Title of Invention	METHOD FOR EFFICIENT FALLBACK OF A CIRCUIT SWITCHED CONNECTION TO GSM LAYER1 DUE TO INTER RAT HANDOVER FAILURE IN THE TARGET RADIO ACCESS TECHNOLOGY
Abstract	The present invention proposes a new method for efficient fallback of GSM layer 1 circuit switched connection after the failure of inter RAT handover. In this method, the GSM synchronization is not lost as the GSM TDMA frames are updated periodically when the UE is on the UMTS RAT executing the handover process. The GSM RAT is resumed after the failure of inter RAT handover in minimum amount of time (about 10 ms) with minimum disruption.

Title of Invention

METHOD FOR EFFICIENT FALLBACK OF A CIRCUIT SWITCHED CONNECTION TO GSM LAYER1 DUE TO INTER RAT HANDOVER FAILURE IN THE TARGET RADIO ACCESS TECHNOLOGY

Abstract

The present invention proposes a new method for efficient fallback of GSM layer 1 circuit switched connection after the failure of inter RAT handover. In this method, the GSM synchronization is not lost as the GSM TDMA frames are updated periodically when the UE is on the UMTS RAT executing the handover process. The GSM RAT is resumed after the failure of inter RAT handover in minimum amount of time (about 10 ms) with minimum disruption.

