I couple of days ago I was wondering what Supplementary Uplink (SUL) for 5G is all about, if it is supported for EN-DC (simultaneous LTE+5G data transmission) and what potential benefits it might have.
I can pretty much understand how SUL would help for 5G standalone operation, let’s say on 3.x GHz. The problem here is the limited uplink transmission power of mobile devices which will be the limiting factor when it comes to cell range. The idea of SUL is to add an uplink transmission path on a lower frequency band that can reach farther. Also, it could increase 5G uplink transmission speeds, because I assume that in TDD operation, most operators will give as much of the 3.x GHz carrier capacity as possible to the downlink.
But would SUL also have benefits for EN-DC operation, i.e. when LTE on, e.g. 1.8 GHz and 5G NR on, e.g. 3.x GHz are used simultaneously? Even without SUL, there is an LTE 1.8 GHz uplink channel which can be used for user and signaling data (see my post on signaling bearers) for the 5G NR gNB as well. So no immediate problem here. But if an implementation wants to keep gNB signaling separate from LTE (see option 2 in the link above), such a measure can help to transmit signaling directly to the gNB even in difficult RF situations. On the other hand, SUL would require to split the uplink transmission power between the LTE and the NR uplink which runs against the idea of extending the cell range.
Another advantage (even for EN-DC operation) would of course be that the uplink transmission speed could be further increased. However, in EN-DC operation, this can already be done by aggregating several LTE carriers in both uplink and downlink direction. Add to this the fact that there is mostly FDD spectrum assignment in Europe in the lower bands, so just using the uplink of a channel while not using the downlink (for LTE) also doesn’t make much sense.
For some technical details have a look at 3GPP TR 37.872 which lists the currently defined band combinations for SUL operations. In particular, have a look at Table 1-3 which lists the ‘LTE-NR DC with SUL band combinations’.
So maybe I’ve missed something here but in the case of European operators I don’t see much of a point in using SUL + EN-DC operation for combined LTE/5G data transmission. Thoughts?