Cellular mobile communications system using frequencies commonly shared by neighboring cells for handoff operations

• US 5,319,795 A
• Filed: 02/27/1992
• Issued: 06/07/1994
• Est. Priority Date: 02/27/1991
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Hamabe, Kojiro, Kanai, Toshihito, Taketsugu, Masanori, Furuya, Yukitsuna
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): PHAM, CHI H
• Application Number:
  US07/843,914
• Time in Patent Office: 831 Days
• Field of Search: 455/33.1-33.4, 455/34.1, 455/54.1, 455/54.2, 455/56.1, 455/67.1, 455/137, 379/59, 379/60, 379/63
• US Class Current: 455/451
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection

Abstract

In a cellular mobile communication system, each base station is allocated common frequencies shared with other base stations in addition to unique frequencies. One of the unique frequencies is assigned by a base station to a mobile terminal in response to a call request. The field strength of a signal from that mobile terminal is detected and one of the common frequencies is assigned to it if the detected field strength becomes lower than a threshold. The base station receives a field strength signal from a neighboring station indicating the field strength of a version of the signal at the neighboring station and compares it with a threshold, and combines the signal of that mobile terminal with the version of the signal received by and relayed from the neighboring station when the field strength of that version of the signal is higher than the threshold. At the given mobile station, a signal from the base station which assigned the common frequency is combined with a signal from the neighboring station. In a modification, the field strength of signals from surrounding base stations is detected by mobile terminals and a start-of-diversity-mode request is transmitted when the field strength of one of the surrounding stations is higher than a threshold and an end-of-diversity-mode request signal is transmitted when the field strength of such a surrounding station is lower than the threshold. The base station responds to these requests for effecting diversity reception.

Claims

1. A cellular mobile communications system for serving mobile terminals within a plurality of cells, the system including a plurality of base stations respectively located at the cell sites;

each of said base stations having unique frequencies and common frequencies shared with other base stations, each of the base stations comprising;

receiver means for receiving a signal from a given mobile terminal;

means for assigning one of the unique frequencies to said given mobile terminal in response to a call request;

field strength detector means for detecting the field strength of the signal received by said receiver means;

means for assigning one of the common frequencies to said given mobile terminal if the detected field strength is lower than a first threshold value;

comparator means for receiving a field strength signal from a neighboring base station indicating the field strength of a version of the signal of said given mobile terminal at said neighboring base station and comparing the field strength signal with a second threshold value;

diversity combiner means for receiving said version of the signal of said given mobile terminal from said neighboring base station and combining the same with the signal of said given mobile terminal received by said receiver means when the field strength signal is determined by said comparator means to be higher than said second threshold value, and

each of said mobile terminals comprising means for tuning to frequencies assigned by the base stations and diversity combiner means for combining a signal from the base station which assigned said common frequency with a signal from said neighboring base station.

2. A cellular mobile communications system for serving mobile terminals within a plurality of cells, the system including a plurality of base stations respectively located at the cell sites;

each of said base stations having unique frequencies, a first group of common frequencies and a second group of common frequencies, said common frequencies of the first and second groups being shared with other base stations, each of the base stations comprising;

receiver means for receiving a signal from a given mobile terminal;

means for assigning one of the unique frequencies to said given mobile terminal in response to a call request;

field strength detector means for detecting the field strength of the signal received by said receiver means;

means for assigning one of the common frequencies of the first group to said given mobile terminal if the detected field strength is lower than a first threshold value;

comparator means for receiving a field strength signal from a neighboring base station indicating the field strength of a version of the signal of said given mobile terminal at said neighboring base station and comparing the field strength signal with a second threshold value;

diversity combiner means for receiving said version of the signal of said given mobile terminal from said neighboring base station and combining the same with the signal of said given mobile terminal received by said receiver means when the field strength signal is determined by the comparator means to be higher than said second threshold value; and

means for assigning one of the common frequencies of the second group to said given mobile terminal when the field strength signal is determined by said comparator means to be lower than the field strength detected by said field strength detector means, and

each of said mobile terminals comprising means for tuning to frequencies assigned by the base stations and diversity combiner means for combining a signal from the base station which assigned said common frequencies of the first and second groups with a signal from said neighboring base station.

3. A cellular mobile communications system as claimed in claim 2, wherein the signal from the base station is transmitted to the given mobile terminal at one of the common frequencies of the first group and the signal from said neighboring base station is transmitted to the given mobile terminal at one of the common frequencies of the second group.

4. A cellular mobile communications system for serving mobile terminals within a plurality of cells, the system including a plurality of base stations respectively located at the cell sites,

each of the mobile terminals comprising:

means for detecting the field strength of signals transmitted from surrounding base stations;

means for transmitting a start-of-diversity-mode request signal when the field strength of one of said surrounding base stations is higher than a threshold value;

means for transmitting an end-of-diversity-mode request signal when the field strength of said one of the surrounding base stations is higher than the field strength of another base station but lower than said threshold value; and

means for tuning to frequencies assigned by the base stations,

each of said base stations having unique frequencies and common frequencies shared with other base stations, each base station comprising;

receiver means for receiving a first version of a signal from a given mobile terminal;

means for assigning one of the unique frequencies to a given mobile terminal in response to a call request;

means for assigning one of the common frequencies to said given mobile terminal in response to said start-of-diversity-mode request signal; and

diversity combiner means for receiving a second version of the signal of said given mobile terminal from a neighboring base station and combining the first and second versions of the signal of said given mobile terminal during a period starting in response to said start-of-diversity-mode request signal and terminating in response to the end-of-diversity-mode request signal.

5. A cellular mobile communications system for serving mobile terminals within a plurality of cells, the system including a plurality of base stations respectively located at the cell sites,

each of the mobile terminals comprising:

means for detecting the field strength of signals transmitted from surrounding base stations;

means for transmitting a start-of-diversity-mode request signal when the field strength of one of said surrounding base stations is higher than a threshold value;

means for transmitting an end-of-diversity-mode request signal when the field strength of said one of the surrounding stations is higher than the field strength of another base station but lower than said threshold value and a change-of-frequency request signal when the field strength of said one of the surrounding base stations is lower than the field strength of said another base station; and

means for tuning to frequencies assigned by the base stations,

each of said base stations having unique frequencies, a first group of common frequencies and a second group of common frequencies, the common frequencies of both groups being shared with other base stations, each base station comprising;

receiver means for receiving a first version of a signal from a given mobile terminal;

means for assigning one of the unique frequencies to said given mobile terminal in response to a call request;

means for assigning one of the common frequencies of the first group to said given mobile terminal in response to said start-of-diversity-mode request signal and assigning one of the common frequencies of the second group in response to said change-of-frequency request signal; and

diversity combiner means for receiving a second version of the signal of said given mobile terminal from a neighboring base station and combining the first and second versions of the signal of said given mobile terminal during a period starting in response to said start-of-diversity-mode request signal and terminating in response to the end-of-diversity-mode request signal.

6. A cellular mobile communications system as claimed in claim 5, wherein the signal from the base station is transmitted to a given mobile terminal at one of the common frequencies of the first group and the signal from said neighboring base station is transmitted to the given mobile terminal at one of the common frequencies of the second group.