GB2387291A

GB2387291A - Maintaining system clock accuracy of a mobile apparatus whilst maximising battery life of the apparatus

Info

Publication number: GB2387291A
Application number: GB0207618A
Authority: GB
Inventors: Ashley Wheeler
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2002-04-02
Filing date: 2002-04-02
Publication date: 2003-10-08
Anticipated expiration: 2022-04-02
Also published as: GB2387291B; GB0207618D0

Abstract

Apparatus for maintaining the clock frequency of a second communications apparatus 14 within a specified tolerance of a reference frequency derived from a primary communications apparatus is described. The apparatus comprises a receiver 16 , a system clock 8 , a receiver scheduler 28 and a system clock error processor 20 . The receiver 16 receives signals transmitted by a primary communications apparatus over a communications medium. The system clock 18 derives the transmission frequency of signals to be transmitted by the secondary communications apparatus 14 to the primary communications apparatus. The receiver scheduler 28 controls the idle mode reception rate of the secondary communications apparatus. When the idle mode reception rate would fall below a minimum reception rate required to maintain the system clock 18 of the secondary communications apparatus 14 within a specified tolerance, the receiver scheduler 28 schedules at least one additional signal transmitted by the primary communications apparatus for reception by the secondary communications apparatus 14 . The additional signal is used to boost the idle mode reception rate of the secondary communications apparatus 14 to at least the minimum sample rate. The system clock error processor 20 processes the received signals to determine an error correction signal for correction of a system clock 18.

Description

Correction of clock frequency This invention relates to a method of, and apparatus for, maintaining the clock frequency of a secondary communications apparatus within a specified tolerance of a reference frequency of a primary communications apparatus. Typically, the primary communications apparatus is a telecommunication base station or cell and the secondary communications apparatus is a mobile telephone.

In order to communicate with a base station or cell of a-cellular network, a mobile telephone must be able to transmit data at the frequency required for transmissions to that cell. The mobile telephone is, therefore, provided with a system clock and uses the frequency of the clock to control its transmission frequency and reception frequency. The system clock is usually a crystal oscillator and.is used to provide a local reference for the telephone handset's internal time base to control when certain activities take place and to determine the frequencies to be used in radio transmission of a transmission signal to the base station and reception of a reception signal from the base station.

A problem arises because although the frequency of the system clock is considered fixed at a known, nominal frequency it will in fact vary from this nominal fixed frequency depending on the choice of the crystal and variations in the crystal's cut, temperature, age, mechanical or shock experienced by the crystal and the voltage supplied to the crystal. Typically, the frequency of the system clock will drift such that, compared to a true reference frequency, there will be

a frequency error of the order of 1 to ~ 20 parts per million (ppm) (corresponding to 1 to 20 x 10 -6% error) .

Any error in the frequency of the system clock causes an error of the same magnitude in the transmission frequency of signals transmitted by the mobile apparatus. For example, an error in the system clock frequency of X ppm will cause an error of X ppm in the frequency of all transmitted bursts from the mobile apparatus to the base station. Such errors cause a deterioration in the communication between the mobile apparatus and the base station and may cause interference with neighbouring cells if the transmission frequency were allowed to drift too far off the nominal transmission frequency.

3GPP (Third Generation Partnership Project) standards for both the GSM (Global System for Mobile) and UMTS (Universal Mobile Telephone Service) require the frequency error present in all signals transmitted by the mobile apparatus to the base station to be less than 0.1 ppm when compared to the frequency of the signals received by the mobile apparatus from the base station. The apparent frequency error in the received signals due to base station frequency error and any Doppler shift due to relative movement of the mobile apparatus to the base station can be ignored. In order to achieve this level of accuracy of transmission, the mobile apparatus uses its system clock to determine the reception frequency at which it expects to receive signals from the base station, the "expected reception frequency", and the reception frequency of received signals, the "actual reception frequency". Any difference between the frequencies of the expected and actual reception signals is assumed to be due to an error in the system clock of the mobile apparatus.

Compensation using the frequency of the signals received from the base station is applied to the system clock to try to re-synchronize the system clock with the reference clock and hence to ensure accuracy of the transmission frequency of signals transmitted by the mobile apparatus to the base station.

