Abstract:
Techniques for limiting cell reselection in response to a variable channel are disclosed. In one aspect, a measurement of received pilot power from a base station is used as an indication of channel quality. In another aspect, hysteresis is applied to limit cell reselection, wherein the hysteresis is greater in relatively higher quality channel environments and lower in relatively lower channel quality environments. Various other aspects are also presented. These aspects have the benefit of reducing cell reselection, thus increasing time spent in low-power mode, thereby reducing power consumption and increasing standby time.

Description:
RELATED APPLICATIONS 
   This application claims benefit of U.S. Provisional Patent Application No. 60/406,455, filed Aug. 27, 2002 and entitled “WCDMA PHYSICAL LAYER REQUIREMENTS FOR CELL SELECTION AND RESELECTION.” 

   FIELD 
   The present invention relates generally to communications, and more specifically to a novel and improved method and apparatus for limited cell reselection based on pilot power. 
   BACKGROUND 
   Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity. 
   A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards. Non-CDMA systems include the AMPS and GSM systems. 
   When a mobile station first acquires a system, one or more available base stations or cells are identified by the mobile station. Typically a single cell is selected as the serving cell. When the mobile station is not actively communicating for a voice call or data session, it is said to be in idle mode. The mobile station monitors signals from the serving cell to determine if an event, such as an incoming call, would require the mobile station to leave idle mode. As the connnunication channel between the serving cell and the mobile station changes, the mobile station must periodically test the channel to determine if that cell is still suitable. For example, the communication channel will change as the mobile station moves between cells within a system or between one or more neighboring systems. Occasionally a new serving cell will be selected, a process known as cell reselection in W-CDMA, or idle mode handoff in IS-95 or cdmu2000. 
   During idle mode, a mobile station may enter a low-power state, or “sleep” state, to reduce power consumption. A mobile station may discontinue reception during sleep state to allow receive circuitry to be disabled. Periodically, the mobile station must come out of sleep state, or “wake up”, reacquire the serving cell, and monitor signals therefrom to determine if an incoming call is being directed to the mobile station. During this time, a cell reselection process may be performed to determine if cell reselection is necessary. It is desirable for the mobile station to go back to the sleep state as soon as possible if it is to remain in idle mode, in order to conserve power. If a cell reselection is performed, the mobile station will remain awake longer, in order to process the reselection. 
   As channel conditions change, for example, due to movement of the mobile station within the system, or changes in the interference experienced, the serving cell may need to be adapted accordingly. Two or more base stations may, at any given time, have comparable quality metrics. Slight changes in the channel environment may cause the ranking of these base stations to change. However, reselecting a cell with comparable quality to the current cell may yield only marginal improvement, or none at all. Since cell reselection increases the time required before returning to sleep, it is desirable to limit reselection that would yield marginal or no improvement. 
   Furthermore, variations in the relative quality of the channel may cause variations in the desire for cell reselection. In a relatively good channel environment, cell reselection may be less desirable, since the current serving cell provides ample communication quality. On the other hand, in a relatively poor channel environment, gains from cell reselection may improve communication performance. Thus, limiting cell reselection, adapted appropriately for changing channel quality conditions, can increase time spent in a low-power, or sleep, mode, thus reducing idle mode power and increasing standby time. There is therefore a need in the art for limiting cell reselection in response to a variable channel. 
   SUMMARY 
   Embodiments disclosed herein address the need for limiting cell reselection in response to a variable channel. In one aspect, a measurement of received pilot power from a base station is used as an indication of channel quality. In another aspect, hysteresis is applied to limit cell reselection, wherein the hysteresis is greater in relatively higher quality channel environments and lower in relatively lower channel quality environments. Various other aspects are also presented. These aspects have the benefit of reducing cell reselection, thus increasing time spent in low-power mode, thereby reducing power consumption and increasing standby time. 
