Overall Paging mechanism for LTE BL/CE is similar to legacy LTE. So I strongly recommend you to read through the legacy LTE Paging first. The three formulas below are the legacy ones, and only the third of them is new.
What differs is where the paging message is looked for rather than when. A legacy UE watches the whole carrier and a BL/CE UE watches one narrowband. The same arithmetic that picks a frame and a subframe therefore has to pick a narrowband as well.
Followings are the topics to be covered in this page.
- Paging Parameters in SIB2
- Paging Occasion, Paging Frame and Paging Narrow Band
- Which subframe the paging occasion falls in
- What changed after Release 13
- Reference
Paging Parameters in SIB2
Everything a UE needs to find its paging occasion arrives in one message, and the decode below is that message. Two of its branches matter here, and both are marked in it.
Captured from a live network, not quoted from a specification. The field names follow
+-sib2 ::= SEQUENCE [00] +-ac-BarringInfo ::= SEQUENCE OPTIONAL:Omit +-radioResourceConfigCommon ::= SEQUENCE | +-rach-ConfigCommon ::= SEQUENCE | +-bcch-Config ::= SEQUENCE | +-pcch-Config ::= SEQUENCE | | +-defaultPagingCycle ::= ENUMERATED [rf64] | | +-nB ::= ENUMERATED [halfT] | +-prach-Config ::= SEQUENCE | +-pdsch-ConfigCommon ::= SEQUENCE | +-pusch-ConfigCommon ::= SEQUENCE | +-pucch-ConfigCommon ::= SEQUENCE | +-soundingRS-UL-ConfigCommon ::= CHOICE [release] | +-uplinkPowerControlCommon ::= SEQUENCE | +-ul-CyclicPrefixLength ::= ENUMERATED [len1] | +-EXTENSION ::= SEQUENCE [0001] | +-VERSION-BRACKETS1 ::= SEQUENCE OPTIONAL:Omit | +-VERSION-BRACKETS2 ::= SEQUENCE OPTIONAL:Omit | +-VERSION-BRACKETS3 ::= SEQUENCE OPTIONAL:Omit | +-VERSION-BRACKETS4 ::= SEQUENCE [1110111] OPTIONAL:Exist | +-bcch-Config-v1310 ::= SEQUENCE OPTIONAL:Exist | +-pcch-Config-v1310 ::= SEQUENCE [1] OPTIONAL:Exist | | +-paging-narrowBands-r13 ::= INTEGER (1..maxAvailNarrowBands-r13[16]) [1] | | +-mpdcch-NumRepetition-Paging-r13 ::= ENUMERATED [r1] | | +-nB-v1310 ::= ENUMERATED [one64thT] OPTIONAL:Exist | +-freqHoppingParameters-r13 ::= SEQUENCE [000000] OPTIONAL:Exist | +-pdsch-ConfigCommon-v1310 ::= SEQUENCE OPTIONAL:Omit | +-pusch-ConfigCommon-v1310 ::= SEQUENCE [000] OPTIONAL:Exist | +-prach-ConfigCommon-v1310 ::= SEQUENCE [11] OPTIONAL:Exist | +-pucch-ConfigCommon-v1310 ::= SEQUENCE [11100] OPTIONAL:Exist
Two blocks are marked in the decode, and they are the legacy paging configuration and the coverage enhanced one. The blue block is pcch-Config, which a legacy UE reads and a BL/CE UE reads as well. The red block is pcch-Config-v1310, which only a BL/CE UE reads.
Nothing in the red block replaces the blue one. The first field adds a dimension the legacy configuration has no field for, the second adds repetition, and the third extends the nB enumeration downward rather than overriding it.
- defaultPagingCycle : rf32, rf64, rf128, rf256
- nB : fourT, twoT, oneT, halfT, quarterT, oneEighthT, oneSixteenthT, oneThirtysecondT
- mpdcch-NumRepetition-Paging-r13 : r1, r2, r4, r8, r16, r32, r64, r128, r256
- nB-v1310 : one64thT, one128thT, one256thT
The four lines above are the enumerations, and the listings below are where they come from. Two information elements carry them. PCCH-Config is the legacy one, unchanged since Release 8, and PCCH-Config-v1310 is the extension a BL/CE UE also reads.
