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Outage Probability of Hybrid Decode-Amplify-Forward Relaying Protocol for Buffer-Aided Relays

  • The University of Oklahoma Tulsa OK USA


Buffer-aided cooperative relaying is often investigated either using decode and forward (DF) or amplify and forward (AF) relaying rules. However, it is seldom investigated using the hybrid decode-amplify-forward (HDAF) relaying rule. In this work, the performance of signal-to-noise ratio (SNR) based HDAF relaying rule is followed for buffer-aided cooperative relaying. Relay with the best possible corresponding channel is determined for reception or transmission. When source to relay hop is the most powerful, data is forwarded to chosen relay and its SNR is compared against the predefined SNR threshold at the relay. If it is greater than the threshold, the decoded data is saved in the corresponding buffer. Otherwise, the amplified data is saved in the respective buffer. When relay to destination link is the most powerful, data is forwarded to the destination. The famous Markov chain analytical model is used to illustrate the progression of the buffer state and to get the outage probability expression. Mathematical and simulation outcomes support our findings and prove that the outage probability performance of the proposed technique beats the existing SNR based buffer-aided relaying protocols based on DF and AF relaying rules by 2.43 dBs and 8.6 dBs, respectively.
Outage Probability of Hybrid
Decode-Amplify-Forward Relaying Protocol for
Buffer-Aided Relays
Hina Nasir1,2, Nadeem Javaid3, Waseem Raza4, Muhammad Imran5, Nidal Naseer6
1International Islamic University, Islamabad 44000, Pakistan
2Air University, Islamabad 44000, Pakistan
3COMSATS University Islamabad, Islamabad 44000, Pakistan
4The University of Lahore, Lahore 54000, Pakistan
5College of Applied Computer Science, King Saud University, Saudi Arabia
6College of Engineering, Alfaisal University, Saudi Arabia
Abstract—Buffer-aided cooperative relaying is often investi-
gated either using decode and forward (DF) or amplify and
forward (AF) relaying rules. However, it is seldom investigated
using the hybrid decode-amplify-forward (HDAF) relaying rule.
In this work, the performance of signal-to-noise ratio (SNR)
based HDAF relaying rule is followed for buffer-aided cooperative
relaying. Relay with the best possible corresponding channel is
determined for reception or transmission. When source to relay
hop is the most powerful, data is forwarded to chosen relay and
its SNR is compared against the predefined SNR threshold at
the relay. If it is greater than the threshold, the decoded data is
saved in the corresponding buffer. Otherwise, the amplified data
is saved in the respective buffer. When relay to destination link
is the most powerful, data is forwarded to the destination. The
famous Markov chain analytical model is used to illustrate the
progression of the buffer state and to get the outage probability
expression. Mathematical and simulation outcomes support our
findings and prove that the outage probability performance of
the proposed technique beats the existing SNR based buffer-aided
relaying protocols based on DF and AF relaying rules by 2.43
dBs and 8.6 dBs, respectively.
Index Terms—cooperative relaying, buffer-aided, relay selec-
tion, hybrid decode-amplify-forward.
Cooperative relaying (CR) allows the source’s data to be
transmitted to the target with the cooperation of intermediate
relays. It has wide applicability in ad-hoc and sensor networks
and their services. The regular CR system picks one relay
to send the source signal to the target in the consecutive
time-slots [1]. This relaying model offers many advantages
in terms of throughput, capacity, coverage, etc. when matched
to the non-cooperative relaying model, however, it has some
weaknesses. The choice of a single relay is a bottleneck
because the excellent source-to-relay (SR) hop may not
guarantee the excellent relay-to-destination (RD) hop.
Besides, the set transmission schedule, i.e., the alternate order
of transmission of source and relay, limits the diversity gain
of the system.
The obstacles as stated earlier are eased by the introduction
of data buffers at the relays [2]. The buffers grant freedom
to pick separate relays for receiving and transmitting data
by storing the received data at the relay and transmitting
it whenever the RDchannel is favorable. They offer
advantages like increased diversity gain, increased goodput,
increased capacity, etc., as compared to the traditional CR
system. For a buffer-aided CR system, the dominant link on the
grounds of link state is picked for data transmission. When the
dominant link is from SRchannels, data is delivered to the
picked relay and saved in the corresponding buffer. Besides,
when the dominant link is from RDside, the corresponding
buffer sends data to the destination.
To design a relaying scheme for a buffer-aided system, the
designers keep the following main challenges in mind. First
is the acquisition of channel state information (CSI). Since
the best link in terms of link quality is to be decided in
each time-slot, the CSI needs to be known. Secondly, buffer
status monitoring is needed as the full buffers are incapable of
receiving data and empty buffers are incapable of transferring
data. Therefore, it is essential to keep track of full and empty
buffers to know the link availability. Furthermore, the delay
introduced by incorporation of buffers is a challenging task
because data has to wait inside the buffers un till the respective
RDchannel is selected.