   The invention provides methods and system elements that implement various aspects, embodiments, and features of the invention, as described in further detail below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
       FIG. 1  is a general block diagram of a wireless communication system capable of supporting a number of users; 
       FIG. 2  depicts an exemplary embodiment of a mobile station equipped for cell selection and reselection; 
       FIG. 3  is a flowchart of an embodiment of a method of cell reselection; 
       FIG. 4  depicts a graph of a hysteresis function of received pilot power for use with a cell reselection method; and 
       FIGS. 5A and 5B  are a flowchart of an embodiment of an alternate method of cell reselection. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a diagram of a wireless communication system  100  that may be designed to support one or more CDMA standards and/or designs (e.g., the W-CDMA standard, the IS-95 standard, the cdma2000 standard, the HDR specification). In an alternative embodiment, system  100  may also deploy any wireless standard or design other than a CDMA system, such as a GSM system. In the exemplary embodiment, system  100  may contain base stations conforming to the W-CDMA standard as well as the GSM standard. 
   For simplicity, system  100  is shown to include three base stations  104  in communication with two mobile stations  106 . The base station and its coverage area are often collectively referred to as a “cell”. In IS-95 systems, a cell may include one or more sectors. In the W-CDMA specification, each sector of a base station and the sector&#39;s coverage area is referred to as a cell. As used herein, the term base station can be used interchangeably with the terms access point or NodeB. The term mobile station can be used interchangeably with the terms user equipment (UE), subscriber unit, subscriber station, access terminal, remote terminal, or other corresponding terms known in the art. The term mobile station encompasses fixed wireless applications. 
   Depending on the CDMA system being implemented, each mobile station  106  may communicate with one (or possibly more) base stations  104  on the forward link at any given moment, and may communicate with one or more base stations on the reverse link depending on whether or not the mobile station is in soft handoff. The forward link (i.e., downlink) refers to transmission from the base station to the mobile station, and the reverse link (i.e., uplink) refers to transmission from the mobile station to the base station. 
   For clarity, the examples used in describing this invention may assume base stations as the originator of signals and mobile stations as receivers and acquirers of those signals, i.e. signals on the forward link. Those skilled in the art will understand that mobile stations as well as base stations can be equipped to transmit data as described herein and the aspects of the present invention apply in those situations as well. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. 
   As described above, cell selection is the process that allows a mobile station to select a suitable cell of a particular system. A mobile station first accesses a system (during power-up, for example), and selects a base station, or serving cell, with which to establish communication, in accordance with certain cell selection conditions. A mobile station may be in idle mode, that is, without an active call or data session in progress. In idle mode, the mobile station can intermittently monitor the serving cell to, among other things, determine if an incoming call is being directed to the mobile station. In a typical wireless communication system, a mobile station will maintain communication with, or “camp” on, a single cell while in idle mode. Periodically, a mobile station can monitor the serving cell to determine if the selection conditions are still being met. Surrounding base stations can be monitored to determine if the mobile station should attempt to reselect to another cell. Cell reselection is the process that allows the mobile station to detect and camp on a more “suitable” cell than the current serving cell. Cell reselection is a term used in the W-CDMA standard, and the process is similar to idle handover in IS-95 or IS-2000 systems. In each of these three exemplary systems, cell selection and reselection is autonomously completed by the mobile station. Thus, a mobile station can roam in and between various systems, updating the current serving cell as appropriate, with a minimal amount of signaling required between the mobile station and the various cells or base stations within the systems. 
   A mobile station commonly maintains various lists of base stations or cells in and around its current location. An active set is the set of cells with which the mobile station is communicating. It is common for the active set to contain only a single cell, the serving cell, while the mobile station is in idle mode, although the active set can contain any number of cells within the scope of the present invention. A list of neighboring cells may be kept in a neighbor list. A set of cells that should be periodically monitored, for example, when the serving cell no longer meets the selection conditions, can be kept in a monitored list. Various techniques for determining active sets, neighbor lists, monitored lists, and the like are well known in the art, and any such technique can be deployed within the scope of the present invention. 
   In  FIG. 1 , a mobile station  106  may be operable with one or more communication systems  100 , conforming to one or more standards, examples of which are given above. A system standard commonly provides requirements for cell selection, reselection and the like. In some cases, procedures are not specified directly, but performance criteria are set forth that must be adhered to by base stations or mobile stations within the system. An exemplary cell selection procedure conforming to the W-CDMA standard, and operable with multiple system formats (including W-CDMA and GSM) is detailed below with respect to  FIGS. 5A and 5B . However, the principles of the present invention are not limited to W-CDMA systems, and can be applied to any communication system. 
