Device and method for performing handoff from async mobile communication system to sync mobile communication system

Abstract: Disclosed is a method for performing a handoff from an async base station to a sync base station when a mobile station with async and sync demodulation modules travels from a present cell of the async base station to a cell of the sync base station, the method including the steps of: the mobile station driving the sync demodulation module for a given idle sleep time to acquire and maintain the timing of the sync base station, during an operation in the cell of the async base station; during the handoff, the mobile station measuring the strengths of pilot signals from the adjacent sync base stations based on the acquired timing, and sending the measurement results to the async base station; the mobile station receiving from the async base station information necessary to establishment of a traffic channel with the sync channel; and the mobile station performing handoff to the sync base station according to the information necessary to establishment of the traffic channel.

Description: The present invention relates generally to a device and method for performing handoff in a mobile communication system, and more particularly, to a novel device and method for implementing handoff when a mobile station travels from a cell of the async mobile communication system to a cell of the sync mobile communication system. The async mobile communication system may be, for example, UMTS adopted as the European standards and the sync mobile communication system may be IMT-2000 adopted as the American standards. These two systems are becoming increasingly harmonized and there is thus a need for various technologies that are compatible with both systems. One of such technologies is related to handoff that may happen between the sync mobile communication system and the async mobile communication system. The handoff is a technology that enables users to continuously receive a call service without interruption when a mobile station travels from a present cell to an adjacent cell during the call service in a mobile communication system. The handoff is classified into soft hand off and hard hand off. In the soft hand off, the mobile station maintains a call using both a channel assigned by a target base station and a channel assigned by the present base station in service. Eventually, the mobile station abandons one of the two channels, whose quality has a value lower than a threshold. In the hard hand off, a channel assigned by the present base station in service is first released and then, connection to an adjacent base station is attempted. Until now, development of the handoff has been concentrated on the sync mobile communication system. But, with the emergence of the async mobile communication system, research has been undertaken regarding handoff between sync and async mobile communication systems. The mobile station usually acquires information about adjacent cells and informs the base station of the information when a pilot signal received from one of the adjacent cells has strength higher than a threshold or a handoff direction message is received from the base station. The information sent to the base station is used as information about the handoff performed when the mobile station travels from a present cell to the adjacent cell during a call service. The base station transmits the information about the handoff through a paging channel (for the sync communication system) or a broadcasting channel (for the async communication system). A hard handoff takes place normally when the mobile station travels from a cell of the async mobile communication system to a cell of the sync mobile communication system. For the hard handoff, the mobile station interrupts a call service from the async mobile communication system while it acquires information about the adjacent cells. Conventionally, the mobile station has to perform the following procedure in order to interpret information about the sync mobile communication system. First, the mobile station interprets a sync signal message stored in the sync signal frame transferred from the sync channel of the sync mobile communication system. The transmission bit per 80 ms frame of the sync signal frame is 96 bits, and the sync signal message including information the mobile station can communicate with the sync mobile communication system, has a length of 221 bits. Thus the mobile station needs at least 240 ms (80 ms x 3) for interpreting the message. The above-mentioned specifications are included in the TIA/EIA-IS-2000.5 standards that define the sync mobile communication system. Hereinafter, a base station of the sync mobile communication system will be referred to as "sync base station" and a base station of the async mobile communication system will be called "async base station". FIG. 2 illustrates a procedure for the mobile station in communication with a present async base station to acquire information about the adjacent sync base stations. Referring to FIG. 2, the mobile station receives from an async base station a direction message to detect information about sync base stations adjacent to the async base station, in step 201. Then, the mobile station sets to detect information about the adjacent sync base stations in step 203, and detects pilot signals from the adjacent sync base stations in step 205. The mobile station determines in step 207 whether a pilot signal having a highest peak value is detected. Upon failure to detect such a pilot signal of highest peak value, the mobile station returns to step 205. If a highest peak value of a pilot signal is detected, the mobile station proceeds to step 209 to receive sync frames through the forward sync channel of the sync base station from which the pilot signal having the highest peak value has been detected. In this case, the mobile station has to receive at least three sync frames from the sync base station in order to receive all sync signal messages. For example, the mobile station takes at least 240 ms in receiving the sync frames from the sync base station having a channel structure shown in FIG. 2 and, during the frame reception time, interrupts the communication with the async base station. Taking a long time in performing the procedure of FIG. 2 may therefore result in a detrimental effect such as a loss of data communicated between the async base station and the mobile station. Moreover, the mobile station does not necessarily receive messages at the time when it begins to receive the sync frames. If including the time of waiting for the time in the unit of 240 ms, the mobile station has to interrupt communication with the async base station for about 513.3 ms on the assumption that there is no error in the received frames. Therefore, a loss of data communicated between the sync base station and the mobile station is inevitable if the mobile station takes a long time in performing the procedure of FIG. 2. This does not meet the compressed mode defined in the async system in order to monitor other frequency bands suggested in the async standards. FIG. 1 illustrates an exemplary construction of the respective channels communicated between in a mobile station and a base station, and a channel communication device for the respective channels in a code division multiple access (CDMA) communication system, which is one of the sync mobile communication systems according to the prior