GB2421660A

GB2421660A - Clock calibration in a mobile communications device

Info

Publication number: GB2421660A
Application number: GB0428228A
Authority: GB
Inventors: Richard Ormson; Hamid Falaki; Nigel Legg; Darioush Downer
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2004-12-23
Filing date: 2004-12-23
Publication date: 2006-06-28
Anticipated expiration: 2024-12-23
Also published as: GB2421660B; GB0428228D0

Abstract

A master clock is turned off at instant 1 for a portion of a pre-determined time (e.g. a paging cycle). The master clock is turned back on at instant 2, when the sleep clock is turned off, and counts the clock cycles until instant 3 (the end of the paging cycle). As the number of master clock cycles is already known for the entire paging cycle, by subtracting the main clock cycles between instant 2 and 3 then the main clock cycles between instants 1 and 2 can be calculated. This can then be compared to the number of sleep clock cycles counted by the sleep clock for the same period and hence cross calibration can be performed. This decreases battery power as only one clock is active at a time and also conventional hardware calibration arrangements are not required.

Description

cLOCK CALIBRATION IN A

MOBILE RADIO COMMUNICATIONS DEVICE

The present invention relates to a method of achieving initial clock calibration within a mobile radio communications device, and to such a device arranged to operate in accordance with such a method It has long been recognised that power savings can be achieved within a mobile radio communications device through the adoption of two clocks operating at different speeds. The primary, and faster, of the two clocks is generally arranged to drive the time base of the device when in operation. However, when the device is an idle mode, it can be arranged to switch to a lowpower, so-called sleep, mode during which the slower so-called sleep clock replaces the faster clock as the means for maintaining the time base The faster clock, which generally requires power in the order of milliamps, can be powered down in favour of the slower sleep clock which has a power requirement in the order of micro amps However, since the sleep clock is not particularly accurate, it has to be calibrated against the faster clock generally at regular intervals and it has previously been identified that the use of a standard hardware calibration timer for cross calibrating the two clocks serves to limit the efficiency in speed of operation of the device and requires dedicated hardware and also serves to reduce the battery life of the device.

Such a problem has been addressed in, for example, UK Patent Application GB-A-2397465 of the applicant and which proposes an arrangement for calibrating the sleep clock against the air interface of the network.

However, even in the known arrangement of the above-mentioned UK patent application, an initial calibration between the two clocks is required and this is provided by a conventional hardware calibration arrangement Thus, the advantages available by way of GB-A-2397465 are limited since the aforementioned conventional hardware calibration arrangement is still required It would therefore be advantageous if a method and arrangement could be provided which can achieve such initial calibration between the two clocks and without requiring use of such dedicated calibration hardware.

The present invention therefore seeks to provide for a method of achieving initial calibration and a related mobile radio communications device, having advantages over known such methods and devices According to a first aspect of the present invention, there is provided a method of achieving initial calibration of a main clock with a sleep clock within a mobile radio communications device arranged for communication via an air interface, the sleep clock being arranged to operate at a slower speed than the said main clock, and the method comprising.

synchronizing the main clock frequency to the air interface clock frequency, synchronising the device time-base to the air interface timebase, identifying a sleep period of the device in terms of a number of sleep clock cycles, commencing the sleep period at a first instant on the air interface time-base; restarting, after completion of the sleep period, the main clock and time-base counter at a second instant on the air interface time-base; repeating synchronization of the device time-base to the air interface at a third instant on the air interface time-base subsequent to the said second instant, determining a first number of main clock cycles representing cycles that should have occurred between the said first and third instants, determining the number of main clock cycles occurring between the said second and third instants; determining a second number of main clock cycles representing the cycles occurring between the first and second instants on the basis of the difference between said first number and the determined number of main clock cycles occurring between the said second and third instants, wherein the cross calibration between the main clock and the sleep clock is determined on the basis of the number of sleep clock cycles within the sleep period and the said second number of main clock cycles.

