Abstract:
A wireless mobile communication device comprises first and second wireless interfaces that are individually powered from a battery. A controller sets the first wireless interface in a power-ON state for establishing a first wireless link with a first base station, makes a determination whether a handover is necessary from the first base station to a second base station, sets the second wireless interface in a power-ON state in response to the determination that the handover is necessary for making a search for the second base station, and sets the first wireless interface in a power-OFF state when the second wireless interface has established a second wireless link with the second base station. A wireless LAN module is connected to the controller for establishing a session with a remote communication device via the first and second wireless links.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power saving diversity mode wireless LAN mobile communication device. 
     2. Description of the Related Art 
     A communication system using diversity mode wireless LAN terminals is shown and described in Japanese Patent Publication P2000-13393A. In this prior art system, a wired LAN terminal and a wireless LAN terminal. Each terminal is provided with a pair of diversity antenna systems to establish a wireless link The wired LAN terminal is an access point of the wireless LAN terminal to a LAN cable. A desktop computer is connected to the wireless LAN terminal. One of the wireless links is used as a working transmission system and the other as a spare or backup transmission system. When the working transmission system fails to meet a specified bit error rate, protection switching occurs between the working and backup systems. Since the wireless LAN terminal is a fixed terminal, the backup system is constantly warmed up by the mains power supply in readiness for protection switching. However, a substantial amount of power is dissipated when the spare unit is being warmed up. 
     In the case of a wireless LAN cellular mobile terminal that establishes a session with a remote terminal using a wireless link to a base station, it is necessary to periodically perform handover from one base station to the next when the session proceeds as the terminal is on a vehicle travelling at high speed. To ensure against interruptions, the diversity method is advantageous for the wireless LAN cellular mobile terminal since it allows one of the transmission systems to operate for establishing a session with a base station and allows the other transmission system to operate for making a search for the next base station while the session proceeds. However, due to the limited battery power, the diversity mode wireless LAN cellular mobile terminal would encounter difficulty in continuing a session for an extended period of time. 
     A power saving diversity mode wireless communication device is disclosed in Japanese Patent Publication 1996-107381. According to the prior art, a plurality of wireless interfaces are set in a power-ON state and one of the wireless interfaces is selected to establish a channel with a remote terminal and the quality of the channel is monitored. When the monitored quality of the channel is higher than a threshold value, the other wireless interfaces are set in a power-OFF state. If the quality of the channel drops below the threshold value, the other wireless interfaces are set in a power-ON state and the same process is repeated. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a power saving diversity mode wireless LAN mobile communication device which eliminates the need to warm-up wireless interfaces. 
     According to a first aspect of the present invention, there is provided a wireless mobile communication device comprising a battery, first and second antennas, first and second wireless interfaces operable with the antennas, the first and second wireless interfaces being arranged to be individually powered from the battery, a controller for setting the first wireless interface in a power-ON state for establishing a first wireless link with a first base station, making a determination whether a handover is necessary from the first base station to a second base station, setting the second wireless interface in a power-ON state in response to the determination that the handover is necessary for making a search for the second base station, and setting the first wireless interface in a power-OFF state when the second wireless interface has established a second wireless link with the second base station, and a wireless LAN module connected to the controller for establishing a session with a remote communication device via the first and second wireless links. 
     According to a second aspect, the present invention provides a power saving method for a wireless mobile communication device, wherein the device comprises a battery, first and second antennas, first and second wireless interfaces operable with the antennas, and a wireless LAN module for establishing a session with a remote communication device via wireless links established by the first and second wireless interfaces, wherein the first and second wireless interfaces are arranged to be individually powered from the battery, the method comprising the steps of (a) setting the first wireless interface in a power-ON state for establishing a first wireless link with a first base station, (b) making a determination whether a handover is necessary from the first base station to a second base station, (c) setting the second wireless interface in a power-ON state in response to the determination that the handover is necessary and making a search for the second base station, and (d) setting the first wireless interface in a power-OFF state when the second wireless interface has established a second wireless link with the second base station. 