The buffer-aided CR schemes can are usually classified
into fixed and non-fixed transmission models. In a fixed
transmission model, the dominant SRlink is picked for
communication in the odd time-slot and the received data from
the source is saved in the particular relay buffer. For the next
time-slot, the dominant RDlink is picked for communi-
cation and the corresponding relay sends to the destination
from its buffer [3]. In comparison, in a non-fixed transmission
model, any link can be dominant for data transmission in a
given time-slot [4]. The maximum obtainable diversity gains
in fixed and non-fixed transmission models are Kand 2K,
respectively, where Kis the count of relays.
The famous works in the existing literature addressing the
aforementioned challenges in buffer-aided CR are max-max
[3] and max-link [4] relay selection schemes. These schemes
978-1-5386-8088-9/19/$31.00 ©2019 IEEE
obey fixed and non-fixed transmission models, respectively.
Link state is the only metric in the choice of the most
suitable relay and are based on decode and forward (DF)
relaying. Most of the literature on buffer-aided CR is based
on max-max and max-link relay selection schemes following
DF relaying strategy. In [5], the authors added a direct link
between source and destination in the system model of the
max-link scheme and obtained satisfying diversity gain and
delay performance. The authors in [6] proposed the hybrid of
schemes proposed in [3] and [4] and achieved full diversity
gain of 2Kusing fixed transmission regulation. Another effort
on relay selection based on link quality only is made in [7].
The authors prioritized RDover SRto reduce queuing
delay and achieved the ideal delay of two time-slots.
The relay choice based only on the link state significantly
reduced the outage probability of the system, however, the
outage probability can be more improved if buffer status is
taken into consideration. Full buffers are incapable to receive
data and empty buffers are incapable to transmit data. Thus,
the count of links is decreased. The reduction in the number of
links reduces the diversity gain of the system. The authors in
[8] recommended the buffer status based relay determination
scheme based on the non-fixed transmission model. They
assigned weight to each link based on the available and
occupied buffer space of the corresponding buffer. The link
with the greatest weight is picked for data transmission. The
max-weight scheme achieved reduced outage probability in
comparison to the max-link scheme, however, in case of links
with same weights, random selection is made which may not
guarantee the best link selection. Based on this, the authors in
[9] proposed link priority and link quality as a second selection
metric in relay selection. Another effort on buffer status based
relay selection is made in [10]. In this scheme, the authors
avoid full and empty buffers by taking link quality and buffer
status in relay selection. They also achieved reduced queuing
delay by prioritizing RDlink. The same as the previous
attempt, a buffer situation based relay determination scheme
following fixed transmission is given in [11]. The authors made
relay selection on the basis on smallest in shortest out buffer
status and attained diversity gain equal to the number of relays
at a tiny buffer size. The authors in [12] exploit the broadcast
nature of the wireless network and activate multiple SRlinks
to reduce delay. Although the delay is reduced to a significant
amount in these schemes, however, they compromise on the
diversity gain. There is a diversity-delay trade-off in buffer-
aided cooperative communication. Delay negotiates for the
diversity gain. A low complexity based relay determination
scheme is proposed in [13] to reduce delay by giving priority
to the RDlink while preserving the diversity gain. The
authors imposed a threshold for the number of packets in
a buffer. For the minimum of one SRtransmission, the
RDlink is given priority if the buffer occupancy meets the
The authors in [14] explored that many of the present works
in buffer-aided relay determination is based on DF relaying.
They proposed amplify and forward (AF) relaying based max-
link scheme and achieved enhanced diversity and coding gain.
This scheme is enhanced in [15] using buffer status based relay
determination to enhance the outage probability performance
of the system.
Form the literature as mentioned earlier, it is evident that
most of the work in buffer-aided CR either focuses on DF
relaying or on AF relaying. In DF relaying, the transmission
or reception at relay only happens if the signal is decode-
able at the receiving end. The signal is decode-able only if
its quality is greater than the predefined metric such as a
specific signal to noise ratio (SNR) threshold. If the signal
is corrupted, relays remain silent for both reception and trans-
mission. In HDAF, instead of remaining silent on corrupted
signal quality, relay adaptively switches between the AF or DF
mode to improve the system performance. The HDAF relaying
is mostly explored for buffer-less cooperative relaying. In
[16], the authors presented the SNR based incremental HDAF
cooperative relaying protocol for three node network. The
relays prefer to remain quiet or transmit either in AF or DF
mode, depending on the signal condition. The authors in [17],
[18] examined the performance gain of HDAF protocol over
AF and DF protocols for multiple relays cooperative network.
An HDAF model with the nth best-relay determination scheme
is presented in [19]. In this scheme, the best relay sends to
the destination indiscriminately. However, when the best relay
is not available, the nth best relay is considered.
