Quick Read
One definition and the points that matter, for each core 5G topic. When one needs more than a minute, the full article is a click away.
5G overview
The system level: what 5G is, how it is deployed, and the capabilities built on top.
What is 5G?
5G is the IMT-2020 generation: a new radio, NR, and a new service-based core, 5GC, specified by 3GPP from Release 15 to meet ITU-R's IMT-2020 requirements.
- Three usage scenarios — eMBB, URLLC and mMTC — with conflicting design targets.
- Headline targets: 20 Gbit/s downlink peak, 1 ms user-plane latency, a million devices per km².
- NSA delivers NR capacity on the LTE core; the full feature set needs SA.
- mMTC was met largely by LTE-M and NB-IoT, not by NR.
NSA vs SA
Two ways to deploy 5G. NSA (Option 3x) adds an NR gNB as secondary node to an LTE eNB on the EPC; SA (Option 2) connects the gNB directly to the 5G Core.
- NSA runs on EN-DC: LTE carries the control plane, NR adds user-plane capacity.
- Slicing, QoS flows, VoNR, RedCap and edge UPF need SA, because they are 5G Core features.
- Throughput is comparable; the SA business case is the core, not the radio.
5G Core
The 5G Core is a service-based architecture: network functions such as the AMF, SMF and UPF expose services over HTTP/2 and discover each other through the NRF.
- AMF handles access and mobility, SMF handles sessions, UPF forwards packets.
- Control and user planes separate over N4 (PFCP), so UPFs scale and move to the edge independently.
- Radio-facing and packet-path interfaces (N1–N4, N6) stay reference points.
- Slicing falls out of dynamic NF discovery rather than being bolted on.
Network slicing
Network slicing runs several logical networks over shared infrastructure, each identified by an S-NSSAI: an 8-bit slice/service type plus an optional 24-bit differentiator.
- The UE requests slices at registration; the network returns the Allowed NSSAI per registration area.
- Each PDU session belongs to exactly one slice; several slices mean several sessions.
- Selecting a slice reserves nothing — isolation comes from RAN, transport and core resource management.
MEC
MEC places application compute and a local UPF near the RAN, so traffic breaks out to an edge data network instead of travelling to a central data centre.
- It shortens transport latency and backhaul load; air-interface latency is unchanged.
- 3GPP gets traffic there with UPF selection, local data networks and AF traffic influence.
- SSC modes decide what happens to the session anchor when the UE moves.
- ETSI MEC defines the application platform; 3GPP defines how traffic reaches it.
NTN
NTN adapts NR, from Release 17, to satellites and high-altitude platforms, where long delay, large Doppler and moving beams break terrestrial assumptions.
- LEO trades coverage for latency; GEO covers continents with a round trip near 540 ms.
- UEs pre-compensate timing and Doppler using GNSS and broadcast ephemeris.
- Timing advance and HARQ timers were extended to survive the delay.
- Transparent payloads keep the gNB on the ground; regenerative payloads put it in orbit.
5G-Advanced
5G-Advanced is 3GPP's name for Releases 18, 19 and 20: the same NR radio and 5G Core, extended with features for efficiency, intelligence and new device classes.
- Not a new generation: most features arrive as software on existing networks, plus new UEs.
- Release 18 opened AI/ML for the air interface, network energy saving, L1/L2-triggered mobility and eRedCap.
- Later releases make the study items normative and trial concepts that 6G will build on.
Massive MIMO
Massive MIMO uses arrays with tens of transceiver chains — 32T32R, 64T64R — to serve several users on the same time-frequency resources, separated in space.
- The capacity gain is MU-MIMO; beamforming alone only improves one link.
- It is a cell-capacity feature: gains grow with load, not peak rate.
- TDD reciprocity via SRS gives full channel knowledge with overhead independent of array size.
- FDD relies on codebook feedback, which limits practical port counts.
Beamforming
Beamforming applies per-element phase and amplitude weights so transmissions add constructively towards a receiver, giving array gain of about 10·log₁₀(N) dB.
- RF-chain count sets the architecture: analog, digital or hybrid.
- FR1 radios go digital; FR2 goes hybrid, because full RF chains at mmWave are impractical.
- SSB beam sweeping bootstraps initial access, with up to 64 beams in FR2.
- Beam management and failure recovery run continuously, not as exceptions.
5G NR
The radio, in the order that makes each topic easier than the last.
What is 5G NR
NR is the 5G radio access technology: an OFDM air interface whose numerology scales subcarrier spacing and slot length together, across FR1 and FR2.
- Numerology µ sets 15 × 2^µ kHz subcarrier spacing; slots shorten as spacing grows.
- FR1 carries coverage and capacity; FR2 buys bandwidth with a much harder link budget.
- Symbol-level TDD flexibility and mini-slots are where NR's latency gains come from.
- Bandwidth parts decouple device capability from carrier width.
NR architecture
NR architecture has three domains — UE, NG-RAN and 5G Core — joined by the air interface and the NG interface, with a gNB that can be split into CU and DU.
