US20070129075A1 - Method for Seamless Inter-Frequency Hard Handover in Radio Communication System - Google Patents

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.

Description

RELATED APPLICATIONS
• This application is a continuation of U.S. patent application Ser. No. 10/466,053 filed on Jul. 9, 2003, pending, which is a non-provisional application of International Application No. PCT/KR01/02314, filed Dec. 31, 2001.
TECHNICAL FIELD
• The present invention relates to a handover method in a radio communication system; and, more particularly, to a method capable of implementing a seamless inter-frequency hard handover in a direct sequence code division multiple access (DS-CDMA) system and a computer readable recording medium for executing the method.
BACKGROUND ART
• It is widely known that a soft handover, in which a frequency change does not occur, can be used generally for handover between base stations of one wireless communication service provider in a CDMA cellular system since all base stations of the one wireless communication service provider can use a same frequency for the soft handover.
• The soft handover is a method for maintaining a communication link by simultaneously transceiving communication signal with both a source base station and a neighbor base station without changing a communication frequency when a mobile station is located at a cell boundary of the two base stations, i.e., when the mobile station moves from a coverage of the source base station to a coverage of the neighbor base station, and then disconnecting the communication link with the source base station, if a signal intensity of the source base station is weaken below a standard signal intensity as maintaining continuously the communication link with the neighbor base station. The above-mentioned soft handover provides the unstrained handover by eliminating an instant-disconnection, which is a chronic problem of an analog system, decreases a probability of call loss and maintains high quality communication.
• However, the soft handover is not applicable in case a certain wireless communication service provider allocates different number of frequencies to the neighbor base stations according to a call density by considering an economy of a network design and an efficiency of investment, i.e., the neighbor base stations use different frequencies. That is, if the mobile station using a specific frequency of the source base station is moving to the cell of the neighbor base station, which does not equip the specific frequency, the soft handover cannot be applicable. Furthermore, the soft handover cannot be applicable between base stations possessed by two wireless communication providers using different frequencies although they adopt the same CDMA scheme. For these cases, the hardware handover has to be used.
• The hard handover needs to be performed between frequencies in case of the handover from a wideband-CDMA (W-CDMA) time division duplex (TDD) to a W-CDMA frequency division duplex (FDD) or handover from global system for mobile communications (GSM) to the W-CDMA FDD.
• Referring toFIG. 1, an example of the hard handover between asource base station2 and atarget base station3 is explained as follows.
• As shown inFIG. 1, thetarget base station3 does not support communication frequencies f₁and f₁₁of a currentmobile station1.
• InFIG. 1, in case of themobile station1 having a dual-mode receiver, themobile station1 can measure a signal intensity of a new frequency f₂while demodulating downlink signals through the currently established frequency f₁and acquire synchronization of signals transmitted from thetarget base station3.
• Such the dual-mode receiver needs an additional hardware for radio frequency (RF) compared to a single-mode receiver and thus the complexity of a mobile device is increased.
• To overcome above-mentioned problems, a compressed mode is defined in an asynchronous W-CDMA (FDD) standard (Release '99) of 3rd generation partnership project (3GPP), which was released at September 2000.
• FIG. 2 illustrates an example of compressed mode transmission.
• In the 3GPP standard, a frame has a length of 10 msec and consists of 15 slots.
• For a transmission gap (TG)region7 in a compressed frame, data transmission is not permitted. Instead of permitting the data transmission, a rate of frame errors of the compressed frame is maintained identical to that of anormal frame5 by keeping a transmitting power at aslot region6 in the compressed frame higher than a power of thenormal frame5.
• Themobile station1 having the single-mode receiver can search the signal intensity of the new frequency f₂in downlink on the handover situation shown inFIG. 1 by using the compressed mode ofFIG. 2. That is, it is possible to search the signal intensity by dropping the current established communication frequency f₁, changing to a frequency f₂and measuring the signal intensity of f₂in the TG region and after the TG region is over, demodulating the call channel of the frequency f₁.
• In the 3GPP (FDD) standard, the compressed mode is defined at not only the downlink but also the uplink. The downlink and uplink can be operated simultaneously as the compressed mode and only one of the downlink and the uplink can be operated as the compressed mode. A reason of defining the compressed mode in the uplink is for prevention of interference to the downlink when themobile station1 measures a frequency of the uplink and the downlink of a neighbor system such as 3GPP TDD or GSM. Therefore, even though themobile station1 employs the dual-mode receiver, the uplink needs to be operated as the compressed mode in case that themobile station1 measures the downlink of other system using a frequency similar to the frequency of the uplink.
• In shortly, it is possible that themobile station1, which satisfies the 3GPP (FDD) standard, monitors a new frequency f₂of the downlink before disconnecting the current established call channel completely in the handover situation inFIG. 1 and call disconnection of the downlink can be avoided although there occurs the hard handover to the new frequency f₂since the synchronization of the downlink transmitted from thetarget base station3 can be acquired by using a synchronization channel of f₂and a common pilot channel.
• On the other hand, in case of the uplink, since the target base station receives no signal before themobile station1 drops the current established frequency f₁′ and transmits signals by using a new frequency f₂′, i.e., the hard handover occurs, the synchronization of the uplink needs to be started at thetarget base station3 from a moment that the hard handover occurs. There occurs call disconnection since at least one frame is required to acquire the synchronization of the uplink even if an outperformed searcher is used in thetarget base station3.
