Radio Spectrum | Teleinf Edu

Radio Spectrum and Frequency Sharing

Wireless systems transmit information by modulating electromagnetic waves. The complete range of those waves is the electromagnetic spectrum. The bands discussed here, from about 3 kHz to 300 GHz, form the used by radio, television, mobile networks, satellite links, and Wi-Fi.

Radio spectrum is a shared set of frequency ranges with limited separation between simultaneous transmissions. If every transmitter used any frequency at any power, receivers would see harmful interference instead of a usable signal. International rules and national regulations therefore define which services may use a band, with what bandwidth and power, at which locations, and under which coordination conditions. A highway analogy can illustrate the sharing problem, but the engineering controls are frequency assignments, channel bandwidths, emission limits, and interference protection criteria.

Frequency Bands and Their Uses

For administration and engineering, the radio spectrum is divided into frequency bands. Wavelength, antenna dimensions, atmospheric absorption, channel bandwidth, and propagation conditions influence an application's design. Higher frequencies have shorter wavelengths and can support wider channels when regulations and technology provide them, but blockage and atmospheric loss often increase. Lower frequencies generally propagate farther and penetrate some obstacles more effectively, while available channel bandwidth and antenna dimensions can constrain capacity.

VLF (Very Low Frequency) & LF (Low Frequency)

Frequencies: $3-30 \text{ kHz}$ (VLF), $30-300 \text{ kHz}$ (LF). These are the longwave bands.

Properties:Long wavelengths support groundwave propagation over large distances by following the Earth's curvature. VLF signals can also penetrate seawater to limited depths, which is useful for selected submarine links.

Uses: These bands support selected navigation systems, time-signal broadcasts such as WWVB in North America, and long-range military communication, including submarine links. Data rates are low because the usable bandwidth is narrow.

MF (Medium Frequency)

Frequencies: $300 \text{ kHz}$ to $3 \text{ MHz}$ . This is the medium wave band.

Properties: Groundwave propagation can provide regional coverage during the day. After sunset, changes in the ionosphere can support skywave reception over a longer path, although the result depends on frequency, time, and ionospheric conditions.

Uses: The classic home of AM radio broadcasting, as well as marine communication and navigation beacons.

HF (High Frequency)

Frequencies: $3-30 \text{ MHz}$ . Known as the shortwave band.

Properties: Skywave propagation can refract or reflect HF signals through the ionosphere. With suitable frequency and ionospheric conditions, multiple hops can support intercontinental communication.

Uses: International shortwave broadcasting, amateur (ham) radio, aviation communication, and over-the-horizon military communication.

VHF (Very High Frequency)

Frequencies: $30-300 \text{ MHz}$ .

Properties: Propagation is primarily line-of-sight at ordinary terrestrial distances. Hills and large buildings can obstruct the direct component, while antenna height and diffraction influence the coverage area.

Uses: FM radio broadcasting, over-the-air television in national VHF channel plans, including channels 2-13 in the United States, air traffic control, and private radio systems used by police and fire departments.

UHF (Ultra High Frequency)

Frequencies: $300 \text{ MHz}$ to $3 \text{ GHz}$ .

Properties: UHF links are often line-of-sight at ordinary terrestrial distances and usually have shorter geometric coverage than comparable VHF links. The shorter wavelength permits smaller antennas and wider channels. Lower UHF frequencies can penetrate buildings more effectively than higher microwave frequencies, subject to building materials and network design.

Uses: UHF contains region-specific terrestrial television allocations, many cellular bands used by 4G LTE and 5G, GPS, Wi-Fi at $2.4 \text{ GHz}$ , Bluetooth, and microwave ovens. Television channel numbers such as 14-51 refer to particular national plans rather than the whole UHF band.

SHF (Super High Frequency) & EHF (Extremely High Frequency)

Frequencies: $3-30 \text{ GHz}$ (SHF), $30-300 \text{ GHz}$ (EHF). These are the microwave and millimeter-wave bands.

Properties:At the upper microwave and millimetre-wave bands, propagation becomes quasi-optical. The direct path is readily blocked by buildings, foliage, rain, or a person's hand at some frequencies. These bands can support wide channels and narrow, directional beams, while atmospheric absorption becomes important at selected frequencies such as the oxygen band near 60 GHz.

