5G – Part 4 – LTE-NR Carrier Aggregation – DC_3A-7A-20A-n78A

Part 3 of my series on 5G has given an overview of 5G-New Radio Option 3 Non-Standalone, which I think is how 5G is likely to be deployed in practice first: In loose combination with LTE. Option 3 basically puts a 5G gNB next to a 4G eNB base station. The LTE eNB controls connectivity and can add the 5G cell to the connection when the mobile detects that it is available. In effect this is quite similar to LTE carrier aggregation today, where several LTE carriers can be bundled to increase the available bandwidth. The big difference to LTE carrier aggregation is, however, that the combination of one or more LTE carriers and one 5G-NR carrier is not as tight as LTE Carrier Aggregation. Instead, a mechanism referred to as Dual Connectivity is used. Here, the schedulers in the eNB and gNB are independent of each other and have their own state. As a result, the mobile device talks to two independent schedulers and receives two data streams that it has to combine back into a single data stream for upper layers. In other words, some IP packets arrive via the LTE carriers and some arrive via the 5G-NR carrier. In the uplink direction the principle is the same. What I didn’t cover in the last post is which LTE/5G band combinations are foreseen in the specifications and there was quite a bit of a surprise there for me!

A LTE/5G-NR Dual Connectivity combination

Information about the intended combinations for the first networks can be found in TR 37.863-01, 02, 03, 04, 05 with the double digit appendices indicating how many LTE carriers are bundled in combination with one 5G-NR carrier. Let’s take a simple example shown in the first screenshot of a table in 37.863-01. This table shows how to bundle one LTE carrier in LTE band 3 (1800 MHz) that can be up to 20 MHz wide with a 5G-NR carrier in NR band 78 (n78) which is the 3300-3800 MHz range that can be up to 100 MHz wide. This band is foreseen in Europe as the first sub-6 GHz band for 5G. So here we go, the first 5G band that is going to be used in Europe is referred to as n78!

The major surprise for me was how this band will be used! Have a look at the ‘duplex mode’ column on the right. While band 3 will be used for the LTE carrier in Frequency Division Duplex (FDD) mode, the 5G-NR carrier will use Time Division Duplex (TDD), i.e. uplink and downlink will be on the same channel! From my point of view this makes a lot of sense as even today there is still a significant imbalance of downlink vs. uplink traffic. I would imagine that the 5G-NR carrier will be configured for the maximum amount of time for download traffic which means that most of the uplink traffic will be handled on the LTE side. Having the uplink traffic on the lower frequency band is better anyway as it makes it easier for the mobile device to transmit data with its small antenna and limited transmission power.

Another interesting point is the subcarrier spacing which is 15 kHz for the LTE channel and 15, 30 or 60 kHz for the NR carrier. For a 100 MHz channel, only 30 or 60 kHz subcarriers can be used. Note that while LTE carriers can only have a bandwidth of 20 MHz, NR carriers in band n78 are defined for up to 100 MHz. The Dual Connectivity configuration described just now would be referred to as DC_3A-n78A and would bundle up to 120 MHz of spectrum. But this is just the beginning, the spec shows other combinations with up to 5 LTE carriers to bundle a total of 200 MHz of sub 6-GHz spectrum. One combination which we might see in practice is DC_3A-7A-20A-n78A which would bundle up to 20 MHz each in bands 3, 7 and 20 (1800, 2600 and 800 MHz) and up to 100 MHz in NR band n78 (3500 MHz) for a total of 160 MHz. In practice, many network operators are limited to 10 MHz on band 20 so the total channel bandwidth would be 150 MHz.

4G/5G Modem design in TR 37.863

As mentioned before, Dual Connectivity is not the same as Carrier Aggregation because the 4G and 5G sides are independent of each other. This is reflected very nicely in the second screenshot also taken out of TR 37.863-01 that shows a potential modem architecture of a future 5G device. As can be seen on the right of the screenshot, there are two modem blocks, one for the 4G modem (that also handles 2G and 3G) and a separate block for the new 5G modem. Also, the radio front-ends are separated. Note the dotted lines to the antennas which indicate that the 4G and 5G front-ends can share antennas to allow the 5G side to use 4×4 MIMO in the downlink direction and for 2 transmit antennas in the uplink direction.

And finally the TRs also contain Dual Connectivity combinations for LTE/5G-NR for millimeter wave bands which have been named n257 for the 28 GHz range, n258 for the 26 GHz range and n260 for the 39 GHz range. All mmWave bands specified so far will use TDD and can have a bandwidth of up to 400 MHz when used with 120 kHz subcarriers! Like for the sub-6 GHz range NR described above, these bands are also intended to be used with Dual Connectivity and LTE as an anchor.

For more details have a look at TR 37.863 or, if you like it more fancy and condensed, head over to niviuk.free.fr, a great site to which I’ve linked for various other radio related topics in the past as well!