Method and apparatus for measurement directed hard handoff in a CDMA system

• US 5,917,811 A
• Filed: 05/22/1996
• Issued: 06/29/1999
• Est. Priority Date: 05/22/1996
• Status: Expired due to Term

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Holcman, Toni Lee, Munsinger, David B., Ziv, Noam A., Weaver, Lindsay A. Jr., Baker, Kenneth R.
• Primary Examiner(s): Pham, Chi H.
• Assistant Examiner(s): LUTHER, WILLIAM A
• Application Number:
  US08/652,726
• Time in Patent Office: 1,133 Days
• Field of Search: 370/331, 370/332, 370/335, 370/336, 370/337, 370/491, 370/500, 370/479, 455/422, 455/436, 455/437, 455/438, 455/439, 455/440, 455/442, 455/443, 375/200, 375/205, 375/206
• US Class Current: 370/332
• CPC Class Codes: H04W 36/00835 Determination of neighbour cell lists
• CPC Class Codes: H04W 36/0085 Hand-off measurements
• CPC Class Codes: H04W 36/32 by location or mobility data, e.g. speed data

Abstract

In a communications network, a network user communicates using a remote unit with another user via at least one base station. The network is comprised of first and second mobile switching control stations respectively controlling communications through a first set of base stations including a first base station and through a second set of base stations including a second base station. To direct communications between the remote unit and the first and second base stations the first base station measures a round trip delay of an active communication signal between the first base station and the remote unit. The remote unit measures a first phase offset of a pilot signal received from a first candidate base station and reports it to the first mobile switching center via the first base station. The first mobile switching center calculates a candidate round trip delay between the remote unit and the first candidate base station based on the first phase offset and the round trip delay of the active communication signal. A active communication control unit accesses a measurement directed hard handoff table to determine a location of the remote unit based on the round trip delay corresponding to the first active communication signal and the candidate round trip delay.

Claims

1. In a communications network in which a network user communicates through a remote unit with another user via at least one base station, said network including first and second mobile switching centers for respectively controlling communications through a first set of base stations including a first base station and through a second set of base stations including a second base station, a method for directing communications between said remote unit and said first and second base stations comprising the steps of:

measuring at said first base station a round trip delay of an active communication signal between said first base station and said remote unit;

measuring at said remote unit a first phase offset of a pilot signal received from a first candidate base station;

reporting by said remote unit to said first mobile switching center via said first base station said first phase offset;

calculating at said first mobile switching center a candidate round trip delay between said remote unit and said first candidate base station based on said first phase offset and said round trip delay of said active communication signal;

entering a measurement directed hard handoff state by an active communication control unit in said first mobile switching center wherein said measurement directed hard handoff state enables said step of calculating; and

accessing by said active communication control unit a measurement directed hard handoff table to determine, based on said round trip delay corresponding to said active communication signal and said candidate round trip delay, whether the location of said remote unit corresponds to an area where a handoff is desirable;

wherein said measurement directed hard handoff table comprises an entry designating a type of action that should be taken based on a location of said remote unit.

2. The method of claim 1 further comprising the step of transmitting from each of said first set of base stations a common pilot sequence wherein each of said first set of base stations transmits said common pilot sequence at an identifying channel offset.

3. The method of claim 2 wherein said step of calculating said candidate round trip delay is computed as

4. The method of claim 1 wherein said first candidate base station is controlled by said first mobile switching center.

5. The method of claim 1 wherein said first candidate base station is controlled by said second mobile switching center.

6. The method of claim 1 wherein said first candidate base station is controlled by said second mobile switching center and said pilot signal from said first candidate base station is generated by a pilot beacon unit.

7. The method of claim 3 wherein said first candidate base station is controlled by said second mobile switching center and said pilot signal from said first candidate base station is generated by a pilot beacon unit.

8. The method of claim 1 further comprising the steps of:

storing at said remote unit a list of active base stations comprising an entry corresponding to each base station with which active communication is established;

storing at said remote unit a list of candidate base stations comprising an entry corresponding to each base station through which active communication may be possible but is not established and said list of candidate base stations has an entry for said first candidate base station; and

storing at said remote unit a list of neighbor base stations from which said list of candidate base stations is chosen.

