I. J. Communications, Network and System Sciences, 2008, 3, 207-283
Published Online August 2008 in SciRes (http://www.SciRP.org/journal/ijcns/).
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 3, 207-283
A Multi-User Cooperative Diversity for Wireless
Local Area Networks
Jun CHEN
1
, Karim DJOUANI
2
1
LISSI Lab., University Paris 12 and CEDRIC-CNAM Paris, France
2
Member, IEEE, F’SATIE/TUT Pretoria, South Africa and LISSI Lab., University Paris 12, France
E-mail:
1
chen_ju@auditeur.cnam.fr,
2
djouani@univ-paris12.fr
Received on November 12, 2007; revised and accepted on May 3, 2008
Abstract
In this paper, an idea of using space-time block coding (STBC) in multi-user cooperative diversity has been
exploited to improve the performance of the transmission in wireless local area networks. The theoretical
and simulation results show that, using STBC approaches can always achieve the better performance than
existing techniques without introducing the space-time coding. By analyzing the throughput and frame error
ratio (FER) of the two different STBC cooperative schemes, we find the trade-off between throughput and
reliability. The location of the relay is crucial to the performance, which supposes a rule for future cross-
layer design.
Keywords: Multiple-input-multiple-output (MIMO), Cooperation, Space-time Block Coding (STBC).
1. Introduction
Diversity is a powerful technique to mitigate fading and
improve robustness to interference [1], which refers to
the method by conveying the signal to the receiver over
multiple independently signal fading channels. The
conventional view of transmit diversity is that a single
wireless terminal transmits using an array of multiple-
antennas so that the paths from each antenna to the
destination with independently fading. The recent
research work in this area is the space-time coding (STC)
techniques that have been developed for multi-antenna
arrays. STC is an effective coding technique that uses
transmit diversity to combat the detrimental effects in
wireless fading channels [7]. Unfortunately, transmit
diversity methods based on multiple-input-multiple-
output (MIMO) approach are not applicable to many
wireless systems because of the size, complexity, power
or other constraints, as for instance, ad-hoc networks and
sensor networks. On account of these reasons,
cooperation between wireless terminals has been recently
proposed as a means to provide transmit diversity as
which shown in Figure 1, where S, R and D represent
source, relay and destination terminal, respectively. A
new method introduced in [2] and [3] to realize space
diversity gain has been studied under the name of
cooperative diversity. Traditional cooperative diversity
transmits the same signals through two different channels
as Figure 2. In the first time slot, the source
communicates to the relay and to the destination at the
same time; in the second time slot, just the relay
retransmits the signal received at the first time-slot to the
destination. The relay may simply forward the signal
received from the source terminal or retransmit the
estimates of the received symbols, obtained by detection.
We call it as repeat cooperation. In this paper, we present
a paradigm for cooperative diversity, which we term
space-time block coding (STBC) cooperation [21],
integrating user cooperation with STBC.
We summarize here the relevant contributions in the
area of the cooperative diversity. Relay channels and space-
time code form the basis for our study. The classical three-
terminal communication channels originally examined by
van der Meulen [5]. For the channels with multiple
information sources, Kramer and van Wijngaarden [6]
consider a multiple access channel in which the sources
communicate to one destination and share one relay.
Laneman et al. examines the mode of user cooperation
diversity [2,3] and analyzes space time coding
cooperative diversity in nonergodic settings using outage
probability as a performance measure [4]. They
A MULTI-USER COOPERATIVE DIVERSITY FOR WIRELESS LOCAL AREA NETWORKS 267
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 3, 207-283
Figure 1. Single-relay cooperative diversity model.
Figure 2. Time sequence of 2 time slots repeat cooperative
diversity.
