WO2004039017A1

WO2004039017A1 - SEARCHING GSM FCCHs FROM A CDMA SYSTEM

Info

Publication number: WO2004039017A1
Application number: PCT/KR2003/002265
Authority: WO
Inventors: Hyeon-Woo Lee; Guangzhao Tian; Xiaoqiang Li
Original assignee: Beijing Samsung Telecom R&D Center; Samsung Electronics Co Ltd
Current assignee: Beijing Samsung Telecom R&D Center; Samsung Electronics Co Ltd
Priority date: 2002-10-24
Filing date: 2003-10-24
Publication date: 2004-05-06
Anticipated expiration: 2005-04-24
Also published as: AU2003272131A8; AU2003272131A1; CN1492611A

Abstract

A timing scheme is defined for searching GSM Frequency Correction Channel from CDMA system includes following steps: Set a Searching Window in each CDMA traffic frame; Search for FCCH of adjacent GSM cell through the Searching Window; If the searching succeeds, terminate the searching; otherwise, restart the searching. When the width d of Searching Window is fixed, in the average conditions, the timing scheme provided by this invention makes the number of required Searching Windows minimal and cause the minimal influence on the system performance. For the same width of Searching Window d, in the average conditions, the time needed is minimal and the searching speed is the fastest from the start point to the success of searching. And the number of parameters needed to be set is minimal, which makes the MS implementation easier for MS manufacturers.

Description

Timing scheme for Searching GSM Frequency Correction Channel from

CDMA2000 System

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of mobile communications, and especially relates to a method for searching GSM Frequency Correction Channel from CDMA system.

2. Description of the Prior Art

Global System of Mobile Communication (referred to as GSM) is a digital mobile communication system of the 2^nd generation that is deployed in large scale over the whole world. CDMA is the mobile communication technology and system of the 3th generation researched and developed mainly by Qualcomm Company, USA. Although both have been widely deployed, they are incompatible with each other due to historic causes. So up to now, Mobile Station only supporting GSM operation can not roam into CDMA network and vice versa.

In order to enable dual-mode Mobile Station supporting both GSM and CDMA to roam from CDMA network to GSM network, a series of operations shall be accomplished by the dual-mode Mobile Station, including search, measurement and switching, etc, between the two systems.

In existing physical layer standards of CDMA (3GPP2 C.S0002), measurement between different systems has been defined, but the definition is not only specified to the measurement of GSM system. Some main points therein are given in the following.

When a Mobile Station (referred to as MS) receives a Candidate Frequency Searching Reference Message (abbreviated as CFSRM) from Base Station (referred to as BS), MS will search and measure BSs in adjacent cells once or periodically. BSs in adjacent cells can be BS working at different frequency band in CDMA Mode, BS working in Analogue Mode, or BS working in GSM mode.

In the following, we only discuss the problem of searching and measuring adjacent cell BS in GSM mode. After receiving and processing above CFSR message, MS will obtain the required information of adjacent GSM cell BSs, including Absolute Radio Frequency Channel Number (referred to as ARFCN) of Broadcast Control Channel carrier frequency (referred to as BCCH carrier frequency), on which Forward Common Control Channel resides.

The dual-mode MS stops receiving traffic data on Forward Traffic Channel from BS and also stops sending data on Reverse Traffic Channel. Then MS stops the operations in CDMA mode, and tunes the radio frequency module to target frequency and performs measurements on the cell. After finishing the measurement, the MS tunes the RF module back to original working frequency of CDMA, continues with the interrupted CDMA operation; and reports measurement results of adjacent cell to the upper layer as the reference data for following switching procedure

As the operation of CDMA is interrupted for a period of time, a time period of interruption will occur, which is called Searching Window. Obviously, considering that the data js transmitted and received continuously in CDMA mode, such interruption will influence the performance of data transmitting and receiving.

For reverse data transfer, compensatory approach is defined in existing standard: that is, in reverse traffic channel, in which 20 milliseconds frame is used as an unit; if an interruption of d milliseconds is required for searching adjacent cells during a specified frame, the transmission power for the remaining part of the frame is boosted to compensate the losses of transmission power of that frame, to reduce the error rate of received data at the receiver side (the BS). The value of boosted power is dependent on d. The larger d is, the larger the boosted power is.

