Optimization of Packet Length for Two Way Relaying with Energy Harvesting

Optimization of Packet Length for Two Way Relaying
with Energy Harvesting
Ghassan Alnwaimi *,Hatem Boujemaa **, Kamran Arshad ***
(*) King Abdulaziz University, Kingdom of Saudi Arabia
(**) University of Carthage, Sup’Com, COSIM Laboratory, Tunisia
(***) College of Engineering, Ajman University
galnwaimi@kau.edu.sa,boujemaa.hatem@supcom.tn, k.arshad@ajman.ac.ae
May 16, 2019
Abstract
In this article, we suggest optimizing packet length for two way relaying with
energy harvesting. In the first transmission phase, two source nodes N1 and N2 are
transmitting data to each others through a selected relay R. In the second phase,
the selected relay will amplify the sum of the signals received signals from N1 and
N2. The selected relay amplifies the received signals using the harvested energy
from Radio Frequency (RF) signals transmitted by nodes N1 and N2. Finally, N1
will remove, from the relay’s signal, its own signal to be able to decode the symbol
of N2. Similarly, N2 will remove, from the relay’s signal, its own signal to be able to
decode the symbol of N1. We derive the outage probability, packet error probability
and throughput at N1 and N2. We also optimize packet length to maximize the
throughput at N1 or N2.
Index Terms : Cooperative systems, Optimal packet length, Rayleigh fading channels.
1 Introduction
In Two-Way Relaying (TWR), two nodes N1 and N2 simultaneously transmit data to
each other using a selected relay [1-5]. The communication process contains two phases.
In the first one, N1 and N2 transmit data to some relays. Each relay will receive the sum
of signals transmitted by N1 and N2. In the second phase, a selected relay amplifies the
received signal. Then, N1 will remove, from the relay’s signal, its own signal to be able
to decode the symbol of N2. Similarly, N2 will remove, from the relay’s signal, its own
signal to be able to decode the symbol of N1.
Two way relaying for Multiple Input Multiple Output (MIMO) systems has been con-
sidered in [1-5]. Receive and transmit diversity improves the performance of TWR. At the
receiver, the best antenna can be selected (Selection Combining SC). The corresponding
International Journal of Computer Networks & Communications (IJCNC) Vol.11, No.3, May 2019
DOI: 10.5121/ijcnc.2019.11307 97

Signal to Noise Ratio (SNR) is the maximum of SNRs over all antennas. It is also possi-
ble to combine the signals of all antennas using Maximum Ratio Combining (MRC). The
SNR will be the sum of all SNRs [1-5]. TWR with Energy Harvesting (EH) consists to use
the Radio Frequency (RF) signal to charge the battery of nodes [6-10]. Relays with EH
capabilities has been studied in [6-10]. In order to enhance the throughput especially at
low SNRs, channel coding is required in TWR [11-13]. Secure two way relaying has been
suggested in [14-20]. Security aspects of TWR should be studied to avoid data recovery
by a malicious node.
The main contribution of the paper is to optimize packet length so that the throughput
at node N1 or N2 is maximized. In all previous studies, a Fixed Packet Length (FPL) is
used [1-20]. This is the ﬁrst paper to suggest an Optimal Packet Length (OPL) for TWR
with Energy Harvesting.
The system model is presented in section 2. Section 3 gives the Cumulative Dis-
tribution Function (CDF) of SNR. Section 4 derives the PEP while section 5 gives the
expression of OPL. Some numerical results are given in section 6. Conclusions are pre-
sented in section 7.
2 System model
The system model is shown in Fig. 1. There are two nodes N1 and N2 communicating
information to each other through a relay R. Node N1 transmits data to node N2 and at
the same time node N2 is also communicating data to N1 through relay R. N1 and N2
transmit over the same channel.
98

N1
N2
R
Multi Access Phase
Broadcast Phase
Energy Harvesting Multi Access Phase Broadcast Phase
αT (1-α)T/2 (1-α)T/2
Figure 1: Two way relaying with Energy harvesting.
The frame with duration T is decomposed in three parts :
- The ﬁrst slot with duration αT is dedicated to energy harvesting. Relay R harvests
energy from RF signal transmitted by nodes N1 and N2.
The harvested energy is written as
E = β(P1|hN1R|2
+ P2|hN2R|2
)αT = β(E1 |hN1R|2
+ E2|hN2R|2
)αp, (1)
where 0< α < 1 is harvesting duration percentage, Pi (resp. Ei) is the transmit power
(resp. symbol energy) of node Ni and hN1R (respectively hN2R) is channel coeﬃcient
between nodes N1 (respectively N2) and R. p = T/Ts is the number of symbols per frame
T. We have EX = TsPX
- During the second time slot with duration (1 − α)T/2, N1 and N2 transmit data to
node R over the same channel. This is the multiple access phase. The received signal at
R is written as
yR(j) =
√
E1x1(j)hN1R +
√
E2x2(j)hN2R + nR(j) (2)
where Ei is the transmitted energy per symbol of node i with 1≤ i ≤ 2, xi(j) is the j-th
transmitted symbol by node Ni and nR(j) is an Addivite White Gaussian Noise (AWGN)
99

