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The well-known Shannon-Hartley law tells us that there is an absolute limit on the error-free
bit rate that can be transmitted within a certain bandwidth at a given signal to noise ratio
(SNR). Although it is not obvious, this law can be restated (given here without proof) by
saying that for a given bit rate, one can trade off bandwidth and power. On this basis then, a
certain digital communications system could be either bandwidth limited or power limited,
depending on its design criteria.
Practice also tells us that digital communication systems designed for HF are necessarily
designed with two objectives in mind; slow and robust to allow communications with weak
signals embedded in noise and adjacent channel interference, or fast and somewhat subject to
failing under adverse conditions, however being able to best utilize the HF medium with
good prevailing conditions.
Taken that the average amateur radio transceiver has limited power output, typically 20 - 100
Watts continuous duty, poor or restricted antenna systems, fierce competition for a free spot
on the digital portion of the bands, adjacent channel QRM, QRN, and the marginal condition
of the HF bands, it is evident that for amateur radio, there is a greater need for a weak signal,
spectrally-efficient, robust digital communications mode, rather than another high speed,
wide band communications method.
Recent Developments using PSK on HF
It is difficult to understand that true coherent demodulation of PSK could ever be achieved in
any non-cabled system since random phase changes would introduce uncontrolled phase
ambiguities. Presently, we have the technology to match and track carrier frequencies
exactly, however tracking carrier phase is another matter. As a matter of practicality thus, we
must revert to differentially coherent phase demodulation (DPSK).
Another practical matter concerns that of symbol, or baud rate; conventional RTTY runs at
45.45 baud (a symbol time of about 22 ms.) This relatively-long symbol time have been
favored as being resistant to HF multipath effects and thus attributed to its robustness.
Symbol rate also plays an important part in determining spectral occupancy. In the case of a
45.45 baud RTTY waveform, the expected spectral occupancy is some 91 Hz for the major
lobe, or +/- 45.45 on each side of each the two data tones. For a two tone FSK signaling
system of continuous-phase frequency-shift keying (CPFSK) paced at 170 Hz, this system
would occupy approximately 261 Hz.
Signal space representation
• Band pass Signal
• Real valued signal S(f) Ù S* (-f)
• finite bandwidth B Ù infinite time span
• f c denotes center frequency
• Negative Frequencies contain no Additional Info
Characteristics: