5G NR Initial Access

5G NR Initial Access

How 5G NR initial Cell Search process work?

  • For the initial access case (idle mode), the UE should start by the cell search procedure. By using the PSS, SSS, and specific synchronization algorithms, the UE can estimate and correct the frequency and time offsets. 

 

Cell Search

The UE searches for a specific frequency based on a synchronization raster defined in 3GPP TS 38.104.

Range of frequencies (MHz)

SS block frequency position SSREF

GSCN

Range of GSCN

0 – 3000

N * 1200 kHz + M * 50 kHz,

N = 1:2499, M ϵ {1,3,5} (Note)

3N + (M-3)/2

2 – 7498

3000 – 24250

3000 MHz + N * 1.44 MHz,
N = 0:14756

7499 + N

7499 – 22255

24250 – 100000

24250.08 MHz + N * 17.28 MHz,
N = 0:4383

22256 + N

22256 – 26639

NOTE:  The default value for operating bands that only support SCS spaced channel raster(s) is M=3.

Synchronization raster entries for each operating band

NR operating band

SS Block SCS

SS Block pattern
(note)

Range of GSCN

(First – – Last)

n1

15 kHz

Case A

5279 – <1> – 5419

n2

15 kHz

Case A

4829 – <1> – 4969

n3

15 kHz

Case A

4517 – <1> – 4693

n5

15 kHz

Case A

2177 – <1> – 2230

30 kHz

Case B

2183 – <1> – 2224

n7

15 kHz

Case A

6554 – <1> – 6718

n8

15 kHz

Case A

2318 – <1> – 2395

n12

15 kHz

Case A

1828 – <1> – 1858

n14

15 kHz

Case A

1901 – <1> – 1915

n18

15kHz

CaseA

2156 – <1> – 2182

n20

15 kHz

Case A

1982 – <1> – 2047

n25

15 kHz

Case A

4829 – <1> – 4981

n26

15 kHz

Case A

2153 – <1> – 2230

n28

15 kHz

Case A

1901 – <1> – 2002

n29

15 kHz

Case A

1798 – <1> – 1813

n30

15 kHz

Case A

5879 – <1> – 5893

n34

15 kHz

Case A

5030 – <1> – 5056

n38

15 kHz

Case A

NOTE 2

30 kHz

Case C

6437 – <1> – 6538

n39

15 kHz

Case A

4706 – <1> – 4795

n40

30 kHz

Case C

5762 – <1> – 5989

n41

15 kHz

Case A

6246 – <3> – 6717

30 kHz

Case C

6252 – <3> – 6714

n48

30 kHz

Case C

7884 – <1> – 7982

n50

30 kHz

Case C

3590 – <1> – 3781

n51

15 kHz

Case A

3572 – <1> – 3574

n53

15 kHz

Case A

6215 – <1> – 6232

n65

15 kHz

Case A

5279 – <1> – 5494

n66

15 kHz

Case A

5279 – <1> – 5494

30 kHz

Case B

5285 – <1> – 5488

n70

15 kHz

Case A

4993 – <1> – 5044

n71

15 kHz

Case A

1547 – <1> – 1624

n74

15 kHz

Case A

3692 – <1> – 3790

n75

15 kHz

Case A

3584 – <1> – 3787

n76

15 kHz

Case A

3572 – <1> – 3574

n77

30 kHz

Case C

7711 – <1> – 8329

n78

30 kHz

Case C

7711 – <1> – 8051

n79

30 kHz

Case C

8480 – <16> – 8880

n90

15 kHz

Case A

6246 – <1> – 6717

30 kHz

Case C

6252 – <1> – 6714

n91

15 kHz

Case A

3572 – <1> – 3574

n92

15 kHz

Case A

3584 – <1> – 3787

n93

15 kHz

Case A

3572 – <1> – 3574

n94

15 kHz

Case A

3584 – <1> – 3787

 

SYNCHRONIZATION SIGNALS :

  • Similar to 4G networks, the synchronization signals PSS and SSS are used in 5G NR systems to help the UEs to get radio frame boundaries and to detect the cell identity (ID .
  • In 5G NR systems, each radio cell is identified by a cell ID from 1008 IDs that are arranged into 336 different groups.
  • Each group is identified by the cell ID group,N(1,ID) 0 to 335 , and consists of three different sectors, which are specified by the cell ID sector, N(2)
    ID 2 0 to 2.
  •  

PSS:

  • Similar to 4G, 5G NR PSS is a physical layer-specific signal and helps UE to get radio frame boundary &  to detect the cell ID sector.
  • NR PSS consisit of one of the three 127 symbols m- sequence and is allocated on the first symbol of each SSB and on 127 subcarriers.

Please check our PSS article for more detail description(https://cafetele.com/5g-nr-primary-synchronization-signalpss/)

SSS:

  • Similar to 4G SSS, 5G NR SSS issued to detect the cell ID group,
  • However the 5G NR PSS consists of one of 336 127 symbol gold sequences and is allocated on the third symbol of each SSB and on 127 subcarriers.
  • For more detail about SSS please check this link https://cafetele.com/secondary-synchronization-signal-nr/

 

  • After decoding PSS, the UE can detect the cell ID sector, then by using the detected cell ID, the UE can decode SSSand detect the cell ID group, hence it can finally detect the CELL ID.

  • After detecting cell ID, the UE should identify the candidate SSB within the SS burst (set of L SSB).

  • After detecting the PSS/SSS UE knows, in addition to the physical cell ID, the timing of the PBCH. The PBCH carrying MIB, and integrated part of SS/PBCH block, is used for signalling the MOST essential system related to access frequency position and timing

  •  

check PBCH section over here https://cafetele.com/pssssspbch-overview/

 

Master information Block: https://cafetele.com/mib-master-information-block/

SI Acquisition: 

A gNodeB can deliver SI in periodic broadcast mode or on-demand delivery mode. Therefore, a UE can also acquire SI in two ways.

The UE searches for a cell, parses the MIB, and checks the cell status.

  • If the value of CellBarred is barred, the UE stops SI acquisition.
  • Otherwise, the UE goes to the next step.

The UE attempts to parse SIB1 using the parameters carried in the MIB.

  • If the UE parses SIB1 successfully, it stores the related information and performs the subsequent step.
  • Otherwise, it stops SI acquisition.

The UE attempts to acquire other SIBs according to their broadcast mode indicated in SIB1.

  • If the periodic broadcast mode is used for other SIBs, the UE attempts to receive and parse SI in the OSI search space indicated in SIB1.

 

Note: Complete Article will update soon till RRC connection release or I will make part 2.

 

Comments

  • Vijay Anand
    August 14, 2020

    Hi,

    What is difference in 5G initial access in Standalone and NON standalone. What are the parameters involved . How to tune

    • fernando manik
      August 14, 2020

      there are couple things. one example, in standalone, there will be SIB. In Non standalone, only MIB… also in standalong there is paging.. in non standalone, no paging… there are some more..

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