WO2007008380A2

WO2007008380A2 - Mobile station, system and method for use in cellular communications

Info

Publication number: WO2007008380A2
Application number: PCT/US2006/024697
Authority: WO
Inventors: Menahem Manny Raif; Imanuel Aminov; Inna Kogan; Boaz Or-Shraga
Original assignee: Motorola, Inc.
Priority date: 2005-07-06
Filing date: 2006-06-26
Publication date: 2007-01-18
Also published as: WO2007008380A3; GB0513754D0; GB2428164A; GB2428164B

Abstract

A mobile station in a cellular communication system applies a cell monitoring mode in which signals from base transceiver stations are received to determine which of the base transceiver stations is to be selected by the mobile station as a serving base station, the mobile station including a movement detector and a controller for activating application of the monitoring mode when the mobile station is detected to be in motion and for de-activating application of the monitoring mode when the mobile station is detected to be stationary, wherein the movement detector comprises an estimator estimating a current signal quality level of a received signal and a processor determining a value of a parameter which is related to a variance of the current signal quality level from an average value of the received signal quality level and to determine if the value of said parameter is greater than a threshold value.

Description

MOBILE STATION, SYSTEM AND METHOD FOR USE IN CELLULAR

COMMUNICATIONS

FIELD OF THE INVENTION

The present invention relates to a mobile station, a system and a method for use in cellular communications. In particular, the invention relates to a method of operation therein relating to monitoring of signals from candidate serving cells or sites of the system.

BACKGROUND OF THE INVENTION

A cellular communication system is one in which mobile or portable user terminals, such as mobile telephones or portable radios, herein collectively referred to as 'mobile stations' or 'MSs', can communicate via a network infrastructure which generally includes various fixed installations such as base transceiver stations or ^λBTSs' . Each BTS has one or more transceivers which serve MSs in a given region or area, known as a 'cell' or 'site', by radio communication. The cells of neighbouring BTSs are often overlapping. Signals sent from MSs to their serving BTS are known as 'uplink' signals. Signals sent from a BTS to MSs are known as 'downlink' signals. Uplink and downlink signals may be sent in different channels, e.g. with different carrier frequencies.

Generally, it is desirable for the MSs to be served by the BTS which can provide the best service which includes good signals to and from the MS. Since MSs can move from one region to another it is well known for the MSs to operate a procedure to monitor signals from different BTSs, to assist in a determination of which BTS can best serve the MS and of whether it would be worthwhile to switch from a current serving BTS to another one and if appropriate to carry out such a switch. In the art, the procedure to make determinations regarding a possible switch is known as a ^λcell re- selection' procedure. The procedure to carry out a switch using such a determination is known as 'handover' or 'handoff ' .

Such a cell re-selection procedure is operated in various cellular systems, e.g. in TETRA systems, i.e. those that operate in accordance with TETRA (Terrestrial Trunked Radio) standards defined by the European Telecommunications Institute (ETSI) . In TETRA systems, each BTS continuously broadcasts to MSs served by that BTS information relating to neighbour BTSs which could serve the MSs. The information is broadcast on a control channel . The information includes channel frequencies and other data which allows each of the MSs easily to search for other BTSs in order to carry out the monitoring of downlink signals from such BTSs as candidate serving BTSs. Each MS runs an algorithm which produces and records link measurements between the MS and the candidate BTSs including the current serving BTS and its neighbour BTSs. These measurements are based on estimated received signal strength indication (RSSI) values. The algorithm uses the measurements plus some service parameters (such as local site trunking availability, subscriber class served, class of security available, and level of cell services available) to build a list of selected neighbour cells in a preferred order. When the MS determines that the link with the current serving BTS should be exchanged for a better link it abandons the link with current serving BTS and undergoes handover to the selected alternative BTS which is top of the compiled list. Since the monitoring procedure in which a MS monitors signals from candidate BTSs to assist in cell re-selection involves actively tuning the receiver of the MS to a number of different downlink signals it requires considerable expenditure of energy. In many MSs, the monitoring procedure is set as a default to be triggered to run automatically as soon as the RSSI (received signal strength indication) falls to a value less than a monitoring threshold. This can be wasteful of energy if the MS is stationary and unlikely to need to undergo handover. Many MSs are powered by a battery and user requirements demand maximisation of the length of time for which the MS will operate, both in active calls and in standby mode, before the battery needs recharging. Thus, it is highly undesirable to waste energy by operating the monitoring procedure in situations when the procedure is unnecessary.

