AU736384B2 - Method for performing handoff in optical repeater installed zone in CDMA mobile telecommunication network system - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): SAMSUNG ELECTRONICS CO., LTD.

Invention Title:, METHOD FOR PERFORMING HANDOFF IN OPTICAL REPEATER INSTALLED ZONE IN CDMA MOBILE. TELECOMMUNICATION NETWORK SYSTEM The following statement is a full description of this invention, including the best method of performing it known to me/us: 1A P9017/ST2 (99-29497) METHOD FOR PERFORMING HANDOFF IN OPTICAL REPEATER INSTALLED ZONE IN CDMA MOBILE TELECOMMUNICATION NETWORK SYSTEM

PRIORITY

This application claims priority to an application entitled "Method for Performing Handoff in Optical Repeater Installed Zone in CDMA Mobile Telecommunication Network System" filed in the Korean Industrial Property 10 Office on July 21, 1999 and assigned Serial No. 99-29497, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to a handoff method in a CDMA mobile telecommunication network system, and in particular, to a method for performing a handoff in an optical repeater installed zone in a CDMA mobile telecommunication network system.

2. Description of the Related Art A general CDMA (Code Division Multiple Access) mobile telecommunication network system performs a handoff to secure continuity of a call even while a mobile station travels between neighbor base stations. Further, in order to expand a coverage (or service zone) of a mobile telephone, a relatively inexpensive optical repeater having proper performance is installed in a shadow zone or outer zone in association with the base station of the corresponding zone.

-1- P9017/ST2 (99-29497) FIG. 1 shows a cell structure of a mobile telecommunication network system in which an optical repeater is installed in association with a base station.

Referring to FIG. 1, the mobile telecommunication network system includes a base station (BS) A, a base station B, a base station C and an optical repeater associated with the base station C. That is, the base station C is interconnected to the optical repeater through an optical line. However, the optical line has a unique line delay due to its physical property, and the line delay prevents a handoff from being normally performed when a specific mobile station (MS) 10 travels to the optical repeater zone, passing by several base stations. That is, when the mobile station travels the traveling route shown in FIG. 1, although a handoff between the base stations is normally performed, a handoff to the optical repeater cannot be normally performed. This is because in the conventional mobile telecommunication network system, while traveling between the base stations, the mobile station does not properly update the handoff-related system information (or parameters) but maintains the handoff-related system information of the base station where the call was initially stet up.

ooo Eo This problem will be described with reference to FIG. 2. First, a mobile station (MS) is provided with search window system information (SRCHWlN_A, SRCH_WIN_N, SRCH_WIN_R) from the base station where the call was initially set up, and stores the received information therein. The system information parameter values define a boundary of a search window required in detecting a pilot signal from a neighbor base station. Here, the search window parameter values are set as a PN (Pseudo Noise) sequence offset length when the mobile station acquires the pilot signal, and the size and center of the search window are independently specified in the active set, candidate set and remaining set within the pilot. For the base station A, the parameter value of the search window system information is set to be very narrow. However, for the base station C, the parameter value is set to be very wide, so as to solve the delay -2- P9017/ST2 (99-29497) problem of the optical line connected to the optical repeater. Further, even for the base station B which neighbors to the optical repeater, the parameter value is set to be wide. In this situation, a handoff is normally performed when the mobile station travels from the base station A, where the call was initially setup, to the base station B. However, even when the mobile station continues to travel to the optical repeater's service zone, the mobile station maintains the previous parameter value according to the handoff of the base station A, where the call was initially setup. In this case, if the mobile station travels to a specific base station service zone, the handoff will be normally performed. However, if the 10 mobile station travels to the service zone of the optical repeater which has a unique line delay, the mobile station may depart from the previous search window value, so that the handoff may not be normally performed. That is, a pilot signal of the base station C which is a mother base station of the optical repeater falls outside the length range according to the search window parameter value that the mobile station maintains, due to the delay property of the optical line. In this case, the mobile station fails to detect the pilot signal and thus releases (or drops) the call, so that the handoff is not normally performed. That is, when the mobile station passes by the optical repeater's service zone along the traveling route stated above, the handoff may not be normally performed, thereby causing a release of the call.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a method for performing a normal handoff even when a mobile station travels in an optical repeater installed zone in a CDMA mobile telecommunication network system.

To achieve the above object, there is provided a method for performing a handoff in a CDMA (Code Division Multiple Access) mobile telecommunication network system. The method comprises comparing search window system -3i' 'I P9017/ST2 (99-29497) information parameter values of a base station where a call is initially set up, with search window system information parameter values of a handoff target base station, and transmitting the higher parameter values to a mobile station; transmitting reference point system information of the handoff target base station to the mobile station; and performing a handoff depending on the transmitted system information.

