5G NR Initial Access Procedure — Complete Visual Guide (103 Steps)

This interactive animated course explains the complete 5G NR initial access procedure — how your phone connects to a 5G network from power-on to the first data packet. Covering 29 chapters and 103 visual scenes with 3GPP-accurate technical animations.

Chapter 1: Introduction — What Happens When You Turn On Your 5G Phone

When you power on a 5G device, the UE (User Equipment) must complete the initial access procedure to establish a connection with the 5G NR (New Radio) network. This journey involves 17 major steps spanning RF scanning, synchronization, random access, security, and session establishment.

Chapter 2: RF Front-End — Antenna, LNA, and Baseband Processing

The RF front-end module receives electromagnetic waves through the antenna. The Low Noise Amplifier (LNA) boosts the weak signal. The mixer performs frequency down-conversion, and the ADC converts the analog signal to digital samples for baseband processing.

Chapter 3: Frequency Scanning — GSCN Raster and NR-ARFCN

Instead of scanning every possible frequency, the UE uses the GSCN (Global Synchronization Channel Number) raster — predefined positions where SSBs can be found. Below 3 GHz: N×1200 + M×50 kHz spacing. 3-24.25 GHz: 3000 + N×1.44 MHz spacing. FR2 mmWave: 24250.08 + N×17.28 MHz spacing. Each frequency maps to a unique NR-ARFCN (Absolute Radio Frequency Channel Number).

Chapter 4-5: PSS and SSS Detection — Finding the Physical Cell ID

The Primary Synchronization Signal (PSS) uses 3 M-sequences (length 127) to determine NID2 (0, 1, or 2). The UE correlates received samples against all three sequences. The Secondary Synchronization Signal (SSS) uses Gold sequences to determine NID1 (0 to 335). Together, PCI = 3 × NID1 + NID2, giving 1008 unique Physical Cell IDs.

Chapter 6: SSB Structure — SS/PBCH Block

The SSB (SS/PBCH Block) occupies 4 OFDM symbols and 240 subcarriers (20 RBs). Symbol 0: PSS (127 subcarriers). Symbol 1: PBCH + DMRS. Symbol 2: SSS (127 subcarriers). Symbol 3: PBCH + DMRS. The SSB is transmitted in burst sets with Lmax = 4 (below 3 GHz), 8 (3-6 GHz), or 64 (FR2) beams.

Chapter 7: PBCH Decoding — MIB Extraction

The PBCH carries the Master Information Block (MIB) — 32 bits containing: SFN (System Frame Number), subcarrier spacing (15 or 30 kHz for FR1), PDCCH configuration for CORESET#0, cell barred indicator, and DMRS position. QPSK modulation with polar coding provides robust decoding even at low SNR.

Chapter 8: Beam Management — LTE vs 5G NR

Unlike LTE's cell-wide transmission, 5G NR uses beam-based transmission. Analog beamforming steers a narrow beam. Digital beamforming creates multiple beams simultaneously. Hybrid beamforming combines both for massive MIMO. The UE measures RSRP across SSB beams and selects the best beam index for RACH.

Chapter 9-10: SSB Burst Set and MIB

SSB burst sets repeat with periodicity of 5, 10, 20, 40, 80, or 160 ms (default 20 ms). Positions within the half-frame depend on subcarrier spacing and Lmax. The MIB provides the critical pointer to CORESET#0 via pdcch-ConfigSIB1.

Chapter 11: CORESET#0 — Control Resource Set

CORESET#0 defines where the UE searches for PDCCH carrying SIB1 scheduling. Table 13-1 in 3GPP TS 38.213 maps the 4-bit pdcch-ConfigSIB1 to number of RBs, number of symbols, and offset. The search space uses aggregation levels 4, 8, or 16.

Chapter 12: SIB1 — System Information Block 1

SIB1 contains essential system information: PLMN identity list, cell selection parameters (Q-RxLevMin, Q-QualMin), frequency band indicator, RACH configuration (preamble format, PRACH configuration index, power ramping step, RA-ResponseWindow), paging configuration, and pointers to other SIBs.

