Single-sideband modulation

In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a refinement of amplitude modulation which uses transmitter power and bandwidth more efficiently. Amplitude modulation produces an output signal that has twice the bandwidth of the original baseband signal. Single-sideband modulation avoids this bandwidth doubling, and the power wasted on a carrier, at the cost of increased device complexity and more difficult tuning at the receiver.

Radio transmitters work by mixing a radio frequency (RF) signal of a specific frequency, the carrier wave, with the signal to be broadcast. The result is a set of frequencies with a strong peak signal at the carrier frequency, and smaller signals from the carrier frequency plus the maximum frequency of the signal, and the carrier frequency minus the maximum frequency of the signal. That is, the resulting signal has a spectrum with twice the bandwidth of the original input signal. In conventional AM radio, this signal is then sent to the radio frequency amplifier, and then to the broadcast antenna. Due to the nature of the amplification process, the quality of the resulting signal can be defined by the difference between the maximum and minimum signal energy. Normally the maximum signal energy will be the carrier itself, perhaps twice as powerful as the mixed signals.

SSB takes advantage of the fact that the entire original signal is encoded in either one of these sidebands. It is not necessary to broadcast the entire mixed signal, a suitable receiver can extract the entire signal from either the upper or lower sideband. This means that the amplifier can be used much more efficiently. A transmitter can choose to send only the upper or lower sideband, the portion of the signal above or below the carrier. By doing so, the amplifier only has to work effectively on one half the bandwidth, which is generally easier to arrange. More importantly, with the carrier suppressed before it reaches the amplifier, it can amplify the signal itself to higher energy, it is not wasting energy amplifying a signal, the carrier, that can (and will) be re-created by the receiver anyway.

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