Abstract:
A power management system for a mobile unit wherein the frequency of scanning a neighboring cell may be controlled. A mobile unit consumes power in standby mode by periodically scanning the base station in each neighboring cell to determine which base stations are providing a usable signal. When the signal provided by the mobile unit&#39;s servicing base station diminishes, the mobile unit informs the cellular network of the base stations providing usable signals to assist in the handover. By detecting the rate of change of the signal strength received from a base station, the mobile unit may change the scanning rate of each neighboring cell. Alternatively, the mobile unit can estimate the relative speed it is traveling through a cell. If traveling slowly through the cell or if the signal strength is not changing over time, the need for a rapid handover diminishes. The mobile unit may also increase the scanning rate if traveling rapidly through the cell.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to the field of wireless communication systems. More specifically, the invention relates to power management systems for mobile units. 
     2. Description of the Related Art 
     The use of wireless communication systems is growing with users now numbering well into the millions. However, one inconvenience associated with routine use of a mobile unit is the constant need to recharge and replace depleted batteries. Even users who make and receive a few telephone calls such that their mobile units operate mostly in a standby mode (awaiting calls) experience the annoying and frequent problem of depleted battery power. 
     As a mobile unit travels through a cellular network, the mobile unit moves through service areas known as cells. Each cell is a specific geographic region containing a base station. When moving from one cell to another, the base station servicing the mobile unit changes from the base station of one cell to the base station of another cell. In many analog cellular systems, this change is controlled by the base stations and the switch is called “handoff.” However, in conventional digital cellular systems, the mobile unit assists in determining when the serving base station should be changed, and the switch is termed “handover.” 
     In digital cellular systems, the mobile unit does not have to be served by the nearest base station. From signal strength measurements, the mobile unit can determine which base stations are providing signals capable of adequately servicing the mobile unit. This information is then sent to a mobile switching center to determine which base station will serve the mobile unit. Due to loading requirements, it may be advantageous for a more remote base station to serve the mobile unit, provided the received signal strength from the remote base station is adequate. 
     A large portion of battery power consumed in common standby modes is attributable to determining proper handover. While in the standby mode, the mobile unit is periodically activated to scan the signal strength of each neighboring cell. For example, in the Global System for Mobile Communications (GSM) wireless communication networks, a mobile unit receives and decodes the signal strength of each neighboring cell about once every thirty seconds. 
     At any given time, a mobile unit may have between 6 to 12 neighboring cells. Because of the requirement to scan each cell approximately every 30 seconds, the mobile unit may be activated every 2-5 seconds. Each activation and scan consumes a significant amount of battery power, thereby reducing the standby time of the mobile unit. 
     Improvements in battery technology, while helpful, have done little to avoid the seemingly ever-present need to recharge and replace mobile unit batteries. What is needed is a system to conserve battery power by minimizing the power consumed scanning neighboring cells. 
     SUMMARY 
     The present invention reduces power consumed by a mobile unit in the standby mode by reducing the frequency the mobile unit scans neighboring cells to determine signal strength. Reduced scanning of neighboring cells consumes less power and advantageously increases the standby mode lifetime of a mobile unit battery. 
     For example, scanning each neighboring cell every 30 seconds ensures a mobile unit traveling through a cell knows which base stations are providing usable signals. In this manner, when the mobile unit crosses a cell boundary, or loses the required signal strength from its servicing base station, the mobile switching center can handover the mobile unit to a new base station. 
     Generally, a handover occurs when a mobile unit exceeds the range of its servicing base station. Therefore, if a mobile unit is stationary, or moving at a slow rate of speed, there is less need to monitor the neighboring base stations. 
     One embodiment of the invention detects the speed the mobile unit is moving and adjusts the frequency of scanning each neighboring cell accordingly. For example, if a mobile unit is moving slowly through a cell, the time period between scanning for each neighboring cell can be increased. 
     The speed of the movement through a cell may be determined by the change in signal strength. Every time a mobile unit communicates with the base station, the received signal strength is measured. If a mobile unit is not moving through the cell, the signal strength should remain fairly constant. However, as the mobile unit moves away from the base station, the signal strength decreases. 
     One embodiment of the invention determines the rate of change in the measured signal strength of the mobile unit. If the rate of change is low, the mobile unit increases the amount of time between each scan of a neighboring cell. 
     One embodiment of the invention is a wireless communication system comprising a plurality of base stations which transmit signals and a mobile unit which intermittently detects the signals transmitted by the plurality of base stations. A signal strength detector then determines the quality of the signals received by the mobile unit, and a processor calculates the speed at which the mobile unit is moving from one of the plurality of base stations based on the rate of change of the quality of the signals received by the mobile unit. The processor adjusts the frequency in which the mobile unit detects the signals transmitted by at least one of the plurality of base stations based upon the speed of the mobile unit. 
