rach overview and analysis

March 21, 2007, Presentation to Verizon, Motorola Confidential Proprietary

MOTOROLA and the Stylized M Logo are registered in the US Patent & Trademark Office. All other product or service names are the property of their respective owners. © Motorola, Inc. 2006.

RACH Overview and Analysis

Craig Long

Kurt Kallman•Much data “re-used” (stolen) from NAT “LTE PRACH” presentation

•Overview info taken from “Nomor 3GPP Newsletter – December 2007 Overview LTE RACH”

March 21, 2007, Presentation to Verizon


Motorola Confidential Proprietary 2

Revision History

•Rev 0Initial Draft11/10/2008

•Rev 0.1Updates after initial review with Kurt K11/11/2008




Outline

• Purpose Understand how RACH works in LTE Understand how RACH is specified in LTE Understand TDD Impacts

• RACH Functional Overview A high level functional overview of the LTE RACH process

• RACH Physical Overview A high level overview of the PHY channel implementation of RACH

• Standards A discussion of the LTE RACH information contained in applicable standards 36.331 RRC

RACH related Connection establishment/change RACH related parameters

36.321 MAC MAC RACH specification

36.213 Phy Layer Procedures PRACH procedure

36.211 PHY PRACH format



RACH Functional Overview

From “Nomor 3GPP Newsletter – December 2007 Overview LTE RACH”




RACH Triggers

• There are five events that will trigger random access procedure. Initial access from RRC_IDLE;

Contention/Non-Contention based RACH: Contention only Initial access after radio link failure;

Contention/Non-Contention based RACH: Contention only Handover requiring random access procedure;

Contention/Non-Contention based RACH: Either, as specified by eNB DL data arrival during RRC_CONNECTED when UL synchronisation

status is “nonsynchronised”; Contention/Non-Contention based RACH: Either, as specified by eNB

UL data arrival during RRC_CONNECTED when UL synchronisation status is “nonsynchronised” or there are no PUCCH resources for SR available available.

Contention/Non-Contention based RACH: Contention

How does eNB know UE is “nonsynched”?



Contention Based RACH Overview





Contention Based RACH Message Diagram

(PRACH)

(PDCCH contains pointer to RAR)(PDSCH contains RAR)

(PUSCH)

(PDCCH contains pointer to Message)(PDSCH contains Message)




Contention RACH – message 1 (RACH Preamble)

• The CONTENT of the RACH preamble consists of a Zadoff-Chu sequence that is specified by parameters broadcast by the eNB One of N (usually 64) preambles selected randomly by the UEMore details under the 36.321 RACH detail description

• The RA-RNTI associated with the RACH preamble is a combination of the subframe ID and frequency resource used to transmit the RACH preambleThere are no “RA-RNTI” bits in the preamble sent over the airThe eNB can decode the “RA-RNTI” from the subframe and

frequency resource




Contention RACH – Message 2 (Random Access Response)

• Upon reception of the UE RACH, the eNB will return a Random Access Response (RAR)The PDCCH will identify the resource blocks on the DL-SCH that

carry the contents of the RAR The RAR is addressed to the RA-RNTI used by the UE for the initial

RACH This is how the UE knows which RAR is intended for it

The RAR contains Timing info An UL grant on which the UE will respond A temporary Cell-RNTI (C-RNTI) for the UE to use in it’s response




Contention RACH – Message 3

• The eNB will send data on the PUSCH in the resource blocks identified by the RAR grant in message 2

For initial access: Containing at least NAS UE ID identifier but no NAS message; Conveys the RRC Connection Request generated by the RRC layer and transmitted via

CCCH; RLC Transparent Mode (see RLC spec): no segmentation (if RLC is involved);

After radio link failure: Conveys the RRC Connection Re-establishment Request generated by the RRC layer and

transmitted via CCCH; RLC Transparent Mode (see RLC spec): no segmentation (if RLC is involved); Does not contain any NAS message.

After handover, in the target cell: Conveys the ciphered and integrity protected RRC Handover Confirm generated by the RRC

layer and transmitted via DCCH; Conveys the C-RNTI of the UE (which was allocated via the Handover Command); Includes an uplink Buffer Status Report when required.

For other events: Conveys at least the C-RNTI of the UE.

What is this?




Contention Resolution

• Contention Resolution is required becauseIn message 1, multiple UEs may have randomly selected the same

time/frequency resources (RA-RNTI) and RA Preamble for preamble transmission

The enB may still decode the preamble on this RA-RNTI even if multiple UE transmitted on the same RA-RNTI/RAPID (Random Access Preamble ID)

In message 2, the eNB addressed the UL grant to the RA-RNTI. If multiple UEs had sent on the same RA-RNTI/RAPID (and assuming the eNB decoded one of them), then multiple UEs will hear the RAR and accept the temporary C-RNTI and UL grant

Therefore, multiple UEs may send message 3, each using the same C-RNTI, but each containing different message contents

In message 4, the eNB “echos” the message it decoded in message 3. Only one UE’s message content can possibly be in message 4; all other UEs will declare RACH failure




Contention RACH – Message 4

• The Contention Resolution Message is an “echo” of message 3, addressed as shown below

Early contention resolution shall be used i.e. eNB does not wait for NAS reply before resolving contention

I think this means that even when the UE sends a NAS message in message3 that the eNB responds with a contention resolution message without waiting for a NAS response

Not synchronised with message 3; HARQ is supported; Addressed to:

The Temporary C-RNTI on L1/L2 control channel for initial access and after radio link failure

The C-RNTI for UE in RRC_CONNECTED; HARQ feedback is transmitted only by the UE which detects its own UE

identity, as provided in message 3, echoed in the RRC Contention Resolution message.



Non-Contention Based RACH





Non-Contention Based RACH

•From where does the eNB get the “dedicated preambles” that will not conflict with other eNBs? Can’t be from the general RA preamble pool, as those can all be selected by UEs

(PRACH?)




