Base station synchronization for handover in a hybrid gsm/cdma network

{BASE STATION SYNCHRONIZATION FOR HANDOVER IN A HYBRID GSM/CDMA NETWORK}

<p>FIELD OF THE INVENTION The present invention relates generally to wireless telecommunications, and more particularly to improved cellular telephone networks.</p>

<p>In many countries of the world, GSM (Global System for Mobile) telecommunications is used in cellular telephone networks. GSM provides a useful range of network services and standards. Existing GSM networks are based on time-division multiple access (TDMA) digital communication technology. In a TDMA-based cellular network, each mobile subscriber unit communicates with only one base station at any moment. When a subscriber moves from one cell to another, a "hard handover" occurs in which the base station communicating with the subscriber cuts the link between itself and the subscriber and a new base station takes over.</p><p>Code-division multiple access (CDMA) is an advanced digital communication technology that not only provides a more reliable, fading-free link between cellular telephone subscribers and base stations, but also uses radio bandwidth more efficiently than TDMA. The latest CDMA standard is IS-95, established by the Telecommunications Industry Association (TIA). In this standard, a "soft handover" (or handoff) function is provided in which a subscriber unit temporarily communicates with two or more base stations simultaneously when moving from one cell to another. This soft handover is made possible by the code division method, and reduces the possibility of communication interruption that may occur frequently in hard handover.</p><p>In PCT patent application PCT/US96/20764, a wireless telecommunication system is described that implements GSM network services and protocols using a CDMA air interface (ie, basic RF communication protocol). Using this system, even some of the TDMA base stations (BSS) and existing GSM network subscriber units can be replaced or supplemented by the corresponding CDMA devices. The CDMA BSS of this system is adapted to communicate with the GSM Mobile Switching Center (MSC) via the standard GSM A-interface. Thus, the core of the GSM network service is maintained, and the transition from TDMA to CDMA is transparent to users.</p><p>In addition, a hybrid cellular communication network incorporating GSM and CDMA components is described in PCT patent applications WO 95/24771, WO 96/21999, and Secondary on Universal Personal Communications in Ottawa, 1993. It is described in a paper by Tscha et al. entitled "A Subscriber Signaling Gateway between CDMA Mobile Station and GSM Mobile Switching Center", pages 181-185 of the International Conference Proceedings. In these publications, the specific issues of how to efficiently handover subscriber units between different base stations in such a hybrid network are not addressed.</p><p>In PCT patent application PCT/US97/00926, inter-system handover methods between CDMA and TDMA BSSs in a hybrid GSM/CDMA telecommunications system are described. GSM/TDMA BSS generates a pilot beacon signal according to CDMA technology. During a telephone call, the subscriber unit detects pilot signals and notifies the base station controller that pilot signals have been detected. The subscriber unit is then handed over from CDMA to TDMA BSS without disconnection.</p>

<solutionproblem><p>It is an object of the present invention to provide a method and apparatus for use in a mixed TDMA/CDMA cellular communication network.</p><p>Another object of some aspects of the present invention is to provide an improved method and apparatus for handover of a subscriber unit between a TDMA base station and a CDMA base station without interruption of communication.</p></solutionproblem><meansproblemsolution><p>In preferred embodiments of the present invention, a mixed GSM/CDMA cellular communication system includes both a TDMA base station and a CDMA base station, which are jointly controlled by a Mobile Switching Center (MSC). A system of this type is generally described in the aforementioned PCT patent applications. A subscriber unit (also referred to herein as a mobile station (MS)) of such a system communicates with base stations of both types by appropriately switching between the two types of interfaces, using GSM network protocols, preferably over TDMA and CDMA air interfaces. can do. A feature of the preferred embodiments of the present invention is that such a communication system builds on the existing GSM/TDMA infrastructure while adding CDMA BSSs, so that it may require little change to the existing infrastructure.</p><p>To determine when a handover should take place, an MS communicating with a current base station of one type (CDMA or TDMA) monitors RF signals from another base station, which is a base station of another type (TDMA or CDMA). may be Through the message sequence between the current base station and the MS, the MS can obtain appropriate synchronization information about the new base station, and re-report this information to the current base station. That information is used by the system to allow the MS to establish an air interface with its new base station, so that communication between the MS and the network is minimal and handover occurs.</p><p>In the context of this patent application, such handover between base stations is referred to as "mobile-assisted handover". The busy handover function by a mobile station is used in GSM systems and CDMA systems known in the prior art. Measure and report on it. However, in the hybrid GSM/CDMA system proposed so far, the mobile station, at any moment, receives a signal from one of them but not both CDMA or TDMA (or associated with the TDMA base station in the aforementioned PCT patent application PCT/US97/00926). Since it was considered capable of receiving only CDMA beacons), it cannot provide this type of handover. By providing a handover function during a call by a mobile station according to the principles of the present invention, handover can be performed more smoothly and reliably than in the case where it is not.</p><p>In some preferred embodiments of the present invention, the MS switches between TDMA and CDMA operation during a phone call according to instructions received from the base station with which the unit is communicating. Before handover, the MS receives signals from both the TDMA base station and the CDMA base station, and re-reports to the base station related to the signal the MS is receiving. This reported information is re-reported to the BSC and used by the BSC to initiate a handover. Preferably, the MS transceiver has a single radio transceiver so that the MS can communicate with either, but not both, a TDMA or CDMA base station at any moment. (However, as mentioned above, according to the principle of IS-95, a unit can communicate with more than one CDMA base station at a time.) Also, GSM/TDMA base stations have their own synchronous clock, which allows them to communicate with those base stations. MSs are synchronized to that clock, but CDMA base stations are inter-synchronized to the actual time. Thus, when switching between a TDMA station and a CDMA station, in each case, the MS synchronizes its operation to the appropriate clock signal after acquiring synchronization with little to no disconnection of the phone call.</p><p>In some of these preferred embodiments, the MS is communicating with a CDMA base station, and a decision is made for the unit to be handed over to the GSM/TDMA base station. The CDMA transmission by the MS transceiver is temporarily stopped, during which the unit normally performs a GSM neighbor scan according to the GSM standard, acquires synchronization, and then synchronizes to the TDMA base station. Preferably, the CDMA transmission is interrupted for a single frame, typically 20 ms, creating a latency slot according to the IS-95 standard. By identifying the TDMA base station and exchanging the appropriate messages, a traffic channel is established between the base stations, and the MS switches to the TDMA base station with little to no telephony disconnection performed by the MS.</p><p>In some other of these preferred embodiments, the MS is communicating with a TDMA base station, and a decision is made for the unit to be handed over to the CDMA base station. In order to synchronize with the CDMA station, preferably, the MS obtains the date and time by receiving the correct date and time from its TDMA base station, where the GSM network is provided with the necessary equipment to generate and broadcast the date and time. Preferably, the network comprises a cell broadcast system (CBS) according to the GSM standard, which is provided for example by a Global Positioning System (GPS) or one of the CDMA base stations. It is used to receive the date and time received from , and broadcast this date and time to MSs over the network. Alternatively, the MS temporarily stops receiving TDMA to obtain and synchronize the date and time of the CDMA station. Thus, some signal degradation may occur due to TDMA time slot(s) lost in this way, but compared to otherwise, busy handovers by mobile stations from TDMA to CDMA are typically more reliable, and the MS less inconvenience to users of</p><p>Although preferred embodiments are described herein with reference to MSs having a single transceiver for TDMA and CDMA, the principles of the present invention are useful when using different types of system hardware and subscriber units, and particularly when separate or only partially integrated. A similar application is possible when using a subscriber unit with integrated TDMA and CDMA transceivers.</p><p>According to preferred embodiments of the present invention, in a mobile wireless telecommunications system comprising base stations of a first type operating according to a first air interface, and base stations of a second type operating according to a second air interface, the A method of handing over a mobile station of a system from a first base station of a first type to a second base station of a second type, the method comprising:</p><p>establishing a communication link between the mobile station and the first base station via the first air interface;</p><p>receiving data from the mobile station in response to a signal received by the mobile station from the second base station over a second air interface with little disruption of a communication link with the first base station; and</p><p>A method is provided comprising handing over a mobile station from a first base station to a second base station in response to data received from the mobile station.</p><p>Preferably, receiving the data comprises receiving signal strength measurements, and handing over the mobile station comprises handing over the mobile station by comparing signal strength measurements from the first base station and the second base station. include Preferably, receiving the data comprises applying a weighting factor to the measure of signal strength, and applying the weighting factor comprises varying the factor according to network conditions of the system. Also, the step of applying the weighting factor preferably includes transmitting the weighting factor to the mobile station over the communication link, and the mobile station applying the weighting factor to the measurement.</p><p>Preferably, receiving the data comprises receiving an identifier of the second base station based on the mobile station decoding a signal received over the second air interface.</p><p>In a preferred embodiment, a frequency list of base stations of the second type of system is transmitted from the first base station to the mobile station so that the mobile station receives a signal at a frequency in the list.</p><p>Preferably, handing over the mobile station comprises transmitting a handover command from the first base station. In a preferred embodiment, handing over the mobile station comprises transmitting an initial transmission to a second air interface in response to the handover command, the method comprising: the initial transmission being successful over the second air interface if not received, reacquiring the communication link over the first air interface.</p><p>Preferably, transmitting the handover command comprises transmitting over the first air interface a command encapsulating parameters related to the second air interface. Most preferably, the step of sending the command comprises sending a command according to the GSM standard encapsulating parameters specified according to the IS-95 standard, wherein the encapsulated parameters include the IS-95 long code. .</p><p>Preferably, establishing the communication link and receiving the data in response to the signal comprises establishing the link and receiving the signal at the mobile station using a single RF transceiver within the mobile station.