# A Random Subcarrier-Selection Method Based on Index Modulation for Secure Transmission
## 4 Simulation Results
### The Simulation Results of BER Performance  
In order to evaluate the secure performance of the proposed scheme RSCS-IM, the 3-D performance surface of BER versus the direction angle and distance was simulated. Furthermore, we simulated the performance of PSS Plus RP to make a comparison with our scheme. Meanwhile, in order to illustrate the coupling effect of a uniform FDA, and to further illustrate the importance of randomly selecting subcarriers, this paper also  
Figure 6: Simulation comparison of the random degree. (**a**) \(N_{s}=512\). (**b**) \(N_{s}=1024\). Figure 7: The computational complexity of RSCS-IM and PSS Plus RP. (**a**) \(N_{s}=512\). (**b**) \(N_{s}=1024\). simulated the 3-D performance surface of BER versus the direction angle and distance of uniform FDA [7]. Figure 8 is the 3-D performance surface of BER versus the direction angle and distance of a uniform FDA. As shown in Figure 8, besides the desired position, there were other positions where the BER was low as well. If eavesdroppers were located at these positions, the transmission was not secure. This phenomenon is known as the coupling effect of a uniform FDA. Figure 9 depicts the 3-D BER performance surface versus the direction angle and distance when the number of total subcarriers \(N_{s}=512\). Compared with Figure 8, Figure 9 depicts that low bit error rate occurred only in a small area around the desired position. The other positions could not restore the original signal correctly. Therefore, the random selection of subcarriers was important for weakening the coupling effect and, thus, achieving safe and accurate wireless transmission. When \(N_{s}=1024\), Figure 10 depicts the 3-D BER performance surface versus the direction angle and distance. Figure 10a is similar to Figure 9a. Comparing Figure 10b with Figure 9b, the BER performance at Eve was slightly improved because of the increased  
Figure 8: 3-D performance surface of a uniform FDA. Figure 9: 3-D performance surface of BER versus the direction angle and distance (\(N_{s}=512\)). (**a**) 3-D performance surface of BER using PSS Plus RP; (**b**) 3-D performance surface of BER using RSCS-IM. weight of bits used for index selection. In terms of the desired position, there was a certain BER gain when employing RSCS-IM, comparing Figure 10b with Figure 10a. Figure 11 depicts the BER performance versus \(E_{b}/N_{0}\) of [PERSON]. As shown in Figure 11, the theoretical and simulation curves were basically consistent. Specifically, regardless of \(N_{s}=512\) or \(N_{s}=1024\), the BER performance of [PERSON] was consistent when employing PSS Plus RP since it did not include IM. The BER performance of [PERSON] was only related to the symbol modulation method. Additionally, when \(N_{s}=512\) and BER was \(10^{-2}\), there was more than 1 dB gain when employing RSCS-IM rather than PSS Plus RP. When \(N_{s}=1024\), there was an extra gain in the BER performance of [PERSON] compared with \(N_{s}=512\) in high \(E_{b}/N_{0}\), since SE was improved. While in low \(E_{b}/N_{0}\), there was a distinct degradation in the error probability of greedy detection. So the BER performance of [PERSON] was basically consistent in low \(E_{b}/N_{0}\) when \(N_{s}=512\) and \(N_{s}=1024\). Likewise, since the error probability of greedy detection was very high in low \(E_{b}/N_{0}\), the BER performance of [PERSON] employing RSCS-IM was worse than employing PSS Plus RP. From Figure 9 we can see that the BER performance of the other positions, other than the desired position, was of small variation. So, when taking an arbitrary position (\(60^{\circ},50\) m) as an eavesdropper (Eve), the BER performance versus SNR using RSCS-IM with different \(N_{T}\) is shown in Figure 12. The number of total subcarriers \(N_{s}=512\). As for Eve, the BER performance versus SNR was always about \(\frac{1}{2}\) when employing PSS Plus RP. As the other positions could demodulate the index bits correctly, the BER performance employing RSCS-IM was less than \(\frac{1}{2}\).