Figure 3. Time sequence of 2 time slots multi-user
cooperative diversity.
demonstrated the extent to which space-time coding
cooperative diversity achieves higher diversity order than
repetition-based schemes for larger spectral efficiencies
in theorem. The model they analyzed is a selective
orthogonal amplify and forward (OAF) protocol, where
source transmits the vector of encoded data in the first
time slot and relay retransmits the received vector by
adjusting the power. The non-orthogonal amplify-and-
forward (NAF) scheme was proposed by Nabar et al.
[8,9] for the single-relay channel, where source transmits
all the time but the relay only transmits on even time
slots. They consider three different time-division
multiple-access-based cooperative protocols that vary the
degree of broadcasting and receive collision in either the
amplify-and-forward (AF) or decode-and-forward (DF)
modes. And the results indicate that optimal space-time
code design in the single relay case consists of satisfying
the classical rank and determinant criteria for co-located
antennas. These academic works sustain the possibility,
existence and benefits for deploying space-time coding
cooperative diversity protocols in practice.
This paper examines a new 2×2 full-rate space-time
code (Golden-Code) [12] in the single-relay cooperative
NAF model. For source transmits in both two time slots,
this protocol can achieve a higher throughput than that of
the OAF protocol. And we here consider these two types
of cooperative protocols and compare the performance
between Golden-Code and the classical Alamouti code
[10]. Besides the distinct benefits of the space-time code,
we can see the trade-off between throughput and
reliability during the transmission by analyzing the
results of throughput and frame error rate. At the last part,
we give a basic idea about the selection of relay.
Organization of the paper. This paper continues as
follows: Section 2 outlines the multi-user cooperative
diversity model. Section 3 explains STBC cooperative
diversity. Section 4 shows the performance analysis by
the simulation results. Section 5 summarizes our
conclusions.
2. Multi-User Cooperative Diversity Model
We consider wireless network in which two terminals are
communicating with a base station. The channel between
each terminal and the base station are independent of
each other, and independent of the channel between the
terminals. All channels are subject to flat (frequency non-
selective) fading in order to isolate the benefits of spatial
diversity. Considering the multi-user cooperative
diversity model, signal is to be transmitted from the
source terminal S to the destination terminal D with the
assistance of the relay terminal R. All the terminals are
equipped with single antenna. Throughout the paper we
assume that a terminal cannot transmit and receive
simultaneously. the channels SD, SR and RD are
known to the destination terminal.
The signal transmits procession is like following:
During the first time slot, the source communicates with
the relay and destination. In the second time slot, both
the relay and source communicate with the destination.
Figure 3 shows the detail of the time sequence.
In the AF relaying method [1], the relay simply
amplifies and retransmits the signal received from the
source (the signal received at the relay is distorted by
fading and additive noise). No demodulation or decoding
of the received signal is performed at relay in this case.
The signals received by the destination and relay in
the first time slot can be defined as
sdsdsdsd
nxhwy +=
1 (1)
and
srsrsrsr
nxhwy
+
=
1
(2)
respectively, where w
2
sd
and w
2sr
are the average signal
energies received by destination over channel S
D and
S
R, respectively [9]. h
sd
and h
sr
are the random,
complex-valued and unit-power channel gains between S
D and S
R. n
sd
CN(0, N
sd
, n
sr
CN(0,Nsr) is the
268 J. CHEN ET AL.
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 3, 207-283
additive noises, and in general w
2sd
w
2sr
.
The energy of received signal (3) is given by
(
)
(
)
(
)
srsrsrsrsrsrsr
NhwnExhwEyE +=+=
22
22
1
2
(3)
In order to retransmit the signal with the same power as
the sender did, the gain β for the amplification is
srsrsr
Nhw +
=
22
1
β
(4)
Then, the destination receives a superposition of relay
and source during the second time slot:
rdsrrdrdsdsd
nyhvxhwy
+
+
=
β
22
(5)
where v
2rd
is the average signal energy received at the
destination through channel R
D, the definition of h
rd
and n
rd
are the similar to h
sr
and n
sr
.
