Abstract:
A mobile station enhances the ability to complete a handoff by maintaining channels on the secondary base stations at low levels. The mobile station searches all active base stations to determine if a lock on the dedicated channel can be maintained. If a lock can be maintained, the mobile station provides feedback to the base station to adjust the power of the secondary channels. The power is adjusted so that the secondary channels are maintained at a low power level, typically a barely receivable level.

Description:
TECHNICAL FIELD 
     This invention relates to wireless communication systems, and more particularly to signal power control within wireless communication systems. 
     BACKGROUND 
     Cellular telephones may operate under a variety of standards including the code division multiple access (CDMA) cellular telephone communication system as described in TIA/EIA, IS-95, Mobile station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, published July 1993. CDMA is a technique for spread-spectrum multiple-access digital communications that creates channels through the use of unique code sequences. In CDMA systems, signals can be and are received in the presence of high levels of interference. The practical limit of signal reception depends on the channel conditions, but CDMA reception in the system described in the aforementioned IS-95 Standard can take place in the presence of interference that is 18 dB larger than the signal for a static channel. Typically, the system operates with a lower level of interference and dynamic channel conditions. 
     A mobile station using the CDMA standard constantly searches a Pilot Channel of neighboring base stations for a pilot that is sufficiently stronger than a threshold value. As the mobile station moves from the region covered by one base station to another, the mobile station promotes certain pilots from the Neighbor Set to the Candidate Set, and notifies the base station or base stations of the promotion from the Neighbor Set to the Candidate Set via a Pilot Strength Measurement Message. The base station determines an Active Set according to the Pilot Strength Measurement Message, and notifies the mobile station of the new Active Set via a Handoff Direction Message. When the mobile station commences communication with a new base station in the new Active Set before terminating communications with the old base station, a “soft handoff” has occurred. 
     One problem with a “soft handoff” is that RF resources are used inefficiently while mobile stations are in soft handoff. To solve this problem, some providers have proposed using fast cell selection mechanisms. However, such mechanisms have several problems, including an imbalance in the forward and reverse link, a selection feedback delay, and reduced reliability. What is desired is a system that includes the advantages of both the soft handoff and the fast cell selection mechanism while reducing the negative aspects of each system. 
     SUMMARY 
     The present invention enhances the ability of a mobile station to complete handoff by maintaining channels on the secondary base stations at low levels. The mobile station searches all active base stations to determine if a lock on the dedicated channel can be maintained. If a lock can be maintained, the mobile station provides feedback to the base station to adjust the power of the secondary channels. The power is adjusted so that the secondary channels are maintained at a low power level, typically a barely receivable level. 
     One aspect of the invention is a method of controlling power in a wireless communication system comprising detecting a signal and instructing the transmitter to reduce the signal power if the signal power exceeds a predetermined threshold. The method also comprises instructing the transmitter to increase the signal power if the signal power does not exceed the predetermined threshold. The threshold may be, among other things, a receive signal strength indicator (RSSI), a bit error rate, or a finger correlation of a null or preamble signal. 
     Another aspect of the invention is a method of controlling the power level of a channel comprising selecting a power measurement metric, comparing the metric to a threshold value, and adjusting the power level of the channel based on the comparison. The channel may include usable data or may include null data. 
     Another aspect of the invention is a mobile station for use in a wireless communication system. The mobile station comprises a signal detector and a power adjustment circuit. The power adjustment circuit provides instructions on a power setting based on the detected signal. The power setting instructions are provided via a closed-loop feedback channel. 
    
