## 2 Connection Options and Topology Optimization after Site Densification  
With the deployment of pico-cells and 5G introduction, the distance between neighboring sites is continuously decreasing. Meanwhile, the capacity demands of the sites are increasing. As a result, more and more locations require fiber-optical connection or microwave links with increased access transmission capacity [1; 2; 3; 4; 5; 6; 7; 8]. The rapid network expansion inherently provides the possibility of optimization by means of topology re-planning. Thanks to the site densification, the hop lengths of the micro- and millimeter-wave access links can be reduced. A recent case study estimated significant hop-length reduction with 5G deployment [9]. The shorter links can either use higher millimeter-wave frequencies where available bandwidths are wider; alternatively, when the same hardware is re-used, the average output power can be regulated by ATPC, and elevated modulation can provide higher bit rates [9; 10; 11; 12; 13; 14]. Thus, the access capacity can be increased and, with careful re-design, the possible radio interference can simultaneously be reduced. Figure 2 shows two connection scenarios: how 5G and LTE cells can be connected to the LTE and 5G cores in the early 5G deployment phase, and then to a common 5G core in the mature phase of 5G. More connection options are discussed in Reference [3]. As an example, the \(\upmu\)/mmW links of an urban access network are shown (dark lines) in Figure 3, as seen in the link planning tool. Line-of-sight (LoS) information for possible new radio hops is plotted with light-green lines. High sites having fiber-optical access points are considered as transmission hubs (e.g., the locations 102A and 270C in Figure 3) and are target end-points of new links deployed after site densification. Route diversity can be achieved with short radio chains composed of a few \(\upmu\)/mmW hops and fiber-optical loop-back. Alternatively, the entire loop can be fiber-optic and, from the aggregation points, radio links can access the sites in star or in tree topology [8; 15]. It should be mentioned also that very long radio chains of multiple hops increase the delay and, therefore, cannot be tolerated by low latency 5G services. Finally, a full mesh topology composed of \(\upmu\)/mmW radios, free-space optical (FSO) links, and fiber-optical cables provides the mobile anyhaul [16; 17; 18]. Satellite links are very rarely used in Europe due to the high associated installation and lease costs and the relatively low capacity provided. Twisted pair or coaxial cables are limited in the bandwidth distance product; therefore, metallic cables were step-by-step replaced by optical fibers. Lightweight optical fibers are already widely used for remote antenna heads as feeder lines, and there is a significant research activity of radio-over-fiber (RoF) systems to deliver combined \"fiber/wireless\" networks for a reduced cost, with easier installation to support 5G deployments [5; 17; 18; 19; 20; 21; 22; 23]. Figure 2: Different 5G interfaces: (**a**) in early 5G phase; (**b**) in mature 5G phase. The evolving networking scenario is shown in Figure 4. Due to low latency requirements, cloudification, and 5G slicing, the cloud core networks tend to be more distributed toward the edge sites. Current distributed radio access networks tend to be more centralized, and the role of fiber optics in cloud RANs (radio access networks) is increasing [17; 18; 19; 20; 21; 22; 23]. However, the microwave and millimeter-wave links still continue to form an essential portion of the mobile anyhaul [1; 3; 4; 6; 7; 9]. Business analysts forecast an initial 50%/50% split of fiber and \(\upmu\)/mmW access for 5G. Later on, this split may change to 75% optical fiber and 25% radio, when new optical access points--often called PoPs (points of presence)--are added to the networks. This paper focuses on the radio links of the mobile anyhaul.