Full Text	FIELD OF TECHNOLOGY This invention in general relates to wireless technology. More particularly this invention relates to a method for efficient fallback of a circuit switched connection to GSM Layerl User Equipment/Mobile Equipment due to inter RAT handover failure in the target radio access technology. PRESENT STATE OF ART The 3GPP specifications impose certain timing restrictions for inter RAT handover procedure accomplishments together with the description of how the inter RAT handover procedure has to be carried out in the mobile. This invention assumes that the mobile is currently in GSM RAT with a dedicated circuit switched connection. This invention has references to 3GPP March 2002 version, Release 99 set of Technical Specifications. They are: • 3GPP TS04.18 Version 8.a.0 RR protocol specification, • 3GPP TS05.10 Version 8.8.0.Radio subsystem synchronization • 3GPP TS05.08 Version 8.10.0 Radio subsystem link control • 3GPP TS 25.133 Version 3.6.0 Requirements for Support of Radio Resource Management (FDD) When there is an Inter RAT Handover to UTRAN then the GSM RAT in the UE needs to relinquish the control of the hardware to UMTS RAT. This shall mean that the UE shall disconnect the GSM cell that the UE is synchronized before attempting to connect to UMTS RAT and that the UE will lose the synchronization information with the GSM cell. But this information is required if the UMTS RAT is not able to successfully connect to the UMTS cell and the UE has to revert back to the old GSM channel configuration that existed before the handover started. To achieve successful reactivation of GSM channels in the absence of synchronization, it is required to restore the synchronization information and provide the connection. The timing requirements as specified in the technical standards 3GPP 05.10 and 3GPP 25.133 to complete the Inter RAT handover procedures are: Casel: Switching over from GSM to UTRAN shall be completed within 190 ms if the UE is already synchronized to the FDD cell and 270 ms if the UE is not already synchronized to the FDD cell. Case 2: Switching over from UTRAN to GSM shall be completed within 90 ms if the UE is already synchronized to the GSM cell and 190 ms if the UE is not already synchronized to the GSM cell. Handover from UTRAN to a GSM cell shall be completed within the following time limits. From the above, it is clear that the time allowed for synchronizing back to a GSM cell is 100ms. Operation of Related Art Method 1: The UE shall carry out the following functions for RAT handover process and reestablishment of connection to GSM RAT in case of RAT handover failure: a) suspend the normal operation except for RR management; b) disconnect the main signaling link via local end release; c) disconnect the traffic channels, if any; d) pass on the handover to RRC; e) RRC attempts to establish the connection; f) complete the handover procedure; g) if failure to establish the connection in UMTS revert back to GSM RAT; h) re-connect to GSM channels and send Handover Failure to the network. The step (h) above involves synchronizing to the GSM cell first and then reestablishing the connection. A typical design of connecting back to the GSM involves the following steps: I. Carry out FCH/SCH search on the BCCH frequency; II. Switch to the traffic channels or the signaling channels depending on when the stage handover attempt took place to UTRAN; III. Send the Handover Failure message and continue on the TCH or SDCCH as is the case. The time involved in this method can be arrived at as follows: The UE has to carry out FCH detection and SCH detections, which in the worst case take 22 TDMA frames (100 ms). Now that the UE has synchronized to the cell it takes a minimum of one TDMA frame to configure the TCH or SDCCH channel. Thus the total time required in this method is at least 100ms+ 4.612ms = 104.612ms. Method 2: Another way of achieving this synchronization is by storing some of the parameters as listed below: The following parameters are assumed: • Current TDMA frame number denoted by N • TCH frequency offset • the timing offset for bit synchronization and • the time offset within the current TDMA frame number at which the GSM RAT gets shut down, denoted by s1 • a timestamp shall also be recorded to mark the time at which the GSM RAT is shut off, denoted by t1 • now when the HANDOVER failure is indicated by UMTS Access Stratum, the GSM RAT shall be reactivated and another timestamp shall be recorded to mark the start of the GSM RAT, denoted by t2. Now to synchronize to the network, the UE shall arrive at the new FN that is currently running in the cell in the GSM RAT based on the difference in the timestamps t1, t2, the frame number N, and the time offset s1 that was stored at the beginning of Inter RAT Handover Procedure. The time offset s2, with reference to the start of the TDMA frame when the GSM RAT is resumed after processing the handover failure command is calculated based on si, stored when the GSM RAT was shut off and the difference in the timestamps t1 and t2. Since the GSM RAT knows the frequency of the dedicated channel that was allocated before the handover started this shall complete the synchronization to the old GSM channel. This shall take at least 3 TDMA frames after the handover failure command is processed. But this synchronization may not be reliable because the start position of TDMA frame may not come at exactly the same position at which it was coming before the handover started. This way, the total handover completion time comes out to be at least about 20 ms. (the time required to compute the current frame number from the values stored and then the time required to switch to TCH/SDCCH channels). TDMA frames along the time axis - t1 is the time at which the GSM RAT was shut down; N is the frame number in which the GSM RAT was shut down; s1 is the time offset with respect to the start of the TDMA frame when the GSM RAT was shut down; t2 is the time at which the GSM RAT was started, after the processing of the handover failure command; N+m is the frame number in which the GSM RAT is started; N+m+1 is the frame number in which the first TDMA frame interrupt shall come when the GSM RAT is started; FN dur is the time duration of 1 TDMA frame; FN dur s2 is the time offset with respect to the start of the TDMA frame when the GSM RAT is started after the processing of the handover failure command; N+m+4 is the frame number at which the GSM RAT has synchronized back to the old GSM channel configuration that existed before the handover started; FN2 is the frame number when the frame counters shall be updated when the GSM RAT is started. Known quantities: t1,t2. N,s1, FN2 = N+m+2 m = [t2-(t1+ FNdur-s1)]DIVFNdur + 1 FN2 =[t2-(t1 +FNdur-s1)]DIVFNdur + 1 +2 + N s2 = FNdur- [t2-(t1 + FNdur-s1)] % Fndur Total time taken = t2 -11 + s2 + 3* FNdur LIMITATIONS In method 1, the total time required is at least 100ms+ 4.612ms = 104.612ms. In method 2, we shall be able to save the FCH/SCH scan time which is about 22 TDMA frames i.e. about 100 ms when compared to Method 1. This means, in this method, by doing away with FCH/SCH scan, the GSM channels can be reactivated in about 20-ms which is 5 times faster than Method 1. In case the degradation in quality of service is low due to handover attempts and that is not perceptible to the mobile user, the time taken for the handover process should be as low as possible and hence an attempt is made to evolve a method wherein very low time duration for the handover process (of about 10 ms) is achieved. OBJECTS OF THE INVENTION The primary object of this invention to invent a method for efficient fallback of a circuit switched connection to GSM layerl user equipment. It is another object of the invention to invent a method as wherein GSM RAT is resumed after failures of inter RAT handover with minimum disruption. It is another object of the invention to invent a method wherein GSM RAT is resumed after failure of inter RAT handover in minimum amount of time (about 10 ms). It is another object of the invention to invent a method which provides reliable and accurate fallback to GSM RAT after the failure of inter RAT handover in minimum amount of time with minimum disruption. SUMMARY OF INVENTION The proposed invention relates to a case where a handover from GSM radio access technology to UMTS radio access technology (WCDMA in specific) is attempted and reverted back to GSM technology on not being able to connect to WCDMA technology in UMTS for an on-going dedicated connection in Samsung's dual mode mobile handset that supports GSM TDMA and UMTS W-CDMA radio access technologies. The inter RAT handover should happen keeping the disruption to the ongoing service as minimum as possible. In such a scenario, this proposed invention describes an efficient way of achieving the synchronization to the GSM RAT so that the call can still be supported. The purpose of this invention is to reduce the total time required (104.612ms) to a bare minimum by efficiently handling the synchronization information In GSM Layer 1. In this invention