IEEE Wireless Communications? August 2006
84
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ADVANCES IN SMART ANTENNAS
INTRODUCTION
The burgeoning demand for mobile data net-
works has highlighted some constraints on its
future growth,Wireless links have always had
orders of magnitude less bandwidth than their
wireline counterparts,Mobile users have always
chafed at this limitation,which essentially forces
them to use applications in a manner reminis-
cent of wireline networks of decades past,albeit
freeing them from a desktop,Newer technolo-
gies such as multiple-input multiple-output
(MIMO) systems are starting to increase the
number of bits per second per hertz of band-
width through spatial multiplexing,and to
improve the robustness/range of the wireless link
for a given data rate through space-time coding
and beamforming,However,all these improve-
ments come at the cost of multiple RF front
ends at both the transmitter and the receiver.
Furthermore,the size of the mobile devices may
limit the number of antennas that can be
deployed,Even when MIMO technology is feasi-
ble,wireless engineers are running into another
roadblock,the inefficient way the electromagnet-
ic spectrum has been allocated to different class-
es of users,mainly for historical or regulatory
reasons,Thus,while large portions of the spec-
trum are grossly underused,the popular unli-
censed bands are very crowded,Given this
limitation,for unlicensed bands,the issue of
interference from having too many users has
become as important as how much bandwidth
can be squeezed from it.
This article outlines one way to address these
problems by using the notion of cooperation
between wireless nodes,In cooperative commu-
nications,multiple nodes in a wireless network
work together to form a virtual antenna array.
Using cooperation,it is possible to exploit the
spatial diversity of the traditional MIMO tech-
niques without each node necessarily having mul-
tiple antennas,Multihop networks use some
form of cooperation by enabling intermediate
nodes to forward the message from source to
destination,However,cooperative communica-
tion techniques described in this article are fun-
damentally different in that the relaying nodes
can forward the information fully or in part.
Also the destination receives multiple versions of
the message from the source,and one or more
relays and combines these to obtain a more reli-
able estimate of the transmitted signal as well as
higher data rates,The main advantages of coop-
erative communications are:
Higher spatial diversity,resistance to both
small scale and shadow fading
Higher throughput/lower delay,higher achiev-
able data rates,fewer retransmissions,and
lower transmission delay
Reduced interference/lower transmitted
power,better frequency reuse in a
cellular/WLAN deployment
Adaptability to network conditions,oppor-
tunistic use and redistribution of network
energy and bandwidth
The past few years have seen tremendous
interest in cooperative communications,mostly
at the physical layer,However,significant
research challenges still exist,some of which we
outline in this article,
The goal of this article is to provide new
PEI LIU,ZHIFENG TAO,ZINAN LIN,ELZA ERKIP,AND SHIVENDRA PANWAR,
POLYTECHNIC UNIVERSITY
ABSTRACT
“Denise and her husband Mitch are at opposite
ends of a living room at a crowded party,Denise
tries to attract Mitch’s attention and shouts some-
thing at him,All Mitch can hear is the word ‘Let’s.’
Celine,in the middle of the room,who overhears
Denise and notices their predicament,repeats to
Mitch the part she hears,‘Go home.’ This time,all
Mitch hears is the word ‘home.’ Mitch finally fig-
ures out that his wife wants to go home.” This
analogy from everyday life vividly portrays the
essential element of cooperative wireless com-
munications,namely,utilizing information over-
heard by neighboring nodes to provide robust
communication between a source and its desti-
nation,Cooperative communication exhibits var-
ious forms at different protocol layers and
introduces many opportunities for cross-layer
design and optimization,some of which will be
explored in detail in this article.
COOPERATIVE WIRELESS COMMUNICATIONS,
A CROSS-LAYER APPROACH
The work is partially supported by the National Science
Foundation under grant no,0520054,and the Wireless
Internet Center for Advanced Technology (WICAT),an
NSF Industry/University Cooperative Research Center.
In cooperative
communications,
multiple nodes in a
wireless network
work together to
form a virtual
antenna array,
Using cooperation,
it is possible to
exploit the spatial
diversity of the
traditional MIMO
techniques without
each node having
multiple antennas.
ERKIP LAYOUT 8/3/06 1:24 PM Page 84
IEEE Wireless Communications? August 2006
85
cross-layer research directions in order to illus-
trate the feasibility and performance of coopera-
tive wireless networking,We first describe the
notion of physical-layer cooperation and cooper-
ative diversity,However,in order to realize a
fully cooperative network,research at the physi-
cal layer should be coupled with higher layers of
the protocol stack,in particular,the MAC sub-
layer and the network layer,We describe how
physical-layer cooperation can be integrated with
the MAC sublayer for dramatic improvements in
throughput and interference,We also outline
some of the challenges in extending the notion
of cooperative diversity to the network layer.
MOTIVATION FOR
COOPERATIVE COMMUNICATION
In this section we introduce the basic concepts
underlying cooperative communications,Coop-
erative techniques utilize the broadcast nature of
wireless signals by observing that a source signal
intended for a particular destination can be
“overheard” at neighboring nodes,These nodes,
called relays,partners,or helpers,process the sig-
nals they overhear and transmit towards the des-
tination,The relay operations can consist of
repetition of the overheard signal (obtained,for
example,by decoding and then re-encoding the
information or by simply amplifying the received
signal and then forwarding),or can involve more
sophisticated strategies such as forwarding only
part of the information,compressing the over-
heard signal,and then forwarding,We refer the
reader to [1] for a detailed overview of relaying
methods,The destination combines the signals
coming from the source and the relays,enabling
higher transmission rates and robustness against
channel variations due to fading,We note that
the spatial diversity arising from cooperation is
not exploited in current cellular,wireless LAN,
or ad hoc systems; only one copy of the signal,
whether it comes from the mobile directly or
from a relay,is processed at the destination.
Hence,cooperative relaying is substantially dif-
ferent than traditional multihop or infrastructure
based methods.
This notion of cooperation dates back to the
relay channel model in information theory exten-
sively studied in the 1970s by Cover and El
Gamal [2],but we owe the recent popularity to
[3–5],which showed the benefits of cooperative
relaying in a wireless environment,In order to
illustrate the idea of cooperation and coopera-
tive diversity at the physical layer,we consider
the cooperative coding scheme used in [6,7],Let
us consider an isolated source S who wants to
communicate with a destination D with the help
of a cooperative relay R,as illustrated in Fig,1a.
Here,d
i
denotes the distances between the
nodes.
For direct transmission (i.e.,if the relay R is
not utilized),each channel block,or packet,con-
tains B data bits and r parity bits for forward
error correction (FEC),leading to a total of N =
B + r coded bits,as shown at the top of Fig,1b.
For ease of exposition,we have r ≥ B,We assume
that cyclic redundancy check (CRC) is employed
for error detection,In order to cooperate,S
divides its channel block into two and only trans-
mits in the first half,as shown at the bottom of
Fig,1b,Hence,in the cooperative mode S ends
up sending only half of its coded bits,These bits
are received both by the destination and by the
relay R,The relay observes a higher coding rate
and thus a weaker FEC,Nevertheless,it attempts
to decode the underlying B data bits,If R has
the correct information (which can be checked
using the CRC),it re-encodes and sends the
remaining N/2 parity bits in the second half of
S’s time slot,Otherwise,R informs S that there
was a failure in decoding,and S continues trans-
mission,Therefore,when R decodes correctly,
a73 Figure 1,a) Cooperative system for an isolated link; b) time division in cooperative coding; c) two user cooperative coding perfor-
mance for d
1
= 1,d
2
= 0.5 and d
3
= 0.5,(13,15,15,17) convolutional code,100-byte frame size.
S transmits directly to D
N coded bits
S
d
2
d
3
d
1
D
R
(a)
(b)
Average total received SNR at the destination (dB)
0
10
–2
10
–3
F
rame error rate
10
–1
10
0
5 10
(c)
d
1
= 1.0,d
2
= 0.5,d
3
= 0.5
15 20 25
Direct transmission
Cooperative coding
S transmits R relays for S
N/2 coded bits N/2 coded bits
ERKIP LAYOUT 8/3/06 1:24 PM Page 85
IEEE Wireless Communications? August 200686
the destination will receive half the coded bits
from S and the remaining ones from R,thus cre-
ating spatial diversity,The question is how often
this happens and how it affects the overall error
performance.
Figure 1c illustrates simulation results for
frame error rate (FER) versus the total trans-
mit signal-to-noise ratio (SNR) for the scenario
where the relay is located halfway between the
source and destination (i.e.,d
1
= 1.0,d
2
= 0.5,
and d
3
= 0.5),Note that direct transmission
and cooperative coding use the same total
power and bandwidth (we consider a low-mobil-
ity environment),Hence,along with path loss,
we assume all links experience independent
slow Rayleigh fading that stays constant for the
duration of each packet,The nodes use convo-
lutional coding and each node has the same
average power constraint,We observe from the
figure that for an error rate of 10
–3
we obtain
about 18 dB improvement in SNR with cooper-
ation,Also,the FER for cooperative coding
decreases at a much faster rate than direct
transmission; in fact,cooperation is able to
achieve two full levels of diversity similar to a
MIMO system with two transmit antennas and
one receive antenna.
The above example considers one particular
cooperative scheme to obtain diversity,yet it
shows the potential of cooperation at the physi-
cal layer,Indeed,there is a rich literature on
physical-layer cooperation that investigates many
aspects,such as cooperative protocols for two or
more users,performance bounds for cooperative
systems,resource allocation for cooperation,and
partner-choice strategies,Using a cross-layer
approach between physical and MAC layers,this
article investigates how these gains can be
attained in a wireless network.
BENEFITS OF COOPERATIVE NETWORKING
From the perspective of the network,coopera-
tion can benefit not only the nodes involved,but
the whole network in many different aspects.
