FHSS - Computer Definition
A signal modulation technique employed in radio communications, FHSS transmits short bursts of data over a range of frequency channels within the wideband carrier. Each transmission is assigned a 10-bit pseudorandom binary code sequence, which comprises a series of ones and zeros in a seemingly random pattern known to both the transmitter and receiver.The original code sequence is mathematically self-correlated to yield a code that stands out from all others, at least on average. The paired transmitters and receivers recognize their assigned and correlated code sequences, which look to all others as pseudorandom noise (PN). FHSS phase-modulates the carrier wave with a continuous string of PN code symbols, or chips, resulting in a chip rate that can be much faster than the bit rate.Thereby, the noise signal occurs with much greater frequency than the original data signal and spreads the signal energy over a much wider band.The transmitter and receiver hop from one frequency to another in a carefully choreographed hop sequence under the control of the centralized base station antenna. Each transmission dwells on a particular frequency for a very short period of time (no more than 400 milliseconds for FCC-controlled applications), which may be less than the time interval required to transmit a single data packet, or symbol, or even a single bit. So, the chip rate can be faster than the bit rate. A large number of other transmissions also may share the same range of frequencies simultaneously, with each using a different hop sequence.The potential remains, however, for the overlapping of packets.The receiving device can distinguish each packet in a packet stream by reading the various codes prepended to the packet data transmissions, and treating competing signals as noise. Code division multiple access (CDMA) and Bluetooth employ FHSS, which is much like the original SS technology patented by Hedy Lamarr in 1942. See also Bluetooth, CDMA, carrier, channel, chip, chip rate, frequency, hop sequence, modulation, PN, signal, symbol, and wideband. See also Lamarr, Hedy.