This document provides an overview of two fundamental mechanisms in LTE access networks: random access and buffer status reporting. It describes the random access procedure used by UEs to connect to the network, including the exchange of preambles, responses, and temporary identifiers. It also explains the buffer status reporting procedure, where UEs indicate to the base station the amount of data waiting to be transmitted so that uplink resources can be allocated. Key parameters for both mechanisms are defined in 3GPP specifications to optimize performance and control signaling in the network.

1. LTE 4TH GENERATION NETWORK
RANDOM ACCESS AND
BUFFER 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

2. AGENDA
1. RANDOM ACCESS PROCEDURE
2. BUFFER STATUS REPORTING PROCEDURE
3. REFERENCE

3. 1 RANDOM ACCESS PROCEDURE

4. SIMPLIFIED VIEW OF PREAMBLE SEQUENCE UE TO ENB
PREAMBLE 1 PREAMBLE 2 PREAMBLE 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

5. 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.

6. 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, SEARCH
L1
Master Information Block
SYSTEM INFORMATIONbcch/bch
System Information Block 1
CELL ACCESS INFORMATIONbcch/dl-sch
PRACH PREAMBLE ASSIGNMENT
RA-RNTI = 1 + T_ID + 10*F_ID
Msg1 rach/prach
System Information Block 2
PREAMBLE INFORMATION[PRACH CONFIGURATION]bcch/dl-sch
RANDOM ACCESS RESPONSE
TEMPORARY C-RNTI
Msg2 dl-sch/pdsch
RRC CONNECTION REQUEST
PTMSI 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 SETUP
ccch/dl-sch/pdsch
RRC CONNECTION COMPLETE
UE & NW now communicate via
the allocated CRNTI
RRC / NAS signaling ON
RRC_CONNECTED ul-dcch/ul-sch/pusch

7. UE Contention Resolution Identity MAC control element
MAC 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).

8. 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.

9. 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

10. 2 BUFFER STATUS REPORTING PROCEDURE

11. 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.

12. 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.

13. MAC PDU
MAC PDU SUBHEADER FOR DL-SCH
MAC PDU SUBHEADER FOR UL-SCH
LCID LENGTH 5 BITS [0..31]

14. 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).

15. 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

16. 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.’

17. 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 0
timeStart retxBSR
ul-sch/pusch
Timer expiry
UL GRANT NOT RECEIVED

18. 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.

19. 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.

20. 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)

21. 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.

22. 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.

23. 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

24. 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

25. 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

26. THANK YOU FOR READING THIS OVERVIEW !
ありがとうございました !