Description in webhttp://lteuniversity.com/get_trained/expert_opinion1/b/prakash/archive/2013/04/17/discontinuous-reception-drx-in-rrc-idle-part-1.aspxOne of the important activities performed by UE during RRC Idle mode is listening to paging message. The UE should listen to the paging message to know about incoming calls, system information change, and ETWS (Earthquake and Tsunami Warning Service) notification for ETWS capable UEs. In LTE, the UE needs to monitor PDCCH continuously (every 1ms) for P-RNTI to listen to paging message and this drains UE battery power heavily. Instead, if the UE is allowed to monitor P-RNTI in PDCCH, only at predetermined period say every 60ms or 100ms, it will reduce UE’s power consumption. The process of monitoring PDCCH discontinuously for P-RNTI to listen to paging message during RRC idle state is known as Discontinuous Reception (DRX) in RRC_Idle.What is Idle mode for LTE UE?

No NAS Signaling connection exists between UE and core network At least one PDN Connection exists UE is registered with MME & EMM context exists at MME UE location is known to the MME with an accuracy of a list of tracking areas No RRC connection exists between UE and eNB The eNB has no context information

Paging in Idle ModePaging is a procedure to transmit paging message to the UE in RRC Idle mode or EMM registered state. Paging message may be triggered either by MME in core network or eNB in Radio Access Network.Discontinuous Reception for PagingPaging information for the UE is being carried on the PDSCH in the resource blocks indicated by the PDCCH. Paging indication on the PDCCH is single fixed indicator (FFFE) called the Paging RNTI (P-RNTI). Different group of UEs monitor different sub frames for their paging messages.UEs in idle mode monitor the PDCCH channel for P-RNTI used for paging to receive paging message from eNB. The UE needs to monitor the PDCCH only at certain UE specific paging occasion, i.e. only at specific subframe in specific radio frame. At other times the UE may go for sleep mode to save battery power.Let us discuss how can UE find out paging frame and paging occasion for its paging message.Paging Frame (PF): One Paging Frame (PF) is one Radio Frame in which the UE is looking for paging message. One Paging Frame may contain one or multiple Paging Occasion(s).Paging Occasion (PO): It is a subframe where there may be P-RNTI transmitted on PDCCH addressing the paging message for the UE. There is always only one paging occasion for each UE in a DRX cycle.To find out PF (Paging Frame) and PO (Paging Occasion), the UE uses DRX parameters broadcasted in the system information SIB2.

 Significance of DRX Parameters:DRX cycle: Indicates the number of radio frames in the paging cycle. Larger the value of this parameter, lesser the UE battery power consumption. Whereas smaller the value of DRX cycle, increase in the UE battery power consumption. DRX cycle may be cell specific or UE specific parameter. DRX cycle is cell specific when it is configured at eNB and broadcasts to all UEs in a cell in system information SIB2 as default paging cycle. DRX cycle is UE specific when MME may configure DRX cycle and provide it to UE in NAS signaling as’ UE specific DRX cycle’ and to eNB in PAGING S1 AP message as ‘Paging DRX’ for MME initiated paging message. Both UE and eNB use minimum of default and dedicated DRX cycle.

 An UE with DRX cycle of 128 radio frames needs to wake up every 1.28 sec and look for paging message.nB: This cell specific parameter indicates number of Paging occasions in a cell specific DRX cycle. Configuration of nB value depends on paging capacity required in a cell. The larger the value of nB is configured to, the larger the paging capacity. Likewise, the smaller the paging capacity when the value of nB is smaller.