The invention is advantageous in that no additional specific hardware is required for the initial cross calibration between the sleep clock and the main, faster, clock and such calibration can readily be achieved through reference to the air interface time-base In particular, the invention is advantageous in relating the sleep clock of the device to the air interface clock Preferably the step of performing the synchronization of the main clock to the air interface after completion of the sleep period is the same as the previous step of synchronizing the main clock to the air interface time-base Advantageously, identification of the sleep period comprises the step of receiving information broadcast from the network that indicates the paging interval Further, the identification of the sleep period is likewise based on a nominal frequency allotted to the sleep clock signal.

Advantageously, the method includes the step of locking the main clock frequency to the air interface time-base.

In particular, such locking is achieved by way of automatic frequency control Advantageously, the mobile radio communications device comprises an at least dual mode communications device arranged to communicate by way of at least two radio access technologies.

Advantageously, the method of the present invention is arranged to be employed with a plurality of such radio access technologies.

As such, the method can employ a common single sleep timer for use by all radio access technologies In one embodiment, the mobile radio communications device can be arranged to operate in accordance with the GSM radio access technology In this manner, the synchronization of the main clock to the air interface time-base is achieved by way of a frequency correction and synchronisation burst search.

it will be appreciated that the invention is advantageously used prior to connection of the device to a new network Advantageously, prior to the synchronisation of the main clock to the air interface time-base, the method includes the steps of performing a network search such that the aforementioned method is performed prior to camping-on a network located in accordance with such said search.

Advantageously, the sleep period is determined to coincide with a paging interval In accordance with a second embodiment however, the sleep period can be determined to be less than a paging interval and advantageously by determined amount Preferably, the difference between the sleep period and the paging interval is determined to be sufficient for allowing the synchronization of the handset time-base after the aforesaid second and third instant.

in this manner, the initial calibration can be achieved before the device receives its next paging signal According to another aspect of the present invention there is provided a mobile radio communications device having a main clock and a sleep clock arranged to operate at a slower speed than the main clock, and comprising means for synchronising the main clock frequency to an air interface clock frequency, means for synchronising the device time-base to the air interface time base, means for identifying a sleep period of the device in terms of a number of sleep clock cycles, means for commencing the sleep period at a first instant on the air interface time-base, means for restarting, after completion of the sleep period, the main clock and time- base counter at a second instant on the air interface time-base, means for repeating the synchronization of the device time-base to the air interface time-base at a third instant on the air interface time-base and subsequent to the second instant, means for determining a first number of the main clock cycles that should have occurred between the said first and third instants, means for determining the number of main clock cycles occurring between the said second and third instants, means for determining a second number of main clock cycles occurring between the first and second instants on the basis of the difference between said first number and the determined number of main clock cycles arising between the said second and third instants, whereby cross calibration is then achieved on the basis of the number of main clock cycles and the number of sleep clock cycles defined in the sleep period As will be appreciated, the mobile radio communications device of the present invention can be arranged to include means for providing the functionality of the various method steps described above The present invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which Fig I comprises a schematic block diagram of a cellular telephone handset adapted for use in accordance with an embodiment of the present invention, Fig 2 is a timing diagram to illustrate a method according to one embodiment of the present invention; and Fig 3 is a further timing diagram arranged to illustrate a method according to another embodiment of the present invention.

Turning first to Fig. I, there is illustrated a cellular phone handset 10 having schematically illustrated communication functionality 12 including a time-base generating circuit 14 and a digital signal processor (DSP) equaliser 16 The time-base circuitry 14 is arranged to provide for the local time-base within the handset 10 under the control of signals derived either from a master clock 18, or a sleep clock 20 by means of control lines 22, 24 respectively.

Once running as required, each of the clocks 18, 20 employs clock-routing lines 26, 28 for driving the handset 10 Communication by way of the cellular phone handset 10 is achieved via an antenna 30 and it will be appreciated that a local time-base is created by means of the time-base generating circuit 14 within the handset 10. This local time-base can be compared with the air interface time-base exhibited in an incoming signal 32 received at the antenna 30 The DSP equaliser 16 is arranged to receive the incoming signal from the antenna 30 by way of RF circuitry 34 Further, the DSP equaliser 16 is arranged to determine the offset between the expected time of arrival of signals within a paging block and a time when such signals actually did arrive As will be appreciated, and as is known from GB-A-2397465, the handset 10 can be arranged to employ this offset parameter in order to recalibrate the sleep clock 20 as required.

