HK1075982B - Handoff in a hybrid communication network - Google Patents

Description

Handover in a hybrid communication network

RELATED APPLICATIONS

This application claims priority to U.S. provisional application serial No. 60/340,242, entitled "method and Apparatus for influencing manual Between Different cellular communication Systems", filed on 12/7/2001; U.S. application attorney docket No. 020043, entitled "Method and Apparatus for influencing manual Between differential Communication Systems", filed on day 2, month 14, 2002; and U.S. provisional application serial No. 60/350,401, entitled "GSM Authentication, Encryption and OtherFeature Support in a CDMA 1x Network Using a GSM-1x MSC", filed on month 1, 17, 2002.

I. Field of the invention

The present invention relates generally to a method and system for efficient handover between different cellular communication systems.

Description of the related Art

The so-called Code Division Multiple Access (CDMA) modulation technique is one of several techniques for facilitating communications in which a large number of system users are present. CDMA has significant advantages over these other modulation techniques, although other techniques exist as well, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and modulation techniques such as amplitude companded single sideband modulation (ACSSB) AM. The use of CDMA techniques in a Multiple Access Communication system is disclosed in U.S. patent No. 4,901,307, entitled "Spread Spectrum Multiple Access Communication system using Satellite Or Terrestrial reagents," assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference.

In U.S. patent 4,901,307, a multiple access technique is described in which a large number of mobile telephone system users, each having a transceiver, communicate using Code Division Multiple Access (CDMA) spread spectrum communication signals through satellite repeaters or terrestrial base stations (also known as cell-sites).

In conventional cellular telephone systems, the available frequency band is divided into a plurality of channels, typically one channel having a bandwidth of 30KHz when using analog FM modulation techniques. The system service area is geographically divided into cells of unequal size. The available frequency channels are divided into a number of groups, each group typically comprising an equal number of channels. Frequency groups are allocated to cells in a manner that minimizes the likelihood of co-channel (co-channel) interference. For example, consider a system where there are 7 frequency groups and the cells are hexagons of equal size. The set of frequencies used in a cell will not be used in the 6 nearest or surrounding neighbors of the cell. Also, frequencies in one cell will not be used in the 12 next nearest neighbors.

In conventional cellular systems, the handoff method implemented is intended to allow a call or other type of connection (i.e., data link) to continue as a mobile station crosses the boundary between two cells. A handover from one cell to another is initiated when a receiver in a cell base station handling a call or connection notices that the received signal strength from the mobile station falls below a predetermined threshold value. A low signal strength indication means that the mobile station is definitely close to the cell border. When the signal level falls below a predetermined threshold, the base station requests the system controller to determine whether a neighboring base station receives a mobile station signal having a better signal strength than the current base station.

The base station controller sends a message with a handover request to neighboring base stations in response to the query of the current base station. The base stations adjacent to the current base station use a specific scanning receiver that searches for signals from the mobile station on a specific channel. When there is a neighboring base station reporting a sufficient signal level to the system controller, a handover will be attempted.

The handover begins when an idle channel from the set of channels used by the new base station is selected. A control message is sent to the mobile station instructing it to switch from the current channel to the new channel. At the same time, the system controller hands off the call from the first base station to the second base station.

In conventional systems, if the handover to the new base station is unsuccessful, the call will not continue. Handover failures can result from a number of reasons. Handover may fail because no free channels are available for the communication call in the neighboring cell. The failure of the handover may also be due to another base station reporting the hearing of the mobile station in question, in fact the base station is actually listening to a different mobile station using the same channel in another completely different cell. The reporting error will cause the call to be handed off to a wrong cell, typically one with insufficient signal strength to maintain communication. Furthermore, if the mobile station fails to listen to the command to switch channels, the switch will also fail. Practical operational experience has shown that handover failures occur frequently due to system reliability issues.

Another common problem in conventional telephone systems occurs when a mobile station approaches a boundary between two cells. In this case, fluctuation (fluctuation) occurs in the signal levels in both base stations. This fluctuation in signal level results in a "ping-pong" phenomenon in which repeated requests are made to switch back and forth between the two base stations. These additional unwanted handoff requests increase the likelihood that the mobile station will incorrectly listen to the channel switch command or will fail to listen to the command altogether. Furthermore, ping-ponging increases the likelihood that a call will no longer continue if the call is inadvertently transferred to a cell in which all channels are currently in use and therefore unavailable to receive the handover.

