![]() In 1899 Guglielmo Marconi experimented with frequency-selective reception in an attempt to minimise interference. The FCC also prescribes a minimum number of frequency channels and a maximum dwell time for each channel. Both FHSS and direct-sequence spread-spectrum (DSSS) systems can transmit at 1 watt, a thousandfold increase from the 1 milliwatt limit on non-spread-spectrum systems. In the US, FCC part 15 on unlicensed spread spectrum systems in the 902–928 MHz and 2.4 GHz bands permits more power than is allowed for non-spread-spectrum systems. The transmitter and receiver can use fixed tables of frequency-hopping patterns, so that once synchronized they can maintain communication by following the table. The transmitter's data is identified by a special sequence of data that is unlikely to occur over the segment of data for this channel, and the segment can also have a checksum for integrity checking and further identification. The receiver can then find the transmitter by picking a random channel and listening for valid data on that channel. One approach is to have a guarantee that the transmitter will use all the channels in a fixed period of time. One of the challenges of frequency-hopping systems is to synchronize the transmitter and receiver. While providing no extra protection against wideband thermal noise, the frequency-hopping approach reduces the degradation caused by narrowband interference sources. But because transmission occurs only on a small portion of this bandwidth at any given time, the instantaneous interference bandwidth is really the same. The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. A type of multiple access is achieved allowing hundreds of transmitter/receiver pairs to be operated simultaneously on the same band, in contrast to previous FM or AM radio-controlled systems that had limited simultaneous channels. FHSS technology is also used in many hobby transmitters and receivers used for radio-controlled model cars, airplanes, and drones. Some walkie-talkies that employ FHSS technology have been developed for unlicensed use on the 900 MHz band. eFCC CFR 47 part 15.247 covers the regulations in the US for 902–928 MHz, 2400–2483.5 MHz, and 5725–5850 MHz bands, and the requirements for frequency hopping. In the US, since the Federal Communications Commission (FCC) amended rules to allow FHSS systems in the unregulated 2.4 GHz band, many consumer devices in that band have employed various FHSS modes. United States military radios that use frequency hopping include the JTIDS/MIDS family, the HAVE QUICK Aeronautical Mobile communications system, and the SINCGARS Combat Net Radio, Link-16. This key is generated by devices such as the KY-57 Speech Security Equipment. Military radios generate the frequency-hopping pattern under the control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share in advance. Spread-spectrum signals are highly resistant to deliberate jamming unless the adversary has knowledge of the frequency-hopping pattern. FHSS signals add minimal interference to narrowband communications, and vice versa. FHSS transmissions can share a frequency band with many types of conventional transmissions with minimal mutual interference.Jamming is also difficult if the pattern is unknown the signal can be jammed only for a single hopping period if the spreading sequence is unknown.Signals are difficult to intercept if the frequency-hopping pattern is not known.FHSS signals are highly resistant to narrowband interference because the signal hops to a different frequency band.įHSS offers four main advantages over a fixed-frequency transmission: Interference at a specific frequency will affect the signal only during a short interval. Signals rapidly change ("hop") their carrier frequencies among the center frequencies of these sub-bands in a determined order. ![]() The frequency band is divided into smaller sub-bands. FHSS is used to avoid interference, to prevent eavesdropping, and to enable code-division multiple access (CDMA) communications. The changes are controlled by a code known to both transmitter and receiver. ![]() ![]() Frequency-hopping spread spectrum ( FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many frequencies occupying a large spectral band. ![]()
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