7 Paging

38.3043GPPNRRelease 17TSUser Equipment (UE) procedures in idle mode and in RRC Inactive state

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

7.1 Discontinuous Reception for paging

The UE may use Discontinuous Reception (DRX) in RRC_IDLE and RRC_INACTIVE state in order to reduce power consumption. The UE monitors one paging occasion (PO) per DRX cycle. A PO is a set of PDCCH monitoring occasions and can consist of multiple time slots (e.g. subframe or OFDM symbol) where paging DCI can be sent (TS 38.213 [4]). One Paging Frame (PF) is one Radio Frame and may contain one or multiple PO(s) or starting point of a PO. A L2 U2N Relay UE monitors the paging occasions of its PC5-RRC connected L2 U2N Remote UEs. In this case, the DRX cycle and UE ID mentioned in this clause refer to those of the L2 U2N Remote UE.

In multi-beam operations, the UE assumes that the same paging message and the same Short Message are repeated in all transmitted beams and thus the selection of the beam(s) for the reception of the paging message and Short Message is up to UE implementation. The paging message is same for both RAN initiated paging and CN initiated paging.

The UE initiates RRC Connection Resume procedure upon receiving RAN initiated paging. If the UE receives a CN initiated paging in RRC_INACTIVE state, the UE moves to RRC_IDLE and informs NAS. However, if a L2 U2N Relay UE in RRC_INACTIVE state receives a CN initiated paging for a L2 U2N Remote UE, the L2 U2N Relay UE does not move to RRC_IDLE state.

NOTE 0a: The L2 U2N Remote UE does not need to monitor the PO in order to receive the paging message.

NOTE 0b: While the SDT procedure is ongoing in RRC_INACTIVE state, the UE monitors the PO in order to receive only the Short Message as specified in TS 38.331 [3].

The PF and PO for paging are determined by the following formulae:

SFN for the PF is determined by:

(SFN + PF_offset) mod T = (T div N)*(UE_ID mod N)

Index (i_s), indicating the index of the PO is determined by:

i_s = floor (UE_ID/N) mod Ns

The PDCCH monitoring occasions for paging are determined according to pagingSearchSpace as specified in TS 38.213 [4] and firstPDCCH-MonitoringOccasionOfPO and nrofPDCCH-MonitoringOccasionPerSSB-InPO if configured as specified in TS 38.331 [3]. When SearchSpaceId = 0 is configured for pagingSearchSpace, the PDCCH monitoring occasions for paging are same as for RMSI as defined in clause 13 in TS 38.213 [4].

When SearchSpaceId = 0 is configured for pagingSearchSpace, Ns is either 1 or 2. For Ns = 1, there is only one PO which starts from the first PDCCH monitoring occasion for paging in the PF. For Ns = 2, PO is either in the first half frame (i_s = 0) or the second half frame (i_s = 1) of the PF.

When SearchSpaceId other than 0 is configured for pagingSearchSpace, the UE monitors the (i_s + 1)^th PO. A PO is a set of ‘S*X ‘ consecutive PDCCH monitoring occasions where ‘S’ is the number of actual transmitted SSBs determined according to ssb-PositionsInBurst in SIB1 and X is the nrofPDCCH-MonitoringOccasionPerSSB-InPO if configured or is equal to 1 otherwise. The [x*S+K]^th PDCCH monitoring occasion for paging in the PO corresponds to the K^th transmitted SSB, where x=0,1,…,X-1, K=1,2,…,S. The PDCCH monitoring occasions for paging which do not overlap with UL symbols (determined according to tdd-UL-DL-ConfigurationCommon) are sequentially numbered from zero starting from the first PDCCH monitoring occasion for paging in the PF. When firstPDCCH-MonitoringOccasionOfPO is present, the starting PDCCH monitoring occasion number of (i_s + 1)^th PO is the (i_s + 1)^th value of the firstPDCCH-MonitoringOccasionOfPO parameter; otherwise, it is equal to i_s * S*X. If X > 1, when the UE detects a PDCCH transmission addressed to P-RNTI within its PO, the UE is not required to monitor the subsequent PDCCH monitoring occasions for this PO.

