Glossary
412 terms, each defined in one or two sentences, with links to where the idea is explained properly.
0–9
- 1 dB compression point (P1dB)
- The input or output power at which an amplifier's gain has dropped 1 dB below its small-signal value. It marks the onset of strong compression.
- 3GPP release
- A frozen, numbered set of 3GPP specifications that implementers build against. Release 8 brought LTE, Release 15 the first full set of 5G standards and Release 17 NTN and RedCap.
- 3rd Generation Partnership Project (3GPP)
- The partnership of regional standards organisations that writes the GSM, UMTS, LTE, 5G NR and future 6G specifications. Its work is organised in numbered releases.Hub: Careers and industry. Related: 3GPP release, Long Term Evolution (LTE), 5G New Radio (NR), ETSI
- 5G Core (5GC)
- The core network defined for 5G in a service-based architecture, with separate functions for access and mobility, sessions and the user plane. Standalone NR connects to it.
- 5G New Radio (NR)
- The 3GPP radio interface for 5G, first specified in Release 15, with flexible numerology, LDPC and polar coding and operation from below 1 GHz to the millimetre-wave bands. TS 38.300 gives the overall description.
- 5G-Advanced
- The 3GPP name for 5G evolution from Release 18 onwards. Release 18 work covers topics such as energy efficiency, AI/ML, extended reality and further NTN integration.
- 6 GHz band
- Spectrum from 5.925 GHz to 7.125 GHz that the FCC opened for unlicensed use in the United States in April 2020, with standard-power devices under automated frequency coordination. Other regulators have opened different portions.
- 6G
- The informal name for the generation after 5G, which ITU-R calls IMT-2030. At the time of writing 3GPP plans Release 20 for 6G studies and Release 21 for the first normative 6G specifications.
A
- Access point (AP)
- The Wi-Fi device that forms a basic service set, connects stations to the wired network and coordinates their access.
- Adaptive data rate (ADR)
- A LoRaWAN mechanism in which the network server tells each device which spreading factor and transmit power to use based on its link quality.
- Additive white Gaussian noise (AWGN)
- A noise model in which independent Gaussian samples with a flat spectrum are added to the signal. It is the baseline channel for comparing modulation and coding schemes before fading is considered.Hub: Wireless foundations. Related: Thermal noise, Bit error rate (BER), Signal-to-noise ratio (SNR)
- Adjacent channel interference
- Interference from a transmitter on the neighbouring channel, caused by its leakage and by the victim receiver's limited selectivity.
- Adjacent channel leakage ratio (ACLR)
- The ratio of a transmitter's power in its assigned channel to the power it leaks into the neighbouring channel, in dB. 3GPP sets minimum ACLR values for base stations and devices.
- AI-native air interface
- A research direction in which parts of the physical and MAC layers, such as channel estimation, CSI compression or detection, are learned rather than hand-designed. 3GPP studied AI/ML for the NR air interface in Release 18 (TR 38.843), covering CSI feedback, beam management and positioning.
- Alamouti code
- A space-time block code for two transmit antennas that achieves full transmit diversity with simple linear decoding and no channel knowledge at the transmitter.
- Aliasing
- The folding of signal energy from above half the sample rate into the band below it when a signal is sampled too slowly. Once it happens it cannot be undone, so an anti-aliasing filter must come before the ADC or decimator.
- Amplitude and phase-shift keying (APSK)
- A modulation that places symbols on concentric rings rather than a square grid. The ring layout tolerates power amplifier compression better, which is why satellite systems use it.
- Amplitude modulation (AM)
- Analogue modulation in which the carrier amplitude follows the message signal. It is still used for broadcast and aviation voice.
- Analogue beamforming
- Beamforming with phase shifters in the RF path and one shared converter chain, so only one beam can be formed per chain at a time. It is cheap and power-efficient for large mmWave arrays.
- Analogue-to-digital converter (ADC)
- The converter that samples an analogue signal and outputs digital values. Its sample rate sets the bandwidth captured and its resolution and noise set the dynamic range.
- Angle of arrival (AoA)
- The direction from which a signal reaches an antenna array, estimated from phase differences between elements. Bluetooth direction finding and UWB both use it.
- Antenna array
- A set of antenna elements arranged in a known geometry and fed so their signals can be combined. Element spacing of about half a wavelength avoids grating lobes.
- Antenna gain
- How much an antenna concentrates power in its best direction compared with a reference antenna, including its efficiency. It is given in dBi against an isotropic radiator or in dBd against a half-wave dipole.
- Atmospheric absorption
- Loss caused by oxygen and water vapour in the air, which peaks at particular frequencies such as the oxygen line near 60 GHz. ITU-R P.676 gives a method for calculating it.
- Automated frequency coordination (AFC)
- A database service that tells a standard-power 6 GHz device which frequencies and power levels it may use at its location, to protect incumbent fixed links.
- Automatic gain control (AGC)
- A loop that adjusts receiver gain so the signal fills the ADC range without clipping. Bursty signals need a fast AGC that settles within the preamble.
B
- Back-off
- Running a power amplifier below its saturated output so that signal peaks stay in the linear region. More back-off means less distortion and lower efficiency.
- Bandwidth
- The width of the frequency range a signal or channel occupies, in hertz. It caps the symbol rate a link can carry and sets how much noise power the receiver collects.
- Bandwidth part (BWP)
- A contiguous set of resource blocks within an NR carrier, with its own numerology, that a device is configured to use. It lets a device work in less than the full carrier width to save power or support mixed numerologies.
- Beacon frame
- A management frame an access point sends periodically to announce its network, capabilities and timing.
- Beam management
- The NR procedures by which a base station and device find, track and switch between narrow beams, using sweeps of reference signals and measurement reports.
- Beamforming
- Setting the phase and amplitude of the signal on each element of an antenna array so the contributions add in chosen directions. It raises gain towards the user and can place nulls on interferers.Hub: MIMO and beamforming. Related: Antenna array, Precoding, Analogue beamforming, Digital beamforming
- Beamwidth
- The angular width of an antenna's main lobe, usually measured between the points where gain is 3 dB below the peak. Higher gain means a narrower beam.
- Belief propagation
- An iterative decoding method that passes probability messages between variable and check nodes of a code's graph. LDPC decoders use it or simplified versions such as min-sum.
- Bent pipe
- See transparent payload.
- BER
- See bit error rate.
- Binary phase-shift keying (BPSK)
- A modulation with two symbols of opposite phase, carrying one bit per symbol. It needs the lowest Eb/N0 of the common schemes for a given bit error rate.
- Bit error rate (BER)
- The fraction of received bits that are wrong. It is plotted against Eb/N0 on a log scale to compare modulation and coding schemes.
- BLE
- See Bluetooth Low Energy.
- BLE advertising
- Short broadcast packets a Bluetooth LE device sends on the advertising channels so others can discover it or read its data without a connection.
- BLE PHYs
- The Bluetooth LE physical layer options: LE 1M at 1 Mbit/s, LE 2M at 2 Mbit/s, and LE Coded at 500 kbit/s or 125 kbit/s for longer range, according to the Bluetooth SIG.
- Block code
- A code that maps each fixed-length block of information bits to a longer codeword independently of other blocks. Hamming, Reed-Solomon, LDPC and polar codes are block codes.
- Block error rate (BLER)
- The fraction of transport or code blocks that fail their CRC after decoding. Cellular link adaptation steers the MCS to hold BLER near a target value.
- Bluetooth Low Energy (BLE)
- The low-power Bluetooth radio in the 2.4 GHz ISM band, using 40 channels at 2 MHz spacing from 2.402 GHz to 2.480 GHz, of which 3 are for advertising and 37 for data, according to the Bluetooth SIG. It uses GFSK modulation.
- Bluetooth SIG
- The Bluetooth Special Interest Group, which develops the Bluetooth specifications and runs product qualification.Hub: Careers and industry. Related: Bluetooth Low Energy (BLE)
C
- Carrier aggregation
- Combining several component carriers, possibly in different bands, into one wider allocation for a single device. It arrived with LTE-Advanced in Release 10, and NR supports it from Release 15.
- Carrier frequency
- The centre frequency of the radio wave onto which information is modulated. It sets the wavelength, which in turn governs antenna size and much of the propagation behaviour.
- Carrier frequency offset (CFO)
- The difference between the transmitter's carrier frequency and the receiver's local oscillator, plus any Doppler shift. Uncorrected, it rotates the constellation and causes inter-carrier interference in OFDM.
- Cascaded noise figure
- The overall noise figure of a chain of stages, given by the Friis noise formula. Each stage's contribution is divided by the gain ahead of it, so the first stage dominates.
- Cell-free massive MIMO
- A research architecture in which many distributed access points jointly serve all users in an area, with no cell boundaries. It aims to give more uniform service than cellular layouts.