</p><p>In a preferred embodiment, one of the first and second air interfaces comprises a TDMA interface, and the other interface comprises a CDMA interface, wherein the TDMA interface preferably comprises a GSM interface, wherein the CDMA interface comprises GSM network messages. set to deliver. Preferably, the CDMA interface is based on the IS-95 standard.</p><p>Preferably, establishing the communication link comprises managing the first air interface using a single radio resource management protocol layer, and handover of the mobile station comprises managing the first air interface using the single radio resource management protocol layer. 2 managing the air interface.</p><p>More preferably, the step of receiving data from the mobile station defines an overlapping area between a first area serviced by the first air interface and a second area serviced by the second air interface, wherein the mobile station is located in the overlapping area triggering the mobile station to receive the data.</p><p>In a preferred embodiment, the first air interface comprises a CDMA interface, and the second air interface comprises a GSM/TDMA interface, and receiving data from the mobile station comprises gating the mobile station to interrupt the CDMA communication link. Thus, the GSM/TDMA signal is received and decoded. Preferably, gating the mobile station comprises aborting CDMA communications for the duration of the IS-95 frame, and receiving the data includes decoding signals of the GSM frequency correction channel and synchronization channel by the mobile station. based on the second base station's identifier.</p><p>In another preferred embodiment, the first air interface comprises a GSM/TDMA interface and the second air interface comprises a CDMA interface, and wherein receiving data from the mobile station comprises controlling the mobile station to interrupt the communication link. , to receive and decode CDMA signals.</p><p>Preferably, receiving the data comprises communicating the temporal information via a GSM/TDMA interface. More preferably, the step of forwarding the time information comprises broadcasting the time information through a system using a GSM cell broadcast service, wherein the step of broadcasting the time information comprises: a first type base station of the system; and receiving the date and time and corresponding GSM frame number from the communicating transceiver. Preferably, the mobile station decodes the synchronization channel of the CDMA signal to obtain the date and time.</p><p>Alternatively or additionally, receiving the data includes forwarding a GSM cell broadcast service message to the mobile station to initiate the search by the mobile station for a signal from the base station of the second type. Preferably, forwarding the GSM cell broadcast service message to the mobile station comprises forwarding the message to be received by the mobile station while the mobile station operates in a dedicated mode.</p><p>Preferably, receiving data from the mobile station comprises receiving an identifier of a CDMA pilot beam decoded by the mobile station. More preferably, the method comprises mapping the second base station to the GSM base station to control the handover.</p><p>Preferably, the step of controlling the mobile station receives a CDMA signal during a first TDMA timeslot and decodes the signal during a next TDMA timeslot while communicating with the base station over a TDMA interface to generate data to be received by the base station. including controlling the</p><p>According to a preferred embodiment of the present invention, there is provided a method for transmitting temporal information to a mobile station in a GSM wireless telecommunication system, comprising:</p><p>inputting date and time information into the system; and</p><p>A method is further provided comprising broadcasting the information to a mobile station via the system.</p><p>Preferably, the GSM wireless telecommunications system comprises a cell broadcast system, and the step of broadcasting the time information comprises broadcasting the information through the cell broadcast system. Preferably, broadcasting the time information comprises broadcasting a message to be received by the mobile station while the mobile station is operating in a telephony mode.</p><p>More preferably, broadcasting the time information comprises receiving the time and date and corresponding GSM frame number from a transceiver in communication with the system, the method using the time information to synchronize the mobile station to a CDMA transmission. includes steps.</p><p>In a preferred embodiment, the method includes determining a location of a mobile station in response to transmitting temporal information to a plurality of base stations in the system.</p><p>Preferably, the step of entering the date and time comprises establishing a data call from the transceiver having the date and time information to the cell broadcast center, wherein the step of establishing the data call preferably includes receiving the time information from the GPS device. including the steps of Alternatively, establishing a data call includes receiving temporal information from a CDMA cell associated with the GSM system.</p><p>According to a preferred embodiment of the present invention, in a GSM mobile radio telecommunications system comprising a first base station subsystem and a second base station subsystem, wherein at least one of the subsystems operates according to a CDMA air interface, the system A method for handover of a mobile station from a first base station subsystem to a second base station subsystem, comprising:</p><p>mapping one or more of the first and second subsystems operating in accordance with a CDMA air interface to a GSM/TDMA subsystem;</p><p>establishing a communication link between the mobile station and the first base station subsystem, such that the mobile station receives a first signal from the first base station subsystem;</p><p>receiving data from the mobile station in response to a second signal received by the mobile station from a second base station subsystem without breaking a communication link with the first base station subsystem;</p><p>comparing the strengths of the first and second signals as if in fact both the first and second base station subsystems were GSM/TDMA subsystems; and</p><p>A method is provided comprising handing over a mobile station from a first base station subsystem to a second base station subsystem in response to the comparison of signal strengths.</p><p>Preferably, mapping one or more of the subsystems operating in accordance with the CDMA air interface comprises assigning a GSM frequency and location to the subsystem.</p><p>More preferably, the steps of establishing the communication link and handover of the mobile station comprise passing messages between the first and second subsystems and the mobile switching center of the system via the GSM A-interface. Preferably, both the first and second base station subsystems operate in accordance with a CDMA air interface, wherein the step of handing over the mobile station does not substantially violate the A-interface protocol and the new IS-95 long over the A-interface. and passing the code.</p><p>Preferably, receiving data from the mobile station comprises applying a weighting factor to the second signal, wherein comparing the strengths of the signals comprises comparing the weighted signal, and applying a weighting factor The step of performing includes communicating the weighting factor to the mobile station, the mobile station applying the weighting factor to the second signal. Preferably, the step of applying the weighting factor comprises varying the factor according to the network state of the system.</p><p>According to a preferred embodiment of the present invention, it is used in a mobile telecommunication system,</p><p>a base station of a first type for transmitting and receiving a first signal according to a first air interface;</p><p>a base station of a second type for transmitting and receiving a second signal according to a second air interface; and</p><p>While maintaining a communication link with the base station of the first type over the first air interface, receiving a second signal from the base station of the second type over the second air interface, and in response to the second signal, to the base station of the first type Also provided is a wireless communication apparatus having a mobile station handed over from a first base station to a second base station in response to the transmitted data by transmitting the data.</p><p>Preferably, the data transmitted by the mobile station includes a measure of signal strength such that the mobile station is handed over in response to a signal strength comparison of the first and second signals. Preferably, a weighting factor is applied to the measure of signal strength, wherein the weighting factor varies according to the network state of the system. Preferably, the weighting factors are transmitted to the mobile station over a communication link, and the mobile station applies the weighting factors to the measurements.</p><p>More preferably, the mobile station decodes the second signal to determine the identifier of the second type base station.</p><p>Preferably, the base station of the first type transmits to the mobile station a frequency list of mobile stations of the second type of the system, such that the mobile station receives the second signal on a frequency in the list.</p><p>Preferably, the base station of the first type transmits a handover command to the mobile station, whereby the mobile station is handed over from the first base station to the second base station. In a preferred embodiment, the initial transmission is sent over the second air interface in response to the handover command, and if the initial transmission over the second air interface is not successfully received, the mobile station reestablishes the communication link over the first air interface. acquire</p><p>Preferably, the handover command encapsulates parameters relating to the second air interface. Most preferably, the command is actually transmitted according to the GSM standard and encapsulates parameters defined according to the IS-95 standard, wherein the encapsulated parameters include the IS-95 long code.</p><p>More preferably, the mobile station has a single RF transceiver that communicates with both the base station of the first type and the second type.</p><p>In a preferred embodiment, one of the first and second air interfaces comprises a TDMA interface, and the other interface comprises a CDMA interface, wherein the TDMA interface preferably comprises a GSM interface and the CDMA interface comprises a GSM network message. are set to deliver Preferably, the CDMA interface is based on the IS-95 standard. More preferably, the mobile station manages both the first and second air interfaces using a single radio resource management protocol layer.</p><p>Preferably, when the mobile station is located in an overlapping area between a first area serviced by the first air interface and a second area serviced by the second air interface, the base station is configured to receive the second signal over the second air interface. Trigger the mobile station.</p><p>In a preferred embodiment, the first air interface comprises a CDMA interface and the second air interface comprises a GSM/TDMA interface, wherein the base station of the first type receives the GSM signal by gating the mobile station to interrupt the communication link; to decode.</p><p>Preferably, the mobile station drops the link for the duration of the IS-95 frame.</p><p>More preferably, the mobile station processes the second signal to decode the signals of the GSM frequency correction channel and the synchronization channel.</p><p>In another preferred embodiment, the first air interface comprises a GSM/TDMA interface, the second air interface comprises a CDMA interface, and the base station of the first type is configured to interrupt the communication link to receive and decode the CDMA signal. Control the mobile station.</p><p>Preferably, the base station of the first type communicates the time information to the mobile station via a GSM/TDMA interface. Preferably, the apparatus comprises a GSM cell broadcast center, wherein the center communicates temporal information via the system to the mobile station using a GSM cell broadcast service, wherein the cell broadcast center is a base station of the first type of the system. Receives the time information and the corresponding GSM frame number from the transceiver communicating with the</p><p>Alternatively or additionally, the mobile station decodes the synchronization channel of the CDMA signal to obtain the date and time.</p><p>Preferably, the GSM cell broadcast center forwards a cell broadcast service message to the mobile station to initiate a search for a second signal by the mobile station, wherein the mobile station sends the cell broadcast service message while the mobile station is operating in a call mode. receive</p><p>Alternatively or additionally, the mobile station processes the CDMA signal to identify a CDMA pilot beam.