Abstract:
A mobile station provides for a more robust reacquisition process with the base station. The mobile station performs an evaluation of the signal conditions from the base station. The mobile station may perform this evaluation at the end of a call, upon release, or immediately after release. If the signal is strong, the mobile station will jump directly back to the synchronization channel. However, if the signal has deteriorated, the mobile station may determine it is more appropriate to reacquire the pilot signal of a base station. The mobile station may also use the signal characteristics to determine whether to perform a full acquisition, a quick acquisition, or a wide acquisition. The mobile station may also use a history to assist in the reacquisition process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional application No. 60/214,111, filed Jul. 26, 2000. 
    
    
     TECHNICAL FIELD 
     This invention relates to wireless communication systems, and more particularly to enhance reacquisition of the network 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. 
     In current CDMA systems, mobile stations acquire the pilot signal of a base station. The pilot signal has identifying spreading code used by the mobile station. The mobile stations use the pilot signal to synchronize themselves with the base station so the mobile stations can recognize any of the other channels necessary. Once a mobile station acquires the pilot signal of a base station, the mobile station may communicate with the base station over the appropriate channels of the wireless communication system. 
     After completing a call, or upon call release, the mobile station currently resynchronizes with the base station using the sync channel. Under strong signal conditions, this poses no problems. However, when the signal conditions deteriorate, the mobile station may be unable to resynchronize with the pilot base station. What is needed is a system that allows resynchronization under strong signal conditions but allows for reacquisition when signal conditions deteriorate. 
     SUMMARY 
     The present invention creates a more robust reacquisition process for the mobile station. The mobile station performs an evaluation of the signal conditions from the base station. The mobile station may perform this evaluation at the end of a call, upon release, or immediately after release. If the signal is strong, the mobile station will jump directly back to the synchronization channel. However, if the signal has deteriorated, the mobile station may determine it is more appropriate to reacquire the pilot signal of a base station. The mobile station may also use the signal characteristics to determine whether to perform a full acquisition, a quick acquisition, or a wide acquisition. The mobile station may also use a history to assist in the reacquisition process. 
     One aspect of the invention is a method of reacquisition in a wireless communication system. The method comprises obtaining a metric indicating of signal conditions and comparing the metric to a first predetermined threshold. If the metric meets or exceeds the first predetermined threshold, the method attempts connection with the synchronization channel. If the metric does not meet the first predetermined threshold, the method performs reacquisition. The method may also compare the metric to a second predetermined threshold and attempt narrow reacquisition if the metric meets or exceed the second predetermined threshold. 
     Another aspect of the invention is a method of enhancing reacquisition in a wireless communication system. The method comprises determining signal strength and connecting to the synchronization channel if the signal strength meets or exceeds a first threshold. The method then performs narrow reacquisition if the signal strength is between the first threshold and a second threshold. If the signal strength is at or below the second threshold, the method performs full reacquisition. The method may further creating a history log of reacquisition performance and determine the first threshold and the second threshold using historical data in the history log. 
     Another aspect of the invention is a mobile station for use in a wireless communication system. The mobile station comprises a signal quality indicator which determines signal quality. A reacquisition logic circuit then determines the appropriate reacquisition procedure based on the signal quality. 
    
