# A System-Level Methodology for the Design of Reliable Low-Power Wireless Sensor Networks
## 6 Resulting Design Methodology and Case Studies
### Case Studies
#### 6.2.1 99% Reliability Target  
Like in [7], the reliability in this work is directly linked to the packet success probability, \(P\). Therefore, in order to evaluate the data transfer reliability of a point-to-point link, an application's requirement of 0.99 in terms of packet success probability is assumed, which corresponds to a 99% reliability figure of merit. This level of reliability is required by applications, such as utility-to-consumer real time pricing, outage management, and automated feeder switching in a smart grid [5; 66]. Also, a forward and feedback channel bit error probability, \(P_{e}=10^{-3}\), is assumed.  
Figure 22 Shows that the requirement is met by the studied FEC code. When using BR, only two retransmissions are needed (\(R=3\)). However, the 2-Rep-ACK approach, which is also affected by the imperfect feedback channel, requires three retransmissions (\(R=4\)), as two retransmissions (\(R=3\)) are not enough. In fact, for a feedback channel having the same packet error probability as the forward's (\(L=1\)), a large number of 1-Rep-ACK transmissions (i.e., \(R>20\)) is required, as shown in Figure 23. As such, for identical forward and feedback error probabilities, it is appropriate to consider \(L>1\) for the ACK transmissions.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Parameter** & **Value** \\ \hline \multirow{3}{*}{\
\begin{tabular}{c} **Inputs (based on data** \\ **sheet information,** \\ **measurements,** \\ **and application** \\ **requirements)** \\ \end{tabular} } & \(N_{A}\) & \(-\)154.27 dBm/Hz \\  & \(N_{fg}\)(CC1310) [58] & 7 dB \\  & \(B\bar{E}\bar{R}\) (requirement) & 0.1\% \\  & \(P_{t}\) & \(-\)10 dBm \\  & \(d\) (requirement) & 95 m \\  & \(\frac{E_{t}}{N_{opt}}\) (at 0.1\% \(B\bar{E}\)) [59; 60] & 10.3 dB \\  & \(PL\) (This model at 95 m) & 80.5 dB \\ \hline \multirow{3}{*}{
\begin{tabular}{c} **Results** \\ **end** } & \(P_{r}\) (Equation (14)) & \(-\)90.5 dBm \\  & \(SINR\) (Equation (15)) & 56.77 dBm \\  & \(D_{R}\) (Equation (16)) & 44.668 kbps \\ \hline \hline \end{tabular}
\end{table}
Table 3: Example explaining how to estimate the achievable data rate.  
The overall energy consumption per measurement \(E_{sys}\) is given by  
\[E_{sys}=E_{TRX}+E_{MCU}+E_{SENSOR}. \tag{32}\]  
Moreover, the latency of the wireless link \(t_{sys}\) can be calculated using  
\[t_{sys}=t_{TRX}+t_{PROCESS}+t_{SENSOR}. \tag{33}\]  
The energy per measurement results presented in Figure 24a were obtained using Equation (32). It is assumed that the sensor's and MCU's energies are not changed from one data transfer scheme to another. Moreover, the results in Figure 24b were obtained using Equations (33). Both figures show that when the required reliability is around 99%, using FEC is the least expensive choice, in terms of energy consumption and latency.  
Therefore, by way of example, without applying this methodology, a designer could have used the regular SAW-ARQ and not met the required reliability. Moreover, if BR were arbitrarily used (R = 3) to ensure the 99% reliability requirement, the energy consumption and latency would increase by around 75% and 32%, respectively, when compared with opting for FEC.  
Figure 23: Meeting 99% reliability target by using FEC, BR, and 1-Rep-ACK retransmissions (\(L=1\)).  
Figure 22: Meeting 99% reliability target by using FEC, BR, and 2-Rep-ACK retransmissions (\(L=2\)).