EN-DC (E-UTRAN New Radio - Dual Connectivity) secondary node addition refers to the process of adding a 5G NR (New Radio) cell as a secondary node to a connected LTE (Long-Term Evolution) cell, which serves as the primary node.
This dual connectivity setup enables the User Equipment (UE) 
to simultaneously connect to both 4G and 5G networks, allowing for more efficient use of network resources and improved overall performance.
The process of EN-DC secondary node addition involves the following steps:
- Measurement Configuration
:
The primary LTE cell configures the UE to measure the signal strength and quality of nearby 5G NR cells. The UE then reports these measurements back to the network.
Example: Your 5G-capable smartphone measures the signal strength of nearby 5G NR cells 
while still connected to the 4G network.
- Secondary Node Addition Decision
:
Based on the measurement reports, the LTE cell decides whether to add a 5G NR cell as a secondary node. Factors such as signal strength, network load, and user requirements are considered.
Example: The network determines that adding a 5G NR cell 
will improve your smartphone’s performance and decides to establish a secondary node.
- RRC Reconfiguration
:
The primary LTE cell sends an RRC (Radio Resource Control) Reconfiguration message to the UE, instructing it to configure and establish a connection with the selected 5G NR cell.
Example: Your smartphone receives instructions from the LTE network to establish a connection with the 5G NR cell .
- Dual Connectivity Setup
:
The UE establishes a connection with the 5G NR cell as a secondary node while maintaining its connection with the LTE cell as the primary node.
Example: Your smartphone successfully connects to both the 4G LTE and 5G NR cells, enabling seamless use of resources from both networks.
By leveraging EN-DC secondary node addition, users can enjoy faster data speeds 
, lower latency, and enhanced connectivity, as their devices can access resources from both 4G and 5G networks simultaneously.
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