Radio spectrum
The radio spectrum is the radio frequency (RF) portion of the electromagnetic spectrum, consisting of electromagnetic waves with frequencies ranging from approximately 3 kHz to 300 GHz that propagate through space without wires.[1] This range is divided into nine frequency bands—Very Low Frequency (VLF), Low Frequency (LF), Medium Frequency (MF), High Frequency (HF), Very High Frequency (VHF), Ultra High Frequency (UHF), Super High Frequency (SHF), Extremely High Frequency (EHF), and Tremendously High Frequency (THF)—each suited to specific applications based on propagation characteristics and wavelength.[1] Essential for modern society, the radio spectrum enables a wide array of radiocommunication services, including broadcasting, mobile telephony, satellite communications, radar, navigation, and radio astronomy, supporting everything from emergency response to global connectivity.[2][3] Its finite nature makes efficient management critical to prevent harmful interference and accommodate growing demands from technologies like 5G and emerging 6G networks.[4] Internationally, the spectrum is coordinated by the International Telecommunication Union (ITU) through its Radiocommunication Sector (ITU-R), which maintains the Radio Regulations—a binding treaty updated every four years at World Radiocommunication Conferences (WRCs) to allocate frequencies to services on a global or regional basis (divided into three regions).[4][1] Nationally, in the United States, the Federal Communications Commission (FCC) allocates non-federal spectrum for commercial and personal uses, while the National Telecommunications and Information Administration (NTIA) manages federal allocations for government operations, with both referencing the ITU's framework in the United States Table of Frequency Allocations.[2] Allocations distinguish between primary services (protected from interference) and secondary services (tolerant of it), ensuring equitable and rational use worldwide.[1]Fundamentals
Definition and Physical Limits
The radio spectrum refers to the portion of the electromagnetic spectrum occupied by radio waves, defined by frequencies ranging from approximately 3 kHz to 300 GHz, which correspond to wavelengths from about 100 kilometers to 1 millimeter. This range encompasses electromagnetic waves suitable for wireless communication and sensing, distinguished from lower-frequency phenomena like audio signals and higher-frequency infrared radiation. While physical radio waves can extend to extremely low frequencies (ELF, 3–30 Hz) for specialized propagation, the regulated radio spectrum per ITU Radio Regulations covers allocations from 8.3 kHz to 3000 GHz, with practical applications typically up to 300 GHz as of 2025.[5][2] The lower frequency limit of 3 kHz aligns with the onset of viable long-distance transmission via ground waves and early ionospheric interactions, though ELF waves (3–30 Hz) can resonate in the Earth-ionosphere waveguide for global propagation despite enormous wavelengths. Below 3 Hz, antenna sizes become impractically large (exceeding planetary scales), and propagation efficiency drops sharply due to insufficient interaction with the ionosphere. At the upper end, 300 GHz marks a practical limit where atmospheric absorption—primarily by water vapor, oxygen, and other gases—intensifies, creating high attenuation that restricts line-of-sight ranges to kilometers or less, compounded by technological challenges in generating, amplifying, and detecting signals at such frequencies as of 2025.[6] A fundamental physical property of the radio spectrum is the inverse relationship between frequency f and wavelength \lambda, given by the equation \lambda = \frac{c}{f}, where c \approx 3 \times 10^8 m/s is the speed of light in vacuum; this relation underscores how lower frequencies yield longer wavelengths conducive to diffraction and ground-wave propagation, while higher frequencies enable narrower beams but suffer greater free-space path loss. Natural constraints further shape usability: atmospheric windows—bands like 1–10 GHz and select millimeter-wave slots (e.g., 57–71 GHz)—offer low absorption for terrestrial links, whereas ionospheric ionization aids sky-wave reflection below 30 MHz, and water vapor sharply attenuates signals above 100 GHz in humid conditions.[7] These limits ensure the spectrum's allocation prioritizes viable propagation paths while mitigating environmental interference. The overall range and allocations have evolved through international conferences since the early 20th century, with major revisions in events like the 1947 Atlantic City Conference and subsequent World Radiocommunication Conferences (WRCs).[8]Relation to Electromagnetic Spectrum
