The IoT network consists of the source node,
K AF-SWIPT relays, and the destination node, are shown in
Figure 1. Each node has a single antenna. The source is a base station (BS) which may consist of the data gateway and the external/central systems of the IoT network [
11,
12,
13,
15,
16,
17]. In order to increase the operation efficiency of the relay node, each relay node operates in SWIPT mode. A RF signal received at each relay is split using the PS ratio where a part of the signal is stored in battery in each relay via energy harvesting (EH). Then, the information processing (i.e., amplification and forwarding) is performed using the remaining RF signals, and then the transmission is performed to the next node with the harvested energy.
We assume that the source node provides the channel state information (CSI) for all communication nodes. However, each relay and the destination node only knows the CSI for their communication channel. It is assumed that there is no direct link between the current node and the next second or more nodes. For example, there is no direct link between the first node and the third node.
The received RF signal at node
from the previous node is given as
where
is the channel coefficient between the current node and the previous node, and
∼
(0,
) represents the antenna noise at the current node. The channel
is defined as
, where
is the large-scale fading coefficient,
is the attenuation constant for a distance
,
is the pathloss exponent,
is distance between the transmit and receive nodes and
∼
(0,1) is the Rayleigh fading component. Next, the signal for EH and harvested energy at the
kth relay node are written, respectively, as
where
is the processing power for information decoding at destination node,
is the transmit energy at source node, and signal for information transmission at the
kth relay is represented by
where
is the additional noise introduced by the information decoding (ID) circuitry. The signal-to-noise ratio (SNR) is expressed as
The signal to be transmitted to the
kth relay is represented by
where
is amplification factor, and
is the information signal at source node.
We now consider the problem of optimizing the PS ratio
at the relay nodes and energy at the source node
. We aim to minimize the source transmit power, under the SNR of destination node constraint and PS ratio for each of the multihop links as