For illustration purposes,we choose to explain
only a few potential benefits below.
HIGHER SPATIAL DIVERSITY
As a simple example,Fig,2a shows a small net-
work of four mobile nodes,If the channel quali-
ty between mobile nodes S and D degrades
severely (e.g.,due to shadow or small-scale fad-
ing),a direct transmission between these two
nodes may experience an intolerable error rate,
which in turn leads to retransmissions,Alterna-
tively,S can exploit spatial diversity by having a
relay R
1
overhear the transmissions and then
forward the packet to D as discussed above,The
source S may resort to yet another terminal R
2
for help in forwarding the information,or use R
1
and R
2
simultaneously [8],Similar ideas apply to
larger networks as well,Therefore,compared
with direct transmission,the cooperative
approach enjoys a higher successful transmission
probability,We note here that cooperative com-
munications has the ability to adapt and to miti-
gate the effects of shadow fading better than
MIMO since,unlike MIMO,antenna elements
of a cooperative virtual antenna array are sepa-
rated in space and experience different shadow
fading.
HIGHER THROUGHPUT-LOWER DELAY
At the physical layer,rate adaptation is achieved
through adaptive modulation and adaptive chan-
nel coding,Many MAC protocols have intro-
duced rate adaptation to combat adverse channel
conditions,For instance,when a high channel-
error rate is encountered due to a low average
SNR,the wireless LAN standard IEEE 802.11
switches to a lower transmission rate so as to
guarantee a certain error rate,The power of
cooperation is evident when it is applied in con-
junction with any rate adaptation algorithm,In
Fig,2a,specifically,if Rate
2
and Rate
3
are higher
than Rate
1
such that the total transmission time
for the two-hop case through R
2
is smaller than
that of the direct transmission,cooperation read-
ily outperforms the legacy direct transmission,in
terms of both throughput and delay perceived by
the source S,Furthermore,for relays such as R
1
and R
2
,it turns out that their own individual
self-interest can be best served by helping others.
a73 Figure 2,a) Cooperation in a network; b) illustration of the delay and throughput improvement achieved by cooperation in the time
domain.
(b)
S transmits directly to D
Time T
1
R
1
transmits its
own traffic to D
Time T
2
(a)
Rate
2 Rate
3
Rate
4
Rate
5
Rate
1
R
1
R
2
D
S
S transmits
Time T
3
Time T
4
R
1
transmits its
own traffic to D
R
1
relays for
S to D
Time T
2
ERKIP LAYOUT 8/3/06 1:24 PM Page 86
IEEE Wireless Communications? August 2006 87
As further illustrated in Fig,2b,the intermediate
node R
1
that cooperates enjoys the benefit of
lower channel-access delay,which in turn can be
translated into higher throughput,It is worth-
while to note that Fig,2b also draws a rough
analogy with the cooperative scheme discussed
above (Fig,1b) and illustrates that rate adapta-
tion can further improve the benefits of coopera-
tion in a network setting.
LOWER POWER CONSUMPTION AND LOWER
INTERFERENCE/EXTENDED COVERAGE
The diversity,error rate,and throughput gains
obtained through cooperation can be traded in
for power savings at the terminals,Alternatively,
cooperation leads to an extended coverage area
when the performance metric (error rate,
throughput,etc.) is fixed.
The advantage of cooperation also leads to
reduced interference when the network is
deployed in a cellular fashion to reuse a limited
bandwidth,With the improvement of through-
put,we can reduce the average channel time
used by each station to transfer a certain amount
of traffic over the network,Therefore,the sig-
nal-to-interference ratio (SIR) between proximal
cells using the same channel can be reduced,and
a more uniform coverage can be achieved,As
wireless network deployments become ever more
dense,a reduction of SIR will directly lead to a
boost in network capacity,Indeed,the problem
of dense deployment has already been reported
for IEEE 802.11 b/g networks,which have only
three nonoverlapping channels.
ADAPTABILITY TO
NETWORK CONDITIONS
The cooperative communication paradigm allows
wireless terminals to seamlessly adapt to chang-
ing channel and interference conditions,The
choice of relays,cooperation strategy,and the
amount of resources available for cooperation
can be opportunistically decided,For example,
in Fig,2a,if the source S has some information
about the current channel gains,packet-loss
rates,traffic conditions,interference,or remain-
ing battery energy of nodes in the network,it
may choose to transmit its information directly
to its destination D,using R
1
or R
2
or both in a
cooperative fashion,depending on which trans-
mission mode results in better performance (in
terms of error rates,throughput,or power),This
way,a surplus of resources such as battery ener-
gy or bandwidth at a particular node can be uti-
lized by other nodes in the network in a manner
that will benefit everyone,including the relay
node itself.
Although originating from physical-layer
cooperation,all the aforementioned benefits
cannot be fully realized until proper mechanisms
have been incorporated at higher protocol layers
(e.g.,MAC,network) and the necessary informa-
tion is made available from the lower layer (e.g.,
PHY),Indeed,a cross-layer approach has to be
followed to reap all the benefits of cooperation.
As we illustrate via the cooperative MAC proto-
col described in the following section,an addi-
tional three-way handshake procedure and a new
signaling message have to be introduced to the
MAC layer,and information on channel condi-
tions for related wireless links should be made
available to the upper layers so that the coopera-
tion can be fully enabled,Another example of a
cross-layer approach to cooperation,which
involves interaction between the application
layer and the physical layer,is provided in [9] for
transmission of video signals over wireless links.
COOPMAC,A COOPERATIVE
MEDIUM ACCESS CONTROL
As described above,cooperation at the physical
layer uses the broadcast nature of the wireless
medium and overheard information to improve
the performance,Unfortunately,conventional
wireless medium access control (MAC) proto-
cols have long treated this feature as a problem,
rather than something that can be exploited,The
methodology of cooperation,however,embraces
this concept,and thus creates a new paradigm
for MAC protocol design in wireless network.
We present a new MAC protocol called
CoopMAC
1
for IEEE 802.11 wireless LANs,
which exploits both the broadcast nature of the
wireless channel and cooperative diversity,As
we demonstrate,the CoopMAC protocol fully
capitalizes on the notion of cooperation,and
realizes some of the key benefits previously high-
lighted,such as higher throughput,lower delay,
better coverage,and reduced interference,In the
end,we briefly discuss a preliminary CoopMAC
implementation.
Zhao and Valenti also consider a MAC pro-
tocol [11] for exploiting cooperative diversity,
but it is based upon a conceptual generalization
of the hybrid automatic repeat request scheme
(hybrid-ARQ),instead of the widely deployed
802.11 protocol,Recently,there have been
attempts to explore the benefits of virtual MIMO
at the network level,by pursuing a cross-layer
approach spanning the physical,MAC,and net-
working (e.g.,routing) layers [12],However,the
proposed scheme is based on the assumption
that multiple nodes can be perfectly synchro-
nized,Although the protocol mechanism pro-
posed herein bears some resemblance to that
described in [13],the two protocols address fun-
damentally different issues in two distinct prob-
lem spaces,More specifically,rate adaptation is
the main focus of [13],while cooperative diversi-
ty is incorporated in the protocol introduced
here.
COOPMAC PROTOCOL DESCRIPTION
When a source node has a new MAC proto-
col data unit (MPDU) to send,it can either
transmit directly to the destination,or use an
intermediate helper for relaying,whichever con-
sumes less total air time,The air time is com-
pared using cached information on the feasible
data rates between the three nodes,The feasible
data rate is the largest data rate that guarantees
a predetermined average error rate threshold for
an average channel SNR.
Beyond its normal function,a request to
send (RTS) message is also used by CoopMAC
to notify the node that has been selected for
1
A preliminary version of
the CoopMAC protocol
was described in [10].
As wireless network
deployments become
ever more dense,
a reduction of SIR
will directly lead to a
boost in network
capacity,Indeed,the
problem of dense
deployment has
already been
reported for IEEE
802.11 b/g
networks,which
have only three
nonoverlapping
channels.
ERKIP LAYOUT 8/3/06 1:24 PM Page 87
IEEE Wireless Communications? August 200688
cooperation,Moreover,CoopMAC introduces a
new message called helper-ready to send (HTS),
which is used by the helper to indicate its avail-
ability after it receives the RTS message from
the source,If the destination hears the HTS
message,it issues a clear to send (CTS) message
to reserve channel time for a two-hop transmis-
sion,Otherwise,it still sends out the CTS,but
only reserves channel time for a direct transmis-
sion.
If both HTS and CTS are received at the
source,the data packet should be transmitted to
the relay first,and then forwarded to the desti-
nation by the relay,If the source does not receive
an HTS,it should then initiate a direct transmis-
sion to the destination.
A normal ACK is used to acknowledge a
correct reception,regardless of whether the
packet is forwarded by the relay,or is directly
transmitted from the source,If necessary,
retransmission is attempted,again in a coopera-
tive fashion.
It is crucial that each node obtains and con-
stantly updates its information about the avail-
ability of potential relays,The CoopMAC
protocol deals with this issue mainly through
maintaining a table called the CoopTable in its
management plane,Each entry in the CoopT-
able corresponds to a potential relay,and con-
tains such information as the ID (e.g.,48-bit
MAC address) of the potential relay,the latest
time at which a packet from that potential relay
is overheard by the source,and the data rate
used for direct transmission between the poten-
tial relay and destination,and between the cur-
rent node and the potential relay,A set of
protocols have been defined in CoopMAC to
properly establish,manage,and update the table
in a timely manner.
