5G in practice, finally! This summer has seen a number of 5G NR network launches, all of them using the ‘Non-Standalone (NSA)’ option 3 configuration. In Europe and Asia, network operators have started deploying the new radio network in the 3.5 GHz band, also referred to as band n78. In the US, different bands are used and some operators are even experimenting with mmWave bands. So while 5G for the moment is just a ‘bolt-on’ to 4G, ‘handover’ scenarios between cell sites are handled a bit differently from how it is done today in pure LTE networks today. Let’s have a closer look!
On the LTE part of a 4G/5G connection, handovers to a cell at the same or a different site are triggered by a measurement report of the mobile device in which it reports that a neighboring cell is received with a significantly better signal than the currently serving cell. A difference of about 3 db (twice the signal strength) of a neighboring cell is a typical threshold. The serving LTE eNB then contacts the eNB that manages the better cell and performs a handover over the X2 interface that logically connects the two base station sites. If the target eNB agrees to the handover, the UE receives an RRCReconfiguration message with the details of the target cell. During and after the handover, the user plane, i.e. the user’s IP packets, still flow via the old eNB and traverse the X2 interface to the new eNB to minimize packet loss. Only once the handover of the air interface connection has been completed is the routing in the access network cleaned up and data flows directly between the new cell site and the Serving Gateway in the core network.
So far so good, this is the basic procedure. Most LTE networks, however, use carrier aggregation today to increase data rates so additional steps are necessary. When the UE reports a better neighbor cell, a common option is to perform a handover to the target cell with only a single carrier and then request the UE to perform measurements on other potential carriers that could be added in the new cell. If the UE reports a sufficient signal strength for those carriers, the eNB then goes ahead and activates carrier aggregation again. In other words, LTE carrier aggregation is lost during a handover and added again later. It is also possible to just blindly add additional carriers which is faster but obviously has some drawbacks as well.
For 5G NR handovers, things unfortunately do not stop here. If the mobile reports only a stronger 5G signal from a different cell site but no stronger signal of the LTE part of the connection, the network can decide to only hand-over the 5G part of the connection to another cell site while leaving the LTE connection in place. In effect the 4G and 5G parts of the link are then served over two different cell sites. This works because LTE and 5G NR are not as tightly coupled as the different carriers in LTE carrier aggregation. In effect, LTE and NR use independent eNB/gNB packet schedulers and such a split scenario is referred to as an ‘Inter-Site Anchor’.
Another variant in practice can be that the UE only reports a stronger LTE signal from a neighboring cell site but does not report a stronger 5G NR signal from a different location. In this case the simplest variant is to hand over the LTE part, discontinue the 5G connection and add it again once the UE is in the new cell. That’s a bit crude however. A more elegant solution is to hand-over the LTE anchor to a different site while leaving the 5G part of the connection with the current base station location. Again we have an ‘Inter-Site Anchor’ scenario and part of the data for the user traverses the logical X2 interface between two base station sites.
Inter-Site Anchoring scenarios might look complicated at first but actually they are not. As the LTE and NR part of a radio link are handled by two different entities that are connected over the X2 interface it doesn’t matter if the 5G leg of a connection is handed-over inside a base station site over a virtual or physical X2 interface leg, or over a physical X2 interface leg to another base station site. From an eNB/gNB software point of view it is pretty much the same.
It is of course also possible that the UE reports a stronger LTE and a stronger 5G NR cell simultaneously and the network then hands over the LTE and NR leg to another cell site simultaneously. I haven’t seen this one in practice yet, however. For the details have a look at 3GPP TS 37.340
Now one might argue that such cases must be rare in practice as band n78 (3.5 GHz) does not reach as far as most LTE bands and thus, the 5G signal should become too weak before a handover on the LTE layer occurs. In practice, however, this is usually not the case. A follow up post will shed some light on this.