NR - RACH
Why RACH ?
 It is used for uplink synchronization
 First message that network detects from UE
 Vary depends on NSA and SA
 To obtain the resource for msg3
When RACH procedure called?
 Initial access
 RRC Connection Re-establishment
 Handover
 Beam failure recovery
 Request for Other SI
 time alignment at Scell
 To recover from RRC_INACTIVE state
PRACH Transmitter
Matlab Code
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% code
% Cell Configuration
ueConfig=struct('NULRB',6,'DuplexMode','FDD','CyclicPrefix','Normal');
prachConfig=struct('Format',0,'SeqIdx',0,'PreambleIdx',0,'CyclicShiftIdx',1,'HighSpeed',0,'TimingOffset',0,'FreqIdx',0,'FreqOffset',0);
ue.NULRB = 6;
ue.DuplexMode = 'FDD';
% Sequence
N_ZC = 839;
sequence = lteZadoffChuSeq(129, 839);
% DFT -> Subcarrier Mapping -> IFFT
preambleDFT = fft(sequence, N_ZC);
preamble864subcarriers = [ zeros(13,1); preambleDFT; zeros(12,1)];
preambleMapped = [ zeros(336,1); preamble864subcarriers; zeros(336,1)]; % Only ' conjugate the complex numbers as well!!!
preambleIFFT = ifft(preambleMapped,1536);
% ADD CP and Guard Band
CPpreambleGP = [preambleIFFT(end-198+1:end); preambleIFFT; zeros(186,1)];
% MATLAB LTE Toolbox Generation
[matlabPreamble, info]=ltePRACH(ueConfig,prachConfig);
disp('--- MATLAB PREAMBLE ----');
[index,offset] = ltePRACHDetect(ue,prachConfig,matlabPreamble,(0:63).')
disp('--- OWN PREAMBLE ----');
[index,offset] = ltePRACHDetect(ue,prachConfig,CPpreambleGP,(0:63).')
PRACH Format
 Long format
 839 length
 For Large cell(Sub 6GHz generally, FR1)
 SCS 1.25KHz and 5KHz
 Short Format
 139 Length
 For small cell, Indoor deployment
 SCS 15KHz and 30KHz for FR1
 SCS 60KHz, 120KHz for FR2
RACH procedure in SA
RACH procedure in NSA
Preamble generation
 38.211 6.3.3.1
 https://www.sharetechnote.com/html/5G/5G_RACH.html#Preamble_Sequ
ence_Generation
Types
 Contention based
 Non Contention based
LTE Vs NR
 BeamForming : UE should detect best beam for RACH process
Parameters
PRACH content in SIB1
 PRACH Preamble Format
 Time Resources
 Frequency Resources
 index to logical root sequence table
 Cyclic Shift(Ncs)
 Set Type (unrestricted, restricted set A or restricted set B)
Msg2(RAR)
 gNB sends a DCI scrambled with RA-RNTI
 Content
 Backoff indicator
 Random Access PREAMBLE ID(RAPID)
 TA
 UL Grant
 TC-RNTI
PRACH issues
 Why PRACH is not detected by network ?
 Incorrect preamble time and frequency location(t_id, f_id), sequence
 incorrect SSBperRACHocassion mapping
 Msg1 Collision in high density cells
 Msg1 detection failure due to low SNR at gNB
 UE transmitting RACH at lower power
 msg2 reception failure
 cell/beam is congested and gNB might not have resources to allocate PDCCH for msg2
PDSCH which might cause PDCCH blocking
 Might have sent UE with a Back-off indication
 RAPID mismatch
Why DFT block is needed in PRACH
signal generation?
 Frequency Selectivity and Interference Rejection in frequency domain
Example
Cell Search Procedure
 Frequency Scan
Frequency Scan
 Channel Raster
 Synchronization Raster
SS Block and Periodicity
PSS Signal Detection
SSS signal detection
Cell ID detection
MIB detection
SIB1 detection
 1.In RACH Failure there is a reason of RAPID mismatch.How UE can measure
this?
2.In RACH Failure there is a reason if PRACH sequence delay is greater than the
Cyclic shift value.How this PRACH sequence delay is calculated?
SA : Preamble generation
 RACH Legnth(LRA) :