PHY-MAC cross layer cooperative protocol supporting physical layer network coding

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PHY-MAC cross layer cooperative protocol supporting physical layer network coding. This paper propose a PHY-MAC crosslayer cooperative protocol which can support PNC for multi-rate cooperative wireless networks with bidirectional traffic. The design objective of the proposed protocol is to increase the transmission reliability, throughput, and energy efficiency, as well as to reduce the transmission delay.
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VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43
PHY-MAC Cross-Layer Cooperative Protocol Supporting
Physical-Layer Network Coding
Quang-Trung Hoang*, Xuan Nam Tran
Le Quy Don Technical University, Hanoi, Viet Nam
Abstract
Cooperative communication has known as an e ective solution to deal with the channel fading as well as to
improve the network performances. Further, by combining the cooperative relaying technique with the physical-
layer network coding (PNC), cooperative networks will obtain more benefits to improve the throughput and
network resource utilization. In order to leverage these benefits, in this paper, we propose a PHY-MAC cross-
layer cooperative protocol which can support PNC for multi-rate cooperative wireless networks with bidirectional
trac. The design objective of the proposed protocol is to increase the transmission reliability, throughput, and
energy eciency, as well as to reduce the transmission delay. Simulation results show that the proposed protocol
outperforms the previous cooperative protocol as well as the traditional protocol in terms of network performance.
2015 Published by VNU Journal of Sciences.
Manuscript communication: received 01 June 2015, revised 20 June 2015, accepted 25 June 2015
Correspondence: Xuan Nam Tran, namtx@mta.edu.vn
Keywords:
Cross-Layer MAC, Cooperative MAC, Physical-Layer Network Coding, Alamouti-DSTBC.
1. Introduction
wireless
networks
is
no
longer
unidirectional
but mostly bidirectional.
A typical example of
Nowadays, the increase in the number of
people using mobile devices has leveraged the
development of wireless networks. With the
increased requirements in the quality of service
for various applications, technical solutions
need to be developed to improve the network
performance such as the channel capacity,
end-to-end throughput, transmission reliability,
energy eciency, and the network coverage.
Cooperative transmission has been known as an
e ective method to exploit spatial diversity to
enhance the quality of wireless channels at the
bidirectionaltracisthepeer-to-peerapplication
such as voice and video communications. A
challenging problem for the bidirectional trac
is how to design the data exchange protocol
eciently. In order to deal with this problem,
cooperative relaying has been known as a
promising technique in the wireless ad hoc
networks [1]. In the more recent researches,
cooperative relaying has also been proposed to
combine with network coding (CC) to achieve
more performance benefits, in particular, with the
bidirectional trac [2]–[5].
physical layer.
In the cooperative transmission
In wireless ad hoc networks, network coding
multiple single-antenna devices can collaborate
can be implemented by two ways: (i) using the
with one another to share their antennas with
conventional network coding (CNC) in which
neighbouring partners in order to form a virtual
the relay implements data decoding of received
multiple-input multiple-output (MIMO) system.
packets in two individual transmission time slots
Recent development of data communication
[6]; (ii) using the physical-layer network coding
applications
has
shown
that
the
trac
in
(PNC) in which the relay decodes data packets
30
Q.T. Hoang, X.N. Tran / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43
received simultaneously from the two end nodes
 The physical-layer design of the protocol
[7]. Compared with CNC, PNC has advantage in
can
be
adapted
to
various
cooperative
reducing the number of transmission phases and
diversity schemes depending on the channel
thus helps to increase the end-to-end throughput
conditions.
In
our
protocol,
more
than
as well as to reduce the delay [8]–[10].
one optimal relay node can be selected and
Most of recent researches on the bidirectional
communication simply focused on combining
PNC and the cooperative relaying [10]–[14]. In
[10] Shiqiang et al. have proved that the PNC-
based medium access control (MAC) protocol,
namely PNC-MAC, has more advantages than
the CNC-based MAC one in terms of the end-
to-end throughput and delay. However, the
drawback of this protocol is that it does not have
partitioned in one or two relaying groups.
