The wireless age
Subsequent to the mathematical theory of electromagnetic waves formulated by James Clerk Maxwell in 1873 and the demonstration of the existence of these waves by Heinrich Hertz in 1887, Guglielmo Marconi made history by using radio waves for transatlantic wireless communications in 1901. In 1906, amplitude modulation (AM) radio was invented by Reginald Fessenden for music broadcasting. In 1913, Edwin H. Armstrong invented the superheterodyne receiver, based on which the first broadcast radio transmission took place at Pittsburgh in 1920. Land-mobile wireless communication was first used in 1921 by the Detroit Police Department. In 1929, Vladimir Zworykin performed the first experiment of TV transmission. In 1933, Edwin H. Armstrong invented frequency modulation (FM). The first public mobile telephone service was introduced in 1946 in five American cities. It was a half-duplex system that used 120 kHz of FM bandwidth. In 1958, the launch of the SCORE (Signal Communication by Orbital Relay Equipment) satellite ushered in a new era of satellite communications. By the mid-1960s, the FM bandwidth was cut to 30 kHz. Automatic channel trunking was introduced in the 1950s and 1960s, with which full-duplex was introduced. The most important breakthrough for modern mobile communications was the concept of cellular mobile systems by AT&T Bell Laboratories in the 1970s.