U.S. Pat. No. 5,101,501, entitled "Method And System For Providing A software Communication In A CDMA Cellular Telephone System", assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. This patent discloses a method and system for providing communications to a mobile station through more than one base station during a handoff in which the communications in a cellular system are not disrupted by an eventual handoff from the base station corresponding to the cell that the mobile station is leaving to the base station corresponding to the cell that the mobile station is entering. This type of handoff is referred to as a "soft" handoff between cell base stations communicating with a mobile station, where two or more base stations or sectors of a base station concurrently transmit to the mobile station. Techniques using such "soft" handoffs have been found to significantly reduce the impact of ping-pong phenomena in which handoff requests are repeated between a pair of base stations.

An improved Soft Handoff technique is disclosed In U.S. Pat. No. 5,267,261, entitled "Mobile Stationed Soft Handoff In A CDMA Cellular Communications System," assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. The soft handoff technique is improved by measuring at the mobile station the strength of the "pilot" signal transmitted by each base station in the system. These pilot strength measurements provide assistance in the soft handoff process by helping to identify viable base station handoff candidates.

Improved soft handoff techniques provide for the mobile station to monitor the strength of pilot signals from neighboring base stations. When the measured signal strength reaches a given threshold, the mobile station transmits a signal strength message to the system controller via the base station with which the mobile station is communicating. A command message is sent from the base station controller to a new base station and also to the mobile station to establish simultaneous communication with the current base station via the new base station. When the mobile station detects that the signal strength corresponding to at least one base station with which the mobile station is communicating has fallen below a predetermined level, the mobile station reports the measured signal strength indicating the corresponding base station to the system controller via the base station with which it is communicating. The command message from the base station controller to the identified base station and the mobile station terminates communication through the corresponding base station, while communication through another base station or other base stations continues.

Although the foregoing techniques are well suited for call transmissions between cells in the same cellular system, a more difficult situation arises when a mobile station enters a cell served by a base station from another cellular system. One complicating factor of such "intersystem" handovers is that neighboring cellular systems often have different characteristics. For example, adjacent cellular systems typically operate on different frequencies and may maintain different base station output power levels or pilot strengths. These differences effectively preclude attempts by the mobile station to perform pilot strength comparisons, etc., using existing mobile-assisted soft handoff techniques.

When there are no resources available for making soft intersystem handovers, the timing of the handover of a call or connection from one system to another becomes critical if uninterrupted service is to be maintained. That is, the intersystem handover must be performed at a time that is most likely to result in successful delivery of the call or connection between the systems. In such a handoff, referred to herein as a hard handoff, communication between a mobile station and one system must be stopped before communication between the mobile station and the other system can begin. The handover should only occur when the following conditions, including:

(i) there is a free channel in the new cell,

(ii) the mobile station is indeed within range of the new cell base station, but before it loses contact with the current cell base station, an

(iii) The mobile station is located in a position that is guaranteed to receive a command to switch channels.

Ideally, each such hard inter-system handoff would be performed in a manner that minimizes the potential for "ping-pong" handoff requests between base stations of different systems. However, such handoffs are difficult because existing handoff procedures do not identify when, by which base station, the mobile station may be provided with new frequency, channel information, and instructions to send the existing call or connection.

These and other upcoming existing inter-system handover techniques affect the quality of cellular communications and performance may further degrade as more and more cellular systems compete with each other. There is therefore a need for a handover technique that reliably leads to a handover of a call or connection between base stations of different cellular communication systems.

U.S. patent No. 5,697,055, entitled Mobile Station Assisted Soft Handoff inas CDMA Cellular Communication System, assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference, describes a method and System for inter-System Handoff for a Mobile Station communicating between base stations of first and second Cellular systems. At the mobile station, a parameter quantifiable of a signal transmitted by a second base station of the second system is measured. The mobile station transmits a signal quality message to the first mobile switching control station via the first base station of the first system when the measured value of the quantifiable parameter passes a first predetermined level.

A channel request message is then transmitted from the first mobile switching control station to a second mobile switching control station in the second system. At the second base station, a quantifiable parameter received from the mobile station is also measured. The second base station establishes communication with the mobile station when the value of the measured quantifiable parameter passes a predetermined level. Alternatively, the signal strength of a first pilot signal transmitted by a first base station is measured at the mobile station. When the measured signal strength of the first pilot signal becomes weaker than the second predetermined level, a handover request message is next transmitted to the second base station, and thus mobile station communication is established. The voice link provided between the mobile switching control stations allows forwarding an existing connection between the first and second cellular system fingers and enables the action of soft intersystem handover (performance).

