Physical Layer Parameters - LTE Guidelines

- At initial network access (UE wants to access the network in order to establish RRC

Connection)

- To send a scheduling request (in case there is no dedicated PUCCH resource configured for

scheduling request)

- To establish uplink synchronization when uplink or downlink data arrives in RRC connected

state and the uplink is not yet synchronized

- At handover (so the target eNB can measure uplink timing)

- After radio link failure (so the radio link can be re-established)

The random access procedure is triggered by the RRC layer. It involves MAC and physical layer

procedures and makes use of the RACH uplink transport channel mapped to the PRACH uplink

physical channel. Since the random access procedure carries only limited control information, there is

no logical channel associated to this procedure.

2.2 Physical Layer Parameters

The physical layer procedure consists of transmitting a random access preamble (msg1) on the

PRACH uplink channel, and receiving a random access response (msg2) on PDCCH/PDSCH.

On the physical layer, the PRACH uplink channel is used to send the random access preamble.

In the frequency domain, PRACH has 6 PRBs allocated (regardless of the system bandwidth) with a

position depending on the system configuration (prach-FreqOffset). In the time domain, PRACH can

be allocated to one or more sub-frames which depends on the system configuration

(prach-21

ConfigIndex).

Note that the eNB is not prohibited to schedule PUSCH in the time-frequency resources allocated for

PRACH.

The random access preamble is a Zadoff-Chu sequence which makes it possible for the eNB to

differentiate between UEs and calculate uplink timing. Each LTE cell has a set of 64 available

sequences derived from the cell specific root sequence(s) (rootSequenceIndex) via cyclic shifting. A

group of the sequences may be reserved for contention free procedure; the rest can be used for

contention based random access. The contention based subset can actually be divided into two groups

A and B. Letting the UE chose one preamble from either group based on UL granting requirements to

hint the eNB about its next UL transmission requirements.

The random access preamble has a cyclic prefix appended. Duration of the cyclic prefix and of the

random access sequence is configurable.

prach-FreqOffset 2.2.1

Definition: Index of the first physical resource block (PRB) allocated for PRACH

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IE Value Engineering Units

Allowed Range 0 … 𝑁_𝑅𝐵^𝑈𝐿- 6 0 … 𝑁_𝑅𝐵^𝑈𝐿- 6 physical resource blocks

Recommended 1 (5 MHz) 2-3 (10 MHz)

1 RB (5 MHz) 2-3 RB (10 MHz)

Setting Tradeoff: n/a

Dependencies/Constraints: Parameter is restricted by the UL system bandwidth and PUCCH

configuration.

Traceability: TS36.211 Sect. 5.7.1

RRC Message Structure:

SIB2  RadioResourceConfigCommon  PRACH-Config 

PRACH-ConfigInfoprach-6

FreqOffset.

Notes: In the frequency domain PRACH has always 6 PRBs allocated, regardless of the UL system

bandwidth (𝑁𝑅𝐵𝑈𝐿, which is the UL system bandwidth expressed in RBs).

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prach-ConfigIndex 2.2.2

Definition: Determines preamble format: a) Prach cyclic prefix duration, b) preamble sequence

length. It also defines PRACH resource allocation in the time domain (system frame(s) and

sub-3

frame(s))

IE Value Engineering Units

Allowed Range

[0 … 63] For mapping see TS36.211 Table 5.7.1-2

Preamble format: Format 0, 1, 2 or 3 System frame: Even or Any

Subframe: 1, 2, 3 or 5 selected subframes or 10 subframes

Recommended 3, 4, 5 3, 4, 5

Setting Tradeoff (see notes also notes):

- Preamble format: use of PRACH long cyclic prefix duration is beneficial for large cells

(where larger delay spreads are expected), but it may cause un-necessary overhead for a

small cell size deployments. Use of long preamble sequence makes access preamble reception

transmission more robust but occupies certain part of the consecutive subframe.

- PRACH resource allocation - the time domain:

If too few time resources are allocated, random access occasions will be infrequent, causing

delays in starting the random access procedure. This will result in longer call set up times

and handover execution times.

