GB2402301A

GB2402301A - Increasing the standby time of a mobile

Info

Publication number: GB2402301A
Application number: GB0311578A
Authority: GB
Inventors: Graham Peace
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2003-05-20
Filing date: 2003-05-20
Publication date: 2004-12-01
Anticipated expiration: 2023-05-20
Also published as: GB2402301B; GB0311578D0

Abstract

A mobile determines the accuracy of the internal clock synchronisation based on periodical monitors network signals including the network clock, and selects the rate of monitoring based on the result. If the accuracy is within a predefined timing limit the mobile enters a sleep period to save power. The network clock time may be determined by reading the paging cycle, which may include channel readings, serving and neighbouring cell measurements. The different network signals may be monitored at different rates, but at least one may be equal to the split paging value.

Description

An Apparatus for Increasing the Standby Time of a Mobile Communication

Device The present invention relates to an apparatus for increasing the standby time of a mobile communication device.

In order to communicate reliably with a communications network a mobile communication device (unit) must operate at the same frequency as the network on which it is communicating. The frequency at which the unit operates is controlled by an internal clock. This clock is typically a crystal oscillator which controls the frequency at which signals are transmitted from the unit and received by the unit. Therefore, in order that the unit can receive, decode and transmit signals and information to and from the network, via a base station, its internal clock must remain synchronized with the network. When the internal clock is not synchronized to the network the unit can not receive and interpret information from the network.

In practice, during operation the frequency of the internal clock will vary with respect to the network. Variations in the frequency of the clock are caused by imperfections in the crystal oscillator within the mobile unit and changes in the ambient temperature. In order to keep the internal clock in synchronization with the network, the mobile unit monitors signals from the network which provide information regarding the frequency at which the network is operating. The mobile unit includes software to compare the network frequency with the frequency of the internal clock. The frequency of the internal clock is then adjusted to synchronism with the network clock.

In certain conditions the unit can remain in synchronization with the base station for extended time periods. Typically this occurs when the unit is stationary. Movement of the unit can distort signals to and from the base station and cause the timing offset between the unit and base station to increase. The signals are distorted due to the effects of Doppler shift and changes in propagation delay between the mobile and base station. During these 45463-CB time periods it is important that the unit monitors the signals from the base station at regular intervals in order that the internal clock is regularly adjusted synchronized to the network.

The reception and processing of the signals from the network causes a large drain on the battery power of the unit. The higher the rate at which signals are received and processed by the unit, the faster the battery will drain. Battery power saving in mobile devices is a particular concern with third generation (3G) systems due to the increased power requirements of 3G applications.

We have appreciated that taking measurements from the network in order to synchronism the internal clock is a large drain on battery power. However, it is essential that sufficient measurements are taken in order that the clock frequency of the mobile unit remains synchronized to the frequency of the network.

We have also appreciated that there are time periods during use when the unit will remain tightly synchronized to the network, and during these periods the unit is not required to sample the network frequency at a high rate. Equally, there are periods when the unit is not well synchronized to the network and during these periods measurements are required at a higher rate.

Embodiments of the present invention extends the battery life in a mobile communication device by controlling the rate at which measurements of the network are taken by the unit in dependence on the accuracy of the synchronization of the internal clock with respect to the network clock. The embodiment includes means to identify the timing difference (offset) between the internal clock of the unit and the network. When the offset is within a predefined value (tolerance) the internal clock is considered to be accurately synchronized with the network. If the unit remains within this tolerance for a predefined time period, the unit enters an extended discontinuous reception (DRX) sleep period in which measurements from the network are taken at a reduced rate.

Once the offset exceeds the tolerance level the unit increases the rate at which measurements are taken in order to maintain accurate synchronization with the network. By reducing the rate at 45463-GB which measurements are taken during the extended DRX sleep period the battery life of the unit is extended.

The present invention is 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 is a flow diagram showing the known process by which a mobile unit is attached to the network on activation.

Figure 2 is a flow diagram showing how the present invention controls the rate at which measurements are taken.

Figure 1 shows the steps taken by the mobile unit immediately after it is activated in order to attach itself to the network.

At 10 the mobile unit is powered up. At 15 the mobile unit performs a scan of all of the channels (carrier frequencies) of the networks supported by the mobile unit. At 20 the mobile unit uses the result of this scan in order to determine the most suitable cell to attempt to select. At 30 the unit finds a carrier frequency for the network to be selected and decodes its sync burst information to determine if this is a permitted network on which to attach. If it is not permitted, steps 10 to 20 are repeated until a permitted network cell is found. Once a permitted network has been found, the mobile unit attaches (camps onto the serving cell) to the network at and enters either GSM circuit switched or GPRS packet switched idle mobile. The mode entered is determined from system information broadcast by the network to the mobile stations located in the cell and the network operating mode. The unit advises the network of the split paging value at which it will be monitoring the network at 50.