NOTE 1: A PO associated with a PF may start in the PF or after the PF.

NOTE 2: The PDCCH monitoring occasions for a PO can span multiple radio frames. When SearchSpaceId other than 0 is configured for paging-SearchSpace the PDCCH monitoring occasions for a PO can span multiple periods of the paging search space.

The following parameters are used for the calculation of PF and i_s above:

T: DRX cycle of the UE.

If eDRX is not configured as defined in clause 7.4:

– T is determined by the shortest of the UE specific DRX value(s), if configured by RRC and/or upper layers or provided in PC5-RRC signalling in case of a L2 U2N Relay UE, and a default DRX value broadcast in system information. In RRC_IDLE state, if UE specific DRX is not configured by upper layers, the default value is applied.

In RRC_IDLE state, if eDRX is configured by upper layers, i.e., T_{eDRX, CN}, according to clause 7.4:

– If T_{eDRX, CN} is no longer than 1024 radio frames:

– T = T_{eDRX, CN};

– else:

– During CN configured PTW, T is determined by the shortest of UE specific DRX value, if configured by upper layers, and the default DRX value broadcast in system information.

In RRC_INACTIVE state, if eDRX is configured by RRC, i.e., T_{eDRX, RAN} , and/or upper layers, i.e., T_{eDRX, CN}, as defined in clause 7.4:

– If both T_{eDRX, CN} and T_{eDRX, RAN} are no longer than 1024 radio frames, T = min{T_{eDRX, RAN}, T_{eDRX, CN}}.

– If T_{eDRX, CN} is no longer than 1024 radio frames and no T_{eDRX, RAN} is configured, T is determined by the shortest of UE specific DRX value configured by RRC and T_{eDRX, CN}.

– If T_{eDRX, CN} is longer than 1024 radio frames:

– If T_{eDRX, RAN} is not configured:

– During CN configured PTW, T is determined by the shortest of the UE specific DRX value (s), if configured by RRC and/or upper layers, and a default DRX value broadcast in system information. Outside the CN configured PTW, T is determined by the UE specific DRX value configured by RRC;

– else if T_{eDRX, RAN} is no longer than 1024 radio frames:

– During CN configured PTW, T is determined by the shortest of the UE specific DRX value, if configured by upper layers and T_{eDRX, RAN}, and a default DRX value broadcast in system information. Outside the CN configured PTW, T is determined by T_{eDRX, RAN}.

N: number of total paging frames in T

Ns: number of paging occasions for a PF

PF_offset: offset used for PF determination

UE_ID:

If the UE operates in eDRX as specified in clause 7.4:

– 5G-S-TMSI mod 4096

else:

– 5G-S-TMSI mod 1024

Parameters Ns, nAndPagingFrameOffset, nrofPDCCH-MonitoringOccasionPerSSB-InPO, and the length of default DRX Cycle are signaled in SIB1. The values of N and PF_offset are derived from the parameter nAndPagingFrameOffset as defined in TS 38.331 [3]. The parameter firstPDCCH-MonitoringOccasionOfPO is signalled in SIB1 for paging in the BWP configured by initialDownlinkBWP. For paging in a DL BWP other than the BWP configured by initialDownlinkBWP, the parameter first-PDCCH-MonitoringOccasionOfPO is signaled in the corresponding BWP configuration.

If the UE has no 5G-S-TMSI, for instance when the UE has not yet registered onto the network, the UE shall use as default identity UE_ID = 0 in the PF and i_s formulas above.

5G-S-TMSI is a 48 bit long bit string as defined in TS 23.501 [10]. 5G-S-TMSI shall in the formulae above be interpreted as a binary number where the left most bit represents the most significant bit.

In RRC_INACTIVE state, if the UE supports inactiveStatePO-Determination and the network broadcasts ranPagingInIdlePO with value "true", the UE shall use the same i_s as for RRC_IDLE state. Otherwise, the UE determines the i_s based on the parameters and formula above.