a method is proposed in which this time is reduced to 10 ms which is less than 1/10^*^ of the time taken by a typical design that is described above. By keeping this time to such a low duration, the degradation in the quality of the service due to handover attempts is at the minimum possible level which will not even be perceptible to the mobile user. Accordingly the present invention comprises a method for efficient fallback of a circuit switched connection to GSM layerl user equipment wherein: a) the GSM TDMA frame counters are updated regularly even when the UE is on the UMTS RAT executing the Handover process whereby the synchronization information with the GSM is retained; b) after receiving the HANDOVER FAILURE command, the GSM RAT, using the synchronization information, tunes the RF to the ARFCN that It was allocated in the GSM cell before the Handover started; c) the bit synchronisation is achieved by applying the last timing offset value calculated while on TCH/SDCCH and the UE achieves synchronization in less than 2 TDMA frames; and d) thereby, the HANDOVER occurs with minimum disruption and takes less than 10 ms to connect back to the GSM channels and the dedicated connection is continued without disruption enabling the user not to lose an on-going speech call. The other objects, features and advantages of the present invention will be apparent from the accompanying drawings and the detailed description as follows. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1 shows typical protocol architecture of GSM, WCDMA dual mode mobile. As shown in Figure 1, the RR and RRC modules exchange layer 3 protocol messages as well as some information relating to radio resource management. The interface between GSM Layer 1 and WCDMA Layer 1 is to exchange hardware control and its timing. Figure 2 shows the typical TDMA frame structure along the time axis. It represents the time at which the GSM was shut down and the time at which the GSM is reactivated after the RAT handover failure. It can be seen from the figure that the total time taken for reactivating the GSM RAT is roughly 3 TDMA frames (time taken for synchronization) including the time offset s1 plus the difference between the time stamps t1 and t2. This works out to about 20 ms. The 3 TDMA frames used for synchronization are avoided in the new method and that synchronization information is made available during the handover process. In figure 2, t1 is the time at which the GSM RAT was shut down; N is the frame number in which the GSM RAT was shut down; s1 is the time offset with respect to the start of the TDMA frame when the GSM RAT was shut down; t2 is the time at which the GSM RAT was started, after the processing of the handover failure command; N+m is the frame number in which the GSM RAT is started; N+m+1is the frame number in which the first TDMA frame interrupt shall come when the GSM RAT is started. DETAILED DESCRIPTION OF THE INVENTION A preferred embodiment of the present invention will now be explained with reference to the accompanying drawings. It should be understood however that the disclosed embodiment is merely exemplary of the invention, which may be embodied in various forms. The following description and drawings are not to be construed as limiting the invention and numerous specific details are described to provide a thorough understanding of the present invention, as the basis for the claims and as a basis for teaching one skilled in the art how to make and/or use the invention. However in certain instances, well-known or conventional details are not described in order not to unnecessarily obscure the present invention in detail. This invention proposes the most efficient way of reverting back to the GSM RAT on the failure of handover attempt in UMTS RAT. With this method, the synchronization can be achieved in the shortest possible time and with a high level of accuracy. In this method, the GSM RAT will be only updating the TDMA frame counters and storing the values of the following parameters; • TCH frequency offset; • The timing offset for bit synchronization; and • The timing advance value being used along with the Tx power level. This shall allow updating of the GSM TDMA frame counters regularly as it would normally do when the GSM RAT is active. This way, though the UE is on the UMTS RAT executing the Handover procedure, it shall not lose the synchronization information with the GSM network. Now when the HANDOVER FAILURE command is received the GSM RAT knows all about the synchronization because all it has to do is to tune the RF to the ARFCN that it was allocated in the GSM cell before the Handover started. It shall take about one TDMA frame duration after the HANDOVER FAILURE command is processed to connect back to the GSM channels. The bit synchronisation can be achieved by applying the last timing offset value calculated while on TCH/SDCCH. Bit synchronization can be achieved in this way - when the UE is on a normal TCH, it receives the timing advance value to be used in each downlink SACCH block which means every 480 ms the UE shall update its timing advance value. In cases where the Inter RAT Handover is completed (including the case of reconnecting back to GSM channels on Handover Failure at UMTS) in the same SACCH period as the Handover Command is received as a FACCH block i.e. within a SACCH period of 480 ms then the UE shall not lose the bit synchronization and use the already stored timing advance and Tx power level values. OthenA/ise, i.e. when the reactivation of GSM channels takes place in a different SACCH period than when the GSM RAT was shut off due to Inter RAT Handover, it shall send the Handover Access Bursts to complete the handover procedure. This is the most efficient way of connecting back to GSM channels because the time taken to complete the connection to GSM channels is exactly the same as the time taken by an intra-cell GSM handover. As information pertaining to TDMA frame counters is available to UE, it can switch to the TCH within 2 TDMA frames. Thus UE shall, in this case, achieve synchronization in less than 2 TDMA frames. It will also be obvious to those skilled in the art that other control methods and apparatuses can be derived from the combinations of the various methods and apparatuses of the present invention as taught by the description and the accompanying drawings and these shall also be considered within the scope of the present invention. Further, description of such combinations and variations is therefore omitted above. It should also be noted that the host for storing the applications include but not limited to a computer, phnter or a multi function device. Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are possible and are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom. GLOSSARY OF TERMS AND THEIR DEFINITIONS CS Circuit Switched FCH Frequency Correction Channel FN Frame Number GSM Global System for Mobile Communication LAPDm Link Access Procedures for Data Link Layer (for mobile applications) MAC Medium Access Control MS Mobile Station RAT Radio Access Technology RLC Radio Link Control RR Radio Resource SCH Synchronization Channel SDCCH Standalone Dedicated Control Channel TCH Traffic Channel TDD Time Division Duplex TDMA Time Division Multiple Access UE User Equipment UMTS Universal Mobile Telecommunications System RRC Radio Resource Control UTRAN UMTS Terrestrial Radio Access Network WCDMA Wideband Code Division Multiple Access WE CLAIM 1. A method for efficient fallback of a circuit switched connection to GSM layerl user equipment wherein: a) the GSM TDMA frame counters are updated regularly even when the UE is on the UMTS RAT executing the Handover process whereby the synchronization information with the GSM is retained; b) after receiving the HANDOVER FAILURE command, the GSM RAT, using the synchronization information, tunes the RF to the ARFCN that it was allocated in the GSM cell before the Handover started; c) the bit synchronisation is achieved by applying the last timing offset value calculated while on TCH/SDCCH and the UE achieves synchronization in less than 2 TDMA frames; and d) thereby, the HANDOVER occurs with minimum disruption and takes less than 10 ms to connect back to the GSM channels and the dedicated connection is continued without disruption enabling the user not to lose an on-going speech call. 2. The method as claimed in claim 1, wherein GSM RAT is resumed after failure of inter RAT handover with minimum disruption as the time taken for the handover process is low and that the degradation in the quality of the service due to handover attempts is at the minimum possible level which will not even be perceptible to the mobile user. 3. The method as claimed in preceding claims, wherein GSM RAT is resumed after failure of inter RAT handover in minimum amount of time of about 10 ms. 4. A method for efficient fallback of circuit switched connection to GSM Layerl in case of failure of inter RAT handover to the target radio access technology substantially as herein described and exemplified in the accompanying drawings.

