Our understanding of spectrum has changed a great deal since Marconi first spanned the Atlantic with his “wireless telegraph message”. In 1902, he used the whole spectrum available at the time to send a few bits per second over thousands of square kilometres.
The spark transmitter used for Marconi’s telegraph occupied all electromagnetic frequencies available to existing receivers. As a result, no one could use radio for communication within a 3500 km radius of the transmitting station in England.
If other users wanted to send messages in that area, they needed to coordinate their transmissions in different “time slots” in order to share the medium. This technique became known as Time Division Multiple Access or TDMA.
Users located further than 3500 km from Marconi’s transmitter could use the spectrum because the power of radio waves decreases as we move farther away from the transmitter. Reusing the spectrum in different geographical areas is called Space Division Multiple Access or SDMA.
Marconi was later able to build a transmitter that could restrict emissions to just a range of frequencies, and a receiver that could be “tuned” to a particular frequency range. This allowed many users to transmit in the same area (space) and at the same time. This process of assigning different frequencies to different users became known as Frequency Division Multiple Access or FDMA. With FDMA, radio became a practical means of communication, and the only technology capable of reaching ships in open seas.
National agencies were created to coordinate allocation of frequencies to different users. Since radio waves are not stopped by national borders, however, international agreements were also needed. The international organization that had been created to regulate the transmission of telegrams among different countries was commissioned to allocate use of electromagnetic spectrum. Today, the International Telecommunications Union (ITU), is the oldest United Nations agency, and issues recommendations for frequency use to 193 United Nations (UN) members.
The use of spectrum for military applications raised a new issue. “Jamming” refers to intentional interference in spectrum to impede communication . To make jamming more difficult, a new technique was developed in which the information to be transmitted was combined with a special mathematical code. Only receivers with knowledge of that particular code could interpret the information. The coded signal was transmitted at low power but using a very wide interval of frequencies. This technique was later adapted to civilian applications is called Code Division Multiple Access or CDMA. Today CDMA is used extensively in modern communications systems.
In summary, spectrum can be shared among many users by assigning different time slots, different frequency intervals, different regions of space, or different codes. A combination of these methods is used in the latest cellular systems.