To the best of the author’s awareness, the buffer-aided
HDAF is still primarily to explore. In [20], the authors
explored HDAF on buffer-aided incremental cooperative re-
laying. The authors used link state as the single metric in relay
determination. This scheme is based on the fixed transmission
model with the consideration of the direct link connecting
source and target in the system model. The authors obtained
the expression for the outage probability for finite and infinite
length buffers. The scheme achieved the diversity gain equal
to Kand delay of 1+KL/2time-slots, where, Lis the buffer
In this work, we propose a buffer-aided SNR based HDAF
scheme based on non-fixed transmission regulation. The
scheme selects the dominant link on the grounds of link qual-
ity. The link with the highest SNR is selected for transmission
or reception. If the SRlink is dominant, data is forwarded
from source to the corresponding relay and compared against
the SNR threshold. If it is higher than the threshold, data is
decoded and saved in the buffer. In contrast, if it is less than
or equal to the threshold, data is amplified and saved in the
buffer. When the corresponding RDlink is dominant, the
decoded or amplified data saved in the buffer is transmitted
to the destination. We obtain the theoretical equation for the
outage probability by analyzing the infinite and finite buffers
at the relays. Markov chain based analytical model is used
to model the development of the buffer state and to calculate
the outage probability. The analytical outcomes are confirmed
using thorough Monte Carlo simulations.
Paper organization is as follows. In Section II, we give the
system model, relay selection and outage probability inves-
tigation of the proposed work. In Section III, the numerical
results of the proposed scheme are given. Conclusive remarks
are presented in Section IV.
TABLE I: Numerical notations
Symbol Description
R Set of Relays
S Source
D Destination
SRSource-to-relay channel
RDRelay to destination channel
K Number of relays
L Buffer size
ψ(LRk)Number of packets in buffer
γSR SNR of SRchannel
γRD SNR of RDchannel
Z HDAF threshold at relay
γth Threshold at destination
roinformation rate
Csr Number of open SRlinks
Crd Number of open RDlinks
. . .
Fig. 1: System model for the buffer-aided HDAF scheme
The system design under study is a dual-hop cooperative
relaying network of source S, destination Dand a set Rof
Knumber of relays indicated by R={R1, R2,· · · , RK}as
shown in Fig. 1. The detailed numerical notation is given in
Table I. The direct communication link between Sand Dis
not possible as it is in deep fade and as considered in many of
the existing works [3], [4], [8], [8]–[11], [14]. All nodes are
provided with a separate antenna device and do not support
simultaneous reception and transmission, i.e., they work in
half-duplex mode. Relays are provided with both AF or DF
hardware. Depending upon the channel state, they either work
as AF relay or DF relay. Every relay has a fixed sized data
buffer of maximum Lpackets space to save the received data.
Buffers support first in first out policy to process data. The
count of elements in a buffer LRk is denoted by ψ(LRk)where
0ψ(LRk)L. Only a neither empty nor full buffer can
accept and transmit data. When a packet goes into the buffer,
buffer ψ(LRk)gets a unit increment, likewise, when a packet
departs from the buffer, ψ(LRk )gets a unit decrement. An
SRlink is supposed to be ’open’ if its respective buffer is
not full and RDlink is supposed to be open if its respective
buffer is not empty. The open link means it is available and
open for selection.
Let the channel coefficients between SR(RD) hop is
indicated by hSR (hRD ). It is believed that all channels support
independent and identically distributed (i.i.d) Rayleigh fading
where the envelop fading signal for a particular hop is fixed
for a certain time-slot and differs individually from one time-
slot to another. The instantaneous SNRs of SR(RD)
hop is given as γSR =Ps|hSR |2/No(γRD =Pr|hRD|2/No).
Where, Ps(Pr) is the transmission power of the source
(relay) node and Nois noise variance of additive white
Gaussian noise with unit mean assumed for the channels.
Transmission rate is assumed to be robits/s/Hz. The average
SNRs of SR(RD) hop is ¯γSR =PsE(|hSR|2)/No
(¯γRD =PrE(|hRD|2)/No), where, E(.)is the statistical
average operator. In case of Rayleigh distribution, for any γ,
the probability distribution function (PDF) is expressed as:
fγi(γ) = 1
and cumulative distribution function (CDF) is depicted as
Fγi(γ) = P r(γiγ)=(eγ/¯γi+ 1) .(2)
According to the proposed transmission scheme, the dis-
tributed method adopted in [21] is adopted to exchange SNR
information among the relays. The relays are able to decide
whether itself is best for reception or transmission of data.
The data transfer is from Sto Dthrough an intermediary
node called relay. Since each relay has a data storing facility,
we can pick the most dominant link for data transmission
among all open links. The dominant link is the strongest link
among all open links in terms of link quality. When the SR
link is dominant, data is forwarded from Sto the correspond-
ing R. Upon reception at the relay, the experienced SNR is
matched against the predefined SNR threshold indicated by Z,
where, Z= 2ro1. If received SNR is higher then Z, it means
that the signal is surely decode-able at the relay. Therefore, the
DF relaying method is adopted and the decoded data is saved
in the buffer. In contrast, if the received SNR is less than
Z, it specifies that data cannot be decode-able and hence, it
is amplified using the AF relaying method and saved in the
buffer without decoding. The obtained signal at the relay is
analytically represented as:
where, xsis the signal transmitted from source, ySRkis the
signal received at relay Rk,nSRkis the channel noise.