    
     DESCRIPTION OF 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 the components of an exemplary wireless communication system used by one embodiment of the present invention. 
     FIG. 2 illustrates a mobile station in communication with a primary sector and secondary sectors according to one embodiment of the present invention. 
     FIG. 3 is a flowchart illustrating the power adjustment process used by the mobile station according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates components of an exemplary wireless communication system. A mobile switching center  102  communicates with base stations  104   a - 104   k  (only one connection shown). The base stations  104   a - 104   k  (generally  104 ) broadcasts data to and receives data from mobile stations  106  within cells  108   a - 108   k  (generally  108 ). The cell  108  is a geographic region, roughly hexagonal, having a radius of up to 35 kilometers or possibly more. 
     A mobile station  106  is capable of receiving data from and transmitting data to a base station  104 . In one embodiment, the mobile station  106  receives and transmits data according to the Code Division Multiple Access (CDMA) standard. CDMA 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 CDMA standard, additional cells  108   a ,  108   c ,  108   d , and  108   e  adjacent to the cell  108   b  permit mobile stations  106  to cross cell boundaries without interrupting communications. This is so because base stations  104   a ,  104   c ,  104   d , and  104   e  in adjacent cells assume the task of transmitting and receiving data for the mobile stations  106 . The mobile switching center  102  coordinates all communication to and from mobile stations  106  in a multi-cell region. Thus, the mobile switching center  102  may communicate with many base stations  104 . 
     Mobile stations  106  may move about freely within the cell  108  while communicating either voice or data. Mobile stations  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 station  106 . 
     One example of such a mobile station  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 scans certain frequencies (frequencies known to be used by CDMA) to synchronize 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 CDMA network. 
     When detecting a call, the cellular telephone 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 cellular telephone receives, stores and examines paging message data, and determines whether the data contains a mobile station identifier matching an identifier of the cellular telephone. If a match is detected, the cellular telephone establishes a call with the mobile switching center  102  via the base station  104 . If no match is detected, the cellular telephone enters an idle state for a predetermined period of time, then exits the idle state to receive another transmission of paging message data. 
     FIG. 2 illustrates a mobile station in communication with a primary sector and secondary sectors according to one embodiment of the present invention. For purposes of illustration, FIG. 2 only shows a portion of the wireless communication system of FIG. 1 comprising an active set  200  of base stations  104   b ,  104   d , and  104   e . The active set  200  includes the information necessary for the mobile station  106  to maintain a call. Each of the base stations  104   b ,  104   d , and  104   e  in the active set  200  transmits to the mobile station  106  on a forward dedicated channel  210 ,  215 , and  225 . The mobile station  106  communicates back to the base stations  104   b ,  104   d , and  104   e  over closed-loop feedback channels  205 ,  220 , and  230 . The forward channels  210 ,  215 , and  225  and the closed-loop feedback channels  205 ,  220 , and  230  may be any channel used in the wireless cellular system. These include, but are not limited to, the TCH, the DCCH, the SCH, and the SCCH as per IS2000-A. 
     At any time, one of the base stations  104   b ,  104   d , and  104   e  is selected as the active or primary station and the remaining base stations are the secondary stations. The primary station is used for demodulation and the secondary stations in the active set  200  are maintained in case the mobile station  106  needs to change the primary station. The primary station transmits the dedicated channel  210 ,  215 , or  225  at a power level designed to enable demodulation with the mobile station  106 . The power level may be full power or may be power controlled. The secondary stations transmissions over the dedicated channel  210 ,  215 , or  225  are power controlled by the mobile station  106  via the closed-loop feedback channels  205 ,  220 , and  230 . The secondary stations may transmit the same data that is being sent by the primary stations, or may transmit null or preamble data. If the mobile station  106  desires to change primary stations, the mobile station  106  may send a coded message on the closed-loop feedback channels  205 ,  220 , and  230 . The mobile station  106  may simply power down the current primary station and power up the new primary station. 
     FIG. 3 is a flowchart illustrating a cycle of the power adjustment process  300  used by the mobile station  106 . The process  300  begins at a START state  305 . Proceeding to state  310 , the mobile station  106  searches each of the active cells for a signal. The mobile station  106  searches for the desired channel, which may be the TCH (traffic channel), the DCCH (dedicated control channel), the SCH (synchronization channel), and the SCCH (specific cell channel). The signal may be transmitting data, or if a secondary station, may transmit null or preamble information instead of data. 
     Proceeding to state  315 , the mobile station  106  determines if the signal is detected. If there is no signal detected, then the base station  104  that is supplying the signal may be transmitting at a power level that is too low for the mobile station  106  to receive. In this case, the mobile station  106  proceeds along the NO branch to state  320 . In state  320 , the mobile station instructs the appropriate base station  104  to increase power. This instruction may be transmitted via the closed-loop feedback channels  205 ,  220 , and  230 . After receiving the instruction to increase power, the base station retransmits the signal at a higher power level. The mobile station  106  then returns to state  310  to again search for the signal. The mobile station  106  remains in this loop until the power level is high enough to detect. 
     Returning to state  315 , if the signal is detected, the mobile station  106  proceeds along the YES branch to state  325 . In state  325 , the mobile station  106  determines if the signal power is above a specific threshold. The mobile station  106  uses a variety of indicators to determine signal power. Among these indicators is a bit error rate, a symbol error rate, a frame error rate, a receiver quality indicator (RX Quality), a receive signal strength indicator (RSSI), also known as RX Level, determining the period of time of a finger lock is maintained, or having the finger correlation of a null/preamble signal above a certain threshold. The mobile station  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 station  106  with an expected measurement accuracy. The mobile station  106  uses the RX Quality to determine the overall potential for error. 
     Another measurement that may be used by the mobile station  106  is RSSI. RSSI provides a known value based upon the measured strength of the signal at the mobile station  106 . A stronger signal at the mobile station  106  indicates less likelihood for error. Table 2 provides sample values for RSSI based upon the signal strength at the mobile station  106 . Each specific value for RSSI correlates to the strength of the signal (in measured decibels (dBm)) at the mobile station  106  reciever. 
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 RSSI 
                 Level at Receiver (dBm) 
               
