Patent Publication Number: US-9432887-B2

Title: Apparatus and method for high priority search on RRC state change

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application is a continuation application of U.S. patent application Ser. No. 13/468,854, filed on May 10, 2012, and is related to U.S. patent application Ser. No. 13/466,979, filed on May 8, 2012, now abandoned, and U.S. patent application Ser. No. 13/466,993, filed on May 8, 2012, all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present application relates generally to wireless devices and, more specifically, to performing neighboring cell searches on state changes. 
     BACKGROUND OF THE INVENTION 
     Conventional wireless networks support priority-based reselection (PBR) operations, in which a user equipment (or mobile device) accessing a first cell re-selects to a second cell. A priority-based reselection operation uses priority information in its algorithm to choose the second cell. The priority information may be common priority information that is common to multiple user equipment or may be dedicated priority information that is device-specific to a particular user equipment. 
     In a priority-based reselection algorithm, an initial ranking of neighboring cells may be done based on assigned priorities rather than on relative or absolute radio measurement. It is possible to assign device-specific priority information, which persists across multiple cells, including cells of different radio access technologies (RATs). Reselection algorithms are typically based on measured properties of a signal associated with a particular cell. For example, for a GSM cell, measurements of the signal transmitted on the broadcast control channel (BCCH) frequency for that cell may be used for a PBR algorithm. 
     In earlier reselection algorithms, a wireless device often made such measurements for multiple (possibly all) candidate cells. This often required measurements and comparisons of values for cells using different radio access technologies. As the numbers and types of cells increase (e.g., introduction of new radio access technologies), such a re-selection algorithm becomes progressively more complex. First, there are more candidate neighbor cells to evaluate. Also, it become harder to define appropriate ways of comparing measurements of cells that operate using different radio access technologies. 
     Additionally, the mobility of user mobile devices increases the number of reselection and handover operations and increases the number and frequency of cell searches accordingly. Previous changes in Release 10 (Rel-10) of 3GPP mandate a high priority search within a short time period of entering CELL_PCH state, URA_PCH state, or IDLE state. While this potentially speeds up a reselection to a higher priority layer by avoiding the possibility that the user equipment waits so long to perform the search that it enters a non-idle state first, it also imposes unreasonable searching requirements on user equipment that enter and leave these states frequently. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
         FIG. 1  illustrates an exemplary wireless network according to one implementation of the disclosure. 
         FIG. 2  illustrates user equipment according to one implementation of the disclosure. 
         FIG. 3  illustrates the operation of exemplary user equipment that performs cell search according to one implementation of the disclosure. 
         FIG. 4  illustrates the operation of exemplary user equipment, which performs cell searches according to an alternative implementation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 through 4 , discussed herein, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless user equipment. 
     In 3GPP Release 8, which coincided with the first specifications of the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), a new priority-based reselection (PBR) algorithm was defined based on priority levels. In a PBR algorithm, cells are grouped into different priority layers. For any given neighbor cell, a determination is made as to whether it fulfills certain reselection requirements. If it meets the requirements, then no further evaluation of cells will lower priorities is necessary. Comparison of multiple cells is only necessary if multiple cells within a priority level meet the criteria. In the 3GPP priority-based reselection algorithm, it is not permitted to have cells using different radio access technologies (RATs have the same priority level). This means there is no requirement to compare measurements of cells of different RATs when using the reselection algorithm. Examples of different (RATs) are Global System for Mobile Communications (GSM), UTRAN and E-UTRAN. 
     Two sets of thresholds for candidate neighbor cells are specified. The set of thresholds to be used dependent on whether the neighbor cell has a higher or lower priority than the serving cell. For candidate neighbor cells with a higher priority than the serving cell, the reselection criteria do not include any criteria associated with the serving cell. Reselection to a candidate neighbor cell with a lower priority than the serving cell is possible only if the serving cell measurements meet some criteria—typically that the strength and/or quality of the serving cell signal has dropped below some threshold(s). 