Moreover, when the dominant link is from RDside, the
buffer transmits from the connected relay to D. Analytically,
the received signal in DF mode is expressed as:
where, yRkDis the received signal at D,xris the decoded,
corrected and transmitted signal from relay and nRkDis the
channel noise. For a relay to work in AF mode, the signal
received at Dis given as:
where, Gis the gain factor defined as:
A. Relay Selection
According to the proposed buffer-aided HDAF protocol, in
a given time-slot, the dominant link is picked from all open
links on either sides. The relay selection is analytically stated
R= arg max
where, Ris the selected relay.
B. Outage Probability Investigation
In this part, we are interested in calculating the outage
probability of the buffer-aided HDAF scheme. In order to
assure the signal reception is successful at the target, we
establish SNR threshold γth = 22ro1, which is the minimum
threshold below which the signal is not decode-able at the
destination. We first find the outage probability considering
infinite buffer size. Then, we move towards the practical case
of finite buffers using Markov modeling.
C. Infinite Buffer Size at Relays
The outage probability of buffer-aided HDAF utilizing the
law of total probability is defined as,
Pout =P r(γSR > Z )PDF +P r (γSR Z)PAF ,(8)
where, PDF (PAF ) is the outage probability of DF (AF)
method. The first term in (8) defines that the signal at relay
is decode-able and relay operates in DF method. The terms
P r(γSR > Z )and P r(γS R Z)are respectively expressed
P r(γSR > Z ) = 1 [(1 eZ/¯γSR )Csr ],(9)
P r(γSR Z) = (1 eZ/¯γSR )Csr ,(10)
where, Csr and Crd are the count of open links on SR
and RDsides, respectively. It is to state here that in the
case of infinite (huge) buffer size, the corresponding links of a
relay are always open for data transmission. Therefore, Csr =
Crd =K.
The probability to operate in DF mode is given by,
PDF =P r(γDF
Dγth|γS R > Z),(11)
where, γDF
Dis the SNR at the destination in case of DF
relaying protocol. Using the law of conditional probability,
PDF =P r(γRD γth , γSR > Z)
P r(γSR > Z ).(12)
Since, γSR and γRD are independent of each other,
PDF =P r(γRD γth )P r(γSR > Z )
P r(γSR > Z )
= (1 eγth/¯γRD )Crd .(13)
The second term in (8) defines that the relay is unable to
decode the signal and it works in AF mode with the probability
to operate expressed as,
PAF =P r(γAF
Dγth|γS R Z),(14)
where, γAF
Dis the end-to-end equivalent SNR at the destina-
tion for AF relaying protocol defined in [22], [23] as,
γSR +γRD + 1 .(15)
Putting (15) in (14), we get,
PAF =P r(γSR γRD
γSR +γRD+1 , γSR Z)
P r(γSR Z)(16)
Now, we represent γSR as random variable xand γRD as
random variable y. The probability of HDAF to operate in AF
mode is derived as,
0fγRD (y)fγSR (x)dydx
P r(γSR Z).(17)
Using order statistics, the CDF of γSR following Rayleigh
distribution is given as
FγSR (x) = (1 ex/¯γS R )Csr ,(18)
differentiating w.r.t x, we get PDF of γSR as
fγSR (x) = (1 ex/¯γSR )Csr1Csr ex/¯γSR
Using Binomial expansion,
fγSR (x) = Csr
m=0 Csr 1
m(1)mex/¯γSR emx/¯γSR .
Similarly, PDf of γRD using Binomial expansion is given as
fγRD (y) = Crd
n=0 Crd 1
n(1)ney/¯γRD eny/¯γRD .
Putting (20) and (21) in (17), we get,
PAF =1
(1 eZ/¯γSR )Csr ×
(n+ 1)¯γSR
n=0 Csr 1
mCrd 1
m+ 1(1 e(m+1)Z/¯γSR )
e(m+1)x/¯γSR e
(xγth) ¯γRD dx . (22)
Putting (13), (9), (10) and (22) in (8), we get the outage
probability for the presented buffer-aided HDAF system.
D. Finite and Homogeneous Buffer Size at Relays
Now, we move towards the realistic case of finite and homo-
geneous buffer size at the relays. Markov modeling is utilized
to get the outage probability. The total number of states of
the Markov chain is (L+ 1)K. Let AR(L+1)K×(L+1)K
denote the state transition matrix of the Markov chain. Each
entry Aij =P(sjsi) = P(Xt+1 =si|Xt=sj)in Ais a
probability to transit from state sjto siat time tand (t+ 1),
respectively. The probability of transition relies on the state of
buffer. A relay with (Ψ(LRk) = Lor Ψ(LRk) = 0) cannot
accept or transmit data, respectively. Let csr and crd be the
binary variables for the link availability of SRand RD
hops, respectively. cq= 1, if link is available and cq= 0, if
link is not available, where q∈ {sr, rd}. The link availability
of a relay Rkcan be found as,
csr(Rk) = (1,if 0ψ(LRk)L1,
0,otherwise, (23)
crd(Rk) = (1,if 1ψ(LRk)L,
0,otherwise. (24)
Hence, the total number of open links at SRand RD
sides which compete in the proposed dominant link election
process are respectively expressed as,
Csr =
Crd =
The state transition matrix entries are denoted by:
Aij =
out,if si=sj,
Csr (1 PAct
RD ),if siUsj
Crd PAct
RD ,if siUsj
where, Usj
SR and Usj
RD are the set of states to which sjcan
transit when SRand RDhops are picked, respectively.
