GB2397465A

GB2397465A - Clock calibration in a mobile radio communications device

Info

Publication number: GB2397465A
Application number: GB0300919A
Authority: GB
Inventors: Jean-Francois Deprun; Richard Ormson
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2003-01-15
Filing date: 2003-01-15
Publication date: 2004-07-21
Anticipated expiration: 2023-01-15
Also published as: GB2397465B; GB0300919D0

Abstract

A method of calibrating a sleep clock of a mobile radio communications device includes the steps of determining drift between the air interface timebase and the local timebase as controlled during sleep periods by the sleep clock, and adding the determined value of drift to a preceding calibration value to arrive at a calibration value for calibrating the sleep clock. The method allows the mobile device to remain in sleep mode for longer periods of time thus conserving more battery power, and the fast clock is not required for calibration.

Description

MOBILE RADIO COMMUNICATIONS DEVICE

AND OPERATING METHOD

The present invention relates to a mobile radio communications device 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 important 1 0 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 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, and in a 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 phone handsets are arranged to rely 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 timebase within the mobile handset is maintained by the 32kHz sleep clock and this serves to advantageously reduce power consumption.

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.

It is known to employ a variety of parameters in order to determine when to recalibrate the slow clock to the fast clock although these are not themselves without disadvantages. 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 timer can be set simply to provide intervals at which recalibration should be initiated. There are however some disadvantages exhibited by such known arrangements.

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 paging block introduces inherent limitations when comparing operation of such devices with the requirements of Third Generation Partnership Project (3GPP) specifications.

Also, if an interval timer has been employed, possibly in situations 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 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.

Such known recalibration, once commenced, also requires the fast clock to be active when it is not need for any purpose other than re-calibration.

This process therefore exhibits a power consumption overhead.

The present invention seeks to provide for a method of recalibrating a sleep clock within a mobile radio communications device, and a 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 calibrating a sleep clock of a mobile radio communications device including the steps of determining drift between the air interface timebase and the local timebase as controlled during sleep periods by the sleep clock, and adding the determined value of drift to a preceding calibration value so as to arrive at a calibration value for calibrating the sleep clock.

The invention is advantageous in that it provides for a method of calibrating the slow sleep clock so as to allow the handset to remain in sleep mode for longer time periods. In particular, the advantages can be achieved without requiring additional hardware.

The method of the present invention advantageously uses outputs from software that are already available within the handset and advantageously can eliminate the calibration operation as a distinct event so as to allow the handset to remain in deep sleep mode for longer.

Thus, as a further advantage, clock calibration can be provided on a more frequent basis without additional power consumption overheads and such that the sleep clock retains a high degree of accuracy.

The fast clock is therefore not required for the calibration steps according to the present invention.

Advantageously, the drift that represents the offset between the timebases can be represented in terms of symbols.

According to one particular feature of the present invention, the value of the drift is represented as a number of fast clock cycles and, advantageously, from identification of the number of fast clocks within one symbol.

Preferably the said previous calibration is represented by determination of the number of fast clock cycles in the sleep period.

Advantageously, an initial calibration step can be employed in order to cross calibrate the slow clock to the fast clock.

Further, the calibration procedure can be performed every paging cycle.

Still further, the calibration period can advantageously be the same as the sleep period and eliminate fractional count errors.

In order to arrive at an offset value, the method can advantageously average the offset determined over a four burst paging reception.

According to another aspect of the present invention, there is provided a mobile radio communications device having a sleep clock and including means for recalibrating the sleep clock and which includes means for determining drift between the air interface timebase and the local timebase as controlled during sleep periods by the said sleep clock, and means for adding the determined value of the drift to a preceding calibration value and so as to arrive at a calibration value for calibrating the sleep clock.

Such a device therefore advantageously exhibits the advantages noted above and can also be arranged to operate in accordance with the additional method steps noted.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing which comprises a schematic 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 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.

Once running as required, each of the clocks 18,22 employs clock routing lines 40,42 for driving the device.

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 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 recalibrate the sleep clock 22 as required. However, between 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.