- Centre frequency
- The RF frequency an SDR is tuned to, which appears at 0 Hz in the complex baseband stream. Recordings must store it alongside the samples.
- CFO
- See carrier frequency offset.
- Channel bonding
- Combining adjacent Wi-Fi channels into one wider channel for higher throughput. Wider channels collect more noise and are more exposed to interference.
- Channel estimation
- Working out the channel's complex gain from known pilot or reference symbols, then interpolating it across time and frequency for the data. Its accuracy limits equalisation and precoding.
- Channel model
- A mathematical or statistical description of how a propagation environment changes a signal, used to test designs in simulation. 3GPP TR 38.901 defines the models used for NR studies from 0.5 GHz to 100 GHz.
- Channel quality indicator (CQI)
- A report from the device to the base station that indicates the highest modulation and coding scheme it could receive at the target error rate.
- Channel reciprocity
- The property that the propagation channel is the same in both directions on the same frequency. TDD systems use it to estimate the downlink channel from uplink sounding, after calibrating the radio hardware.
- Channel state information (CSI)
- Knowledge of the channel's gains and phases between antennas, either measured directly or summarised in reports. It drives precoding, rank selection and link adaptation.
- Chase combining
- A HARQ method in which each retransmission repeats the same coded bits and the receiver adds the soft values. It gains SNR but no new parity.
- Chirp spread spectrum (CSS)
- A modulation in which each symbol is a frequency sweep across the channel, with data encoded in the starting point of the sweep. It tolerates Doppler and multipath well.
- Clipping
- Limiting a signal's peaks to reduce PAPR before the power amplifier. It adds in-band distortion and out-of-band emissions, so it is usually followed by filtering.
- Clustered delay line (CDL) model
- A channel model whose paths are grouped in clusters with defined delays, powers and angles of arrival and departure. The angles make it suitable for testing multi-antenna and beamforming designs.
- Code block segmentation
- Splitting a large transport block into code blocks that fit the maximum size the channel code supports, each with its own CRC. In NR the maximum depends on the LDPC base graph.
- Code rate
- The ratio of information bits to coded bits. A rate 1/3 code sends three coded bits for each information bit.
- Codebook
- A predefined set of precoding matrices known to both ends. The device reports the index of the best entry instead of the full channel, which keeps feedback small.
- Coding gain
- The reduction in required Eb/N0 that a code provides at a given error rate, compared with sending the same data uncoded.
- Cognitive radio
- A radio that senses its environment and adapts its frequency, power or waveform to use spectrum others are not using at that time and place.
- Coherence bandwidth
- The frequency separation over which the channel response stays roughly the same. It is inversely proportional to the delay spread.
- Coherence time
- The time over which the channel response stays roughly constant. It is inversely proportional to the Doppler spread, so it shrinks as speed and carrier frequency rise.
- Complex baseband
- A representation of a band-pass radio signal as a complex-valued signal centred on 0 Hz. Its real and imaginary parts are the in-phase and quadrature components, which lets software process the signal at a sample rate set by its bandwidth rather than its carrier frequency.
- Conformance testing
- Testing a device or base station against the test cases in the standard, usually with calibrated instruments and network emulators, before certification.
- Connection interval
- The period between connection events in a Bluetooth LE link, when the two devices wake and exchange packets. Longer intervals save energy at the cost of latency.
- Constellation diagram
- A plot of the possible symbols of a digital modulation as points in the I-Q plane. Received samples plotted the same way show noise, phase errors and distortion at a glance.
- Control resource set (CORESET)
- A configured region of resource blocks and symbols in which an NR device searches for PDCCH. Unlike the LTE control region it need not span the full carrier.
- Convolutional code
- A code that produces output bits from a sliding window over the input stream using shift registers. LTE uses a tail-biting convolutional code for its broadcast channel and downlink control information.
- Correlation
- Sliding a known template along a received signal and summing the products at each lag. A peak marks where the template appears, which is the basis of detection and synchronisation.
- Costas loop
- A carrier recovery loop for suppressed-carrier signals such as BPSK and QPSK that derives its phase error from the I and Q branches. GNU Radio includes one as a standard block.
- Coverage enhancement
- Repeating transmissions many times so that a receiver can combine them and decode at very low SNR. NB-IoT and LTE-M use it to reach devices in basements and remote sites.
- CSI reference signal (CSI-RS)
- A configurable NR downlink reference signal that devices measure to report channel state and to support beam management and tracking.
- CSMA/CA
- Carrier sense multiple access with collision avoidance, the Wi-Fi channel access method. A station listens before transmitting and waits a random backoff if the medium is busy.
- Cyclic prefix (CP)
- A copy of the end of each OFDM symbol placed in front of it. If the channel's delay spread is shorter than the CP, multipath causes no ISI and the channel acts as a circular convolution.
- Cyclic redundancy check (CRC)
- A short checksum computed by polynomial division and appended to a block of data. Receivers use it to detect errors and to decide whether to send an acknowledgement.
D
- dBc
- A level in decibels relative to the carrier power. It is used for spurs, harmonics, phase noise and adjacent-channel leakage.
- dBi
- Antenna gain in decibels relative to an isotropic radiator. Gain in dBd is converted to dBi by adding the gain of a half-wave dipole.
- dBm
- Power in decibels relative to 1 mW, so 0 dBm is 1 mW and 30 dBm is 1 W. Absolute powers in a link budget are written in dBm.
- dBW
- Power in decibels relative to 1 W. A value in dBW is 30 dB lower than the same power in dBm.
- DC offset
- A constant offset at 0 Hz in a direct-conversion receiver, caused by LO leakage self-mixing and circuit mismatch. On an SDR display it shows as a spike at the tuned centre frequency.
- Decibel (dB)
- A logarithmic unit for a power ratio, equal to 10 log10 of the ratio. Gains and losses in dB add, which makes link calculations a matter of addition.
- Decimation
- Lowering the sample rate of a signal by an integer factor after low-pass filtering it. SDR receivers decimate the ADC stream so the host only handles the bandwidth it needs.
- Decision-directed tracking
- Using the receiver's own symbol decisions as if they were pilots to keep updating phase, timing or channel estimates. It works once the error rate is low and fails quickly when it is not.
- Delay spread
- A measure of how far apart in time the multipath copies of a signal arrive, commonly given as the RMS delay spread. Larger delay spread means a narrower coherence bandwidth.
- Demodulation reference signal (DMRS)
- An NR reference signal sent inside a data or control allocation so the receiver can estimate the channel for that allocation. It is precoded with the data, so it captures the effective channel.
- DFT-spread OFDM (DFT-s-OFDM)
- OFDM in which data symbols pass through a DFT before subcarrier mapping, which gives a single-carrier-like signal with lower PAPR. NR calls this step transform precoding and offers it on the uplink.
- Digital beamforming
- Beamforming in which every element has its own converter chain and the weights are applied to the digital samples. It allows many simultaneous beams at the cost of power and data rate.
- Digital predistortion (DPD)
- Applying an inverse model of the power amplifier's non-linearity to the digital signal before the DAC, so the amplified output comes out close to linear. It lets the amplifier run with less back-off and higher efficiency.
- Digital-to-analogue converter (DAC)
- The converter that turns digital samples into an analogue waveform in a transmitter. Its output is followed by a reconstruction filter that removes images.
- Dipole antenna
- An antenna made of two conductors fed at the centre, most often half a wavelength long in total. It has a doughnut-shaped pattern with nulls along its axis.
- Direct-conversion (zero-IF) receiver
- A receiver that mixes the RF signal straight to complex baseband with an I/Q mixer. It needs few filters and suits integrated transceivers, but suffers from DC offset and I/Q imbalance.
- Direct-sequence spread spectrum (DSSS)
- Spreading each data bit over many shorter chips from a pseudo-random code, which widens the bandwidth. The receiver correlates with the same code to recover the data and suppress narrowband interference.
- Direct-to-device (D2D satellite)
- Satellite service that reaches ordinary phones or IoT modules without a dish, either through 3GPP NTN or through proprietary systems using mobile spectrum.
- Diversity
- Sending or receiving the same information over several independently fading branches, in space, time, frequency or polarisation. The chance that all branches fade together is small, so the error rate falls faster with SNR.
- Doppler pre-compensation
- Shifting the transmit frequency and timing in advance, from known satellite ephemeris and the device's own position, so the signal arrives at the satellite close to nominal.
- Doppler shift
- The change in received frequency caused by relative motion between transmitter, receiver or reflectors. Its maximum value is the speed divided by the wavelength.
- Doppler spread
- The range of Doppler shifts present when multipath components arrive from different directions relative to the motion. It sets how quickly the channel changes.
- Downlink
- The direction from the network to a user device, for example from a base station or access point to a phone.