Due to the broadcast nature of the channel,
the destination will receive the signals transmit-
ted by both the source and the relay,If the desti-
nation is capable of combining these two copies
to decode the original information,then cooper-
ative diversity can be fully leveraged,Receiver
combining,not supported by any existing wire-
less hardware,can be implemented in the next-
generation wireless baseband chip,Given the
constraint of using existing hardware,we have
developed a backward compatible mode of
CoopMAC,which does not perform receiver
combining and therefore only requires a driver
or firmware upgrade.
Without diversity combining,If no combining
capability is supported at the destination,the
packet should be transmitted on both the first
and second hop at the highest physical layer rate
that the respective link can sustain.
With diversity combining,When receiver
combining is enabled,the relay now can forward
packets at a rate equal to or greater than the
one that it adopts in CoopMAC where combin-
ing is not possible,More specifically,the trans-
mission rate between the source and relay is
chosen so as to guarantee a desired error proba-
bility at the relay,Although the destination can-
not fully decode the packet after the first-hop
transmission,this received signal will be stored.
If the relay can successfully receive the packet,it
then forwards the packet to the destination,The
transmission rate on the second hop is the high-
est one that meets a predetermined average
error rate at the destination,once the destina-
tion combines the source and relay signals.
The diversity combining capability allows
CoopMAC to leverage both the spatial diversity
and the coding gain,thereby resulting in even
better performance than the protocol without
receiver combining,Using the coded coopera-
tion framework described above,the helper pro-
vides different coded bits than the source,
leading to a better error performance than repe-
tition coding.
It is worthwhile to note that although the
protocol architecture and signaling mechanism
defined above are applicable both with and with-
out diversity combining at the receiver,the
relay-selection scheme may not yield an optimal
choice for CoopMAC with receiver combining
any longer,because it does not take the possible
a73 Figure 3,Network capacity comparison,a) saturation capacity; b) network capacity gain with respect to 802.11g.
Number of stations
50
7
8
Capacity (
M
b/
s)
9
10
11
12
13
14
10 15 20
(a) (b)
25 30 35 40
Number of stations
5
0%
10%
Capacity gain (percentage)
20%
30%
40%
50%
60%
10 15 20 25 30 35 40
CoopMAC with receiver combining
CoopMAC without receiver combining
802.11g
CoopMAC without receiver combining
CoopMAC with receiver combining
ERKIP LAYOUT 8/3/06 1:24 PM Page 88
IEEE Wireless Communications? August 2006 89
rate increase on the second hop into considera-
tion,In addition,the relay has to be aware of
the average link quality (e.g.,average SNR)
between the source and destination so that it can
properly select a higher transmission rate on the
second hop,The information can be easily con-
veyed to the relay by sandwiching a,shim” link
quality field between the legacy MAC header
and the MAC payload in the data packets that
the source transmits.
Although CoopMAC bears some superficial
resemblance to conventional ad hoc routing pro-
tocols,they are in essence completely different.
First and foremost,forwarding in CoopMAC is
an essential means to accomplish the goal of
leveraging cooperative diversity,Secondly,all
the associated operations occur in the MAC
layer,which enjoys a shorter response time and
more convenient access to the physical layer
information,as compared to the traditional net-
work layer routing,In addition,no channel con-
tention is needed when the relay forwards the
packets to the destination,thereby leading to a
shorter delay for the relay and a more efficient
channel utilization as well,Last but not the least,
no routing protocol that we are aware of has
adopted the receiver-combining technique to tap
into the potential of cooperative diversity.
THROUGHPUT,DELAY,AND
ENERGY EFFICIENCY OF COOPMAC
To evaluate the performance of CoopMAC,we
have developed an event-driven custom simula-
tor using the programming language C to faith-
fully model all the critical MAC and physical
layer features of IEEE 802.11 and CoopMAC.
The parameters in the performance evaluation
assume the default values specified for an IEEE
802.11g network operating in a typical office
environment with low user mobility.
Figures 3–5 depict the simulation results for a
saturated network with a payload size of 1500
bytes,The MSDU size of 1500 bytes has been
chosen in the simulation because the data pack-
ets usually assume such a length in wireless
LANs,as widely reported in recent traffic pat-
tern research [14],Saturation here refers to a
MAC-level condition,where each station always
has packets to transmit at any time instant,Note,
however,that the MAC-level saturation does not
necessarily imply that the physical wireless chan-
nel is always occupied,as all the stations have to
perform backoff according to the random-access
MAC protocol.
As demonstrated in Fig,3,both flavors of
CoopMAC can achieve a much higher network
capacity than the legacy IEEE 802.11g,Between
the two versions of CoopMAC,the one with
receiver-combining capability can deliver more
throughput,as was anticipated above.
Another highly desirable feature of Coop-
MAC that Figs,3a and 3b reveal is that both the
network capacity and the capacity gain for Coop-
MAC with respect to 802.11g increase as the
number of nodes in the network grows,This
improvement primarily stems from the increas-
ing availability of relays as the network becomes
more populated.
For a wide variety of network sizes,Fig,4
portrays the simulation results for the average
channel access delay,which essentially is the
duration from the time a packet becomes the
head-of-line (HOL) packet until the time the
packet is successfully transmitted,The corre-
sponding delay improvement over 802.11g is
shown in Fig,4b,It is evident that data packets
in CoopMAC experience significantly less delay
than in legacy IEEE 802.11g.
It is also worthwhile to note that the same
trend in throughput and delay improvement can
be observed for networks operating in a medi-
um-to-low-traffic regime,In addition,even more
improvement can be achieved when a larger
frame size is used,Due to space limitations,we
will not present additional results in this article
a73 Figure 4,Channel access delay comparison,a) mean channel access delay; b) improvement of mean channel access delay with respect
to 802.11g.
Number of stations
5
0
0.01
Aver
a
ge
cha
nnel a
ccess d
elay (s)
0.02
0.03
0.04
0.05
0.06
10 15 20 25
(a) (b
30 35 40
Number of stations
5
0%
5%
Impr
ove
me
nt of a
ve
r
ag
e
channe
l
ac
c
e
ss de
la
y (
percentage)
10%
15%
20%
25%
30%
35%
40%
45%
50%
10 15 20 25 30 35 40
802.11g
CoopMAC without receiver combining
CoopMAC with receiver combining
CoopMAC with receiver combining
CoopMAC without receiver combining
ERKIP LAYOUT 8/3/06 1:24 PM Page 89
IEEE Wireless Communications? August 200690
for the nonsaturation condition or for larger
frame sizes.
In addition to conventional measures like
throughput and delay,we have also evaluated
the energy efficiency of CoopMAC,since power
conservation is always a key concern for wireless
networks,Figure 5a depicts the energy consump-
tion per user in terms of total amount of energy
needed to successfully deliver a bit for each user
(i.e.,joules/bit/user),which includes the energy
consumed in transmission,reception and chan-
nel sensing,Figure 5b shows the percentage
improvement with respect to 802.11g,We
observe that as the number of nodes increases,
the improvement in per user energy efficiency
achieved by CoopMAC also grows,This is pri-
marily due to the fact that although CoopMAC
requires nodes to receive and retransmit traffic
for each other,it also enables them to spend less
time listening to the medium,Ultimately,this
saving outweighs the new energy expense,and
leads to an increase in energy efficiency,We
refer the readers to [15] for details of the ener-
gy-consumption model.
INTERFERENCE REDUCTION IN A DENSE NETWORK
The deployment of wireless networks has grown
increasingly dense,leading to concerns that
deployments may become interference-limited.
For instance,there are 11 channels defined in
the 2.4 GHz spectrum for operation of IEEE
802.11 WLANs in the United States,However,
in order to avoid interference between adjacent
cells,only three mutually nonoverlapping chan-
nels can be used at the same time.
In the following discussion,we focus only on
co-channel interference for a cellular deploy-
ment of IEEE 802.11 with a reuse factor of three.
Note that while a node is transmitting packets in
a particular cell,there will be six proximal cells
in which parallel transmissions generate co-chan-
nel interference,Our simulation calculates the
signal-to-interference-plus-noise ratio (SINR)
for each point in a cell by randomly choosing the
locations of six interfering nodes in the six proxi-
mal cells and assuming path loss as well as
Rayleigh fading,The maximum feasible data
rate is estimated based on the SINR and the
error rate threshold requirement.
Figure 6 compares the interference for 802.1
ig MAC and CoopMAC in a multicell environ-
ment with a frequency reuse factor of three,All
three systems are under the same traffic load in
all cells,From these figures,another advantage
of cooperation becomes apparent,CoopMAC
without receiver combining decreases the aver-
age interference by 21.5 percent,while receiver
combining enables another 12.5 percent reduc-
tion,Since both versions of CoopMAC are more
efficient in terms of throughput,the transmission
time for the same amount of traffic using the
CoopMAC protocol is less than that of the lega-
cy system,therefore reducing total energy radiat-
ed to the network,Due to the lower background
interference,the sustainable regions for all four
rates supported by IEEE 802.llg are extended
[15].
IMPLEMENTATION
In order to further validate the design of Coop-
MAC and demonstrate the feasibility of an incre-
mental deployment,we have made efforts to
implement the CoopMAC protocol using off-
the-shelf IEEE 802.1 lb network interface cards
(NICs) on a Linux platform [15],Since no exist-
ing hardware can perform the receiver-combin-
ing function,only the CoopMAC protocol
without diversity combining can be implemented.
In fact,due to the constraint in accessing the
firmware on the chip,we had to take an emula-
tion approach at the driver level for the Coop-
MAC version without receiver combining,which
unfortunately incurs additional protocol over-
head,Nevertheless,as demonstrated in the
experiment,CoopMAC can still reduce the aver-
a73 Figure 5,Energy efficiency comparison,a) average energy consumption per bit per user; b) average user energy efficiency gain with
respect to 802.11g.
