GB2433332A

GB2433332A - Determining timing accuracy of a receiver oscillator

Info

Publication number: GB2433332A
Application number: GB0525576A
Authority: GB
Inventors: Harri Valio; Samuli Pietila; Tuomo Honkanen
Original assignee: Nokia Oyj
Current assignee: Nokia Oyj
Priority date: 2005-12-16
Filing date: 2005-12-16
Publication date: 2007-06-20
Also published as: WO2007068501A3; GB0525576D0; WO2007068501A2

Abstract

A method for determining timing accuracy of an oscillator in an electronic device, wherein the electronic device is operable in power-on and power-off modes. The method comprising; the electronic device receiving external timing information, comparing the oscillator to the external timing information to produce a first comparison result, comparing the oscillator to said external timing information to produce a second comparison result, wherein the electronic device changes its operation mode from power-on to power-off mode between said first and second comparison results, and calibrating said oscillator based on said first and second comparison results. The electronic device may be a mobile phone.

Description

Determining timing accuracy of a receiver

FIELD OF THE INVENTION

The invention relates to a method for enhancing time keeping accuracy of an electronic device, especially a sleep crystal of the electronic device. The invention equally relates to a corresponding electronic device, mobile phone device, system and software program product.

BACKGROUND OF THE INVENTION

In positioning systems based on satellite positioning, such as a GPS, a positioning receiver attempts to detect its position based on signals from at least four orbiting satellites, using triangulation techniques. GPS satellites transmit code division multiple access (CDMA) signals which are received by the receiver. Embedded in each CDMA signal are pseudo random noise (PRN) code sequences 1023 bits (chips) in length and transmitted continuously at a rate of 1.023 Mchip/s. Superimposed upon the PRN code by binary phase shift keying (BPSK) modulation is navigation data at 50 bit/s structured in 5 subframes of 300 bits each. A chip is equivalent to one transmitted bit of the PRN code and is 1 microsecond in duration. A data hit is 20 ms in length during which the PRN sequence will have been repeated 20 times. Timing information spanning from carrier phase to PRN bit position down to the edges of the 50 bit/s modulation and subframes that contain ephemeris and other satellite vehicle data, are all precisely timing related and derived from the satellite vehicle's onboard atomic clock.

Assuming that the precise time and its relationship to the PRN code sequences are known to the earth-based GPS receiver then the shift in time of the received PRN code sequence can be used to determine the radio frequency (RF) propagation delay or time of flight which can be used to calculate the distance between the satellite and the receiver simply by multiplying this by the speed of light. Since the satellites' position as a function of GPS time is transmitted in the 50 bit/s navigation data, precise orbital position at any instant in time can be calculated provided the receiver also knows precise GPS time.

If three satellites' x, y and z co-ordinates are combined mathematically then in theory it is possible to determine the location of the GPS receiver through triangulation techniques. This assumes that the GPS receiver knows precise GPS time synchronised to the satellite atomic clocks. Because cost and size prohibit the use of such highly accurate time references in a consumer level receiver, a fourth satellite measurement is used by the GPS receiver which allows time to be solved as another variable along with x, y and z variables in four simultaneous equations. This is also known as a position, velocity and time (PVT) solution. The result of the solution of these equations is a position fix and GPS time known to sub-microsecond accuracy. Precise time is therefore a product of obtaining a position fix.

If further position fixes are required, then the precise time must be accurately maintained. If the receiver is not tracking the satellite signals continuously, an internal clock signal of the position receiver is used to determine the time elapsed since the previous fix. If there is inaccuracy in the clock signal, then depending on the size of the inaccuracy resynchronisation to the nearest bit edge or even frame edge of the navigation data may be required before another position fix can take place. This can be very costly in terms of the time and energy required for this re-acquisition. Furthermore, in low signal areas such as in buildings or a dense forest, a time error of greater than 0.5 ms could mean an inability to maintain tracking because of the higher signal strength needed to determine the position of the bit edges.

Reducing the power consumption of positioning devices is a major concern of designers, particularly if a GPS receiver is to be incorporated in a mobile phone handset.

Lower power consumption means that a smaller battery may be used, and therefore the smaller and lighter the mobile phone handsets can be designed.