- Downlink control information (DCI)
- The message on the PDCCH that tells a device which resources, MCS and HARQ process to use for a downlink or uplink transmission. NR encodes it with a polar code.
- DPD
- See digital predistortion.
- Drive test
- Measuring network coverage and performance by driving a route with test phones or scanners that log RSRP, SINR, throughput and events.
- Duplexer
- A three-port filter that lets an FDD radio transmit and receive through one antenna at the same time by separating the two bands.
- Duplexing
- The method a link uses to separate the two directions of communication. The two common schemes are frequency division and time division.
- Duty cycle
- The fraction of time a transmitter is active. In some licence-exempt bands regulators cap it, which limits how often a device may send.
- Dynamic range
- The span between the weakest signal a receiver can detect and the strongest it can handle without unacceptable distortion, in dB. Spurious-free dynamic range uses the level of the largest spur as the upper limit.
- Dynamic spectrum sharing (DSS)
- Running LTE and NR on the same carrier, with the scheduler splitting resources between them from moment to moment.
E
- Eavesdropping
- Passive interception of radio transmissions. Link-layer encryption protects the content, but metadata such as timing and identifiers may still leak.
- Eb/N0
- Energy per bit divided by noise power spectral density, a normalised SNR that allows fair comparison of schemes with different rates and bandwidths. It equals SNR times bandwidth over bit rate.
- Effective isotropic radiated power (EIRP)
- The transmit power an isotropic antenna would need to produce the same power density as the real antenna in its main beam. In dB terms it is the power into the antenna in dBm plus the antenna gain in dBi, less any feed loss.
- Effective number of bits (ENOB)
- The resolution an ideal ADC would need to match the measured signal-to-noise-and-distortion of a real one. It is always lower than the nominal bit count.
- eMTC
- See LTE-M.
- Enhanced mobile broadband (eMBB)
- One of the three IMT-2020 usage scenarios in ITU-R M.2083, covering high data rates for people using mobile devices.
- eNodeB
- The LTE base station. It hosts the radio protocols and connects to the core network and to neighbouring eNodeBs.
- Ephemeris
- Data that describe a satellite's orbit so its position and velocity can be computed at any time. NR NTN broadcasts it so devices can correct their timing and frequency.
- Equalisation
- Undoing the distortion the channel adds to a signal so that symbols can be decided correctly. In OFDM it is one complex division per subcarrier, while single-carrier links need a filter that spans the delay spread.
- Error vector magnitude (EVM)
- The RMS distance between received constellation points and their ideal positions, as a percentage or in dB. Standards use it as a transmitter quality limit for each modulation order.
- Es/N0
- Energy per symbol divided by noise power spectral density. It equals Eb/N0 times the number of information bits carried per symbol.
- ETSI
- The European Telecommunications Standards Institute, a European standards body and one of the 3GPP organisational partners. It also publishes 3GPP specifications under its own numbering.Hub: Careers and industry. Related: 3rd Generation Partnership Project (3GPP)
- Evolved Packet Core (EPC)
- The all-IP core network behind LTE, which handles mobility, sessions, authentication and the connection to external packet networks.
- Extended cyclic prefix
- A longer cyclic prefix for channels with large delay spread, at the cost of fewer symbols per slot. TS 38.211 allows it in NR only with 60 kHz subcarrier spacing.
- Extended discontinuous reception (eDRX)
- A cellular sleep scheme that stretches the interval between paging checks, trading reachability delay for battery life.
- Extremely large-scale MIMO (XL-MIMO)
- Research on arrays so large in wavelengths that users fall in the radiating near field, where beams can focus on a point rather than a direction.
- Eye diagram
- An oscilloscope-style plot of many symbol periods of a signal overlaid on each other. A wide-open eye shows little noise and ISI, and the best sampling instant is where it opens most.
F
- Fade margin
- Extra link margin set aside to cover fading and shadowing so that the link meets a target availability. Its size comes from the fading statistics and the outage probability you accept.
- Fading
- Variation of received signal strength over time, frequency or position caused by multipath and movement. Small-scale fading changes over distances of a wavelength, while shadowing changes over tens of metres.
- False base station
- An unauthorised transmitter that imitates a legitimate cell to make devices attach, used to track identities or force a downgrade to weaker security.
- Far field
- The region far enough from an antenna that the wavefront is close to planar and the radiation pattern no longer changes with distance. Antenna gain and patterns are defined there.
- Fast Fourier transform (FFT)
- An efficient algorithm for computing the discrete Fourier transform, needing on the order of N log N operations for N points. Every OFDM modem and spectrum display relies on it.
- Feeder link
- The link between a satellite and its ground gateway. The link between the satellite and user devices is the service link.
- FFT size
- The number of points in the inverse FFT that generates each OFDM symbol. The sample rate equals the FFT size times the subcarrier spacing, and unused bins at the edges form guard bands.
- Field-programmable gate array (FPGA)
- A chip of reconfigurable logic that many SDRs use between the converters and the host for decimation, filtering and timing. Moving processing into it reduces the data rate the host must handle.
- Finite impulse response (FIR) filter
- A digital filter whose output is a weighted sum of a finite number of past inputs. FIR filters can have exactly linear phase, which keeps pulse shapes intact.Hub: Signals and spectra. Related: Infinite impulse response (IIR) filter, Pulse shaping, Decimation
- Flat fading
- Fading in which the whole signal bandwidth sees the same channel gain, because the bandwidth is smaller than the coherence bandwidth. The channel can then be corrected with one complex coefficient.
- Flowgraph
- A directed graph of signal-processing blocks connected by sample streams, as used in GNU Radio. Sources produce samples, sinks consume them, and the scheduler moves data between blocks.
- Forward error correction (FEC)
- Adding structured redundancy to data before transmission so the receiver can correct errors without asking for a resend. The code rate sets how much redundancy is added.Hub: Channel coding. Related: Code rate, Low-density parity-check (LDPC) code, Polar code, Coding gain
- Fourier transform
- A mathematical operation that expresses a signal as a sum of complex sinusoids, mapping it from time to frequency. The discrete Fourier transform is its sampled, finite-length version.
- Frame synchronisation
- Finding where a packet or frame starts in the received sample stream, usually by correlating against a known preamble or sequence.
- Free-space path loss
- The path loss between isotropic antennas with nothing in the way, caused only by the spreading of the wavefront. ITU-R P.525 gives it as 32.4 + 20 log10(f) + 20 log10(d) dB with f in MHz and d in km.
- Frequency division duplex (FDD)
- A duplexing scheme in which uplink and downlink run at the same time on two separate frequency bands. The device needs a duplexer or filter to keep its own transmitter out of its receiver.
- Frequency hopping
- Changing the carrier frequency in a pattern known to both ends, so that interference or fading on one channel affects only some packets. Bluetooth hops among its data channels.
- Frequency modulation (FM)
- Analogue modulation in which the carrier frequency follows the message signal. Broadcast FM is a common first receiver project for SDR users.
- Frequency range 1 (FR1)
- The NR frequency range from 410 MHz to 7125 MHz, as defined in TS 38.104 and TS 38.101-1 Release 19. Most NR deployments, such as band n78 at 3300-3800 MHz, are in FR1.
- Frequency range 2 (FR2)
- The NR millimetre-wave range, split into FR2-1 from 24250 MHz to 52600 MHz and FR2-2 from 52600 MHz to 71000 MHz in TS 38.104 Release 19. FR2-2 was added when Release 17 extended NR up to 71 GHz.
- Frequency synthesiser
- A circuit, usually built around a PLL, that generates a tunable LO frequency locked to a crystal reference.
- Frequency-selective fading
- Fading in which different parts of the signal bandwidth see different gains, because the delay spread is large compared with the symbol period. It causes intersymbol interference in single-carrier links and is handled per subcarrier in OFDM.
- Frequency-shift keying (FSK)
- A modulation that sends data by switching the carrier between discrete frequencies. Its constant envelope suits cheap, efficient power amplifiers.
- Fresnel zone
- An ellipsoidal region around the direct path between two antennas within which obstacles affect the received signal. Point-to-point links are planned to keep most of the first Fresnel zone clear.
- Friis transmission equation
- The formula for received power in free space: transmit power times both antenna gains times the square of wavelength over 4π times distance. In dB it becomes a sum of gains and losses, which is the basis of a link budget.
- Functional split
- The choice of where to divide base station processing between the radio unit, distributed unit and central unit. Lower splits move more processing to central hardware but need more fronthaul capacity.
G
- G/T (figure of merit)
- A receiving station's antenna gain divided by its system noise temperature, in dB/K. It sets the uplink or downlink SNR in a satellite link budget.
- Gardner timing error detector
- A timing error detector that uses two samples per symbol and works without first knowing the carrier phase. It is a common choice for PSK and QAM receivers.
