 in Figure 10.24. The principles of dual connectivity are as follows: The UE decides the uplink data split between LTE and 5G. The threshold for 5G usage is defined by the network parameters. The 5G base station decides the downlink split between LTE and 5G radios. The Layer 1 and 2 control plane is needed in both radios. While LTE is used for the uplink user plane, the 5G uplink must still deliver Radio Link Control (RLC) Acknowledgments (ACK/NACK) on the PUSCH as well as RLC status Good signal area Weak signal area 5G at 3.5 GHz 5G at 3.5 GHz Downlink Uplink LTE at 1.8 GHz LTE at 1.8 GHz Downlink Uplink Downlink 5G at 3.5 GHz Downlink 5G at 3.5 GHz Uplink 5G at 3.5 GHz Uplink LTE at 1.8 GHz = User and control plane = Control plane Figure 10.24 Dual connectivity for uplink coverage. 10.6 Coverage for Sub-6-GHz Band 5G at 3.5 GHz 5G PDSCH 3.5 GHz 5G PUSCH 3.5 GHz 5G PUCCH and SRS 3.5 GHz Dual connectivity 5G + LTE 5G PDSCH 3.5 GHz 5G PUSCH 3.5 GHz (only RLC ACK) 5G PUCCH and SRS 3.5 GHz LTE downlink LTE uplink (user plane) Figure 10.25 Physical channel locations and coverage. reports, the SRS for 5G downlink beamforming, and the PUCCH for layer 1 hybrid auto- matic repeat request (HARQ) ACK/NACK. The physical channel locations are shown in Figure 10.25. Fortunately, the 5G uplink control channels have better coverage than data channels. Therefore, it is feasible to use LTE for the user plane while still running the control plane in 5G. Dual connectivity allows inter-site operation where 5G and LTE connections use dif- ferent sites. That capability can be useful in the early phase of 5G deployment where single 5G site can cover multiple LTE sites. The concept is shown in Figure 10.26. Note that in theory, coverage is shorter at higher bands. But 5G coverage at 3.5 GHz can be larger than LTE at 1800 MHz because there is no/low inter-cell interference at 3.5 GHz. LTE networks are mostly interference limited because they are designed to provide great indoor coverage. Therefore, one 5G cell at 3.5 GHz could potentiall