Inter-frequency measurement and handover for wireless communications

• US 6,845,238 B1
• Filed: 04/06/2000
• Issued: 01/18/2005
• Est. Priority Date: 09/15/1999
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Muller, Walter G. A.
• Primary Examiner(s): Trost, William
• Assistant Examiner(s): D AGOSTA, STEPHEN M
• Application Number:
  US09/545,872
• Time in Patent Office: 1,748 Days
• Field of Search: 455/436, 455/437, 455/438, 455/442, 455/443, 455/444, 370/331, 370/464, 370/450, 370/555.1, 370/553.1
• US Class Current: 455/436
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection
• CPC Class Codes: H04W 36/30 by measured or perceived connection quality data

Abstract

A telecommunications network performs an inter-frequency hard handover for a connection with a user equipment unit (UE) by switching either from a cell or a current active set of base stations on a first frequency to a virtual active set of base stations on another (new) frequency. The inter-frequency hard handover can be an inter-frequency handover within a same system, or an inter-system handover. The virtual active set of base stations is maintained at the user equipment unit (UE), and is updated in accordance with one of several updating implementations of the invention. In a first mode of the invention for implementing virtual active set updates, the network authorizes the user equipment unit (UE) to report to the network the occurrence of certain network-specified events which are acted upon by the network for communicating virtual active set update information to the user equipment unit (UE). In a second mode of the invention, the network authorizes the user equipment unit (UE) to perform an autonomous virtual active set update upon occurrence of certain network-specified events, with inter-frequency events being reported from the equipment unit (UE) to the network and the network issuing an inter-frequency handover command. Advantageously, events which trigger intra-frequency measurements can be reused for reporting inter-frequency measurements. In another of its aspects, the present invention provides the network with a quality estimate for a current active set as well as a quality estimate for the virtual active set. The quality estimate can be utilized in a context of a handover from one UTRAN frequency to another UTRAN frequency, or even in the context of an inter-system handover (e.g., a handover between a UTRAN system and a GSM system, for example). The quality estimate can be utilized to trigger a change or switch of frequencies/systems. Certain thresholds employed in the quality estimate-utilizing handovers provide hysteresis protection.

Claims

1. A user equipment unit which, while communicating with a telecommunications network using one of a cell or a current active set of base stations on a fist frequency, maintains a virtual active set of plural base stations on a second frequency and, while communicating with the telecommunications network using one of the cell or the current active set of base stations on the first frequency, performs measurements respecting signals on the second frequency for the respective plural base stations of the virtual active set, whereby the user equipment unit can switch to the virtual active set of plural base stations upon performance of an inter-frequency handover, wherein when a determination is made that an inter-frequency handover should be performed, the user equipment unit can switch to the virtual active set of plural base stations without making measurements respecting the signals on the second frequency after the determination and before using the virtual active set of plural base stations as a new active set.

2. The apparatus of claim 1, wherein the measurements made at the user equipment unit is triggered either periodically, immediately, or in response to a predetermined event.

3. The apparatus of claim 1, wherein in response to a measurement trigger criteria, the user equipment unit performs and reports inter-frequency measurements for the second frequency.

4. The apparatus of claim 3, wherein the measurement trigger criteria which causes the user equipment unit to perform and report inter-frequency measurements for the second frequency is the same criteria which is employed to cause the user equipment unit to perform and report intra-frequency measurements for the first frequency.

5. The apparatus of claim 3, wherein the measurement trigger criteria is one of being periodic, immediate, or in response to a predetermined event.

6. The apparatus of claim 1, wherein the signal for which the user equipment unit performs the measurement is a physical control channel on the second frequency.

7. The apparatus of claim 1, wherein the cells which belong to the virtual active set on the second frequency are cells which would be considered in the active set on the second frequency if the user equipment unit were to use the second frequency for active traffic.

8. A method of operating a user equipment unit which communicates with a telecommunications system, the method comprising tie user equipment unit:

while using one of a cell or a current active set of base stations on a first frequency, both

(1) maintaining a virtual active set of plural base stations on a second frequency; and

(2) performing measurements respecting signals on the second frequency for the respective plural base stations of the virtual active set;

when a determination is made that an inter-frequency handover should be performed, switching to the virtual active set of plural base stations without making measurements respecting the signals on the second frequency after the determination and before using the virtual active set of plural base stations as a new active set.

