In general, the frequency bands used in 4G/LTE communication are 800MHz or 2.1GHz, while those used in 5G/NR communication are 3.5GHz or 28GHz.
As a result, due to the shorter reach of waves in 5G communication, the coverage is smaller compared to LTE communication. Therefore, in order to establish a 5G network, 5G base stations need to be densely deployed to enable handovers between base stations and ensure uninterrupted communication.
However, it was not easy to densely deploy 5G base stations nationwide from the beginning for the commercialization of 5G. Therefore, operators came up with the concept of using the existing LTE network as a base for 5G usage.
In countries like South Korea and Japan, nationwide networks are built entirely on LTE. South Korea has excellent basic coverage of LTE even in mountains and rural areas. Therefore, the basic coverage is provided by LTE, and 5G base stations are installed in specific areas requiring high capacity to cover those areas with high data rates. From the user's perspective, they receive basic coverage through LTE and can receive very high data services in specific hotspots.
This is called Dual Connectivity, where LTE base stations and 5G base stations operate together, meaning it is Not Standalone (NSA). This indicates that the new radio technology called NR cannot operate alone and always operates together with LTE.
This is generally referred to as Option 3. There are derived options such as Option 3A and 3X within Option 3.
The solid line represents the communication of actual user plane data, while the dotted line represents the communication of control signals. The terminal can receive data from both LTE base stations and NR base stations. The terminal is connected to both LTE and NR.
In Option 3, user data is transmitted (S1) from the LTE base station to the LTE core network EPC. The LTE base station acts as a switch. If there are users in areas where services can be provided at high data rates, the LTE base station splits specific data and transmits it to the NR base station (X2). In Option 3, 5G terminals can receive data from both LTE base stations and 5G base stations.
However, the operation of Option 3 can be a significant burden on the LTE base station side. Basically, LTE base stations are already deployed as established technology. However, for Option 3 operation, LTE base stations must be able to perform additional functions of switching data received from EPC to NR. This can be a burden on the LTE base station side. Therefore, in practice, it operates in a different form rather than Option 3. This led to the emergence of Options 3A and 3X during the development process.
In Option 3A, user data is still transmitted from the LTE core network EPC to the LTE base station, but an additional line (S1-U) has been added for direct transmission of user data from EPC to the NR base station. The data to be transmitted by the NR base station comes directly from the EPC. This is Option 3A, which reduces the burden on LTE base stations' switching function.
However, if we compare Option 3 with Option 3A, Option 3 had options that allowed the terminal to select the better base station between LTE base stations and NR base stations depending on radio conditions for user data transmission. For example, there were options in Option 3 to split data depending on the situation, such as using NR base stations when the load on LTE base stations is excessive and using LTE base stations when the load on NR base stations is excessive. In Option 3A, such options cannot be provided.
Therefore, Option 3X was introduced to complement this. In Option 3X, the NR base station still receives data from the EPC, but an additional red solid line (X2) has been added. Through split bearers, data from the NR base station can be sent to the LTE base station. In Option 3X, the NR base station acts as a switch in a mirror structure with Option 3. Since the NR base station is a new base station, putting the switching function on the NR base station is less burdensome than putting it on the LTE base station. This is because LTE base stations maintain the existing state as already installed base stations, and it is easier to add such a function to new NR base stations.
In practice, the commercialization in April 2019 was based on Option 3. All three major telecom companies in South Korea, LG Uplus, SKT, and KT, have now commercialized their networks based on Option 3X.
Option 3 was the first standard to be developed as an early drop.