The electromagnetic spectrum encompasses a continuous range of electromagnetic radiation characterized by varying wavelengths and frequencies, from the longest wavelengths and lowest frequencies in the radio portion to the shortest wavelengths and highest frequencies in gamma rays. The radio spectrum occupies the lowest frequency segment, typically from about 3 kHz to 300 GHz, corresponding to wavelengths from kilometers down to millimeters, and is distinct from adjacent regions such as microwaves (which overlap with higher radio frequencies but extend to 300 GHz), infrared (0.7–1000 μm), visible light (400–700 nm), ultraviolet (10–400 nm), X-rays (0.01–10 nm), and gamma rays (below 0.01 nm).[9][10] Radio waves exhibit unique propagation behaviors compared to higher-frequency portions of the spectrum, enabling longer-range transmission under certain conditions. In the radio domain, signals can travel via ground waves that follow the Earth's curvature over moderate distances, sky waves that reflect off the ionosphere for global reach at lower frequencies, and line-of-sight paths dominant at higher radio bands, whereas microwaves and beyond increasingly suffer from atmospheric absorption, scattering by particles, or require unobstructed paths due to their shorter wavelengths.[11][12][13] Several environmental factors influence radio wave propagation, including ionospheric refraction, which bends high-frequency (HF, 3–30 MHz) and very high-frequency (VHF, 30–300 MHz) signals back to Earth, facilitating long-distance communication, and tropospheric ducting in the ultra-high frequency (UHF, 300 MHz–3 GHz) range, where atmospheric temperature inversions trap waves in refractive layers, extending beyond-horizon coverage. Free-space path loss, a fundamental limitation in all radio propagation, arises from the spreading of waves and is quantified by the formula: \text{FSPL} = \left( \frac{4\pi d f}{c} \right)^2 where d is distance, f is frequency, and c is the speed of light, resulting in greater loss at higher frequencies and longer distances.[14][15][16] Attenuation mechanisms further shape radio signal behavior, with ground conductivity affecting low-frequency ground waves by enabling or hindering surface propagation, atmospheric gases like oxygen causing peak absorption around 60 GHz (up to 15 dB/km), and rain fade impacting higher microwave bands through water droplet scattering and absorption, which can exceed 10 dB/km in heavy precipitation. These effects contrast with negligible atmospheric influence on lower radio frequencies but intensify toward infrared and visible regions, where molecular absorption dominates.[11][17] Technologically, the extended wavelengths of radio waves necessitate larger antenna structures for efficient radiation and reception, as resonant designs like half-wavelength dipoles (length \lambda/2) scale directly with wavelength to achieve optimal impedance matching and gain, making low-frequency antennas impractically large compared to compact designs feasible for microwaves and higher frequencies.[18]Band Designations
ITU Designations
The International Telecommunication Union (ITU), through its Radiocommunication Sector (ITU-R), establishes a standardized nomenclature for radio frequency bands to facilitate global coordination and consistency in spectrum management.[19] This system divides the radio spectrum into 12 principal bands, numbered 0 to 12, spanning from extremely low frequencies to tremendously high frequencies, using descriptive names that reflect their relative positions in the spectrum.[19] The designations emphasize the use of the hertz (Hz) as the primary unit of frequency, with wavelength ranges provided for completeness, promoting uniformity in technical documentation and regulatory practices worldwide.