out is the outage probability for no development in the buffer
state. It is defined in (8). PAct
RD is the probability that RD
hop is activated when its SNR is greater than the SNR of
SRhop. PAct
RD is derived as,
RD =P r(X < Y ) = Z
FγSR (x)fγRD (x)dx . (28)
Considering Markov chain as a-periodic, irreducible and col-
umn stochastic, the vector for the probability of steady state
is given by [24]:
π= (Q+AI)1q, (29)
where, π= [π1, ..., π(L+1)K]T,Qij = 1 i, j and
q= [1,1, ..., 1]T, and IR(L+1)K×(L+1)Kis the identity
matrix. The structure of the Markov chain states that when
there is no development in the buffer state, an outage happens.
Accordingly, the system’s outage probability is analytically
displayed as [4]:
Pout =
out =diag(A)π . (30)
The outage probability assessment of the proposed scheme
is given in this segment. The proposed scheme is referred as
‘Maxlink-HDAF’ in the plots. Maxlink-HDAF is compared
with the existing schemes, i.e., ‘Maxlink-DF’ [4], ‘Maxlink-
AF’ [14] and ‘Maxmax-HDAF’ [20]. The parameter, rois
set to 1bits/sec/Hz everywhere as followed in [3], [4]. The
results are presented for derived analytical expressions and
verified utilizing thorough Monte-Carlo simulations, i.e., 106
iterations. All the outcomes are based on symmetric channel
0 2 4 6 8 10 12 14 16 18
SNR (dB)
Outage probability
Maxlink-HDAF (simulation)
Maxlink-HDAF (theory)
Maxmax-HDAF (theory)
Maxmax-HDAF (simulation)
Fig. 2: Outage probability of Maxlink-HDAF and Maxmax-
HADF schemes for K= 3 and L= 3
The investigation of the outage probability of the proposed
Max-link HDAF scheme and Maxmax-HDAF scheme against
average SNR are displayed in Fig. 2. The outcomes are
presented for K= 3 and L= 3. The Maxlink-HDAF scheme
outperforms the Maxmax-HDAF scheme because the latter
follows the fixed transmission model that limits its diversity
gain. While Maxlink-HDAF tends to obtain diversity gain
approximately equal to 3. In this case, the full diversity gain
is not obtained because of the tiny buffer size. There is a huge
chance of buffer overflow which lowers the number of open
links. It is seen that the simulation and theoretical and out-
comes coincide with each other that proves our investigation.
Fig. 3 shows the investigation of the outage probability of
the Maxlink-HDAF scheme with Maxlink-DF and Maxlink-
AF schemes against the average SNR. The results are pre-
sented for L= 3 and K= 3. The enhancement in the result of
0 5 10 15
SNR (dB)
Outage Probability
Maxlink-HDAF (simulation)
Maxlink-HDAF (theory)
Maxlink-DF (theory)
Maxlink-DF (simulation)
Maxlink-AF (theory)
Maxlink-AF (simulation)
Fig. 3: Outage probability of Maxlink-HDAF, Maxlink-DF and
Maxlink-AF schemes for K= 3 and L= 3
Maxlink-HDAF is evident. For the design obeying DF relaying
solely, when the SNR at relay or destination is lower than the
predefined threshold, the system runs into the outage. In HDAF
protocol, instead of pushing the system into the outage, the
relay switches from DF mode to AF mode and stores the data.
That is why the outage probability is improved. The Maxlink
in DF mode is better than the AF mode. Because in AF mode,
data is decode-able at the destination only. Therefore, there
is a high chance that packet is stored with weak SNR and
amplified accompanying noise and transmitted to the target.
The improvement of HDAF mode is slightly better than DF
mode because HDAF uses DF mode when SNR is higher than
the threshold at the relay defined as Z.
This research focuses on the buffer-aided relay selection
using the HDAF relaying rule. The link state, i.e., SNR
is solely used in relay selection. The outage probability is
investigated for the case of symmetric channel conditions.
The following conclusions are derived. The proposed scheme
using HDAF mode achieved a better outage probability in
comparison to SNR based buffer-aided relay selection schemes
using DF or AF relaying rules solely by 2.4 dBs and 8.6 dBs,
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... In hybrid decode-amplify-forward (HDAF), instead of remaining silent on poor signal quality, the relay switches between the AF or DF mode to enhance the system performance. [44] studies the performance of SNR-based HDAF relaying policy for BA ...
... The OP of the proposed policy is superior to the existing SNR-based BA relaying schemes based on DF and AF rules by 2.43 dB and 8.6 dB, respectively [44]. ...