In RRC_INACTIVE state, if eDRX value configured by upper layers is no longer than 1024 radio frames, the UE shall use the same i_s as for RRC_IDLE state.

In RRC_INACTIVE state, if eDRX value configured by upper layers is longer than 1024 radio frames, during CN PTW, the UE shall use the same i_s as for RRC_IDLE state.

7.2 Paging Early Indication

7.2.1 Paging Early Indication reception

The UE may use Paging Early Indication (PEI) in RRC_IDLE and RRC_INACTIVE states in order to reduce power consumption. If PEI configuration is provided in system information, the UE in RRC_IDLE or RRC_INACTIVE state supporting PEI (except for the UEs expecting multicast session activation notification) can monitor PEI using PEI parameters in system information according to the procedure described below.

If lastUsedCellOnly is configured in system information of a cell, the UE monitors PEI in the cell only if the UE most recently received RRCRelease without noLastCellUpdate in this cell. Otherwise (i.e., if lastUsedCellOnly is not configured in system information of a cell), the UE monitors PEI in the camped cell.

The UE monitors one PEI occasion per DRX cycle. A PEI occasion (PEI-O) is a set of PDCCH monitoring occasions (MOs) and can consist of multiple time slots (e.g. subframes or OFDM symbols) where PEI can be sent (TS 38.213 [4]). In multi-beam operations, the UE assumes that the same PEI is repeated in all transmitted beams and thus the selection of the beam(s) for the reception of the PEI is up to UE implementation.

The time location of PEI-O for UE’s PO is determined by a reference point and an offset:

– The reference point is the start of a reference frame determined by a frame-level offset from the start of the first PF of the PF(s) associated with the PEI-O, provided by pei-FrameOffset in SIB1;

– The offset is a symbol-level offset from the reference point to the start of the first PDCCH MO of this PEI-O, provided by firstPDCCH-MonitoringOccasionOfPEI-O in SIB1.

If one PEI-O is associated with POs of two PFs, the two PFs are consecutive PFs calculated by the parameters PF_offset, T, Ns, and N. The first PF of the PFs associated with the PEI-O is provided by (SFN for PF) – floor (i_PO/Ns)*T/N, where SFN for PF is determined in clause 7.1, i_PO is defined in clause 10.4a in TS 38.213[4], T, Ns, and N are determined in clause 7.1.

The PDCCH MOs for PEI are determined as specified in TS 38.213 [4] according to pei-SearchSpace, pei-FrameOffset, firstPDCCH-MonitoringOccasionOfPEI-O and nrofPDCCH-MonitoringOccasionPerSSB-InPO if configured as specified in TS 38.331 [3]. When SearchSpaceId = 0 is configured for pei-SearchSpace, the PDCCH MOs for PEI are same as for RMSI as defined in clause 13 in TS 38.213 [4]. UE determines first PDCCH MO for PEI-O based on pei-FrameOffset and firstPDCCH-MonitoringOccasionOfPEI-O, as for the case with SearchSpaceId > 0 configured.

When SearchSpaceId = 0 is configured for pei-SearchSpace, the UE monitors the PEI-O according to searchSpaceZero. When SearchSpaceId other than 0 is configured for pei-SearchSpace, the UE monitors the PEI-O according to the search space of the configured SearchSpaceId.

A PEI occasion is a set of ‘S*X’ consecutive PDCCH monitoring occasions, where ‘S’ is the number of actual transmitted SSBs determined according to ssb-PositionsInBurst in SIB1, and X is the nrofPDCCH-MonitoringOccasionPerSSB-InPO if configured or is equal to 1 otherwise. The [x*S+K]^th PDCCH MO for PEI in the PEI occasion corresponds to the K^th transmitted SSB, where x=0,1,…,X-1, K=1,2,…,S. The PDCCH MOs for PEI which do not overlap with UL symbols (determined according to tdd-UL-DL-ConfigurationCommon) are sequentially numbered from zero starting from the first PDCCH MO for PEI in the PEI-O. When the UE detects a PEI within its PEI-O, the UE is not required to monitor the subsequent monitoring occasion(s) associated with the same PEI-O.