For forward data transfer, no compensatory measurement is taken. And the transmission gap in the search window is simply regarded as data loss and will be processed by upper layers.

The method for searching and measuring between the systems by interrupting the transmission/reception of service data is illustrated in Figure 2.

It should be noted as following: a) The standards emphasis that, in the above searching and measuring process, MS behaves by itself and BS doesn't know that MS is performing cell search and measurement. Thus, it is impossible for BS to provide the required relative timing of GSM to MS ! b) Although power boosting can compensate the performance losses during the call, the additional power will inevitably affect other CDMA MSs by adding additional burst interference.. c) Analysis shows that during one CDMA2000 service frame (20 millisecond ), at most one Searching Window is permitted, and Searching Windows above two will result in inevitable information losses in the reverse traffic channel. In addition, simulation results shows that excessively frequent Searching Windows will do more harm than help to the performance, and implementation will become very complex.

Existing standards only give relatively general definitions for the process and method of inter-system measurement (including measurements for CDMA system on different frequency, Analogue system and GSM system), while detailed implementation method hasn't been given (e.g. the timing method adopted for searching), needless to say, the detailed descriptions specified to

GSM system are not given.

Actually, GSM system has its own structure, so specifed strategy must be adopted for searching and measuring GSM cells. For the convenience of later description, it is necessary to introduce the structure of Forward Common Control Channel of GSM here.

The Forward Common Control Channel of GSM cell is transmitted continuously on a specified frequency point, which is called BCCH carrier frequency.

GSM is the time-division system. In Forward Common Control Channel, frames such as Frequency Correction Channel (referred to as FCCH) and Synchronization Channel (referred to as SCH) are multiplexed into a multi-frame for transmission.

Figure 3 shows the multi-frame structure of Forward Common Control Channel continuously transmitted on BCCH carrier frequency. The length of a multi-frame is about 231 milliseconds, including 51 TDMA frames, each which is about 4.63 milliseconds long. We can see that FCCH and SCH are transmitted 5 times in each multi-frame. The interval between each is 10 or 11 TDMA frames, or 46.3 or 50.93 milliseconds.

For a MS that attempts to switch to the GSM cell, some important information of the BS, e.g. Receiving Signal Strength Indication (referred to as RSSI), Frequency Correction Information, BS Identification Code (referred to as BSIC), etc., must be obtained from this common channel. The frequency correction information is contained in the first timeslot of FCCH. The length of the timeslot is about 0.58 milliseconds, which is marked with red block in Figure 3; and BSIC is included in SCH. Both must be obtained through cell search.

In fact, any MS that tries to switch to the GSM cell must tune its RF module to the BCCH carrier frequency; searches for FCCH and SCH with certain searching scheme to obtain information mentioned above, and process it for inter-system switching in next step.

Obviously, no definition is given specifically for GSM searching in existing standard, nor the detailed FCCH searching process has been considered.

SUMMARY OF THE INVENTION

The object of this invention is to provide a method of searching tentatively GSM cell for GSM/CDMA dual-mode MS that works in CDMA mode.

To realize the above object, A method for searching GSM Frequency Correction Channel (FCCH) from CDMA system, comprising following steps:

Setting a searching window in CDMA traffic frame;

Searching for the FCCH of the adjacent GSM cell in the Searching Window; If the searching succeeds, terminate the searching; otherwise, restart the searching in the other Searching Window of the other traffic frame.

In the timing method in this invention, when the width d of Searching Window is the fixed, under average conditions, the number of Searching Windows needed is minimized, thereby results in minimal influence on the system performance. For same width d of Searching Window, under average conditions, the time needed is also minimal and the searching speed is the highest from the start point to the success of search. And the number of parameters needed to be set is minimal, which makes the MS design easier for MS manufacturers.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

Figure 1 The method of combining Searching Windows in adjacent frames in the searching of GSM FCCH;

Figure 2 The inter-system searching method of interruption window and power compensation defined in CDMA standard;

Figure 3 The Multi- frame structure of GSM Forward Common Control Channel;

Figure 4 Composition of the method by Tentatively Searching GSM FCCH; Figure 5 The flowchart of GSM FCCH searching with Combined Searching Windows.