Number of stations
(a)
50
0
0.5
Aver
a
ge
us
e
r e
ne
rg
y
co
n
su
mp
ti
o
n p
er
bit
(
J/
b
/us
e
r)
1
1.5
2
2.5
3
3.5
4
4.5
x10
-6
10 15 20 25 30 35 40
Number of stations
(b)
5
0%
10%
Im
pr
ov
e
me
n
t i
n a
ve
r
ag
e
u
s
er
e
ne
r
gy
co
n
su
mp
ti
o
n p
er
bit
(
pe
r
ce
n
ta
g
e)
20%
30%
40%
50%
10 15 20 25 30 35 40
CoopMAC without receiver combining
CoopMAC with receiver combining
802.11g
CoopMAC without receiver combining
CoopMAC with receiver combining
ERKIP LAYOUT 8/3/06 1:24 PM Page 90
IEEE Wireless Communications? August 2006 91
age file-transfer times significantly below the
original value,and a more significant perfor-
mance improvement can be achieved when the
entire CoopMAC without receiver combining is
completely implemented in firmware,In addi-
tion,an even higher performance gain would be
possible if the CoopMAC with receiver combin-
ing can be realized in a baseband chip.
CONCLUSION AND FUTURE WORK
By introducing collaboration from nodes that
otherwise do not directly participate in trans-
mission,cooperative communication introduces
a new paradigm for wireless communication,It
enables a tremendous improvements in robust-
ness,throughput,and delay; a significant reduc-
tion in interference; and an extension of
coverage range,To fully leverage the concept
of cooperation,the entire protocol stack —
from physical layer to networking protocols —
should be carefully reengineered or even
redesigned.
To illustrate the necessity of a cross-layer
design approach,we have explored cooperation
at both layers 1 and 2 of the OSI protocol stack,
and have proposed a new MAC protocol for
IEEE 802.11 networks which we call CoopMAC.
In particular,the CoopMAC protocol has an
option to enable the capability of diversity com-
bining at the receiver,where two versions of the
same data are jointly decoded to recover the
original packet,As verified by extensive simula-
tions,the CoopMAC protocol,both with and
without receiver-combining capability,can
achieve substantial performance improvements,
without incurring appreciable additional com-
plexity in system implementation,Compared
with the noncombining version,the CoopMAC
protocol with receiver-combining capability
pushes cooperation to an even higher level and
reaps additional benefits.
To further exploit cooperation gains at the
network layer for highly adaptive and scalable ad
hoc networks,many research challenges remain.
Given the increasing number of cooperating
nodes listening to each transmission,packet for-
warding can now be done in a more opportunis-
tic way than has been traditionally considered in
ad hoc networks,Indeed,the notions of routing
and routing protocols may change when cooper-
ation is fully integrated in the link layer,Cooper-
ative partners should be carefully selected along
the route so that optimality at both the link and
path levels can be accomplished,while spatial
reuse in ad hoc networks is not compromised.
Similar subtle cross-layer design issues abound in
ad hoc networks,and the implications of node
cooperation,including cooperative routing algo-
rithms and the scalability of network capacity
with the number of nodes in a network,deserve
further investigation.
REFERENCES
[1] G,Kramer,M,Gastpar,and P,Gupta,“Cooperative
Strategies and Capacity Theorems for Relay Networks,”
IEEE Trans,Info,Theory,vol,51,Sept,2005,pp.
3037–63.
[2] T,Cover and A,E,Gamal,“Capacity Theorems for the
Relay Channel,” IEEE Trans,Info,Theory,vol,IT-25,
Sept,1979,pp,572–84.
[3] A,Sendonaris,E,Erkip,and B,Aazhang,“User Cooper-
ation Diversity Part I,System Description,” IEEE Trans.
Commun.,vol,51,Nov,2003,pp,1927–38.
[4] A,Sendonaris,E,Erkip,and B,Aazhang,“User Cooper-
ation Diversity Part II,Implementation Aspects and Per-
formance Analysis,” IEEE Trans,Commun.,vol,51,Nov.
2003,pp,1939–48.
[5] J,N,Laneman,D,Tse,and G,W,Wornell,“Cooperative
Diversity in Wireless Networks,Efficient Protocols and
Outage Behavior,” IEEE Trans,Info,Theory,vol,50,
Dec,2004,pp,3062–80.
[6] A,Stefanov and E,Erkip,“Cooperative Coding for Wire-
less Networks,” IEEE Trans,Commun.,vol,52,Sept.
2004,pp,1470–76.
[7] T,E,Hunter and A,Nosratinia,“Diversity through
Coded Cooperation,” IEEE Trans,Wireless Commun.,
vol,5,Feb,2006,pp,283–89.
[8] J,N,Laneman and G,W,Wornell,“Distributed Space-
Time-Coded Protocols for Exploiting Cooperative Diver-
sity in Wireless Networks,” IEEE Trans,Info,Theory,vol.
49,Oct,2003,pp,2415–25.
[9] H,Shutoy,Y,Wang,and E,Erkip,“Cooperative Source
and Channel Coding for Wireless Video Transmission,”
Proc,IEEE Int’l,Conf,Image Processing,Atlanta,GA,
Oct,2006 (to appear).
[10] P,Liu,Z.Tao,and S,Panwar,“A Cooperative MAC Pro-
tocol for Wireless Local Area Networks,” Proc,IEEE ICC,
June 2005.
[11] B,Zhao and M,C,Valenti,“Practical Relay Networks:
A Generalization of Hybrid-ARQ,” IEEE JSAC,vol,23,
Jan,2005,pp,7–18.
[12] G,Jakilari et al.,“A Framework for Distributed Spa-
tiotemporal Communications in Mobile Ad Hoc Net-
works,” Proc,IEEE INFOCOM 2006,Barcelona,Spain,
Apr,2006.
a73 Figure 6,Interference (W),frequency reuse factor = 3,traffic = 500 p/s,transmission power = l μW,a)
802.11g; b) CoopMAC without receiver combining; and c) CoopMAC with receiver combining.
x10
-7
2
1.8
1.6
1.4
1.2
1
(a) (b) (c)
To further validate
the design of
CoopMAC and
demonstrate the
feasibility of an
incremental
deployment,
we have made
efforts to implement
the CoopMAC
protocol using
off-the-shelf IEEE
802.1 lb network
interface cards
(NICs) on a Linux
platform
ERKIP LAYOUT 8/3/06 1:24 PM Page 91
IEEE Wireless Communications? August 200692
[13] H,Zhu and G,Cao,“rDCF,A Relay-Enabled Medium
Access Control Protocol for Wireless Ad Hoc Networks,”
Proc,IEEE INFOCOM,Mar,2005.
[14] J,Yeo,M,Youssef,and A,Agrawala,“Characterizing
the IEEE 802.11 Traffic,The Wireless Side,” Univ,of
Maryland,College Park,res,rep,CS-TR-4570,Mar.
2004,http://www.csumd.edu/moustafa/papers/CS-IR-
4570.pdf
[15] P,Liu et al.,“CoopMAC,A Cooperative MAC for Wire-
less LANs,” Tech,rep.,http//catt.poly.edu/CATT/pan
war.html
BIOGRAPHIES
PEL LIU [S’01] (pliu@photon.poly.edu) completed his B.S.
and M.S,degrees in electrical engineering at Xi’an Jiaotong
University,China,in 1997 and 2000,respectively,He is a
Ph.D,candidate in the Department of Electrical and Com-
puter Engineering of Polytechnic University,Brooklyn,NY.
His research interests are in wireless communications and
wireless networks.
ZHIFENG TAO [S’00] (jeff.tao@ photon.poly.edu) received a
B.E,degree in communication and information engineering
from Xi’an Jiaotong University,P,R,China,in 2000,Since
then,he has been a Ph.D,candidate in the Department of
Electrical and Computer Engineering at Polytechnic Univer-
sity,He also received his M.S,degree in telecommunication
networking from Polytechnic University in May 2002,His
current research interests include wireless networking,
medium access control,quality of service,and cooperative
communications.
ZINAN LIN [S’00] (zlin03@utopia.poly.edu) received a B.E
degree in information engineering from Zhejiang Universi-
ty,Hangzhou,China,in 1998,and an M.E,degree in elec-
trical engineering from Nanyang Technological University
(NTU),Singapore,in 2001,From 1998 to 2000 she was
awarded a research scholarship by NTU and worked as a
research assistant in the Center for Signal Processing,
School of Electronic and Electrical Engineering,NTU,Since
2001,she has been Ph.D,candidate in the Department of
Electrical and Computer Engineering,Polytechnic Universi-
ty,Brooklyn,NY,Her general research interests include
wireless communications and digital signal processing,
especially channel coding,diversity techniques,and CDMA
sequence design.
ELZA ERKIP [S’93,M’96,SM’05] (e1za@poly.edu) received
M.S,and Ph.D,degrees in electrical engineering from Stan-
ford University in 1993 and 1996,respectively,and a B.S.
degree in electrical and electronic engineering from Middle
East Technical University,Turkey,in 1990,She joined Poly-
technic University in Spring 2000,where she is currently an
associate professor of electrical and computer engineering.
During 1996–1999 she was with the Department of Electri-
cal and Computer Engineering of Rice University,She
received the 2004 Communications Society Stephen O,Rice
Paper Prize in the field of communications theory,the NSF
CAREER award in 2001,and the IBM Faculty Partnership
Award in 2000,She is the Technical Program Co-Chair of
the 2006 Communication Theory Workshop,She is also an
Associate Editor of IEEE Transactions on Communications
and a Publications Editor for IEEE Transactions on Informa-
tion Theory,Her research interests are in wireless commu-
nications,information theory,and communication theory.
