# Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface
## 1 Introduction
The transmission of optical signals in the A-RoF technique is very spectral-effective and allows signals to be built up on the edge of the network or in the edge cloud to the final form (software defined radio (SDR) technique). The mast with antennas operating in AAU mode is, in this case, a simple device for amplifying and radiating the RF signal. This solution also fits very well with digital (DBF), analog (ABF) and hybrid (HBF) beamforming on the wireless side of the network. However, it should be taken into account that the signal coming from the A-RoF interface has the structure of an analog signal and is strongly influenced by phenomena occurring in the optical path. In the work [21], the author proposed the introduction of two new Splits/Options, in relation to the 3 GPP model [13] to support transport in the IFoF (Option 9) and RFoF (Option 10) formats (Figure 1). In the future B5G/6G mobile systems, it is planned to use high millimeter frequencies up to 100 GHz. High radio frequency provides mass and broadband radio access, but over a not very large area. Transporting the radio signals in the RFoF format at such a high radio frequency requires special measures, as the optical path is dispersive. We mean sections of links or optical paths, which should not exceed 20 km, but the influence of phenomena, especially related to the relative delay, is noticeable. Massive access and local or regional use of the RFoF interface necessitates the use of simple and the same modulation and demodulation techniques, so the solution based on IM-DD modulation seems to be optimal. During the IM modulation, a signal in DSB format appears. The sidebands carry information, but at the point of the direct detection (DD) receiver, they are subject to interference, significantly lowering the receiver sensitivity. This phenomenon is strictly dependent on the RF radio frequency, the length of the optical fiber path and the chromatic dispersion coefficient occurring in the optical channel. For the first time, the effect of the dispersion-induced power penalty (DIPP) phenomenon on the reception of an optical signal in the DSB format was presented in [22]. This was confirmed in many subsequent experiments [23; 24; 25; 26; 27]. In the meantime, new formats for modulating radio signals have emerged, and the demand for RoF solutions for use in mobile wireless communication systems has increased. Methods and techniques have been presented in many publications to mitigate or compensate for the effect of DIPP on the efficient reception of modulated optical signals carrying microwave radio signals. The publication of [28] proposed a technique to mitigate the influence of DIPP by appropriate biasing of the electrooptical Mach-Zehnder modulator (MZM) either at the maximum or at the minimum transmission bias points of the operation of an external MZM. The authors of [29] proposed the use of an FBG taper, whose task is narrowband compensation of chromatic dispersion. This is a good solution, but in most optical paths, it is implemented in a static manner with dispersion-compensating fiber (DCF). In the case of fiber-optic access networks, chromatic dispersion compensation is usually not used, which requires it to be taken into account when designing the link. An alternative to static compensators can be dynamic tunable dispersion-compensation module (TDCM) compensation [30; 31]. Single-sideband (SSB) modulation was proposed in [32; 33] as an effective way to avoid the sideband interference effect at the photodetector point. This method, however, increases the complexity of the modulator system and lowers the sensitivity of the direct photodetection. A method of mitigating the influence of the DIPP was also proposed by appropriately selecting the modulation index and introducing phase imbalance in the signal modulated in the dual-sideband optical carrier suppression (DSB-OCS) format [34].