RELATED APPLICATION
The present invention claims the benefit of the Indian Provisional Application No. 2750/CHE/2013 titled "METHOD AND SYSTEM FOR PERFORMING RESELECTION OF A CELL IN A HETEROGENEOUS NETWORK ENVIRONMENT" by Samsung R&D Institute India - Bangalore Private Limited filed on 13th June 2013, which is herein incorporated in its entirety by reference for all purposes.
FIELD OF THE INVENTION
The present invention generally relates to the field of wireless communication, and particularly relates to methods and systems for enabling user equipments to perform cell reselections based on RAT priority and signal conditions.
BACKGROUND OF THE INVENTION
User equipments (UEs) moving through a heterogeneous network may need to be handed over between different cells when being in a connected mode or may need to perform cell reselection when being in an idle mode. The cell selection and reselection processes are run autonomously by the UE in idle mode according to standardized reselection procedures and related network configured parameters. For instance, E-UTRAN and UTRAN (Rel-8 and onward) supports a priority based cell reselection procedure for inter-frequency and inter-RAT reselections (3GPP TS 36.304 / 35.304) referred to as the Absolute Priority method. A camping UE, i.e. , a UE in idle mode being served by one cell, may also need to detect autonomously its mobility state (TS 36.304 / 25.304), and if in high- or medium-mobility state, the UE may apply speed dependent scaling rules to the reselection parameters if

configured by the network.
In current reselection mechanism defined by 3GPP GSM specs, the candidate cell for reselection are selected by comparison with serving cell for some predefined time (usually 5 sec). The existing reselection mechanism considers the neighbor cell for reselection that meets the suitability criteria "first". However this mechanism does not consider if the reselecting cell is the "best" cell among other reselection candidates.
There are multiple scenarios in this problem can occur. In one scenario, as multiple instances of SI2quater are required (3-7 instances are observed in field) because one SI2quater is not sufficient to carry the neighbor cell information belonging to other RATs (WCDMA and LTE) and respective reselection parameters. So, depending on which instance of SI2quater is received by the UE first, it starts the measurement of those cells first. Hence it can cause a delay in receiving the neighbor cell information of highest priority RAT.
The other scenario, when the UE is camped on a GSM cell and moving from GSM only area to GSM/WCDMA/LTE area and there are GSM cells present with better signal strength compared to serving cell in the neighborhood. It is possible that the UE will start the 5 second reselection timer for GSM cells before the IRAT cells. In these cases UE will trigger GSM to GSM reselection, even though IRAT cells are suitable and suitability timer was running for that cell, which cause a delay in reselection to higher priority IRAT cells.
As explained above, there can be multiple scenarios where UE will reselect lower priority cells even when higher priority cells are suitable and 5 second reselection timer is running for them are available. The current 3GPP

specifications have provided signal related threshold parameters to accelerate higher priority RAT. However these methodologies have generally suffered from various drawbacks including, for example, increased complexity for the mobile terminal and/or the serving cell or the failure to address all use cases or scenarios.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent upon a reading of the specification and a study of the drawings.
SUMMARY OF THE INVENTION
The primary objective of the present invention is to provide a method and system for enabling cell reselection procedure for user equipments in idle mode.
Another objective of the present invention is to provide a method and system for performing cell reselection based on RAT priority, signal strength and reselection timer value parameters associated with the target cells for reselection.
Another objective of the present invention is to provide a method and system for optimizing the number of cell reselections and avoid the un-necessary reselections when mobile is camped on GSM cells.
The various embodiments herein provide a method and system for enabling cell reselection by user equipment in wireless communication. The method comprising the steps of detecting, by the user equipment, one or more target