SHIVENDRA S,PANWAR [S’82,M’85,SM’00]
(panwar@catt.poly.edu) received a B.Tech,degree in electri-
cal engineering from the Indian Institute of Technology,Kan-
pur,in 1981,and M.S,and Ph.D,degrees in electrical and
computer engineering from the University of Massachusetts,
Amherst,in 1983 and 1986,respectively,He joined the
Department of Electrical Engineering at the Polytechnic Insti-
tute of New York,Brooklyn,NY (now Polytechnic University),
where he is a professor in the Electrical and Computer Engi-
neering Department,Currently he is the director of the New
York State Center for Advanced Technology in Telecommuni-
cations (CATT),He spent the summer of 1987 as a visiting
scientist at the IBM T,J,Watson Research Center,Yorktown
Heights,NY,and was a consultant to AT& T Bell Laborato-
ries,Holmdel,NJ,His research interests include the perfor-
mance analysis and design of networks,Current work
includes video systems over peer-to-peer networks,switch
performance,and wireless networks,He has served as Secre-
tary of the Technical Affairs Council of the IEEE Communica-
tions Society (1992–1993) and is a member of the Technical
Committee on Computer Communications,He is a co-editor
of two books,Network Management and Control,Vol,II,
and Multimedia Communications and Video Coding (Plenum,
1994 and 1996,respectively),and co-author of TCP/IP Essen-
tials,A Lab-Based Approach (Cambridge University Press,
2004),He is a co-recipient of the 2004 IEEE Communications
Society Leonard G,Abraham Prize in the Field of Communi-
cations Systems.
Cooperative partners
should be carefully
selected along the
route so that
optimality at both
the link and path
levels can be
accomplished,while
spatial reuse in ad
hoc networks is not
compromised.
ERKIP LAYOUT 8/3/06 1:24 PM Page 92
84
1536-1284/06/$20.00? 2006 IEEE
Rate
2 Rate
Rate
4
Rate
Rate
1
R
1
R
2
ADVANCES IN SMART ANTENNAS
INTRODUCTION
The burgeoning demand for mobile data net-
works has highlighted some constraints on its
future growth,Wireless links have always had
orders of magnitude less bandwidth than their
wireline counterparts,Mobile users have always
chafed at this limitation,which essentially forces
them to use applications in a manner reminis-
cent of wireline networks of decades past,albeit
freeing them from a desktop,Newer technolo-
gies such as multiple-input multiple-output
(MIMO) systems are starting to increase the
number of bits per second per hertz of band-
width through spatial multiplexing,and to
improve the robustness/range of the wireless link
for a given data rate through space-time coding
and beamforming,However,all these improve-
ments come at the cost of multiple RF front
ends at both the transmitter and the receiver.
Furthermore,the size of the mobile devices may
limit the number of antennas that can be
deployed,Even when MIMO technology is feasi-
ble,wireless engineers are running into another
roadblock,the inefficient way the electromagnet-
ic spectrum has been allocated to different class-
es of users,mainly for historical or regulatory
reasons,Thus,while large portions of the spec-
trum are grossly underused,the popular unli-
censed bands are very crowded,Given this
limitation,for unlicensed bands,the issue of
interference from having too many users has
become as important as how much bandwidth
can be squeezed from it.
This article outlines one way to address these
problems by using the notion of cooperation
between wireless nodes,In cooperative commu-
nications,multiple nodes in a wireless network
work together to form a virtual antenna array.
Using cooperation,it is possible to exploit the
spatial diversity of the traditional MIMO tech-
niques without each node necessarily having mul-
tiple antennas,Multihop networks use some
form of cooperation by enabling intermediate
nodes to forward the message from source to
destination,However,cooperative communica-
tion techniques described in this article are fun-
damentally different in that the relaying nodes
can forward the information fully or in part.
Also the destination receives multiple versions of
the message from the source,and one or more
relays and combines these to obtain a more reli-
able estimate of the transmitted signal as well as
higher data rates,The main advantages of coop-
erative communications are:
Higher spatial diversity,resistance to both
small scale and shadow fading
Higher throughput/lower delay,higher achiev-
able data rates,fewer retransmissions,and
lower transmission delay
Reduced interference/lower transmitted
power,better frequency reuse in a
cellular/WLAN deployment
Adaptability to network conditions,oppor-
tunistic use and redistribution of network
energy and bandwidth
The past few years have seen tremendous
interest in cooperative communications,mostly
at the physical layer,However,significant
research challenges still exist,some of which we
outline in this article,
The goal of this article is to provide new
PEI LIU,ZHIFENG TAO,ZINAN LIN,ELZA ERKIP,AND SHIVENDRA PANWAR,
POLYTECHNIC UNIVERSITY
ABSTRACT
“Denise and her husband Mitch are at opposite
ends of a living room at a crowded party,Denise
tries to attract Mitch’s attention and shouts some-
thing at him,All Mitch can hear is the word ‘Let’s.’
Celine,in the middle of the room,who overhears
Denise and notices their predicament,repeats to
Mitch the part she hears,‘Go home.’ This time,all
Mitch hears is the word ‘home.’ Mitch finally fig-
ures out that his wife wants to go home.” This
analogy from everyday life vividly portrays the
essential element of cooperative wireless com-
munications,namely,utilizing information over-
heard by neighboring nodes to provide robust
communication between a source and its desti-
nation,Cooperative communication exhibits var-
ious forms at different protocol layers and
introduces many opportunities for cross-layer
design and optimization,some of which will be
explored in detail in this article.
COOPERATIVE WIRELESS COMMUNICATIONS,
A CROSS-LAYER APPROACH
The work is partially supported by the National Science
Foundation under grant no,0520054,and the Wireless
Internet Center for Advanced Technology (WICAT),an
NSF Industry/University Cooperative Research Center.
In cooperative
communications,
multiple nodes in a
wireless network
work together to
form a virtual
antenna array,
Using cooperation,
it is possible to
exploit the spatial
diversity of the
traditional MIMO
techniques without
each node having
multiple antennas.
ERKIP LAYOUT 8/3/06 1:24 PM Page 84
IEEE Wireless Communications? August 2006
85
cross-layer research directions in order to illus-
trate the feasibility and performance of coopera-
tive wireless networking,We first describe the
notion of physical-layer cooperation and cooper-
ative diversity,However,in order to realize a
fully cooperative network,research at the physi-
cal layer should be coupled with higher layers of
the protocol stack,in particular,the MAC sub-
layer and the network layer,We describe how
physical-layer cooperation can be integrated with
the MAC sublayer for dramatic improvements in
throughput and interference,We also outline
some of the challenges in extending the notion
of cooperative diversity to the network layer.
MOTIVATION FOR
COOPERATIVE COMMUNICATION
In this section we introduce the basic concepts
underlying cooperative communications,Coop-
erative techniques utilize the broadcast nature of
wireless signals by observing that a source signal
intended for a particular destination can be
“overheard” at neighboring nodes,These nodes,
called relays,partners,or helpers,process the sig-
nals they overhear and transmit towards the des-
tination,The relay operations can consist of
repetition of the overheard signal (obtained,for
example,by decoding and then re-encoding the
information or by simply amplifying the received
signal and then forwarding),or can involve more
sophisticated strategies such as forwarding only
part of the information,compressing the over-
heard signal,and then forwarding,We refer the
reader to [1] for a detailed overview of relaying
methods,The destination combines the signals
coming from the source and the relays,enabling
higher transmission rates and robustness against
channel variations due to fading,We note that
the spatial diversity arising from cooperation is
not exploited in current cellular,wireless LAN,
or ad hoc systems; only one copy of the signal,
whether it comes from the mobile directly or
from a relay,is processed at the destination.
Hence,cooperative relaying is substantially dif-
ferent than traditional multihop or infrastructure
based methods.
This notion of cooperation dates back to the
relay channel model in information theory exten-
sively studied in the 1970s by Cover and El
Gamal [2],but we owe the recent popularity to
[3–5],which showed the benefits of cooperative
relaying in a wireless environment,In order to
illustrate the idea of cooperation and coopera-
tive diversity at the physical layer,we consider
the cooperative coding scheme used in [6,7],Let
us consider an isolated source S who wants to
communicate with a destination D with the help
of a cooperative relay R,as illustrated in Fig,1a.
Here,d
i
denotes the distances between the
nodes.
For direct transmission (i.e.,if the relay R is
not utilized),each channel block,or packet,con-
tains B data bits and r parity bits for forward
error correction (FEC),leading to a total of N =
B + r coded bits,as shown at the top of Fig,1b.
For ease of exposition,we have r ≥ B,We assume
that cyclic redundancy check (CRC) is employed
for error detection,In order to cooperate,S
divides its channel block into two and only trans-
mits in the first half,as shown at the bottom of
Fig,1b,Hence,in the cooperative mode S ends
up sending only half of its coded bits,These bits
are received both by the destination and by the
relay R,The relay observes a higher coding rate
and thus a weaker FEC,Nevertheless,it attempts
to decode the underlying B data bits,If R has
the correct information (which can be checked
using the CRC),it re-encodes and sends the
remaining N/2 parity bits in the second half of
S’s time slot,Otherwise,R informs S that there
was a failure in decoding,and S continues trans-
mission,Therefore,when R decodes correctly,
a73 Figure 1,a) Cooperative system for an isolated link; b) time division in cooperative coding; c) two user cooperative coding perfor-
mance for d
1
= 1,d
2
= 0.5 and d
3
= 0.5,(13,15,15,17) convolutional code,100-byte frame size.
S transmits directly to D
N coded bits
S
d
2
d
3
d
1
D
R
(a)
(b)
Average total received SNR at the destination (dB)
0
10
–2
10
–3
F
rame error rate
10
–1
10
0
5 10
(c)
d
1
= 1.0,d
2
= 0.5,d
3
= 0.5
15 20 25
Direct transmission
Cooperative coding
S transmits R relays for S
N/2 coded bits N/2 coded bits
ERKIP LAYOUT 8/3/06 1:24 PM Page 85
IEEE Wireless Communications? August 200686
the destination will receive half the coded bits
from S and the remaining ones from R,thus cre-
ating spatial diversity,The question is how often
this happens and how it affects the overall error
performance.
