Method for Seamless Inter-Frequency Hard Handover in Radio Communication System

• US 20070129075A1
• Filed: 07/21/2006
• Published: 06/07/2007
• Est. Priority Date: 01/10/2001
• Status: Abandoned Application

• Current Assignee: Electronics and Telecommunications Research Institute
• Original Assignee: Electronics and Telecommunications Research Institute
• Inventors: Bang, Seung-Chan, Kim, Jae-Heung, Kim, Chang-Joo, Kim, Il-Gyu
• Application Number:
  US11/459,134
• Publication Number:
  US 20070129075A1
• Time in Patent Office: Days
• US Class Current: 455/436
• CPC Class Codes: H04W 36/0069 in case of dual connectivity, e.g. decoupled uplink/downlink
• CPC Class Codes: H04W 56/00 Synchronisation arrangements

Abstract

A method for seamless inter-frequency hard handover in a radio communication system is disclosed. The method for seamless inter-frequency hard handover includes the steps of: a) at a mobile station, blocking a first uplink carrier frequency used for communication, transmitting a direct sequence spread preamble signal through a second uplink carrier frequency for a short time, and continuously performing the communication through the first uplink carrier frequency; and b) at a target base transceiver station, acquiring an uplink synchronization of a mobile station based on the preamble before performing handover.

Claims

1. A method for performing seamless inter-frequency hard handover in a radio communication system, comprising the steps of:

a) a mobile station, disconnecting a first uplink carrier frequency used for communication, transmitting a direct sequence spread preamble signal through a second uplink carrier frequency for a short time, and continuously performing the communication through the first uplink carrier frequency;

b) a target base station, acquiring an uplink synchronization of the mobile station by using the direct sequence spread preamble; and

c) performing the hard handover by using the uplink synchronization.

2. The method as recited in claim 1, further comprising the steps of:

d) the target base station, after acquiring the uplink synchronization of the preamble, transmitting an AI as an acknowledgment of the acquisition of the uplink synchronization by using a new downlink frequency; and

e) the mobile station, detecting the AI.

3. The method as recited in claim 2, further comprising the step of f) re-transmitting the preamble by increasing a transmission power if the mobile station does not receive the AI from the target base station.

4. The method as recited in claim 2, further comprising the step of f) in case a base station controller does not know a frame offset, which is a difference between a connection frame number (CFN) of the mobile station and a system frame number (SFN) of the target base station, calculating the frame offset by using the SFN used for transmitting the AI and a CFN used for receiving the AI, which are transmitted from the target base station and the mobile station, respectively, and transmitting the calculated frame offset to the target base station.

5. The method as recited in claim 1, further comprising the step of d) in case a base station controller knows a frame offset, the target base station, acquiring the uplink synchronization by using the preamble and reporting the acquisition of the uplink synchronization only to the base station controller.

6. The method as recited in claim 5, wherein the step a) includes the steps of:

a1) the base station controller, transmitting a transport channel frame number (TGCFN), at which the mobile station starts an uplink preamble transmission, a slot number (TGSN) and compressed mode pattern parameters, TGL1, TGL2, TGD, TGPL1, TGPL2 and TGPRC, to the mobile station after a time0 at which the mobile station is communicating with a source base station through the first carrier frequency and acquires the synchronization of a new downlink, the target base station has resources to support the mobile station but does not acquire the uplink synchronization and the base station controller knows the frame offset and a chip offset; and

a2) the mobile station, transmitting the preamble during a transmission gap (TG) of a corresponding compressed frame through the second uplink carrier frequency by using compressed mode information,

the step b) including the steps of;

b1) the base station controller, transmitting the TGCFN, the TGSN, the mode parameters, TGL1, TGL2, TGD, TGPL1, TGPL2 and TGPRC, a scrambling code identification (SCID), the frame offset and the chip offset to the target base station, thereby allowing the target base station to search the preamble transmitted from the mobile station in a preset time;

b2) the target base station, searching the preamble by using the compressed mode information, the frame offset, the chip offset and the SCID; and

b3) the target base station, after achieving the preamble searching, reporting the success of searching the preamble to the base station controller by the target base station, and

the step c) including the steps of;

c1) the base station controller, instructing the source base station and the mobile station to perform the handover and at the same time, transmitting orthogonal variable spreading factor (OVSF) information, which is used for establishing a new link, a transport channel frame number, of the mobile station, at which the handover is performed, CFN and information for an uplink transmission power, which is used by the mobile station just after the handover is performed;

c2) the base station controller, instructing the target base station to perform the handover and at the same time, transmitting the CFN at which the handover is performed; and

c3) the mobile station and the target base station, establishing communication there between through the new link at the CFN.