     When the mobile terminal continues the established session, the method further includes the steps of (e) making a determination whether a handover is necessary from the second base station to a third base station, (f) setting the first wireless interface in a power-ON state in response to the determination that the handover is necessary and making a search for the third base station, and (g) setting the second wireless interface in a power-OFF state when the first wireless interface has established a third wireless link with the third base station. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described in detail further with reference to the following drawings, in which: 
         FIG. 1  is a block diagram of a power saving diversity mode wireless LAN mobile terminal of the present invention; 
         FIG. 2  is a flowchart of the operation of the diversity controller of  FIG. 1  according to a first embodiment of the invention; 
         FIG. 3  is a flowchart of the operation of the diversity controller of  FIG. 1  according to a second embodiment of the invention; 
         FIGS. 4A and 4B  are graphic illustrations for describing the operation of the second embodiment; 
         FIG. 5  is a block diagram of a power saving diversity mode mobile telephone of the present invention; 
         FIG. 6  is a flowchart of the operation of the diversity controller of  FIG. 5  according to a third embodiment of the invention; and 
         FIG. 7  is a flowchart of the operation of the diversity controller of  FIG. 5  according to a fourth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , there is shown a wireless LAN mobile communication device of the present invention. The communication device is implemented with a notebook computer provided with a pair of antennas  11  and  12 . The antennas  11  and  12  are connected to an antenna switch  13  that selectively establishes a connection to wireless interfaces  30 A and  30 B under the control of a diversity controller  17 , to which a wireless LAN module  18  is connected. Wireless LAN module  18  is designed according to the IEEE 802.11. 
     Each of the wireless interfaces  30 A,  30 B comprises a wireless transceiver  14 , a field strength detector  15  and a sync detector  16 . In each wireless interface, the field strength detector  15  is connected to the transceiver  14  to produce an output representing the field strength of a signal transmitted from a base station, and the sync detector  16  is also connected to the transceiver to produce an output indicating that the mobile terminal is synchronized to a base station. 
     Diversity controller  17  initially sets one of the wireless interfaces  30 A and  30 B in a power-ON state by supplying a DC voltage through a power switch  24  from a rechargeable battery  23  and establishes a connection to a base station. Once a connection is established, the diversity controller  17  directs the wireless LAN module  18  to begin a session with a remote data terminal using the active wireless transceiver  14 . WLAN module  18  is connected to a data processor  19  which is, in turn, connected to an I/O interface  20  to which the keyboard  21  and the display screen  22  of the notebook computer are connected. All circuit elements of the mobile terminal are connected through power lines (as indicated by thick dotted lines) to the rechargeable battery  23 . 
     While the session proceeds, the diversity controller  17  monitors the output of the field strength detector  15  of the active wireless interface and determines whether or not a cell search is required for a handover to a new base station so that continuity is guaranteed for the session. If this is the case, the diversity controller sets the other wireless interface in a power-ON state and initiates a search for a new base station so that both wireless interfaces operate simultaneously for a brief period of time while the session with the old base station is nearing its end. 
     Details of the operation of the diversity controller  17  will be described below with reference to the flowcharts of  FIGS. 2 and 3 . 
     In  FIG. 2 , at step  101 , the diversity controller  17  sets the wireless interface  30 A in a power-ON state and operates it as a cell-search wireless interface (CWI) and sets the wireless interface  30 B in a state termed as an inactive wireless interface (IWI). 
     At step  102 , the diversity controller  17  performs a cell search using the wireless interface  30 A and monitors the output of field strength detector  14 A for detecting a base station (step  103 ). If the output of field strength detector  14 A indicates a base station is detected, the diversity controller  17  checks to see if the output of sync detector indicates that the mobile terminal is synchronized to the base station (step  104 ). If the decision at step  104  is negative, flow returns to step  102  to continue the cell search. 
     If the mobile terminal is synchronized to the detected base station, flow proceeds from step  104  to step  105  to set the cell-search wireless interface (CWI) in a session wireless interface (SWI) to begin a session with a remote data terminal. 