- User plane: SDAP maps QoS flows, PDCP secures, RLC segments, MAC schedules, PHY transmits.
- RRC terminates in the gNB; NAS terminates in the AMF, carried transparently.
- The F1 split puts RRC, SDAP and PDCP in the CU; RLC, MAC and PHY in the DU.
Frame structure
NR keeps LTE's 10 ms frame and 1 ms subframe, but slot length depends on numerology: 1/2^µ ms, carrying 14 OFDM symbols (12 with extended cyclic prefix at 60 kHz).
- Slot formats mark each symbol as downlink, uplink or flexible, configured in layers.
- Mapping type B mini-slots start at any symbol and run 2–7 symbols.
- Slot aggregation stretches a transmission across several slots for coverage.
- All timing is an integer multiple of Tc, about 0.5 ns, locked to LTE's.
Numerology
Numerology µ scales NR subcarrier spacing as 15 × 2^µ kHz. Symbols per slot stay at 14 and the subframe stays at 1 ms, so a slot lasts 1/2^µ ms.
- Powers of two keep the grids aligned, so one carrier can host several numerologies.
- Small µ favours coverage and delay spread; large µ favours latency, Doppler tolerance and high carrier frequencies.
- 30 kHz in FR1 and 120 kHz in FR2 are the practical defaults.
Resource grid
An NR resource block is 12 subcarriers, addressed through four coordinate systems: common resource blocks from Point A, bandwidth parts, and physical and virtual resource blocks.
- CRBs are absolute, anchored at Point A for every numerology.
- A UE can have up to four bandwidth parts configured and one active, switched by DCI, timer or RRC.
- PRBs count from the BWP start; VRBs are what the DCI names.
- BWPs enable power saving, narrowband devices and mixed numerologies.
PUSCH
PUSCH carries uplink data, plus uplink control when the two share a slot. Its design is driven by a power-limited UE transmitter more than by the channel.
- Transform precoding selects DFT-s-OFDM: lower PAPR and better coverage, single layer only.
- CP-OFDM supports multiple layers where the link budget allows.
- Grant-based access costs a scheduling round trip; configured grants remove it by reserving resources.
- Fractional path-loss compensation deliberately holds back cell-edge UEs to protect neighbouring cells.
PDSCH
PDSCH carries downlink user data. The gNB picks an MCS and a resource allocation, and the transport block size formula turns them into a byte-aligned block for LDPC.
- Processing: CRC, LDPC with redundancy versions, scrambling, then layer and resource-element mapping.
- Mapping skips DM-RS, SSB, CSI-RS, the scheduling PDCCH and configured reserved resources.
- A pre-emption indication tells the UE which resources URLLC traffic took.
- Slot aggregation repeats the block with rotating redundancy versions for coverage.
PUCCH
PUCCH carries uplink control — HARQ-ACK, scheduling requests and CSI — when the UE has no PUSCH to multiplex it onto. NR defines five formats.
- Two axes: short (1–2 symbols) or long (4–14), and up to 2 bits or more.
- Format 0 carries no DM-RS; the information sits in the cyclic shift.
- Long formats buy energy for cell-edge UEs, not extra payload.
- Format 4 is Format 3 with spreading, so UEs can share resources.
PRACH
Random access is NR's one unscheduled uplink transmission, breaking the circularity of needing a grant to ask for a grant.
- Msg1 is a preamble on a PRACH occasion tied to an SSB beam, carrying no identity.
- Msg2 returns timing advance, a Msg3 grant and a TC-RNTI.
- Msg4 resolves contention; without it, two UEs could end up sharing one identity.
- Two-step RACH saves a round trip by transmitting payload before timing advance is known.
SRS
SRS is an uplink reference signal the gNB uses to measure a UE's channel — and, in TDD, to build the downlink precoder through reciprocity.
- One SRS transmission replaces heavy CSI feedback for large antenna arrays.
- Reciprocity covers the propagation channel, not the transceivers, so calibration is required.
- Antenna switching sounds receive antennas the UE does not transmit from.
- Usages: beam management, codebook, non-codebook and antenna switching.
SSB
The SSB packs PSS, SSS and PBCH into 4 OFDM symbols and 240 subcarriers. It is the first thing a UE finds, and the reference for beams across NR.
- A burst set holds up to 4, 8 or 64 SSBs in a 5 ms half-frame, one per beam.
- Burst sets repeat every 5 to 160 ms.
- Cases A–E set where blocks start in the half-frame for each SSB subcarrier spacing.
- The SSB index acts as a beam index for measurement, RACH occasion selection and paging.
Scheduling
Every slot, the MAC scheduler decides which UEs transmit, on which resources, at what MCS and on which layers; logical channel prioritisation then fills the resulting transport block.
- Who, how much, how hard and where are vendor algorithms; LCP is standardised.
- LCP serves each channel up to its prioritised bit rate, then by strict priority.
- Uplink scheduling is blind: an SR is one bit, a BSR is grouped and quantised.
- Configured grants remove the scheduling-request round trip.