GB2413036A - Automatic timebase correction for non-serving base stations - Google Patents

Abstract

An apparatus for synchronising a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations comprising a means for receiving data from the at least one base station, a means for determining the timing of transmissions from the base station, a means for creating a reception window to monitor a further transmission from the base station, a means for measuring the time of receipt of the further transmission, a means for calculating the error between the determined timing of transmission and the measured time of receipt, and a means for adjusting the timing of a subsequent reception window in dependence on the calculated error. The timing of the window is adjusted by calculating the expected time of arrival of a subsequent signal using the aligned time base and applying the error to the expected time of arrival to determine the timing of the subsequent reception window. Thus it is predicted that the measured error will be repeated and so the window timing adjusted accordingly.

Description

1 2413036

APPARATUS FOR AUTOMATIC TIMEBASE CORRECTION

The present invention relates to an apparatus for automatic timebase correction and, in particular, to an apparatus for automatically correcting timebases for non- serving base stations.

A mobile communication device receives signals from a number of local base stations during operation.

Specifically, the device receives a large amount of system information from a first base station and smaller amounts of information from a plurality of further local base stations in neighbouring cells. The first base station is known as the serving base station and the further base stations are known as non-serving base stations.

Signals from non-serving base stations are monitored to enable the device to maintain synchronization with the transmission cycle of these base stations. It is important to maintain synchronization with the transmission cycles from non-serving base stations since, if the device moves out of range of the primary base station or moves to a position where it is more appropriate to receive a signal from a neighbouring base station, the device can switch to communicate with one of the further base stations and receive the full transmission cycle from that base station.

This is the case when the device moves into a neighbouring cell.

Communication signals transmitted from base stations are transmitted in cycles and include information regarding the frequency of the network clock as well as information regarding incoming calls or SMS messages. The different parts of information occupy specific blocks within the transmission cycle.

The device receives a large amount of system information in transmissions from the serving base station.

The device synchronizes its internal clock with the network clock as received in the transmission and receives operating information from the serving base station within the transmission. In contrast, the device only requires a fraction of the information transmitted from the non- serving base stations. Since the device synchronizes its clock and receives other general information from the serving base station, this information is not required to be received from the non-serving base stations. In fact, the only information that the device requires from the non-serving base stations is information specific to that base station, for example, the identity code of the base station, which occurs cyclically and can thus only be received at certain times, and the strength of signal being received from that base station, which can be measured at any time.

It is inefficient for the device to monitor the complete transmission signal from non-serving base stations since it only requires a fraction of the information within the transmission. Instead, the device establishes a regular reception window for each base station. The timing and duration of the window is synchronized with the required portion of the transmission signal. In order to synchronism, the device aligns its timebase, which is the internal "cyclic" counter, with the incoming signal. The device also establishes a window within the timebase to match the required incoming information. The timebase indicates the current position in the transmission cycle and should be synchronized with the incoming signal. The device maintains a separate timebase for each cell with which synchronization is required.

In GSM, a transmission cycle is split into 51 blocks of information. Generally the information required from non-serving cells is confined to around l or blocks per non serving cell every 30 seconds. By contrast, a minimum of 4 blocks are received from the serving cell every 2 seconds.

Since base stations transmit information in regular, periodic, cycles, the required information always occurs in the same position in the cycle. Therefore, if the distance between the base stations and the device is constant, the timing of the window is regular. However, any change in the distance between the mobile device and the base station results in the required blocks arriving at the device earlier or later than expected. In this case, the reception window is not synchronized with the incoming data. In order to align the window, the device must synchronize its timebase with each incoming signal and position the window correctly within the timebase to receive the required blocks of information. The position of the window within the timebase is not changed.

If the distance between the device and the base station is reduced, the blocks are received sooner than expected. To account for this change in timing, the device must advance its reception window for the particular base station. Therefore, the timebase must be advanced in order that the window is synchronized. Conversely, when the distance between the device and the base station is increased, the time taken for the blocks to reach the device is increased and the timebase must be delayed in order that the window is delayed. The device must ensure that the timebase for each base station remains synchronized with the corresponding transmission cycle in order that the reception window is timed correctly to receive the required blocks of information from each base station transmission.

As mentioned above, the internal clock of the mobile is permanently locked to the network clock via the transmission from the serving base station. For this reason, any delay or advance of the timebase due to movement of the device with respect to the serving base station is automatically accounted for by the device. In explanation, as the device moves with respect to the serving base station, i.e. there is a rate of change of position; the apparent frequency of the network clock signal is altered due to the effects of Doppler shift. Movement away from the base station will reduce the apparent clock frequency while movement towards the base station will increase the apparent frequency.