Such know cross calibration can be employed to establish an initial calibration value which assists with the generally iterative method of the invention. The calibration counter module 34 is connected by way of clock routing lines 36,38 to the clocks 22 and 18 respectively and is arranged to count the number of fast clock pulses within a given number of slow clock pulses. The determination of the calibration value is made within a CPU 20 which is connected between the DSP equaliser 16 and the sleep clock 22.

Once such initial calibration has been achieved, subsequent recalibration advantageously employs the method of the present invention.

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 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 in recalibrating the sleep clock 22 is required.

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

As discussed further below, the CPU is arranged to arrive at a calibration value to deliver to the sleep clock 22 on the basis of aforesaid offset value, the number of fast clock cycles in a symbol, the sleep period in 32kHz clock cycles and the number of fast clock cycles in that number of 32kHz cycles as determined at the last calibration.

Use of the offset parameter as a clock calibration in its own right can prove advantageous for a variety of reasons.

First cross calibration to the fast clock is no longer necessary each time recalibration is required. As an example, current cross calibration procedures typically take 0.25 seconds and draw 50mA. On average the recalibration will occur every 6 minutes. Thus, for a typical handset standby of hours, the cross calibration procedure is responsible for (180*60/6)*(0.25/3600)*50mAh =6.25 mAh of consumption. For a 650mAh battery for example, this is approximately 1% so that avoiding the cross calibration operation, can lead to an extension of standby time by 1%.

Since the calibration procedure of the present invention will no longer draw any appreciable current, it can be performed every paging cycle. This has two particular benefits in that one can eliminate some complexity from the software as no decision is required on whether or not to calibrate, and a more accurate timebase is achievable overall.

Also, the calibration period becomes the same length as the sleep period, and this can serve to eliminate fractional count errors that are seen when using the conventional method. That is, when calibrating, the number of fast clock pulses in a defined number of slow clock pulses are counted.

Although this count is accurate to +/-1, calibration generally takes 0.25 seconds, while the paging interval is larger than this. Thus the error is multiplied by the ratio of the paging interval to the calibration period.

As proposed in the present invention, the use of the offset parameter to calibrate the slow clock can prove relatively simple. The following data is readily available within the terminal.

An offset in terms of symbols at the end of a sleep period between the timebase derived from the slow clock within the handset and the timebase received by the handset over the air interface. The number of fast clock cycles in a symbol. Also, a symbol represents a set period of time and the relationship between symbols and fast clock cycles is well defined. The sleep period in terms of 32kHz clock cycles, and the number of fast clock cycles in that number of 32kHz cycles as determined at the last calibration The new calibration value is therefore simply determined as the previous calibration plus the offset in fast clock cycles. Of course, the offset can be negative.

As mentioned above, the process is iterative and so will require an initial calibration. There is a variety of options for this. However, with current hardware, it could be provided using the cross calibration method.

Claims

Claims 1. A method of calibrating a sleep clock of a mobile radio

communications device including the steps of determining drift between the air interface timebase and the local timebase as controlled during sleep periods by the sleep clock, and adding the determined value of drift to a preceding calibration value so as to arrive at a calibration value for calibrating the sleep clock.
2. A method as claimed in Claim 1, wherein the drift is represented as an offset in terms of symbols.
3. A method as claimed in Claim 2, wherein the drift is identified as a number of fast clock cycles from identification of the number of fast clocks in one symbol.
4. A method as claimed in Claim 1, 2 or 3, wherein the said previous calibration is identified on the basis of the number of fast clocks in the sleep period.
5. A method as claimed in any one or more of Claims 1-4, and including the step of providing an initial calibration by means of cross calibrating the sleep clock to the fast master clock.
6. A method as claimed in any one or more of Claims 1-5, and wherein the calibration procedure is performed every paging cycle.
7. A method as claimed in any one or more of the preceding claims, wherein the calibration period coincides with a sleep period.
8. A method as claimed in any one or more of the preceding claims, wherein an average offset value over a plurality of bursts of the paging reception is employed as the offset value used in calibrating the sleep clock.
9. A method as claimed in any one or more of the preceding claims and including means for determining whether recalibration is required responsive to comparison of the offset value relative to a threshold value.
10. A mobile radio communications device having a sleep clock and including means for recalibrating the sleep clock and which includes means for determining drift between the air interface timebase and the local timebase as controlled during sleep periods by the said sleep clock, and means for adding the determined value of the drift to a preceding calibration value and so as to arrive at a calibration value for calibrating the sleep clock.
11. A device as claimed in Claim 10, wherein the drift is represented as an offset in terms of symbols.
12. A device as claimed in Claim 11, wherein the drift is identified as a number of fast clock cycles from identification of the number of fast clocks in one symbol.
13. A device as claimed in Claim 10, 11 or 12, wherein the said previous calibration is identified on the basis of the number of fast clocks in the sleep period.
14. A device as claimed in any one or more of Claims 10 to 13 and including means for providing an initial calibration by cross calibrating the sleep clock to the fast master clock.
15. A device as claimed in any one or more of Claims 10 to 14 and arranged such that the calibration procedure is performed every paging cycle.
16. A device as claimed in any one or more of Claims 10 to 15, and arranged such that the calibration period coincides with a sleep period.
17. A device as claimed in any one or more of Claims 10 to 16, and including means for determining an average offset value over four bursts of the paging reception for use as the offset value used in calibrating the sleep clock.
18. A mobile radio communications device substantially as hereinbefore described with reference to, and as illustrating in the accompanying drawing.

