A narrowband multi-ary FSK transmission principle using sampling function waveform modulation

Conventional FSK uses shift frequencies of integer times the frame rate. If shift frequencies using non-integers (NIFSK) could be used, the frequency utilization efficiency would improve due to the capability of multi-ary transmission. However, since a framed sequence consisting of NIFSK includes abrupt transients, the waveform suffers from considerable distortion when it is transmitted via a bandlimited path. To solve this problem, we propose sampling function waveform modulation system (SAFWM). SAFWM uses a modified sampling function waveform that is immune to intersymbol interference generated in the process of conveying each sampled value of an FSK signal. The transmission bandwidth is only slightly larger than that of zero to the maximum shift frequency. At the receiver, the sampled values are restored by correlation detection with the same waveforms, and are transformed into the original FSK. To accurately restore a waveform of transmitted NIFSK signals, frame extension is introduced. Distortionless transmission performed with this method is confirmed by computer simulation. This principle can be applied not only for FSK/PSK but also for FSK-SS (spread spectrum) and FH-SS, resulting in improving their frequency utilization efficiencies.