A

AAS

Active Antenna System — antenna array with integrated radios per element, enabling massive MIMO and beamforming.

ACK

Acknowledgment — UE indicates successful PDSCH decoding via PUCCH or PUSCH.

AMC

Adaptive Modulation and Coding — gNB selects MCS based on UE-reported CQI.

Antenna Port

Logical channel defined by its associated reference signal — TS 38.211 §4.4.1.

ARFCN

Absolute Radio Frequency Channel Number — integer identifying a channel center frequency.

B

BCCH

Broadcast Control Channel — logical channel carrying MIB and SIBs.

BFR

Beam Failure Recovery — UE detects beam degradation, requests new beam via dedicated PRACH.

BG1 / BG2

LDPC Base Graphs — BG1 for high-rate codes (K≤8448), BG2 for low-rate (K≤3840).

BLER

Block Error Rate — fraction of failed transport-block decodes; target typically 10% before HARQ.

BWP

Bandwidth Part — contiguous slice of carrier (≤275 PRBs) UE actively monitors. TS 38.211 §4.4.5.

C

C-RNTI

Cell-RNTI — UE's unique RNTI within a cell, assigned at RRC setup.

CB

Code Block — segment of a transport block (max 8448 bits BG1 / 3840 bits BG2) for LDPC encoding.

CB-G

Code Block Group — for HARQ-CB-level retransmission, several CBs grouped per ACK/NACK.

CCE

Control Channel Element — basic resource unit of PDCCH (1 CCE = 6 REGs = 72 REs).

CDM

Code Division Multiplexing — port multiplexing within a DMRS group via orthogonal cover codes.

CORESET

Control Resource Set — the 1-3 OFDM symbol region where UE searches for PDCCH.

CP

Cyclic Prefix — guard interval prepended to each OFDM symbol, lengths defined per μ.

CP-OFDM

Cyclic-Prefixed OFDM — default DL waveform; UL waveform optionally Type 1.

CQI

Channel Quality Indicator — UE-reported MCS hint for the channel (4 bits, 16 entries).

CRB

Common Resource Block — cell-wide PRB index counted from Point A.

CRC

Cyclic Redundancy Check — error detection code; 16-bit for small TBs, 24-bit for large.

CSI

Channel State Information — UE feedback containing RI, PMI, CQI for MIMO scheduling.

CSI-RS

CSI Reference Signal — gNB-transmitted pilots for UE channel sounding (port range 3000+).

D

DAI

Downlink Assignment Index — counter in DCI tracking PDSCH allocations for HARQ codebook.

DCI

Downlink Control Information — scheduling message on PDCCH (formats 0_x for UL, 1_x for DL, 2_x group).

DCI 0_0

Compact UL grant for fallback (smallest DCI).

DCI 0_1

UE-specific UL grant — full feature set.

DCI 1_0

Compact DL assignment for fallback.

DCI 1_1

UE-specific DL assignment with TCI, BWP, MCS — most-used data DCI.

DCI 2_0

Slot Format Indicator — group-common signaling of D/U/F symbols.

DFT-s-OFDM

DFT-spread OFDM — alternative UL waveform with low PAPR for cell-edge.

DL

Downlink — gNB → UE direction.

DMRS

Demodulation Reference Signal — UE-specific pilots for channel estimation; Type 1 or Type 2.

DRX

Discontinuous Reception — UE sleeps between paging cycles to save battery.

E

eCPRI

enhanced CPRI — Ethernet-based fronthaul transport replacing legacy CPRI.

eMBB

enhanced Mobile Broadband — high-throughput 5G use case.

eType II

Enhanced Type II Codebook (Rel-16) — frequency-domain compressed Type II MIMO feedback.

F

F1 Interface

Open interface between O-CU and O-DU in O-RAN architecture.

FDD

Frequency Division Duplex — separate UL and DL bands.

FFT

Fast Fourier Transform — converts time-domain samples to frequency-domain subcarriers.

FR1

Frequency Range 1 — 410 MHz to 7.125 GHz (sub-6 GHz, includes C-band).

FR2

Frequency Range 2 — 24.25 to 52.6 GHz (mmWave bands n257/n258/n260/n261).

FR2-2

Frequency Range 2-2 (Rel-17) — 52.6 to 71 GHz, requires μ=4/5/6 numerology.

Frame

10 ms reference cycle, 10 subframes per frame (invariant across all numerologies).

G

gNB

Next-generation NodeB — 5G NR base station.

GSCN

Global Sync Channel Number — sparse raster on which SSB centers can sit.

H

Half-Frame

5 ms — SSB burst window and paging frame indicator boundary.

HARQ

Hybrid Automatic Repeat Request — combines FEC (LDPC) with retransmission and soft-combining.

