## 3 Experiments and Results  
The feasibility of the proposed MBAPA technique was verified experimentally in multi-IFoF transmission by intensity-modulated and direct detection (IM/DD). In the experiment, as shown in Figure 7, randomly generated bits were mapped to symbols depending on each modulation format of subcarriers (QPSK, 16 QAM, 64 QAM, and 256 QAM). Bandwidth and spacing of subcarriers were set for generating OFDM signal. To have 20 MHz bandwidth per single IF, the number of subcarriers per band was 1200. The subcarrier spacing of the OFDM signal was set to be 15 kHz. The spacing between bands was set as 10 MHz, and 24 and 48 multi-IF bands were generated. The optical signal was transmitted through 10 km and 20 km optical single-mode-fiber. The received optical power at the photodetector (PD) was about \(-1.5\) dBm, and the RF spectra of the signal, which was converted from optical to electrical signal at PD, were measured. The proposed MBAPA signal can suppress the nonlinearity of the system. It was assumed that the IMD components were generated by transfer functions of RF amplifier and LD and by power saturation at PD. Figure 8a,b shows the transmitted signal spectrum with the conventional technique and with the proposed technique, respectively. After 10 km transmission, two nonlinear distortions were generated and interfered with each other in a spectrum, as shown in Figure 9.  
Figure 8: The power spectral density of transmitted signal (**a**) without the technique and (**b**) with the technique 1. Figure 7: Experimental setup. In Figure 9, using the proposed technique, the IMD power was reduced by about 10 dB. Figure 10 shows the constellations of the first IF band, which was modulated by QPSK, and the third IF band modulated by 256 QAM after a 10 km transmission with 24-IFs. Figure 10a,b shows QPSK constellation diagrams, respectively, without the proposed technique and with the proposed technique. In the case of QPSK, the EVM value without the technique was 7%, and The EVM value with the technique was 11%. QPSK is a low-order modulation that requires less signal power. Therefore, the proposed technique reduces the surplus of the signal power, improves the SNR, which causes deterioration of the EVM. The EVM values of both signals satisfy the EVM requirement of the QPSK format. Figure 10c,d is the 256 QAM constellation; (c) is constellation without the proposed technique, and (d) is with the proposed technique. In the case of 256 QAM, unlike QPSK, the proposed technique improved EVM performance from 10.8% to 3.5%. The received signal without the technique could not satisfy the EVM requirement to receive by nonlinear distortion noise. On the other hand, with the proposed technique, the EVM requirement is satisfied, so the signal with the technique is suitable for multi-IFoF transmission. In addition, in order to verify the effect of the technique on the fiber nonlinearity, experiments were conducted by increasing the transmission distance. The dispersion effect is more severe as increasing the transmission distance. As already written above, the dispersion mixed with the chirp effect of laser-generated the IMD2 more. As Figure 11 shows, the constellation of the 20 km transmitted signal with the same received optical power about \(-1.5\) dBm. Similar to Figure 10, the EVM from (a) without the technique to (b) with the technique deteriorated from 6.5% to 13.4%, and both EVM values satisfied the EVM requirement. In addition, when using the proposed technique from (c) to (d), the EVM improved from 9.2% to 4%. It was proven that the IMD2 component generated by the dispersion could be effectively reduced by the proposed technique. Figure 10: Constellation of the received signal after 10 km transmission with 24-IFs (**a**) quadrature phase-shift keying (QPSK) without the technique, (**b**) QPSK with the technique, (**c**) 256 QAM without the technique and (**d**) 256 QAM with the technique. Figure 9: Received 24-IF bands signal spectrum after 10 km transmission.