Dual Connectivity (4G/5G) in Non-Standalone Modes
Dual connectivity indicates that the control and synchronization between the site and the UE are performed by the 4G network, while the 5G network is a complementary RAN tethered to the 4G anchor.
In this model, the 4G anchor establishes the critical link using the existing 4G network with the overlay of 5G data/control.
In the 5G NSA option-3A (for example) network topology mobility will be handled by LTE radio anchors (control and user planes).
5G NSA option-3A leverages the LTE legacy coverage to ensure continuity of existing service delivery and the progressive rollout of 5G
cells.
5G/4G NSA UE Requirments
UE should support simultaneous dual uplink transmissions of LTE (1TX/1RX) and NR (2TX/2RX) carriers in all possible combinations of standardized bands and radio access technologies (FDD, TDD, SUL, SDL).
This will be the technical barrier of getting multiple separate radios and bands functioning in a small device.
LTE TDD network operation may be synchronous, in which case the operating modes will be constrained to Tx1/Tx2 and Rx1/Rx2, or asynchronous which will require Tx1/Tx2, Tx1/Rx2, Rx1/Tx2, Rx1/Rx2.
LTE FDD, the TDD/FDD inter-band operation will require simultaneous Tx1/Rx1/Tx2 and Tx1/Rx1/ Rx2.
In all cases, since control plane information is transported by LTE radio bearers, it is critical to ensure that LTE anchor point uplink traffic is protected.
Source: Skyworks