If too many time resources are allocated then there will be more chances that the PRACH

may collide with PUSCH, and it may lead to lower throughput.

- Set the PRACH Configuration Index to 3,4,5 for three cells of any site to minimize PRACH

to PRACH interference. The implicit corresponding preamble format is Format 0.

Dependencies/Constraints: None.

Traceability: TS36.211 Sect. 5.7.1

RRC Message Structure:

SIB2  RadioResourceConfigCommon  PRACH-Config 

PRACH-ConfigInfoprach-21

ConfigIndex.

Notes: The random access preamble is composed of a cyclic prefix and a sequence part. Preamble

format is defining the length of each part, as described in TS36.211 Sect. 5.7.1:

Cyclic prefix Sequence

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Preamble Format

TCP TSEQ

0 3168 TS 24576 TS

1 21024 TS 24576 TS

2 6240 TS 2x24576 TS

3 21024 TS 2x24576 TS

Where Ts = 1/(15000 x 2048) seconds is the basic time unit.

Note that for formats 1, 2 and 3 TCP+TSEQ > 1 ms resulting the preamble ending out of the sub-frame.

Also, due to propagating delay the preamble may end out of the sub-frame even for format 0 which is

shorter than 1 ms. It is up to the eNB implementation to handle this either via not scheduling PUSCH

in the consecutive subframe or to schedule PUSCH and to cope with the interference.

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rootSequenceIndex 2.2.3

Definition: Index of the physical Zadoff-Chu root sequence to be used for PRACH in the cell

IE Value Engineering Units

Allowed Range [0 … 837] For mapping see TS36.211 Table 5.7.2-4 Recommended Variable (Each cell should have

individual root sequence(s).

Setting Tradeoff: n/a

Dependencies/Constraints: None.

Traceability: TS36.211 Sect. 5.7.2

RRC Message Structure:

SIB2  RadioResourceConfigCommon  PRACH-Config  rootSequenceIndex

Notes: The RACH Root Sequence (sometimes also referred to as Logical Root Sequence Index or

Logical Root Sequence Number) is mapped to a Physical Root Sequence Number. Each LTE cell has

one root sequence assigned.

In order to differentiate UEs sending random access preambles at the same time, there are 64 different

preamble sequences available in the cell. The 64 preamble sequences are derived as cyclic shifts of

the Zadoff-Chu root sequence assigned to the cell. In case the 64 preambles can not be generated from

a single root sequence due to larger zero correlation zone setting (see parameter

zeroCorrelationZoneConfig), further sequences will be generated as cyclic shifts of the next root

sequence. The degree of orthogonalitly of preamble sequences obtained from different root sequences

is deteriorated.

The algorithm for performing the cyclic shifting is described in TS36.211 Sect. 5.7.2. It is making use

of two configuration parameters: Ncs and the High-Speed-Flag (for explanation of these parameters

see the respective section).

Each cell should have individual root sequence value to use the Zadoff-Chu cross-correlation

properties.

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N

_CS

configuration (zeroCorrelationZoneConfig) 2.2.4

Definition: Parameter used to calculate cyclic shifts of the root Zadoff-Chu sequence in the cell

IE Value Engineering Units

Allowed Range [0 … 15] For mapping see TS36.211 Table 5.7.2-2 Recommended 12*

Setting Tradeoff: For large Ncs settings, a larger cyclic shift of the ZC sequence is expected. This is

efficient for larger coverage and larger delay spreads, but reduces the total number of available

preamble sequences available in a cell and hence enforces the use of additional root sequence(s)

which result in the loss of degree of the orthogonality between the root sequence families. The other

way around, i.e the smaller Ncs, will have the contrary effect as the one described above.

Dependencies/Constraints: *The corresponding recommended 𝑁_𝐶𝑆 value 𝑁_𝐶𝑆 = 119 for

unrestricted set is based on a 15 km cell size that determines propagation delay and a maximum delay

spread. As results, ⌊839/119⌋ = 7 preambles per root sequence can be generated and 10 root

sequences are needed to produce 64 preambles. For restricted set, the corresponding recommended

value 𝑁_𝐶𝑆 = 119 = 158. ⌊839/158⌋ = 5 preambles per root sequence can be generated and 13 root

sequences are needed to produce 64 preambles.