     One embodiment of the invention is a method of conserving power in a wireless communication system. The method comprises the acts of measuring the quality of a plurality of signals received from one of a plurality of base stations and then calculating the speed of a mobile unit from the signal quality measurements. The mobile unit then adjusts the frequency the signals are detected based upon the speed of the mobile unit. 
     In one embodiment of the invention, a wireless communication system comprises a speed sensor which determines the speed of a mobile unit based upon the rate of change of the strength of signals received by a mobile unit. A scan inhibitor causes the mobile unit to inhibit,detection of at least one message from a base station when the speed of the mobile unit is below a set level. 
     One embodiment of the invention is a method of saving power in a communications system. The method comprises the acts of determining the speed of a receiving unit and altering a periodic interval for detecting a transmission based upon the speed of the receiving unit. 
     In one embodiment of the invention, a wireless communication system comprises means for determining a change over time of a signal and means for adjusting the rate of detecting the signal based upon the change over time. 
     One embodiment of the invention is a method of saving power in a communications system. The method comprises the acts of measuring signal changes as a function of time and then altering a periodic interval for detecting a transmission based upon the measured signal changes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
     FIG. 1 illustrates components of a wireless communication system appropriate for use with an embodiment of the invention. 
     FIG. 2 illustrates a series of cells in a wireless communication system. 
     FIG. 3 illustrates a block diagram of a mobile unit according to one embodiment of the invention. 
     FIG. 4 illustrates one embodiment of a wireless communication signal data transmitted by a base station. 
     FIG. 5 illustrates one embodiment of the acts performed by a mobile unit to establish a scanning rate of neighboring cells. 
     FIG. 6 illustrates acts performed by a mobile unit to determine the change of signal strength according to one embodiment of the invention. 
     FIG. 7 illustrates acts performed by a mobile unit to determine the change of signal strength according to one embodiment of the invention. 
     FIG. 8 illustrates acts performed by a mobile unit to adjust the scanning rate of neighboring cells according to one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates components of a wireless communication system. A mobile switching center  102  communicates with a base station  104 . The base station  104  broadcasts data to and receives data from mobile units  106  within a cell  108 . The cell  108  is a geographic region, roughly hexagonal, having a radius of up to 35 kilometers or possibly more. 
     The mobile unit  106  is capable of receiving data from and transmitting data to a base station  104  in compliance with the Global System for Mobile communications (GSM). GSM is a communication standard permitting mobile users of wireless communication devices to exchange data over a telephone system wherein radio signals carry data to and from the wireless devices. Under the GSM standard, additional cells adjacent to the cell  108  permit mobile units  106  to cross cell boundaries without interrupting communications. This is because base stations  104  in adjacent cells assume the task of transmitting and receiving data for the mobile units  106 . The mobile switching center  102  coordinates all communication to and from mobile units  106  in a multi-cell region, thus the mobile switching center  102  may communicate with many base stations  104 . 
     The mobile units  106  may move about freely within the cell  108  while communicating either voice or data. The mobile units  106  not in active communication with other telephone system users may, nevertheless, scan base station  104  transmissions in the cell  108  to detect any telephone calls or paging messages directed to the mobile unit  106 . 
     One example of such a mobile unit  106  is a cellular telephone used by a pedestrian who, expecting a telephone call, powers on the cellular telephone while walking in the cell  108 . The cellular telephone synchronizes communication with the base station  104 . The cellular telephone then registers with the mobile switching center  102  to make itself known as an active user within the GSM network. 
     As discussed in further detail below, the mobile unit  106  scans data frames broadcast by the base station  104  to detect any telephone calls or paging messages directed to the cellular telephone. In this call detection mode, the mobile unit  106  receives, stores and examines paging message data, and determines whether the data contains an identifier matching an identifier of the mobile unit  106 . If a match is detected, the mobile unit  106  establishes a call with the mobile switching center  102  via the base station  104 . If no match is detected, the mobile unit  106  enters an idle state for a predetermined period of time, then exits the idle state to receive another transmission of paging message data. 
     A common implementation of the GSM system uses frequencies in the 900 megahertz (MHz) range. In particular, mobile units  106  transmit in the 890-915 MHz range and base stations  104  transmit in the higher 935-960 MHz range. Each 25 MHz range is divided into 125 radio frequency channels, each having a width of 200 kilohertz (kHz). The direction of communication from a mobile unit  106  to a base station  104  is referred to as uplink, and the direction from a base station  104  to a mobile unit  106  is referred to as downlink. 