     Searching for Neighbor Cells 
     In UTRAN, there is an existing requirement in Section 4.2.2.3 (Measurement of Inter-Frequency FDD Cells) of 3GPP Technical Specification 25.133 to search for cells and, if found, to measure the cells. Priority Information for UTRA FDD carrier frequencies may be provided in the measurement control system information. Then, if
 
Srxlev ServingCell &gt;S prioritysearch1 , and
 
Squal ServingCell &gt;S prioritysearch2 ,
 
then the user equipment (UE) in Idle, Cell_PCH or URA_PCH shall search for any higher priority UTRA inter-frequency cells at least every time period T higher   _   priority   _   search . An example of the parameter T higher   _   priority   _   search  is described in section 4.2.2 of 3GPP TS 25.133.
 
     If the user equipment (UE) is not in CELL_PCH or URA_PCH state, or IDLE state, then the UE shall search of UTRA FDD layers of higher priority within time period (T higher   _   layer   _   start ) upon entering into any of these states. If less than one (1) second has elapsed since the UE camped on the current serving cell when the UE enters into any one of these states, then the value of parameter T higher   _   layer   _   start  is one DRX cycle, plus 1 second. Otherwise, the value of T higher   _   layer   _   start  is one DRX cycle. If higher priority UTRA cells are found by the higher priority search, the higher priority UTRA cell may be measured at least every (N carrier −1)*T measureFDD  seconds. 
     If, after detecting a cell in a higher priority search, it is determined that reselection has not occurred, then the user equipment (UE) is not required to continuously measure the detected cell to evaluate the ongoing possibility of reselection. However, the specified minimum measurement filtering requirements may still be met by the UE before the UE makes any determination to stop measuring the cell. 
     The relevant text of the first change request related to the above modifications to change to 3GGP TX 25.133 stated the reasons for the change:
         “Cell reselection requirements for higher priority layer search when UE changes from active states to idle, URA_PCH or CELL_PCH are unclear regarding the time instant when higher priority search is to be triggered. In case of frequency state transitions between active and semi-idle states caused by some applications&#39; traffic pattern this can lead to a situation in which a higher priority layer is not selected or rarely selected for cell reselection.” RP-110778, RAN#52, “Cell Reselection Requirements for Higher Priority Layer”, SOURCE: Nokia Siemens Networks, Nokia, Renesas Mobile Europe Ltd, TeliaSonera, MediaTek).       

     Mobility States 
     In UTRAN and E-UTRAN, the user equipment (UE) may be associated with a mobility state. For example, in UTRAN, the UE may be in a higher-mobility state if ordered by the network or if the number of cell reselections in the recent past exceeds a threshold. For example, in 3GPP TS 25.304 Section 502050101A (High-Mobility State When HCS Is Not Used), the high-mobility state is applicable in the non-hierarchical cell structure (non-HCS) case, if the parameters non-HCS_T CRmax , non-HCS_N CR , and non-HCS-T CRmaxhyst  are broadcast in system information. If, in non-HCS environment, the number of cell reselections during time period non-HCS_T CRmax  exceeds non-HCS_N CR , or if the network via RRC signaling has ordered the UE to consider itself to be in high-mobility state, then the high-mobility state has been detected. When the number of cell reselections during time period non-HCS_T CRmax  no longer exceeds non-HCS_N CR , the UE continues in high-mobility state. If the criteria for entering high-mobility state are not detected during time period non-HCS-T CRmaxhyst , then the UE exits high-mobility state. If the UE is non-HCS environment and in high-mobility state, the UE shall apply the speed dependent scaling rules as defined in Section 5.2.6.1.4 of 3GPP TX 25.304. 
     However, according to conventional search requirements, if a device goes from connected to idle frequently (e.g., more frequently than one every 60*N layers  seconds), the device will perform high priority measurements more frequently than a device that is idle over the same period. If there are a high number of higher priority layers (or a given layer includes a high number of discrete frequencies), this may introduce significant excessive battery consumption. 
     Thus, while the 3GPP standard may address the potential problem that a device switching between states may never (or very rarely) perform higher priority layer searches, it also may introduce excessive measurement requirements. Also, the UE may not have received the complete system information for the serving cell (e.g., SIB 11, 11bis, 19), which contain neighbor cell information and priority information. As a result, the existing requirement may mean that the UE performs a high-priority search without knowing: a) which high-priority frequencies are in the local area; and/or b) if it is using common priorities signaled in system information, which frequencies are high priority. 