9. The method of claim 1 wherein said active communication control unit is a selector.

10. The method of claim 1 wherein a first border set of base stations controlled by said first mobile switching center each have a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center further comprising the steps of:

storing at said remote unit a list of active base stations comprising an entry corresponding to each base station with which active communication is established; and

executing the step of entering said measurement directed hard handoff state only when said list of active base stations comprises an entry corresponding to one of said first border set of base stations.

11. The method of claim 1 wherein a first border set of base stations controlled by said first mobile switching center each have a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center further comprising the steps of:

storing at said remote unit a list of active base stations comprising an entry corresponding to each base station with which active communication is established; and

executing the step of entering said measurement directed hard handoff state only when each entry on said list of active base stations corresponds to said first border set of base stations.

12. The method of claim 1 wherein a first border set of base stations controlled by said first mobile switching center each have a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center further comprising the steps of:

wherein said remote unit stores a list of active base stations comprising an entry corresponding to each base station with which active communication is established; and

executing the step of entering said measurement directed hard handoff state only when said list of active base stations comprises a single entry and said single entry corresponds to one of said first border set of base stations.

13. The method of claim 1 further comprising the step of accessing by said active communication control unit a measurement directed hard handoff table to determine a location of said remote unit based on said round trip delay corresponding to said active communication signal and said candidate round trip delay.

14. The method of claim 1 wherein said type of action that should be taken is a handoff from said first base station which is communicating with said remote unit using code division multiple access (CDMA) to said second base station operating using an alternative modulation technology.

15. The method of claim 14 wherein said alternative modulation technology is frequency modulation (FM.).

16. The method of claim 14 wherein said alternative modulation technology is time division multiple access (TDMA.).

17. The method of claim 1 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said second base station communicating at a second frequency using CDMA.

18. The method of claim 1 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said first base station communicating with said remote unit using an alternative modulation technology.

19. The method of claim 1 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said first base station communicating at a second frequency using CDMA.

20. The method of claim 1 wherein said active communication signal is comprised of a set of multipath signals each having a different delay and wherein said round trip delay of said active communication signal is the round trip delay corresponding to the earliest arriving multipath signal of said set of multipath signals.

21. The method of claim 1 wherein said step of accessing is executed when said round trip delay of said active communication signal changes by a predetermined amount.

22. The method of claim 1 wherein said step of accessing is executed whenever said step of reporting by said remote unit is repeated.

23. The method of claim 1 wherein said active communication signal is a spread spectrum signal which is spread by a pseudorandom noise (PN) sequence and wherein said round trip delay of said active communication signal is measured in PN chips.

24. The method of claim 3 wherein said active communication signal is a spread spectrum signal which is spread by a pseudorandom noise (PN) sequence and wherein said round trip delay of said active communication signal is measured in PN chips, said identifying channel offset is measured in PN chips, and said candidate round trip delay is measured in PN chips.

29. The method of claim 24 wherein said alternative modulation technology is frequency modulation (FM.).

76. The method of claim 1 wherein said first base station defines a first coverage area and wherein said first coverage area contracts in size when the number of remote units communicating with said first base station increases.

77. The method of claim 1 further comprising the step of deploying at said first base station a base station breathing function for balancing a forward link coverage area to a reverse link coverage area.

78. The method of claim 1 wherein a first border set of base stations controlled by said first mobile switching center each have a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center, further comprising the step of deploying at said first base station, if said first base station is one of said first border set of base stations, a border base station breathing function for balancing a forward link coverage area to a reverse link coverage area which is different from a base station breathing function deployed by other base stations of said first set of base stations.

25. In a communications network in which a network user communicates through a remote unit with another user via at least one base station, said network including first and second mobile switching centers for respectively controlling communications through a first set of base stations including a first base station and through a second set of base stations including a second base station, a method for directing communications between said remote unit and said first and second base stations comprising the steps of:

measuring at said first base station a round trip delay of an active communication signal between said first base station and said remote unit;

entering a measurement directed hard handoff state by an active communication control unit located in said first mobile switching center; and

accessing by said active communication control unit a measurement directed hard handoff table to determine based on said round trip delay corresponding to said active communication signal whether a location of said remote unit corresponds to an area where a handoff is desirable;

wherein said measurement directed hard handoff table comprises an entry specifying a type of action that should be taken based on said location of said remote unit.

26. The method of claim 25 further comprising the step of transmitting from each of said first set of base stations a common pilot sequence wherein each of said first set of base stations transmits said common pilot sequence at an identifying channel offset.

