Combined Frequency and Time-Shift Keyed Transmission Systems

Consideration is given to a transmission technique which is based on the combination of frequency-shift keying (FSK) and time-shift keying (TSK). In the resulting frequency and timeshift keyed (FTSK) transmission, the two possible states of a binary waveform are represented by the order of two pulses which occur on different frequencies, for instance, f
1
and f
2
. For example, if the ordered pair
(f_{1}, f_{2})
represents a Mark, then
(f_{2}, f_{1})
represents a Space. The power spectrum, as well as the performance of both coherent and noncoherent systems in an interference environment, is investigated. The following cases are considered: 1) white noise interference, with and without fading and 2) CW interference. It is shown that with white noise interference and no fading, the performance of a coherent FTSK system is identical to that of coherent FSK and TSK systems operating at the same data rate. Under similar conditions, the performance of a noncoherent FTSK system would be slightly degraded (less than 1 db for digit error probabilities less than 10
^-1
) compared to that of noncoherent FSK and TSK systems. FTSK's dual frequency diversity is shown to provide a substantial antifading capability to both coherent and noncoherent transmissions. Furthermore, it is observed that FTSK offers the potential for: 1) complete rejection of CW and other forms of narrow-band interference, 2) simplified postdetection signal coupling circuits, 3) enhanced performance of digit synchronization circuits and 4) error detection and correction.