    
     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 is a block diagram showing features of a mobile station according to one embodiment of the invention. 
     FIG. 3 is a flowchart illustrating the reacquisition process 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 By 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 its 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 shows a block diagram of the mobile station  106  and the processing that occurs in that mobile station  106 . The processor  200  is driven by a program stored in a memory  205 . Parameters for the mobile station  106  may also be stored in another part of memory shown here as  210 . The memory  210  stores various conditions including information obtained from searches to base stations. 
     The processor  200  executes a process  300  shown in FIG.  3 . The process  300  begins at a START state  305 . Proceeding to state  310 , the mobile station  106  enters the acquisition mode to acquire the pilot signal. The mobile station  106  may acquire the pilot signal using a full acquisition or a narrow acquisition. A full acquisition has the mobile station  106  synchronize with the base station  104  using the pilot signal without any known variables. A narrow acquisition may use known variables of the pilot signal such as offset and timing information. Of course, when first synchronizing with a pilot signal, the mobile station  106  uses the full acquisition. If the mobile station  106  is reacquiring the pilot signal, the narrow acquisition may be used. 
     The mobile station  106  performs acquisition using the search element. The search element may be configured using a number of parameters. These parameters allow the searcher to restrict the range of several factors, including the pseudo-noise (PN) space that it searches, how much tolerance there is for fading, and how much tolerance there is for frequency error. The terms “wide” and “narrow” refer to the ranges of these parameters, and in general refers to the PN space. A narrow acquisition might focus on only PN space around the last known pilot whereas a wide acquisition might search all of PN space for all pilots. Similarly, a narrow acquisition could describe a situation where the acquisition does not tolerate much frequency error or other factor. 
     Proceeding to state  315 , the mobile station  106  synchronizes with the base station  104  using the information in the pilot signal. The mobile station  106  uses the received pilot signal to synchronize its spreading sequences and phase align its local oscillator (LO). After synchronization, the mobile station  106  can freely communicate with the base station  104 . 
     Proceeding to state  320 , the mobile station  106  enters a page mode. In the page mode, the mobile station  106  monitors the base station  104  awaiting communications. The base station  104  may address a mobile station  106  in the page mode to, among other things, inform the mobile station  106  of incoming calls. While in the page mode, the mobile station  106  may also enter an idle mode to conserve power. 
     Proceeding to state  325 , the mobile station  106  activates a call with the base station  104 . The call activation may be the result of an incoming call and a page by the base station  104 , or an outgoing call initiated by the mobile station  106 . The mobile station  106  remains in state  325  during the call process. 
     At the end of the call, the mobile station  106  proceeds to state  330  to determine the signal strength from the base station  104 . The mobile station  106  may determine the signal strength at the end of the call, upon release, or shortly after call release. When the mobile station  106  determines the signal strength may be predetermined and may vary among mobile stations  106 . The mobile station  106  uses a variety of indicators to determine signal power. A known standard measurement of forward link system coverage has been to monitor the signal strength of the pilot signal as a function of the total interference density in the CDMA carrier band, popularly known as Ec/Io. Other indicators include 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  receiver. 
     
       
         
               
               
               
             
           
               
                   
                 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 
               
               
                   
                   
               
             
          
         
       
     
     After determining the signal strength, the mobile station  106  proceeds to state  335 . In state  335 , the mobile station determines a confidence level in the signal. This may be accomplished by having a predefined threshold for a quality metric programmed into the mobile station  106 . If the metric meets or exceeds this threshold, then the signal quality is assumed to be good and the mobile station  106  is confident in the signal. However, if the metric is below the threshold, the signal quality is presumed to be weak and the mobile station  106  indicates a lack of confidence in the signal. 
     If the signal quality is good and the mobile station is confident in the signal, the process  300  proceeds along the YES branch to state  315  where the mobile station resynchronizes with the base station  104 . The mobile station  106  then reenters the paging mode to stay in communication with the wireless system. 
     Returning to state  335 , if the signal quality metric is below the threshold indicating a lack of confidence in the signal, the process  300  proceeds along the NO branch to state  340 . In state  340 , the signal quality metric is compared against a second threshold level to determine if a lower level of confidence can be assumed. The second threshold level is lower than the threshold level used to determine signal confidence in state  335 . If the signal quality exceeds the second threshold, the mobile station  106  proceeds along the YES branch to state  350 . In state  350 , the mobile station  106  is set up to perform a narrow acquisition. In a narrow acquisition, the signal quality is high enough to use some known information such as offset and timing information to simplify the acquisition process. 
     Returning to state  340 , if the signal quality metric falls below the second threshold, the process  300  proceeds along the NO branch to state  345 . In state  345 , the mobile station  106  has a lack of confidence in the prior signal and is therefore set for a full acquisition. The full acquisition procedure has the mobile station  106  completely reacquire a base station  104  without using any previously known information. After the mobile station  106  is set for either full acquisition in state  345  or narrow acquisition in state  350 , the process  300  proceeds to state  310  where the acquisition process is performed. The mobile station  104  remains in the process  300  as long as the mobile station is active in the wireless communication system. 
     The mobile station  106  may also compile a history log recording the results of the resynchronization or reacquisition attempts. The history log may indicate whether the attempts to resynchronize succeeded. Based on this historical data, the mobile station  106  may adjust the values of the threshold values to enhance the success rate of reconnecting with the base station  104 . 
     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.