Olivia MFSK
Olivia MFSK is an amateur radioteletype protocol, using multiple frequency-shift keying (MFSK) and designed to work in difficult (low signal-to-noise ratio plus multipath propagation) conditions on shortwave bands.
It is commonly used by amateur radio operators to reliably transmit ASCII characters over noisy channels using the high frequency (3–30 MHz) spectrum.
The tests proved that the protocol works well and can allow regular intercontinental radio contacts with as little as one watt RF power.
Since 2005 Olivia has become a standard for digital data transfer under white noise, fading and multipath, flutter (polar path) and auroral conditions.
As a result, amateur radio operators have voluntarily decided upon channelization for this mode.
To accommodate for different conditions and for the purpose of experimentation the bandwidth and the baud rate can be changed.
[2] For the second FEC layer: every ASCII character is encoded as one of 64 possible Walsh functions (or vectors of a Hadamard matrix).
In order to avoid simple transmitted patterns (like a constant tone) and to minimize the chance for a false lock at the synchronizer the characters encoded into the Walsh function pass through a scrambler and interleaver.
This stage simply shifts and XORs bits with predefined scrambling vectors and so it does not improve the performance where the white (uncorrelated) noise is concerned, but the resulting pattern gains certain distinct characteristics which are of great help to the synchronizer.
The receiver synchronizes automatically by searching through possible time and frequency offsets for a matching pattern.
To adapt the system to different propagation conditions, the number of tones and the bandwidth can be changed and the time and frequency parameters are proportionally scaled.
The modulation layer of the Olivia transmission system in the default mode sends one of 32 tones at a time.
The two layers (MFSK+Walsh function) of the FEC code can be treated as a two dimensional code: the first dimension is formed along the frequency axis by the MFSK itself while the second dimension is formed along the time axis by the Walsh functions.
The scrambling and simple bit interleaving is applied to make the generated symbol patterns appear more random and with minimal self-correlation.
The listed audio files both are encoded with the message: "Welcome to Wikipedia, the free encyclopedia that anyone can edit."
This reduced character set does not print out in both upper and lower case (such as RTTY).
