PRACH Basic-1

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In this section we will learn about PRACH and its basics as well as interview questions.

To obtain the UL synchronization, a preamble is send by UE to gNB over PRACH channel. As LTE, in 5G NR also there are 64 preambles defined in each time frequency PRACH accession.

After the cell search process, UE has obtained DL synchronization with cell, so UE can receive DL data. But UE can perform UL transmission only when it obtains UL synchronization with the cell. The UE forms the connection with cell and obtains UL synchronization through random access procedure. The main purpose is:

  • Achieve uplink synchronization between eNB and UE
  • Obtain the resource from Msg3 (obtain uplink grant, the uplink resource request)
  • Assign CRNTI(unique identity) to Device (TCRNTI will promote as CRNTI once contention resolved)

It is not capable if UE is using broadcasting or always on synchronization mechanism. The UL synchronization process should meet the below criteria:

  • The process of synchronization should happen only when there is immediate necessary.
  • The synchronization should be dedicated to only a specific device(UE).
The fundamental difference here in 5G NR RACH from LTE RACH would lie just before RACH preamble gets transmitted. This is because of beam forming which would be supported by default in NR. When NR operates on beam forming mode, UE needs to detect and select the best beam for RACH process. The beam selection process could be the fundamental difference between LTE and NR RACH process.

Before the initiation of RA (random access) procedure layer1 i.e physical layer at UE receives set of PBCH/SS blocks.

Based on RSRP measurement of SS/PBCH blocks decides on 1 PBCH/SS index and then performs RA procedure. The relationship between SS/PSCH block and PRACH resources is totally based on higher layer parameter ssb-perRACH-OccasionAndCB-PreamblesPerSSB.

How does Network know exactly when UE will transmit the RACH? 

Network knows when UE will transmit the RACH even before UE sends it as Network tells (using PRACH Configuration Index) UE when the UE is supposed to transmit the RACH.

How to determine which preamble format to use?

PRACH Configuration Index determines the Preamble Format that is to be used. 

Who determines PRACH Configuration index?

eNB determines it through prach-Configuration IE in SIB2 (In the Case of LTE. for ENDC, MSG (LTE) initial attach).

From where UE can find RACH parameter information during HO?

UE can find RACH info from RRCConnectionReconfig msg. 

How many RACH-Preamble can be used?
64 PRACH preambles (which is divided in Preambles group A & B as well as contention free for HO) are available theoretically in total, but the number of the preambles available in a specific condition is determined by a couple of SIB2 parameters (for LTE Case). 

How UE decides about where and when it needs to send RACH Request:-

UE decides it on the basis of parameters received in SIB2 PRACH configurations (for LTE Case).
UE can use six Resource blocks for sending the RACH Request.

How many types of Msg3 on CCCH?

There are two types of Msg3 on CCCH. RRCconnectionRequest ( which used for initial access and random access) RRCconnectionReestablishmentReq ( which used for after RLF detected).

In the next section we will learn more about RACH. 
So keep reading……. And Stay tune……………

Pinal Dobariya………..

3GPP TS 38.321
3GPP TS 38.300
3GPP TS 38.213


  1. I guess most of the content is based on LTE. Would you mind extending to NR also?


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