A wireless communication device and method

IS 2 255 151 T3

DESCRIPTION

Wireless communication device and procedure.

Background of the invention

I. Field of the invention

The present invention relates to a wireless communication device. More particularly, the present invention relates to a wireless communication device and a method for automatically switching between wireless communication systems.

II. Description of Related Art

The use of code division multiple access (CDMA) modulation techniques is one of several existing techniques to facilitate communications in a wireless communication system, in which a large number of users of the system are present. Other multiple access communication system techniques are known in the art, such as time division multiple access (TDMA) and frequency division multiple access (FDMA). An example of a TDMA communication system is the pan-European global mobile communication system (GSM). An example of an analog FDMA system is the Advanced Mobile Phone System (AMPS) currently used in the United States for cellular communications.

However, the CDMA spread spectrum modulation technique has significant advantages over these other techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in US Patent No. 4,901,307, published February 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", and assigned to the assignee of the present invention, the content of which is incorporated herein by reference.

The CDMA signal, being inherently a broadband signal, offers a form of frequency diversity by spreading the signal energy over a large bandwidth. Consequently, frequency selective fading affects only a small part of the bandwidth of the CDMA signal. Diversity in space or path is achieved by providing multiple signal paths over simultaneous links from a mobile user through two or more cell sites. Furthermore, path diversity can be achieved by exploiting the multipath environment through spread spectrum processing, which allows a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in US Patent No. 5,101,501, issued March 31, 1992, entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM" and in US Patent No. 5,109,390, issued on April 28, 1992, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated herein by reference.

In a CDMA system, the detrimental effects of fading can be further controlled, to some extent, by controlling the transmitter power. A system for controlling the power of cell sites and mobile units is disclosed in US Patent No. 5,056,109, issued October 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM ”, serial number 07 / 433,031, filed on November 7, 1989 and also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in US Patent No. 5,103,459, published April 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM ”and assigned to the assignee of the present invention, the content of which is incorporated herein by reference.

In the field of wireless communications, such as cellular communications, wireless local loop communications, and personal communications services (PCS) communications, base stations communicate with remote subscriber units, such as portable radiotelephones. For simplicity, the term "mobile station" will be used herein to refer to such remote subscriber units, although it will be understood that some remote subscriber stations, such as wireless local loop phones, do not typically roam the environment. wireless, but generally remain stationary.

Typically, in any geographic service area, there will be more than one wireless communication service provider. For example, in cellular systems in the United States, there are commonly two service providers, one whose system is designated as system "A" and the other whose system is designated as system "B". For currently projected PCS services in the United States, there are many more service providers, denoted by blocks "A" through "F," covering the same geographic service area. The frequency spectrum available for each geographic service area is divided among these wireless telecommunication service providers . Each service provider typically uses its own base stations and other network equipment.

Under the various wireless communication rules, including the Telecommunications Industry Association (TIA) / Electronic Industries Association (EIA) provisional rule IS-95, entitled “Mobile Station - Base Station

IS 2 255 151 T3

Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System ”, there are portable dual-mode CDMA / AMPS radiotelephones that can communicate with a CDMA base station or an AMPS base station. In addition, other existing or developing industrial rules stipulate dual-mode operation among other modulation and multiplexing systems, such as dual-mode CDMA PCS and AMPS band operation, dual-mode CDMA PCS band and CDMA cellular band, dual-mode GSM and AMPS and various other combinations of known modulation and multiplexing systems.

As can be easily deduced from the number of service providers occupying the same position and the number of communication protocols that can be used, there are a large number of possible communication systems that can operate in a single geographic area, each of the which have a variable degree of coverage. For example, since the AMPS system was the first FM-based analog cellular system to gain wide acceptance in the United States market, AMPS communication systems currently provide nearly 100% coverage in densely populated areas of the United States. . However, due to the deployment of other alternative communication systems, such as CDMA cellular and CDMA PCS systems, the total coverage area of ​​these systems is increasing rapidly. Therefore, it is envisaged the coexistence of many communication systems in the same location, which will present different and overlapping degrees of coverage.