art. The respective channels shown in FIG. 1 are illustrated focusing on a transmitter. To describe the channel construction of a base station, a controller 101 enables/disables an operation of the individual channel generators, processes a message communicated between a physical layer and the base station, and communicates messages with the upper layer. Pilot channel generator 103, sync channel generator 104, and paging channel generator 107 are devices for generating common channel information shared among the users in a single cell or a plurality of cells. Dedicated control channel (DCCH) generator 102, fundamental channel (FCH) generator 108 and supplemental channel (SCH) generator 109 are devices for generating dedicated channel information assigned differently to the users. The dedicated control channel generator 102 processes various control messages received on a forward dedicated control channel DCCH and sends them to a mobile station. The messages on the forward dedicated control channel include radio link protocol (RLP) frames or various control messages used in the IS-95B, and medium access control messages (MAC) related to a packet data service control, i.e., assigning or releasing supplemental channels. Power control signals can be transmitted on the dedicated control channel instead of the fundamental channel, in which case the power control signals are included in the control messages. On the forward dedicated control channel, the dedicated control channel generator 102 negotiates with the base station in regard to a data rate to be used for a supplemental channel or, if orthogonal codes are used for the supplemental channel, gives a direction to change the orthogonal codes. The forward dedicated control channel is spread with one unused orthogonal codes among those not assigned to the pilot channel generator 103, sync channel generator 104, or paging channel generator 107. The RLP frame provides a service for successful transmission of an octet stream. The RLP may be classified into transparent RLP and non-transparent RLP. The transparent RLP does not retransmit an erroneously transmitted frame but informs the upper layer of the time and position of the erroneously transmitted frame. The non-transparent RLP involves error correction. The pilot channel generator 103 processes information received on a forward pilot channel and sends the received information to the mobile station. The forward pilot channel always transmits logic signals of all 0's or 1's. It is assumed herein that the pilot channel transmits logic signals of all 0's. The pilot channel signal enables the mobile station to rapidly acquire initial synchronization for new multiple paths and estimate channels. The pilot channel is spread with one specific orthogonal code previously assigned thereto. The sync channel generator 104 processes information received on a forward sync channel and sends the received information to the mobile station. Information on the sync channel enables every mobile station in a cell to acquire initial time and frame synchronizations. The forward sync channel is spread with one specific Walsh code previously assigned thereto. The paging channel generator 107 processes information received on a forward paging channel and sends the received information to the mobile station. Information on the paging channel is all information necessary prior to establishment of traffic channels. The forward paging channel is spread with one of orthogonal codes previously assigned thereto. The fundamental channel generator 108 processes information received on a forward fundamental channel and sends the received information to the mobile station. Information on the forward fundamental channel may include a variety of control messages (L3 signaling) used in the IS-95B and power control signals, other than the voice signal. If necessary, such information may include RLP frames and MAC messages. The fundamental channel has a data rate of 9.6 kbps or 14.4 kbps and, according to circumstances, has a variable data rate such as 4.8 kbps or 7.2 kbps as 1/2 of the given data rate; 2.4 kbps or 3.6 kbps as 1/4 of the data rate; or 1.2 kbps or 1.8 kbps as 1/8 of the data rate. Such a variable data rate must be detected by the receiving unit. The forward fundamental channel is spread with orthogonal codes not assigned to the pilot channel generator 103, sync channel generator 104, or paging channel generator 107. The supplemental channel generator 109 processes information received on a forward supplemental channel and sends the received information to the mobile station. Information on the forward supplemental channel includes RLP frames, packet data and the like. The supplemental channel generator 109 has a data rate of more than 9.6 kbps. The supplemental channel generator 109 has a scheduled data rate, i.e., the base station communicates with the mobile station at a data rate determined under negotiation with the mobile station through the dedicated control channel. The forward supplemental channel is spread with orthogonal codes not assigned to the pilot channel generator 103, sync channel generator 104, or paging channel generator 107. The fundamental channel and the supplemental channel become traffic channels. An adder 110 adds in-phase channel transmission signals on the forward link from dedicated control channel generator 102, fundamental channel generator 108 and supplemental channel generator 109 to transmission signals from pilot channel generator 103, sync channel generator 104 and paging channel generator 107. An adder 111 adds together quadrature-phase channel transmission signals output from dedicated control channel generator 102, fundamental channel generator 108 and supplemental channel generator 109. A spreading modulator 112 multiplies the transmission signals from the adders 110 and 111 by a spreading sequence and ascent frequency converts the transmission signals. A receiver 113 frequency converts the respective channel signals of the mobile station on the reverse link with a base band and then despreads the signals through multiplication of the converted signals by a spreading sequence. The constructions of the reverse link channel receivers provided in the base station are omitted in FIG. 1. Now to describe the channel construction of the mobile station, a controller 114 enables/disables the operation of the individual channel generators, processes a message communicated by the mobile station, and communicates messages with the upper layer. A dedicated control channel generator 115 processes various control messages received on a reverse dedicated control channel and sends them to a base station. The messages on the reverse dedicated control channel include radio link protocol (RLP) frames or various control messages used in the IS-96B, and medium access control messages (MAC) related to a packet data service control, i.e., assigning or releasing supplemental channels. For a reverse link, power control signals are not separately transmitted on the dedicated control channel because they are inserted in a pilot channel for transmission. On the reverse dedicated control channel, the dedicated control channel generator 115 negotiates with the base station in regard to a data rate to be used for a supplemental channel. The reverse dedicated control channel generator 115 spreads the individual channels with unique orthogonal codes previously assigned thereto to discriminate the channels and spreads