     FIG. 2 illustrates one example of a series of cells  108   a - 108   n  in a wireless communication system. The cells  108   a - 108   n  are generally hexagonal, although they may be other shapes including circular, square, oval, oblong, or any other polygon. The size of each cell  108   a - 108   n  may vary depending on location. For example, in densely packed urban areas, a cell  108   k  may be small but in a more rural area the size of a cell  108   a  increases. Each of the cells  108   a - 108   n  has a corresponding base station  104   a - 104   n.    
     In FIG. 2, the mobile unit  106  is located in the cell  108   b . While the mobile unit  106  is in cell  108   b , it is likely being served by the base station  104   b , although due to loading and other requirements, it may be served by any base station  104  providing a useable signal. While in one cell  108 , the mobile unit  106  periodically checks the signal strength of the base stations  104  in each neighboring cell  108 . For example, while the mobile unit  106  is in the cell  108   b , the mobile unit  106  monitors the signal strength of base stations  104   a ,  104   c ,  104   d , and  104   h . If the mobile station  106  travels into cell  108   h , the mobile switching center  102  may cause the mobile station  106  to handover to base station  104   h . In this circumstance, the mobile station  106  then periodically monitors the signal strength of base stations  104   b ,  104   c ,  104   d ,  104   e ,  104   g , and  104   i.    
     Scanning each neighbor cell to check the signal strength consumes power. Millions of consumers use mobile units  106 , such as portable, hand-held cellular telephones, that rely on batteries for power. Even consumers who initiate and receive relatively few telephone calls on their cellular telephones must frequently recharge and replace batteries because of the power consumed by the cellular telephone while in standby operation (waiting for an incoming call). 
     The present invention substantially reduces the power consumed by the mobile unit  106  in scanning neighbor cells and consequently increases battery lifetime. To reduce power consumption, one embodiment of the invention does not scan each neighbor cell at the rate prescribed by the base station  104 . Rather, the mobile unit  106  detects the signal quality and if the quality is sufficient, skips a number of scanning cycles. Advantageously, the embodiment substantially reduces neighbor cell scanning and extends the lifetime of a single battery charge. 
     FIG. 3 illustrates one embodiment of the mobile unit  106 . The mobile unit  106  downlinks the signals from the base station  104  at a transceiver  120  via an antenna  115 . The transceiver  120  may also uplink information to the base station  104 . Alternatively, a separate receiver and transmitter may be used in place of the transceiver  120 . After receiving the signals, the transceiver  120  relays the signals to a processor  125 . In one embodiment, a microprocessor performs the function of the processor  125 . Of course, other types of processors may be used including conventional general purpose single- or multi-chip microprocessors, digital signal processors, embedded microprocessors, microcontrollers and the like. 
     The processor  125  converts the signals into data and performs the functions requested by the signal. This may include an indication that a call is pending. The mobile unit  106  may inform the user of a pending call by a variety of methods, including ringing, vibrating or flashing lights. During the pendency of a call, the data transmitted and received by the mobile unit  106  may include voice and data. 
     The data created by the processor  125  may be temporarily or permanently stored in a storage medium  130 . The storage medium  130  may comprise any method of storing information. For example, the storage medium  130  may comprise an electrically erasable programmable read-only memory (EEPROM), read-only memory (ROM), random access memory (RAM), hard disks, floppy disks, laser disc players, digital video devices, compact discs, video tapes, audio tapes, magnetic recording tracks, and other techniques to store data. 
     The data from the storage medium  130  may be transmitted through a decoder  140  to a speaker  150 . The decoder  140  may comprise a digital-to-analog converter or the like. The decoded data may then be played through the speaker  150  to be heard by the user. 
     The user may also direct voice into the microphone  145  of the mobile unit  106 . The voice data passes through an encoder  135  and may be stored by the storage medium  130  prior to processing by the processor  125 . The encoder  135  may comprise an analog-to-digital converter or the like. The processor  125  maintains two-way communication with the transceiver  125 , and therefore the voice data may be sent from the mobile unit  106  to the base station  104 . 
     FIG. 4 illustrates wireless communication data transmitted by a base station  104  and structured in data frames, sometimes called time-division multiple access (TDMA) frames, according to the GSM standard. TDMA is a type of multiplexing where two or more channels of information are transmitted over the same link by allocating a different time interval (“slot” or “slice”) for the transmission of each channel. That is, the channels take turns to use the link. Of course, the present invention is not limited to the GSM standard or TDMA frames, and may include systems using code-division multiple access, statistical time division multiplexing, spread spectrum, a single communications channel or the like. For ease of understanding, the present invention will be described with reference to a GSM based system. 
     The GSM specification provides eight time slots (or physical channels) in each 200 kHz radio channel. An entire data frame has a duration of 4.615 milliseconds. Each time slot has a time length of 577 microseconds (4,615 / 8=577). Because a mobile unit  106  may use only one time slot in any data frame, it must transmit information within the  577  microsecond time slot duration. 
