In the light of increasing spectral congestion in the land mobile radio bands below 1 GHz,a renewed interest in Single Sideband (SSB) modulation has been observed world-wide. In this context, much of the work described in this thesis has been directed towards assessing and developing audio signal processing techniques that enable the spectrum and power efficient SSB systems to acheive a comparable performance with that of existing wideband modulation systems. Statistical properties of the land mobile-radio propagation are presented showing that the received signal can be adversely affected by unwanted random phase and envelope modulations. Their effect upon AM, FM and SSB systems are discussed. A novel space diversity system employing a technique termed as Feedforward Signal Regeneration (FFSR) is presented which simultaneously performs the operation of an equal gain combining and suppression of both the random envelope and phase modulations. In connection with the use of FFSR techniques in SSB systems, a phase-locked Transparent-Tone-In-Band (TTIB) pilot-tone configuration which allows the transmission of a "transparent" pilot-tone, positioned centrally within the audio bandwidth, is discussed. Finally, efficient coherent and non-coherent data transmission in the mobile-radio environment is shown to be possible by the use of the TTIB/FFSR diversity system.