This invention only takes effect when split paging is supported by the network on the serving cell. The mobile is able to determine if split paging is supported from the broadcast information received from the network. In the case where split paging is not supported in the cell, the minimum number of paging 45463-GB - 4 channels which can be read is based on the bs_pa_mfrms parameter broadcast by the network. This has a value in the range 2 to 9.

The minimum number of paging channels are read when this value is equal to 9. This allows the mobile to enter discontinuous reception (DRX) for 9GSM 52 frame multiframes (which equates to a period of 9 * 51 * 4.615ms = 2.118 seconds). Thus the mobile station may enter a low power sleep period for a maximum of 2.118 seconds under this regime.

When split paging is supported by the network cell, the minimum number of paging channels which can be read (and hence the maximum DRX sleep period attainable) is one paging channel in each 64 multiframe 52 frame period for packet idle mode (or one paging channel in each 64 multiframe 51 frame period for GSM circuit switched idle mode). This equates to one paging channel read every 15.359 seconds (1 * 64 * 52 * 4.615ms) for GPRS idle mode or 15.063 second (1 * 64 * 51 4.615ms) for GSM circuit switched idle mode and equates to a split paging cycle value of 1 (in either mode).

However, as the network power and synchronization measurements are made during the paging channel read, in order to maintain synchronization with the network whilst mobile, the minimum split paging value permissible by the mobile unit is 8 (which equates to a DRX sleep period of 8 * 52 * 4. 615ms = 1.92 seconds or 2.118 seconds in GSM idle mode). This invention utilises the opportunity that the mobile unit is able to select its own split paging value as follows: À the mobile unit informs the network (during the attach procedure) that it will be using a split paging value of 1, thus giving the mobile unit the ability to sleep for the maximum permissible period.

À the mobile unit determines if it is stationary or mobile.

À when the mobile unit is stationary, the paging channel is read at a rate of 15.359 seconds (in GPRS packet idle mode) of 15.063 seconds (in GSM circuit switched idle mode) À when the mobile unit has detected that it is no longer stationary, the rate of paging channel reads is increased to match a split paging value of 8 (in GPRS idle mode) or 45463 -GB bs_pa_mfrms rate of 9 (in GSM idle mode). This new split paging value is not communicated to the network as this is unnecessary.

Figure 2 shows the operating procedure for an embodiment of the present invention. At 110 the unit identifies the network frequency. Information regarding the frequency of the network clock is included in the paging channel which the mobile unit is monitoring.

The measurement of the network frequency is made by layer 1 of the protocol stack (open systems interconnection - OSI). The protocol stack is the standard reference model for communication between two users in a network. The OSI includes seven layers of related functions where each layer is responsible for a different task in dealing with incoming or outgoing information. The top layer is the user interface (MMI) and the lowest layer, layer 1, is the layer responsible for controlling the radio resources and transmits and receives messages during communication with the network.

In operation, layer 1 receives information from the paging channel. The protocol stack then processes the information and identifies the network frequency. At 120 software within the unit compares the frequency of the internal clock with the network frequency. The unit calculates the difference (timing offset) between the received network frequency and the frequency of the internal clock at 130.

The mobile unit is programmed with a predefined timing offset value (tolerance) which is acceptable to the unit. If the timing offset is within the predefined tolerance, the synchronization between the internal clock and network is considered to be acceptable and no adjustment of the internal clock is required. At the software determines if the timing offset is within the defined tolerance level.

If, at 140, the timing offset is within the predefined tolerance level the internal clock is not adjusted.

45463-GB 6 - At 140, if the timing offset exceeds the predefined limit then the internal clock is adjusted to synchronise with the network at 150. At 155, the unit continues to take measurements at the normal rate. During normal operation at 155 the unit will adopt an intermediate split page value, for example 8. This intermediate value allows sufficient monitoring of the paging cycle to enable the unit to remain synchronized to the network while taking measurements at a sufficiently low rate so that the battery life of the unit is not unacceptably short. These readings are taken on paging channels which are equally spaced within the paging cycle. The split paging value is also used to control the rate at which measurements are taken on the serving cell and neighbouring cells in order to identify the strongest signal. Measurements can be taken at the same rate as the split paging value or at another rate which uses the split paging value as a control frequency.

