Simultaneous Broadcasting of Analog FM and Digital Signals by Separating Co-Channel FM Signals

A new method is presented to broadcast digital signals simultaneously with the existing analog frequency modulation (FM) radio in the same band. The method involves special signal design at the transmitter and analytic technique of separating two co-channel FM signals at the receiver side. The digital signal is first removed by adding/subtracting the received signal in an orthogonal frequency-division multiplexing (OFDM) symbol basis, resulting in two co-channel FM signals. Then, the two co-channel FM signals are separated within one of two possibilities. The phase ambiguity can be removed naturally by computing the distance between the estimates and the original received signal. After obtaining the FM signal estimates, the digital signals can be obtained by subtracting the estimated analog FM signals from the original received signals. Iterative methods can be adopted to further improve the system performance. Simulation results show that the bit error rate of the recovered digital signal does not depend on the power level difference between the host analog FM and the digital signal and is only 4-dB loss compared with clean OFDM digital systems.