• Moreover, since, according to the 3GPP W-CDMA (FDD) scheme, corresponding base stations operate in asynchronization, thetarget base station3 cannot detect a round trip delay between themobile station1 and thetarget base station3 and therefore, a time for acquiring synchronization in thetarget base station3 may be more than several frames since a search window size becomes very large, which a searcher has to search, in case that a coverage area of the base station is huge. In this case, several frame disconnection may happen and current call disconnection also may be happened in more serious case. Also, in this case, a power may not be controlled properly, so that a capacity of the uplink of thetarget base station3 may be incredibly decreased.
• In the 3 GPP W-CDMA standard (Release '99), it is possible to perform the handover only in case a difference between a system frame number (SFN) of thetarget base station3 and a connection frame number (CFN) of themobile station1 is known to the network. Therefore, themobile station1 needs to detect the SFN information of thetarget base station3 by demodulating a common channel of the downlink of thetarget base station3 before performing the handover and transmit the SFN information and a frame offset, which is the difference between the CFN of themobile station1, to thebase station controller4. Thereby allowing thebase station controller4 to decide an exact handover time, resulting in performing the handover. Above-mentioned operations are well performed in the soft handover between same frequencies. However, in case of the hard handover between different frequencies, themobile station1 should use the compressed mode of the downlink for acquiring the SFN information of thetarget base station3.
• However, in the standard (Release '99), it is impossible to acquire the SFN information by using the compressed mode since at least 50 msec continuous demodulating time is required in the downlink for acquiring the SFN information. In case of the hard handover, since themobile station1 has to acquire the SFN information after being completely disconnected with the current established frequency and being connected to a new frequency, there may occur at least S0 msec additional call disconnection.
• The above-mentioned problems are not limited to the inter-frequency hard handover in the W-CDMA FDD and they may happen when a multimode device having the dual-mode receiver or the single-mode receiver such as GSM/WCDMA FDD multimode device or W-CDMA TDD/W-CDMA FDD multimode device performs the hard handover from the GSM system to the W-CDMA FDD system or from the W-CDMA TDD system to the W-CDMA FDD system.
• As mentioned above, the disconnection is inevitable for performing the inter-frequency hard handover defined in the 3GPP W-CDMA FDD standard. Specially, in case of the mobile station having the single mode receiver, since the compressed mode is used in the downlink for searching signals of the target base station so the frame offset between the target base station and the mobile station is not known to the network. In this case, the disconnection problem becomes more serious since at least 50 msec disconnection is generated during performing the inter-frequency handover. This is indicated as a problem in the 3GPP. Therefore, a handover method, which performs the inter-frequency hard handover without disconnection, is required for addressing the problems in the asynchronous W-CDMA standard.
DISCLOSURE OF THE INVENTION
• It is, therefore, an object of the present invention to provide a method capable of implementing a seamless inter-frequency hard handover in a wireless communication system such as a DS-CDMA system and a computer readable recording medium storing instructions for executing the method.
• In accordance with an aspect of the present invention, there is provided a method for performing seamless inter-frequency hard handover in a radio communication system, including 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.
• In accordance with another aspect of the present invention, there is also provided 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, including 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 acquisition indicator (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.
• The present invention transmits a preamble (or a pilot) through new frequency f₂′ in a transmission gap (TG) by using an uplink compressed mode before a mobile station completely disconnects a currently established communication in an inter-frequency hard handover situation described inFIG. 1.
• The present invention provides a method for seamless inter-frequency hard handover by acquainting a synchronization of an uplink by using a preamble (or pilot) transmitted from a target base station before a currently established communication is completely disconnected.
• In the present invention, the target base station transmits an acquisition indicator (AI) through downlink for a fast response of acquisition of the preamble (or pilot) transmitted from a mobile station in the transmission gap (TG).
• The present invention also prevents an additional call disconnection in case a base station controller does not know a frame offset, which is a difference between SFN of the target base station and CFN of the mobile station, by providing a method that a network knows the frame offset before performing the hard handover.
• In a difference way of a convention inter-frequency hard handover having problem of at least more than one frame call disconnection, the present invention provides a method for seamless inter-frequency hard handover by transmitting a preamble (or pilot) through new frequency with a compressed mode, acquainting a synchronization of an uplink by using a preamble (or pilot) transmitted from a target base station before a currently established communication is completely disconnected.
• The present invention also performs the hard handover quickly in a network by the target base station transmits the acquisition indicator (AI) according to a received preamble (or pilot) transmitted from a mobile station in a transmission gap (TG).
• In a difference way of a convention inter-frequency hard handover having problem of at least more than 50 msec call disconnection in case that a base station controller dose not know a frame offset, which is a difference between SFN of a target base station and CFN of a mobile station, the present invention provides a method for seamless inter-frequency hard handover by providing a method that a network knows the frame offset before performing the hard handover.
• The present invention can be implemented to not only an inter-frequency hard handover in W-CDMA FDD system but also a hard handover to W-CDMA FDD or from GSM to W-CDMA FDD in W-CDMA TDD system.