27. The method of claim 25 wherein said active communication control unit is a selector.

28. The method of claim 25 wherein said type of action that should be taken is a handoff from said first base station which is communicating with said remote unit using code division multiple access (CDMA) to said second base station operating using an alternative modulation technology.

30. The method of claim 28 wherein said alternative modulation technology is time division multiple access (TDMA.).

31. The method of claim 25 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said second base station communicating at a second frequency using CDMA.

32. The method of claim 25 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said first base station communicating with said remote unit using an alternative modulation technology.

33. The method of claim 25 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said first base station communicating at a second frequency using CDMA.

34. The method of claim 25 wherein said active communication signal is comprised of a set of multipath signals each having a different delay and wherein said round trip delay of said active communication signal is the round trip delay corresponding to the earliest arriving multipath signal of said set of multipath signals.

35. The method of claim 25 further comprising said step of executing said step of accessing when said round trip delay of said active communication signal changes by a predetermined amount.

36. A communications network comprising:

a remote unit through which a network user communicates with another user via at least one base station wherein said remote unit measures a first phase offset of a pilot signal received from a first candidate base station;

a first set of base stations including a first base station wherein said first base station measures a round trip delay of an active communication signal between said first base station and said remote unit;

a first mobile switching center for controlling communications through said first set of base stations wherein said first mobile switching center receives a message from said remote unit comprising said first phase offset and receives said round trip delay of said active communication signal from said first base station and calculates a candidate round trip delay between said remote unit and said first candidate base station therefrom;

an active communication control unit in said first mobile switching center which enters a measurement directed hard handoff state in which said active communication control unit calculates said candidate round trip delay;

a measurement directed hard handoff table which is accessed by said active communication control unit to determine, based on said round trip delay corresponding to said active communication signal and said candidate round trip delay, whether the location of said remote unit corresponds to an area where a handoff is desirable; and

wherein said measurement directed hard handoff table comprises an entry designating a type of action that should be taken based on a location of said remote unit.

37. The network of claim 36 wherein each of said first set of base stations transmits a common pilot sequence and wherein each of said first set of base stations transmits said common pilot sequence at an identifying channel offset.

38. The network of claim 37 wherein said first mobile switching center calculates said candidate round trip delay as:

space="preserve" listing-type="equation">RTD.sub.can1 =RTD.sub.ref +2*(pilotphase.sub.can -ChannelOffset.sub.can1 *PilotInc)

where;

RTD_can1 =said candidate round trip delay;

RTD_ref =said round trip delay of said active communication signal;

PilotPhase_can1 =said first phase offset; and

ChannelOffset_can1 * PilotInc=an identifying channel offset corresponding to said first candidate base station.

39. The network of claim 36 wherein said first candidate base station is controlled by said first mobile switching center.

40. The network of claim 36 wherein said first candidate base station is controlled by said second mobile switching center.

41. The network of claim 36 wherein said first candidate base station is controlled by said second mobile switching center and said pilot signal from said first candidate base station is generated by a pilot beacon unit.

42. The network of claim 36:

wherein said remote unit stores a list of active base stations comprising an entry corresponding to each base station with which active communication is established;

wherein said remote unit stores a list of candidate base stations comprising an entry corresponding to each base station through which active communication may be possible but is not established and said list of candidate base stations has an entry for said candidate base station; and

wherein said remote unit stores a list of neighbor base stations from which said list of candidate base stations is chosen.

43. The network of claim 36 wherein said active communication control unit is a selector.

44. The network of claim 36 wherein said remote unit stores a list of active base stations comprising an entry corresponding to each base station with which active communication is established, further comprising a first border set of base stations controlled by said first mobile switching center each have a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center wherein said active communication control unit enters said measurement directed hard handoff state only when said list of active base stations comprises an entry corresponding to one of said first border set of base stations.

45. The network of claim 36 wherein said remote unit stores a list of active base stations comprising an entry corresponding to each base station with which active communication is established, further comprising a first border set of base stations controlled by said first mobile switching center each have a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center wherein said active communication control unit enters said measurement directed hard handoff state when each entry on said list of active base stations corresponds to said first border set of base stations.

46. The network of claim 36 wherein said remote unit stores a list of active base stations comprising an entry corresponding to each base station with which active communication is established further comprising a first border set of base stations controlled by said first mobile switching center each have a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center wherein said active communication control unit enters said measurement directed hard handoff state only when said list of active base stations comprises a single entry and said single entry corresponds to one of said first border set of base stations.

