scheme are equal to those of a NCD scheme. Both processing
delay at the partner mobile and propagation d elay difference
between the direct and the partner transmissions are taken
into account at the base station (BS) receiver. Furthermore,
the proposed scheme is simpler to implement than the CD
schemes reported in the literature, since it only needs the
additional hardware to receive the uplink signal of the partner.
It also can be easily switched between CD and NCD modes
based on the inter-user signal-to-noise ratio (iSNR). Bit error
probability (BEP) and diversity gain achieved by the proposed
CD scheme is studied, and a cooperative region for a user to
find its partner with specified bit error performance is defined
and analyzed.
The rest of the paper is o rganized as follows. In section
II, the system architecture is d escribed and a CD scheme is
proposed. The BEP of the system is derived and analyzed in
Section III. Section IV studies the diversity gain and coding
gain of the proposed scheme. The region in which the partner
can have cooperation, called the cooperative region, is derived
in Section V. Conclusions are given in section VI.
II. S
YSTEM ARCHITECTURE
Consider a cooperative wireless communication system
in which each mobile user cooperates with another mobile
user (the partner) to transmit information in the uplink. The
transmission frame format and the signal constellation of the
QPSK modulation scheme are shown in Fig. 1. Fig. 1(a)
shows a frame of information symbols transmitted by user
1 in channel 1 and user 2 in channel 2, respectively. In
this case, the multiple access channels may be frequency
division multiple access (FDMA) or time division multiple
access (TDMA). Since the individual users use their own
multiple access channels, the relaying u ser can transmit and
receive simultaneously. In the first symbol interval, each user
transmits its own information only. In successive symbol
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c
2007 IEEE
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