 With this we can infer that a maximum of 1 radio frame is available per DRX cycle of the UE for paging. Also, the minimum of 1 sub frame per radio frame (nB ≤ 1T) and maximum of 4 sub frames (for nB=4T) are available for paging message for the UE.Calculation of Paging Frame NumberBoth the eNB and the UE calculate the Paging Frame number by the following relation:

 The eNB will receive UE_ID from the MME in PAGING S1AP message as “UE Identity Index Value” for MME initiated paging message.The UE will find out the UE_ID by using the relation IMSI mod 1024 (Ref: TS 36.304 section 7.1)The IMSI is given as sequence of digits of type Integer (0..9). The IMSI shall, in the formulae above, be interpreted as a decimal integer number, where the first digit given in the sequence represents the highest order digit.For example: IMSI = 12 (digit1=1, digit2=2). In the calculations, this shall be interpreted as the decimal integer "12", not "1x16+2 = 18".Calculation of Paging Occasion (Paging Sub frame Number in a Paging Frame)

 Paging Occasion for the UE is obtained from the predefined subframe pattern (36.304 section 7.2, shown below for quick reference) based on the value of Ns, i_s. For example, if Ns=2 and i_s = 1, UE interprets that sub frames 4 and 9 are configured for paging in the cell and subframe 9 is paging occasion for it.Subframe Patterns (Ref: TS 36.304 section 7.2)FDD

 TDD (All DL/UL Patterns)

Example in webhttp://lteinwireless.blogspot.kr/2012/12/paging-in-lte.html

S-GW:1. DL data arrives for the UE2. Creates DL Data Notification message and forwards to MME

MME:   1. When UE is in ECM-Idle state, the UE location is known to MME on a per TA basis. Therefore, MME has to page all eNB's within a group of TA.2. MME starts timer 3413when:    a. Paging is for PS data    b. UE is addressed by S-TMSI3. List of TAI: informs the eNB to broadcast the Paging messages in the mentioned TAI's4. Sends DL Data Notification ACK message to S-GW

eNB:1. Receives S1AP: Paging and constructs RRC: Paging message2. RRC: Paging message may contain multiple PAGING RECORDS to page multiple UE's

UE:1. Wakes up every Paging occasion and searches for P-RNTI within the PDCCH transmission2. If UE finds the P-RNTI then it proceeds to decode the PDSCH information which is present in PDCCH3. UE decodes RRC:Paging from the PDSCH Resource Block within which the PAGING message is sent4. If UE doesn't find it's own UE identity then it returns to monitor the Paging Occasion.5. When UE finds it's identity in the message it triggers the Random Access Procedure. followed by establishing the RRC Connection.6. If PAGING is for PS domain then UE NAS layer triggers SERVICE REQUEST otherwise, if the PAGING is for CSFB then UE triggers Extended SERVICE REQUEST.

Thought:How does UE determine the Paging Frame and Paging Occasion? ORHow does eNB know when to send the Paging message so that UE will be awake to receive the message?

During this specific occurrence (Frame and Occasion), UE will wake up to check if there is any PAGING intended to it. If there is no information, UE will be go back to sleep. This happens when UE is in IDLE mode after registration. The related parameters are sent through SIB2. Therefore,a synchronization exist between the UE and eNB and both of them know when to look for Paging(UE) and when to transmit the Paging (eNB).

Paging Frame calulation:Formula for calculating Paging Frame:::  SFN mod T=  (T div N) * (UE_id mod N)

where, - T: DRX cycle of the UE. T is determined by the shortest of the UE specific DRX value, if allocated by upper layers, and a default DRX value broadcast in system information. If UE specific DRX is not configured by upper layers, the default value is applied. - nB: 4T, 2T, T, T/2, T/4, T/8, T/16, T/32.- N: min(T,nB)- Ns: max(1,nB/T)- UE_ID: IMSI mod 1024Parameters broadcasted in SIB2:Default Paging cycle=T; (value=rf128)nB= Broadcasted in SIB2 (value=1T)

Example:Calculation:

a.       IMSI of UE (in DECIMAL)= 262022008880715 b.      N=128 (Look for the formula above to determine the value of N)c.       UE_id = 262022008880715 mod 1024= 587d.      (UE_id mod N)= 587 mod 128=75e.      (T div N)= 128 / 128 = 1

Therefore, SFN mod T =75So, whenever SFN mod T will be 75, UE will wake to read if there is a Paging message addressed to it. 

From this particular SFN the frame is derived. Now, the frame is identified but we also need to know that on which sub frame the Paging information is present. To determine, this Paging Ocassion is used.