</p><p>Preferably, the mobile station receives a CDMA signal during a first TDMA time slot and processes the signal during a subsequent TDMA time slot to generate data for transmission to the base station while communicating with the base station over a TDMA interface.</p><p>According to a preferred embodiment of the present invention, there is provided an apparatus for transmitting temporal information to a mobile station in a GSM wireless telecommunication system, the apparatus comprising a cell broadcast center for broadcasting the information to the mobile station using the GSM cell broadcast system. is provided</p><p>Preferably, the apparatus comprises a transceiver for communicating with the system and transmitting a date and time and a corresponding GSM frame number to a cell broadcast center, wherein the transceiver establishes a data call through the system to the cell broadcast center, the cell Sends the date and time and the corresponding frame number to the broadcast center.</p><p>Preferably, the mobile station uses the time information to synchronize to the CDMA transmission.</p><p>More preferably, the mobile station receives the information from the cell broadcast system while operating in the call mode.</p><p>According to a preferred embodiment of the present invention, there is provided an apparatus for inputting date and time information to a communication controller in a wireless telecommunication system, </p><p>a clock signal receiver for receiving time information from a clock source; and</p><p>Further provided is an apparatus comprising a wireless transceiver that receives temporal information from the clock signal receiver and establishes a data call through the system to the communication controller to communicate the information to the communication controller.</p><p>Preferably, the communication controller comprises a GSM cell broadcast center, wherein the wireless transceiver receives the GSM frame number from the base station of the system and forwards the frame number along with the time information to the cell broadcast center.</p><p>Preferably, the clock signal receiver comprises a radio receiver for receiving temporal information from a CDMA communication cell, wherein the radio transceiver comprises the radio receiver.</p><p>Alternatively, the clock signal receiver comprises a GPS device.</p><p>In addition, according to a preferred embodiment of the present invention, </p><p>mobile station and;</p><p>An apparatus for mobile wireless telecommunications in a GSM telecommunications system is provided, comprising first and second base station subsystems for transmitting, to a mobile station, first and second signals, at least one of which is a CDMA signal; </p><p>Both the first and second subsystems are mapped from the GSM system to the GSM base station subsystem,</p><p>The mobile station is handed over by comparing the strengths of the first and second signals received by the mobile station as practically both the first and second base station subsystems operate according to the GSM/TDMA air interface.</p><p>Preferably, the subsystem transmitting the CDMA signal is provided with the GSM frequency and location of the system. More preferably, the messages are communicated between the mobile switching center of the system and the first and second subsystems via the GSM A-interface, wherein the first and second signals include CDMA signals. Preferably, the new IS-95 long code is passed from the second subsystem to the first subsystem via the A-interface in order to handover the mobile station without actually violating the A-interface.</p><p>Preferably, before the signal strengths are compared, the mobile station applies a weighting factor to the second signal.</p><p>According to a preferred embodiment of the present invention, there is provided a mobile station for use in a wireless remote system comprising a CDMA and TDMA base station,</p><p>a single mobile radio transceiver communicating with CDMA and TDMA base stations; and</p><p>A modem unit that encodes signals to be transmitted by a mobile station transceiver, CDMA encodes a signal for communication with a CDMA base station, TDMA encodes a signal for communication with a TDMA base station, and decodes signals received by the mobile station Also provided is a mobile station comprising:</p><p>Preferably, the modem unit encodes the signal according to the GSM air interface layer protocol.</p><p>More preferably, the mobile station receives and processes signals from the other of the CDMA and TDMA base stations with little to no disruption of the communication link existing between the mobile station and the one of the CDMA and TDMA base stations.</p><p>Further, according to a preferred embodiment of the present invention, there is provided a method for delivering a message to a plurality of mobile stations operating in a call mode in a GSM wireless telecommunication system, the method comprising:</p><p>broadcasting the message to the mobile station via a cell broadcast service; and</p><p>A method is provided comprising receiving the message at a mobile station without substantially disrupting the mobile station's telephony operation.</p><p>Preferably, broadcasting the message comprises transmitting time information, or alternatively or additionally, broadcasting a discovery trigger message.</p><p>According to a preferred embodiment of the present invention, </p><p>a cell broadcast center that broadcasts a message through the cell broadcast system; and</p><p>An apparatus for mobile wireless telecommunications in a GSM telecommunications system is additionally provided that includes a mobile station that receives a message while communicating in a talk mode without interrupting the talk mode communication.</p><p>Preferably, the cell broadcast center broadcasts date and time information or, alternatively or additionally, broadcasts a discovery trigger message.</p><p>According to a preferred embodiment of the present invention, there is provided a mobile station for use in a wireless telecommunications system comprising CDMA and TDMA base stations, the mobile station comprising:</p><p>one or more mobile radio transceivers in communication with CDMA and TDMA base stations; and</p><p>Process signals received by a mobile station and transmitted by one or more transceivers according to a communication protocol stack comprising a single radio resource management protocol layer that controls communication with both CDMA and TDMA base stations, wherein for communication with a CDMA base station, A mobile station is additionally provided comprising a modem unit that encodes and processes to TDMA-encode for communication with a TDMA base station.</p><p>Preferably, the radio resource management protocol layer performs almost all functions of the GSM air interface layer 3 RR sublayer.</p><p>More preferably, the radio resource management protocol layer controls mobile station handover from one base station to another base station.</p><p>Further, in accordance with a preferred embodiment of the present invention, in a GSM mobile wireless telecommunications system comprising base station subsystems operating in accordance with a CDMA air interface, at least several base station subsystems enable communication between the base station subsystems and the mobile stations of the system. As a method of controlling,</p><p>transmitting and receiving signals between one of the base station subsystems and the mobile station via a CDMA air interface; and</p><p>A method comprising controlling the transmission and reception by using a radio resource management communication protocol layer that performs almost all functions of the GSM air interface layer 3 RR sublayer is provided.</p><p>Also preferably, the system comprises base station subsystems operating in accordance with a TDMA air interface, the method comprising:</p><p>and transmitting and receiving signals between one of the base station subsystems and the mobile station over a TDMA air interface, wherein controlling the transmission and reception controls the transmission and reception of signals over both the CDMA and TDMA air interfaces. using the single radio resource management communication protocol layer to</p><p>More preferably, the method comprises handing over a mobile station between TDMA and CDMA base stations, wherein the handover is controlled by a radio resource management communication protocol layer.</p></meansproblemsolution><effectiveness><p>A method and apparatus are provided for handover of a mobile station (40) of a system from a first base station (30) of a first type to a second base station (36) of a second type. A communication link between the mobile station 40 and the first base station 30 is established via the first air interface. Data is received from the mobile station 40 in response to a signal received by the mobile station 40 from the second base station 36 over the second air interface without substantially breaking the communication link with the first base station. In response to data received therefrom, the mobile station 40 is handed over from the first base station 30 to the second base station 36 .</p></effectiveness>

<p>DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.</p><p>Overview of Hybrid GSM/CDMA System Operation</p><p>1 shows a hybrid GSM/CDMA cellular communication system 20 in accordance with a preferred embodiment of the present invention. The system 20 is installed around a public land mobile network (PLMN) 22, which is based on the GSM communication standard as described below. The infrastructure for such a network already exists and is widely used in many countries, and the present invention takes advantage of the fact that CDMA services can be introduced in phases in relation to such a network without the need to significantly change the existing infrastructure. The PLMN 22 has one or more mobile-service switching centers (MSCs) 24, or it is possible to have several such centers (only one MSC is shown here for clarity of explanation), which center controls network operation within a geographic area. In particular, the MSC 24, in addition to the function of linking the PLMN 22 to a public switched telephone network (PSTN) and/or a packet data network (PDN) 48, is capable of locating subscriber units and handing subscriber units between base stations. Responsible for overclocking. The PLMN also has a Network Management Center (NMC) 26 and a Cell Broadcast Center (CBC) 28 . Hereinafter, these functions will be described.</p><p>System 20 includes a plurality of mobile stations (MS) 40, which mobile stations are configured via a wireless RF link at one or more allowed cellular communication frequencies and a plurality of base station subsystems (BSS) 30, 32 via a PLMN ( 22) and communicate with The MS 40, also known as a subscriber unit, can communicate with both the GSM BSS 30 using a practical standard GSM TDMA signaling protocol and the CDMA BSS 32 using a CDMA-based communication method to be described later. Also, in a standard GSM system, it is common for mobile stations to receive broadcasts from CBC 28 only in idle mode, but MS 40 can receive such broadcasts via BSS 30 during a call. and this will be further described below. For clarity, only one MS 40, GSM BSS 30 and CDMA BSS 32 are shown in FIG. 1, but in practice, it is common for the system 2 to have a plurality of such system components. .</p><p>Both GSM BSS 30 and CDMA BSS 32 communicate with and are controlled by MSC 24 . Communication between the GSM BSS 30 and the MSC 24 is in effect conforming to the GSM standard. The CDMA BSS 32 is modified to communicate with the PLMN 22 according to the GSM standard while relating to the IS95 CDMA standard, and in particular to communicate with the MSC 24 via the GSM standard A-interface, which is shown in Figs. 3a and 3b. It will be further described below with reference. The BSS 32 also communicates with the CBC 28 to receive messages broadcast over the air, and includes a Radio Operations and Maintenance Center (OMC-R) 38 . The OMC-R communicates with the NMC 26 via a GSM-standard Q3 interface, preferably using an information model based on the specifications of the GSM 12.XX series. Optionally, the BSS 32 may be linked to a general packet data service (GPRS) 50 , such as that proposed by the European Telecommunications Standards Institute (ETSI). Alternatively or additionally, the BSS 32 may be directly connected to the PSTN/PDN and preferably linked to the Internet via it, for transmission of packet data (however, for simplicity, such a connection is not shown in FIG. 1 ). didn't).