Due to the advantages of the CDMA system outlined above in general terms, many users of CDMA / AMPS dual-mode mobile stations prefer to use the CDMA service whenever it is available and to use the AMPS service only when the CDMA service is not available. Also, a private user of a CDMA cellular and CDMA PCS dual-mode portable radiotelephone may prefer to use PCS services over cellular services for a number of reasons. For these reasons, dual-mode mobile stations designed according to the IS-95 rule generally allow the user to select a preferred mode of operation (eg, CDMA or AMPS), and then the mobile station operates according to that mode. Other rules may allow the same user preferences or may "pre-program" a particular system priority.

In any case, provided that the user of a dual-mode mobile station is in a geographical area that has good coverage for one of the non-preferred communication systems (for example, AMPS) and imperfect coverage for the preferred system (for CDMA example), the user will generally want to make a seamless transition between the two systems that does not require directing undue attention to the portable radiotelephone.

Within the art, it is generally assumed that the forward link (from base station to mobile station) fades along with the reverse link (from mobile station to base station). However, due to the complexities of network planning, the forward and reverse links can be somewhat out of balance. That is, the mobile station may not be able to receive the forward link in certain localized areas where a base station must otherwise be able to receive the reverse link. It is often said that this first case is a case of "direct link limitation". A forward link limiting condition can be caused by an obstruction or reflection in the forward link path that may be less detrimental to the reverse link, due to the difference in frequency between the forward link and the reverse link. In contrast, the forward link may have sufficient power to be received by the mobile station in certain localized areas where the base station cannot receive the reverse link. The latter case is often said to be a case of "reverse link limitation". A reverse link limitation condition can also be caused by differences in the propagation path, or perhaps because the service provider has increased the apparent radiated power (ERP) of the base station to try to avoid interference from other stations. base belonging to an alternative system.

To illustrate the unwanted effects of a forward link limiting condition, consider the case where a CDMA / AMPS dual mode mobile station operates in CDMA mode and enters a strong fading area for the CDMA forward link ( e.g. a deep valley) and consequently loses the CDMA forward link (i.e. cannot receive or demodulate the pilot signal from the base station), although you still have a strong enough forward link signal from a nearby AMPS system. If the user of a conventional dual-mode mobile station tries to originate a call during this severe fading, the call is unsuccessful and the user sees the indication "no service" and "call failed" appear on the screen, while the mobile station of Dual mode tries to get the service back. Once the service has been obtained, the user of the conventional dual-mode mobile station will be requested to restart the call origin by dialing the number again.

Regarding the reverse link limitation condition, consider the case where the CDMA / AMPS dual mode mobile station operates in the CDMA mode and enters a strong fading area for the CDMA reverse link (for example, inside a building or near the coverage limit of a cell) and is consequently unable to transmit any messages to the CDMA base station, although it could transmit them to an AMPS base station if it were operating in AMPS mode. It will further be assumed that the CDMA forward link still has enough power in this case to be demodulated by the dual-mode mobile station. In a conventional dual-mode mobile station, the display will present an indication of the signal strength (eg, a group of signal bars), which will be due to the forward link. However, if the user attempts to originate a call during this condition, the call will fail and the user will receive the indication "call failed" on the screen. Therefore, even if the user receives a successful service indication due to the relative power of the forward link, they will still be unable to communicate with the CDMA base station.

IS 2 255 151 T3

In this reverse link limitation situation, the dual-mode mobile station will be unable to confirm a paging, originate a call, or even register with the CDMA base station, even though it has received a successful service indication. Furthermore, in a conventional dual-mode mobile station, the mobile station will not try to obtain the non-preferred system (AMPS), since it can successfully demodulate the preferred system (CDMA) and therefore the user will not be able to originate or receive no call, unless you have taken some manual action to force the phone to go to the non-preferred system (for example, by switching the mobile station to “AMPS only” mode and then dialing the outgoing phone number again). Also, the user will be unaware of the reverse link limitation condition, because the dual-mode mobile station will indicate that the service is satisfactory.

Therefore, when the conventional dual-mode mobile station is in the forward link limitation condition or the reverse link limitation condition, it is necessary for the user to take at least some positive action to redial a telephone number. if there was an unsuccessful call attempt, and, in the worst case, the user needs to access a user preferences menu to manually switch the mobile station to an alternate system. Thus, the need arises for a mobile station that automatically and seamlessly switches to an alternative system available when it is in a forward link limitation condition or a reverse link limitation condition, and that does not require the user to take any positive action to select the alternate system or restart the origin of a call.

