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1. INGON Jd eel ie AV Id 0 SOYVOGATH el oet 101 U S Patent Mar 18 1997 Sheet 3 of 3 5 613 208 se Fis 13 40 A0 ENTER SLEEP MODE CAR MOUNTED 7 YES JUMP TO NEXT 545 SCAN CHANNEL MEASURE RSSI Si ASSI MEAN VALUE gt 9308mM 1T NO NEW CC RSSI BETTER THAN OLD NO NEW AC RSSI BETTER THAN OLD 7 S21 24 YES AU CURRENT TRAFFIC AREA 2 CURRENT TRAFFIC AREA 2 S25 YES 23 SAVE AC AS START AC USE NEW CC AS FOR CALL SET UP CURRENT GC 5 613 208 1 CHANNEL SCAN IN CELLULAR TELEPHONE SYSTEM BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to a channel scanning method and apparatus for use in a cellular telephone system and more particular to a method and apparatus for scanning control and access channels during a period of reduced battery consumption 2 Discussion of Related Art Cellular telephone systems are designed to permit radio communications with mobile stations which by their mobil ity can cross boundaries between radio zones or cells the cells being defined as areas serviced by relatively immobile base stations within the cells Because of their need for mobility the mobile stations typically are battery operated whether vehicle mounted portable or pocket telephones The battery life becomes an important factor in cellular systems and consequentl2. radio channels said mobile station comprising receiving means for receiving radio transmissions from at least one base station demodulating means for demodulating radio transmis sions from at least one base station received by said receiving means a control channel message detector for extracting control information from at least a specific control message received by said receiving means and demodulated by said demodulating means said specific control message informing said mobile station that no paging signals are to be expected during a predetermined time a signal level meter for determining the signal strength of a signal received by said receiving means a controller for controlling scanning of said radio chan nels when said specific control message is received demodulated and detected and during said predeter mined time to determine which of said channels has the best characteristics and for storing said radio chan nels having the best characteristics 11 A mobile station according to claim 10 wherein said specific control message is a battery saving order directing a group of mobile stations to power down at least one preselected function and wherein at least one mobile station of said group of mobile stations upon receipt of said battery saving order instead performs said scanning step during said predetermined time 12 A mobile station according to claim 10 wherein said predetermined time is longer than a fr
3. signal strength value is then sent to the microprocessor based controller 130 An RF demodulator 127 which receives the receiver carrier frequency signal from the receiving fre quency synthesizer 125 and the radio frequency signal from the receiver 126 demodulates the radio frequency carrier signal thus generating an intermediate frequency IF The intermediate frequency signal is then demodulated by an IF demodulator 128 which stores the original information for example z 4 DQPSK modulated digital information or ana log information A controller 130 includes a control channel message detector 133 which is sent control channel mes sages and supplies detected control information to a micro processor in the controller 130 The microprocessor based controller 130 controls the mobile station activity and the base station communication and also handles the terminal keyboard input and display output 131 Decisions by the controller 130 are made in accordance with received messages and measurements that are made The keyboard and display unit 131 enable infor mation to be exchanged between the user and the base station Also included in the mobile station is a speaker 119 for generating sound corresponding to a received voice signal If the present invention is implemented in a digital system then a speech coder may be placed between the microphone 101 and the controller 130 and a decoder may be connected between the speaker 119 and the cont
4. state in which the mobile station remains ready for reception or initiation of a call In the battery savings mode the mobile station is not ready for reception of a call and the cellular system will not attempt to initiate calls with the group of mobile stations it has ordered to be in the battery savings mode Calls are put on hold in the mobile switching center until the end of the battery savings period for the group into which the called mobile station belongs Specifically the mobile station may close the receiver for the period indicated in the information field of a BSO With reference to the accompanying flowchart of FIG 3 the present invention will now be described This flow chart is followed every time a BSO is received Upon receipt of a BSO by a mobile station step S10 it is determined whether the mobile station is car mounted and therefore has a greater likelihood of being relatively fast moving between cells step 11 If the mobile station is not car mounted the time since the last channel scan is deter mined to be either greater or less than a predetermined time T typically 1 minute step 12 If time since the last scan is less than the predetermined time T then the mobile station enters the battery savings mode step S13 based on the assumption that the recent channel scan results are still accurate However if the time since the last channel scan is greater than the predetermined time T or if the mobile stati