It is known in the prior art to detect motion of a MS by use of a GPS (Global Positioning System) receiver to assist in selective operation of the monitoring procedure described above. However, such a technique has a number of drawbacks. Installation of the GPS receiver in the MS significantly adds to cost. GPS receivers do not work well in certain environments, e.g. in an enclosed building or vehicle, or in a wooded area etc. Additionally, obtaining location updates from GPS signals can be relatively slow.

SUMMARY OF THE INVENTION

According to the present invention in a first aspect there is provided a mobile station as defined in claim 1 of the accompanying claims.

According to the present invention in a second aspect there is provided a system as defined in claim 14 of the accompanying claims.

According to the present invention in a third aspect there is provided a method of operation as defined in claim 15 of the accompanying claims.

Further features of the invention are defined in the accompanying dependent claims and are disclosed in the embodiments of the invention to be described.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an arrangement of terminals in a mobile communication system. FIG. 2 is a block schematic diagram of an arrangement of components in a mobile station included in the arrangement of FIG . 1.

FIGS . 3 and 4 are graphs of results obtained for variance of RSSI measurements in a stationary mobile station.

FIGS . 5 and 6 are graphs of results obtained for variance of RSSI measurements in a moving mobile station. FIG. 7 is a flowchart of an algorithm run in a processor of the mobile station of FIG. 2.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates an arrangement of units in a mobile (cellular) radio communication system 100 operating in accordance with TETRA standards . The system 100 includes a plurality of BTSs (base transceiver stations) , three of which are shown namely BTSs 101, 102 and 103. Each of the BTSs 101, 102 and 103 serves MSs (mobile stations) within its cell or service area. The BTS 101 generally serves a cell 104. The BTS 102 generally serves a cell 105. The BTS 103 generally serves a cell 106. Three MSs, namely MSs 107, 108 and 109 are shown within the cell 104 served by the BTS 101. Three further MSs, namely MSs 110, 111 and 112 are shown within the cell 105 served by the BTS 102. Three further MSs, namely MSs 113, 114 and 115 are shown within the cell 106 served by the BTS 103. The MSs 107, 108 and 109 are able to communicate with each other via their serving BTS 101. The MSs 107, 108 and 109 are also able to communicate with MSs served by other BTSs via the BTS 101 and the appropriate other BTS (s) via a link between the BTSs. Thus, the MS 109 can communicate with the MS 110 via the BTS 101 and the BTS 102.

Each of the MSs 107 - 109, 110 - 112 and 113 - 115 shown in FIG. 1 may be a portable device such as a portable radio, a mobile telephone or a wireless enabled computer. Alternatively, each of these MSs may be a radio communications unit fitted in a vehicle.

