oreover, 3G and 4G technologies operated at sub-6 GHz frequencies, which have more favorable propagation characteristics and allow deployment of heterogenous networks (HetNets). These HetNets combine macro cells for area coverage with small cells to meet capacity demand at hot spots (Figure 5.2.1). By adapt- ing wireless capacity to the spatial traffic distribution, adequate service can be provided in an economic manner. It was possible to grow the network incrementally by restrict- ing the initial rollout to a macro-cellular layer while adding capacity through cell split or small-cell fill-ins with increasing traffic demand. Incremental deployment therefore per- mitted market-wide service from day one while stretching deployment expenses over years. For mm Wave access, a densified small-cell network is necessary to provide coverage even where demand is low. Network densification implies high cost for backhaul connectivity. To match the large wireless access capacity, backhaul connections require high-capacity fiber (in some cases microwave backhaul can be used, but for mmWave access microwave backhaul capacity is unlikely to be sufficient), which exacerbates the economic burden. There is obviously no path to incrementally evolve the network in compliance with traffic growth. Apart from high propagation loss, mmWaves also exhibit more prominent shadowing in the presence of obstructions such as cars and buildings due to diminished diffraction at these short wavelengths. The prominent shadowing can be addressed with deployment of more cells, which further increases network density and backhaul cost. Macro-cell for area coverage Small cells for Densified high traffic zones small cell network Sub-6 GHz deployment mmWave deployment Figure 5.2.1 Sub-6 GHz access can be deployed as HetNets while mmWave access requires a densified small-cell network. 5 NG-RAN Evolution Fiber link Fiber link Fiber link Fiber link Fiber link Fiber link Without IAB: Separate fiber link needed With IAB: One fiber link shared among for each c