cells for reselection of a serving cell based on a plurality of reselection parameters, wherein the plurality of reselection parameters comprises at least one of a priority, signal strength and reselection timer value associated with the one or more target cells. The method further includes initiating a preset timer for one or more target cells satisfying the reselection parameters, storing a plurality of target cells for which the preset timer value is greater than a threshold; checking if signal conditions of a serving cell is above a predefined threshold and triggering the cell reselection to a first target cell if the signal condition of the serving cell is above the predefined threshold.
According to an embodiment, the method further comprises of selecting a second target cell having the highest priority and signal strength for cell reselection from the plurality of target cells if the signal condition of the serving cell is above the predefined threshold.
According to an embodiment, the method further comprises of verifying if the priority of the second target cell is higher than the first target cell, delaying the reselection to the first target cell and wait for the preset timer value to expire for the second target cell, verifying if the second target cell is suitable for reselection after the expiry of the preset timer and triggering the reselection to the second target cell if the second target cell is suitable for reselection.
According to an embodiment, the method further comprises of checking for the plurality of target cells in a decreasing order of priority if the second target cell is not suitable for reselection after the expiry of the preset timer, verifying if another target cell is available for reselection with suitably running timer and having the preset timer value greater than the maximum threshold, and triggering the reselection to the available third target cell having a highest RAT priority and signal strength.

According to an embodiment, the method further comprises of triggering the reselection of the first target cell by the user equipment when the first target cell and the second target cell have a same priority which is higher than the priority of the serving cell.
According to an embodiment, the method further comprises triggering the reselection of the first target cell by the user equipment when the first target cell and the second target cell is of same priority and the power level of the second target cell is less than the first target cell.
According to an embodiment, the method further comprises triggering the reselection of the second target cell by the user equipment during at least one of the first target cell, the second target cell and the serving cell is of same priority, the preset timer is at its maximum threshold value for the first target cell and the power level of the second target cell is higher than the first target cell by the predefined threshold value.
According to an embodiment, the cell reselection is initiated from a 2G network to at least one of a 3G network, 4G networks, an LTE network or any later-technology network.
According to an embodiment, the user equipment is in idle mode or active data transfer mode when user equipment is allowed to do the autonomous reselection.
The foregoing has outlined, in general, the various aspects of the invention and is to serve as an aid to better understanding the more complete detailed description which is to follow. In reference to such, there is to be a clear

understanding that the present invention is not limited to the method or application of use described and illustrated herein. It is intended that any other advantages and objects of the present invention that become apparent or obvious from the detailed description or illustrations contained herein are within the scope of the present invention.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1 is a schematic representation of a cell reselection procedure by a user equipment in a heterogeneous network environment, according to an embodiment of the present invention.
Figure 2 is a flow diagram illustrating a method for enabling cell reselection by user equipment in idle mode, according to an embodiment of the present invention.
Figure 3 is a flow diagram illustrating a method for enabling cell reselection by user equipment in idle mode, according to another embodiment of the present invention.
Although specific features of the present invention are shown in some drawings and not in others. This is done for convenience only as each feature may be combined with any or all of the other features in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a method and apparatus for enhancing cell reselection by user equipments in idle mode in wireless communication. The

method herein improves the existing idle mode reselection procedure from GSM to GSM/IRAT which helps the UE to reselect to best cell in terms of RAT priority and signal conditions. In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.
Figure 1 is a schematic representation of a cell reselection procedure by a user equipment in a heterogeneous network environment, according to an embodiment of the present invention. According to Figure 1, there exists a serving network 104 to which the user equipment 102 is currently camped on and a plurality of target cells 106 acting as target cells for reselection by the user equipment 102. When the user equipment 102 is switched on, it will attempt to select a suitable cell to camp on. The user equipment 102 then transmits signals to and receives signals from the cell on which it camps. The user equipment 102 also receives a broadcast list of neighboring cells and builds up a list of neighbor cells in its vicinity which allows the user equipment 102 to determine with which other cell(s), the UE is capable of communicating. From time to time, the user equipment 102 performs a cell reselection process.
Generally, cell reselection may occur for a variety of reasons e.g. because of a decrease in the quality of the signal to or from the cell on which it is camped