Some GPS receivers keep a highly accurate system clock running for maintaining GPS time between fixes but its high frequency means that the oscillator alone will account for current consumption in the range of milliamps and this could easily be for 95% of the time for navigation and location based applications. This is clearly undesirable due to the amount of energy used, and therefore the battery size required.

Generally the real time clock (RTO) oscillator used in a GPS receiver would be derived from a 32768 Hz quartz crystal operating continuously within the cellular engine of a mobile phone handset and is often referred to a sleep oscillator or sleep crystal since it is often the only clock running in sleep mode or power-off of the cellular engine. The term cellular engine' refers to the active electronics, including any operating software of a cellular telephone. Correspondingly, GPS engine' refers to the active electronics of a GPS receiver, including operating GPS software. In sleep mode power consuming circuitry of the receiver is switched off except for that needed for accurate time keeping and some data storage.

When the receiver is in power-on mode, the receiver is operating normally. Satellite positioning receivers use sleep periods to reduce power consumption. The performance of an oscillator is defined in terms of its accuracy and stability. Accuracy is a measure of how well an oscillator can be tuned to a specified frequency.

Quartz crystal is normally selected for RTC of a satellite positioning receiver because of its low power consumption and that it operates continuously, however there is one major drawback. Such low frequency crystals can typically vary in frequency by 3 ppm (parts per million) per degree temperature change and have a ppm initial error. A temperature change between fixes could be expected due to the cellular RF power amplifier or battery charging circuitry in the cellular terminal, and could therefore mean that the GPS engine fails to re-acquire the satellite signals. It would be preferred that the cellular engine informs the GPS engine of a potential increased time keeping error based on its knowledge that the change in operating mode will cause a temperature change. The GPS receiver can then widen its time bin search accordingly. However, this will take longer and consume more power.

Fig. 1 illustrates one prior art solution of precise time keeping in a receiver. The sleep crystal is calibrated by measuring its frequency against an external reference frequency during power on periods of the receiver. The calibration is done so that at the beginning of each power on period a first measurement is made and at the end of the same sleep period a second measurement is made. Each measurement compares the external timing information to the sleep crystal. Eased on these two measurements, the calibration of the sleep crystal can be performed. Sleep and power-on periods follow each other as can bee seen from Fig. 1. After each sleep period, the receiver goes into power-on mode and the receiver needs to determine accurate time in order to quickly resynchronise to the satellite or base station signals.

In order to monitor accurate time over the sleep period, the sleep crystal should he as accurate as possible.

However, the sleep crystal quality is often poor, because high quality sleep crystals are often too expensive to be inserted in mass products. Therefore, the sleep period cannot be extended to be as long as wanted but the maximum sleep period often depends on the sleep crystal quality.

The major deficiency of the prior art solution described in Fig. 1 is that, for instance H inaccuracies are only partly taken into account. Therefore, the inaccuracies in the measurement that are caused for instance by the comparison HW or jitter in the signals have to be multiplied in order to get the final accuracy. If the gap between the first and the second measurement is for instance 1 s and sleep period is 10 s, then the errors that come from the measurement have to multiplied by 10 to get the accuracy when the system powers up again.

SUMMAPY OF THE INVENTION

The applicant has recognised that there is a need for an improved time keeping method in a receiver.

According to a first aspect of the invention there is provided a method for determining timing accuracy of an oscillator in an electronic device, wherein the electronic device is operable in power-on and power-off modes, the method comprising; receiving external timing information, comparing the oscillator to the external timing information to produce a first comparison result, comparing the oscillator to the external timing information to produce a second comparison result, wherein the electronic device changes its operation mode from power-on to power-off mode between the first and second comparison results, and calibrating the oscillator based on the first and second comparison results to provide a calibration result.

A method in accordance with an embodiment of the invention has the advantage that it provides an improved oscillator calibration and therefore the oscillator accuracy can be compromised and also the power-off periods can be longer. The power consumption of the receiver can be reduced.

Further, the oscillator in accordance with an embodiment of the invention is a sleep crystal of a satellite positioning system receiver.

Further advantage provided by improved oscillator calibration is that satellite acquisition requires less time because of better time keeping accuracy.

Other aspects of the invention are in the claims appended hereto.