     If the mobile terminal is moving at high speed, cell-search operation must be performed continuously using the other wireless interface to hand over the session to a new base station so that continuity of data transmission is ensured. This requires two wireless interfaces to be simultaneously activated on a continued basis. However, the continued use of the two wireless interfaces would result in a rapid loss of battery power or require a heavy, high capacity battery. In order to operate the two wireless interfaces in a battery-saving mode, the diversity controller  17  activates the inactive wireless interface at an appropriate timing to perform a cell-search operation. 
     In a first embodiment, the diversity controller  17  compares the output of the field strength detector  15  of the session wireless interface to a threshold value at step  106  and determines if the detected field strength is lower than the threshold value (step  107 ). If the detected field strength is higher than the threshold value, flow returns to step  106  to repeat the comparison. 
     If the detected field strength is lower than the threshold value, it is determined that a handover is imminent and flow proceeds from step  107  to step  108  to set the inactive wireless interface (IWI) in a cell-search mode (CWI). Therefore, the wireless interface  30 B, which was initially set in an inactive mode, is activated. At step  109 , the diversity controller  17  sets the wireless interface  30 B to initiate a cell search for a new base station, while the wireless interface  30 A is continuing the session, and determines if a new base station is detected (step  110 ). 
     If a new base station is detected, flow proceeds from step  110  to step  111  to monitor the output of the sync detector  16 B to determine if the mobile terminal is synchronized to the new base station. If not, steps  109  to  111  are repeated. If the mobile terminal is synchronized to the new base station, the diversity controller  17  proceeds to step  112  to set the session-mode wireless interface (i.e.,  30 A) in an inactive state (IWI) and set the cell-search mode wireless interface (i.e.,  30 B) in a session-mode wireless interface (SWI), and returns to step  106 . 
     Therefore, the session is handed over from the old base station to the new base station. It is seen that the two wireless interfaces  30 A and  30 B are simultaneously set in a power-ON state only for a brief interval of time that is necessary to perform a handover. As the session proceeds with the new base station, steps  106  and  107  are repeated on the output of the field strength detector  15 B. If the field strength drops below the threshold, steps  108  to  112  will be executed to hand over the session to a further new base station using the wireless interface  30 A. 
     In a second embodiment, shown in  FIG. 3 , the diversity controller  17  checks the output of the field strength detector  15  of the session wireless interface at periodic intervals and stores the detected field strength values in memory, and determines from the stored strength values the timing at which the field strength will drop below the threshold. 
     Specifically, following step  105 , the diversity controller  17  sets a variable “i” equal to 1 (step  201 ) and monitors a timer and determines whether the timer has reached a check timing (step  202 ). If so, flow proceeds to step  203  to detect the field strength by storing the output of the field strength detector  15  of the initially activated wireless interface. This field strength check is repeated until the variable “i” equals a predetermined value “m” (step  204 ) by incrementing the variable “i” (step  206 ). 
     When the field strength check is repeated m times, the diversity controller  17  proceeds from step  204  to step  206  to use the stored field strength values to estimate the timing at which the field strength will drop below a predetermined threshold value. 
     As illustrated in  FIG. 4A , if the stored field strength values at times t 0 , t 1  and t 2  show a tendency that they are decaying with time and their rate of decay is low, the diversity controller determines that the field strength will drop below the threshold immediately after time t 5 . If the field strength values stored at times t 0 , t 1 , and t 2  show that they are decaying rapidly as illustrated in  FIG. 4B , the diversity controller determines that the field strength will drop below the threshold within the interval between times t 2  and t 3 . 
     At step  207 , the diversity controller  17  compares the estimated timing to the next check timing. If the estimated timing is greater than the next check timing, it is determined that the field strength will not drop below the threshold at the next check timing and flow returns to step  201 . If the estimated timing is nearer than the next check timing, it is determined that the field strength will drop below the threshold at the next check timing and flow proceeds to step  108 . 
