GB2393610A

GB2393610A - Clock calibration in a mobile communications device

Info

Publication number: GB2393610A
Application number: GB0222367A
Authority: GB
Inventors: Richard Ormson
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2002-09-26
Filing date: 2002-09-26
Publication date: 2004-03-31
Anticipated expiration: 2022-09-26
Also published as: GB0222367D0; GB2393610B

Abstract

There is provided a method of controlling calibration of a sleep clock 20 by a master clock 18 within the mobile device 10 e.g. GSM handset. The method includes determining the drift between an air interface timebase contained in a received signal 30 and the local timebase 14, which in sleep mode is controlled by the 32Khz slow sleep clock. During active periods, the timebase is driven by higher power consumption high frequency clock 18. The slow sleep clock 18 is calibrated if the drift exceeds a threshold, for example one symbol width.

Description

MOBILE RADIO COMMUNICATIONS DEVICE

AND OPERATING METHOD

The present invention relates to a mobile radio communications device 5 and a method of operation thereof.

With improved functionality of mobile radio communication devices, and in particular with a reduction in size thereof, improved efficiency of operation and related power saving within the device become increasingly requirements.

For example, for mobile phone handsets, when the handset is not in a communication mode, a standby mode is required wherein communication signals can be received as and when sent, and a communication channel established. Even in such a standby mode however, there is a limited number 15 of operations that have to be performed. Such operations, although requiring a finite amount of time, also require that the handset be in a fully powered-up, or so-called "awake" state. For the remainder of the standby time however, the mobile phone handset can be placed into a low power so-called "sleep" state. The longer that the handset can remain in its low power sleep state, the more efficient its power usage will be and so the longer the handset can remain in a standby mode before, for example, battery recharging or replacement is required. In order to assist with power saving, current mobile 25 phone handsets are arranged to provide on a relatively slow clock in order to maintain the local timebase, in contrast to the high-speed clock that normally drives the timebase. Such a slow clock, usually running at 32kHz has a power requirement in the order of microamps, whereas the high-speed clock has a power requirement in the order of milliamps. Thus, when in a sleep mode, the 30 timebase within the mobile handset is maintained by the 32kHz sleep clock so as to save power.

( However, there are disadvantages arising in the use of such a slow sleep clock in that the clock lacks accuracy and recalibration is required at intervals against the fast clock.

5 It is known to employ a variety of parameters in order to determine when to recalibrate the slow clock to the fast clock. For example, it is known to monitor the temperature drift of the 32kHz crystal to identify whether such drift exceeds a threshold value indicative of the need to recalibrate the clock.

Alternatively, it is known to monitor for a failure to decode a paging block, or a 10 timer can be set simply to provide intervals at which recalibration should be initiated. There are however some disadvantages exhibited by such known arrangements. For example, these arrangements require additional hardware, for example to monitor a temperature at the crystal, which disadvantageously serves to increase cost and power requirements. Also, the failure to decode a 15 paging block introduces inherent limitations when comparing operation of such device with the requirements of Third Generation Partnership Project (3GPP) specifications.

Also, if an interval timer has been employed, possibly in situations 20 where it is not possible to measure the temperature at the 32kHz crystal, this often leads to a default arrangement in which the timer is set to initiate recalibration at relatively short intervals, for example in the order of 10 seconds, in order to guarantee that the clock is maintained in calibration for all conditions. Such frequent control of the recalibration disadvantageously 25 increases the power requirement of the device and, in some instances, the resulting consumption of the interval timer accounts for in the order of 7% of the power drawn by the device when in standby mode.

The present invention seeks to provide for a method of controlling 30 recalibration of a sleep clock within a mobile radio communications device, and related mobile radio communications device, having advantages over known such methods and devices.

( According to one aspect of the present invention, there is provided a method of controlling the calibration of a sleep clock to a master clock within a mobile radio communications device, including the steps of determining drift 5 between the air interface timebase and the local timebase as controlled during sleep periods by the sleep clock, and initiating recalibration of the slow clock to the fast clock if the said drift is greater than a threshold value.

The invention is advantageous in providing for an accurate and efficient 10 means for reducing the frequency of at least one of the operations for which the handset must be awakened when a standby mode.

The invention advantageously allows for the determination of clock drift between the sleep clock and the master clock without the need for the 15 employment of additional hardware. As a particular advantage, the method of the present invention can be achieved through the employment of output signals derived from operating software that is already present in known mobile radio communication devices.

20 Advantageously, the threshold value relates to the symbol offset between expected time arrival, and actual time of arrival of signals at the handset. In one embodiment, the determination of drift between the air interface 25 timebase and the local timebase is conducted a plurality of times and an average value is then calculated and compared with the threshold value.

As a further feature, the consistency within which the threshold value is exceeded can also be monitored.

In particular, the parameter indicating the drift between the timebase of the air interface and the local timebase as controlled by the sleep clock is

( derived from digital signal processor operation within the mobile radio communications device.

According to another aspect of the present invention, there is provided 5 a mobile radio communications device having a sleep clock for a power saving mode and a master clock for use in an active mode, means for calibrating the sleep clock to the master clock, means for determining drift between the air interface timebase and the local timebase as controlled by the sleep clock, and means for initiating calibration if the said drift is greater than a threshold value.

As will therefore be appreciated, the method and device of the present invention are particularly advantageous in that they can achieve the greatest power saving for modes of operation when paging block intervals are shortest.