In practice, however, a complication arises. The frequency of a signal received by the mobile apparatus may be altered as it travels from the base station to the mobile apparatus over the communication medium due to noise, interference and other path propagation conditions between the base station and the mobile apparatus. These noise and interference effects result in spreading the frequency of the received signals from their original transmission frequency. The noise and interference effects follow a normal distribution. The standard deviation of this distribution is larger than the frequency error allowed by the appropriate communication standards. These noise and interference effects must, therefore, be minimized if the mobile apparatus is to use the received signals to synchronize its system clock with the reference clock. It should also be noted that when the mobile apparatus is moving, the frequency of signals received from the base station by the mobile apparatus will additionally be affected by Doppler shift. Under such non-static conditions, the accuracy of the transmit frequency of the mobile apparatus is allowed to vary more widely than under static conditions to compensate for the effects of Doppler shift. Thus, the mobile apparatus is not expected to compensate for Doppler shift when correcting the system clock using received signals transmitted by the base station to adjust the system clock to a reference clock.

Noise and interference effects can be minimized using a number of independent samples of the received signals to determine the error in the frequency of the system clock. Mathematically, averaging samples whose values are distributed according to a normal distribution has the effect of reducing the standard deviation according #2 to the equation: #m2 = # where . #m equals standard deviation of the average samples, # equals the standard deviation of the distribution of samples received from the base station by the numerical apparatus and N equals the number of samples over which the average is taken.

The effects of noise and interference may be minimized by choice of a sufficiently large number of samples, N.

When these effects are minimized, the true error in the frequency of the system clock may be determined by comparing the local frequency of the system clock with the average frequency of the received samples.

However, the choice of the value of N determines the length of time required to accumulate the number of samples needed to perform the averaging operation. The frequency error present in the system clock is not fixed but varies over time due to environmental effects on the crystal. The aim in using the received signals transmitted from the base station to correct the frequency of the system clock is to compensate for these time varying effects before they cause the frequency of the system clock to drift outside the accuracy required by the communication standards. Currently the required accuracy is 0.1 ppm. Thus, the value of N must be sufficiently large to minimize the effect of noise and interference over the propagation path of the communication medium whilst being

sufficiently small to ensure that the required number of samples are received quickly enough to ensure that error updates (based on the average set of samples) are calculated in time to compensate for the time varying effects on the frequency of the crystal used as the system clock before the system clock frequency has drifted out of tolerance.

In order to maximize the battery life of the mobile apparatus, the rate at which the mobile apparatus receives signals from the base station depends on the current operational state of the mobile apparatus. For example, if the mobile apparatus is supporting a dedicated connection to the base station over 200 signals per second are received by the mobile apparatus whereas if the mobile apparatus is idle it may receive as few as 0.26 signals per second. To maximize handset battery life, the mobile apparatus minimizes radio reception and transmission activities whenever possible. Thus, overall, the rate at which signals are received by the mobile apparatus is minimized and varies according to network configuration and use of the mobile apparatus. Under many network conditions, it is possible to calculate a value of N that satisfies both the requirement that the noise and interference effects are minimized whilst allowing a sufficient number of samples to be collected in a time scale which prevents the system clock from drifting from the frequency of the reference clock more than the allowed accuracy of the relevant communication standard. Thus, the frequency of transmissions by the mobile apparatus may be kept within the overall 0.1 ppm error limit required by the communication standard using the corrected clock frequency of the system clock. The optimal value of N will vary according to the

particular fixed and time bearing frequency error properties of the crystal used in the mobile apparatus.

We have appreciated that, under certain network configurations, it is not possible to estimate a satisfactory value of N. That is, in order to maximise battery life the signal reception rate of the mobile apparatus is so low that the time taken to gather the N samples required to derive a frequency error estimate meeting noise and interference levels required is much longer than the time taken for the system clock to drift outside the 0.1 ppm tolerance mandated by the relevant communication standard. Reducing the value of N to speed up the error estimate calculation process would mean that the effects of noise and interference over the communications medium would force the accuracy of the estimate of the frequency of the reference clock to fall such that the clock frequency of the mobile apparatus cannot be corrected to meet the standard.

Under these circumstances, it is not possible to select a suitable value of N that is large enough to remove noise and interference effects and small enough to ensure that frequency corrections to the system clock may be made fast enough to compensate for time varying errors in the crystal frequency output such that the transmission frequency of the mobile apparatus is maintained within the tolerance required.

We have appreciated that in some circumstances it may be possible to choose a higher specification crystal oscillator for the system clock of the mobile apparatus allowing a value of N to be found which satisfies the residual ppm error requirement of the communication standard. However, crystals with the desired accuracy and robustness to time varying errors are very

expensive and for this reason would not be commercially acceptable for the majority of mobile telephones.