In order however to achieve initial calibration within the handset 10, i. e at the time of connecting to a new network, or at the time of first power-up, the handset 10 also includes a central processor unit 36 which is arranged to monitor synchronization of the time-base generating circuitry 14 to the air interface time-base, and also to determining the number of clock cycles arising in each of the master clock 18 and sleep clock 20 during the initial calibration procedure and as discussed further below Although not illustrated it should of course be appreciated that the master clock 18 connects to the central processor unit, the DSP and the RF and time-base circuitry As discussed above, the present invention seeks to provide for an initial calibration value relating the sleep clock 20, which is generally referred to as a 32kHz sleep clock, of the handset 10 with the air interface clock of a particular radio access technology The example discussed further below relates to communication on a GSM network but it should be appreciated that the concept of the present invention is equally applicable to any radio access technology and generally one that exhibits a periodic synchronization signal.

As is known, the 32kHz sleep clock is arranged to operate with a sleep timer that serves to counter down a generally pre-programmed number of 32kHz, i e slow clock 20, cycles, before generating an interrupt signal serving to wake the faster master clock 18 and subsequently the reminder of the handset 10 for frill operational functionality.

Such an arrangement is generally standard within single mode cellular phone handsets and can also be found within dual mode handsets although it is likewise also found that separate sleep timers can be implemented for each radio access technology within such a dual mode handset. However, it is considered that the implementation of such separate sleep timers is ineffIcient in terms of hardware and also serves to produce non- optimal power saving figures.

Thus, within the context of the illustrated embodiment of the present invention, should dual mode operability be relevant, only one sleep timer is envisaged for all possible radio access technologies In seeking to eliminate the hardware requirements of initial calibration currently known in the art, the present invention concentrates on the generation of a calibration value from the signals present on the air interface for the handset Further, and as noted above, the invention proposes the use of the sleep timer but, in doing so, it has been found necessary to overcome potential problems in that the timer is only operational in sleep mode and is therefore isolated from direct cross- reference to the air interface activities during non-sleep modes.

However, the invention can overcome such potential problems and as illustrated further below by way of reference to a specific example.

Assuming that the handset 10 has already been powered up, and it is required to connect to a new network while in an idle mode, the handset 10 is arranged to perform a network search in the normal manner.

Once the handset 10 has located a network and decoded the broadcast information, for example that found on the broadcast control channel (BCCH) for a GSM device, then, in accordance with the following embodiment of the present invention, and before camping-on the network, i e. before letting the network know that it can be paged, the following steps are performed It should however be appreciated that the steps need not follow the specific order suggested below.

First, the handset's time-base, as driven by way of the master clock 18, is synchronized to the air interface time-base by reference to frame number, bit number etc In accordance with a GSM-enabled handset, this is achieved by performing a frequency correction and synchronisation burst search. However, in other radio access technologies, similar equivalent mechanisms are employed. Once such synchronisation to the air interface time-base has been achieved, automatic frequency control (AFC) lock is then achieved for the master clock 18 of the handset 10 In some arrangement such AFC lock occurs automatically when a handset first receives from a serving cell of the network but, in according to the principal embodiment of the present invention, it is nevertheless important to be sure that some form of frequency lock is provided for Also, the method relies upon identification of the sleep period measured in an approximate number of sleep clock cycle and said sleep period preferably is arranged to coincide with the paging interval for the relevant network it is appreciated that this number of sleep clock cycles representing the sleep period can only be approximated since, at this stage, the sleep clock is not calibrated However, such approximation can readily be achieved through knowledge of the aforementioned paging interval in use on a particular network and which is decoded from the broadcast channel, and further from knowledge of the nominal frequency of the sleep clock, which is usually considered to be 32 768kHz.