     As shown in FIG. 4, a data frame  202  has eight time slots  204  (or physical channels). The time slots  204  carry bit-oriented control information, voice information or data. Generally, the first time slot of each frame  206  holds bit-oriented control information. Control information is used in a GSM-based system to broadcast synchronization information and system parameters, to notify mobile units  106  of pending telephone calls or page messages, and to grant mobile units  106  access to other physical channels. 
     The time slots carrying control information are formatted in groups of 51 time slots (i.e., the first time slot of each of 51 successive frames) referred to as a multiframe  208 . Downlink information transmitted to a mobile unit  106  by a base station  104  is formatted in multiframes  208 . In accordance with the GSM standard, a multiframe  208  may include four types of control information: (1) a frequency correction channel  210  which provides the mobile unit  106  with the frequency reference of the GSM system; (2) a synchronization channel  212  which supplies the mobile unit  106  with the key (or training sequence) needed to demodulate the information coming from the base station  104  and also contains a frame number, as well as the base transceiver station identity code; (3) a broadcast control channel  214  which informs the mobile unit  106  about specific system parameters it may need to identify the network or to gain access to the network, including location area code, operator identification, information on which frequencies of the neighboring cells may be found, different cell options, and access to other parameters; and (4) a common control channel  216  which supports the establishment of a link between a mobile unit  106  and a base station  104 . 
     A common control channel  216  may have different uses. A common control channel  216  may be a paging message  218   a  or  218   b , referred to collectively as paging messages  218 . The paging messages  218  provide information indicating whether a telephone call is currently pending for a particular mobile unit  106 . A common control channel  216  may also be an access grant channel through which a mobile unit  106  acquires information identifying which channel to use for communication needs. 
     The frequency correction channel  210  and the synchronization channel  212  each consist of bit-oriented data in a time slot. The broadcast control channel  214  uses four time slots to carry information. In addition, the common control channel  216  also uses four time slots to carry information. For example, a common control channel  216  used as a paging message  218  uses four time slots of bit-oriented data  220 , each time slot  220  carrying 156.25 bits. 
     The process of establishing a scanning rate for neighboring cells by a mobile unit  106  is illustrated in FIG.  5 . The process is shown generally by flowchart  500 . The mobile unit  106  initializes in a start state  505 . Proceeding to state  510 , the mobile unit  106  determines the default scanning rate for neighboring cells by obtaining this information from the base station  104 . The default scanning rate is the rate prescribed by the service provider. This rate varies from system to system, but is generally on the order of once every 30 seconds. This means the mobile unit  106  scans each neighboring cell every 30 seconds. For example, if the mobile unit  106  has 6 neighboring cells, the mobile unit  106  scans a neighboring cell approximately every 5 seconds. 
     Proceeding to state  520 , the mobile unit  106  determines the speed it is traveling through a cell  108  or the rate of change of the received signal strength. The speed determined by the mobile unit  106  may not be the actual speed of the mobile unit  106 , but rather the relative speed the mobile unit  106  is traveling through a cell  108 . For example, a mobile unit  106  traveling at 30 miles per hour directly through a cell  108  may have a higher relative speed than a mobile unit  106  traveling at 80 miles per hour up a hill near the center of the cell  108 . 
     The mobile unit  106  may determine the speed or the rate of change of the received signal strength using several techniques. The speed a mobile unit  106  travels through a cell  108  may be determined using a Global Positioning System (GPS). Such systems are well known and can identify the location of an object. The speed of a mobile unit  106  may be obtained using GPS by taking several readings and calculating the change in location over time. 
     FIG. 6 illustrates the process according to one embodiment used to determine the speed the mobile unit  106  is traveling through a cell  108  or rate of change of the received signal strength of state  520 . The process begins at start state  600 . Proceeding to state  605 , the mobile unit  106  records the level or quality of the signal received. Because many factors may influence the signal quality each time it is measured, one embodiment averages several measurements of the signal quality detected by the mobile unit  106 . 
     The mobile unit  106  uses a variety of indicators to determine signal quality. Among these indicators is a bit error rate, a receiver quality indicator (RX Quality), and a receive signal strength indicator (RSSI), known in the GSM and the digital cellular embodiment as RX Level. The mobile unit  106  uses these indicators to determine the signal quality. 
     In particular, the bit error rate is the number of erroneous bits in a data transmission. The RX Quality is a value assigned by the network indicating the quality of the received signal based upon the bit error rate. The RX Quality figure provides a mobile unit  106  with an expected measurement accuracy. The mobile unit  106  uses the RX Quality to determine the overall potential for error. The values assigned for RX Quality according to the GSM standard based upon the bit error rate are presented in Table 1. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Expected MU- 
               
