LTE 4TH GENERATION NETWORKRANDOM ACCESS ANDBUFFER STATUS REPORTING

BASED ON 3GPP 36.321 LTE RELEASE 12.06.0 [FDD/TDD]

AN OVERVIEW OF 2 FUNDAMENTAL MECHANISMS IN LONG TERM EVOLUTION ACCESS NETWORK

PREPARED: VINCENT DAUMONT RAN TECHNOLOGY ARCHITECT

AGENDA

1. RANDOM ACCESS PROCEDURE

2. BUFFER STATUS REPORTING PROCEDURE

3. REFERENCE

1 RANDOM ACCESS PROCEDURE

SIMPLIFIED VIEW OF PREAMBLE SEQUENCE UE TO ENB

PREAMBLE 1 PREAMBLE 2PREAMBLE 3

1) UE says ‘Hello, can you hear me ?’

2) Meanwhile, ENB says ‘I can only hear noise…’

3) ENB: I can see a preamble (3)

4) ENB: sure, I will give you an access to EUTRAN

5) ENB: UE, I assign you a temporary radio network identifier,

after all you are not alone out there !

• Power Ramp up Step

• RA Response Window

• Preamble Trans Max

preamble

parameters

time

RANDOM ACCESS PROCEDURE

What is RA ?

• Whenever UE accesses the network either for the first time or at mobility it may read

the MIB and various SIB information transmitted by eNB to all UE’s.

• Random Access is the procedure which allows UE to connect to EUTRAN. It is the very

first ‘entry point’ to further access the network and its services.

• Before UE is able to send Initial Layer 3 Message [data services/MME] the ‘Random

Access procedure’ will be initiated (either by MAC / UE or by RRC / e.g. Paging).

• There are two type of RA: Contention based and non-contention based. In our study

case contention based RA is presented.

PREAMBLE SEQUENCE[PRACH CONFIGURATION]

SYSTEM INFORMATION

ACQUISITION / BCCH

RAR MAC PAYLOAD-TA information [Adjustment]

-UL Grant (for UL SCH usage)

-Temporary C-RNTI (Cell RNTI)

PRACH PREAMBLE where

T_id = start sub-frame number

F_id = Frequency Offset [SIB2]

Radio Access Radio Network Temporary Id

UE

E-UTRAN mechanism to prevent collision with other UE’s access network:

• Msg1: derive RA-RNTI from SIB2 and own sub-frame at the time

msg1 is sent (for RA Response)

• Msg2: MAC assigns a temporary C-RNTI value to UE, it uses the

RA-RNTI[msg1] on the PDCCH to unambiguously identify the time-

frequency used by the UE accessing the NW.

• Msg3: UE unveils either its PTMSI or a unique random value + the

temporary C-RNTI[msg2].

• Msg4: MAC completes the UE’s Random Access, it sends down the

UE Contention Resolution Identity.

• MAC set C-RNTI with the value of T C-RNTI and discard it

E-UTRAN

DL SYNCHRONIZATIONIDLE_MODE, SEARCHL1

Master Information BlockSYSTEM INFORMATIONbcch/bch

System Information Block 1CELL ACCESS INFORMATIONbcch/dl-sch

PRACH PREAMBLE ASSIGNMENTRA-RNTI = 1 + T_ID + 10*F_ID

Msg1 rach/prach

System Information Block 2PREAMBLE INFORMATION[PRACH CONFIGURATION]bcch/dl-sch

RANDOM ACCESS RESPONSETEMPORARY C-RNTI

Msg2 dl-sch/pdsch

RRC CONNECTION REQUESTPTMSI OR RANDOM VALUE

TEMPORARY C-RNTI MAC CE

Msg3 ul-cch/ul-sch/pusch

CONTENTION RESOLUTION MESSAGEMsg4

UE Contention Resolution Identity

{Initial L3 Message transmitted / CCCH SDU}

ccch/dl-sch/pdsch

RRC CONNECTION SETUPccch/dl-sch/pdsch

RRC CONNECTION COMPLETE

UE & NW now communicate via

the allocated CRNTI

RRC / NAS signaling ON

RRC_CONNECTEDul-dcch/ul-sch/pusch

UE Contention Resolution Identity MAC control elementMAC HEADER [DL-SCH]

