FIELD OF THE INVENTION 
     This invention relates to mobile station control states and, more particularly, to a method and apparatus for transitioning a mobile station between packet data service control states based on available power at the mobile station. 
     BACKGROUND OF THE INVENTION 
     Major cellular system types include those operating according to the Global System for Mobile Communication (GSM) standard, the TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual Mode Wide Band Spread Spectrum Cellular System (IS-95A), the TIA/EIA/IS-136 Mobile Station-Base Station Compatibility Standard (IS-136), and the TIA/EIA 553 Analog Standard (AMPS/TACS). Other major cellular systems include those operating in the personal communications system (PCS) band according to the ANSI-J-STD-008 1.8-2.0 GHz standard or those operating according to the GSM-based PCS 1900 (1900 MHz frequency range) standard. IS-95A is currently being updated as IS-95B in the document TIA/EIA-3693. 
     Currently, each of the major cellular system standards bodies is implementing data services into its digital cellular specifications. A packet data service specification has been finalized for GSM and IS-95A. Packet data service specifications compatible with the IS-136 and IS-95B standards are also being prepared. 
     A third-generation CDMA system is also being developed to provide more sophisticated and improved data services than provided by IS-95 and eventually to replace IS-95. In the proposed standard for third-generation CDMA, known as cdma2000 ITU-R RTT, it has been proposed that third-generation systems include packet data services that utilize one or more control states that a mobile station may be in when engaged in a data service. The control states are states in which a mobile station can have varying physical and logical channel configurations assigned to it, depending on the present data transmission situation. The third-generation CDMA control states are intended to be utilized when packet data services for particular mobile stations have varying quality of service (QoS) requirements. 
     For example, when no data has been transmitted for a certain period of time, a mobile station may transition from an active state, in which dedicated forward and reverse control and traffic channels are each maintained, to a control hold state in which only a dedicated forward control channel is maintained. The control hold state allows fast reassignment through the forward control channel and frees up system traffic channel resources. Again, after a certain period of time in the control hold state when no data has been transmitted, the mobile station may transition from the control hold state to a suspended state. In the suspended state, all dedicated channels are released and the mobile station monitors only the forward common control channel. From the suspended state, the mobile station may transition back to the control hold state if it is determined that data is to be transmitted within a certain period of time, or the mobile may transition to a null state if data is not to be transmitted within a certain period of time. Each of the control states requires the mobile station to expend a certain amount of power that depends on the type of channels assigned in that state and the time spent in that state. QoS requirements may be used to determine the time period for transitioning between control states and to determine which states are allowable for a mobile station. By defining the time periods and allowable states in a particular way, a mobile station may have faster access to channel resources and less delay in its packet application to satisfy certain QoS requirements while minimizing power consumption and freeing up system resources. While QoS requirements may be the major factor in determining the transition periods and allowable control states, basing the transition periods and allowable control states solely on QoS requirements may not be the most efficient way of controlling transitions between control states. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for transitioning a mobile station between control states in a telecommunications system based on available power at the mobile station. The method and system allows a mobile station to signal the system to indicate that power available to the mobile station is less than or greater than a predetermined threshold and that parameters for controlling transitions between control states should be adjusted accordingly. For example, if a mobile station battery power falls below a predetermined threshold, the time periods for triggering transitions from selected control states that require greater mobile station power expenditure to maintain may be decreased, or the mobile station may be controlled to transition immediately from a current state to another state. The system may trade off packet service delay with mobile station power expenditure to prolong useful life of a battery in the mobile station. If the battery is charged and available power rises again above the predetermined threshold, the time periods for triggering transitions from the selected control states may be increased, or certain states prohibited in a low-power condition may become allowable again. 
     In an embodiment of the invention, the method and apparatus is implemented in a system having packet data service with states including an active state, a suspended state and a control hold state. The active state is associated with a timer Tactive, and the control hold state is associated with a timer Thold. The timer associated with each state is activated upon the termination of each data transmission and determines how long a mobile station will remain in that state if no data is transmitted or received within the time period duration for which the timer runs. According to the embodiment, a mobile station involved in transmitting and receiving data through the packet service transmits an indication to the system that the mobile station&#39;s battery power has fallen below or risen above a predetermined threshold level of full charge, for example, X%, in order to allow the system to modify the time period durations for Tactive and Thold. Tactive and Thold may be set to initial values used for a battery having full power. 
     Upon receiving the indication from the mobile station that battery power has fallen below X%, the system may then decrease the time period duration for the Tactive and Thold timers. As the mobile station is involved in the packet data service and data transmission ceases for periods of time, the mobile station will spend less time in the active and control hold states and more time in the suspended state. This preserves the remaining battery power. 