US Patent No. 5,420,911 discloses a cellular mobile radio telephone system, in which mobile stations can select analog or digital control channels. Analog and dual-mode mobile stations are equipped to become multi-mode mobile stations that monitor both analog and digital control channels, depending on the selected mode. Although forward link multimode mobile stations must use analog control channels for call access, they can examine paging and digital control channels to receive direct link paging and control communications from the terrestrial system. Two-way multimode mobile stations have the same capabilities as forward link mobile stations, but are also equipped to transmit control information to the ground station through the reverse link digital control channel. If no digital control channel is available, multimode mobile stations examine primary and secondary groups of analog control channels. In this way, analog and dual-mode mobile stations can take advantage of digital control channel monitoring and digital paging channels, including multi-channel paging and battery-saving capabilities.

Summary of the invention

In one aspect, the present invention provides, as set forth in the appended claims, a wireless communication device comprising: a processor for generating a first signaling message to be transmitted to a first communication system presenting a first pilot signal and a transceiver, coupled to said processor, for transmitting said first signaling message to said first communication system and for receiving said first pilot signal and a first confirmation message from said first communication system, in response to the first signaling message; wherein said processor is arranged to automatically generate a second signaling message for transmission to a second communication system displaying a second pilot signal, in response to the fact that said transceiver does not receive said first confirmation message and receives said first pilot signal.

In another aspect, the present invention provides a method for automatically switching between a first communication system displaying a first pilot signal and a second communication system displaying a second pilot signal, the method being performed by means of a wireless communication device. and comprising the following steps: receiving said first pilot signal; generating a first signaling message for transmission to said first communication system; transmitting said first signaling message to said first communication system; automatically generating a second signaling message for transmission to said second communication system if a first confirmation message in response to the first signaling message is not received from said first communication system, and said first pilot signal is received.

Preferably, the wireless communication device comprises a processor for generating a first signaling message for transmission to a first communication system presenting a first pilot signal, and a transceiver for transmitting the first signaling message to the first communication system. In response to the first signaling message, the first communication system should generate a first confirmation message. The transceiver receives the first pilot signal and should also receive the first confirmation message from the first communication system. However, if the first confirmation message is not received, but the first pilot signal is received (a fact that indicates that the wireless communication device is in a reverse link limitation situation), the processor automatically generates a second signaling message. for transmission to a second communication system displaying a second pilot signal. Therefore, the wireless communication device tries to automatically obtain the second communication system after failing its attempt to close the reverse link with the first communication system.

IS 2 255 151 T3

In a preferred embodiment, the wireless communication device further comprises a user interface, such as a keyboard and associated circuitry, for generating a first set of dialed digits, and a memory for storing the first set of dialed digits, including said first and said second signaling message the first set of dialed digits. For example, the first set of dialed digits can be a desired destination telephone number. The destination phone number is stored in memory so that the processor can later retrieve it and generate the second signaling message. In this case, the first and second signaling messages will be call origin messages and, therefore, the processor will carry out an automatic call origin “retry” with the second communication system, retrieving the dialed digits from memory. , and the user will not need to take any action.

To this end, in the preferred embodiment, the processor comprises a call processing module that makes it possible to generate the first and second signaling messages and generate an origin hold signal, and a user interface module that makes it possible to store the first set of dialed digits in memory and provide the first set of dialed digits to the call processing module in response to the originating hold signal. The call processing module gives said origin hold signal the logical value "TRUE" (true) when said wireless communication device is obtaining the first or the second communication system, and the logical value "FALSE" (false) when said wireless communication device has obtained said first or second communication system. Therefore, when the user interface module detects the change from TRUE to FALSE of the originating hold signal, it resets the pending call origin that was held during the get attempt.

Therefore, the wireless communication device automatically and seamlessly switches to an available alternate system when in a forward link limitation condition or a reverse link limitation condition, without the need for the user to take any positive action. to select the alternate system or restart a call source.

Brief description of the drawings

The features, objectives, and advantages of the present invention will become apparent from the following detailed description of an embodiment of the present invention, taken in conjunction with the drawings, in which similar reference signs are used to identify similar elements, and in which:

Figure 1 is a selected component block diagram of an exemplary mobile station of the present invention and Figure 2 is a high-level state diagram of the operation of the present invention.

