## 5 Experimental Results
### Performance and Computational Complexity of the Low Complexity Demodulation Method
#### 5.2.1 Performance Evaluation of Low Complexity Demodulation Method  
Since the SET method and the TCST method use thresholds, we first searched optimal thresholds for each SNR and antenna (1, 2, 4, and 8). Here, the optimum threshold meant a maximum value at which performance degradation did not occur when compared with an ED method in a designated SNR environment. Because the number of sub-carriers used for Euclidean distance calculation decreased as the thresholds value increased, it was effective in reducing computational complexity. Figures 7 and 8 show flowcharts for searching optimal thresholds in SET method and TCST method. In Figure 7, the SET method calculated Euclidean distance using only the sub-carriers whose energy exceeded the thresholds, and \(m\)-th data were demodulated to the smallest Euclidean distance (\(r_{m}\)). Then we calculated the BER and compared it with the BER of ED method. As shown in Figure 8, the TCST method searched an index of preamble having the maximum received power and an index of preamble data having the smallest received power for each antenna from the received preambles. Subcarriers in which the difference between the selected preamble CSI subcarriers exceeded the threshold were used for demodulation process. After demodulating the data, the BER performance was evaluated and compared with the ED method. SET and TCST methods were repeated until maximum thresholds were found that did not cause performance degradation compared to BER performance of ED method. In the SET method, \(\alpha=\) transmitted signal power \(\times\) 0.1, and in the TCST method, \(\alpha=0.1\). Tables 1 and 2 show the average number of subcarriers applied to the Euclidean distance and optimal threshold for each SNR and antenna in the SET method and TCST method. Regardless of the number of antennas, the higher SNR, the larger threshold and the fewer the average number of subcarriers applied to the Euclidean distance, but the BER performance came out effectively. The thresholds shown in the Tables 1 and 2 were normalized using the number of antennas and SNR. Also, each threshold was rounded to four decimal places. As the number of antennas increased, the number of sub-carriers used for calculations decreased. Figure 7: Flowchart for searching the optimal thresholds (SET (subcarrier energy-based threshold) method). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & \multicolumn{5}{c}{**\# of Antennas**} \\ \cline{3-6}  & & **1** & **2** & **4** & **8** \\ \hline \multirow{5}{*}{**SNR**} & \multirow{2}{*}{**20**} & Threshold & 1 & 1 & 1 & 0.75 \\  & & Average \# of subcarrier & 36 & 25 & 13 & 7 \\  & & Threshold & 0.316 & 0.158 & 0.08 & 0.039 \\  & & Average \# of subcarrier & 45 & 45 & 45 & 45 \\  & & Threshold & 0.1 & 0.05 & 0.025 & 0.125 \\  & & Average \# of subcarrier & 49 & 50 & 50 & 50 \\  & & Threshold & 0.031 & 0.015 & 0.007 & 0.004 \\  & & Average \# of subcarrier & 51 & 51 & 51 & 51 \\  & & Threshold & 0.01 & 0.005 & 0.03 & 0.083 \\  & & Average \# of subcarrier & 51 & 51 & 49 & 38 \\  & & Threshold & 0.003 & 0.031 & 0.119 & 0.193 \\  & & Average \# of subcarrier & 51 & 50 & 42 & 26 \\  & & Threshold & 0.02 & 0.12 & 0.25 & 0.538 \\  & & Average \# of subcarrier & 51 & 47 & 33 & 9 \\  & & Threshold & 0.063 & 0.198 & 0.64 & 0.988 \\  & & Average \# of subcarrier & 50 & 43 & 17 & 3 \\  & & Threshold & 0.08 & 0.25 & 1 & 1 \\  & & Average \# of subcarrier & 49 & 41 & 10 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Data used in Euclidean distance calculation and optimal threshold for each SNR (signal to noise ratio) and number of antennas (SET method). Figure 8: Flowchart for searching the optimal thresholds (TCST (tag’s channel state-based threshold)). Figure 9 shows the BER performance of the SET method. Figure 9a-d shows the BER performance of the SET method when applying the optimal thresholds of each SNR and the BER performance of the existing ED method.