with variance N0. A Rayleigh block fading channel is assumed where the channel remains
constant over all the time frame with duration T.
- During the third time slot with duration (1 − α)T/2, R transmits amplifies the
received signal to nodes N1 and N2. This is the broadcast phase.
Relay R uses the harvested energy E to amplify the received signal yR(j) to N1 and
N2. The transmit symbol energy of R is equal to the harvested energy E devided by the
number of transmitted symbols during (1−α)T/2 seconds i.e. (1−α)T/(2Ts) = (1−α)p/2
with p = Ts/T:
ER =
E
(1 − α)p/2
=
β(E1|hN1R|2
+ E2|hN2R|2
)αp
(1 − α)p/2
= 2
αβ
(1 − α)
(E1|hN1R|2
+ E2|hN2R|2
) (3)
Using (43), the amplification factor G used by relay R is written as
G =
√
ER
E1|hN1R|2 + E2|hN2R|2 + N0
(4)
2.1 SNR at node N1
The received signal at N1 is written as
y1(j) = GhRN1 yR(j) + n1(j), (5)
where n1(j) is an AWGN with variance N0.
Using (43), we deduce
y1(j) = GhRN1 [
√
E1x1(j)hN1R +
√
E2x2(j)hN2R + nR(j)] + n1(j),
=
√
E1GhRN1 x1(j)hN1R +
√
E2GhRN1 x2(j)hN2R
+GhRN1 nR(j) + n1(j). (6)
Node N1 removes the self interference,
√
E1GhRN1 x1(j)hN1R, since it knows the value
of symbol x1(j). After removing self interference, we obtain
y1(j) =
√
E2GhRN1 x2(j)hN2R + GhRN1 nR(j) + n1(j). (7)
The SNR at N1 is written as
Γ1 =
E2G2
|hRN1 |2
|hN2R|2
N0 + N0G2|hRN1 |2
. (8)
Using the expression of amplification factor G (45), we deduce
Γ1 =
E2|hRN1 |2
|hN2R|2
N0
G2 + N0|hRN1 |2
=
|hRN1 |2
E2|hN2R|2
N0
ER
[N0 + E1|hN1R|2 + E2|hN2R|2] + N0|hRN1 |2
. (9)
100

We assume that channels are reciprocal i.e. hN1R = hRN1 . By neglecting the term in
N2
0 and using (44), the SNR at node N1 lower bounded by
Γ1 >
2 αβ
N0(1−α)
E2|hRN1 |2
|hN2R|2
1 + 2 αβ
(1−α)
|hRN1 |2
(10)
This upper bound is tight at high average SNR as the term N2
0 can be neglected.
We can write
Γ1 >
a1X1X2
1 + a2X1
, (11)
where
a1 = 2
αβ
N0(1 − α)
E2, (12)
a2 = 2
αβ
(1 − α)
, (13)
X1 = |hRN1 |2
, (14)
and
X2 = |hN2R|2
. (15)
2.2 SNR at node N2
The received signal at N2 is written as
y2(j) = GhRN2 yR(j) + n2(j), (16)
where n2(j) is an AWGN with variance N0.
Using (43), we deduce
y2(j) = GhRN2 [
√
E1x1(j)hN1R +
√
E2x2(j)hN2R + nR(j)] + n2(j),
=
√
E1GhRN2 x1(j)hN1R +
√
E2GhRN2 x2(j)hN2R
+GhRN2 nR(j) + n2(j). (17)
Node N2 removes the self interference,
√
E2GhRN2 x2(j)hN2R, since it knows the value
of symbol x2(j). After removing self interference, we obtain
y2(j) =
√
E1GhRN2 x1(j)hN1R + GhRN2 nR(j) + n2(j). (18)
The SNR at N2 is written as
Γ2 =
E1G2
|hRN2 |2
|hN1R|2
N0 + N0G2|hRN2 |2
. (19)
101