47. The network of claim 36 further comprising a measurement directed hard handoff table which is accessed by said active communication control unit to determine a location of said remote unit based on said round trip delay corresponding to said active communication signal and said candidate round trip delay.

48. The network of claim 36 wherein said type of action that should be taken is a handoff from said first base station which is communicating with said remote unit using code division multiple access (CDMA) to said second base station operating using an alternative modulation technology.

49. The network of claim 48 wherein said alternative modulation technology is frequency modulation (FM.).

50. The network of claim 48 wherein said alternative modulation technology is time division multiple access (TDMA.).

51. The network of claim 36 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said second base station communicating at a second frequency using CDMA.

52. The network of claim 36 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said first base station communicating with said remote unit using an alternative modulation technology.

53. The network of claim 36 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said first base station communicating at a second frequency using CDMA.

54. The network of claim 36 wherein said active communication signal is comprised of a set of multipath signals each having a different delay and wherein said round trip delay of said active communication signal is the round trip delay corresponding to the earliest arriving multipath signal of said set of multipath signals.

55. The network of claim 36 wherein said measurement directed hard handoff table is accessed when said round trip delay of said active communication signal changes by a predetermined amount.

56. The apparatus of claim 36 wherein said measurement directed hard handoff table is accessed whenever said first mobile switching center receives said message from said remote unit comprising said first phase offset.

57. The network of claim 36 wherein said active communication signal is a spread spectrum signal which is spread by a pseudorandom noise (PN) sequence and wherein said round trip delay of said active communication signal is measured in PN chips.

58. The network of claim 37 wherein said active communication signal is a spread spectrum signal which is spread by a pseudorandom noise (PN) sequence and wherein said round trip delay of said active communication signal is measured in PN chips, said identifying channel offset is measured in PN chips, and said candidate round trip delay is measured in PN chips.

79. The network of claim 36 wherein said first base station defines a first coverage area and wherein said first coverage area contracts in size when the number of remote units communicating with said first base station increases.

80. The network of claim 36 wherein said first base station deploys a base station breathing function for balancing a forward link coverage area to a reverse link coverage area.

81. The network of claim 36 further comprising a first border set of base stations controlled by said first mobile switching center, each of said first border set of base stations have a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center and wherein said first base station is one of said first border set of base stations and said first base station deploys a first base station breathing function for balancing a forward link coverage area to a reverse link coverage area which is different from a base station breathing function deployed by other base stations of said first set of base stations.

59. A communications network comprising:

a remote unit through which a network user communicates with another user via at least one base station;

a first and second mobile switching centers for respectively controlling communications through a first set of base stations and through a second set of base stations including a second base station;

a first base station which is a member of said first set of base stations which measures a round trip delay of an active communication signal between said first base station and said remote unit;

an active communication control unit located in said first mobile switching center having a measurement directed hard handoff state in which said active communication control unit accesses a measurement directed hard handoff table to determine based on said round trip delay corresponding to said active communication signal whether a location of said remote unit corresponds to an area where a handoff is desirable;

wherein said measurement directed hard handoff table comprises an entry specifying a type of action that should be taken based on said location of said remote unit.

60. The network of claim 59 wherein each of said first set of base stations transmits a common pilot sequence and wherein each of said first set of base stations transmits said common pilot sequence at an identifying channel offset.

61. The network of claim 59 wherein said active communication control unit is a selector.

62. The network of claim 59 wherein said type of action that should be taken is a handoff from said first base station which is communicating with said remote unit using code division multiple access (CDMA) to said second base station operating using an alternative modulation technology.

63. The network of claim 62 wherein said alternative modulation technology is frequency modulation (FM.).

64. The network of claim 62 wherein said alternative modulation technology is time division multiple access (TDMA.).

65. The network of claim 59 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said second base station communicating at a second frequency using CDMA.

66. The network of claim 59 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said first base station communicating with said remote unit using an alternative modulation technology.

67. The network of claim 59 wherein said type of action that should be taken is a handoff from said first base station which is communicating at a first frequency with said remote unit using CDMA to said first base station communicating at a second frequency using CDMA.

68. The network of claim 59 wherein said active communication signal is comprised of a set of multipath signals each having a different delay and wherein said round trip delay of said active communication signal is the round trip delay corresponding to the earliest arriving multipath signal of said set of multipath signals.

