5G EN-DC – Split Bearer In Downlink Only

In most situations the limiting factor when transmitting data over the air between a mobile device and a network is the uplink transmission power of the mobile device. In practice that means that when the mobile device is unable to increase its transmission power any further, it can still focus the transmission power on a narrower 4G channel and thus increase its range. But how will this work in early 5G networks that will use the dual connectivity (EN-DC) approach in which data can flow over 4G and 5G simultaneously?

The approach to have data flowing over 4G and 5G at the same time is referred to as a split-bearer. While this is great for the downlink direction, will this also make sense in the uplink direction when the UE transmit power reaches its limit?

This lead me to the question if it would be possible to configure a split bearer only for the downlink direction but not for the uplink direction. This seems to be possible as the downlink and the uplink channel on the air interface are configured independently of each other in an RRC-Reconfiguration Message when a split bearer is activated. A closer look at 3GPP TS 38.331 and 36.323 reveals the following interesting parameters:

ul-DataSplitThreshold

splitSRB-WithOneUL-Path

splitDRB-WithUL-Both-MCG-SCG

The first parameter is used during RRC reconfiguration for the network to tell the device to split its uplink data between 4G and 5G and at which usage percentage of the primary path to start splitting its data between primary and secondary path.

The other two parameters are part of the MRDC information element in the UE capabilities message the network can request. Both are optional and allow the UE to tell the network if a split uplink is supported from its side or not.

So in summary, it seems to be quite possible to configure a split downlink to increase downlink throughput while configuring a ‘normal’ uplink to ensure the UE does not run out of transmit power. Whether this will be used in practice or not remains yet to be seen.

For some additional reading, have a look at Sharetechnote on this topic.