Paging Ocassion calculation:Formula for calculating the PO::: i_s = floor(UE_ID/N) mod Ns

a.       Ns=1 (Look for the formula above to determine the value of N)b.      floor (587/128)= 4

i_s = 0

Index i_s points to PO from the sub-frame pattern defined in the below table will be derived from following calculation: Table taken from 3GPP for FDD:Ns PO when i_s=0 PO when i_s=1 PO when i_s=2 PO when i_s=31 9 N/A N/A N/A2 4 9 N/A N/A4 0 4 5 9

The Ns is 1 and PO would be 9 since i_s =0 in this example. Also, Paging Frame is derived as 75.

Therefore, from this example, whenever SFN mod T will be equal to 75, UE will look for P-RNTI in sub-frame 9 of that particular Radio Frame.

Example for each nB, Number of paging occasions per DRX cycleT_C

T_UE T

DRX cycl

e nB N N_

SUe_ID_mod

_NNum of PO during DRX Cycle frame

(ms)

256 128 12

81,28

0 4T 128 4 75 512

256 128 12

81,28

0 2T 128 2 75 256

256 128 12

81,28

0 T 128 1 75 128

256 128 12

81,28

0 T/2 64 1 11 64256 128 12

81,28

0 T/4 32 1 11 32256 128 12

81,28

0 T/8 16 1 11 16256 128 12

81,28

0T/16 8 1 3 8

256 128 12

81,28

0T/32 4 1 3 4

T_C Cell specific DRX cycle, signaled by eNodeB via SIB2T_UE Cell specific DRX cycle, signaled by MMET DRX cycle (paging cycle) of UEn_B Number of paging occasions per DRX cycle, signaled by eNodeB via

SIB2The larger the value of nB is configured to, the larger the paging capacity. Likewise, the smaller the paging capacity when the value of nB is smaller.

N Number of paging framesN_S Number of paging subframesue_ID_mod_N

Paging frame SFN modular

i_s index to Paging Occasion from subframe pattern

36.304 Paging7.1 Discontinuous Reception for pagingThe UE may use Discontinuous Reception (DRX) in idle mode in order to reduce power consumption. One Paging Occasion (PO) is a subframe where there may be P-RNTI transmitted on PDCCH addressing the paging message. One Paging Frame (PF) is one Radio Frame, which may contain one or multiple Paging Occasion(s). When DRX is used the UE needs only to monitor one PO per DRX cycle.PF and PO is determined by following formulae using the DRX parameters provided in System Information:

PF is given by following equation:SFN mod T= (T div N)*(UE_ID mod N)

Index i_s pointing to PO from subframe pattern defined in 7.2 will be derived from following calculation:

i_s = floor(UE_ID/N) mod NsSystem Information DRX parameters stored in the UE shall be updated locally in the UE whenever the DRX parameter values are changed in SI. If the UE has no IMSI, for instance when making an emergency call without USIM, the UE shall use as default identity UE_ID = 0 in the PF and i_s formulas above.The following Parameters are used for the calculation of the PF and i_s:

- T: DRX cycle of the UE. T is determined by the shortest of the UE specific DRX value, if allocated by upper layers, and a default DRX value broadcast in system information. If UE specific DRX is not configured by upper layers, the default value is applied.

- nB: 4T, 2T, T, T/2, T/4, T/8, T/16, T/32.- N: min(T,nB)- Ns: max(1,nB/T)- UE_ID: IMSI mod 1024.