Thanks to this arrangement, the process of
cooperative relaying node selection can be
implementedeasily. Especially,incasethere
are two cooperative relaying groups, we can
usethespatialdiversityschemebasedonthe
Alamoutidistributedspatial-timeblockcode
(DSTBC) [16] to improve the transmission
reliability.
a proper mechanism for reducing problems of
hidden nodes in the network. Compared with
the PNC-MAC protocol, the ANC-ARA protocol
proposed in [14] has di erence in that it does
not need to know the queue status information of
the neighboring nodes. Instead, it uses a special
mechanismtoavoidtheproblemofhiddennodes.
 By letting the optimal relays in the same
priority group send a signaling pulse of the
same format the relay contending collision
is avoided. As a result, the relay-contending
time duration is reduced and the system
throughput is thus improved.
The proposed cross-layer protocol in [15] uses
 The MAC layer of the proposed protocol
PNCtosupportthebidirectionaltraceciently.
is designed to support two main functions:
Comparedwiththeprotocolsin[10]and[14],this
(i)
adaptive
relay
selection
mechanism
protocol considers the protocol overheads as well
supporting
the
bidirectional
trac;
(ii)
as the contending time duration among optimal
PNC is initiated by the cooperative relay
relay nodes in the design to increase the network
nodes
only
if
the
bidirectional
trac
is
performance.
However,
this
PNC
supported
occurred.
By
this
design,
the
proposed
protocol still faces a problem of collisions during
protocol can adapt itself flexibly to network
optimal
relay
selection.
Clearly,
a
collision
environment variations to increase the end-
avoidance solution will help to increase further
to-end bidirectional throughput.
network
performance
in
terms
of
end-to-end
throughput or delay.
Our main contributions can be summarized as
Motivated by the above problem, in this paper
follows:
we propose an improved cross-layer cooperative
MAC protocol which can support PNC and
avoid the problem of collisions happened during
the optimal relay selection process. The
proposed protocol is designed to work in three
 A cooperative diversity transmission
model based on optimal relay groups with
the improved transmission reliability is
proposed for cooperative wireless networks.
modes: directional transmission, cooperative
transmission for the unidirectional trac, and
cooperative relaying based on PNC for the
bidirectional trac. However, in this paper we
will focus mainly on the last one. Compared with
 The MAC layer protocol supporting PNC
with the improved overall performance
of network is introduced for multi-rate
cooperative wireless networks.
the protocols in [10]–[15], our proposed protocol
 An analytical model of energy eciency is
has the following advantages:
introduced for the proposed protocol.
T +T
O P
i i i i
Q.T. Hoang, X.N. Tran / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43
31
The remainder of the paper is organized as
3. ProposedPNC-supportedPHY-MACcross-
follows.
Sect.
2 presents the network model
layer cooperative protocol
under
consideration.
Sect.
3
describes
the
proposed protocol. The performance analysis of
3.1. Operations at the PHY layer
the proposed protocol is presented in Sect. 4.
Simulation results are shown in Sect. 5. Finally,
conclusions are drawn in Sect. 6.
Assume that the PHY layer can support L
di erent data rates r1;r2;::;rL (for example, L =
8 in the IEEE 802.11a standard). Each network
node uses a certain data rate if its estimated SNR
2. System model
is above a corresponding threshold l; l 2 ( 1 <
We consider a cooperative wireless network as
illustrated in Fig. 1. The network consists of a
source (S), a destination (D) placed apart at a
distance of d, and a set of N intermediate nodes
which are distributed randomly between S and
D. All network nodes are equipped with only
one single antenna and have limited transmitting
power. The two end nodes are assumed to
exchangedatawitheachotherinthebidirectional
mode using the basic rate of R0 = 2Mbps.
Channelsbetweeneachpairofnodesareassumed
independent and a ected by flat slow Rayleigh
fading plus log-normal shadowing.