The UE Contention Resolution Identity MAC Control Element

is identified by MAC PDU sub-header LCID 28, Its length

is 48-bit (6 octets *8 bits)

UE Contention Resolution Identity: This

field contains the uplink CCCH SDU

(Initial Layer 3 Message).

RRC SYSTEMINFORMATIONBLOCKTYPE2

RadioResourceConfigCommon information element

prach-Config PRACH-ConfigSIB

PRACH-ConfigSIB ::= SEQUENCE {

rootSequenceIndex INTEGER (0..837),

prach-ConfigInfo PRACH-ConfigInfo

}

PRACH-Config ::= SEQUENCE {

rootSequenceIndex INTEGER (0..837),

prach-ConfigInfo PRACH-ConfigInfo

}

RadioResourceConfigCommon information element

PRACH-ConfigInfo ::= SEQUENCE {

prach-ConfigIndex INTEGER (0..63),

highSpeedFlag BOOLEAN,

zeroCorrelationZoneConfig INTEGER (0..15),

prach-FreqOffset INTEGER (0..94)

}

Specify common radio resource configurations in the system information

This IE contains the necessary RACH parameters to help UE to select the right PRACH resources and permit a successful

Random Access procedure.

PRACH mask index value

PRACH resources are selected by UE at RA procedure.

For more details refer to

TS 36.211 Physical Channel and Modulation chapter 5.7

Physical random access channel describes in details the whole preamble assignment.

PHYSICAL RANDOM ACCESS CHANNEL IN BRIEF

2 BUFFER STATUS REPORTING PROCEDURE

BUFFER STATUS REPORTING

What is BSR ?

• When UE has data to send up it relies on the network side i.e. the eNB to grant UL resources.

• Because eNB serves more than one Ue, eNB shared resources shall be used in an optimal way,

it allocates resource to UE’s only when this becomes necessary. Therefore there is a procedure

in place to guarantee that UE is granted resources (UL Grant) when it asks for it.

• This is called Buffer Status Reporting, the procedure is implemented on the MAC level.

• When data in buffer are to be transmitted to the network, UE sends a MAC PDU BSR by

which it requests eNB to grant UL resources, as an answer eNB sends down UL Grant.

• Once granted, UE transmits the data contained in its buffers.

BUFFER STATUS REPORTING

‘Uplink buffer status reports (BSR) are needed to provide support for QoS-

aware packet scheduling. In E-UTRAN uplink buffer status reports refer to the

data that is buffered in for a group of logical channel (LCG) in the UE. Four

LCGs and two formats are used for reporting in uplink:

- A short format for which only one BSR (of one LCG) is reported

- A long format for which all four BSRs (of all four LCGs) are reported.

Uplink buffer status reports are transmitted using MAC signaling’

• BSR and Scheduling Request i.e. request UL-SCH resource for new transmission

is described in chapter 5.4.4 of TS-36.321.

MAC PDU

MAC PDU SUBHEADER FOR DL-SCH

MAC PDU SUBHEADER FOR UL-SCH

LCID LENGTH 5 BITS [0..31]

MAC CONTROL ELEMENT BUFFER STATUS REPORT

• Buffer Status Report (BSR) MAC control elements consist of either:

- Short BSR and Truncated BSR format: one LCG ID field and one corresponding Buffer Size

field

or

- Long BSR format: four Buffer Size fields, corresponding to LCG IDs 0 to 3.

• The BSR formats are identified by MAC PDU sub-headers with LCIDs.

