Radiotelephone locating and handoff using alternative criteria

• US 5,513,246 A
• Filed: 11/12/1991
• Issued: 04/30/1996
• Est. Priority Date: 12/07/1990
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Chambert, Georg W. R., Bodin, Stig R., Jonsson, Sven-Olof L.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Shankar, Vijay
• Application Number:
  US07/790,170
• Time in Patent Office: 1,631 Days
• Field of Search: 379/59, 379/60, 455/33.1, 455/33.2, 455/34.1
• US Class Current: 455/443
• CPC Class Codes: H04W 36/30 by measured or perceived connection quality data
• CPC Class Codes: Y02D 30/70 in wireless communication networks

Abstract

In a cellular mobile radiotelephone system having a plurality of cells providing radio coverage by a plurality of base stations and having a plurality of mobile stations, the location of a mobile station with respect to cell boundaries is identified using measurements, performed at the mobile station or at selected cells, of transmissions from the cells or transmissions from the mobile station. When a cell is found that meets a specified standard with respect to a call connection in progress according to a first radio-signal criterion, a handoff candidate cell is selected according a second different radio-signal criterion. When no cell is found that meet the specified standard, a cell is selected according to the first radio-signal criterion. Handoff to the selected cell is attempted if that cell is not presently handling that call. More particularly, the first radio-signal criterion may be signal strength and the second radio-signal criterion may be path loss. When a cell is found that meets the signal-strength criterion, the cell is determined to have sufficient signal strength margin so that the path loss criterion may be used instead. As mentioned above, use of the path loss criterion has the advantages of conserving power, reducing interference, and allowing effective cell boundaries to remain fixed. When no cell is found that meets the signal-strength criterion, the call connection is determined to be signal-strength limited such that the signal strength criterion is used to select the cell having the greatest signal strength.

Claims

1. In a cellular mobile radiotelephone system having a plurality of cells each providing radio coverage by a base station and having a plurality of mobile stations each associated with one of said cells, a method of call handoff from a first cell to a second cell by using a radio signal criterion based on a path loss relationship between said first and second cells in order to prevent a call in progress being dropped before said handoff, said method comprising the steps of:

determining a number of candidate cells for handoff which meet a specified standard according to a signal strength criterion;

selecting one of said determined cells if this one cell in addition meets said first radio signal path-loss criterion;

when no cell is found that meets said specified standard, selecting a cell according to said radio-signal strength criterion and disregarding said radio signal path-loss criterion; and

attempting handoff to said selected cell.

2. The method of claim 1 wherein said specified standard requires that a radio link from a candidate cell to a mobile station with respect to which handoff is being decided and a radio link in the opposite direction from the mobile station to said candidate cell both have signal strength margin of a predetermined mount.

3. A call handoff apparatus in a cellular mobile radiotelephone system having a plurality of cells each providing radio coverage by a base station and having a plurality of mobile stations each associated with one of said cells, said apparatus comprising:

means for determining a number of candidate cells for a hand-off which meet a specified standard according to a signal strength criterion;

means for, when a cell is found that meets said specified standard, selecting a cell according to a radio-signal path-loss criterion;

means for, when no cell is found that meets said specified standard, selecting a cell according to said radio-signal strength criterion and disregarding said radio path-loss criterion; and

means for attempting handoff to said selected cell.

4. The apparatus of claim 3 wherein said specified standard requires that a radio link from a candidate cell to a mobile station with respect to which handoff is being decided and a radio link in the opposite direction from the mobile station to said candidate cell both have signal strength margin of a predetermined amount.

5. In a cellular mobile radiotelephone system, a method of call handoff comprising the steps of:

ranking cells according to a radio-signal strength criterion;

when one or more cells meet a specified standard according to said radio-signal strength criterion, ranking said one or more cells according to a radio-signal path loss criterion;

forming a final list by ranking said one or more cells ranked according to radio-signal path-loss ahead of cells that did not meet said specified standard according to radio-signal strength criterion; and

attempting handoff of said call handoff to a highest-ranked cell on said final list when a cell handling said call connection is not highest ranked.

6. In a cellular mobile radio system including a mobile station and number of base stations each serving a specific service area, said base stations including a communicating base station communicating call information with said mobile station and serving one area and a neighboring base station serving another area, wherein said one area and a neighboring area have a common area in which communications both between the communicating base station and the mobile station and between the neighboring base station and the mobile station have sufficient signal level, a method of locating said mobile station in the system, comprising the steps of:

a) using a first criterion when said mobile station is within said common area, said first criterion involving determining the radio path loss of radio signals between the communicating base station and the mobile station, and

b) using a second criterion when said mobile station is outside said common area, said second criterion involving determining the strength of the signals between the communicating base station and the mobile station.

7. In a cellular mobile radio system including a mobile station and number of base stations each serving a specific service area, said base stations including a communicating base station communicating call information with said mobile station and serving one area and a neighboring base station serving another area, said communicating base station serving said one area with a lower radio signal power than said neighboring base station serves said another area with said one area and said another area having a common area in which communications both between the communicating base station and the mobile station and between the neighboring base station and the mobile station have sufficient signal level, a method of locating said mobile station in the system, comprising the steps of:

a) using a first criterion when said mobile station is within said common area, said first criterion involving determining the radio path loss of radio signals between the communicating base station and the mobile station, and

b) using a second criterion when said mobile station is outside said common area, said second criterion involving determining the strength of the signals between the communicating base station and the mobile station.

8. A cell handoff method in a cellular mobile radio telephone system having a plurality of cell sites and a mobile station, comprising the steps of:

a) determining associated up-link signal strength at a plurality of cell sites of transmissions from said mobile station and associated down-link signal strength at said mobile station of transmissions from said plurality of cell sites;

b) designating cell sites whose associated up-link and down-link signal strengths are both at least a sufficient level;

c) determining path loss between said mobile station and said designated cell sites;

d) ordering said designated cell sites in order of increasing path loss;

e) designating cell sites whose associated up-link and down-link signal strengths both exceed a minimum level;

f) ordering said cell sites designated in step (e) in order of increasing difference between said sufficient level and a lesser one of said up-link signal strength and said down-link signal strength;

g) ordering the cell sites designated in step b) ahead of the cell sites designated in step e) while preserving the order among the cell sites designated in step b) and preserving the order among the cell sites designated in step e); and

h) attempting handoff of said mobile station call to a cell site ordered first among cell sites ordered as in step g) when said cell site is not already handling said call.

9. The call handoff method of claim 8 wherein the cell sites designated in step b) are designated in a first list, the cell sites designated in step e) are designated in a second list, and said ordering step g) comprises concatenating said first and second lists.