This arrangement works well when both systems are CDMA based systems and are both capable of soft handoff, leaving the problem of how to effect an intersystem handoff when one or more systems are not capable of such soft handoff. For example, the standard known as GSM has no mechanism for soft handover. Thus, there is a problem in handing off a call using the air interface from the CDMA network to the GSM network. Furthermore, authentication for GSM cannot be done because the CDMA2000 mechanism cannot send the data required to do GSM authentication. The encryption in GSM differs from the encryption in CDMA 2000.

One way to solve the above problem is to improve GSM to enable efficient handover to a non-GSM system, such as a CDMA system. However, GSM has been relatively established for a long time and operators are reluctant to make expensive modifications to existing equipment to accommodate a neighbouring incompatible system. If new messages are added to the air interface to support dual mode mobile stations, improvements must be made to support these new messages. This is reluctant to do from an operator's perspective.

Another problem with switching between CDMA systems and GSM systems is that CDMA and GSM authentication use two different methods and keys. The authentication methods for GSM and CDMA 1X are basically the same, but the keys have different sizes. CDMA 1X has additional procedures such as unique challenge-response (challenge) and billing methods that prevent channel hijacking (hijaking) and playback (replay) attacks, respectively.

Summary of The Invention

The present invention solves the above problems.

According to an aspect of the present invention there is provided a method of efficiently handing over a first base station in a first cellular communication system controlled by a first mobile switching control station to a second base station in a different second cellular communication system controlled by a second mobile switching control station, the method comprising: measuring at the mobile station a parameter of a signal transmitted by the belonging first base station; measuring, at the mobile station, a parameter of a signal transmitted by the second base station; transmitting a signal quality message from said mobile station to said first mobile switching control station via said first base station when said parameter reaches a predetermined condition; generating a channel request message at said first mobile switching control station for information to be used by said second mobile switching control station; transmitting information from said first mobile switching control station to said mobile station; generating, at the mobile station, a channel request message for a second mobile switching control station from information from the first mobile switching control station; and transmitting the channel request message from the mobile station to the second mobile switching control station.

According to another aspect of the present invention, there is provided a mobile station including: a first transceiver chain for receiving and transmitting signals with a first base station in a first cellular communication system; a second transceiver chain for receiving and transmitting signals with a second base station in a second cellular communication system; and a controller for: measuring a parameter of a signal transmitted by the first base station; measuring a parameter of a signal transmitted by the second base station; transmitting signal quality information from the mobile station to the first cellular communication system via the first base station when the parameter reaches a predetermined condition; receiving information of a channel request message for a second cellular communication system from the first base station; generating a channel request message for said second cellular communication system from said information from said first base station; and transmitting the channel request message to the second mobile station.

The above and other features of the present invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of an exemplary embodiment of the invention given in connection with the accompanying drawings.

Brief Description of Drawings

In the drawings:

FIG. 1 is a schematic illustration of a cellular system;

FIG. 2 is a schematic illustration of a boundary between two cellular systems;

FIG. 3 is a schematic illustration of a dual mode mobile station; and

fig. 4 is a schematic illustration of data exchange in a GSM system;

fig. 5 is a schematic illustration of a single mode mobile station.

Detailed description of an embodiment of the invention

Fig. 1 is a schematic illustration of an example cellular telephone system. The illustrated system may be used with any kind of multiple access modulation technique for facilitating communication between a typically large number of system mobile users or mobile phones and base stations. These multiple access communication system techniques include: time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and AM modulation schemes such as amplitude companded single sideband modulation. CDMA spread spectrum modulation techniques, such as disclosed in the aforementioned U.S. patent No. 4,901,307, have significant advantages over other modulation techniques used in multiple access communication systems and are therefore preferred.

In a typical CDMA system, each base station transmits a unique pilot signal that includes transmitting a "pilot carrier" on a corresponding pilot channel. The pilot signal is an unmodulated, direct-sequence spread spectrum signal that is transmitted by each base station at all times using a common Pseudorandom Noise (PN) spreading code. The pilot signal, in addition to providing a phase reference for demodulation and a measurement reference for signal strength in handoff determinations, allows the base station to obtain originating system synchronization, i.e., timing. The pilot signals transmitted by each base station may often be the same PN spreading code, but they have different code phase offsets.

In the system shown in fig. 1, a system controller and switch 10, also known as a Mobile Switching Center (MSC), typically includes interface and processing circuitry (not shown) for providing system control for a plurality of base stations 12, 14 and 16. The controller 10 also controls the routing of telephone calls from the Public Switched Telephone Network (PSTN) to the appropriate base stations for transmission to the appropriate mobile stations. The controller 10 also includes controls routing of calls from the mobile stations to the PSTN through at least one base station site. The controller 10 may direct calls between mobile users through appropriate base stations because these mobile stations typically do not communicate directly with one another.

The controller 10 may be coupled to the base station in a number of ways, such as a dedicated telephone line, a fiber optic link, or a microwave communication link. Three such base stations, 12, 14 and 16, and an example mobile station 18, which includes a cellular telephone, are illustrated in fig. 1. Arrows 20a and 20b define possible communication links between base station 12 and mobile stations 18. Arrows 22a and 22b define possible communication links between the station 14 and the mobile station 18. Similarly, arrows 24a and 24b define possible communication links between the base station 16 and the mobile station 18.

The base station service area or cell is designed to be geographically shaped such that a mobile station will typically be closest to one base station. When the mobile station is idle, no call is occurring and the mobile station keeps monitoring the pilot signals from each of the neighboring base stations. Pilot signals, as shown in fig. 1, are transmitted by base stations 12, 14, and 16 to mobile station 18 over communication links 20b, 22b, and 24b, respectively. The mobile station then determines which cell it is in by comparing the pilot signal strengths from these particular base stations.

In the example shown in fig. 1, the mobile station 18 may be considered to be closest to the base station 16. When the mobile station 18 initiates a call, a control message is transmitted to the closest base station, here base station 16. The base station 16, upon receiving the call request message, signals the system controller 10 and transmits a call number. The system controller 10 then connects the call to the intended recipient via the PSTN.

If the call is originated in the PSTN, the controller 10 transmits call information to all base stations in the area. In response, the base station transmits a paging message to the intended receiving mobile station. When the mobile station hears the paging message, it responds with a control message transmitted to the nearest base station. The control message signals the system controller that this particular base station is communicating with the mobile station. The controller 10 then routes the call to the mobile station through the nearest base station.

If the mobile station 18 moves out of the coverage area of the originating base station, base station 16, the call may continue by routing the call through other base stations. During handover, there are a number of different methods of handing over an originating call or routing through other base stations.

In a base station originated handoff method, the originating base station, base station 16, notices that the signal transmitted by mobile station 18 falls below a certain threshold level. The base station 16 will then transmit a handover request to the system controller 10, and the system controller 10 will pass the request to all neighbour base stations 12, 14 of the base station 16. The request transmitted by the controller includes information related to the channel, including the PN code sequence used by the mobile station 18. The base stations 12 and 14 tune a receiver to the channel being used by the mobile station and measure signal strength, typically using digital techniques. If one of the base station 12 and 14 receivers reports a signal strength that is stronger than that reported by the originating base station, a handoff is made to that base station.

Or the mobile station itself may initiate a handoff known as mobile-assisted. Each base station transmits a pilot signal that identifies the base station, among other things. The mobile station is equipped with a search receiver for searching for pilot signals transmitted by neighboring base stations 12 and 14, in addition to performing other functions. If the strength of the pilot signal of one of the neighbouring base stations 12 and 14 is found to be stronger than a given threshold, the mobile station 18 transmits a message to the current base station 16 about this phenomenon.

The interaction procedure between the mobile station and the base stations allows the mobile station to communicate through one or more of the base stations 12, 14 and 16. In this procedure, the mobile station identifies and measures the signal strength of the pilot signal it receives. The information is communicated to the MSC through the base station with which the mobile station is communicating. The MSC, upon receiving this information, initiates or terminates the connection between the mobile station and the base station, thereby effecting a mobile-assisted handoff.

The foregoing procedure may also be understood as a "soft" handover when a mobile station is communicating via more than one base station at the same time. During soft handoff, the MSC may combine or select signals received from each base station with which the mobile unit communicates as the mobile unit moves between different cells. In a similar manner, the MSC can forward (relay) signals from the PSTN to each of the base stations with which the mobile unit is communicating. Mobile assisted handovers are further complicated if the mobile station happens to be within the coverage area of two or more base stations of the same cellular system, i.e. not controlled by the same MSC.