FIG. 2 is a block schematic diagram showing more detail of one form of the MS 109. The other MSs shown in FIG. 1 may be constructed and may operate in a similar manner. The main functional operations of the MS 109 are controlled by a controller 201 which operates in conjunction with a timer 209 which synchronises operations within the MS 109 and a memory 210 which stores data and programs used within the MS 109. A signal processor 202 processes information included in RF signals sent and received by a transceiver 203. The signal processor 202 extracts and processes information from a received RF signal detected by the transceiver 203 and passes the information to an appropriate output transducer. Similarly, the signal processor 102 receives and processes input information for transmission from an appropriate input transducer and delivers the information to the transceiver 203 for transmission in the form of an RF signal by the transceiver 203. The MS 109 includes an output transducer which is an audio output 204, e.g. a speaker, which converts signals received which represent speech information into an output audible form for delivery to a user. The MS 109 also includes an input transducer which is an audio input 205 which converts an input audio signal, e.g. in the form of speech, into an electrical form in a well known manner. The electrical signal is delivered to the processor 202 described above. A data connector 213 provides an output for data received in an RF signal at the transceiver 203 and delivered from the processor 202. The data connector 213 also provides an input for delivery of data to the processor 202 for sending as an RF transmission by the transceiver 203. The data connector 213 may provide a connection to one or more peripheral devices (not shown), e.g. it may comprise a USB data connection. A keypad 212 serves as a user interface and allows a user to enter control signals for delivery to the controller 201 to operate functions of the MS 109. A display 207 operated by a display driver 206 under control of the controller 201 provides displayed information to a user of the MS 109 in a known manner. A battery 211 provides electrical power to all operational components of the radio 125. The transceiver 203 provides RF communications to, and receives RF communications from, the serving BTS 101. The MS 109 has two modes relating to monitoring of signals from its serving BTS 101 and other BTSs such as the BTS 102 and the BTS 103 to assist in cell-re- selection. These modes are (i) a 'monitoring' mode when monitoring takes place automatically at regular intervals; and a ^λnon-monitoring mode' wherein monitoring to assist cell re-selection is disabled. In the monitoring mode, the transceiver 203 receives signals from other BTSs such as the BTS 102 and the BTS 103 as well as from the BTS 101.

The transceiver 203 is connected to an RSSI estimator 208 and provides a sample of a received signal to the RSSI estimator 208. The RSSI estimator 208 operates to estimate a current value of the RSSI

(received signal strength indication) of the currently received signal. The RSSI estimator 208 is connected to a VAR (variance) estimator 214 and to a mode selection processor 215 and provides a signal indicating a current value of the estimated RSSI of the received signal to the VAR estimator 214 and to the mode selection processor 215. The VAR estimator 214 operates in a manner described later to estimate a current value of a parameter ^λVAR' (variance) . The VAR estimator 214 is connected to the mode selection processor 215 and provides a signal indicating a current value of the VAR to the mode selection processor 215. The mode selection processor 215 operates an algorithm using the current value of the VAR to determine whether the MS 109 is currently in motion or is stationary and then to select the mode of the MS 109 to be the monitoring mode or the non-monitoring mode accordingly. Output signals provided by the mode selection processor 215 indicating required changes in the mode selected by the mode selection processor 215 are delivered to the controller 201 for application of the appropriate mode by the controller 201. The RSSI estimator 208, the VAR estimator 214 and the mode selection processor 215 operate in a manner to be described in more detail as follows.

The RSSI estimator 208 samples the received downlink signal from the serving BTS 101 at frequent intervals, e.g. 6 times per second. Each of these samples may be obtained in a known manner by taking in-phase (I) and quadrature phase (Q) components of the received downlink signal, quantising the I and Q components into a block of M discrete incremental samples, e.g. where M is from 500 to 10000, and calculating a value of current RSSI using the following relationship:

Equation 1

where Ji and Q± are the values of I and Q components in the individual incremental samples . Thus the value of RSSI, as defined in Equation 1, obtained for one block of incremental samples represents one RSSI sample. The RSSI estimator 214 applies further calculation steps as follows. The RSSI estimator 214 calculates a value of a local average L of the received RSSI values . For example, the local average L may be calculated in real time every second using six values of RSSI obtained per second.

Each time the RSSI estimator 208 calculates a new value of the local average L it also calculates a value of a running average R. The running average R is the average of the last r values of the local average L, including the new value obtained, where r is a suitable integer, e.g. in the range 2 to 10. For example, r may be 5. A signal indicating each value of the running average R is provided by the RSSI estimator 208 to the VAR estimator 214. The VAR estimator 214 calculates a block average μ for a block of the last N values of the running average R using the following relationship:

