y collect traffic from many LTE cells from those users that are in favorable conditions. If we simply assume that the LTE network design includes 20 dB indoor loss, the outdoor coverage area can be over 10x larger. That means one 5G cell can collect all outdoor users, and some indoor users, from the coverage area of 10 LTE cells. Figure 10.26 Inter-site dual connectivity. LTE only LTE + 5G LTE only 10 5G Performance Some synchronization is required between LTE and 5G base stations for asyn- chronous dual connectivity, but the requirements are relatively relaxed. The maximum timing difference between LTE and 5G is allowed to be 250 us for the 30 kHz 5G case; see [8]. This requirement is relaxed compared to the TDD network synchronization requirement of 3 us. 10.6.3.2 Low Band 5G with Carrier Aggregation Low band usage for 5G is one alternative to make 5G coverage better both in the uplink and in downlink. If there are no empty spectrum blocks available, spectrum sharing between 5G and LTE needs to be considered. The traditional refarming from 2G to 3G and to LTE was a slow process. Similar frequency domain refarming can be utilized also in the transition from LTE to 5G. There is another alternative available as well because LTE and 5G frame structures are SO similar. That is dynamic spectrum sharing (DSS) in the time and frequency domains, which allows one to have full bandwidth for LTE and for 5G from the control channel point of view while the data channel resources are shared dynamically between the two radios. DSS requires fast coordination between LTE and 5G packet schedulers. The different low band 5G options are illustrated in Figure 10.27. DSS brings benefits because it allows the allocation to be changed instantaneously between LTE and 5G. On the other hand, DSS has a lower spectral efficiency due to overhead. Figure 10.28 shows the channel structure in the time and frequency domains for the shared LTE-5G carrier. 5G capacity is impacted by LTE Cell Reference Signals (CRSs) and PDCCH overheads. There is l