If the offset is within the predefined tolerance at 140, the unit checks whether the internal clock has remained synchronized to the network for a predefined time period at 160. If the internal clock remains synchronized with the network for this predefined time period, the system is considered to be stable and is placed into an extended discontinuous reception (DRX) sleep period at 170. This prolonged state of synchronization with the network is most likely to occur while the unit is stationary and therefore can be interpreted as an indication that the unit is stationary. However, it is possible that the invention may enter a DRX sleep period when not stationary.

If the internal clock does not remain synchronized to the network for this predefined time period at 160 the clock is adjusted at 150 and continues taking measurements at the normal rate at 155.

If the unit enters the extended DRX sleep period at 170 the software reduces the rate at which measurements are taken at 180.

Typically, the split page value may be reduced to the minimum split paging value of 1, however any reduced value may be used. The rate at which measurements are taken on the serving cell and neighbouring cells may also be reduced. Therefore, whilst the unit is in the extended DRX sleep state it continues to monitor the frequency of 45463-GB t the network and calculate whether the timing offset remains within the tolerance level at 190. As long as the timing offset remains within the tolerance level the unit will remain in the extended DRX sleep state.

During the extended DRX sleep state the unit continues to monitor neighbouring cells in order that it may perform its cell reselection calculations and may reselect a neighbour cell should the serving cell communication be lost.

If the timing offset exceeds the tolerance level at 190 this indicates that the unit has become less well synchronized with the network, the software will exit the extended DRX sleep period at 200. The internal clock is then adjusted to be synchronised with the network at 150 and measurements continue to be taken at the normal rate. In particular, the unit will make additional serving cell sync burst reads in order that the unit can realign with the paging cycle.

The unit will then continue to read the paging channel and take serving and neighbour cell measurements at the normal DRX rate until the software detects that the serving cell timing offset has returned to within the predefined tolerance for the predefined period. At this stage the unit will move back into the extended DRX sleep period.

It will be obvious to those skilled in the art that the present invention is not limited to use in mobile telephones. The invention may be used in any communications device which is linked to a communications network.

45463-GB - 8

Claims

Claims 1. A method for extending the battery life of a mobile

communication device comprising the steps of; periodically monitoring signals from the network including a network clock, determining the accuracy of synchronization between an internal clock in the mobile communication device and the network clock and determining the rate at which monitoring of signals from the network takes place in dependence on the accuracy of the synchronization.
2. A method for extending the battery life of a mobile communication device according to claim 1 including the further step of determining the accuracy of synchronization by comparing the timing difference between the internal clock and the network clock.
3. A method for extending the battery life of a mobile communication device according to claim 1 or 2 including the further step of entering a sleep period if the accuracy of synchronization is within a predefined limit.
4. A method for extending the battery life of a mobile communication device according to claim 1, 2 or 3 including the step of reading the timing of the network clock from a paging cycle.
5. A method for extending the battery life of a mobile communication device according to claim 1, 2, 3 or 4 where signals from the network include paging channel readings, serving cell measurements and neighbouring cell measurements.
6. A method for extending the battery life of a mobile communication device according to claim 1, 2, 3, 4 or 5 where different signals from the network are monitored at different rates.

45463,B - 9 -
7. A method for extending the battery life of a mobile communication device according to any preceding claim where at least one signal from the network is monitored at a rate equal to the split paging value.
8. An apparatus for extending the battery life of a mobile communication device comprising; means for periodically monitoring signals from the network including a network clock, means for determining the accuracy of synchronization between an internal clock in the mobile communication device and the network clock and means for determining the rate at which signals from the network are monitored in dependence on the accuracy of the synchronization.
9. An apparatus for extending the battery life of a mobile communication device according to claim 8 wherein the accuracy of synchronization is determined by comparing the timing difference between the internal clock and the network clock.
10. An apparatus for extending the battery life of a mobile communication device according to claim 8 or 9 wherein the device enters a sleep period if the accuracy of synchronization is within a predefined limit.
11. An apparatus for extending the battery life of a mobile communication device according to claim 8, 9 or 10 wherein the timing of the network clock is read from a paging cycle.
12. An apparatus for extending the battery life of a mobile communication device according to claim 8, 9, 10 or 11 wherein the signals from the network include paging channel readings, serving cell measurements and neighbouring cell measurements.

45463-GB
13. An apparatus for extending the battery life of a mobile communication device according to claim 8, 9, 10, 11 or 12 where different signals from the network are monitored at different rates.
14. An apparatus for extending the battery life of a mobile communication device according to any of claims 8 to 13 wherein at least one signal from the network is monitored at a rate equal to the split paging value.
15. A method for extending the battery life of a mobile communication device substantially as herein described with reference to the accompanying drawings.
16. An apparatus for extending the battery life of a mobile communication device substantially as herein described with reference to the accompanying drawings.

45463-GB