5. ALTRE E E E OT US005613208A United States Patent n9 11 Patent Number 5 613 208 Blackman et al 45 Date of Patent Mar 18 1997 54 CHANNEL SCAN IN CELLULAR FOREIGN PATENT DOCUMENTS TELEPHONE SYSTEM 2085686 5 1994 Canada 75 Inventors Hans Blackman Malmoe Sven 0234201 3 1287 European Pat OR Tryding Lund both of Sweden OTHER PUBLICATIONS 73 Assignee Ericsson Inc Research Triangle Park Service Manual for Ericsson Mobile Phone 1941 EN LZB N C 126 1219 RIA available from Ericsson Mobile Communi cations AB CM S S 223 70 Lund Sweden 21 ADPISCIN 254918 Primary Examiner Reinhard J Eisenzopf 22 Filed Jun 6 1994 Assistant Examiner Lee Nguyen Attorney Agent or Firm Burns Doane Swecker amp 3I Inf CLP a ere ere ee ee dotis H04Q 9 00 Mathis LLP 52 U S CL se 455 34 1 455 34 2 455 38 3 455 343 57 ABSTRACT 58 Field of Search 455 32 1 33 1 F 455 34 1 34 2 38 3 54 1 54 2 56 1 58 2 A method and apparatus for controlling scanning of radio 68 70 161 1 343 379 61 63 channels by mobile stations in a cellular radiotelephone us See d system including transmitting from a base station a control 56 References Cited signal e g a battery savings order to a group of mobile stations for informing the group of mobile stations that no U S PATENT DOCUMENTS pages will be issued directed to them for a predetermined 4419261 10 1984 Oda et al serssssssssneseseeeecr
6. a cellular mobile radio system comprising many more mobile stations than ten In particular them are normally many more mobile stations than there are base Stations Also illustrated in FIG 1 is a mobile switching center a k a mobile switching office or exchange The mobile switching center MSC illustrated in FIG 1 is connected to all ten illustrated base stations by cables The mobile switch ing center is connected by cables also to a fixed public switching telephone network or similar fixed network with ISDN facilities All cables from the mobile switching center to base stations and cables to the fixed network are not illustrated Further other media may be used instead of cables for base to mobile switching center communications e g fixed radio links As illustrated in FIG 2 a mobile station includes a microphone 101 which in turn is connected to a controller 130 The controller 130 may be in the form of function blocks such as shown in FIG 2 of U S Pat No 5 119 502 or as shown in the Service Manual for the Ericsson Mobile Phone 1941 cited above or any suitable circuitry having sufficient computing power to carry out the present inven tion A receiver carrier frequency signal is generated in accor dance with a selected receiving channel by a receiving frequency synthesizer 125 Incoming radio frequency sig nals are received by a receiver 126 the strength of which are measured by a signal level meter 129 The received
7. ame length 13 A mobile station according to claim 12 wherein said controller identifies whether a radio channel being scanned is one of a control channel and an access channel and stores a radio channel determined to have superior characteristics according to whether said radio channel is a control channel or an access channel 14 A mobile station according to claim 12 wherein said characteristics include signal strength 10 15 20 25 30 35 10 15 A mobile station according to claim 10 wherein said predetermined time is one of 5 10 15 20 25 30 and 35 seconds 16 A mobile station according to claim 10 wherein during said predetermined period a call is placed to a mobile station will be put on hold until the predetermined time has lapsed 17 A mobile station capable of communicating with at least one base station over a plurality of radio channels said mobile station comprising receiving means for receiving radio transmissions from at least one base station demodulating means for demodulating radio transmis sions from at least one base station received by said receiving means a control channel message detector for extracting control information from at least a specific control message received by said receiving means and demodulated by said demodulating means said specific control message informing said mobile station that no paging signals are to be expected during a predetermined time a signal l
8. annel If the access channel is not part of the current traffic area then it does not store the new access channel and the procedure returns to step S15 This is because the mobile station is not registered in the neighbor ing traffic area and to be registered which would require a registration procedure a k a roaming such as used in the conventional NMT system Otherwise the burden that would 5 613 208 7 be placed on the exchanges would be too great if the mobile stations registered with a new traffic area every time they detected a stronger channel from a neighboring traffic area particularly travelling along the boarder between two service areas If however the access channel is part of the current traffic area then the access channel is saved for the start access channel which may be used later for call set up step S23 and thereafter the system returns to step S15 to search for an even better access channel If it is determined that the channel being scanned is a control channel step 20 then it is determined whether the new control channel is better than the old control channel step S24 If it is not then the mobile station returns to step S15 If the new control channel is better than the old control channel then it is determined whether the control channel is part of the current traffic area step S25 If it is not then again the mobile station returns to step S15 If the new better control channel is part of t
