Learning Roadmap — where to start

Absolute beginner — start here

If you're new to mobile networks entirely, read these four first, in order. They give you the vocabulary and the big picture before you touch any one layer in depth.

1. 5G NR Overview — what 5G is and the pieces involved.
2. 5G System Architecture — how the radio (NG-RAN) and core (5GC) split the work.
3. Spectrum: FR1, FR2, Bands & Duplexing — where 5G lives in the airwaves.
4. Radio Frame, Subframe, Slot & Symbol — how the air interface is organised in time.

Prefer to start with 4G? Jump to the LTE beginner path below — LTE is in many ways simpler and a great on-ramp to 5G.

Beginner path — 5G NR

A complete first pass through 5G NR, from "how a phone finds a cell" to "how it registers and gets data flowing." Read in this order:

1. 5G NR Overview
2. 5G System Architecture (NG-RAN + 5GC)
3. Spectrum & Bands
4. Radio Frame / Slot / Symbol
5. Resource Grid: RE, RB & BWP
6. SSB & Cell Search — how a phone finds a cell.
7. Physical Channels
8. Logical, Transport & Physical Channels
9. Random Access (RACH) — the phone's first words to the network.
10. RRC States (Idle / Inactive / Connected)
11. End-to-End Registration Call Flow — everything above, tied together step by step.

Beginner path — LTE 4G

The same journey for LTE. If you already did the 5G path, this is a fast, illuminating compare-and-contrast.

1. LTE / 4G Overview
2. EPS Architecture (E-UTRAN + EPC)
3. EPS Network Elements (MME/S-GW/P-GW/HSS/PCRF)
4. Spectrum: E-UTRA Bands, FDD/TDD
5. Frame Structure (Type 1 FDD & Type 2 TDD)
6. Resource Grid: RE, RB & Bandwidth
7. Cell Search: PSS/SSS & PBCH/MIB
8. Physical Channels & Signals
9. Random Access (Contention & Non-Contention)
10. RRC States (IDLE & CONNECTED)
11. End-to-End Attach Call Flow

Interview-prep shortlist

If you're preparing for a RAN / protocol-stack interview and time is short, these are the topics that come up most — the "how does X actually work, step by step, with the exact messages and IEs" questions. Know these cold:

1. SSB & Cell Search — PSS/SSS, PCI, MIB/SIB1.
2. Random Access — 4-step vs 2-step, contention resolution.
3. HARQ — soft combining, RV, DL vs UL (and LTE's sync UL HARQ).
4. Scheduling — dynamic vs SPS/CG, SR→BSR→grant, scheduler types.
5. PDCCH & DCI Formats — the highest-density interview area.
6. PDSCH and PUCCH & UCI.
7. RRC States and RRC Procedures.
8. Measurements & Events (A1–A6).
9. Xn Handover — the message flow with exact IEs.
10. Registration Call Flow — end to end.
11. 5G QoS Model — 5QI / QFI / ARP.
12. PDCP Ciphering & Integrity and the Key Hierarchy.

Deep-dive tracks by layer

Once you have the basics, go deep on the area you work in. Each track is an ordered reading list.

PHY track (5G). Waveforms → Frame/Numerology → Resource Grid & BWP → CORESET & Search Space → PDCCH & DCI → Time-Domain Alloc & SLIV → Freq-Domain Alloc & RIV → PDSCH → PUSCH → PUCCH & UCI → Reference Signals → MIMO → Channel Coding → Modulation & MCS → Link Adaptation → Throughput → CSI Feedback → Power Control → Beam Management → BWP Switching.

Protocol-stack track (L2, 5G). Channel Mapping → MAC PDU & CEs → HARQ → Scheduling → LCP → BSR/PHR/SR → DRX → RLC Modes → Segmentation → ARQ & Status → PDCP → Ciphering & Integrity → ROHC → PDCP Duplication → SDAP → QoS Flow → DRB → Radio Bearers.

Mobility track (5G). RRC States → Measurements & Events → Measurement Gaps & SMTC → Cell Reselection → Handover Overview → Xn Handover → NG (N2) Handover → Conditional HO & DAPS → RLM & RLF.

Core & NAS track (5G). System Architecture → NAS Overview → Registration & Authentication → PDU Session Management → NGAP (N2) → 5G QoS Model → Network Slicing → Security Key Hierarchy → End-to-End Registration.

Voice track. 5G Voice Options (VoNR / EPS Fallback) → NAS & RRC Voice Signalling → VoNR Call Flow → VoNR with ROHC → VoNR KPIs. LTE side: VoLTE / CSFB / SRVCC → VoLTE Call Flow → CSFB → SRVCC.

LTE deep-dive. LTE has the same tracks with lte- pages — see the LTE 4G menu, or start from the LTE vs 5G comparison to map what you know in one onto the other.