The handset 10 is then arranged to enter its sleep mode so as to commence the sleep period in accordance with its normal sleep operation Reference is now made to Fig. 2 which provides an illustration of this particular embodiment of the present invention Fig 2 illustrates a timing diagram of the operation of the master 18 and sleep 20 clocks in relation to each other and in relation to the air interface time-base At Instant I on the air interface time-base, the handset 10 enters the sleep period as noted and the fast clock 18 is switched off as indicated.

At the time of instant I, i e. the beginning of the sleep period, it will be appreciated that the time-base of the handset 10 as previously driven by the master clock 18 is in a synchronised state with regard to the air interface time-base.

The handset 10 then remains asleep before the sleep period, which in accordance with this embodiment and as noted above, is coexistent with the paging interval, and subsequently wakes up at Instant 2 on the air interface time-base once the number X of slow clock cycles has elapsed and as was determined above from the knowledge of the paging interval and use on a network and the nominal frequency of the sleep clock 20.

As illustrated, this is Instant 2 on the air interface time-base in Fig 2.

At this time, the faster clock 18 is woken and serves to re-establish control of the handset time-base. It should be appreciated that, also at this instant 2, a second synchronization operation is commenced by the master clock 1 8 and so as to re- establish the time-base with regard to the master clock. This re- synchronisation operation that starts at Instant 2 is the same as that used previously by the master clock 18 and which lead to the synchronization achieved at the time of entry into the sleep mode at Instant I At some later moment in time, for example when the next synchronisation burst appears on the air interface, the air interface time-base can be established accurately once more by completing the resynchronization of the master clock 1 8 with the air interface time-base that commenced at Instant 2 Such re-synchronisj0 is illustrated at Instant 3 within the timing diagram of Fig 2 and involves completion of the synchronization process started at Instant 2 From a determination of instants 1, 2 and 3, as illustrated in the diagram of Fig 2, the sleep clock 20 can be calibrated to the air interface clock as explained further below and even though Instants 1 and 3 are known on the air interface, the difference between Instants I and 2 is only defined within the handset 10.

It should be appreciated that once the aforementioned calibration is known, the handset then serves to camp on the network and can enter idle mode and any further recalibration required can be performed, for example, in accordance with the subject matter of GB-A-23 97465 Returning however to the initial cross-calibration offered by the present 1 5 invention, Instants I and 3 are known points on the air interface time-base and, as noted, the master clock is AFC synchronized to match the air interface frequency and so the number of master clock cycles arising between Instants I and 3, identified as A in Fig 2, can readily be determined.

Further, the time between Instants 2 and 3 comprises a known number of master clock cycles and this number, in Fig 2, is identified as B clock cycles It should be appreciated that the master clock cycles are directly related to the air interface clock speed due to the operation of the AFC mechanism before the handset 10 entered its sleep mode. Since the sleep period is in the order of one paging cycle, there is insufficient time for the AFC to be significantly in error and such an arrangement therefore serves to provide for accuracy within the method of the present invention As mentioned above, the time between Instants 1 and 2 on the air interface time-base is only defined within the handset in terms of a sleep clock 20 cycles as estimated above on the basis of the known paging interval and the nominal frequency of the sleep clock 20 This is identified at Fig 2 as X slow clock cycles as noted.

However, this sleep period, which is equivalent to X sleep clock cycles, i.e. the period between Instants 1 and 2, is also defined externally with reference to the air interface since, as will be appreciated from Fig. 2 the period between Instants I and 2, measured in master clock cycles, is clearly the difference between A master clock cycles and B master clock cycles Thus, the period between instants I and 2 can then be determined in terms of both the number of master (A-B) clock cycles, and the number of X sleep clock cycles such that the cross calibration between the master 18 and sleep clock 20 can then be readily achieved.