9. d selecting the control channel with the strongest signal strength the word error rate of signals received at the mobile station may decrease thereby increasing the probability of correctly receiving messages on the control channel Before describing further details of the present invention an example of the construction of a cellular mobile radio system in which the present invention can be used will be discussed with reference to FIG 1 FIG 1 is a schematic diagram illustrating ten cells C1 to C10 in a cellular mobile radio telephone system Normally the method according to the present invention would be implemented in a cellular mobile radio system comprising many more cells than ten For purposes of this discussion the system depicted herein is considered to be an isolated portion of a larger system For each cell C1 to C10 there is a respective base station B1 to B10 FIG 1 illustrates base stations situated in the 10 15 20 25 30 35 45 50 55 60 65 4 vicinity of the respective cell centers and having omni directional antennas The base stations of adjacent cells may however be collocated in the vicinity of cell borders and have directional antennas FIG 1 also illustrates ten mobile stations e g vehicle mounted portable and pocket telephones M1 to M10 which are movable within a cell and from one cell to another cell The method according to the present invention may be implemented in
10. e 455 33 3 Period upon receipt of which the mobile stations begin 4 618 998 10 1986 Kawamura 455 77 scanning during the predetermined period radio channels to 4 736 461 4 1988 Kawasaki et al ee 455 343 identify which radio channels have superior characteristics 4 903 319 2 1990 Kasai et al sss 455 33 1 and storing the channels identified to have superior charac 5 119 502 6 1992 Kalin et al sss 455 342 teristics for later use as respective access and control chan 5 197 093 3 1993 Knuth et al sss 379 61 nels 5 212 803 5 1993 Uddenfeldt et al 455 33 1 5 241 542 8 1993 Natarajan et al sss 370 95 3 5 375 254 12 1994 Owen sse 455 54 1 17 Claims 3 Drawing Sheets SLEEP MODE x JUMP TO NEXT SCAN CHANNEL MEASURE RSSI ASSI MEAN VALUE gt 93d8m SIT 19 20 CURRENT LA HE YES 23 SAVE AC AS START AC FOR CALL SET UP HEN CC RSSI BETTER THAN 0107 524 m3 CURRENT TRAFFIC AREA 2 2 TES S26 USE KEW EC AS CURRENT CC U S Patent Mar 18 1997 Sheet 1 of 3 5 613 208 f 4 5 613 208 Sheet 2 of 3 Mar 18 1997 U S Patent 9cl YIMFITY JOLV 1NGONT0 Jd Let Gel vet YIM dN YTMOd e UE E 6el 431 JN 13437 WW2 S JOLV TOON T0 JI 40127110 J9F 5 7N TINN VHI 10YLNOI YIZISTHINAS 4287003407 INIAIFITE YIZISTHINAS 428300301 IMILLINSNVEL Y777041N09 40L
11. evel meter for determining the signal strength of a signal received by said receiving means a controller for controlling scanning of said radio chan nels when said specific control message is received demodulated and detected and during said predeter mined time to determine which of said channels has the best characteristics and for storing said radio chan nels having the best characteristics wherein said controller determines whether a mobile station is mounted to a vehicle determines an amount of time elapsed since a previous channel scanning operation and enters a sleep mode if said elapsed amount of time is less than a predetermined amount of time and said mobile station is not vehicle mounted and initiates said channel scanning if said mobile station is vehicle mounted or said elapsed amount of time is greater than said predetermined amount of time R0 Kk k 0
12. he current traffic area then the new control channel is stored as the current control channel step S26 and the system returns to step S15 to search for an even better control channel It can be seen from the foregoing description that the BSO acts as or a form of a rescan order to cause the mobile station to rescan preselected radio channels during a BSO period wherein the system will not transmit pages directed to the mobile stations to which the BSO was directed Also the present invention takes into consideration the amount of time since the last scanning operation and whether the mobile station is likely to be relatively fast moving between cells in determining whether to initiate a channel scanning operation The present invention is usable in any suitable radio communications system transmitting over a plurality of control channels including analog or digital systems TDMA CDMA which involve fixed stations broadcasting power reduction orders to battery operated stations From the foregoing description of the specific embodi ments others can readily modify and or adapt for various applications such specific embodiments without departing from the general nature of the invention and therefore such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalence of the disclosed embodiments It is to be understood that the phraseology and terminology employed herein is for the pu
13. l for Ericsson Mobile Phone 1941 EN LZB 126 1219 R1A available from Erics son Mobile Communications AB CM S 8 223 70 Lund Sweden herein incorporated by reference A typical cellular mobile radio telephone system is con trolled by at least one mobile switching center also known as a mobile telephone switching office or exchange at least one base station and at least one mobile station The mobile switching center constitutes an interface between the radio system and the public switching telephone network The base station transmits information between the mobile sta tions and the mobile switching centers Calls to and from mobile subscribers are switched by the mobile switching center The mobile switching center also provides all sig nalling functions needed to establish the calls In order to obtain radio coverage of a geographical area sometimes referred to as a mobile telephone exchange MTX area a number of base stations are normally required This number may range from in an exceptional case one base station up to one hundred or more base stations in normal systems The area is divided into cells where each cell may either be serviced by a base station or may share a base station with a number of other cells Each cell has an associated control 65 channel over which control non voice information is com municated between the mobile units in that cell and the base station Generally speaking the control channel inc
