spectrum

In radio-frequency engineering, spectrum refers to the portion of the electromagnetic range allocated for a particular purpose. A transmitter occupies a band of frequencies centered on a carrier frequency, and the width of that band is called bandwidth. The Federal Communications Commission in the United States, the ITU internationally, and comparable bodies in each country divide the usable radio spectrum into slices and assign them to broadcasters, cellular operators, military users, and industrial applications. A mobile phone network, for instance, might operate in the 700 MHz, 850 MHz, 1.9 GHz, or 2.4 GHz bands, each band split further into narrower channels.

Spectrum is not necessarily contiguous. A broadcaster might hold licenses to non-adjacent frequency blocks, or a company might operate across multiple separate bands. The term "spectrum" can mean the frequencies themselves, the right to use them (a license), or the actual electromagnetic energy present in a given frequency range. In practice, when an engineer says a signal occupies 20 MHz of spectrum, they mean the signal's energy spreads across a 20 MHz-wide band, not that it fills every frequency perfectly uniformly.

Spectrum sharing and interference

Because spectrum is finite and valuable, regulators control who can broadcast on which frequencies and with how much power. Two transmitters operating on the same frequency in the same area will interfere with each other, garbling both signals. Licensed spectrum gives exclusive rights to a band; unlicensed spectrum (such as the 2.4 GHz industrial, scientific, and medical band) is open to anyone but users must tolerate interference from other devices. Modern systems use techniques like frequency-hopping, spread-spectrum modulation, and beam steering to share spectrum more efficiently or reduce the impact of interference.

Spectrum efficiency is measured in bits per second per hertz (bit/s/Hz). As demand for wireless data has exploded, engineers have developed more sophisticated modulation schemes and coding to extract more throughput from the same amount of spectrum. A 20 MHz cellular channel using older technology might carry 5 Mbps; the same channel with advanced modulation can carry 100 Mbps or more.

The radio spectrum extends from extremely low frequency (ELF, below 3 kHz) through microwave, millimeter-wave, and into the terahertz region. Different parts of the spectrum behave differently: lower frequencies propagate farther and penetrate obstacles better, but require larger antennas; higher frequencies allow smaller antennas and higher data rates but are absorbed by rain, foliage, and walls. Engineers select frequency bands based on the trade-offs required by their application.