# Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface
## 1 Introduction
Another way to eliminate the phenomenon of deep sideband interference in the photodetector can be the introduction of interleaving light intensity modulation (IM-DD) with phase modulation (PM-DD) [35; 36]. In this paper, we propose a solution based on a deep analysis of the DIPP interference characteristics and the introduction of a tunable optical carrier source, according to the flexible DWDM system grid [37]. Such an approach requires the use of a suitable transceiver insert on the optical transmitter side with the ability to tune the laser. If the signal is introduced into the optical path of the DWDM network, the laser tuning must follow the system's optical channel grid, which fits well with the Xhaul architecture supported by the Open RAN Alliance [38]. At the moment, this solution cannot be classified as cheap, but it should be assumed that the increased demand for the development of laser techniques and the wide implementation of flexible DWDM systems in the Xhaul domain will mean that transceivers/transponders equipped with lasers working in accordance with the DWDM grid will in time cease to be a luxury product. The results of the first studies in this area were presented at the international scientific conference \"Optical Fibers and Their Applications\", which took place in Poland in 2020 [39]. The paper consists of six sections. The first section presents a broad introduction showing the advisability of researching the optical signals modulated in the A-RoF format, with the rapidly developing D-RoF format. Section 2 describes the system, the methods used, and the network components. In this section, Section 2.1 gives a detailed description of the fiber-optic fronthaul path parameters, and Section 2.2 presents the theoretical basis for the DSB-RFoF signal. Section 3 is devoted to the description of the DIPP phenomenon and the analysis of the interference characteristics formed at the photo-receiver side with direct detection. This section is divided into four subsections, where Section 3.1 deals with the calculations related to the DIPP phenomenon occurring in the Option 10 interface, Section 3.2 deals with the calculations related to the selection of the optical channel for the vRF service, Section 3.3 contains the calculations of the relative delay within the CP-OFDM  
Figure 1: Functional splits proposed by 3 GPP [13] in NG-RAN with an example of IF/RF extensions proposed by the author for A-RoF functions introduced into the distributed unit (DU) and the radio unit (RU) (green blocks and options) [21]. Optical BH/MH/FH and their maximal links could be realized in mobile 5G systems [40]. symbol, and Section 3.4 is devoted to the proposed algorithm for selecting an optical channel. Section 4 presents the results of the verification simulations carried out using the VPIphotonics platform, which show the possibilities and limitations of the proposed \"Option 10\" [21] split working in the DSB-RFoF format. The last two sections present a broad discussion of the obtained results of calculations and simulations, as well as conclusions and predictions for further research.