OEO100010 LTE Idle Mode Behavior ISSUE1.00.pdf

LTE Idle Mode Behavior

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Confidential Information of Huawei. No Spreading Without Permission

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A UE that is powered on but does not have an RRC connection to the radio network is defined as being in idle mode. In the case of idle mode management, the eNodeB sends configurations by broadcasting system information, and accordingly, UEs select suitable cells to camp on. Idle mode management can increase the access success rate, improve the quality of service, and ensure that UEs camp on cells with good signal quality.

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In Idle state, the system can perform the following process 

PLMN selection: A UE selects a public land mobile network (PLMN) and registers on the PLMN

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Cell selection: A UE selects a cell in the chosen PLMN and camps on the cell so that the UE can monitor system information and paging messages to obtain service.

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Cell reselection: When camped on a cell, a UE regularly searches for a better cell according to the cell reselection criteria, for the purpose of correctly receiving system information and successfully initiating services.

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Tracking Area registration: A UE registers its location with the Evolved Packet Core (EPC) through a TA registration procedure so that the EPC can page the UE within the TA and request the UE to initiate services.



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Cell reservation and access control: Cell reservation and access control are two mechanisms for operators to control their networks. In the cell reservation mechanism, specific cells can be reserved for operators' use only. In the access control mechanism, the UEs that can initiate calls in a cell are determined on the basis of the classes of the UEs.

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System information broadcast and paging: In system information broadcast, the information such as parameters related to cell selection and reselection is broadcast to UEs through system information over the Broadcast Control Channel (BCCH). In paging, paging messages are used to inform all UEs in a cell of a change in the system information and to carry paging-related information.




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Access classes are applicable to access of UEs on the radio interface. There are 16 access classes, numbered from 0 to 15. The information about access classes 0 to 9 and 11 to 15 is stored in Universal Subscriber Identity Modules (USIMs), and the information about access class 10 is signaled over the radio interface to UEs.

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Access classes 0 to 9 are ordinary classes. Each UE is randomly allocated one out of the ten classes. UEs of these classes can obtain service in their HPLMN and VPLMNs.

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Access classes 11 to 15 are special classes. They are allocated to specific high-priority users. Each UE may be allocated one or more out of the five classes. These special access classes are described as follows: 

If the Equivalent Home PLMN (EHPLMN) list is absent, access classes 11 and 15 are applicable in the HPLMN only.

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Class 11: for PLMN use

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Class 15: for PLMN operation

Access classes 12 to 14 are applicable in the HPLMN and VPLMNs of the home country only. 

Class 12: for security services

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Class 13: for public utilities

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Class 14: for emergency services



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The service that UEs in idle mode can obtain is categorized into the following types: 

Limited service: A UE can initiate only emergency calls in an acceptable cell. Such a cell should meet the requirements that the cell is not barred and the cell selection criteria are fulfilled. The UE camps on the acceptable cell to obtain limited service when the cell is part of at least one TA on the list of forbidden TAs for roaming or when the cell does not belong to the registered PLMN.

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Normal service: A UE obtains normal service in a suitable cell. Such a cell should meet all the following requirements: 

The cell is part of the selected PLMN, the registered PLMN, or a PLMN on the EPLMN list.

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The cell is part of at least one TA that belongs to a PLMN fulfilling the previous requirement but is not on the list of forbidden TAs for roaming.

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The cell is not barred.

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The cell selection criteria are fulfilled.

Operator service: A UE obtains operator service in a reserved cell, which is used by the operator only. In the case of access class 11 or 15 in the HPLMN or an EHPLMN, a UE may consider reserved cells as candidates during cell selection or reselection. In other cases, a UE regards reserved cells as barred during cell selection or reselection.




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When a UE is powered on or recovers from lack of coverage, the UE first selects the last registered PLMN and attempts to register on that PLMN. If the registration on the PLMN is successful, the UE shows the selected PLMN on the display, and now it can obtain service from an operator. If the last registered PLMN is unavailable or the registration on the PLMN fails, another PLMN can be automatically or manually selected according to the priorities of PLMNs stored in the USIM.

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PLMN Selection in Automatic Mode 

When a UE fails to register on its last registered PLMN, the UE attempts to select another PLMN. In automatic network selection mode, the UE selects a PLMN in the following order: 

HPLMN or an EHPLMN

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Each PLMN in the "User Controlled PLMN Selector with Access Technology" data file in the USIM, in order of priority, according to the UE's support for Radio Access Technologies (RATs)

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Other PLMNs with signals of high received quality, in a random order. The high quality of signals is defined as the Reference Signal Received Power (RSRP) of a cell in the PLMN being greater than or equal to -110 dBm.