BRIEF DESCRIPTION OF THE DRAWINGS
• The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:
• FIG. 1 is a view illustrating a typical wireless communication system for performing an inter-frequency hard handover;
• FIG. 2 is a diagram showing a structure of a typical compressed mode transmission;
• FIG. 3 is a diagram depicting an example of an uplink transmission, which transmits a preamble through a new frequency by using a compressed mode of a single frame configuration for a seamless inter-frequency hard handover in accordance with an embodiment of the present invention;
• FIG. 4 is a diagram providing an example of an uplink transmission, which transmits a preamble through a new frequency by using a compressed mode of a double frame configuration for a seamless inter-frequency hard handover in accordance with an embodiment of the present invention;
• FIG. 5 is a diagram representing an example of uplink and downlink transmission in case of a mobile station having a dual-mode receiver for a seamless inter-frequency hard handover in accordance with an embodiment of the present invention;
• FIG. 6 is a diagram illustrating an example of uplink and downlink transmission in case of a mobile station having single-mode receiver for a seamless inter-frequency hard handover in accordance with an embodiment of the present invention;
• FIG. 7 shows a signal flow between systems in case a target base station does not transmit an AI through a downlink when a network knows a frame offset for a seamless inter-frequency hard handover in accordance with the present invention;
• FIG. 8 is a diagram illustrating a compressed mode pattern of the 3GPP W-CDMA employed in the present invention;
• FIG. 9 exemplifies a timing chart showing that a target base station decides a searching region for a preamble transmitted from a mobile station having a dual-mode receiver in case a frame offset is known to a network for a seamless inter-frequency hard handover in accordance with the present invention;
• FIG. 10 presents a signal flow between systems in case a target base station transmits AI through a downlink when a frame offset is known to a network for a seamless inter-frequency handover in accordance with the present invention;
• FIG. 11 is a timing chart in case a target base station transmits an AI for detecting a preamble when a frame offset is known to a network for a seamless inter-frequency handover in accordance with the present invention;
• FIG. 12 illustrates a signal flow chart in case one class of AI and one class of preamble is transmitted when a frame offset is not known to a network for a seamless inter-frequency handover in accordance with the present invention;
• FIG. 13 is a timing chart illustrating that a target base station searches a preamble in a corresponding region of each frame from a moment it receives a preamble search order in case a frame offset is not known to a network for a seamless inter-frequency handover in accordance with the present invention;
• FIG. 14 is a view illustrating that a base station controller calculates a frame offset by using an SFN from a target base station and a CFN from a mobile station in case the frame offset is not known to a network for a seamless inter-frequency handover in accordance with an embodiment of the present invention;
• FIG. 15 is a view describing that a base station controller calculates a frame offset by using an SFN of a target base station and a CFN of a mobile station in case the frame offset is not known to a network for a seamless inter-frequency handover in accordance with another embodiment of the present invention;
• FIG. 16 is a signal flow chart in case two classes of AI and two classes of preamble are transmitted when a frame offset is not known to a network for a seamless inter-frequency handover in accordance with the present invention;
• FIG. 17 shows an example of operations of a mobile station and a target base station in case two classes of AI and two classes of preamble are transmitted when a frame offset is not known to a network for a seamless inter-frequency handover in accordance with the present invention; and
• FIG. 18 provides a flowchart illustrating operations of a mobile station and a base station using the method ofFIG. 16 for a seamless inter-frequency handover in accordance with the present invention.
MODES FOR CARRYING OUT THE INVENTION
• With reference to the accompanying drawings, the preferred embodiments of the present invention will be described in detail hereinafter.
• FIG. 1 shows an example of performing a hard handover between two base stations, i.e., asource base station2 and atarget base station3. As shown inFIG. 1, thetarget base station3 does not support communication frequencies f₁and f₁′ of amobile station1.
• FIGS. 3 and 4 exemplify preamble transmission methods of an uplink, which use an uplink compression mode of the present invention for implementing a seamless inter-frequency hard handover in a same situation is inFIG. 1.
• FIG. 3 shows a single frame compression mode andFIG. 4 represents a double frame compression mode.
• In the present invention, themobile station1 transmits apreamble8 through a new frequency f₂′ in a TG region by using the uplink compressed mode before disconnecting the current established frequency f₁′ completely as shown inFIGS. 3 and 4. At this moment, thepreamble8 transmitted from themobile station1 is a direct sequence bandwidth spread signal and a code sequence used for bandwidth spread needs to be known to thetarget base station3 before the preamble is transmitted.
• For seamless inter-frequency hard handover, before performing the hard handover, thetarget base station3 acquires the synchronization of signals transmitted from themobile station1 by using thepreamble8 of the uplink at which themobile station1 transmits signals through the new frequency f₂′ by using the uplink compressed mode.
• In the present invention, themobile station1 has “Guard Time” before 9 or after 10 transmitting thepreamble8 in the uplink compressed mode, and thetarget base station3 and abase station controller4, i.e., a network should know the “Guard Time” (T_grd)9 before thepreamble8 transmitted. This has to be newly defined in the 3GPP standard.
• In the present invention, after thetarget base station3 acquires the synchronization of signal transmitted from themobile station1 by using thepreamble8 of the uplink transmitted through the frequency f₂′, thetarget base station3 can transmit an acknowledgment (ACK) of acquiring the synchronization (through a wired network) to thebase station controller4 or through the downlink of the frequency f₂to themobile station1.