               
                   
               
             
             
               
                 0 
                 Less than −110 
               
               
                 1 
                 −110 to −109 
               
               
                 2 
                 −109 to −108 
               
               
                 . . . 
                 . . . 
               
               
                 . . . 
                 . . . 
               
               
                 . . . 
                 . . . 
               
               
                 62  
                  −49 to −48 
               
               
                 63  
                 above −48 
               
               
                   
               
             
          
         
       
     
     The precise threshold value may be preset in the mobile station  106  or may be adjusted based on historical data. If the signal is above the selected threshold (i.e. RSSI bit error rate), than the mobile station proceeds along the YES state  330 . In state  330 , the mobile station instructs the appropriate base station  104  to decrease power. This instruction may be transmitted via the closed-loop feedback channels  205 ,  220 , and  230 . After receiving the instruction to decrease power, the base station retransmits the signal at a lower power level. The mobile station  106  then returns to state  310  to again search for the signal. 
     Returning to state  325 , if the signal is not above the selected threshold, the mobile station  106  proceeds along the NO branch to state  335 . In state  335 , the mobile station  106  determines if the signal power is below a specific threshold. The mobile station  106  may use the same threshold for the power adjustment as was used by the mobile station  106  in state  325 , or may use a separate threshold for determining low power. If the signal is below the threshold, the mobile station  106  proceeds along the YES branch to state  320 . In state  320 , the mobile station instructs the appropriate base station  104  to increase power. This instruction may be transmitted via the closed-loop feedback channels  205 ,  220 , and  230 . After receiving the instruction to increase power, the base station retransmits the signal at a higher power level. The mobile station  106  then returns to state  310  to again search for the signal. 
     Returning to state  335 , if the signal power is not below the threshold, the mobile station proceeds along the NO branch to state  340 . In state  340 , the mobile station has determined the power is set at a barely receivable level and maintains this power level. The mobile station  106  may send a signal to the base station  104  indicating the desire to maintain the power level, or may not send any signal to the base station  104 . In absence of instructions from the mobile station  106 , the base station  104  may maintain the last power level. Of course, depending on the threshold selected, the signal may be maintained at an unusable level as opposed to the barely receivable level. The precise level to maintain the power may be set by selecting the appropriate power metric and threshold. After indicating the power level is to be maintained, the mobile station  106  proceeds to an END state  345 . 
     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.