               
                 RX 
                 Corresponding Bit 
                 Range of Actual 
                 Reporting-Accuracy 
               
               
                 Quality 
                 Error Rate (%) 
                 Bit Error Rate (%) 
                 Probability (%)  
               
               
                   
               
             
             
               
                 0 
                 Below 0.2 
                 Below 0.1 
                 90 
               
               
                 1 
                 0.2 to 0.4 
                 0.26 to 0.30 
                 75 
               
               
                 2 
                 0.4 to 0.8 
                 0.51 to 0.64 
                 85 
               
               
                 3 
                 0.8 to 1.6 
                 1.0 to 1.3 
                 90 
               
               
                 4 
                 1.6 to 3.2 
                 1.9 to 2.7 
                 90 
               
               
                 5 
                 3.2 to 6.4 
                 3.8 to 5.4 
                 95 
               
               
                 6 
                  6.4 to 12.8 
                  7.6 to 11.0 
                 95 
               
               
                 7 
                 Above 12.8 
                 Above 15 
                 95  
               
               
                   
               
             
          
         
       
     
     Another measurement that may be used by the mobile unit  106  is RX Level (also known as RSSI in analog systems). RX Level provides a known value based upon the measured strength of the signal at the mobile unit  106 . A stronger signal at the mobile unit  106  indicates less likelihood for error. Table 2 provides values for RX Level based upon the signal strength at the mobile unit  106 . Each specific value for RX Level correlates to the strength of the signal (in measured decibels (dBm)) at the mobile unit (MU)  106  receiver. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 RX Level 
                 Level at MU Receiver (dBm)  
               
               
                   
                   
               
             
             
               
                   
                  0 
                 Less than −110 
               
               
                   
                  1 
                 −110 to −109 
               
               
                   
                  2 
                 −109 to −108 
               
               
                   
                 . . . 
                 . . . 
               
               
                   
                 . . . 
                 . . . 
               
               
                   
                 62 
                 −49 to −48 
               
               
                   
                 63 
                 above −48  
               
               
                   
                   
               
             
          
         
       