However, it is important to note that the operation of the abovementioned method applied primarily to a situation in which the handset is capable of entering sleep mode It is known that some handsets inhibit sleep under certain circumstances For example when the handset 10 is initially powered on, sleep is not allowed for several minutes after such power-up in order to allow the temperature of the components within the handset 10 to stabilize. While the handset 10 is warming up after initial power-up, the crystal parameters of both the master and sleep clocks are known to change rapidly and so a cross-calibration between the two clocks is not possible Initiation of a sleep period is therefore illogical since there is little point in calibrating a rapidly drifting 32kHz clock when calibration cannot give sufficient accuracy for the time-base to be rebuilt after a sleep period Thus, since sleep is therefore to be inhibited during initial handset warm up, it will generally mean that, subsequent to power-up, the handset will then have camped onto a network before a first sleep period is possible In this case, it would not then be possible to perform a calibration for the paging interval length as arises in accordance with the above-mentioned embodiment of the present invention since this would cause a paging signal to be missed A further embodiment of the present invention is therefore illustrated in Fig. 3 and which seeks to overcome such potential problems possibly arising at the time of initial power-up of the handset 10 As will be appreciated this further embodiment of the present invention provides for calibration over a shorter period than that disclosed above in relation to Fig 2 It has been appreciated that the FCB signals that can be employed for calibration generally occur every 10 frames, while the paging interval is generally in the order of 102-459 frames This further embodiment of the present invention therefore involves a procedure in which, subsequent to receipt of a paging signal, the handset enters a sleep mode for a period that is determined, for example, to be 20 frames less than the paging interval. At the time of "wake-up", a FCB search is performed in order to synchronise the master clock 18 to the air interface and it is acknowledged that this will take a maximum often frames in view of the FCB interval noted above.

From the results of the FCB search, the time base is rebuilt and, in a manner similar to that illustrated above in relation to Fig. 2, initial calibration can be achieved before it is again the time to receive a paging signal from the network.

Sleep mode can then be entered in the normal manner using such an initial calibration value.

It will be appreciated that this further embodiment of the present invention is generally similar to the procedure illustrated with reference to Fig. 2 with the exception that it is performed for a shorter period and that this period falls within adjacent paging signals Turning to Fig 3, this further embodiment of the present invention is illustrated with reference to receipt of two paging signals Page 1 and Page 2, the control of the sleep period to be in the order of 20 frames less than the paging interval between the signals Page 1 and Page 2, and the initial synchronization and resynchronisatjon of the master clock 18 within the 20 frame period noted within Fig 3, and thus before the next paging signal Page 2 arrives.

While it is appreciated that the above-mentioned examples have been provided in relation to a GSM network, it is considered that the invention is equally applicable to other radio access technologies since, with regard to all cellular communication systems, the frequency correction information is generally broadcast more frequently than the paging signals Thus, it will be appreciated that the present invention can provide for initial calibration in any situation either before, or after, the handset is camped onto a network and is thereby monitoring receipt of the paging signals Remaining with GSM time-base reconstruction however, an intermediate step can be provided between instants 2 and 3 within Fig. 2 and can serve to re-establish an approximate GSM time-base and then note the start value in frames and quarter bits Also, before the actual GSM time-base is re-established at Instant 3 in Fig 2, a final value of the approximate time-base can advantageously be recorded. The difference between the aforementioned starting value and final values of the approximate time-base can then be used to calculate the number of fast clock cycles since the handset 10 actually woke up. Knowing this value, the calibration factor can be established as noted above It will therefore be appreciated that the present invention is particularly applicable to dual mode handsets where, although calibration hardware may be present, such hardware may not be accessible to every radio access technology Thus, without use of the mechanism described herein, additional hardware or signalling would then be required in order to achieve such initial calibration for all radio access technologies The present invention therefore advantageously allows for initial calibration to be provided over all radio access technologies of a dual mode handset without requiring any additional calibration hardware, or without requiring any architectural changes and, of course, the method can allow for initial calibration within a single, dual or multi mode handset and without requiring any calibration hardware js

