Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1999-11-04
2004-05-11
Appiah, Charles (Department: 2686)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S343200
Reexamination Certificate
active
06735454
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention generally relates to mobile communication systems and in particular to techniques for activating a high frequency clock following a sleep period within a mobile station of a mobile communications system employing slotted paging.
II. Description of the Related Art
Certain state of the art wireless communication systems, such as Code Division Multiple Access (CDMA) Systems, employ slotted paging to allow mobile stations to conserve battery power. In a slotted paging mode, paging signals are transmitted from a base station to particular mobile stations only within assigned paging slots separated by predetermined intervals of time. Accordingly, each individual mobile station may remain within a sleep mode during the period of time between consecutive paging slots without risk of missed paging signals. Whether any particular mobile station can switch from an active-mode to a sleep mode depends, however, upon whether the mobile station is currently engaged in any user activity such as processing input commands entered by the user or processing a telephonic communication on behalf of the user. Assuming that the mobile station is not currently engaged in any processing on behalf of the user, the mobile station automatically powers down selected internal compounds during each period of time between consecutive slots. One example of a slotted paging system is disclosed in U.S. Pat. No. 5,392,287, entitled “Apparatus and Method for Reducing Power Consumption in a Mobile Receiver”, issued Feb. 21, 1995, assigned to the assignee of the present invention and incorporated by reference herein.
Thus, within the slotted paging mode, a mobile station reduces power consumption by disconnecting power from selected internal components during a sleep period between consecutive slots. However, even during the sleep period, the mobile station must reliably track the amount of elapsed time to determine when the next slot occurs to permit receive components of the mobile station to power up in time to receive any paging signals transmitted to the mobile station within the slot. One solution to this problem is to operate a high frequency clock throughout the sleep period and to track the amount of elapsed time using the high frequency clock. This solution allows the sleep period to be very precisely tracked. However, considerable power is consumed operating the high frequency clock and optimal power savings therefore are not achieved during the sleep period.
Hence, it would be desirable to instead employ a low frequency, low power clock during the sleep period to reduce power consumption. However, clock signals provided by low frequency, low power clocks typically suffer from considerable frequency drift such that the amount of elapsed time during the sleep period cannot be precisely determined by counting cycles of the low power, low frequency clock. Frequency drift within a mobile station can be particularly significant if there are temperature variations within the mobile station caused by, for example, heat generated by the operation of components of the mobile station or by changes in ambient conditions. For example, during an extended telephone call, internal components of the mobile station may heat to 87 degrees Celsius. During an extended period of inactivity between telephone calls, the internal components may cool to an ambient temperature of, perhaps, 25 degrees Celsius. Moreover, if the user places the mobile telephone in either a very hot or very cold location, corresponding temperature changes within the mobile station may occur. Typical low power, low frequency clock signal generators are affected significantly by even slight changes in temperature and are even more strongly affected by the wide variations in temperature that can occur in a mobile telephone. Indeed, the amount of frequency drift within a typical low power, low frequency clock signal used in a mobile station may be so great that, if used by itself to calculate elapsed time within the sleep period, there is a significant risk that the mobile station will not be reactivated in time to power up components to detect a paging signal transmitted within a next paging slot. Accordingly, important paging signals maybe missed, possibly resulting in missed phone calls and the like. Thus the timing accuracy provided by a low frequency, low power clock signal is typically poor.
Another significant problem with using low frequency clock signals to track elapsed time within a sleep period is the relative lack of precision provided by the low frequency clock. The lack of precision can result in a considerable off-set between the initiation of the sleep period and a first counted cycle of the low frequency clock signal and also a considerable off-set between a last counted cycle of the low frequency clock and the actual end of the sleep period. More specifically, a counter is typically employed to count either rising edges or falling edges of the low frequency clock signal to track elapsed time within the sleep period and, once the number of cycles of the low frequency clock corresponding to the length of the sleep period have been counted, the high frequency clock is then re-activated. However, nearly an entire cycle of the low frequency clock may elapse between the beginning at the sleep period and the first edge of the low-frequency clock signal detected by the counter. The initial off-set can have a duration anywhere from zero to one full cycle of the low frequency clock or, in some systems, possibly even more. With conventional systems, it is not possible to determine the duration of the initial off-set. The uncertainty in the duration of the initial offset further increases the amount of error in the determination of elapsed time within the sleep period resulting in an even greater risk that the next paging slot will be missed. In an exemplary system wherein the high frequency clock operates at 9.68 megahertz and the sleep clock operates at 32 kilohertz, there are about 300 cycles of the high frequency clock within each cycle of the sleep clock. Therefore, even if the system can reliably compensate for frequency drift, the high frequency clock may still need to be activated as many as 300 cycles of the high frequency clock earlier than necessary to thereby account for the unknown duration of the intial offset. Also, because the re-activation of the high frequency clock at the end of the sleep period is synchronized with transitions in the low frequency clock, the degree of precision by which the high frequency clock can be re-activated is limited by the precision of the low frequency clock. For example, even if the system reliably and precisely determines that the correct duration of the sleep period is 853.44 cycles of the sleep clock, the system will need to re-activate the high frequency clock no later than the detected transition of the 853rd cycle and therefore will not properly account for the remaining fractional number of cycles, i.e. the remaining 0.44 cycles. With about 300 cycles of the high frequency clock occurring within each cycle of the sleep clock, in the example the high frequency clock is therefore turned on an additional 130 cycles earlier than necessary. In another example, if the correct duration of the sleep period is 853.99 cycles of the sleep mode clock, the high frequency clock will be turned on nearly 300 cycles earlier than necessary.
Hence, when using a low-frequency clock signal to track time during a sleep period, the mobile station is usually configured to return to an active mode well in advance of a next expected paging slot to thereby overcome possible timing errors cause by frequency drift in the low frequency clock and to compensate for the relative lack of precision in the low frequency clock. For example, if paging slots occur every 26.67 milliseconds, the mobile station may be programmed to activate the high frequency clock after only, for example, 25 milliseconds of sleep to ensure that the next paging slot is not missed. Hen
Easton Kenneth David
Sankuratri Raghu
Yu Nicholas K.
Appiah Charles
Brown Charles D.
Qualcomm Incorporated
Seo Howard H.
Wadsworth Philip R.
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