- GATT
- The Generic Attribute Profile, the Bluetooth LE framework that organises a device's data into services and characteristics that a client can read, write or subscribe to.Hub: Short-range radio, Bluetooth and UWB. Related: Bluetooth Low Energy (BLE)
- Gaussian frequency-shift keying (GFSK)
- FSK in which the data pulses pass through a Gaussian filter before modulating the frequency, which narrows the spectrum. Bluetooth Low Energy uses it.
- Geostationary orbit (GEO)
- An orbit about 35 786 km above the equator in which a satellite appears fixed in the sky. It gives wide coverage but a long propagation delay.
- Global navigation satellite system (GNSS)
- A satellite constellation such as GPS, Galileo, GLONASS or BeiDou that lets receivers compute position and time. NR NTN devices use their GNSS fix for pre-compensation.
- gNB
- The NR base station. It can be split into a central unit and distributed units, with radio units at the antenna.
- GNU Radio
- A free and open-source toolkit for building signal-processing flowgraphs from blocks, with a C++ core and Python bindings. It works with simulated signals, recordings and many SDR front ends.
- GNU Radio Companion (GRC)
- The graphical editor for GNU Radio, in which you place and connect blocks and then generate a Python flowgraph from the picture.
- Gray coding
- A bit-to-symbol mapping in which neighbouring constellation points differ in one bit. The most likely symbol errors then cause only one bit error.
- Guard band
- Unused spectrum at the edges of a channel that lets filters roll off and keeps emissions out of neighbouring channels. In OFDM it is made by leaving edge subcarriers empty.
H
- Hamming distance
- The number of positions in which two codewords differ. A code whose minimum distance is d can correct up to (d − 1)/2 errors, rounded down.Hub: Channel coding. Related: Block code
- Handover
- Transferring a connected device from one cell to another without dropping the connection, triggered by measurements such as RSRP.
- A collision case in which two stations can both reach an access point but cannot hear each other, so carrier sensing fails to stop them transmitting together.
- High-altitude platform station (HAPS)
- A radio station on an aircraft or balloon in the stratosphere. TS 38.104 lists NR bands that can be applied for HAPS operation.
- Hybrid automatic repeat request (HARQ)
- A retransmission scheme that combines FEC with acknowledgements, and keeps failed attempts so they can be combined with the retransmission. LTE, NR and IEEE 802.11 amendments use it in different forms.Hub: Channel coding. Related: Chase combining, Incremental redundancy, Cyclic redundancy check (CRC)
- Hybrid beamforming
- A split between analogue phase shifting across sub-arrays and digital precoding across a smaller number of converter chains. Most mmWave base stations use some form of it.
- Hyper-reliable and low-latency communication (HRLLC)
- The IMT-2030 usage scenario in ITU-R M.2160 that extends URLLC to tighter reliability, latency and timing requirements.
I
- I/Q imbalance
- Gain and phase mismatch between the I and Q paths of a quadrature mixer. It creates a mirror image of the signal across 0 Hz in the baseband spectrum.
- ICNIRP
- The International Commission on Non-Ionizing Radiation Protection, which publishes guidelines for limiting exposure to electromagnetic fields. Many national regulators base their limits on them.Hub: Spectrum, security and regulation. Related: Specific absorption rate (SAR)
- IEEE 802.11
- The IEEE family of standards for wireless LAN MAC and PHY, maintained by the IEEE 802.11 working group and extended by lettered amendments such as 802.11ax and 802.11be.
- IEEE 802.15.4
- The IEEE standard for low-rate wireless personal area networks that underlies Zigbee and Thread, and that also defines the UWB PHYs used for ranging.
- IEEE 802.15.4z
- The amendment that enhanced the IEEE 802.15.4 UWB PHYs and MAC for secure, more accurate ranging, with changes to the high and low pulse repetition frequency modes.
- Image frequency
- The unwanted input frequency that a mixer translates to the same output frequency as the wanted signal, on the other side of the LO. It must be filtered out before mixing or rejected with an I/Q structure.
- IMT-2020
- The ITU-R name for the fifth generation of International Mobile Telecommunications, whose vision is set out in Recommendation ITU-R M.2083.
- IMT-2030
- The ITU-R framework for the sixth generation of IMT, set out in Recommendation ITU-R M.2160. It defines six usage scenarios, including integrated sensing and communication and artificial intelligence and communication.
- Incremental redundancy
- A HARQ method in which each retransmission sends different coded bits, lowering the effective code rate as attempts accumulate.Hub: Channel coding. Related: Hybrid automatic repeat request (HARQ), Chase combining, Rate matching
- Infinite impulse response (IIR) filter
- A digital filter that feeds back past outputs as well as past inputs. It reaches a sharp response with fewer coefficients than an FIR filter but does not have linear phase.Hub: Signals and spectra. Related: Finite impulse response (FIR) filter
- Integrated sensing and communication (ISAC)
- Using one radio system and waveform both to communicate and to sense the environment, for example to detect, range or track objects. It is one of the IMT-2030 usage scenarios in ITU-R M.2160.
- Inter-carrier interference (ICI)
- Leakage between OFDM subcarriers when orthogonality is lost, for example through carrier frequency offset, Doppler or phase noise. It acts as extra noise on every subcarrier.
- Interference
- Unwanted signals from other transmitters that fall within a receiver's bandwidth. Unlike thermal noise it is often structured and can sometimes be cancelled or avoided.
- Interleaving
- Reordering coded bits before transmission so that a burst of errors from a deep fade is spread over many codewords. The receiver reverses the order before decoding.
- Intermodulation distortion
- Unwanted products at sum and difference frequencies created when two or more signals pass through a non-linear stage. Third-order products land close to the wanted signals and are hard to filter.
- Interpolation
- Raising the sample rate of a signal by inserting samples and filtering out the spectral images that result. Transmit chains interpolate up to the DAC rate.
- Intersymbol interference (ISI)
- Distortion in which energy from one symbol spills into the decision instants of neighbouring symbols. It comes from multipath, bandwidth limits or poor pulse shaping.
- IQ samples
- Pairs of in-phase (I) and quadrature (Q) values that together form one complex baseband sample. SDRs deliver them as streams of complex numbers, commonly 32-bit floats or 16-bit integers per component.
- ISI
- See intersymbol interference.
- ISM band
- Bands designated in the Radio Regulations for industrial, scientific and medical equipment, such as the 2.4 GHz band, which licence-exempt radios share with microwave ovens and other users.
- Isotropic radiator
- An ideal antenna that radiates equally in all directions. It does not exist physically but is the reference for antenna gain in dBi.
- ITU Radiocommunication Sector (ITU-R)
- The part of the International Telecommunication Union that manages the international Radio Regulations and publishes Recommendations on propagation, sharing and mobile systems. It defines the IMT families.
J
- Jamming
- Deliberate transmission of interference to deny a receiver use of a channel. Regulators restrict or ban the use of jammers, and the rules differ by country.
L
- Latency
- The time a packet takes to cross a link or a network, usually stated one-way or round trip. In radio systems it includes frame alignment, scheduling, processing and retransmission delays as well as propagation.
- LDPC
- See low-density parity-check code.
- LDPC base graph
- A small prototype matrix that is expanded by a lifting size into the full parity-check matrix of an NR LDPC code. TS 38.212 defines two base graphs and selects between them by transport block size and code rate.
- Least-squares (LS) channel estimation
- Estimating the channel at each pilot by dividing the received symbol by the known transmitted symbol. It is simple and unbiased but noisy at low SNR.
- libiio
- Analog Devices' library for Linux Industrial I/O devices, used to stream samples from and configure the ADALM-Pluto and AD936x-based boards, locally or over a network.
- Licence-exempt spectrum
- Spectrum that anyone may use without an individual licence, provided their equipment meets technical rules on power and access. Wi-Fi, Bluetooth and LoRaWAN mostly use it.
- Licensed spectrum
- Spectrum a regulator grants for exclusive use by a licensee in an area, often by auction. It gives operators control of interference.
- Line of sight (LOS)
- A propagation condition in which there is an unobstructed direct path between the two antennas. A LOS link has a strong dominant component and usually fades less.
- Link adaptation
- Changing modulation, coding, rank and power to suit current channel conditions. It keeps the block error rate near a target while maximising throughput.
- Link budget
- An accounting of every gain and loss between transmitter and receiver, in dB, to find the received power and the margin over the receiver's sensitivity. It covers transmit power, antenna gains, cable losses, path loss and fade margins.
- Link margin
- The amount by which received power exceeds the receiver sensitivity, in dB. It is the reserve that absorbs fading, shadowing and modelling error.
- Local oscillator (LO)
- The oscillator that drives a mixer to set the frequency translation. Its accuracy and phase noise carry straight through to the signal.