N-i μ=j _NrY^jμ^. ^mu u „n. ^g__Ax ^vg__R. ^SS^I_i ^• Equation 2

where Running_Avg_RSSIi is an individual value of the running average R and JV is the number of values of the running average R taken in a block average μ. The number N may for example be an integer in the range 10 to 100, in particular 20 to 60. We have found N = 30 to be satisfactory. The VAR estimator 214 also calculates a current variance VAR or σ², where σ is the standard deviation, of the current running average R from the current block average μ using the following relationship:

, ∑⁽R^unnin_g_^Av_g_RSSI_i-_μf _Equatiθn 3 σ² = ^J=(L N-I

The VAR estimator 214 provides to the mode selection processor 215 an output signal indicating each value of the VAR calculated in the manner described above. The mode selection processor 215 compares each value of the VAR obtained with threshold values tha and th_s, where tha is a pre-determined dynamic threshold and th_s is a predetermined static threshold lower than tha- In one embodiment of the invention if the MS 109 is currently in the non-monitoring mode, then if the value of VAR is less than tha the MS 109 remains in that non-monitoring mode. If, however, the value of VAR reaches or exceeds the dynamic threshold value tha, the mode requires switching to activate the monitoring mode. The mode selection processor 215 therefore issues a trigger signal to the controller 201 to cause the monitoring mode to begin. The monitoring mode continues until the value of the VAR falls below the static threshold th_Ξ. When this happens the processor 215 issues another signal to the controller 201 to cause the monitoring mode to end and the non-monitoring mode to begin. Thus the monitoring mode is activated or de-activated according to the value of the VAR relative to the thresholds tha and th_s. If the value of the VAR is relatively high, it is an indication that the MS 109 is currently in motion. If the value of the VAR is relatively low, it is an indication that the MS 109 is substantially stationary. The values of the thresholds tha and th_s. are chosen to distinguish suitably between a low value and a high value of the VAR. Thus, the monitoring mode is activated and de-activated according to whether the processor 215 detects whether the MS 109 is moving or stationary according to the calculated value of the VAR. The thresholds tha and th_s. are chosen to be different in order to provide a hysteresis effect which prevents rapid switching of the mode back and forth between monitoring and non-monitoring. The thresholds tha and th_s may be selected by a designer according to operating parameters of the MS 109 and may even be changed dynamically during operation if those parameters change. We have found values of 3 and 2 to be suitable examples of the (dimensionless) thresholds thd and th_Ξ. FIG. s 3 to 6 illustrate use in practice of the parameter VAR = σ² to provide actual motion detection of an MS. Firstly, the MS was operated in a stationary state and measurements of RSSI were taken and values of VAR were calculated in the manner described above. The results obtained are shown in FIGS. 3 and 4. In FIG. 3, a plot 300 is shown of the values of VAR obtained. The horizontal axis shows the running number of periodic measurements of RSSI. FIG. 4 is a related histogram. The horizontal axis of FIG. 4 shows the VAR values of the plot 300 sorted into ten incremental bands 0.16 wide covering the range VAR = 0 to VAR = 1.6. The vertical axis of FIG. 4 shows the number of instances in the plot 300 of values falling in each of the incremental bands. In FIG. 3 it is seen that the value of the VAR is always less than 2 and always not greater than 1.6. In FIG. 4 it is seen that a major proportion of the values of the VAR obtained are for VAR less than 1. Thus, the values of the VAR are all less than the thresholds tha and th_s if these are set to 3 and 2 respectively .

Next, the MS was operated in a moving state, i.e. whilst being carried by a moving vehicle travelling in an urban environment with a maximum speed of 50 km per hour. The vehicle carrying the MS then entered a highway and travelled along the highway with a maximum speed of 120 km per hour. The travel path was up to 3 km from a BTS (travelling at times both toward and away from the BTS) . During the travel of the MS, the value of the VAR for the MS was calculated in the manner described above and a plot 500 of the results obtained is shown in FIG. 5. A related histogram is shown in FIG. 6. The axes of the graphs in FIG.s 5 and 6 correspond respectively to those of the graphs in FIG.s 3 and 4 although the scales are different for the VAR in FIG. 5 (compared with FIG. 3) and for incremental VAR values in FIG. 6 (compared with FIG. 4) . The speed of the moving MS increased at a point X which occurred after 420 measurements of RSSI when the vehicle carrying it entered a highway.