Referring now to fig. 2, which illustrates a method of performing handoff between base stations of different systems, fig. 2 shows a schematic form of a cellular communications network 30 including a CDMA cellular system (e.g., IS-951X) controlled by a CDMA mobile switching center MSCc and a GSM cellular system controlled by a GSM mobile switching center MSCg. In fig. 2, 5 exemplary base stations B1A to B5A located in cells C1A to C5A, respectively, of a CDMA system and 5 base stations B1B to B5B located in cells C1B to C5B, respectively, of a GSM system are illustratively shown. Although cells C1A through C5A and cells C1B through C5B are shown as circles for ease of illustration, it should be understood that cells will generally be designed in other shapes and will in fact have shapes that depend on the terrain and ground in which they are located. In the following description, the cells C1A through C3A and C1B through C3B will be referred to as "border" cells since these cells are located approximately on the border between the first and second cellular systems. This naming method allows the remaining cells in each system to be conveniently referred to as "interior" cells.

The following description will be made with reference to a mobile station that is capable of receiving and reacting to signals from base stations in CDMA as well as GSM cellular systems. However, it is contemplated that any other type of communication system may be used, such as CDMA one, CDMA2000, CDMA 20001 x, CDMA 20003 x, high data rate principles, CDMA 1xEV, CDMA 1xEVDO, TDMA, TDSCDMA, W-CDMA, GPRS, or others. To this end, in one embodiment, the mobile station is configured with a dual band transceiver having a receive chain capable of adjusting the different operating frequencies of the two cellular systems. A schematic illustration of one such mobile station is given in fig. 3. As shown, the mobile station 40 includes an antenna 42 connected through a duplexer 44 to a CDMA transmit and receive chain 46 and a GSM transmit receive chain 48. The transmit/receive chains 46, 48 are conventionally used for CDMA and GSM systems, respectively. The output of the chain is suitably modulated and converted to conventional baseband circuitry 50. The transmit/receive chains 46, 48 are controlled by a controller 52, the controller 52 switching between the two chains in response to, among other things, command signals from the CDMA or GSM system. Thus, in this embodiment, the two chains are not active at the same time. In another embodiment, both chains may be active at the same time.

In another embodiment, a mobile station is configured with a single transceiver having a receive chain that is tunable to one of two cellular systems. A schematic illustration of one such mobile station is shown in figure 5. As shown, the mobile station 53 includes an antenna 54. The duplexer 55 is connected to a CDMA transmit and receive chain 56 (if it is a CDMA handset). In addition, the mobile station 53 is connected to a GSM transmit and receive chain 57. The transmit/receive chains 56, 57 are conventionally used for their respective CDMA and GSM systems. The chain output is suitably modulated and converts the data to a conventional baseband circuit 58 and receives the data for transmission from the baseband circuit 58. The transmit/receive chains, whether chain 56 or chain 57, are controlled by a controller 59.

Returning to fig. 2, the CDMA mobile switching center (MSCc) controls the routing of telephone calls from the public switched telephone network (PTSN) to the appropriate base stations B1A through B5A for transmission to designated mobile stations. The CDMA mobile switching center MSCc also controls the routing of calls from mobile stations in the coverage area of the first cellular system, through at least one base station, to the PSTN. The GSM mobile switching center MSCg controls the operation of the base stations B1B to B5B in a similar manner and routes calls between the PTSN and the GSM cellular system. Control messages and the like are communicated between MSCc and MSCg over an intersystem data link 34.

When a mobile station is located within an interior cell of a CDMA system, the mobile station is typically programmed to monitor pilot signal transmissions from each of the neighboring (i.e., interior and/or border) base stations. The mobile station then determines which interior cell it is in by comparing the strength of the pilot signals transmitted from the surrounding base stations. As the mobile station approaches the boundary of an interior cell, mobile-assisted handoff may begin in a manner such as described in the above-referenced U.S. patent No. 5,267,261.

There are different cases when a mobile station is located at the boundary of one of the cells C1A through C3A or C1B through C3B. For example, consider a case where a mobile station is located in cell C2A and is moving towards cell C2B. In this case, the mobile station may begin receiving available signal levels from base station B2B and then report to base station B2B and to any other base station with which the mobile station is communicating. The time at which a useful signal level is being received by a mobile station or base station may be determined by measuring one or more quantifiable parameters of the received signal (e.g., signal strength, signal-to-noise ratio, frame erasure rate, bit error rate, and/or associated time delay). The mechanism is similar to that described in the above-identified U.S. patent No. 5,697,055.