[19] The ITU band nomenclature originated in the mid-20th century and has evolved through periodic revisions by ITU-R study groups to accommodate technological advancements. First adopted in 1953 and revised multiple times, including in 1978 to expand higher-frequency coverage, in 2015 to refine wavelength metrics, and in October 2025 (ITU-R V.431-9) to formalize the tremendously high frequency (THF) band and update low-frequency names, the system reflects ongoing adaptations to emerging applications.[19] A notable update from prior studies came through the 2019 World Radiocommunication Conference (WRC-19), which advanced allocations for millimeter-wave (mmWave) frequencies within the existing extremely high frequency (EHF) band (30-300 GHz), supporting high-capacity wireless systems like 5G.[20] These conferences ensure the nomenclature remains relevant amid spectrum demands. The 12 ITU bands are logarithmically scaled, with each spanning approximately one decade in frequency (from 3 × 10^n Hz to 3 × 10^{n+1} Hz), allowing proportional bandwidth growth as frequencies increase.[19] This scaling accommodates diverse propagation characteristics: lower bands like extremely low frequency (ELF, 3-30 Hz) enable long-range, ground-wave propagation for applications such as submarine communications, while higher bands like THF offer wide bandwidths but suffer from atmospheric attenuation, suiting short-range, high-data-rate links.[19] The band names follow a mnemonic pattern—"TLF" for Tidal Low Frequency, "ELF" for Extremely Low Frequency, "SLF" for Super Low Frequency, "ULF" for Ultra Low Frequency, "VLF" for Very Low Frequency, "LF" for Low Frequency, "MF" for Medium Frequency, "HF" for High Frequency, "VHF" for Very High Frequency, "UHF" for Ultra High Frequency, "SHF" for Super High Frequency, "EHF" for Extremely High Frequency, and "THF" for Tremendously High Frequency—derived from the initials to aid memorization.[19]| Band No. | Designation | Frequency Range | Wavelength Range |
|---|---|---|---|
| 0 | TLF | 0.3–3 Hz | 100,000–1,000,000 km (sub-gigametric) |
| 1 | ELF | 3–30 Hz | 10,000–100,000 km (super-megametric) |
| 2 | SLF | 30–300 Hz | 1,000–10,000 km (megametric) |
| 3 | ULF | 0.3–3 kHz | 100–1,000 km (sub-megametric) |
| 4 | VLF | 3–30 kHz | 10–100 km (super-kilometric) |
| 5 | LF | 30–300 kHz | 1–10 km (kilometric) |
| 6 | MF | 0.3–3 MHz | 100–1,000 m (hectometric) |
| 7 | HF | 3–30 MHz | 10–100 m (decametric) |
| 8 | VHF | 30–300 MHz | 1–10 m (metric) |
| 9 | UHF | 300 MHz–3 GHz | 10–100 cm (decimetric) |
| 10 | SHF | 3–30 GHz | 1–10 cm (centimetric) |
| 11 | EHF | 30–300 GHz | 1–10 mm (millimetric) |
| 12 | THF | 300–3,000 GHz | 0.1–1 mm (sub-millimetric) |
IEEE and Regional Standards
The IEEE Standard for Letter Designations for Radar-Frequency Bands, originally developed to standardize terminology for radar applications, assigns letter designations to specific frequency ranges spanning from 1 GHz to 110 GHz. These designations originated during World War II as part of U.S. military efforts to classify radar frequencies using code letters for security purposes, later formalized by the IEEE to reduce confusion in technical communications. The standard was first issued in 1976 and revised in 2002 (IEEE Std 521-2002), with the current version being IEEE Std 521-2019, which maintains the core band definitions while updating references for modern applications.[22] The IEEE radar bands are defined as follows, with approximate frequency ranges and corresponding free-space wavelengths:| Band | Frequency Range (GHz) | Wavelength Range (cm) |
|---|---|---|
| L | 1–2 | 30–15 |
| S | 2–4 | 15–7.5 |
| C | 4–8 | 7.5–3.75 |
| X | 8–12 | 3.75–2.5 |
| Ku | 12–18 | 2.5–1.67 |
| K | 18–27 | 1.67–1.11 |
| Ka | 27–40 | 1.11–0.75 |
| V | 40–75 | 0.75–0.4 |
| W | 75–110 | 0.4–0.27 |
- A: 0–0.25 GHz (wavelength >120 cm)
- B: 0.25–0.5 GHz (120–60 cm)
- C: 0.5–1 GHz (60–30 cm)
- D: 1–2 GHz (30–15 cm)
- E: 2–3 GHz (15–10 cm)
- F: 3–4 GHz (10–7.5 cm)
- G: 4–6 GHz (7.5–5 cm)
- H: 6–8 GHz (5–3.75 cm)
- I: 8–10 GHz (3.75–3 cm)
- J: 10–20 GHz (3–1.5 cm)
- K: 20–40 GHz (1.5–0.75 cm)
- L: 40–60 GHz (0.75–0.5 cm)
- M: 60–100 GHz (0.5–0.3 cm)
Comparisons of Designation Systems
