## 4 Laboratory Experiment
### Experiment Results of the Secondary System  
Figure 7 shows the in-phase and quadrature (IQ) constellation upon the application of BAA with/without STPA at \(\text{SIR}_{\text{total}}=0\) dB, \(G=20\) dB, and \(N_{\text{High}}=7\). In BAA without STPA, the IQ constellation of the received signal is spread due to the unsuppressed interference; the resulting bit error rate (BER) was 0.5. BAA with STPA can effectively suppress the interference and thus the IQ constellation converged for both high and low-level subcarriers. Meanwhile, the amplitude of the IQ constellation at the low-level subcarriers was smaller than that at high-level subcarriers. It is obvious but enlarging the value of \(G\) degrades the reception performance of the low-level subcarriers. Figure 8a shows the BER performance as a function of \(\text{SIR}_{\text{total}}\) of the secondary system with a comparison to the simulation results. This figure shows the fundamental BER performance in the case that CMA, PI and PI-CMA are applied to all subcarriers without STPA. Simulation was conducted in the equivalent baseband domain assuming ideal conditions, without quantization nor oversampling. The \(\text{SIR}_{\text{total}}\) offset of the experiment results to the simulated ones is within 2 dB except for the original PI. Since the BAA is applied to oversampled signals whose envelope is not constant, interference suppression ability is slightly weakened. In addition to that, discrepancy between two results of PI becomes larger than that of CMA and PI-CMA. It is because of excessive interference signal level to the receiver inputs. In the lower \(\text{SIR}_{\text{total}}\) region, large power of the interference signal is input and the desired signal power is accordingly reduced so as not to exceed the dynamic range of the receiver inputs. It thus causes bit resolution shortage for the desired signal and the interference suppression performance of PI to deteriorate. Meanwhile, it can be confirmed that PI-CMA exhibits improved BER performance compared to the original PI. CMA utilizing PI as an initial weight can greatly enhance interference suppression performance. The legacy CMA and PI-CMA cannot suppress interference signal when \(\text{SIR}_{\text{total}}\) is around 0 dB. Figure 8b shows the BER performance of the secondary system with STPA-BAA with parameters \(G=20\) dB and \(N_{\text{High}}=3\) and 7. The proposed scheme effectively works in the region around \(\text{SIR}_{\text{total}}=0\) dB and improves the BER performance. Here, we define the operational region of the secondary system as the range to which \(\text{SIR}_{\text{total}}\) holds the BER under \(10^{-5}\). In the case of Figure 8b, the upper and lower bounds that satisfy requirements are 11 and \(-9\) dB, respectively; operational \(\text{SIR}_{\text{total}}\) range is thus derived as 20 dB. Focusing on the upper bound, BER behavior of the experiment matches that of the simulation well and they exhibit almost the same upper bound \(\text{SIR}_{\text{total}}\) value. In the higher \(\text{SIR}_{\text{total}}\) region, interference signal level is slight, so that the sufficient  
Figure 7: In-phase and quadrature (IQ) constellation by BAA at \(\text{SIR}_{\text{total}}=0\) dB (\(G=20\) dB and \(N_{\text{High}}=7\)). dynamic range can be kept wherein BAAs suppress interference well. Although BER behaviors of the experiment and simulation results are different especially in the lower \(\text{SIR}_{\text{total}}\) region, they almost coincide and satisfactory operational \(\text{SIR}_{\text{total}}\) range can be ensured. The operational \(\text{SIR}_{\text{total}}\) range of the secondary system is revealed by the subcarrier power ratio, \(G\), and the high-level subcarrier number, \(N_{\text{High}}\). Figure 9 shows the upper and lower bounds of the operational \(\text{SIR}_{\text{total}}\) as a parameter of \(G\) when \(N_{\text{High}}\) is set to 7.