the signals from the users with unique PN codes to discriminate the users. Thus different orthogonal codes are assigned to a dedicated control channel, a pilot channel, an access channel, a fundamental channel and a supplemental channel in order to discriminate the respective channels and the respective orthogonal codes used for every channel are shared among the users. For example, an orthogonal code used for the dedicated control channel is shared among all users to discriminate the dedicated control channel. The reverse dedicated control channel has a fixed data rate of 9.6 kbps, which prevents any performance deterioration in determining the data rate and eliminates a data rate determination circuit, reducing complexity of the receiver. Also, the reverse dedicated control channel has the same data rate as the basic data rate of voice signals, i.e., 9.6 kbps, thus maintaining the same service diameter as the basic voice service. A pilot channel generator 116 processes information received on a reverse pilot channel and sends the received information to the base station. Like the forward pilot channel, the reverse pilot channel enables rapid acquisition of initial synchronization for new multiple paths and channel estimation. The reverse pilot channel also transmits reverse power control information by adding power control signals to the pilot signal at a defined time. An access channel generator 117 processes information received on a reverse access channel and sends the received information to the base station. The information on the access channel includes control messages and all information about the mobile station required to the base station prior to establishment of a traffic channel. A fundamental channel generator 118 processes information received on a reverse fundamental channel and sends the received information to the base station. Information on the reverse fundamental channel normally includes voice signals. Such information may include a variety of control messages (L3 signaling) used in the IS-95B as well as voice signals. If necessary, the information may include RLP frames and MAC messages. For a reverse link, power control signals are not separately transmitted on the access channel because they are inserted in the pilot channel for transmission. The fundamental channel has a fixed data rate of 9.6 kbps or 14.4 kbps and, according to circumstances, has a variable data rate such as 4.8 kbps or 7.2 kbps as 1/2 of the given data rate; 2.4 kbps or 3.6 kbps as 1/4 of the data rate; or 1.2 kbps or 1.8 kbps as 1/8 of the data rate. Such a variable data rate must be detected by the receiving unit. The reverse fundamental channel generator 117 spreads the individual channels with unique orthogonal codes previously assigned thereto to discriminate the channels and spreads the signals from the users with unique PN codes to discriminate the users. Thus different orthogonal codes are assigned to a pilot channel, an access channel, a fundamental channel and a supplemental channel in order to discriminate the respective channels and the respective orthogonal codes used for every channel are shared among the users. For example, an orthogonal code used for the fundamental channel is shared among all users to discriminate the fundamental channel. A supplemental channel generator 119 processes information received on a reverse supplemental channel and sends the received information to the base station. Information on the reverse supplemental channel includes RLP frames, packet data and the like. The supplemental channel generator 118 has a data rate of more than 9.6 kbps. The supplemental channel generator 118 has a scheduled data rate, i.e., the base station communicates with the mobile station at a data rate predetermined under negotiation with the mobile station through the dedicated control channel. The reverse supplemental channel spreads the individual channels with unique orthogonal codes previously assigned thereto to discriminate the channels and spreads the signals from the users with unique PN codes to discriminate the users. The fundamental channel and the supplemental channel will become traffic channels. An adder 120 adds together transmission signals on the reverse link received from the dedicated control channel generator 115 and the pilot channel generator 116. An adder 121 adds together transmission signals on the reverse link received from access channel generator 117, fundamental channel generator 118 and supplemental channel generator 119. A spreading modulator 112 multiplies the transmission signals from the adders 120 and 121 by a spreading sequence and ascent frequency converts the transmission signals. A receiver 123 frequency converts the respective channel signals of the mobile station on the reverse link with a base band and then despreads the signals through multiplication of the converted signals by a spreading sequence. The constructions of the reverse link channel receivers provided in the mobile station are omitted in FIG. 1. In the CDMA communication system, as shown in FIG. 1, the base station comprises controller 101, dedicated control channel generator 102, pilot channel generator 103, sync channel generator 104, paging channel generator 107, fundamental channel generator 108 and supplemental channel generator 109. And, the mobile station comprises controller 114, dedicated control channel generator 115, pilot channel generator 116, access channel generator 117, fundamental channel generator 118 and supplemental channel generator 119. For the output form of the individual channel generators in the base station, the signals from dedicated control channel generator 102, fundamental channel generator 108 and supplemental channel generator 109 are two channel signals, i.e., in-phase channel component and quadrature-phase channel component, while only one channel signal is generated from pilot channel generator 103, sync channel generator 104 and paging channel generator 107. It is assumed herein that the only one channel component is the in-phase channel component. Unlike the channel generators of the base station, those of the mobile station generate only one channel component. Thus the outputs of the dedicated control channel generator 115 and the pilot channel generator 116 of the mobile station are added up and fed into the spreading modulator 122 as an in-phase channel, and the outputs of the remaining channel generators 117, 118 and 119 are added up and fed into the spreading modulator 122 as a quadrature-phase channel. When using the access channel, the output of the pilot channel generator 116 is an in-phase channel input and the output of the access channel generator 117 is a quadrature-phase channel input, since the access channel generator 117 generates the output prior to generation of the traffic channel. FIG. 3 illustrates a handoff procedure according to prior art when the mobile station travels from a cell of the async base station to a cell of the sync base station shown in FIG. 1. Referring to FIG. 3, in step 301, mobile station B receives from async base station A a message including information about another base stations adjacent to the async base station A through a broadcast channel or a paging channel. In step 302, the mobile station measures the reception strengths of pilot signals transferred from the adjacent base stations and sends a message including the measurement results of the pilot signals to the async base station A through