Different designation systems for radio bands have evolved independently to serve specific needs, such as international regulation by the International Telecommunication Union (ITU), radar applications standardized by the Institute of Electrical and Electronics Engineers (IEEE), and electronic countermeasures (ECM) defined by NATO/EU/US frameworks. These systems often overlap in frequency coverage but differ in nomenclature, boundaries, and focus, leading to potential confusion in cross-system referencing.[30][31][24] A key discrepancy lies in their foundational approaches: the ITU employs a logarithmic scale based on powers of ten for wavelength or frequency, dividing the spectrum into broad bands like VHF (30–300 MHz) and UHF (300–3000 MHz) to facilitate global allocation. In contrast, IEEE uses a letter-based system originating from World War II military radar secrecy, with bands like L (1–2 GHz) and S (2–4 GHz) tailored to microwave frequencies above 1 GHz. NATO/ECM designations, also letter-based, emphasize ECM and radar jamming, with ranges like J (10–20 GHz) that consolidate multiple IEEE bands but start from lower frequencies with A (0–0.25 GHz). These differences result in non-aligned boundaries; for instance, the ITU's UHF band (300–3000 MHz) encompasses the entire IEEE L (1–2 GHz) and part of S (2–4 GHz up to 3 GHz), while NATO's D (1–2 GHz) and E (2–3 GHz) partially overlap this but use distinct lettering. Similarly, the ITU's SHF (3–30 GHz) spans IEEE's S (partial), C (4–8 GHz), X (8–12 GHz), Ku (12–18 GHz), K (18–27 GHz), and Ka (27–40 GHz), whereas NATO's J (10–20 GHz) and K (20–40 GHz) group these into fewer, broader categories focused on military applications.[30][31][24]| Frequency Range (GHz) | ITU Band | IEEE Band | NATO/ECM Band | Notes on Primary Uses and Overlaps |
|---|---|---|---|---|
| 0.03–0.3 | VHF | N/A | A/B | Broadcasting and FM radio; ITU and NATO apply, but IEEE designations start at 1 GHz.[30][24] |
| 0.3–1 | UHF | N/A | C | Mobile communications; ITU and NATO apply, but IEEE starts at 1 GHz.[30][24] |
| 1–2 | UHF | L | D | Radar and satellite; Overlap across all three, with ITU UHF encompassing IEEE L and NATO D.[30][24] |
| 2–4 | UHF/SHF | S | E/F | Weather radar and Wi-Fi; ITU UHF/SHF split at 3 GHz mismatches IEEE S and NATO E/F boundaries.[30][24] |
| 4–8 | SHF | C | G | Aviation radar; Full overlap in SHF/C/G, used for terminal Doppler weather radar.[30][24] |
| 8–12 | SHF | X | I (partial) | Military surveillance; IEEE X within ITU SHF, NATO I covers up to 10 GHz with J extending to 20 GHz.[30][24] |
| 12–18 | SHF | Ku | J | Satellite TV; ITU SHF includes IEEE Ku, NATO J consolidates 10–20 GHz for ECM.[30][24] |
| 18–30 | SHF | K/Ka (partial) | J/K (partial) | Millimeter-wave radar; Boundaries vary, with ITU SHF up to 30 GHz spanning multiple IEEE and NATO letters.[30][24] |
| 30–300 | EHF | V/W/mm | K/L/M | 5G and experimental; Broad ITU EHF covers IEEE high-end radar bands and NATO upper ECM ranges.[30][24] |
| 300–3,000 | THF | N/A | N/A | THz applications like 6G; Newly designated in ITU V.431-9 (2025); IEEE and NATO typically up to 110 GHz and 100 GHz, respectively. |
Spectrum Allocation and Regulation
International Framework
The international framework for radio spectrum management is led by the International Telecommunication Union (ITU), a United Nations specialized agency responsible for coordinating global telecommunications standards. The ITU's Radiocommunication Sector (ITU-R) specifically handles the management of the radio-frequency spectrum and satellite orbits to ensure their efficient, rational, and equitable use worldwide, preventing harmful interference among nations.[4] This role is enshrined in the ITU Constitution and Convention, which empower ITU-R to develop regulations through collaborative processes involving member states and sector members.[36] Central to this framework is Article 5 of the Radio Regulations, which outlines the international Table of Frequency Allocations, dividing the spectrum from 8.3 kHz to 3,000 GHz into bands assigned to radiocommunication services on a primary, secondary, or other basis.[37] Allocations are primarily to services such as fixed (point-to-point communications), mobile (including land, maritime, and aeronautical), and broadcasting, with over 40 services defined in total; these can be exclusive or shared, often conditioned by footnotes that specify operational restrictions, additional allocations, or interference protection criteria.