14. l messages Depending on the access type the mobile station will then receive an adequate message from the base station If the access type is origination or paging the mobile station is assigned a free traffic channel by the base station and the mobile station takes over the control of the traffic channel and leaves the system access mode The mobile station then tunes the transmitting frequency synthesizer 124 and the receiving frequency synthesizer 125 to the frequen cies associated with the chosen traffic channel A time alignment procedure is controlled by the base station and is based on time delay measurements which are performed at the base station on the received signal From this moment on the exchange of messages between the base station and the mobile station are transferred over e g a fast associated control channel FACCH and a slow associated control SACCH A brief description of the NMT specification for a hand held mobile station with a battery saving function follows For more details reference is made to NMT Doc 900 3 herein incorporated by reference A battery saving mobile station is defined as the hand held mobile station or the like which can process received battery saving orders BSO s The BSO s typically address a group of mobile stations wherein all mobile stations within the system are assigned to one of a plurality of groups according to the last digit in the mobile station subscriber number for i
15. lude access channels and control channels 5 A method according to claim 3 wherein said charac teristics include signal strength 6 A method according to claim 1 wherein said predeter mined period is one of 5 10 15 20 25 30 and 35 seconds 7 A method according to claim 1 wherein during said predetermined period a call is placed to a mobile station will be put on hold until the predetermined period has termi nated 8 A method for controlling scanning of radio channels by mobile stations in a cellular radiotelephone system wherein said mobile stations are operationally connected to at least one base station over a plurality of radio channels the method comprising the steps of transmitting from a base station a control signal to at least one mobile station for informing said at least one mobile station that no pages will be issued directed to said at least one mobile station for a predetermined period receiving in said at least one mobile station said control signal scanning during said predetermined period in said at least one mobile station said radio channels to identify which radio channels have superior characteristics storing in said mobile station radio channels determined to have superior characteristics determining whether a mobile station is mounted to a vehicle determining an amount of time elapsed since a previous channel scanning operation and entering a sleep mode if said elapsed amount
16. ludes a dedicated channel at a known frequency over which certain information is communicated from the base station to 5 613 208 3 mobile stations an access channel for unidirectional trans missions of information from the base station to the mobile stations and one or more traffic channels for bidirectional communications between the mobile stations and the base station These various channels may share the same fre quency or they may operate at different respective frequen cies In the NMT system duplex channels are used so that a conversation can progress simultaneously in both directions Three types of transmissions normally take place on the control channels between the mobile stations and the base stations First when a mobile station is originating a call it sends an access request to the base station whose control channel has the strongest signal This access request informs the base station that the requesting mobile station needs to be assigned a voice channel i e traffic channel over which the call can be connected Second when a mobile station is paged by a base station over the control channel indicating that the base station has a call to be completed to the mobile subscriber the paged mobile station sends back a paging response on the access channel Finally when a mobile station travels from one cell to another or from one traffic area to another or for other reasons the mobile station may send a registratio
17. n access over the access channel to identify itself and its presence to the telephone exchange associated with the cell To facilitate paging that is searching for a mobile unit a service area may be divided into traffic areas each consist ing of a number of cells When a mobile station leaves one traffic area and enters another it is requested by the mobile switching center to repeat its new location In this way the mobile station can be searched for over a limited number of base stations A mobile station in an idle state i e with no active call taking place will listen for pages and other information on the control channel having the strongest signal strength The mobile station will stay tuned to the access channel until it either initiates an access e g a registration call access or paging response or performs a rescan of all the control channels within its range to determine if a new control channel has a stronger signal strength than the current control channel In cellular radio systems since the mobile station may move the quality of the received signal may deteriorate if the mobile station leaves the base station s coverage area As the quality of the received signal is reduced the ability to correctly detect and interpret data messages is also reduced This decreases the probability of for example receiving incoming calls By performing a rescan of all control channels which might be 21 channels for example an