So the equation (5) can be rewritten as:
nxhwvxhwy
rdsrrdsdsd
~
122
++=
β
(6)
where
(
)
N
CNn
0
,0
~
with
N
N
h
v
N
rdsrrdrd
β+=
2
22
0
As the summary, the transmission function of this
cooperative diversity is y = Hx + n (7)
where
=
=
2
1
2
1
,x
x
x
y
y
y
is the received signal vector and transmitted signal vector,
respectively;
+
=
rdsrrdrd
sd
nnhv
n
n
β
(8)
is the noise vector; and H is the 2×2 channel matrix
given by
=
sdsdrdsrrdsr
sd
hwhhvw
w
H
β
0
(9)
Assuming that the channel coefficient matrix H is
known or can be estimated, Maximum Likelihood (ML)
decoding can be used at receiver to fully explore the
diversity advantage of the scheme. In equation (9), the
noise of first time slot and second time slot do not have
the same powers, the ML estimation can not be used
directly. One solution is normalizing the received noise
by a parameter
ρ
as follows:
+
=
n
n
x
x
hwhhvw
hw
y
y
sd
sdsdrdsrrdsr
sdsd
~
0
2
1
2
1
ρ
ρβρρ
(10)
where
rdsrrdrd
rd
NNhv
N
+
=
22
β
ρ
(11)
Then, equation (10) can be noted as
y
~
=
H
~
x+
n
~
.
Assuming that the channel coefficient matrix
H
~
is
known or can be estimated, the ML estimate of the
transmitted packets is presented as follows:
2
~
~
minarg
ˆ
F
x
xHnx −=
(12)
where
‖·‖
F
represents the Frobenius-2 norm, and x
takes all possible finite values depending on the signal
constellation.
3. STBC Cooperation Model
STC is a method employed to improve the reliability of
data transmission in wireless systems by using multiple
transmit antennas. It relies on redundant copies of a
signal to the receiver in the hope that at least some of
them may survive the physical path between transmission
and reception. Space time codes may be split into two
main types: Space-time trellis coding (STTC) [16] and
STBC [17]. We are only concerned here with STBC
which acts on a block of data at once (similarly to block
coding) and provide only diversity gain, but are much
less complex in implementation terms than STTC.
Alamouti coding [10] and Golden-Code [12] are typical
examples of STBC.
3.1. Repeat Cooperation
Firstly, we present the model shown in Figure 2, repeat
cooperation transmits the same signals through two
different channels. In the first time slot, the source
communicates to the relay and to the destination at the
same time; in the second time slot, just the relay
retransmits the signal received at the first time slot to the
destination. Then, the transmission function can be noted
as follows:
rdsrrdrdrdsrsrrd
sdsdsd
nnhvxhhwvy
nxhwy
++=
+=
ββ
12
11
(13)
The cooperative transmission function can be written
as
nhxy+=
1
(14)
where
+
=
=
=
rdsrrdrd
sd
rdsrsrrd
sdsd
nnhv
n
n
hhwv
hw
h
y
y
y
β
β
,
2
1
3.2. Alamouti Coding Cooperation
Alamouti proposed a simple MIMO scheme that
achieves a full diversity gain [17] with a simple ML
A MULTI-USER COOPERATIVE DIVERSITY FOR WIRELESS LOCAL AREA NETWORKS 269
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 3, 207-283
decoding algorithm. The transmit signals are modulated
using an M-ary modulation scheme, then the encoder
takes a block of two modulated signals s
1
and s
2
in each
encoding operation and sends it to the transmit antennas
according to the code matrix:
=
=
*
12
*
21
43
21
ss
ss
xx
xx
C
(15)
where * denotes complex conjugate. In this code matrix,
the first column represents the first time slot
(transmission period) in a 2
×
2 MIMO system [11] and
the second column represents the second time slot. The
first row corresponds to the signals transmitted from the
first antenna and the second row corresponds to the
signals transmitted from the second one. This implies
that the signals are transmitting both in space (across two
antennas) and time (two transmission intervals), that is to
say, space-time coding.
