Abstract:
A method for rapidly reacquiring a timing reference after a sleep period in a wireless telephone receiving pilot signals from one or more base stations in a wireless network, wherein the wireless telephone has a master timer (local time reference), a fast, accurate clock source, a slow, less accurate but power efficient clock source and a memory. The method includes the steps of entering the sleep period by storing one or more parameters related to the pilot signals and how the pilot signals are changing over time, calculating a prediction of the parameters of the pilot signals after a sleep period based on the stored parameters, storing the prediction, starting the slow clock, and stopping the master timer (local time reference) and fast clock source. The method further includes the steps of ending the sleep period by generating a wake-up interrupt by the slow clock after the sleep period, restarting the master timer and fast clock source responsive to the wake-up interrupt, and reacquiring pilot signals using the predictions stored previously. A time correction factor can then be computed that closely aligns the received pilot signals on the prediction which is then weighted and the master timer is adjusted by the weighted time correction.

Description:
FIELD OF THE INVENTION 
     This invention relates to reacquiring a timing reference from a wireless network after a sleep mode in a wireless telephone. 
     BACKGROUND OF THE INVENTION 
     A tension exists between the diverse goals of minimizing size and weight of a wireless telephone (also called cell phones, mobile stations or mobile telephones) and the amount of time that the wireless telephone may be used without recharging the battery. Generally, the greater the battery capacity, the larger and heavier the battery is. While many advances have been made in battery technology to address this issue, efforts are being made in other areas of wireless telephone technology to conserve battery energy and hence lengthen the useful time of the wireless telephone between recharging. 
     In direct spread spectrum, code division multiple access (CDMA) wireless technology, there is recognition of a need for battery energy conservation. To this end, the IS95 CDMA standard specifies a “sleep mode” for the wireless telephone, wherein the components that consume the most power are turned off. When the wireless telephone is idle (that is, not on call and not receiving instructions from the wireless system), the wireless telephone is only listening on the paging channel for instructions or a page from the wireless system. Paging messages for a particular wireless telephone can occur from once every 1.28 seconds to once every 163.84 seconds. The wireless telephone can thus turn off the power to many of its components during the other times. This technique provides a very powerful method by which a battery-operated wireless telephone can conserve battery energy when idle. 
     A problem arises, however, when the components of the wireless telephone come out of the sleep mode. As is known in the art, most wireless communication systems use a centralized time reference to maintain synchronization of modulation and demodulation. Every wireless telephone includes an oscillator as frequency reference and input to a master timer or local system time reference. This oscillator is, in many cases, a temperature-compensated crystal oscillator that maintains precise alignment of the wireless telephone&#39;s clock to the system&#39;s reference clock. It is desirable to disable (power off) the temperature-compensated crystal oscillator because it uses a relatively large amount of power when active. However, to begin modulating and demodulating signals again, the master timer (local time reference) must be resynchronized to the network time reference when the oscillator is turned back on. One method for resynchronization is to acquire the synchronization channel as in the wireless telephone power-up routine. Such synchronization may take most of the time of the sleep period. In the worst case, acquisition of the synchronization channel may take more power than the master timer requires if it remained powered-on during the sleep period. 
     This invention is directed to solving one or more of the above-described problems. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of this invention, a method is disclosed for rapidly reacquiring a timing reference after a sleep period in a wireless telephone receiving pilot signals that have a plurality of parameters from one or more base stations in a wireless network. The wireless telephone has a local time reference (master timer) synchronized to a timing reference in the wireless network, a fast, accurate clock, a slow, less accurate but more power efficient clock and a memory. The method includes the steps of storing one or more of the pilot signals&#39; parameters, calculating predicted parameters of the pilot signals after the sleep period based on the stored parameters, loading the prediction into the master timer, starting the slow clock source, and stopping the master timer (local time reference) and fast clock source. The method further includes the steps of generating a wake-up interrupt by the slow clock after the sleep period, restarting the master timer and fast clock source responsive to the wake-up interrupt, and reacquiring pilot signals. A time correction factor is then computed that closely aligns the received pilot signals with the prediction. The master timer (local time reference) is adjusted by the weighted time correction. 
