In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of signal modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modulation, it uses transmitter power and bandwidth more efficiently. Amplitude modulation produces an output signal the bandwidth of which is twice the maximum frequency of the original baseband signal. Single-sideband modulation avoids this bandwidth increase, and the power wasted on a carrier, at the cost of increased device complexity and more difficult tuning at the receiver.
Basic concept In conventional amplitude modulation (AM), an audio signal controls the amplitude of a radio-frequency carrier, producing a carrier plus two mirror-image sidebands. Each sideband contains a complete copy of the original information, while the carrier itself conveys none. Consequently, an AM signal occupies a bandwidth equal to twice the highest audio frequency and expends a large fraction of the transmitted power on the carrier and redundant sideband. This spectral structure of AM is described in classic radio texts, including Everitt's treatments of modulation theory. Single-sideband modulation (SSB) is derived directly from AM by removing this redundancy. Since either sideband alone contains the entire modulating signal, SSB transmits only one sideband and usually suppresses the carrier. Compared with AM, SSB requires approximately half the bandwidth and uses transmitter power more efficiently. SSB signals are typically generated at low power using filtering or phase-cancellation techniques and then amplified linearly. Because the carrier is suppressed, SSB reception requires reinsertion of a locally generated carrier and greater frequency stability than AM. AM can be generated and received with relatively simple equipment, while SSB is used primarily where efficiency and range are important. In amateur radio, prior to widespread digital voice, most HF voice operation moved from AM to SSB.
History The first U.S. patent application for SSB modulation was filed on December 1, 1915, by John Renshaw Carson. The U.S. Navy experimented with SSB over its radio circuits before World War I. SSB first entered commercial service on January 7, 1927, on the longwave transatlantic public radiotelephone circuit between New York and London. The high power SSB transmitters were located at Rocky Point, New York, and Rugby, England. The receivers were in very quiet locations in Houlton, Maine, and Cupar, Scotland. SSB was also used over in carrier telephony over long-distance telephone lines, using a technique known as frequency-division multiplexing (FDM). FDM was pioneered by telephone companies in the 1930s. With this technology, many simultaneous voice channels could be transmitted on a single physical circuit, for example in L-carrier. With SSB, channels could be spaced (usually) only 4,000 Hz apart, while offering a speech bandwidth of nominally 300 Hz to 3,400 Hz. Amateur radio operators began serious experimentation with SSB after World War II. The Strategic Air Command established SSB as the radio standard for its aircraft in 1957. It has become a de facto standard for long-distance voice radio transmissions since then. In December 1956, the Proceedings of the IRE devoted a full issue to single-sideband transmission, covering its history, theory, and practical implementation across spectrum management, transmitters, receivers, filtering, power amplifiers, and military, commercial, and amateur applications, including Oswald's historical review and Weaver's third method of SSB generation and detection.
Mathematical formulation
Single-sideband has the mathematical form of quadrature amplitude modulation (QAM) in the special case where one of the baseband waveforms is derived from the other, instead of being independent messages:
where s ( t ) {\displaystyle s(t)\,} is the message (real-valued), s ^ ( t ) {\displaystyle {\widehat {s}}(t)\,} is its Hilbert transform, and f 0 {\displaystyle f_{0}\,} is the radio carrier frequency. To understand this formula, we may express s ( t ) {\displaystyle s(t)} as the real part of a complex-valued function, with no loss of information:
s ( t ) = Re { s a ( t ) } = Re { s ( t ) + j ⋅ s ^ ( t ) } , {\displaystyle s(t)=\operatorname {Re} \left\{s_{\mathrm {a} }(t)\right\}=\operatorname {Re} \left\{s(t)+j\cdot {\widehat {s}}(t)\right\},}
where j {\displaystyle j} represents the imaginary unit. s a ( t ) {\displaystyle s_{\mathrm {a} }(t)} is the analytic representation of s ( t ) , {\displaystyle s(t),} which means that it comprises only the positive-frequency components of s ( t ) {\displaystyle s(t)} :
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