69. The network of claim 59 wherein said active communication signal accesses said measurement directed hard handoff table when said round trip delay of said active communication signal changes by a predetermined amount.

70. In a communications network in which a network user communicates through a remote unit with another user via at least one base station, said network including first and second mobile switching centers for respectively controlling communications through a first set of base stations including a first base station and through a second set of base stations including a second base station, a method for directing communications between said remote unit and said first and second base stations comprising the steps of:

measuring at said first base station a round trip delay of an active communication signal between said first base station and said remote unit;

measuring at said remote unit a first phase offset of a pilot signal received from a first candidate base station;

reporting by said remote unit to said first mobile switching center via said first base station said first phase offset; and

calculating at said first mobile switching center a candidate round trip delay between said remote unit and said first candidate base station based on said first phase offset and said round trip delay of said active communication signal;

storing at said remote unit a list of active base stations comprising an entry corresponding to each base station with which active communication is established;

storing at said remote unit a list of candidate base stations comprising an entry corresponding to each base station through which active communication may be possible but is not established and said list of candidate base stations has an entry for said first candidate base station; and

storing at said remote unit a list of neighbor base stations from which said list of candidate base stations is chosen;

measuring at said remote unit a power level of a pilot signal corresponding to a first entry on said list of neighbor base stations and if said power level exceeds an add threshold said first entry on said list of neighbor base stations is added to said list of candidate base stations; and

wherein if said remote unit is communicating with one of a first border set of base stations controlled by said first mobile switching center, each of said first border set of base stations having a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center, said remote unit has a lower value of said add threshold than other remote units communicating with at least one base station of said first set of base stations.

71. In a communications network in which a network user communicates through a remote unit with another user via at least one base station, said network including first and second mobile switching centers for respectively controlling communications through a first set of base stations including a first base station and through a second set of base stations including a second base station, a method for directing communications between said remote unit and said first and second base stations comprising the steps of:

measuring at said first base station a round trip delay of an active communication signal between said first base station and said remote unit;

measuring at said remote unit a first phase offset of a pilot signal received from a first candidate base station;

reporting by said remote unit to said first mobile switching center via said first base station said first phase offset; and

calculating at said first mobile switching center a candidate round trip delay between said remote unit and said first candidate base station based on said

first phase offset and said round trip delay of said active communication signal;

storing at said remote unit a list of active base stations comprising an entry corresponding to each base station with which active communication is established;

storing at said remote unit a list of candidate base stations comprising an entry corresponding to each base station through which active communication

may be possible but is not established and said list of candidate base stations has an entry for said first candidate base station; and

storing at said remote unit a list of neighbor base stations from which said list of candidate base stations is chosen;

measuring by said remote unit a power level of said first base station and if said power level is less than a drop threshold an entry on said list of active base stations corresponding to said first base station is removed; and

wherein if said remote unit is communicating with one of a first border set of base stations controlled by said first mobile switching center, each of said first border set of base stations having a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center, said remote unit has a lower value of said drop threshold than other remote units communicating with at least one of said first set of base stations.

72. In a communications network in which a network user communicates through a remote unit with another user via at least one base station, said network including first and second mobile switching centers for respectively controlling communications through a first set of base stations including a first base station and through a second set of base stations including a second base station, a method for directing communications between said remote unit and said first and second base stations comprising the steps of:

measuring at said first base station a round trip delay of an active communication signal between said first base station and said remote unit;

measuring at said remote unit a first phase offset of a pilot signal received from a first candidate base station;

reporting by said remote unit to said first mobile switching center via said first base station said first phase offset; and

calculating at said first mobile switching center a candidate round trip delay between said remote unit and said first candidate base station based on said first phase offset and said round trip delay of said active communication signal;

storing at said remote unit a list of active base stations comprising an entry corresponding to each base station with which active communication is established;

storing at said remote unit a list of candidate base stations comprising an entry corresponding to each base station through which active communication may be possible but is not established and said list of candidate base stations has an entry for said first candidate base station;

storing at said remote unit a list of neighbor base stations from which said list of candidate base stations is chosen;

measuring at said remote unit a power level of said first base station and if said power level is less than a drop threshold for longer than a predetermined amount of time, an entry on said list of active base stations corresponding to said first base station is removed; and

wherein if said remote unit is in communication with one of a first border set of base stations controlled by said first mobile switching center, each of said first border set of base stations having a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center, said remote unit has a longer value of said predetermined amount of time than other remote units communicating with at least one base station of said first set of base stations.