     The present disclosure describes improved apparatus and methods that enable user equipment to do a high priority search less frequently while ensuring reselection to higher priority layers within a reasonable delay. Thus, existing 3GPP specifications may be modified so that a search is only required if the most recent search was carried out more than T higher   _   priority   _   search  seconds ago. In such an implementation, the parameter T HPF2   _   SEARCH  is considered to be equal to T higher   _   priority   _   search . The previous search may have been performed: 1) for the same purpose (i.e., higher priority cell search), 2) for a broader purpose (e.g., cell search for higher or lower priority cells), or 3) during any RRC state. Another condition may be that the UE has not changed serving cell since the previous search. 
     In one implementation, the parameter T HPF2   _   SEARCH  may be, for example, 60*N layers  seconds. Other implementations may have different values. Advantageous values may be, for example, in the range 1&lt;&lt;T HPF2   _   SEARCH ≦60*N layers  seconds. As a further enhancement, the parameter T HPF2   _   SEARCH  may be dependent on mobility (e.g., on the mobility state of the UE). For a higher mobility UE, the value of T HPF2   _   SEARCH  would be set lower. This minimizes the risk that the UE has moved too far since the previous search and that the previous search results are invalid. 
     In another implementation T HPF2   _   SEARCH  for a given layer depends on how recently any cell on that layer has been detected. If one or more cells on a layer have been detected recently (e.g., within a most recent period of 3 minutes), then T HPF2   _   SEARCH  for that layer is reduced (relative to T HPF2   _   SEARCH  for other layers, or relative to T HPF2   _   SEARCH  for that layer when no cells have been detected on that layer recently). 
     The foregoing improvement may be described (in respect of a UE camped on a UTRAN cell) by modifying 3GPP 25.133 in Section 4.2.2.3 (Measurement of Inter-frequency FDD Cells) as follows (additions are underlined):
         If priority information for UTRA FDD carrier frequencies is provided in the measurement control system information and Srxlev ServingCell &gt;S prioritysearch1  and Squal ServingCell &gt;S prioritysearch2  then the UE shall search for any higher priority UTRA inter-frequency cells at least every T higher   _   priority   _   search  where T higher   _   priority   _   search  is described in section 4.2.2. If the UE is not in CELL_PCH, URA_PCH or IDLE state then the UE shall search for UTRA FDD layers of higher priority within T higher   _   layer   _   start  upon entering into any of these states unless its most recent search for UTRA FDD layers of higher priority was within the last T higher   _   priority   _   search  seconds. If 1 second has not elapsed since the UE camped on the current serving cell when the UE enters into any of these states, T higher   _   layer   _   start  is one DRX cycle plus 1 second; otherwise . . . .       

     Additionally, 3GPP 25.133 may be modified in Section 4.2.2.5.2 (Cell Reselection Based On Priority Information) as follows (additions are underlined):
         If Srxlev ServingCell &gt;S prioritysearch1  and Squal ServingCell &gt;S prioritysearch2  then the UE shall search for GSM BCCH carrier at least every T higher   _   priority   _   search  where T higher   _   priority   _   search  is described in section 4.2.2. If the UE is not in CELL_PCH, URA_PCH or IDLE state then the UE shall search for GSM layers of higher priority within T higher   _   layer   _   start  upon entering into any of these states unless its most recent search for GSM layers of higher priority was within the last T higher   _   priority   _   search  seconds. If 1 second has not elapsed since the UE camped on the current serving cell when the UE enters into any of these states, T higher   _   layer   _   start  is one DRX cycle plus 1 second; otherwise . . .       

     Finally, 3GPP25.133 may be modified in Section 4.2.2.5.a (Measurement of Inter-RAT E-UTRA Cells) as follows (additions are underlined):
         If Srxlev ServingCell &gt;S prioritysearch1  and Squal ServingCell &gt;S prioritysearch2  then the UE shall search for E-UTRA layers of higher priority at least every T higher   _   priority   _   search  where T higher   _   priority   _   search  is described in section 4.2.2. If the UE is not in CELL_PCH, URA_PCH or IDLE state then the UE shall search for E-UTRA layers of higher priority within T higher   _   layer   _   start  upon entering into any of these states unless its most recent search for E-UTRA layers of higher priority was within the last T higher   _   priority   _   search  seconds. If 1 second has not elapsed since the UE camped on the current serving cell when the UE enters into any of these states, T higher   _   layer   _   start  is one DRX cycle. The minimum rate at which the UE is required to search for and measure such layers may be reduced in this scenario to maintain UE battery life.       