     In accordance with a further aspect of this invention, the wireless telephone includes a counter in the slow clock and the step of generating a wake-up interrupt comprises generating the wake-up interrupt when the counter reaches a predetermined number. In accordance with an additional aspect of this invention, one of the pilot signals&#39; parameters is a time offset and the step of storing one or more of the pilot signals&#39; parameters includes storing the time offset. 
     In accordance with another aspect of this invention, the step of calculating predictions of the pilot signals comprises calculating a prediction of each pilot signal&#39;s time offset after the sleep period and further may include weighting the time corrections based on the signal strength of the reacquired pilot signals. 
     In accordance with another aspect of this invention, the step of weighting the time corrections comprises weighting the time corrections based on the arrival time of each of the reacquired pilot signals, wherein earlier arriving pilot signals are assigned more weight than later arriving pilot signals. 
     In accordance with a further aspect of this invention, the step of adjusting the master timer comprise averaging the weighted time corrections and adjusting the master timer by advancing or delaying it by the amount of the time correction. 
     In accordance with another aspect of this invention, the step of calculating predicted parameters of the pilot signals after a sleep period comprises basing the prediction on factors internal to the wireless telephone, wherein the internal factors include the age of the slow clock, the current state of the slow clock&#39;s power supply voltage and the temperature of the slow clock. 
     In accordance with a further aspect of this invention, the step of calculating predicted parameters of the pilot signals after a sleep period comprises basing the predictions on factors external to the wireless telephone, such as movement of the wireless telephone, arrival time of reflective pilot signals and the course of the wireless telephone. In accordance with another aspect of this invention, the wireless telephone&#39;s position and course may be measured by a global positioning system or based on the latitude and longitude of the transmitting base stations. In accordance with a further aspect of this invention, the master time measures time in chips and the step of adjusting the master timer comprises advancing or delaying the master timer one chip at a time. 
     In accordance with a different aspect of this invention, a wireless telephone is disclosed that rapidly reacquires a timing reference after a sleep period wherein the wireless telephone receives pilot signals having a plurality of parameters from one of more base stations in a wireless network. Wireless telephone comprises a slow clock configured to time the sleep period and generate a wakeup interrupt at the end of the sleep period, a master timer configured to provide a timing reference for the wireless telephone and a digital signal processor configured to demodulate the pilot signals using the timing reference and to determine parameters in pilot signals. The wireless telephone further includes a processor which is configured to calculate a prediction of the parameters of the pilot signals after the sleep period based on the parameters determined by the digital signal processor, start the sleep period, end the sleep period responsive to the interrupt, compare parameters of pilot signals delivered by the digital signal processor to the prediction and adjust the master timer to align the timing reference of the master timer to the pilot signals. 
     In accordance with another aspect of this invention, a wireless telephone further includes a memory for storing a history of parameters of pilot signals and wherein the processor is configured to calculate the prediction of the parameters of the pilot signals after the sleep period based on the parameters determined by the digital signal processor in the history of parameters. 
     In accordance with a further aspect of this invention, the processor is configured to turn on power to the slow clock prior to the sleep period and turn off power to the slow clock after the sleep period and to turn off power to the master timer at the beginning of the sleep period and turn on power to the master timer at the end of the sleep period. 
     In accordance with yet another aspect of this invention, the master timer includes a counter and the processor is configured to adjust the master timer by speeding up or slowing down the counter. The wireless telephone further includes a fast timing reference that operates at a multiple of the timing reference and the processor is configured to speed up the master timer by advancing the counter according to the fast timing reference. Further, the processor is configured to adjust the master timer by slowing down the counter by the timing reference. 
     In accordance with a further aspect of this invention, the wireless telephone may include a global positioning system wherein the processor is configured to determine the position of the wireless telephone and to calculate the prediction of parameter of pilot signals based on the position. Further, the processor can store a plurality of positions over time and base the calculation of the prediction of parameters on the stored plurality of positions. 