7. The method as recited in claim 1, further comprising the step of d) in case a base station controller knows a frame offset, the target base station, transmitting an AI through a new downlink of the second carrier frequency after acquiring the uplink synchronization by using the preamble and reporting the acquisition of the uplink synchronization the base station controller.

8. The method as recited in claim 7, wherein the step a) includes the steps of:

a1) the base station controller, transmitting a transport channel frame number (TGCFN), at which the mobile station starts an uplink preamble transmission, a slot number (TGSN), compressed mode pattern parameters, TGL1, TGL2, TGD, TGPL1, TGPL2 and TGPRC, and orthogonal variable spreading factor (OVSF) code information to be used as a channel spread code of the AI through the new downlink to the mobile station after a time0 at which the mobile station is communicating with a source base station through the first carrier frequency and acquires the synchronization of a new downlink, the target base station has resources to support the mobile station but does not acquire the uplink synchronization and the base station controller knows the frame offset and a chip offset; and

a2) the mobile station, transmitting the preamble during a transmission gap (TG) of a corresponding compressed frame through the second uplink carrier frequency by using compressed mode information,

the step b) including the steps of;

b1) the base station controller, transmitting the TGCFN, the TGSN, the compressed mode parameters, TGL1, TGL2, TGD, TGPL1, TGPL2 and TGPRC, a scrambling code identification (SCID), the frame offset and the chip offset to the target base station, thereby allowing the target base station to search the preamble transmitted from the mobile station is a preset time;

b2) the target base station, searching the preamble by using the compressed mode information, the frame offset, the chip offset and the SCID;

b3) the target base station, after achieving the preamble searching, transmitting an AI spread to the OVSF through the new downlink as a response of the success of searching the preamble; and

b4) the target base station, after achieving the preamble searching, reporting the success of searching the preamble to the base station controller, and

the step c) including the steps of;

c1) the base station controller, instructing the source base station and the mobile station to perform the handover and at the same time, transmitting a connection frame number (CFN) of the mobile station, at which the handover is performed, and information for an uplink transmission power, which is used by the mobile station just after the handover is performed;

c2) the base station controller, instructing the target station to perform handover and at the same time, transmitting the CFN at which the handover is performed; and

c3) the mobile station and the target base station, establishing communication there between through a new link at the CFN, wherein the OVSF code, which is used for transmitting the AI, is used for a communication channel of the new downlink.

9. The method as recited in claim 2, wherein, if the mobile station employs a single-mode receiver, a last frame of the first carrier frequency for the communication established between the mobile station and a source base station and a first frame after the preamble is transmitted to the target base station are operated as compressed frames and, between the two frames, there is a guard time for a frequency conversion of the mobile station.

10. The method as recited in claim 4, wherein the step a) includes the steps of:

a1) for making the base station controller know the frame offset just before instructing the mobile station and the target base station to perform the handover, the base station controller, transmitting a transport channel frame number (TGCFN), at which the mobile station starts an uplink preamble transmission, a slot number (TGSN), compressed mode pattern parameters, TGL1, TGL2, TGD, TGPL1, TGPL2 and TGPRC, and an orthogonal variable spreading factor (OVSF) code information to be used as a channel spread code of the AI through a new downlink to the mobile station after a time0, at which, the mobile station is communicating with a source base station through the first carrier frequency and acquires the synchronization of the new downlink, the target base station has resources to support the mobile station but does not acquire the uplink synchronization and the base station controller knows a chip offset but does not know the frame offset; and

a2) the mobile station, transmitting the preamble during a transmission gap (TG) of a corresponding compressed frame through the second uplink carrier frequency by using compressed mode information,

the step b) including the steps of;

b1) the base station controller, transmitting TGCFN, the TGSN, the compressed mode parameters TGL1, TGL2, TGD, TGPL1, TGPL2 and TGPRC, a scrambling code identification (SCID) and a chip offset to the target base station, thereby allowing the target base station to search the preamble transmitted from the mobile station in a preset time;