Full Text

FIELD OF TECHNOLOGY
This invention in general relates to wireless technology. More particularly this invention relates to a method for efficient fallback of a circuit switched connection to GSM Layerl User Equipment/Mobile Equipment due to inter RAT handover failure in the target radio access technology.
PRESENT STATE OF ART
The 3GPP specifications impose certain timing restrictions for inter RAT handover procedure accomplishments together with the description of how the inter RAT handover procedure has to be carried out in the mobile.
This invention assumes that the mobile is currently in GSM RAT with a dedicated circuit switched connection.
This invention has references to 3GPP March 2002 version, Release 99 set of Technical Specifications. They are:
• 3GPP TS04.18 Version 8.a.0 RR protocol specification,
• 3GPP TS05.10 Version 8.8.0.Radio subsystem synchronization
• 3GPP TS05.08 Version 8.10.0 Radio subsystem link control
• 3GPP TS 25.133 Version 3.6.0 Requirements for Support of Radio Resource Management (FDD)
When there is an Inter RAT Handover to UTRAN then the GSM RAT in the UE needs to relinquish the control of the hardware to UMTS RAT. This shall mean that the UE shall disconnect the GSM cell that the UE is synchronized before attempting to connect to UMTS RAT and that the UE will lose the synchronization information with the GSM cell. But this information is required if the UMTS RAT is not able to successfully connect to the UMTS cell and the UE has to revert back to the old GSM channel configuration that existed before the handover started.