     The present invention as described above can be equally used in a mobile telephone provided with wireless LAN functionality as shown in  FIG. 5 . 
     The mobile telephone has a pair of antennas  41  and  42  connected to an antenna switch  43  that selectively establishes a connection to wireless interfaces  30 A and  30 B under the control of a diversity controller  47 , to which a wireless LAN/telephone interface  48  is connected. Similar to  FIG. 1 , each of the wireless interfaces  30 A,  30 B comprises a wireless transceiver  44 , a field strength detector  45  and a sync detector  46 . 
     Diversity controller  47  initially sets one of the wireless interfaces  30 A and  30 B in a power-ON state by supplying a DC voltage through a power switch  55  from a rechargeable battery  54  and establishes a connection to a base station. Once a connection is established, the diversity controller  47  directs the wireless LAN/telephone interface  48  to begin a session with a remote data terminal using the active wireless transceiver  44 . WLAN/telephone interface  48  is connected to a CPU  49  which is, in turn, connected to a display panel  50  and a user interface  51  such as a keypad and a microphone and a speaker. Mobile telephone further includes a protocol table  52  for storing the identification codes of a plurality of protocols that require that no interruption should occur during a session. An application table  53  is further provided for storing the identification codes of a plurality of application programs that require that no interruption should occur during a call. All circuit elements of the mobile telephone are connected through power lines to the rechargeable battery  54 . 
     Diversity controller  47  is capable of reading the identity of a protocol from the header of an IP packet and comparing the identity of the protocol to the identification codes of the protocols stored in the protocol table  52 . If the protocol used in a session is the RTP (Real-time Transport Protocol), for example, it is necessary to avoid interruption when the mobile telephone is roaming. 
     Diversity controller  47  is further capable of detecting the identity of an application program being used and comparing the identity of the application program to the identification codes stored in the application table  53 . If the application being used is the VoIP (Voice over IP), it is necessary to avoid interruption when the mobile telephone is roaming. 
     While the session proceeds, the diversity controller  47  monitors the output of the field strength detector  45  of the active wireless interface and determines whether or not a cell search is required for a handover to a new base station so that continuity is guaranteed for the session. If this is the case, the diversity controller sets the other wireless interface in a power-ON state and initiates a search for a new base station so that both wireless interfaces operate simultaneously for a brief period of time while the session with the old base station is nearing its end. 
     Diversity controller  47  operates in the same manner to the diversity controller of  FIG. 1  with reference to the flowcharts of  FIGS. 2 and 3 . In addition, the diversity controller  47  operates according to the flowcharts of  FIGS. 6 and 7 . 
     In  FIG. 6 , steps corresponding to those of  FIG. 2  are marked with the same reference numerals and the description thereof is omitted.  FIG. 6  differs from the  FIG. 2  flowchart by the inclusion of step  301  in which the diversity controller  47  compares the identity of the protocol of the current session to those of the protocol table  52  which require that no interruption should occur when a session is proceeding. If the identity of the current protocol corresponds to one of the identities stored in the protocol table  52 , such as the RTP protocol, the decision at step  301  is affirmative and flow proceeds to step  106  to compare the output of field strength detector  45  of the active wireless interface to the threshold. Depending on the result of comparison at step  107 , steps  108  to  112  are executed in the same manner as described in connection with  FIG. 2 . 
       FIG. 7  differs from the  FIG. 6  flowchart in that step  301  is replaced with step  401  in which the diversity controller  47  compares the identity of the application program currently in use to those of the application table  53  which require that no interruption should occur when a call is in progress. If the identity of the current application corresponds to one of the identities stored in the application table  53 , such as the VoIP protocol, the decision at step  401  is affirmative and flow proceeds to step  106  to compare the output of field strength detector  45  of the active wireless interface to the threshold. Depending on the result of comparison at step  107 , steps  108  to  112  are executed in the same manner as described in connection with  FIG. 2 . 
     It is seen that, instead of steps  106  and  107 , steps  201  to  204  can equally be used in  FIGS. 6 and 7 .