If both systems are CDMA systems, the handover mechanism described in U.S. Pat. No. 5,697,055 may be used to effect a handover between cell C2A and cell C2B. A problem, however, is that there is currently no mechanism for implementing a handoff of a call using the air interface from a CDMA network to a GSM network. GSM authentication cannot be performed because the CDMA mechanism cannot transfer the data required to perform GSM authentication. The encryption in GSM differs from the encryption in CDMA. If new messages are added to the air interface to support dual mode mobile stations, modifications must be made to support these new messages. This is undesirable.

One way to address this problem is to use a generic message containing instructions to enable the mobile station to send from the CDMA network to the GSM network. The generic message must be able to carry the necessary data to enable GSM authentication and encryption. Preferably, other supplementary features in other GSM's also need to be supported by the generic message. In other words, the established GSM protocol must remain intact to minimize any changes to the existing system. Part of the handover operation includes establishing subscriber identification and a need to maintain data confidentiality (encryption) of the signaling and physical connections as the handover proceeds. The definition of subscriber identification authentication and the operational requirements are given in GSM 02.09.

The authentication procedure is also used to set the encryption key. Thus, the authentication procedure is performed after the network establishes subscriber identification but before the channel is encrypted. Two network functions are necessary to achieve this, i.e. the authentication process itself, and the management of authentication and keys in the system.

In view of this, it is contemplated to use a channel "tunneling" mechanism, which can operate at any time (in the handover case as well as in the non-handover case) and can be unidirectional or bidirectional. One type of tunneling mechanism is known as ADDS (application data transfer service) messages and short data burst (burst) messages to transparently transport GSM parameters in CDMA systems, which are not typically checked by the GSM base station controller BSC but are required by dual mode mobile stations. The use of both ADDS messages and data burst messages allows a generic payload (payload) to be sent between mobile services switching centers (MSCs) or other network elements (e.g., SMS, location positioning server, OTASP) of the network. The system takes advantage of this to transfer GSM information end-to-end between the network and the mobile station without requiring any changes to the CDMA BSC or BTS.

In the network arrangement shown in fig. 2, the ADDS message is used to carry GSM handover data, such as timing information and authentication data from the MSCc to the mobile station via the BSCc. The mobile station then uses a message called MAP (mobile application protocol) to carry the handover data to the MSCg in the GSM network. This requires only a small change to the MSCg to enable it to interpret the data in the MAP message and control the mobile station accordingly. Other options for transmitting data are of course possible.

When the mobile station is located at the boundary between the CDMA and GSM systems (e.g., located within cell C2A and approaching cell C2B), the mobile station initiates the handoff procedure, sending a message back to the MSCc informing the MSCc that the mobile station is about to handoff to the GSM system.

A cell database (not shown) may be used as part of the handover procedure. This database is used to provide the mobile station with basic information about the GSM network to enable it to handover between the CDMA MSC and GSM as needed.

There are two types of handover in GSM systems, called synchronous handover and asynchronous handover. Asynchronous switching is preferred for ease of implementation. The mobile station will tell GSM that the handover will be an asynchronous handover. After the handover command is received by the mobile station and the mobile station first sends some access burst data to the GSM base station controller BSCg until it receives back a MAP handover message, which is passed back to the CDMA MSC so that GSM authentication data is generated and provided to the mobile station. GSM has procedures for asynchronous handover and helps BSCg to acquire bursty data used for mobile station timing. The ADDS message thus includes an "action time" message specifying a particular time at which the handover is to begin. The mobile station starts normal transmission only when data is received.

Another problem with switching between CDMA and GSM is that authentication for CDMA and GSM uses two different methods and keys. The method of authentication is basically the same in GSM and CDMA 1x, but the keys have different sizes. CDMA 1 has additional procedures such as Unique Challenge response (Unique Challenge) and billing method for preventing channel hijacking and replay attacks, respectively. In order to use the CDMA physical layer in a GSM system without significant changes to the GSM MSCg, the GSM authentication method should be reused at the CDMA physical layer. This provides the advantage that the system does not need to support two different types of authentication centers, two types of SIM cards, etc.

The authentication procedure involves a series of exchanges between the system and the mobile station. The system transmits an unpredictable number RAND to the mobile station. Next, the mobile station computes a result SRES, also called signature of the RAND number, using an algorithm called the a3 algorithm. The a3 algorithm uses RAND and an individual subscriber authentication key Ki to calculate SRES. The subscriber authentication key Ki is assigned when the customer first orders a service and is stored in a SIM (subscriber identity module) card and in the Home Location Register (HLR) of the system. Ki is the private key in the encryption and is therefore never sent to the network. Finally, the mobile station transmits the signature SRES to the system where it checks validity.

