Mobile station assisted soft handoff in a CDMA cellular communications system

• US 5,640,414 A
• Filed: 04/11/1994
• Issued: 06/17/1997
• Est. Priority Date: 03/05/1992
• Status: Expired due to Term

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Blakeney, Robert D. II, Tiedemann, Edward G. Jr., Karmi, Gadi, Weaver, Lindsay A. Jr.
• Primary Examiner(s): Cain, David C.
• Application Number:
  US08/226,222
• Time in Patent Office: 1,163 Days
• Field of Search: 375/1, 375/68, 379/60, 379/59, 455/33, 455/34, 455/56, 380/34, 370/18, 370/21, 370/22
• US Class Current: 375/130
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection
• CPC Class Codes: H04W 36/304 due to measured or perceived resources with higher communication quality

Abstract

In a code division multiple access (CDMA) spread spectrum cellular communication system in which a mobile station user communicates with another system user via at least one base station, wherein each base station transmits a common pilot signal of a different code phase with respect to other base stations in said system, a method for directing communications between said mobile station user and said base stations. The mobile station monitors the signal strength of pilots and reports the measured signal strength to a system controller via the base station through which it is communicating. Command messages from the system controller to a new base station and the mobiles station establishes communication through the new base station in addition to the communication through the current base station. When the mobile station detects the falling below a predetermined level of the signal strength of a pilot corresponding to at least one of the base stations through which the mobile station is communicating, the mobile station reports the measured signal strength indicative of the corresponding base station to the system controller via the base stations through which it is communicating. Command messages from the system controller to the identified base station and the mobiles station terminates communication through the corresponding base station while communications through the other base station or base stations continue.

Claims

1. In a cellular communication system in which a mobile user communicates with other users via at least one base station, wherein each of a plurality of base stations within said system transmits a unique pilot signal, a mobile station transceiver comprising:

a pilot signal strength measurement circuit for measuring strength of base station transmitted pilot signals and for generating a first signal strength message when one or more of said measured pilot signals, from base stations not currently in communication with said mobile station transceiver, exceed a first predetermined level;

a mobile station transmitter for transmitting said first signal strength message to at least one of said base stations with which said mobile user is currently in communication; and

a mobile station receiver for receiving a first direction signal from at least one of said base stations with which said mobile user is currently in communication;

wherein in response to said first received direction signal communication is established between said mobile station transceiver and at least one of said base stations from which were received said measured pilot signals exceeding said first predetermined level.

2. The mobile station transceiver of claim 1 wherein said pilot signal strength measurement circuit includes means for generating a second signal strength message when one or more of said measured pilot signals, from at least one of said base stations currently in communication with said mobile station transceiver, fail below a second predetermined level.

3. The mobile station transceiver of claim 2 wherein said mobile station transmitter operates to transmit said second signal strength message to at least one of said base stations currently in communication with said mobile station transceiver, and wherein said mobile station receiver includes means for receiving a second direction signal from at least one of said base stations with which said mobile unit transceiver is currently in communication, said mobile station transceiver including a controller for terminating communication, in response to said second received direction signal, with at least one of said base stations from which were received said measured pilot signals below said second predetermined level.

4. In a cellular communication system in which a mobile user communicates through a mobile station transceiver with other users via at least one base station, wherein each base station in said system transmits a unique pilot signal and wherein each base station is in communication with a system controller, said mobile station transceiver comprising:

a pilot signal strength measurement circuit for measuring signal strength of base station transmitted pilot signals from base stations included in a first list identifying a first set of said base stations;

a comparator for comparing said measured pilot signal strengths to a first predetermined level;

means for generating a second list identifying a second set of base stations by including within said second list those of said base stations on said first list for which said measured pilot signal strengths exceeded said first predetermined level;

a mobile station transmitter for transmitting said second list to at least a first of said base stations with which said mobile station transceiver is currently in communication;

a mobile station receiver for receiving, from at least one of said base stations with which said mobile unit transceiver is currently in communication, a third list identifying a third set of base stations available for communication with said mobile station transceiver, said third list being generated by said system controller in accordance with said second list of base stations and on the basis of base station availability information; and

a mobile station controller for directing said mobile station transceiver to establish communication with said third set of base stations.

5. The mobile station transceiver of claim 4 wherein said pilot signal strength measurement circuit operates to measure signal strength of pilot signals transmitted by each base station of said second and third sets of base stations, and wherein said mobile station transmitter includes means for transmitting a particular base station listing of said second list to at least one of said base stations with which said mobile station transceiver is currently in communication, wherein a pilot signal strength measurement of a particular base station corresponding to said particular base station listing exceeds by a predetermined amount a pilot signal strength measurement of a base station corresponding to a base station listing of said third list.