IMSI is given as sequence of digits of type Integer (0..9), IMSI shall in the formulae above be interpreted as a decimal integer number, where the first digit given in the sequence represents the highest order digit.For example:

IMSI = 12 (digit1=1, digit2=2)In the calculations, this shall be interpreted as the decimal integer "12", not "1x16+2 = 18".7.2 Subframe PatternsFDD:

Ns PO when i_s=0 PO when i_s=1 PO when i_s=2 PO when i_s=31 9 N/A N/A N/A2 4 9 N/A N/A4 0 4 5 9

TDD (all UL/DL configurations):Ns PO when i_s=0 PO when i_s=1 PO when i_s=2 PO when i_s=31 0 N/A N/A N/A2 0 5 N/A N/A4 0 1 5 6

36.331 RadioResourceConfigCommonRadioResourceConfigCommon is an information element of SIB2.The IE RadioResourceConfigCommonSIB and IE RadioResourceConfigCommon are used to specify common radio resource configurations in the system information and in the mobility control information, respectively, e.g., the random access parameters and the static physical layer parameters.

RadioResourceConfigCommon information element

-- ASN1START

RadioResourceConfigCommonSIB ::= SEQUENCE {rach-ConfigCommon RACH-ConfigCommon,bcch-Config BCCH-Config,pcch-Config PCCH-Config,prach-Config PRACH-ConfigSIB,pdsch-ConfigCommon PDSCH-ConfigCommon,pusch-ConfigCommon PUSCH-ConfigCommon,pucch-ConfigCommon PUCCH-ConfigCommon,soundingRS-UL-ConfigCommon SoundingRS-UL-ConfigCommon,uplinkPowerControlCommon UplinkPowerControlCommon,ul-CyclicPrefixLength UL-CyclicPrefixLength,...

}

RadioResourceConfigCommon ::= SEQUENCE {rach-ConfigCommon RACH-ConfigCommon OPTIONAL, -- Need ONprach-Config PRACH-Config,pdsch-ConfigCommon PDSCH-ConfigCommon OPTIONAL, -- Need

ONpusch-ConfigCommon PUSCH-ConfigCommon,phich-Config PHICH-Config OPTIONAL, -- Need ONpucch-ConfigCommon PUCCH-ConfigCommon OPTIONAL, -- Need

ONsoundingRS-UL-ConfigCommon SoundingRS-UL-ConfigCommon OPTIONAL,-- Need ONuplinkPowerControlCommon UplinkPowerControlCommon OPTIONAL, -- Need ONantennaInfoCommon AntennaInfoCommon OPTIONAL, -- Need ONp-Max P-Max OPTIONAL, -- Need OPtdd-Config TDD-Config OPTIONAL, -- Cond TDDul-CyclicPrefixLength UL-CyclicPrefixLength,...

}

BCCH-Config ::= SEQUENCE {modificationPeriodCoeff ENUMERATED {n2, n4, n8, n16}

}

PCCH-Config ::= SEQUENCE {defaultPagingCycle ENUMERATED {

rf32, rf64, rf128, rf256},nB ENUMERATED {

fourT, twoT, oneT, halfT, quarterT, oneEighthT,oneSixteenthT, oneThirtySecondT}

}

UL-CyclicPrefixLength ::= ENUMERATED {len1, len2}

-- ASN1STOP

RadioResourceConfigCommon field descriptionsp-MaxPmax to be used in the target cell. If absent the UE applies the maximum power according to the UE capability.modificationPeriodCoeffActual modification period, expressed in number of radio frames= modificationPeriodCoeff * defaultPagingCycle. n2 corresponds to value 2, n4 corresponds to value 4, n8 corresponds to value 8 and n16 corresponds to value 16.defaultPagingCycleDefault paging cycle, used to derive ‘T’ in TS 36.304 [4]. Value rf32 corresponds to 32 radio frames, rf64 corresponds to 64 radio frames and so on.nBParameter: nB is used as one of parameters to derive the Paging Frame and Paging Occasion according to TS 36.304 [4]. Value in multiples of defaultPagingCycle ('T'). A value of fourT corresponds to 4 * defaultPagingCycle, a value of twoT corresponds to 2 * defaultPagingCycle and so on.UL-CyclicPrefixLengthParameter: Uplink cyclic prefix length see 36.211 [21, 5.2.1] where len1 corresponds to normal cyclic prefix and len2 corresponds to extended cyclic prefix.

Conditional presence

Explanation

TDD The field is optional for TDD, Need ON; it is not present for FDD and the UE shall delete any existing value for this field.