Furthermore, the change in position with respect to the base station will affect the time at which the signals are received. However, the timing change of the reception is always exactly cancelled by the Doppler effects. Therefore, the timebase is always exactly synchronized with the transmission from the serving base station. In other words, the timebase has been adjusted but this adjustment has been dealt with automatically by the device due to the frequency lock to the incoming signal.

In contrast, since the base stations from neighbouring cells are not colocated with the base station of the serving cell, the change in position of the device with respect to the serving base station will not be cancelled.

Therefore, the movement with respect to the serving base station will produce a further error in the timebase for the non-serving base station.

Therefore, separate errors in the timebase, and hence in the timing of the reception windows, for the non-serving base stations will arise due to change in position of the device with respect to the serving and nonserving base stations.

Known systems overcome the problems associated with errors in the timebase of non-serving base stations by extending the duration of the reception window for each non- serving base station. Such systems open the reception window before the time at which they expect to receive the required information and close the window after the time that they expect to finish receiving the information. Thus, if the signal arrives sooner or later than expected, due to movement of the device, the required information is still received. Once the signal has been received, the timebase corresponding to each non-serving base station is adjusted in order to maintain synchronization with the transmission from the non-serving base station.

Increasing the duration of the reception window in order to account for any movement is inefficient and, in extreme situations, the device may fail to receive the signal if it arrives outside the reception window.

Furthermore, increasing the window size requires an increase In the size of the demodulation software in the digital signal processor (DSP), which requires an increase in the size of the buffer. As the buffer size increases, the processing power required by the device increases exponentially. This results in an increase in the number of components and power requirements of the device, which consequently increases the cost of the device.

Typically, synchronization signals from neighbouring base stations are monitored every thirty seconds. If the device is moving at a high speed during this period, for example if the user is on a train, this movement may require a significant change in the internal timebase. A possible way to improve synchronization would be to monitor the neighbouring base stations more regularly, for example every two seconds. However, increasing the number of monitoring events increases the power consumption of the device.

We have appreciated that mobile communication devices must maintain synchronization of their reception windows with non-serving base stations in order to receive the required information which is transmitted within specific blocks of the transmission cycle. A change in location and movement of the device with respect to both non-serving and serving base stations generates errors in the timebase for the non-serving base stations. These errors lead to ambiguity in the precise time that a required block will be received by the handset. We have also appreciated that increasing the duration of the reception window for neighbouring cells is an inefficient solution to this problem due to the increased power, component and cost requirements associated with this solution.

Embodiments of the present invention dynamically adjust the timing of reception windows for receiving signals from non-serving base stations. Embodiments make use of any previous adjustments that have been made to remove the error in the timebase for non-serving base stations to predict the likely time of arrival of future signals from the same non- serving base stations. Therefore, the error in the timing of future signal receptions from non-serving base stations is reduced. In contrast, known systems only update the timebase for non-serving cells after an error is detected, and no predictive technique is applied to reduce this error in future signal receptions. Since the present invention predicts the likely timebase error in the next signal reception from a non-serving cell, actual timebase errors in the received signal are reduced. This reduction in error reduces the need for the reception window size to be increased in order to ensure receipt of the required signal.

Therefore, the need for increasing buffer sizes and increasing the complexity of the handset is reduced.

The invention is now defined in its various aspects in the appended claims, to which reference should now be made.

An embodiment of the present invention will now be described in detail by way of example with reference to the accompanying drawings in which: Figure l shows the position of a mobile communication device in relation to local cells and base stations.

Figure 2 shows a mobile communication device communicating with a serving base station and a non-serving base station.

Figure 3 is a flow diagram identifying the steps for implementing an embodiment of the present invention when initially activated.

Figure 4 is a flow diagram identifying steps for implementing an embodiment of the present invention.

Figure 5 is a block diagram showing the components within an embodiment of the present invention.

Figure l shows a device lo positioned within a geographical cell 20, served by a base station SBS. The cell 20 is surrounded by a number of neighbouring cells 30 to 80, each of which is served by a base station NSBS l to NSBS 6. The device establishes a primary communication link with SBS. SBS is the serving base station with which the communication device lo establishes a primary communication link. Each of the base stations in the surrounding cells NSBS l to NSBS 6 are classed as neighbouring base stations and the device l0 monitors the transmissions from these base stations periodically.

