# Cross-Layer Energy Optimization for IoT Environments: Technical Advances and Opportunities
## 4 Energy Efficient Techniques in the IoT
### Energy Efficient Techniques at Network/Communication Layer
#### 4.3.3 Energy Efficient Communication Techniques  
An Energy Efficiency Communications Approach for Delay Minimizing in Internet of Things [48] optimizes end-to-end delay and network lifetime from a theoretical perspective and establishes the relationships between the minimum per-hop forwarding distance \(r_{0}\), the transmission range \(r\), the node density \(\rho\) and the duty cycle that affect the node energy and network delay as well as the network lifetime. [PERSON] et al. suggested a technique called Fast data collection for the node Farthest away from the sink and Slow data collection for the node Closest to the Sink (FFSC) that adopts a direct forwarding routing technique for nodes located at a large distance from the sink after determining the availability of forwarding nodes. In the region near the sink, which has higher energy constraints, data is only forwarded after the selection of the optimal relay node to minimize energy consumption and enhance network lifetime. FFSC saves energy of node and minimizes delay using a duty cycle technique. If the number of forwarding set nodes for a node is \(n\) and duty length is \(t\), then the duty cycle of every node is \(\frac{1}{t}\), and the expected delay of nodes is calculated as:  
\[D_{n}^{t}=\sum_{i=0}^{t-2}i\bigg{(}1-\frac{1}{t}\bigg{)}^{in}\bigg{[}1-\bigg{(} 1-\frac{1}{t}\bigg{)}^{n}\bigg{]}+(t-1)\bigg{(}1-\frac{1}{t}\bigg{)}^{(t-1)n} \tag{21}\]  
Total delay of the entire network can be formulated as:  
\[D^{r_{0}}=2\pi\int_{0}^{R}\bigg{[}\frac{X}{e_{X}^{r_{0}}}\bigg{]}\bigg{(} \sum_{i=0}^{t-2}i\bigg{(}1-\frac{1}{t}\bigg{)}^{in_{2}^{r_{0}}}\bigg{[}1- \bigg{(}1-\frac{1}{t}\bigg{)}^{n_{d}^{r_{0}}}\bigg{]}+(t-1)\bigg{(}1-\frac{1} {t}\bigg{)}^{(t-1)n_{d}^{r_{0}}}\bigg{)}Xd_{X} \tag{22}\]  
where \(d\) and \(X\) are the distance from source to sink, and the distance of particular node from the sink, respectively. \(e_{X}^{r_{0}}\) is expected reward of each hop. Energy consumption of a node \(X\) distance away from the sink is formulated as:  
\[E_{X}^{r_{0}}=P_{LPL}^{X}t+P_{R}^{X}\varnothing_{R}^{rx}+P_{T}^{X}\varnothing_{ T}^{rx} \tag{23}\]  
where \(P_{LPL}^{X}\), \(P_{R}^{X}\), \(P_{T}^{X}\) and \(\varnothing_{R}^{rx}\) are the power associated with low power listening operation of node, average power to transmit a packet, power used to receive a packet and amount of data loaded by node. Considering \(\varnothing_{T}^{rx}=\varnothing_{R}^{rx}+1\), Therefore total energy consumption in the entire network is expressed as:  
\[E^{r_{0}}=2\pi\int_{0}^{R}E_{X}^{r_{0}}Xd_{X} \tag{24}\]  
Bacterial Relay for Energy Efficient Molecular Communications (MCvB) [49] has been proposed to deliver information while considering energy efficiency using bacterial mobile relays for molecular communication in the Internet-of-Nano-Things. This technique first derives the probability of end-to-end delivery using a propagation model of information pickup and information delivery stages after introducing system parameters. This provides an energy model while deriving an energy efficiency expression considering all communication stages. These communication stages are: encoding, encapsulation, propagation, decapsulation and decoding (Figure 16). If \(E^{tot}\) is energy consumption for whole communication mechanism including these all stages as:  
\[E^{tot}=N\times E^{p}+M\times E^{b}+K\times E^{m}\times T+J\times\Big{(}E^{ d}+E^{e}\Big{)} \tag{25}\]  
where \(N\times E^{p}\) is the total energy consumption in encoding stages, having \(N\) plasmids, \(M\times E^{b}\) is energy consumption in encapsulation stage having \(M\) plasmids, \(K\times E^{m}\times T\) energy consumption in propagation stages and \(J\times\Big{(}E^{d}+E^{e}\Big{)}\) is the energy consumption in the decapsulation and decoding stage. This technique has a drawback in the case of delay tolerant and short distance communication.