N. Zhao et al.: Exploiting Interference for EH: A Survey, Research Issues, and Challenges
time slots, one for information transmission and the other for
power transfer. Thus, the received signal is fed back to the
ID and EH terminals periodically through a time switcher at
each receiver. 0 < α < 1 is the percentage of transmission
time in each frame, and 1 − α is the percentage of EH
time. Fig. 3 depicts a single-antenna case. When multiple
antennas are equipped at each receiver, the PS or TS method
can be similarly performed at each antenna, and the divided
ID signals and EH signals of these antennas at a specific
receiver are fed back to the ID and EH terminals using the
same coefficient ρ or α.
Comparing TS and PS modes, the TS mode is much
simpler to realize than the PS mode [7]. Nevertheless, it is
shown theoretically that the PS mode can achieve much
better tradeoffs between transmission rate and harvested
energy [7], [52]. Besides, accurate time synchronization
between the transmitter and receiver is required in the
TS mode, and the receiver should adjust its switching function
according to the time information of transmitter. Therefore,
in the practical systems, the PS mode is more suitable to be
utilized than the TS mode, and most of the research works are
based on the PS mode.
FIGURE 4. Logic map of the research on exploiting interference for
energy harvesting.
Interference is ubiquitous and adequate in the wireless
networks, including the interference between multi-users and
the dedicated interference like jamming signal and artificial
noise [45], [54]. Thus the interference can be exploited as
a perpetual energy source for wireless EH systems, and the
logic map of the topic in this paper, i.e., exploiting interfer-
ence for EH, is presented in Fig. 4. In the rest of this paper, we
will mainly concentrate on the aspect of interference-based
wireless EH systems.
III. EXPLOITING INTERFERENCE FOR
WIRELESS ENERGY HARVESTING
Interference is ubiquitous in wireless networks, and it used
to be deemed as a harmful factor that can affect the QoS of
the information transmission [2]. Many techniques have been
developed to mitigate the influence of interference on the
performance of wireless networks, e.g., orthogonal multiple
access, interference alignment [4], [5], interference cancel-
lation [55], dirty paper [56], etc. On the other hand, inter-
ference is also a kind of RF signal, which can carry energy
together with information. Thus the harmful interference can
be exploited as a helpful energy source for wireless energy
harvesting. A pictorial illustration of exploiting ambient inter-
ferences for wireless EH is shown in Fig. 5.
FIGURE 5. Pictorial illustration of exploiting ambient interferences for
wireless energy harvesting.
In the figure, it is shown that the signals transmitted
by other users will affect the transmission of a specific
transceiver as interferences, which can be harvested by the
receiver to support its operation and recharge the battery. This
is especially suitable in the scenarios of hyper-dense small-
cell networks, where plenty of users coexist in a limited area,
and the interferences from other users are very strong. One
key challenge to exploit interference for wireless EH is that
the interference should be perfectly eliminated to recover the
transmitted information and harvest energy. Besides, when
there exists an adversarial jammer that intends to disrupt the
legitimate transmission of the network, its transmit power is
usually very high, and the jamming signal can be properly
managed and harvested to support the legitimate receiver.
When the security of wireless networks is considered, arti-
ficial noise can be generated to disrupt the potential eaves-
droppers, without affecting the legitimate transmission. Thus
the AN can be re-utilized as energy source that is harvested
by the legitimate receivers.
In this section, the classifications and applications of wire-
less EH systems exploiting interference are demonstrated in
detail as in Table 2, including receiver architecture, antenna
dimension, network topology, and interference management.
A. RECEIVER ARCHITECTURE
There are mainly three kinds of receiver architectures
of exploiting interference for wireless EH [7], i.e.,
power splitting [25], [27]–[37], [40], [42], [45], [57]–[65],
time switching [38], [41], [43], [44], [65]–[70], and sep-
arated ID and EH [71]–[79], which are discussed as
follows.
10406 VOLUME 5, 2017