[0os78],[Lus7SJ,[We178]. All three systems transmit a low level pilot tone, usually
-7.5 dB to -15 dB below the peak envelope power of the single sideband signal.
They essentially differ in the spectral positioning of the low level pilot tone.
One system transmits a low level carrier along with the sideband signal (tone-inband),
while the other two place a pilot tone above or within the SSB band.
The tone-in-band SSB system offers many advantages which make it particularly
suited to the mobile radio environment. In this technique, a small portion
of the audio spectrum is removed from the central region of the audio band
using a notch ifiter, and a low level pilot tone is inserted in its place. This has the
advantage of maintaining the low bandwidth property of the SSB signal, while
at the same time providing good adjacent channel protection. Due to very high
correlation between the fades experienced by the pilot tone and the audio signals,
a tone-in-band system makes it possible to employ some form of feedforward
automatic gain and frequency control to mitigate the effects of multipath
induced fading.
