Mobile Telecom Networks

Initialy transmission in both UL and DL directions is done via source gNB. The UE sends measurement report to inform the gNB that signal from other cell is better than the one in the current cell. After report reception the source gNB takes a decision to trigger handover procedure towards target gNB. The target gNB starts to prepare resources to serve this UE and when ready acknowledges this to source gNB. The source gNB sends handover command message with information about target cell. At the same time the source gNB suspends the data transmission and starts forwarding data to target gNB. The UE needs to perform random access procedure towards the target cell. After successful random access procedure the UE confirms the handover towards target gNB. Target gNB starts the DL data transmission and informs the CN about completed handover. Data tunnels from Core Network are switched to target gNB. The target gNB informs source one that it may release UE resources.

 

  Figure 1 Xn interface protocol stack for control plane (left side) and user plane (right side).

There are couple options how the Dual Connectivity could be configured in 4G and 5G. First of all Dual Connectivity in 4G shall be mentioned. Two eNodeB connected with X2 interface. Both of those connected to EPC. Figure 1 LTE DC As for the migration towards 5G several more options are available: EN-DC The first option is the eNodeB connected to gNodeB with X2 and both of nodes are connected to EPC via S1 interfaces. However the gNodeB in this option is only using S1-U towards SGW. The eNodeB has the MN role and the gNodeB has the SN role. This option is called EN-DC (E-UTRA-NR Dual Connectivity). This and three other options described below are grouped in MR-DC (Multi-Radio Dual Connectivity). Figure 2 EN-DC Overview NE-DC In this option called NE-DC (NR-E-UTRA Dual Connectivity) the gNodeB is MN and the eNodeB is SN. Nodes are connected with Xn. Figure 3 NE-DC Overview NGEN-DC Yet another option one is called NGEN-DC (NG-RAN E-UTRA-NR Dual Connectivity). It is the...

Dual Connectivity allows the phone to be connected simulatenously to two radio base stations. What can be achievied with DC? The phone may send and received data from both nodes at the same time. User plane tranmission is aggregated (Figure 1). Figure 1 User Plane aggregation in Dual Connectivity It shall be mentioned that two nodes in the RAN shall be connected directly. What is more one node in DC relation will be called Master Node (MN) and the other one will be called Secondary Node (SN). The signalling connection is anchored in the MN. In a one of the cases this will allow to have User Plane and Control Plane separation as it is shown in Figure2. Figure 2 User Plane and Control Plane separation One other case is when the direction of transmission could be separated. The transmission in the Uplink direction (UL) can be done through SN and tranmission in Downlink direction (DL) can be done through MN. This is depicted in Figure 3 Figure 3 UL/DL separation

Simplified 5G network architecture presented in Figure 1. Not all function or nodes or entities are presented. Figure 1 Simplified 5G Network Architecture Table 1 Network Elements Name Description AMF Access and Mobility Management Function AUSF Authentication Server Function SMF Session Management Function UPF User Plane Function UDM User Data Management PCF Policy Control Function PDN Packet Data Network UDR Unified Data Repository

Mobility is one of the key aspects in the mobile telecom networks. We would like to travel with our phones and has a continuous access to the used service. Moving around the city while still talking to your friend is what we need, want and expect from the mobile network. The phone measures the strength and quality of the radio signal received from the base station. Information about what exactly shall be measured is sent from the gNB to the UE using RRC protocol.  Sometimes it may happen that the phone will find the cell that it was not supposed to find. In such case the phone will report this new "unknown" cell to the gNB. When several phones do the same the gNB may decide to add this "unknown" cell as a known neighbor cell and next time the gNB will tell other devices to actually measure radio signal for this particular cell. This is part of SON functionality called ANR. Measurements There is couple defined Measurement Events. Definitions for those are ...