     
     Proceeding to state  610 , the mobile unit  106  begins a timer. The timer may be included in the processor  125  of the mobile unit  106  as shown in FIG.  3 . The timer may also be provided by the base station  104 . Alternatively, instead of starting a timer, the mobile unit  106  may record a first time from a clock. This enables the mobile unit  16  to calculate the elapsed time after recording a second time from the clock. 
     Proceeding to state  620 , the mobile unit  106  again reads the signal level. Proceeding to state  625 , the mobile unit  106  determines if the signal level read in state  620  exceeds a predetermined level. This may be, for example, higher than the signal read in state  605 . The mobile unit  106  may also look for a specific increase in one of the indicators, such as an increase in RX Level. The amount of change needed may be set upon initialization of the mobile unit  106  or may be dynamically adjusted by the processor  125  based upon the previous rate of change of signal strength measured. This may be, for example, a 6 dB increase in the signal strength or an increase in RX Level by 5. If the level is not exceeded, the mobile unit  106  proceeds along the NO branch back to state  620 . In state  620 , the mobile unit  106  again reads the signal level and proceeds to state  625 . The mobile unit  106  continues to read the signal level until the predetermined change is exceeded. 
     After the signal strength measured in state  620  exceeds the predetermined level, the mobile unit  106  proceeds along the YES branch to state  630 . In state  630 , the mobile unit  106  stops the timer and records the stop time or the elapsed time, depending on whether a clock or timer is used. If a clock is used, the mobile unit  106  determines the elapsed time by recording the start time and stop time, then subtracting the start time from the stop time. 
     Proceeding to state  635 , the mobile unit  106  determines if the elapsed time is less then a first predetermined period of time X. The amount of the first predetermined period of time X may be programmed into the mobile unit  106 , established upon initialization of the mobile unit  106 , or may be dynamically adjusted by the processor  125 . The value of X may vary and is effected by the amount of signal change required in state  625 . In one embodiment, the value of X is 2 seconds. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. 
     If the elapsed time is less then X, the mobile unit proceeds along the YES branch to state  640 . In state  640 , the mobile unit  106  sets the multiplier factor to A. The multiplier factor is a multiple that the mobile unit  106  uses to modify the scanning rate for neighboring cells. The value to assign to A could vary, and may be programmed into the mobile unit  106 , established upon initialization of the mobile unit  106 , or may be dynamically adjusted by the processor  125 . In one embodiment of the invention, the value of A is equal to 1. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. After setting the multiplier factor to a value of A, the mobile unit  106  proceeds to end state  670 . 
     Returning to state  635 , if the elapsed time is not less then X, the mobile unit proceeds along the NO branch to state  645 . In state  645 , the mobile unit  106  determines if the elapsed time is less then a second predetermined period of time Y. The amount of the second predetermined period of time Y may be established in the same manner as the first predetermined period of time X, by programming into the mobile unit  106 , establishing upon initialization of the mobile unit  106 , or dynamically adjusting by the processor  125 . In one embodiment, the value of Y is 4 seconds. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. 
     If the elapsed time is less then Y, the mobile unit  106  proceeds along the YES branch to state  650 . In state  650 , the mobile unit  106  sets the multiplier factor to B. In one embodiment of the invention, the value of B is equal to 2. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. After setting the multiplier factor to a value of B, the mobile unit  106  proceeds to end state  670 . 
     Returning to state  645 , if the elapsed time is not less then Y, the mobile unit proceeds along the NO branch to state  655 . In state  655 , the mobile unit  106  determines if the elapsed time is less then a third predetermined period of time Z. The value of the third predetermined period of time Z is determined in the same manner as the periods of time X and Y. In one embodiment, the value of Z is 8 seconds. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. 
     If the elapsed time is less then Z, the mobile unit  106  proceeds along the YES branch to state  660 . In state  660 , the mobile unit  106  sets the multiplier factor to C. In one embodiment of the invention, the value of C is equal to 4. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. After setting the multiplier factor to a value of C, the mobile unit  106  proceeds to end state  670 . 
     Returning to state  655 , if the elapsed time is not less then Y, the mobile unit proceeds along the NO branch to state  665 . In state  665 , the mobile unit  106  sets the multiplier factor to D. In one embodiment of the invention, the value of D is equal to 8. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. After setting the multiplier factor to a value of D, the mobile unit  106  proceeds to end state  670 . Of course, the number of predetermined periods of time and multiplier factors may vary depending on how precise of a change is desired. 
     The number of stages of elapsed time, represented by states  635 ,  645 , and  655  may vary depending upon the sensitivity desired. In the above example, three levels of sensitivity are used, resulting in four possible multipliers. If a more precise rate of change of signal strength is desired, the difference between each amount of time may be decreased. For example, eight stages may be used, with the elapsed time being for each stage having a small one second increase. 