• The fields LCG ID and Buffer Size are defined as follow:

- LCG ID: The Logical Channel Group ID field identifies the group of logical channel(s) which

buffer status is being reported. The length of the field is 2 bits.

- Buffer Size: The Buffer Size field identifies the total amount of data available across all

logical channels of a logical channel group after all MAC PDUs for the TTI have been built. The

amount of data is indicated in number of bytes, it shall include all data that is available for

transmission in the RLC layer and in the PDCP layer (RLC and MAC headers not considered in

buffer size).

EX. OF MAPPING OF LOGICAL CHANNELS TO LOGICAL CHANNEL GROUPS LC/LCG

LOGICAL CHANNELS

CCH . . .

SRB1 DTCH . .

SRB2 DCCH . STCH

QUALITY OF SERVICE QoS DEPENDENCY

RADIO BEARER RB1 RB2 . . RB'n

LCG ID 0 1 2 3

MAC CONTROL ELEMENT: BUFFER STATUS REPORT

BS LENGTH 6 BITS [0..63] POSSIBILITIES

BUFFER SIZE LEVELS FOR BSR EXTENDED BUFFER SIZE LEVELS FOR BSR

Depending on optional parameter extendedBSR-Sizes

[0 1 1 1 0 0] = 28

Buffer size requested by UE to eNB

BSR AND TRIGGER EVENTS

• UE has UL data to transmit for a specific LCG, Regular BSR.

• UE got the UL Grant and the padding data is larger than the size of BSR CE and the

sub-header, Padding BSR.

• periodicBSR-Timer if configured expires, periodic BSR

• retxBSR-Timer expires (see next slide)

• ‘The MAC entity shall transmit at most one Regular/Periodic BSR in a TTI. If the MAC

entity is requested to transmit multiple MAC PDUs in a TTI, it may include a padding

BSR in any of the MAC PDUs which do not contain a Regular/Periodic BSR.

• A Padding BSR is not allowed to cancel a triggered Regular/Periodic BSR. A Padding

BSR is triggered for a specific MAC PDU only and the trigger is cancelled when this

MAC PDU has been built.’

BSR

BSR

Timer expiry

SEND NEW BSR

& Restart timerBSR

UL GRANT

UL GRANT

BSR NOT RECEIVED ?

RRC control timer retxBSR-timer

UEE-UTRAN UL BUFFER

UL BUFF 0timeStart retxBSR

ul-sch/pusch

Timer expiryUL GRANT NOT RECEIVED

BSR RELATED PARAMETERS

• TS 36.331 RRC Release 12 - RRC Information Elements & Parameters

• RRC control of BSR-related parameters

[periodicBSR-Timer, retxBSR-Timer, logicalChannelSR-ProhibitTimer,

logicalChannelSR-Mask]

• Controlling SR triggering on a logical channel basis when an uplink grant is configured.

• N.B.

For Regular BSR, if the BSR is triggered due to data becoming available for transmission for a

logical channel for which logicalChannelSR-ProhibitTimer is configured by upper layers

logicalChannelSR-ProhibitTimer is either started or restarted. Otherwise if running it is to be

stopped.

MAC-MAINCONFIG INFORMATION ELEMENT

MAC-MainConfig ::= SEQUENCE {

…

extendedBSR-Sizes-r10 ENUMERATED

{setup} /If setup configured, the BSR index indicates extended BSR size levels/

ul-SCH-Config SEQUENCE {

…

periodicBSR-Timer PeriodicBSR-Timer-r12

retxBSR-Timer RetxBSR-Timer-r12,

…

}

logicalChannelSR-Config-r12 CHOICE {

…

setup SEQUENCE {

logicalChannelSR-ProhibitTimer-r12

ENUMERATED

{sf20, sf40, sf64, sf128, sf512,

sf1024,sf2560, spare1}

}

}

Also present in IE UE-EUTRA-Capability

information element.