Non-Contention Based RACH Messaging

The procedure as illustrated in figure 2 is characterized by the following steps:

1. RA Preamble Assignment on DL dedicated signaling: HO command generated by target eNB and sent via source eNB for

handover; MAC signalling (L1/L2 control channel or MAC control PDU is FFS) in case of

DL data arrival. 2. Random Access Preamble on RACH

Use the preamble received from message 1 3. Random Access Response

Within a flexible window of message 1 No HARQ Addressed to RA-RNTI on L1/L2 control channel; Containing at least Timing Alignment, Initial Uplink Grant for handover case

and Timing Alignment for DL data arrival case, RA-preamble identifier



RACH Physical Overview

A high level overview of the Physical RACH implementation




Contention Based RACH Message Diagram

•RACH utilizes the following physical channels

PRACHPhysical Random Access Channel

PDCCHPhysical Downlink Control Channel

PDSCHPhysical Downlink Shared Channel

PUSCHPhysical Uplink Shared Channel

(PRACH)

(PDCCH contains pointer to RAR)(PDSCH contains RAR)

(PUSCH)

(PDCCH contains pointer to Message)(PDSCH contains Message)




RACH Physical Channels

• PRACH Dedicated channel for UE random access opportunities

Designated resource blocks for random access opportunities Specified format for random access transmissions Described in more detail in the following slides, and in the 36.211 section of this presentation

• PDCCH The DL control channel is used by the eNB during RACH to identify the resource

blocks on the downlink shared channel (PDSCH) that contain data for the UE (RAR and Resource Contention Resolution messages)

No further description of PDCCH contained in this package

• PDSCH Carries the RAR and Resource Contention Resolution messages to the UE No further description of PDSCH contained in this package

• PUSCH Carries message3 to the eNB No further definition of PUSCH contained in this package




PRACH Physical Implementation – RA Opportunities

• The physical random access channel occurs periodically in the UL frame

The number of random access opportunities is configurable and depends on capacity, latency and performance goals

RACH parameters are transmitted on the Broadcast Channel (BCH) and can be changed in a semi-static manner

• Each physical random access channel (PRACH) occupies 1.08 MHz (6 resource blocks)

Freq

uenc

y

10ms radio frame

0 1 2 3 4 5 6 7 8 9

PUSCH PUCCH

Example PRACH Configuration 12

PRACH




RA Transmissions

• Each RACH transmission consists of a cyclic prefix and a Random Access Preamble

Preamble format is specified in the broadcast channel and sets the length of the CP and the length and content of the preamble sequence

Preamble formats that require multiple consecutive RACH opportunities are possible

A set of 64 possible preambles are provided to the UE in the broadcast channel For contention based RACH, the UE selects one of these preambles at random

For non-contention based RACH, the eNB specifies the preamble Does the eNB reserve specific PRACH opportunities for non-contention?

Are certain preambles reserved for non-contention?

TCP

TRA

TGT TPRE

Time

Random Access




Standards Describing RACH

• 36.331 RRC Specification Defines UE states Contains descriptions of processes that initiate RACH

Initial access from idle (RRC_Connection_Request) Initial access from radio link failure (RRC_Connection_ReEstablishment_Request) Handover

Contains Broadcast message parameters Including system wide parameters used in RACH processing

• 36.321 MAC Specification Specifies the RACH process and associated MAC Control Elements, PDUs,

and parameters

• 36.213 Phy Layer Procedures Specifies the random access procedure at the PHY layer

• 36.211 PHY specification Defines how the RACH information is formatted for transmission



36.331-830 RRC

UE States/Transitions

Broadcast RACH Parameters




4.2.1 UE states and state transitions including inter RAT

• RRC_IDLE: A UE specific DRX may be configured by upper layers. UE controlled mobility; The UE:

Monitors a Paging channel to detect incoming calls; Performs neighbouring cell measurements and cell (re-)selection; Acquires system information.

• RRC_CONNECTED: Transfer of unicast data to/from UE. At lower layers, the UE may be configured with a UE specific DRX. Network controlled mobility, i.e. handover and cell change order with network

assistance (NACC) to GERAN; The UE:

Monitors control channels associated with the shared data channel to determine if data is scheduled for it;

Provides channel quality and feedback information; Performs neighbouring cell measurements and measurement reporting; Acquires system information.

RACH purpose and messaging depends upon UE connection state




4.2.2 Signalling radio bearers

• "Signalling Radio Bearers" (SRBs) are defined as Radio Bearers (RB) that are used only for the transmission of RRC and NAS messages. More specifically, the following three SRBs are defined:

SRB0 is for RRC messages using the Common Control Channel (CCCH) logical channel;

SRB1 is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using Dedicated Control Channel (DCCH) logical channel

RRC_Connection establishes SRB1 (Craig Long comment, see section 5.3.3.1) SRB2 is for NAS messages, using DCCH logical channel. SRB2 has a lower-priority

than SRB1 and is always configured by E-UTRAN after security activation.

• In downlink piggybacking of NAS messages is used only for one dependant (i.e. with joint success/ failure) procedure: bearer establishment/ modification/ release. In uplink NAS message piggybacking is used only for transferring the initial NAS message during connection setup.

NOTE: The NAS messages transferred via SRB2 are also contained in RRC messages, which however do not include any RRC protocol control information.

• Once security is activated, all RRC messages, including those containing a NAS or a non-3GPP message, are integrity protected and ciphered by PDCP. NAS independently applies integrity protection and ciphering to the NAS messages

RACH used to establish SRB1 in certain cases




5.3.1.3 Connected mode mobility

• In RRC_CONNECTED, the network controls UE mobility, i.e. the network decides when the UE shall move to which cell (which may be on another frequency or RAT). The network triggers the handover procedure e.g. based on radio conditions, load. To facilitate this, the network may configure the UE to perform measurement reporting (possibly including the configuration of measurement gaps). The network may also initiate handover blindly, i.e. without having received measurement information from the UE.

• For mobility within E-UTRA, handover is the only procedure that is defined. Before sending the handover command to the UE, the source eNB prepares one or more target cells. The target eNB generates the message used to perform the handover, i.e. the message including the AS-configuration to be used in the target cell. The source eNB transparently (i.e. does not alter values/ content) forwards the handover message/ information received from the target to the UE. When appropriate, the source eNB may initiate data forwarding for (a subset of) the radio bearers.

• After receiving the handover command, the UE attempts to access the target cell at the first available RACH occasion, i.e. the handover is asynchronous. Consequently, when allocating a dedicated preamble for the random access in the target cell, E-UTRA shall ensure it is available from the first RACH occasion the UE may use. Upon successful completion of the handover, the UE sends a handover confirmation.

RACH used as part of handover




RACH Parameters (1 of 2)

rootSequenceIndex Parameter: Root-sequence-index , see TS 36.211, table 5.7.2-4 and 5.7.2-5

prach-ConfigurationIndex Parameter: PRACH configuration index . For FDD, see TS 36.211 [21, 5.7.1: table 5.7.1-1 and 5.7.1-2] (providing mapping of Preamble format and PRACH configuration to PRACH Configuration Index). For TDD, see TS 36.211 [21, table 5.7.1-3]

highSpeedFlag Parameter: FFS, see TS 36.211, 5.7.2.TRUE corresponds to Restricted set and FALSE to Unrestricted set

zeroCorrelationZoneConfig Parameter: NCS configuration, see TS 36.211, [21, 5.7.2: table 5.7.2-2]

ra-PreambleIndex Explicitly signalled Random Access Preamble in [36.321].ra-ResourceIndex Explicitly signalled PRACH resource in [36.321]. Frequency

resource index in [36.211]. Only applicable to TDD

PRACH-Configuration information elements

RACH-ConfigDedicated field descriptions

From 36.331-830:




RACH Parameters (2 of 2)

numberOfRA-Preambles Number of non-dedicated random access preambles [36.321]. Value is an integer. Default value is 64. Value n4 corresponds to 4, n8 corresponds to 8 and so on.

sizeOfRA-PreamblesGroupA Size of the random access preambles group A [36.321]. Value is an integer. If the parameter is not signalled, the value is equal to numberOfRA-Preambles . Value n4 corresponds to 4, n8 corresponds to 8 and so on.

powerRampingStep Parameter: POWER_RAMP_STEP [36.321]. Value in dB. Default value is [FFS]. Value dB0 corresponds to 0 dB, dB2 corresponds to 2 dB and so on.

preambleInitialReceivedTargetPower Parameter: PREAMBLE_INITIAL_RECEIVED_TARGET_POWER [36.321]. Value in dBm. Default value is -104 dBm. Value dBm-120 corresponds to -120 dBm, dBm-118 corresponds to -118 dBM and so on.

preambleTransMax Parameter: PREAMBLE_TRANS_MAX [36.321]. Value is an integer. Default value is [FFS]. Value n1 corresponds to 1, n2 corresponds to 2 and so on.

ra-ResponseWindowSize Duration of the RA response window [RA_WINDOW_BEGIN — RA_WINDOW_END] [36.321]. Value in subframes. Default value is [FFS]. Value sf2 corresponds to 2 subframes, sf3 corresponds to 3 subframes and so on.

mac-ContentionResolutionTimer Parameter: Contention Resolution Timer [36.321]. Value in subframes. Default value is [FFS]. Value sf8 corresponds to 8 subframes, sf16 corresponds to 16 subframes and so on.

maxHARQ-Msg3Tx Parameter: max-HARQ-Msg3-Tx [36.321], used for contention based random access. Value is an integer. Default value is [FFS].

partitionPLThreshold Parameter PARTITION_PATHLOSS_THRESHOLD [36.321]. Value range and step size are [FFS].

RACH-ConfigCommon field descriptions



RACH Procedures

36.321-830 MAC Protocol Spec




RACH Triggers and associated key RACH parameters (1 of 2) – This info is not in 36.321, but helpful to understand it

• There are five events that will trigger random access procedure. Initial access from RRC_IDLE;

MAC/PDCCH Initiated: MAC (eNB does not know about UE yet) Contention/Non-Contention based RACH: Contention only UE ID

Needs new C-RNTI from eNB CCCH SDU in Message3? Yes (RRC_Connection_Request)

Initial access after radio link failure; MAC/PDCCH Initiated: MAC (Lost link with eNB) Contention/Non-Contention based RACH: Contention only UE ID

set the c-RNTI to the C-RNTI used in the source cell (handover failure case) or used in the cell in which the trigger for the re-establishment occurred (other cases); (36.331 section 5.3.7.4)

CCCH SDU in Message3? Yes (RRC_Connection_Reestablishment_Request) Handover requiring random access procedure;

MAC/PDCCH Initiated: PDCCH (All handovers commanded by eNB) Contention/Non-Contention based RACH: Either, as specified by eNB UE ID

Target C-RNTI passed to UE as part of handover messaging (prior to RACH on new cell. UE will have to put this C-RNTI inot the message3 buffer so the target eNB utilizes the appropriate C-RNTI)

CCCH SDU in Message3? No, SRB1 movement handled between the eNBs




RACH Triggers and associated key RACH parameters (2 of 2)

• There are five events that will trigger random access procedure. DL data arrival during RRC_CONNECTED when UL synchronisation

status is “nonsynchronised”; MAC/PDCCH Initiated: PDCCH (DL data arrives at eNB) Contention/Non-Contention based RACH: Either, as specified by eNB UE ID

Existing C-RNTI. UE will have to put this C-RNTI into the message3 buffer so the target eNB utilizes the appropriate C-RNTI)

CCCH SDU in Message3? No, SRB1 still active UL data arrival during RRC_CONNECTED when UL synchronisation

status is “nonsynchronised” or there are no PUCCH resources for SR available available.

MAC/PDCCH Initiated: MAC (UL data arrives at UE) Contention/Non-Contention based RACH: Contention UE ID

Existing C-RNTI. UE will have to put this C-RNTI into the message3 buffer so the target eNB utilizes the appropriate C-RNTI)

CCCH SDU in Message3? No, SRB1 still active




Section 5.1.1 RACH Init – Input Parameters Required at MAC (and 36.331 reference)

Parameter Required by MAC Comment 36.331 ReferenceP-RACH Resources Available and corresponding RA-RNTIs

RA-RNTI is calculated from the subframe number and frequency resource, so RA-RNTI = PRACH Resource Selected

RA_Response_Window_Size ra-ResponseWindowSizeGroup A Preambles Group B Preambles

Calculated from “#OF_RA_Preambles” and “#OF_RA_Preambles_GroupA”

NumberofRA-PreamblesSizeofAR-PreamblesGroupA

Partition Pathloss Threshold Message Size Group A

Used to select GroupA or GroupB Preambles PartitionPLThresholdMessageSizeGroupA NOT in 36.331 – assume overlooked for now

Power_Ramp_Step Size or incremental power boost for preamble re-transmission PowerRampingStep

Preamble_Transmissions_Max Maximum number of preamble re-tries PreambleMaxTransPreamble_Initial_Received_Target_Power Target Power for the first preamble attempt preambleInitialReceivedTargetPowerMaximum_Message3_HARQ_Transmissions

Don't understand this one yetmaxHARQ-Msg3Tx

Clear HARQ BufferSet_Preamble_Xmission_Ctr=1

Set Backoff=0msecBegin RACH Process

Go To Resource Selection




Section 5.1.2 – Resource Selection

PRACH Rsrc

specified?

C-RNIT MAC CE or CCCH

SDU Previously sent?

Msg-Size >Msg_Size_GrpAAnd Pathloss >

Pathloss_Thresh

GrpBExists?

Choose GrpA

Choose GrpB

Randomly Select Preamble from Group

If TDD and >1 PRACHresource available in

this TTI, randomly Select PRACH resource

Resource Selection

Complete – Go To

Transmission

Y

Y

Y

N

N

N

N

Y

Begin ResourceSelection

Choose same RA preamble set as before

If the UE has previously performed a

RACH, re-use those RACH resources

If the UE has not previously performed a RACH, select RACH

resources




Section 5.1.3 – TransmissionPreamble

Xmission_Ctr= Preamble_Max

+ 1?