   It is typically desirable for a mobile station to minimize power consumption, which yields increased standby or “talk” times for a given battery configuration, or allows a reduced cost and/or smaller battery. For the purpose of discussion, the embodiments herein are described in the context of idle mode operation, a mode in which low power operation is desirable. In the exemplary W-CDMA embodiment, cell reselection (as defined herein) occurs during idle mode. Active mode operation provides for cell handoff according to a different set of procedures. However, this distinction is for clarity of discussion only. Those of skill in the art will readily adapt the principles disclosed herein to any system or mode in which limiting cell reselection is desirable. 
   In the exemplary W-CDMA embodiment, a mobile station can enter “sleep” mode, which can be interchanged with the term idle mode herein. In sleep mode, the mobile station sequences through one or more Discontinuous Reception (DRX) cycles until the mobile station receives or initiates an incoming call or data transmission. During each DRX cycle, the mobile station goes to sleep, discontinuing reception, and disabling as much circuitry as possible to achieve a low power state. For example, radio frequency (RF) and baseband processing components may be shut off during this time. During each DRX cycle, the mobile station “wakes up” to monitor the current serving cell. The mobile station may need to search for and locate the serving cell if the communication channel has changed or the timing between the base station and mobile station has drifted. The serving cell can be measured to determine if the selection conditions are still being met. Periodically the monitored set may need to be searched to determine if reselection is in order. Once the current serving cell is located (and possibly updated through reselection), the mobile station can monitor the serving cell (for example, a paging channel), and return to sleep for the next DRX cycle if active communication is not required. Reducing the portion of a DRX cycle during which a mobile station must remain awake allows for a corresponding savings in power. Since cell reselection takes additional time, during which the mobile station is awake, power can be reduced by limiting cell reselection to only those times when reselection would provide substantial benefits. Embodiments described herein provide for cell reselection limitation that dynamically adapts to changes in the communication channel experienced by the mobile station. 
   In the exemplary embodiment, a system exhibiting features included in the W-CDMA and GSM standards is deployed. Those of skill in the art will recognize that the principles disclosed herein can be extended to a variety of alternate systems. 
     FIG. 2  depicts a portion of an embodiment of a mobile station  106 . Signals are received at antenna  210  and delivered to receiver  220 . Receiver  220  performs processing according to one or more wireless system standards, such as the cellular standards listed above. Receiver  220  performs various processing such as Radio Frequency (RF) to baseband conversion, amplification, analog to digital conversion, filtering, and the like. Various techniques for receiving are known in the art. Note that some or all of the components of receiver  220  can be disabled or put in a low-power state to conserve power while sleeping in idle mode. 
   The mobile station  106  can communicate with a base station  104  by tuning receiver  220  according to the parameters associated with the base station. Receiver  220  may be directed to periodically tune to an alternate frequency to measure the channel quality of a cell on an alternate frequency, including those on alternate systems. Receiver  220  may be used to measure channel quality of the current serving cell, as well neighbor base stations, although a separate signal strength estimator  280  is shown in  FIG. 2  for clarity of discussion (detailed below). 
   Signals from receiver  220  are demodulated in demodulator  230  according to one or more communication standards. In the exemplary embodiment, a demodulator capable of demodulating W-CDMA and GSM signals is deployed. Additional standards, such as IS-95 or cdma2000, could also be supported in an alternate embodiment. Demodulator  230  may perform RAKE receiving, equalization, combining, deinterleaving, decoding, and various other functions as required by the format of the received signals. Various demodulation techniques are known in the art. 
   Message decoder  240  receives demodulated data and extracts signals or messages directed to the mobile station  106  by the system  100  through one or more base stations  104 . Message decoder  240  decodes various messages used in setting up, maintaining and tearing down a call (including voice or data sessions) on a system. Messages may include neighbor cell information. Messages may include various parameters for use in cell selection and reselection, detailed further below. Various other message types are known in the art and may be specified in the various communication standards being supported. The messages are delivered to processor  260  for use in subsequent processing. Some or all of the functions of message decoder  240  may be carried out in processor  260 , although a discrete block is shown for clarity of discussion. 