- Log-likelihood ratio (LLR)
- The natural log of the ratio of the probabilities that a bit is 0 or 1 given the received signal. Its sign gives the hard decision and its size gives the confidence.
- Long Term Evolution (LTE)
- The 3GPP 4G radio standard, first specified in Release 8, with an OFDMA downlink and an SC-FDMA uplink. It uses a 15 kHz subcarrier spacing and turbo codes for its data channels.Hub: Cellular 4G LTE. Related: eNodeB, Evolved Packet Core (EPC), Single-carrier FDMA (SC-FDMA), Turbo code
- LoRa
- A proprietary chirp spread spectrum modulation from Semtech that forms the physical layer under LoRaWAN. Its processing gain lets it decode signals below the noise floor.
- LoRa Alliance
- The industry association that develops the LoRaWAN specification and runs device certification.Hub: Careers and industry. Related: LoRaWAN
- LoRaWAN
- The LoRa Alliance's MAC-layer protocol and system architecture for LoRa networks. End devices reach gateways in a star-of-stars topology, and gateways forward packets over IP to a network server.
- LoRaWAN gateway
- A receiver that demodulates LoRa packets from any device in range and forwards them over IP to the network server, and relays downlinks the other way.
- Low Earth orbit (LEO)
- The lowest orbits used for communication satellites, well below MEO. They give short delay and lower path loss, but the satellites move fast across the sky, which brings large Doppler shifts and frequent handovers.
- Low-density parity-check (LDPC) code
- A block code defined by a sparse parity-check matrix and decoded iteratively by message passing. NR uses LDPC codes for its uplink and downlink shared channels and paging channel, per TS 38.212.
- Low-noise amplifier (LNA)
- The first amplifier after the antenna in a receiver, designed for low noise figure. Its gain lowers the noise contribution of every stage after it.
- Low-power wide-area network (LPWAN)
- A class of networks for battery-powered devices that send small amounts of data over long distances. LoRaWAN, NB-IoT and LTE-M are the main examples.
- LTE-M
- A reduced-bandwidth LTE device category for machine-type communication, also called eMTC or Cat-M1. It supports mobility and voice, unlike NB-IoT.
M
- Massive machine-type communication (mMTC)
- The IMT-2020 usage scenario in ITU-R M.2083 for very large numbers of low-cost, low-power devices that send small amounts of data.
- Massive MIMO
- MIMO with many more base station antennas than users served at once. The large array gives high beamforming gain and lets simple linear precoding separate many users.
- Matched filter
- The receive filter whose impulse response is the time-reversed conjugate of the expected pulse. It gives the highest possible SNR at the sampling instant in white noise.
- Maximum coupling loss (MCL)
- The largest total loss between transmitter and receiver antenna ports at which a link still works. It is the usual coverage metric for cellular IoT.
- Maximum ratio combining (MRC)
- A receive-diversity method that weights each branch by the conjugate of its channel gain before summing. It maximises the output SNR in noise-limited conditions.
- Medium access control (MAC) layer
- The protocol layer above the PHY that decides who transmits when and on which resources. It handles scheduling, retransmissions and multiplexing of logical channels.
- Medium Earth orbit (MEO)
- Orbits between low Earth orbit and geostationary altitude, used by navigation constellations and some broadband systems.
- Millimetre wave (mmWave)
- Radio frequencies whose wavelength is between 1 mm and 10 mm, which corresponds to 30 GHz to 300 GHz. In 5G usage the term is applied loosely to the NR bands above 24 GHz, which suffer high path loss and blockage and rely on beamforming.
- Minimum-shift keying (MSK)
- Continuous-phase FSK with the smallest frequency separation at which the two tones stay orthogonal over a symbol. It has a constant envelope and a compact spectrum.
- Mixer
- A circuit that multiplies an RF signal by a local oscillator to shift it in frequency. Receivers use mixers to down-convert and transmitters to up-convert.
- MMSE channel estimation
- Channel estimation that uses the channel's correlation statistics and the noise level to minimise mean squared error. It beats least squares at low SNR but needs those statistics.
- MMSE equaliser
- An equaliser that balances removal of ISI against noise enhancement by including the noise level in its weights. At high SNR it approaches the zero-forcing solution.
- Mobile network operator (MNO)
- A company that holds spectrum licences and runs a public mobile network.
- Modulation and coding scheme (MCS)
- An index that selects the modulation order and code rate for a transmission from a table in the standard. The scheduler picks it from CQI reports and HARQ feedback.
- Mueller-Müller timing error detector
- A timing error detector that uses one sample per symbol and the symbol decisions. It needs carrier phase lock first.
- Multi-link operation (MLO)
- A Wi-Fi 7 feature that lets a device and access point use links in more than one band at once or switch between them, for load balancing and lower latency.Hub: Wi-Fi. Related: Wi-Fi 7 (IEEE 802.11be)
- Multi-user MIMO (MU-MIMO)
- MIMO in which a base station or access point serves several users on the same time and frequency resources with different spatial beams. It needs accurate channel knowledge to keep the users' signals apart.
- Multipath
- Arrival of the transmitted signal at the receiver over several paths with different delays, amplitudes and phases. The copies add constructively or destructively, which causes fading and intersymbol interference.
- Multiple-input multiple-output (MIMO)
- Using several antennas at both transmitter and receiver to exploit the spatial dimension of the channel. It provides diversity, spatial multiplexing or beamforming gain depending on how the signals are processed.
N
- Narrowband IoT (NB-IoT)
- A 3GPP cellular IoT radio introduced in Release 13 with a channel bandwidth limited to 200 kHz, per TS 36.300. It can run standalone, in an LTE guard band or inside an LTE carrier.
- Near field
- The region close to an antenna where the field structure still changes with distance and the plane-wave approximation fails. Very large arrays at high frequencies can put users inside it.
- Near-field communication (NFC)
- A very short-range link that couples two devices through their magnetic fields, used for payments, access cards and pairing.Hub: Short-range radio, Bluetooth and UWB. Related: Near field
- Network digital twin
- A software model of a live network, kept up to date with measurements, used to test changes before applying them. Ray tracing is often used for the radio part.
- Network slicing
- Running several logical networks with different service characteristics on one shared physical network. It relies on the 5G Core and standalone NR.
- New Radio
- See 5G NR.
- Noise figure
- How much a component or receiver degrades the SNR passing through it, in dB. It is 10 log10 of the noise factor, the ratio of input SNR to output SNR at 290 K.
- Noise floor
- The total noise power at the receiver input within the signal bandwidth, which is thermal noise plus the noise figure. Signals much below it cannot be recovered without processing gain.
- Noise temperature
- The temperature at which a resistor would produce the same noise power as the component or antenna being described. It is a convenient scale for low-noise systems such as satellite ground stations.
- Non-line of sight (NLOS)
- A propagation condition in which the direct path is blocked and energy arrives only by reflection, diffraction and scattering. NLOS links have higher path loss and deeper fades.
- Non-standalone (NSA)
- An NR deployment in which an LTE carrier and the LTE core handle control while NR adds capacity. It was the first NR option 3GPP completed, in late 2017, ahead of the rest of Release 15.
- Non-terrestrial network (NTN)
- A 3GPP network in which satellites or high-altitude platforms carry the radio link to devices. Support for NR and IoT over NTN was introduced in Release 17 and extended in later releases.
- NR operating band
- A numbered band, written n followed by a number, that defines the uplink and downlink frequency ranges and the duplex mode NR may use there. TS 38.101-1, TS 38.101-2 and TS 38.104 list them.
- Numerology
- In NR, the combination of subcarrier spacing and cyclic prefix length indexed by μ. Larger μ means shorter symbols and slots, which suits high bands and low latency, while smaller μ tolerates longer delay spread.
- Nyquist rate
- Twice the highest frequency in a real signal, which is the minimum sample rate that lets the signal be reconstructed without aliasing. A complex baseband signal of bandwidth B needs a complex sample rate of at least B.
O
- O-RAN
- The architecture and specifications from the O-RAN Alliance, founded in February 2018, for open, interoperable and virtualised radio access networks. It defines open interfaces between radio units, distributed units, central units and RAN intelligent controllers.
- OFDM symbol
- One block of samples produced by an inverse FFT over all subcarriers, plus its cyclic prefix. It is the basic time unit of an OFDM frame.
- On-off keying (OOK)
- The simplest amplitude modulation, in which the carrier is switched on for a 1 and off for a 0. Low-cost remotes and some wake-up radios use it.
- Open fronthaul
- The O-RAN interface between a radio unit and a distributed unit, specified so that units from different vendors can work together.
- Orthogonal frequency-division multiple access (OFDMA)
- Multiple access in which different users get different groups of subcarriers, and often different time slots, within one OFDM signal. LTE and NR downlinks and IEEE 802.11ax use it.