</p><p>Communication between the CDMA BSS 32 and the MS 40 is established over a CDMA "wireless interface", preferably generally conforming to the IS-95 standard for CDMA communications. A BSS 32 is installed around a base station controller (BSC) 34, which controls and communicates with a number of base station transceivers (BTS) 36 . Each BTS transmits RF signals to, and receives RF signals from, MSs 40 within the geographic area or cell served by that particular BTS. During a phone call, when the MS moves from the cell of one CDMA BTS 36 to the cell of another CDMA BTS, a "soft handover" (or handoff) occurs between those BTSs, as is known in CDMA technology.</p><p>However, there may be service areas of system 20 where CDMA service is weak or congested, or where CDMA service is not provided (ie, without CDMA BTS 36 ). If the MS 40 moves into such an area during a phone call, the MS is handed over from the CDMA BTS to the BTS associated with the GSM BSS 30 without interruption. Similarly, when the MS 40 moves from an area served only by the GSM BSS 30 to the cell of the CDMA BTS 36 during a call, it is desirable for the MS to be handed over from the GSM to the CDMA BSS. A method for performing handover between the CMDA BSS 32 and other CDMA BSSs, as well as handovers between such CDMA and GSM/TDMA services is further described below. With the architecture of system 20 and such methods as shown in FIG. 1 , MS 40, within the areas serviced by system 20, maintains service in the TDMA area, while providing access to CDMA service. also receive benefits for The transition between the CDMA and TDMA domains is virtually transparent to users of MS 40, since only the lower level RF interface is changed during the transition while adhering to the higher level GSM network protocol throughout the system.</p><p>2A is a block diagram schematically illustrating a communication protocol stack between the MS 40 and the BSS 30, 32, according to a preferred embodiment of the present invention. Since the MS 40 communicates with the GSM BSS 30 via the GSM Um interface based on the TDMA air interface, there is no need to modify the BSS 30 or the GSM Layer 1 and Layer 2 standard interface protocols to accommodate the MS 40. none. MS 40 communicates with CDMA BSS 32 via a CDMA Um interface that is based on a slightly modified CDMA IS-95 air interface. The subscriber units known in the prior art can operate over either the GSM Um interface or the CDMA Um interface, but cannot operate over both modes.</p><p>To support both of these interfaces, the MS 40 is equipped with a mobile device unit (ME) 42 (FIG. 1), which has two radio transceivers: one configured for TDMA operation and one configured for CDMA. or a single transceiver that can be dynamically switched between TDMA and CDMA. The ME includes a mobile terminal unit (MT) capable of supporting a terminal device (TE) 46 for input/output of voice and/or data. The MS 40 also includes a Subscriber Identity Module (SIM) 44 according to the GSM standard.</p><p>2B is a schematic block diagram illustrating an MS 40 having a single wireless transceiver within the ME 42, in accordance with a preferred embodiment of the present invention. MS 40 is installed around a modem unit 59 that includes a DSP core 60 capable of generating and processing both TDMA and CDMA signals. Preferably, the core 60 comprises an ASIC having a SIM 44 port and including stand-alone CDMA transmit/receive processing functions supported by GSM timing logic 64 and a GSM hardware accelerator (or DSP) 62 . equipped with the device. Core 60 receives the input and passes the output to TE 46 . In this case, TE 46 denotes an audio microphone and speaker, and the core 60 not only performs a vocoding function on an audio signal, as is known in the prior art, but also a D/A and A/D conversion function. carry out Depending on whether the MS 40 is accessing the GSM BSS 30 or the CDMA BSS 32, either GSM or CDMA vocoding is applied. Alternatively or additionally, the core 60 may be configured to operate with a TE 46 that provides digital data input/output, such as a FAX device.</p><p>The core 60 outputs digital data in either the TDMA or CDMA format to the mixed-signal output device 66 . Device 66 processes and converts the data into analog baseband form for input to RF transmitter 68 . The duplexer 70 transmits the resulting RF signal to the GSM or CDMA base station as appropriate through the antenna. A signal received from the base station passes through a duplexer 70, through an RF receiver 72, through a mixed-signal input unit 74 that performs a baseband conversion function and an AGC function, and then arrives at the core 60 . Preferably, the transmitter 68 , the receiver 72 and the mixed-signal devices 66 , 74 are controlled by the core 60 .</p><p>RF transmission and reception by the MS 40 is preferably performed in a frequency band of GSM 900 MHz or 1800 MHz for compatibility with the existing GSM device, particularly the BSS 30 . If MS 40 includes only the single transceiver shown in FIG. 2B, then the CDMA device of system 20 must also be suitably configured to operate in that frequency range.</p><p>Referring again to FIG. 2A , whether MS 40 physically includes one transceiver or two transceivers, the MS, in order to operate against GSM BSS 30 and CDMA BSS 32, respectively, has its own It shall support the dual air interface Layer 1 and Layer 2 of the protocol stack. The CDMA air interface between MS 40 and CDMA BSS 32 is CDMA Layer 1 operating according to the standard IS-95 protocol, and GSM-CDMA with IS-95 operation modified to accommodate the requirements of GSM network services. Layer 2 is provided. GSM-CDMA Layer 2 includes functions such as message sorting, prioritization and fragmentation, and suspending and resuming communications, which are typically supported by standard GSM Layer 2, but CDMA IS-95 not supported by The air interface layer 1 and layer 2 against the GSM BSS 30 conform to the virtually unmodified GSM standard.</p><p>The standard GSM protocol includes a third air interface layer (RIL3) which includes three sublayers on top of GSM layer 1 and layer 2. The lowest layer of these three RIL3 sublayers is a Radio Resource (RR) management layer, and supports Mobile Management (MM) and Connection Management (CM) sublayers above it. The RIL3 sublayers of the GSM BSS 30 are virtually unchanged with respect to the GSM standard, and likewise the GSM MM sublayers and the CM sublayers of the MS 40 remain virtually unchanged. The CM sublayer supports signaling for call processing in addition to GSM supplementary service and short message service (SMS). The MM sublayer supports the signaling necessary for encryption key management, authentication, and locating of the MS 40 .</p><p>To support the MM sublayer and the CM sublayer, the GSM-CDMA RR sublayer is introduced in the MS 40 and BSS 32 protocol stacks. The GSM-CDMA RR sublayer manages radio resources and manages the radio link between the MS 40 and the BSS 30, 32, and the dual GSM and CDMA sublayers (layers 1 and 2) of the MS 40 protocol stack. is aware of the existence of By doing so, the appropriate lower layers of the MS stack communicate with the standard RIL3-RR sublayer of the BSS 30 via the GSM Um interface, or with the BSS (via the CDMA Um interface), depending on commands received from the BSS with which the MS is communicating. 32) and communicates with the GSM-CDMA RR sublayer. The MM sublayer and the CM sublayer are not handled by the BSS 32, but act as a relay between the MS 40 and the MSC 24 for processing virtually transparent to the CDMA air interface layer below. The RR sublayer of the MS stack also controls handover between the corresponding air interfaces defined in Layer 1 and Layer 2 under instructions from the MSC 24 and BSSs and assists in cell selection for the handover.</p><p>Regardless of the air interface in use, the GSM-CDMA RR sublayer supports the standard GSM RIL3-MM sublayer and the CM sublayer above it. Preferably, the RR sublayer provides a complete radio resource management function defined by GSM standards 04.07 and 04.08. Although the "RR" layer itself is not specified by the CDMA IS-95 standard, the GSM-CDMA RR sublayer described herein also maintains full IS-95 radio resource functionality.</p><p>According to the GSM standard, the function of the RR sublayer includes both standby mode operation and call mode services (ie, services performed during a phone call). The standby mode operation of the RR sublayer includes standby handover and automatic cell selection between GSM cells and CDMA cells, as well as between CDMA cells and between GSM cells, according to cell switching instructions as specified by the GSM standard. The RR sublayer also performs broadcast channel processing, and RR connection establishment, as specified by the GSM and CDMA standards, in standby mode.</p><p>In the call mode, the RR sublayer performs the following services.</p><p> Routing service, service request, message transmission, and almost all other functions specified by the GSM standard.</p><p> CDMA TO CDMA Soft and "softer" handovers and change of call channels (handovers), including hard handovers as described below.</p><p> Mode settings for RR channels including transmission mode, channel type and coding/decoding/transcoding mode.</p><p> MS parameter management based on IS-95 standard.</p><p> MS classmark management based on GSM standard.</p><p>Although the functions of the RR sublayer are only schematically listed above, additional details and functions may be added based on the published GSM and CDMA standards.</p><p>3A is a block diagram schematically illustrating a protocol stack used when signaling an interface between MS 40, CDMA BSS 32 and GSM MSC 24, in accordance with a preferred embodiment of the present invention. These interfaces allow MS 40 to communicate with GSM MSC 24 over a CDMA air interface. The operation of these interfaces, and in particular the flow of messages through these interfaces, is described in detail in the aforementioned PCT patent application PCT/US96/20764. When the MS 40 communicates with the MSC 24 via the GSM BSS 30, the protocol stack conforms to the virtually unaltered GSM standard.</p><p>As mentioned above, the MS 40 exchanges signals with the CDMA BSS 32 over the CDMA Um interface, where the MS and the BSS protocol stack have been modified to include the GSM-CDMA RR sublayer and Layer 2. In Figure 3a, a relay layer that carries RIL3-CM and MM signaling between MS 40 and MSC 24 with little processing by BSS 32 is explicitly shown in the BSS 32 protocol stack. . Other layers related to this Um interface have been described with reference to FIG. 2A .</p><p>The CDMA BSS 32 communicates with the GSM MSC 24 via a standard, virtually unmodified GSM A-interface. This interface is based on the GSM SS7 protocol and the BSS Application Part (BSSAP) protocol, which are known in the prior art and preferably conform to the GSM 08.08 standard. BSSAP is the MSC 24, which requires processing and interpretation of information related to single calls and resource management, in addition to the transmission of mobility management messages and call control between the MSC 24 and the MS 40. ) and BSS (32) support procedures (procedures). BSS 32 appropriately converts the CDMA Layer 1 protocol, GSM-CDMA Layer 2 protocol, and GSM-CDMA RR protocol exchanged between the BSS and MS 40 into the SS7 protocol and BSSAP protocol for transmission to the MSC 24 . and vice versa.</p><p>Since the CDMA BSC 34 communicates with the GSM MSC 24 using the standard A-interface, there is virtually no need to modify the core of the GSM MSC to add the CDMA BSS 32 to the GSM system 20. does not Also, since both the GSM/TDMA BSS 30 and the CDMA BSS 32 communicate with the MSC over the A-interface in virtually the same way, the MSC 24 provides a link between the GSM/TDMA BSS 30 and the CDMA BSS 32 . There is no need to know that there is a difference in reality. Preferably, the cells associated with the BTSs 36 of the BSS 32 are mapped by the MSC 24 in substantially the same way as the GSM/TDMA cells, so that, according to the GSM standard, the GSM absolute radio frequency channel number (ARFCN) ) and base station identification code (BSIC) values. From the MSC 24's point of view, even a handover between a GSM BSS 30 and a CDMA BSS 32, or even a handover between two different CDMA BSSs, is between two GSM BSSs in a conventional GSM/TDMA-based system. There is no difference between handover and handover. The BSIC of CDMA cells is assigned to be distinct from conventional GSM cells in system 20 .</p><p>3B is a block diagram schematically illustrating a protocol stack associated with passing voice data between MS 40 and MSC 24 over CDMA BSS 32, in accordance with a preferred embodiment of the present invention. The voice data between MS 40 and BSS 32 is coded and decoded by a CDMA vocoder, which may have any standard IS-95 vocoder protocol known in the art. BSS 32 converts CDMA Layer 1 to GSM E1 TDMA signals and converts CDMA vocoded data to PCM A-law companded voice data, in accordance with the requirements of the A-Interface standard. The MSC 24 can in this way communicate with the MS ( 40 ) and receive data from MS 40 .