     Therefore, it is an object of this invention to provide a system that can rapidly reacquire the timing synchronization after a sleep mode. 
     It is another object of this invention to provide a system and method that minimized power consumption while maintaining operability. 
     It is a further object of this invention to provide a method for rapidly reacquiring the timing synchronization and maximize the sleep period. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of this invention may be obtained from a consideration of the following detailed description, in conjunction with the drawings, in which: 
     FIG. 1 is a block diagram of a wireless telephone that can rapidly reacquire a timing reference after a sleep mode in accordance with an exemplary embodiment of this invention; 
     FIG. 2 is a flow chart of operation of the wireless telephone of FIG. 1 prior to the sleep mode; 
     FIG. 3 is the operation of the wireless telephone of FIG. 1 after the sleep mode; 
     FIG. 4 is an exemplary sliding correlator of FIG. 1; and 
     FIG. 5 is an example of predicted correlation results versus actual correlation results in reacquiring the timing reference. 
    
    
     DETAILED DESCRIPTION 
     A sleep mode in a wireless telephone requires that the wireless telephone be able to resynchronize to the wireless network&#39;s time as rapidly as possible in order to maximize the benefit of the sleep mode. To this end, a database of internal and external parameters which may affect a slow (but energy efficient) clock is kept. Internal parameters may include such things as the age of the crystal source, battery age, ambient temperature, etc. External parameters include relative movement of the wireless telephone of the present invention to the pilot signals and a relation to geography in general. The wireless telephone predicts the pilot signals&#39; parameters (e.g., time and frequency offset) at the end of the sleep mode and then shuts down or halts a number of internal devices (except for the slow clock and a sleep mode controller). At the end of the sleep mode, the wireless telephone reacquires the pilot signals and resynchronizes the master timer by determining how far off the estimation is from the reacquired current pilot signals. 
     FIG. 1 shows a block diagram of a wireless telephone shown generally at  10 . At the heart of the wireless telephone  10  is a microprocessor  12 , which coordinates, controls and operates the wireless telephone  10  using programs and data stored in memory  14 . Microprocessor  12  also controls a display  16  and a keypad  18  that displays information to the user and receives input from the user, respectively. 
     A digital signal processor  20  provides a connection between a speaker and a microphone, as are known in the art (not shown here for clarity), and the signal modulation/demodulation portion of the wireless telephone  10 . The digital signal processor  20  coordinates its actions with a master timer  22  which, in this exemplary embodiment, includes a temperature-compensated, crystal oscillator. Master timer  22  increments counter  24  in “chips,” as is known in the art, as the basic unit of time. A signal from master timer  22  is delivered to a transmit modulator  26 , a searcher  28 , and rake receivers  30 ,  32  and  34 . 
     The transmit modulator  26  modulates symbols representing the data on speech to be transmitted prepared by digital signal processor  20  with the known “access code” (derived from the “spreading code,” “long code” and other mobile specific parameters) and transmits the resulting signal to the digital to analog converter (D/A)  36 . In the opposite direction, an analog to digital converter (A/D)  38  delivers a digital representation of waveforms to searcher receiver  28  and rake receivers  30 ,  32  and  34 . RF transceiver  40  sends and receives analog wave forms between wireless telephone  10  and a wireless network (not shown but well known in the art) over antenna  42 . 
     In a typical RF communication system, a transmitted signal may travel from a transmitter to a receiver over multiple paths, for example, a direct path and also one or more reflected paths. Each path (which may be considered a separate “channel”) is subject to the effects of fading, Doppler shift, etc. Moreover, the combination of channels at the receiver can result in additional fading. A CDMA wireless telephone  10  includes more than one rake receiver, illustrated here as three rake receivers  30 ,  32  and  34 , each of which demodulate a received channel. The output of the rake receivers  30 ,  32  and  34  are constructively combined and delivered to digital signal processor  20 . However, in order to constructively combine the signals, the rake receivers  30 ,  32  and  34  must know the delay of each channel. (Two of the rake receivers are delayed so that all three signals align when combined.) Typically, rake receivers  30 ,  32  and  34  operate in conjunction with a searcher  28 . The searcher  28  analyzes received signals to determine the several delayed versions of the signal of interest. The strongest channels are assigned to the rake receivers  30 ,  32  and  34  and the two delays are set. 
