5G NR Cell Scan and RACH Procedure Poster

5G NR Cell Scan and RACH Procedure Poster

When you power on a 5G New Radio (NR) device, it must perform a series of sequential physical layer and control plane procedures before it can transmit or receive any user traffic. This step-by-step guide explains how a device scans the frequency band, detects and camps on a cell, decodes system parameters, and establishes a connection using the Random Access Channel (RACH).

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5G NR Initial Connection Procedure Flow

Below is the step-by-step timeline showing how a User Equipment (UE) transitions from power-on to a connected state.

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Detailed Step Breakdown

Step 1: Power ON

Upon powering up, the UE’s internal modem initializes the protocol stack and activates the RF transceiver.

Step 2: Cell Scan Start

The UE scans pre-configured frequency channels (referred to as the Synchronization Raster) within the supported bands to detect the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS).

Step 3: Decoding the Master Information Block (MIB)

Once a synchronization signal is detected, the UE decodes the Physical Broadcast Channel (PBCH) containing the Master Information Block (MIB). The MIB provides:

• The most significant bits of the System Frame Number (SFN).
• The common subcarrier spacing for subsequent control messages.
• The frequency offset ($k_{SSB}$) used to align with the resource grid.

Step 4: Obtaining System Information Block 1 (SIB1)

Using the CORESET#0 and SearchSpace#0 configurations decoded from the MIB, the UE monitors the Physical Downlink Control Channel (PDCCH) to locate the downlink scheduling assignment for System Information Block 1 (SIB1). It then decodes SIB1 over the PDSCH.

Step 5: Cell Scan End

The UE gathers all SIB1 information from detected cells to prepare for ranking.

Step 6: Ranking and Camping

The UE evaluates the signal strength (RSRP/RSRQ) of the detected cells. Using the cell selection criteria ($S$-criteria) broadcast in SIB1, the UE ranks available cells and camps on the best-ranked cell. Camping allows the UE to monitor paging and system information updates.

Step 7: Decoding SIB1 of the Selected Cell

After camping, the UE reads the full, detailed configurations from the selected cell's SIB1 to prepare for uplink communication.

Step 8: Reading RACH Configuration

The UE extracts the rach-ConfigCommon parameter set from SIB1, which contains:

• The available physical Random Access (PRACH) resources and formats.
• Preamble grouping parameters.
• Power ramping parameters for transmission retries.

Step 9: Initiating the RACH Procedure

Armed with the RACH configuration, the UE selects an available PRACH preamble slot and transmits its initial access request. Depending on network capability, the UE will trigger a 4-Step RACH (Msg1) or a 2-Step RACH (MsgA).

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References

• 3GPP TS 38.304: User Equipment (UE) procedures in Idle mode and RRC Inactive state.
• 3GPP TS 38.213: Physical layer procedures for control.

Further Reading

• 5G-NR Master Information Block (MIB)
• SSB Location in Time Domain
• 2-Step RACH in Release 16 of 5G NR (Part 1)
• 5G NR RACH Trigger Reasons with Details