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Ever since the idea of buffers was incorporated in wireless communications, buffer-aided relaying has become an emerging breakthrough in the world of transmitting and receiving signals. Equipping the relays with buffers adds a new degree of freedom capable of enhancing numerous quality-of-service (QoS) metrics including throughput, outage probability, power efficiency and physical-layer security. The QoS enhancement is achieved by compromising the end-to-end delay that is inflicted by storing the packets in the relays' buffers until a suitable source-relay or relay-destination link is selected. In this context, the selection of a relay for transmission/reception is important since it governs the QoS-delay trade-off that can be contemplated. In this survey, the authors review and analyse various link selection protocols in buffer-aided relaying systems. These relaying strategies are categorised and contrasted according to their performance levels, limitations, applications, system model assumptions and complexity. Finally, they discuss current challenges, and highlight future research directions.
... In [19], a hybrid decode-amplify-forward relaying policy is proposed. In this relaying policy, SNR of the signal received at the relay is compared with the threshold SNR, if the received signal SNR is high, the relay will act as DaF relay. ...
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Cooperative device-to-device (D2D) wireless systems use relays such as amplify-and-forward (AaF) and decode-and-forward (DaF). The AaF relay is simple to implement but amplifies noise as well. Conventional DaF relay does decoding and acts as a regenerative relay, hence deliver better noise performance. In this study, the authors generalise the cooperative D2D model by designing power adaptive decode-and-forward relays. In it, the probabilistically selected relay subject to an average relay transmit power constraint does decoding and jointly adapt its gain and transmit power as a function of the channel gains to the destination link. For the proposed cooperative D2D system model, the authors derive the optimal relaying policies which set relay gains to (i) minimise the fading averaged symbol error rate; (ii) maximise fading averaged spectral efficiency and (iii) maximise fading averaged energy efficiency. Then they present a comprehensive performance analysis of the optimal policies. The mathematical analysis includes derivations of exact expressions and their upper bound expressions. To further extend the FAvSER analysis, they investigate diversity order in an insightful scaling regime. Then they present extensive numerical results to validate the derived analytical expressions and to show that the proposed optimal relaying policies outperform the benchmark policies.
... It involves a relay node which helps in transmitting data from source to intended destination. SWIPT combined with cooperative relaying offers benefits in terms of increased diversity gain as well as increased lifetime of battery constrained devices by recharging node's battery [5][6][7][8][9]. Some of the earliest works on cooperative energy harvested SWIPT relaying include [10] and [11]. ...
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In this work, a generalized approach is proposed to review the performance of relaying designs with energy harvesting capability. The unified modeling of generalized energy harvesting relaying (GEHR) design covers the non-energy harvesting designs and the well-known energy harvesting designs, i.e., time-based relaying (TR) and power-based relaying (PR). Moreover, the hybrid of both TR and PR designs is also catered. We find the mathematical representations for the outage probability, ergodic capacity and average throughput in Rayleigh fading channels for amplify-and-forward (AF) and decode-and-forward (DF) relaying modes. The closed-form expressions are derived for the outage probability. To validate that GEHR design is a generalization of TR and PR designs, we study the individual cases of GEHR design from the perspectives of schematic diagram, signal analysis and the performance evaluation parameters comparison. Furthermore, the GEHR design is studied for the mixed Rayleigh-Rician fading channels. We considered two sub-cases of mixed fading, i.e., in case 01, source to relay (SR) link is considered as Rayleigh channel and relay to destination (RD) link is considered as a Rician channel. Conversely, in case 02, SR link is taken as Rician channel and RD link is a Rayleigh channel. We find the mathematical expressions for the ergodic capacity, outage probability and average throughput for DF and AF relaying for both cases of mixed fading channels. The analytical results in both channel configurations are presented for throughput and verified using extensive Monte-Carlo simulations. The results show that the proposed GEHR design can be set to work as not only for the conventional TR and 2 PR designs but also for hybrid of them. Furthermore, with some slight modifications in the proposed design, it can work as a conventional non-energy harvesting cooperative relaying model. Index Terms Ergodic capacity, outage probability, average throughput, delay tolerant and delay limited communication , mixed Rayleigh-Rician fading channels.
... Moreover, the average throughput is also derived for the proposed scheme. A part of this work has been published in [88]. proposed scheme is termed as the max-score. ...
