Method and system for providing a different frequency handoff in a CDMA cellular telephone system

• US 5,649,000 A
• Filed: 12/12/1994
• Issued: 07/15/1997
• Est. Priority Date: 11/16/1994
• Status: Expired due to Term

• Current Assignee: Pil Woo Park
• Original Assignee: Electronics and Telecommunications Research Institute
• Inventors: Kim, Myoung-Jin, Chung, Yeoung-Jee, Lee, Dong-Wook
• Primary Examiner(s): Cumming, William
• Assistant Examiner(s): MAUNG, NAY AUNG
• Application Number:
  US08/354,436
• Time in Patent Office: 946 Days
• Field of Search: 379/59, 379/60, 455/33.1, 455/33.2, 375/200, 375/202, 375/203, 375/205
• US Class Current: 455/436
• CPC Class Codes: H04W 36/30 by measured or perceived connection quality data
• CPC Class Codes: H04W 36/36 by user or terminal equipment

Abstract

A cellular system transfers a call from a source base station to the target base station. A mobile unit measures the strength of pilot signals emanating from surrounding base stations. In response to the result of the measurement, a different frequency hand off operation is initiated by the mobile unit when all pilot signals are lower than a threshold or is initiated by a system controller with consideration being given to the frequency band occupation state of surrounding base stations and the strength information reported from the mobile unit. In a handoff operation, the mobile unit reduces a maximum data rate to modulate user information with a current used frequency and to modulate a different frequency preposition code with a primary frequency so that neighboring base stations may measure the intensity of the preposition code and the system controller can determine how the different frequency hand-off should be operated.

Claims

1. A CDMA cellular telephone communication system having a mobile unit communicating to a source cell-site, a system controller for directing communications between said mobile unit and said cell-site, wherein the mobile unit requires a communication transfer from the source cell-site to a target cell-site selected from a number of proximal cell-sites, said source cell-site communicating user information with said mobile unit through a predetermined frequency channel different from a primary frequency channel, said proximal cell-sites not using said predetermined frequency channel, said communication system comprising:

control means, at the mobile unit, for determining if the mobile unit is to initiate a different frequency handoff based upon a comparison of intensity of pilot signals received from said source cell-site and said proximal cell-sites with a threshold value, for limiting a user data rate, and for controlling a data positioning device and a frequency synthesizer to operate in the different frequency handoff;

means, at the mobile unit, for deciding a transmitting position of said user information data to be transferred to said source cell-site and a different frequency handoff preposition code under control of the control means;

means, at the mobile unit, for modulating the data and said different frequency handoff preposition code with different frequencies;

means, at the cell-site, for demodulating said user information data if the cell site is in a normal mode or measuring intensity of said different frequency handoff preposition code if the cell site is in the different frequency handoff mode from said mobile unit;

channel control means, at the cell-site, for notifying said means for demodulating a receiving position of received data or for measuring intensity of a different frequency preposition code;

means, at the system controller, for receiving the different frequency handoff request from said mobile unit, determining the different frequency handoff, controlling said source cell-site and the cell-sites in a range around said source cell-site to measure the intensity of the received different frequency handoff preposition code, selecting the target cell-site on a basis of measurements of said intensities from said cell-sites, and cutting off a call between the mobile unit and the proximal cell-sites except the target cell-site when in said handoff mode.

2. The communication system of claim 1, wherein:

said control means, upon determining said different frequency handoff, reduces a maximum data rate and commands a data transceiver means to operate in a different frequency handoff mode, whereas if the handoff is withdrawn or the handoff is over said control means recovers to a previous maximum data rate and commands said data transceiver to operate in a normal mode.

3. The communication system of claim 1, wherein:

the mobile unit further comprises a local oscillator frequency synthesizer, means for inserting in a different frequency handoff mode a different frequency handoff preposition code into said user information, for regulating said local oscillator frequency synthesizer for modulating said user information with said predetermined frequency and modulating said preposition code with the primary frequency or in the normal mode said synthesizer for modulating only said user information with the predetermined frequency.

4. In a CDMA cellular telephone communication system having a mobile unit communicating to a source cell-site, a system controller for controlling a different frequency handoff between proximal cell-sites, wherein said mobile unit requires a communication transfer from the source cell-site to a target cell-site selected from a number of said proximal cell-sites, said source cell-site communicating user information with said mobile unit through a predetermined frequency channel different from a primary frequency channel, said proximal cell-sites not using said predetermined frequency channel, and said target cell-site being controlled by the system controller as the source cell-site, a method for achieving handoff between the source cell-site and the target cell-site including the steps of:

A) comparing, at said mobile unit, strengths of pilot signals received from said source cell-site and said proximal cell sites with a threshold value;

B) sending, at the mobile unit, a different frequency handoff request to the system controller via said source cell-site when the strength of all the pilot signals are lower than the threshold value, said different frequency handoff request containing strength information of pilot signals emanating from the proximal cell-sites;

C) sending, at the system controller, a handoff acknowledgement to said mobile unit via the source cell-site when the system controller can not find a proper target cell-site which is using said predetermined frequency channel, said acknowledgement containing an instruction about a different frequency handoff;

D) sending, at the system controller, a mobile unit assignment to the proximal cell-sites within a predetermined range;

E) modulating and transmitting, at the mobile unit, user information and a different frequency handoff preposition code with said predetermined frequency and said primary frequency in a different time slot, respectively;

F) sending, at the proximal cell-sites within a range, an information message of a strength of the preposition code to the system controller, said information message containing information of energy in said preposition code;

G) selecting, at the system controller, the target cell-site based upon said strength, and transferring a different frequency handoff grant to said mobile unit via the source cell-site wherein the handoff grant contains information on a frequency which said mobile unit will use to communicate with said target cell-site, and, said different frequency handoff grant containing information for the selected target cell-site;

H) transferring, at the mobile unit, communications from the source cell-site to the selected target cell-site using the frequency which is contained in said different, handoff frequency and modulating and transmitting at said mobile unit only user information;

I) sending, at the mobile unit, a handoff end message to the system controller via said target cell-site;

J) sending, at the system controller, a clear message to drop call service to all proximal cell-sites but said target cell-site; and

K) sending, at said controller, a mode change message for said target cell-site to demodulate said user information instead of message strength of said different frequency handoff preposition code.