TECHNICAL FIELD

This invention relates to time tracking, and in particular to a method and an apparatus for allowing mobile equipment to maintain time tracking, even after the process of handing over from one cell to another cell of a cellular communications system has taken place.

BACKGROUND OF THE INVENTION

It is recognized that it is useful for mobile equipment to track time precisely.

The Global Positioning System (GPS) is one of several actual or proposed systems, in which a constellation of satellites broadcast signals which can be freely used by ground or air equipment to determine the current location and velocity of the equipment, and the current time. The system uses the principle of tri-lateration to determine the user position.

It is well known that GPS chips can be included in mobile communications devices such as mobile phones and the like, so that the device can use information relating to its current location, in order to provide additional services to the user. Assisted GPS (A-GPS) is a system that can be used in a mobile communications device, in which some information is delivered to the GPS chipset via the mobile communication network. The GPS chipset then does not need to decode this information from the GPS satellite signals, which results in improved time-to-first-fix, time-to-fix and sensitivity. An improved time-to-fix also results in a reduced power consumption.

Fine time assistance (FTA) is the name given to a procedure of providing highly accurate GPS time information to the GPS chip, by which accuracies of 5μs are currently achievable. Fine time assistance has been shown to further improve time-to- first-fix, time-to-fix, location accuracy and sensitivity.

The FTA procedure involves the mobile equipment (ME) generating a strobe (that is, a pulse on an input/output signal line of the GPS chip in the ME) at a known GPS time. Typically, when the GPS chip detects the occurrence of a strobe, it resets and starts a high precision counter. The highly accurate GPS time can then simply be determined by adding the counter value to the GPS time-of-strobe, which is passed to the GPS chip via a communication interface.

For the ME to accurately generate a strobe at a known GPS time, the ME must have knowledge of the cellular-GPS time relationship, or CGTR, of its currently camped cell. A CGTR is represented as two time-stamps, namely a cellular time stamp and a GPS time stamp, which represent the same instant in the time continuum. The CGTR of the currently camped cell is an input to the FTA procedure and is commonly referred to as the reference CGTR.

However, this has the limitation that the CGTR is valid for only one cell, and cannot be used if the ME is handed over to another cell.

US2005/0146462 discloses a system for determining a time relationship between an air interface time of a base station and a satellite time for a satellite constellation.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method for maintaining timing information in a mobile communications device operating in a cellular communication network. A relationship is determined between time as measured in a first cell of the cellular communication network and time as measured in a timing system. At a clock start time, a time count is started, and this time count is stopped at a clock stop time as measured in a second cell of the cellular communication network. The counted time is determined, and the clock stop time as measured in the timing system is determined by adding the counted time to the clock start time as measured in the timing system. Then, from the clock stop time as measured in the second cell of the cellular communication network and the clock stop time as measured in the timing system, a relationship is determined between time as measured in the second cell of the cellular communication network and time as measured in the timing system. The step of determining the relationship between time as measured in a first cell of the cellular communication network and time as measured in a timing system may be performed by obtaining information from the cellular communication network.

Alternatively, the step of determining the relationship between time as measured in a first cell of the cellular communication network and time as measured in a timing system may be performed by determining said relationship in the mobile communications device without obtaining information from the cellular communication network.

The time count may be started by enabling a counter that is connected to receive clock signals.

The clock start time may be defined as a predetermined time as measured in a first cell of the cellular communication network.

According to a second aspect of the present invention, there is provided a timing circuit, for use in a mobile communications device, the timing circuit being adapted to perform the method according to the first aspect of the invention.

The timing circuit may comprise a counter, and a clock, wherein the counter is connected to receive signals from the clock, and to count the received signals when enabled.

The timing circuit may further comprise logic circuitry adapted to enable the counter in response to a first probe at the clock start time, and to disable the counter in response to a second probe at the clock stop time.

According to a third aspect of the present invention, there is provided a mobile communications device, comprising a timing circuit according to the second aspect of the invention.

The timing system may be a satellite positioning system, such as the Global Positioning System (GPS). This has the advantage that the ME is able to deliver highly accurate timing information, for example GPS timing information to a GPS chipset, or to other time critical devices, without the need for assistance from an external source, such as a mobile network, and is able to provide valid information even in the event of a handover from one cell to another. In the specific case of a GPS system, the ability to provide precise GPS timing information to the GPS chipset is beneficial, as it reduces the number of tasks that the GPS chipset needs to perform, resulting in faster time-to-fix, improved location accuracy and improved GPS sensitivity.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a schematic illustration of a part of a mobile communications network in accordance with an aspect of the present invention.