or because the cell on which it is camped is unable to provide a service which the user equipment 102 requires. Prior to undertaking cell reselection, the user equipment 102 periodically measures neighboring cells. These measurements generally give an indication of the quality of signal from a given cell. The measurements tend to be defined in various standards and may relate to the signal-to-noise ratio, signal strength, error rate etc. A multi-mode mobile communication device carries out measurements of cells in a plurality of radio access networks. When the cell carries out cell reselection, the cell performs a ranking of the cells in its neighboring cell list in order. These neighboring cells are referred to as the reselection candidates. The user equipment 102 then attempts cell reselection with the best cell among the reselection candidates based on the RAT priority, signal strength and adjusted timer value.
Figure 2 is a flow diagram illustrating a method for enabling cell reselection by user equipments in idle mode, according to an embodiment of the present invention. At step 202, the user equipment detects one or more target cell for reselection of a serving cell based on a plurality of reselection parameters. The reselection parameters herein comprises at least one of a priority, signal strength and reselection timer value associated with the one or more target cells. At step 204, initiate a preset timer for one or more target cells satisfying the reselection parameters. At step 206, check whether the preset timer is expired for any of the target cells. If the timer is expired for any of the target cells, then at step 208, store a plurality of target cells for which the preset timer value is greater than a threshold. If the timer for any of the target cells is not expired, then step 204 is repeated. At step 210, a check is performed to verify whether the DSC counter value of the serving cell is less than the maximum threshold value or not. If the DSC counter value of the serving cell is less than the maximum threshold value, then trigger the cell reselection to

the first target cell at step 214. If the DSC counter value of the serving cell is greater than the maximum threshold value, then at step 212, select a second target cell having the highest priority and signal strength from the plurality of target cells for reselection.
The UE first measures the 2G/IRAT cells and checks each cell to understand if the cell is suitable for cell reselection based on the reselection parameters. The UE then starts a 5 second timer for a plurality of target cells which satisfy the suitability criteria. In an exemplary embodiment, consider an UE camped on a 2G cell SG1. The UE performs measurement for 2G/IRAT cells and finds a neighbor cell Ncell (it can be 2G, 3G, 4G or LTE cell) to be suitable. Consider that the 5 second timer T5Sec_NCeii is expired for the NCell. In this case, the UE checks for other reselection candidate cells for which 5 second suitability timer T5sec_Nx is running before triggering the reselection to cell NCell. After the check is complete, the cells for which the value of the suitability timer T5sec_Nx ^ TThreshoid_evaiuation (say 3 seconds) are stored in NX={nx1, nx2,..}. The UE receives the priority of neighbor cell (according to RATs) in SI2 quarter. If the Inter Radio Access Technology (IRAT) priority is not given in SI2 quarter, RR gets the multi-mode UE's RAT priority from upper layers. (Non Access Stratum (NAS) has the priority of each RAT being supported by multimode UE).
Based on the priority of the neighbor cells, UE compares the value of a DSC counter with a maximum value to select a cell for reselection. In a first case, if the DSC counter value of the serving cell is less than the maximum value, then the UE triggers the reselection to NCell. (Since serving cell is facing the decoding failure, the reselection is not delayed as it causes the DSF and due to which the UE loses the service). In the second case, if the UE determines that the DSC counter is equal to maximum value, then the UE selects the

highest priority cell with highest power level (in case more than one cell of same priority are available) from NX (say NX1).
Figure 3 is a flow diagram illustrating a method for enabling cell reselection by user equipment in idle mode, according to another embodiment of the present invention. At step 212, a second target cell having the highest priority and signal strength is selected for reselection. It is then verified at step 302, whether the priority of the second target cell is higher than the first target cell. If the priority of the second target cell is not higher than the first target cell, then at step 304, then check whether the first target cell and second target cell has the same priority and is higher than the priority of the serving cell or not. If the priority of the second target cell is higher than the first target cell, then at step 306, the reselection to the first target cell is delayed and the UE is made to wait until the preset timer value for the second target cell is expired. At step 308, check whether the second target cell is suitable for the reselection after the expiry of the preset timer. If the second target cell is suitable for the reselection after the expiry of the preset timer, then trigger the reselection of the second target cell at step 310. If the second target cell is not suitable for the reselection after the expiry of the preset timer, then at step 312, check a plurality of target cells in a decreasing order of priority. At step 314, it is verified whether another target cell is available for reselection with suitably running timer and which is having a preset timer value greater than the maximum threshold value. If another target cell is not available for reselection, then at step 316, reselection is not triggered and the UE continues in the same serving cell. If there is another cell available for reselection with suitably running timer and which is having a preset timer value greater than the maximum threshold value, then at step 318, the reselection to the available target cell is triggered.