     According to this invention, a sleep mode controller  50  operates in conjunction with the microprocessor  12 , a fast clock source  52  and a slow clock source  54 . The fast clock source  52  operates at a speed faster than the master timer  22 , as is known in the art. The slow clock source  54 , as will be described further below, includes an oscillator that is slower and less accurate than the oscillator in master timer  22 , but is more energy efficient. 
     An exemplary method that operates in the mobile station of FIG. 1 will now be described in connection with the diagrams of FIGS. 2 and 3. Processing starts in circle  200  wherein mobile station  10  is actively demodulating signals, such as pilot signals, from one or more base stations. Processing moves to box  202  where representations of the parameters of the pilot signals (complex vectors describing the pilot signals) are stored in memory  14 . Such parameters include, but are not limited to, frequency offset and timing offset. To this end, digital signal processor  20  configures the searcher  28  to perform a correlation search of the pilot signal from the serving base station, pilot signals from surrounding neighbor base stations and any other previously identified pilot signal sets at predefined time offsets. The digital signal processor  20  stores the information in a data base in memory  14 . Thus, pilot signal parameters are accumulated that represent the time offset and signal strength of the pilot signals and how the time offset and signal strength change over time. Processing then moves to decision diamond  204  where a determination is made whether a sleep mode is possible. If the sleep mode is not possible (i.e., the mobile station is on a call or is receiving instructions from the base station), then processing returns to box  202 . 
     If the microprocessor  12  determines that the sleep mode is possible in decision diamond  204 , then processing proceeds to box  206  where the microprocessor  12  calculates the appropriate wake-up time for the wireless telephone  10 . This data is loaded into sleep mode controller  50 . In box  206 , the digital signal processor  20  calculates and stores a prediction of the pilot signals&#39; parameters after the sleep mode is completed by using the information stored in the database and the length of the sleep mode, and advantageously other data. Such calculations include, but are not limited to, calculating a frequency offset of one or more pilot signals at the end of the sleep period by determining how the frequency offsets and other data will change over the sleep period by extrapolating over the period of time that the master timer will be in the sleep mode. A further calculation may be to determining the timing offset of one or more pilot signals at the end of the sleep period by determining how the timing offset of the pilot signals have changed over time and extrapolating the change of timing offset over the time period of the sleep mode. Similar calculations may be made on other signal parameters. 
     Other data that can be used in the calculation may include internal factors, including the age of the crystal of the slow clock source  54 , current battery age, oscillator&#39;s supply voltage, temperature of the unit, and previous calibration results at known voltage/temperature conditions/age. External factors that affect the timing of the pilot signals include movement of the wireless telephone. Further, an approximate position and course for the wireless telephone  10  may be estimated. This estimation may be computed based on an optional Global Positioning System (GPS) receiver (not shown but well known in the art) in the wireless telephone  10  or the change in time of arrival of signals from several sufficiently geographically separated base stations (wherein the base station latitude/longitude is provided in higher level protocol messaging, as is known in the art). The course and speed can further be used to refine the prediction because the signals from base stations that the wireless telephone  10  is heading towards arrives earlier in time and signals from base stations that the wireless telephone  10  is heading away from arrive later. 
     Processing moves to action box  208  where the microprocessor  12  starts the slow clock  54  and enables the slow clock  54  to time the sleep controller  50 . A counter  56  is incremented by slow clock  54  at its oscillation rate, as known in the art. Microprocessor  12  then loads the wake-up time into the sleep controller  50 . The microprocessor  12  also loads the prediction into the master timer  22  (to be used at wake-up) and then turns off the master timer  22 . Microprocessor  12 , in action box  210 , turns off the power to the fast clock source  54 . Microprocessor  12  then halts to wait for the wake-up interrupt. In block  212 , the mobile station  10  is in sleep mode. Thus, only the slow clock source  54  and the counter  56  in the sleep mode controller  50  are powered and active in sleep mode. 
