Control of UE connectivity including carrier transition before transition from standalone connectivity to dual connectivity

1. A method for controlling connectivity of a user equipment device (UE), the method comprising:

serving by a first access node the UE with standalone connectivity on a first connection, wherein the first connection is on a first carrier;

during the serving, detecting by the first access node a trigger for transitioning the UE from being served with the standalone connectivity by the first access node on the first connection to being served instead with dual connectivity by the first access node on the first connection and by a second access node on a second connection; and

responsive to at least the detecting, (i) reconfiguring, by the first access node, the first connection from being on the first carrier to being instead on a second carrier, based on the second carrier being lower in frequency than the first carrier, and (ii) after the reconfiguring, transitioning, by the first access node, the UE from being served with the standalone connectivity to being served with the dual connectivity.

2. The method of claim 1 , wherein detecting the trigger for transitioning the UE from being served with the standalone connectivity to being served instead with the dual connectivity is based on a quality of service level of a communication in which the UE will be engaging.

3. The method of claim 2 , wherein detecting the trigger comprises detecting setup for the UE of a bearer having a predefined threshold low packet-delay budget.

4. The method of claim 1 , wherein detecting the trigger for transitioning the UE from being served with the standalone connectivity to being served instead with the dual connectivity comprises detecting that a communication in which the UE will be engaging is latency-sensitive.

5. The method of claim 1 , wherein the first carrier is defined in a first band, wherein the second carrier is defined in a second band, and wherein the second band is lower in frequency than the first band.

6. The method of claim 1 ,

wherein, before the reconfiguring, the first connection is on only the first carrier, and

wherein, after the reconfiguring, the first connection is on only the second carrier.

7. The method of claim 1 ,

wherein, before the reconfiguring, the first connection is on the first carrier as a primary component carrier of carrier-aggregation, and

wherein, after the reconfiguring, the first connection is on the second carrier as the primary carrier of the carrier-aggregation.

8. The method of claim 1 ,

wherein the first and second connections are on different radio access technologies than each other,

wherein, before the reconfiguring, the first connection has the first carrier as a primary component carrier of carrier-aggregation and has the second carrier as one of one or more secondary component carriers of the carrier-aggregation, and

wherein, after the reconfiguring, the first connection has the second carrier as the primary carrier of the carrier-aggregation and has the first carrier as one of one or more secondary component carriers of the carrier-aggregation.

9. The method of claim 1 , wherein the transitioning after the reconfiguring is responsive to the reconfiguring.

10. The method of claim 1 , further comprising, before the reconfiguring, determining that the UE has predefined threshold weak signal strength on the first connection, wherein the reconfiguring is additionally responsive to the determining.

11. The method of claim 10 , wherein the predefined threshold weak signal strength comprises predefined threshold low reference-signal-receive-power (RSRP).

12. The method of claim 1 , wherein transitioning the UE from being served with the standalone connectivity to being served instead with the dual connectivity comprises:

engaging in signaling by the first access node to coordinate setup for the UE of the second connection; and

engaging in signaling by the first access node to trigger setup of a split bearer for the UE.

13. The method of claim 1 , wherein the first connection is a 4G Long Term Evolution (4G LTE) connection, the second connection is a 5G New Radio (5G NR) connection, and the dual connectivity is EUTRA-NR Dual Connectivity (EN-DC).

14. The method of claim 1 , wherein the first and second connections are on different radio access technologies than each other, and wherein detecting the trigger is based on deep packet inspection.

15. A non-transitory computer-readable medium embodying program instructions executable by a processing unit to cause a first access node to carry out operations including:

serving a user equipment device (UE) with standalone connectivity on a first connection, wherein the first connection is on a first carrier;

during the serving, detecting a trigger for transitioning the UE from being served with the standalone connectivity by the first access node on the first connection to being served instead with dual connectivity by the first access node on the first connection and by a second access node on a second connection; and

responsive to at least the detecting, (i) reconfiguring the first connection from being on the first carrier to being instead on a second carrier, based on the second carrier being lower in frequency than the first carrier, and (ii) after the reconfiguring, transitioning the UE from being served with the standalone connectivity to being served with the dual connectivity.

16. The non-transitory computer-readable medium of claim 15 , wherein the first and second connections are according to different radio access technologies than each other.

17. The non-transitory computer-readable medium of claim 15 , wherein detecting the trigger for transitioning the UE from being served with the standalone connectivity to being served instead with the dual connectivity is based on a quality of service level of a communication in which the UE will be engaging.

18. The non-transitory computer-readable medium of claim 15 , wherein the transitioning after the reconfiguring is responsive to the reconfiguring.

19. The non-transitory computer-readable medium of claim 15 , wherein the operations additionally include determining, before the reconfiguring, that the UE has predefined threshold weak signal strength on the first connection, wherein the reconfiguring is additionally responsive to the determining.

20. The non-transitory computer-readable medium of claim 15 ,

wherein, before the reconfiguring, the first connection is on the first carrier as a primary component carrier of carrier-aggregation, and

wherein, after the reconfiguring, the first connection is on the second carrier as the primary carrier of the carrier-aggregation.