Figure 1c illustrates simulation results for
frame error rate (FER) versus the total trans-
mit signal-to-noise ratio (SNR) for the scenario
where the relay is located halfway between the
source and destination (i.e.,d
1
= 1.0,d
2
= 0.5,
and d
3
= 0.5),Note that direct transmission
and cooperative coding use the same total
power and bandwidth (we consider a low-mobil-
ity environment),Hence,along with path loss,
we assume all links experience independent
slow Rayleigh fading that stays constant for the
duration of each packet,The nodes use convo-
lutional coding and each node has the same
average power constraint,We observe from the
figure that for an error rate of 10
–3
we obtain
about 18 dB improvement in SNR with cooper-
ation,Also,the FER for cooperative coding
decreases at a much faster rate than direct
transmission; in fact,cooperation is able to
achieve two full levels of diversity similar to a
MIMO system with two transmit antennas and
one receive antenna.
The above example considers one particular
cooperative scheme to obtain diversity,yet it
shows the potential of cooperation at the physi-
cal layer,Indeed,there is a rich literature on
physical-layer cooperation that investigates many
aspects,such as cooperative protocols for two or
more users,performance bounds for cooperative
systems,resource allocation for cooperation,and
partner-choice strategies,Using a cross-layer
approach between physical and MAC layers,this
article investigates how these gains can be
attained in a wireless network.
BENEFITS OF COOPERATIVE NETWORKING
From the perspective of the network,coopera-
tion can benefit not only the nodes involved,but
the whole network in many different aspects.
For illustration purposes,we choose to explain
only a few potential benefits below.
HIGHER SPATIAL DIVERSITY
As a simple example,Fig,2a shows a small net-
work of four mobile nodes,If the channel quali-
ty between mobile nodes S and D degrades
severely (e.g.,due to shadow or small-scale fad-
ing),a direct transmission between these two
nodes may experience an intolerable error rate,
which in turn leads to retransmissions,Alterna-
tively,S can exploit spatial diversity by having a
relay R
1
overhear the transmissions and then
forward the packet to D as discussed above,The
source S may resort to yet another terminal R
2
for help in forwarding the information,or use R
1
and R
2
simultaneously [8],Similar ideas apply to
larger networks as well,Therefore,compared
with direct transmission,the cooperative
approach enjoys a higher successful transmission
probability,We note here that cooperative com-
munications has the ability to adapt and to miti-
gate the effects of shadow fading better than
MIMO since,unlike MIMO,antenna elements
of a cooperative virtual antenna array are sepa-
rated in space and experience different shadow
fading.
HIGHER THROUGHPUT-LOWER DELAY
At the physical layer,rate adaptation is achieved
through adaptive modulation and adaptive chan-
nel coding,Many MAC protocols have intro-
duced rate adaptation to combat adverse channel
conditions,For instance,when a high channel-
error rate is encountered due to a low average
SNR,the wireless LAN standard IEEE 802.11
switches to a lower transmission rate so as to
guarantee a certain error rate,The power of
cooperation is evident when it is applied in con-
junction with any rate adaptation algorithm,In
Fig,2a,specifically,if Rate
2
and Rate
3
are higher
than Rate
1
such that the total transmission time
for the two-hop case through R
2
is smaller than
that of the direct transmission,cooperation read-
ily outperforms the legacy direct transmission,in
terms of both throughput and delay perceived by
the source S,Furthermore,for relays such as R
1
and R
2
,it turns out that their own individual
self-interest can be best served by helping others.
a73 Figure 2,a) Cooperation in a network; b) illustration of the delay and throughput improvement achieved by cooperation in the time
domain.
(b)
S transmits directly to D
Time T
1
R
1
transmits its
own traffic to D
Time T
2
(a)
Rate
2 Rate
3
Rate
4
Rate
5
Rate
1
R
1
R
2
D
S
S transmits
Time T
3
Time T
4
R
1
transmits its
own traffic to D
R
1
relays for
S to D
Time T
2
ERKIP LAYOUT 8/3/06 1:24 PM Page 86
IEEE Wireless Communications? August 2006 87
As further illustrated in Fig,2b,the intermediate
node R
1
that cooperates enjoys the benefit of
lower channel-access delay,which in turn can be
translated into higher throughput,It is worth-
while to note that Fig,2b also draws a rough
analogy with the cooperative scheme discussed
above (Fig,1b) and illustrates that rate adapta-
tion can further improve the benefits of coopera-
tion in a network setting.
LOWER POWER CONSUMPTION AND LOWER
INTERFERENCE/EXTENDED COVERAGE
The diversity,error rate,and throughput gains
obtained through cooperation can be traded in
for power savings at the terminals,Alternatively,
cooperation leads to an extended coverage area
when the performance metric (error rate,
throughput,etc.) is fixed.
The advantage of cooperation also leads to
reduced interference when the network is
deployed in a cellular fashion to reuse a limited
bandwidth,With the improvement of through-
put,we can reduce the average channel time
used by each station to transfer a certain amount
of traffic over the network,Therefore,the sig-
nal-to-interference ratio (SIR) between proximal
cells using the same channel can be reduced,and
a more uniform coverage can be achieved,As
wireless network deployments become ever more
dense,a reduction of SIR will directly lead to a
boost in network capacity,Indeed,the problem
of dense deployment has already been reported
for IEEE 802.11 b/g networks,which have only
three nonoverlapping channels.
ADAPTABILITY TO
NETWORK CONDITIONS
The cooperative communication paradigm allows
wireless terminals to seamlessly adapt to chang-
ing channel and interference conditions,The
choice of relays,cooperation strategy,and the
amount of resources available for cooperation
can be opportunistically decided,For example,
in Fig,2a,if the source S has some information
about the current channel gains,packet-loss
rates,traffic conditions,interference,or remain-
ing battery energy of nodes in the network,it
may choose to transmit its information directly
to its destination D,using R
1
or R
2
or both in a
cooperative fashion,depending on which trans-
mission mode results in better performance (in
terms of error rates,throughput,or power),This
way,a surplus of resources such as battery ener-
gy or bandwidth at a particular node can be uti-
lized by other nodes in the network in a manner
that will benefit everyone,including the relay
node itself.
Although originating from physical-layer
cooperation,all the aforementioned benefits
cannot be fully realized until proper mechanisms
have been incorporated at higher protocol layers
(e.g.,MAC,network) and the necessary informa-
tion is made available from the lower layer (e.g.,
PHY),Indeed,a cross-layer approach has to be
followed to reap all the benefits of cooperation.
As we illustrate via the cooperative MAC proto-
col described in the following section,an addi-
tional three-way handshake procedure and a new
signaling message have to be introduced to the
MAC layer,and information on channel condi-
tions for related wireless links should be made
available to the upper layers so that the coopera-
tion can be fully enabled,Another example of a
cross-layer approach to cooperation,which
involves interaction between the application
layer and the physical layer,is provided in [9] for
transmission of video signals over wireless links.
COOPMAC,A COOPERATIVE
MEDIUM ACCESS CONTROL
As described above,cooperation at the physical
layer uses the broadcast nature of the wireless
medium and overheard information to improve
the performance,Unfortunately,conventional
wireless medium access control (MAC) proto-
cols have long treated this feature as a problem,
rather than something that can be exploited,The
methodology of cooperation,however,embraces
this concept,and thus creates a new paradigm
for MAC protocol design in wireless network.
We present a new MAC protocol called
CoopMAC
1
for IEEE 802.11 wireless LANs,
which exploits both the broadcast nature of the
wireless channel and cooperative diversity,As
we demonstrate,the CoopMAC protocol fully
capitalizes on the notion of cooperation,and
realizes some of the key benefits previously high-
lighted,such as higher throughput,lower delay,
better coverage,and reduced interference,In the
end,we briefly discuss a preliminary CoopMAC
implementation.
Zhao and Valenti also consider a MAC pro-
tocol [11] for exploiting cooperative diversity,
but it is based upon a conceptual generalization
of the hybrid automatic repeat request scheme
(hybrid-ARQ),instead of the widely deployed
802.11 protocol,Recently,there have been
attempts to explore the benefits of virtual MIMO
at the network level,by pursuing a cross-layer
approach spanning the physical,MAC,and net-
working (e.g.,routing) layers [12],However,the
proposed scheme is based on the assumption
that multiple nodes can be perfectly synchro-
nized,Although the protocol mechanism pro-
posed herein bears some resemblance to that
described in [13],the two protocols address fun-
damentally different issues in two distinct prob-
lem spaces,More specifically,rate adaptation is
the main focus of [13],while cooperative diversi-
ty is incorporated in the protocol introduced
here.
COOPMAC PROTOCOL DESCRIPTION
When a source node has a new MAC proto-
col data unit (MPDU) to send,it can either
transmit directly to the destination,or use an
intermediate helper for relaying,whichever con-
sumes less total air time,The air time is com-
pared using cached information on the feasible
data rates between the three nodes,The feasible
data rate is the largest data rate that guarantees
a predetermined average error rate threshold for
an average channel SNR.
Beyond its normal function,a request to
send (RTS) message is also used by CoopMAC
to notify the node that has been selected for
1
A preliminary version of
the CoopMAC protocol
was described in [10].
As wireless network
deployments become
ever more dense,
a reduction of SIR
will directly lead to a
boost in network
capacity,Indeed,the
problem of dense
deployment has
already been
reported for IEEE
802.11 b/g
networks,which
have only three
nonoverlapping
channels.