Detailed description of the preferred embodiment

Although the embodiment of the present invention described herein will be disclosed with reference to an example of a CDMA / AMPS dual-mode mobile station, it should be noted that the present invention is applicable to any wireless communication device that can communicate with more than one communication system, whether the two communication systems use the same modulation or multiplexing protocols or not. For example, the present invention is applicable to any wireless communication device that is capable of "roaming" between various communication systems.

In Figure 1, selected components of an example mobile station 100 that may be used with the present invention are illustrated. On the direct link (from the base station to the mobile station), the antenna 102 captures RF energy and routes it to the transceiver (XCVR) 104. The XCVR 104 performs frequency reduction and demodulation of the received signal and passes the resulting signal to processor 107. Processor 107 receives the demodulated forward link signal from XCVR 104 and processes the signal according to conventional procedures known in the art and further described in the patents listed above. With reference to the present invention, the processor 107 also performs tasks such as determining which base station of which wireless communication system should be obtained in response to user preferences stored in non-volatile memory 111, obtaining the direct link from the selected base station and generating signaling messages to be transmitted to the base station. Processor 107 can be any conventional microprocessor known in the art that has been programmed to perform the tasks of the present invention described herein. Although processor 107 includes many other processing modules, call processing module 106 and user interface module 108 are the most relevant to the present invention and are therefore the only ones illustrated in Figure 1.

The call processing module 106 receives and processes signaling messages from the base station and, on the other hand, controls the actions taken by the mobile station 100 in response to the signaling from the base station. For example, the call processing module 106 receives and acts on signaling messages such as registration requests, supplemental information messages such as paging channel assignments, paging messages, and handover address messages.

IS 2 255 151 T3

In response to instructions from call processing module 106, user interface module 108 controls various user interfaces, such as display 110 and keyboard 112, which can be any of the conventional displays and keyboards known in the art. . For example, when the base station indicates by signaling that the mobile station 100 is out of its "home" system and is therefore "roaming", the user interface module 108 may enable the "roaming" indication in display 110. In another example, upon obtaining the pilot channel from the base station, user interface module 108 may enable a service indication on display 110. User interface module 108 may control other user interfaces that are not illustrated (to make Figure 1 clearer and simpler), due to their limited relevance to the present invention.

On the reverse link (from mobile station to base station), the user of mobile station 100 can enter a destination telephone number via keypad 112 to initiate a call origin. The user interface module 108 temporarily stores the dialed digits in memory 109 and, unless instructed to "hold" the dialed digits as indicated below, provides them to the call processing module 106 which generates a message. source to be transmitted to the base station by the XCVR 104, through the antenna 102. In the present invention, the call processing module 106 and the user interface module 108 cooperate to implement the method of the present invention, as described with reference to Figure 2. In particular, mobile station 100 automatically and seamlessly switches to an available alternate system when it is in a forward link limitation condition or a reverse link limitation condition, with no need for the user to take any positive action to select alternate system or restart a call source.

In Figure 2, a high-level state diagram of the operation of an example mobile station employing the present invention is depicted. Although the diagram in Figure 2 corresponds to a mobile station that complies with the IS-95 rule, its content is equally applicable to other wireless communication rules, whether cellular, mobile, PCS or other, and regardless of the modulation or multiplexing technology used (code division multiple access or CDMA, time division multiple access or TDMA, global mobile communications system or GSM based on TDMA or advanced mobile phone system or AMPS).

In Figure 2, five separate major operating states are illustrated: system no get state 200, initialization state of mobile station 202, idle state of mobile station 204, access state to system 206, and control state of the traffic channel over mobile station 208. Various relevant transitions between these major operating states are further illustrated. However, it should be noted that, in any mobile station suitable for use with the present invention, there may be many more states and transitions that have not been illustrated in Figure 2 to simplify and clarify the description.

The first state of the mobile station is the system not obtaining state 200. The system not obtaining state 200 can be reached in many ways, such as the transition between "gaps" in the coverage areas of the wireless service, and also from any of the other states indicated after the loss of the direct link. When in the system 200 not getting state, the display 110 of Figure 1 may display the indication "NO SERVICE".