Fig. 4 illustrates how authentication is achieved in a GSM MSC, where the authentication key is called Ki and is 128 bits long. The network generates a random number (RAND) which is also 128 bits long. RAND and Ki are input to the a3 algorithm for computing a 32-bit result (SRES) from the input data. The RAND number is also transmitted to the mobile station by means of an over-the-air message. In the GSM system, each mobile station comprises a smart card, the so-called SIM (for identification module) card. The standard SIM commands for authentication are specified in GSM 11.11. These commands are only allowed to be executed when they do not interfere with the correct functioning of the GSM application. If the SIM is removed from the mobile station during the call, the call is terminated immediately, as defined by GSM 11.11.

The SIM in the mobile station also calculates SRES by applying the a3 algorithm to the received RAND number and a locally stored copy of Ki. The result of the settlement is also SRES and should be the same as the SRES calculated by the network. The result SRES is therefore sent by the mobile station to the network where it is compared with the SRES calculated by the network. The mobile station is authenticated if the values of the two SRES are the same. In the system shown in fig. 2, the RAND number is sent over the air interface using an ADDS message and the result SRES is sent back.

The SRES value is also used in an algorithm called A8 to calculate a 64-bit encryption or key Kc. The Kc key generated by GSM authentication and the encryption algorithm generated in the SIM of the mobile station are applied to the CDMA physical layer instead of the private long code mask typically generated using the CDMA CAVE algorithm. The 64-bit Kc key is uniquely mapped to the 42-bit private long code and thus used as the basis for a "private long code mask" to provide voice privacy. The private long code mask is transmitted around the CDMA message and is interpreted as no difference from what is generated from the CAVE algorithm. Using this voice privacy method allows the system to maintain a unique authentication center and a unique SIM type in a hybrid CDMA/GSM network.

GSM encrypts at the frame level. Each frame is encrypted using the frame number and a 64-bit Kc key, which is derived as discussed in connection with fig. 4. The frame number and Kc mask are applied to each frame. In CDMA 1x systems, encryption is performed using a 42-bit private long code. In the hybrid system shown in fig. 2, the Kc key is used to derive a 42-bit private long code mask, and a mapping algorithm maps between Kc and the private long code mask. The mapping is done in the MSCc which in turn simply tells the BSC which private long code to use.

65 ADDS operation allows the transfer of transparent services between terrestrial network elements (e.g., MSC, SMS, PDC) and the mobile station. The system uses this operation to send authentication information RAND to the MS and back to the SRES to the MSC. The ADDS message operates from MSCc to BSCc and allows data to be sent to the mobile station on the paging channel. ADDS transfer operations from BSCc to MSCc and allows data to be sent from the mobile station to the network over the access channel. The ADDS transfer operation goes from MSCc to BSCc or BSCc to MSCc and allows data to be sent between the mobile station and the network over the traffic channel. The ADDS parameter is defined as the "ADDS user part" which includes a 6-bit "data burst type" indicating the application data message type. The ADDS operation uses the ADDS user part parameters to contain the service specific data. The authentication operation uses the ADDS user part to carry authentication data. The described system uses a new data burst type called "GSM map authentication", which is in turn interpreted by the mobile station.

It should be noted that the exemplary embodiments may be implemented at or accessible to a receiver for whenever a database exists that holds information pertaining to an authentication procedure. The processor of the example embodiment may be used to implement one cryptographic scheme on one side and another cryptographic scheme on another side. The underlying implementation of the example embodiments may be performed without the need to physically connect to an intermediate resource, since the communication between the different parts is done over a wireless medium.

Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans will appreciate the interchangeability of hardware and software in these circuits, and how best to implement the described functionality for each particular application. The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented as an Application Specific Integrated Circuit (ASIC), a programmable logic device, discrete gate or transistor logic, discrete hardware components such as registers in a FIFO, a processor executing a set of firmware instructions, any conventional programmable software and a processor, a Field Programmable Gate Array (FPGA) or other programmable logic device, or any combination thereof. The processor is preferably a microcontroller, but in the alternative, the processor may be any conventional processor, controller, microprocessor, or state machine. The software may be located in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hardware, a removable disk, a CD-ROM, a DVD-ROM, registers, or any other magnetic or optical storage medium. Those skilled in the art will further appreciate that the data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

The invention has been described above in connection with the preferred embodiment, it being understood that this embodiment is by way of example only. Various modifications and changes may be made by those skilled in the art without departing from the principles and scope of the invention, which is set forth in the claims and their equivalents.