• FIGS. 5 and 6 show an example that thetarget base station3 transmits an acknowledgment for the synchronization in the form of an acquisition indicator (AI) to themobile station1 by using the downlink of frequency f₂after acquiring the synchronization of signals transmitted from themobile station1 by using thepreamble8 of the uplink transmitted through the frequency f₂′.
• FIG. 5 shows an example of data transmitting between a mobile station and a base station in case of the mobile station having a dual-mode receiver andFIG. 6 shows an example of data transmitting between a mobile station and a base station in case of the mobile station having a single-mode receiver.
• As illustrated inFIGS. 5 and 6, the AI transmitted by thetarget base station3 is not related with a structure of the receiver of the mobile station, whereas a structure of frames transmitted from thesource base station2 and a role of the frames at receiver of the mobile station are depending on the structure of the receiver of the mobile station. That is, in case themobile station1 has the single-mode receiver, thesource base station2 transmits the compressed frame in the downlink as shown inFIG. 6 as a frame, which is corresponding to a location of theAI11 transmitted from thetarget base station3, on the other hand, case of themobile station1 has the dual-mode receiver, a normal frame is transmitted as shown inFIG. 5.
• As shown inFIGS. 5 and 6, thetarget base station3 transmits the AI after a certain period time is passed by considering the processing time after receiving thepreamble8 from themobile station1 and themobile station1 has to know a start point of transmitting the AI.
• When demodulating the AI, the mobile station with the dual-mode receiver or the single-mode receiver can perform a non-coherent demodulation or can perform a coherent demodulation by using a common pilot channel transmitted through the downlink of the frequency f₂from thetarget base station3. Although it is not shown in the drawings, thetarget base station3 transmits other channel of the downlink of the frequency f₂such as a common pilot channel, a synchronization channel and data channel for othermobile station1 and so on in a code division scheme.
• In case themobile station1 has the single-mode receiver, themobile station1 uses the common pilot channel of thetarget base station3, which is received through the frequency f₂in theTG region7, to coherently demodulate the AI.
• In case themobile station1 has the single-mode receiver1, the “Guard Time”13 has to exist between the end of the AI transmission and the end of the TG region.
• In the uplink preamble transmission method of the present invention shown in FIGS.3 to6, themobile station1 can transmit a same preamble several times by repeating the compressed mode several times so as to increase the probability of the target base station acquiring the synchronization. At this moment, the compressed mode pattern is transmitted from thebase station controller4 to themobile station1 and thetarget base station3. Also, in the AI transmission of the downlink described inFIGS. 5 and 6, thetarget base station3 can transmit the AI several times to increase the probability of themobile station1 detecting the AI. Parameters related to the compressed mode pattern are defined in the 3GPP TS25. 525 standard.
• FIG. 7 is a diagram illustrating a signaling procedure between themobile station1, thesource base station2, thetarget base station3 and thebase station controller4 during performing the hard handover in case that the frame offset is known to thebase station controller4 and thetarget base station3 does not transmit the acquisition indication (AI) for the acquisition of the downlink preamble to themobile station1, wherein the frame offset is a difference between the SFN of thetarget base station3 and the CFN of themobile station1.
• InFIG. 7, it is assumed that the frame offset is known to thebase station controller4 but thetarget base station3 does not transmit the AI to the downlink. Referring toFIG. 7, attime014, themobile station1 and thesource base station2 are communicating through the frequency f₁(f₁′); thetarget base station3 has resources to support themobile station1 through the frequency f₂(f₂′); the frame offset and a chip offset were known to thebase station controller4 which OVSF_targetfrom thetarget base station3, the mobile station acquired the synchronization of the downlink of thetarget base station3; and thetarget base station3 does not acquire the synchronization of the uplink.
• That is, at thetime014, themobile station1 andsource base station2 are communicating through f₁(downlink) and f₁′ (uplink); thetarget base station3 has reported to thebase station controller4 that there are resources at the f₂(f₂′) link to support themobile station1 and also thetarget base station3 has reported an orthogonal variable spreading factor (OVSF) code of the downlink to be used by themobile station1 at the downlink to thebase station controller4 after performing the handover.
• As mentioned above, thebase station controller4 already knows the frame offset and the chip offset at thetime014. Herein, the frame offset represents the difference between the SFN of thetarget base station3 and the CFN of themobile station1 and it is defined in the standard TS25. 402 (Release '99) of 3GPP. Also, the SFN is a frame number of a common control channel of the downlink and has a range of 0 to 4095 and the CFN is a transport channel frame number, having a range of 0 to 255 and is determined after a communication link between themobile station1 and a base stations is established.
• Thebase station controller4 can know the frame offset by receiving the frame offset from the mobile station before thetime014 in case themobile station1 has a dual-mode receiver or analogizing the frame offset from information reported from other base stations. The chip offset is a difference between frame boundary of a transport channel of themobile station1 and a frame boundary of a common control channel of thetarget base station3 and has a range of 0 to 38399 chips. Themobile station1 measures the chip offset by using the dual-mode receiver or using the compressed mode in case of a single mode receiver and the measured chip offset transmitted to thebase station controller4. The chip offset is defined in TS25. 402 (Release '99).
• Also, at thetime014, themobile station1 has acquired the synchronization of the downlink of thetarget base station3 while thetarget base station3 has not acquired the synchronization of the uplink.
