Spectrally Efficient Digital Transmission Using Analog FM Radios

This paper explores the feasibility of using 4 level FM to enable digital transmission with a spectrum efficiency on the order of two bits per cycle of transmitted bandwidth when used with Frequency Modulated (FM) radios. A hybrid computer simulation was used to assess the compatibility of 4 level FM with the transfer characteristics of commercial FM radios and also to evaluate the performance of 4 level FM modulation under conditions of radio misalignment. It is shown that 4 level FM is able to provide good spectral efficiency without requiring modification or replacement of most current FM radios, It is also shown that 4 level FM performance is relatively insensitive to the transmission degradations typically encountered in fielded systems.