lte paging

HSPA 72 Users Per CellLTE Paging
Introduction
The MME is responsible for sending paging records to the eNode B using an S1 Application Protocol (S1AP): Paging message
The MME sends the paging records to all eNode B with cells belonging to the relevant tracking area (idle mode UE location is known on a tracking area basis)
Paging records can originate from either the CS or PS core network domains
CS pages traverse the SGs interface to reach the MME from the MSC
The eNode B collects, schedules and broadcasts the individual paging records
accounts for paging frame and paging occasion belonging to individual UE
MME
Transfers up to 16 paging records during a paging occasion
* © Nokia Siemens Networks LTE Training / CJo
DRX
UE in RRC Idle mode use Discontinuous Reception (DRX) to reduce power consumption
The DRX cycle determines how frequently UE check for paging messages
The default DRX cycle duration is broadcast in SIB 2
The default DRX cycle is defined using the defPagCyc parameter
defined in terms of radio frames (rf)
defPagCyc
(LNCEL)
Name
Range
Description
Default
The default paging cycle defines the cell-specific paging DRX cycle duration. It also determines the maximum paging DRX duration applicable in the cell.
128rf (2)
Paging Frames (I)
UE specific Paging Frames occur when the following is true:
SFN mod T= (T div N) × (UE_ID mod N)
T is the DRX cycle duration in radio frames
N is give by Min(T, nB)
nB belongs to the set {4T, 2T, T, T/2, T/4, T/8, T/16, T/32}
UE_ID is the IMSI unless a USIM is not present, then UE_ID = 0
Thus, UE are divided in ‘N’ groups according to their UE Identity
all UE with equal values for ‘UE_ID mod N’ share the same paging frames
Paging Frames (II)
The value of nB is defined by the pagingNb parameter
The value of nB determines the rate at which Paging Frames occur from the cell perspective, i.e. there is an impact upon paging capacity
pagingNb
(LNCEL)
Name
Range
Description
oneT (2), halfT (3), quarterT (4), oneEighthT (5), oneSixteenthT (6), oneThirtySecondT (7)
Default
Paging nB defines the number of possible paging occasions per radio frame, i.e. the density of paging occasions. This parameter is used to calculate the number of paging occasions within one paging DRX duration, which in turn is used to calculate the paging occasion.
quarterT (4)
nB = T/4
nB = T/8
nB = T/16
Paging Occasions
UE attempt to decode paging messages during specific subframes within their Paging Frames
defined by the Paging Occasion
The Paging Occasion defines a single subframe for each UE
The Paging Occasion is determined using the table below
Ns = Max(1, nB / T)
i_s = Floor(UE_ID / N) mod Ns
Within the timescales of RL40, only Ns = 1 is supported (see range of pagingNb parameter)
This then means that only i_s = 0 is supported so paging messages are always broadcast during the last subframe of a Paging Frame
Ns
Paging Capacity (I)
The upper limit of paging capacity within a cell can be quantified as the number of paging records per second
The DRX cycle does not impact the upper limit of paging capacity within a cell
The nB variable (defined by pagingNb) does impact paging capacity
A maximum of 16 paging records can be accommodated within each paging message
The upper limit upon paging capacity is then defined by the rate at which Paging Frames occur (1 Paging Occasion per Paging Frame with RL40)
SCTP: Stream Control Transmission Protocol. For IP signalling. Ensures reliable, in-sequence transport of messages with congestion control Similar to TCP but with advantages:
Multi-homing support, where one (or both) endpoints of a connection can consist of more than one IP address, enabling transparent fail-over between redundant network paths.
Transaction-oriented, it transports data in one or more messages instead of in byte streams ( TCP)
GTP: GPRS Tunnelling Protocol ( same as for UMTS Rel 99): user plane traffic
Paging Capacity (II)
In practice, paging congestion will start to be experienced prior to reaching the upper limits of paging capacity shown on the previous slide
due to the random nature of paging arrivals for each Paging Frame
As the paging load increases, some Paging Frames will experience congestion while others will be less congested
Paging Frames experiencing congestion changing randomly over time
The eNode B buffers up to 16 paging records for each Paging Frame
any additional paging records are discarded
Discarded paging records can be quantified using the counter shown below:
M8008C2
DISC_RRC_PAGING
Other Counters
There is a counter to quantify the number of received S1AP: Paging messages
This counter has an object of ‘eNode B’ rather than cell
There is a cell level counter to record the number of paging records broadcast by a cell
And a cell level counter to record the number of paging messages broadcast by a cell
M8000C11
S1_PAGING
M8008C1
RRC_PAGING_REQUESTS
M8008C3
RRC_PAGING_MESSAGES
S1-AP Paging message
MME forwards the S1-AP: Paging message to the eNode B
Specifies the UE identity and list of Tracking Area Identities
Core Network Domain is also specified
The UE Identity Index
allows the eNode B to calculate the Paging Frame without having to signal the IMSI across the S1 interface
it is defined as IMSI mod 1024
The UE Paging Identity is typically S-TMSI, but can also be the IMSI
RRC Paging Message (I)
PDSCH resource allocations for the RRC Paging message are allocated using Downlink Control Information (DCI) format 1C
DCI format 1C is transmitted using the PDCCH
the CRC bits are scrambled by the P-RNTI when the resource allocation is for the PCCH logical channel
The high level structure of PCCH messages is shown below:
Only 2 message types have been defined for the PCCH logical channel
c1 (paging message)
message class extension
RRC Paging Message (II)
4 optional information elements
Paging record includes UE identity and core network domain
UE identity can be S-TMSI or IMSI
RRC Paging Message (III)
The S-TMSI is the concatenation of MME Code (MMEC) and M-TMSI
The M-TMSI is a bit string of length 32
The MMEC is a bit string of length 8
Example RRC Paging Message - Header
0100 0000 1
40 80 1C E0 43 42 00 01 CF 00 D3 AE 00
PCCH-MessageType CHOICE
Optional Information Element Bitmap
Bit 3: ETWS Indication
Bit 4: non-Critical Extension
etc.
HEX dump from UE logging tool
Example RRC Paging Message – Paging Records
40 80 1C E0 43 42 00 01 CF 00 D3 AE 00
000
01
0 = no extension
0 = no extension
Example RRC Paging Message - Padding
40 80 1C E0 43 42 00 01 CF 00 D3 AE 00
000 0000
7 bits of padding
Padding is added by the RRC layer to make the overall message an integer number of bytes
Resultant Paging Message Size
Based upon the preceding analysis, the size of a paging message can be expressed as:
Paging Message Size = 8 × ROUNDUP((9 + 44 × Num_Paging_Records) /8 ,0) bits
This equation assumes that UE identities are signalled using the S-TMSI rather than IMSI
Resultant sizes are shown in the next slide
DCI format 1C supports a limited subset of transport block sizes
the Paging message is rounded up to the nearest transport block size using additional padding
Transport Block Sizes supported by DCI format 1C
40
56
72
120
136
144
176
208
224
256
280
296
328
336
392
488
552
600
632
696
776
840
904
1000
1064
1128
1224
1288
1384
1480
1608
1736
Resultant Paging Message Size
The physical layer add a 24 bit CRC to the transport block and then completes channel coding
Channel coding uses the Coding Rate (CR) specified using the maxCrPgDl parameter
maxCrPgDl
(LNCEL)
Name
Range
Description
Default
The parameter defines the maximum code rate for paging. This maximum code rate is taken into account during PDSCH scheduling.
0.12
The default value of 0.12 means that high quantities of redundancy are added before transmitting the Paging message across the air-interface
coding rates of 0.7 are typical for the transfer of application data
the value of 0.12 helps to make paging more reliable
QPSK is always used on the PDSCH when transferring a Paging message
2 bits of information per modulation symbol
1 modulation symbol per Resource Element
PDSCH Load
Resource Block figures assume 132 Resource Elements per Resource Block pair within a subframe
Based upon 2 OFDMA symbols allocated to the PDCCH, and 2×2 MIMO
Figures illustrate the requirement for a large number of Resource Blocks when the paging load is very high
Number of Paging Records
Paging Message Size (bits)
Transport Block Size (bits)
# Bits after Channel Coding
The End

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