   Signals from receiver  220  are also directed to searcher  250 . Searcher  250  can be used to locate various cells available to the mobile station, including the serving cell and neighboring cells. Searcher  250  can be directed by processor  260  to search for cells and to indicate channel quality metrics associated with those cells to processor  260 . Search results can be used to direct demodulator  230  to demodulate various signals, as well as for use in cell selection or reselection. Searcher  250  may be deployed to support searching cells of any type of system supported by mobile station  106 . 
   Signal strength estimator  280  is connected to receiver  220 , and used for making various power level estimates for use in the cell selection or reselection process, as well as for use in various other processing used in communication, such as demodulation. Signal strength estimator  280  is shown as a discrete block for clarity of discussion only. It is common for such a block to be incorporated within another block, such as receiver  220  or demodulator  230 . Various types of signal strength estimates can be made, depending on which signal or which system type is being estimated. In the exemplary embodiment, various pilot signals from one or base stations are used for signal strength estimation, examples of which are detailed below. In general, any type of channel quality metric estimation block can be deployed in place of signal strength estimator  280  and is within the scope of the present invention. The channel quality metrics are delivered to processor  260  for use in cell selection or reselection, as described herein. 
   Data received may be transmitted to processor  260  for use in voice or data communications, as well as to various other components. A mobile station  106  will generally be equipped with modulation and transmission components for transmitting data to one or more base stations. Additional components for supporting voice communications or data applications are well known in the art and are not shown. 
   Processor  260  may be a general purpose microprocessor, a digital signal processor (DSP), or a special purpose processor. Processor  260  may perform some or all of the functions of receiver  220 , demodulator  230 , searcher  250 , signal strength estimator  280 , message decoder  240 , as well as any other processing required by the mobile station. Processor  260  may be connected with special purpose hardware to assist in these tasks (details not shown). Data or voice applications may be external to mobile station  106 , such as an externally connected laptop computer, may run on an additional processor within mobile station  106  (not shown), or may run on processor  260  itself. Processor  260  is connected with memory  270 , which can be used for storing data as well as instructions for performing the various procedures and methods described herein. Those of skill in the art will recognize that memory  270  may be comprised of one or more memory components of various types, that may be embedded in whole or in part within processor  260 . 
     FIG. 3  depicts an embodiment of a method of limiting cell reselection based on received pilot power. In the exemplary embodiment, the method of  FIG. 3  is performed once during each DRX cycle. The process begins in step  310 , after the period during which reception was disabled. The received energy of the current active set, i.e., the active set used during the previous DRX cycle, is measured. Proceed to decision block  320 . 
   In decision block  320 , a determination is made whether or not searching of the monitored set is required. Under certain conditions, if the measured energy of the active set is sufficient, there is no need to determine the quality of alternate base stations indicated in the monitored set, and the process can stop. No cell reselection will be required, and the current active set remains the active set. If searching of the monitored set is indicated, proceed to step  330 . Steps  330 – 370  can be used to make a cell reselection decision, limited to avoid excessive reselection, in accordance with the principles of the present invention. Various techniques for determining the need for additional searching are known in the art, an example of which is detailed below with respect to  FIGS. 5A and 5B . Any technique, known or developed in the future, can be deployed with the cell reselection methods described herein. 
   In step  330 , searching is performed on all scheduled cells in the monitored list, and the energy is measured for each. The definition of the monitored set may vary, and, as such, a subset of the available cells may be searched in this step. For example, a subset of cells on the current frequency may be searched, or on multiple frequencies, or even over multiple systems (neighboring GSM cells, for example). Various methods for determining the desired set of monitored cells for searching in step  330  are known, examples of which are detailed below with respect to  FIGS. 5A and 5B , and any method can be deployed within the scope of the present invention. In the exemplary embodiment, the energy of the pilot of each cell in the monitored list is measured. Once this list of candidate energies is measured, proceed to step  340 . 
   In step  340 , a hysteresis value is applied to the measured energy of the current active set. The hysteresis value may be comprised of one or more components. In the exemplary embodiment, one of the components of the hysteresis value is determined in response to the measured energy in one or more pilots of the active set. Another component, for example, may be signaled by the system to the mobile station. Utilizing at least one component that is determined in response to the current channel conditions, as measured by the mobile station, allows cell reselection to be dynamically adapted in response to the quality of the current active set as seen by the mobile station. Thus, cell reselection can be made more likely when conditions are relatively poor, thus increasing the likelihood of locating a more suitable cell. When conditions are relatively good, reselection can be made less likely, thus avoiding cell reselections that may provide little or no improvement. 