2 <  < L). Similar to the analysis of the
cross-layerPHY-MACprotocolforunidirectional
trac in [17], we define the MAC cooperation
region (CR) as a set of triple rates, C :=
(R1;RC1;RC2)  R3, such that the bidirectional
e ective payload transmission rate (EPTR) in
relaying transmission is always larger than that in
directtransmission. HereR1;RC1;RC2 denotesthe
direct rate, the first hop rate, and the second hop
rate, relatively. In generally, the EPTR is given
by LP , with LP;TO;TP being the payload
length, the overhead time duration, and the
payload time duration respectively. Hence, the
condition for a relay to belong to the cooperation
S
Selected optimal
relay nodes
D
Optimal relay nodes
Weak intermediate nodes
Optimal relay nodes
Relay candidates
Multiple access (MA)
transmission phase
Broadcast (BC)
transmission phase
region is that the transmission delay for the
cooperative bidirectional trac is always less
than that without cooperative relaying.
Inordertoimprovethetransmissionreliability,
we propose two cooperative relaying schemes
which support bidirectional trac. These
schemes are shown in Fig. 2. In our proposed
schemes, depending on the channel conditions
Fig. 1. Network model of the cooperative wireless network.
each relaying group R1 and R2 can have one or
It is further assumed that among N
intermediate nodes, only those capable nodes,
more optimal relays selected by the MAC layer
protocol.
referred to as relay candidates, will participate in
the relay selection process. Those intermediate
nodes with weak channel gain to S and D,
referred to as weak intermediate nodes, will
not participate into the relaying process. The
optimal relay nodes are those relay candidates
which have the same maximal cooperative rate.
Moreover, the selected optimal relay nodes are
the optimal relay nodes which are selected after
thecontentionperiod. AsshowninFig.1, several
3.1.1. Transmission based on one relaying group
In this case, the transmission scheme is
illustratedinFig.2-a. Inthescheme,bidirectional
dataexchangebetweenSandDisperformedover
themultipleaccess(MA)phaseandthebroadcast
(BC) phase. In the MA phase, the two end nodes
S and D transmit simultaneously to R1. The
signal received simultaneously at the i-th relay in
the relaying group R1 is given by
intermediate nodes can be selected as the optimal
relays and the selected optimal relays.
yR1 = hSR1 xS +hDR1 xD +zR1;
(1)
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PHY-MAC cross layer cooperative protocol supporting physical layer network coding. This paper propose a PHY-MAC crosslayer cooperative protocol which can support PNC for multi-rate cooperative wireless networks with bidirectional traffic. The design objective of the proposed protocol is to increase the transmission reliability, throughput, and energy efficiency, as well as to reduce the transmission delay..

Nội dung

VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43 PHY-MAC Cross-Layer Cooperative Protocol Supporting Physical-Layer Network Coding Quang-Trung Hoang*, Xuan Nam Tran Le Quy Don Technical University, Hanoi, Viet Nam Abstract Cooperative communication has known as an eective solution to deal with the channel fading as well as to improve the network performances. Further, by combining the cooperative relaying technique with the physical-layer network coding (PNC), cooperative networks will obtain more benefits to improve the throughput and network resource utilization. In order to leverage these benefits, in this paper, we propose a PHY-MAC cross-layer cooperative protocol which can support PNC for multi-rate cooperative wireless networks with bidirectional trac. The design objective of the proposed protocol is to increase the transmission reliability, throughput, and energy eciency, as well as to reduce the transmission delay. Simulation results show that the proposed protocol outperforms the previous cooperative protocol as well as the traditional protocol in terms of network performance. 2015 Published by VNU Journal of Sciences. Manuscript communication: received 01 June 2015, revised 20 June 2015, accepted 25 June 2015 Correspondence: Xuan Nam Tran, namtx@mta.edu.vn Keywords: Cross-Layer MAC, Cooperative MAC, Physical-Layer Network Coding, Alamouti-DSTBC. 