Frequency-hopping spread spectrum

Lore & Background

The earliest mentions of frequency hopping in open literature are in US patent 725,605, awarded to Nikola Tesla on March 17, 1903, and in radio pioneer Jonathan Zenneck's book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck writes that Telefunken had already tried it. The German military made limited use of frequency hopping for communication between fixed command points in World War I to prevent eavesdropping by British forces. In 1920, Otto B. Blackwell, De Loss K. Martin, and Gilbert S. Vernam filed a patent application for a 'Secrecy Communication System', granted as U.S. Patent 1,598,673 in 1926, describing a method of transmitting signals on multiple frequencies in a random manner for secrecy. In 1942, actress Hedy Lamarr and composer George Antheil received a patent for their 'Secret Communications System', an early version of frequency hopping using a piano-roll to switch among 88 frequencies to make radio-guided torpedoes harder for enemies to detect or jam. They donated the patent to the U.S. Navy. During World War II, the US Army Signal Corps invented a communication system called SIGSALY, which incorporated spread spectrum in a single frequency context, but its existence was not known until the 1980s. In 1957, engineers at Sylvania Electronic Systems Division adopted a similar idea using the transistor. In 1962, the US Navy utilized Sylvania's work during the Cuban Missile Crisis. A practical application was developed by Ray Zinn, co-founder of Micrel Corporation, who developed a method allowing radio devices to operate without the need to synchronize a receiver with a transmitter; he received a patent in 2006 for his 'Wireless device and method using frequency hopping and sweep modes.'

Reader's Guide

FHSS offers four main advantages over fixed-frequency transmission: high resistance to narrowband interference because the signal hops to a different frequency band; difficulty of interception if the frequency-hopping pattern is not known; difficulty of jamming if the pattern is unknown (the signal can be jammed only for a single hopping period); and the ability to share a frequency band with many types of conventional transmissions with minimal mutual interference. The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency, but because transmission occurs only on a small portion of this bandwidth at any given time, the instantaneous interference bandwidth is the same. While providing no extra protection against wideband thermal noise, the frequency-hopping approach reduces the degradation caused by narrowband interference sources. One challenge is synchronizing transmitter and receiver; one approach uses a guarantee that the transmitter will use all channels in a fixed period, allowing the receiver to find the transmitter by picking a random channel and listening for valid data identified by a special sequence and checksum. 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 for non-spread-spectrum systems—1 watt versus 1 milliwatt. Adaptive frequency-hopping spread spectrum (AFH), as used in Bluetooth, improves resistance to interference by avoiding crowded frequencies, but if radio frequency interference is dynamic, AFH's strategy of 'bad channel removal' may not work well.

Did You Know?

• Nikola Tesla was awarded US patent 725,605 on March 17, 1903, which alludes to frequency hopping.
• Hedy Lamarr and George Antheil received a patent in 1942 for a 'Secret Communications System' using a piano-roll to switch among 88 frequencies.
• The German military used frequency hopping in World War I to prevent eavesdropping by British forces.
• In the US, FCC rules allow FHSS systems in the unlicensed 2.4 GHz band to transmit at 1 watt, compared to 1 milliwatt for non-spread-spectrum systems.