- Orthogonal frequency-division multiplexing
- See OFDM.
- Orthogonal frequency-division multiplexing (OFDM)
- A multicarrier scheme that sends data on many closely spaced subcarriers that are orthogonal over one symbol period, generated with an inverse FFT. With a cyclic prefix, a frequency-selective channel reduces to one complex gain per subcarrier.
- Orthogonal time frequency space (OTFS)
- A research modulation that places symbols on a delay-Doppler grid rather than a time-frequency grid. It is studied for high-mobility channels where Doppler spread is large.
- Overflow (dropped samples)
- Loss of receive samples because the host did not read them from the SDR fast enough. UHD reports it by printing the letter O, or in some cases D, to the console.
P
- Path loss
- The reduction in signal power between transmit and receive antennas, expressed in dB. It grows with distance and frequency and with obstacles in the path.Hub: Antennas and propagation. Related: Free-space path loss, Path-loss exponent, Shadowing, Link budget
- Path-loss exponent
- The power n in a model where path loss grows as distance to the n. Free space gives n = 2, and cluttered urban or indoor paths usually fit higher values.
- Peak-to-average power ratio (PAPR)
- The ratio of a signal's peak power to its mean power, in dB. OFDM has a high PAPR because many subcarriers can add in phase, which forces the power amplifier to back off.Hub: OFDM and multicarrier. Related: Power amplifier (PA), Back-off, DFT-spread OFDM (DFT-s-OFDM), Clipping
- Phase noise
- Random fluctuation of an oscillator's phase, seen as skirts around the carrier in the spectrum and given in dBc/Hz at an offset. It rotates constellation points and causes inter-carrier interference in OFDM, more so at high carrier frequencies.
- Phase-locked loop (PLL)
- A feedback loop that adjusts an oscillator until its phase follows a reference. PLLs synthesise local oscillator frequencies in hardware and track carrier phase in software receivers.
- Physical cell identity (PCI)
- The number that identifies a cell at the physical layer, derived from the synchronisation signals. Neighbouring cells need distinct PCIs.
- Physical downlink control channel (PDCCH)
- The downlink channel that carries scheduling commands and other downlink control information to devices.
- Physical downlink shared channel (PDSCH)
- The channel that carries user data and some system information on the downlink. In NR it is LDPC coded and demodulated with its own DMRS.
- Physical layer (PHY)
- The lowest layer of a protocol stack, which turns bits into waveforms and back. It covers coding, modulation, multiplexing, synchronisation and the reference signals the receiver needs.
- Physical random access channel (PRACH)
- The uplink channel a device uses to send a random access preamble when it first connects or regains timing. The preambles are built from Zadoff-Chu sequences.
- Physical uplink control channel (PUCCH)
- The uplink channel for HARQ acknowledgements, scheduling requests and CSI reports when no PUSCH is scheduled.
- Physical uplink shared channel (PUSCH)
- The channel that carries user data, and sometimes control information, on the uplink. In NR it can use CP-OFDM or DFT-s-OFDM.
- Physical-layer security
- Methods that use properties of the radio channel, such as its randomness and reciprocity, to generate keys or limit what an eavesdropper can receive.
- Pilot subcarrier
- A subcarrier that carries known symbols instead of data so the receiver can estimate the channel and track phase. Wi-Fi uses fixed pilot subcarriers, while LTE and NR scatter reference signals across the grid.
- Polar code
- A block code that uses channel polarisation to place information bits on reliable bit positions and fixed bits on unreliable ones. NR uses polar codes for the broadcast channel and for downlink and most uplink control information, per TS 38.212.
- Polarisation
- The orientation of a wave's electric field, for example vertical, horizontal or circular. A mismatch between the polarisations of two antennas adds loss to the link.
- Power amplifier (PA)
- The final transmit stage that raises the signal to the power fed to the antenna. It is usually the biggest consumer of power in a radio and its non-linearity limits EVM and adjacent-channel leakage.
- Power saving mode (PSM)
- A cellular IoT state in which the device stays registered but turns its radio off for long periods and cannot be reached until it wakes.Hub: LPWAN and IoT. Related: Extended discontinuous reception (eDRX), Narrowband IoT (NB-IoT), LTE-M
- Power spectral density (PSD)
- The distribution of a signal's power over frequency, in watts per hertz or dBm/Hz. Thermal noise has a flat PSD, which is why noise power grows with bandwidth.
- Preamble
- A known sequence sent at the start of a packet so the receiver can detect it, set gain and estimate timing, frequency offset and the channel.
- Precoding
- Multiplying the transmitted layers by a matrix chosen from channel knowledge before mapping them to antennas. It covers beamforming, spatial multiplexing and interference suppression between users.Hub: MIMO and beamforming. Related: Codebook, Zero-forcing, SVD precoding, Channel state information (CSI)
- Precoding matrix indicator (PMI)
- The part of a CSI report in LTE and NR that tells the base station which codebook entry the device prefers.
- Private network
- A cellular network deployed for one organisation, such as a factory or port, on licensed, shared or local spectrum.
- Processing gain
- The SNR improvement a receiver gets by spreading a signal over a wider bandwidth or a longer time and then despreading or integrating it. It lets spread-spectrum systems such as LoRa work below the noise floor.
- PSS and SSS
- The primary and secondary synchronisation signals. A device finds them first to get symbol timing, frequency and the physical cell identity.
- Pulse shaping
- Filtering the symbol stream so that each symbol has a band-limited shape. It confines the transmitted spectrum while keeping intersymbol interference at the sampling instants close to zero.
- Puncturing
- Deleting some coded bits before transmission to raise the code rate. The decoder treats the missing bits as unknown.
Q
- Quadrature amplitude modulation (QAM)
- A family of modulations that place symbols on a grid in the I-Q plane, varying both amplitude and phase. Higher orders such as 256-QAM carry more bits per symbol but need a higher SNR.
- Quadrature phase-shift keying (QPSK)
- A modulation with four symbols spaced 90° apart in phase, carrying two bits per symbol. With Gray coding its bit error rate against Eb/N0 equals that of BPSK.
- Quantisation noise
- The error introduced when a continuous amplitude is rounded to one of a finite set of levels by an ADC. For an ideal converter each extra bit lowers it by about 6 dB.
R
- Radiation pattern
- A plot of an antenna's gain against direction, usually shown as horizontal and vertical cuts. It shows the main lobe, sidelobes and nulls.
- Radio access network (RAN)
- The part of a mobile network that connects devices over the air to the core, made up of base stations and their transport. In 5G it is split into radio units, distributed units and central units.
- Radio frame
- The 10 ms time structure of NR and LTE, made of ten 1 ms subframes. In NR each subframe is further divided into slots according to the numerology.
- Radio link
- A path that carries information from a transmitter to a receiver through electromagnetic waves. Its quality depends on transmit power, antennas, propagation loss, noise and interference at the receiver.
- Rain attenuation
- Loss caused by raindrops absorbing and scattering a radio wave, which grows with frequency and rain rate. ITU-R P.838 gives the specific attenuation in dB/km as a function of rain rate.
- Raised-cosine filter
- A pulse-shaping response whose impulse response is zero at every other symbol instant, so it causes no intersymbol interference when sampled correctly. Its roll-off factor sets the excess bandwidth beyond half the symbol rate.
- RAN intelligent controller (RIC)
- An O-RAN platform that hosts applications to monitor and optimise the RAN, split into a near-real-time and a non-real-time controller.Hub: Careers and industry. Related: O-RAN
- Rate matching
- Puncturing or repeating coded bits so that the output exactly fills the resources allocated for a transmission. It also selects which redundancy version is sent on each HARQ attempt.
- Rayleigh fading
- A fading model for many scattered paths with no dominant one, in which the complex channel gain is Gaussian and its amplitude has a Rayleigh distribution. It is the usual model for NLOS urban links.
- Receiver
- The part of a radio that captures a weak signal from the antenna, amplifies and down-converts it, and recovers the transmitted data. Its noise figure and selectivity set how weak a signal it can use.Hub: Wireless foundations. Related: Transmitter, Transceiver, Low-noise amplifier (LNA), Receiver sensitivity
- Receiver sensitivity
- The lowest input power at which a receiver meets a stated performance target, such as a given bit or packet error rate. It equals the noise floor plus the SNR the modulation and coding require.
- Reconfigurable intelligent surface (RIS)
- A panel of many passive elements whose reflection phase can be controlled to steer incoming radio waves towards a receiver. It is mainly a research and trial topic at the time of writing.
- Reduced capability (RedCap)
- A class of lower-complexity NR devices with fewer antennas and narrower bandwidth, introduced in Release 17 for wearables, sensors and similar devices.