The traditional Alamouti coding is designed for a
two-transmit antenna system. Assuming the cooperative
method using one-relay AF channel, we define d
1
= (x
1
,
x
2
) and d
2
= (x
3
, x
4
). Thus, in the first time slot, the
source sends d
1
, the relay and destination receive the
signal; in the second time slot, the source and relay send
d
2
and x
r
to destination respectively. Then the Alamouti
coding cooperative transmission function can be written as
Y = HX +N
(16)
where
=
=
43
21
43
21
,yy
yy
Y
xx
xx
X
are the transmitted and received signal matrix,
respectively; channel matrix H and noise N are given by
=
sdsrrdsrsdrd
sdsd
hwhhwv
hw
H
β
0
1 2
1 13224
sd sd
rd rdsrrdsdrd rdsrrdsd
n n
Nvh nnnvh nnn
β β
=
+ +++
3.3. Golden-Code Cooperation
The Golden-Code is a STBC for 2× 2 MIMO system as
Figure 5, the coding matrix for the model is:
Figure 4. Alamouti coding in 2 × 2 MIMO model.
Figure 5. Golden-code in 2 × 2 MIMO model.
(
)
(
)
() ()
1 23 4
1 2
3 4341 2
1
5
s sss
x x
Cx x
i ssss
α θαθ
αθα θ
+ +
 
 
= =
 
 + +
 
 
 
(17)
where s1, s2, s3, s4
Z[i] are the information signals,
θαθαθθ
iiii −−=−−=
=
+
=1,1,
2
51
,
2
51
and the
factor
5
1
is necessary for energy normalizing purposes
[12].
The Golden-Code achieves the diversity multiplexing
frontier [13], and in [12] the Golden-Code was proposed
as a full rate and full diversity code for 2× 2 MIMO
systems.
To the cooperative method using one-relay AF
channel, we define d1 = {(x1,x2)} and d2 = {(x3, x4)}
which are transmitted in first time slot and second time
slot, respectively. The transmission function is similar to
equation (16).
4. Numeral Results
In this section, some simulations are presented to show
the performances of the presented approaches. In the
following simulations, Rayleigh model is used for the
fading channel [20], each channel multi-path is a zero
mean complex Gaussian random variable, and the
distance between all the terminals is assumed to be same.
Transmission energies follow the hypothesis as Table 1.
Table 1. Transmission energies in simulations.
Protocol 1
st
time slot 2
nd
time slot
Cooperation
2
sd
w
=1.0
2
sr
w
=0.5
2
rd
v
=0.5
MIMO 2
11
w
=2
12
w
=0.5
2
11
v
=2
12
v
=0.5
The throughput was defined as the average number of
available frames that were transmitted in a specific time
slot. We performed a random experiment consisting of
10,000 repeated independent trials. The length of each
frame was fixed to N = 600 bits. Considering the multi-
pack reception, the throughput can more than 1.
4.1. The Throughput Comparison between the
Repeat Cooperation and STBC Cooperation
We conducted comparisons between the STBC
270 J. CHEN ET AL.
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 3, 207-283
cooperation and repeat cooperation scheme. Figure 6 and
Figure 7 show the results of throughput versus SNR, for
6Mbps and 12Mbps transmit rates, respectively. We
observe that all the three schemes can achieve the
maximum throughput with a high SNR (> 25dB). With a
special coding method, Golden-Code cooperation
scheme achieves a much higher throughput than the other
two. Considering the coding matrix of Golden-Code,
each row contains all the 4 original signals, which means
the full-rate of the transmission. The cooperative method
transmits 4 available signals ({s1; s2; s3; s4}) during 2
time slots, which means the maximum value of
throughput is 2.