In this thesis, a suite of schemes is presented to enhance the performance of cooperative communication networks. In particular, techniques to improve the outage probability, end-to-end delay and throughput performances are presented. Firstly, a buffer-aided cooperative communication is studied and analyzed for packet selection and relay selection. A three-node network is considered in the beginning and the phenomenon of packet diversity is taken into consideration to overcome bad channel conditions of the source to relay (SR) and relay to destination (RD) links. The closed-form expressions for the computation of the outage probability along with the delay, throughput and diversity gain are derived. Then, packet selection is studied along with relay selection for buffer-aided amplify and forward (AF) cooperative relaying networks. The proposed protocol is analyzed for both symmetric and asymmetric channel conditions and buffer size using multiple antennas at relays and compared against the existing buffer-aided schemes. Markov chain (MC) is used to derive the closed-form expressions for outage probability, diversity gain, delay and throughput. Next, the performance of SNR based hybrid decode-amplify-forward relaying protocol is observed. When SR link is the strongest, data is transmitted to the selected relay and checked against the predefined threshold at the relay. If it is greater than the threshold, data is decoded and stored in the corresponding buffer. Otherwise, it is amplified and stored in the respective buffer. When RD link is the strongest, data is transmitted to the destination. MC based theoretical framework is used to derive an expression for the outage probability, the average end-to-end delay and throughput. Then, relay selection schemes considering the instantaneous link quality along with buffer status in the relay selection are proposed. A scheme is proposed that simultaneously considers buffer status and link quality. Then, we discuss multiple links with equal weights using a general relay selection factor. It includes the weight of the link as the first metric and the link quality, or priority, as the second metric for different cases of the same weight. The proposed scheme is evaluated for symmetric and asymmetric channel conditions. Moreover, we propose a specific parameter, termed as the bu�er-limit, which controls the selection of SR or RD links and also have its impact on the average delay and throughput. In this scheme, the outage probability is traded with the average end-to-end queuing delay or the average throughput by adjusting the values of the buffer-limit. The MC based framework is employed to derive the closed-form expressions for the outage probability, average end-to-end queuing delay and the average throughput. The suggested schemes are compared to the existing bufferaided relay selection schemes. Lastly, we consider the energy constraint cooperative network and propose a generalized approach to study the performance of energy harvesting relaying schemes. The unified modeling of generalized energy harvesting relaying (GEHR) scheme covers the non-energy harvesting schemes and the well-known energy harvesting schemes, i.e., time switching based relaying (TSR) and power splitting based relaying (PSR). Moreover, the scheme also caters the hybrid of both TSR and PSR schemes. The closed-form expressions for the outage probability and ergodic capacity and average throughput are formulated for non-mixed Rayleigh fading and mixed Rayleigh-Rician fading channels. Each case is analyzed for AF and decode and forward relaying models. Comprehensive Monte-Carlo simulations confirm all theoretical results.
The goal of cooperative communication is to increase spatial diversity and wireless network coverage. However, in a classical relaying setup, where a single best relay is chosen to receive the source's data in the first time slot and retransmit in the second time slot regardless of channel circumstances, channel mismatch occurs. In this paper, we have developed an enhanced relay selection technique at the relay nodes that incorporates buffers to address the aforementioned problem. The outage probability and throughput computations are derived using closed-form formulas. The results obtained show that the enhanced relay selection technique gives lower values in outage probability and higher values in throughput when compared with the conventional technique, providing it a desirable strategy for use in technical communication.
The usage of network coding technology in the network can effectively improve the network performance in many aspects, such as delay, energy consumption, and throughput. Considering that multiple data streams can be encoded together, the network node forwards the data packet to reduce the transmission times of the encoded data packet in the wireless network. In particular, a measurement standard based on the number of pre-coding transmissions is proposed with the help of network coding and multiplexing technology. First, the opportunistic routing strategy between multiple data streams of network coding is applied to optimize the forwarding and transmission mode of data storage, which can effectively reduce the transmission of data packets. Second, the concept of coding is introduced into the relay nodes in the network, and the data distribution strategy is used to reduce the number of data packet transmissions in the network. Finally, a measurement standard for the number of pre-coding transmissions is designed for the resource allocation path in wireless multi-hop networks. The experimental results show that the strategy proposed in this paper can achieve a lower transmission delay compared with the single-path transmission strategies and the multi-path data distribution strategies without considering network coding.
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PhD Defense
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Buffer-aided relays can improve the diversity of multi-hop networks, however, they increase packet delays. Thus, various delay-aware protocols have been developed, but without considering the transmission diversity. Moreover, most works adopt ideal assumptions, such as symmetric links, perfect Channel State Information (CSI) and error-free feedback channels. So, we propose a Low-Complexity (LoCo) link selection algorithm, herein called LoCo - Link. The proposed algorithm may experience delays during CSI updates, and hence, by using outdated CSI its performance may deteriorate. To alleviate this issue, we next propose a distributed version of LoCo - Link (d - LoCo - Link) dealing with outdated CSI. In both algorithms, the source performs broadcasting towards multiple relays; when the packets are transmitted by a relay to the destination, they are discarded from all other relays. This coordination relies on feedback channels. For non error-free feedback channels, we propose a scheme in which the relays listen to the transmission of the best relay and drop duplicate packets. Results show that LoCo - Link surpasses other algorithms, by decreasing the delay in asymmetric networks. Moreover, d - LoCo - Link avoids diversity losses due to outdated CSI, while the effect of non error-free feedback channels is mitigated by taking advantage of the inter-relay channels.
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Applying data buffers at relay nodes significantly improves the outage performance in relay networks, but the performance gain is often at the price of long packet delays. In this paper, a novel relay selection scheme with significantly reduced packet delay is proposed. The outage probability and average packet delay of the proposed scheme under different channel scenarios are analyzed. Simulation results are also given to verify the analysis. The analytical and simulation results show that, compared with non-buffer-aided relay selection schemes, the proposed scheme has not only significant gain in outage performance but also similar average packet delay when the channel SNR is high enough, making it an attractive scheme in practice.