If the UE detects PEI and the PEI indicates the subgroup the UE belongs to monitor its associated PO, as specified in clause 10.4a in TS 38.213 [4], the UE monitors the associated PO as specified in clause 7.1. If the UE does not detect PEI on the monitored PEI occasion or the PEI does not indicate the subgroup the UE belongs to monitor its associated PO, as specified in clause 10.4a in TS 38.213 [4], the UE is not required to monitor the associated PO as specified in clause 7.1.

If the UE is unable to monitor the PEI occasion (i.e. all valid PDCCH MO for PEI) corresponding to its PO, e.g. during cell re-selection, the UE monitors the associated PO according to clause 7.1.

In RRC_INACTIVE state, if the UE supports inactiveStatePO-Determination and the network broadcasts ranPagingInIdlePO with value "true", the UE shall use the same i_PO as for RRC_IDLE state. Otherwise, the UE determines the i_PO based on the formula defined in clause 10.4a in TS 38.213 [4].

7.3 Subgrouping

7.3.0 General

If PEI and subgrouping are configured, UEs monitoring the same PO can be divided into one or more subgroups. With subgrouping, the UE monitors the associated PO if the corresponding bit for subgroup the UE belongs to is indicated as 1 by PEI corresponding to its PO, as specified in clause 10.4a in TS 38.213 [4].

The following parameters are used for the determination of subgroup ID:

– subgroupsNumPerPO: total number of subgroups for both CN assigned subgrouping (if any) and UE_ID based subgrouping (if any) in a PO, which is broadcasted in system information;

– subgroupsNumForUEID: number of subgroups for UE_ID based subgrouping in a PO, which is broadcasted in system information.

UE’s subgroup can be either assigned by CN as specified in clause 7.3.1 or formed based on UE_ID as specified in clause 7.3.2:

– If subgroupsNumForUEID is absent in subgroupConfig, the subgroup ID based on CN assigned subgrouping as specified in clause 7.3.1, if available for the UE, is used in the cell.

– If both subgroupsNumPerPO and subgroupsNumForUEID are configured, and subgroupsNumForUEID has the same value as subgroupsNumPerPO, the subgroup ID based on UE_ID based subgrouping as specified in clause 7.3.2 is used in the cell.

– If both subgroupsNumPerPO and subgroupsNumForUEID are configured, and subgroupsNumForUEID < subgroupsNumPerPO:

– The subgroup ID based on CN assigned subgrouping as specified in clause 7.3.1, if available for the UE, is used in the cell;

– Otherwise, the subgroup ID based on UE_ID based subgrouping as specified in clause 7.3.2 is used in the cell.

If a UE has no CN assigned subgroup ID or does not support CN assigned subgrouping, and there is no configuration for subgroupsNumForUEID, the UE monitors the associated PO according to clause 7.1.

7.3.1 CN assigned subgrouping

Paging with CN assigned subgrouping is used in the cell which supports CN assigned subgrouping, as described in clause 7.3.0. A UE supporting CN assigned subgrouping in RRC_IDLE or RRC_INACTIVE state can be assigned a subgroup ID (between 0 to 7) by AMF through NAS signalling. The UE belonging to the assigned subgroup ID monitors its associated PEI which indicates the paged subgroup(s) as specified in clause 7.2.

7.3.2 UE_ID based subgrouping

Paging with UE_ID based subgrouping is used in the cell which supports UE_ID based subgrouping, as described in clause 7.3.0.

If the UE is not configured with a CN assigned subgroup ID, or if the UE configured with a CN assigned subgroup ID is in a cell supporting only UE_ID based subgrouping, the subgroup ID of the UE is determined by the formula below:

SubgroupID = (floor(UE_ID/(N*Ns)) mod subgroupsNumForUEID) + (subgroupsNumPerPO – subgroupsNumForUEID),

where:

N: number of total paging frames in T, which is the DRX cycle of RRC_IDLE state as specified in clause 7.1

Ns: number of paging occasions for a PF

UE_ID: 5G-S-TMSI mod X, where X is 32768, if eDRX is applied; otherwise, X is 8192

subgroupsNumForUEID: number of subgroups for UE_ID based subgrouping in a PO, which is broadcasted in system information

The UE belonging to the SubgroupID monitors its associated PEI which indicates the paged subgroup(s) as specified in clause 7.2.