BRIEF DESCRIPTION OF THE FIGURES

These and other features of the present invention will by way of example become apparent from the following detailed description when considered in conjunction with the accompanying drawings, in which:

Fig. 1 illustrates a prior art solution for

calibrating a sleep crystal; Fig. 2 illustrates an environment in which the current invention can be applied; Fig. 3 is a block diagram of the receiver in accordance with an embodiment of the invention; Fig. 4 is a flow chart of a method according to an embodiment of the invention; Fig. 5 illustrates the measurements done in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig. 2 illustrates an operational environment in which embodiments of the present invention may exist.

Specifically, in Fig. 2, there is shown a communication device 210, in this case a mobile phone handset, which according to the present embodiment includes a sleep crystal, which acts as an oscillator. The communication device 210 could also be another electronic device, another than a mobile phone, for instance a receiver that includes an oscillator.

Fig. 2 also shows two communication network elements.

First network element is an access point 220, in this case a base station (BS) . The first network element could also he any other access point capable of communicating with the communication device 210. The base station 220 can work according to any existing, for instance OSM, GPRS, EDGE, HSCSD, UMTS, CDMA 2000, 1S95, ViMAX, etc., or future communication network standards. Alternatively, the base station 220 could act as an access point of a wireless local area network, such as any variation of wireless local area network (WLAN) 802.11 standard.

Furthermore, the base station 220 could be connected to the mobile phone handset with any other wireless or wired connectivity method. The second network element is a mobile station switching centre (MSO) 230. The MSC 230 controls the operation of a cellular network.

Furthermore, Fig. 2 comprises four satellites 240.

The base station 220 communicates with the mobile phone handset 210 using any suitable communication means, advantageously the mobile phone handset 210 operates according to the same communication standard as the base station 220. In this embodiment the base station 220 uses RF transmissions in order to transmit signals to the mobile phone handset 210. Accordingly, the mobile phone handset 210 receives transmissions sent by the base station 220. The mobile phone handset 210 may also send signals to the base station 220. Thus, the communication may be two directional. Ease stations and MSCs form part of the cellular communication network, such as a GSM network. The cellular communication network is able to send timing information to the mobile phone handset 210.

The mobile phone handset 210 can observe paging or control channel timing signals transmitted from the base station 220 in order to acquire accurate timing information.

The satellites 240 transmit signals to at least one of the following: the mobile phone handset 210 and/or the communication network. If the satellites 240 transmit signals to the communication network, the communication network may process the signals and then send assistance data to the handset 210. Wireless communication link is used for signal transmissions between the satellites 240 and the handset 210 and between the satellites 240 and the communication network. In Fig. 2 the mobile handset 210 can receive signals from four different satellites.

In this case the handset 210 timing can be synchronised to the satellite timing by, for instance, receiving time of the week (ToW) signal from the 50 bit/second data stream cr by correiatig the PRN codes generated in the receiver with the gold sequence sent by each satellite 240. The coarse/acquisition (C/A) signals sent by the satellites 240 are called gold sequences and they are PRN sequences. The 0/A code repeats after 1023 bits or chips.

The ToW signals received by the satellites 240 allow the receiver to unambiguously determine the time. The mobile phone handset 210 can also receive timing information from the communication network via the base station 220.

For the mobile phone handset 210 also other suitable methods for acquiring the timing synchronisation may exist as known in the art.

Fig. 3 is a block diagram of the mobile phone handset 210 of Fig. 2. The handset 210 functions as a cellular telephone according to, for example, one or many of the following standards: GSM, GPRS, EDGE, HSCSD, UNTS, CDMA 2000, 1S95, etc. Alternatively or in addition to the -cellular operation, the handset 210 can also communicate in accordance with any other existing or coming wireless communication standard such as WLAN, Bluetooth, UWS, iMAX, etc. For receiving and transmitting signals, the handset 210 includes an antenna 301. Two or more separate antennas could also he used, but in this embodiment the same antenna can receive arid transmit signals of cellular and positioning systems. The handset 210 also includes a transceiver unit 302 (TRX) For centrally controlling the functioning of the handset 210 the handset 210 also includes a central processing unit 303 (CPU) . The CPU 303 includes one or more processing units depending on the implementation of the handset 210. For time keeping, the handset includes a sleep crystal 304. The sleep crystal 304 is running even when the handset 210 is in sleep mode.

