Typical Question regarding 5G Random Access Channel (RACH)

RACH is the first channel used by UE to access 5G service in UL.

Q1. What is RACH?

RACH is the first channel used by UE to access 5G service in UL. It is used for UL synch and the UE starts Random Access by sending a RACH Preamble on the predefined RACH location. The preambles have many formats as described in 3GPP TS 38.211.

Q2. When do we need to use the Random Access Channel?

As mentioned, Random Access Channel (RACH) is used for RACH procedure. The RACH procedure is triggered by procedures mentioned in Section 9.2.6 of TS 38.300. They are as follows:

Q3. How many RACH formats are there, and why do we need them?

The RACH preambles have many formats as described in 3GPP TS 38.211. But basically, there are two:

1. Long Formats: 0,1,2, and 3
2. Short Formats: A1,A2, A3, B1, B2, B3, B4, C0,C2

Q4. Why do we need Long and Short RACH preamble formats?