rach

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RACH

The PRACH occupies 6 PRBs in the frequency domain and spans 1 or 2 or 3 sub frames in the time domain, depending on the specific preamble format. In the frequency domain, several subcarriers at both ends of the 6 PRBs are not used to avoid interference with the adjacent PUCCH/PUSCH. In the time domain, the cyclic prefix (CP) and guard time (GT) are used to avoid interference with the previous and next subframes. As it turns out, the GT determines the maximum cell radius.

The Preamble format is defined in LTE (3GPP TS 36.211, section 5.7).

[http://2.bp.blogspot.com/-sluMqvkP5VA/UrF3Bs4GSmI/AAAAAAAAAs4/0XQxbLYI4m0/s1600/preamble+format.png]

Ts is the basic time unit in LTE, Ts = 1/(15000*2048) s = 3.25521 X 10^(-5) ms.

Cell radius supported:

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Format 1:~14km,Format 2: ~30 km, Format 3: ~107 km

random access configuration for preamble format 0-3.

[http://1.bp.blogspot.com/-SnJrbOPtIKU/UrPce6riF7I/AAAAAAAAAtg/9LY5H2db5Ew/s1600/PRACH_config_subframe.png]

PRACH position in frequency domain is given by prach-FreqOffset (present in SIB2). And in time domain is given by prach-ConfigIndex (present in SIB2).

TDD: preamble format 0-4 used.

In TDD usage of preamble format depends on UL/DL configuration used. There might be multiple random access resources in an UL sub frame depending on the UL/DL configuration.

Eg: preamble format 1 requires 2sf and format 3 requires 3sf and num of available UL sf depends on UL/DL config used.

Preamble Format 4 (Short PRACH) is only used in TD-LTE, which can be transmitted in the UpPTS part of the special sub-frame (SF1 and/or SF6). Minimum number of symbols needed for this is 2. Hence this can be applied only for Special sub frame configurations 5-8 with normal CP or configuration 4-6 with extended CP (Refer: 36.211, Table 4.2-1).PRACH configuration indexes 48 to 57 will use Short PRACH

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UpPTS lengths 4384 .Ts and 5120.Ts only. PRACH configuration indexes 48 to 57 will use Short PRACHPRACH POSITION:random access preamble mapping in time and frequency

[http://3.bp.blogspot.com/-Zm8wcPmX4SA/UrPhCp2x_rI/AAAAAAAAAts/qRhhH6pqmWY/s1600/PRACH_tdd.png]

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

TDD PRACH Resource allocation is defined by a quadruple shown below.

[http://1.bp.blogspot.com/-olXSlbZ6das/UrF3EsyLXVI/AAAAAAAAAtE/2g4PUt6wLzk/s1600/RACH_quadraple.png]

Details of prach-ConfigIndex is given in Table: 5.7.1-2 of 36.211.

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Types of RACH preambles (Regular and restricted).

Restricted set used in high speed scenarios. where UE experiences higher

frequency offset due to doppler shift which causes spurious and aliased peaks.

restricted in the preamble set.

The parameter High-speed-flag provided by higher layers determines if unrestricted set or restricted set shall be used.

In high speed scenarios, there may be RAPID mismatches during RACH procedure if

high speed flag is not set (in SIB2). If this flag is set, UE will not use these cyclic

shifts while selecting RACH preambles.

The random access preambles are generated from Zadoff-Chu sequences, generated from one or several root Zadoff-Chu sequences

Preamble generation:

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 rootSequenceIndex, where rootSequenceIndex and for preamble generation 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(total of 838 Zadoff-Chu sequences).

NCS:The cyclic shift can be used to expand the preamble capacity. A small Ncs value generates more preambles.

Numof RA preamble= 839/ NCS : where 839 Is preamble sequence length.

below diagram :The cyclic shift used by UE0 is 0 and the cyclic shift used by UE1 is Ncs and UE2 is 2NCS. At the eNB, the observed cyclic shift of UE0 will not be 0 but some value x because of the transmission

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delay. As long as x is less than Ncs, the auto-correlation between the shifted x and shifted Ncs will be zero, and eNB will be able to distinguish between the accesses from UE0 and UEx (properties of Z-C sequences)

[http://2.bp.blogspot.com/-gQRxNo4bD8s/UsLqIXr_x3I/AAAAAAAAAuY/oZgfdfaPSIY/s1600/preamble1.png]

However, the cyclic shift cannot be configured smaller than expected cell radius, since that will block random accesses from the cell edge and may cause drops during handovers. eg:NCS=119 cell radius ~16Km. NCS=46 cell radius ~5.5Km.

RACH trigger:

Events that will trigger random access procedure.

Initial access from RRC_IDLE; Initial access after radio link failure; Handover requiring random access procedure; DL data arrival during RRC_CONNECTED when UL is nonsynchronised; UL data arrival during RRC_CONNECTED when UL is nonsynchronised or there are no PUCCH resources for SR available available.

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if UE's TA timer has expired, and UE needs to send or receive data, it performs random access

PPRACH = min{PCMAX , PREAMBLE_RECEIVED_TARGET_POWER + PL} [dBm].

where PCMAX is the configured maximum UE transmitted power and PL is the downlink pathloss estimate calculated in the UE. Based on the reference signal power signaled in SIB2 and the measured RSRP,

Random access procedurerandom access procedure at the UE side.

The UE listens to a downlink broadcast signal to obtain the transmission timing. It is also informed of the available signatures, frequency bands and

time slots for a random access.(SIB2)

Open-loop power control can be used to obtain a suitable transmission power. The path loss is estimated from a downlink signal and the UE estimates the

transmission power to achieve a certain SNR target.

The UE selects randomly a signature, a time slot and a frequency band among the available set.

The NodeB correlates the received signal in the RACH sub-frame with all possible signature sequences

The UE monitors a specified downlink channel for response from the NodeB. In case of positive answer: The UE decodes the response and adapts its transmission timing. If the UE does not receive any response from the NodeB: A new attempt is

performed by power ramping method or : the UE selects a new signature after

a random back-off time.

Example:

sib2 : {....prach-Config {rootSequenceIndex 384,prach-ConfigInfo {prach-ConfigIndex 4,highSpeedFlag FALSE,zeroCorrelationZoneConfig 12,prach-FreqOffset 2}...

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}prach-ConfigIndex is 4, meaning PRACH can be transmitted in any SFN and 4th sub-frame.prach-FreqOffset is 2, meaning PRACH starts from 1st RB in frequency domain

UE Logs on RACHVersion = 2

Current SFN SF = 10184

Preamble Sequence = 2

Cycle Shift = 52

Preamble Format = 0

First RB for PRACH Opportunity = 17

Start RB = 17

PRACH Transmit Power = -19 dBm

Logical Root (q) = 129

Posted 18th December 2013 by premchand jodukalluLabels: format4, peamble, PRACH, upPTS

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