DETAILED DESCRIPTION OF THE INVENTION

This invention is proposed in order to solve the problem of existing physical layer standard of CDMA not supporting FCCH searching of adjacent GSM cell. Although this invention contains a complete searching process, it is divided into two aspects for explanation for the convenience of description:

1. A method for tentative searching It should be noted that CDMA standard specifies that BS doesn't know the searching and measurement operations of MS. Thus, it is impossible for BS to provide correspondent information to MS, in which one of the most importance is the relevant timing relation between GSM multi-frame and CDMA traffic channel frames. The effect thus caused is that MS is "blind" when searching GSM FCCH, i.e. it doesn't know when GSM FCCH begins related to CDMA frames of the traffic channel.

In another word, during the period from the Open of the Searching Window by MS (The whole process of stopping the receiving/transmitting operation on CDMA service channel and tuning to GSM BCCH frequency and starting the searching, which is the same hereinafter) for searching to the Close of the Searching Window (the whole process of stopping GSM searching and tuning to original CDMA frequency and continuing with the operation in CDMA mode, which is the same hereinafter), it is impossible to predict whether GSM FCCH will appear, i.e. the probability of FCCH appearing exactly at this period of time is random.

So, the whole process adopts a method of tentative searching, i.e. opening a Searching Window without the prior knowledge of GSM FCCH relevant timing relationship (it is called as one Searching Attempt). If FCCH appears exactly at this time, MS can know the searching success through demodulating specific information in FCCH and the tentative searching of this time succeeds and the whole searching process is terminated at the same time; otherwise, this tentative searching fails and MS must close the Searching Window and return to work in CDMA mode.

After waiting for a specific period of time (the length is determined by the timing scheme), MS opens a Searching Window once again and repeats the tentative searching. Repeat the above processes till successful demodulation of FCCH.

In fact, considering that the probability of 5-times FCCH appearing in the whole GSM multiframe is very small, one time tentative searching can't succeed in obtaining FCCH in most cases, and multiple tentative searching are needed. So the whole process must be performed tentatively.

The time relationship between multiple searching attempt is defined by specific timing scheme. The composition of such a timing schemeis shown in Figure 5, which includes several factors:

1. One or more Searching Windows: because related to Searching Window, the time for appearance of FCCH is random and it can't be guaranteed to find FCCH at the first Searching Window, and multiple Searching Windows are needed in most cases; 2. The starting time S of Searching Window indicates the time offset of Searching Window with respect to the CDMA traffic frame the window resides. The unit is millisecond;

3. The width d of Searching Window, i.e. the time duration of one searching attempt, whose unit is millisecond; 4. Time interval TI is the time between two adjacent Searching Windows, whose unit is millisecond.

2. The timing method based on Combined Searching Window

Based on the tentative GSM FCCH searching, this invention gives a timing scheme that specifies the parameters for above searching process, which makes MS finish searching procedure quickly and cause minimal influence on system performance, whose structure is shown in Figure 1 :

1. Each frame has Searching Window

That is, when MS starts the searching for FCCH channel of a certain GSM cell, each frame, during the period from the start of immediately following CDMA traffic frame, till the time that GSM FCCH is searched successful, will be interrupted for a time gap, which is used for GSM FCCH searching;

2. The width d of Searching Window

According to the requirement of existing standard, the width of Searching Window must be less than one half of the 20-millisecond CDMA traffic frame, i.e. d < 10 milliseconds. At the same time, simulation results show that when d < 1 millisecond, the searching time will be very long, which is actually impractical. So the range of d given in this invention is from 1 millisecond to 10 milliseconds. Within this range, this invention won't make further requirements on the value of Searching Window width d, because according to simulation results, the timing method given in this invention behaves optimal or quasi-optimal for different value of Searching Window widths. Considering the different requirements on d for different equipment manufacturers, no specific requirement is made.

3. Setting the Starting Time S of Searching Window depending on odd or even frame

When setting the Starting Time of Searching Window, odd frame and even frame are distinguished.

The CDMA traffic frame that the first Searching Window locates in is the 1^st frame (odd frame), and the next frame is the 2^nd frame (even frame), and the next frame after that is the 3^rd frame (odd frame), and so on.