     If the mobile station has access to additional power during the packet data service, for example, the mobile station is plugged into a charger, the mobile station may transmit an indication to the system that the mobile station&#39;s battery power has risen again above the threshold level, X%. The system may then restore the time period duration for the Tactive and Thold timers to the initial settings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of portions of a mobile station according to an embodiment of the invention; 
     FIG. 2 is a bock diagram of portions of a base station according to an embodiment of the invention; and 
     FIG. 3 is a flow diagram illustrating process steps performed for transitioning a mobile station between packet data control states according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In an embodiment of the invention, the method and apparatus may be implemented into a cellular system that operates according to the Code Division Multiple Access (CDMA) cellular system standard specified in the document, “The cdma2000 ITU-R RTT Candidate Submission (0.18),” published by the Telecommunications Industry Association, TR-45.5 Subcommittee, Jul. 27, 1998 (cdma2000). The packet data services of the system are modified by the implementation of the method and apparatus for transitioning a mobile station between active, control hold and suspended packet data service control states based on available power according to the embodiment of the invention. The method and apparatus of the invention also has application to all types of telecommunication systems that may use similar packet data services, such as, for example, time division multiple access (TDMA) systems. 
     Referring now to FIG. 1, therein is a block diagram of portions of a mobile station  100  of the embodiment of the invention. Mobile station  100  comprises antenna  101 , duplexer  102 , transmit power amplifier  104 , analog receiver  106 , transmit power controller  108 , searcher receiver  110 , digital data receiver  112 , digital data receiver  114 , diversity combiner/decoder  116 , control processor  118 , digital vocoder  120 , transmit modulator  122 , user interface  124 , and battery monitor  126 . Mobile station  100  may be implemented as any type of terminal having data capability, such as a mobile phone attached to a laptop, a communication type device, or a laptop having built-in transceiving capability. 
     Antenna  101  is coupled to analog receiver  106  through duplexer  102 . Signals received at antenna  101  are input to analog receiver  106  through duplexer  102 . The received signals are then converted to baseband frequency and then filtered and digitized in analog receiver  106  for input to digital data receiver  112 , digital data receiver  114  and searcher receiver  110 . The digitized baseband signal input to digital data receiver  112 , digital data receiver,  114  and searcher receiver  110  may include signals from ongoing calls, including control information and data transmitted on the forward common control channel (F-CCCH), together with the pilot carriers transmitted by the base station of the cell site in which the mobile station is currently located, plus the pilot carriers transmitted by the base stations in all neighboring cell sites. Digital data receiver  112  and digital data receiver  114  perform correlation on the baseband signal with a pseudo random noise (PN) sequence of a desired received signal. The output of digital data receivers  112  and  114  is a sequence of encoded data signals from two independent paths. Searcher receiver  110  scans the time domain around the nominal time of a received pilot signal of a base station for other multi-path pilot signals from the same base station and for other signals transmitted from different base stations. Searcher receiver  110  measures the strength of any desired waveform at times other than the nominal time. Searcher receiver  110  generates signals to control processor  118  indicating the strengths of the measured signals to control processor  118 . 
     The encoded data signals output from digital data receiver  112  and digital data receiver  114  are input to diversity combiner/decoder  116 . In diversity combiner/decoder  116  the encoded data signals are aligned and combined, and the resultant data signal is then decoded using error correction and input to digital vocoder  120 . Digital vocoder  120  then outputs information signals to the user interface  124 . User interface  124  may be a handset with a keypad or another type of user interface, such as a laptop computer monitor and keyboard. 
     For transmission of signals from mobile station  100 , a signal received at user interface  124  is input to digital vocoder  120  in digital form as, for example, data or voice that has been converted to digital form at user interface  124 . In digital vocoder  120  the signal is encoded and output to transmit modulator  122 . Transmit modulator  122  Walsh encodes the signal and then modulates the Walsh encoded signal onto a PN carrier signal, with the PN carrier sequence being the PN carrier sequence of the CDMA channel to which the mobile station is assigned. The PN carrier information is transmitted to mobile station  100  from the system and transferred to control processor  118  from digital data receivers  112  and  114  after being received from the system. Control processor  118  sends the PN carrier information to transmit modulator  122 . The PN modulated signal is then output from transmit modulator  122  to transmit power controller  108 . Transmit power controller  108  sets the level of the transmission power of mobile station  100  according to commands received from control processor  118 . The modulated signal is then output from transmit power controller  108  to transmit power amplifier  104  where the signal is amplified and converted to an RF signal. The RF signal is then output from transmit power amplifier  104  to duplexer  102  and transmitted from antenna  101 . 
     According to the embodiment of the invention, battery monitor  126  monitors mobile station battery power level and sends an indication to control processor  118  at an appropriate time. For example, battery monitor  126  may send a signal to control processor  118  indicating that battery power level has fallen below a predetermined threshold level of full charge, X%, and may also send a signal to control processor  118  indicating that the battery power level has risen above X%. 
