What is Frequency Hoping Spread Spectrum

Frequency hoping spread spectrum is a technique for the radio transmission of electromagnetic signals by jumping the carrier between various frequency channels on a wide frequency band.  This jumping takes place according to a pre-generated pseudorandom sequence; the knowledge of the sequence by the transmitter and the receiver helps them to maintain synchronization with each other.

This technique has the following advantages over the traditional transmission of a carrier on a single frequency channel:-

FHSS signal is much more resistant to narrowband interference than the traditional single frequency signal. This is because of the fact that the FHSS signal is spread out over a vast frequency band and the narrowband interference signal is only able to affect a very small part which leads to only a negligible degradation in the overall quality of the signal. FHSS signal provides a higher level of security as the signal can’t be intercepted by the eaves dropper without the knowledge of the pseudorandom sequence. The signal in such a case only appears as useless background noise. It leads to a much more effective utilization of bandwidth as a number of devices can use the same frequency band at the same time just by using different pseudorandom sequences. Also the FHSS signal causes very little interference to the narrowband operating devices in the frequency range.

A recent advance in the technology has been the use of Adaptive Hopping to further reduce the interference of FHSS signals with the narrowband signals in the bandwidth. Basically the narrowband region is skipped in the carrier jump sequence so that no interference takes place.

The communication operation is very simple to explain in an FHSS system:-

One party initiates the communication by sending a request to the other device on a fixed frequency known as the control channel. The other device if free acknowledges this request and sends a seed number to the initiating party. Both parties then calculate the pseudorandom sequence for the transmission by using the seed number in a pre-defined algorithm. The initiating device than sends a synchronization signal on the first frequency of the calculated sequence to let the other device get ready for communication. The radio transmission than takes place between the two devices according to the calculated sequence. This occurs by both the devices tuning to the frequencies according to the generated sequence.

Many recent wireless technologies like WLAN, Bluetooth etc. utilizes the FHSS transmission method for the advantages it provides over the conventional single frequency carrier systems.

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