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Other PLMNs in order of decreasing signal quality



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When the UE finds a suitable cell in a PLMN during the preceding procedure, the UE attempts to register on the PLMN. If the registration is successful, the UE camps on the suitable cell; otherwise, the UE tries the next PLMN.

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If the registration on none of the PLMNs is successful, the UE enters the limited service state. If no PLMN is available, the UE cannot obtain any service.

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PLMN Selection in Manual Mode 

When a UE fails to register on its last registered PLMN, the UE attempts to select another PLMN. In manual network selection mode, the UE displays a list of PLMNs, in the same order as the PLMNs to be selected in automatic network selection mode.

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In manual mode, the UE can also try forbidden PLMNs. If the UE succeeds in registering on the selected PLMN, the PLMN is no more a forbidden PLMN to the UE. The forbidden PLMNs are determined by the EPC.

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Roaming 

When in roaming state, a UE obtains service from a VPLMN that is different from its HPLMN. For example, the UE obtains service from a PLMN in another country (that is, from an international roaming area). When the UE with the roaming service moves out of the coverage area of its HPLMN, the UE can obtain service from another PLMN according to the PLMN information stored in the USIM. The operator determines whether roaming is allowed for a UE.

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After the UE registers on a VPLMN by roaming, it periodically searches for its HPLMN and attempts to return to the HPLMN. The time interval between consecutive searches for the HPLMN is defined by the operator and stored in the USIM.



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After selecting a PLMN, a UE selects a cell in the PLMN to camp on. When camped on the cell, the UE monitors system information, performs measurements on the serving cell and neighboring cells, and selects a better cell to camp on, according to measurement rules and cell reselection criteria.

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Before a UE selects a cell to camp on, it performs a cell search procedure. After the UE finds a cell, it performs cell selection or reselection, according to the selection or reselection related parameters in the system information or in the RRC Connection Release message. Figure 3-2 shows the cell selection and reselection procedures. A UE can use one of the following two cell selection procedures: Stored Information Cell Selection and Initial Cell Selection. The Stored Information Cell Selection procedure takes precedence over the Initial Cell Selection procedure. The UE uses the latter procedure only when it fails to find a suitable cell by using the former procedure.



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Cell search is a procedure in which a UE achieves time and frequency synchronization with a cell, obtains the physical cell ID, and learns the signal quality and other information about the cell based on the physical cell ID. Before selecting or reselecting a cell, a UE performs a cell search on all carrier frequencies.

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In the Long Term Evolution (LTE) system, Synchronization Channels (SCHs) are specially used for cell search. There are two types of SCH: Primary Synchronization Channel (P-SCH) and Secondary Synchronization Channel (S-SCH). The cell search procedure on SCHs is as follows: 

The UE listens to the P-SCH to achieve clock synchronization with a maximum synchronization error of 5 ms. Physical cell IDs have a one-to-one correspondence with primary synchronization signals. Therefore, the UE acquires the physical cell ID by monitoring the P-SCH

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The UE listens to the S-SCH to achieve frame synchronization, that is, time synchronization with the cell. Cell ID groups have a one-to-one relation with secondary synchronization signals. Therefore, the UE acquires the number of the cell ID group to which the physical cell ID belongs by monitoring the S-SCH.

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The UE listens to the downlink reference signal to acquire the signal quality in the cell.

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The UE listens to the Broadcast Channel (BCH) to acquire other information about the cell.



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During cell selection, a UE needs to check whether a cell fulfils the cell selection criteria. The cell selection is based on the RSRP of the E-UTRAN cell. Before a UE can select a cell to camp on, the RSRP of the cell must be higher than the user-defined minimum receive (RX) level Qrxlevmin of the cell.

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The formula for cell selection decision is as follows: 

Srxlev > 0

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where Srxlev = Qrxlevmeas - (Qrxlevmin + Qrxlevminoffset) - Pcompensation



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The threshold is setting by the following command


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The following parameters are regarding to inter-frequency/RAT measurement

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A UE makes a cell reselection decision according to cell reselection criteria. When making a decision on reselection to an intra-frequency or equal-priority inter-frequency cell, the UE checks whether the signal quality of a neighboring cell is higher than that of the serving cell. The UE evaluates the neighboring cell only after the cell meets the cell selection criteria.