I

IR

Incremental Redundancy — HARQ retransmission with different parity bits, lowering effective code rate.

K

K_SSB

Subcarrier offset between SSB and Point A, signaled in MIB.

L

LBT

Listen Before Talk — required mechanism for unlicensed-band NR-U operation.

LDPC

Low-Density Parity-Check code — channel code for PDSCH/PUSCH; replaces LTE Turbo. TS 38.212 §5.3.2.

Lifting Size Z

Parameter expanding LDPC base graph into the full parity-check matrix; selected from 8 sets.

LLR

Log-Likelihood Ratio — soft bit value passed between belief-propagation nodes in LDPC decoder.

M

MAC-CE

MAC Control Element — fast in-band control (e.g., TCI activation, BWP indication).

MCS

Modulation and Coding Scheme — index into Tables 1/2/3 (TS 38.214 §5.1.3).

MIB

Master Information Block — 32-bit broadcast on PBCH carrying SFN, SCS, k_SSB.

MIMO

Multiple-Input Multiple-Output — multi-antenna transmission for spatial multiplexing.

Mini-slot

Sub-slot scheduling unit (2/4/7 OFDM symbols) for URLLC sub-ms latency.

mMTC

massive Machine-Type Communications — 5G use case for IoT-scale deployments.

mmWave

Millimeter Wave — frequencies above ~24 GHz (FR2 + FR2-2 in 5G).

N

N1, N2

Codebook configuration: N1 horizontal × N2 vertical antenna ports in 2D array.

NAS

Non-Access Stratum — mobility and session-management messages above the gNB.

NCC

Next Hop Chaining Counter — used in 5G AS key derivation.

NDI

New Data Indicator — bit in DCI flipping per HARQ Process when new data starts.

Normal CP

14 OFDM symbols per slot; standard cyclic-prefix length; supported on all μ.

NSA

Non-Standalone — 5G UE anchored on LTE eNB (via EN-DC).

NTN

Non-Terrestrial Networks — satellite-based NR (Rel-17).

O

O-CU

O-RAN Centralized Unit — runs PDCP, RRC, SDAP layers.

O-DU

O-RAN Distributed Unit — runs RLC, MAC, high PHY.

O-RU

O-RAN Radio Unit — runs low PHY (RE map, IFFT, CP, RF).

OCC

Orthogonal Cover Code — used for DMRS port multiplexing within a CDM group.

OFDM

Orthogonal Frequency-Division Multiplexing — base waveform of 5G NR.

OOK

On-Off Keying — simple modulation used in NB-IoT wake-up signals.

P

PAPR

Peak-to-Average Power Ratio — high in CP-OFDM, lower in DFT-s-OFDM.

PBCH

Physical Broadcast Channel — carries MIB; in SSB.

PCI

Physical Cell Identity — 1 of 1008 (336 SSS × 3 PSS) cell identifiers.

PDCCH

Physical Downlink Control Channel — carries DCI; uses Polar coding.

PDCP

Packet Data Convergence Protocol — header compression, ciphering layer.

PDSCH

Physical Downlink Shared Channel — carries user data; uses LDPC.

PHR

Power Headroom Report — UE-reported margin between max Tx power and current usage.

PMI

Precoding Matrix Indicator — UE-recommended precoder index from CSI.

Point A

Absolute frequency anchor — every PRB index is offset from Point A.

Polar Code

Channel code for PDCCH and PBCH; achieves Shannon-bound at small block sizes.

PRACH

Physical Random Access Channel — carries Zadoff-Chu preamble for RACH.

PRB

Physical Resource Block — BWP-relative; 12 contiguous subcarriers × 1 OFDM symbol.

PRG

Precoding Resource-block Group — bundle size for PRB bundling (2/4/wideband).

PRS

Positioning Reference Signal — for DL-TDOA, multi-RTT, AOA positioning (Rel-16+).

PSS

Primary Synchronization Signal — 3 length-127 sequences in SSB.

PT-RS

Phase Tracking Reference Signal — tracks phase noise, important at mmWave + high-order QAM.

PUCCH

Physical Uplink Control Channel — formats 0/1/2/3/4 carrying ACK/NACK, SR, CSI.

PUSCH

Physical Uplink Shared Channel — UL data; uses LDPC.

Q

QAM

Quadrature Amplitude Modulation — 16/64/256/1024-QAM constellation orders supported.

QCL

Quasi-Co-Location — two ports share large-scale channel properties; Types A/B/C/D.

QPSK

Quadrature Phase Shift Keying — 4-point constellation (2 bits/symbol).

R

RA-RNTI

Random Access RNTI — temporary identifier scrambling RAR's PDCCH.

RACH

Random Access Channel — UE's first uplink for cell access, handover, BFR.