To obtain the service, the processor 107 (Figure 1) determines that the mobile station 100 enters the initialization state of the mobile station 202. The initialization state of the mobile station 202 consists of the following four substates (although in Figure 2 only the first is illustrated for clarity):

1) system determination substate;

2) pilot channel obtaining substate;

3) get synchronization channel substate and

4) timing change substate.

In response to the fact that it has entered the initialization state of mobile station 202, call processing module 106 gives the logical tag "HOLD_ORIG", which is short for "hold origins", the logical value "TRUE ”. Although the value of HOLD_ORIG is TRUE, the origins of calls that the user has initiated (for example, by dialing a telephone number on the keyboard 112) and that are detected by the user interface module 108 are not acted upon immediately, rather, they are temporarily "held" in memory 109 pending the result of the initialization state of mobile station 202. This is in contrast to a conventional mobile station, where origination attempts that occur during mobile station initialization result in the immediate display of the "call failed" indication, as the wireless service has not yet been established.

In system determination substate 214, mobile station 100 selects which system and which system channel to use. This system determination is usually influenced by a set of preferences (eg "CDMA only", "AMPS only" or "CDMA first, then AMPS") that may be stored in non-volatile memory 111. This determination of the system can generally be carried out according to the procedures described in detail in the pending US patent application Serial No. 08 / 509,719, entitled "METHOD AND APPARATUS FOR

IS 2 255 151 T3

SYSTEM DETERMINATION IN A MULTI-MODE SUBSCRIBER STATION ”, filed July 31, 1995 and assigned to the assignee of the present invention and which is incorporated herein by reference. In addition, the determination of the system can be carried out, in general, according to the procedures described in the pending US patent application, serial number 08 / 626,744, entitled "METHOD AND APPARATUS FOR PERFORMING PREFERRED SYSTEM SELECTION", filed on September 27. March 1996, assigned to the assignee of the present invention and incorporated herein by reference.

In the pilot channel obtain substate (not shown), mobile station 100 obtains the pilot channel of the selected system. In an IS-95 compliant system, the pilot channel is an unmodulated direct sequence spread spectrum signal transmitted continuously by each CDMA base station. The pilot channel enables mobile stations to obtain the forward CDMA channel timing, provides a phase reference for coherent demodulation, and provides a means for making signal strength comparisons between base stations.

To obtain the pilot channel, the mobile station 100, controlled by the processor 107, can execute the obtaining procedures described in detail in the pending US patent application, Serial No. 08 / 687,694, entitled "METHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM ”, filed July 26, 1996 and assigned to the assignee of the present invention, and which is incorporated herein by reference. Alternatively, mobile station 100 may perform other acquisition procedures well known in the art, depending on the type of system desired to obtain.

However, if mobile station 100 fails to obtain the pilot channel of a preferred CDMA system when it is in the initialization state of mobile station 202, it returns to the system determination substate and attempts to obtain the pilot channel of a control system. alternative communication, according to the set of preferences stored in non-volatile memory 111 indicated above. The unsuccessful attempt to initially obtain the pilot channel may constitute one of the cases of the forward link limitation situation described above.

Upon successful obtaining of the pilot channel, the mobile station 100 enters the obtaining synchronization channel substate, where it receives and processes messages from the synchronization channel. In an IS-95 compliant CDMA system, the sync channel uses the same PN sequence and offset as the pilot channel and can be demodulated as long as the pilot channel is tracked. The sync channel contains, among other things, the identification of the base station and the system time. In the timing change substate, the mobile station synchronizes its system timing with that of the base station, based on the system time received on the synchronization channel.

Upon successful obtaining and initialization of the system, the call processing module 106 will reassign HOLD_ORIG the value FALSE, thereby allowing all call origins that have been detected and stored by the user interface module 108 to proceed normally. . This situation is in contrast to that of a conventional mobile station, in which origination attempts that occur during mobile station initialization result in the immediate display of the "call failed" indication, due to the wireless service not being supported. has established yet. Therefore, the dialed digits of an origination attempt that occurs during the initialization state of the system 202 are temporarily stored in memory 109 until a system is obtained, at which point they are transmitted to the call processing module 106. for inclusion in the dialed digit portion of an originating message transmitted by the XCVR 104 (Figure 1). The temporary storage of dialed digits in memory 109 until a system is obtained avoids the need for the user of mobile station 100 to re-enter the dialed digits via keypad 112 or take any other action (such as pressing the keypad). redial) to complete a premature call origination attempt that occurs before wireless communication has been established.