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This paper investigates the buffer-aided relay selection problem for a decode-and-forward cooperative wireless network with K relays. We propose a new relay selection scheme, that incorporates the status of the relay buffers and the instantaneous strength of the wireless links. Specifically, each link is assigned with a weight related to the buffer status, then the best relay is selected with the largest weight among all the qualified source-relay and relay-destination links. We derive the closed-form expression for the outage probability as well as the diversity gain by introducing several Markov chains to model the evolution of the buffer status. The analysis shows that the proposed scheme can achieve the optimal diversity gain, 2K, for a small L (i.e., L 3), an improvement in comparison with the existing max-link scheme that achieves the optimal diversity gain only when L is sufficiently large, where L denotes the buffer size of each relay. The provided theoretical and numerical results confirm the performance gain of the proposed relay selection scheme over the existing max-link scheme.
In this letter, we discuss multiple links with equal weights, in buffer size based relay selection schemes in cooperative wireless networks. A general relay selection factor is defined, which includes the weight of the link as the first metric and the link quality, or priority, as the second metric for different cases of the same weight. The Markov chain based theoretical framework is employed to evaluate the outage probability, delay and throughput of the system. The proposed scheme is evaluated for symmetric and asymmetric channel conditions. The link quality based second selection metric achieves lower outage probability, while the link priority based selection shows significant improvements in terms of delay and throughput. Theoretical results are validated through extensive Monte carlo simulations.
In this paper, we propose a novel buffer-state-based amplify-and-forward relaying protocol for dual-hop cooperative networks. The proposed protocol is designed to amalgamate the following three concepts: the exploitation of multiple source-to-relay broadcast channels, the introduction of thresholding at relay and destination nodes, and buffer-state-based relay selection. Our numerical results demonstrate that the proposed protocol is capable of attaining a lower end-to-end delay and a lower outage probability than previous amplify-and-forward relay selection schemes while simultaneously imposing no additional costs.
This paper investigates the secrecy outage performance of buffer-aided multirelay multiple-input multiple-output cooperative systems in the presence of a passive eavesdropper. Due to the unavailability of the channel state information of eavesdropper's channel, a buffer-aided joint transmit antenna and relay selection scheme based on the main channel is proposed to enhance the secrecy performance. Specifically, we model the evolution of the relay buffers as a Markov chain and derive new exact and asymptotic closed-form expressions for the secrecy outage probability, which provides an efficient way to assess the effect of system parameters on the secrecy outage probability. Moreover, simple asymptotic results are further exploited under two special scenarios, i.e., L → ∞ and L → ∞ (where L denotes the size of the relay buffers), for characterizing the achievable secrecy diversity gain, the secrecy coding gain, and the secrecy diversity-multiplexing tradeoff. Our results reveal that: 1) a secrecy diversity gain of N <sub xmlns:mml="" xmlns:xlink="">R</sub> Mmin(N <sub xmlns:mml="" xmlns:xlink="">S</sub> , N <sub xmlns:mml="" xmlns:xlink="">D</sub> ) is achieved when L → ∞, however, when L → ∞, the secrecy diversity gain increases to NRM(N <sub xmlns:mml="" xmlns:xlink="">S</sub> + N <sub xmlns:mml="" xmlns:xlink="">D</sub> ), where N <sub xmlns:mml="" xmlns:xlink="">S</sub> , N <sub xmlns:mml="" xmlns:xlink="">R</sub> , and ND represent the number of antennas at the source, each of M relays and the destination, respectively. 2) The eavesdropper's channel does not affect the secrecy diversity gain but only the secrecy coding gain in both the two scenarios.
Motivated by the recent buffer-aided relaying protocol that selects the best available link at each time slot, we herein introduce an additional degree of freedom to the protocol by simultaneously exploiting multiple links between the source node and the multiple buffer-aided relay nodes, which is enabled owing to the broadcast nature of wireless channels. More specifically, the proposed schemes are designed to allow multiple relay nodes to receive a source packet through source-to-relay broadcast channels, resulting in multiple copies of the source packet, which are stored in relay node buffers. As the explicit benefits of its increased design degree of freedom, the proposed protocols attain a significantly lower end-to-end packet delay than the conventional buffer-aided relaying protocols, which is achieved without imposing any substantial penalty on the achievable outage probability. Furthermore, the proposed protocol is capable of reducing the overhead required for monitoring the available links and buffer statuses of the relay nodes. Based on the Markov chain model, we derive the theoretical bounds of the outage probabilities of the proposed protocols.
Motivated by the recent max-link protocol and the max-max protocol, both of which were developed for a simple dual-hop buffer-aided cooperative network, we present a novel hybrid buffer-aided cooperative protocol that attains the benefits of high reliability and reduced packet delay. More specifically, we incorporate a two-stage relay selection strategy, which is capable of attaining a low outage probability comparable to the max-link protocol. In addition, the proposed protocol maintains a beneficial end-to-end packet delay, which is lower than that of the max-link protocol in the realistic scenario supporting finite source-packet transmissions. Furthermore, by introducing a periodic Markov chain model, we derive the theoretical outage probability of our hybrid buffer-aided scheme under the realistic assumption of finite-buffer relays. Our analytical and simulation results demonstrate that the proposed protocol benefits from the aforementioned high-diversity reliable performance and the reduced end-to-end packet delay.