Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many distinct frequencies occupying a large spectral band. The changes are controlled by a code known to both transmitter and receiver. FHSS is used to avoid interference, to prevent eavesdropping, and to enable code-division multiple access (CDMA) communications.

With frequency hopping spread spectrum (FHSS), the signal is broadcast over a seemingly random series of radio frequencies, hopping from frequency to frequency at fixed intervals. A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message. Would-be eavesdroppers hear on ly unintelligible blips. Attempts to jam the signal on one frequency succeed only It knocking out a few bits of it.

In FHSS, the transmitter hops between available narrowband frequencies within a specified broad channel in a pseudo-random sequence known to both sender and receiver.

A short burst of data is transmitted on the current narrowband channel, and then the transmitter and receiver tune to the next frequency in the sequence for the next burst of data. In most systems, the transmitter will hop to a new frequency more than twice per second.

Because no channel is used for long and the odds of any other transmitter being on the same channel at the same time are low, FHSS is often used as a method to allow multiple transmitter and receiver pairs to operate in the same space on the same broad channel at the same time.

Fig: Frequency-hopping spread spectrum system: A typical block diagram for a frequency hopping system is shown in the above figure. For transmission, binary data are fed into a modulator using some digital-to-analog encoding scheme, such as frequency-shift keying (FSK) or binary phase-shift keying (BPSK). The resulting signal Sd(t) is centered on some base frequency. A pseudonoise (PN), or pseudorandom number, source serves as an index into a table of frequencies; this is the spreading code referred to previously. Each k bits of the PN source specifies one of the 2k carrier ""requencies. At each successive interval (each k PN bits), a new carrier frequency c(t) is selected. This frequency is then modulated by the signal produced from thE initial modulator to produce a new signal set with the same shape but now cent ~red on the selected carrier frequency. On reception, the spread spectrum signal i; demodulated using the same sequence of PN-derived frequencies and then demodulated to produce the output data.