In all odd frames, the offset of Searching Window from the starting time of the frame of is (20 - d) milliseconds, i.e. at the end of the frame.

In all even frames, the offset of Searching Window from the starting time of the frame of is 0 millisecond, i.e. at the head of the frame.

4. Setting Searching Windows Interval depending on odd frame or even frame

Similar with those in 3, the setting the Searching Windows Interval TI between two adjacent frames is also determined by the frame being odd frame or even frame. The Searching Windows Interval between the 1^st frame and the 2^nd frame is d milliseconds, and that between the 2^nd and the 3^rd is (40-d) milliseconds, and that between the 3^rd and the 4^th is d milliseconds, and so on.

5. Combining Searching Windows for use Through the settings in 3 and 4, the Searching Window in adjacent frames (the 1^st frame and the 2^nd frame, the 3^rd frame and the 4^th frame, and so on) is adjacent in time and can be used as a combination, which makes the searching process be performed in a new and larger Searching Window with a width of 2 times d millisecond, and can speed up the searching procedure and reduce the searching times.

Figure 5 describes the operation flow of tentative searching for FCCH channels of adjacent GSM cells by using timing scheme of Combined Searching Window for dual-mode MS working in CDMA mode:

501 Searching preparation: after MS selects and tunes to the GSM BS whose FCCH needs to be searched for, the searching process starts. The next 20-millisecond CDMA traffic frame is the first searching frame;

502 Searching in odd frames: in each odd frame, the transmission power on Reverse Traffic Channel will be boosted during the period from the start of the frame to (20-d) milliseconds. The remaining d milliseconds of the fame is the first part for a searching attempt. During this period, Searching Window is opened, i.e. MS both stops sending data on Reverse Traffic Channel and stops receiving information from Forward Traffic Channel, and tunes the RF module to the BCCH carrier frequency of GSM cell, to start a searching attempt, which will keep on until the even frame immediately following it;

503 Searching in even frame: in each even frame, the duration from the start of the frame to d milliseconds is the second part of the searching attempt. During this period, no matter whether FCCH is searched successfully, Searching Window will be closed. That is, MS stops the searching on GSM cell frequency, tunes the RF module back to the original working frequency in CDMA mode, and continues with the interrupted operation in CDMA mode. By far, the entire searching attempt is finished. 504 Decision process: during the period of Combined Searching Window on odd frames and even frames, a Searching Windows of two times d milliseconds is resulted, and MSs keep son trying to demodulate specific characters, i.e. frequency correction information. If the demodulation succeeds, it indicates that the searching attempt succeeds, and MS will directly go to step 505; otherwise, it indicates that the tentative searching fails and steps 502 and 503 are repeated as a new searching attempt. This process is performed repeatedly until the successful demodulation of frequency correction information.

505 Search successfully: after one or more searching attempt, GSM FCCH is successfully found and the whole process is finished.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

WHAT IS CLAIMED

1. A method for searching GSM Frequency Correction Channel (FCCH) from CDMA system, comprising following steps: Setting a searching window in CDMA traffic frame;

Searching for the FCCH of the adjacent GSM cell in the Searching Window;

If the searching succeeds, terminate the searching; otherwise, restart the searching in the other Searching Window of the other traffic frame.

2. The method as defined in Claim 1, wherein 20-millisecond CDMA traffic frame contains one Searching Window.

3. The method as defined in Claim 2, wherein the width d of the Searching Window is larger than 1 millisecond and smaller than 10 milliseconds.

4. The method as defined in Claim 1, wherein the traffic frames are divided into: the 1^st CDMA traffic frame directly following the start point is the 1^st frame and is an odd frame; the 2^nd frame is an even frame, and so on, The fame number increases by 1 each time when a frame pass by.

5. The method as defined in Claim 4, wherein the Searching Window for odd frame is at the end of the frame, and the searching window for even frame is on the head of the frame.

6. The method as defined in Claim 4, wherein the interval between the Searching Window in all even frames and that in the preceding frame, the odd frame, is d milliseconds, and the interval between it and that in the next odd frame is 40-d milliseconds;

7. The method as defined in Claim 1, wherein it includes the combined Searching Windows in adjacent odd frame and even frame, and in the duration of two times d is used as a new and larger Searching Window length, only one searching attempt is performed.