condary cell 5G connection anchor (Master) 28 GHz with up to 400 MHz carrier 3.5 GHz with up to 100 MHz carrier Control Plane 5G UE User Plane Figure 6.52 5G-5G dual connectivity operation. 6 5G Physical Layer Uplink TDM switched between LTE and 5G LTE eNB 5G gNB Continuous downlink data flow 1TX mode 5G UE Figure 6.53 LTE and 5G dual connectivity in difficult band combination configuration (with exact second-order harmonics coinciding with the part of the 5G band in use). TDM uplink is shown in Figure 6.53. The different dual connectivity options from the architectural perspective were covered in Chapter 5. In the downlink direction, the support for dual connectivity requires more from the receiver when compared to the LTE downlink intra-site carrier aggregation. One needs to be able to keep synchronization to two independent base stations which have inde- pendent timing, and a key challenge is also how to share decoding resources between LTE and 5G as discussed in more detail in Chapter 13. There is also an additional option that could be considered with dual connectivity, called Supplemental Uplink (SUL). The SUL principle employs an additional uplink car- rier which would be transmitted typically on a lower frequency band than 5G, and this could be transmitted only when the 5G uplink is not used on the regular band. There are also different alternatives for SUL on the lower band, either with TDM or FDM with the LTE uplink operation. As SUL is an optional UE feature and SUL elements in the 3GPP specification are still experiencing corrections in 2019, it is not foreseen to be deployed in the near term. 6.15 5G Data Rates With 5G, the supported data rate by the UE is not dependent on a particular UE category, but rather it is dependent on the supported frequency band, at least in Release 15. For example, with the 3.5 GHz band, the UE supports the following: 100 MHz bandwidth Four receiver antennas 256-QAM modulation (downlink) These basically define the UE-supported data rate to be on the order of 2 Gbps. It is