   The hysteresis value can be any function of the received signal strength of the currently selected cell. In the exemplary embodiment, the hysteresis value, H s , is determined according to equation 1: 
                   H   s     =     {           2   ⁢           ⁢   dB         when             Ec     p   ,   s         I   o       &lt;       -   12     ⁢           ⁢   dB                   -     (     10   +       Ec     p   ,   s         I   o         )       +     2   ⁢           ⁢   dB           when             -   12     ⁢           ⁢   dB     &lt;       Ec     p   ,   s         I   o       &lt;       -   10     ⁢           ⁢   dB                 4   ⁢           ⁢   dB         when             Ec     p   ,   s         I   o       &gt;       -   10     ⁢           ⁢   dB                       (   1   )               
where Ec p,s /I o  is the chip energy over interference (E c /I o ) of the Common Pilot Channel (CPICH), measured by the searcher (in dB). H s  is plotted in  FIG. 4 . Those of skill in the art will recognize that equation 1 is exemplary only, and that any function of received signal strength can be used to determine hysteresis within the scope of the present invention.
 
   Once the hysteresis value is added to the energy of the currently selected cell in step  340 , proceed to step  350 . In step  350 , the cell energies of the monitored set and the current active set are ranked according to measured energy (including the hysteresis-modified energy of the active cell). Proceed to decision block  360 . In decision block  360 , if the current active cell&#39;s energy, as modified, is the highest ranked energy, no cell reselection is required and the process can stop. If not, proceed to step  370  to perform cell reselection to the cell from which the highest energy has been received. After cell reselection is completed, the process terminates. 
   In the exemplary embodiment, the method embodiment described with respect to  FIG. 3  is performed once during each DRX cycle. The mobile station “wakes up” from the low power state during which receiving is deactivated, determines whether cell reselection is in order, in accordance with the method described, performs any additional processing required (such as monitoring the paging channel to determine if an incoming call is being directed to the mobile station, for example), and then reenters the low power state, or “sleeps”, for another DRX cycle, if appropriate. The reduction in cell reselections, based on channel information available to the mobile station, allows the mobile station to avoid remaining in the higher-power “awake” state any longer than necessary, thus reducing power consumption and increasing battery life or standby time. At the same time, the mobile station maintains communication with a suitable base station during idle mode, as cell reselection is adapted in response to changing channel conditions experienced by the mobile station. 
     FIGS. 5A and 5B  depict an exemplary method of cell selection suitable for mobile stations operable with both W-CDMA and GSM systems, among others. Cell selection and reselection is based on computation of the S criteria, S qual  and S rxlev , given in equations 2 and 3: 
                   S   qual     =         Ec   p       I   o       -     Q     qual   ⁢           ⁢   min                 (   2   )                 S   rxlev   =Ec   p   −Q   rxlev min −max( UE   —   TXPWR _MAX_RACH− P _MAX,0)  (3) 
   where: 
   
       
       
         
           Ec p /I o  is the measured CPICH Ec/Io by the searcher (in dB); 
           Q qual min  is the minimum required quality level of the cell; 
           Ec p  is the measured CPICH Received Signal Code Power (RSCP) (in dBm); 
           Q rxlev min  is the minimum required received level in the cell (in dBm); 
           UE_TXPWR_MAX_RACH is the max power the UE, or mobile station, may use when transmitting on the Random Access Channel (RACH); and 
           P_MAX is the maximum transmit power of the UE, or mobile station. 
         
       
     
  
   The W-CDMA standard mandates that the mobile station perform cell selection on the serving cell by computing the S criteria every DRX cycle. The mobile station is expected to monitor the results to ensure that the conditions for cell selection are met. If the cell selection criteria S do not fulfill cell selection requirements for a preset number, N, of consecutive DRX cycles, the mobile station should initiate measurements on the neighbor cells (i.e. the monitored list) and attempt to reselect to another cell, regardless of other rules limiting the mobile station from measuring other cells. The exemplary method embodiment depicted in  FIGS. 5A and 5B  incorporate the W-CDMA standard provisions as well as various aspects of the present invention. The method can be performed during each DRX cycle subsequent to the mobile station coming out of the low power state during which reception was disabled. 
