## 1 Introduction
The signal is affected by IMD noises, including IMD2 and IMD3, which are generated by nonlinear devices and through the fiber. Because of these complex IMD noises, it will be very difficult to estimate the channel. Therefore, it is hard to reduce signal distortions by equalization and signal pre-distortion requiring accurate channel information. These techniques are not suitable to mitigate IMD noise in MFN. Existing studies have conducted just tone experiments for mitigating nonlinear components [6, 7, 8, 9, 11] or have assumed that all IF bands have the same modulation format. Furthermore, in conventional techniques, all IF bands have the same power regardless of modulation format, as shown in Figure 1. However, because each IF band supports different devices and applications, each IF band passes through different channels and has different modulation formats. After receiving the signal at the receiver, the SNR is measured and informed the channel to the transmitter. Accordingly, the transmitter sets the modulation format, and power loading is performed with the water-pouring algorithm [15]. In the case of the CPRI, the generated orthogonal frequency-division multiplexing (OFDM) signal is converted to the time domain through the inverse Fourier transform (IFT), and the converted signal is digitized again to the baseband digital signal, then the signal is transmitted to the RU. However, since this conversion leads to explosive growth in data traffic if the OFDM signal is directly transmitted to the RU using the IFoF transmission, IMD noise is generated, and this IMD noise distorts the signal. At the RU, the distorted optical signal is downconverted to the RF frequency, and the RF signal is amplified again and transmitted to each user through the wireless channel. As shown in Figure 2a, at the RU, the distorted optical signal is downconverted to the RF frequency, and the RF signal is amplified again and transmitted to each user through the wireless channel. Since the CPRI signal is converted to the OFDM signal at  
Figure 1: Conventional signal spectrum of (**a**) transmitted signal and (**b**) received signal. the RU, the IMD noise is not considered in the MFN using CPRI, and also, the transmitted signal power, which affects IMD noise power, was not a major consideration in the MFN. However, in the case of IFoF transmission, as shown in Figure 2b, after downconversion of the received signal, the signal is amplified without removing IMD noise at the RU, so the noise is also amplified and transmitted with the signal. Because of using an analog signal in IFoF transmission, the power of the nonlinear component affected by the signal power can be considered as the main distortion, and the generated IMD is amplified without being removed from the RU stage, which can continuously affect the signal distortion. In this paper, we propose a spectral power allocation technique dependent on the modulation format and verify the feasibility of system linearity enhancement of multi-IFoF-based MFN. The signal spectral power was allocated for each IF band according to modulation order, which is dependent on the required signal-to-noise power ratio (SNR), which can be converted from the error vector magnitude (EVM) requirement. Since the IMD noise power is affected by the transmitted signal power, when the total transmitted signal power is changed, the IMD noise power would be significantly changed compared to the signal power variation. We have experimentally demonstrated the reduction of IMD noise using the proposed IF power allocation. Transmission performance enhancement using the technique was verified experimentally by the EVM improvement of the received signal. This technique was briefly proposed at the Asian communication and photonics (ACP) conference 2019 and is described in detail in this paper [16].