1. Introduction wireless networks is no longer unidirectional but mostly bidirectional. A typical example of Nowadays, the increase in the number of people using mobile devices has leveraged the development of wireless networks. With the increased requirements in the quality of service for various applications, technical solutions need to be developed to improve the network performance such as the channel capacity, end-to-end throughput, transmission reliability, energy eciency, and the network coverage. Cooperative transmission has been known as an eective method to exploit spatial diversity to enhance the quality of wireless channels at the physical layer. In the cooperative transmission multiple single-antenna devices can collaborate with one another to share their antennas with neighbouring partners in order to form a virtual multiple-input multiple-output (MIMO) system. Recent development of data communication bidirectionaltracisthepeer-to-peerapplication such as voice and video communications. A challenging problem for the bidirectional trac is how to design the data exchange protocol eciently. In order to deal with this problem, cooperative relaying has been known as a promising technique in the wireless ad hoc networks [1]. In the more recent researches, cooperative relaying has also been proposed to combine with network coding (CC) to achieve more performance benefits, in particular, with the bidirectional trac [2]–[5]. In wireless ad hoc networks, network coding can be implemented by two ways: (i) using the conventional network coding (CNC) in which the relay implements data decoding of received packets in two individual transmission time slots [6]; (ii) using the physical-layer network coding applications has shown that the trac in (PNC) in which the relay decodes data packets 30 Q.T. Hoang, X.N. Tran / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43 received simultaneously from the two end nodes The physical-layer design of the protocol [7]. Compared with CNC, PNC has advantage in can be adapted to various cooperative reducing the number of transmission phases and diversity schemes depending on the channel thus helps to increase the end-to-end throughput conditions. In our protocol, more than as well as to reduce the delay [8]–[10]. Most of recent researches on the bidirectional communication simply focused on combining PNC and the cooperative relaying [10]–[14]. In [10] Shiqiang et al. have proved that the PNC-based medium access control (MAC) protocol, namely PNC-MAC, has more advantages than the CNC-based MAC one in terms of the end-to-end throughput and delay. However, the drawback of this protocol is that it does not have a proper mechanism for reducing problems of hidden nodes in the network. Compared with the PNC-MAC protocol, the ANC-ARA protocol proposed in [14] has dierence in that it does not need to know the queue status information of the neighboring nodes. Instead, it uses a special mechanismtoavoidtheproblemofhiddennodes. The proposed cross-layer protocol in [15] uses PNCtosupportthebidirectionaltraceciently. Comparedwiththeprotocolsin[10]and[14],this protocol considers the protocol overheads as well as the contending time duration among optimal relay nodes in the design to increase the network performance. However, this PNC supported protocol still faces a problem of collisions during optimal relay selection. Clearly, a collision avoidance solution will help to increase further network performance in terms of end-to-end throughput or delay. Motivated by the above problem, in this paper we propose an improved cross-layer cooperative MAC protocol which can support PNC and avoid the problem of collisions happened during the optimal relay selection process. The proposed protocol is designed to work in three modes: directional transmission, cooperative transmission for the unidirectional trac, and cooperative relaying based on PNC for the bidirectional trac. However, in this paper we will focus mainly on the last one. Compared with the protocols in [10]–[15], our proposed protocol has the following advantages: one optimal relay node can be selected and partitioned in one or two relaying groups. Thanks to this arrangement, the process of cooperative relaying node selection can be implementedeasily. Especially,incasethere are two cooperative relaying groups, we can usethespatialdiversityschemebasedonthe Alamoutidistributedspatial-timeblockcode (DSTBC) [16] to improve the transmission reliability. By letting the optimal relays in the same priority group send a signaling pulse of the same format the relay contending collision is avoided. As a result, the relay-contending time duration is reduced and the system throughput is thus improved. The MAC layer of the proposed protocol is designed to support two main functions: (i) adaptive relay selection mechanism supporting the bidirectional trac; (ii) PNC is initiated by the cooperative relay nodes only if the bidirectional trac is occurred. By this design, the proposed protocol can adapt itself flexibly to network environment variations to increase the end-to-end bidirectional throughput. Our main contributions can be summarized as follows: A cooperative diversity transmission model based on optimal relay groups with the improved transmission reliability is proposed for cooperative wireless networks. The MAC layer protocol supporting PNC with the improved overall performance of network is introduced for multi-rate cooperative wireless networks. An analytical model of energy eciency is introduced for the proposed protocol. Q.T. Hoang, X.N. Tran / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43 31 The remainder of the paper is organized as 3. ProposedPNC-supportedPHY-MACcross-follows. Sect. 2 presents the network model layer cooperative protocol under consideration. Sect. 3 describes the proposed protocol. The performance analysis of the proposed protocol is presented in Sect. 4. Simulation results are shown in Sect. 5. Finally, conclusions are drawn in Sect. 6. 2. System model We consider a cooperative wireless network as illustrated in Fig. 1. The network consists of a source (S), a destination (D) placed apart at a distance of d, and a set of N intermediate nodes which are distributed randomly between S and D. All network nodes are equipped with only one single antenna and have limited transmitting power. The two end nodes are assumed to exchangedatawitheachotherinthebidirectional mode using the basic rate of R0 = 2Mbps. Channelsbetweeneachpairofnodesareassumed independent and aected by flat slow Rayleigh fading plus log-normal shadowing. 3.1. Operations at the PHY layer Assume that the PHY layer can support L dierent data rates r1;r2;::;rL (for example, L = 8 in the IEEE 802.11a standard). Each network node uses a certain data rate if its estimated SNR is above a corresponding threshold l;l 2 (1 < 2 < < L). Similar to the analysis of the cross-layerPHY-MACprotocolforunidirectional trac in [17], we define the MAC cooperation region (CR) as a set of triple rates, C := (R1;RC1;RC2) R3, such that the bidirectional eective payload transmission rate (EPTR) in relaying transmission is always larger than that in directtransmission. HereR1;RC1;RC2 denotesthe direct rate, the first hop rate, and the second hop rate, relatively. In generally, the EPTR is given by LP , with LP;TO;TP being the payload length, the overhead time duration, and the payload time duration respectively. Hence, the condition for a relay to belong to the cooperation Selected optimal relay nodes S D Optimal relay nodes Weak intermediate nodes Optimal relay nodes Relay candidates Multiple access (MA) transmission phase Broadcast (BC) transmission phase region is that the transmission delay for the cooperative bidirectional trac is always less than that without cooperative relaying. Inordertoimprovethetransmissionreliability, we propose two cooperative relaying schemes which support bidirectional trac. These schemes are shown in Fig. 2. In our proposed Fig. 1. Network model of the cooperative wireless network. It is further assumed that among N intermediate nodes, only those capable nodes, referred to as relay candidates, will participate in the relay selection process. Those intermediate nodes with weak channel gain to S and D, referred to as weak intermediate nodes, will not participate into the relaying process. The optimal relay nodes are those relay candidates which have the same maximal cooperative rate. Moreover, the selected optimal relay nodes are the optimal relay nodes which are selected after thecontentionperiod. AsshowninFig.1, several schemes, depending on the channel conditions each relaying group R1 and R2 can have one or more optimal relays selected by the MAC layer protocol. 