     It is noted that the proposed changes herein are presented in the context of a UTRAN system. However, it will be understood by those of ordinary skill in the art that the apparatuses and methods described herein may readily apply to other RATs, such as GSM and E-UTRAN. 
       FIG. 1  illustrates an exemplary wireless network  100  according to one implementation of the present disclosure. Wireless network  100  includes base station (BS)  111 , BS  112 , and BS  113 . BS  111 , BS  112  and BS  113  may communicate with each other via wireless links or by a wireline backbone network (e.g., optical fiber, DSL, cable, T1/E1 line, etc.). By way of example, in  FIG. 1 , each of the base stations  111 - 113  is configured to communicate with other base stations using Internet protocol (IP) network  130 , which may be, for example, the Internet, a proprietary IP network, or another data network. Each of base stations  111 - 113  is also configured to communicate with a conventional circuit-switched telephone network (not shown), either directly or by means of network  130 . 
     BS  111  provides wireless broadband access to network  130  to a first plurality of user equipments (UEs) within a coverage area of BS  111 . The first plurality of UEs includes user equipment (UE)  121  and UE  122 , among others. BS  112  provides wireless broadband access to network  130  to a second plurality of UEs within a coverage area of BS  112 . The second plurality of UE 2  includes UE  121 , UE  122 , UE  123 , and UE  124 , among others. BS  113  provides wireless broadband access to network  130  to a third plurality of UEs within a coverage area of BS  113 . The third plurality of UEs includes UE  121 , UE  124 , and UE  125 , among others. It is noted that UE  121  is able to access all three of base stations  111 - 113 , whereas UE  125  is only able to access BS  113  and UE  123  is only able to access BS  112 . UE  122  and UE  124  can each access two base stations. 
     Each of base stations  111 - 113  may provide different levels of service to UEs  121 - 125  according to priority levels (common and/or dedicated) associated with each UE. For example, BS  111  may provide a T1 level services to UE  121  and may provide a fractional T1 level service to UE  122 . UEs  121 - 125  may use the broadband access to network  130  to access voice, data, video, video teleconferencing, and/or other broadband services. Each one of the UEs  121 - 125  may be any of a number of types of wireless devices, including a wireless-enabled laptop computer, a personal data assistant, a notebook, a mobile phone, a tablet, or another wireless-enabled device. 
     It is noted that the term “base station” may be commonly used in some types of networks, such as CDMA2000 systems or some 3GPP systems. But “base station” is not universally used in all types of radio access technology (RAT). In some types of networks, the term “base station” may be replaced by “eNodeB”, or “eNB”, or “access point.” For the purposes of simplicity and consistency, the term “base station” is used in this disclosure document, and in the claims in particular, to refer to the network infrastructure device that provides wireless access to user equipment. 
     Similarly, the term “user equipment” may be commonly used in some types of networks, but not in others. In some types of networks, the term “user equipment” may be replaced by “subscriber station”, “mobile station”, “remote terminal”, “wireless terminal” or the like. For the purposes of simplicity and consistency, the term “user equipment”, or “UE”, is used in this disclosure document, and in the claims in particular, to refer to the remote wireless device that accesses the network infrastructure device (i.e., the base station). 
       FIG. 2  illustrates an exemplary user equipment (UE)  121  that performs high priority cell searches according to the present disclosure. UE  121  comprises at least one antenna  205 , radio frequency (RF) transceiver (XCVR)  210 , transmitter baseband (TX BB) processing circuitry  215 , microphone  220 , and receiver baseband (RX BB) processing circuitry  225 . UE  121  also comprises speaker  230 , main controller  240 , input/output (I/O) interface (IF)  245 , keypad  250 , display  255 , and memory  260 . Memory  260  stores basic operating system (OS) program  261 , search algorithm  262 , and other data (not shown). 