Rank

Number of MIMO layers transmitted (1 to min(N_tx, N_rx, channel rank)).

RAR

Random Access Response — Msg2 in 4-step RACH; carries T_TA command and UL grant.

RB

Resource Block — generic 12-SC unit (used as RB in spec, often interchangeable with PRB).

RE

Resource Element — 1 subcarrier × 1 OFDM symbol (smallest unit).

RedCap

Reduced Capability UE (Rel-17) — for IoT-class devices with smaller bandwidth/MIMO.

REG

Resource Element Group — 12 REs in 1 OFDM symbol (PDCCH allocation unit).

RI

Rank Indicator — UE-reported MIMO rank from CSI.

RIC

RAN Intelligent Controller — O-RAN component for AI/ML-driven optimization (near-RT and non-RT).

RLC

Radio Link Control — segmentation, ARQ between MAC and PDCP.

RMSI

Remaining Minimum System Information — synonymous with SIB1 contents.

RNTI

Radio Network Temporary Identifier — UE's temporary handle for various procedures.

RoE

Radio over Ethernet — alternative fronthaul encapsulation to eCPRI.

RRC

Radio Resource Control — control-plane protocol above MAC.

RSRP

Reference Signal Received Power — UE measurement of SSB or CSI-RS power.

RSRQ

Reference Signal Received Quality — RSRP / total received power ratio.

RV

Redundancy Version — 4 values (0,1,2,3) selecting different starts in rate-matched bits.

S

SA

Standalone — 5G UE camped only on NR (no LTE anchor).

SCS

Subcarrier Spacing — = 2^μ × 15 kHz; defines numerology.

SDAP

Service Data Adaptation Protocol — QoS flow ↔ DRB mapping.

SDT

Small Data Transfer (Rel-16) — payload sent during RACH without full RRC.

SFI

Slot Format Indicator — 56 predefined formats (TS 38.213 Table 11.1.1-1).

SFN

System Frame Number — 10-bit counter (0..1023), wraps every 10.24 s.

SIB1

System Information Block 1 — RMSI, contains CORESET#0, scheduling info, full system params.

SLIV

Start and Length Indicator Value — encodes mini-slot start symbol and length in DCI.

Slot

Time-domain unit = 1 ms / 2^μ; 14 OFDM symbols (Normal CP) or 12 (Extended CP).

SMO

Service Management Orchestrator — top-level O-RAN management.

Soft Combining

HARQ technique merging soft information from prior decode attempts; Chase or IR.

SR

Scheduling Request — UE requests UL grant via PUCCH Format 0/1.

SRS

Sounding Reference Signal — UE-transmitted UL pilot for gNB to estimate UL channel.

SS Burst

Set of L SSBs (4/8/64) transmitted within a 5 ms half-frame.

SSB

Synchronization Signal Block — PSS + SSS + PBCH on 240 SCs × 4 sym.

SSS

Secondary Synchronization Signal — 336 length-127 sequences in SSB.

T

T_c

Basic time unit — 1 / (480 kHz × 4096) ≈ 0.509 ns.

TA

Timing Advance — UE Tx offset compensating round-trip delay; T_TA = (N_TA + N_TA,offset) · T_c.

Tanner Graph

Bipartite graph representation of LDPC parity check matrix; basis of belief-propagation decoding.

TBS

Transport Block Size — number of bytes in a transport block (TS 38.214 §5.1.3.2).

TCI

Transmission Configuration Indicator — labeled QCL tuple; up to 128 configured, 8 active, 1 selected.

TDD

Time Division Duplex — UL and DL share same band, separated in time.

TPC

Transmit Power Control — closed-loop UL power adjustment via DCI.

TPMI

Transmit Precoding Matrix Indicator — gNB-signaled precoder for UL codebook-based PUSCH.

TRS

Tracking Reference Signal — special CSI-RS configuration for fine timing/frequency tracking.

U

UCI

Uplink Control Information — ACK/NACK, SR, CSI; carried on PUCCH or piggy-backed on PUSCH.

UE

User Equipment — mobile device.

UL

Uplink — UE → gNB direction.

URLLC

Ultra-Reliable Low-Latency Communications — 5G use case targeting 99.999% / sub-ms.

V

V2X

Vehicle-to-Everything — NR sidelink communication for automotive.

VRB

Virtual Resource Block — logical RB; can map to PRBs in localized or distributed fashion.

Z

Z (lifting)

See Lifting Size Z.

Zadoff-Chu

Constant-amplitude zero-autocorrelation sequence used for PRACH preambles and DMRS.

ZC

Zadoff-Chu — see above.

Μ

μ (mu)

Numerology index — SCS = 2^μ × 15 kHz, μ ∈ {0..6}.