Instruct PHY to Transmit

TransmissionComplete –Go To “RAR Reception”

Y

N

Transmission Begin

Indicate RA problem To Upper Layers

Set Preamble_Pwr=Init_Tgt_Pwr +

(Last_Xmission_Pwr)*Pwr_Ramp_Step




Section 5.1.3 – Random Access Response (RAR) Reception• The UE monitors the PDCCH for an RAR containing it’s RA-RNTI within the

RA_Window RA-RNTI = t_ID+10*f_ID

t_ID = the index of the first subframe of the PRACH resource on which the preamble was transmitted {0,1,2,…,9}

f_ID = the index of the PRACH resource within that subframe {0,1,2,…,5}

SFn-1 SFnSFn+1 SFn+2

SFn+3 SFn+m+3

…PreambleXmission

RA

_Window

_Begi

n RA

_Window

_End

RA_Window_Size = m subframes

RA_Window




If this is not initial access, U

E already has C

-RN

TI and needs to pass that C

-RN

TI back to eNB

Section 5.1.4 – RAR Success Case

Backoff Indicator inSubHeader?

Backoff= 0msec

Process Timing AlignmentSend UL grant to lower layers

N

N

Y

RAR for this RA-RNTI Decoded In

Window?

RA Preamble ExplicitlySignaledTo MAC?

RAR Reception Complete

Set Backoff parameter asSpecified in RAR(see table 7.1.2)

Y

Set temp C_RNTI = valueRcvd in RAR, effective NLT

UL grant time

1st SuccessfulRAR in

this RACH?Is this RACHFor CCCH*?

Instruct Mux Layer “include a C_RNTI MAC Cntl Elem in Subsequent transmissions

Get MAC PDU from MuxLayer; put in MSG3 buffer

Y

Y

N

Go To RAR Fail

N

Y

If UE has alredy successfully

completed a R

AC

H, C

-RN

TI is established. SH

OU

LD SA

Y R

AC

H, not R

AR

above????

UE initial access; m

ust signal via C

CC

H to establish

RR

C_C

onnect

RAPID = Xmit RAPID

?

Y

RA Preamble is explicitly signaled to MAC only for PDCCH commanded RACH, therefore UE

identity is known and RAR can complete here.

Manage the UE MAC identity




Section 5.1.4 – RAR Failure Modes

RACH FailError Response

FFS

Y

RACHInit by MAC?

RACHInit by

PDCCH Order?

N

Preamble_Xmission_Ctr

< Max?

Inc Preamble_Xmission_Ctr

Inc Preamble_Xmission_Ctr

Select random Backoff between 0 and

Backoff parameter

Delay subsequent RACHby backoff parameter

Re-Try RACH (Go to preamble

resource selection)

No RAR in windowOr no match with RA-RNTI/RAPID

Y

N

Y

N

Why do we not check the preamble xmission counter for MAC initiated RACH? Note counter is checked prior to transmission.




UL C-RNTI MAC CE means UE was already in RRC_Connected,

and did not use the temporary C-RNTI for the UL. If the eNB

returnes this specific C-RNTI, then this UE was recognized.

Section 5.1.5 – Contention ResolutionPDCCH Rcvd?

Did UL incl C-RNTI MACCntl Elem?

Contention Resolution Successful

N

N

Y

RACH initVia MAC &

This C-RNTI & UL grant inclIn PDCCH?

Y

Y

RACH initVia PDCCH

and addressed to C-RNTI?

Y

UL incl CCCH SDU and PDCCH addr to

C-RNTI?

MAC PDU Decode

success?

MAC PDU = what the UE

sent?

Y

Contention Resolution

UnSuccessful

Y

NN

N

This is the response to a CCCH SDUIn the UL. DL PDU must decode to match the UL PDU

N

Y

N

UL C-RNTI MAC CE means UE was already in RRC_Connected,

and did not use the temporary C-RNTI for the UL. If the eNB

returnes this specific C-RNTI, then this UE was recognized.

Should never take this leg?




Section 6.1.5 – MAC PDU for RAR

• A MAC PDU consists of a MAC header and one or more MAC Random Access Responses (MAC RAR)

• A MAC PDU header consists of one or more MAC PDU sub-headers; each subheader corresponding to a MAC RAR except for the Backoff Indicator sub-header

• Note: I believe the entire MAC PDU is sent in the DLSCH, only the pointer to this MAC PDU is sent on the PDCCH

MAC RAR 1 ...

E/R/RAPID subheader 1

MAC header

MAC payload

...

MAC RAR 2 MAC RAR n

E/R/RAPID subheader 2

E/R/RAPID subheader n

MAC PDU consisting of a MAC header and MAC RARs

Each MAC subheader “points” to a MAC RAR




Section 6.2.2/6.2.3 – RA Formats and Parameters

E = Extension1 = another sub-header coming

0 = MAC RAR1 next

T = Type1 = RAPID

0 = BI

R = Reserved

BI = Backoff IndicatorIdentifies overload condition in the cell

RAPID = Random Access Preamble ID

TA = Timing Advance

RAPIDE T Oct 1

BIE R Oct 1RT

TA Oct 1

TA

UL Grant

UL Grant

Temporary C-RNTI

Temporary C-RNTI

UL Grant Oct 2

Oct 3

Oct 4

Oct 5

Oct 6

R

E/T/RAPID MAC sub-header

E/T/R/R/BI MAC sub-header

MAC RAR




36.321-830 MAC Protocol Spec Questions/Issues

• What is the BackoffTimer for? Retries

• What is Message3 Buffer? [Bill Shores Comment] The Random Access procedure consists of a preamble transmission from the UE

(Message1), then a RA Response from the eNB which includes an uplink grant on the PUSCH (Message2), then an uplink data transmission on the assigned PUSCH resource block(s) (Message3), then possibly a downlink contention resolution response from the eNB (Message4). Message3 would typically include a CCCH SDU (see below) or uplink signaling/bearer data multiplexed with a buffer status report

• What is the “measurement gap”? [Bill Shores Comment] Measurement Gaps are used to allow the UE to take measurements of neighbor cells (e.g.,

to facilitate Handover decisions). Periodic measurement intervals may be configured via RRC. For many functions (like air interface packet scheduling), it is important that the eNB is aware of the UE's "unavailability" during these gaps. I suspect that the statement above is meant to imply that if the UE is in the midst of a Random Access procedure, then it should forego any inter-cell measurement activities.

• When is RACH explicitly signaled to the MAC layer? When does MAC layer initiate? What about PDCCH RACH command?

[Bill's Comment] I believe that this covers the case in which the UE does not yet have a RRC connection. The CCCH SDU carries the common channel signaling (SRB0) to establish a dedicated connection (SRB1). In this case, Message3 contains the CCCH SDU and Message4 echoes it back to the UE as a method for performing contention resolution. I'm not sure about this, so I have copied Javed for his input. BTW, the PDCCH order is used by the eNB when downlink data arrives, but the UE's time alignment timer has expired (i.e., the RRC connection is still active, but the lower layers are possibly out of time alignment).

• Need to understand RNTI management



36.213 Phy Layer Procedures

Random Access Procedure




36.213 Section 6.1 – Scope of Phy Random Access Procedure

• From the physical layer perspective, the L1 random access procedure encompasses the transmission of random access preamble and random access response. The remaining messages are scheduled for transmission by the higher layer on the shared data channel and are not considered part of the L1 random access procedure. A random access channel occupies 6 resource blocks in a subframe or set of consecutive subframes reserved for random access preamble transmissions. The eNodeB is not prohibited from scheduling data in the resource blocks reserved for random access channel preamble transmission.