a reverse dedicated channel. Then, the async base station A analyzes the message on the reverse dedicated channel to determine whether there is a target async base station. If a target async base station exists, the async base station A confirms the handoff; otherwise, it sets parameters T, T 0 and N for detecting the reception strength of the pilot signals from the adjacent sync base stations, where T 0 is a time to detect the pilot signal of a sync base station, T is a time interval for detecting the pilot signal of the sync base station, and N is a parameter defining the number of times for detecting the pilot signal of the sync base station. In step 303, the mobile station B receives a direction message on a forward dedicated control channel to measure the reception strength of the pilot signals of the async and sync base stations adjacent to the async base station A, and a message including the parameters. Upon receiving the message on the forward dedicated control channel, the mobile station B measures the reception strengths of the pilot signals from the sync and async base stations adjacent to the async base station A based on the parameters T, T 0 and N. In step 306, the mobile station B detects a pilot signal received from the individual sync base stations adjacent to the async base station A. Here, the pilot signal enables the mobile station B to estimate the channels and rapidly acquire initial synchronization for new multiple paths. Besides detection of the pilot signal, the mobile station B analyzes in step 306 a sync message received from a sync base station such as sync base station C through a forward sync channel to recognize the sync base station C, and acquires system information about the sync base station C. The sync message includes system information necessary to communication with the sync base station C, such as system ID number, network ID number, PN_OFFSET value, long code information 320msdlgndml, and paging channel data rate. For example, the sync channel frame used in the IS-95 system is 80 ms in length with a data rate of 96 bits and comprises three sub frames having a length as long as one period of a short code. Here, the sync message including the system information about the sync base station C has a length of more than 200 bits including a message length field and CRC. Even when the message is less than 96 bits in length, the 80ms sync frame necessarily sends 96 bits by adding the surplus bits to the message. Thus the mobile station B must receive at least three 80ms sync frames in order to receive all sync messages including the system information. Without errors in the sync messages, it takes at least 240 ms for the mobile station B to recognize the sync base station C and receive information of the sync base station C. In step 304, the mobile station B sends a message, including the measurement results of the reception strength of the pilot signals received from the adjacent base stations and information about the sync message, to the async base station A through a reverse dedicated channel. Then, the async base station A analyzes the received message on the reverse dedicated channel and sends the measurement results to the upper network. The upper network checks the existence of the target sync base station C and sends to the async base station A a handoff direction message including information necessary to the handoff. In step 305, the mobile station B receives the handoff direction message including information about traffic channels for communication with the target sync base station C, through the forward dedicated channel from the async base station A. Once receiving the handoff direction message, the mobile station B prepares to receive traffic data from the sync base station C with reference to the traffic channel information included in the message. In step 308, the mobile station B receives null traffic or the like on a forward fundamental channel from the sync base station C to ensure stability of channels. The mobile station B receives in step 309 a traffic message on the forward fundamental channel from the sync base station C while moving to a cell of the target sync base station C, thereby switching a call service from the async base station A to the sync base station C. Thereafter, the mobile station B sends a preamble on a reverse fundamental channel to inform that transmission is successful, in step 310, and sends a handoff complete message on the reverse fundamental channel to the sync base station C, in step 311. With the above-described forward channel structure of the conventional sync mobile communication system, the mobile station B must receive at least three sync frames on the forward sync channel of the sync mobile communication system. For example, a sync mobile communication system having the channel structure shown in FIG. 2 has the minimum reception time of 240 ms. Thus it will take at least 240 ms for the mobile station B to acquire system information for communication with the sync base station C while traveling from a cell of the async base station A to a cell of the target sync base station C. During the reception time, the mobile station interrupts communication with the async base station A. That is, taking a long time in performing the procedure of FIG. 3 results in a detrimental effect such as a loss of data communicated between the async base station and the mobile station. EP 1 102 506 A1 discloses a method for performing handoff between an asynchronous base station and synchronous base station. To this end a mobile station should obtain information relating to the synchronous base station to be connected during an idle period. This information can be obtained from a forward synchronous base station, a pilot channel and a traffic channel transmitted from the synchronous base station. A mobile station communicating with an asynchronous base station searches information relating to a synchronous base station in a compressed mode in order to perform a handoff to the synchronous base station. A common channel is used for informing the mobile station of the information relating to the synchronous base station. The synchronous base station transmits its timing information, i.e. a zero offset, a long code state and a synchronous channel super frame timing of the synchronous base station through the common channel. A common channel between a synchronous base station and a mobile station is established. Monitoring information of neighbor base stations is transmitted to the asynchronous base station. It is determined whether there is an asynchronous base station satisfying a handoff condition. If there is an asynchronous base station satisfying this condition, the handoff to the neighbor asynchronous base station is performed. If not, the mobile station requests a handoff to the synchronous base station and after receiving a compressed mode message from the radio resource control, the mobile station operates in a compressed mode. The compressed mode message informs the mobile station on a length and period of an idle period. The mobile station should obtain timing information of the synchronous base station, the strongest cell selection using the pilot offsets according to the neighbor lists and the long code state used in the traffic channel during the idle period of the compressed mode. In order to obtain information necessary for the handoff within the idle period, an additional common channel is set and transmitted to all