A reliable method for broadcasting digital audio simultaneously with existing analog FM radio is presented. According to this linear combining scheme, the digital data sequence is adaptively modulated so as to achieve high data rates while ensuring that the interference incurred on the analog FM signal remains at acceptably low levels. The digital transmission is based on adaptive orthogonal frequency division multiplexing. Implementable, near optimum algorithms for insertion of the maximum number of carriers at all times are presented, which incur negligible distortion on the analog FM signal. The digital receiver response of the analog FM signal (known to the transmitter) is computed and canceled from the aggregate signal prior to signal transmission. As a result of this precanceling process the digital transmission is free from analog FM interference. We present simulation results on acoustic signals where the average digital data use up to 50% of the 200 kHz power spectrum with digital signal power levels 25-35 dB below the analog signal. A control channel is used for the positions of the modem carriers.
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