It can be seen from FIGs . 5 and 6 that the value of the VAR is always 4 or greater and is mainly greater than 10. The value of the VAR increased to over 20 when the speed of travel of the MS increased at point X. Thus, if the values of the thresholds tha and th_s referred to earlier are set at 3 and 2 respectively, both of these thresholds are below all of the values of the VAR plotted for the moving MS in FIG.s 5 and 6. Also, as noted above, the values of the VAR which were obtained when the MS was stationary are all less than the thresholds th_d and th_s if these are set as 3 and 2 respectively. Thus, comparison of a value of the VAR with the thresholds tha = 3 and th_s = 2 provides a satisfactory distinction between whether the MS is in motion or is stationary.

FIG. 7 is a flowchart showing an algorithm 700 which may be operated by the mode selection processor 215 in accordance with another embodiment of the invention. In the following description of the algorithm 700, reference is made to a threshold 'FRT' (fast reselect threshold) which is a pre-determined threshold value of RSSI used in a TETRA MS such as the MS 109 to indicate, if the received signal has a value of RSSI less than FRT, that cell a re-selection procedure should be begun. The algorithm 700 begins at a start step 701. In a step 703 a new VAR result is received from the VAR estimator 214. In a decision step 705 a determination is made as to whether or not the received VAR value is valid. Generally, the running VAR result is placed in a buffer and this step provides a check as to whether or not the last VAR value placed in the buffer is an updated one. If the decision produced in decision step 705 is 'N' ('no' or 'false', i.e. that the VAR value is not valid), a step 717 follows in which the algorithm 700 ends indicating no change is needed in the current mode (i.e. 'monitoring' or 'not monitoring') of the MS 109. If the decision produced in decision step 705 is ^λY' (^vyes' or 'true', i.e. that the VAR value is valid), a decision step 707 follows. In decision step 707 a determination is made as to whether (i) the monitoring mode has already been activated; AND (ii) the new value of VAR obtained is equal to or less than th_s defined earlier. If the determination in step 707 produces a ^ΛN' result, a decision step 709 follows. If the determination in step 707 produces a ^NY' result, a decision step 711 follows .

In decision step 709 a determination is made as to whether (i) the current mode is non-monitoring; AND (ii) the new value of VAR obtained is equal to or greater than tha defined earlier,. OR (iii) the current value of RSSI is less than the FRT. If the determination in step 709 produces a 'Y' result, a step 715 follows. By step 715 monitoring is started by a trigger signal being issued to the controller 201. The algorithm 700 ends after step 715 at step 717 in which the present state, i.e. monitoring which has been selected, continues. If the determination in step 709 produces an ^λN' result the algorithm ends at step 717. In decision step 711 a determination is made as to whether the current received value of RSSI is less than the FRT. If the determination in step 711 produces a ^XY' result, the algorithm 700 ends in step 717. If the determination in step 711 produces an ^λN' result, a step 713 follows. By step 713 monitoring is stopped by issue of a signal to the controller 201. The algorithm 700 then ends at step 717 in its current state, i.e. with no monitoring. Thus, the algorithm 300 uses values of the VAR to make comparisons with the thresholds th_s and th_d to make decisions as to whether monitoring should be started or stopped. The algorithm 700 also compares the current value of RSSI with the fast re-select threshold, FRT, and when a weak signal is indicated by RSSI being less than FRT, uses this indication as an alternative trigger to start monitoring even if a stationary state of the MS is detected from the VAR value. The algorithm 700 also stops monitoring when a signal is indicated by the value of RSSI being not less than the FRT.