The handset 210 also comprises a memory 305. The memory 305 may have random access (RPM) and read only memory (ROM) parts. Suitable data can be stored in that memory.

The handset 210 also includes a cellular engine 306 for providing communication capabilities with the cellular communication network, such as GSM network. For communicating with the satellites, the handset 210 comprises a positioning engine (pos engine) 307.

Furthermore, the handset 210 contains input/output (I/O) means 308. Input means may be for instance a keyboard but it can also be a touch pad or a touch screen. A microphone may also be provided as an input means for receiving voice information. Output means may be provided for instance by a display, such as a liquid crystal -11 -display (LCD) . A loudspeaker may also be provided as an output means for outputting speech or sound. Other suitable input/output means are also possible.

Fig. 4 illustrates a flow chart of a method for calibrating the sleep crystal 304 of the handset 210.

Here a general description is given and all the method steps will be explained in more detail later. The sleep crystal of the handset 210 can also be called a sleep oscillator and it is always turned on, even when the handset 210 is in sleep mode. In sleep mode power consuming circuitry is switched off except for that needed for accurate time keeping and some data storage.

At step 401 the handset 210 receives external timing information. At step 402, the external timing information is compared to the sleep crystal of the handset 210 to produce a first comparison result. At step 403, the external timing information is compared to the sleep crystal of the handset 210 to produce a second comparison result. At step 404, the sleep crystal of the handset 210 is calibrated based on the first and second comparison results. At step 405, the position of the handset 210 is determined using the timing information of the calibrated sleep crystal.

The operation of the handset 210 of Figs. 2 and 3 will now be described in more detail with reference to the flow chart of Fig. 4 and Fig. 5. At step 401, the handset 210 receives external timing information. According to the first embodiment of the invention, the external timing information is received from the satellites 240.

The satellites may operate according to the following standards: Global Positioning System (GPS), Russian GLONASS or European alternative Galileo, which is not yet -12 -in operation, or any other satellite based positioning system. ccording to the first aspect of the first embodiment, the external timing signal is a satellite positioning system time, such as a GPS time. The satellite positioning time can be solved by receiving the ToW signal sent by the satellites 240. The operation in which the time is solved is also known as position, velocity and time (PVT) solution.

According to a variation of the first embodiment, the external timing information is a satellite code phase.

Satellite code phases can be used as the satellite spreading chipping rate is known accurately and Doppler effect to the chip rate can be removed as Doppler is also solved in the PVT calculation as is known in the art.

At step 402, the first comparison result is obtained for calibrating he sleep crystal of the handset 210. The comparison result compares the external timing information to the sleep crystal cycle count. The first comparison 510 result is obtained when the handset is in power-on mode. This is also shown in Fig. 5. The sleep crystal cycle count is measured against satellite positioning system time or satellite code phase and the result is saved into memory 305. Also phase information of the sleep clock can be measured and saved to increase measurement accuracy.

Next the handset 210 goes into sleep mode. When the handset 210 next time changes its operational mode from sleep to power-on mode, a second measurement is done (step 403) to obtain a second comparison result 520.

Again the external timing information is compared to the sleep crystal. When the handset 210 is in power-on mode, -13 -it needs to determine accurate time in order to quickly resynchronise to the satellite or base station signals.

In power-on mode, the handset 210 can perform normal positioning tasks. Now processing algorithm has two measurements: Sleep crystal cycle count arid phase at time instant 1 or satellite code phase 1 Sleep crystal cycle count and phase at time instant 2 or satellite code phase 2 At step 404, the sleep crystal 304 of the handset 210 can be calibrated based on the first and second comparison results. By calibration it is meant that the frequency of the sleep crystal is calculated and then it can be deduced how much the frequency of the sleep crystal differs from the desired frequency. Sleep crystal frequency (f) can be calculated by using the equation: f = [(Sleep crystal cycle count and phase 2) -(Sleep crystal cycle count and phase 1)] / (elapsed time) Now it can be calculated how long the sleep period lasted in reality. This means that the actual sleep period can be calculated based on the external timing information.

