GB2410864A

GB2410864A - Apparatus for automatic time base correction.

Info

Publication number: GB2410864A
Application number: GB0402644A
Authority: GB
Inventors: Richard Ormson
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2004-02-06
Filing date: 2004-02-06
Publication date: 2005-08-10
Anticipated expiration: 2024-02-06
Also published as: GB2410864B; GB0402644D0

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 (510) for receiving data from the at least one base station, a means for determining the timing of transmissions from the base station, a clock (510), a means for synchronising the frequency of the clock with a transmission received from a further base station, a means for creating a reception window to monitor transmissions from the base station, the reception window being synchronised with the reception of the transmission, a means (540) for comparing the frequency of the transmission with the frequency of the clock, a means for adjusting the reception window in dependence on the difference in frequency in order to maintain synchronisation with the reception of the transmission.

Description

24 1 0864

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 basestations is information specific to that basestation, 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 synchronise 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 compare the observed frequency of the signal received from the non serving base station with that of the internal clock, i.e. the observed frequency of the signal from the serving base station. Any difference in these observed frequencies is due to the absolute movement of the device with respect to the serving base station being different from the absolute movement with respect to the non-serving base station.

This difference in frequency can be converted into a net radial velocity. Using this net radial velocity and the known time until the next monitoring of the transmission from the non-serving base station, the change in relative distance of the device between the serving and nonserving base stations during this time interval is calculated.

This distance is converted into a change in timebase for the non-serving cell. The timebase for the non-serving cell can then be adjusted automatically in order to synchronies the reception window with the incoming transmission from the non-serving cell.

The advantage of this arrangement is that the frequency offset measurements can be made on any of the receptions from a given neighbour cell. As noted above, the signals requiring timebase accuracy are much less numerous than those that do not. In known devices, the timebase is only corrected when one of the operations requiring an accurate timebase is made, this is because these are the only measurements capable of reporting a timebase offset. However, using the frequency offset measurement of the present invention the correction is possible on a much shorter interval simply because there are many more occasions on which such measurements can be made. This leads to an accurate timebase when the next cyclic measurement is required.

Embodiments of the invention automatically update the timebase for nonserving base stations and provide a reliable prediction of when the required blocks of information will be received. Thus the timebase is automatically synchronized with the incoming transmission.

This automatic timebase correction reduces the need for the reception window size to be increased in order to receive 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 1 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.

Figure 4 is a block diagram showing the components within an embodiment of the present invention.

Figure 1 shows a device 10 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 1 to NSBS 6. The device establishes a primary communication link with SBS. SBS is the serving base station with which the communication device 10 establishes a primary communication link. Each of the base stations in the surrounding cells NSBS 1 to NSBS 6 are classed as neighbouring base stations and the device 10 monitors the transmissions from these base stations periodically.

Figure 2 is a diagram showing the communication device 10 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 l0 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 nonserving 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.

The device then determines the frequency of the transmission cycle from each non-serving base station and the position within the transmission cycle of any information that the device is required to receive. The device then determines the timing at which it must receive the transmission from each non-serving base station in order to receive the required information. The timing of this required window is stored with respect to the internal clock at 350. This timing with respect to the internal clock is the timebase and is unique to each non- serving base station. The device then recommences communication with the serving base station.

At 360, the device identifies that it is time to receive a transmission from a non-serving base station.

On receipt of this signal, the device compares the frequency of the received signal with the frequency of the internal clock, i.e. the observed frequency of the transmission from the serving base station. The device determines the frequency difference between these signals at 380. If the frequency of the internal clock and the non-serving base station are identical, the device determines that there is no net radial velocity or movement of the device with respect to the non-serving base station. Therefore, the device is either stationary or is moving at the same absolute velocity with respect to both base stations. If there is no frequency difference at 380, the timebase associated the non-serving base station is maintained at 380.

Since both base stations are transmitting at the same frequency, any frequency difference between the internal clock and the signal received from the non-serving base station is due to a difference in the absolute radial motion of the device with respect to the serving and non- serving base stations. Thus the observed Doppler shifts for each signal are different. The Doppler shift in frequency corresponds to a velocity according to the equation: observed frequency = absolute frequency *(1 (observer's velocity / velocity of light)) Therefore, the difference in observed frequencies corresponds to a net radial velocity. The net radial velocity is a combination of the radial velocity relative to the serving cell and the radial velocity relative to the neighbour cell. It cannot be defined in terms of either one of these, but has to be in terms of both. It makes no difference as far as the correction is concerned since only the magnitude and sign of its value are of importance.