Using the expression of amplification factor G (45), we deduce
Γ2 =
E1|hRN2 |2
|hN1R|2
N0
G2 + N0|hRN2 |2
=
|hRN2 |2
E1|hN1R|2
N0
ER
[N0 + E1|hN1R|2 + E2|hN2R|2] + N0|hRN2 |2
. (20)
We assume that channels are reciprocal i.e. hN2R = hRN2 . By neglecting the term in
N2
0 and using (44), the SNR at node N1 lower bounded by
Γ2 > Γlow
2 =
2 αβ
N0(1−α)
E1|hRN2 |2
|hN1R|2
1 + 2 αβ
(1−α)
|hRN2 |2
(21)
This upper bound is tight at high average SNR as the term N2
0 can be neglected.
We can write
Γlow
2 =
a1X1X2
1 + a2X2
, (22)
where
a1 = 2
αβ
N0(1 − α)
E2 (23)
a2 = 2
αβ
(1 − α)
(24)
X1 = |hRN1 |2
(25)
and
X2 = |hN2R|2
(26)
2.3 Two way relaying in the presence of multiple relays
Fig. 2 shows the principle of TWR in the presence of K relays. The selected relay offers
the largest SNR at node N1 or N2. When the selected relay maximizes the SNR at node
N1, the CDF of SNR is the products of CDF of SNRs of different relays
FΓ1 (x) =
K∏
k=1
FΓk
1
(x) (27)
where Γk
1 is the SNR at node N1 when relay Rk is the active relay. Γk
1 is given in (9).
102

N1
N2
R1
Multi Access Phase
Broadcast Phase
Energy Harvesting Multi Access Phase Broadcast Phase
αT (1-α)T/2 (1-α)T/2
R2
RK
Selected relay
Figure 2: Two way relaying with Energy harvesting in the presence of K relays.
3 CDF of SNR
3.1 CDF of SNR at node N1
The SNR at node N1 is lower bounded by
Γ1 > Γlow
1 =
a1X1X2
1 + a2X1
. (28)
The CDF of SNR is upper bounded by
FΓ1 (x) < FΓlow
1
(x) = P(Γlow
1 ≤ x). (29)
We have
P(Γlow
1 ≤ x) =
∫ +∞
0
[1 − P(Γlow
1 > x|X1 = u)]fX1 (u)du
=
∫ +∞
0
[1 − P(
a1uX2
1 + a2u
> x)]fX1 (u)du (30)
103

where fX1 (u) is the Probability Density Function (PDF) of X1.
For Rayliegh fading channels, X1 is exponentially distributed with mean
1
λ1
= E(X1) = E(|hRN1 |2
). (31)
We deduce
P(Γlow
1 ≤ x) =
∫ +∞
0
[1 − P(X2 >
x(1 + a2u)
a1u
)]λ1e−uλ1
du (32)
=
∫ +∞
0
[1 − e
−
x(1+a2u)λ2
a1u
]λ1e−uλ1
du (33)
We use the following result grad
∫ +∞
0
e− b
4v
−av
dv =
√
b
a
K1(ab), (34)
where K1(x) is the modified Bessel function of first order and second kind.
We finally obtain
FΓ1 (x) < P(Γlow
1 ≤ x) = 1 − 2λ1e
−
a2xλ2
a1
√
λ2
x
a1λ1
K1(2
√
λ1
xλ2
a1
). (35)
3.2 CDF of SNR at node N2
The SNR at node N2 is lower bounded by
Γ2 > Γlow
2 =
a1X1X2
1 + a2X2
. (36)
The CDF of SNR is upper bounded by
FΓ2 (x) < FΓlow
2
(x) = P(Γlow
2 ≤ x). (37)
We have
P(Γlow
2 ≤ x) =
∫ +∞
0
[1 − P(Γlow
2 > x|X2 = u)]fX2 (u)du
=
∫ +∞
0
[1 − P(
a1X1u
1 + a2u
> x)]fX2 (u)du (38)
We deduce
P(Γlow
2 ≤ x) =
∫ +∞
0
[1 − P(X1 >
x(1 + a2u)
a1u
)]λ2e−uλ2
du
=
∫ +∞
0
[
1 − e
−λ1
x(1+a2u)
a1u
]
λ2e−uλ2
du (39)
104