The length of the sleep period in time domain can be calculated because the number of sleep crystal oscillations during the sleep period is known. The precise duration of the sleep period is important when performing acquisition of satellites. An error of 1 ps in the sleep cycle duration can cause an error of one chip in the satellite code phase acquisition. The elapsed time can he obtained from the difference of time instants 1 and 2. Alternatively, the elapsed time can be obtained -14 -from the satellite code phases. As the measurement is done over the sleep period the inaccuracies in the measurement H have less effect to the measurement. Also errors that the sleep crystal 304 undergoes during the sleep and power-on period are taken into account as the measurement period is continuously taking into account all crystal cycles. Further measurements can be done as can be seen in Fig. 5.

Even if the actual measurements are done during power-on periods, the sleep crystal 304 cycles are taken into account also during sleep periods to determine the accuracy of the sleep crystal 304. The measurements can take place at any time instant during the power-on mode.

In Fig. 5, the measurements are done shortly after the handset 210 goes into power-on mode. The calibration time is long and errors that happen during the sleep period are taken into account.

According to the second embodiment of the invention, communication system, such as GSN, GPRS, EDGE, HSCSD, UMTS, CDNA 2000, 1S95, Eluetooth, WLAN, ViMAX, frame and bit structure serves as an external timing information.

In this example a cellular communication system is used, but the communication system can also be other than a cellular system. According to the second embodiment of the invention, the receiver does not have to be a satellite positioning receiver, but it can rather be any receiver that can operate in power-on and power-off modes. The measurements are done in the same way as in the first embodiment, but now the external timing information is received from the cellular network.

Now the measurements are: -15 - * Sleep crystal cycle count and phase at cellular frame and phase 1 * Sleep crystal cycle count and phase at cellular frame and phase 2 As the target now is to synchronise the sleep crystal 304 with the cellular frame structure after sleep period, the number of sleep crystal cycles during a cellular frame (or bit or similar) needs to be solved: Cycles / Cellular frame = (Sleep crystal cycle count and phase 2 -Sleep crystal cycle count and phase 1) / (Cellular frame and phase 2 -Cellular frame and phase 1).

The sleep crystal 304 can now be calibrated based on the first and second calibration measurements. The frequency (f) of the sleep crystal can be calculated because the length of the cellular frame is known.

In both embodiments, between the first and two measurements, the receiver goes into sleep mode and therefore the gap between the two measurements is long enough. This means that the inaccuracies in the measurement HW have less effect on the measurement.

Errors that the sleep crystal 304 undergoes during the sleep and power-on period are taken into account as the measurement period is continuous taking into account all sleep crystal cycles. The first and second calibration measurements do not necessarily have to be taken during two consecutive power-on modes.

The frequency of the sleep crystal 304 can be evaluated with the proposed method already during the initial -16 -longer power-on period that the device has before the first sleep period. t'hen sleep periods are started, then the first sleep periods could be shorter so that the effects of new inaccuracies caused by sleep period to the sleep crystal 304 can be minimised. The sleep periods can then be made longer for instance step by step as the sleep period effect on the sleep crystal 304 is also taken into account.

Now that the frequency error of the sleep crystal 304 over the first calibration period is known based on the first 510 and second 520 comparison results, more measurements can be done in the same way as explained earlier. Based on the second comparison 520 and the third comparison 530, the frequency error of a second calibration period can be determined. Similarly based on the third comparison 530 and a fourth comparison 540, a frequency error of a third calibration period can be determined and in the same way frequency errors of any subsequent calibration periods can be calculated. It is now possible to average the frequency errors over several calibration periods to obtain an average frequency error of the sleep crystal 304. Therefore, if the frequency error is averaged over several measurements, the result may be more reliable than just one individual frequency error calculation.

It is also possible to subtract the frequency errors over different periods from each other to detect whether there is a systematic error in the frequency errors of the sleep crystal 304. For instance, the frequency error over the first period subtracted by the frequency error over the second period gives us a number A. Frequency error over the second period subtracted by the frequency error -17 -over the third period gives as a number B. To get a third result C, we need to subtract the frequency error over the forth calibration period from the frequency error over the third period. Now by comparing the numbers A, B and C, it can be deduced if the frequency error tends to increase to a certain direction. If this is the case, it can he deduced that error might be systematic. And if there is for instance one individual frequency error over a certain calibration period that differs from other results, it can be filtered out as it can be assumed that the frequency calibration did not somehow succeed.

Finally at step 405, the position of the handset 210 can be determined using the timing information of the calibrated sleep crystal.