Chapter 13-17: RACH Procedure — 4-Step Random Access (Msg1 to Msg4)

Msg1 (Preamble): UE selects a Zadoff-Chu preamble from 64 available sequences and transmits on PRACH with initial power. If no response, power ramping increases transmission power by the configured step size. Msg2 (RAR): gNB responds within the RA-Response Window with Timing Advance command, UL grant for Msg3, TC-RNTI, and backoff indicator. The RA-RNTI is calculated from the time-frequency resource. Msg3: UE transmits RRC Setup Request on PUSCH using the Msg2 grant, including UE identity (either 5G-S-TMSI or random value for contention-based RACH). Msg4: gNB resolves contention by echoing the UE identity. If matched, the UE is assigned a C-RNTI and sends HARQ ACK.

Chapter 18: RRC Setup — Radio Resource Control

After RACH completion, the gNB sends RRC Setup message configuring SRB1 (Signaling Radio Bearer 1) with RLC-AM mode. The UE responds with RRC Setup Complete, which also carries the NAS Registration Request piggy-backed in the dedicatedNAS-Message IE.

Chapter 19-20: NAS Registration — 5G Core Network

The Registration Request contains: 5GS registration type (initial, mobility, periodic), SUCI (Subscription Concealed Identifier) encrypted with the home network public key, UE security capabilities (supported NAS and AS algorithms), requested NSSAI (network slicing), and 5GMM capability. The AMF forwards to UDM/AUSF via NAS transport.

Chapter 21: 5G-AKA Authentication

The UDM generates authentication vectors using subscriber key K and RAND. AUSF computes XRES* and sends authentication challenge. UE computes RES* and returns it. If RES* matches XRES*, mutual authentication succeeds. Anchor key KAUSF is derived for NAS and AS security.

Chapter 22: NAS Security — Ciphering and Integrity

NAS Security Mode Command activates ciphering (NEA1/NEA2/NEA3) and integrity protection (NIA1/NIA2/NIA3) for all subsequent NAS messages. The UE derives NAS keys from KAMF. Security Mode Complete is the first integrity-protected uplink NAS message.

Chapter 23: Registration Accept

Registration Accept from AMF includes: 5G-GUTI (Globally Unique Temporary Identifier), Allowed NSSAI (permitted network slices), TAI list (Tracking Area Identity list for mobility), T3512 periodic registration timer value, and registration result.

Chapter 24-26: PDU Session Establishment

PDU Session Establishment Request contains requested PDU type (IPv4, IPv6, IPv4v6), SSC mode, requested QoS rules, and S-NSSAI. SMF selects UPF via NRF lookup. PCF provides PCC rules with QoS parameters. SMF programs UPF with N3/N9 GTP-U tunnels. PDU Session Establishment Accept carries: IP address, authorized QoS rules (QFI, 5QI), Session-AMBR, DNN, and S-NSSAI.

Chapter 27-28: RRC Reconfiguration and Data Flow

RRC Reconfiguration establishes DRBs (Data Radio Bearers) mapped to QoS flows via SDAP (Service Data Adaptation Protocol). PDCP provides ciphering and integrity for user plane. The complete data path: UE → DRB → gNB → N3 GTP-U tunnel → UPF → N6 → Internet. Your phone is now connected to 5G!

Chapter 29: Summary — The Complete 17-Step Journey

From RF scanning to data flow: (1) GSCN scan, (2) PSS detection, (3) SSS detection, (4) PBCH decode, (5) Beam selection, (6) CORESET#0, (7) SIB1, (8) Msg1 preamble, (9) Msg2 RAR, (10) Msg3 RRC Setup Request, (11) Msg4 contention resolution, (12) RRC Setup, (13) Registration, (14) 5G-AKA, (15) NAS security, (16) Registration Accept, (17) PDU session — and the 5G icon appears on your screen.