Traceability: TS36.211 Sect. 5.7.2

RRC Message Structure:

SIB2  RadioResourceConfigCommon  PRACH-Config  PRACH-ConfigInfo 

zeroCorrelationZoneConfig

Notes: The Zero Correlation Zone Config (NCS configuration) value is an index, which maps to an

NCS value. The mapping is different for restricted and unrestricted set (see the parameter High Speed

Flag).

The Ncs value is used to calculate the cyclic shift (CV) to generate the Zadoff-Chu preamble

sequence. The cyclic shift to be applied is CV = v*NCS where v = 0 … 63 describes the preamble

index (see description under PRACH Preamble Index).

Note that NCS value has an impact of the cell size.

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highSpeedFlag 2.2.5

Definition: Defines the use of restricted sets of cyclic shifts of the Zadoff-Chu root sequence used for

high speed moving UEs.

IE Value Engineering Units

Allowed Range BOOLEAN TRUE, FALSE

Recommended TRUE if in high-speed mobility, FALSE otherwise

TRUE if in high-speed mobility, FALSE otherwise

Setting Tradeoff: If the parameter is set to FALSE, the unrestricted set will be used. The UE can use

all the sequence available every integer multiple of Ncs. However, some sequences are not optimized

for high Doppler shift due to high-speed mobility, and the correlation peak at eNB will reflect the

wrong sequence, hence the false detection cannot be avoided.

If the parameter is set to TRUE, the restricted set will be used. The UE can only use the specific

sequences which are designed for high-speed mobility. The correlation peak at eNB will be able to

reflect the correct sequence.

Dependencies/Constraints: None.

Traceability: TS36.211 Sect. 5.7.2

RRC Message Structure:

SIB2  RadioResourceConfigCommon  PRACH-Config  PRACH-ConfigInfo 

highSpeedFlag

Notes: Setting this parameter to TRUE will result in the use of restricted sets when calculating cyclic

shifted sequences. The use of restricted set is optimized for high speed mobility cases with relatively

large Doppler shift.

If the parameter is set to FALSE, unrestricted set will be used. Also please note that

- For unrestricted set, the UE basically can use the any sequence every integer multiples of

Ncs.

- For restricted set, the UE can only use some specific sequences which can compensate the

high Doppler shit in high-speed mobility wireless channel. Otherwise, the correlation peak

will reflect the wrong sequence and hence a false detection will happen.

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2.3 MAC Layer Parameters

In general, the MAC layer is the one controlling the random access procedure.

There are two types of random access procedures: contention based and contention free.

In the contention based case the UE is using a randomly chosen preamble index; hence there is a

possible collision situation when multiple UEs attempt to use the same index at the same time (i.e. in

the same sub-frame). When this occurs, it needs to be resolved by the so called contention resolution

procedure.

In the contention free case the UE gets a preamble ID assigned by the network.

The contention based procedure is used in all cases except handover, where the contention free

procedure may be used instead.

ra-PreambleIndex 2.3.1

Definition: PRACH preamble index value, optionally signaled to the UE at handover for contention

free random access to target eNB

IE Value Engineering Units

Allowed Range [0 … 63] 0 … 63

Recommended Recommended for contention-free handover.

Setting Tradeoff: The parameter value has no significance. However, using it is recommended in

order to have shorter interruption at handover when contention-free procedure is enabled. Please note

that a number of preamble indexes from the 64 possible are reserved for contention free or dedicated

access.

Dependencies/Constraints: The value 0 shall not be used (TS36.321 Sect. 5.1.2). Value has to be

larger than numberOfRA-Preambles (i.e. out of the non-dedicated preambles pool).

Traceability: TS36.321 Sect. 5.1.1 and 5.1.2; TS36.331 Sect. 6.3.2

RRC Message Structure:

RRCConnectionReconfiguration  MobilityControlInfo  rach-ConfigDedicated 

ra-22

PreambleIndex

Notes: When triggering handover the network may signal to the UE the PRACH preamble index and

the PRACH mask index. In this case the UE MAC layer will trigger a contention free random access

procedure using the received preamble index and mask.