of asynchronous base stations neighbor to the synchronous base station. According to the zero offset, the timing information, the long code state and the super frame timing information of the synchronous channel received from the synchronous base station through the common channel, the handoff from the asynchronous base station to the synchronous base station can be performed. WO 99/20 074 A1 describes a method and apparatus for performing idle handoff in a multiple access communication system. WO 00/74 275 A1 describes an apparatus and method for implementing handoff in a mobile communication system with short sync channel. WO 94/29 981 A1 describes a technique for non-continuous transmission for seamless handover in DS-CDMA systems. It is, therefore, the object of the present invention to provide a device and method for acquiring and maintaining the timing information of a sync system prior to searching for the adjacent sync systems or receiving a handoff indication message by a mobile station that is in communication with an async system. This object is achieved by the subject matter of the independent claims. Preferred embodiments are the subject matter of the dependent claims. According to an aspect of the present invention, there is provided a device and method for the async system previously sending information about the adjacent sync base stations to the mobile station and the mobile station acquiring pilot offset, zero offset PN code period and PN long code of the sync base station prior to receiving a handover indication message. There is provided a method for performing a handoff from an async base station to a sync base station when a mobile station with async and sync demodulation modules travels from a present cell of the async base station to a cell of the sync base station, the method including the steps of: the mobile station driving the sync demodulation module for a given idle sleep time to acquire and maintain the timing of the sync base station, during an operation in the cell of the async base station according to a measurement request about the sync base stations of the async station , the mobile station measuring the strengths of pilot signals from the adjacent sync base stations based on the acquired timing, and sending the measurement results to the async base station; the mobile station receiving from the async base station traffic channel information necessary to establish a traffic channel with the sync base station; and the mobile station performing handoff to the sync base station according to the traffic channel information. The above object, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: FIG. 1 is a diagram illustrating the construction of a base station in a sync mobile communication system according to prior art; FIG. 2 is a diagram illustrating a procedure for measuring information about a sync mobile communication system by a mobile station in communication with an async mobile communication system according to prior art; FIG. 3 is a diagram showing a procedure for the mobile station performing a handoff from an async base station to a sync base station according to prior art; FIG. 4 is a diagram illustrating the construction of a dual mode mobile station that is able to operate in both sync and async modes according to the present invention; FIG. 5 is a diagram illustrating a procedure for acquiring synchronization with a sync mobile communication system by the mobile station in communication with an async mobile communication system according to the present invention; and FIG. 6 is a diagram illustrating a procedure for the mobile station performing a handoff from an async base station to a sync base station. Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. Like reference numbers denote the same components in the drawings. By way of example, the present invention is illustrated in terms of a length of frames transmitted on the respective channels, a coding rate, and the number of data and symbols output from the blocks of the respective channels. It will be understood by those skilled in the art that the present invention is readily implemented without these particulars and with various changes. The term "pseudo-noise (PN) short code" as used herein means a code with a chip period of 2 15 (26.2 ms). As the timing varies from one base station to another, the mobile station has to search for the short code timing of a target cell whenever it receives a handoff request message. Here, the short code timing (i.e., PN offset) is information indicating how much the base station deviates from the reference time (or offset zero time). The mobile station can measure the reception strengths of the pilot signals from the base station simply from the knowledge about the reference time and the PN offset. The term "PN long code" as used herein means a code with a chip period of 2 42 , which code is used to transmit forward traffic channels and reverse signals. The mobile station is in itself hard to acquire synchronization with this code. Thus the system has to provide the timing of the PN long code in the sync channel during the initialization step. FIG. 4 is a diagram illustrating the construction of a mobile station that is able to communicate with both async and sync systems according to the embodiment of the present invention. Referring to FIG. 4, switches 401 and 408 switch between DS and MC modules according to the operational mode of the mobile station. A first radio frequency (RF/IF) section 402 multiplies a received signal from the async base station via the switch 401 by a carrier frequency signal from a first phase locked loop (PLL) 410 to perform frequency down-conversion to a baseband signal. A first baseband processor 403 demodulates the baseband signal from the first RF/IF section 402 by despreading. The first baseband processor 403 also provides a reference clock to a local generator 409 via the switch 408. The first RF/IF section 402 and the first baseband processor 403 are integrally referred to as an async module (or direct spread (DS) module). A controller 404 controls the overall operation of the mobile station. Also, the controller 404 processes a message from the base station and generates a message to be sent to the base station. A second RF/IF section 405 multiplies a received signal from the sync base station via the switch 401 by a carrier frequency signal from a second PLL 411 to perform a frequency down-conversion to a baseband signal. A second baseband processor 406 demodulates the baseband signal from the second RF/IF section 406 by despreading. The second baseband processor 406 also provides a reference clock to the local generator 409 via the switch 408. The second baseband processor 406 includes a PN generator 407 and operates according to the timing of the sync system acquired for a given time period during communication with the async system in the present invention. The operation of the PN generator 407 continues even while the mobile station is in communication with the async system. The second RF/IF section 405 and the second baseband processor 406 are integrally referred to as a sync module (or multi carrier (MC) module). The local generator 409 supplies a local oscillation signal to the first PLL 410 and the second PLL 411 based on the reference clock from the first baseband processor 403 and the second baseband processor 406. As described above, the mobile station has first and second RF/IF sections 402 and 405 and