The present invention aims to alleviate the problem of maintaining the system clock accuracy whilst maximizing battery life of the mobile apparatus. The present invention, in its various aspects, is defined in the independent claims below. Preferred features of the embodiment are set out in the dependent claims.

A preferred embodiment of the invention will now be described in further detail with reference to the following drawings: Figure 1 is a schematic diagram showing mobile communications between a secondary communications apparatus and a primary communications apparatus over a communications medium; Figure 2 is a schematic diagram of a secondary communications apparatus according to an embodiment of the invention; Figure 3 shows a process for measuring and correcting the internal system clock frequency error in a cellular handset; and Figure 4 shows a process for measuring and correcting the internal system clock frequency according to an embodiment of the invention.

In the preferred embodiment of the invention the apparatus for, and method of, maintaining the clock frequency of a secondary communications apparatus within a specified tolerance of a reference frequency of a primary communications apparatus is for use in a

mobile communications network. In effect, the apparatus is system clock error correction apparatus.

As shown in Figure 1, a mobile phone user using a secondary communications apparatus, such as a mobile telephone handset 14, connects to the communications network by communicating over a communications medium 12, in the case of mobile communications a radio link, to the nearest primary communications apparatus or base station 10, located in that particular cell. Each secondary communications apparatus 14 must be able to derive a transmission frequency for communicating with the appropriate base station or primary communications apparatus 10. It is important that the transmission frequency of the secondary communications apparatus 14 to the primary communications apparatus 10 is within a specified tolerance in order to meet the required communications standards and to minimize interference and error over the communications network. The transmission frequency is derived using the local system clock provided in the secondary communications apparatus.

As shown in Figure 2, the secondary communications apparatus 14 includes a receiver 16, system clock 18, system clock error processor 20, modulator 24, transmitter 26 and receiver scheduler 28. The receiver 16 is coupled to an antenna provided on the secondary communications apparatus 14 and receives signals transmitted by a primary communications apparatus over a communications medium. The receiver is coupled to the system clock error processor 20 which is in turn coupled to the system clock 18 and to the receiver scheduler 28. The system clock error processor 20 performs steps 34, 36' and 38' shown in figure 4 and described below with reference to figure 4.

- 9 The system clock 18 is adjustable with adjustment of the system clock 18 being controlled by the system clock error processor 20.

The receiver 16 receives signals transmitted by the primary communications apparatus or base station intended for reception by the secondary communications apparatus or mobile telephone handset. It may also receive data representing the minimum sample rate required by the handset 14 to maintain the clock frequency of the system clock of the handset within a specified tolerance of the perceived reference frequency of the base station. To conserve battery power in the mobile telephone handset 14, the signal reception rate is minimized when the mobile telephone is not in use i.e. in an idle mode. When data is to be transmitted from the mobile telephone handset 14 to the base station 10, the data is captured by any suitable means (not shown), and modulated by modulator 24 using the frequency of the system clock 18 to derive the transmission frequency. The modulator 24 is therefore coupled to the system clock 18. The modulated data is transmitted over the communications medium 12 to the base station 10 via a transmitter 26 coupled to the modulator 24. In order that the data for transmission is modulated at the correct frequency, the frequency of the system clock must be maintained within a given tolerance.

The receiver scheduler 28 controls when the handset receives signals on the antenna. It is coupled to the receiver 16 and to the system clock error processor 20. In contrast to known handsets, a handset embodying the present invention provides a connection from the receiver scheduler 28 to the system clock error processor 20.

Figure 3 shows a known process for measuring and correcting the internal system clock frequency in a GSM handset in an idle state. A receiver 16 is set up to receive a signal. As soon as a signal is received, it is de-modulated using frequencies derived from the system clock to determine an actual reception frequency 32. The-actual reception frequency is subtracted from an expected reception frequency calculated from the system clock to generate a frequency error for each sample 34. In known systems, the de-modulated frequency components of the various samples representing the frequency error in the samples are averaged with the previous frequency error samples using a sliding window to determine the system clock frequency error 36. The system clock frequency error is compared with a stored threshold value and if the system clock frequency error is greater than a predetermined threshold, the system clock frequency is adjusted 38.

In known handsets, the receiver scheduler performs a power saving function by minimising the number os signals received. No account is taken of the need to measure and control frequency error. The receiver scheduler simply schedules receive events based on known patterns of transmissions from the base station but minimised to help reduce handset power consumption. Using modern communications standards, the reception rate of signals by a mobile telecommunications handset may, therefore, be below the minimum rate required to maintain the system clock within the tolerance required by the communications standard.