     In an example of the present invention using the process of FIG. 6, a mobile unit  106  moves slowly through a cell  108 . Because the slow moving mobile unit  106  changes cells  108  less often, there is less need to frequently scan each neighboring cell  108 . In this circumstance, the mobile unit  106  begins at start state  600  and proceeds to state  605  to read the current signal level from its servicing base station  104 . Proceeding to state  610 , the mobile unit  106  starts a timer. Proceeding to state  620 , the mobile unit  106  receives subsequent signals from the base station  104  and checks the signal level. Proceeding to state  625 , the mobile unit  106  checks if the signal exceeds a predetermined level. Once the signal level reaches a predetermined level, the mobile unit  106  proceeds along the YES branch of state  625  and stops the timer and records the elapsed time according to state  630 . In this case, because the mobile unit  106  is moving slowly through the cell  104 , it is likely to take a long time for the signal level to change, say 15 seconds. Proceeding to state  635 , the mobile unit  106  checks if the elapsed time is less than the first predetermined period of time X, which is set at 2 seconds. In this example, because 15 seconds is longer than 2 seconds, the mobile unit  106  proceeds along the NO branch of state  635  to state  645 . In state  645 , the mobile unit  106  checks if the elapsed time of 15 seconds is less than the second predetermined period of time Y, set at 4 seconds. Again, in this example, 15 seconds is longer than Y so the mobile unit  106  proceeds along the NO branch of state  645  to state  655 . In state  655 , the mobile unit  106  checks if the elapsed time is less than the third predetermined period of time Z, set at 8 seconds. Again, in this example, 15 seconds is longer than Z, so the mobile unit  106  proceeds along the NO branch of state  655  to state  665 . In state  665 , the multiplier factor is set to a level D, in this case  8 . Using this multiplier factor, the slow moving mobile unit  106  will change the scanning rate for neighboring cells  108  from say every 30 seconds to every 240 seconds. Applying the multiplier factor to the scanning rate will be described below. By scanning each neighboring cell only every 240 seconds, the mobile unit  106  activates less often and battery power is conserved. 
     FIG. 7 illustrates the process according to another technique used to perform the activities indicated by state  520  to determine the rate of change of the received signal strength. In FIG. 6, the mobile unit  106  determined the amount of time required for the signal strength to increase a set amount. An alternative approach illustrated in FIG. 7 determines the actual change in signal strength after a predetermined period of time. The process begins at start state  700 . Proceeding to state  705 , the mobile unit  106  records the level of the signal received. The mobile unit  106  may use a variety of indicators to determine signal quality as described above. These indicators include a bit error rate, a receiver quality indicator (RX Quality), a receive signal strength indicator (RSSI), or a RX Level. 
     Proceeding to state  710 , the mobile unit  106  begins a timer or records a start time from a clock. The timer or clock may be included in the processor  125  of the mobile unit  106  as shown in FIG. 3, or may be provided by the base station  104 . 
     Proceeding to state  720 , the mobile unit  106  again reads the timer or clock to determine the amount of time elapsed. The amount of time elapsed in state  720  is compared to determine if it exceeds a predetermined level. This may be, for example, 5 seconds. The amount of elapsed time needed may be set upon initialization of the mobile unit  106  or may be dynamically adjusted by the processor  125  based upon the previous rate of change of signal strength measured. The mobile unit  106  remains in state  620  until the predetermined amount of time elapses. 
     Proceeding to state  725 , the mobile unit  106  records the current signal level. As stated above, this may be a direct reading of the signal strength in a unit like decibels, or the mobile unit  106  may use any of the available indicators to determine the strength of the signal. 
     Proceeding to state  730 , the mobile unit  106  calculates the change in the signal levels measured in state  705  and state  730 . The change in the signal levels may be recorded in the storage medium  130  for future use. 
     Proceeding to state  735 , the mobile unit  106  determines if the change in the levels is greater then a first predetermined change R. The amount of the first predetermined change R may be programmed into the mobile unit  106  by the user, established upon initialization of the mobile unit  106 , or may be dynamically adjusted by the processor  125 . The value of R may vary and is effected by the amount of elapsed time required in state  720 . In one embodiment, the value of R is 6 decibels. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. 
     If the change in levels is greater then R, the mobile unit  106  proceeds along the YES branch to state  740 . In state  740 , the mobile unit  106  sets the multiplier factor to A. The multiplier factor is an amount that the scanning rate will eventually modify the scanning rate for neighboring cells. The value to assign to A could vary, and may be programmed into the mobile unit  106 , established upon initialization of the mobile unit  106 , or may be dynamically adjusted by the processor  125 . In one embodiment of the invention, the value of A is equal to 1. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. After setting the multiplier factor to a value of A, the mobile unit  106  proceeds to end state  770 . 
     Returning to state  735 , if the change in levels is not greater then R, the mobile unit proceeds along the NO branch to state  745 . In state  745 , the mobile unit  106  determines if the change in levels is greater then a second predetermined change S. The amount of the second predetermined change S may be established in the same manner as the first predetermined change R, by programming into the mobile unit  106  by the user, establishing upon initialization of the mobile unit  106 , or dynamically adjusting by the processor  125 . In one embodiment, the value of S is 4 decibels. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. 
     If the change in levels is greater then S, the mobile unit  106  proceeds along the YES branch to state  750 . In state  750 , the mobile unit  106  sets the multiplier factor to B. In one embodiment of the invention, the value of B is equal to 2. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. After setting the multiplier factor to a value of B, the mobile unit  106  proceeds to end state  770 . 
     Returning to state  745 , if the change in levels is not greater then S, the mobile unit proceeds along the NO branch to state  755 . In state  755 , the mobile unit  106  determines if the change in levels is greater then a third predetermined change T. The value of the third predetermined change T is determined in the same manner as the changes in levels R and S. In one embodiment, the value of T is 2 decibels. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. 
     If the change in levels is greater then T, the mobile unit  106  proceeds along the YES branch to state  760 . In state  760 , the mobile unit  106  sets the multiplier factor to C. In one embodiment of the invention, the value of C is equal to 4. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. After setting the multiplier factor to a value of C, the mobile unit  106  proceeds to end state  770 . 