9. The method of claim 8, further comprising triggering the measurements made at the user equipment unit either periodically, immediately, or in response to a predetermined event.

10. The method of claim 8, further comprising the user equipment unit performing and reporting inter-frequency measurements for the second frequency in response to a measurement trigger criteria.

11. The method of claim 10, wherein the measurement trigger criteria which causes the user equipment unit to perform and report inter-frequency measurements for the second frequency is the same criteria which is employed to cause the user equipment unit to perform and report intra-frequency measurements for the first frequency.

12. The method of claim 10, wherein the measurement trigger criteria is one of being periodic, immediate, or in response to a predetermined event.

13. The method of claim 8, further comprising the network providing at least one member of the virtual active set of base stations on the second frequency in a virtual active set update procedure.

14. The method of claim 8, further comprising the user equipment unit receiving from the network an authorization message that allows the user equipment unit to update autonomously the virtual active set of base stations.

15. The method of claim 14, wherein the authorization message includes one of an event or a parameter that can trigger the update of the virtual active set of base stations without the user equipment unit first having to send a measurement report to the network.

16. The method of claim 8, wherein the signal for which the user equipment unit performs the measurement is a physical control channel on the second frequency.

17. The method of claim 8, wherein the cells which belong to the virtual active set on the second frequency are cells which would be considered in the active set on the second frequency if the user equipment unit were to use the second frequency for active traffic.

18. A method of operating a telecommunications network which communicates with a user equipment unit, the user equipment unit, while using one of a cell or a current active set of base stations on a first frequency, both (1) maintaining a virtual active set of plural base stations on a second frequency;

and (2) performing measurements respecting signals on the second frequency for the respective plural base stations of the virtual active set;

the method comprising

when a determination is made that an inter-frequency handover should be performed, switching to the virtual active set of plural base stations without making measurements respecting the signals on the second frequency after the determination and before using the virtual active set of plural base stations as a new active set.

19. The method of claim 18, further comprising the network providing, information regarding the virtual active set of base stations on the second frequency in a measurement control message.

20. The method of claim 19, further comprising including the measurement control message in a DCCH control channel.

21. The method of claim 19, further comprising including in the measurement control message further one of a measurement parameter to be measured and a predetermined measurement event which triggers a measurement.

22. The method of claim 18, further comprising:

maintaining the virtual active set of base stations on the second frequency by a second operator which differs from a first operator which maintains the current active set of base stations on the first frequency.

23. The method of claim 18, wherein the virtual active set of base stations on the second frequency comprises a second network system which differs from a first network system provided on the first frequency.

24. The method of claim 23, wherein the second network system is universal mobile telecommunications (UMS) and the first network system is a system having soft intra-frequency handover.

25. The method of claim 23, wherein the second network system is a Global System for Mobile (GSM) system and the first network system is universal mobile telecommunications (UMTS).

26. The method of claim 18, further comprising using a frequency quality estimate to determine when to switch to the virtual active set of base stations.

27. The method of claim 26, wherein the frequency quality estimate is provided by the equation:

$Q_{{\mathrm{carrier}}_j}=10·<br><br>\mathrm{Log}<br><br>\text{\hspace{1em}<br><br>}<br><br>M_{{\mathrm{carrier}}_j}=10·<br><br>\mathrm{Log}<br><br>\left(W_j·<br><br>\left(\underset{i=1}{\overset{N_{\mathrm{Aj}}}{\sum <br><br>}}<br><br>M_{i_j}\right)+\left(1-W_{j<br><br>\text{\hspace{1em}<br><br>}}\right)·<br><br>M_{{\mathrm{Best}}_j}\right)$

wherein;

Q_{frequency j} is the estimated quality of the active set on frequency j;

M_{frequency j} is the estimated quality of the active set on frequency j.

M_i is a measurement result of cell i in the active set.

N_A is the number of cells in the active set;

M_Best is the measurement result of the strongest cell in the active set;

W is a parameter with the value range 1-0 sent from the network to the user equipment unit;

wherein when W=0 only the measurement results from the best cell on frequency j is used; and

when W=1 only the sum of the measurement results from the cells in the active set is used.

28. The method of claim 26, wherein the frequency quality estimate is based on two factors:

(1) a carrier Radio Signal Strength Indication (RSSI); and

(2) whether the Base Transceiver Station Identity Code/Base Station Identifier Code (BSIC) has been confirmed or not.

