Controlling handover between dual-connectivity service and standalone service, with dynamic handover threshold

1. A method for controlling handover of a user equipment device (UE) between a first cell site that provides dual-connectivity service on a first radio access technology (RAT) and a second RAT and a second cell site that provides standalone first-RAT service, wherein the handover includes transitioning the UE from source first-RAT service to target first-RAT service, the method comprising: setting a handover threshold to use as a basis to evaluate whether target first-RAT coverage detected by the UE is sufficiently stronger than source first-RAT coverage detected by the UE, wherein setting the handover threshold comprises setting the handover threshold to a value selected based on whether the UE has detected that second-RAT coverage of the first cell site is threshold strong, and wherein setting the handover threshold to a value selected based on whether the UE has detected that the second-RAT coverage of the first cell site is threshold strong comprises selecting as the handover-threshold value a first handover-threshold value if the UE has not detected that the second-RAT coverage of the first cell site is threshold strong and selecting as the handover-threshold value a second handover-threshold value less than the first handover-threshold value if the UE has detected that the second-RAT coverage of the first cell site is threshold strong; and applying the set handover threshold as a basis to control whether to invoke the handover of the UE.

2. The method of claim 1 , wherein the handover is from the second cell site to the first cell site, and wherein the UE is served with the standalone first-RAT service in the second cell site.

3. The method of claim 2 , wherein the defined threshold is a B1 threshold.

4. The method of claim 2 , further comprising:

determining a ratio of first-RAT bandwidth to second-RAT bandwidth; and

configuring the second handover-threshold value to be lower than the first handover-threshold value by a delta that is based on the determined ratio.

5. The method of claim 4 , wherein delta is set proportionally to the determined ratio.

6. The method of claim 1 , wherein the first RAT is 4G LTE, wherein the second RAT is 5G NR, and wherein the dual-connectivity service is EN-DC service.

7. A method for controlling handover of a user equipment device (UE) from a first cell site in which the UE is served with dual-connectivity on a first radio access technology (RAT) and a second RAT to a second cell site that provides standalone first-RAT service and does not provide service on the second RAT, the method comprising:

applying a handover threshold to determine whether first-RAT coverage detected by the UE from the second cell site is sufficiently stronger than first-RAT coverage detected by the UE from the first cell site, including (i) using as the handover threshold a first handover-threshold value, (ii) determining that the UE has not yet detected that second-RAT coverage of the first cell site is threshold weak, (iii) responsive to determining that the UE has not yet detected that second-RAT coverage of the first cell site is threshold weak, increasing the handover threshold to a second handover-threshold value higher than the first handover-threshold value, and (iv) using as the handover threshold the second handover-threshold value;

based on applying the handover threshold, controlling whether to invoke the handover of the UE from the first cell site to the second cell site.

8. The method of claim 7 , further comprising:

determining a ratio of first-RAT bandwidth to second-RAT bandwidth; and

configuring the second handover-threshold value to be higher than the first handover-threshold value by a delta that is based on the determined ratio.

9. The method of claim 7 , wherein the first RAT is 4G LTE, wherein the second RAT is 5G NR, and wherein the dual-connectivity is EN-DC.

10. A method for controlling handover of a user equipment device (UE) from a first cell site in which the UE is served with standalone service on a first radio access technology (RAT) to a second cell site that provides dual-connectivity on the first RAT and a second RAT, the method comprising:

setting a handover threshold based on whether the UE has detected that second-RAT coverage of the second cell site is threshold strong, including (i) if the UE has not detected that the second-RAT coverage of the second cell site is threshold strong, then setting the handover threshold to a first handover-threshold value, and (ii) if the UE has detected that the second-RAT coverage of the second cell site is threshold strong, then setting the handover threshold to a second handover-threshold value lower than the first handover-threshold value; and

applying the set handover threshold as a basis to determine whether first-RAT coverage detected by the UE from the second cell site is sufficiently stronger than first-RAT coverage detected by the UE from the first cell site, so as to control whether to invoke the handover of the UE from the first cell site to the second cell site.

11. The method of claim 10 , further comprising:

determining a ratio of first-RAT bandwidth to second-RAT bandwidth; and

configuring the second handover-threshold value to be lower than the first handover-threshold value by a delta that is based on the determined ratio.

12. The method of claim 11 , wherein the delta is determined proportionally to the ratio.

13. The method of claim 10 , wherein the first RAT is 4G LTE, wherein the second RAT is 5G NR, and wherein the dual-connectivity is EN-DC.