- Reed-Solomon code
- A block code over multi-bit symbols that corrects bursts of errors well. It appears in storage, digital broadcasting and as an outer code in concatenated schemes.
- Reference signal
- Known symbols inserted at defined positions in the time-frequency grid for channel estimation, synchronisation and measurement. NR defines several types, each with its own purpose.
- Reference signal received power (RSRP)
- The average power of the cell-specific reference signal resource elements a device measures, in dBm. It is the main input to cell selection and handover.
- Reference signal received quality (RSRQ)
- A measure that relates RSRP to the total received power in the measurement bandwidth, so it falls as load and interference rise.
- Regenerative payload
- A satellite that demodulates and decodes the signal on board, so some or all base station functions run in orbit.Hub: Satellite and non-terrestrial networks. Related: Transparent payload
- Resource block (RB)
- A group of 12 consecutive subcarriers in LTE and NR, the basic unit for frequency-domain scheduling. Its width in hertz scales with the subcarrier spacing.Hub: OFDM and multicarrier. Related: Resource element (RE), Subcarrier spacing, Bandwidth part (BWP)
- Resource element (RE)
- The smallest unit of the LTE and NR time-frequency grid: one subcarrier for one OFDM symbol.
- Resource unit (RU)
- In IEEE 802.11ax and later, a group of subcarriers that an access point assigns to one station in an OFDMA transmission.
- Rician fading
- A fading model with one strong steady path plus scattered paths, in which the amplitude follows a Rice distribution. The K-factor, the ratio of the steady power to the scattered power, sets how deep the fades are.
- Roll-off factor
- The parameter, usually written α or β between 0 and 1, that sets how much wider a raised-cosine spectrum is than the minimum Nyquist bandwidth. The occupied bandwidth is the symbol rate times one plus the roll-off.
- Root-raised-cosine filter
- A filter whose frequency response is the square root of a raised cosine. Using one at the transmitter and a matched one at the receiver gives an overall raised-cosine response and maximises SNR.
- Round-trip delay (NTN)
- The time for a signal to reach the device via the satellite and return, which in GEO systems is hundreds of milliseconds. NR NTN extends timers and timing advance to cope with it.
- RTL-SDR
- A low-cost receive-only SDR built from a DVB-T television dongle chip with an 8-bit ADC. It is the usual first radio for listening and learning.
- RTS/CTS
- An optional Wi-Fi handshake in which a station sends a short Request to Send and the receiver answers Clear to Send, which reserves the medium and reduces hidden-node collisions.
S
- Sample rate
- The number of samples per second taken from a signal. For complex baseband samples, a rate of fs covers a span of fs hertz centred on the tuned frequency.
- Sampling clock offset
- A mismatch between the transmitter's and receiver's sample clocks, usually given in parts per million. It makes the timing drift over a packet and adds a phase slope across OFDM subcarriers.
- SAW filter
- A surface acoustic wave filter that uses acoustic waves on a piezoelectric substrate to give a sharp RF band-pass response in a small package. Phones use SAW and related BAW filters for band selection.Hub: RF front ends. Related: Duplexer
- Schmidl-Cox synchronisation
- An OFDM timing and frequency offset method that uses a training symbol with two identical halves. Correlating the halves gives the start of the symbol and the phase of the correlation gives the fractional CFO.
- Semantic communication
- A research topic that aims to transmit the meaning or task-relevant content of a message rather than every bit. Definitions vary between research groups at the time of writing.
- Service link
- The radio link between a satellite or HAPS and the user devices it serves.
- Shadowing
- Slow variation of received power around the distance-based path loss, caused by buildings, terrain and other large obstacles. It is usually modelled as a log-normal random variable in dB.
- Shannon capacity
- The highest error-free data rate a channel can support, C = B log2(1 + SNR) for a bandwidth B with additive white Gaussian noise. Practical links fall short of it by a margin that depends on coding and modulation.
- SigMF (Signal Metadata Format)
- An open specification for recording IQ data as a raw binary .sigmf-data file plus a JSON .sigmf-meta file. The metadata holds global fields such as data type and sample rate, a list of captures and optional annotations.
- Signal-to-interference-plus-noise ratio (SINR)
- Signal power divided by the sum of interference and noise power. In dense cellular and Wi-Fi networks interference often dominates, so SINR rather than SNR limits performance.
- Signal-to-noise ratio
- See SNR.
- Signal-to-noise ratio (SNR)
- The ratio of signal power to noise power at a given point in the receiver, usually in dB. It sets the error rate and the highest usable modulation order.
- Single-carrier FDMA (SC-FDMA)
- The name LTE uses for its DFT-spread OFDM uplink. It lowers handset PAPR compared with plain OFDMA.
- Single-user MIMO (SU-MIMO)
- MIMO in which all spatial layers on a resource go to one user. Its gain is limited by the number of antennas at the device.
- Slot
- The NR scheduling unit of 14 OFDM symbols with normal cyclic prefix. A 1 ms subframe holds 2^μ slots, so slots get shorter as subcarrier spacing grows.
- SoapySDR
- A vendor-neutral C++ and Python API with plug-in modules for many SDR devices. Code written against it can switch hardware by changing the device arguments.
- Soft-decision decoding
- Decoding that uses the reliability of each received bit, not only its hard 0 or 1 value. It gives a lower error rate than hard-decision decoding at the same SNR.
- Software radio
- See software-defined radio.
- Software-defined radio (SDR)
- A radio in which most signal processing after the converters is done in software or reprogrammable logic rather than fixed hardware. A general-purpose SDR has a tunable RF front end, ADCs and DACs, and streams IQ samples to a host computer.
- Sounding reference signal (SRS)
- An uplink reference signal the base station measures to estimate the uplink channel, and with TDD reciprocity the downlink channel as well.
- Spatial multiplexing
- Sending independent data streams from different antennas on the same frequency at the same time. In a rich scattering channel the peak rate grows with the smaller of the transmit and receive antenna counts.
- Specific absorption rate (SAR)
- The rate at which radio-frequency energy is absorbed per unit mass of body tissue, in W/kg. Regulators set SAR limits for devices used close to the body and require tests against them.
- Spectral efficiency
- Data rate per unit of bandwidth, in bit/s/Hz. It lets you compare systems with different channel widths on equal terms.
- Spectrogram
- A picture of how a signal's spectrum changes over time, built from a series of short FFTs. It is the quickest way to spot bursts, hopping and interference in a recording.
- Spectrum allocation
- The assignment of frequency bands to radio services, such as mobile, fixed or satellite, in the Radio Regulations and national tables. Allocations differ by country and region.
- Spectrum analyser
- An instrument that displays signal power against frequency. It is the basic tool for checking emissions, spurs and interference.
- Spectrum emission mask
- A limit line on the power a transmitter may emit at each frequency offset from its channel, set by a standard or regulator.
- Spectrum sensing
- Detecting whether a band is occupied, by energy detection or by looking for known signal features.
- Spectrum sharing
- Arrangements that let more than one user or technology use a band, through databases, sensing, priority tiers or coordination.
- Spoofing
- Transmitting a counterfeit signal that a receiver accepts as genuine, such as a fake GNSS signal or a false base station.
- Spreading factor (SF)
- The LoRa parameter that sets how many chips make up a symbol, as a power of two. Each step up roughly doubles time on air and adds sensitivity.
- Spurious emission
- Unwanted emission outside the necessary bandwidth, such as harmonics, mixer products and intermodulation, that is not part of the modulation spectrum. Regulators and standards cap it.
- SS/PBCH block (SSB)
- The NR synchronisation block that carries the primary and secondary synchronisation signals and the broadcast channel, spanning 4 OFDM symbols and 240 subcarriers. Base stations sweep it across beams for initial access.
- SSID
- Service set identifier, the network name an access point advertises and stations use to choose a network.
- Standalone (SA)
- An NR deployment that works without LTE, with NR connected to the 5G Core. Standalone was completed in Release 15 and is needed for features such as network slicing.
- Steering vector
- The set of relative phase shifts a plane wave from a given direction produces across the elements of an array. Applying its conjugate as weights points the beam in that direction.
- Sub-terahertz (sub-THz)
- The upper part of the millimetre-wave range, below the terahertz band, studied in research for very wide channels over short links. Path loss, absorption, phase noise and device efficiency are the main obstacles.
- Subcarrier
- One of the narrowband tones in an OFDM signal, each carrying one modulated symbol per OFDM symbol period.
- Subcarrier spacing
- The frequency gap between adjacent OFDM subcarriers, equal to the inverse of the useful symbol duration. NR defines spacings of 15 × 2^μ kHz for μ from 0 to 6, that is 15 kHz to 960 kHz, in TS 38.211 Release 19.