In a system embodying the present invention, the receiver scheduler 28 additionally monitors the mode of

the secondary communications apparatus 14 and when a mode is detected which would result in too low a sample rate for adequate correction of the system clock frequency, the receiver scheduler 28 schedules at least one additional signal to be monitored by the secondary communications apparatus 14 to boost the sample rate to the minimum sample rate required to obtain the N samples before the system clock is liable to drift out of tolerance. Depending on the state of the secondary communications apparatus 14, the receiver scheduler 28 causes the receiver 16 to receive additional signals resulting in additional samples being received. The additional samples are used to boost the sample rate to the minimum sample rate of data required to be received by the secondary communications apparatus to maintain the clock frequency of the secondary communications apparatus 14 within the specified tolerance of the perceived reference frequency of the primary communications apparatus 10. The perceived reference frequency is effectively the average of the actual reception frequencies of the various samples.

This process is shown in Figure 4. The receiver scheduler 28 monitors the mode or state of the secondary communications apparatus 14 and schedules the apparatus 14 to receive additional signals when necessary 40. Any such additional signals which are received are used in conjunction with the remaining signals received under normal conditions to reduce the noise and interference in step 36. The system clock error processor 20 uses any suitable averaging technique, such as a sliding window average technique, to determine an average frequency error for N samples 36. As in previous systems, if the system clock frequency error is greater than a predetermined threshold, the system clock error processor 20 causes

the frequency of the system clock 18 to be adjusted to maintain the frequency of the system clock within the specified tolerance for the communications system 38' thereby synchronising the system clock frequency of the secondary communications apparatus 14 with the perceived reference frequency of the primary communications apparatus 10. The perceived reference frequency may differ from the actual reference frequency of the primary communications apparatus due to Doppler shift.

During certain modes of operation, the sample rate of received signals at the mobile handset 14 may be large enough that additional signals are not required. When the receiver scheduler 28 detects that the mobile handset 14 is operating under these conditions, no additional signals are scheduled for reception by the receiver 16.

For example, in the table below it can be seen that when a mobile phone 14 is configured to monitor GSM paging channels, 1.89 bursts per second are received in idle mode and a value of N can be found that meets the communications standard. However when the mobile phone 14 is configured to monitor only GPRS paging channels, the burst reception rate can drop to 0.26 bursts per second and no suitable value of N can be found.

Scenario samples Optimum N Residual Allowed Pass /second (calculated) frequency error or error (ppm) (ppm) Fail Idle mode, CS 1.89 50 0-100 0.100 PASS paging

Packet idle 0.26 13 0.186 0.100 FAIL mode, GPRS paging only.

(Network mode 1) Dedicated mode 200 400+ 0.031 0.100 PASS Thus for this particular handset, the mobile phone 14 must ensure that through the reception of additional signals it can maintain its burst reception rate at or above 1.89 bursts per second. These additional signal measurements can be made on common channel transmissions from the base-station 10 (ie channels available to all secondary communications apparatus 14) and could include BCCH or other paging channels.

The increased radio activity required has a negative effect on battery life. Therefore the scheduling of these additional reception activities needs to be carefully considered. An optimal: implementation would schedule the reception of additional signals immediately before or after existing reception activity. This allows the handset to remain in an idle, power-conserving state for the maximum time. By ' doing this, the activities needed for the transition from a power-conserving idle state to a reception capable state (and thus the associated power-drain from the battery) are minimised. The implementation of such a schedule of activities is within the scope of the skilled person and is not, therefore, described in detail here.

With respect to the above description, it is to be realised that the equivalent apparatus and methods are deemed readily apparent to one skilled in the art, and all equivalent apparatus and methods to those illustrated in the drawings and described in the

specification are intended to be encompassed by the present invention. Therefore, the foregoing is considered as illustrative only of the principals of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described.

Accordingly, all suitable modifications and equivalents may be resulted to, f alling within the scope of the invention. It should also be noted that the features described by reference to particular figures and at different points of the description may be used in combinations other than those particularly described or shown. All such modifications are encompassed within the scope of the invention as set forth in the following claims.