• Thebase station controller4, which knows the frame offset, transmits instep15 the transport channel frame number (TGCFN) of starting the compressed mode in the uplink of themobile station1 after thetime014, information for a starting slot number of the TG (TGSN), information for the compressed mode pattern (TGL1, TGL2, TGD, TGPL1, TGPL2) and information for a total length of the compressed-mode TGPRC to themobile station1 before TGCFN.
• FIG. 8 shows parameters for the compressed mode pattern defined in the 3GPP TS25.215. Themobile station1 transmits the uplink preamble at a time decided by the compressed mode pattern received from thebase station controller4. Themobile station1 uses an open loop power control when transmitting the first preamble. That is, themobile station1 determines a transmission power of the first preamble by using the intensity of received signals measured at the downlink f₂of thetarget base station3 before thetime014 or a signal to noise rate (Ec/Io) of received signals at the common pilot channel.
• Also, the controller ofbase station4, which knows the frame offset, transmits a transport channel frame number (TGCFN) of starting uplink compressed mode of themobile station1 after thetime014, information for a starting slot number of each TG (TGSN), information of the compressed mode pattern (TGL1, TGL2, TGD, TGPL1, TGPL2), information of total length of the compressed mode (TGPRC), a frame offset, information for a chip offset and a scrambling code number (SCID) to thetarget base station3 before theTGCFN16 instep16.
• After then, thetarget base station3 acquires the synchronization of the preamble transmitted from themobile station1 by using the frame offset, the chip offset information, the SCID and the compressed mode pattern received from thebase station controller4.
• FIG. 9 provides a timing chart illustrating that thetarget base station3 decides a searching region for the preamble transmitted from themobile station1 by using the information from thebase station controller4.
• Referring toFIG. 9, it is assumed that themobile station1 has the dual-mode receiver.
• At first, thetarget base station3 calculates the SFN corresponding to the TGCFN by using the frame offset and a following equation EQ. 1.
  SFNmod 256=(frame offset+TGCFN)mod 256  EQ. 1
• Since in the equation EQ. 1, the range of the SFN is 4096 and that of the TGCFN is 256, there are 16 SFNs satisfying the equation EQ. 1. For example, if the frame offset is 67 as shown inFIG. 9, the SFNs become 123, 379, 635, . . . .
• A base station searcher selects an SFN whose value is closest to a frame corresponding to a time of receiving a preamble searching order from thebase station controller4 among the 16 possible SFNs as the SFN corresponding to the TGCFN. Therefore, inFIG. 9, “123” is selected as the SFN corresponding to the TGCFN since the “123” is closest to a time “120”, which is a time of receiving the preamble searching order from thebase station controller4. Thetarget base station3 searches the uplink preamble by setting up a searching region from atime17, which is β chips away from boundary of a corresponding frame (e.g., 123 frame inFIG. 9), to a time which is “preamble length+2τ_max”. Herein, β is defined as follows:
  β=chip offset+TGSN×2560+T₀+T_grd  EQ. 2
• In the equation EQ. 2, T₀is a difference between a downlink time (DL DPCH_norm) and an uplink time of themobile station1 and it is defined as 1024 chips in the 3GPP standard. As before mentioned T_grdis a Guard Time before themobile station1 transmits the preamble and the T_grdshould be known to thetarget base station3. β may be larger or smaller than 38400 chips.
• InFIG. 9, 2τ_maxis the maximum round trip delay corresponding to a cell coverage of thetarget base station3 and equal to a search window size of thetarget base station3. If it is assumed that the chip transmission speed is 3.84 Mcps and the cell coverage is 20 km, 2τ_maxbecomes almost 512 chips.
• In case the preamble searching is failed, thetarget base station3 searches the uplink preamble by setting up the search region from atime18, which is β chips away from a boundary of a next frame (e.g., 129 frame inFIG. 9) designated by a compressed mode pattern received from the controller ofbase station4, to a time, which is “preamble length+2τ_max”. Thetarget base station3 repeatedly performs the above-mentioned procedure until the preamble is detected.
• Thetarget base station3 acquires the uplink synchronization by using the configuration ofFIG. 9 and reports it to thebase station controller4 instep19. At this moment, thetarget base station3 also transmits the receiving intensity of the preamble (e.g., E_c/I_ovalue) transmitted from themobile station1. Then, instep20, thebase station controller4 instructs thesource base station2, themobile station1 and thetarget base station3 to perform the handover. At this time, thebase station controller4 transmits a CFN at a time at which the handover starts to thesource base station2, themobile station1 and thetarget base station3 and further transmits OVSF_targetto be used as a channel spread code of a new downlink in the handover to themobile station1.
• Themobile station1, which received the handover instruction from thebase station controller4, stops transmitting the preamble using the compressed mode. Also, themobile station1 disconnects the call with thesource base station2 at the CFN received from thebase station controller4 and starts to communicate with thetarget base station3 by using a new frequency f₂(f₂′) instep21. Before starting to the communicate between thetarget base station3 and themobile station1 at the corresponding CFN, the uplink and the downlink have been already synchronized by the method of the present invention and, therefore, a call disconnection there between does not occur.
• FIG. 10 is a diagram illustrating a signaling procedure between themobile station1, thesource base station2, thetarget base station3 and abase station controller4 during performing the hard handover in case that the frame offset is known to thebase station controller4 and thetarget base station3 transmits the AI for the synchronization acquisition for the uplink to themobile station1.