     Radio frequency transceiver  210  receives from antenna  205  an incoming RF signal transmitted by a base station of wireless network  100 . Radio frequency transceiver  210  comprises receiver circuitry configured to operate in cells associated with one or more types of radio access technology (RAT) networks (e.g., GSM, UTRAN, E-UTRAN, etc.). Radio frequency transceiver  210  down-converts the incoming RF signal to produce an intermediate frequency (IF) or a baseband signal. The IF or baseband signal is sent to RX BB processing circuitry  225 , which may produce a processed baseband signal by, for example, filtering and digitizing the received baseband or IF signal, additional filtering, and, if necessary, demodulation and/or decoding. Receiver baseband (RX BB) processing circuitry  225  transmits the processed baseband signal to speaker  230  (i.e., voice data) or to main controller  240  for further processing (e.g., web browsing). 
     Transmitter baseband (TX BB) processing circuitry  215  may receive analog or digital voice data from microphone  220  or other outgoing baseband data (e.g., web data, e-mail, interactive video game data) from main controller  240 . TX BB processing circuitry  215  may encode, modulate, multiplex, and/or digitize the outgoing baseband data to produce a processed baseband or IF signal. Radio frequency transceiver  210  receives the outgoing processed baseband or IF signal from TX BB processing circuitry  215 . Radio frequency transceiver  210  up-converts the baseband or IF signal to a radio frequency (RF) signal that is transmitted via antenna  205 . 
     Main controller  240  may comprise any device, system or part thereof that controls at least one operation. Such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. Main controller  240  is a microprocessor or a microcontroller. Memory  260  is coupled to main controller  240 . Part of memory  260  may comprise a random access memory (RAM) and another part of memory  260  may comprise a non-volatile memory, such as Flash memory. 
     Main controller  240  executes basic operating system (OS) program  261  stored in memory  260  in order to control the overall operation of UE  121 . In one such operation, main controller  240  controls the reception of forward channel signals and the transmission of reverse channel signals by radio frequency transceiver  210 , RX BB processing circuitry  225 , and TX BB processing circuitry  215 , in accordance with well-known principles. 
     Main controller  240  is capable of executing the other processes and programs resident in memory  260 . Main controller  240  can move data into or out of memory  260 , as required by an executing process. Main controller  240  is also coupled to I/O interface  245 . I/O interface  245  provides UE  121  with the ability to connect to other devices, such as laptop computers and handheld computers. I/O interface  245  is the communication path between these accessories and main controller  240 . Main controller  240  may also be coupled to an input device, such as keypad  250 , and display  255 . The operator of UE  121  uses keypad  250  to enter data into UE  121 . Display  255  may be a liquid crystal display capable of rendering text and/or at least limited graphics from web sites. Alternate examples may use other types of displays (or none). Display  255  may include a touch screen input device that may be used in conjunction with, or in place of, keypad  250 . 
     UE  121  is configured to perform a high priority search in memory  260  according to the principles of the present disclosure. Main controller  240  executes search algorithm  262  and uses internal timers (not shown) to perform high priority searches. 
       FIG. 3  illustrates the operation of exemplary user equipment  121 , which performs cell searches according to one implementation of the disclosure. Initially, it is assumed that UE  121  is not in one of the designated RRC states (i.e., CELL_PCH state, URA_PCH state, or IDLE state). At some point, main controller  240  determines that UE  122  has entered one of the designated states (step  305 ). 
     Next, main controller  310  determines whether or not UE  121  has changed cells since the last high-priority search was performed (step  310 ). If UE  121  has changed cells since the last high-priority search (Yes in  310 ), then main controller  240  performs another high priority search (step  320 ). If UE  121  has not changed cells since the last high-priority search (No in  310 ), then main controller  240  determines whether or not the most recent high-priority search occurred more than the threshold value of parameter T HPF2   _   SEARCH  seconds ago (step  315 ). If the last search occurred more than the threshold value ago (Yes in  315 ), then UE  121  performs a high priority search (step  320 ). If the last search occurred less than the threshold value ago (No in  315 ), then UE  121 , after a suitable time delay, will subsequently re-determine whether or not the most recent high-priority search occurred more than the threshold value of parameter T HPF2   _   SEARCH  seconds ago (loop back to step  315 ). 