This arises since the local sleep clock signal is reset to the air interface clock 15 following reception of a paging block and so it is only the drift of the sleep clock within one paging interval that has to be monitored. Generally, the shorter the paging interval, the shorter the standby time and so a reduction of power consumption when employing shorter paging intervals is particularly desirable. As an example, with reference to GSM there are possible paging intervals of between two and nine multiframes. Assuming a common drift limit for both such intervals and assuming that clock drift remains linear over time, there is therefore clearly a greater likelihood that recalibration will be required 25 when employing paging intervals of nine multiframes, than when paging intervals of two multiframes are employed.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing which comprises a schematic 30 block diagram of a mobile telephone handset adaptively use in accordance with an embodiment of the present invention.

Turning to the figure, there is illustrated a mobile phone handset 10 having schematically illustrated communication functionality 12 including a timebase generating circuit 14 and a Digital Signal Processor (DSP) equalizer 16. The timebase circuitry 14 is arranged to provide for the local timebase 5 within the handset 10 under the control of signals derived from either a master clock 18, or the slower sleep mode clock 22 by means of control lines 24, 26 respectively. A key aspect of the present invention relates to the manner in which it is 10 determined that the sleep clock 22 should be recalibrated to the master clock 18. Of course, once running as required, each of the clocks 18,22 employs clock routing lines 40,42 for driving the device.

15 Communication by way of the mobile handset is achieved via an antenna 28 and it will be appreciated that a local timebase is created by means of the timebase generating circuitry 14 within the handset. This local timebase can be compared with the air interface timebase exhibited in an incoming signal 30 received at the antenna 28.

The DSP equaliser 16 receives the incoming signal from the antenna 28 by way of r.f. circuitry 32. Further, the DSP equaliser 16 is arranged to determine the offset between the expected time of arrival of signals within a paging block and the time when such signals actually did arrive. Currently 25 known DSP equalisers can operate over a window of +/- 2 symbols offset between the expected, and actual time of arrival, of the signal bursts.

The present invention advantageously employs this offset parameter in order to determine when calibration of the sleep clock 22 is required. Between 30 the clocks 18, 22 is provided a calibration counter module 34 and which serves for the recalibration of the sleep clock 22 to the more accurate master clock 18 since the sleep clock 22 proves to be relatively inaccurate. The calibration

( counter module 34 is connected by way of clock routing lines 36,38 to the clocks 22 and 18 respectively. The calibration counter module 34 is arranged to count the number of fast clock pulses within a given number of slow clock pulses. The determination of when to calibrate is made within a CPU 20 which 5 is connected between the DSP equaliser 16 and the master clock 18.

For example, if it is found that the offset is consistently greater than 1 symbol then this can be employed as an indication that the sleep clock has become inaccurate and requires recalibration.

Although it is known that the offset determined at the DSP equaliser 16 may vary by up to one symbol as a result of multipath and signal fading effects, this need not disadvantageously limit the present invention. For example, when monitoring the arrival of a paging block, the arrival of, for 15 example, each of the four bursts can be determined, and then an average taken of the four drift values. Thus, for example, if the resulting average value is greater than 1 symbol, this can be employed as a determination that recalibration of the sleep clock 22 is required.

20 It should of course be appreciated that the invention is not restricted for the details of the foregoing in that the drift between the timebases can be monitored, and the resulting value employed, whether averaged or not, in any scheme thought appropriate.

Claims

( Claims

1. A method of controlling the calibration of a sleep clock to a master clock within a mobile radio communications device, including the 5 steps of determining drift between the air interface timebase and the local timebase as controlled during sleep periods by the sleep clock, and initiating recalibration of the slow clock to the master clock if the said drift is greater than a threshold value.

10

2. A method as claimed in Claim 1 wherein the drift between the said air interfaces in measured by reference to paging blocks received at the device.

3. A method as claimed in Claim 1 or 2, wherein the drift between the 15 air interfaces is measured at a symbol level.

4. A method as claimed in Claim 3, wherein the said threshold value comprises one symbol.

20

5. A method as claimed in any one of Claims 14, wherein the drift between the said air interfaces is determined a plurality of times and an average value taken to determine if the threshold value is exceeded. 25

6. A method as claimed in any one of Claims 1-5, and including the step of determining a plurality of values for the drift between the timebase and further determining the consistency within which the threshold value is exceeded.

30

7. A method as claimed in any one of Claims 1-6, wherein the step of determining the drift between the time basis comprises the step of determining a drift over a paging interval.

(

8. A mobile radio communications device having a sleep clock for a power saving mode and a master clock for use in an active mode, means for calibrating the sleep clock to the master clock, means for 5 determining drift between the air interface timebase and the local timebase as controlled by the sleep clock, and means for initiating calibration if the said drift is greater than a threshold value.

9. A device as claimed in Claim 8, wherein the means for determining 10 drift between the air interface timebase and the local timebase is controlled by the sleep clock comprises a GSP equaliser.

10. A device as claimed in Claim 8 or 9, wherein the means for determining the said drift in the timebases is arranged to determine 15 a plurality of such values.

11. A device as claimed in Claim 10 and including means for averaging the determined values of drift between the timebases and identifying whether the average values exceeds the said threshold.

12. A method of controlling recalibration of a sleep clock to a master clock within a mobile radio communications device, substantially as hereinbefore described, with reference to, and as illustrated in, the accompanying drawing.

13. A mobile radio communications device substantially as hereinbefore described, with reference to, and as illustrated in, the accompanying drawing.