periodicBSR-Timer PeriodicBSR-Timer-r12 (optional)

retxBSR-Timer RetxBSR-Timer-r12

PeriodicBSR-Timer-r12 ::= ENUMERATED {

sf5, sf10, sf16, sf20, sf32, sf40, sf64, sf80,

sf128, sf160, sf320, sf640, sf1280, sf2560,

infinity, spare1} /default value = default MAC main configuration/

RetxBSR-Timer-r12 ::= ENUMERATED {

sf320, sf640, sf1280, sf2560, sf5120,

sf10240,

spare2, spare1}

Where 1 subframe has a duration of 1ms (1 radioframe = 10 subframes = 10ms FDD type 1)

RRC CONTROL PARAMETERS

periodicBSR-Timer

• Timer for BSR reporting. Value in number of sub-frames. Value sf10 corresponds to 10 sub-frames,

sf20 corresponds to 20 sub-frames and so on.

retxBSR-Timer

• Timer for BSR reporting. Value in number of sub-frames. Value sf640 corresponds to 640 sub-

frames, sf1280 corresponds to 1280 sub-frames and so on.

logicalChannelSR-ProhibitTimer

• Timer used to delay the transmission of an SR for logical channels enabled by logicalChannelSR-

Prohibit. Value sf20 corresponds to 20 sub-frames, sf40 corresponds to 40 sub-frames, and so on.

logicalChannelSR-Mask

• Controlling SR triggering on a logical channel basis when an uplink grant is configured.

LOGICALCHANNELCONFIG INFORMATION ELEMENT

ul-SpecificParameters SEQUENCE {

logicalChannelGroup INTEGER (0..3) OPTIONAL

logicalChannelSR-Mask-r9 ENUMERATED {setup} OPTIONAL

logicalChannelSR-Prohibit-r12 BOOLEAN OPTIONAL

• logicalChannelSR-Prohibit-r12: value TRUE indicates that the logicalChannelSR-ProhibitTimer is

enabled for the logical channel. E-UTRAN only (optionally) configures the field (i.e. indicates

value TRUE) if logicalChannelSR-ProhibitTimer is configured.

• logicalChannelSR-Mask (RRC 36.331 9.1.1.2 CCCH configuration, value RELEASE) Controlling

SR triggering on a logical channel basis when an uplink grant is configured.

BSR CONTROL PARAMETERS IE IN RRC PROTOCOL

RRC CONNECTION SETUP / REESTABLISHMENT / RECONFIGURATION

IE RADIO RESSOURCE CONFIG DEDICATED

IE MAC MAIN CONFIG

IE UE EUTRA CAPABILITY

IE LOGICAL CHANNEL CONFIGURATION

RRC HANDOVER PREPARATION INFORMATION

IE AS CONFIGURATION

IE RADIO RESSOURCE CONFIG DEDICATED

ABBREVIATIONS

• LCID: The Logical Channel ID field

Identifies the logical channel instance of the corresponding MAC SDU or the type of the

corresponding MAC control element or padding. There is one LCID field for each MAC

SDU, MAC control element or padding included in the MAC PDU.

• RA-RNTI: Radio Access Radio Network Temporary Identifier used at Preamble pre-

assignment sequence. Uniquely identifies a UE among others.

• Temporary-CRNTI: the temporary Cell Radio Network Temporary Identifier used at

RAR sequence. Assigned by MAC until msg4 CRM is sent to UE.

RNTI values and their usage and associated transport and logical channels

3 REFERENCE

• TS-36.211PHYSICAL CHANNELS AND MODULATION

• TS-36.300 RADIO ACCESS NETWORK OVERALL DESCRIPTION

• TS-36.321MEDIUM ACCESS CONTROL PROTOCOL SPECIFICATION

• TS-36.331 RADIO RESOURCE CONTROL PROTOCOL SPECIFICATION

THANK YOU FOR READING THIS OVERVIEW !

ありがとうございました !