first and second baseband processors 403 and 406, respectively, for async (DS) and sync (MC) systems, which share the controller 404, the generator 409 and the antenna. Thus, the mobile station can select either the DC module or the MC module by way of the switches 401 and 408 according to the operational mode. When the mobile station is in communication with the async system, the first switch 401 is connected to the DS module. If the mobile station intends to acquire the timing of the sync system during an operation in the async system, it temporarily turns the first switch 401 to the MC module and, after acquiring the timing of the sync base station at the PN code generator 407 in the second baseband processor 406, reconnects the first switch 401 to the DS module. After acquiring the timing of the sync base station, the mobile station turns off the second baseband processor 406 and the second PLL 411 but the second RF/IF section 405 and the PN generator 407 and communicates with the async system using the DS module. Only the PN generator 407 continuously operates at the chip rate of the sync system. For that reason, the mobile station can maintain the acquired timing of the sync system during communication with the async system and successively update the timing whenever it searches for a new timing of the sync system. Now, a description will be given regarding a procedure for acquiring the timing of a sync system by a mobile station in communication with an async system and then to a procedure for the mobile station performing a handoff from the async system to the sync system after acquiring the timing of the sync system. FIG. 5 is a diagram illustrating a procedure for acquiring the timing of a sync system by a mobile station during communication with an async system according to the present invention. Referring to FIG. 5, the mobile station receives information about the adjacent cells (i.e., adjacent base stations) from the async base station, in step S501. The information about the adjacent cells can be PN offsets and frequency bands in the case where the adjacent cells are sync base stations. In step S503, the mobile station analyzes the received information about the adjacent cells to determine whether an adjacent sync system exists. If there exists an adjacent sync system, the mobile station turns to step S507 to check whether it has already acquired the timing of the sync system. If acquired the timing of the sync system, the mobile station performs a normal operation in the async system; otherwise, it acquiring the timing of the sync system. If in communication, the mobile station requests the async base station for a compressed mode operation and then acquires the timing of the sync system using the compressed mode. The following description will be however given on the assumption that the mobile station is in the idle mode. In the idle mode, the mobile station determines in step S509 whether the current idle sleep time is longer than the required pilot signal acquisition time T1. The pilot signal acquisition time T1 is properly determined according to the cell searching ability of the mobile station. If the current idle sleep time is longer than the pilot signal acquisition time T1, the mobile station proceeds to step S519 to enter the sync mode (or, enable the MC module) for the idle sleep time and acquire the pilot signal from the adjacent sync base station, thereby acquiring the short code timing of the sync system. Subsequently, the mobile station synchronizes a short code generator in the PN code generator 407 of FIG. 4 with the acquired short code timing. If the current idle sleep time is shorter than the pilot signal acquisition time T 1 or the mobile station is not in the idle mode, the flow of the procedure goes to step S511 in which the mobile station requests the async base station for the pilot signal acquisition time. Then, the mobile station determines in step S513 whether the request is granted, i.e., an acknowledgement (ACK) signal is received from the async base station. Upon receiving the ACK signal, the mobile station acquires the pilot signal for the given time and gets in synchronization with the PN short code, in step S517. Without the ACK signal, the mobile station performs a normal operation in the async system in step S515 and, after an elapse of a defined time, returns to step S511 to request the async base station for the pilot signal acquisition time again. After acquiring synchronization with the PN short code, the mobile station determines in step S521 whether the current idle sleep time is longer than the sync channel demodulation time T2 of the sync system. The required sync channel demodulation time T2 is in the range of about 200 to 500 ms for the IS-95 system. If the current idle sleep time is longer than the sync channel demodulation time T2, the mobile station proceeds to step S531 to enter the sync mode for the idle sleep time and demodulate the sync channel of the sync system. If the current idle sleep time is shorter than the sync channel demodulation time T2, the mobile station proceeds to step S523 to request the async base station for the sync channel demodulation time. Then, the mobile station checks in step S525 whether the request is granted, i.e., an ACK signal is received from the async base station. Upon receiving the ACK signal, the mobile station demodulates the sync channel for the given time, in step S529. Without the ACK signal, the mobile station performs a normal operation in the async system in step S527 and, after an elapse of a defined time, returns to step S523 to request the async base station for the sync channel demodulation time. After demodulating the sync channel, the mobile station synchronizes the PN long code generator in the PN generator of FIG. 4 with the PN long code timing acquired by the demodulation, in step S533. The mobile station continues the operation of step S533 even when the operational mode is switched to the async system. The synchronizations of the PN short code and the PN long code are not necessarily successive. When the idle sleep time is relatively short, the mobile station acquires the timing of the PN short code and, after returning to the async system mode, gets in synchronization with the PN long code in the next sleep interval. If the mobile station requests the async base station for the timing acquisition time, then the async base station temporarily controls the call signal period of the mobile station to assign the required timing acquisition time to the mobile station. The mobile station records in the memory that it is registered in the sync system, in step S535, and performs a normal operation in the async system, in step S537. In FIG. 5, the procedure for acquiring the timing of the sync system using the pilot signal and the sync channel of the sync base station can be performed while the mobile station acquires the initial timing of the async system. The timing of the async system can be acquired during the idle sleep time, or using the compressed mode. Now, a description will be given regarding an operation of the async base station in response to the action of the mobile station as illustrated in FIG. 5. During transmission of information about the adjacent cells to the mobile station, the async base station informs the mobile station whether there exists a sync system among the adjacent cells. Upon receiving a request for the pilot signal acquisition time from the mobile station (in step S511 of FIG. 5), if possible according to the circumstances, the async base station sends an ACK signal to the mobile station and interrupts the signal transmission to the corresponding mobile station for a defined time so as to afford the pilot signal acquisition time to the mobile station. When the mobile station is in communication, the async base station may indicate a compressed mode operation according to the channel environment. The async base station normally sends a non-acknowledgement (NACK) signal to the mobile station in cases where the mobile station is in communication or under bad channel environment, it is not possible to assign the compressed mode for the required time, or it is not possible to assign the pilot signal acquisition time due to the presence of data to send in emergency. Upon receiving a request for the sync channel demodulation time from the mobile station (in step S523 of FIG. 5), if possible according to the circumstances, the async base station sends an ACK signal to the mobile station and interrupts the signal transmission to the corresponding mobile station for a defined time so as to afford the sync channel demodulation time to the mobile station. If the mobile station is in communication at this time, the async base station may indicate a compressed mode operation according to the channel environment. The async base station normally sends an NACK signal to the mobile station in cases where the mobile station is in communication under good channel environment, the compressed mode is not allowed during the required time, or the sync channel demodulation time cannot be assigned due to the presence of data to send in emergency. FIG. 6 is a diagram illustrating a procedure that the mobile station operating in an async system acquires the timing of a sync system and then performs a handoff to the sync system. The handoff process where the mobile station has already acquired the timing of the sync base station as described in FIG. 5 is similar to the general process for performing a handoff to the sync system. Referring to FIG. 6, the mobile station receives a message including information about the adjacent base stations from the async base station through a broadcast channel, in step S601. Here, the async base station sends to the mobile station information about the adjacent sync base stations together with the pilot offset PILOT_OFFSET and the frequency band of the individual sync base stations. In step S602, the mobile station measures the strengths of the pilot signals from the adjacent base stations using the received information about the adjacent base stations and sends a message including the measured strengths of the pilot signals to the async base station through the reverse dedicated channel periodically or by request. During a normal operation in the async system, the mobile station acquires the pilot signal from the sync base station in the process as illustrated in FIG. 5, in step S604. As described above, the mobile station acquires the pilot signal by two methods: the one is switching the operational mode for the idle sleep time to acquire the pilot signal, and the other is separately requesting the async base station for the pilot signal acquisition time and acquiring the pilot signal for the given time. If the mobile station is not in the idle mode, e.g., during communication, it acquires the pilot signal from the sync base station using the compressed mode. Upon acquiring the pilot signal, the mobile station synchronizes the PN short code generator in the PN code generator 407 of the MC module with the PN short code of the sync system. After acquiring the pilot signal, the mobile station performs the procedure of FIG. 5 to demodulate the sync channel from the sync base station and analyze a sync message, in step S605. The sync message includes system information necessary to communication with the base station, such as system ID, network ID, PN_OFFSET value, information about the long code state after 320 ms, paging channel data rate, etc. As described above, the mobile station demodulates the sync channel by two methods: the one is switching the operational mode for the sleep time to demodulate the sync channel, and the other is separately requesting the async base station for the sync channel demodulation time and demodulating the sync channel for the given time. If the mobile station is not in the idle mode, e.g., during communication, it demodulates the sync channel from the sync base station using the compressed mode. Upon acquiring the pilot signal, the mobile station synchronizes the PN long code generator in the PN code generator 407 of the MC module with the PN long code of the sync system. As described above, the mobile station can acquire the timing of the adjacent sync base station for a given time during an operation in the async system. The procedures of steps S604 and S605 are performed before the mobile station receives from the async base station a measurement parameter requesting for the measurement of the sync base station. The async base station analyzes the message received through the reverse dedicated channel and checks whether there exists another async base station to which the mobile station can perform a handoff. If there is another async base station, the async base station decides to perform the handoff; otherwise, the async base station sets parameters T, T0 and N for detection of the received strength of the pilot signal from the adjacent sync base station. The parameter T0 indicates the time to detect the pilot signal from the sync base station, T the time period for detection of the pilot signal from the sync base station, N the number of repeating times for detecting the pilot signal from the sync base station. The mobile station receives a message including a measurement indication for the reception strengths of the pilot signals from the adjacent async and sync base stations and the above-set parameters through a forward dedicated control channel, in step S603. Upon receiving the message on the forward dedicated control channel, the mobile station measures the received strengths of the pilot signals from the adjacent sync and async base stations based on the parameters. Since the mobile station has already acquired the timing of the sync base station in steps S604 and S605, it detects the pilot signals for data collected in the compressed mode using the timing of the sync base station. That is, the mobile station, which has the knowledge about the reference zero offset timing, can measure the strengths of the pilot signals from the candidate cells in a short time. When the maximum of the measured values exceeds the threshold, the timings of the PN short code and the PN long code in the mobile station are updated by an offset given based on the pilot signal. That is, if the actual phase difference between the reference timing and the pilot detection timing is different from an offset given by the base station, the PN short code timing and the PN long code timing in the mobile station are regulated such that the phase difference should be the same as the given offset. The mobile station sends a message including the measured strengths of the pilot signals from the adjacent base stations and the sync message to the async base station through the reverse dedicated channel, in step S606. Then, the async mobile station analyzes the message received on the reverse dedicated channel and sends the measurement