UE Power ON and Cell Selection in 5G Standalone (SA)

In a 5G SA network, when the User Equipment (UE) powers on, it must detect, synchronize with, and evaluate a serving cell before initiating access procedures. This document explores each step of this cell acquisition journey, with particular emphasis on SSB decoding, DCI (Downlink Control Information) parsing, and initial BWP (Bandwidth Part) configuration, all essential before any meaningful communication begins.

Below is the process flow of UE power to Cell Selection in 5G SA network.

Step 1: UE Power ON and PLMN Selection

When the UE powers on, the initial procedures involve both hardware and software readiness, followed by network selection, particularly PLMN selection. This is a critical step in ensuring that the UE connects to an operator network that is both authorized and available.

1.1 UE Power ON Sequence

Upon pressing the power button or triggering a software reboot, the UE performs:

1. Hardware Initialization

   • Boot-up of baseband processor, RF front-end, and memory.

   • Initialization of GNSS, sensors, and other peripherals.

2. USIM Access

   • The UE reads the contents of the USIM to retrieve:

     • IMSI (International Mobile Subscriber Identity)

     • List of Home PLMN (HPLMN) and Preferred PLMNs

     • Access class information (used for barring scenarios)

     • Subscription-specific information (e.g., allowed networks)

3. Loading Protocol Stack

   • Initialization of NR Layer 1 to Layer 3

   • NAS and RRC layer initialization for 5G SA

1.2 PLMN Selection Procedure

a. Types of PLMN Selection

Type	Description
Automatic	UE autonomously selects a PLMN from the preferred or strongest signal list.
Manual	User selects PLMN manually via phone settings.

b. PLMN Selection Flow

c. PLMN Info Broadcast in SIB1

Each cell broadcasts a list of supported PLMNs in SIB1 → plmn-IdentityList:

asn.1

plmn-IdentityList ::= SEQUENCE (SIZE (1..maxPLMN-List)) OF PLMN-IdentityInfo

Each PLMN-IdentityInfo contains:

• MCC (Mobile Country Code)

• MNC (Mobile Network Code)

• trackingAreaCode

• Cell barred / emergency support flags

Example from UE Log:

"plmn-IdentityList": [

  {

    "mcc": "404",

    "mnc": "45",

    "tac": "0x1234",

    "cellBarred": "notBarred"

  }

]

UE matches this list against USIM-stored PLMNs before deciding to camp.

d. PLMN Selection Considerations

• Forbidden PLMN List (FPLMN):

  • UE maintains a list of previously failed PLMNs

  • These are skipped in future automatic selection attempts

• Roaming Allowed Flag:

  • If roaming is not allowed by the USIM, UE avoids non-home PLMNs

• Access Barring:

  • If the cell is barred (cellBarred = barred), UE cannot camp even if PLMN matches

1.3 Scanning Bands and Frequencies

After PLMN logic, UE begins scanning carrier frequencies for SSBs on all bands supported for NR, prioritized by:

• Operator-defined scan list

• Band priority (e.g., n78 > n1)

• Stored frequency list from previous sessions

  
  

Step 2: Cell Search and Synchronization

The first step is for the UE to detect Synchronization Signal Blocks (SSBs) on a carrier frequency.

a. SSB Composition

Each SSB includes:

• PSS (Primary Synchronization Signal) – Time domain symbol position

• SSS (Secondary Synchronization Signal) – Frame number and physical layer cell ID (NID)

• PBCH (Physical Broadcast Channel) – Contains the MIB

SSBs are transmitted in beamformed directions (8 or 64 beams in FR1/FR2), and indexed (0–7 or up to 63) depending on the frequency band.

b. SSB Decoding and Cell ID Derivation

Physical Cell ID = 3 × NID1 + NID2

E.g., If NID1 = 150, NID2 = 2

→ Cell ID = 3 × 150 + 2 = 452

The UE uses this Cell ID to decode PBCH and identify MIB information.

c. SSB Timing (SSB Grid Alignment)

UE aligns to the periodic SSB structure using:

• ssb-Periodicity (e.g., 20ms, 40ms)

• SSB Offset and Subcarrier Offset

• The SSBs occupy specific OFDM symbols and subcarriers based on the subCarrierSpacingCommon in the MIB.

Step 3: PBCH and MIB Decoding

Once synchronization is achieved, the UE decodes PBCH to extract the MIB.