     The predetermined threshold level X% may be set through a user interface menu function that allows a user to set the threshold. The menu function could allow a simple implementation, for example, the function could allow the user to use a yes/no activation of the packet data service power-saving function with a previously-set or default threshold level, or it could allow a user to choose a particular value for the threshold when activating the packet data service power-saving function. Alternatively, the predetermined threshold level could be activated and set by the system operator to a value received from base station  200  in a control message. The setting of threshold levels within battery monitor  126  may be controlled by control processor  118  according to the desired value. Control processor  118  includes appropriate software and/or hardware, including memory for storing control programs, that receives a signal from battery monitor  126  and generates a signal to the system according to the embodiment of the invention. 
     Referring now to FIG. 2, therein is a block diagram of portions of a base station  200  according to an embodiment of the invention. Base station  200  includes a first receiver section  232 , a second receiver section  234 , control processor  222 , diversity combiner/decoder  224 , transmit power controller  226 , digital link  228 , input/output I/O  236 , transmit modulator  230 , control channel transmitter/modulator  220 , transmit power amplifier  210 , and antenna  201 . First receiver section  232  comprises antenna  201 , analog receiver  206 , searcher receiver  212  and digital data receiver  214 . Second receiver section  234  comprises antenna  202 , analog receiver  208 , searcher receiver  216  and digital data receiver  218 . 
     First receiver section  232  and second receiver section  234  provide space diversity for a single signal that may be received at both antennas  201  and  202 . The signals received at antenna  201  are input to analog receiver  206  where the signal is filtered, converted to baseband frequency and digitized to generate a digital signal. The digital signal is then output from analog receiver  206  to searcher receiver  212  and digital data receiver  214 . Searcher receiver  212  scans the time domain around the received signal to verify that digital data receiver  214  tracks the correct signal. Control processor  222  generates the control signals for digital data receiver  214  according to a signal received from the searcher receiver  212 , so that the correct signal is received at digital data receiver  214 . Digital data receiver  214  generates the proper PN sequence necessary to decode the digital signal received from analog receiver  206  and generates weighted output symbols for input to diversity combiner/decoder  224 . Antenna  202 , analog receiver  208 , searcher receiver  216  and digital data receiver  218  of second receiver section  234  function identically to the components of first receiver section  232  to generate a second set of weighted output symbols. The weighted symbols from digital data receiver  214  and digital data receiver  218  are then combined and decoded in diversity combiner/decoder  224  to generate received digital data which is then output through digital link  228  and I/O  236  to the system. 
     When data received from the system is to be transmitted from base station  200  on a traffic channel, the data is received at digital link  228  over I/O  236  and sent to transmit modulator  230 . Transmit modulator  230  then modulates the data using the appropriate Walsh function assigned to the mobile station to which the base station is transmitting. The Walsh modulated data is then spread by a channel PN sequence having the appropriate time shift and input to transmit power controller  226 . Control information and data are also transmitted by base station  200  on the appropriate control channels to mobile stations. Transmit power controller  226  controls the transmission power of base station  200  in response to control signals received from control processor  222 . Base station  200  also controls packet data services and packet data service control states by transmitting control information on the appropriate control channels. Base station  200  generates packet data service control information and signals as triggered by the Tactive and Thold timers of the active and control hold states, respectively, to transition mobile station  100  between the packet data service control states. The power control commands may be generated by software in control processor  222 . The signal output from transmit power controller  226  is input to transmit power amplifier  210  and then transmitted from antenna  204 . Base station  200  may have multiple transmit modulators and transmit power controllers for transmitting to multiple mobile stations. 
     According to the embodiment of the invention, control processor  222  includes software and/or hardware, including memory for storing control programs, that receives data included in a signal transmitted from mobile station  100  indicating that mobile station battery power has fallen below or risen above the threshold X%. According to the data received, control processor  222  will modify Tactive and Thold timer period durations and generate the appropriate commands. 
     Referring now to FIG. 3, therein is a flow diagram illustrating process steps performed for transitioning a mobile system between packet data service control states according to an embodiment of the invention. The process steps of FIG. 3 are performed under the control of control processor  118  of mobile station  100  and control processor  222  of base station  200  according to software and/or hardware configured according to the embodiment of the invention. 
     The process begins at step  300 . The process may either be automatically activated upon power-up of mobile station  100  or upon the entry of mobile station  100  into a system providing the method and apparatus of the invention. Alternatively, the process may be activated by a user menu function during a time when the mobile station is powered on. When the process is activated, battery monitor  126  monitors the battery power of mobile station  100  at step  302 . If no battery power transition below or above the threshold level is detected, the process will remain at step  302 . If a transition of the available battery power below or above the threshold is detected by battery monitor  126 , battery monitor  126  sends a signal to control processor  118  indicating the transition. 