As to amamouti coding scheme, each row of the
coding matrix contains 2 original signals (s
1
and s
2
). In
every time slot, the system transmits one signal and the
conjugated signal of the other one, where s
*
1
and s
*
2
are
surly the redundancy copies of the original signals. That
is why only 2 available signals (s
1
and s
2
) can be obtained
at the destination in this scheme while 4 available signals
({
1
; s
2
; s
3
; s
4
}) can be obtained by using Golden-Code
scheme. Thus, by using two pair of conjugate signals,
Alamouti coding scheme transmits 2 available signals
during 2 time slots of the cooperative period, which
means the maximum value of throughput can no more
than 1 with the increasing of SNR.
Furthermore, as shown in Figure 2, repeat cooperation
transmits one signal during the first time slot and
retransmits the same one in the second. Clearly, repeat
cooperation can just transmit 1 signal during the two
time slots. Thus, its throughput is less than 0.5.
From the simulations, we see that with the help of
STBC gains, the STBC cooperation is outperform repeat
cooperation. And as a reasonable result of analysis and
simulation, the Golden-Code cooperation can clearly
achieve the best throughput among all the three schemes.
This also proves that the design of the space-time code
could impact the performance of the transmission.
Figure 6. Throughput of STBC cooperation and repeat
cooperation schemes (6Mbps).
Figure 7. Throughput of STBC cooperation and repeat
cooperation schemes (12Mbps).
4.2. The FER Comparison between Non-
ooperation, Repeat Cooperation and STBC
Cooperation
The simulation results of FER versus SNR between Non-
cooperation and cooperation schemes demonstrate again
that the use of relay-assisted communication is not
always beneficial when compared to direct transmission
(Non-cooperation scheme) [8]. Figure 8 and Figure 9
reveal that the frame error rate of Non-cooperation
communication is better than that of the simple repeat
cooperation for a high SNR (>35dB).
Further, as expected, cooperation with STBC is
always preferred over Non-cooperation scheme. Thus
from our simulations, we see that, performance using
STBC cooperation improves significantly over Non-
cooperation demonstrating the advantage of using STBC
cooperation. Between the two STBC cooperation
schemes (Alamouti coding and Golden-Code), Alamouti
coding method shows a better performance. As we
discussed, Alamouti coding transmits the redundance
Figure 8. FER versus SNR for Non-cooperation and
cooperation schemes (6Mbps).
A MULTI-USER COOPERATIVE DIVERSITY FOR WIRELESS LOCAL AREA NETWORKS 271
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 3, 207-283
Figure 9. FER versus SNR for Non-cooperation and
cooperation schemes (12Mbps).
Figure 10. FER versus SNR for STBC in cooperation and
MIMO schemes (6Mbps).
signals, a original and a conjugate. This is the reason that
it has a lower error rate in the destination while Golden-
Code just intersperses original signal among all parts of
the transmit signals.
Comparing with the simulation results about the
throughput of these two STBC cooperation schemes, we
see that, Alamouti coding have a lower throughput but a
higher reliability than that of Golden-Code. As a
summary, there is always a trade-off between the
throughput and the reliability.
4.3. The FER Comparison between STB
Cooperation and MIMO Schemes
Figure 10 and Figure 11 show us the FER of cooperation
and MIMO system referring to the different SNRs.
According to the simulation results, the MIMO systems
achieve lower FER than the corresponding cooperative
schemes. This supports that MIMO channels allowing
multiplexing gain [14,15] which is absent in cooperative
relaying channel since time is expended in the latter.
Thus, using MIMO system always obtains the gain of
spatial diversity. And as expected, the Alamouti coding
method has a better performance than the corresponding
Golden-Code method. The simulation result demonstrates
again that there is a trade-off between the throughput and
the reliability.
4.4. Effect via the Movements of the Relay
The main building blocks of a wireless network design
are rate control, power control, medium access
(scheduling) and routing. These building blocks are
divided in layers. Typically, routing is considered in a
routing layer and medium access in a MAC-layer, whereas
power control and rate control are sometimes considered
in a PHY-layer and sometimes in a MAC-layer.