- Superheterodyne receiver
- A receiver that mixes the RF signal down to a fixed intermediate frequency for filtering and gain before final demodulation or digitisation. It gives good selectivity but needs image filtering.
- SVD precoding
- Precoding with the right singular vectors of the channel matrix, which turns a MIMO channel into parallel independent sub-channels. It is the textbook optimum for single-user MIMO with full channel knowledge.
- Symbol
- One discrete waveform state sent for a fixed period, carrying one or more bits. Each point in a constellation diagram is one possible symbol.
- Symbol error rate (SER)
- The fraction of received symbols that are decided wrongly. With Gray coding at high SNR the BER is close to the SER divided by the bits per symbol.
- Symbol rate
- The number of symbols sent per second, in baud. The bit rate is the symbol rate times the number of bits per symbol.
T
- Tapped delay line (TDL) model
- A channel model made of a set of delayed, faded paths with fixed average powers, with no spatial information. TR 38.901 defines TDL profiles for link-level simulation.
- Target wake time (TWT)
- An IEEE 802.11ax mechanism that lets a station and access point agree when the station will wake to exchange frames, so it can sleep in between.
- Thermal noise
- Noise from the random motion of electrons in any conductor above absolute zero, with power kTB. At the 290 K reference temperature used in ITU-R P.372 its density is about −174 dBm/Hz.
- Third-order intercept point (IP3)
- A figure of merit for linearity, defined as the extrapolated power at which third-order intermodulation products would equal the wanted output. A higher IP3 means less intermodulation.
- Thread
- An IPv6-based low-power mesh networking protocol that runs on IEEE 802.15.4 radios.
- Throughput
- The rate of useful data actually delivered over a link, after coding overhead, control signalling and retransmissions are taken out. It is always lower than the peak PHY rate.
- Time division duplex (TDD)
- A duplexing scheme in which uplink and downlink share one frequency band and take turns in time. Because both directions use the same frequencies, the channel is reciprocal once the radios are calibrated.
- Time of flight (ToF)
- The propagation time of a radio signal between two devices. Multiplied by the speed of light it gives distance, about 30 cm per nanosecond.
- Time on air
- How long a packet occupies the channel. In LPWAN it drives energy use, collision probability and compliance with duty-cycle limits.
- Timing advance
- A command that makes a device transmit early by its round-trip propagation delay so that uplink signals from near and far devices arrive aligned at the base station.
- Timing recovery
- Estimating and tracking the best sampling instant for each symbol at the receiver. Common detectors include the Gardner and Mueller-Müller algorithms.
- Transceiver
- A device or chip that contains both a transmitter and a receiver, usually sharing one local oscillator reference and one antenna port. Most SDR front ends and cellular modems are transceivers.
- Transform precoding
- See DFT-spread OFDM.
- Transmission rank
- The number of independent spatial layers sent at once. The receiver reports the rank the channel supports, and the scheduler may choose a lower one.
- Transmission time interval (TTI)
- The time unit over which the scheduler allocates a transport block. In LTE it is one 1 ms subframe, while NR scheduling works in slots and shorter units.
- Transmitter
- The part of a radio that turns data into a modulated signal at the carrier frequency and drives it into an antenna. A typical chain runs coding, modulation, digital-to-analogue conversion, up-conversion and power amplification.
- Transparent payload
- A satellite that filters, frequency-converts and amplifies the signal without demodulating it, so the base station stays on the ground. It is also called a bent pipe.
- Turbo code
- A code built from two convolutional encoders joined by an interleaver and decoded iteratively. LTE uses a rate 1/3 turbo code for its shared data channels, as set out in TS 36.212.Hub: Channel coding. Related: Convolutional code, Interleaving, Low-density parity-check (LDPC) code
- Two-way ranging
- Measuring distance by exchanging timed messages in both directions so that neither device needs a synchronised clock. The reply delay is reported and subtracted from the round-trip time.
- Type approval
- Regulatory certification that a radio model meets a country's technical rules before it can be sold, such as FCC equipment authorisation in the United States.
U
- UHD (USRP Hardware Driver)
- The open-source driver and API that Ettus Research provides for USRP devices. GNU Radio, SoapySDR and srsRAN can all use it.
- Ultra-reliable low-latency communication (URLLC)
- The IMT-2020 usage scenario in ITU-R M.2083 for services that need very high reliability and low latency, such as industrial control.
- Ultra-wideband (UWB)
- Radio that sends very short pulses spread over a very wide bandwidth. The fine time resolution allows accurate distance measurement, which is its main use in phones, cars and trackers.
- Underflow
- A gap in transmission because the host did not deliver samples to the SDR fast enough. UHD reports it by printing the letter U to the console.
- Uplink
- The direction from a user device to the network, for example from a phone to a base station. Uplink power is limited by the device's battery and exposure rules, so it is often the weaker direction.
- User equipment (UE)
- The 3GPP term for a device that connects to the network, such as a phone, modem or IoT module.
- USRP
- Universal Software Radio Peripheral, the family of SDRs from Ettus Research, an NI brand. They are controlled through the UHD driver.
V
- Vector signal analyser
- An instrument that captures IQ data and demodulates it to measure EVM, constellation errors and other modulation quality figures.
- Virtualised RAN (vRAN)
- Running base station software on general-purpose servers instead of dedicated hardware, often with accelerators for the physical layer.
- Viterbi algorithm
- A dynamic-programming method that finds the most likely transmitted sequence through a code trellis. It is the standard decoder for convolutional codes.
- Voice over LTE (VoLTE)
- Voice calls carried as IP packets over LTE using the IP Multimedia Subsystem, rather than falling back to a 2G or 3G network.
W
- Wavelength
- The distance a radio wave travels during one cycle, equal to the speed of light divided by the frequency. At 3 GHz the free-space wavelength is about 10 cm.
- Wi-Fi
- The Wi-Fi Alliance brand for wireless LAN products based on the IEEE 802.11 standards. Generations are named Wi-Fi 6, Wi-Fi 7 and so on, each tied to an 802.11 amendment.Hub: Wi-Fi. Related: IEEE 802.11, Wi-Fi 6 (IEEE 802.11ax), Wi-Fi 7 (IEEE 802.11be), Access point (AP)
- Wi-Fi 6 (IEEE 802.11ax)
- The Wi-Fi generation based on IEEE 802.11ax, High Efficiency WLAN, which brought OFDMA, multi-user MIMO, 1024-QAM and target wake time to Wi-Fi. Wi-Fi 6E is the same technology extended into the 6 GHz band.
- Wi-Fi 6E
- See Wi-Fi 6 and the 6 GHz band.
- Wi-Fi 7 (IEEE 802.11be)
- The Wi-Fi generation based on IEEE 802.11be, Extremely High Throughput, published by IEEE on 22 July 2025. The Wi-Fi Alliance lists 320 MHz channels in the 6 GHz band, 4096-QAM and multi-link operation among its features.
- Wi-Fi Alliance
- The industry body that certifies Wi-Fi products for interoperability and owns the Wi-Fi brand and generation names.
- Wi-Fi HaLow (IEEE 802.11ah)
- The Wi-Fi Alliance name for IEEE 802.11ah, a sub-1 GHz Wi-Fi amendment for longer range, low-power IoT links.
- Windowing
- Multiplying a block of samples by a tapered function before an FFT to reduce spectral leakage. The choice of window trades frequency resolution against the level of the sidelobes.
- WLAN sensing (IEEE 802.11bf)
- The IEEE 802.11bf amendment, published in September 2025, that defines how Wi-Fi devices measure and report channel changes to sense motion and presence.
- World Radiocommunication Conference (WRC)
- The ITU conference that revises the international Radio Regulations, including allocations of frequency bands to services.
- WPA3
- The current Wi-Fi Alliance security certification, the successor to WPA2. According to the Wi-Fi Alliance, WPA3-Personal gives stronger protection against password guessing.
Z
- Zadoff-Chu sequence
- A complex sequence with constant amplitude and zero cyclic autocorrelation at non-zero lags. LTE and NR use Zadoff-Chu sequences for random access preambles and some reference signals.
- Zero-forcing
- A linear precoder or detector that inverts the channel matrix so each stream sees no interference from the others. It amplifies noise when the channel matrix is poorly conditioned.
- Zero-forcing equaliser
- An equaliser that applies the inverse of the channel response. It removes ISI completely but amplifies noise wherever the channel response is weak.
- Zero-IF receiver
- See direct-conversion receiver.
- Zigbee
- A mesh networking stack for low-power devices built on the IEEE 802.15.4 PHY and MAC, common in home and building automation.
Π
- π/2-BPSK
- BPSK with an extra 90° phase rotation applied on alternate symbols, which avoids zero crossings and lowers the peak-to-average power ratio. NR defines it in TS 38.211 for uplink use with transform precoding.