We use (34), to deduce
FΓ2 (x) < P(Γlow
2 ≤ x) = 1 − 2λ2e
−
a2xλ1
a1
√
λ1
x
a1λ2
K1(2
√
λ1
xλ2
a1
). (40)
4 PEP
In this section, we derive the expression of the average Packet Error Probability (PEP).
The PEP can be tightly upper bounded by [21]
PEP ≤
∫ w0
0
fΓ(γ)dγ (41)
where fΓ(γ) is the Probability Density Function (PDF) of SNR Γ and w0 is a waterfall
threshold.
Equation (13) shows that the PEP for a given instantaneous SNR, γ ≤ w0, can be
approximated to 1. However, the PEP for a given instantaneous SNR, γ > w0 can be
approximated to 0 [21].
Hence,
PEP ≤ FΓ(w0), (42)
where FΓ(x) is the Cumulative Distribution Function (CDF) of the received SNR. We
denote Γ = Eb
N0
as the average SNR, where Eb is the transmitted energy per bit, N0 is the
noise Power Spectral Density (PSD) and w0 is a waterfall threshold written as [21],
w0 =
∫ +∞
0
g(γ)dγ (43)
g(γ) is the PEP for a given instantaneous SNR, γ = Γ|h|2
and h is the channel
coeﬃcient.
4.1 PEP for uncoded transmission
For uncoded M-QAM modulation, we have
g(γ) = 1 − (1 − Pes(γ))
N+nd
log2(M)
, (44)
where N is the number of useful information bits per packet, nd is the number of
parity bits per packet and Pes is the Symbol Error Probability (SEP) given as [22]
Pes(γ) ≃ 2(1 −
1
√
M
)erfc
(√
log2(M)3γ
(M − 1)2
)
. (45)
erfc(x) is the complementary error function,
erfc(x) ≤ e−x2
(46)
Using (45) and (46), the SEP is approximated by
105

Pes ≃ a1e−c1γ
(47)
where,
a1 = 2
(
1 −
1
√
M
)
, (48)
c1 =
3 log2(M)
2(M − 1)
(49)
4.2 PEP with Channel Coding
If a convolutional encoding is used, g(γ) can expressed as,
g(γ) = 1 − (1 − PE(γ))
N+nd
log2(M)
, (50)
where
PE(γ) ≤
+∞∑
d=df
adPd(γ) (51)
df and ad are respectively the free distance and the number of trellis with Hamming
weight d. Further,
Pd(γ) ≃ 2
(
1 −
1
√
M
)
erfc
(√
3Rcdγ log2(M)
2(M − 1)
)
. (52)
where Rc is the rate of convolutional encoding.
Using the approximation in (46) and keeping only the ﬁrst term of (23), we have
PE(γ) ≃ a2e−c2γ
(53)
where
a2 = adf
2
(
1 −
1
√
M
)
, (54)
c2 =
3Rcdf log2(M)
2(M − 1)
. (55)
Hence, we can generalize g(γ) as follow,
g(γ) ≃ 1 − (1 − aie−ciγ
)
N+nd
log2(M)
, (56)
where i = 1 in the absence of any channel coding and i = 2 for convolutional coding.
106

4.3 Waterfall Threshold
Using (43), the waterfall threshold is given by
w0 ≃ k1 ln
(
N + nd
log2(M)
)
+ k2 (57)
where the Proof is provided in Appendix A.
k1 =
1
ci
, (58)
k2 =
E + ln(ai)
ci
, (59)
E ≃ 0.577 is the Euler constant.
5 Optimal Packet Length for TWR
The average number of attempts of HARQ protocols is equal to
Tr =
+∞∑
i=1
PEPi−1
(1 − PEP) =
1
1 − PEP
(60)
Therefore, the throughput in bit/s/Hz is expressed as
Thr =
log2(M)N
(N + nd)TsBTr
=
log2(M)N
(N + nd)
(1 − PEP)
≥
log2(M)N
(N + nd)
[1 − FΓ (w0)] (61)
where B = 1/Ts is the used bandwidth and Ts is the symbol period.
The optimal packet length maximizing the throughput can be obtained using the
Gradient algorithm.
N(i + 1) = N(i) + µ
∂Thr(N = N(i))
∂N
(62)
We can write
∂Thr
∂N
=
log2(M)nd
(N + nd)2
[1 − FΓ (w0)] −
log2(M)N
(N + nd)
fΓ (w0)
k1
N + nd
(63)
OPL can be applied to maximize the throughput at node N1 or N2.
107