The algorithm 700 described above with reference to FIG. 7 thus uses the FRT as a secondary control mechanism to trigger monitoring. This helps especially in flat terrain areas, in which a line of site exists between an MS and its serving BTS, even on highways. In such cases, the running average RSSI is expected to change slowly - thus limiting the VAR value. Therefore, the algorithm 700 uses also the FRT as a backup mechanism for use in triggering monitoring in such scenarios. It is to be noted however that the FRT is lower than the monitoring threshold used in the prior art as a default to trigger monitoring, so even when the FRT is used, less energy is used than in the prior art. Use, according to whether or not a MS is detected to be in motion in the embodiments of the invention described above, of selective activation of the monitoring mode of the MS beneficially allows valuable battery energy to be saved by unnecessary monitoring when the MS is stationary. Detecting motion of the MS by calculating the variance of a received signal quality level, e.g. received RSSI, allows the use a GPS receiver to be avoided with benefits in overall lower cost and allows motion to be detected in situations when a GPS receiver would have difficulty in detecting such motion.

Although the various signal processors of the MS 109, particularly the controller 201, the processor 202, the RSSI estimator 208, the VAR estimator 214 and the mode selection processor 215 are shown in FIG. 2 as separate functional units, any two or more of these processors may be combined in a single processing device, as will be apparent to those familiar with the art of digital signal processing.

Claims

1. A communication system, comprising: a mobile station operable in a cellular communication system to apply a cell monitoring mode in which signals from base transceiver stations of the system are received to determine which of the base transceiver stations is to be selected by the mobile station as a serving base transceiver station, the mobile station including a motion detector for detecting whether the mobile station is in motion or is stationary and a controller for activating application of the monitoring mode when the mobile station is detected by the motion detector to be in motion and for de- activating application of the monitoring mode when the mobile station is detected to be stationary, wherein the motion detector comprises an estimator for estimating a current signal quality level of a received signal and a processor for calculating a value of a parameter which is related to a variance of the current signal quality level from an average value of the received signal quality level and for determining if the calculated value of said parameter is equal to or greater than a threshold value .

2. A mobile station according to claim 1 wherein said parameter is the square of the standard deviation of the current signal quality level.

3. A mobile station according to claim 1 wherein the current signal quality level is an RSSI (received signal strength indication) of the received signal and the estimator is operable to estimate a current value of the RSSI of the received signal.

4. A mobile station according to claim 3 wherein the estimator is operable to quantise an I (in-phase) component and a Q (quadrature phase) component of a received signal into a block of M discrete incremental samples, where M is from 500 to 10000, and to calculate a block value of current RSSI for the block using the following relationship:

where Ij and Q± are the values of the I and Q components in the individual incremental samples ; and to find an average of a plurality of the block values.

5. A mobile station according to claim 3 wherein the estimator is operable to estimate a running average of the RSSI from a plurality of recent values of the RSSI of the received signal.

6. A mobile station according to claim 3 wherein the processor includes a variance estimator operable to calculate a running variance from a running average of recently received RSSI values or of recently received running averages of RSSI values .

7. A mobile station according to claim 1 wherein the processor is operable to determine whether the current value of said parameter is greater than a first threshold to activate the monitoring mode and to determine whether the current value of said parameter is less than a second threshold at a smaller value of the parameter than the first threshold to de-activate the monitoring mode.

8. A mobile station according to claim 1 wherein the processor includes a mode selection processor which is operable to determine whether or not the monitoring mode is to be activated.

9. A mobile station according to claim 8 wherein the mode selection processor is operable to determine whether a current value of received signal quality level is less than a re-select threshold value.

10. A mobile station according to claim 9 wherein the mode selection processor is operable to issue an activation signal to activate the monitoring mode when the current value of received signal quality level is determined to be less than the re-select threshold value .

11. A mobile station according to claim 10 wherein the mode selection processor is operable to issue the activation signal when it has determined that a current value of the variance is less than a threshold required to indicate that the mobile station is in motion.

12. A mobile station according to claim 9 wherein the mode selection processor is operable to issue a deactivation signal to de-activate the monitoring mode when the current value of signal quality level is determined to be greater than the re-select threshold value .

13. A mobile station according to claim 12 wherein the mode selection processor is operable to issue the deactivation signal when it has determined that a current value of the variance is greater than a threshold required to indicate that the mobile station is in motion.