So far, the three stations (S, R, D) were positioned
equidistantly and therefore all the three channels had the
fixed distance. Let us denote the distance between source
and destination as d
sd
; distance between source and relay
as d
sr
and distance between relay and destination as d
rd
.
Denote SNR
sd
, SNR
sr
, SNR
rd
as SNR between the source
and destination during the 2 time slots. We have
1
1
1
v
sr
sr
v
rd
rd
v
sd
sd
SNR d
SNR d
SNR d
 
 
 
 
 
 
 
 
 
(18)
where v is the path loss exponent. In the following
analysis, we assume that v = 4 for urban environment [18].
In this section, the relay is moved, so the distance
between the relay and source, the relay and destination
will change at the same time. The effects on the signal
quality when moving the relay between the source and
destination using Golden-Code cooperation with 6Mbps
and 12Mbps transmission rate are shown in Figure 12
and Figure 13, respectively. In the simulations, the
distance between the sender and the destination is set to
one, and therefore the SNRs shown in the X-axis is only
valid for the direct link S
D.
Figure 11. FER versus SNR for STBC in cooperation and
MIMO schemes (12Mbps).
272 J. CHEN ET AL.
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 3, 207-283
Figure 12. Benefit results when the relay is located between
the source and the destination (6Mbps).
Figure 13. Benefit results when the relay is located between
the source and the destination (12Mbps).
The best performance is achieved when the relay is
situated in the middle of the source and destination,
which means the better channel quality at S
R and R
D. And this can be a rule for a relay-selection method at
MAC-layer using the information of PHY-layer.
5. Conclusions
This paper describes STBC cooperation in wireless
communication, a technique that allows single-antenna
mobiles to share their antennas for obtaining some
benefits of multiple-antenna systems. The diversity is
realized by using a third station as a relay and the STBC
methods for information coding. We analyze the
performance of two different types of STBC cooperative
methods (Alamouti coding and Golden- Code) through
the theoretical study and simulations, there is the trade-
off between throughput and reliability during the
transmission. The results show that using the STBC
cooperative diversity can always increase the
performance. Through the analysis of the two methods
with the corresponding MIMO systems, we know that the
performance of MIMOs is always better than that of
cooperation with allowing multiplexing gain. The
location of the relay is crucial to the performance.
The best performance was achieved when the relay is
in the middle of source and destination. And in general
the relay should not be to far from the line between the
two terminals.
We believe several areas of future research on
cooperative communication will be fruitful. Firstly, the
generalization of the one hop space-time coded
cooperation to multi-hop case. Most of the research work
about cooperative communication concerns the single-
hop (single-relay or multi-relay) transmission. Nowadays,
multi-hop ad-hoc network can be found in everywhere,
and the protocol adapted to multi-hop environment
always derives from that of the single-hop. Secondly, the
integration and interaction with higher layer network
protocols can be explored. Recently, the need for
protocol adaptation and code cooperation of wireless
communication system suggested a new concept of
protocol architecture, named cross-layering architecture.
Different protocols implemented at different protocol
layers may be designed to have mutually cooperative
reactions, based on sharing the information between the
different layers. Obviously, a cross-layer approach that is
based on metrics computed at physical layer as SNR and
minimal distance in the decoding process is under
investigation. Such approach will be of a certain interest
for MAC and Network levels, taking advantage of the
information measured or estimated at the physical layer.
Our contribution will concern, mainly, link adaptation
and frames scheduling at MAC level. Lastly,
generalization of the STBC approach to meshed network
while considering multi-channel cooperation, radio
resources management and link adaptation will be our
crucial objective in perspective.
6. Acknowledgement
This work comes within the framework of a project
supported by the Agence Nationale de la Recherche/
R’eseau National de Recherche en T’el’ecommunications
under name RNRT/RADIC-SF/COMSIS and reference
ANR-05-RNRT- 014-01.
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