# Zero Energy IoT Devices in Smart Cities Using RF Energy Harvesting
## 2 Literature
### Simultaneous Wireless Information and Power Transfer (SWIPT)  
The first paper in this area was [13], as claimed by [14]. SWIPT uses a single transmission line for information as well as for energy harvesting. In SWIPT, information decoding (ID) and EH are performed on each node. We review and discuss some papers related to SWIPT. Ref. [14] presented an energy-aware routing algorithm for RF-based EH-WSN, which is based on the SWIPT technique. SWIPT uses a relay node to transmit data and power to nodes at the same time. The relay node decodes and forwards the data between nodes (see Figure 2). Some researchers adopt the amplify-and-forward technique instead of the decode-and-forward. The authors introduced the energy and information allocation problem, then they presented an energy-aware SWIPT routing algorithm called ESWIPTR. The protocol is improved to a distributed synchronous proactive version and an asynchronous proactive table-driven version, respectively. The basic routing algorithm is based on an energy equation called Ecost. ESWIPTR finds the minimum energy cost path for routing through a routing function. The author also presents a distributed version of ESWIPTR using the distributed Bellman Ford protocol. The author considered a WSN with sensor nodes, each equipped with a single antenna. The data flow between the source and destination nodes. Two transmission modes are used between any two (_i, j_) nodes, information transmission (IT) and simultaneous wireless information and power transfer (SWIPT). The IT is used when the battery of the sensor  
Figure 2: SWIPT-based relay architecture. is full, and SWIPT is used when the battery energy is less than the minimum energy requirements. The author addressed three problems in this work, routing, information, and energy allocation. In decode-and-forward (DF) protocols, the receiver node first decodes the information and then forwards it to the next node. Therefore, successful decoding is essential for DF protocols. The author presented a decode as follows:  
\[\gamma_{ij}^{SWIPT}=\frac{p_{ij}|h_{ij}|^{2}P_{ij}}{\sigma_{ij}^{2}-n_{ij}^{2} }\geq R_{min} \tag{1}\]  
where \(P_{ij}\) is the sender node's sending power, \(p_{ij}\) is the power splitting value, \(h_{ij}\) is the channel gain, \(\sigma_{ij}\) is the power of the signal, \(R_{min}\) is the required SNR requirement, and \(n_{ij}\) is the antenna noise. The forward equation is as follows:  
\[E_{ij}^{eh}=\epsilon\big{(}1-p_{ij}\big{)}\left(\big{|}h_{ij}\big{|}^{2}P_{ij} +\sigma_{ij}{}^{2}\right)\geq P_{cj} \tag{2}\]  
where \(P_{cj}\) is the energy harvesting power requirement for forwarding the information to the next hop node. \[P_{cj}=P_{ji},\text{ if }r_{ji}=1 \tag{3}\]  
\(P_{ji}\) is the power cast for forwarding to the neighbor/next node, and \(r_{ji}\) is the link state (active = 1, not active = 0). In routing, the main objective of the author was to find the minimum cost link. The author presented two types of routing algorithms, i.e., centralized and distributed. The routing algorithm is based on the concept that when a node has low energy, it selects the next node for routing. The presented routing algorithms, ESWIPTR, are based on Equations (1)-(3). A Dijkstra-based centralized routing algorithm was used in this work. The Dijkstra algorithm first checks the shortest path between the neighbor nodes and selects the node with the shortest or closest path to the destination. Just like the Dijkstra algorithm, the authors first examine the path with the minimum energy from the source to the destination node and then allocate resources. The minimum energy can be calculated using the Ecost equation. The authors also presented the distributed version of the ESWIPTR algorithm by using the distributed Bellman-Ford protocol. The algorithms are evaluated by convergence rate, the impact of the node density, minimum energy requirements for packet forwarding, and the impact of the barrier. The authors presented another paper [15] for interference-aware routing.