b2) the target base station, searching the preamble in each frame by using the compressed mode information, the chip offset and the SCID from directly after the target base station receives a preamble search instruction from the base station controller;

b3) the target base station, after achieving the preamble searching, transmitting an AI spread by the OVSF through the new downlink as a response of the success of searching the preamble search to the mobile station;

b4) the target base station, reporting an SFN of the target base station, which transmits the AI, to the base station controller; and

b5) the mobile station, reporting a CFN of the mobile station, at which the AI is detected, to the target base station after succeeding to detect the AI and stopping transmitting the preamble, and

the step c) including the steps of;

c1) the base station controller, instructing the source base station and the mobile station to perform the handover and at the same time, transmitting a connection frame number (CFN) of the mobile station, at which the handover is performed, and information for an uplink transmission power, which is used by the mobile station just after the handover is performed;

c2) the base station controller, instructing the target station to perform the handover and, at the same time, transmitting the CFN of the mobile station, at which the handover is performed, and the frame offset to the target base station; and

c3) the mobile station and the target base station, establishing communication there between through a new link at the CFN, wherein the OVSF code, which is used for transmitting the AI, is used for a communication channel of the new downlink.

11. A method for performing a seamless inter-frequency hard handover in a radio communication system in case that a base station controller (or a radio network) dose not know a frame offset, which is a difference between a connection frame number (CFN) of a mobile station and a system frame number (SFN) of a target base station, comprising the steps of:

a) the mobile station, completely disconnecting a first uplink carrier frequency used for communication, transmitting a direct sequence spread preamble (or pilot) through a second uplink carrier frequency for a short time, and continuously performing the communication through the first uplink carrier frequency;

b) the target base station, acquiring an uplink synchronization of the mobile station by using the preamble before performing the hard handover;

c) the target base station, after acquiring the uplink synchronization, transmitting a direct sequence spread AI as a response for the acquisition of the uplink synchronization for a short time through a new downlink frequency;

d) the mobile station, detecting the AI;

e) the base station controller, calculating a frame offset by using the SFN, which is used for transmitting the AI and the CFN, which is used for receiving the AI, and transmitting the calculated frame offset to the target base station; and

f) the base station controller, instructing the mobile station and the target base station to perform the handover.

12. The method as recited in claim 11, wherein the method uses an orthogonal variable spreading factor (OVSF) code, which is used in a communication channel of the downlink when hard handover is performed through a new link, as a channel spreading code of the AI in case that the target base station transmits the AI as the response for the acquisition of the uplink synchronization through the new downlink frequency.

13. The method as recited in claim 12, wherein the target base station transmits the AI at least more than one time to increase the probability of detecting the AI of the mobile station.

14. The method as recited in claim 11, wherein the mobile station uses an uplink compressed mode pattern for the inter-frequency hard handover.

15. The method as recited in claim 14, wherein the mobile station uses a scrambling code operating in a normal mode when transmitting the preamble while using the uplink compressed mode pattern for the inter-frequency hard handover.

16. The method as recited in claim 11, wherein the occurrence of false handover instruction by the base station controller is minimized by classifying the AI to a first AI (AI₁) and a second AI (AI₂), and the preamble to a first preamble (preamble₁) and a second preamble (preamble₂) for decreasing the false detection probability of the target base station and the mobile station in case the base station controller does not know the frame offset.

17. The method as recited in claim 16, wherein the AI₁ and AI₂ are distinguished with each other according to a binary orthogonal transform method using an orthogonal code.

18. The method as recited in claim 16, wherein the preamble₁ and the preamble₂ use an identical scrambling code, and are distinguished with each other according to a binary orthogonal transform method using a orthogonal code.

19. The method as recited in claim 11, wherein the mobile station performs a coherent demodulation to detect the AI by using a common pilot channel (CPICH) transmitted through the frequency through which the AI is transmitted.

20. The method as recited in claim 11, wherein the target base station stores signals received during a transmission gap (TG) of a compressed mode, at which the mobile station transmits the preamble, and then searches the preamble by using signals stored in a time, which the mobile station does not transmit the preamble.