To achieve successful reactivation of GSM channels in the absence of synchronization, it is required to restore the synchronization information and provide the connection.
The timing requirements as specified in the technical standards 3GPP 05.10 and 3GPP 25.133 to complete the Inter RAT handover procedures are:
Casel: Switching over from GSM to UTRAN shall be completed within 190
ms if the UE is already synchronized to the FDD cell and 270 ms if the UE is not already synchronized to the FDD cell.
Case 2: Switching over from UTRAN to GSM shall be completed within 90 ms if the UE is already synchronized to the GSM cell and 190 ms if the UE is not already synchronized to the GSM cell.
Handover from UTRAN to a GSM cell shall be completed within the following time limits.

From the above, it is clear that the time allowed for synchronizing back to a GSM cell is 100ms.
Operation of Related Art
Method 1:
The UE shall carry out the following functions for RAT handover process and reestablishment of connection to GSM RAT in case of RAT handover failure:
a) suspend the normal operation except for RR management;
b) disconnect the main signaling link via local end release;
c) disconnect the traffic channels, if any;

d) pass on the handover to RRC;
e) RRC attempts to establish the connection;
f) complete the handover procedure;
g) if failure to establish the connection in UMTS revert back to GSM RAT;
h) re-connect to GSM channels and send Handover Failure to the network.
The step (h) above involves synchronizing to the GSM cell first and then reestablishing the connection. A typical design of connecting back to the GSM involves the following steps:
I. Carry out FCH/SCH search on the BCCH frequency;
II. Switch to the traffic channels or the signaling channels depending on when the stage handover attempt took place to UTRAN;
III. Send the Handover Failure message and continue on the TCH or SDCCH as is the case.
The time involved in this method can be arrived at as follows:
The UE has to carry out FCH detection and SCH detections, which in the worst case take 22 TDMA frames (100 ms).
Now that the UE has synchronized to the cell it takes a minimum of one TDMA frame to configure the TCH or SDCCH channel.
Thus the total time required in this method is at least 100ms+ 4.612ms = 104.612ms.
Method 2:
Another way of achieving this synchronization is by storing some of the parameters as listed below:
The following parameters are assumed:

• Current TDMA frame number denoted by N
• TCH frequency offset
• the timing offset for bit synchronization and
• the time offset within the current TDMA frame number at which the GSM RAT gets shut down, denoted by s1
• a timestamp shall also be recorded to mark the time at which the GSM RAT is shut off, denoted by t1
• now when the HANDOVER failure is indicated by UMTS Access Stratum, the GSM RAT shall be reactivated and another timestamp shall be recorded to mark the start of the GSM RAT, denoted by t2.
Now to synchronize to the network, the UE shall arrive at the new FN that is currently running in the cell in the GSM RAT based on the difference in the timestamps t1, t2, the frame number N, and the time offset s1 that was stored at the beginning of Inter RAT Handover Procedure.
The time offset s2, with reference to the start of the TDMA frame when the GSM RAT is resumed after processing the handover failure command is calculated based on si, stored when the GSM RAT was shut off and the difference in the timestamps t1 and t2. Since the GSM RAT knows the frequency of the dedicated channel that was allocated before the handover started this shall complete the synchronization to the old GSM channel.
This shall take at least 3 TDMA frames after the handover failure command is processed. But this synchronization may not be reliable because the start position of TDMA frame may not come at exactly the same position at which it was coming before the handover started.
This way, the total handover completion time comes out to be at least about 20 ms. (the time required to compute the current frame number from the values stored and then the time required to switch to TCH/SDCCH channels).
TDMA frames along the time axis -

