Frequency Hopped Spread Spectrum (FHSS)

Frequency hopping relies on frequency diversity to combat interference. This is accomplished by multiple frequency, code selected, FSK. Basically, the incoming digital stream is shifted in frequency by an amount determined by a code that spreads the signal power over a wide bandwidth. In comparison to binary FSK, which has only two possible frequencies, FHSS may have 2*10^20 or more.

The FHSS transmitter is a pseudo-noise PN code controlled frequency synthesizer. The instantaneous frequency output of the transmitter jumps from one value to another based on the pseudo-random input from the code generator (see Fig. 1). Varying the instantaneous frequency results in an output spectrum that is effectively spread over the range of frequencies generated.

Fig.1 FHSS Spectrum

In this system, the number of discrete frequencies determines the bandwidth of the system. Hence, the process gain is directly dependent on the number of available frequency choices for a given information rate.

Another important factor in FHSS systems is the rate at which the hops occur. The minimum time required to change frequencies is dependent on the information bit rate, the amount of redundancy used, and the distance to the nearest interference source.