Figure 2 is a block diagram illustrating a part of a mobile communications device in accordance with an aspect of the present invention.

Figure 3 is a flow chart, illustrating a method in accordance with an aspect of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 shows a part of a mobile communications network. Specifically, Figure 1 shows two base stations 10, 12, which form part of a cellular communications network. It will be apparent to the person skilled in the art that the network will contain many more than these two base stations. Each of the base stations 10, 12 provides coverage for wireless communications devices located in a respective cell, or coverage area. Figure 1 also shows a mobile wireless communications device, in the form of a mobile phone 14, which is able to communicate using the mobile communications network while it is in the coverage area of one of the base stations.

The mobile communications network may be any suitable cellular network, in which each base station transmits signals with known timing relationships. That is, each base station transmits data in frames having predetermined durations. However, the invention may be applied most usefully in networks in which the base stations are not synchronized with each other. Many conventional networks, such as GSM and UMTS networks operate in this way.

As discussed in more detail below, the mobile phone 14 is also able to receive signals from satellites 16 of a satellite positioning system. The invention is described herein with reference to the Global Positioning System (GPS), but other similar systems are known, or proposed, and the invention is equally applicable to such systems.

Further, although Figure 1 shows a mobile phone 14, it will be noted that the invention is applicable to any form of wireless communications device, although it is primarily applicable to mobile communications devices, or more generally to any mobile equipment (ME).

Figure 2 is a schematic block diagram of the mobile phone 14. As is conventional, the mobile phone 14 includes an antenna, RF circuitry, and various components such as a keyboard, a display and signal processing circuitry, that are relevant to its primary communications functions. These are not shown in Figure 2, or described further, except to the extent that they are relevant to an understanding of the present invention.

The mobile phone 14 includes GPS circuitry 20, which is connected to timing circuitry 22. The timing circuitry 22, in turn, includes an ASIC 24, which is connected to a logic block 26. The ASIC 24 and the logic block 26 are connected to a counter 28, which is also connected to receive signals from a clock 30. The counter 28 is a high precision counter, and the clock 30 can be derived from the main platform clock within the mobile phone 14, or an additional clock.

The timing circuitry 22 is also connected such that it receives information about the operation of the mobile phone 14, as described below. In addition, the GPS circuitry 20 is connected to a suitable antenna, such that it can receive suitable signals from satellites in the GPS satellite constellation, as will be apparent to the person skilled in the art.

Figure 3 is a flow chart, illustrating a method in accordance with the present invention. The basic concept behind the invention is that the ME obtains and stores a time relationship between the time as measured according to the satellite system and the time as measured in the cell on which it is currently camped, and maintains this relationship during handovers. This means that, at any time the ME is required to produce a GPS fix, it will be capable of delivering fine time assistance to the GPS chip, without the need for additional network support.

In general terms, the method according to the invention includes the steps of obtaining the time relationship between the time as measured according to the satellite system and the time as measured in the cell on which it is currently camped, and then converting this relationship to a time relationship between the time as measured according to the satellite system and the time as measured in a different cell, which may be the cell with which the ME is performing a handover.

Although the invention is described with reference to its application in a device which can be used in a cellular communication network and in a satellite positioning system, the same principle can be applied to devices that operate in other timing systems which have their own specific time formats, such as other satellite and broadcast communication systems. In such cases, it is possible to obtain a time relationship between the time as measured according to the timing system and the time as measured in the cell on which it is currently camped, and then converting this relationship to a time relationship between the time as measured according to the timing system and the time as measured in a different cell.

Figure 3 illustrates this method in more detail. The process starts at step 40, at which time the ME is camped on a first cell (referred to below as cell A). At step 42, the ME obtains knowledge of the cellular-GPS time relationship (CGTR) for cell A. The CGTR can be obtained in one of two ways. One possibility is that information can be obtained directly from the network in a GPS time assistance message. Alternatively, as shown in Figure 3 and described below, the ME can determine this relationship itself.

This procedure is commonly used in uplink fine time assistance procedures, that is, procedures where the network requests the CGTR from the ME. Specifically, determination by the ME of the CGTR for the cell on which it is camped is performed by estimating a reference CGTR and correcting it against the GPS satellite signals, from which accurate time information is available.