Figure 2 is a diagram showing the communication device lo communicating with the serving base station SBS and a particular non-serving base station NSBS. In use, the device may communicate periodically with a plurality of non- serving base stations. The device maintains a constant communication with the serving base station and receives signals periodically from the non- serving base station.

Figure 3 shows the steps taken by an embodiment of the present invention in order to maintain synchronization of its timebases for non-serving base stations during operation. At 300 the device is activated. At 310 the device conducts a regular network scan procedure.

Typically, during this procedure the device scans all available frequencies for signals from local base stations.

The device decodes any identified signals and assesses the quality of each decoded signal. Quality is defined in different ways depending on the cellular technology. On the basis of the received signal quality and a number of other parameters broadcast by the network, the device determines with which base station it will establish a primary communication link at 320. This base station is the serving base station. The device receives the full transmission signal from this base station. Using the same selection parameters, the device selects a number of non- serving base stations at 330. These base stations may provide signals of reduced strength compared with that from the serving base station. The device will receive only a fraction of the transmission signal from these base stations. At 340 the device commences communication with the serving base station. Once the communication is commenced, the device synchronizes its internal clock with the clock signal received from the serving base station. Therefore, the device locks its internal clock to the observed frequency of the received signal from the serving base station.

At 350 the device determines the timebase associated with signals transmitted from the non-serving base stations.

The timebase is the timing of the reception of the signal from the non serving base station window with respect to the master clock. The device uses the timebase for the non- serving base station in order to predict a time of arrival of the signal from the non-serving base station. At 360 the device opens a reception window for receiving signals from the non-serving base station. The time at which the signal from the non-serving base station is actually received by the device is measured and recorded as the measured time of arrival at 370. At 380 the device compares the measured time of arrival with the expected time of arrival in order to produce a measured error. Any error between in the expected and measured time of arrival is due to movement of the device since the timebase was last determined. Once the error has been calculated, the timebase for the non-serving base station is adjusted using the error at 390. The timebase is adjusted by simply adding the error to the previously calculated timebase in order to produce an adjusted timebase at 390. This initial updating of the timebase realigns the timebase with the signals from the non-serving base station. The device then calculates the expected time of arrival of the next signal using the aligned timebase at 395.

In the next step, the device determines when it will open its reception window to receive the next reception from the particular non-serving base station by predicting the likely error in the expected time of arrival of the subsequent signal. The device predicts that the error between the expected and measured time of arrivals of the reception of the subsequent signal will be identical to the error associated with the last reception. This prediction works on the assumption that the movement of the device in the next interval between receptions will be identical to that within the last interval. In order to determine the time for opening the next reception window, the device adds the measured (ie predicted) error to the expected time of arrival at 400. The device then opens the reception window at the predicted time of arrival and returns to step 360.

By predicting the likely error in the expected time of arrival of the signal, the device reduces the need for the device to extend the size of the reception window in order to account for movement of the device during the interval between receptions.

Figure 4 shows the manner in which the timing of the reception window is dynamically adjusted during use. At 405 the expected time of arrival of the signal from the non- serving base station is calculated using the aligned timebase associated with the particular non-serving base station. At 410, the device calculates the time for opening the reception window. As discussed above, the time for opening the window is calculated by predicting that the device will experience the same movement in the next interval between receptions as that experienced in the most recent interval. The time at which the window is opened is calculated by adding the predicted error to the expected time of arrival. At 420 the reception window is opened to receive the signal from the non-serving base station. At 430 the signal from the non-serving base station is received by the device. The time of arrival of the signal is measured and recorded. At 440 the measured time of arrival is compared with the expected time of arrival of the signal from the non-serving base station to produce a measured error. At 450 the timebase of the non-serving base station is corrected using the measured error in order to align the timebase for the non-serving base station with the received signal. The measured error is then used as a predicted error at 460 and added to the expected time of arrival of the next signal at 420 in order to determine at what time the reception window will be opened to receive a subsequent from the particular non-serving base station.

Figure 5 shows the components in a preferred embodiment of the present invention. The device 500 includes an antenna 510 for receiving signals from local base stations. The device includes a clock 520 which is locked to the frequency of the incoming signal from the serving base station. The clock is responsible for the timing of all measurements and operations of the mobile communication device.

The device includes a timing comparator 530 for comparing the expected time of arrival of required data blocks with the measured time of arrival. The device also includes a means 540 for updating the timebase in dependence on the difference between the measured and expected times of arrival of the data blocks. The timebases associated with all non-serving base stations are stored in a memory 550.