t1 is the time at which the GSM RAT was shut down;
N is the frame number in which the GSM RAT was shut down; s1 is the time offset with respect to the start of the TDMA frame when the GSM RAT was shut down;
t2 is the time at which the GSM RAT was started, after the processing of the handover failure command;
N+m is the frame number in which the GSM RAT is started;
N+m+1 is the frame number in which the first TDMA frame interrupt shall come when the GSM RAT is started;
FN dur is the time duration of 1 TDMA frame;
FN dur s2 is the time offset with respect to the start of the TDMA frame when the GSM RAT is started after the processing of the handover failure command;
N+m+4 is the frame number at which the GSM RAT has synchronized back to the old GSM channel configuration that existed before the handover started;
FN2 is the frame number when the frame counters shall be updated when the GSM RAT is started.
Known quantities: t1,t2. N,s1,
FN2 = N+m+2
m = [t2-(t1+ FNdur-s1)]DIVFNdur + 1
FN2 =[t2-(t1 +FNdur-s1)]DIVFNdur + 1 +2 + N
s2 = FNdur- [t2-(t1 + FNdur-s1)] % Fndur Total time taken = t2 -11 + s2 + 3* FNdur

LIMITATIONS
In method 1, the total time required is at least 100ms+ 4.612ms = 104.612ms. In method 2, we shall be able to save the FCH/SCH scan time which is about 22 TDMA frames i.e. about 100 ms when compared to Method 1. This means, in this method, by doing away with FCH/SCH scan, the GSM channels can be reactivated in about 20-ms which is 5 times faster than Method 1.
In case the degradation in quality of service is low due to handover attempts and that is not perceptible to the mobile user, the time taken for the handover process should be as low as possible and hence an attempt is made to evolve a method wherein very low time duration for the handover process (of about 10 ms) is achieved.
OBJECTS OF THE INVENTION
The primary object of this invention to invent a method for efficient fallback of a circuit switched connection to GSM layerl user equipment.
It is another object of the invention to invent a method as wherein GSM RAT is resumed after failures of inter RAT handover with minimum disruption.
It is another object of the invention to invent a method wherein GSM RAT is resumed after failure of inter RAT handover in minimum amount of time (about 10 ms).
It is another object of the invention to invent a method which provides reliable and accurate fallback to GSM RAT after the failure of inter RAT handover in minimum amount of time with minimum disruption.
SUMMARY OF INVENTION
The proposed invention relates to a case where a handover from GSM radio access technology to UMTS radio access technology (WCDMA in specific) is attempted and reverted back to GSM technology on not being able to connect to

WCDMA technology in UMTS for an on-going dedicated connection in Samsung's dual mode mobile handset that supports GSM TDMA and UMTS W-CDMA radio access technologies.
The inter RAT handover should happen keeping the disruption to the ongoing service as minimum as possible. In such a scenario, this proposed invention describes an efficient way of achieving the synchronization to the GSM RAT so that the call can still be supported.
The purpose of this invention is to reduce the total time required (104.612ms) to a bare minimum by efficiently handling the synchronization information In GSM Layer 1.
In this invention a method is proposed in which this time is reduced to 10 ms which is less than 1/10^*^ of the time taken by a typical design that is described above. By keeping this time to such a low duration, the degradation in the quality of the service due to handover attempts is at the minimum possible level which will not even be perceptible to the mobile user.
Accordingly the present invention comprises a method for efficient fallback of a circuit switched connection to GSM layerl user equipment wherein:
a) the GSM TDMA frame counters are updated regularly even when the UE is on the UMTS RAT executing the Handover process whereby the synchronization information with the GSM is retained;
b) after receiving the HANDOVER FAILURE command, the GSM RAT, using the synchronization information, tunes the RF to the ARFCN that It was allocated in the GSM cell before the Handover started;
c) the bit synchronisation is achieved by applying the last timing offset value calculated while on TCH/SDCCH and the UE achieves synchronization in less than 2 TDMA frames; and