• InFIG. 10, it is assumed that thebase station controller4 knows the frame offset and thetarget base station3 transmits the AI through the downlink. InFIG. 10, at atime014, themobile station1 and thesource base station2 are communicating with each other through f₁(f₁′), thetarget base station3 has resources at the f₂(f₂′) link to support the currentmobile station1; thebase station controller4 knows the frame offset and chip offset and has received OVSF_targetfrom thetarget base station3; themobile station1 has acquired already the synchronization of the downlink of thetarget base station3; and thetarget base station3 does not acquire the synchronization of the uplink yet.
• The signaling procedure ofFIG. 10 is similar to that ofFIG. 7 but thebase station controller4 transmits not only parameters related to a compressed mode pattern but also OVSF_targetcode information to be used for the downlink in the handover, when thebase station controller4 instructs themobile station1 to transmit an uplink preamble instep22. Also, after acquiring the uplink synchronization, instep23, thetarget base station3 transmits the AI for the synchronization through the downlink by using a configuration shown inFIG. 11.
• When transmitting the AI, thetarget base station3 uses an OVSF_targetcode as a channel spread code, wherein the OVSF_targetcode is identical to the OVSF_targetcode sent when instructing themobile station1 to transmit the uplink preamble in thestep22, and themobile station1 uses the OVSF_targetwhen demodulating the AI.
• In a method shown inFIG. 10, in which thetarget base station3 transmits the AI in case thebase station controller4 knows the frame offset, the procedure of searching the uplink preamble performed by thetarget base station3 is identical to a method shown inFIG. 7 in which thetarget base station3 down not transmit the AI.
• FIG. 11 shows an example that thetarget base station3 succeeds in detecting a preamble at a second time after failing to detect preamble at a first time, wherein the preamble is transmitted from the mobile station, having a single-mode receiver.
• Thetarget base station3 transmits the AI after succeeding in detecting the uplink preamble, and the AI is transmitted at asecond point24, which is γ chips away from a first point, β chips away from a boundary of a frame designated by the compressed mode pattern, wherein the designated frame is a 129 frame inFIG. 11. In here, β is defined in the equation EQ. 2 and γ should satisfy a following equation EQ. 3. The γ has to be known to thetarget base station3 and themobile station1 and, therefore, this should be newly defined in the 3GPP standard.
  γ>2τ_max+T_pre  EQ. 3
• Themobile station1 demodulates the AI at times, at which the mobile station is expected to receive the AI, such as25 and26 inFIG. 11, wherein themobile station1 has known β and γ and acquired aframe boundary27 of signals received from thetarget base station3. In case the mobile station has the single-mode receiver, as like inFIG. 11, the compressed mode of downlink is used. Themobile station1, which received the AI does not transmit the preamble any more. If it fails to detect the AI, themobile station1 re-transmits the preamble in a next compressed mode region. After transmitting the preamble, if the preamble is detected again in a next compressed mode, thetarget base station3 re-transmits the AI.
• Thetarget base station3 detecting the preamble reports the detection of the preamble to thebase station controller4 and then thebase station controller4 instructs thesource base station2, themobile station1 and thetarget base station3 to perform the handover. At this moment, thebase station controller4 transmits the CFN at a point of the handover started to thesource base station2, themobile station1 and thetarget base station3. Themobile station1, which received the handover instruction from thebase station controller4, disconnects communication with thesource base station2 at the CFN and starts to communicate with thetarget base station3 by using the new frequency f₂(f₂′). Before starting to communicate with thetarget base station3 and themobile station1 at corresponding CFN, synchronization with the uplink and downlink is acquired by using the method of the present invention and, therefore, the call disconnection there between does not occur.
• In the W-CDMA FDD standard (Release '99) of the 3GPP, if thebase station controller4 does not know the frame offset, which is a difference between the SFN of thetarget base station3 and the CFN of themobile station1, when performing the handover, themobile station1 disconnects the established frequency completely and re-acquires the SFN of thetarget base station3 through a new frequency for performing the hard handover. Therefore, at least 50 msec of additional call disconnection may occur between themobile station1 and thetarget base station3.
• For the case that thebase station controller4 does not know the frame offset, the handover method of the present invention forces the base station controller to know the frame offset before performing the handover by using the AI transmitted from thetarget base station3 to themobile station1. This will be explained in detail with reference toFIG. 12.
• FIG. 12 shows a signaling procedure between themobile station1, thesource base station2, thetarget base station3 and thebase station controller4 during performing the hard handover in case that thebase station controller4 does not know the frame offset.
• InFIG. 12, it is assumed that thebase station controller4 does not know the frame offset. Attime028 inFIG. 12, themobile station1 and thesource base station2 are communicating with each other through the frequency f1 (f1′); thetarget base station3 has resources at the f2 (f2′) link to support themobile station1; thebase station controller4 knows the chip offset but does not know the frame offset, and has received OVSF_targetfrom thetarget base station3; themobile station1 has already acquired the synchronization of the downlink of thetarget base station3; and thetarget base station3 does not acquire the synchronization of the uplink yet.