Claims (24)

1. A method for handing off a mobile station from a first base station in a first cellular communication system controlled by a first mobile switching control station to a second base station in a different second cellular communication system controlled by a second mobile switching control station, the method comprising:

measuring, at the mobile station, a parameter of a signal transmitted by the first base station;

measuring, at the mobile station, a parameter of a signal transmitted by the second base station;

transmitting a signal quality message from the mobile station to the first mobile switching control station through the first base station when the two parameters reach a predetermined condition;

generating, at said first mobile switching control station, information relating to a channel request message for a second mobile switching control station;

sending said information from said first mobile switching control station to said mobile station;

generating, at the mobile station, a channel request message for a second mobile switching control station from the information from the first mobile switching control station; and

transmitting the channel request message from the mobile station to the second mobile switching control station.

2. The method of claim 1, further comprising generating, at the second mobile switching control station, channel information for identifying a channel for the mobile station in the second cellular communication system.

3. The method of claim 2, further comprising establishing communication between the mobile station and the second base station in the identified channel.

4. The method of claim 3, further comprising disconnecting communication between the mobile station and the first base station.

5. The method of claim 1, wherein the parameter corresponds to a signal strength.

6. The method of claim 1, wherein said first cellular communication system is a CDMA system.

7. The method of claim 6, wherein the second cellular communication system is a GSM system.

8. A mobile station, comprising:

a first transceiver chain for receiving and transmitting signals in a first cellular communication system with respect to a first base station;

a second transceiver chain for receiving and transmitting signals in a second cellular communication system with respect to a second base station; and

controller of

Measuring a parameter of a signal transmitted by the first base station;

measuring a parameter of a signal transmitted by the second base station;

transmitting a signal quality message from the mobile station to the first cellular communication system via the first base station when the two parameters reach a predetermined condition;

receiving information from said first base station regarding a channel request message for said second cellular mobile communication system;

generating a channel request message for said second cellular mobile communication system from said information from said first base station; and

transmitting a channel request message to the second mobile station.

9. The mobile station of claim 8 wherein said controller further receives channel information from said second base station for identifying a channel in said second cellular communication system for said mobile station.

10. The mobile station of claim 9 wherein said controller is arranged to respond to said channel information by establishing communication between said mobile station and said second base station in said identified channel.

11. The mobile station of claim 10 wherein said controller is arranged to respond to said channel information by disconnecting communications between said mobile station and said first base station.

12. The mobile station of claim 8, wherein said parameter corresponds to signal strength.

13. The mobile station of claim 8, wherein said first cellular communication system is a CDMA system.

14. The mobile station of claim 13, wherein said second cellular communication system is a GSM system.

15. The mobile station of claim 8, wherein the first transceiver chain is active when the second transceiver chain is inactive.

16. The mobile station of claim 8, wherein the second transceiver chain is active when the first transceiver chain is inactive.

17. An apparatus for handing off a mobile station from a first base station in a first cellular communication system controlled by a first mobile switching control station to a second base station in a different second cellular communication system controlled by a second mobile switching control station, the apparatus comprising:

means for measuring, at the mobile station, a parameter of a signal transmitted by the first base station;

means for measuring, at the mobile station, a parameter of a signal transmitted by the second base station;

means for transmitting a signal quality message from said mobile station to said first mobile switching control station via said first base station when said two parameters reach a predetermined condition;

means for generating information at said first switching control station about a channel request message for a second mobile switching control station;

means for transmitting said information from said first mobile switching control station to said mobile station;

means for generating, at said mobile station, a channel request message for a second mobile switching control station from said information from said first mobile switching control station; and

means for transmitting said channel request message from said mobile station to said second mobile switching control station.

18. The apparatus of claim 17, further comprising means for generating channel information at a second mobile switching control station for identifying a channel for the mobile station in the second cellular communication system.

19. The apparatus of claim 18, further comprising means for establishing communication between the mobile station and the second base station in the identified channel.

20. The apparatus of claim 19, further comprising means for disconnecting communication between the mobile station and the first base station.

21. The apparatus of claim 17, wherein the parameter corresponds to a signal strength.

22. The apparatus of claim 17 wherein said first cellular communication system is a CDMA system.

23. The apparatus of claim 22, wherein the second cellular communication system is a GSM system.

24. The apparatus of claim 22, wherein said second cellular communication system is a GPRS system.