**Frequency Hopping Spread Spectrum (FHSS)** - automatic switching of the transmitted frequency. A short burst of data is transmitted on a narrowband then the transmitter quickly retunes to another frequency and transmits again.

Selection of the frequency to be transmitted (hops) is made in a pseudo-random manner from a set of frequencies over a band that is larger than the information bandwidth. The receiver hops in synchronisation with the code of the transmitter to retrieve the data.

**Direct Sequence Spread Sprectrum (DSSS)** - chops the data into small pieces and spreads them across the frequency domain. It can use Code Division Multiple Access (CDMA).

### Modulation Techniques

**Binary Phase Shift Keying (BPSK)** The phase of the carrier is shifted either 0 degrees or 180 degrees depending on the input data stream. Differential modulation is used to resolve phase ambiguity which could otherwise occur.

Each output data bit is dependant on the level of the present input. A logic '1' at the input inverts the phase of the carrier and a logic '0' maintains the current phase.

**Orthogonal Phase Shift Keying (OPSK)** uses 8 different combinations of the Welsh code to represent each combination of 3 data bits. The Welsh codes are 8-bit codes.

In the Welsh code each 8-bit combination differs from all other valid codes by 4-bits so that each code is easily recognisable even if one bit is missing.

**Binary Frequency Shift Keying (BFSK)** the transmit carrier is shifted between its nominal centre frequency and another adjacent frequency, the 2 frequencies corresponding to the 2 possible values in a binary data stream.

**Multi-Frequency Shift Keying (MFSK)** represents using many different frequencies usually numbering 4, 32 or 64 different carrier frequencies.

**4 Frequency Shift Keying (4FSK)** uses 4 frequencies to denote a different combinations of data bit pairs:-

00 01 10 11
Each pair is called a symbol. As there are two data bits per symbol the data rate is twice the symbol rate.

See also

Scanner Definitions.

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