73. A communications network comprising:

a remote unit through which a network user communicates with another user via at least one base station wherein said remote unit measures a first phase offset of a pilot signal received from a first candidate base station;

a first set of base stations including a first base station wherein said first base station measures a round trip delay of an active communication signal between said first base station and said remote unit;

a first mobile switching center for controlling communications through said first set of base stations wherein said first mobile switching center receives a message from said remote unit comprising said first phase offset and receives said round trip delay of said active communication signal from said first base station and calculates a candidate round trip delay between said remote unit and said first candidate base station therefrom;

a second set of base stations including a second base station;

a second mobile switching center for controlling communications through said second set of base stations;

wherein said remote unit stores a list of active base stations comprising an entry corresponding to each base station with which active communication is established;

wherein said remote unit stores a list of candidate base stations comprising an entry corresponding to each base station through which active communication may be possible but is not established and said list of candidate base stations has an entry for said candidate base station;

wherein said remote unit stores a list of neighbor base stations from which said list of candidate base stations is chosen;

wherein said remote unit measures a power level of a pilot signal corresponding to a first entry on said list of neighbor base stations and if said power level exceeds an add threshold said first entry on said list of neighbor base stations is added to said list of candidate base stations; and

wherein if said remote unit is communicating with one of a first border set of base stations controlled by said first mobile switching center, each of said first border set of base stations having a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center, said remote unit has a lower value of said add threshold than other remote units communicating with at least one base station of said first set of base stations.

74. A communications network comprising:

a remote unit through which a network user communicates with another user via at least one base station wherein said remote unit measures a first phase offset of a pilot signal received from a first candidate base station;

a first set of base stations including a first base station wherein said first base station measures a round trip delay of an active communication signal between said first base station and said remote unit;

a first mobile switching center for controlling communications through said first set of base stations wherein said first mobile switching center receives a message from said remote unit comprising said first phase offset and receives said round trip delay of said active communication signal from said first base station and calculates a candidate round trip delay between said remote unit and said first candidate base station therefrom;

a second set of base stations including a second base station;

a second mobile switching center for controlling communications through said second set of base stations;

wherein said remote unit stores a list of active base stations comprising an entry corresponding to each base station with which active communication is established;

wherein said remote unit stores a list of candidate base stations comprising an entry corresponding to each base station through which active communication may be possible but is not established and said list of candidate base stations has an entry for said candidate base station;

wherein said remote unit stores a list of neighbor base stations from which said list of candidate base stations is chosen;

wherein said remote unit measures a power level of said first base station and if said power level is less than a drop threshold an entry on said list of active base stations corresponding to said first base station is removed; and

wherein if said remote unit is communicating with one of a first border set of base stations controlled by said first mobile switching center, each of said first border set of base stations having a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center, said remote unit has a lower value of said drop threshold than other remote units communicating with at least one of said first set of base stations.

75. A communications network comprising:

a remote unit through which a network user communicates with another user via at least one base station wherein said remote unit measures a first phase offset of a pilot signal received from a first candidate base station;

a first set of base stations including a first base station wherein said first base station measures a round trip delay of an active communication signal between said first base station and said remote unit;

a first mobile switching center for controlling communications through said first set of base stations wherein said first mobile switching center receives a message from said remote unit comprising said first phase offset and receives said round trip delay of said active communication signal from said first base station and calculates a candidate round trip delay between said remote unit and said first candidate base station therefrom;

a second set of base stations including a second base station;

a second mobile switching center for controlling communications through said second set of base stations;

wherein said remote unit stores a list of active base stations comprising an entry corresponding to each base station with which active communication is established;

wherein said remote unit stores a list of candidate base stations comprising an entry corresponding to each base station through which active communication may be possible but is not established and said list of candidate base stations has an entry for said candidate base station;

wherein said remote unit stores a list of neighbor base stations from which said list of candidate base stations is chosen;

wherein said remote unit measures a power level of said first base station and if said power level is less than a drop threshold for longer than a predetermined amount of time, an entry on said list of active base stations corresponding to said first base station is removed; and

wherein if said remote unit is in communication with one of a first border set of base stations controlled by said first mobile switching center, each of said first border set of base stations having a coverage area that abuts a coverage area corresponding to a base station controlled by said second mobile switching center, said remote unit has a longer value of said predetermined amount of time than other remote units communicating with at least one base station of said first set of base stations.