BRIEF DESCRIPTION OF THE DRAWINGS 10 The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: FIG. 1 is a diagram for explaining how to perform a handoff when a mobile station travels between base stations in a mobile telecommunication network system having an optical repeater installed zone; FIG. 2 is a flow diagram illustrating a procedure for transmitting handoff-related messages in a mobile telecommunication network system according to the prior art; FIG. 3 is a flow chart illustrating a procedure for performing a handoff in a mobile telecommunication network system according to an embodiment of the present invention; and FIG. 4 is a flow diagram illustrating a procedure for transmitting handoff-related messages in a mobile telecommunication network system according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they -4- 4 P9017/ST2 (99-29497) would obscure the invention in unnecessary detail.

A handoff between a base station and an optical repeater according to an embodiment of the present invention is performed in the mobile telecommunication network system shown in FIG. 1. In particular, the invention is effective when the mobile station connects a call in a service zone of the base station A of FIG. 1 and then travels to the optical repeater's service zone passing by the base station B. In this case, the invention will be much superior to the prior art.

With reference to FIGS. 3 and 4, a detailed description will now be made of a handoff method performed in an optical repeater installed zone in a CDMA mobile telecommunication network system according to an embodiment of the S. present invention.

FIG. 3 shows a procedure for performing a handoff in a mobile telecommunication network system according to an embodiment of the present invention. In particular, FIG. 3 shows a method for performing a handoff in an optical repeater installed zone by controlling a base station controller by means of a mobile switching center. Referring to FIG. 3, a receiving power level provided from the mobile station to a corresponding base station is measured in step 310. This operation is performed by analyzing a pilot strength measurement message including the receiving pilot level in the CDMA mobile telecommunication network system. In step 320, the mobile switching center determines whether to perform a handoff, depending on the receiving power level from the mobile station, included in the pilot strength measurement message.

That is, the mobile switching center determines whether the mobile station travels from the present service base station to a target base station. The pilot strength measurement message is defined by IS-95 family specifications for the CDMA mobile telecommunication system. If it is determined to perform the

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P9017/ST2 (99-29497) handoff in step 320, the mobile switching center assigns a handoff channel in step 330, and transmits updated handoff parameter value to the mobile station in step 340. At this point, the mobile switching center compares the search window system information (SRCH_WIN_A, SRCH_WIN_N, SRCH_WIN_R), stored in the mobile station, of the base station where the call is initially set up with the search window system information of the handoff target base station, to determine the search window system information having the higher value, and thereafter transmits the determined search window system information to the mobile station as a handoff parameter value. In addition, the mobile switching 10 center transmits a reference point system information of the base station, updated according to occurrence of the handoff to the mobile station, and enables the mobile station to perform the handoff depending on the transmitted system information. The search window system information and the reference point system information are included in an intraffic system parameter message and transmitted through the corresponding base station. The mobile station then stores the search window system information (SRCH_WIN_A, SRCHWIN_N, SRCH_WIN_R) provided from the corresponding base station, and a parameter value of this system information falls within the range of the search window required for searching a pilot signal from the neighbor base station. The stored parameter value of the search window will be set as a PN sequence offset length when the mobile station acquires the pilot signal. The size and center of the search window are independently specified in the active set, candidate set and remaining set within the pilot. A handoff direction message is transmitted to the mobile station in step 350, and the handoff is normally performed in step 360.

When transmitting the handoff direction message in step 350, the system information SRCH_WIN_A corresponding to the active set out of the search window system information for the target base station is transmitted to the mobile station, and the previously stored active set system information is updated again to direct the handoff. At this point, the reference point system information is also retransmitted. As a result, the search window system information -6- P9017/ST2 (99-29497) (SRCH_WIN_A, SRCH_WIN_N, SRCH_WINR) transmitted to the mobile station according to the present invention has the higher parameter value as a proper parameter value in comparison with the target base station, rather than to have a proper parameter value for the base station where the call was initially setup. In addition, when performing a handoff, the mobile station is provided again with the search window system information SRCH_WIN_A of the target base station to have a corresponding parameter value. This is a main feature of the present invention to solve the delay problem of the optical line, which may occur in performing a handoff in the optical line installed zone. That is, since the 10 mobile station updates and stores the delay property-considered parameter value, 0 the handoff will be normally performed. The parameter updating operation is performed whenever a handoff occurs while the mobile station travels to several base stations. Therefore, the mobile station performs a handoff by comparing and updating the search window system information of the base station whenever it is required to perform a handoff.

ee* FIG. 4 shows a procedure for transmitting handoff-related messages in an optical repeater installed zone in a mobile telecommunication network system according to an embodiment of the present invention. Referring to FIG. 4, the mobile station transmits the pilot strength measurement message to a call processing main block CCP and a vocoder call processing block TSB of the base station control system (BSC). The vocoder call processing block then transmits a handoff request message and receives a handoff channel assignment message according thereto. Further, the vocoder call processing block transmits a call configuration setting request message and receives a call configuration response message. In this state, the vocoder call processing block transmits the intraffic system parameter message to the mobile station, including the parameter value updated in step 340 of FIG. 3. In this process, the mobile station updates the search window system information. When the master pilot is changed, the reference point system value is changed to the updated base station value and -7- 11 0 P9017/ST2 (99-29497) included in an extended handoff direction message before being transmitted to the mobile station. Thereafter, the vocoder call processing block transmits a link active message to the call processing main block, and receives a handoff completion message from the mobile station, thus normally completing the handoff.

