MSG-1 is the first message in the Random Acccess Procedure of 5G (NR). It is transmitted by the User Equipment (UE) to the gNodeB (gNB) over the Physical Random Access Channel (PRACH).
MSG1 contains a Random Access Preamble, which is a special signal used by the UE to:
- Request initial access to the network
- Re-establish connection after radio link failure
- Perform handover
- Synchronize uplink timing
Why is MSG1 Required?
MSG1 is essential because:
- The UE doesn’t yet have uplink timing aligned with the gNB.
- It allows the gNB to detect the UE, measure timing offset, and allocate resources.
- It initiates communication when the UE is in RRC_IDLE, RRC_INACTIVE, or during beam failure recovery.
MSG1 Structure (PRACH Preamble)
MSG1 is not a regular/normal message with headers and payload. It’s a waveform generated using Zadoff-Chu sequences. It includes:
| Field | Explanation |
|---|---|
| Preamble Index | Identifies which preamble UE is using (used for contention resolution). |
| Sequence Format | Long (839) or Short (139) depending on cell size and deployment scenario. |
| Subcarrier Spacing | it is not constant varies by frequency range (like FR1: 15/30 kHz, FR2: 60/120 kHz). |
| PRACH Configuration Index | Determines time/frequency resources for PRACH transmission. |
| RA-RNTI | it is used to identify the UE. it is being used during Random Access Procedure only. it stands for “Random Access Radio Network Temporary Identifier” |
How MSG1 is Transmitted
- UE selects a preamble index and PRACH resource based on configuration from SIB1 or RRC.
- UE transmits the PRACH waveform using selected format and power.
- The transmission is blind—UE doesn’t know if gNB received it.
What Happens at gNB After Receiving MSG1?
Once gNB receives MSG1:
- It detects the preamble and estimates timing offset.
- It sends MSG2 (Random Access Response) via PDCCH and PDSCH.
- MSG2 includes:
- Timing Advance
- Temporary C-RNTI
- Uplink grant for MSG3
If multiple UEs send the same preamble (contention-based access), gNB resolves it in later steps (MSG4).
MSG1 in the Full Random Access Procedure
UE gNB
│ │
├── MSG1: PRACH Preamble ─────▶│ (Initial access)
│◀── MSG2: RAR (Timing, Grant) ── ┤
├── MSG3: RRC Setup Request ────▶│
│◀── MSG4: Contention Resolution ──┤