     Next, at step  304 , control processor  118  determines if the transition was a transition to less than the threshold. If, at step  304 , it is determined that the transition was a transition to less than the threshold, the process moves to step  306 . At step  306 , control processor  118  determines if mobile station  100  is currently active on a packet data service. If mobile station  100  is not currently active on a packet data service, the process moves to step  310 . At step  310 , control processor  118  formats and stores a low-power alert message to be transmitted to the system upon setup of a data service. The low-power alert message will allow the system to set Tactive and Thold to power-conserving values when a packet data service is activated. The process then returns to step  302 , where battery monitor  126  monitors the battery of mobile station  100  for a transition to above the threshold level. If, however, at step  306  it is determined that mobile station  100  is currently active on a packet data service, the process moves to step  314 . At step  314 , control processor  118  initiates transmission of a low-power alert message to the base station with which it is currently communicating, such as base station  200 . The low-power alert message may be transmitted by mobile station  100  on the cdma2000 reverse dedicated medium access control channel (r-dmch). An r-dmch is assigned to a mobile station when the mobile station is in the active or control hold states. As an alternative, the reverse dedicated traffic channel (r-dtch) may also be used to transmit the low-power alert message while in the active state. Next, at step  318 , the system decreases Tactive and Thold to power-conserving values. Mobile station  100  will now transition more quickly from the active and control hold states to the suspended states, conserving battery power. The process then returns to step  302 , where battery monitor  126  monitors the battery of mobile station  100  for a transition to above the threshold level. 
     If, however, at step  304 , control processor  118  determines that the transition detected at step  302  was a transition to greater than the threshold, the process moves to step  308 . At step  308 , control processor  118  determines if mobile station  100  is currently active on a packet data service. If mobile station  100  is not currently active on a packet data service, the process moves to step  312 . At step  312 , control processor  118  deletes any low-power alert message that has been stored for transmission to the system upon setup of a data service. Deletion of any stored low-power alert message will cause the system to activate a packet data service with Tactive and Thold set to initial (high power) values. If, however, at step  308 , it is determined that mobile station  100  is currently active on a packet data service, the process moves to step  316 . At step  316 , control processor  118  initiates transmission of a high-power alert message to the base station with which it is communicating, such as base station  200 . The high-power alert message may be transmitted by mobile station  100  on r-dmch when the mobile station is in the active or control hold states. Alternatively, the r-dtch may also be used to transmit the high-power alert message while in the active state. Next, at step  320 , the system increases Tactive and Thold to the initial (high power) values. Mobile station  100  will now transition more slowly from the active and control hold states to the suspended state than when Tactive and Thold are set to low-power values. Mobile station  100  will now spend more time in the active and control hold states providing a better quality of service (QoS) for the data service through quicker return times to the active state, since return to the active state is quicker from the control hold state than from the suspended state. The process then returns to step  302 , where battery monitor  126  monitors the battery of mobile station  100  for a transition to below the threshold level. 
     In an alternative embodiment, when battery monitor  126  detects that the mobile station battery power has fallen below the threshold X%, one or more intermediate control states may be prohibited to the mobile station. In this embodiment, a mobile station operating in the control hold state and detecting a low-power condition would transmit a low-power indication signal requesting an immediate state change to the suspended state from the base station. The base station then would transmit a control message to the mobile station forcing a change of state to the suspended state. 
     It will be apparent to those skilled in the art that various alternative embodiments of the invention are possible. For example, while a single threshold level has been described in the disclosed embodiments, it is possible to implement the invention using more than one threshold level and having more than two possible values for the time period durations for triggering transitions between control states, with the time period durations successively set to lower values for each threshold level the battery power falls below. Also the time period duration for triggering transitions between control states could be set in opposite directions, for example, upon determining that mobile station battery power has fallen below a threshold, an active state timer value could be decreased while a control hold state timer could be increased to provide lower power consumption without degrading quality of service as severely as decreasing both timer values. 
     Thus, although the method and apparatus of the invention has been illustrated and described with regard to presently preferred embodiments thereof, it will be understood that numerous modifications and substitutions may be made to the embodiments described and that numerous other embodiments of the invention may be implemented without departing from the spirit and scope of the invention as defined in the following claims.
A mobile station is transitioned between control states in a telecommunications system based on available power at the mobile station. In an embodiment, a mobile station signals the system to indicate that power available to the mobile station is less than or greater than a predetermined threshold and that timers for controlling transitions between packet data service control states are adjusted accordingly. If power is below the predetermined threshold, the time period durations of transition timers for control states that require higher mobile station power can be reduced. The mobile station will then spend less time in those control states, thereby conserving battery power.