18. A device as claimed in any one or more of the preceding claims and including means for determining whether recalibration is required responsive to comparison of the offset value relative to a threshold value.

19. A method of calibrating a sleep clock of a mobile radio communications device substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.

20. A mobile radio communications device substantially as hereinbefore described with reference to, and as illustrating in the accompanying drawing. l : r l

5. . . . ....

Amendments to the claims have been tiled as follows Claims 1. A method of calibrating a sleep clock of a mobile radio communications device including the steps of determining drift between the air interface timebase and the local timebase as controlled during sleep periods by the sleep clock, and adding the determined value of drift to a preceding calibration value so as to arrive at a calibration value for calibrating the sleep clock, wherein an average offset value over a plurality of bursts of a paging reception is employed as an offset value used in calibrating the sleep clock.

2 A method as claimed in Claim 1, wherein the drift is represented as an offset in terms of symbols.

3.A method as claimed in Claim 2, wherein the drift is identified as a number of fast clock cycles from identification of the number of fast clocks in one symbol.

4 A method as claimed in Claim 1, 2 or 3, wherein the said previous calibration is identified on the basis of the number of fast clocks in the sleep period.

5. A method as claimed in any one or more of Claims 1-4, and including the step of providing an initial calibration by means of cross calibrating the sleep clock to the fast master clock.

6. A method as claimed in any one or more of Claims 1-5, and wherein the calibration procedure is performed every paging cycle.

7. A method as claimed in any one or more of the preceding claims, wherein the calibration period coincides with a sleep period.

l l I r I À ( 8. A method as claimed in any one or more of the preceding claims and including determination of whether recalibration is required responsive to comparison of the offset value relative to a threshold value.

9. A mobile radio communications device having a sleep clock and including means for recalibrating the sleep clock and which includes means for determining drip between the air interface timebase and the local timebase as controlled during sleep periods by the said sleep clock, means for adding the determined value of the drip to a preceding calibration value and so as to arrive at a calibration value for calibrating the sleep clock, and means for determining an average offset value over four bursts of a paging reception for use as the drip value used in calibrating the sleep clock.

10. A device as claimed in Claim 9, wherein the drift is represented as an offset in terms of symbols.

11. A device as claimed in Claim 10, wherein the drift is identified as a number of fast clock cycles from identification of the number of fast clocks in one symbol.

12. A device as claimed in Claim 9, 10 or 11, wherein the said previous calibration is identified on the basis of the number of fast clocks in the sleep period.

13. A device as claimed in any one or more of Claims 9 to 12 and including means for providing an initial calibration by cross calibrating the sleep clock to the fast master clock. l / 1' À r r r I

14. A device as claimed in any one or more of Claims 9 to 13 and arranged such that the calibration procedure is performed every paging cycle.

15. A device as claimed in any one or more of Claims 9 to 14, and arranged such that the calibration period coincides with a sleep period.

16. A device as claimed in any one or more of the preceding claims and including means for determining whether recalibration is required responsive to comparison of the offset value relative to a threshold value.

17. A method of calibrating a sleep clock of a mobile radio communications device substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.