## 1 Introduction  
With the advent of diverse applications and devices, the number of devices per single user and the required data capacity has been increasing significantly, leading to an exponential growth in mobile data traffic [1]. As a result, due to the limitation of data traffic per cell and the increase in the amount of data required per device, cell coverage is reduced to small cells. Many cells, including small cells, were configured to support the required data traffic. To reduce capital expenditure (CAPEX) and operating expenditures (OPEX), the radio access network (RAN) structure changed from a distributed-RAN (D-RAN) structure to a centralized RAN (C-RAN) structure in which digital units (DU) are separated from the distributed cell sites. Because communication was required between the DU and the cell site where there are only radio units (RU) available, the common public radio interface (CPRI) has been standardized and used as the digital optical transmission protocol between the DU and the RU [1, 2]. However, when the signal is converted for digital transmission, the amount of transmission data is dynamically increased. For example, long term evolution (LTE, 4 th generation) signal with 20 MHz bandwidth requires 2.457 Gbps speed of CPRI signal in digital transmission [2]. Likewise, the bandwidth required in 5G (5 th generation) increases further, and the CPRI transmission capacity increases significantly. Thus, there is a limitation in digital transmission systems as the data required by the user increases in the current mobile fronthaul network (MFN). In order to prevent data capacity enlargement and support the required data traffic, Radio over fiber (RoF) transmission as an analog transmission has been proposed as an alternative technology [1, 2, 3, 4, 5]. The RoF transmission has higher spectral efficiency than the CPRI. The enhanced CPRI (eCPRI) technique that developed CPRI has been proposed. In the case of eCPRI, data enlargement is reduced due to ethernet transmission through packet-based optical transmission. However, since eCPRI technology is a packet-based ethernet transmission, it is necessary to convert the data for mobile communication. It makes the system complexity at the RU increase. In the RoF transmission, there is no process of digitizing the signal,so the data capacity enlargement has not occurred. Accordingly, the major drawback of CPRI, data capacity enlargement, does not occur in the RoF transmission. Moreover, to use frequency resources efficiently, multiband intermediate frequency over fiber (multi-IFoF) transmission has been proposed and studied [4,6]. The RoF transmission, utilizing an intermediate carrier frequency, is the most bandwidth-efficient technique in the MFN because it preserves spectral bandwidths of the signals. For transmitting high data capacity, CPRI requires transceivers and WDM components because of data capacity enlargement, but only a single analog optical transceiver is required in the multi-IFoF transmission. Therefore, the IFoF transmission could significantly reduce the required transmission capacity [7]. Though the multi-IFoF transmission has a high-efficiency of frequency resource and has no need for an additional device to convert digital form, IFoF transmission is vulnerable to noise, especially intermodulation distortion (IMD) noise, compared to the digital transmission. Because of the difference in the receiving technique of the analog signal and the digital signal. Several techniques have been used in previous research to reduce IMD noise effects [6, 7, 8, 9, 10, 11, 12, 13, 14]. One technique is the equalizer using many memory taps, another technique is to pre-distort the signal to mitigate the nonlinear channel effect, and the other is employing the additional external modulators into the system [8, 9, 10, 11]. However, as the number of devices in the cell increases and the required bandwidth of each device gets broader, the total bandwidth and data capacity increases.