36.213 Section 6.1 – Prep for PHY Random Access Procedure

• Prior to initiation of the non-synchronized physical random access procedure, Layer 1 shall receive the following information from the higher layers: Random access channel parameters (PRACH configuration, frequency position and

preamble format) Frequency position is given by the parameter nRA

PRB as shown in 5.7.3. This is the starting frequency position. According to the latest agreement, this frequency position is configured by higher layers and can be anywhere Rapeepat email 080910

…and the variable phi, a fixed offset determining the frequency-domain location of the random access preamble within the physical resource blocks…is given in table 5.7.3-1 (36.211-830)

So it appears there is a fixed frequency offset into the resource blocks specified by nRAPRB?

The value of the fixed offset is 7 for format 0-3. Does this imply the first 6 frequencies are unused?

Parameters for determining the root sequences and their cyclic shifts in the preamble sequence set for the cell (index to root sequence table, cyclic shift, and set type

(normal or high-speed set))




36.213 Section 6.1 –Phy Random Access Procedure• Layer 1 procedure

Layer 1 procedure is triggered upon request of a preamble transmission by higher layers.

A preamble index, a target preamble received power (PREAMBLE_RECEIVED_TARGET_POWER), a corresponding RA-RNTI and a PRACH resource are indicated by higher layers as part of the request.

A preamble transmission power PPRACH is determined as PPRACH = min{Pmax, PREAMBLE_RECEIVED_TARGET_POWER + PL}, where Pmax is the maximum allowed power that depends on the UE power class and PL is the downlink pathloss estimate calculated in the UE.

A preamble sequence is selected from the preamble sequence set using the preamble index.

A single preamble is transmitted using the selected preamble sequence with transmission power PPRACH on the indicated PRACH resource.

Detection of a PDCCH with the indicated RA-RNTI is attempted during a window controlled by higher layers (see [8], clause 5.1.4). If detected, the corresponding PDSCH transport block is passed to higher layers. The higher layers parse the transport block and indicate the 20-bit UL-SCH grant to the physical layer, which is processed according to section 6.2.




From “Award Solutions” correspondence

• AS: The length of PHY random access preamble is not always 1ms. It is 1ms for format 0, 2ms for format 1&2, and 3ms for format 3. Obviously, the PRACH overhead increases in order to support large cell size.

[Craig Long:] Do you have a spec reference for the 1ms format 0, 2 ms format1&2, 3ms format 3? I can't find that in 36.211. I did find an indication in 32.213 that the RACH window size is "indicated by higher layers", and that the window size is in terms of subframes. Are you saying the 1ms, 2ms, and 3ms are the expected values for that window size, or are those times specified somewhere?

AS: In 36.213 section 6.1, it says “A random access channel occupies 6 resource blocks in a subframe or set of consecutive subframes reserved for random access preamble transmission.” If adding the time of Tcp and Tseq for format 1,2,3, the total would be 1.484ms, 1.8ms, and 2.284ms. They cannot be transmitted within 1ms, so they must occupies several subframes, round up to the nearest integer number. Also, please check the attached 36.211 contribution for discussion on large cell preamble design.




36.213 section 6.1.1 Timing

• For the L1 random access procedure, UE’s uplink transmission timing after a random access preamble transmission is as follows

RAR for this UE detected If a PDCCH with associated RA-RNTI is detected in subframe n, and the corresponding DL-

SCH transport block contains a response to the transmitted preamble sequence, the UE shall, according to the information in the response, transmit an UL-SCH transport block in the first subframe (>n+5) where an UL-SCH transmission is available.

RAR for another UE detected If a random access response is received in subframe n, and the corresponding DL-SCH

transport block does not contain a response to the transmitted preamble sequence, the UE shall, if requested by higher layers, transmit a new preamble sequence in the first subframe (>n+4) where a PRACH resource is available.

No RAR Detected within window If no random access response is received in subframe n, the UE shall, if requested by higher

layers, transmit a new preamble sequence in the first subframe (>n+3) where a PRACH resource is available.

• RACH Initiated by PDCCH Order In case random access procedure is triggered by the PDCCH indicating downlink data

arrival in subframe n, UE shall, if requested by higher layers, transmit random access preamble in the first subframe (>n+5) where a PRACH resource is available.



36.211 PHY




36.211 section 5.7.1 Time and frequency structure

• The physical layer random access burst consists of a cyclic prefix, a preamble, and a guard time.

• Cyclic prefix is used to assist in efficient frequency domain processing – maximum supported cell size is determined by cyclic prefix and guard time?

• Guard time is used to accommodate round trip propagation delay• Basic random access burst (format 0) spans 1ms with cyclic prefix of 102.7 s,

preamble length of 800 s and guard time of 97.3 s (provides coverage for cell size of ~14.6 km)

Calculation presented in slide xx• For larger cell size, extended burst structure is used with either

Extended cyclic prefix and guard time, but the same preamble length (format 1, 2msec) Extended cylic prefix, guard time, and preamble length (2x800 s) (format 3, 3 msec)

• For TDD special subframe, format 4 has reduced CP and preamble lengths

TCP

TRA

TGT TPRE

Time

Random Access




36.211 section 5.7.1 Preamble Formats

Preamble format

Tcp TSEQ

Maximum Supported Cell Size

(km)

Subcarrier Spacing

0 3168×Ts 24576×Ts 14.6

1.25 kHz1 21024×Ts 24576×Ts 101.8

2 6240×Ts 2×24576×Ts 29.7

3 21024×Ts 2×24576×Ts 101.8

4( TDD only)

448×Ts 4096×Ts 1.4 7.5 kHz

R1 supports only formats 0 and 1. 34310E will add format 4. No support for formats 2 and 3 for 34310E




36.211 section 5.7.1 FDD Preamble Configuration

• A “Preamble Configuration Index” is broadcast by the eNB, and indexes a table (5.7.1-2) that specifiesPreamble formatLocation of preamble opportunity in the uplink frame structure (frame

and subframe number)

• The frequency location within that frame/subframe is given by the parameter nRA

PRB. This is the starting frequency position. This frequency position is configured by higher layers and can be anywhere in the subframe




Example – PRACH Configuration 12Fr

eque

ncy

10ms radio frame

0 1 2 3 4 5 6 7 8 9

PUSCH PUCCH

Frequency position is given by the parameter nRAPRBoffset as shown in 5.7.3. This is

the starting frequency position. According to the latest agreement, this frequency position is configured by higher layers and can be anywhere

- Rapeepat email 080910

nRAPRBoffset

PRACH




36.211 section 5.7.1 TDD Preamble Configuration• A “Preamble Configuration Index” is broadcast by the eNB, and indexes a table