MIB (Master Information Block) Fields:

Field	Purpose
systemFrameNumber	Frame alignment
subCarrierSpacingCommon	Used for CORESET0 decoding
ssb-SubcarrierOffset	Maps SSB to subcarrier grid
dmrs-TypeA-Position	Reference signal position
cellBarred	Indicates if access is restricted
intraFreqReselection	Governs reselection during idle

This enables the UE to locate and decode CORESET0 and DCI formats.

Step 4: DCI Decoding and Initial BWP Configuration

After MIB decoding, the UE needs to read System Information Blocks (SIBs) starting with SIB1. This requires decoding of DCI (Downlink Control Information) formats delivered in PDCCH within CORESET0.

a. DCI Format 1_0 (for SIB1 scheduling)

• Indicates frequency domain allocation and time domain resource assignment for SIB1.

• Scheduled in CORESET0, identified by controlResourceSetZero and searchSpaceZero.

DCI Format 1_0 → PDCCH → points to → PDSCH (carrying SIB1)

b. CORESET0 and SearchSpaces

• These are defined by the network and signaled via implicit rules from MIB.

• UE uses:

  • subCarrierSpacingCommon

  • ssb-SubcarrierOffset

  • controlResourceSetZero

  • searchSpaceZero

c. Initial BWP (Bandwidth Part) Selection

5G NR introduces Bandwidth Parts to reduce UE power consumption and complexity.

What is BWP?

A Bandwidth Part is a portion of the full carrier bandwidth where the UE operates during specific phases (initial access, data transmission, etc.).

Initial BWP Characteristics:

• Defined in SIB1 under initialDownlinkBWP

• Contains:

  • locationAndBandwidth

  • subcarrierSpacing

  • controlResourceSet

  • searchSpaces

Initial BWP Use:

• UE operates in Initial BWP during idle and RRC setup phases.

• Reduced bandwidth (e.g., 20 MHz of 100 MHz carrier) improves UE energy efficiency.

 Step 5: SIB1 Decoding

After locating PDSCH from DCI 1_0, UE decodes SIB1.

Field	Use
cellSelectionInfo → q-RxLevMin	Minimum RSRP threshold
plmn-IdentityList	Supported PLMN IDs
cellAccessRelatedInfo	Access barring and emergency support
servingCellConfigCommon	Config for RACH, UL BWPs
tac	Tracking area code

Step 6: Cell Selection Procedure

UE evaluates whether the cell is suitable for camping:

a. Measured vs Broadcast Thresholds

Qrxlevmeas = RSRP - QrxlevminOffset (if any)

→ Qrxlevmeas ≥ Qrxlevmin → Cell is suitable

Example:

Qrxlevmin = -122 dBm (from SIB1)

Measured RSRP = -100 dBm

Qrxlevmeas = -100

→ -100 ≥ -122 → OK → Selectable cell

b. Cell Selection Algorithm

UE evaluates multiple cells and chooses the best based on:

1.      RSRP

2.      If tie: RSRQ, SINR

3.      Barred status

4.      Matching PLMN

Step 7: Transition to Random Access

Once the cell is deemed suitable:

• UE starts Random Access Procedure:

  • Preamble → RAR → RRC Connection Request → Setup → Complete

• Uses configurations from servingCellConfigCommon in SIB1

• Also uses UL BWP info from SIB1 (initialUplinkBWP)

Summary Table of Key Parameters and Phases

Step	Key Info Elements	Source
SSB Detection	PSS, SSS, PBCH	Physical Layer
MIB Decoding	ssbSubcarrierOffset, cellBarred	PBCH
CORESET0 Mapping	controlResourceSetZero, searchSpaceZero	Derived from MIB
DCI 1_0	SIB1 scheduling	PDCCH
SIB1	PLMN, TAC, RACH config, BWP info	PDSCH
BWP	initialDownlinkBWP, initialUplinkBWP	SIB1
Cell Selection	Qrxlevmin, Qqualmin	SIB1

Conclusion

The UE Power ON and cell acquisition procedure in a 5G SA network is a multi-layered synchronization and decoding operation. Each step — from detecting SSBs to decoding DCI and selecting BWPs — is finely orchestrated to ensure minimal power consumption and maximal efficiency.

By understanding how these procedures interact (especially SSB alignment, DCI decoding, and BWP usage), engineers and testers can better analyze and optimize UE behavior during initial access.

References:

• 3GPP TS 38.304 – NR; User Equipment (UE) procedures in idle mode and RRC inactive state

• 3GPP TS 38.331 – Radio Resource Control (RRC) protocol specification

• 3GPP TS 38.300 – NR; Overall description