What is the Split bearer in ENDC?

Split bearer is the function that allow the split of data going to gNB from the core side into two paths.

One path is the 1st path through the 5G air interface toward the UE.
And the 2nd path is X2 interface with the anchored eNodeB.

So in split bearer UE receiving the data from two paths simultaneously.

The aim of split bearer is to utilize the radio resources in case of multi carriers to improve the UE throughput.

The Main difference between CA nd DC is ,the CA can be implement between between different carriers of same cells but in case of DC is is implemented between different carriers of two different cells.

In case of CA there will be only one C-RNTI and but in case of DC the UE have two C-RNTI one for MCG and one for SCG and get asperated PUCCH resources.

Here are the below points for understanding the Split bearer process in EN-DC.

  1. In case of split bearer ,the split is done at PDCP layer becasue it the single point which controlling the other lower layers RLC,MAC,PHY layers.
  2. Estimate how much the data the UE has to transmit ,this process is tightly depends upon the data volume calculation.
  3. Buffer Status Report (BSR) is a message from UE to eNB (MAC layer command) for this amount of data to send would you give grants enough to transmit all of these data ?The Buffer Status reporting (BSR) procedure is used to provide the serving gNB with information about UL data volume in the MAC entity.
  4. Based on the estimated data volume ue needs to decide either it needs data split or not. The criteria is configured by the parameter “UL data split threshold” in RRC layer. If the the data volume is less than the threshold it wont go for split and in case of more than the configured data volume threshold it will go for split bearer.
  5. Once it decided to split the data the next thing is how to decide how much portion of data transfer to each cell. This determined by the cell group in RRC.
    5.1 If the cell group is set to zero it will mention the MCG (indicating LTE in ENDC),the data volume upto "uplink split threshold "transfer through the LTE cell and the remaining data through NR cell.
    5.2 If the cell group set to 1 than the amount of data up to split threshold transfer through NR cell and remaining data from LTE cell.
  6. There are two more parameters in "ul split thresold"b0 and infinity. If it set to b0 than UE determine the split ratio by its own algorithm and if infinity is set then UE send the 100% of the data via the cell specified in cell group.

Please also check the attached Snaps for parameters.

ENDC parameters

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