• That is, attime028, themobile station1 and thesource base station2 are communicating with each other through the frequencies f₁(downlink) and f₁′ (uplink); thetarget base station3 has reported to thebase station controller4 that there are resources at the f2 (f2′) link to support themobile station1; and an orthogonal variable spreading factor (OVSF) code of the downlink has been already reported to thebase station controller4, wherein the OVSF code will be used in the downlink by themobile station1 after the handover performed.
• As mentioned above, thebase station controller4 knows the chip offset but does not know the frame offset attime028.
• The signaling procedure shown inFIG. 12 is similar to that inFIG. 10. However, thebase station controller4 does not transmit the frame offset information to thetarget base station3 when instructing thetarget base station3 to search an uplink preamble instep30. It is because thebase station controller4 does not know the frame offset information.
• Thetarget base station3, which received the preamble searching instruction in thestep30, performs the preamble searching process for a corresponding region of the frame from the moment of receiving the preamble searching order. This is illustrated inFIG. 13.
• Since thetarget base station3 does not know an SFN of a frame through which themobile station1 transmits the preamble instep31 but knows information for a slot at which the preamble starts, thetarget base station3 sets up searching regions (38,39,40, . . . ) from a boundary of each frame to a point, which is 2τ_maxaway from an offset ofβ_mod38400 and searches the preamble. In here, “_mod38400” is used because, as mentioned, β may be larger than 38400.
• Thetarget base station3, which succeeded in searching the preamble, transmits the AI instep32 to themobile station1 through the downlink, and, at the same time, reports the acquisition of the uplink synchronization to thebase station controller4 instep33. At this time, the SFN corresponding to the AI is also transmitted. After then, themobile station1, which received the AI from thetarget base station3, transmits a CFN of the frame corresponding to the received AI to thebase station controller4 instep34. At this time, thebase station controller4 calculates the frame offset instep35 by using the SFN received from thetarget base station3 and the CFN information received from themobile station1 for the AI transceiving time.
• FIGS. 14 and 15 show a procedure of calculating the frame offset performed in thebase station controller4 by using the SFN information received from thetarget base station3 and the CFN information received from themobile station1.
• After calculating (β+τ)_mod38400, if the calculated value is smaller than the chip offset, thebase station controller4 determines the value of the frame offset as (SFN−CFN−1)mod 256 and, if otherwise, decides the value of the frame offset as (SFN−CFN)_mod256. This is shown in an equation EQ. 4.
  frame offset=(SFN−CFN−1)_mod256 for (β+τ)_mod38400<chip offset
  frame offset=(SFN−CFN)_mod256 for (β+τ)_mod38400·chip offset EQ. 4
• When transmitting the AI, thetarget base station3 uses the OVSF target code, which is identical to the OVSF target used when sending the uplink preamble transmitting instruction in thestep22 to themobile station1, as a channel spread code and themobile station1 uses the same OVSF_targetwhen demodulating the AI.
• After calculating the frame offset, thebase station controller4 instructs thesource base station2, themobile station1 and thetarget base station3 to perform the handover insteps36 and37. At this time, thebase station controller4 provides themobile station1 with the CFN in which the handover is performed and, in turn, themobile station1 disconnects the current communication link with thesource base station2 at the CFN and starts to communicate with thetarget base station3 through a new frequency f₂(f₂′).
• Thebase station controller4 transmits the calculated frame offset and the CFN at which the handover is performed to thetarget base station3 instep37. At this time, thetarget base station3 calculates an SFN corresponding to the CFN at which the handover is performed by using the frame offset received from thebase station controller4 and receives an uplink DPCH at the same time transmitting, a downlink DPCH from the moment corresponding to the calculated SFN instep38.
• As mentioned above, although thebase station controller4 does not know the frame offset, the handover method of the present invention forces thebase station controller4 to know the frame offset before performing the handover by using the AI transmitted from thetarget base station3 to themobile station1 and transmits the frame offset thetarget base station3 together with the handover instruction, so that, it is possible to perform the seamless inter-frequency hard handover.
• FIG. 16 shows a signaling procedure of a method using two classes of preambles and two classes of AIs in each of themobile station1 and the base station to minimize false handover instruction given by thebase station controller4 by minimizing the false detection probability of themobile station1 and thetarget base station3 in case the base station controller does not know the frame offset.
• Themobile station1, which received a preamble transmitting instruction from thebase station controller4 transmits the preamble₁through the frequency f₂′ by using the uplink compressed mode instep41. At this time, thetarget base station3, which received the preamble searching instruction from thebase station controller4 searches the preamble₁according to the above-mentioned procedure described inFIG. 13. After succeeding in searching the preamble, thetarget base station3 transmits an AI₁, which is an acknowledgement for succeeding in searching the preamble₁, the frequency f₂instep42. And then, themobile station1 detects the AI₁, which is transmitted to the frequency f₂₁by using the downlink compressed mode or the dual-mode receiver and, in turn, transmits a preamble₂to a frequency f₂′ at a next compressed frame instep43.
• After transmitting the preamble₂thetarget base station3 detects the preamble₂transmits an AI₂instep44, which is an acknowledgement for achieving to detect the preamble₂, to themobile station1, and reports the acquisition of the uplink synchronization to thebase station controller4 at the same time of notifying an SFN through which the AI₂is transmitted to thebase station controller4 instep45.
