Olivia MFSK - The Technical Details

The Technical Details

Being a digital protocol, Olivia transmits a stream of ASCII (7-bit) characters. The characters are sent in blocks of 5. Each block takes 2 seconds to transmit, thus the effective data rate is 2.5 character/second or 150 characters/minute. The most common transmission bandwidth is 1000 Hz and the baud rate is 31.25 MFSK tones/second. To accommodate for different conditions and for the purpose of experimentation the bandwidth and the baud rate can be changed.

The Olivia transmission system is constructed of two layers: the lower, modulation and forward error correcting (FEC) code layer is a classical multiple frequency-shift keying (MFSK) while the higher layer is a forward error correcting code based on Walsh functions.

Both layers are of similar nature: they constitute a "1-out-of-N" FEC code. For the first layer the orthogonal functions are (co)sine functions, with 32 different frequencies (tones). At a given time only one of those 32 tones is being sent. The demodulator measures the amplitudes of all the 32 possible tones (using a Fourier transform ) and (knowing that only one of those 32 could have been sent) picks up the tone with the highest amplitude. See the equations and graphs behind the MFSK layer here .

For the second FEC layer: every ASCII character is encoded as one of 64 possible Walsh functions (or vectors of a Hadamard matrix). The receiver again measures the amplitudes for all 64 vectors (here comes the Hadamard Transform) and chooses the greatest. See the algorithms and more details here .

For optimal performance the actual demodulators work with soft decisions and the final (hard) decision to decode a character is taken only at the second layer. Thus the first layer demodulator actually produces soft decisions for each of the 5 bits associated to an MFSK tone instead of simply picking up the highest tone to produce hard decisions for those 5 bits.

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. The frequency search range is normally +/- 100 Hz but can be as high as +/- 500 Hz if the user wishes so.