Spread frequency shift keying

A binary modulation and demodulation technique is presented which is particularly suited for channels with nonwhite interferers and/or for channels with different attenuations at the two signalling frequencies. Because spread frequency shift keying is mainly based on classical FSK modem technology it can be easily implemented and it is well suited for low cost applications. The improvement in performance compared to FSK can be up to 15 dB depending on the characteristics of the channel. Finally, the use of a convolutional code leads to an additional gain of up to 4 dBs