ERKIP LAYOUT 8/3/06 1:24 PM Page 87
IEEE Wireless Communications? August 200688
cooperation,Moreover,CoopMAC introduces a
new message called helper-ready to send (HTS),
which is used by the helper to indicate its avail-
ability after it receives the RTS message from
the source,If the destination hears the HTS
message,it issues a clear to send (CTS) message
to reserve channel time for a two-hop transmis-
sion,Otherwise,it still sends out the CTS,but
only reserves channel time for a direct transmis-
sion.
If both HTS and CTS are received at the
source,the data packet should be transmitted to
the relay first,and then forwarded to the desti-
nation by the relay,If the source does not receive
an HTS,it should then initiate a direct transmis-
sion to the destination.
A normal ACK is used to acknowledge a
correct reception,regardless of whether the
packet is forwarded by the relay,or is directly
transmitted from the source,If necessary,
retransmission is attempted,again in a coopera-
tive fashion.
It is crucial that each node obtains and con-
stantly updates its information about the avail-
ability of potential relays,The CoopMAC
protocol deals with this issue mainly through
maintaining a table called the CoopTable in its
management plane,Each entry in the CoopT-
able corresponds to a potential relay,and con-
tains such information as the ID (e.g.,48-bit
MAC address) of the potential relay,the latest
time at which a packet from that potential relay
is overheard by the source,and the data rate
used for direct transmission between the poten-
tial relay and destination,and between the cur-
rent node and the potential relay,A set of
protocols have been defined in CoopMAC to
properly establish,manage,and update the table
in a timely manner.
Due to the broadcast nature of the channel,
the destination will receive the signals transmit-
ted by both the source and the relay,If the desti-
nation is capable of combining these two copies
to decode the original information,then cooper-
ative diversity can be fully leveraged,Receiver
combining,not supported by any existing wire-
less hardware,can be implemented in the next-
generation wireless baseband chip,Given the
constraint of using existing hardware,we have
developed a backward compatible mode of
CoopMAC,which does not perform receiver
combining and therefore only requires a driver
or firmware upgrade.
Without diversity combining,If no combining
capability is supported at the destination,the
packet should be transmitted on both the first
and second hop at the highest physical layer rate
that the respective link can sustain.
With diversity combining,When receiver
combining is enabled,the relay now can forward
packets at a rate equal to or greater than the
one that it adopts in CoopMAC where combin-
ing is not possible,More specifically,the trans-
mission rate between the source and relay is
chosen so as to guarantee a desired error proba-
bility at the relay,Although the destination can-
not fully decode the packet after the first-hop
transmission,this received signal will be stored.
If the relay can successfully receive the packet,it
then forwards the packet to the destination,The
transmission rate on the second hop is the high-
est one that meets a predetermined average
error rate at the destination,once the destina-
tion combines the source and relay signals.
The diversity combining capability allows
CoopMAC to leverage both the spatial diversity
and the coding gain,thereby resulting in even
better performance than the protocol without
receiver combining,Using the coded coopera-
tion framework described above,the helper pro-
vides different coded bits than the source,
leading to a better error performance than repe-
tition coding.
It is worthwhile to note that although the
protocol architecture and signaling mechanism
defined above are applicable both with and with-
out diversity combining at the receiver,the
relay-selection scheme may not yield an optimal
choice for CoopMAC with receiver combining
any longer,because it does not take the possible
a73 Figure 3,Network capacity comparison,a) saturation capacity; b) network capacity gain with respect to 802.11g.
Number of stations
50
7
8
Capacity (
M
b/
s)
9
10
11
12
13
14
10 15 20
(a) (b)
25 30 35 40
Number of stations
5
0%
10%
Capacity gain (percentage)
20%
30%
40%
50%
60%
10 15 20 25 30 35 40
CoopMAC with receiver combining
CoopMAC without receiver combining
802.11g
CoopMAC without receiver combining
CoopMAC with receiver combining
ERKIP LAYOUT 8/3/06 1:24 PM Page 88
IEEE Wireless Communications? August 2006 89
rate increase on the second hop into considera-
tion,In addition,the relay has to be aware of
the average link quality (e.g.,average SNR)
between the source and destination so that it can
properly select a higher transmission rate on the
second hop,The information can be easily con-
veyed to the relay by sandwiching a,shim” link
quality field between the legacy MAC header
and the MAC payload in the data packets that
the source transmits.
Although CoopMAC bears some superficial
resemblance to conventional ad hoc routing pro-
tocols,they are in essence completely different.
First and foremost,forwarding in CoopMAC is
an essential means to accomplish the goal of
leveraging cooperative diversity,Secondly,all
the associated operations occur in the MAC
layer,which enjoys a shorter response time and
more convenient access to the physical layer
information,as compared to the traditional net-
work layer routing,In addition,no channel con-
tention is needed when the relay forwards the
packets to the destination,thereby leading to a
shorter delay for the relay and a more efficient
channel utilization as well,Last but not the least,
no routing protocol that we are aware of has
adopted the receiver-combining technique to tap
into the potential of cooperative diversity.
THROUGHPUT,DELAY,AND
ENERGY EFFICIENCY OF COOPMAC
To evaluate the performance of CoopMAC,we
have developed an event-driven custom simula-
tor using the programming language C to faith-
fully model all the critical MAC and physical
layer features of IEEE 802.11 and CoopMAC.
The parameters in the performance evaluation
assume the default values specified for an IEEE
802.11g network operating in a typical office
environment with low user mobility.
Figures 3–5 depict the simulation results for a
saturated network with a payload size of 1500
bytes,The MSDU size of 1500 bytes has been
chosen in the simulation because the data pack-
ets usually assume such a length in wireless
LANs,as widely reported in recent traffic pat-
tern research [14],Saturation here refers to a
MAC-level condition,where each station always
has packets to transmit at any time instant,Note,
however,that the MAC-level saturation does not
necessarily imply that the physical wireless chan-
nel is always occupied,as all the stations have to
perform backoff according to the random-access
MAC protocol.
As demonstrated in Fig,3,both flavors of
CoopMAC can achieve a much higher network
capacity than the legacy IEEE 802.11g,Between
the two versions of CoopMAC,the one with
receiver-combining capability can deliver more
throughput,as was anticipated above.
Another highly desirable feature of Coop-
MAC that Figs,3a and 3b reveal is that both the
network capacity and the capacity gain for Coop-
MAC with respect to 802.11g increase as the
number of nodes in the network grows,This
improvement primarily stems from the increas-
ing availability of relays as the network becomes
more populated.
For a wide variety of network sizes,Fig,4
portrays the simulation results for the average
channel access delay,which essentially is the
duration from the time a packet becomes the
head-of-line (HOL) packet until the time the
packet is successfully transmitted,The corre-
sponding delay improvement over 802.11g is
shown in Fig,4b,It is evident that data packets
in CoopMAC experience significantly less delay
than in legacy IEEE 802.11g.
It is also worthwhile to note that the same
trend in throughput and delay improvement can
be observed for networks operating in a medi-
um-to-low-traffic regime,In addition,even more
improvement can be achieved when a larger
frame size is used,Due to space limitations,we
will not present additional results in this article
a73 Figure 4,Channel access delay comparison,a) mean channel access delay; b) improvement of mean channel access delay with respect
to 802.11g.
Number of stations
5
0
0.01
Aver
a
ge
cha
nnel a
ccess d
elay (s)
0.02
0.03
0.04
0.05
0.06
10 15 20 25
(a) (b
30 35 40
Number of stations
5
0%
5%
Impr
ove
me
nt of a
ve
r
ag
e
channe
l
ac
c
e
ss de
la
y (
percentage)
10%
15%
20%
25%
30%
35%
40%
45%
50%
10 15 20 25 30 35 40
802.11g
CoopMAC without receiver combining
CoopMAC with receiver combining
CoopMAC with receiver combining
CoopMAC without receiver combining
ERKIP LAYOUT 8/3/06 1:24 PM Page 89
IEEE Wireless Communications? August 200690
for the nonsaturation condition or for larger
frame sizes.
In addition to conventional measures like
throughput and delay,we have also evaluated
the energy efficiency of CoopMAC,since power
conservation is always a key concern for wireless
networks,Figure 5a depicts the energy consump-
tion per user in terms of total amount of energy
needed to successfully deliver a bit for each user
(i.e.,joules/bit/user),which includes the energy
consumed in transmission,reception and chan-
nel sensing,Figure 5b shows the percentage
improvement with respect to 802.11g,We
observe that as the number of nodes increases,
the improvement in per user energy efficiency
achieved by CoopMAC also grows,This is pri-
marily due to the fact that although CoopMAC
requires nodes to receive and retransmit traffic
for each other,it also enables them to spend less
time listening to the medium,Ultimately,this
saving outweighs the new energy expense,and
leads to an increase in energy efficiency,We
refer the readers to [15] for details of the ener-
gy-consumption model.
INTERFERENCE REDUCTION IN A DENSE NETWORK
The deployment of wireless networks has grown
increasingly dense,leading to concerns that
deployments may become interference-limited.
For instance,there are 11 channels defined in
the 2.4 GHz spectrum for operation of IEEE
802.11 WLANs in the United States,However,
in order to avoid interference between adjacent
cells,only three mutually nonoverlapping chan-
nels can be used at the same time.
In the following discussion,we focus only on
co-channel interference for a cellular deploy-
ment of IEEE 802.11 with a reuse factor of three.
Note that while a node is transmitting packets in
a particular cell,there will be six proximal cells
in which parallel transmissions generate co-chan-
nel interference,Our simulation calculates the
signal-to-interference-plus-noise ratio (SINR)
for each point in a cell by randomly choosing the
locations of six interfering nodes in the six proxi-
mal cells and assuming path loss as well as
Rayleigh fading,The maximum feasible data
rate is estimated based on the SINR and the
error rate threshold requirement.
