1. Description in web
http://lteuniversity.com/get_trained/expert_opinion1/b/prakash/archive/2013/04/17/discontinuous-reception-drx-
in-rrc-idle-part-1.aspx
One 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 Mode
Paging 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 Paging
Paging 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.
2. 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 Number
Both the eNB and the UE calculate the Paging Frame number by the following relation:
3. 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)
4. Example in web
http://lteinwireless.blogspot.kr/2012/12/paging-in-lte.html
S-GW:
1. DL data arrives for the UE
2. 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-TMSI
3. List of TAI: informs the eNB to broadcast the Paging messages in the mentioned TAI's
4. Sends DL Data Notification ACK message to S-GW
eNB:
1. Receives S1AP: Paging and constructs RRC: Paging message
2. 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 transmission
2. If UE finds the P-RNTI then it proceeds to decode the PDSCH information which is present in PDCCH
3. UE decodes RRC:Paging from the PDSCH Resource Block within which the PAGING message is sent
4. If UE doesn't find it's own UE identity then it returns to monitor the Paging Occasion.
5. 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?
OR
How 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 1024
Parameters 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= 587
d. (UE_id mod N)= 587 mod 128=75
e. (T div N)= 128 / 128 = 1
Therefore, SFN mod T =75
So, 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.
6. 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=3
1 9 N/A N/A N/A
2 4 9 N/A N/A
4 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 cycle
T_C T_UE T
DRX
cycle
(ms)
nB N N_S Ue_ID_mod_N
Num of PO
during DRX
Cycle frame
256 128 128 1,280 4T 128 4 75 512
256 128 128 1,280 2T 128 2 75 256
256 128 128 1,280 T 128 1 75 128
256 128 128 1,280 T/2 64 1 11 64
256 128 128 1,280 T/4 32 1 11 32
256 128 128 1,280 T/8 16 1 11 16
256 128 128 1,280 T/16 8 1 3 8
256 128 128 1,280 T/32 4 1 3 4
T_C Cell specific DRX cycle, signaled by eNodeB via SIB2
T_UE Cell specific DRX cycle, signaled by MME
T DRX cycle (paging cycle) of UE
n_B Number of paging occasions per DRX cycle, signaled by eNodeB via SIB2
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.
N Number of paging frames
N_S Number of paging subframes
ue_ID_mod_N Paging frame SFN modular
7. i_s index to Paging Occasion from subframe pattern
36.304 Paging
7.1 Discontinuous Reception for paging
The 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 mess
age. 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 Ns
System 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 Patterns
FDD:
Ns PO when i_s=0 PO when i_s=1 PO when i_s=2 PO when i_s=3
1 9 N/A N/A N/A
2 4 9 N/A N/A
4 0 4 5 9
TDD (all UL/DL configurations):
8. Ns PO when i_s=0 PO when i_s=1 PO when i_s=2 PO when i_s=3
1 0 N/A N/A N/A
2 0 5 N/A N/A
4 0 1 5 6
36.331 RadioResourceConfigCommon
RadioResourceConfigCommon 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 ON
prach-Config PRACH-Config,
pdsch-ConfigCommon PDSCH-ConfigCommon OPTIONAL, -- Need ON
pusch-ConfigCommon PUSCH-ConfigCommon,
phich-Config PHICH-Config OPTIONAL, -- Need ON
pucch-ConfigCommon PUCCH-ConfigCommon OPTIONAL, -- Need ON
soundingRS-UL-ConfigCommon SoundingRS-UL-ConfigCommon OPTIONAL, -- Need ON
uplinkPowerControlCommon UplinkPowerControlCommon OPTIONAL, -- Need ON
antennaInfoCommon AntennaInfoCommon OPTIONAL, -- Need ON
p-Max P-Max OPTIONAL, -- Need OP
tdd-Config TDD-Config OPTIONAL, -- Cond TDD
ul-CyclicPrefixLength UL-CyclicPrefixLength,
...
}
9. 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 descriptions
p-Max
Pmax to be used in the target cell. If absent the UE applies the maximum power according to the UE
capability.
modificationPeriodCoeff
Actual 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.
defaultPagingCycle
Default 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.
nB
Parameter: 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-CyclicPrefixLength
Parameter: 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.