</p><p>CDMA TO TDMA Base Station Handover</p><p>4A is a schematic diagram illustrating the details of a system 20 useful in understanding how to busy handover an MS 40 from a CDMA BSS 32 to a GSM BSS 30 in accordance with a preferred embodiment of the present invention. It is a block diagram. Unlike FIG. 1 , the BSS 30 detailed herein includes a BSC 77 and a plurality of BTSs 78 , 80 . 4A illustrates handover of MS 40 from one of the BTSs associated with BSS 32 (referred to herein as BTS 76) to BTS 78 in BSS 30. BSS 32 also includes GSM-CDMA BSC 34 and GSM-CDMA BTS 36, as described with reference to FIG.</p><p>The handover from the CDMA BTS 76 to the TDMA BTS 78 is preferably initiated by the BSS 32 if it is determined that the MS 40 is located where such a handover is needed. This may occur when the signal received from the BTS 76 is weak, when it is known that the MS 40 is approaching the boundary of a CDMA service area, or when the traffic on the CDMA channel is heavy. Alternatively, BSS 32 may instruct MS 40 to occasionally search for signals from BTS 78 (or other GSM BTSs), without any specific instruction to do so.</p><p>4B is a schematic signal flow diagram illustrating signals transferred between the MS 40, the BSS 30, 32 and the MSC 24 in the handover process of FIG. 4A according to a preferred embodiment of the present invention. BSC 34 instructs MS 40 to initiate a gated search for adjacent GSM BTSs, which MS 40 communicates with BTS 76 for a short period of time to search for and receive a TDMA signal. stop Preferably, the MS 40 operates in accordance with the IS-95 standard so that CDMA transmissions can be idle for a frame of 20 ms, during which the GSM TDMA neighbor scan can be performed without substantially interrupting CDMA voice communications. can be done Most preferably, transmission by MS 40 for a frame of 20 ms is paused using the activation/deactivation mechanism defined by section 6.6.6.2.8. of the IS-95B standard. Alternatively, one could introduce such a waiting period under other CDMA standards. Alternatively, as noted above, MS 40 may have independent TDMA transceivers and CDMA transceivers that may be used concurrently for that purpose.</p><p>Preferably, BSC 34 provides MS 40 with a frequency list of adjacent GSM TDMA cells, such as those associated with BTSs 78 and 80 . Such a list is useful in reducing the time required to search and find the BTS 78 because the MS 40 searches only at cell frequencies on the list. The list is updated as the MS 40 moves from one cell to another, and is maintained during handover between TDMA and CDMA base stations.</p><p>When the MS 40 receives a signal at the frequency of the BTS 78, it attempts to decode the GSM frequency correction (FCCH) and synchronization (SCH) channels in the signal. It can take several gated CDMA wait cycles for this decoding to complete. If decoding is successfully achieved, the MS 40 determines the power level of the TDMA signal and reports it to the BSS 32 along with the GSM cell identity. To determine the power level, MS 40 preferably averages the signal period over one period to reduce the effects of MS movement and channel fading. Determining and reporting the TDMA power level is preferably repeated continuously after MS 40 receives a command to do so.</p><p>According to the GSM standard, the power level for each cell monitored by the MS 40 must be determined at least once every 5 seconds, and the corresponding SCH must be decoded at least once every 30 seconds. The power level must be determined for every cell on the list of neighboring cells provided by the BSS 32 . Preferably, the MS decodes the SCH and reports the power level of only the cell that received the best signal. Most preferably, the MS reports to the BSS 32 only if there is a change in the determined power level since the most recent report or any other significant change in signals received by the MS from the monitored cells.</p><p>Based on this information, the BSS determines whether and when handover should be performed. At the appropriate time, the BSS 32 initiates a handover request to the MSC 24 . The MSC forwards the handover request to the GSM BSS 30, which acknowledges the request. Then, when the GSM BSS 30 sends an RR handover command to the MS 40 via the MSC 24 and the CDMA BSS 32, a new traffic channel (TCH) is established between the BSS 30 and the MS. do. At this point the handover is complete, and the MS 40 switches to the BTS 78 . In effect, according to the GSM messaging standard, after a successful handover is reported to the MSC 24, the MSC issues a "clear"</p><p>Preferably, the new traffic channel is established in an asynchronous handover mode according to the allowed GSM handover methods, and the GSM BSS 30 is configured to accommodate such handover. Preferably, the MS 40 responds to the RR handover command with a handover access burst on the primary working control channel (DCCH) of the GSM BSS 30, as indicated by the handover command. The MS then waits to receive an appropriate physical information message from the BSS 30 on its TCH, as specified in GSM standard 04.08, to complete its handover. If the physical information is not received within a predetermined time period, preferably 320 ms according to the T3124 timer of the IS-95 standard, the MS attempts to resume the connection to the CDMA BSS 32 .</p><p>The decision to initiate a handover may be made at any time if the signal from the GSM BTS 78 is stronger than the signal from the CDMA BTS 76, but preferably other criteria apply. For example, it is desirable that a handover be initiated only when the GSM signal is stronger than the CDMA signal by a predetermined weighting factor, since CDMA channels typically provide better transmission quality than GSM channels. The factor may be pre-programmed within the system 20 or set by the user of the MS 40 . It may also be dynamically adjusted according to parameters such as the geographic location of the MS and the relative amount of traffic on the system's CDMA and TDMA channels.</p><p>4C and 4D are blocks schematically illustrating the structure of IS-95B frames 81 and 87 individually used by MS 40 to decode and monitor the power of TDMA cells, in accordance with a preferred embodiment of the present invention. it is do Monitoring frames 81 and 87 are interspersed with normal CDMA communication frames 82 with a repetition rate of no more than one monitoring frame in 480 ms. According to the IS-95B standard, the monitoring frame may have a duration of 20 ms or 40 ms. If necessary, a longer monitoring period may be used. Choosing a shorter (20 ms) frame reduces data loss in a CDMA call that is performed concurrently between MS 40 and BSS 32, but increases the length of time required to complete the decoding and monitoring cycle.</p><p>4C shows a monitoring frame 81 used to acquire FCCH and SCH of a specific TDMA cell of interest. In an initial interval 83, MS 40 adjusts its receiver frequency, typically by adjusting an appropriate phase-locked loop (PLL) to match the frequency of the TDMA cell. In the next interval 84, the MS adjusts its receiver gain for the signal received from the TDMA cell, typically using automatic gain control (AGC). Methods suitable for PLL and AGC tuning are known in the prior art. Preferably, the duration of each interval 83 , 84 is about 1 ms. Then, as described above, the FCCH and SCH of the obtained TDMA cell are decoded for about 15 ms or 35 ms depending on whether the total duration of the frame is 20 ms or 40 ms. Then, for the next CDMA frame 82, the MS 40 readjusts its frequency to its previous (CDMA) settings, and then resynchronizes to the CDMA BTS 76 in the last interval 86.</p><p>4D shows a monitoring frame 87 used to measure the power level of a TDMA cell of interest. As described above, for each such cell, the frequency of the MS 40 is adjusted in an initial interval 83 . Then, the cell power level is determined during the corresponding energy measurement period 88, which preferably has a duration of about 1.4 ms. In the example shown in Figure 4D, the duration of frame 87 is 20 ms, during which power levels can be determined for 7 different cells. Alternatively, if a frame of 40 ms is used, the power levels for 15 different cells can be determined during that frame.</p><p>In an embodiment of another method not shown in the drawing, one monitoring frame may be divided into two or more parts, that is, one part for acquiring FCCH and SCH, and the other part for energy measurement. Another method embodiment may be based on the IS-95C or IS95-Q CDMA standard.</p><p>5A, 5B, 6A and 6B are flowcharts schematically in the form of a state machine for operations involved in performing the handover shown in FIGS. 4A and 4B according to a preferred embodiment of the present invention. 5A and 5B show the states of the MS 40, and FIGS. 6A and 6B show the states of the GSM-CDMA BSS 32. As shown in FIG. In these figures, solid lines represent processes performed using IS-95 gating, as described above, and the MS switches between CDMA and TDMA reception. The dotted line indicates the different state transitions possible when the MS can simultaneously perform CDMA/TDMA operation, in which case, typically (unlike the single-transceiver MS shown in FIG. 2B ) the MS must have dual radio transceivers. . The states of the GSM-TDMA BSS 30 are not shown, as they in fact conform to the GSM standard known in the prior art.</p><p>Specific messages transferred between the MS 40 and the BSS 30 , 32 in the course of the handover process are indicated along lines connecting the relevant states of the BSS 32 and the MS 40 in the figures. Preferably, these messages are in the conventional form of standard IS-95 messages or GSM messages, suitably modified and/or supplemented to convey the additional information that needs to be conveyed in the hybrid GSM-CDMA system 20 . . Although specific exemplary messages and message formats are described herein, virtually any message field appropriately assigned may be used within the constraints of the relevant IS-95 and GSM standards.</p><p>At the beginning of the handover process, in state 100 of MS and state 130 of BSS, MS 40 communicates with BSS 32 via a CDMA traffic channel (TCH). After the BSS issues a search gating command including the gating parameters, it waits for gating to complete in state 134 . MS 40 checks its parameters in state 102 . If the MS is not configured to support those parameters, the MS issues a gating reject message. If the parameters are supported, the MS issues a gating complete message and enters the IS-95 gating state 104 . When a stop gating command is received, MS 40 returns to state 100 .</p><p>Upon receiving the gating complete message, the BSS 32 enters the IS-95 gating state 136 and instructs the MS 40 to initiate monitoring of adjacent cells. (As mentioned above, if the MS is capable of concurrently performing CDMA/TDMA operation, the gating state 103, 136 is not needed, in which case the MS enters directly from state 100 to state 106. .) The BSS then enters state 132, where it waits for monitoring to complete. The MS checks its monitoring command parameters in state 106 . If it is determined that the MS supports its monitoring command parameters, the MS 40 enters the GSM monitoring state 108, in which it periodically decodes and determines the signal strength of adjacent cells, as described above. Likewise, upon receiving confirmation from the MS that it has initiated monitoring of neighboring cells, the BSS 32 enters its respective GSM monitoring state 138 .</p><p>The MS 40 continuously monitors the neighboring cells and reports the results to the BSS 32 in the form of a Pilot Strength Measurement Message (PSMM). When the handover trigger condition is established, i.e., if the signal received by the MS 40 from the BSS 32 is weaker than that of the neighboring cells, the BSS informs the MSC 24 that a handover is necessary and goes to the standby state 140. enter According to the GSM standard, if a handover command is not received within a predetermined period preferably determined by the GSM timer T7, the BSS returns to state 138. When a handover command is received from the MSC, the BSS 32 transfers the RIL3-RR handover command to the MS 40, and then enters another standby state 142, in this state, the layer of the command from the MS. 2 (L2) Wait for acknowledgment. BSS 32 may receive a handover command while in state 138 , in which case it similarly issues a RIL3-RR handover command to MS 40 and enters state 142 .