If the preamble index or the mask index is absent (or the preamble index is 0), a contention based

procedure is initiated (which requires more time to complete). For this case the UE MAC will

randomly choose a preamble index from a given pool and will set the mask index to zero.

[See also notes numberOfRA-Preambles sizeOfRA-PreamblesGroupA and messageSizeGroupA]

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numberOfRA-Preambles 2.3.2

Definition: Number of contention-based random access preambles available in the cell

IE Value Engineering Units

Allowed Range [n4, n8, n12, … n64] 4, 8, 12 … 64

Recommended

64 if only Contention-Based RACH supported

40 – 56 if Contention-Free RACH supported

64 if only Contention-Based RACH supported 40 – 56 if Contention-Free RACH supported

Setting Tradeoff: Too small setting results in low number of contention-based preambles and

increases the collision probability for contention based procedure, resulting in longer random access

procedure duration.

Too high setting results in low number of contention-free preambles and hence may delay handovers

into the cell.

Dependencies/Constraints: Parameter cannot be set to 64 if contention free random access

procedure is used in the network

numberOfRA-Preambles shall not be smaller than sizeOfRA-PreamblesGroupA

Traceability: TS36.321 Sect. 5.1.1; 36.331 Sect. 6.3.2

RRC Message Structure:

SIB2  RadioResourceConfigCommon  RACH-ConfigCommon preambleInfo 

numberOfRA-Preambles

Notes: This parameter defines how many out of the 64 available preambles are non-dedicated. One

out of these non-dedicated preambles shall be randomly chosen by the UE MAC layer in case of

contention-based random access procedure (i.e. when Random Access Preamble Index is not provided

by the network).

The remaining preambles (if any) are the dedicated ones which are used for contention free procedure

(i.e. they could be explicitly assigned by the network).

[See notes under sizeOfRA-PreamblesGroupA]

Please note that If contention-free handover is supported, this value should be function of handover

traffic and can be started with 56 (8 for contention-free handover) and moving lower to 40 based on

handover load.

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sizeOfRA-PreamblesGroupA 2.3.3

Definition: Number of preambles in random access preambles group A

IE Value Engineering Units

Allowed Range [n4, n8, n12, … n60] 4, 8, 12, … 60

Recommended

Implementation dependent.

- If the uplink resource allocation after PRACH is to be optimized, then we need to configure this parameter.

- It depends on the statistic of the first TB size after the successful RACH procedure.

Setting Tradeoff: If this parameter is not configured, then eNodeB does not have enough information

for uplink resource allocation after successful RACH. So a general uplink grant is allocated.

If this parameter is configured, then eNodeB will be able to distinguish the size of the uplink grant

requested by the UE after the successful RACH procedure.

Dependencies/Constraints: sizeOfRA-PreamblesGroupA shall be smaller than

numberOfRA-7

Preambles

Traceability: TS36.321 Sect. 5.1.1; 36.331 Sect. 6.3.2

RRC Message Structure:

SIB2  RadioResourceConfigCommon  RACH-ConfigCommon 

preambleInfo  preamblesGroupAConfig  sizeOfRA-PreamblesGroupA

Notes:

This parameter should be optimized together with messageSizeGroupA and

messagePowerOffsetGroupB.

This Parameter is optional. If it is not signaled, then sizeOfRA-PreamblesGroupA is equal to

numberOfRA-Preambles

Contention-based random access preambles (see numberOfRA-Preambles) are divided into two

groups: group A and B as illustrated below:

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Non-dedicated preambles Dedicated preambles

Preambles group A Preambles group B

Preamble Index 0 1 2 … … 63

sizeOfRA-PreamblesGroupA

numberOfRA-Preambles

Note that if sizeOfRA-PreamblesGroupA is equal to numberOfRA-Preambles then there is no

preambles group B.

If numberOfRA-Preambles is set to 64 then there are no dedicated preambles.

Regarding the selection between groups A and B see notes under messageSizeGroupA

[See also notes under Number of Random Access Preambles].