3.1.1. Transmission based on one relaying group In this case, the transmission scheme is illustratedinFig.2-a. Inthescheme,bidirectional dataexchangebetweenSandDisperformedover themultipleaccess(MA)phaseandthebroadcast (BC) phase. In the MA phase, the two end nodes S and D transmit simultaneously to R1. The signal received simultaneously at the i-th relay in the relaying group R1 is given by intermediate nodes can be selected as the optimal relays and the selected optimal relays. yR1 = hSR1 xS +hDR1 xD +zR1; (1) 32 Q.T. Hoang, X.N. Tran / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43 Relay group hSR1 R1 hR1S S hDR1 hR1D Relay group D R1 DR1 D SR1 R1D R1S R2D S R2S R DR2 hSR2 Relay group (a) Proposed scheme with one relaying group (b) Proposed scheme with two relaying groups Fig. 2. Cooperative relaying model supporting bidirectional trac. where xS and xD are the transmitted signals from S and D, respectively. hSRi and hDRi are the fading coecients of the channels from S and from D to the i-th relay of R1, respectively; zRi is noise at the i-th relay of R1 . In the BC phase, the signals received at S and i-th relay of R1 in two consecutive time slots are respectively given by y1i = hSRi x1 +hDRi x1 +z1; (4) y2i = hSRi x2 +hDRi x2 +z2; (5) D are given respectively as follows: NR1 yS = hRi SCPNC yRi +zi; (2) i=1 NR1 yD = hRi DCPNC yRi +zi; (3) i=1 where, hSRi and hDRi are the Rayleigh fading coecients of the link from S and D to the i-th relay of R1, respectively. z1;z2 are the noise occurred in each time-slot, respectively. Similarly, the signals received at the j-th relay of R2 during two consecutive time-slots of the MA phase are denoted by where NR1 is the number of relays of R1; CPNC() is a function of PNC. In this paper, we use the decoding and forwarding (DF) scheme at the relays and the PNC mapping function as in [7]. 3.1.2. Transmission based on two relaying groups The transmission for transmission scheme is drawn as Fig. 2-b. Assume that R1 and R2 consist of NR1 and NR2 optimal relays, where NR1;NR2 1. In order to improve the transmission reliability of this scheme, we apply the Alamouti DSTBC scheme [16] to our considered transmission scheme. Similartothecaseofonerelayinggroup, the bidirectional data exchange between S and D also takes place over two phases (MA and BC). However, each phase uses two time slots for transmission. In two consecutive time slots of the MA phase, S and D send simultaneously their data vectors: xS = [x1;x2] and xD = [x1 ;x2 ], respectively to relays. The signals received at the yRj = hSRj xS +hDRj x1 +z1; (6) y2 j = hSRj x2 +hDRj x2 +z2: (7) In the BC phase, the selected optimal relays broadcast their PNC encoded signals to both S and D. Since the Alamouti DSTBC scheme is used, the signals received at S during two consecutive time slots are given by y1 = H1CPNCy1i + H2CPNCy2 j +z1; (8) h i h 2 i yS = HS CPNC yRi + HS CPNC yRj +zS; (9) where NR1 NR2 H1 = hRi S and H2 = hRjS; i=1 j=1 and the asterisk is used to denote the complex conjunction; z1;z2 are the noise occurred at the Q.T. Hoang, X.N. Tran / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43 33 source in each time slot, respectively. We also rates according to the IEEE 802.11a standard assume that the links between any two nodes in the network are reversible such that hRi S = hSRi ;hRjS = hSRj . Similar to the source, the signals received at the destination during two consecutive time slots of the BC phase are given by y1 = H1CPNCy1i + H2CPNCy2 j +z1 ; (10) h i h 2 i yD = HD CPNC y i + HD CPNC y j +zD; 2 (11) where [18]. The process of PNC mapping is illustrated in Fig. 3. In the figure, denotes the general binary operation for network-coding arithmetic. That is, applying on mi; mj 2 Mb gives mimj = mk 2 Mb; Mb isasetofpotentialbinary code-words depending on each modulation type. Assuming that the Ms-ary modulation is used, then Ms is a set of the potential modulation symbols. Let be the binary combination operation, then combination of sS; sD 2 Ms yields sS sD = sk 2 Ms, where Ms is the domain after the binary operation; each sk 2 Ms received by the relay node must be mapped to a NR1 NR2 H1 = hRi D and HD = hRjD; (12) i=1 j=1 demodulated symbol mk 2 Mb. 3.2. Operation at the MAC layer z1 ;z2 are the noise at each time slot, respectively. Here, we also assume that hRi D = hDRi ;hRjD = hDRj . Hence, based on the estimated channel status information (CSI), the source and destination can estimate the signals received from the optimal relays in two groups R1 and R2, then decode xS and xD based on the XOR operation. 