## 4 Laboratory Experiment
### Experiment Results of the Primary System  
Finally, we reveal the operational \(\text{SIR}_{\text{total}}\) improvement of the primary system by STPA. Figure 11 shows the BER performances of experiment and simulation as a function of SIR at the primary system. The parameters of the secondary system are \(G=20\) and 24 dB, and \(N_{\text{High}}=1,3,4,7\)  
Figure 10: Operational \(\text{SIR}_{\text{total}}\) range of the secondary system by number of high level subcarrier \(N_{\text{High}}\) (\(G=20\) dB).  
Figure 9: Operational \(\text{SIR}_{\text{total}}\) range of the secondary system by subcarrier power ratio \(G\) (\(N_{\text{High}}=7\)).  
For comparison, BER performance is depicted when the secondary system does not perform STPA. When \(N_{\text{High}}\) is 3 or lower (see Figure 11a), BER performance of the primary system can be improved. It should be noted that performance of the primary system depends on the symbol timing accuracy. Since the interference power within the bandwidth cannot be changed, the primary receiver should precisely detect the symbol timing in the interference dominant situation. As described in Section 3, the prototype receiver utilizes the symbol timing of a successfully estimated subcarrier. If inter-system interference from the secondary system is reduced on certain subcarriers, the primary receiver can extract symbol timing for demodulation and thus BER performance is improved. Meanwhile, when \(N_{\text{High}}\) is larger than 3 (see Figure 11b), BER performance of the experiment becomes worse than that of the simulation. The large number of high-level subcarriers might affect over all BER characteristics of the primary receiver. Although FEC function is expected to compensate its undesired degradation, detailed mechanisms should be further investigated. \(\text{SIR}_{\text{total}}\) improvements at \(\text{BER}=~{}10^{-5}\) in experiments are 2.5 dB at \((G,N_{\text{High}})=(24~{}\text{dB},1)\), 3.0 dB at \((G,N_{\text{High}})=(20~{}\text{dB},3)\) and 7.0 dB at \((G,N_{\text{High}})=(24~{}\text{dB},3)\), respectively. Allowing degradation of interference suppression performance for the proposed scheme, larger value of \(G\) and lower value of \(N_{\text{High}}\) can provide more effective interference reduction for the primary system. In addition, the experiment results match the computer simulation ones well except for the primary system with \(N_{\text{High}}>3\).  
The above experiments verified a fundamental feasibility of the proposed spectrum superposing approach via STPA-BAA using a wired setup. For practical applicability, experiments should be conducted in outdoor environments where multipath components exist. Multipath fading might dynamically fluctuate levels of high and low subcarriers and it weakens interference suppression performance of BAAs. Although our simulative work showed the effectiveness of the proposal [19], more practical performance should be further investigated. In this case, OFDM is preferable since cyclic prefix (CP) can mitigate the inter-symbol interference (ISI) caused by the multipath effect. Here, challenges arise regarding how to extract an exact timing for FFT windowing in interference limited situations. FFT windowing should be performed before BAA processing per subcarrier; therefore, the timing extraction should be done in the presence of large inter-system interference, which causes the detection accuracy to deteriorate. Resolving the above issues, its overall effectiveness will be disclosed under practical application scenarios, e.g., LAA or LTE-U.  
Figure 11: _Cont._