29. The method of claim 26, wherein the network compares the frequency quality estimate to at least one threshold to determine when to switch to the virtual active set of base stations.

30. The method of claim 29, wherein the at least one threshold is chosen to provide hysteresis protection.

31. A user equipment unit which, while communicating with a telecommunications network using one of a cell or a current active set of base stations which operate on a first frequency, performs measurements respecting signals of a second frequency from each of plural base stations which operate on the second frequency, and maintains as a virtual active set plural base stations which operate on the second frequency and which satisfy network-specified criteria, so that when an inter-frequency handover is required the virtual active set can essentially immediately be utilized as the active set for the user equipment whereby the user equipment unit can be in radio communication with each of the plural base stations in the virtual active set;

wherein the virtual active set being essentially immediately utilized involves the user equipment unit switching to the virtual active set of plural base stations without making measurements respecting the signals on the second frequency after a determination that an inter-frequency handover should be performed and before using the virtual active set of plural base stations as a new active set.

32. The apparatus of claim 31, wherein the measurements made at the user equipment unit is triggered either periodically, immediately, or in response to a predetermined event.

33. The apparatus of claim 31, wherein in response to a measurement trigger criteria, the user equipment unit performs and reports inter-frequency measurements for the second frequency.

34. The apparatus of claim 33, wherein the measurement trigger criteria which causes the user equipment unit to perform and report inter-frequency measurements for the second frequency is the same criteria which is employed to cause the user equipment unit to perform and report intra-frequency measurements for the first frequency.

35. The apparatus of claim 31, wherein the measurement trigger criteria is one of being periodic, immediate, or in response to a predetermined event.

36. The apparatus of claim 31, wherein when the network issues an inter-frequency handover command to the user equipment unit, the user equipment unit switches to the virtual active set of base stations.

37. The apparatus of claim 31, wherein the network provides information regarding the virtual active set of base stations on the second frequency in a measurement control message.

38. The apparatus of claim 37, wherein the measurement control message is included in a DCCH control channel.

39. The apparatus of claim 37, wherein the measurement control message further includes one of a measurement parameter to be measured and a predetermined measurement event which triggers a measurement.

40. The apparatus of claim 31, wherein the network provides at least one member of the virtual active set of base stations on the second frequency in a virtual active set update procedure.

41. The apparatus of claim 31, wherein the user equipment unit receives from the network an authorization message that allows the user equipment unit to autonomously update the virtual active set of base stations.

42. The apparatus of claim 41, wherein the authorization message specifies one of an event or a parameter that can trigger the update of the virtual active set of base stations without the user equipment unit first having to send a measurement report to the network.

43. The network of claim 31, wherein the virtual active set of base stations on the second frequency is maintained by a second operator which differs from a first operator which maintains the current active set of base stations on the first frequency.

44. The apparatus of claim 31, wherein the virtual active set of base stations on the second frequency comprises a second network system which differs from a first network system provided on the first frequency.

45. The apparatus of claim 44, wherein the second network system is universal mobile telecommunications (UMTS) and the first network system is a Global System for Mobile (GSM) system.

46. The apparatus of claim 44, wherein the second network system is a system having soft intra-frequency handover and the first network system is universal mobile telecommunications (UMTS).

47. The apparatus of claim 31, wherein the network utilizes a frequency quality estimate to determine when to switch to the virtual active set of base stations.

48. The apparatus of claim 47, wherein the frequency quality estimate Q_{carrier j} of the active set on frequency j is a function of the following parameters:

$W_j,\underset{i=1}{\overset{N_{\mathrm{Aj}}}{\sum <br><br>}}<br><br>M_{\mathrm{ij}},M_{{\mathrm{Best}}_j}$

wherein;

M_i is a measurement result of cell i in the active set;

N_A is the number of cells in the active set;

M_Best is the measurement result of the strongest cell in the active set;

W is a parameter sent from the network to the user equipment unit.

49. The apparatus of claim 47, wherein the frequency quality estimate is based on two factors:

(1) a carrier Radio Signal Strength Indication (RSSI); and

(2) whether the Base Transceiver Station Identity Code/Base Station Identifier Code (BSIC) has been confined or not.

50. The apparatus of claim 47, wherein the network compares the frequency quality estimate to at least one threshold to determine when to switch to the virtual active set of base stations.

51. The apparatus of claim 50, wherein the at least one threshold is chosen to provide hysteresis protection.