     Turning now to FIG. 3, processing after wake-up from the sleep mode  212  is described. After the timer has expired (the counter  56  in the sleep mode controller  22  has reached a predetermined number), processing moves from sleep mode  212  when an interrupt  300  arrives at the microprocessor  12  from the sleep mode controller  50 . Processing moves to action box  302  where the microprocessor  12  restarts the master timer  22 . Processor  12  also restarts searcher  28 , rake receivers  30 ,  32  and  34 , digital signal processor  20 , and any other components that were powered down or halted during the sleep mode. The fast clock source  52  is switched into the place of the slow clock source  54 . The parameters in CDMA that need to be reinitialized (i.e., the parameters that depend upon the master timer  22 ) include the state of the quadrature spreading sequence (short pseudo-noise code), the state of the long pseudo-noise code, frame timing and timing associated with the wireless telephone&#39;s rake receiver demodulation components. Processing moves to action box  304  where the microprocessor  12  initiates a search by configuring the searcher  28  to perform a search to reacquire the pilot signals using the previously-stored parameters, and restarted master timer  22 . 
     Processing continues to action box  306  where, for each reacquired pilot signal, the digital signal processor  20  computes a time correction that most likely aligns the reacquired pilot signal with the prediction. Processing moves to box  308  where, for each reacquired pilot signal, the digital signal processor  20  weights the computed time correction based on the signal strength of the reacquired pilot signal, its position in time (earlier weighted heavier than later), previous stability of the slow clock  54  and consistency of the position of the wireless telephone  10  by biasing the correction in favor of stronger signal, stable clock, etc. Processing moves to action box  310  where the digital signal processor  20  averages the weighted time corrections and adjusts the master timer  22  by the weighted average time correction by advancing or delaying the timer in a chip-wise fashion. Advantageously, such calculation of boxes  306  and  308  may use a sliding correlator  302 , as shown in FIG.  4 . Processing ends in circle  312  where the wireless telephone  10  is again actively demodulating the pilot and access channels. 
     The sliding correlator  302  of FIG. 4 takes newly received baseband signal samples on line  400  and applies the signal samples to a delay line  402  as known in the art. Delay taps  404 - 1 - 404 -X tap the baseband samples at various points on delay line  402 . These samples are then multiplied in multiplying taps  406 - 1 - 406 -X with the predicted segment of, for example, the long code. The products of multiplying taps are added at summer  408  to generate a correlation. The rate at which the long code is multiplied may be advanced or retarded, and the master time is synchronized at the time of the greatest correlation results. 
     Turning now to FIG. 5, a graph of signal strength versus time is shown. When the system comes out of the sleep mode, it discovers pilot signals at positions  500 ,  502  and  504 . As illustrated in this example, the predicted correlation results  506 ,  508  and  510  are behind in time to the search results (from searcher receiver  28 ). Also, the newly acquired pilot signals do not match the predicted parameters of the pilot signals (i.e., shape, magnitude of the signal strength, etc.). This reflects the reality that the signals are constantly changing in relation to the wireless telephone  10 . In the scenario of FIG. 5, the timing prediction is corrected by advancing the system time until the actual correlation best matches the prediction. In this exemplary embodiment, it is preferable that the prediction be behind the true system time after the sleep mode. The rate at which the time can be advanced is a function of the fastest clock (fast clock  52 , FIG.  1 ). If the fast clock  52  is operating at twelve time the rate of the master timer  22 , then the time can be adjusted twelve times as fast. If the hypothesis is ahead of system time, correction can only occur at a rate of one chip per chip period. 
     It is therefore apparent that this invention provides a system that maximizes the idle mode powered-down time. By predicting pilot signals at the end of the sleep period, the system can load these predictions and then search rapidly to correlate the predictions to the newly observed pilot data. The master timer can then be advanced or retarded, depending on the observed pilot signals. It is to be understood that many variations may be devised by those skilled in the art without departing from the scope of this invention. It is intended that such variations be included within the scope of the appended claims.