# Interference-Aware Opportunistic Dynamic Energy Saving Mechanism for Wi-Fi Enabled IoTs
## 5 Performance Evaluation  
In dense networks, the energy consumption is an important criterion for designing and evaluating portable devices due to its impact on battery life. For evaluating the energy consumption of a wireless device, a typical energy consumption metric reflects the energy consumption measured in units of energy per bit of data transferred. In this paper, the energy efficiency for a wireless transceiver is expressed as a closed form function of transmission time, power consumption for each power state, and frame length. In Figure 5, there are four power states: TX, RX, Listen, and Doze. The TX, RX, and Listen states are parts of the active mode; the Doze state is the doze mode. Table 1 shows current consumption simulation parameters for different power satates and bandwidth modes [13]. The proposed dynamic energy saving mechanism was evaluated using a computer simulation with this power model and parameters under the IEEE 802.11 reference channel model [14]. When a wireless device needs to exchange data, the device enters the awake mode. In the awake mode, the device may waste a significant amount of power, even during an idle listening period, if the device does not use power saving techniques. Therefore, wireless consumer electronic devices need efficient power-saving techniques during their awake state. In a wireless receiver, there is a carrier sensing period for idle listening and a non-carrier sensing period for data processing. The energy consumption during the period (\(t_{0}\), \(t_{i}+1\)) can be described as Equation (1), where \(E\) is energy in Joules, and \(V\), \(I\), and \(t\) are the voltage, amperes, and time, respectively. \[E_{i+1}=E_{i}+V\times I_{i}\times(t_{i}+1-t_{i}). \tag{1}\]  
Hence, the average energy per bit is calculated using Equation (2), where \(E_{T}\) is the total energy consumed during time interval \(T\), and \(B_{T}\) is the total number of successfully transmitted bits during time interval \(T\). \[E=E_{T}/B_{T}. \tag{2}\]  
Figure 4: Dynamic energy saving operation; AIFS: arbitration interframe space. Thus, the energy consumption for 1 bit during 1 second can be described as the Equaltion (3). \[E[J/b/s]=(\sum_{i}T_{i}\cdot P_{i})/L_{data}/T_{total}. \tag{3}\]  
Figure 6 shows the simulation results for the power consumption of a WLAN system when the proposed dynamic energy saving mechanism and conventional dynamic bandwidth operation are applied. DES (dynamic energy saving) and DB (dynamic bandwidth) refer to the dynamic energy saving mechanism and the conventional dynamic bandwidth operation, respectively. The proposed DES scheme was compared with the conventional DB scheme for a 20/40/80 MHz signal bandwidth from MCS0 to MCS9 (MCS: modulation and coding scheme). For the settings of this simulation, we assumed that the tested station can adapt the channel bandwidth by utilizing the DB operation for the dynamically changed channel interference conditions. As a result, the DES achieves significantly improved energy efficiency in a narrower channel bandwidth transmission compared with the DB scheme. Figure 7 shows energy consumption over various frame lengths for a 20 MHz channel bandwidth. As a result, a lower frame length consumed more energy due to the increased overhead ratio, such as the inter-frame space, preamble, header, backoff, and control frames. In shorter frame length conditions, a lower rate increased the gap of energy per bit consumption between DES and DB because of the energy saving gain. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{4}{c}{**Current Consumption (Voltage = 1.1 V).**} \\ \hline
**Power State** & **20 MHz BW** & **40 MHz BW** & **80 MHz BW** \\ \hline PHY TX (mA) & 280 & 285 & 290 \\ PHY RX (mA) & 100 & 110 & 130 \\ Listen (mA) & 50 & 55 & 60 \\ Sleep (mA) & 0.003 & 0.003 & 0.003 \\ MAC TX (mA) & 40 & 60 & 80 \\ MAC RX (mA) & 40 & 60 & 80 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Current consumption for different power states and bandwidth modes.