     Returning to state  755 , if the change in levels is not greater then T, the mobile unit proceeds along the NO branch to state  765 . In state  765 , the mobile unit  106  sets the multiplier factor to D. In one embodiment of the invention, the value of D is equal to 8. Of course, the value given in this one embodiment is provided as only an example and one of skill in the art may assign a variety of alternative values. After setting the multiplier factor to a value of D, the mobile unit  106  proceeds to end state  770 . Of course, the number of predetermined change stages and multiplier factors may vary depending on how precise of a change is desired. 
     The number of stages of signal level changes, represented by states  735 ,  745 , and  755  may vary depending upon the sensitivity desired. In the above example, three levels of sensitivity are used, resulting in four possible multipliers. If a more precise rate of change of signal strength is desired, the difference between each change in signal levels may be decreased. For example, six stages may be used, with the change in level between each stage being only one decibel. 
     In one example of the present invention using the process of FIG. 7, a mobile unit  106  moves at an average speed through a cell  108 . The mobile unit  106  may not change the scanning rate of each neighboring cell  108  as much as the slow moving mobile unit  106  described above. In this circumstance, the mobile unit  106  begins at start state  700  and proceeds to state  705  to read the current signal level from its servicing base station  104 . Proceeding to state  710 , the mobile unit starts a timer. Proceeding to state  720 , the mobile waits a predetermined period of time. Proceeding to state  725 , the mobile unit  106  receives a subsequent signal from the base station  104  and records the subsequent signal level. Proceeding to state  730 , the mobile unit  106  calculates the change in the two signal levels. In this case, because the mobile unit  106  is moving at an average speed through the cell  104 , it is likely for the signal level to change moderately, say 5 decibels. Proceeding to state  735 , the mobile unit  106  checks if the change in levels is greater than the first predetermined change R, or 6 decibels in this example. In this example, because the change in levels of 5 decibels is less than the first predetermined change of 6 decibels, the mobile unit  106  proceeds along the NO branch of state  735  to state  745 . In state  745 , the mobile unit  106  checks if the change in levels is greater than the second predetermined change S, or 4 decibels in this example. Because the measured signal change of 5 decibels is greater than the second predetermined change S of 4 decibels, the mobile unit  106  proceeds along the YES branch of state  745  to state  750 . In state  750 , the multiplier factor is set to a level B, in this case 2. Using this multiplier factor, the average moving mobile unit  106  will change the scanning rate for neighboring cells  108  from every 30 seconds to every 60 seconds. Applying the multiplier factor to the scanning rate will be described below. 
     Returning to FIG. 5, the mobile unit  106  proceeds to state  530  to adjust the scanning rate of neighboring cells based upon the speed or rate of change of signal strength measurement obtained in state  520 . FIG. 8 illustrates the process according to one technique used to perform the activities indicated by state  530  to adjust the scanning rate of neighboring cells. The process begins at start state  800 . Proceeding to state  805 , the mobile unit  106  records the current initial scanning rate as prescribed by the system provider. 
     Proceeding to state  810 , the mobile unit  106  reads the multiplier factor obtained from state  520  in FIG.  5 . The multiplier factor may be known by the processor  125  or retrieved from the storage medium  130 . 
     Proceeding to state  815 , the mobile unit  106  multiplies the initial rate times the multiplier factor to obtain a new scanning rate. For example, if the initial scanning was one every 30 seconds, and the multiplier factor is 2, the new scanning rate is once every 60 seconds. 
     Proceeding to state  820 , the mobile unit  106  stores the new scanning rate and begins operation under the new scanning rate. The mobile unit  106  then proceeds to end state  825  and returns to FIG.  5 . After the mobile unit  106  adjusts the scanning rate in state  530 , the mobile unit  106  proceeds to end state  540 . 
     In another example of the present invention using the process of FIG. 7, a mobile unit  106  moves quickly through a cell  108 . The quick moving mobile unit  106  may not change the scanning rate of each neighboring cell  108 . In this circumstance, the mobile unit  106  begins at start state  700  and proceeds to state  705  to read the current signal level from its servicing base station  104 . Proceeding to state  710 , the mobile unit starts a timer. Proceeding to state  720 , the mobile waits a predetermined period of time. Proceeding to state  725 , the mobile unit  106  receives a subsequent signal from the base station  104  and records the subsequent signal level. Proceeding to state  730 , the mobile unit  106  calculates the change in the two signal levels. In this case, because the mobile unit  106  is moving at a fast speed through the cell  104 , it is likely for the change in signal level to be high, say 9 decibels. Proceeding to state  735 , the mobile unit  106  checks if the change in levels is greater than the first predetermined change R, or 6 decibels in this example. Because the measured signal change of 9 decibels is greater than the first predetermined change R of 6 decibels, the mobile unit proceeds along the YES branch of state  735  to state  740 . In state  740 , the multiplier factor is set to a level A, in this case 1. Using this multiplier factor, the average moving mobile unit  106  does not change the scanning rate for neighboring cells  108  and it remains at every 30 seconds. 
     The present invention may also be used to increase the overall speed a mobile unit  106  may travel through a cell  108 . Cellular systems are limited to serving mobile units  106  traveling less than a set speed, currently on the order of 200 miles per hour. Generally, this speed is high enough for the average mobile unit  106 , either a car or pedestrian that will never approach the maximum speed. However, there exists the possibility that some mobile units  106 , for example a rider on a high speed bullet train, may exceed this speed. In these cases, the multiplier factor can be less than one, causing an increase in the scanning rate of neighboring cells. This allows the mobile unit  106  to travel at high speeds through the cellular network using rapid handovers. 
     Numerous variations and modifications of the invention will become readily apparent to those skilled in the art. Accordingly, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The detailed embodiment is to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.