[38] For example, footnotes enable spectrum sharing in the 700 MHz band (698–790 MHz), where the mobile service is primarily allocated globally, but regional footnotes permit coexistence with broadcasting services to support digital terrestrial television transitions without harmful interference.[39] The Table applies worldwide unless specified regionally, promoting harmonization while allowing flexibility for local needs.[40] World Radiocommunication Conferences (WRCs), convened by ITU-R every three to four years, serve as the primary mechanism for reviewing and updating the Radio Regulations, including spectrum allocations and technical provisions.[41] The most recent conference, WRC-23, held in Dubai from 20 November to 15 December 2023, resulted in amendments incorporated into the 2024 edition of the Radio Regulations, which entered into force on 1 January 2025 following ratification by member states.[42] These conferences address emerging technologies, such as 5G/6G expansions and satellite constellations, by revising Article 5 and related articles based on proposals from administrations and study groups.[43] To account for geographical and operational variations, the ITU divides the world into three regions for allocation purposes, with differences in primary/secondary statuses or band usages across them.[39]| Region | Geographic Coverage |
|---|---|
| Region 1 | Europe; Africa; the Middle East west of the Persian Gulf including Iraq; the former Soviet Union; Mongolia |
| Region 2 | The Americas (North, Central, and South America, including the Caribbean) |
| Region 3 | Asia-Pacific (including Australasia and the South West Pacific, excluding Region 1 areas) |
National and Regional Practices
In the United States, the Federal Communications Commission (FCC) has implemented spectrum auctions since 1994, following authority granted by the Omnibus Budget Reconciliation Act of 1993, to allocate licenses efficiently and generate revenue for wireless services.[46] A notable example is Auction 105 in 2020 for the 3.5 GHz Citizens Broadband Radio Service (CBRS) band, which raised approximately $4.58 billion through competitive bidding for Priority Access Licenses.[47] To enable dynamic spectrum sharing in this band among incumbents, priority users, and general access, the FCC mandates the use of automated Spectrum Access Systems (SAS) that coordinate frequencies in real-time, minimizing interference while promoting shared use.[48] In the European Union, the European Telecommunications Standards Institute (ETSI) and the European Conference of Postal and Telecommunications Administrations (CEPT) collaborate to develop harmonized technical conditions for spectrum use, adapting international guidelines for regional deployment. For instance, the 3.4-3.8 GHz band, initially harmonized for mobile/fixed communications networks (MFCN) under ECC Decision (11)06, was updated through CEPT Report 67 to support 5G operations, including power limits and synchronization requirements suitable for wide-area networks.[49] Additionally, the Radio Equipment Directive (RED) 2014/53/EU ensures that devices using radio spectrum meet essential requirements for efficient spectrum utilization, electromagnetic compatibility, and safety before market placement.[50] Other nations demonstrate diverse approaches to spectrum management. In China, the Ministry of Industry and Information Technology (MIIT) has prioritized 6G research and development, allocating resources for experimental pilots in terahertz frequencies above 100 GHz to explore ultra-high-speed communications, with demonstrations achieving data rates exceeding 100 Gbps in laboratory settings as of 2025.[51] In India, the Telecom Regulatory Authority of India (TRAI) determines reserve prices for spectrum auctions based on valuation methods, such as administrative and market-based approaches, to balance affordability and revenue; for example, the 2022 auction set prices for 3.3-3.6 GHz at around INR 30,000 crore per block, influencing operator investments in 5G rollout. Regional organizations facilitate coordinated practices across multiple countries. The Inter-American Telecommunication Commission (CITEL), under the Organization of American States, advises on spectrum allocation in the Americas by harmonizing positions for international forums and promoting equitable access, such as through recommendations on broadband spectrum in the 3.5 GHz band to support regional 5G deployment.