In the idle state of mobile station 204, mobile station 100 monitors the paging channel. The paging channel is a direct channel used for the transmission of paging and control information from a base station to a mobile station. While in the idle state of mobile station 204, mobile station 100 may receive supplemental information messages that convey information specific to the base station as well as system-wide information, receive an incoming call, initiate an origin call, start a record or start a message transmission.

When mobile station 100 initiates a call origin, registration, or other type of message transmission, such as responding to an incoming paging, call processing module 106 generates an access channel message for transmission. to the base station. When an access channel message, illustrated in decision diamond 218, has been generated, mobile station 100 enters the access state to system 206, in which it attempts to transmit one or more access channel messages on the access channel. access. The access channel message may be, for example, a pending call origination attempt that has been held during the initialization substate of system 202 or it may be a call origination attempt that has been initiated during the idle state of mobile station 204 or it may be a supplemental information message, such as a registration message.

In a CDMA communication system, the access channel allows communications on the reverse link from mobile station 100 to base station when mobile station 100 does not use a traffic channel. Each channel

ES 2 255 151 T3 paging is paired with one or more access channels. The base station responds to transmissions on a particular access channel with a message on the associated paging channel. Similarly, mobile station 100 responds to a paging channel message by transmitting on one of the associated access channels.

If the mobile station experiences strong paging channel fading, then another case of the forward link limitation situation described above may occur, and the mobile station can return to the system determination substate of the initialization state of the mobile station 202 and try to obtain the service again with the same CDMA communication system or an alternative system according to the set of system preferences stored in memory. non-volatile 111.

In the access state to the system 206, the mobile station 100 transmits on the access channel by a random access procedure. The entire procedure of sending an access channel message and receiving (or not receiving) an acknowledgment of that access channel message is called "access attempt". An access attempt comprises the transmission of one or more "access probe sequences". Each access probe sequence consists of transmitting one or more “access probes”. Each access poll comprises the access channel message, and the mobile station transmits the same access channel message on each access poll of an access attempt.

Therefore, in an access attempt, the access probes are grouped into sequences of access probes. The first access probe of each access probe sequence is transmitted at a predetermined power level. Each subsequent access probe in that access probe sequence is transmitted at a power level exhibiting a predetermined power increment greater than the previous access probe in said access probe sequence. For example, if the default power level for the first access probe is 7 dB and the default power increment is 2 dB, then the first access probe of all access probe sequences will be transmitted at 7 dB, the second access poll of all access poll sequences will be transmitted at 9 dB, the third poll at 11 dB, and so on until the polling sequence is complete.

If a sequence of access polls does not cause any confirmation from the base station, another identical sequence of access polls will start. The mobile station 100 interrupts the transmission of access probe sequences, thus ending the access attempt, when it receives an acknowledgment from the base station or when it has transmitted a predetermined maximum number of access probe sequences. Termination of the access attempt when the predetermined number of access probe sequences is reached constitutes one of the cases of the reverse link limitation situation described above. It should be noted that access attempts may be made in ways other than those known in the art depending on the nature of the system providing the service.

If the access attempt is successful, as determined by decision diamond 220, and the access channel message is a source message indicating that the mobile station user is originating a call, the mobile station is addressed by the base station to a traffic channel and the mobile station enters the mobile station traffic channel control state 208. When the traffic channel is no longer used, as is the case when it is determined in the decision diamond 222 that the call has concluded, the mobile station returns to the idle state of the mobile station 204.

However, if the access attempt is unsuccessful, because no confirmation from the base station is detected at decision diamond 220, then processor 107 determines, at decision diamond 224, whether this last unsuccessful access attempt it was the N-th unsuccessful access attempt for the same access channel message, with N being an integer greater than one. Otherwise, the call processing module 106 returns the value TRUE to HOLD_ORIG, and enters the system determination substate 214 again. Instead of using a fixed number of retries determined in the decision diamond 224 , it is also possible to use a timer that allows all attempts or only the attempts that are possible in a certain period of time (for example, 20 seconds). In such a case, the decision diamond 224 will fail a call origin attempt after the time has expired, regardless of how many attempts have been made. Alternatively, it is possible to use a combination of these techniques.