If the first target cell and the second target cell has the same priority and is higher than the priority of the serving cell, then at step 320, trigger the reselection to the first target cell. If the first target cell and second target cell do not have the same priority and the priority of first target cell and second target cell is not higher than the priority of the serving cell, then at step 322, it is checked whether the second target cell and the serving cell has the same priority and if the power level of the second target cell is higher than the first target cell. If the second target cell and the serving cell has the same priority and if the power level of the second target cell is higher than the first target cell, then at step 324, then trigger the reselection to the second target cell is triggered. If the second target cell and the serving cell do not have the same priority and if the power level of the second target cell is not higher than the first target cell, then at step 320, the reselection to the first target cell is triggered.
In one example embodiment, the UE checks if NX1 has higher priority than Ncell. If yes, then the reselection to Ncell is delayed and UE is made to wait utill the 5 second timer expires for Nx1 (i.e. the UE continues the neighbor cell measurements and suitability timer. The suitability timer can go beyond 5 seconds). In this case, the UE decides to trigger the reselection to the nx1 cell if the nx1 cell is found suitable. If the UE determines that the NX 1 is suitable for reselection, then the UE triggers the reselection to nxl Now, suppose if the UE determines that the nx1 fails the suitability criteria (before its 5 second timer expiry), then the UE immediately checks all cells in [NCell, {NX}-nx1]. If there are more than one cell for which suitability timer with value greater than 5 second and is running, then the UE selects the cell with highest priority say nx2 (timer value T5second_nx2^ 5). The UE then triggers the reselection to this cell. Else if the UE determines that there are no cells available in [NCell, {NX}-nx1] for which the suitability timer is running, then the

UE does not trigger the reselection to any cell and hence continues in the same cell with deleting all cells in NX.
In another embodiment, if the UE determines that the nx1 has same priority as the NCell and the priority of NCell and nx1 is higher than serving cell SG1, then the UE triggers the reselection to Ncell.
In yet another embodiment, if the UE determines that the nx1 has same priority as NCell and priority of Ncell and nx1 is same as serving cell SG1, then the UE determines the power level of Nx1. If the power level of Nx1 is higher than the NCell by signal_threshold (i.e. on top of cell reselection hysteresis and other parameters defined by specification), then the reselection to NX1 cell is triggered, else the UE triggers the reselection to Ncell.
In case reselection to any cell fails, the UE behaves the same way as defined in the 3GPP specification and searches the GSM/GPRD BA list to find a suitable cell to camp. The values of timer T-mreshoid_evaiuation and signal_threshold are configurable. Also in place of DSC counter, some other parameters such as signal conditions of serving cell can be used to check the sustainability of serving cell.
The present invention is given with precondition that the value of the T_reselection in the network is equal to 5 seconds. This is broadcasted in SI2 quarter (for priority based reselection). If the value of T_reselection is more than 5 seconds, then it depends on the implementation of the UE to either follow the specification or UE can ignore that value to follow the above reselection method.

According to an example embodiment of the present invention, consider UE camped on 2G cell SG1 and finds a suitable neighbor cell NCell whose 5 second timer is expired. At this point of time, the UE checks for other reselection candidate cells for which 5 second timer T5second_Nx is running before triggering the reselection to NCell. When the check for other cells is complete, the UE stores these cells in NX= {nx1, nx2,...}. In one example scenario, if there is a cell available in NX say nxLTE which belongs to highest priority RAT (LTE), then the UE checks if the value of T5sec_nxLTE ^ T-mreshoidj-TE- If the value of T5sec_nxLTE ^ TThreshoid_LTE, then the UE triggers a reselection to the nxLTE cell. In another example scenario, if there is a cell available in NX say nxwcDMA which belongs to higher priority RAT, then the UE checks if the value of T5sec_nxwcDMA ^ TThreshoid_wcDMA- If the value of T5sec_nxwcDMA ^ TThreshoid_wcDMA, then the UE triggers a reselection to nxwcDMA cell. In yet another example scenario, if there is a cell available in NX say nxcsivi which belongs to GSM, then the UE checks if the power level of nxGsivi is higher than NCell by Signal_Threshold. If so, then the UE triggers the reselection to nxGsivi. If there are no cells available in NX, then the UE triggers the reselection to NCell. The value of threshold timer TThreshoid_RAT for each RAT can be such that:
T-Threshold_LTE ^ T-rhreshold_WCDMA ^ 5 Seconds
According to this example embodiment, different predefined timer threshold values are defined for different priority RATs (high priority will have lower timer threshold values). So, at the expiry of lower priority RAT reselection timer (say 5 seconds), UE checks if any suitable cells of high priority RAT is also available. If the high priority RAT is available and the timer value for that higher priority RAT is more than the threshold decided, then the UE immediately triggers the reselection to the high priority RAT. Otherwise, UE continues with the lower priority RAT for which 5 second timer is expired. In

