China Communications • November 2015
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channels instead of viewing the ‘overall outage’
of the entire system as considered in [22-24] and
this work.
In this paper, we consider 3TS-TWR AF
protocol and investigate the interesting topic
of multi-hop D2D communication with the
cooperative strategies. To the best of our
knowledge, there has been no investigation made
on the D2D communication aided by a relay
node utilizing 3TS-TWR AF relaying protocol.
In this scenario, the two D2D source nodes
transmit signals to the relay node in succession
utilizing two time-slots, the relay node combine
the received signals, then amplies and forwards
the processed signal to the two source nodes
in the third time-slot. Due to the aggressive
spectrum reuse, each D2D receiver is hindered
by interference from the cellular link. Therefore,
this work considers the effect of CCI at all the
three nodes Similar to the scenario in [25, 26].
The first issue studied in this paper is that we
investigate the overall outage for the 3TS-TWR
AF protocol D2D communication underlaying
a cellular infrastructure and derive the closed
form expression of overall outage. Computer
simulations demonstrate that the derived
expressions are accurate and can be used to
evaluate the system performance. The second
issue we study is how to obtain the optimum
power allocation coefficient that is used to
combine the received signals at the relay. This
issue is very important particularly for the 3TS-
TWR AF protocol, because the relay should
optimally combine the two received signals in
order to optimize the system performance.
The remainder of the paper is organized as
follows. In Section II, we present system model
of 3TS-TWR AF protocol D2D communication
underlaying a cellular network. In Section III,
we analyze the overall outage probability for
3TS-TWR AF protocol D2D communication. In
Section IV, we develop the power optimization
method of the proposed system. In Section
V, simulation results are given to validate our
analysis. Finally, we conclude the paper in
Section VI.
II. SYSTEM MODEL
We consider a D2D communication employing
3TS-TWR AF protocol aided by another D2D
user. As illustrated in Fig.1, the isolated cell
scenario includes a cellular user (CUE), a pair
of D2D user and an assistant D2D user (i.e., D_
A, D_B and D_R) over flat-fading Rayleigh
channel. The channels are assumed reciprocal,
invariant in three consecutive equal time slots.
All terminals are equipped with a single antenna
and operate in a half-duplex mode. By sharing
the same radio resources with cellular users, D_
A and D_B exchange the information via D_R
utilizing 3TS-TWR AF protocol. In the uplink
resource sharing mode, the power level required
for the D2D link is much lower than that of
CUE, thus the interference from D2D to BS is
negligible and the interference from CUE to D2D
becomes crucial. Therefore our attention is paid
to suppressing the intra-cell interference from the
CUE.
As shown in Fig.1, the exchange of
information via a two-way AF relaying takes
place in three time-slots. During the first time
slot of D2D communication, D_A transmits unit-
energy message symbols x
A
to the relay. At the
same time CUE transmits unit-energy messages
symbols x
C
to the BS. Thus the received signal
at the relay corrupted by co-channel interference
from CUE and noise can be written as
1Ar A Ar A C Cr C r
y Ph x Ph x n=++
(1)
Where P
A
and P
C
a
re the transmit powers of
terminals D_A and CUE respectively, n
r1
Fig.1
D2D communication aided by 3TS-TWR AF protocol
1 time-slot
interference
BS
D_R
D_A
D_B
2 time
-
slot
3 time-slot
3 time
-
slot
CUE