(5.7.1-3) that specifies Preamble format

34310E will add preamble format 4 to the previously supported preamble formats 1 and 2 Supported PRACH density value

PRACH attempts per 10msec Version Index

Different “versions” of the same PRACH density will provide the same PRACH density but in staggered subframes

This allows a multi-cell eNB to spread PRACH decoding resources evenly among multiple cells and avoid peal loading problems

The version index is for information only; it is not used in determining the PRACH opportunities – that is embedded in the configuration index

Note that since feature 34310E will not support format 2 and format 3 preamble formats Preamble configuration indeces 32-47 in tables 5.7.1-3 and 5.7.1-4 are not supported

• The “Preamble Configuration Index” also indexes a table (5.7.1-4) that specifies Time and frequency resources per TDD UL/DL configuration for PRACH attempts

Each index specifies one or more 4-tuples (fRA, t0RA, t1

RA, t2RA), where

fRA = frequency resource within the time index» see example in following slides

t0RA = 0,1,2 indicates whether the resource is reoccurring in all radio frames, in even radio frames, or in odd radio frames, respectively

t1RA = 0,1 indicates whether the random access resource is located in first half frame or in second half frame, respectively » 1st half = subframes 0 – 4» 2nd half = subframes 5 – 9

t2RA= the uplink subframe number where the preamble starts, counting from 0 at the first uplink subframe between 2 consecutive downlink-to-uplink switch points, with the exception of preamble format 4 which is always transmitted in UpPTS and is denoted as (*).




36.211 section 5.7.1 TDD Preamble Configuration – Table 5.7.1.4 Illustration

t0RA indicates whether the resource is reoccurring in all radio frames, in even radio frames, or in odd radio frames, respectivelyt1RA indicates whether the random access resource is located in first half frame or in second half frame, respectivelyt2RA is the uplink subframe number where the preamble starts, counting from 0 at the 1st uplink subframe between 2 consecutive DL-to-UL switch points

Indicates the first uplink subframe between 2 consecutive downlink-to-uplink switch points

Config

DwPTS GP UpPTS DwPTS GP UpPTS



DwPTS GP UpPTS

DwPTS GP UpPTS

DwPTS GP UpPTS


1st half radio frame 2nd half radio frame

downlinkdownlink special uplink uplink uplink downlink special uplink uplink

downlink downlink

6

downlink downlink downlink downlinkdownlink special uplink downlink

downlink

5

downlink special uplink uplink downlink downlink downlink downlink downlink

downlink downlink

4

uplink downlink downlink downlinkdownlink special uplink uplink

downlink

3

downlink special uplink downlink downlink downlink special uplink downlink

uplink downlink

2

downlink downlink special uplinkdownlink special uplink uplink

uplink

1

downlink special uplink uplink uplink downlink special uplink uplink

subframe8 subframe9

0

subframe0 subframe1 subframe2 subframe3 subframe4 subframe5 subframe6 subframe7




TDD Preamble Configuration Example: NUL

RB = 25, PRACH Configuration 12, UL/DL conf 0, NRAPRBoffset = 19

• (0,0,0,1) - labeled PRACH Opportunity A in the diagram

fRA = 0 frequency resource index (see calculation in RACH vs Cell Size/xls) t0

RA=0 = all radio frames t1

RA=0 = 1st half of radio frame t2

RA=1 = 1st subframe after the first uplink between 2 UL/DL point

• (0,0,0,2) - labeled PRACH Opportunity B in the diagram fRA = 0 frequency resource index (see calculation in RACH vs Cell Size/xls) t0


RA=0 = 1st half of radio frame t2

RA=2 = 2nd subframe after the first uplink between 2 UL/DL points

• (0,0,1,1) - labeled PRACH Opportunity C in the diagram fRA = 0 frequency resource index (see calculation in RACH vs Cell Size/xls) t0


RA=1 = 2nd half of radio frame t2

RA=1 = 1st subframe after the first uplink between 2 UL/DL points

• (0,0,1,2) - labeled PRACH Opportunity D in the diagram fRA = 0 frequency resource index (see calculation in RACH vs Cell Size/xls) t0


RA=1 = 2nd half of radio frame t2

RA=2 = 2nd subframe after the first uplink between 2 UL/DL pointsNote: since all 4-tuples have t0RA=0, the diagram applies to all frames




TDD Preamble Configuration Example: NUL

RB = 25, NRAPRBoffset = 19, PRACH Configuration 12, UL/DL conf 0.

Config


2423222120191817161514131211109876543210

subframe0 subframe1 subframe2 subframe3 subframe4 subframe5 subframe6 subframe7 subframe8

uplink downlink special uplinkdownlink special uplink uplink uplink uplink

subframe9

0

PRACH Opportunit

y D

PRACH Opportunit

y C

PRACH Opportunit

y A

PRACH Opportunit

y B




36.211 section 5.7.1 TDD Preamble Timing• Preamble timing within the subframe

The start of the random access preamble formats 0-3 shall be aligned with the start of the corresponding uplink subframe at the UE assuming a timing advance of zero and the random access preamble format 4 shall start 5158*Ts before the end of the UpPTS at the UE




36.211 section 5.7.1 TDD Preamble Resource Selection

• The random access opportunities for each PRACH configuration shall be allocated in time first and then in frequency if and only if time multiplexing is not sufficient to hold all opportunities of a PRACH configuration needed for a certain density value without overlap in time. The order the 4-tuples are listed in the table is the priority order in

which they should be utilized

• For preamble format 0-3, the frequency multiplexing shall be done according to

otherwise,

266

02mod if,2

6

RARAoffsetPRB

ULRB

RARARA

offsetPRBRAPRB fnN

ffnn





• Example # UL Resource blocks = 25

Chart of the PRACH resource block as a function of fRA and offset

Note that the purple cells indicate ineligible parameters for this configuration To get 5 or 6 frequency resources into the 25 resource blocks available, the

maximum offset allowed is 7 To get 3 or 4 frequency resources into the 25 resource blocks available, the

maximum offset allowed is 13 The max offset is 19 (must allow for 6 consecutive resource blocks for the PRACH

nRAPRBoffset : 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

fRA

0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 191 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 02 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 253 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -64 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 315 7 6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12

nRAPRB=for NUL

RB=25





• For preamble format 4, the frequency multiplexing shall be done according to

otherwise),1(6

02mod)2()2mod( if,6 1

RAULRB

RASPfRARAPRB fN

tNnfn

•Where nf is the system frame number and where NSP is the number of DL to UL switch points within the radio frame.•Each random access preamble occupies a bandwidth corresponding to 6 consecutive resource blocks for both frame structures.