3.1.3. PNC for multirate adaptive modulation The main goal of designing the MAC layer of the proposed protocol is to minimize the overhead time and the bidirectional payload transmission time while supporting the adaptive relay selection. The operation of the proposed MAC layer scheme is illustrated in Fig. 4. The operation of the proposed MAC layer is described as follows Source Initiation: After a back-o interval, the source establishes the link to the In order to work in the multirate destination node using the request-to-send communication mode, network nodes need (RTS) and clear-to-send (CTS) exchange to use adaptive modulation. As a result, the handshake. In order to start, the source PNC scheme needs to be realized appropriately broadcasts the RTS frame to both the for several modulation types. In this paper, destination and intermediate nodes. we adopt the PNC modulation–demodulation mapping principle proposed in [7] for the adaptive modulation with set of transmission Destination Response: If the destination receives the RTS frame correctly, it broadcasts the CTS frame to both the source and intermediate nodes after a SIFS (Short S R D Inter-Frame Spacing) interval. In the case mS (nb bits) Modulation mapping mk = mD mS  mD (nb bits) Modulation Demodulation mapping mapping the destination also has its own data to send to the source, the information of the payload length Lds is included into the CTS frames, if not the length Lds is set to null. MPSK/MQAM symbol (sS) sk = sS sD MPSK/MQAM symbol (sD) Intermediate Node Processing: When the intermediate node overhears the RTS and CTS frames exchanged between the source Fig. 3. The PNC mapping principle. and the destination, it estimates the CSI 34 Q.T. Hoang, X.N. Tran / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43 NAV RTS Source CTS Destination Datasd ACKS Time Datads ACKD Time NAV Optimal relay (RTS) Relay selecting contention DataPNC ACKPNC group 1. Time a) Bidirectional communications with one relaying group. NAV RTS Source CTS Destination Datasd ACKS Time Datads ACKD Time Optimal relay group 1. Optimal NAV (RTS) Relay selecting contention ACKPNC DA-STBC Time DataPNC relay group 2. Time b) Bidirectional communications with two relaying groups. Fig. 4. The operation of the proposed MAC-layer protocol. to determine its cooperative rate allocation 3.3. Optimal relay selection in the cooperation region CR. If the intermediate node satisfies the condition of CR, it participates in the process of the optimal relay selecting contention. As mentioned in Section 3.1, in order to select the optimal relay using the distributed method, the optimal grouping algorithm works as follows. Given the direct transmission rate R1, there exist M potential cooperative rates Rh. A set of Relay Transmission: If a relay node is these cooperative rates are partitioned into G selected for the process of bidirectional cooperative communication, it uses transmission operations as in Fig. 2-a or Fig. 2-b. In contrast, it releases the relay contending process, and holds the waiting status. dierent priority groups, each consists of ng relay members, where each member can be assigned to a dierent m priority level according to its identified data rate, so M = g=1 ng. Each relay candidate can determine its priority allocation in CR according to the g-th group-priority index and the m-th member-priority index. Based on Destination Acknowledgement: After the source and destination have correctly received the data, they simultaneously send their ACKS and ACKD frames to the optimal relays after a SIFS interval. These relays then broadcast the ACKPNC frame to both the source and destination. these parameters, the MAC-layer protocol selects the optimal relay node through control and/or signaling messages. The process of optimal relay selecting contention is shown in Fig. 5 and is described as follows: Step 1: If a relay candidate finds its data rate allocation in CR, it decides to broadcast the Q.T. Hoang, X.N. Tran / VNU Journal of Science: Comp. Science & Com. Eng., Vol. 31, No. 2 (2015) 29–43 35 FB FB

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