results to the upper network. The upper network checks the existence of a sync base station to which the mobile station performs a handoff, and sends to the async base station a handoff indication message including information necessary to the handoff. The mobile station receives the handoff indication message, including traffic channel information for communication with the sync base station, from the async base station through the forward dedicated channel, in step S607. The handoff indication message includes information necessary to establishment of traffic channels, such as traffic channel frequency band, channel division code, frame offset, service option, etc. Upon receiving the handoff indication message, the mobile station is ready to receive the traffic data of the sync base station with reference to the traffic channel information included in the message, and receives null traffic data or the like from the sync base station through a forward fundamental channel to check the stability of the channel, in step S608. In step S609, the mobile station receives a traffic message from the sync base station through the forward fundamental channel while it travels to the cell of the sync base station. Thus the call in connection to the async base station gets in contact with the sync base station. Subsequently, the mobile station sends a preamble on the reverse fundamental channel in order to report the normal transmission state, in step S610, and then a handoff completion message to the sync base station, in step S611. The procedure of FIG. 6 is similar to that of FIG. 3 as previously described in the prior art, excepting that upon detecting the sync base station among the adjacent cells in step S601, the mobile station acquires and maintains in itself the timing of the sync system as described in FIG. 5 in steps S603 and S604 before the async base station requests for the measurement parameters of the sync base station, and then readily measures the strength of the pilot signal with the previously synchronized timing by the measurement request from the async base station. As described above, the present invention allows the mobile station to acquire and maintain the timing of the sync system prior to receiving a handoff indication or an adjacent cell search indication in the mobile communication system in which both async and sync systems coexist. Consequently, upon receiving a search indication message for the adjacent sync cells from the async base station, the mobile station which has the knowledge of the zero offset time can search for the candidate cells with a small window size and send the reliable search results to the base station in a shortest time, thereby reducing any potential risk of call disconnection. Furthermore, upon receiving a handoff indication to the sync system, the mobile station which has the knowledge of the PN long code information and the pilot offset of the target cell can perform a handoff between async and sync systems in the level of the hard handoff occurring in the sync system. This secures a stable handoff without call disconnection.

Claims: A method for performing a handoff from an async base station (6001) to a sync base station (6003) when a mobile station (6002) with async (402, 403, 410) and sync (405, 406, 407, 411) demodulation modules travels from a present cell of the async base station (6001) to a cell of the sync base station (6003), the method comprising the steps of: the mobile station driving the sync demodulation module for a given idle sleep time to acquire and maintain the timing of the sync base station, during an operation in the cell of the async base station; according to a measurement request about the sync base stations of the async station, the mobile station measuring the strengths of pilot signals from the adjacent sync base stations based on the acquired timing, and sending (606) the measurement results to the async base station (6001); the mobile station (6002) receiving from the async base station traffic channel information necessary to establish a traffic channel with the sync base station; and the mobile station performing handoff (607, 608, 610, 611) to the sync base station according to the traffic channel information. The method as claimed in claim 1, wherein the timing of the sync base station includes PN short code timing and PN long code timing (533). The method as claimed in claim 1 or 2, further comprising the step of updating the timing based on the timing when the maximum value of the measured strengths of the pilot signals exceeds a threshold value. The method as claimed in one of claims 1 to 3, further comprising the steps of: the async base station (6001) informing the mobile station (6002) entering the cell of the async base station that there exist adjacent sync base stations (6003); and during the handoff, the async base station sending information (603) about the adjacent sync base stations to the sync base station; wherein the mobile station measures the strengths of pilot signals from the adjacent sync base stations based on the acquired timing of the sync base station. The method as claimed in claim 4, wherein the timing acquisition step comprises the steps of: acquiring (512) the pilot signal of the sync base station for the given idle sleep time to acquire the reference timing of a PN short code; and demodulating (531) the sync channel of the sync base station for the given idle sleep time to acquire (533) the timing of a PN long code. The method as claimed in claim 5, further comprising the step of, when the pilot signal cannot be acquired for the idle sleep time, requesting (511) the async base station for a pilot acquisition time and, if granted, acquiring (517) the pilot signal for the given time allowed by the async base station to acquire the timing of a PN short code. The method as claimed in claim 5, further comprising the step of, when the sync channel cannot be demodulated for the idle sleep time, requesting (523) the async base station for a sync channel demodulation time and, if granted, demodulating (529) the sync channel for the given time allowed by the async base station to acquire the timing of a PN long code. A mobile station device (6002) adapted to handoff between an asynchronous system and a synchronous system, the mobile station device comprising: an async demodulation module (402, 403, 410) for converting a highfrequency signal from an async base station (6001) to a baseband signal and demodulating the baseband signal by despreading; a sync demodulation module (405, 406, 407, 411) for converting a highfrequency signal from a sync base station to a baseband signal and demodulating the baseband signal by despreading; a switch (401) for switching the received signal between the async and sync demodulation modules; and controller means (404) adapted to drive the sync demodulation module for a given idle sleep time interval, said sync demodulation module adapted for being driven to acquire the timing of the sync base station during an operation of the mobile station device in the cell of the async base station, and adapted to maintain the acquired timing of the sync base station even after switching to the sync demodulation module.

Inventors: LEE KYUNG-HA, PARK SUNG-BOK, LEE, KYUNG-HA, PARK, SUNG-BOK, Park, Sung-Bok.c/o Samsung Electronics Co.,LTD
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO. LTD.
Application year: 2000
Main classification: H04Q7
IPC codes: H04B7, H04W36