   The process begins in step  502 , where a search is performed to locate and measure the energy of the pilot of the active set. Proceed to step  504  to compute the S criteria, given in equations 2 and 3 above. Then proceed to decision block  506 , to test if cell selection has failed for the current active set. Cell selection fails when the S criteria, S qual  and S rxlev , are less than zero. In this example, cell selection must fail for N consecutive DRX cycles to initiate a cell reselection. If cell selection has not failed, or has not failed for the requisite N cycles, proceed to decision block  508 . If cell selection has failed for N consecutive cycles, proceed to step  510  to begin the reselection process. 
   In decision block  508 , even if cell selection has not failed for N consecutive DRX cycles, as tested in decision block  506 , if other criteria are met, the cell reselection process may still be initiated. Any measurement criteria can be deployed for initializing a cell reselection process, as will be recognized by one of skill in the art. 
   In the exemplary embodiment, three separate types of monitored cells may be scheduled for searching to determine if cell reselection should proceed (described below). If the serving cell&#39;s S qual , S qual,s , falls below the parameter S intrasearch , then intra-frequency measurements of monitored cells will be scheduled. S intrasearch  may be specified over-the-air by the system. Note that in the exemplary embodiment, a floor is set on S intrasearch  by setting it to the maximum of any signaled S intrasearch  and S min , where S min  is computed using the equation for S qual  with CPICH E c /N o  set to −8 dB. If S qual,s  falls below the parameter S intersearch , then inter-frequency measurements of monitored cells will be scheduled. S intersearch  can be signaled over-the-air by the system. If S qual,s  falls below the parameter S inter-RAT , then alternative Radio Access Technologies (RATs), or inter-RAT measurements of monitored cells will be scheduled. One example inter-RAT search may be to search for neighboring GSM cells, although any alternative system could be scheduled for search within the scope of the present invention. Note that the monitored set need not contain cells of any particular type. The monitored set may contain any combination or subsets of the three types. It follows that the scheduled set of cells may be a subset of the monitored set, regardless of the satisfaction of the three tests just described. If any of the above criteria are satisfied, then proceed to step  510  to begin the process of determining whether cell reselection will take place, as described below. If none of the above criteria are satisfied, then proceed to step  554 , where the Radio Resource Controller (RRC) is signaled that no cell reselection is necessary. The process can then stop. 
   In step  510 , all the scheduled intra-frequency, inter-frequency, and inter-RAT monitored cells are searched. Which cells to search can be determined using the criteria just described with respect to decision block  508 . Note further that, if no parameter S intrasearch  has been specified by the system, then all intra-frequency cells in the monitored list will be searched. If no parameter S intersearch  has been specified by the system, then all inter-frequency cells in the monitored list will be searched. If no parameter Sinter-RAT has been specified by the system, then all inter-RAT cells in the monitored list will be searched. Proceed to step  512 . 
   In step  512 , the quantity Q meas,n  is computed for the n neighbor cells scheduled from the monitored list. Q meas,n  is an indicator of the received signal power from each neighbor, and in the exemplary embodiment, it may be computed in two different ways, as described further below. Those of skill in the art will recognize that any power measurement indicator can be used throughout this embodiment, within the scope of the present invention. During this step  512 , Q meas,n  is an absolute power measurement, measured in dBm, which is useful when comparing results between the serving cell, intra-frequency neighbor cells, inter-frequency neighbor cells, as well as inter-RAT neighbor cells. It is computed as the Received Signal Code Power (RSCP) of the CPICH for W-CDMA cells, and the Received Signal Strength Indicator (RSSI) for GSM cells. RSCP is computed as E c /I o +I o , which normalizes the results for varying interference levels across multiple frequencies. Once Q meas,n  is computed for all the searched neighbors, proceed to step  514 . Q meas,n  can alternately be computed as CPICH E c /I o , measured in dB, which will be detailed below with respect to step  544 . 