case, only high priority RAT cell is found, UE behaves as per existing mechanism and timer values as decided by network (T_reselection). In this method, different threshold timer values are used for different RAT (as per priority) and hence there is no delay in the reselection of lower priority RAT.
According to another example embodiment, consider a UE camped on 2G cell SG1. The UE receives the neighbor cell information for different RATs in an SI2 quarter message. Instead of checking the suitability of each cell belonging to different RAT till T_reselection (5second), the UE uses different suitability timers for each RAT. In one case, if the UE finds an LTE neighbor cell, say nxLTE to be suitable, then the UE will start the suitability timer T-mreshoid_LTE for the cell nxLTE- After expiry of the timer, UE triggers the reselection to cell nxLTE- In the second case, if the UE finds a WCDMA neighbor cell, say nxwcDMA to be suitable, then the UE starts the suitability timer TThreshoid_wcDMA for the cell nxWcDMA- After expiry of the timer, UE triggers the reselection to the cell nxwcDiwv In a third case, if the UE finds a GSM neighbor cell, say nxGsivi, to be suitable, then the UE starts the suitability timer T_reselection (5 second) for the cell nxGsivi- After expiry of the timer count, UE triggers the reselection to the cell nxGsivi- The value of threshold timer T-mreshoid_RAT for each RAT can be such that:
TThreshold_LTE ^ T-rhreshold_WCDMA ^ 5 Seconds
In this example embodiment, the UE initiates different predefined suitability timers (not threshold timer values as in previous example embodiment) based on the priority of that cell (RAT). Whenever the respective timer expires for that cell/RAT, UE triggers the reselection on that cell.
In another example embodiment, consider UE camped on 2G cell SG1 and it

has received the neighbor cell information for different RATs. Another approach is that, UE tries to search the neighbor cells belonging to higher priority RATs (provided by network in the system information) more frequently. This helps in those cases where the length of DRX cycle is too long. Whenever UE finds a GSM neighbor cell to be suitable, UE extends the wake-up cycle and searches the cells of higher priority RAT to find suitable cells belonging to higher priority RATs. This measurement method improves the delay gap which occurs (due to DRX cycle) in starting the suitability timer of higher priority RAT cells compared to GSM cells. Instead of checking the suitability of each cell belonging to different RAT till T_reselection (5 second), UE uses the different suitability timers for each RAT. In first case, If UE finds an LTE neighbor cell, say nxLTE to be suitable, then the UE starts the suitability timer TThreshoid_LTE for the cell nxLTE- After expiry of this timer, UE triggers the reselection to this cell. In the second case, if the UE finds a WCDMA neighbor cell, say nxwcDMA to be suitable, then the UE starts the suitability timer TThreshoid_wcDiviA for the cell nxwcDiwv After expiry of this timer, UE triggers the reselection to this cell. In third case, if the UE finds a GSM neighbor cell, say nxGsivi, to be suitable, then the UE starts the suitability timer T_reselection (5 second) for this cell. After expiry of this timer, UE triggers the reselection to this cell. The value of threshold timer TThreshoid_RAT for each RAT can be such that:
T-Threshold_LTE ^ T-rhreshold_WCDMA ^ 5 Seconds
The embodiments herein enables the UE to consider the best cell for reselection among the reselection candidates in terms of RAT priority, signal strength and adjusted reselection timer value. The present invention also optimizes the number of reselections and avoids the unnecessary reselections when the user equipment is camped on GSM cells.