The Doppler shift equations for the signals from each base station can be combined to produce the equation: freq difference / observed freq of signal from sbs = (net radial velocity)/ velocity of light where frequency difference is the difference between observed frequency of the signals received from the non serving and serving base stations, sbs is the serving base station (where the observed frequency from the sbs is the clock frequency) and net radial velocity is the difference in velocity of the device with respect to the serving base station and non-serving base station.

Thus, by measuring the frequency difference between the signal received from the serving and non-serving base station, an instantaneous net radial velocity can be calculated at 400. The device knows the time period until it will receive the subsequent transmission from the neighbour base station. Therefore, the device can calculate a net change in radial distance at the time of the next radio reception at which a timebase correction can be made at 410. The distance is calculated from the known time period and net radial velocity. The change in tlmebase caused by the change in distance can then be calculated at 420. At 430 the corrected tlmebase is stored in the memory of the device ready for the next reception from the non-serving base station.

Assuming that radio waves from the base station travel at a constant speed of 3 x 108 metres/second, any change in timing is proportional to the change in distance travelled by the waves. Table 1 shows examples of the correction required to be applied to a tlmebase in different radio technologies for particular changes of distance.

Distance moved Correction Distance moved Correction applied to GSM applied to timebase UMTS timebase Minus 277 Plus 1 quarter Minus 19. 5 Plus 1 quarter metres bit metres chip Plus 277 Minus 1 Plus 19.5 Minus 1 metres quarter bit metres quarter chip Table 1: Correction of timebase required for different distances moved by the device for different radio technologies The distances in the table are simply the distances travailed by radio wave in the correction period. Sloce radio waves travel at 3 x 108 metres/second, durlog one UMTS quarter chip (65 x 10-9 seconds) the radio signal travels about 19.5 metres. For radio access technologies (RATS) other than GSM and UMTS, similar corrections could be calculated based on their network frequencies.

The granularity of the timebase may be adjusted for different devices. For example, it can be seen from Table 1 that the handset must move at a considerable rate for a correction to be required if a short measurement interval is used. For example, if the handset is moving at 200 metres per second in a GSM system and the measurement interval is one second, then a correction will never happen. However, in general, the measurements requiring an accurate timebase occur at an interval of about 30 seconds. Clearly the timebase should have been corrected within this interval.

This first embodiment of the present invention assumes that the device will be moving at a constant velocity for the entire time between measurements. The embodiment assumes that the net velocity of the device as measured instantaneously is representative of the movement between measurements. In order to increase the accuracy of the timebases, the frequency of signals from neighbour cells can be monitored more regularly. Since the device only requires a measurement of the frequency of the signal, these measurements can be made whenever the device measures the signal strength of the signal from the non serving base station. As these measurements must be made anyway, the only additional effort is to calculate the frequency error between the signal received by the neighbour signal and the internal clock. The energy expended in making this calculation is significantly less than that expended in processing an extended receive window and hence the method provides a net power saving.

However, of course, as the time period between measurements increases, the power requirements of the mobile unit also increase. Therefore, different embodiments may employ different time periods between monitoring the frequency of the signal.

Further embodiments may also determine the regularity of measurements based on the change in net radial velocity of the device between successive measurements. If there is a large change in the net velocity then the period between measurements can be reduced and if no change is detected then the period can be extended up to the full 30 seconds.

Figure 4 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 observed frequency of the 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 means for determining the frequency of the received signal 530 and a means for comparing the frequency of a received signal from a non- serving base station with the frequency of the internal clock 540. The frequency comparator is linked to a means for calculating the new timebase 550. The timebases associated with all non-serving base stations are stored in a memory 560.

It will be obvious to those skilled in the art that the present invention provides a means in which the timebases for signals from neighbouring base stations can be automatically updated in dependence on the observed frequency of the transmission from the non-serving base stations. Embodiments of the present invention remove the requirement for extending the reception window for reception of signals from non-serving base stations in order to account for movement of the device.

Claims

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, the data transmission having an observed frequency; determining the timing of 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; comparing the observed frequency of the transmission with the frequency of a clock, the frequency of the clock being synchronized with a transmission received from a further base station; and adjusting the timing of the reception window in dependence on the comparison of frequencies in order to maintain synchronization with the reception of the transmission.

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 where the step of adjusting the reception window is performed with respect to the 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 window comprises the steps of calculating a velocity from the difference in frequency and determining the timing of the reception window from the velocity.

4. An apparatus for synchronizing a reception window at a S 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 clock; a means for synchronizing the frequency of the clock with a transmission received from a further base station, a means for creating a reception window to monitor transmissions from the base station, the reception window being synchronized with the reception of the transmission; a means for comparing the frequency of the transmission with the frequency of the clock; a means for adjusting the reception window in dependence on the difference in frequency in order to maintain synchronization with the reception of the transmission.

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 5 further comprising a means for calculating the difference in frequency of the transmission and the clock.

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

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