In the system initialization state 202, the mobile station 100 preferably attempts to obtain a different communication channel or system than the one that caused the last unsuccessful attempt. It will be assumed, for example, that there are two communication systems in the same geographic area - a CDMA PCS system and an analog AMPS system. It will further be assumed that the CDMA PCS system is operating with two different CDMA channels - channel number 400 and channel number 425 - and that the AMPS system is operating with analog channel "B". If an unsuccessful access attempt notice is received for the source message while the service is being provided on the CDMA PCS channel number 425, the system determination substate 214 will preferably try to obtain the analog AMPS system on the analog channel "B" or the CDMA PCS system on CDMA channel number 400.

If the analog AMPS channel "B" is obtained during the initialization state of system 202, the call processing module 106 returns HOLD_ORIG to FALSE. When HOLD_ORIG is detected to change from TRUE to FALSE, the user interface module 108 retrieves the dialed digits from memory 109 and resets the call source. If the call origin is successful this time, indicating that the base station has confirmed the call origin in the decision diamond 220, the mobile station 100 ends the call normally on a traffic channel and returns to the state of idle of mobile station 204. Therefore, the user

ES 2 255 151 T3 does not need to dial the phone number, even if service is lost in one system and re-obtained in another later. In one embodiment, measures are taken to periodically perform a rescan of the service on the original preferred CDMA PCS channel number 425. For example, the processor 107 may activate a timer for one minute and then return to the initialization state of the system 202 to obtain the preferred system again after the timer expires. Alternatively, the one minute timer can be reset after each source in the analog AMPS system.

However, if the AMPS analog channel "B" has not been obtained during the initialization state of the system214, the mobile station 100 can continue to scan and finally obtain the service on the alternative CDMA PCS channel number 400. Similar to the example that Just presented, when HOLD_ORIG is detected to change from TRUE to FALSE, the user interface module 108 retrieves the dialed digits from memory 109 and resets the call origin.

The access attempt is now assumed to have failed, as determined by decision diamond 220, in the alternate system that was obtained first (either AMPS analog channel “B” or CDMA PCS channel number 400). . This will be the second failure for the same call origination attempt, that is, for the same dialed digits entered once by the user. If in decision diamond 224 N = 2, the call origin will fail and the user interface module 108 will present a call failure on screen 110 at block 226. Therefore, the present invention allows N access attempts on different systems for the same call origin before prompting the user to take any action to remedy the situation (i.e. redial the phone number, switch to an area with better coverage, etc.). The number N and therefore the number of "retries" can be fixed or it can be configurable by the user. For example, the manufacturer of mobile station 100 may decide that only a single retry of the described source message is appropriate and may set the value of N.

It should be noted, also this time, that the present invention is not only applicable to the particular CDMA and analog AMPS system examples presented herein, but also to any mobile station that can operate with more than one communication system. For example, if there are two CDMA PCS systems in a given geographic area, the present invention can be used to provide a retry from CDMA to CDMA. Furthermore, it should be noted that the specific user preferences stored in non-volatile memory 111 and used during the system determination substate 214 to determine which systems are to be obtained are not critical to the present invention. Non-volatile memory user preferences can consist of a table of preferred systems or a table of systems arranged in order of preference or simply a list of probable systems that may be available.

As described, the present invention reduces the percentage of failed calls by allowing pending call origin attempts to be held until service is obtained. In the event that the maximum number of access sequences is exceeded (reverse link limitation) or unsuccessful attempts to obtain the forward link from the preferred communication system (forward link limitation), it will not be necessary for the customer to reset the source of call, thereby converting a potential failed call into a successful call.

Furthermore, the present invention is not limited to call origin retries. The present invention can also be used to automatically and seamlessly perform system change in a reverse link limiting situation for a failure of any access channel message, thereby allowing mobile station 100 to rapidly switch to a alternative system, and you sign in to it, when you are unable to sign in to the preferred system. This will avoid the undesirable effect of the mobile station 100 losing an incoming call, due to its inability to register with the preferred system. Even if the mobile station 100 is able to register with the preferred system before entering the reverse link limitation situation, the automatic system change of the present invention can be used to have the phone search for an alternative system to the service when it is unable to receive an acknowledgment for any other supplementary message transmitted on the access channel.