18. nstance Upon receipt of a BSO the mobile stations within a group may close down or turn off certain preselected functions other than the display in visual indicators and user initiated operations An extra display may be incorporated to signify the occasion of a battery saving period The preselected functions are closed down for a time period set by the BSO such periods being 0 5 10 15 20 25 30 or 35 seconds for example This time period is longer than a frame length as evident from the Service Manual for the Ericsson Mobile Phone 1941 cited above at page 12 wherein a frame of 64 bits is transmitted at a speed of 1200 bits per second indicating a frame length of less than 54 000ths of a second Immediately after the battery saving period has expired the mobile station is locked to the same channel it was locked before receiving the BSO This aspect of the con ventional NMT specification is modified in accordance with 10 20 30 35 40 45 50 55 60 65 6 the present invention After the battery saving period has expired the mobile station is immediately able to receive any information on the memorized control channel If the memorized control channel is inadequate the mobile station begins searching for a new control channel according to conventional NMT standards The present invention reduces or eliminates the need for this procedure The battery savings mode should be distinguished from the standby
19. o only user initiated functions Conventionally a mobile telephone does not continuously scan for the strongest control channel When a call is placed an NMT telephone randomly selects a starting channel among a group of prestored access channels AC s to begin an access channel scan The mobile station will select and use the first access channel whose signal strength is suffi ciently high The selected access channel is then used to initiate a call link set up procedure To save battery life in the NMT system for example base stations send out battery saving orders BSO s to groups of mobile stations The groupings are designated by the mobile system The system by issuance of BSO s orders groups of mobile stations to go into a sleep mode wherein the greater part of the circuitry of the mobile station becomes inactive to further reduce battery consumption A typical BSO period is a few seconds and the length of the BSO period is usually determined by the call frequency in the system e g the load at the mobile switching center which services a group of base stations If a call is placed to a mobile station during 15 20 25 30 45 50 55 60 2 the period designated by the BSO the call will be put on hold in the mobile switching center which controls several base stations until the BSO period has terminated SUMMARY OF THE INVENTION During the battery saving period designated by a battery saving orde
20. of time is less than a predetermined amount of time and said mobile station is not vehicle mounted or initiating said scanning step if said mobile station is vehicle mounted or said elapsed amount of time is greater than said predetermined amount of time 9 A method for controlling scanning of radio channels by mobile stations in a cellular radiotelephone system wherein said mobile stations are operationally connected to at least one base station over a plurality of radio channels the method comprising the steps of transmitting from a base station a control signal to at least one mobile station for informing said at least one mobile station that no pages will be issued directed to said at least one mobile station for a predetermined period receiving in said at least one mobile station said control signal scanning during said predetermined period in said at least one mobile station said radio channels to identify which radio channels have superior characteristics wherein said scanning step further comprises the step of identifying whether a radio channel being scanned is one of a control channel and an access channel having superior characteristics and storing in said mobile station radio channels determined to have superior characteristics wherein said storing step includes the step of storing a radio channel 5 613 208 9 10 A mobile station capable of communicating with at least one base station over a plurality of
21. on is car mounted the method then retunes to the next scan channel among a group of scan channels stored in the mobile station step S15 Once the mobile station is fixed to the next channel to be measured a received signal strength indicator RSSI is determined in accordance with known techniques step S16 If the mean value of the RSSI is not greater than e g 93 dBm 1 uV step S17 then the mobile station returns to step S15 and retunes to the next scan channel However if the mean value of the RSSI is greater than 93 dBm then the mobile station proceeds to read and identify Fast Fre quency Shift Keying FFSK data on the channel being scanned step S18 Once this data is acquired the mobile station proceeds to identify whether the channel being scanned is an access channel or a control channel step S19 If it is neither an access channel nor a control channel the mobile station returns to step S15 to be retuned to the next scan channel However if the scan channel is an access channel step S20 then it is determined whether the access channel RSSI is better than the old access channel RSSI step S21 If it is not the mobile station returns to step S15 However if the new access channel RSSI is better than the old access channel RSSI then it is determined whether the new access channel is part of the current traffic area step 22 rather than a neighboring traffic area by the traffic area code sent on the calling ch