     Alternative Solution 
     It is noted that the high priority search algorithm described above may occur during CELL_PCH state, URA_PCH state, or IDLE state. However, in other cases, the most recent search may have occurred in a connected mode state. It is possible to take advantage of the fact that the UE, when moving to connected mode (e.g., CELL_FACH, CELL_DCH) due to data ctivity for even a short period of time, may perform a search of the higher priority cells while in the connected mode. This may require one of the following to be available: 1) measurement gaps available for inter-frequency or inter-RAT neighbor cell measurements during the connected mode, or 2) more than one RF receiver available for parallel inter-frequency or inter-RAT neighbor cell measurements. 
     Assuming that the UE does not change cell while in CELL_FACH or CELL_DCH, the UE may still have valid higher priority search information that could be reused upon entering IDLE, CELL_PCH or URA_PCH states. In particular while being in CELL_FACH or CELL_DCH state, UE  121  may detect if any higher priority cells are available and may store this information in memory  260  before or when leaving the connected state. For efficiency reasons, it is assumed by a preferred implementation would reuse the latest priority measurements performed in connected state. In case of detecting a high priority cell and no cell change in connected state, UE  121  may continue to measure and evaluate this higher priority cell for priority-based reselection as required by Section 4.2.2.3 of 3GPP 25.133. 
     An advantage of this implementation is that UE  121  may effectively reuse the high priority search result performed in connected state while not being in connected state. As a result, UE  121  may skip performing an additional high priority search operation upon entering IDLE, CELL_PCH or URA_PCH state, thereby resulting in battery savings. It is noted that the purpose of the measurements in connected mode may or may not be specifically for the purpose of identifying high priority cells. The measurements may have been, for example, as a pre-requisite to performing reporting of measured neighbor cells. 
       FIG. 4  illustrates the operation of exemplary user equipment  121 , which performs cell searches according to an alternative implementation. Initially, UE  121  moves to CELL_FACH or CELL_DCH (step  405 ). Next, UE  121  performs a higher priority search in the connected state. UE  121  detects either: 1) no higher priority cells available, or 2) high priority cells available (step  410 ). Therefore, UE  121  moves to IDLE, CELL_PCH or URA_PCH state when data activity is finished (step  415 ). Finally, UE  121  skips a high priority search when entering non-connected state and depending on the results of the detection in the second step, preferably further taking into account the elapsed time since the detection in the second step: 1) UE  121  does not perform any higher cell measurements if no higher priority cells detected in connected state; and 2) UE  121  does not perform higher cell measurements and priority based cell reselection evaluation if higher priority cells were detected in connected state (step  420 ). IF the elapsed time since the detection in the second step exceeds a limit, then the UE  121  performs higher cell measurements and priority based cell reselection evaluation regardless of the outcome of the detection. 
     The above-disclosed implementations are by way of example only. The scope of the present disclosures and the claims below are not limited to the exemplary implementations. In particular, the exemplary search algorithms are not limited to priority-based reselection (PBR) searches. In alternate implementations, other types of search algorithms may be used. Additionally, the present disclosure and claims are not limited to a UTRAN network. Those skilled in the art will appreciate that the apparatus and methods disclosed herein may be readily adapted for use in other types of RAT networks. Furthermore, the present disclosure and claims are not limited to searches for high-priority cells. In alternate implementations, the apparatuses and methods disclosed herein may be readily adapted for use in searches for other classifications of potential neighbor cells. 
     In the exemplary implementations described above, the algorithm may be implemented in respect of all higher priority layers jointly, on a per-RAT basis, or on a per-carrier frequency basis. For example, in respect of the algorithm illustrated in  FIG. 4 , if one higher priority E-UTRAN carrier frequency, f 1  has been searched recently in CELL_FACH or CELL_DCH, and another higher priority E-UTRAN carrier frequency, f 2 , has not been searched then on entry to IDLE, or CELL_PCH, or URA_PCH, the UE may omit or refrain from searching on carrier frequency f 1 , and search only on carrier frequency f 2 . 
     SIB reading—In one implementation, UE delays the high priority search (which would otherwise be required within T higher   _   layer   _   start  seconds, as defined in the existing specification, until UE  121  has received all necessary system information for the serving cell. 
     Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.