6. The mobile station transceiver of claim 5 wherein said mobile station receiver further comprises:

means for receiving, from at least one of said base stations with which said mobile station transceiver is currently in communication, an updated third list comprising said particular base station listing, said updated third list being generated by said system controller on the basis of availability of resources at said particular base station;

means for removing said particular base station listing from said second list; and

means for establishing communication with said particular base station.

7. The mobile station transceiver of claim 4 further comprising:

means for generating a fourth list identifying a fourth set of base stations, said fourth list identifying all base stations unlisted in said first, second, or third lists, said pilot signal strength measurement circuit being operative to measure signal strength of said pilot signals transmitted by each base station of said fourth set of base stations;

means for comparing said signal strength measurements of said fourth set of base stations to said first predetermined level; and

means for removing a second particular base station listing from said fourth list and placing said second particular base station listing in said second list if the signal strength measurement of said second particular base station corresponding to said second particular base station listing exceeds said first predetermined level.

8. The mobile station transceiver of claim 7 further comprising:

means for removing, from said first list, each base station listing that has been in said first list for more than a predetermined time interval and for placing each such base station listing from said first list in said fourth list.

9. The mobile station transceiver of claim 4 further comprising:

means for removing, from said second list, each base station listing having a corresponding signal strength measurement less than a second predetermined level for at least a predetermined time interval and for placing each such base station listing from said second list in said first list.

10. The mobile station of claim 7 further comprising:

means for removing, for each base station having a corresponding signal strength measurement less than a second predetermined level for at least a predetermined time interval, if said first list contains more than a predetermined number of base station listings, each corresponding base station listing from said second list and placing each such base station listing in said fourth list.

11. In a cellular telephone system in which a mobile system user and another system user communicate user information signals therebetween via at least one of a plurality of geographically separated cell-sites each defining a respective geographic service area, said cellular telephone system including a system controller for directing communications between system users of said cellular telephone system, a mobile station transceiver utilized by said mobile system user for communicating with said another system user via said at least one of said plurality of cell-sites, said mobile station transceiver comprising:

a mobile station controller for determining, while said mobile system user is in a service area of one cell-site and communicating user information signals with said another system user via said one cell-site, a transition of said mobile system user from said one cell-site service area to a service area of another cell-site, said system controller being operative to generate a handoff request in response to said determination of said transition into said service area of said another cell-site;

a mobile station transmitter for transmitting said handoff request to said one cell-site identifying said another cell-site; and

means for concurrently communicating said user information signals between said mobile station transceiver and said one and another cell-sites, said concurrent communication being established by said system controller in accordance with said handoff request.

12. The system of claim 11 wherein each cell-site transmits a pilot signal indicative of said transmitting cell-site, said mobile station transceiver further comprising:

a scanning receiver for receiving said cell-site transmitted pilot signals, measuring signal strength of each received pilot signal, comparing pilot signal strength measurements, and providing a signal strength signal indicative of a received pilot signal of greatest measured signal strength and cell-site of origin; and

a processor for receiving said signal strength signal and for, when said signal strength signal indicates when said another cell-site transmitted pilot signal is of a signal strength greater than said one cell-site transmitted pilot signal, generating said handoff request.

13. In a cellular communication system in which a user at a mobile station communicates with another system user via at least one base station, wherein each base station in said system transmits a unique pilot signal, a method for directing communications between said mobile station user and said base stations comprising the steps of:

measuring at said mobile station signal strength of base station transmitted pilot signals and generating a signal strength message when one or more of said measured pilot signals, from base stations not currently in communication with said mobile station, exceed a first predetermined level;

establishing, upon generation of said signal strength message, communication between said mobile station and at least one of the one or more base stations from which were transmitted said one or more measured pilot signals exceeding said first predetermined level.

14. The method of claim 13 further comprising the steps of:

communicating said signal strength message, from said mobile station to at least one base station said mobile station is in communication with; and

communicating to said mobile station, via said at least one base station said mobile unit is in communication with, a direction message indicating said mobile station is to establish communications through each of said one or more base stations from which were transmitted said one or more measured pilot signals exceeding said first predetermined level.

15. The method of claim 13 in which said mobile station user communicates with said another system user via said at least one base station using spread spectrum communication signals.