Before we start explaining RACH procedure we can start with a very basic question.

What are the steps needed for a UE to register with a 5G network?

 

The high level answer:

-UE power on

-select cell/GNodeb (Downlink synchronization)

-RACH procedure (uplink Synchronization)

-RRC Setup

 

 

in other words: in order to connect a UE to 5G network, it must be synchronized in downlink as well as in uplink. Downlink synchronization is done through a decoding successfully SSB, for obtaining uplink synchronization and RRC connection, UE must perform RACH random access procedure which will be explained briefly in the following article:

 

Definition:

What is RACH 

RACH stands for for random access channel

What is the purpose of RACH random access procedure?

1-Obtain the resources for message 3

2-Achieve UP link synchronization between UE and eNB