spreadspectrum

Spread spectrum is a method of transmitting a signal by spreading its energy over a wider bandwidth than the minimum necessary to send the information. The technique is designed to improve resistance to interference and eavesdropping, enable multiple users to share the same spectrum, and sometimes lower the probability of detection.

Two common forms are direct-sequence spread spectrum (DSSS) and frequency-hopping spread spectrum (FHSS). In DSSS, the data stream is multiplied by a pseudorandom code at a much higher rate, spreading the signal across a wider band. The receiver, using the same code, despreads the signal to recover the original data. In FHSS, the carrier hops among a predefined set of frequencies according to a pseudorandom sequence, so the signal occupies different frequencies over time. Both methods require synchronization between transmitter and receiver and increase the occupied bandwidth by a spreading factor.

Applications of spread spectrum include military communications, where jamming resistance is valuable, and civilian systems such as GPS, which uses spread-spectrum signals to maintain robustness in noisy environments. Some wireless technologies have used spread-spectrum principles—Bluetooth employs frequency hopping, while older Wi-Fi standards like 802.11b used direct-sequence spread spectrum. In mobile networks, direct-sequence CDMA is used to allow multiple users to share the same spectrum.

Historically, spread spectrum traces to early 20th-century concepts and popularized by Hedy Lamarr and George Antheil's 1941 proposal for frequency hopping, with later technical development for practical communication systems. Regulatory frameworks often permit spread-spectrum use in unlicensed bands under rules that control spectral density and coexistence with other services.