Wireless telephone that rapidly reacquires a timing...

Details Wireless telephone that rapidly reacquires a timing... Wireless telephone that rapidly reacquires a timing...

1998-12-03

2001-04-03

Eisenzopf, Reinhard J. (Department: 2745)

Telecommunications

Transmitter and receiver at separate stations

Plural transmitters or receivers

C455S067700, C455S574000, C455S343200, C370S311000

Reexamination Certificate

active

06212398

ABSTRACT:

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's clock to the system'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' 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' parameters is a time offset and the step of storing one or more of the pilot signals' 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'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'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'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 paramete

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