Claims

1 A method of achieving initial calibration of a main clock with a sleep

clock within a mobile radio communications device arranged for communication via an air interface, the sleep clock being arranged to operate at a slower speed than the said main clock, and the method comprising synchronizing the main clock frequency to the air interface clock frequency, synchronjsing the device time-base to the air interface time-base; identifying a sleep period of the device in terms of a number of sleep clock cycles, commencing the sleep period at a first instant on the air interface time-base, restarting after completion of the sleep period, the main clock and time-base counter at a second instant on the air interface time-base, repeating synchronization of the device timebase to the air interface at a third instant on the air interface timebase subsequent to the said second instant, determining a first number of main clock cycles representing cycles that should have occurred between the said first and third instants, determining the number of main clock cycles occurring between the said second and third instants, determining a second number of main clock cycles representing the cycles occurring between the first and second instants on the basis of the difference between said first number and the determined number of main clock cycles occurring between the said second and third instants, wherein the cross calibration between the main clock and the sleep clock is determined on the basis of the number of sleep clock cycles within the sleep period and the said second number of main clock cycles 2 A method as claimed in Claim 1, wherein the step of performing the synchronization of the main clock to the air interface after completion of the sleep period is the same as the preceding step of synchronizing the main clock to the air interface time-base 3 A method as claimed in Claim I or 2, wherein identification of the sleep period comprises the step of identifying paging signals received at the device so as to determine the paging interval 4 A method as claimed in Claim 3, wherein the identification of the sleep period is likewise based on a nominal frequency allotted to the sleep clock signal A method as claimed in Claim 1, 2, 3 or 4, and including the step of locking the main clock frequency to the air interface time-base.

6, A method as claimed in any one or more of Claims Ito 5, wherein the mobile radio communications device comprises a dual mode communications device 7 A method as claimed in Claim 6 and is arranged to be employed with both of the radio access technologies 8 A method as claimed in Claim 6 or 7, and arranged to employ a common sleep timer for use by both radio access technologies 9 A method as claimed in any one or more of the preceding claims and performed prior to connection of the device to a new network A method as claimed in any one or more of the preceding claims and performed prior to camping-on a network located in accordance with a network search II A method as claimed in any one or more of the preceding claims, wherein the sleep period is determined to coincide with a paging interval.

12 A method as claimed in any one or more of Claims Ito 10, wherein the sleep period is determined to be less than a paging interval 13 A method as claimed in Claim 12, wherein the difference between the sleep period and the paging interval is determined to be sufficient to allow the synchronization of the main clock after the aforesaid second instant 14. A mobile radio communications device having a main clock and a sleep clock arranged to operate at a slower speed than the main clock, and comprising means for synchronising the main clock frequency to an air interface clock frequency, means for synchronising the device time-base to the air interface time base, means for identifying a sleep period of the device in terms of a number of sleep clock cycles, means for commencing the sleep period at a first instant on the air interface time- base, means for restarting, after completion of the sleep period, the main clock and timebase counter at a second instant on the air interface time-base, means for repeating the synchronization of the device time-base to the air interface time-base at a third instant on the air interface time-base and subsequent to the second instant, means for determining a first number of the main clock cycles that should have occurred between the said first and third instants, means for determining the number of main clock cycles occurring between the said second and third instants, means for determining a second number of main clock cycles occurring between the first and second instants on the basis of the difference between said first number and the determined number of main clock cycles arising between the said second and third instants, whereby cross calibration is then achieved on the basis of the number of main clock cycles and the number of sleep clock cycles defined in the sleep period is A device as claimed in Claim 14 and including means for locking the main clock frequency to the air interface time-base 16. A device as claimed in Claim 14 or 15 and comprising a dual mode communications device.

17 A device as claimed in Claim 16, and comprising a common sleep timer for use by both radio access technologies of the dual mode device 18 A method of achieving initial calibration of a main clock with a sleep clock within a mobile radio communications device substantially as hereinbefore described with reference to Fig I, Fig 2 and Fig. 3 of the accompanying drawings 1 9 A mobile radio communications device substantially as hereinbefore described with reference to, and as illustrated in, Fig. I, Fig. 2 and Fig. 3 of the accompanying drawings.