   In step  514 , for each searched cell, compute a ranking value R n , defined as Q meas,n -Qoffset n . Qoffset n  is specified over-the-air by the system, and is set to Qoffset 1   n  when CPICH RSCP or RSSI (for GSM cells) is used to compute Q meas,n , as in step  512 . Qoffset n  is set to Qoffset 2   n  when CPICH E c I o  is used to compute Q meas,n , as in step  544 , detailed below. Qoffset is specified in dBm for Qoffset 1  and in dB for Qoffset 2 . While use of Qoffset may assist in reducing undesirable cell reselection, particular use of the parameter is not specified by the standard, thus it may not be deployed effectively for limiting cell reselection. Furthermore, it is not responsive to the current state of the communication channel between the serving cell and the mobile station, so it cannot adapt to provide varying levels of cell reselection sensitivity. Proceed to decision block  516 . 
   Decision block  516  provides a branch for the process depending on how step  514  was entered. If Q meas,n  was computed using RSCP (or RSSI), as specified in step  512 , then proceed to step  518 . If not, i.e., step  514  was reached through step  544 , proceed to step  530 . 
   In step  518 , set n=0. In the loop formed by steps  520 – 526 , n will be used as an index to test the quality of the searched neighbor cells. Proceed to decision block  520 , where the S criteria, defined above in equations 2 and 3 are calculated for the n th  neighbor. If S qual  or S rxlev  for the n th  neighbor are less than zero, then that neighbor does not meet cell selection criteria. Note that S qual,n  is not computed for GSM cells, so the test for S rxlev  is used exclusively. Proceed to step  522  and remove the cell from the list of searched neighbor cells. In addition, a timer value Tsr,n is cleared, which is an indicator of how long a cell has met the selection criteria, the use of which is detailed below. If the cell selection criteria is met for the neighbor, proceed to step  523  and increment Tsr,n. 
   From either step  522  or  523 , proceed to decision block  524  to determine if there are any additional neighbors in the list. If so, proceed to step  526 , increment n, and return to decision block  520  to test the next neighbor with the cell selection criteria. If not, proceed to step  530 . 
   In step  530 , compute the ranking value, R s , for the serving cell. R s  is computed as Q meas,s +Qhyst, where Q meas,s  is computed for the serving cell using the energy metric used to measure the neighbor cells, i.e., as defined for step  514  or step  544 . Qhyst is set to Qhyst 1  when the measurement quantity used for ranking is CPICH RSCP, where Qhyst 1  can be signaled over-the-air by the system. Qhyst is set to the maximum of Qhyst 2  and H s  when the measurement quantity is CPICH E c /I o . Qhyst 2  can be signaled over-the-air by the system. Hs can be computed according to any hysteresis equation, as will be recognized by those skilled in the art. In the exemplary embodiment H s  is computed according to equation 1. Thus, in this embodiment, the system can introduce hysteresis by supplying Qhyst 2  over-the-air. However, a floor on the hysteresis value is introduced by H s . As described above with respect to  FIG. 3 , using H s  allows limiting cell reselection in response to changing channel conditions, making reselection less likely when the channel is relatively good, and more likely when the channel is relatively poor. Once the ranking value R s  is computed for the serving cell, proceed to step  532 . 
   In step  532 , rank the serving cell and the measured neighbor cells according to their ranking values, R s  and R n . Proceed to step  534  to select the best cell (identified below with subscript bc) as the cell with the highest rank. Proceed to decision block  536 . 
   In decision block  536 , if the best cell is the current serving cell, then proceed to step  554 , and indicate that no cell reselection is needed, as described above. If the best cell is a cell other than the current serving cell, additional steps are taken to determine whether a cell reselection should take place. Proceed to decision block  540  to begin that portion of the process. 
   In decision block  540 , if the best cell is a GSM cell, proceed to decision block  546 . If not, proceed to decision block  542 . 
   In decision block  542 , if the prior pass through step  514  used CPICH RSCP for Q meas,n , i.e., step  514  was entered from step  512 , and there are multiple frequencies in the ranked monitored list, then the list will be re-ranked using CPICH E c /I o . In this case, the process continues to step  544 . If step  544  has already been entered during the process for the current DRX cycle, or there is only one frequency in the ranked list corresponding to the monitored set, then the process continues to decision block  546 . Recall that RSCP=E c /I o +I o . I o  may be different across different frequencies, so using RSCP normalizes this difference when an absolute power level is desired. If only one frequency is used in the ranked list, then a second pass using E c /I o  will not alter the ranking generated with RSCP. 