21. The method as recited in claim 16, wherein the step c) includes the steps of:

c1) the target base station, transmitting the first AI (AI₁) the mobile station through the new downlink frequency as a response for the success of searching the first preamble;

c2) the target base station, detecting the second preamble (preamble₂) in a next compressed frame after transmitting the first AI (AI₁);

c3) the target base station, transmitting the second AI (AI₂) to the mobile station when detecting the second preamble (preamble₂), reporting a success of acquiring the uplink synchronization to the base station controller and at the same time of noticing a SFN at which the second AI (Al₂) is transmitted; and

c4) the target base station, detecting the first preamble again when it failed to detect the second preamble (preamble₂),

the step d) including the steps of;

d1) the mobile station, transmitting the second preamble (preamble₂) in the next compressed frame with the same power as used before when succeeding to detect the first AI (AI₁) and transmitting the first preamble (preamble₁) again when failing to detect the first AI (AI₁);

d2) the mobile station, reporting the detection of the second AI (AI₂) to the base station controller in case that the mobile station is success to detect the second AI (AI₂) after transmitting the second preamble (preamble₂) and, at the same time, transmitting a CFN, at which the second AI (AI₂) is detected to the base station controller; and

d3) the mobile station, reporting the failure of detecting the second AI (AI₂) to the base station controller in case that the mobile station fails to detect the second AI (AI₂), after transmitting the second preamble (preamble₂)

the step e) including the steps of;

e1) the base station controller, calculating the frame offset when it received the CFN and the SFN from the mobile station and the target base station, respectively;

e2) the base station controller, instructing a home base station and the mobile station to perform the handover and, at the same time, transmitting the CFN of the mobile station, which performs the handover, and information for an uplink transmission power to be used by the mobile station directly after performing the handover; and

e3) the base station controller, instructing the target base station to perform the handover and, at the same time, transmitting the frame offset and the CFN of the mobile station, which performs the handover, and

the step f) including the steps of;

f1) establishing a new communication link between the mobile station and the target base station at the CFN, wherein a downlink of the new communication link uses an orthogonal variable spreading factor (OVSF) code, which is used in transmitting the AI; and

f2) repeating the steps c1) to e3) in case that the base station controller receives a message of failing to detect the AI₂ from the mobile station, does not receive the SFN for a certain time after receiving the CFN, does not receive the CFN for a preset time after receiving the SFN, or does not receive any information from the mobile station and the target base station until a predetermined time is expired.

22. A computer readable record medium for storing instructions for executing seamless inter-frequency handover, comprising the functions of:

a) a mobile station, disconnecting a first uplink carrier frequency used for communication, transmitting a direct sequence spread preamble signal through a second uplink carrier frequency for a short time, and continuously performing the communication through the first uplink carrier frequency;

b) a target base station, acquiring an uplink synchronization of the mobile station by using the direct sequence spread preamble; and

c) performing the hard handover by using the uplink synchronization.

23. The computer readable record medium as recited in claim 22, further comprising the functions of:

d) the target base station, after acquiring the uplink synchronization of the preamble, transmitting an AI as an acknowledgment of the acquisition of the uplink synchronization by using a new downlink frequency; and

e) the mobile station, detecting the AI.

24. The computer readable record medium as recited in claim 23, further comprising the function of f) re-transmitting the preamble by increasing a transmission power if the mobile station does not receive the AI from the target base station.

25. The computer readable record medium as recited in claim 22, further comprising the function of d) forcing the base station controller to know a frame offset just before the base station controller (or a radio network) instructs the mobile station and the target base station to perform the handover in case that the base station controller does not know the frame offset, which is a difference between a connection frame number (CFN) of the mobile station and the a system frame number (SFN) of the target base station, thereby preventing an additional call disconnection.

26. The computer readable record medium as recited in claim 22, further comprising the function of d) in case a base station controller knows a frame offset, the target base station, acquiring the uplink synchronization by using the preamble and reporting the acquisition of the uplink synchronization only to the base station controller.

27. The computer readable record medium as recited in claim 22, further comprising the function of d) in case a base station controller knows a frame offset, the target base station, transmitting an AI through a new downlink of the second carrier frequency after acquiring the uplink synchronization by using the preamble and reporting the acquisition of the uplink synchronization the base station controller.