[52] Cross-border challenges arise from differing national implementations, often leading to interference disputes. A prominent case involved 5G deployments in the C-band (3.7-3.98 GHz), where potential interference with aviation radio altimeters prompted concerns; the U.S. Federal Aviation Administration (FAA) resolved this in 2022 by issuing airworthiness directives requiring altimeter modifications or operational restrictions near airports, allowing safe coexistence after coordination with wireless carriers.[53]Band Plans and Frequency Assignments
Band plans and frequency assignments detail the precise subdivision of allocated spectrum into channels or sub-bands for specific uses, ensuring efficient and interference-free operations as governed by the ITU Radio Regulations. These plans are developed through international coordination and national implementations, specifying center frequencies, bandwidths, and operational parameters for services like fixed, mobile, broadcasting, satellite, and emergency communications. The ITU's Article 5 provides the foundational Table of Frequency Allocations, updated at World Radiocommunication Conferences, with the 2024 edition reflecting WRC-23 outcomes.[42] National regulators, such as the FCC in the United States, then create detailed band plans within these allocations.[28] For fixed and mobile services, the 800 MHz band exemplifies cellular assignments, where the ITU allocates 806–960 MHz globally to these services on a primary basis. In Region 2 (Americas), the United States specifies 851–869 MHz for mobile downlink in cellular systems, paired with 806–824 MHz for uplink, supporting 2G to 5G technologies with channel bandwidths up to 25 MHz per carrier.[54] Power limits typically range from 50–200 W effective radiated power (ERP) for base stations, depending on sub-band and deployment.[55] Broadcasting assignments prioritize wide-area coverage. The medium frequency (MF) band for amplitude modulation (AM) radio is designated 535–1605 kHz worldwide, with 9 or 10 kHz channel spacing and power up to 50 kW for high-power stations.[56] Frequency modulation (FM) occupies 87.5–108 MHz in the VHF band across Regions 1 and 2, using 200 kHz channels and ERP limits of 100 kW maximum. Digital terrestrial television, such as DVB-T, utilizes UHF bands like 470–694 MHz in many regions, with 6–8 MHz channels and effective isotropic radiated power (EIRP) up to 10 kW.[57] Satellite communications rely on paired bands for fixed-satellite service (FSS). The C-band includes 3.7–4.2 GHz for downlink and 5.925–6.425 GHz for uplink, allocated worldwide with primary status, supporting transponder bandwidths of 36–72 MHz and EIRP densities regulated to avoid interference.[58] The Ku-band, used for direct-to-home broadcasting, features 11.7–12.75 GHz downlink and 13.75–14.5 GHz uplink in Regions 1 and 3, with channel plans accommodating 27–36 MHz transponders and power flux density limits of -95 dBW/m²/MHz.[59] Emergency services have protected narrow-band assignments. The 406–406.1 MHz band is exclusively allocated to the mobile-satellite service (Earth-to-space) for emergency position-indicating radiobeacons (EPIRBs), enabling global distress signaling via COSPAS-SARSAT satellites, with a 3 kHz bandwidth and maximum EIRP of 37 dBm (5 W).| Frequency Range | Service | Region/Notes | Power Limits |
|---|---|---|---|
| 535–1605 kHz | Broadcasting (AM) | Worldwide; 9/10 kHz spacing | Up to 50 kW ERP |
| 87.5–108 MHz | Broadcasting (FM) | Regions 1/2; 200 kHz channels | Up to 100 kW ERP |
| 806–824 / 851–869 MHz | Mobile (cellular) | Region 2 (e.g., US); paired uplink/downlink | 50–200 W ERP base stations |
| 3.7–4.2 GHz (downlink) / 5.925–6.425 GHz (uplink) | Fixed-satellite (C-band) | Worldwide; FSS primary | -95 dBW/m²/MHz PFD |
| 11.7–12.75 GHz (downlink) / 13.75–14.5 GHz (uplink) | Fixed-satellite (Ku-band, DTH) | Regions 1/3; FSS primary | -95 dBW/m²/MHz PFD |
| 406–406.1 MHz | Mobile-satellite (EPIRBs) | Worldwide; exclusive for distress | 37 dBm (5 W) EIRP max |