36.211 section 5.7.2 Preamble Generation (1 of 2)

• The random access preambles are generated from Zadoff-Chu (ZC) sequences

ZC sequence generation described in 36.211, but I don’t understand it

• The network configures which root sequences to utilize to generate the preambles

The random access preambles are generated from Zadoff-Chu sequences with zero correlation zone, generated from one or several root Zadoff-Chu sequences. The network configures the set of preamble sequences the UE is allowed to use

Each root sequence is orthogonal (or at least low correlation) with every other root sequence (Craig comment)

• One root ZC sequence can generate multiple orthogonal preambles via cyclic shifting.

The number of orthogonal preambles that can be generated from one root sequence depends on the supported cell size.

Each cyclic shift of a root sequence is orthogonal (or low correlation) with every other cyclic shift. Therefore each UE can use a different cyclic shift of a root sequence and be separated from the others by the eNB.

However, the cyclic shifts can look identical if the round trip delay is longer than the distance between cyclic shifts. Therefore the bigger the cell the fewer cyclic shifts you can use from each root sequence (more distance between cyclic shifts)

Each root sequence is 839 bits long, so the number of preambles for each root sequence is 839/cyclic shift distance.




36.211 section 5.7.2 Preamble Generation (2 of 2)

• Each cell is configured with 64 preambles There are 64 preambles available in each cell. The set of 64 preamble sequences in a cell is found

by including first, in the order of increasing cyclic shift, all the available cyclic shifts of a root Zadoff-Chu sequence with the logical index RACH_ROOT_SEQUENCE, where RACH_ROOT_SEQUENCE is broadcasted as part of the System Information. Additional preamble sequences, in case 64 preambles cannot be generated from a single root Zadoff-Chu sequence, are obtained from the root sequences with the consecutive logical indexes until all the 64 sequences are found. The logical root sequence order is cyclic: the logical index 0 is consecutive to 837. The relation between a logical root sequence index and physical root sequence index is given by Tables 5.7.2-4 and 5.7.2-5 for preamble formats 0 – 3 and 4, respectively.

• High mobility is supported through regular preambles with a restricted set of cyclic shifts

eNB broadcasts the High Speed Flag when high mobility is desired




Preamble Generation – Cyclic Shift Selection (1 of 3)

Supportable Cell size (km)

13 0.78 64 115 1.06 55 218 1.47 46 222 2.03 38 226 2.59 32 232 3.42 26 338 4.26 22 346 5.37 18 459 7.18 14 576 9.54 11 693 11.9 9 8119 15.52 7 10167 22.19 5 13279 37.77 3 22419 57.23 2 32839 115.63 1 64

Nzc/N CS (sequence length div by cyclic

shifts)N CS

No of root sequences required to

generate 64 preambles





• Each root ZC sequence is 839 bits long (NZC)

• Actual preamble sequences are generated via cyclic shifts of the root ZC sequence

• NCS (ZeroZoneCorrelation broadcast by eNB) specifies the number of cyclic shifts between preamble sequences

Column 1 in the previous table

• Therefore, the number of preambles per root ZC sequence is 839/NCS

column 3 in the previous table

• Each cell is configured with 64 preambles, so the number of root ZC sequences required to generate 64 preambles is 64/(893/NCS)

column 4 in the previous table





• Cell size is restricted by the number of cyclic shifts between preamblesDifferential delay between preamble transmission of 2 UEs will

appear at the eNB as cyclically shifted preambles Since the UE is not in sync during RACH, the effective delay is 2X the

delay between the eNB and the UE That delay (converted to distance via the speed of light) is the max cell

size. A guard band of a couple of cyclic shifts is used for buffer

This value is column 2 in the previous table An excel spreadsheet with this calculation is available from Craig Long




RACH Timing (frame structure 1), single subframe RACH

RACH Window

SubFrame = 30720Ts = 1msec

eNB

UE Tx 1 way Prop delay

1 way Prop delay CP PreambleeNB Rx

RACH Window

RACH Window time >= 2*PropagationDelay + Tcp + Tseq Solving for Propagation Delay:PropagationDelay <= (RACH Window time - Tcp - Tseq)/2

Cell Size is also restricted by the length of the RACH preamble and the propagation delay between the UE and eNB, as shown above




RACH Timing (frame structure 1), multi-subframe RACH

RACH Window

SubFrame = 1msec

eNB

UE Tx 1 way Prop delay

1 way Prop delay CP PreambleeNB Rx

RACH Window

RACH Window time >= 2*PropagationDelay + Tcp + Tseq Solving for Propagation Delay:PropagationDelay <= (RACH Window time - Tcp - Tseq)/2

SubFrame = 1msec




Section 5.7.3 Baseband Signal Generation

• Don’t understand this yet – work on it



Backup/Old




Cell Size Issues (Craig Long – unverified)

• There are many factors which limit cell size RACH delay

An unsynchronized (no timing advance) UEs ability to send a RACH preamble and have it arrive at the eNB within the RACH window is dependent on the size of the preamble and the round trip delay of the cell

Cyclic Shift Orthogonality Each UE will utilize a different cyclic shift of a root ZC sequence. However, the

delay (and delay spread) of the channel will make the ZC sequence of a far UE look just the same as the ZC sequence of a close UE that has been cyclically shifted.

In order to prevent this, the cyclic shift of the UEs must be kept further apart than the round trip delay time + delay spread

SNR will determine the eNBs probability of detecting the RACH sequence. For larger cells, users at the edge require more signal power (hence a longer preamble) to meet the required probability of detection

Not discussed in this package. See R1-072135 RACH_LargeCell.doc




• From TS 36.211-830 section 5.7.1 The start of the random access preamble shall be aligned with the start of the

corresponding uplink subframe at the UE assuming a timing advance of zero Preamble includes the CP

Each random access preamble occupies a bandwidth corresponding to 6 consecutive resource blocks for both frame structures

RACH preamble is sent over 1.08 MHz BW This is equivalent to 6 consecutive resource blocks

• From TS 36.211-830 section 5.7.2 (table 5.7.2-1) Preamble format 0-3 has preamble sequence length 839 Preamble format 4 has preamble sequence length 139

• From “Random Access Design for e-UMTS As a result, Zadoff-Chu sequence with cyclic shift was selected as the

preamble for E-UTRA. For the baseline preamble length of 800μs, this corresponds to a sequence of length 863 samples.




UpPTS RACH Cell Size Calculation: From “TDD Design for LTE.pdf”

• From Table 3, it is seen that there are only two values for UpPTS duration (one or two OFDM symbols). As a result, UpPTS usage by the UE is limited to either sounding reference signals or random access (RACH) transmission. Random access requires UpPTS length of two OFDM symbols. When one OFDM symbol is allocated to the UpPTS, only sounding reference signals transmission is possible.

• Random access on the UpPTS is limited by the length of the UpPTS and therefore not applicable to all deployment scenarios. An illustration of the random access transmission in the UpPTS is shown in Figure 6. Random access begins 4832×Ts seconds, where Ts = 1/(15000×2048), before the endof the UpPTS with a duration of 4544×Ts seconds. This leaves a guard period of 288×Ts seconds which allows for a maximum supported cell size of approximately 1.4 km. For larger cell sizes, RACH will have to be supported in regular uplink subframes to provide sufficient guard period.