• Themobile station1 detects the AI₂received through the frequency f₂by using the downlink compressed mode or the dual-mode receiver and reports the detection of AI₂to thebase station controller4 at the same time of notifying, CFN through which the AI₂is also received to thebase station controller4 instep46. And then, thebase station controller4 calculates the frame offset by using the equation EQ. 4 with the SFN transmitted from thetarget base station3 and the CFN transmitted from themobile station1 and invokes thesource base station2, thetarget base station3 and themobile station1 to perform the handover in thesteps36 and37. At this moment, the calculated frame offset is transmitted to thetarget base station3. Steps after transmitting the calculated frame offset are same as the steps inFIG. 12.
• FIG. 17 shows an example of an AI transmission of a base station and a preamble transmission of themobile station1 when the method inFIG. 16 is used.
• Themobile station1 uses an open loop power P0 to transmit a preamble₁at the first time and in case of failing to receive the AI₁, increases the power up to P1 to re-transmit the preamble₁. In case of failing to receive the AI₁, the above mentioned steps are performed repeatedly and in case of achieving to receive the AI₁, a preamble₂is transmitted by using a power (P2) identical to the power by which the preamble, is thereby transmitted.
• When using the method described inFIG. 16, in case that thebase station controller4 does not know the frame offset, a transmission cycle of the preamble of themobile station1 has to be constant. “T” inFIG. 17 represents the transmission cycle of the preamble. That is, thebase station controller4, which does not know the frame offset, should transmit a compressed mode pattern satisfying by the above requirement to themobile station1 and thetarget base station3.
• FIG. 18 is a flowchart explaining operations of the mobile station and the base station, which use the method ofFIG. 16.
• At first, themobile station1, which received a preamble transmission instruction from thebase station controller4 instep29, transmits the preamble₁by using the uplink compressed mode instep41 and then, detects the AI₁instep48. In thestep48, if the AI₁is not detected, themobile station1 re-transmits the preamble₁in thestep41, after increasing the power as much as A instep47. On the other hand, if the AI₁is detected, at a next compressed mode, themobile station1 transmits the preamble₂by using a power identical to the power by which the preamble₁is finally transmitted instep43.
• After transmitting the preamble₂if the detection of the AI₂is achieved instep49, themobile station1 transmits a CFN at which the AI₂is received to thebase station controller4 in step of46 and, if the detection of the AI₂is failed in the step of49, themobile station1 notices the failure of detecting the AI₂to thebase station controller4 instep50.
• Meanwhile, thetarget base station3, which received a preamble search instruction from thebase station controller4 instep30, searches the preamble₁in every search region instep51 by using the procedure ofFIG. 13. Thetarget base station3 transmits the AI₁instep42 for the search region, at which the preamble₁is detected instep54, to themobile station1. Therefore, the AI₁may be transmitted more than one time during the T inFIG. 17. Instep52, thetarget base station3 detects the preamble₂from a next expected point corresponding to the search region at which the AI₁is transmitted instep52. At thestep52, if the preamble₂is detected, the AI₂is transmitted instep44 to themobile station1 and the searching process is terminated. And a SFN, at which the AI₁is transmitted, is transmitted to the base station controller. If the preamble₁or the preamble₂are not detected, the above-mentioned procedures are performed repeatedly.
• The base station controller calculates a frame offset value when it receives both of the CFN and SFN from themobile station1 and the base station and produces a handover instruction. In case that the base station controller receives a message of the failure to detect the AI₂from themobile station1, does not receive the SFN for a certain duration after receiving the CFN, does not receive the CFN a predetermined duration after receiving the SFN, or receives no information for a time of TGPRC from thetarget base station3 and themobile station1, the above-mentioned all steps are preformed again.
• The above-mentioned steps of the method of the present invention can be implemented as a program and can be stored in a computer readable recording medium such as CD-ROM, RAM, ROM, floppy disk, hard disk and magneto-optical disk.
• The present invention, as mentioned above, transmits a preamble (or a pilot), which is direct sequence spread to a new frequency for a short time by using an uplink compressed mode or a similar method to the uplink compressed mode before a mobile station completely disconnects a currently established communication in an inter-frequency hard handover situation described inFIG. 1. As a result, the present invention makes the seamless inter-frequency hard handover possible by allowing the target base station to acquire the uplink synchronization before the mobile station completely disconnects the currently established communication. Furthermore, in case that thebase station controller4 does not know the frame offset, which is a difference between the SFN of the target base station and the CFN of the mobile station, the present invention can prevent a call disconnection by forcing the network to know the frame offset by using an AI, which is transmitted through a downlink by using a new frequency from the target base station, just before performing the hard handover.
• While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims (27)

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 inclaim 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 inclaim 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 inclaim 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 inclaim 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 inclaim 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 inclaim 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 inclaim 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 inclaim 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 inclaim 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 inclaim 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 inclaim 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 inclaim 11, wherein the mobile station uses an uplink compressed mode pattern for the inter-frequency hard handover.

15. The method as recited inclaim 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 inclaim 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 inclaim 16, wherein the AI₁and AI₂are distinguished with each other according to a binary orthogonal transform method using an orthogonal code.

18. The method as recited inclaim 16, wherein the preamble₁and the preamble₂use 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 inclaim 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 inclaim 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 inclaim 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 AI₂from 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 inclaim 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 inclaim 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 inclaim 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 inclaim 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 inclaim 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.

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