d) thereby, the HANDOVER occurs with minimum disruption and takes less than 10 ms to connect back to the GSM channels and the dedicated connection is continued without disruption enabling the user not to lose an on-going speech call.
The other objects, features and advantages of the present invention will be apparent from the accompanying drawings and the detailed description as follows.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows typical protocol architecture of GSM, WCDMA dual mode mobile. As shown in Figure 1, the RR and RRC modules exchange layer 3 protocol messages as well as some information relating to radio resource management. The interface between GSM Layer 1 and WCDMA Layer 1 is to exchange hardware control and its timing.
Figure 2 shows the typical TDMA frame structure along the time axis. It represents the time at which the GSM was shut down and the time at which the GSM is reactivated after the RAT handover failure. It can be seen from the figure that the total time taken for reactivating the GSM RAT is roughly 3 TDMA frames (time taken for synchronization) including the time offset s1 plus the difference between the time stamps t1 and t2. This works out to about 20 ms.
The 3 TDMA frames used for synchronization are avoided in the new method
and that synchronization information is made available during the handover
process.
In figure 2,
t1 is the time at which the GSM RAT was shut down;
N is the frame number in which the GSM RAT was shut down;
s1 is the time offset with respect to the start of the TDMA frame when the GSM RAT was shut down;

t2 is the time at which the GSM RAT was started, after the processing of the handover failure command;
N+m is the frame number in which the GSM RAT is started;
N+m+1is the frame number in which the first TDMA frame interrupt shall come when the GSM RAT is started.
DETAILED DESCRIPTION OF THE INVENTION
A preferred embodiment of the present invention will now be explained with reference to the accompanying drawings. It should be understood however that the disclosed embodiment is merely exemplary of the invention, which may be embodied in various forms. The following description and drawings are not to be construed as limiting the invention and numerous specific details are described to provide a thorough understanding of the present invention, as the basis for the claims and as a basis for teaching one skilled in the art how to make and/or use the invention. However in certain instances, well-known or conventional details are not described in order not to unnecessarily obscure the present invention in detail.
This invention proposes the most efficient way of reverting back to the GSM RAT on the failure of handover attempt in UMTS RAT.
With this method, the synchronization can be achieved in the shortest possible time and with a high level of accuracy. In this method, the GSM RAT will be only updating the TDMA frame counters and storing the values of the following parameters;
• TCH frequency offset;
• The timing offset for bit synchronization; and
• The timing advance value being used along with the Tx power level.

This shall allow updating of the GSM TDMA frame counters regularly as it would normally do when the GSM RAT is active. This way, though the UE is on the UMTS RAT executing the Handover procedure, it shall not lose the synchronization information with the GSM network.
Now when the HANDOVER FAILURE command is received the GSM RAT knows all about the synchronization because all it has to do is to tune the RF to the ARFCN that it was allocated in the GSM cell before the Handover started.
It shall take about one TDMA frame duration after the HANDOVER FAILURE command is processed to connect back to the GSM channels. The bit synchronisation can be achieved by applying the last timing offset value calculated while on TCH/SDCCH.
Bit synchronization can be achieved in this way - when the UE is on a normal TCH, it receives the timing advance value to be used in each downlink SACCH block which means every 480 ms the UE shall update its timing advance value.
In cases where the Inter RAT Handover is completed (including the case of reconnecting back to GSM channels on Handover Failure at UMTS) in the same SACCH period as the Handover Command is received as a FACCH block i.e. within a SACCH period of 480 ms then the UE shall not lose the bit synchronization and use the already stored timing advance and Tx power level values. OthenA/ise, i.e. when the reactivation of GSM channels takes place in a different SACCH period than when the GSM RAT was shut off due to Inter RAT Handover, it shall send the Handover Access Bursts to complete the handover procedure.
This is the most efficient way of connecting back to GSM channels because the time taken to complete the connection to GSM channels is exactly the same as the time taken by an intra-cell GSM handover.

As information pertaining to TDMA frame counters is available to UE, it can switch to the TCH within 2 TDMA frames. Thus UE shall, in this case, achieve synchronization in less than 2 TDMA frames.
It will also be obvious to those skilled in the art that other control methods and apparatuses can be derived from the combinations of the various methods and apparatuses of the present invention as taught by the description and the accompanying drawings and these shall also be considered within the scope of the present invention. Further, description of such combinations and variations is therefore omitted above. It should also be noted that the host for storing the applications include but not limited to a computer, phnter or a multi function device.
Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are possible and are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.