</p><p>When the MS 40 receives the RIL3-RR handover command, the MS checks the handover command parameters in state 110 . If the MS 40 supports the handover command parameters, the MS sends an L2 acknowledgment to the BSS 32 and enters the CDMA pause state 112 . If the parameters are not supported, the MS 40 provides a handover failure message and returns to state 108 . In this case, or if no acknowledgment is received within a predetermined period preferably determined by the GSM timer T8, the BSS 32 sends a handover failure message to the MSC 24 and returns to state 138. .</p><p>If the parameters are supported and the handover command indicates to the GSM-TDMA BSS 30 that the MS should be handed over, then the MS sends a handover access message and then goes from state 120 to BSS 30 Waiting for physical information from (If the handover command specifies that the MS should be handed over to another CDMA BSS, the MS enters state 114, as further described below with reference to FIGS. 12 and 13 .) In the meantime, the BSS 32 waits for a "clear" command in state 144, sending periodically "clear request" messages to the MSC 24 .</p><p>When the physical information is received, the handover is successfully completed, and the MS 40 enters the GSM traffic channel communication state 124 . When BSS 32 receives its clear command, it enters state 148, in which it releases radio resources allocated to a communication channel with MS 40, and sends a "clear complete" message. do. The BSS enters the SCCP release state 150 , releases call resources used while communicating with the MSC 24 in this state, and terminates the connection with the MS 40 in the end state 152 .</p><p>However, if the MS 40 does not receive physical information within a certain period given by the expiration of the GSM T3124 timer, the MS enters state 122, where it attempts to reacquire the CDMA BSS 32 and state state 122. (100) is entered. When the handover failure message is issued to the BSS 32 , the BSS 32 enters the corresponding CDMA reacquisition state 146 . If reacquisition fails, BSS 32 issues a clear request, returns to state 144, and eventually exits from that state to state 152, as described above. The MS moves to standby (126).</p><p>TDMA TO CDMA Base Station Handover</p><p>7 is a schematic block diagram illustrating the signal flow of the system 20 (FIG. 1) related to providing temporal information to the associated GSM BSCs and BTSs of the system, in accordance with a preferred embodiment of the present invention. Essentially, no time information is provided in the GSM BSS of the system 20, since no time information is required according to the GSM standard. However, according to IS-95, CDMA base stations need to be synchronized, as such synchronization is essential for identification and decoding of signals and for soft handover between cells. Thus, in case of busy handover of MS 40 from TDMA BTS 78 to CDMA 76 (opposite the direction of the handover arrow in FIG. 4A ), the time information needs to be provided by the system 20 . there is</p><p>According to the method of FIG. 7, there is no need to change the MSC 24, the GSM BSS 30, or the BTSs 78 and 80 in hardware or software, and CBC (a standard part of the PLMN that broadcasts time information through the system) 28) can be used to provide date information to the system 20 . Typically, CBC 28 provides Cell Broadcast Service (CBC) in accordance with GSM interface standards 03.41 and 03.49, and unacknowledges broadcasts of generic short messages to geographic areas defined within system 20. can do. These messages are received by the MS when the MS is on standby, ie the MS is not engaged in a phone call. However, in order to provide date and time information, the MS 40, as already prescribed by the GSM standard, not only when the MS is in the standby mode, but also when the MS is in the talk mode, i.e., during a phone call (but It is desirable to be able to receive CBS messages (even at the cost of data loss due to itself). The use of CBS to provide temporal information to MS 40 is particularly desirable when the MS includes only a single radio transmitter and receiver, as shown in FIG. 2B. In the case of using dual radios, that is, a CDMA radio and a TDMA radio, time information can be received through the CDMA radio while using the TDMA radio for making a phone call.</p><p>* In a preferred embodiment of the present invention, as described above with reference to FIG. 4B , the CBS is also used to initiate the discovery of neighboring cells by the MS 40 .</p><p>A special MS 60 equipped with a global positioning system (GPS) receiver 161 is located within one or more GSM/TDMA cells in the system 20 for which temporal information is required. In FIG. 7, MS 160 receives time information from receiver 161 and combines the time with the current TDMA frame number, based on synchronization signals transmitted by BTS 78, according to the GSM standard. make it Alternatively, MS 160 may be configured to receive temporal information from a CDMA BSS, in which case GPS receiver 161 is not required. MS 160 establishes a data call to CBC 28 via BTS 78, BSC 77, MSC 24 and PSTN/PDN 48 to this CBC, including the cell identifier, current date and time and Transmits the correspondence of the frame number. Alternatively, MS 160 may communicate the information in any other suitable manner, such as using GSM SMS. The CBC 28 then transmits the information to the cell via the CBS so that the MS 40 receives the date and time even if the MS 40 is operating in GSM/TDMA mode. Therefore, when the MS 40 is to be handed over to the CDMA BTS 76, there is no need to obtain synchronization/time information from the CDMA BTS, so that the handover can proceed quickly and smoothly.</p><p>Providing the system 20 with a date and time has advantages for the GSM portion of the system itself, regardless of CDMA handover. For example, MS 40 can transmit its date and time to different GSM BTSs 78, 80, which can be used to determine the location of the MS by measuring the timing delay from that MS to each BTS.</p><p>8 is an overlapping GSM/TDMA cell 162 in network 20, illustrating aspects of a busy handover from a GSM BTS 78 to a CDMA BTS 76, in accordance with a preferred embodiment of the present invention; A schematic map of a CDMA cell 164 . The operator of the system 20 knows that a TDMA/CDMA handover may occur if the MS 40 is located within any one of the cells 1-5 shown in FIG. Accordingly, the CBC 28 broadcasts a CBC message containing the following information and instructions to all dual-mode (GSM/CDMA) MSs within these cells.</p><p> Let the MS initiate a search for a CDMA signal (seek trigger)</p><p>- Frequency of CDMA BTSs in overlapping and adjacent cells</p><p>- GSM mapping of CDMA cells 94 according to GSM MSC 24</p><p> An identifier of the temporal information with the current TDMA frame number, preferably derived from the MS 90, although other methods may also be used to provide the temporal information.</p><p>Optionally, as described above, a factor by which the strength of the CDMA signal must be multiplied for comparison with the TDMA signal.</p><p>There is no need to broadcast such a message within cells 6-10. Also, only the dual-mode MS is programmed to receive and interpret the message, and normal GSM/TDMA MSs ignore the message. Unlike the hybrid GSM/CDMA system proposed in the prior art, by CBS message to collect information and provide it to the GSM BSS 30 and MSC 24 to assist handover to one of the CDMA BSSs. Dual-mode MSs are triggered.</p><p>9 is a block diagram illustrating a signal flow diagram in system 20 related to busy handover from BTS 78 to BTS 76, in accordance with a preferred embodiment of the present invention. As mentioned with reference to FIG. 7 , the handover starts with the transmission of a discovery trigger and other information. A search trigger is transmitted periodically by the BTS 78 whenever the MS 40 is within one of the GSM cells 1-5 (FIG. 8), or in response to some other pre-programmed condition. .</p><p>Upon receiving the trigger, MS 40 cuts off TDMA traffic from itself and BTS 78 for a short period, preferably about 5 ms, and tunes its receiver to the appropriate CDMA frequency. Thereafter, when the MS resumes communication with the BTS 78, the MS uses the CDMA signal it received to identify the BTS transmission it received, e.g., the BTS pilot beam from the BTS 76. try to decode As noted above, CDMA BTS 76 is mapped within system 20 as if it were a GSM-TDMA BTS. Accordingly, the MS 40 reports, along with the BTS 76's GSM system map identifier, indicating the power of the signal received from the BTS 76 (optionally multiplied by the relative CDMA/TDMA weighting factors mentioned above). Retransmit the message to the GSM BTS 78. From the perspective of the GSM BSS 30 and MSC 24, there is virtually no difference between the message transmitted by the MS 40 in this case and the message transmitted as a result of the original GSM neighbor scan.</p><p>This measurement and reporting process continues until the BSS 30 determines that the MS 40 should be handed over to the BTS 76 . At this point, the BSS 30 delivers a message to the MSC 24 indicating that a handover is required. When the MSC 24 forwards the handover request to the BSS 32 , the BSS 32 forwards an acknowledgment back to the BSS 30 via the MSC 24 . The BSS 32 allocates hardware and software resources to the communication traffic channel to be established with the MS 40, and initiates null data transmission to the MS to establish the channel. The GSM BSS 30 then passes to the MS 40 a handover command, preferably a RIL3-RR command encapsulating the IS-95 parameters needed to establish a CDMA traffic channel with the CDMA BTS 76. . The parameters contained in such a message are further described below with reference to FIGS. 13 and 14a-d. After that, a new traffic channel is established, and when the handover is completed, the BSS 30 disconnects the previous TDMA traffic channel.</p><p>Therefore, by the above-described process, the handover during the call from the GSM/TDMA BSS 30 to the CDMA BSS 32 can be performed at high speed with minimal service disruption and high reliability during the handover during a call. For this handover, the GSM cell in the system receives the temporal information, and the CDMA cell is mapped to the GSM system with minimal hardware cost without virtually reprogramming the existing GSM system components.</p><p>A similar TDMA-CDMA handover process can be performed even in the absence of time information in the GSM BSS 30 . In this case, when the MS 40 acquires the pilot channel signal associated with the BTS 76, the MS 40 must tune to the BTS's CDMA sync channel and decode the channel to derive the temporal information. This operation takes about 480 ms and causes only noticeable but tolerable disturbances in the voice service during a call. Alternatively, as described above, a similar handover process can be performed using an MS with two transceivers, one for TDMA and one for CDMA.</p><p>10A, 10B and 11 are flowcharts schematically illustrating the operations of the MS 40 and the BSS 32 in the form of a state machine when performing the handover shown in FIG. 9 according to a preferred embodiment of the present invention. 10A and 10B relate to MS 40 , and FIG. 11 relates to BSS 32 . BSS 30, in effect, operates according to the GSM standard known in the art.</p><p>The MS 40 starts in an initial state 170, in which the MS is communicating with the BSS over a GSM traffic channel (TCH), within a particular cell associated with the BSS 30 . When the MS moves into a new cell, the MS enters state 172 , in which the MS receives and reads a message from the CBC 28 . If there is no CBC message to prepare the MS 40 handover to the CDMA BSS (eg, there is no CDMA BSS in the area), the MS returns to the GSM TCH state 174, from which It may be handed over to GSM-TDMA BSS.</p><p>When prompted by the appropriate CBC message, the MS 40 enters the overlay state 176, in which it acquires the temporal information as described above and sends a pilot strength measurement message (PSMM) to the BSS 30. In standard GSM-TDMA operation, there is generally a free time slot of 6 ms once every 120 ms. During these freetime slots, the MS 40 stops TDMA transmissions to search for pilot beams of adjacent GSM-CDMA cells, such as those associated with the BSS 32 . If no pilot is found, the MS enters state 180, in which it adjusts its frequency and attempts to search for an appropriate GSM frequency correction channel (FCCH). Alternatively, if a pilot is found, the MS enters state 182, where it adjusts its frequency to the required degree and measures the CDMA signal strength. During subsequent slots, MS 40 attempts to decode the CDMA pilot to identify the cell associated with that pilot, communicating over its current GSM-TDMA traffic channel. The results are reported to the BSS 30 .