It will be obvious to those skilled in the art that the present invention provides a means in which the timing of a reception window for receiving a signal from a non- serving base station is established using a measured error between the expected and measured time of arrival of the last received signal. The timing of the reception window is established using the principle that any error between the expected time of receipt of a signal and the measured time of receipt will be repeated in the subsequent reception. By predicting the error in forthcoming reception times, embodiments of the present invention reduce the requirement for extending the reception window to account for movement of the device between receptions of signals from base stations.

Claims (8)

1. A method for synchronlsing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations comprising the steps of; receiving a data transmission from the at least one base station; determining the timing of the transmissions from the base station; creating a reception window to monitor transmissions from the base station, the reception window being synchronized with the reception of the transmission; receiving a further data transmission within the reception window at a measured time of receipt; calculating the error between the determined timing of the transmission and the measured time of receipt; and adjusting the timing of a subsequent reception window in dependence on the measured error between the determined timing of the transmission and the measured time of receipt.

2. A method for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations according to claim l wherein the step of adjusting the reception window is performed with respect to an internal clock.

3. A method for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations according to claim l or 2 wherein the step of adjusting the timing of subsequent reception windows includes the steps of; aligning a timebase associated with the base station with the received signal; calculating the expected time of arrival of a subsequent signal using the aligned timebase; and applying the error between the determined timing of the transmission and the measured time of receipt to the expected time of arrival to determine the timing of the subsequent reception window.

4. An apparatus for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations comprising; a means for receiving data from the at least one base station; a means for determining the timing of transmissions from the base station; a means for creating a reception window to monitor a further transmission from the base station; a means for measuring the time of receipt of the further transmission; a means for calculating the error between the determined timing of transmission and the measured time of receipt; and a means for adjusting the timing of a subsequent reception window in dependence on the calculated error.

S. An apparatus for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations according to claim 4 further comprising an internal clock.

6. An apparatus for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations according to claim 4 or 5 wherein the timing of the window is adjusted by calculating the expected time of arrival of a subsequent signal using the aligned timebase; and applying the error between the determined timing of the transmission and the measured time of receipt to the expected time of arrival to determine the timing of the subsequent reception window.

7. A method for synchronizing a reception window at a mobile communication device substantially as herein described with reference to the accompanying figures.

8. An apparatus for synchronizing a reception window at a mobile communication device substantially as herein described with reference to the accompanying figures.

8. An apparatus for synchronizing a reception window at a mobile communication device substantially as herein described with reference to the accompanying figures. 15. .

Amendments to the claims have been filed as follows:

1. A method for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations comprising the steps of; receiving a data transmission from the at least one base station; determining the timing of the transmissions from the base station) creating a reception window to monitor transmissions from the base station, the reception window being synchronized with the reception of the transmission; receiving a further data transmission within the reception window at a measured time of receipt; calculating the error between the determined timing of the transmission and the measured time of receipt; and adjusting the timing of the next reception window in dependence on the measured error between the determined timing of the transmission and the measured time of receipt of the last received transmission.

2. A method for synchronizing a reception window at a mohi.le communication device with received transmissions from at least one of a plurality of base stations according to claim l wherein the step of adjusting the timing of the next reception window is performed with respect to an internal clock.

3. A method for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations according to claim l or 2 wherein the step of adjusting the timing of the next reception window includes the steps of; ( ' calculating the expected time of arrival of the next transmission using the aligned timebase; and applying the error between the determined timing of the transmission and che measured time of receipt of the last transmission to the expected time of arrival of the next transmission to determine the timing of the next reception window.

4. An apparatus for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations comprising; a means for receiving data from the at least one base station; a means for determining the timing of transmissions from the base station; a means for creating a reception window to monitor a further transmission from the base station; a means for measuring the time of receipt of the further transmission; a means for calculating the error between the determined timing of transmission and the measured time of receipt of the last received transmission; and a means for adjusting the timing of the next reception window in dependence on the calculated error.

5. An apparatus for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations according to claim 4 further comprising an internal clock.

6. An apparatus for synchronizing a reception window at a mobile communication device with received transmissions from at least one of a plurality of base stations according to claim 4 or 5 wherein the timing of the next reception 11, .. . ... i:

window is adjusted by calculating the expected time of arrival of the next transmission using the aligned timebase; and applying the error between the determined timing of the transmission and the measured time of receipt of the last received transmission to the expected time of arrival to determine the timing of the next reception window.

7. A method for synchronizing a reception window at a mobile communication device substantially as herein described with reference to the accompanying figures.