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messageSizeGroupA 2.3.4

Definition: Maximum message size when preambles group A shall be used

IE Value Engineering Units

Allowed Range b56, b144, b208, b256 56 bits, 144 bits, 208 bits, 256 bits

Recommended

Implementation dependent.

- If the uplink resource allocation after PRACH is to be optimized, then we need to configure this parameter.

- It depends on the statistic of the first TB size after the successful RACH procedure.

Setting Tradeoff: Too small setting results in selecting group B, and hence allocating un-necessary

large grant even for small msg3.

Too high setting will result in selecting group A, and hence allocating small grant for large msg3.

Dependencies/Constraints: The sizeOfRA-PreamblesGroupA has to be also present

Traceability: TS36.321 Sect. 5.1.2; 36.331 Sect. 6.3.2

RRC Message Structure:

SIB2  RadioResourceConfigCommon  RACH-ConfigCommon 

preambleInfo  preamblesGroupAConfig  messageSizeGroupA

Notes:

This parameter should be optimized together with sizeOfRA-PreamblesGroupA and

messagePowerOffsetGroupB.

The parameter messageSizeGroupA together with messagePowerOffsetGroupB defines the threshold

for preambles group selection. Preambles group A always exists. Preambles group B is optional, and

it shall be used by UEs which intend to send larger size msg3 (RRC Connection Request). This

separation gives the possibility to the eNB to assign different grant for msg3 transmission depending

on the expected msg3 size.

The selection rules are the following:

Preambles group B shall be used by the UE if:

1.) Preambles group B exists

and

2.) msg3 size (including data + MAC header + eventual MAC control elements) is greater

than messageSizeGroupA

and

3.) If messagePowerOffsetGroupB < PCMAX – preambleInitialReceivedTargetPower –

deltaPreambleMsg3 – PL

(i.e. messagePowerOffsetGroupB < power headroom for msg3 transmission)

Otherwise preambles group A shall be used.

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messagePowerOffsetGroupB 2.3.5

Definition: Minimum UE power headroom where preambles group B may be used

IE Value Engineering Units

Allowed Range minusinfinity, dB0, dB5, dB8, dB10, dB12, dB15, dB18

-∞, 0dB, 5dB, 8dB, …

Recommended

Implementation dependent.

- If the uplink resource allocation after PRACH is to be optimized, then we need to configure this parameter.

- It depends on the statistic of the first TB size after the successful RACH procedure.

Setting Tradeoff: If parameter is too low then a UE with small power headroom (i.e. in far cell

scenario) may still select group B and hence gets un-necessary large grant (which may not be able to

use anyway)

If parameter is set to too high then a UE may be un-necessary prevented from selecting group B and

hence it has to work with smaller grant.

Dependencies/Constraints: Values of preambleInitialReceivedTargetPower and deltaPreambleMsg3

has to be taken into account

Traceability: TS36.321 Sect. 5.1.1; 36.331 Sect. 6.3.2

RRC Message Structure:

SIB2  RadioResourceConfigCommon  RACH-ConfigCommon 

preambleInfo  preamblesGroupAConfig  messagePowerOffsetGroupB

Notes: Parameter defines the threshold for preambles group selection. See notes under

messageSizeGroupA.

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ra-PRACH-MaskIndex 2.3.6

Definition: PRACH mask index defines the allowed preamble transmission occasions out of the ones

available in the system. It is optionally signaled to the UE at handover or by PDCCH order.

IE Value Engineering Units

Allowed Range

(For mapping see TS36.321 Table 7.3-1)

Recommended

Set it to nonzero if access load is high

Setting Tradeoff: If the parameter is set to non-zero value, the eNodeB will apply the additional

constraint in which subframe the UE can send the PRACH during the handover or through PDCCH

order. This additional constraint is applied based on the subframes defined by prach-ConfigIndex. As

such the chances of collision can be reduced in loaded network. If the parameter is set to zero, there

is no additional constraint for PRACH.

Dependencies/Constraints:

- Values 13 and above are “reserved”

- PRACH Resource Index shall not be greater than the number of preamble occasions

In document LTE Guidelines (Page 19-42)