</p><p>As described above, based on the results reported by MS 40 , at the appropriate point in time, MSC 24 forwards a handover request to BSS 32 . The BSS enters the ready state 190, in which it allocates resources, allocates a long code, and establishes an SCCP connection with the MSC in preparation for handover. After sending the appropriate acknowledgment message to the MSC, the BSS 32 enters state 191, in which it sends null forward traffic frames to the MS 40 and waits for reception of reverse traffic from the MS. However, if the BSS fails to allocate resources, the BSS reports a handover failure and exits to the end state 197 .</p><p>Based on the parameters encapsulated in the acknowledgment message from the BSS 32, the RIL3-RR Handover Command message identifying the GSM-CDMA destination cell associated with the BSS 32 and passing the necessary handover parameters is a GSM-TDMA It is transmitted from the BSS 30 to the MS 40 . MS 40 enters state 183 , in which state it verifies that its handover parameters are supported, and if the verification succeeds, suspends its GSM-TDMA operation in state 184 . (If the verification fails, the MS reports the failure and returns to state 176.) The MS then enters state 185, in which state from the BSS 32, preferably the IS- It waits for reception of a predetermined number of good frames, the number determined by the 95 counter N11m. If good frames are received, the MS sends back to the BSS a number of preamble frames (short dummy frames used to establish the traffic channel) specified by the NUM-PREAMBLE parameter in the handover command message, and the service option adjustment status ( 186) is entered. BSS 32 detects the preamble frames and reports to the MSC that the CDMA traffic channel has been established, then enters state 192, where it waits for the completion of the handover.</p><p>When the MS 40 and the BSS 32 cannot establish communication, the handover to the BSS 32 is stopped, and the MS 40 and the BSS 32 return to their initial states. The MS 40 attempts to reacquire the GSM BSS 30 in state 188 , and if the attempt is successful, returns to the GSM TCH state 170 . If reacquisition fails, the MS exits to standby mode (189). In either case, after the BSS 32 receives the clear command and releases all the resources it has allocated to the MS 40 in the state 193, the BSS 32 exits to the end state 197.</p><p>However, assuming that the handover has completed successfully, the BSS 32 enters a service option adjustment state 194 corresponding to the state 186 of the MS 40 . The service request is issued by the BSS 32 , and the BSS waits for a service response from the MS 40 in the standby state 195 . When a service response is received, MS 40 and BSS 32 enter respective CDMA traffic channel (TCH) states 187 and 196, and the call proceeds normally over the CDMA channel.</p><p>CDMA TO CDMA Base Station Handover</p><p>12 is a schematic block diagram illustrating handover between two different CDMA BSSs 201 and 203 within system 20, in accordance with a preferred embodiment of the present invention. BSS 201 includes a BSC 202 and a plurality of BTSs 206,208, and BSS 203 includes a BSC 204 and a plurality of BTSs 210,212. The BSSs 201 and 203 are substantially similar to the BSS 32 shown in FIG. 1 described above, are interchangeable, and communicate with the GSM MSC 24 via the GSM A-interface. The figure shows the MS 40 in handover from the BTS 208 to the BTS 210 under the control of the MSC 24 . From a system point of view, a handover is made between two CDMA BSSs, but this is a handover between two GSM BSSs, each of which BTSs 208 and 210 are mapped to a GSM cell by the MSC 24 .</p><p>13 is a schematic diagram illustrating signal flow between components of the system 20 shown in FIG. 12 during handover, in accordance with a preferred embodiment of the present invention. Before initiating the handover, if the BSS 201 issues a search trigger to the MS 40, the MS searches the CDMA transmit frequencies of neighboring cells, preferably using IS-95 gating, as described above. . When the MS 40 reports to the BSS 201 that it is receiving a signal from the BTS 210 that has a higher power level than that of the BTS 208, a handover is triggered.</p><p>Upon receiving the report from MS 40, BSS 201 sends a Handover Request message to MSC 24 specifying the GSM cell identifier of BTS 210 as a new cell assignment required for handover. The message usually conforms to the GSM standard. The communication CDMA data rate between MS and BSS may be 8 kbit/sec (rate set 1) or 14.4 kbit/sec (rate set 2) according to the IS-95 standard, and the IS-95 data rates are respectively GSM 1/2 It is preferably conveyed in the message by indicating a rate traffic channel and a full-rate traffic channel. When the GSM traffic channel rate is passed to the BSS 203, the BSS interprets the rate and selects an appropriate IS-95 data rate.</p><p>When the MSC 24 sends a handover request to the BSS 201, the BSS responds to the MSC by sending an acknowledgment encapsulating a RIL3-RR handover command message, which in turn to the BSS 201. is transmitted to Thus, all messages between BSS 201 and 203 conform to A-interface requirements and CDMA parameters related to IS-95, such as mapping of CDMA's 13K QCELP vocoder type identifier to GSM full rate vocoder, for example. are mapped to the corresponding GSM parameters. Handover requests, acknowledgments and commands are passed by the MSC 24 virtually unchanged.</p><p>Upon receiving the handover command, the old BSS 201 sends an RR handover command message to the MS 40 to be handed over to the new BSS 203 . The message to the MS 40 encapsulates the following CDMA parameters necessary for handover according to the IS-95 standard, which parameters are not limited to the following.</p><p>- Preferably distributed by the BSS 203 from a pool of available numbers, such that the mask values used in the common service area are as far apart as possible from each other and that no two MSs in that area have the same mask. A new long code mask. An exemplary method of long code mask distribution is described below with reference to FIGS. 14A-D . In the standard IS-95 cellular system, the MS's longcode mask is fixed and delivered to the new BS during handover, but the GSM standard provides a message that can be used to forward the longcode mask to the new BS (203). I never do that. Thus, BS 203 needs to distribute its new longcode mask and forward the message back to MS 40 via BS 201, preferably in an RR handover command, as described herein.</p><p> A means for the MS to set the power level of the signal to transmit to the BSS 203, providing a correction factor to be used by the MS 40 in an open-loop power estimation, preferably as specified in the IS-95 standard. Nominal power level parameters, usually NOM-POWER and NOM-PWR-EXT.</p><p>Delay of forward traffic channel frames and reverse traffic channel frames transmitted to and received from MS 40 with respect to the system timing of system 20, preferably a parameter indicating the delay at intervals of 1.25 ms frame offset. This frame offset is conveyed from the BSS 201 to the BSS 203 in a handover command message. An optional ACTIVE_TIME parameter may also be included to indicate when this delay should be inserted.</p><p> A code channel similarly conveyed from BSS 201 to BSS 203 to indicate the Walsh function to be used to encode the forward traffic channel from BSS 203 to MS 40, according to the IS-95 standard.</p><p>- Layer 2 acknowledgment numbering that can be used by the BSS 203 to reset the acknowledgment processing by the protocol layer 2 of the MS 40, preferably at the time specified in the handover command message.</p><p> Forward traffic channel power control parameters, used by the BSS 203 to reset the TOT_FRAMES and BAD_FRAMES made by the MS 40, for the purpose of reporting the forward channel error statistics to the BSS.</p><p>* Number of preambles indicating the number of preamble frames to be transmitted by MS 40 to BSS 203 after the MS receives N11m good frames from the BSS, as described above with reference to FIG. 10B .</p><p> New band class (frequency domain) and frequency (within this domain) of the cell associated with the BSS 203 to which the MS 40 is currently assigned.</p><p>The parameters listed above are not comprehensive, but are shown only as representative samples of information to be transmitted in the handover command message. Similarly, other IS-95 parameters may be included in the message. More generally, the method exemplified by the handover command described above in which data relating to one air interface (GSM/TDMA or CDMA) in system 20 is conveyed by a message transmitted over the other air interface, It can be used in a similar manner to carry other types of messages and data.</p><p>When the RR handover command is sent to the MS 40 , a new traffic channel is established between the BSS 203 and the MS 40 . To establish the channel, the BSS 203 sends traffic channel frames to the MS 40, which responds with the appropriate number of preamble frames specified by the handover command message. Thereafter, according to the de facto GSM messaging standard, after successful handover to the MSC 24 is reported, the MSC issues an appropriate "clear" command to the previous BSS 201, and the BSS responds with a "clear complete" command. do.</p><p>14A-D are block diagrams schematically illustrating 42-bit longcode masks distributed by the BSS 203 in connection with the handover shown in FIG. 12, in accordance with a preferred embodiment of the present invention. 14A shows a mask 220 used in an access channel, FIG. 14B shows a mask 222 used in a paging channel, and FIG. 14C shows a mask 224 used in a primary (forward and reverse) traffic channel , FIG. 14D shows a mask 226 used for secondary (forward and reverse) traffic channels. Such auxiliary channels are used for multi-channel medium data rate (MDR) communication, as specified by, for example, the IS-95B standard.</p><p>Preferably, the access channel mask 220 includes an access channel number 228, a paging channel number 230, a base station identification number (ID) 232 of the BSS 203, and a pilot beam offset 234; , all of which are effectively assigned according to the IS-95 standard. The paging channel number and pilot beam offset are similarly included in the paging channel mask 222 .</p><p>Traffic channel masks 224 and 226 represent a common longcode mask format. Preferably, they contain a unique 16-bit number 236 and a base station ID 232 selected from the pool assigned to the BSS 203 . Pool number 236 is assigned such that no two MSs have the same longcode mask, as described above. For superior call security, a personal longcode mask may be used instead of the masks 224 and 226 . An example of creating such masks, using the GSM encryption code Kc, is assigned to the assignee of the present invention and is filed October 21, 1998, entitled "Encryption Support in a Hybrid GSM/CDMA Network", which is incorporated herein by reference only. described in the patent application.</p><p>The operation of BSS 201 and BSS 203 when performing the handover shown in FIG. 12 can be schematically represented by state machines substantially similar to those shown by FIGS. 6A/6B and 11 , respectively. The operation of MS 40 in this handover is generally similar to that shown in FIGS. 5A and 5B , up to state 112 where CDMA communication with BSS 201 is suspended. As MS 40 establishes a new traffic channel with CDMA BSS 203, it goes through states 114, 116, 118, which are respectively equivalent to states 185, 186, 187 shown in FIG. 10B. do. If the MS 40 fails to acquire a new traffic channel while in state 114 , the MS goes to state 122 and attempts to reacquire the old BSS 201 .</p><p>The method described above mainly relates to hard handover between two different BSSs 201 and 203 under the control of the MSC 24 . Preferably, in system 20, soft handover according to the IS-95 standard may also be performed between BTSs involved in a single BSC, such as BTSs 206 and 208 shown in FIG. Optionally, between the BSSs from the BTS 208 to the BTS 210 (not shown in the figure), even if the BSC 202 is properly linked to the BSC 204, typically by a connection separate from the MSC 24 (not shown). - Soft handover may occur. In that case, the BSS 203 notifies the MSC 24 that a handover has occurred, so that the new location of the MS 40 is properly registered.</p><p>One of the problems present when attempting to measure the amount of power transmitted from a GSM system is that the timing of the GSM system must be determined. For example, when attempting to perform a handover from a system using a CDMA multicarrier (MC) air interface to a GSM system, such as a GSM system, as provided in 3rd generation CDMA systems, commonly known as "3G" systems, Only when the timing of the GSM system has been determined, power measurements can then be made and reported. This is because, due to the frequency reuse method used in GSM, the MS making the measurement must be able to read the sync channel while the base station identification code (BSIC) is being transmitted. Such a BSIC is transmitted approximately every 10 GSM frames (approximately every 46 milliseconds). According to the GSM industry standard requirements, the MS shall report the BSIC along with the Measured Average Power Level (RXLEV) for each GSM signal to be measured. One way to determine the timing is to provide the MS 40 from the MC base station (MC-BS) with information comprising a GSM frame number that uniquely identifies the temporal instant at which the sync channel is transmitted by the GSM-BSS. . A frame number valid in one GSM-BSS at a specific time is not the same as a valid number in another GSM-BSS in the same system. This is intentionally done to allow GSM MSs to monitor neighboring cells during the TDMA wait period. Thus, at any time instant, the GSM frame number in each GSM-BSS is different.