GLOSSARY OF TERMS AND THEIR DEFINITIONS
CS Circuit Switched
FCH Frequency Correction Channel
FN Frame Number
GSM Global System for Mobile Communication
LAPDm Link Access Procedures for Data Link Layer (for mobile applications)
MAC Medium Access Control
MS Mobile Station
RAT Radio Access Technology
RLC Radio Link Control
RR Radio Resource
SCH Synchronization Channel
SDCCH Standalone Dedicated Control Channel
TCH Traffic Channel
TDD Time Division Duplex
TDMA Time Division Multiple Access
UE User Equipment
UMTS Universal Mobile Telecommunications System
RRC Radio Resource Control
UTRAN UMTS Terrestrial Radio Access Network
WCDMA Wideband Code Division Multiple Access

WE CLAIM
1. A method for efficient fallback of a circuit switched connection to GSM layerl
user equipment wherein:
a) the GSM TDMA frame counters are updated regularly even when the UE is on the UMTS RAT executing the Handover process whereby the synchronization information with the GSM is retained;
b) after receiving the HANDOVER FAILURE command, the GSM RAT, using the synchronization information, tunes the RF to the ARFCN that it was allocated in the GSM cell before the Handover started;
c) the bit synchronisation is achieved by applying the last timing offset value calculated while on TCH/SDCCH and the UE achieves synchronization in less than 2 TDMA frames; and
d) thereby, the HANDOVER occurs with minimum disruption and takes less than 10 ms to connect back to the GSM channels and the dedicated connection is continued without disruption enabling the user not to lose an on-going speech call.

2. The method as claimed in claim 1, wherein GSM RAT is resumed after failure of inter RAT handover with minimum disruption as the time taken for the handover process is low and that the degradation in the quality of the service due to handover attempts is at the minimum possible level which will not even be perceptible to the mobile user.
3. The method as claimed in preceding claims, wherein GSM RAT is resumed after failure of inter RAT handover in minimum amount of time of about 10 ms.

4. A method for efficient fallback of circuit switched connection to GSM Layerl in case of failure of inter RAT handover to the target radio access technology substantially as herein described and exemplified in the accompanying drawings.

Documents:

1047-che-2003-abstract.pdf

1047-che-2003-claims duplicate.pdf

1047-che-2003-claims original.pdf

1047-che-2003-correspondence others.pdf

1047-che-2003-description complete duplicate.pdf

1047-che-2003-description complete original.pdf

1047-che-2003-drawings.pdf

1047-che-2003-form 1.pdf

1047-che-2003-form 19.pdf

1047-che-2003-form 26.pdf

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Patent Number

201595

Indian Patent Application Number

1047/CHE/2003

PG Journal Number

08/2007

Publication Date

23-Feb-2007

Grant Date

04-Aug-2006

Date of Filing

23-Dec-2003

Name of Patentee

SAMSUNG INDIA SOFTWARE OPERATIONS PRIVATE LIMITED

Applicant Address

SAMSUNG ELECTRONICS COMPANY LIMITED, KOREA BAGMANE LAKEVIEW, BLOCK 'B'NO.66/1, BAGMANE TECH PARK, C V RAMAN NAGAR, BYRASANDRA BANGALORE-560093, KARNATAKA, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	SREENATH, YALARMATHI VENKATA	SAMSUNG INDIA SOFTWARE OPERATIONS PRIVATE LIMITED BAGMANE LAKEVIEW, BLOCK 'B'NO.66/1, BAGMANE TECH PARK, C V RAMAN NAGAR, BYRASANDRA BANGALORE-560093, KARNATAKA, INDIA
2	NIGAM, ANSHUMAN	SAMSUNG INDIA SOFTWARE OPERATIONS PRIVATE LIMITED BAGMANE LAKEVIEW, BLOCK 'B'NO.66/1, BAGMANE TECH PARK, C V RAMAN NAGAR, BYRASANDRA BANGALORE-560093, KARNATAKA, INDIA
3	SHUKLA, AKASH	SAMSUNG INDIA SOFTWARE OPERATIONS PRIVATE LIMITED BAGMANE LAKEVIEW, BLOCK 'B'NO.66/1, BAGMANE TECH PARK, C V RAMAN NAGAR, BYRASANDRA BANGALORE-560093, KARNATAKA, INDIA
4	VENUGOPAL, SRUNGARAM KRISHNA SRINIVASA	SAMSUNG INDIA SOFTWARE OPERATIONS PRIVATE LIMITED BAGMANE LAKEVIEW, BLOCK 'B'NO.66/1, BAGMANE TECH PARK, C V RAMAN NAGAR, BYRASANDRA BANGALORE-560093, KARNATAKA, INDIA

PCT International Classification Number

H04Q7/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA