Paging Study in LTE

Introduction

Paging is a procedure of transmitting paging messages to UEs in RRC_IDLE mode or informing all UEs in RRC_CONNECTED mode and RRC_IDLE mode about an SI message change .

         Triggering of Paging

                MME Triggering

                     •To initiate mobile terminated PS call
                     •To initiate mobile terminated CS fallback call

                 NodeB Triggering

         •To trigger LTE UE to re-acquire system information

Paging on the Uu Interface

•UEs in RRC_IDLE mode use DRX to receive paging messages in order to  reduce power consumption.

•UE in RRC IDLE mode checks for paging once every DRX cycle.

•Paging occasion within the paging frame defines specific subframe  during which an LTE UE checks for paging message.

•One PF is one radio frame which may contain one or multiple POs.

•One PO is a subframe where the Paging Radio Network Temporary Identifier (P-RNTI) is contained. 

•The PO is transmitted over the PDCCH. According to 3GPP specifications, the

 P-RNTI value is fixed. UEs read paging messages over the Physical Downlink Shared Channel (PDSCH) according to the P-RNTI.

Notes

Ø  MME is responsible for the initiation of LTE paging procedure. MME does this by forwarding S1AP paging message to one or more eNodeB.

Ø  The LTE paging procedure is applicable to UE in ECM IDLE State. UE in this state are in RRC IDLE mode and do not have S1 connectivity with MME. 

Ø  UE in idle state known to MME by its TAC . Paging message is forwarded to TAC or  TAL(Tracking area List) which UE is located.

Ø  Paging message can include multiple paging records to page multiple UEs

Paging Description over Uu interface

·         UE searches for P-RNTI within PDCCH of subframe belong to paging occasion. P-RNTI  indicates that UE may have a paging message on PDSCH.

·         IF UE found its P-RNTI on the PDCCH , then UE listen  to PDSCH RB where in paging message has been sent.

Note 1

·         There is paging information sent over PDCCHàlike P-RNTI

·         There is paging message sent over PCCH(logical)àDL-SCH(Transport)à PDSCH(physical) à paging message contains UE ID (UE ID may be IMSI or S-TMSI for the paged UEs .

Note 2

P-RNTI(paging-Radio network temporary identifier )

·         The P-RNTI is the 4G complement of the paging indicator.

·         It does not refer to a particular UE, but to a group of UEs.

·         UE decodes PDSCH RBs and checks UE identity in all the records.

1-       If UE do not find its identity in paging record then it will return back to sleeping mode and then it will  check PDCCH for P-RNTI at next paging occasions. 

2-       If the UE find its identity, it will trigger random access procedure to establish RRC connection.

·         UE sends RRC connection request message and eNodeB responds with RRC connection setup message.

Ø  If the LTE paging procedure is for PS data call, UE includes service request NAS message within RRC connection setup complete message.

Ø  If the paging procedure is for CS fallback call, UE includes extended service request NAS message within RRC connection setup complete message.

Paging Discard per eNodeB

•When the CPU usage of the main control board or baseband board is equal to or greater than threshold for certain continuous time, the eNodeB starts flow control based on service priorities . The eNodeB preferentially discards PS paging messages in paging message flow control

SINR Calculation in LTE

• S: indicates the power of measured usable signals. Reference signals (RS) and physical downlink shared channels (PDSCHs) are mainly involved

• I: indicates the average interference power - the power of measured signals or channel interference signals from other cells in the current system
•  N: indicates background noise, which is related to measurement bandwidths and receiver noise coefficients

SINR is a measure of signal quality as well but it is not defined in the 3GPP specs but defined by the UE vendor.

It is not reported to the network. SINR is used a lot by operators, and the LTE industry in general, as it better quantifies the relationship between RF conditions and Throughput. UEs typically use SINR to calculate the CQI (Channel Quality Indicator) they report to the network.

It is a common practice to use Signal-toInterference Ratio (SINR) as an indicator for network quality. It should be however noted that 3GPP specifications do not define SINR and  therefore UE does not report SINR to the network. SINR is still internally measured by most UEs and recorded by drive test tools.

Unfortunately UE chipset and RF scanner manufacturers have implemented SINR measurement in various different ways which in the authors’ field experience are not always easily comparable. While at first it may seem that defining SINR should be unambiguous, in case of LTE downlink this is not the case. This is because different REs within a radio frame carry different physical signals and channels each of which, in turn, see different interference power depending on inter-cell radio frame synchronization.

Timing Advance (TA) in LTE

In LTE, when a UE wants to establish RRC connection with eNB, it transmits a Random Access Preamble, eNB estimates the transmission timing of the terminal based on this. Now eNB transmits a Random Access Response which consists of timing advance command, based on that UE adjusts the terminal transmit timing.

The timing advance is initiated from E-UTRAN with MAC message that implies and adjustment of the timing advance.

3GPP TA Requirements

• Timing Advance adjustment delay
UE shall adjust the timing of its uplink transmission timing at sub-frame n+6 for a timing advancement command received in sub-frame n.
• Timing Advance adjustment accuracy 
The UE shall adjust the timing of its transmissions with a relative accuracy better than or equal to ±4* TS seconds to the signalled timing advance value compared to the timing of preceding uplink transmission. The timing advance command is expressed in multiples of 16* TS and is relative to the current uplink timing.

Maintenance of Uplink Time Alignment

The UE has a configurable timer timeAlignmentTimer which is used to control how long the UE is considered uplink time aligned

- if the Random Access Preamble was not selected by UE MAC then UE applies the Timing Advance Command and starts or restarts timeAlignmentTimer.

- else if the timeAlignmentTimer is not running then UE applies the Timing Advance Command starts timeAlignmentTimer; when the contention resolution is considered not successful  then UE stops timeAlignmentTimer.

- else ignore the received Timing Advance Command.

Timing Advance Command MAC Control Element

The Timing Advance Command MAC control element is identified by MAC PDU subheader with LCID value =  11101 (Timing Advance Command) .

It has a fixed size and it consists of a single octet as show below.

Timing Advance Command MAC control element has following fields.

How to calculate TA in LTE?

The eNB measures the required timing advance based on the received UE signal arrival time. It commands the UE to adjust the transmission time. This is performed on a per-need basis.

It is signaled by means of a special MAC control element; LCID = 11101. The signaled granularity is 16 Ts. The value range for adjustment is 8 bit and related to the current UL timing.

As Ts. = 1 / (2048 x 15000) sec = 1 / 30720000 sec the granularity is given by 0.52 μsec (corresponding to 78 m).

If UE is in-sync the timing advance is valid. Otherwise the RACH-procedure establishes a valid timing advance. An 11 bit value (range 0,1..1282) is signaled to establish the initial offset.

Source: 3GPP specifications (36.133 & 36.321)

Random Access procedure in LTE

Random Access Definition

vRandom access in LTE establishes or restores uplink synchronization between a UE and an eNodeB.

Classifications Scenarios

 vIn contention-based random access, the access may fail because a random access channel (RACH) may not be allocated to the UE.
vIn non-contention-based random access, the eNodeB allocates a dedicated RACH to the UE to ensure successful access. If dedicated RACHs are insufficient, the eNodeB instructs the UE to initiate contention-based random access.

RACH Optimization

RACH Resource Adjustment

RACH resources include a physical random access channel (PRACH) configuration index and preamble groups

ØPRACH Configuration Index:

§The PRACH configuration index indicates the number of PRACHs in each radio frame and the subframe number of each PRACH.

ØPreamble Groups:

§Random access preambles in a cell are grouped into random preambles and dedicated preambles, which are used for contention-based random access and non-contention-based random access, respectively.

RACH Power Control Adjustments

—PRACH power control parameter adjustment is performed based on the random access information reported by the UE, (which will be explained in details in Power Control Session)

PRACH False Alarm Detection

—If a UE does not send a preamble but the eNodeB detects a preamble from the UE during a random access procedure, this falsely detected preamble is called a PRACH false alarm. For a falsely detected preamble, the eNodeB does not send a Random Access Response message or increment related counters.

Random access scenarios

qScenario 1: Initial RRC connection setup

To switch from the RRC_IDLE state to the RRC_CONNECTED state, a UE initiates random access.

qScenario 2: RRC connection reestablishment

When a radio link failure (RLF) occurs, the UE needs to reestablish an RRC connection. In this scenario, the UE initiates random access.

qScenario 3: Handover

During a handover, a UE initiates random access in the target cell.

qScenario 4: Downlink data arrival

When an eNodeB needs to send downlink data to a UE in the RRC_CONNECTED state and finds that the UE is out of uplink synchronization, the eNodeB instructs the UE to initiate random access.

qScenario 5: Uplink data arrival

                When a UE in the RRC_CONNECTED state needs to send uplink data to an eNodeB and finds that it is out of uplink synchronization, the UE initiates random access. 

Random Access Preambles 

—During random access, the eNodeB allocates a random access preamble to a UE. The UE sends the random access preamble to the eNodeB to initiate a random access request. The random access preamble is a burst, which consists of a cyclic prefix (CP), a preamble sequence, and an extra part in the time domain and six resource blocks in the frequency domain. TCP denotes the length of a CP, and TSEQ denotes the length of a preamble sequence.

 Preamble Frame Format

—The Guard Period is required since the eNB does not know when the preambles will arrive.

—The below figures illustrate an example with two UEs. The first is next to the eNB therefore there is very little delay. In contrast UE “B” is some distance from the eNB, as such the initial access preamble is delayed, i.e. there is a round trip delay. The eNB must allocate a large enough window such that the preambles from UE at the edge of the cell don’t arrive outside of this window.

Preamble Sequence Grouping

CONTENTION BASED

Contention Based Random Access Procedure

—The following slides describe the four steps shown below, which involves random access preamble transmission, random access response, scheduled uplink transmission, and contention resolution transmission.

1- Random Access Preamble Transmission

ØIn contention-based random access, the UE directly sends a random access preamble to the eNodeB if the PRACH configuration has been specified and has not expired. If the PRACH configuration has not been specified or has expired, the UE must obtain the PRACH configuration first.

ØThe UE selects random group B if the following conditions are met:

1.Random group B exists.

2.The size of Msg3 (the third message transmitted in the random access procedure shown above) is larger than the corresponding threshold configured for random group A.

3.The path loss of the UE is less than the threshold.

vNote: If any of the preceding conditions are not met, the UE selects random group A.

ØAfter a random group is determined, the UE selects a preamble from the group randomly.

ØThe UE sends a random access preamble on the newly arriving PRACH with the power PPRACH. The preamble usually consists of six bits, where five bits indicate an RA-RNTI of a UE and one bit indicates the size of Msg3. RA-RNTI stands for random access - radio network temporary identifier.

2-Random Access Response

—Upon receiving the preamble, the eNodeB applies for a temporary cell RNTI (C-RNTI) and uplink and downlink resources for scheduling. Then, the eNodeB sends a random access response over the downlink shared channel (DL-SCH) for each UE.

—The response contains the RA-preamble identifier, timing alignment information, initial uplink grant, and temporary C-RNTI. One DL-SCH can carry random access responses to multiple UEs.

—After the UE sends the preamble, it monitors the physical dedicated control channel (PDCCH) and waits for a random access response within a random access response window:

ØIf the UE receives a response that contains an RA-preamble identifier matching the transmitted random access preamble, the response is successful.

ØIf the UE does not receive a response or fails to verify the response reception, the response fails.

qIn this case, if the number of random access attempts is smaller than the maximum, the UE attempts random access again. Otherwise, random access fails. The maximum number of random access attempts of the UE can be obtained from SIB2.

3- Scheduled Uplink Transmission

—After receiving a successful response, the UE sends scheduled uplink transport block over the uplink shared channel (UL-SCH).

—The information in the transport block sent by the UE varies according to the following random access scenarios:

ØIn initial RRC connection setup;

The RRC Connection Request message (including NAS UE_ID) is transmitted over the CCCH in TM at the RLC layer. The message is not segmented.

ØIn RRC connection reestablishment;

The RRC Connection Reestablishment Request message (excluding NAS information) is transmitted in TM at the RLC layer. The message is not segmented.

ØIn contention-based random access due to no dedicated preamble after a handover;

The RRC Handover Confirm message and C-RNTI are transmitted over the DCCH. If required, a buffer status report (BSR) is also carried.

ØIn other scenarios;

At least the C-RNTI of the UE is transmitted.

4- Contention Resolution Transmission

—After the UE sends Msg3 (Scheduled transmission),  a contention resolution timer starts. The contention resolution timer can be obtained from SIB2.

—Within the timer period, the eNodeB performs contention resolution at the MAC layer and informs the UE of the resolution by the C-RNTI on the PDCCH or by the information element (IE) UE Contention Resolution Identity on the DL-SCH.

—The UE monitors the PDCCH before the timer expires. The UE considers the contention resolution as successful, notifies upper layers, and stops the timer if one of the following conditions is met:

ØThe UE obtains the C-RNTI from the PDCCH.

ØThe UE obtains the temporary C-RNTI over the PDCCH, the MAC packet data unit (PDU) is successfully decoded, and the MAC PDU contains information matching the CCCH service data unit (SDU) transmitted in Msg3.

—If the contention resolution is successful, the contention-based random access procedure ends. If the contention resolution timer expires, the UE considers the contention resolution as failed. Then, the UE performs random access again if the number of random access attempts is smaller than the maximum. If the number of random access attempts is not smaller than the maximum, the random access procedure fails.

NON-CONTENTION BASED

—Unlike contention-based random access, non-contention-based random access does not involve contention and conflict resolution because random access preambles are allocated by the eNodeB. Other procedures in non-contention-based random access are similar to those in contention-based random access. Below figure shows the non-contention-based random access procedure.

Please find the below link as a reference for attach procedure

http://www.eventhelix.com/lte/attach/LTE-RRC-Connection-Setup-Messaging.pdf

Attach Procedure

RACH: UE à eNodeB: Random Access Preamble 

—•The terminal picks a preamble to send the random access message

— •The preambles in LTE are defined from a Zadoff-Chu sequence

— •The preamble consists of the cyclic prefix and a sequence

— •The sequence identifies the UE that is initiating the random access

—• The type of the UE and the UE ID value are included in the message

— RA-RNTI is used as a temporary identifier during the random access procedure 

DL-SCH: UEß eNodeB: Random Access Response 

•—The eNodeB responds with a Random Access Response on the DL-SCH channel

—•The UE is addressed with the RARNTI that was sent in the Random Access Preamble

— •The message carries a Timing Advance that is used to adjust the UE transmitter timing

•—This adjustment will synchronize the UE transmitter so that the transmissions from the UE are received within the receive timing window

•—The message may carry an uplink resource assignment

—•The message also assigns a C-RNTI that will be used to address the UE.

UL-SCH: UE à eNodeB RRC Connection Request 

—•The UE has received the Random Access Response based on the RA-RNTI.

Ø The Random Access Response assigns a C-RNTI and resources for transmission of the RRC Connection Request

•The message identifies the UE with the C-RNTI

•The message contains the UE-Identity

Ø IMSI is sent in the message if this is the first attach to the network.

Ø If the terminal had attached previously, the S-TMSI is included in the message.

•—The message also contains the establishment cause. •

ØIn this example, the RRC Connection Request is sent with “Mobile Originated Signaling” cause.

vNote that the eNodeB may optionally send a contention resolution message on receipt of this message.

DL-SCH: UE ß eNodeB RRC Connection Setup

—•The message identifies the signaling radio bearer (SRB).

—•The configuration parameters carried in the message are described in the next points.

UL-SCH: UE à eNodeB RRC Connection Setup Complete 

—UE sends this message on receipt of the RRC Connection Setup message.

— “Dedicated Info NAS” is used to transfer UE specific NAS layer information between the network and the UE. The RRC layer is transparent for this information.

— The message may optionally contain registered MME.

—The RRC Connection Setup Complete may also carry octets for a NAS message exchanged between the UE and the MME.