## 5 Experimental Results
It is worth noting that as shown in the last two rows of Table 2 the proposed architecture shows better area and energy figures than the recently proposed turbo decoder architecture for WSN applications described in [9]. As highlighted in [36], several standards have been proposed for WSNs. It can be interestingly noted that most of them rely on the physical layer of the IEEE 802.15.4 standard. Thus, to evaluate the gain of the proposed architecture in a WSN environment we assume typical parameters taken from the IEEE 802.15.4 standard, namely \(f=2.4\) GHz and \(B=80\) MHz and we fix \(d=50\) m. Moreover, employing an ultra-low-power low-noise-amplifier, as the one proposed in [37], we can fix \(F=3.8\) dB. In the following we investigate the energy saving obtained for a path loss exponent equal to three and four respectively, to model either typical indoor environments and outdoor urban/suburban foliated areas [38] or dense outdoor urban environments [39]. From Equation (12) the energy per bit required by an uncoded system ranges from tens of nJ/bit to few \(\mu\)J/bit depending on the considered path loss exponent value. As a consequence, to obtain a more significant information we compute the percentage of saved energy per bit with respect to the energy per bit of an uncoded system (\(\Delta E/E_{TX,U}\)) as a function of the BER. The percentage of saved energy as function of the BER for all the results shown in Table 1 is depicted in Figures 6 and 7 for \(n=3\) and \(n=4\) respectively. As it can be observed, both for \(n=3\) and \(n=4\) at a BER of \(10^{-4}\) the percentage of saved energy is more than the 50% and, in the best case, it achieves the 80%. It is worth poin \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{**Arch.**} & \multirow{2}{*}{\(\mathbf{N}\)} & **Tech.** & **Area** & \(\mathbf{A_{90}}\) & \(\mathbf{f_{clk}}\) & \(\mathbf{T}\) & \(\mathbf{P_{dec}}\) & & \(\mathbf{E}\) & \(\mathbf{E_{I}}\) & \multirow{2}{*}{\(\Phi\)} \\  & & **[nm]** & **[mm\({}^{2}\)]** & **[mm\({}^{2}\)]** & **[MHz]** & **[Mb/s]** & **[mW]** & **[**pJ/b]** & **[pJ/b/it]** & \\ \hline
[14] & FPA & 1,024 & 160 & 52.5 & 16.6 & 64 & 1,000 & 690 & 64 & 690 & 11 & 404 \\
[18] & PPA & 64,800 & 90 & 13.1 & 13.1 & 270 & 180 & 853 & - & 4,740 & - & - \\
[19] & PPA & 2,304 & 130 & 4.8 & 2.3 & 214 & 955 & 397 & 10 & 416 & 42 & 141 \\
[32] & PPA & 64,800 & 90 & 4.1 & 4.1 & 300 & 90 & - & 30 & - & - & - \\
[34] & PPA & 1,944 & 130 & 7.4 & 3.5 & 111 & 250 & 76 & 8 & 304 & 38 & 197 \\
[35] & PPA & 2,048 & 65 & 7.15 & 13.7 & 300 & 6,680 & 1,030 & 8 & 154 & 19 & 95 \\ \hline
[9] & SA (a) & 6,144 & 90 & 0.35 & 0.35 & 333 & 1.03 & 4.17 & 5 & 4,049 & 810 & 198 \\ This & SA & 576 & 90 & 0.13 & 0.13 & 20 & 0.25 & 0.67 & 10 & 2,696 & 270 & 25 \\ \hline \hline \end{tabular} \({}^{\text{(a) Serial turbo decoder architecture for WSN applications.}}\)  
\end{table} 
Table 2: Comparison of different architectures. reaches the error floor region, the percentage of saved energy is maximum and then it decreases. Thus, the best energy saving performance is achieved in the waterfall region of the code. Figure 6: Percentage of energy per bit saved as a function of the BER for \(n=3\). Figure 7: Percentage of energy per bit saved as a function of the BER for \(n=4\).