The invention also relates to a corresponding software code, which can he used to implement at least some parts of the method according to the embodiments described above. The invention equally relates to a corresponding software program product in which a software code can be stored.

In the handset 210 all inventive features could be incorporated into a single module. The module should at least include the sleep crystal and a processor for implementing the described method steps. According to the first embodiment, the module also includes the satellite positioning engine 307.

The invention also relates to the handset 210, which comprises means for implementing the methods described above. The handset 210 also comprises the module described above. The handset 210 can he a mobile phone -18 -device.

Furthermore the invention relates to a system in which the handset 210 can be used. According to the first embodiment, the system comprises at least the handset 210 and the satellite 240. According to the second embodiment, the system comprises at least the handset 210 and the base station 220.

It is to be noted that the described embodiments can be varied in many ways and that these are just exemplary embodiments of the invention.

Claims

-19 -Claims 1. method for determining timing accuracy of an oscillator

in an electronic device, wherein the electronic device is operable in power-on arid power-off modes, the method comprising; receiving external timing information, comparing said oscillator to said external timing information to produce a first comparison result, comparing said oscillator to said external timing information to produce a second comparison result, wherein said electronic device changes its operation mode from power--on to power off-mode between said first and second comparison results, and calibrating said oscillator based on said first and second comparison results to provide a calibration result.

2. The method according to claim 1, wherein said first and second comparison results are obtained when said electronic device is in power-on mode.

3. The method according to claim 1, wherein said external timing information is received from a satellite.

4. The method according to claim 1, wherein said oscillator is a sleep crystal of a satellite positioning system receiver.

5. The method according to claims 1, wherein said first and second comparison results are obtained by comparing the oscillator cycle count and phase to satellite code -20 -phase.

6. The method according to claims 1, wherein said first and second comparison results are obtained by comparing the oscillator cycle count and phase to a time of the week signal of a satellite.

7. The method according to claim 1, wherein said calibration comprises determining frequency of said oscillator.

8. The method according to claims 1 or 7, wherein said calibration further comprises determining the length of a sleep period in the time domain.

9. The method according to claim 1, wherein the time difference between said first and second comparison results defines a calibration period and the results of several calibration periods can be averaged.

10. The method according to claim 9, wherein some of the averaged calibration results can be filtered out.

11. The method according to claim 1, wherein some of the calibration results can he subtracted from each other.

12. The method according to claim 1, wherein said external timing information is received from a communication system.

13. The method according to claims 1 or, wherein said oscillator is a sleep crystal of a mobile phone handset.

14. The method according to claim 1, wherein said first -21 -and second comparison results are obtained by comparing the oscillator cycle count and phase to communication system frame number and phase.

15. A computer program product, comprising program code sections for carrying out the steps of anyone of the claims 1-14.

16. An electronic device comprising an oscillator, wherein said electronic device is operable in power-on and power-off modes, said electronic device comprising; receiver for receiving external timing information, means for comparing said oscillator to said external timing information to produce a first comparison result, means for comparing said oscillator to said external timing information to produce a second comparison result, and means for calibrating said oscillator based on said first and second comparison results, wherein said electronic device changes its operation mode from power-on to power-off mode between said first and second comparison results.

17. A system comprising a first electronic device and a second device for providing timing information for said first electronic device, wherein said first electronic device is operable in power-on and power- off modes, said first electronic device comprising; receiver for receiving external timing information, means for comparing said oscillator to said external timing information to produce a first comparison result, means for comparing said oscillator to said external -22 -timing information to produce a second comparison result, and means for calibrating said oscillator based on said first arid second comparison results, wherein said first electronic device changes its operation mode from power-on to power-off mode between said first and second comparison results.

18. A system according to claim 17, wherein said second device is a satellite of a satellite positioning system.

19. A system according to claim 17, wherein said second devioe is an access point of a communication system.

20. A mobile phone device comprising an oscillator, wherein said mobile phone device is operable in power-on and power-off modes, said mobile phone device comprising; receiver for receiving external timing information, means for comparing said oscillator to said external timing information to produce a first comparison result, means for comparing said oscillator to said external timing information to produce a second comparison result, and means for calibrating said oscillator based on said first and second comparison results, wherein said mobile phone device changes its operation mode from power-on to power-off mode between said first and second comparison results.