</p><p>According to an embodiment of the method and apparatus disclosed herein, the provided information includes:</p><p>(1) CDMA time</p><p>(2) Indicator of the number of GSM channels to search</p><p>(3) a received signal strength threshold, and</p><p>(4) Information on each channel to be searched</p><p>includes</p><p>In one embodiment of the disclosed method and apparatus, the information about each channel includes: </p><p>(1) Frequency band including the channel to be searched</p><p>(2) the frequency of the channel to be searched (such as "AFRCN" specified in industry standards related to GSM communication systems);</p><p>(3) an identification code associated with that channel (such as a base station identification code (BSIC) specified in industry standards related to GSM communication systems);</p><p>(4) the frame number (such as the GSM frame number stipulated by industry standards related to GSM communication systems) transmitted at the identified CDMA time; and</p><p>(5) a specific portion of a frame transmitted at that identified CDMA time;</p><p>includes</p><p>In another embodiment of the disclosed method and apparatus, the first three bits of the BSIC identifying the network color code are transmitted once for every channel to be searched.</p><p>The following describes how that information is used to reduce the amount of time required to determine if there is an appropriate candidate base station from which a handover can be made.</p><p>15 is a flowchart illustrating a process that occurs when the MC-BS 1501 wants to determine whether it is beneficial to perform a handover. The process described below, shown in FIG. 15 , may be performed in response to a determination that the signal currently supporting communication to the MS is too weak, or may be performed according to another triggering event.</p><p>The process begins with a Candidate Frequency Search Request message 1503 transmitted from MC-BS 1501 to MS 1505 . In one embodiment of the disclosed method and apparatus, the candidate frequency search request message has the following format including the fields shown in Tables 1-3.</p><p>table 1</p><p><img file="KR20070103489A_D0001.tif" /></p><p>According to this embodiment, each field is defined by an industry standard for a CDMA2000 system. However, in one embodiment of the disclosed method and apparatus, an additional search mode is defined. This additional discovery mode requests discovery for GSM channels.</p><p>When the search mode field requests a search for a GSM channel, the following field is transmitted.</p><p>table 2</p><p><img file="KR20070103489A_D0002.tif" /></p><p>The definitions of the fields shown in Table 2 are as follows.</p><p>SF_TOTAL_EC_THRESH Frequency Total Pilot Ec Threshold in Service.</p><p>If the mobile station does not use the measurement of the total Ec of pilots in the serving frequency active set of the GSM frequency in the periodic search procedure of the GSM frequency, the base station sets this field to '11111'. Otherwise, the base station</p><p>(10 × log<sb>10</sb> (<i>total</i><i>_</i><i>ec</i><i>_</i><i>thresh</i>) + 120)/2 」</p><p>set to, where <i>total</i><i>_</i><i>ec</i><i>_</i><i>thresh</i> is defined by the following rule: the total Ec of the pilots in the serving frequency active set is <i>total</i><i>_</i><i>ec</i><i>_</i><i>thresh</i> greater than, the mobile station is not looking for a GSM frequency.</p><p>SF_TOTAL_EC_IO_THRESH Frequency In Service Total Pilot Ec/Io Threshold</p><p>If the mobile station does not use the measurement of the total Ec/Io of pilots in the serving frequency active set of the GSM frequency in the periodic search procedure of the GSM frequency, the base station sets this field to '11111'. Otherwise, the base station sets this field</p><p>(-20 × log)<sb>10</sb> (<i>total</i><i>_</i><i>ec</i><i>_</i><i>io</i><i>_</i><i>thresh</i>)」</p><p>set to, where <i>total</i><i>_</i><i>ec</i><i>_</i><i>io</i><i>_</i><i>thresh</i> is defined by the following rule: the total Ec/Io of the pilots in the serving frequency active set is <i>total_ec_io_thresh</i> greater than, the mobile station is not looking for a GSM frequency.</p><p>GSM_RXLEV_THRESH GSM RXLEV threshold</p><p>The base station sets this field to the minimum GSM RXLEV the mobile station is allowed to report on. This GSM RXLEV is specified in section 8.1.4 of GSM 05.08.</p><p>GSM_T_REF_INCL with GSM time reference</p><p>This field indicates whether a GSM time reference is included in this message.</p><p>If a GSM time reference is specified in this message, the base station sets this field to '1'. Otherwise, the base station sets this field to '0'.</p><p>CDMA_TIME A selected point in CDMA time at which the MC-BS knows the frame number and frame portion transmitted by each of the GSM-BSSs that the MC-BS asks the MS to search for.</p><p>If GSM_T_TEF_INCL is set to '1', the base station sets this field to the CDMA system time in units of 80 ms (modulo 64) referred to by the GSM-FRAME. If the USE_TIME field is set to '0', the base station omits this field.</p><p>NUM_GSM_CHAN Number of GSM channels</p><p>The base station sets this field to the number of GSM ARFCNs to be searched for.</p><p>GSM_FREQ_BAND GSM frequency band</p><p>According to an embodiment of the disclosed method and apparatus, the following values are transmitted to indicate a specific GSM frequency band.</p><p>table 3</p><p><img file="KR20070103489A_D0003.tif" /></p><p>ARFCN absolute radio frequency channel number</p><p>The base station sets this field to the absolute radio frequency channel number to search as specified in section 2 of GSM 05.05.</p><p>BSIC_VERIF_REQ Base Transceiver Station Identification Code Verification Request</p><p>When the base transceiver station identification code check is requested for the corresponding ARFCN, the base station sets this field to '1'. Otherwise, the base station sets this field to '0'.</p><p>BSIC base transceiver station identification code.</p><p>If BSIC_VERIF_REQ is set to '1', the base station sets this field to the base transceiver station identification code of the GSM channel to be searched as specified in section 4.3.2 of GSM 03.03. If BSIC_VERIF_REQ is set to '0', the base station omits this field.</p><p>GSM_FRAME The GSM frame number of the frame transmitted on that channel at the time identified within the corresponding CDMA time field.</p><p>If GSM_T_REF_INCL is set to '1', the base station sets this field to a valid GSM frame number at the time specified by CDMA_TIME in the GSM target base station as specified in section 3.3.2.2 of GSM 05.02. If GSM_T_REF_INCL is set to '0', the base station omits this field.</p><p>GSM_FRAME_FRACT A GSM frame fragment transmitted on that channel at the time identified in the corresponding CDMA time field.</p><p>If GSM_T_REF_INCL is set to '1', the base station sets this field to the GSM target base station with a range of 0 to (2^9-1) of 1/2^9 fragments of a valid GSM frame at the time identified by CDMA_TIME. set by number. The GSM frame duration is specified as 24/5200 seconds in section 4.3.1 of GSM 05.02. If the GSM_T_REF_INCL field is set to '0', the base station omits this field.</p><p>Upon receiving the candidate frequency search request message 1503, the MS 1505 preferably estimates the amount of time required for the MS 1505 to perform the requested search. Such estimation can be performed by any known method. The estimator is sent to the MS-BS in a candidate frequency search response message 1507 .</p><p>According to one embodiment of the disclosed method and apparatus, the MC-BS 1501 responds to the candidate frequency search response message 1507 by determining whether to perform a search and, if so, how to perform the search. do. For example, in one embodiment, the MS-BS 1501 indicates whether the MS 1505 should start performing a search at a predetermined start time (specified in the control message) and the search is one-time, continuous or periodic. Transmit a candidate frequency search control message indicating whether to perform</p><p>The MS 1505 responds to the control message by performing a search based on the received information. The MS 1505 uses the provided timing information (ie, the value provided in the CDMA time field) to determine when to search for each GSM signal that the MS-BS 1501 has requested the MS 1505 to search for. Identifies the time at which the part of the identified GSM frame was transmitted.</p><p>Preferably, the MS 1505 searches for each GSM only at the time that a GSM signal identifying information such as a BSIC is transmitted. The MS 1505 can then measure the signal quality and compare the BSIC to the BSIC associated with the channel that the MS 1505 was requested to search for. If a match exists, the MS 1505 reports the quality of the signal transmitted on the channel that the MS 1505 was requested to search for (such as the amount of power in the signal, signal-to-noise ratio, or other measure of signal quality). .</p><p>When the MS 1505 determines the quality of the signal transmitted on each channel that the MS 1505 is requested to search for, the MS 1505 creates a Candidate Frequency Search Report message 1511 . A candidate frequency search report message 1511 is then transmitted from the MS 1505 to the MC-BS 1501 . Depending on the content of the control message, the MS 1505 may transmit the report message 1511 repeatedly.</p><p>If MS-BS 1501 determines that the handover condition is complete, MS-BS 1501 sends the messages 1513 to GSM-BSS 1515, causing GSM-BSS 1515 to hand Be prepared to accept the over. One method used to send the messages to the GSM-BS 1515 is to encapsulate the information in a standard GSM handover message. The handover message may include timing information on when to find a synchronization channel in case there is in fact a drift in GSM timing with respect to CDMA timing. As such messages are known in the prior art, they are not described in detail here for the sake of brevity.</p><p>When the GSM-BSS 1515 receives the Handover Ready message 1513 , an MC-MAP GSM Handover Command message 1517 is sent to the MS 1505 in the conventional GSM format. The MS 1505 and GSM-BSS then exchange system acquisition and access messages 1519 in an essentially conventional manner. The MS 1505 then provides a handover complete message 1521 to the GSM-BSS 1515 . Thereafter, the GSM-BSS 1515 and the MC-BS 1501 exchange a handover completion message 1523 .</p><p>If the MS 1505 can quickly identify signals transmitted from the GSM-BSS 1515 , the MS 1505 can determine when to monitor signals transmitted by other GSM-BSS 1515 of interest. have. Also, since the candidate frequency search request message 1503 contains information about each of the channels that the MS 1505 has been requested to search for, the search for signals related to each of these channels is performed in several timeslots (these Each duration is only 0.5 milliseconds). Thus, with the disclosed method and apparatus, the MS 1505 performs a search for a handover candidate without spending much time (a few milliseconds in total) starting from the point at which the MS 1505 receives CDMA signals. can be done</p><p>Although the embodiments disclosed above refer to GSM systems, the disclosed methods and apparatus are equally applicable to any TDMA system in which information is transmitted during well-defined time slots.</p><p>Although preferred embodiments have been described above with reference to a specific hybrid GSM/CDMA system, the principles of the present invention may be similarly applied to achieve busy handover in other hybrid communication systems. Also, although the preferred embodiments refer to specific TDMA-based and CDMA-based communication standards, the methods and principles described above may be used in connection with other data encoding and signal modulation methods. The scope of the present invention encompasses not only the complete systems and communication processes described above, but also various novel components of these systems and processes and combinations and partial combinations thereof.</p><p>Accordingly, the preferred embodiments described above are merely cited as examples, and the complete scope of the present invention is limited only by the claims.</p>