At 10:08 PM 1/16/06, Bill Coleman wrote:
>In thinking of narrow shifts, you can theoretically get down to
>extremely small values. If you think of FSK as two OOK symbols spaced
>a short distance apart, there comes a point when the sidebands of the
>one symbol start to encroach in the spectrum of the opposite symbol.
For non-coherent FSK, the minimum shift that will give orthogonality
is a shift equal to the baud rate.
Orthogonality (very loosely speaking) means that the nulls of the
keying sidebands of the mark tone lines up with the peaks of the
keying sidebands of the space tone so they don't interfere with each other.
For coherent FSK (which in practice this would be generated as
coherent AFSK), the minimum shift that will give orthogonality is
half the baud rate. This shift is commonly called "Minimum Shift Keying (MSK)"
Shifts that are any integral multiple of half the baud rate are also
orthogonal; other shifts are not orthogonal.
A shift of about 0.77 times the baud rate, although it is not
orthogonal, gives the lowest bit error rate for a given energy per
bit to noise spectral density ratio (Eb/No); Eb/No is proportional
to the signal to noise ratio. Even though it's a disadvantage to be
non-orthogonal, the extra shift increases the detected signal more
than enough to compensate; the optimum shift is 0.83 dB better than
the orthogonal minimum shift.
Wider shifts are suboptimal because the receive filter must be wider
and therefore passes more noise.
For 45 baud RTTY, these shifts would be:
Minimum shift keying: 23 Hz
Optimum BER /(Eb/No): 35 Hz
Minimum shift (non-coherent FSK): 45 Hz
If the only noise source present was Gaussian white noise, there'd be
no reason to use more than 45 Hz shift FSK or 35 Hz shift AFSK for 45 baud.
The reasons for using a wider shift is the nature of the channel
errors at HF; these are burst errors caused by selective fading,
impulsive noise bursts or interference.
By transmitting a wider shift and using a receiver that employs
separate mark and space filters with a decision circuit, it is
possible to maintain copy even though the mark or space may be
momentarily obliterated by a channel error.
This isn't the optimal approach for recovering maximum signal energy
from Gaussian noise, but it's the correct choice at HF where
non-Gaussian sources of error predominate.
73,Mike K1MK
Michael Keane K1MK
k1mk@alum.mit.edu
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