# Optical Channel Selection Avoiding DIPP in DSB-RFoF Fronthaul Interface
## 1 Introduction
They are purely digital networks for the delivery of content for final distribution over the wireless mobile radio links. Midhaul and fronthaul networks are used somewhat differently, as they act as networks distributing digital control signals and radio signals with varying degrees of processing. At this point, the radio-over-fiber (RoF) technique [4] is used, which consists in transmitting radio signals from the baseband (BBoF), intermediate-frequency (IFoF) and radio-frequency (RFoF) over an optical path or optical link, such as an optical fiber [5]. Such a method of increasing the transmission range of a radio signal was first defined in [6], where a unit for introducing a radio signal onto an optical carrier and a unit for inverting the process are defined. Radio signals can be transported over a fiber-optic medium in a digitized version, in the digitized-radio-over-fiber (D-RoF) technique, or in an analog version, in the analog-radio-over-fiber (A-RoF) technique [7]. The D-RoF technique is widely used in radio access systems and in microwave transmission systems. In this respect, the most recognizable is the common public radio interface (CPRI) [8], which has monopolized the market in this field and allows the delivery of digitized radio baseband signals. This interface produces very high bit rates, which is noticeable in the case of antenna installations based on multi-element matrices [9; 10]. In order to eliminate the monotype of the D-RoF interface, new solutions were introduced to increase the dynamics of this link. The most recognizable dynamic D-RoF interfaces include eCPRI [11] and the next generation fronthaul interface (NGFI) [12]. An initial outline of these solutions was presented by 3 GPP [13] and 5 GPPP [3] for applications in the 5G mobile systems. Both solutions were prepared so that radio signal transport services could be implemented in the radio-over-ethernet (RoE) mode. This is a good and optimal complement to the shortcomings of CPRI but requires the use of synchronous packet networks [14; 15; 16; 17]. The popularization of the implementation of the NGFI family interfaces in 5G systems initiated the trend of looking for solutions that reduce the costs of transporting the digitized radio signals prepared in the edge cloud to an active antenna unit or remote radio head (AAU/RRH) [18]. This resulted in the emergence of a new concept of an open radio access network (O-RAN) [19]. The main players in this field dealing with standardization include O-RAN Alliance, 3 GPP and IEEE. The openness of mobile networks in the radio domain ensures the cooperation of 5G systems with local and regional networks belonging to various local operators but meeting specific requirements at the level of real-time packet transport. Fulfilling these requirements is more difficult the higher the split/option of the supported network interface is [11; 12]. The A-RoF technique is still used in the field of research, as it is based on the transport of signals over optical fibers in the original version. Such a solution is particularly useful when there is a need to transmit a highly processed radio signal at the intermediate carrier (IFoF) or radio broadcast carrier (RFoF) level. The technique of analog transport of radio signals can be implemented in the fiber-to-the-antenna (FTTA) architecture or with the use of an all-optical transport network. Proposed solutions enabling the simultaneous operation of D-RoF and A-RoF interfaces are presented in [20; 21]. Optical systems using coarse/dense WDM (C/DWDM) techniques with all-optical nodes and elastic optical networks (EONs) provide a very good network structure suitable for transporting A-RoF signals. On this basis, it can be concluded that all-optical networks fit well with the concept of O-RAN because their appearance in the local and regional domains ensures the transport of optical signals modulated in any modern and classic formats, as long as they are compatible with the grid of optical channels of a given link.