Transmission of data and digital speech over land mobile radio

During the last few years there has been increased interest in the transmission of digital voice over land mobile radio. The problem with digital voice is that quality of the reconstructed voice is directly related to the bit rate and hence bandwidth. The mobile radio is severely bandlimited so that achievable bit rates and voice quality are also limited. Furthermore, the mobile radio channel suffers multipath fading. This thesis describes a spectrum efficient method of transmitting digital voice over a bandlimited radio frequency (R.F.) fading channel. A circuit which simulates multipath fading is described. Using this fading simulator, experiments are set up to determine the error performance of Differential Phase Shift Keying (DPSK) and Fast Frequency Shift Keying (FFSK) modems when digital information is transmitted in a fading channel. It is shown that direct R.F. modulation (like FFSK) utilizes the spectrum more efficiently than subcarrier modulation (here represented by DPSK) and hence allows higher transmission rates for a given channel bandwidth. Finally, speech is digitized using the continuously variable slope Delta Modulation (CVSD) codec and transmitted by FFSK. The recovered speech is judged subjectively and found to be quite natural. Thus very good quality speech is obtained at 16kb/s even under fading conditions.