The above operation will be described in more detail with reference to FIG. 1.

10 Referring to FIG. 1, for the base station A, the search window system information values (SRCH_WIN_A, SRCH_WIN_N, SRCH_WIN_R) are set relatively low. For the base station B which is a neighboring base station of the optical repeater base station, the search window system information values (SRCH_WIN_A, SRCH_WIN_N, SRCH_WIN_R) are set relatively high. The optical repeater base station is connected to its mother base station C via the optical line. For the base station C connected to the optical repeater base station, the search window system information values (SRCH_WIN_A, SRCH_WIN_N, SRCH_WIN_R) are set relatively high. Here, due to the delay of the optical line, it is not possible to detect the pilot signal from the optical repeater base station using the search window range of the mobile station which has received the search window system information values (SRCH_WIN_A, SRCHWIN_N, SRCH_WIN_R) set in the base station A. In this state, the mobile station sets up a call in the service zone of the base station A. In the conversation state where the call is set up, the mobile station travels to the base station B. At this point, since the mobile station has the search window system information values (SRCH_WIN_A, SRCH_WIN_N, SRCH_WIN_R) of the base station A, it may fall within the pilot search window range of the base station B. Therefore, normal active set registration is performed. In the registration state, when a pilot signal of the base station B is activated, the base station control system of the mobile switching center searches the search window parameter values of all the activated -8- P9017/ST2 (99-29497) q base stations, determines the search window system information values (SRCH_WIN_A, SRCH_WIN_N, SRCH_WIN_R) having the highest value according to the search results, and transmits the determined value to the mobile station through the intraffic system parameter message thereby to perform updating operation. At this point, when the mobile station continues to move to a position where the base station A is dropped, the base station control system of the mobile switching center transmits the extended handoff direction message to the mobile station. A reference point value T_ADD included in the extended handoff direction message becomes a value of the base station B which is the 10 newly updated master base station. The mobile station is provided again with the system information value SRCH_WIN_A according to the handoff as a value of the base station B. In this state, when the mobile station moves to the optical repeater base station passing by the base station B, the mobile station has the search window value of the base station B, so that a pilot from the optical repeater base station will fall within the range of the search window. Therefore, activation will be normally performed and thus the handoff will normally be performed without release of the call. In this manner, the mobile telecommunication network system according to the present invention solves a "handoff failure problem which may occur in an optical repeater installed zone due to a delay of the optical line.

While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

For the purposes of this specification it will be clearly understood that the word "comprising" means "including but not limited to", and that the word "comprises" has a corresponding meaning.

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Claims (8)

1. A method for performing a handoff in a CDMA (Code Division Multiple Access) mobile telecommunication network system, comprising the steps of: comparing search window system information parameter values of a base station where a call is initially set up, with search window system information parameter values of a handoff target base station, and transmitting the higher parameter values to a mobile station; 10 transmitting reference point system information of the handoff target base station to the mobile station; and performing a handoff depending on the transmitted system information.

2. The method as claimed in claim 1, wherein the search window system information is included in an intraffic system parameter message which satisfies a protocol defined by IS-95 family specifications.

3. The method as claimed in claim 1, wherein the reference point system information is included in an extended handoff direction message which satisfies a protocol defined by IS-95 family specifications.

4. The method as claimed in claim 3, wherein the extended handoff direction message includes active set search window system information according to the handoff performed in the target base station. A method for performing a handoff in a CDMA mobile telecommunication system, comprising the steps of: determining whether to perform a handoff by analyzing a receiving power level of a mobile station; assigning a handoff channel for a target base station according to the P9017/ST2 (99-79497) determination results, and comparing search window system information of the mobile station with search window system information of the base station; transmitting the system information selected according to the search window system information comparing results, and reference point system information according thereto to the mobile station to update the information; and performing a handoff according to the updated system information.

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6. The method as claimed in claim 5, wherein in the step the 10 system information transmitted to the mobile station is included in an intraffic system parameter message specified by IS-95 family specifications.

7. The method as claimed in claim 5, wherein the step (d) comprising the step of receiving active set search window system information according to a handoff of the target base station. D

8. The method as claimed in claim 7, wherein the active set search window system information is included in an extended handoff direction message. Dated this 2 1 s t day of July 2000 SAMSUNG ELECTRONICS CO., LTD. By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia 11