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The cell-ranking criteria R_s for the serving cell and R_n for neighboring cells are defined as follows: 

R_s = Qmeas,s + Qhyst

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R_n = Qmeas,n - CellQoffset

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Qmeas_s is the measured RSRP of the serving cell, expressed in units of dBm

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Qhyst is the hysteresis for the serving cell used in the ranking criteria, expressed in units of dB.

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Qmeas_n is the measured RSRP of the neighboring cell, expressed in units of dBm

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CellQoffset is the offset for the neighboring cell used in the ranking criteria, expressed in units of dB

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During cell reselection, the UE needs to check whether access to that cell is allowed according to the cellAccessRelatedInfo Information Element (IE) in the SIB1. If the cell is barred, it must be excluded from the candidate list, and the UE does not consider the cell as a candidate for cell reselection. If the cell is unsuitable because it is part of the list of forbidden TAs for roaming or it does not belong to the registered PLMN or an EPLMN, the UE does not consider this cell and other cells on the same frequency as candidates for reselection for a maximum of 300 seconds.



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The following parameters are regarding to inter-RAT/different priority cell reselecsion



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The UE performs TA updates in the following situations. That is, the UE sends a TA update request when any of the following conditions is met 

The UE detects that it has entered a new TA when it finds that the TAI in the system information is different from any TAI stored in its USIM.

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The periodic TA update timer expires.

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The UE performs reselection to an E-UTRAN cell from another RAT system.

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The information on UE capabilities stored in the EPC changes.



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The MIB is transmitted over the BCH through an independent RRC message. The scheduling period of the MIB is 40 ms. The BCH is predefined by transport format. Therefore, the UE receives the MIB on the BCH without obtaining other information on the network.



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The scheduling period of the SIB1 is fixed to 80 ms. The first transmission of the SIB1 is scheduled in subframe 5 of radio frames for which the SFN mod 8 = 0, and repetitions are scheduled in subframe 5 of the later radio frames for which SFN mod 2 = 0 in each period.



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The scheduling periods of other SIBs are determined by the SibxPeriod (x = 2, 3, …, 8) parameter. The mapping of an SIB to the SI is affected by the scheduling period of the SIB, number of SIBs, and bandwidth resources.



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UEs in RRC_IDLE mode use DRX to receive paging messages in order to reduce power consumption. The positions of paging messages are fixed on the Uu interface, which are indicated by the Paging Frames (PFs) and Paging Occasion (PO) subframes. One PF is one radio frame, which may contain one or multiple POs, as shown above. One PO is a subframe where the Paging Radio Network Temporary Identifier (P-RNTI) is contained. The PO is transmitted over the PDCCH. UEs read paging messages over the Physical Downlink Shared Channel (PDSCH) according to the P-RNTI.

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The PF number and PO subframe number can be calculated according to the IMSI of the UE, DRX cycle, and number of PO subframes in a DRX cycle. Frame numbers are stored in the SI related to the DRX parameters of the UE. When the DRX parameters change, PF number and PO subframe number change accordingly.

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The SFN of a PF is derived from the following formula: 

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SFN mod T = (T div N) x (UE_ID mod N)

The subframe number i_s of a PO is derived from the following formula: 

i_s = (UE_ID/N) mod Ns



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The meaning of parameters is following: 

T is the length of a DRX cycle. It is determined by the shortest UE-specific DRX cycle. The value of T can be either signaled from the NAS or specified by the DefaultPagingCycle parameter. If the NAS indicates the DRX cycle, the UE compares the value signaled from the NAS and the value of DefaultPagingCycle and uses the smaller value between them. If the NAS does not indicate the DRX cycle, the value of T is determined by the DefaultPagingCycle parameter and sent to the UE through an SI message.

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N is min(T,NB). The NB parameter specifies the number of PO subframes in a DRX cycle. Based on the actual configuration on the eNodeB, NB can be set to 4T, 2T, T, T/2, T/4, T/8, T/16, or T/32.

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Ns is max(1,NB/T). UE_ID is IMSI mod 1024. If a UE without IMSI makes an emergency call, the UE should use the default UE_ID value of 0.




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OEO100010 LTE Idle Mode Behavior ISSUE1.00.pdf

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