First, the ME signaling stack is requested to generate a hardware timing strobe at an arbitrary cellular time in the near future (typically 100ms). This is known as the cellular time-of-strobe. However, since there is no reference CGTR available at this time, the GPS time at this instant (i.e., the GPS time-of-strobe) cannot be determined. Instead, the GPS time-of-strobe is estimated. The estimated GPS time-of-strobe and the strobe itself are passed to the GPS chipset 20, which performs the necessary measurements of the GPS satellite signals to accurately track the true GPS time. From this information, the GPS chipset 20 can determine the precise error offset on the estimated GPS time-of-strobe, resulting in the precise GPS time-of-strobe. The cellular time-of-strobe and this precise GPS time-of-strobe together define the CGTR of the current cell.

As is known, the CGTR of the current cell can be used to provide assistance to the GPS circuitry 20. When a handover is expected, it is useful to know the CGTR of the new cell (referred to below as cell B). However, since cellular times between neighboring cells are not synchronized, the CGTR for cell A is not the same as the CGTR for cell B and, indeed, there is no known relationship between the CGTR for cell A and the CGTR for cell B. It is therefore advantageous for the ME to be able to obtain the CGTR of cell B, based on knowledge of the CGTR of cell A.

The following process is therefore performed, when it is determined by the timing circuitry 22, based on an input from a controller of the ME, that a handover is about to occur, but before the handover occurs.

In step 44, the counter 28 is reset. Specifically, it is ensured that a Counter Enable line 32 from the logic block 26 to the counter 28 is LOW (i.e. that the counter 28 is not counting the pulses generated by the clock 30), and then a Reset signal is sent from the ASIC 24 to the counter 28 to set the counter value to zero.

In step 46, the start of a time count is scheduled for an arbitrary yet known cellular time (CellTimeO), as measured in the current cell, cell A. At this arbitrary time, in step 48, a "start-counter" rising edge is applied from the logic block 26 to the counter 28 on the Counter Enable line 32, and this starts the counter 28. Specifically, as shown in step 50, the counter counts the time elapsed since the clock start time by counting the clock ticks received from the clock 30. The "start-counter" rising edge is performed by scheduling a timing strobe from the ASIC 24 to the logic block 26, and using logic in the logic block 26 to latch HIGH the Counter Enable line 32 when this strobe is received. The ability to generate a timing strobe is already known, and is used in existing FTA procedures.

The clock start time as measured in cell A of the cellular system (CellTimeO) is thus known. The clock start time as measured in the GPS system (GPSTimeO) can therefore be determined by using the known relationship between time as measured in cell A of the cellular system and time as measured in the GPS system, i.e. the CGTR of cell A, as obtained in step 42.

At this point the ME is precisely tracking GPS time as it has knowledge of GPS time at one particular instant, and a counter tracking the elapsed time since this instant.

At some time, the ME then performs a handover from cell A to cell B, according to a conventional procedure, which will not be described further herein, it being noted simply that the ME no longer detects the precise timing of signals from cell A, but is able to detect the precise timing of signals from cell B.

At step 52, the ME schedules the stop of the time count for an arbitrary yet known cellular time (CellTimel ), as measured in the new cell, cell B. In step 54, at this arbitrary time instant, a "stop-counter" falling edge is applied from the logic block 26 to the counter 28 on the Counter Enable line 32, and this stops the counter 28. This "stop-counter" falling edge is performed by scheduling a timing strobe from the ASIC 24 to the logic block 26, and using logic in the logic block 26 to latch LOW the Counter Enable line 32 when this strobe is received. As mentioned above, the ability to generate a timing strobe is a known feature used in existing FTA procedures.

At step 56, the ASIC 24 reads from the counter 28 the counter value, representing the elapsed time between the count start time and the count stop time, denoted DeltaGPS.

Then, in step 58, the CGTR of cell B is obtained from the knowledge of the GPS time and cellular time of the "stop-counter" falling edge. Specifically, the CGTR for cell B is the relationship between this time instant (CellTimel ) as measured in cell B and as measured in the satellite system (GPSTimel ). The value of CellTimel is known because it was the scheduled stop time of the count, while the value of GPSTimel is determined by adding the time count to the start time of the count, as measured in te satellite system, i.e. GPSTimel = GPSTimeO + DeltaGPS. The CGTR of cell B is then stored in the ME for use in subsequent GPS fixes.

Thus, even though cellular times between neighbouring cells are not synchronized, and hence there is no predetermined relationship between the CGTR for cell A and the CGTR for cell B, knowledge of a CGTR determined on cell A can still be used to determine the CGTR for cell B when the ME is handed over into cell B. As a result, if the network requires the CGTR from the ME for use in uplink FTA procedures, the ME can provide this, without needing to undergo a GPS fix procedure to deliver it.

There is thus provided a method for maintaining timing information that can usefully be applied to a mobile communications device.