Spectrum is the range of electromagnetic frequencies available for transmission. It spans from radio waves to gamma rays. Telecom uses the radio portion, from about 3 kHz to 300 GHz. Spectrum is a finite resource. It is allocated by governments. It is auctioned to carriers. It is shared by unlicensed devices.

Spectrum is divided into bands. Low bands below 1 GHz travel far and penetrate buildings. Mid bands from 1 to 6 GHz balance coverage and capacity. High bands above 24 GHz carry lots of data but travel short distances. Carriers want a mix of all three.

Spectrum bands

• Low band: coverage, rural areas
• Mid band: balance, urban and suburban
• High band: capacity, dense areas
• Licensed: exclusive, carriers
• Unlicensed: shared, Wi-Fi and Bluetooth

Spectrum is valuable. Governments raise billions auctioning it. Carriers fight over it. Engineers invent ways to use it more efficiently. Modulation, multiplexing, and MIMO squeeze more data into the same frequencies. Spectrum is the oil of the wireless world. It is scarce, valuable, and contested.