   In step  544 , as mentioned preciously, CPICH RSCP will be used for Q meas,n . Proceed to step  514  and re-compute the ranking values R n  for the monitored list (as modified for any cells which failed the cell selection criteria in the loop formed by steps  518 – 524 , described above. Then the process proceeds to decision block  516 , where the path to step  530  will be taken since RSCP is no longer the measurement value used for the ranked list. The monitored cells and the serving cell (modified with hysteresis in step  530 ) will be re-ranked in step  532 , and the best cell selection will be made again in step  534 . In some cases, a different best cell will prevail, and decision block  536  and possibly decision block  540  will be revisited and performed as described above. If the current serving cell becomes the new best cell, then the process proceeds to step  554 , described above, and cell reselection will not be needed. If the same best cell is still selected, or another best cell, different from the current serving cell, is selected, then the process will proceed to step  546 . The process may branch through decision block  540  if the best cell is a GSM cell, or through decision block  542  if the best cell is not a GSM cell. The process will not branch from decision block  542  to step  544  during this second pass, as described above. 
   As described above, when the best cell is different than the serving cell, decision block  546  will be reached regardless of whether a second pass was made, or whether or not the best cell is a GSM cell. In decision block  546 , Tsr,bc must be greater than the parameter Treselection, which indicates the minimum time a cell must have met cell selection criteria before it can be selected (via the reselection process) as the new current serving cell. Treselection can be transmitted over-the-air from the system. If the minimum time indicated by Treselection has been met by the best cell, proceed to step  552 . In step  552 , an indication is made to the RRC that cell reselection should be made using the best cell as the new serving cell. Then the process can stop. 
   If the best cell does not meet the minimum time required for satisfaction of the cell selection criteria, proceed to step  548 . In step  548 , the next best cell, according to the ranking, is selected. If the next best cell is the current serving cell, no cell reselection is needed, and the process proceeds to step  554 , described above, and then terminates. If the next best cell is not the current serving cell, proceed back to decision block  546  to test the minimum time requirement for the next best cell. The loop formed by steps  546 – 550  will continue until a cell is identified for reselection (and step  552  is reached), or the current cell is selected and no cell reselection is perfomed (i.e. step  554  is reached). 
   If the best cell does not meet the minimum time required for satisfaction of the cell selection criteria, proceed to step  548 . In step  548 , the next best cell, according to the ranking, is selected. If The next best cell is the current serving cell, no cell reselection is needed, and the process proceeds to step  554 , described above, and then terminates. If the next best cell is not the current serving cell, proceed back to decision block  546  to test the minimum time requirement far the next best cell. The loop formed by steps  546 – 550  will continue until a cell is identified for reselection (and step  552  is reached), or the current cell is selected and no cell reselection is performed (i.e. step  554  is reached). 
   The embodiment depicted in  FIGS. 5A and 5B  are just one example of a method of limiting cell reselection utilizing aspects of the invention described herein. This particular example demonstrates the applicability of these aspects to a mobile station equipped to interoperate with multiple systems, i.e. W-CDMA and GSM systems. Those of skill in the art will readily expand the principles taught with respect to  FIGS. 5A and 5B  to any number of combinations of systems, parameters, and hysteresis equations, including those known in the art as well as those conceived in the future. Those of skill in the art will recognize that the embodiment of  FIGS. 5A and 5B  is one specific example of the more general method embodiment described in  FIG. 3 . Various modifications can be made to either embodiment within the scope of the present invention and will be readily adaptable by those of skill in the art. 
   It should be noted that in all the embodiments described above, method steps can be interchanged without departing from the scope of the invention. The descriptions disclosed herein have in many cases used signals, parameters, and procedures associated with the W-CDMA and GSM standards, but the scope of the present invention is not limited as such. Those of skill in the art will readily apply the principles herein to various other communication systems. Furthermore, channel quality metrics can be used other than those described, and can be measured using pilot signals from various neighboring cells, or any other signal for which a metric indicating the quality of the channels between those various cells and the mobile station. These and other modifications will be apparent to those of ordinary skill in the art. 
   Those of skill in the art will understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
   Those of skill will further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
   The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
   The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
   The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.