From “TDD Design for LTE.pdf”

CP Preamble Sequence

UpPTS Duration

Tcp=448*Ts seq=4096*Ts 288*Ts614*Ts??

288*Ts = 9.38uSec which supports a round trip delay (cell size) of(9.38uSec*3E8m/sec)/2 = 1.41 Km

TS 36.211-830 section 5.7.1 says preamble starts 5158*Ts before end of subframeAssuming CP and Preamble Sequence are the same length, guard time becomes5158-448-4096 = 614*Ts = 20uSec which supports a round trip delay (cell size) of(20uSec*3E8m/sec)/2 = 3 Km




Random Access Configuration (FDD)

PRACH configuration System frame number Subframe number0 Even 1

1 Even 4

2 Even 7

3 Any 1

4 Any 4

5 Any 7

6 Any 1, 6

7 Any 2 ,7

8 Any 3, 8

9 Any 1, 4, 7

10 Any 2, 5, 8

11 Any 3, 6, 9

12 Any 0, 2, 4, 6, 8

13 Any 1, 3, 5, 7, 9

14 Any 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

15 Even 9




Cyclic Shift Values

configuration value

Unrestricted set Restricted set

0 0 15

1 13 18

2 15 22

3 18 26

4 22 32

5 26 38

6 32 46

7 38 55

8 46 68

9 59 82

10 76 100

11 93 128

12 119 158

13 167 202

14 279 237

15 419 -

configuration value0 2

1 4

2 6

3 8

4 10

5 12

6 15

Preamble Format 0-3

Preamble Format 4

36.211-830 tables 5.7.2-2 and 5.7.2-3




• A random access preamble is generated from

u – root ZC index Ncs – cyclic shift length v – integer value

• Supportable cell size is based on the cyclic value Ncs

• Number of root ZC sequences required to generate 64 preambles depends on the cell size

Preamble Selection and Cell Size

)mod)(()( ZCCS, NvNnxnx uvu

NCSSupportable Cell size

(km)

No of root sequences required

to generate 64 preambles

13 0.78 1

15 1.06 2

18 1.47 2

22 2.03 2

26 2.59 2

32 3.42 3

38 4.26 3

46 5.37 4

59 7.18 5

76 9.54 6

93 11.90 8

119 15.52 10

167 22.19 13

279 37.77 22

419 57.23 32

839 115.63 64




PRACH Root Sequence Index

• Logical root sequence number is provided on the SIB mapped to physical root sequence number u in Tables 5.7.2-4 and 5.7.2-5

• Selection of root sequence number is part of the cell planning process Cross correlation between different root sequences is low (1/sqrt(Nzc)),

however, cubic metric is different for different sequences Number of physical root sequences used per cell depends on the cyclic shift

length and preamble format Example: PRACH Format 0, Ncs = 32, so one root sequence can generate at

most 26 cyclic shifts (26 preambles) 839 bits/32 bit cyclic shift = 26.2 = 26 full sequences

So 3 root sequences are required to generate 64 preambles 64/26 = 2.4 root sequences, implies 3 sequences required

• 838 possible root sequence indices for Format 0-3

• 138 possible root sequence indices for Format 4




Example of RACH Receiver Design

CP Removal

Sub-carrierDe-

MappingDFT

DFT

Size-NZC

Size-NDFT

Received Signal

IDFT Energy Detection

PreambleNZC symbols

1.0 ms

PREAMBLECP

PREAMBLECP

FFT Receive Window

User close to the base station

User at the cell edge




Preliminary PRACH Performance

Number of RX antennas

Propagation conditions

Frequency offset

SNR [dB]Burst

format 0Burst

format 1Burst

format 2Burst

format 3Burst

format 42 AWGN 0 [-13.4] [-13.3] [-15.6] [-15.6] TBD

ETU 70 270 Hz [-5.7] [-5.3] [-7.9] [-7.8] TBD

4 AWGN 0 [-16.0] [-15.6] [-18.1] [-18.0] TBD

ETU 70 270 Hz [-10.1] [-9.7] [-12.3] [-12.2] TBD

Normal Mode

High-Speed Mode

Number of RX antennas

Propagation conditions

Frequency offset

SNR [dB]Burst format 0 Burst

format 1Burst

format 2Burst

format 32 AWGN 0 [-13.2] [-13.1] [-15.4] [-15.4]

ETU 70 270 Hz [-5.1] [-4.9] [-7.1] [-7.3]

AWGN 625 Hz [-11.6] [-11.4] [-13.6] [-13.7]

AWGN 1340 Hz [-12.7] [-12.6] [-14.8] [-14.9]

4 AWGN 0 [-15.9] [-15.5] [-17.9] [-18.0]

ETU 70 270 Hz [-9.8] [-9.4] [-11.9] [-11.7]

AWGN 625 Hz [-14.0] [-13.7] [-16.1] [-16.2]

AWGN 1340 Hz [-15.3] [-14.8] [-17.2] [-17.2]




RACH Description from TS36.213-830 (Phy Procedures) Prior to initiation of the non-synchronized physical random access procedure, Layer 1 shall receive

the following information from the higher layers: Random access channel parameters (PRACH configuration, frequency position and preamble format) Parameters for determining the root sequences and their cyclic shifts in the preamble sequence set for the

cell (index to root sequence table, cyclic shift (), and set type (unrestricted or restricted set))

• 6.1 Physical non-synchronized random access procedure From the physical layer perspective, the L1 random access procedure encompasses the

transmission of random access preamble and random access response. The remaining messages are scheduled for transmission by the higher layer on the shared data channel and are not considered part of the L1 random access procedure. A random access channel occupies 6 resource blocks in a subframe or set of consecutive subframes reserved for random access preamble transmissions. The eNodeB is not prohibited from scheduling data in the resource blocks reserved for random access channel preamble transmission.

The following steps are required for the L1 random access procedure: Layer 1 procedure is triggered upon request of a preamble transmission by higher layers. A preamble index, preamble transmission power (PREAMBLE_TRANSMISSION_POWER), associated RA-

RNTI, random access window ([RA_WINDOW_BEGIN—RA_WINDOW_END]) and PRACH resource are indicated by higher layers as part of the request.

A preamble sequence is then selected from the preamble sequence set using the preamble index. A single preamble transmission then occurs using the selected preamble sequence with transmission power

PREAMBLE_TRANSMISSION_POWER on the indicated PRACH resource. If an associated PDCCH with RA-RNTI is detected within the random access response window then the

corresponding DL-SCH transport block is passed to higher layers. If the random access response window has past then the physical random access procedure is exited.

• 6.1.1 Timing 6.1.1.1 Synchronized 6.1.1.2 Unsynchronized

rach overview and analysis

Documents