22. r a mobile station in accordance with the present invention scans for a new better control channel and or access channel and stores the identified better channel for future use The mobile station switches to listening to the best control channel at the end of the BSO period and stores the best access channel for faster call initiation or set up Other factors involved in initiating the channel scan include a determination of whether the mobile station is vehicle mounted and a determination as to the last time a channel scan has taken place BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described with refer ence to the accompanying drawing in which FIG 1 is a schematic diagram illustrating an example of a cellular mobile radio system illustrating the relationship of the systems cells a mobile switching center and mobile stations FIG 2 is a block diagram illustrating a mobile station in a cellular mobile radio system according to FIG 1 and FIG 3 is a flowchart of a channel scanning method in accordance with the present invention DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS To facilitate consideration of the present invention a brief description of a typical cellular mobile radio telephone system is provided Further details as to the hardware and operation of an exemplary mobile cellular system can be found in U S Pat No 5 119 502 issued to Kallin et al on Jun 2 1992 and in the Service Manua
23. roller 130 along with other system components as disclosed in U S Pat No 5 119 502 for example If the present inven tion is implemented in a system such as the analog NMT 5 613 208 5 system a duplex filter and a single antenna or plural antennas with an antenna switch may be incorporated such as shown in the Service Manual for the Ericsson Mobile Phone 1941 cited above One of the primary tasks performed in the system access mode of the mobile station is the generation of an access message in the mobile station and preparation of a suitable traffic channel for information exchange The access chan nels available to the mobile which were updated during the idle mode and during the BSO period are examined in a manner similar to the measuring of the dedicated control channels as previously described A ranking of the signal strength of each is made and the channel associated with the strongest signal is chosen The transmitting frequency syn thesizer 124 and the receiving frequency synthesizer 125 are tuned accordingly and a service request message is sent over the selected channel via a RF modulator 122 a power amplifier 123 and an antenna in order to inform the base station about the type of access wanted e g call origination page response registration request or order confirmation After completion of this message the amplifier 123 of the mobile station is turned off and the mobile station may wait for further contro
24. rpose of description and not of limitation What is claimed is 1 A method for controlling scanning of radio channels by mobile stations in a cellular radiotelephone system wherein said mobile stations are operationally connected to at least one base station over a plurality of radio channels the method comprising the steps of transmitting from a base station a control signal to at least one mobile station for informing said at least one mobile station that no pages will be issued directed to said at least one mobile station for a predetermined period receiving in said at least one mobile station said control signal scanning during said predetermined period in said at least one mobile station said radio channels to identify which radio channels have superior characteristics and storing in said mobile station radio channels determined to have superior characteristics 2 A method according to claim 1 wherein said control signal is a battery saving order directing a group of mobile 10 15 20 25 30 35 45 50 55 65 8 stations to power down at least one preselected function and wherein at least one mobile station of said group of mobile stations upon receipt of said battery saving order instead performs said scanning step during said 3 A method according to claim 1 wherein said predeter mined period is longer than a frame length 4 A method according to claim 3 wherein said radio channels inc
25. y power consumption is an impor tant issue for mobile stations In order to increase battery life various mechanisms for decreasing power consumption have evolved One such mechanism is to impose one or more of an idle mode a standby mode and a sleep mode on mobile telephones wherein in these modes various functions and or portions of the mobile telephone circuitry are turned off to decrease overall power consumption In the idle mode a mobile telephone operating within e g the Nordic Mobile Telephone NMT system is locked on a control channel CC sometimes referred to as a calling channel to be ready to receive a page which is the first step in initiating a telephone link with a base station The mobile station is not actively engaged in a voice communications link i e a conversation Under the NMT standard in the standby mode the mobile station rests on a calling channel with valid traffic area number and calling channel prefix In the standby mode the mobile station is ready for reception orinitiation of a call If the mobile station loses a lock on the calling channel the signal strength falls below a given limit it starts a scanning procedure for searching for a new stronger calling channel If the user initiates a call the mobile station leaves the standby mode and enters the procedure for initiating a call according to NMT standards In a sleep mode the mobile station is responsive to the fewest functions limited t
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