Claims

Claims 1. A method of maintaining the clock frequency of a secondary communications apparatus within a specified tolerance of a perceived reference frequency of a primary communications apparatus, the method comprising the steps of: a) monitoring the idle state sample rate of the secondary communications apparatus; b) comparing the idle state sample rate with a minimum sample rate, the minimum sample rate representing the data rate required to be received by a secondary communications apparatus from a primary communications apparatus to maintain the clock frequency of the secondary communications apparatus within a specified tolerance of the perceived reference frequency of the primary communications apparatus;

c) scheduling at least one, additional signal transmitted by the primary communications apparatus for reception by the secondary communications apparatus to boost the idle state sample rate of the secondary communications apparatus to at least the minimum sample rate when the idle state sample rate falls below the minimum sample rate; d) using the signals received by the secondary communications apparatus to estimate an average reception frequency for signals transmitted from the primary communications apparatus; and e) adjusting the clock frequency of the secondary communications apparatus to the average reception frequency to reduce the effects of any frequency drift of the clock frequency relative to the perceived reference frequency of the primary communications apparatus.

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2. A method according to claim 1, further comprising the step of using the clock frequency of the secondary communications apparatus to determine the signal frequency for transmission of signals to the primary communications apparatus.
3. A method according to either claim 1 or claim 2 wherein the step of using the signals received by the secondary communications apparatus to generate an average reception frequency further comprises: a) selecting from the received signals at least N independent samples; b) estimating the frequency of each of the N independent samples; and c) determining the average reception frequency for the primary communications apparatus from the average frequency of the at least N samples.
4. A method according to any of the preceding claims, wherein the step of determining a minimum sample rate of data required to be received by a secondary communications apparatus from a primary communications apparatus further comprises: a) estimating the frequency error of signals transmitted by the primary communications apparatus to the secondary communications apparatus; b) using the estimated frequency error to determine the minimum sample rate of data required to be received by the secondary communications apparatus.
5. A method according to any of the preceding claims, wherein the step of determining a minimum sample rate of data required to be received by a

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secondary communications apparatus from a primary communications apparatus further comprises determining the clock frequency drift rate of the mobile apparatus and using the clock frequency drift rate to determine the minimum sample rate.
6. A method according to any of the preceding claims, wherein the minimum sample rate is determined as a function of the number of samples, N, required to perform noise reduction on the received signals and the clock frequency drift rate of the system clock.
7. A method according to any of the preceding claims, wherein the step of scheduling at least one additional signal for reception by the secondary communications apparatus comprises scheduling a signal which occurs substantially immediately before or substantially immediately after signals received by the secondary communications apparatus in idle mode.
8. Apparatus for maintaining the clock frequency of a secondary communications apparatus within a specified tolerance of a reference frequency derived from a primary communications apparatus, the apparatus comprising: a) a receiver for receiving signals transmitted by a primary communications apparatus over a communications medium; b) a system clock, coupled to the transmitter, for deriving the transmission frequency of signals to be transmitted by the secondary communications apparatus to the primary communications apparatus;

c) a receiver scheduler, coupled to the receiver, to control the idle mode reception rate and, when the

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idle mode reception rate would fall below a minimum reception rate required to maintain the system clock within a specified tolerance, to schedule at least one additional signal transmitted by the primary communications apparatus for reception by the secondary communications apparatus to boost the idle mode reception rate to at least the minimum sample rate; and d) a system clock error processor, coupled to a receiver and to the system clock, for processing the received signals to determine an error correction signal for correction of the system clock.
9. Apparatus according to claim 8 further comprising a decoder, coupled to the receiver and to the system clock error processor, for decoding the received signals to determine a frequency estimate of the reception frequency of transmission from the primary communications apparatus.
10. Apparatus according to either claim 8 or claim 9, wherein the receiver scheduler is configured to schedule the reception of signals which occur substantially immediately before or substantially immediately after signals received by the secondary communications apparatus during normal operation.
11. Apparatus according to claim 8, wherein the system clock error processor is coupled to the receiver scheduler for controlling the scheduling of the at least one additional signal.

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12. Secondary communications apparatus comprising apparatus according to any of claims 9 to 12, a transmitter and a modulator, coupled to the system clock of the apparatus and to the transmitter, wherein the modulator modulates transmission data at a frequency determined by the system clock and the modulated transmission data is transmitted by the transmitter over a communications medium for reception by a primary communications apparatus.
13. Secondary communications apparatus according to claim 12, wherein the communications medium is wireless.
14. A computer program comprising program instructions which, when loaded into a computer, constitute the apparatus of any of claims 8 to 13.
15. A computer program comprising program instructions for carrying out the method of any of claims 1 to 7.
16. Apparatus substantially as hereinbefore described with reference to any of figures 1,2 or 4.
17. A method substantially as hereinbefore described with reference to any of figures 1,2 or 4.