6 Theoretical and simulation results
Simulation results were obtained using MATLAB as a simulation environment.
Simulation results were performed by measuring the Packet Error Rate (PER) to
deduce the throughput. The packet error rate is the number of erroneous packets/number
of transmitted packets. We made simulation until 1000 packets are erroneously received.
Fig. 3 and 4 show the throughput at node N1 for α = 1/3, a QPSK modulation for
average SNR 10 and 20 dB. The distance between all nodes is equal to 1. We notice that
we can maximize the throughput by choosing the packet length. Also, the throughput
increases as the number of relays increase due to cooperative diversity. In fact, we always
select the relay with the largest SNR. Finally, by comparing Fig. 3 and 4, we observe that
packet length should be increased as the average SNR increases. There is good accordance
between theoretical and simulation results.
0 100 200 300 400 500 600 700 800 900 1000
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Packet Length N
Throughputinbit/s/Hz
Two−Way Relaying with Energy Harvesting, QPSK, SNR=10 dB
Theory : 1 Relay
Sim : 1 Relay
Theory : 2 Relays
Sim : 2 Relays
Theory : 2 Relays
Sim : 3 Relays
Theory : 4 Relays
Sim : 4 Relays
Figure 3: Throughput at node N1 versus packet length at SNR=10 dB : 64 QAM modu-
altion.
108

0 100 200 300 400 500 600 700 800 900 1000
0
1
2
3
4
5
6
Packet Length N
Two−Way Relaying with Energy Harvesting, 64 QAM, SNR=20 dB
Theory : 1 Relay
Sim : 1 Relay
Theory : 2 Relays
Sim : 2 Relays
Theory : 2 Relays
Sim : 3 Relays
Theory : 4 Relays
Sim : 4 Relays
Figure 4: Throughput at node N1 versus packet length at SNR=20 dB: 64 QAM modu-
altion.
Fig. 5 shows that OPL oﬀers higher throughput than Fixed Packet Length (FPL)
as studied in [1-20]. These results correspond to throughput of N1 for α = 1/3. They
were obtained using MATLAB for a 64 QAM modulation. In fact, the proposed optimal
packet length allows maximizing the throughput. If the SNR is low, the packet length is
decreased. However, at high SNR, we can increase packet length.
109

0 5 10 15 20 25 30
0
1
2
3
4
5
6
Eb
/N0
(dB)
Two−Way Relaying Energy Harvesting, 64 QAM, 3 Relays
Theory Proposed Optimal Packet Length
Sim :Proposed Optimal Packet Length
Theory : N=230
Sim : N=230
Theory : N=110
Sim : N=110
Figure 5: Throughput at node N1 for OPL and FPL :64 QAM modualtion.
Fig. 6 shows the OPL for QPSK, 16 QAM and 64 QAM modulation. We observe that
packet length should be increases when we use a small modulation such as QPSK. When
64 QAM modulation is used, packet length should be reduced since the PEP is high.
Also packet length should be increased (respectively decreased) at high (respectively low)
SNR.
110

0 5 10 15 20 25 30
100
200
300
400
500
600
700
800
900
1000
Eb
/N0
(dB)
OptimalPacketLengthN Two−Way Relaying with Energy Harvesting, 3 Relays
64 QAM
16 QAM
QPSK
Figure 6: Optimal packet length versus SNR.
7 Conclusion
In this paper, we suggested enhancing the throughput of Two Way Relaying (TWR) with
energy harvesting. We derive the best packet length that yields the largest throughput at
node N1 or N2. Our study is valid for energy harvesting systems where the relay harvest
energy from RF signals transmitted by source nodes N1 and N2. We have shown that the
proposed TWR with best packet length oﬀers better throughput than previous studies.
Also, the throughput can be enhanced by increasing the number of relays. The proposed
optimal packet length can be used in Wireless Sensor Networks (WSN) with two way
relaying.
111

Appendix A : We have
w0 =
∫ +∞
0
[1 − (1 − aie−ciu
)
N+nd
log2(M)
]du (64)
We deduce
w0 =
1
ci
∫ ai
0
[1 − (1 − y)
N+nd
log2(M)
]
dy
y
(65)
Therefore, we have
w0 =
1
ci
∫ 1
1−ai
1
1 − x
[1 − x
N+nd
log2(M)
]dx (66)
We obtain
w0 =
1
ci
∫ 1
1−ai
N+nd
log2(M)
−1
∑
k=0
xk
dx. (67)
We deduce
w0 =
1
ci
N+nd
log2(M)
∑
k=1
(
1
k
−
(1 − ai)k
k
). (68)
For N+nd
log2(M)
>> 1, we can write
N+nd
log2(M)
∑
k=1
1
k
= ln(
N + nd
log2(M)
) + E, (69)
and
N+nd
log2(M)
∑
k=1
(1 − ai)k
k
≃
+∞∑
k=1
(1 − ai)k
k
= −ln(ai). (70)
Combining (68), (69) and (70), we obtain (57).
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