Figure 6 compares the interference for 802.1
ig MAC and CoopMAC in a multicell environ-
ment with a frequency reuse factor of three,All
three systems are under the same traffic load in
all cells,From these figures,another advantage
of cooperation becomes apparent,CoopMAC
without receiver combining decreases the aver-
age interference by 21.5 percent,while receiver
combining enables another 12.5 percent reduc-
tion,Since both versions of CoopMAC are more
efficient in terms of throughput,the transmission
time for the same amount of traffic using the
CoopMAC protocol is less than that of the lega-
cy system,therefore reducing total energy radiat-
ed to the network,Due to the lower background
interference,the sustainable regions for all four
rates supported by IEEE 802.llg are extended
[15].
IMPLEMENTATION
In order to further validate the design of Coop-
MAC and demonstrate the feasibility of an incre-
mental deployment,we have made efforts to
implement the CoopMAC protocol using off-
the-shelf IEEE 802.1 lb network interface cards
(NICs) on a Linux platform [15],Since no exist-
ing hardware can perform the receiver-combin-
ing function,only the CoopMAC protocol
without diversity combining can be implemented.
In fact,due to the constraint in accessing the
firmware on the chip,we had to take an emula-
tion approach at the driver level for the Coop-
MAC version without receiver combining,which
unfortunately incurs additional protocol over-
head,Nevertheless,as demonstrated in the
experiment,CoopMAC can still reduce the aver-
a73 Figure 5,Energy efficiency comparison,a) average energy consumption per bit per user; b) average user energy efficiency gain with
respect to 802.11g.
Number of stations
(a)
50
0
0.5
Aver
a
ge
us
e
r e
ne
rg
y
co
n
su
mp
ti
o
n p
er
bit
(
J/
b
/us
e
r)
1
1.5
2
2.5
3
3.5
4
4.5
x10
-6
10 15 20 25 30 35 40
Number of stations
(b)
5
0%
10%
Im
pr
ov
e
me
n
t i
n a
ve
r
ag
e
u
s
er
e
ne
r
gy
co
n
su
mp
ti
o
n p
er
bit
(
pe
r
ce
n
ta
g
e)
20%
30%
40%
50%
10 15 20 25 30 35 40
CoopMAC without receiver combining
CoopMAC with receiver combining
802.11g
CoopMAC without receiver combining
CoopMAC with receiver combining
ERKIP LAYOUT 8/3/06 1:24 PM Page 90
IEEE Wireless Communications? August 2006 91
age file-transfer times significantly below the
original value,and a more significant perfor-
mance improvement can be achieved when the
entire CoopMAC without receiver combining is
completely implemented in firmware,In addi-
tion,an even higher performance gain would be
possible if the CoopMAC with receiver combin-
ing can be realized in a baseband chip.
CONCLUSION AND FUTURE WORK
By introducing collaboration from nodes that
otherwise do not directly participate in trans-
mission,cooperative communication introduces
a new paradigm for wireless communication,It
enables a tremendous improvements in robust-
ness,throughput,and delay; a significant reduc-
tion in interference; and an extension of
coverage range,To fully leverage the concept
of cooperation,the entire protocol stack —
from physical layer to networking protocols —
should be carefully reengineered or even
redesigned.
To illustrate the necessity of a cross-layer
design approach,we have explored cooperation
at both layers 1 and 2 of the OSI protocol stack,
and have proposed a new MAC protocol for
IEEE 802.11 networks which we call CoopMAC.
In particular,the CoopMAC protocol has an
option to enable the capability of diversity com-
bining at the receiver,where two versions of the
same data are jointly decoded to recover the
original packet,As verified by extensive simula-
tions,the CoopMAC protocol,both with and
without receiver-combining capability,can
achieve substantial performance improvements,
without incurring appreciable additional com-
plexity in system implementation,Compared
with the noncombining version,the CoopMAC
protocol with receiver-combining capability
pushes cooperation to an even higher level and
reaps additional benefits.
To further exploit cooperation gains at the
network layer for highly adaptive and scalable ad
hoc networks,many research challenges remain.
Given the increasing number of cooperating
nodes listening to each transmission,packet for-
warding can now be done in a more opportunis-
tic way than has been traditionally considered in
ad hoc networks,Indeed,the notions of routing
and routing protocols may change when cooper-
ation is fully integrated in the link layer,Cooper-
ative partners should be carefully selected along
the route so that optimality at both the link and
path levels can be accomplished,while spatial
reuse in ad hoc networks is not compromised.
Similar subtle cross-layer design issues abound in
ad hoc networks,and the implications of node
cooperation,including cooperative routing algo-
rithms and the scalability of network capacity
with the number of nodes in a network,deserve
further investigation.
REFERENCES
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a73 Figure 6,Interference (W),frequency reuse factor = 3,traffic = 500 p/s,transmission power = l μW,a)
802.11g; b) CoopMAC without receiver combining; and c) CoopMAC with receiver combining.
x10
-7
2
1.8
1.6
1.4
1.2
1
(a) (b) (c)
To further validate
the design of
CoopMAC and
demonstrate the
feasibility of an
incremental
deployment,
we have made
efforts to implement
the CoopMAC
protocol using
off-the-shelf IEEE
802.1 lb network
interface cards
(NICs) on a Linux
platform
ERKIP LAYOUT 8/3/06 1:24 PM Page 91
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war.html
BIOGRAPHIES
PEL LIU [S’01] (pliu@photon.poly.edu) completed his B.S.
and M.S,degrees in electrical engineering at Xi’an Jiaotong
University,China,in 1997 and 2000,respectively,He is a
Ph.D,candidate in the Department of Electrical and Com-
puter Engineering of Polytechnic University,Brooklyn,NY.
His research interests are in wireless communications and
wireless networks.
ZHIFENG TAO [S’00] (jeff.tao@ photon.poly.edu) received a
B.E,degree in communication and information engineering
from Xi’an Jiaotong University,P,R,China,in 2000,Since
then,he has been a Ph.D,candidate in the Department of
Electrical and Computer Engineering at Polytechnic Univer-
sity,He also received his M.S,degree in telecommunication
networking from Polytechnic University in May 2002,His
current research interests include wireless networking,
medium access control,quality of service,and cooperative
communications.
ZINAN LIN [S’00] (zlin03@utopia.poly.edu) received a B.E
degree in information engineering from Zhejiang Universi-
ty,Hangzhou,China,in 1998,and an M.E,degree in elec-
trical engineering from Nanyang Technological University
(NTU),Singapore,in 2001,From 1998 to 2000 she was
awarded a research scholarship by NTU and worked as a
research assistant in the Center for Signal Processing,
School of Electronic and Electrical Engineering,NTU,Since
2001,she has been Ph.D,candidate in the Department of
Electrical and Computer Engineering,Polytechnic Universi-
ty,Brooklyn,NY,Her general research interests include
wireless communications and digital signal processing,
especially channel coding,diversity techniques,and CDMA
sequence design.
ELZA ERKIP [S’93,M’96,SM’05] (e1za@poly.edu) received
M.S,and Ph.D,degrees in electrical engineering from Stan-
ford University in 1993 and 1996,respectively,and a B.S.
degree in electrical and electronic engineering from Middle
East Technical University,Turkey,in 1990,She joined Poly-
technic University in Spring 2000,where she is currently an
associate professor of electrical and computer engineering.
During 1996–1999 she was with the Department of Electri-
cal and Computer Engineering of Rice University,She
received the 2004 Communications Society Stephen O,Rice
Paper Prize in the field of communications theory,the NSF
CAREER award in 2001,and the IBM Faculty Partnership
Award in 2000,She is the Technical Program Co-Chair of
the 2006 Communication Theory Workshop,She is also an
Associate Editor of IEEE Transactions on Communications
and a Publications Editor for IEEE Transactions on Informa-
tion Theory,Her research interests are in wireless commu-
nications,information theory,and communication theory.
SHIVENDRA S,PANWAR [S’82,M’85,SM’00]
(panwar@catt.poly.edu) received a B.Tech,degree in electri-
cal engineering from the Indian Institute of Technology,Kan-
pur,in 1981,and M.S,and Ph.D,degrees in electrical and
computer engineering from the University of Massachusetts,
Amherst,in 1983 and 1986,respectively,He joined the
Department of Electrical Engineering at the Polytechnic Insti-
tute of New York,Brooklyn,NY (now Polytechnic University),
where he is a professor in the Electrical and Computer Engi-
neering Department,Currently he is the director of the New
York State Center for Advanced Technology in Telecommuni-
cations (CATT),He spent the summer of 1987 as a visiting
scientist at the IBM T,J,Watson Research Center,Yorktown
Heights,NY,and was a consultant to AT& T Bell Laborato-
ries,Holmdel,NJ,His research interests include the perfor-
mance analysis and design of networks,Current work
includes video systems over peer-to-peer networks,switch
performance,and wireless networks,He has served as Secre-
tary of the Technical Affairs Council of the IEEE Communica-
tions Society (1992–1993) and is a member of the Technical
Committee on Computer Communications,He is a co-editor
of two books,Network Management and Control,Vol,II,
and Multimedia Communications and Video Coding (Plenum,
1994 and 1996,respectively),and co-author of TCP/IP Essen-
tials,A Lab-Based Approach (Cambridge University Press,
2004),He is a co-recipient of the 2004 IEEE Communications
Society Leonard G,Abraham Prize in the Field of Communi-
cations Systems.
Cooperative partners
should be carefully
selected along the
route so that
optimality at both
the link and path
levels can be
accomplished,while
spatial reuse in ad
hoc networks is not
compromised.
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