7.4 Paging in extended DRX

The UE may be configured by upper layers and/or RRC with an extended DRX (eDRX) cycle T_{eDRX, CN} and/or T_{eDRX, RAN}. The UE operates in eDRX for CN paging in RRC_IDLE or RRC_INACTIVE states if the UE is configured for eDRX by upper layers and eDRX-AllowedIdle is signalled in SIB1. The UE operates in eDRX for RAN paging in RRC_INACTIVE state if the UE is configured for eDRX by RAN and eDRX-AllowedInactive is signalled in SIB1. If the UE is configured with an extended DRX cycle no longer than 1024 radio frames, it monitors POs as defined in 7.1 with configured eDRX cycle. Otherwise, a UE configured with eDRX monitors POs as defined in 7.1 during a periodic Paging Time Window (PTW) configured for the UE. The PTW is UE-specific and is determined by a Paging Hyperframe (PH), a starting position within the PH (PTW_start) and an ending position (PTW_end). PH, PTW_start and PTW_end are given by the following formula:

The PH for CN is the H-SFN satisfying the following equations:

H-SFN mod T_{eDRX_CN}= (UE_ID_H mod T_{eDRX_CN}), where

– UE_ID_H: 13 most significant bits of the Hashed ID.

– T_{eDRX_CN}: UE-specific eDRX cycle in Hyper-frames, (T_{eDRX_CN}= 2, …, 1024 Hyper-frames) configured by upper layers.

PTW_start denotes the first radio frame of the PH that is part of the PTW and has SFN satisfying the following equation:

SFN = 128 * i_{eDRX_CN}, where

– i_{eDRX_CN} = floor(UE_ID_H /T_{eDRX_CN}) mod 8

PTW_end is the last radio frame of the PTW and has SFN satisfying the following equation:

SFN = (PTW_start + L*100 – 1) mod 1024, where

– L = Paging Time Window (PTW) length (in seconds) configured by upper layers

Hashed ID is defined as follows:

Hashed_ID is Frame Check Sequence (FCS) for the bits b31, b30…, b0 of 5G-S-TMSI.

5G-S-TMSI = <b47, b46, …, b0> as defined in TS 23.003 [23].

The 32-bit FCS shall be the ones complement of the sum (modulo 2) of Y1 and Y2, where

– Y1 is the remainder of x^k (x³¹ + x³⁰ + x²⁹ + x²⁸ + x²⁷ + x²⁶ + x²⁵ + x²⁴ + x²³ + x²² + x²¹ + x²⁰ + x¹⁹ + x¹⁸ + x¹⁷ + x¹⁶+ x¹⁵ + x¹⁴ + x¹³ + x¹² + x¹¹ + x¹⁰ + x⁹ + x⁸ + x⁷ + x⁶ + x⁵ + x⁴ + x³ + x² + x¹ + 1) divided (modulo 2) by the generator polynomial x³² + x²⁶ + x²³ + x²² + x¹⁶ + x¹² + x¹¹ + x¹⁰ + x⁸ + x⁷ + x⁵ + x⁴ + x² + x + 1, where k is 32; and

– Y2 is the remainder of Y3 divided (modulo 2) by the generator polynomial x³² + x²⁶ + x²³ + x²² + x¹⁶ + x¹² + x¹¹ + x¹⁰ + x⁸ + x⁷ + x⁵ + x⁴ + x² + x + 1, where Y3 is the product of x³² by "b31, b30…, b0 of S-TMSI or 5G-S-TMSI", i.e., Y3 is the generator polynomial x³² (b31*x³¹ + b30*x³⁰ + … + b0*1).

NOTE: The Y1 is 0xC704DD7B for any 5G-S-TMSI value. An example of hashed ID calculation is in Annex A.