Evolution of LTE Towards 5G
In the original version of LTE (3GPP R8), a UE only communicated with a single eNB, by means of a single cell. LTE-A (R10) introduced CA (Carrier ggregation), in which a UE supports a higher data rate by communicating with multiple cells on multiple carrier frequencies. All the cells are controlled by a single eNB. There is one PCell (Primary Cell), which carries traffic and RRC signalling messages, and optionally one or more SCells (Secondary Cells), which carry traffic alone. The PCell supports both UL and DL, but the SCells can be DL-only.
DC (Dual Connectivity) was introduced in 3GPP R12. When using DC, a UE communicates with two eNBs, which communicate over the X2 interface but have separate schedulers. The UE receives traffic and RRC signalling
messages from an MeNB (Master eNB). This is typically a macrocell on a low carrier frequency, which can only support a low data rate, but which has a wide coverage area and delivers the information with high reliability. At the same time, the UE receives traffic from an SeNB (Secondary eNB). This is typically a small cell on a high carrier frequency, which supports a much higher data rate, but which has a low coverage area and a low reliability. When using DC, the UE can be in a state of carrier aggregation towards either the MeNB, or the SeNB, or both. The SeNB has one PSCell (Primary SCG Cell), which supports both UL and DL, and optionally one or more SCells, which can be DL-only.
5G NSA (Non Standalone) deployment is the first stage in the introduction of 5G, and is a natural enhancement of DC. Usually, the UE receives traffic and RRC signalling messages from an MeNB, with a low data rate but high reliability. At the same time, the UE receives traffic from a Secondary gNB (Next Generation Node B), with a much higher data rate. This allows the UE to benefit from the higher data rates of 5G, without the need for wide-area 5G coverage, and initially without the need to roll out the 5G Core Network.