Uses: Applications include satellite communication, Wi-Fi in the $5 \text{ GHz}$ and $6 \text{ GHz}$ bands, point-to-point microwave links for cellular backhaul, weather radar, and high-band 5G services.

International and National Spectrum Governance

Radio waves can cross national borders, so an authorized transmission in one country can interfere with a service in another. Spectrum governance therefore combines international coordination with national administration and enforcement.

The International Level: ITU:
The coordinates international use of the radio spectrum through the Radio Regulations. The ITU divides the world into three regulatory regions and holds World Radiocommunication Conferences (WRCs) every three to four years. Member administrations negotiate allocations and technical conditions for services such as mobile, satellite, and broadcasting. The treaty provides a framework for national regulators, while each administration remains responsible for its own assignments and enforcement.
The National Level: FCC and Ofcom:
Within the ITU framework, each country administers its national spectrum through one or more authorities. In the United States, non-federal use is managed by the Federal Communications Commission (FCC), while federal use is coordinated by the NTIA. In the United Kingdom, the regulator is Ofcom. National authorities are responsible for:
- Allocation: Designating specific frequency bands for certain types of services.
- Assignment: Granting specific users (individuals or companies) the right to use particular frequencies within those bands through licenses.
- Enforcement: Monitoring the spectrum to ensure users comply with the terms of their licenses and do not cause harmful interference.

Models of Spectrum Access

Frequency bands can be administered through different access models. Three useful categories are exclusive or coordinated licensing, unlicensed access under technical rules, and shared or dynamic access.

1. Licensed Spectrum (Command and Control)

In this model, a regulator grants a defined authorization for a frequency block, service, and geographic area. The authorization may provide protection from specified interference, but it does not remove every coordination obligation. This approach is common for cellular networks, broadcasting, and other services that require predictable coverage and large infrastructure investment.

Mechanism: Licenses can be awarded through competitive auctions in which companies bid for defined rights. FCC Auction 107 for flexible-use licenses in the 3.7-3.98 GHz C-band closed in February 2021 with net winning bids above $81.1 billion.
Pros: A defined authorization can provide interference protection and accountability, which supports investment in network infrastructure.
Cons: License fees and technical conditions can restrict entry. A licensee may also leave assigned capacity underused, a situation described as spectrum hoarding.
Examples: Many cellular bands (4G LTE, 5G), FM/AM radio broadcast bands, and satellite communication bands.

2. Unlicensed Spectrum (Spectrum Commons)

In this model, devices may operate without an individual frequency assignment when they comply with the applicable technical rules. Those rules can specify power, bandwidth, emissions, sensing, and sharing behaviour. The result is a best-effort environment in which a user generally has no guarantee of capacity or protection from another compliant device.

Mechanism: Regulators set aside specific bands, including ISM (Industrial, Scientific, and Medical) bands and other unlicensed allocations, then define technical rules for devices that operate within them.
Pros: The low barrier to entry supports experimentation and deployment. Wi-Fi and Bluetooth developed at large scale without an individual license for each home or device.
Cons: Prone to congestion and interference, as there is no central coordination. The performance is not guaranteed, and you have limited recourse if your neighbor's device is interfering with yours.
Examples: Subject to national rules, the $2.4 \text{ GHz}$ band supports Wi-Fi, Bluetooth, microwave ovens, and cordless phones, while the $5 \text{ GHz}$ and $6 \text{ GHz}$ bands support Wi-Fi 5, 6, and 6E.

3. Shared / Dynamic Spectrum Access

Shared or dynamic access combines protected incumbent use with additional users under coordination rules. A licensed band may be unused at a particular location or time, so a secondary system can use that opportunity when the incumbent is protected.

Mechanism: A database coordinator called a Spectrum Access System (SAS) uses device location, operating parameters, and incumbent information to authorize frequencies and power levels. Devices must register or query the SAS according to the service rules.
Pros: Coordination can make use of capacity that would otherwise remain unused at a particular place or time, while protecting higher-priority users.
Cons: The method requires accurate databases, reliable coordination, device compliance, and procedures for incumbent protection.
Example: The prime example is the Citizens Broadband Radio Service (CBRS) in the $3.5 \text{ GHz}$ band in the United States. It creates a three-tiered sharing system: incumbent users, including federal radar systems, Priority Access License (PAL) users who purchase licenses for parts of the band, and General Authorized Access (GAA) users who operate under SAS coordination, accept interference from higher tiers, and must avoid causing harmful interference to them.