Following is based on
PCCH-Config ::= SEQUENCE {
defaultPagingCycle ENUMERATED {
rf32, rf64, rf128, rf256},
nB ENUMERATED {
fourT, twoT, oneT, halfT, quarterT, oneEighthT,
oneSixteenthT, oneThirtySecondT}
}
PCCH-Config-v1310 ::= SEQUENCE {
paging-narrowBands-r13 INTEGER (1..maxAvailNarrowBands-r13),
mpdcch-NumRepetition-Paging-r13 ENUMERATED {r1, r2, r4, r8, r16, r32, r64, r128, r256},
nB-v1310 ENUMERATED {one64thT, one128thT, one256thT}
OPTIONAL -- Need OR
}
PCCH-Config-v1700 ::= SEQUENCE {
ranPagingInIdlePO-r17 ENUMERATED {true}
}
Only one of the three extension fields is optional. Both paging-narrowBands-r13 and mpdcch-NumRepetition-Paging-r13 are mandatory inside PCCH-Config-v1310. A cell that sends the extension at all therefore has to say how many narrowbands it pages on and how far the MPDCCH repeats.
The optional one is nB-v1310, and it only extends a range. The legacy nB stops at one thirty second of T. The extension adds a sixty fourth, a hundred and twenty eighth and a two hundred and fifty sixth. A cell that needs no value that sparse leaves the field out.
The two elements are read together : defaultPagingCycle comes from PCCH-Config in both cases, and only nB can be overridden by the extension.One field has no legacy counterpart : paging-narrowBands-r13 exists because a BL/CE UE cannot watch the whole carrier.Repetition is a cell wide setting : mpdcch-NumRepetition-Paging-r13 is one value for the cell, not one per coverage enhancement level.The specification spells one enumerator differently : 36.331 writes oneThirtySecondT with a capital S, where the list above writes oneThirtysecondT.
Paging Occasion, Paging Frame and Paging Narrow Band
Three names are needed because the answer has three parts. A UE has to know which radio frame to wake in, which subframe of that frame to watch, and which narrowband to watch it on. The definitions below come from 36.304 clause 7.1, and the formulas after them compute one part each.
- Paging Occasion (PO) is a subframe where there may be P-RNTI transmitted on MPDCCH. When the P-RNTI PDCCH is transmitted repeatedly, PO refers to the starting subframe of MPDCCH repetitions
- 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
- Paging NarrowBand(PNB) is one narrowband, on which the UE performs the paging message reception
nB is a value, not an index : 36.304 writes it as a multiple or a fraction of T, and N and Ns both follow from it.N and Ns multiply to nB : one counts frames and the other counts occasions inside a frame, and T is the boundary between them.The UE_ID modulus is sixteen times the legacy one : 16384 rather than 1024, because the identity also has to choose a narrowband.A UE with no IMSI uses zero : 36.304 has an emergency call without a USIM take UE_ID = 0 in all three formulas.
PF : SFN mod T= (T div N)*(UE_ID mod N)
, N = min(T, nB) , where T is the DRX Cycle
, nB = index indicating pcch-Config.nB and pcch-Config-v1310.nB-v1310 {0,1,2,3,4,5,6,7,8,9,10}
PO : i_s = floor(UE_ID/N) mod Ns
PNB = floor(UE_ID/(N*Ns)) mod Nn
, Nn = [SIB2] pcch-Config-v1310.paging-narrowBands-r13
UE_ID = IMSI mod 16384
IMSI = Integer value of IMSI String. IMSI string is the digit 0~9 in Decimal.
Ex > IMSI = '0123456789' is 123456789 (Dec)
Three symbols in those formulas are not defined above, and 36.304 clause 7.1 defines all three. The table below is that clause, written out.
Symbol | 36.304 definition | What it means here |
T | The DRX cycle of the UE | The shorter of the UE specific value, when upper layers allocate one, and defaultPagingCycle broadcast in SIB2 |
nB | 4T, 2T, T, T/2, T/4, T/8, T/16, T/32, T/64, T/128 or T/256 | A value rather than an index. The first eight come from pcch-Config.nB and the last three from pcch-Config-v1310.nB-v1310 |
N | min(T, nB) | How many paging frames the cycle is spread over. It can never exceed the cycle length itself |
Ns | max(1, nB/T) | How many paging occasions each paging frame holds. It is 1 whenever nB is T or smaller |
Nn | The number of paging narrowbands | paging-narrowBands-r13 from pcch-Config-v1310, which is what the PNB formula reduces modulo |
UE_ID | IMSI mod 16384, when the P-RNTI is monitored on MPDCCH | Sixteen times the legacy modulus. A UE monitoring the P-RNTI on PDCCH uses IMSI mod 1024 |
From 36.304 v19.2.0 clause 7.1. The UE_ID row is the one that changes for a BL/CE UE. The identity now has to spread UEs over narrowbands as well as over frames and subframes.
The UE_ID row repays a second look. A legacy UE reduces its IMSI modulo 1024, and a BL/CE UE reduces it modulo 16384, which is sixteen times larger. The extra factor is there because the same identity now has to pick a narrowband as well, and maxAvailNarrowBands-r13 reaches 16.
The relationship between N and Ns is worth stating plainly too. Their product is nB, so a larger nB is spent either on more paging frames or on more occasions inside each frame, and T decides which. Below T the cycle gains frames, above T it gains occasions within them.
Which subframe the paging occasion falls in
The formulas above give a frame, an index and a narrowband, and the index is not a subframe number. 36.304 clause 7.2 turns is into a subframe through a pattern table, and the table a BL/CE UE uses depends on how wide the cell is.
That split is the part worth knowing. A cell wider than 3 MHz uses the same pattern a legacy UE uses. A cell of 1.4 MHz or 3 MHz uses a different pattern, and in FDD that pattern has only one subframe in it.
Ns | FDD, system bandwidth above 3 MHz | FDD, system bandwidth 1.4 or 3 MHz |
1 | Subframe 9 | Subframe 5 |
2 | Subframes 4 and 9 | Subframe 5 for both values of is |
4 | Subframes 0, 4, 5 and 9 | Subframe 5 for all four values of is |
From 36.304 v19.2.0 clause 7.2, the FDD patterns. The wider column is also what a UE monitoring the P-RNTI on PDCCH uses, so only the narrow column is specific to a small BL/CE cell.
In a narrow FDD cell every paging occasion is therefore subframe 5, whatever is came out of the formula. The index still does work, because it feeds the PNB calculation through Ns, but it no longer selects a subframe.
TDD splits the same way and keeps more variety. Above 3 MHz the occasions are subframes 0, 1, 5 and 6, depending on Ns and is. At 1.4 or 3 MHz they reduce to subframes 1 and 6.
One consequence follows from the definition of a paging occasion for a BL/CE UE. The occasion is the starting subframe of the MPDCCH repetitions rather than a subframe the message fits inside. The transmission continues from there for as many subframes as mpdcch-NumRepetition-Paging-r13 allows.
The pattern depends on the cell bandwidth : a BL/CE UE in a wide cell uses the legacy table, and only a narrow cell uses the other.A narrow FDD cell pages in subframe 5 and nowhere else : all four values of is give the same subframe.The index still matters when the subframe does not : Ns appears in the PNB formula, so is keeps separating UEs across narrowbands.The occasion is a starting point, not a slot : the MPDCCH repeats onward from it.
What changed after Release 13
Everything above is Release 13 and all of it still holds. The three formulas read the same in 36.304 v19.2.0 as they do here, and PCCH-Config-v1310 has not gained a field. What later releases added sits beside paging rather than inside it, and every addition has the same purpose.
That purpose is to stop the UE decoding an MPDCCH it did not need to decode. A paging occasion costs the receiver whether or not the message is for this UE, and with repetition it costs many subframes. Each release has therefore added a cheaper way to learn that nothing is coming.
Release | What was added | Field |
Rel-13 | Idle mode extended DRX, which stretches the cycle far beyond the 256 radio frames defaultPagingCycle can express | eDRX-Allowed-r13 |
Rel-15 | A wake up signal before the paging occasion, so a UE that sees nothing can skip the MPDCCH entirely | wus-Config-r15 |
Rel-16 | A group wake up signal, which splits the UEs of one occasion into groups so that fewer of them wake | gwus-Config-r16 |
Rel-17 | A network indication that RAN paging uses the paging occasion the UE computes for idle mode | ranPagingInIdlePO-r17, in PCCH-Config-v1700 |
Compared against 36.331 v19.3.0 and 36.304 v19.2.0, with each release dated by the suffix of the field that configures it in 36.331.
The group wake up signal is the one that touches the arithmetic on this page. 36.304 computes a group index with the same shape as the PNB formula, reducing floor(UE_ID/(N times Ns)) modulo a group count. The identity that already chose a frame, an occasion and a narrowband now chooses a group as well.
The Release 13 formulas are unchanged : PF, is and PNB read the same in 36.304 v19.2.0 as on this page.Every addition avoids a decode rather than shortening one : extended DRX and the two wake up signals all aim at the same cost.The wake up signals reuse the paging identity : the group index is computed from UE_ID the same way the narrowband is.Nothing was added to PCCH-Config-v1310 : the three fields in the decode above are still the whole of it.
Reference
The clauses the formulas and the field names on this page come from.
[1] 3GPP TS 36.304 v19.2.0 - clause 7.1 for the PF, i_s and PNB formulas and the parameters they use, clause 7.2 for the subframe patterns
[2] 3GPP TS 36.331 v19.3.0 - the PCCH-Config, PCCH-Config-v1310 and PCCH-Config-v1700 information elements, and the eDRX-Allowed-r13, wus-Config-r15 and gwus-Config-r16 fields
[3] 3GPP TS 36.211 v19.3.0 - the MPDCCH common search space the paging occasion starts