The present embodiments have been described with reference to specific example embodiments; it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the various embodiments. Furthermore, the various devices, modules, and the like described herein may be enabled and operated using hardware circuitry, firmware, and/or software embodied in a machine readable medium. Although the embodiments herein are described with various specific embodiments, it will be obvious for a person skilled in the art to practice the invention with modifications. However, all such modifications are deemed to be within the scope of the claims. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the embodiments described herein and all the statements of the scope of the embodiments which as a matter of language might be said to fall there between.

WE CLAIM:
1. A method for enabling cell reselection by a user equipment in wireless
communication, the method comprising steps of:
detecting, by the user equipment, one or more target cells for reselection of a serving cell based on a plurality of reselection parameters;
initiating a preset timer for one or more target cells conforming to the reselection parameters;
storing a plurality of target cells for which the preset timer value is greater than a threshold;
checking if signal conditions of a serving cell is above a predefined threshold; and
triggering the cell reselection to a first target cell if the signal condition of the serving cell is above the predefined threshold;
wherein the plurality of reselection parameters comprises at least one of a priority, signal strength and reselection timer value associated with the one or more target cells.
2. The method as claimed in claim 1, further comprising:
selecting a cell having a highest priority and signal strength from the plurality of target cells as a second target cell for cell reselection if the signal condition of the serving cell is above the predefined threshold.
3. The method as claimed in claim 2, further comprising:
verifying if the priority of the second target cell is higher than the first target cell;
delay the reselection to the first target cell and wait for the preset timer value to expire for the second target cell;
verify if the second target cell is suitable for reselection after the expiry

of the preset timer; and
trigger the reselection to the second target cell if the second target cell is suitable for reselection.
4. The method as claimed in claim 3, further comprising:
checking for the plurality of target cells in a decreasing order of priority if the second target cell is not suitable for reselection after the expiry of the preset timer;
verifying if another target cell is available for reselection with suitably running timer and having the preset timer value greater than the maximum threshold; and
triggering the reselection to the available target cell having a highest RAT priority and signal strength.
5. The method as claimed in claim 2, further comprising triggering the reselection of the first target cell by the user equipment when the first target cell and the second target cell have a same priority which is higher than the priority of the serving cell.
6. The method as claimed in claim 2, further comprising triggering the reselection of the first target cell by the user equipment when the first target cell and the second target cell is of same priority and the power level of the second target cell is less than the first target cell.
7. The method as claimed in claim 2, further comprising triggering the reselection of the second target cell by the user equipment during at least one of:
the first target cell, the second target cell and the serving cell is of

same priority;
the preset timer is at its maximum threshold value for the first target cell; and
the power level of the second target cell is higher than the first target cell by the predefined threshold value.
8. The method as claimed in claim 1 , wherein the cell reselection is initiated from a 2G network to at least one of a 3G network, 4G network, an LTE network or any later-technology network.
9. The method as claimed in claim 1, wherein the user equipment is in idle mode or active data transfer mode when user equipment is allowed to do the autonomous reselection.
10.A wireless communication system, the wireless communication system comprising a user equipment including at least one device module adapted for:
detecting, by the user equipment, one or more target cells for reselection of a serving cell based on a plurality of reselection parameters;
initiating a preset timer for one or more target cells conforming to the reselection parameters;
storing a plurality of target cells for which the preset timer value is greater than a threshold;
checking if signal conditions of a serving cell is above a predefined threshold; and
triggering the cell reselection to a first target cell if the signal condition of the serving cell is above the predefined threshold; and

selecting a cell having a highest priority and signal strength from the plurality of target cells as a second target cell for cell reselection if the signal condition of the serving cell is above the predefined threshold.
11. The system of claim 1, wherein the user equipment further comprises at least one module adapted for:
verifying if the priority of the second target cell is higher than the first target cell;
delay the reselection to the first target cell and wait for the preset timer value to expire for the second target cell;
verify if the second target cell is suitable for reselection after the expiry of the preset timer; and
trigger the reselection to the second target cell if the second target cell is suitable for reselection.