Frequency hopping spread spectrum (FHSS) is a technique that many ultra low power (ULP) radio protocols use to overcome the problems of interference in the crowded 2.4GHz band. (For instance, Nordic products in the nRF24L Series and µBlue™ Bluetooth low energy families employ protocols that use forms of FHSS.) In essence the technique works by sub-dividing the 2.4GHz band (which actually extends from 2.4 to 2.48GHz) into channels of varying frequency. The transmitter broadcasts on a particular channel and if the receiver detects interference, both jump to a different – and hopefully clearer – frequency.
FHSS is simple, elegant, and effective. And you’d think it was modern – but like a lot of today’s wireless technology, FHSS dates back to the turn of the 19th century.
No danger of disturbance
Nikola Tesla, the prolific Serbian-American inventor and radio pioneer, filed a U.S. patent, granted on March 17, 1903 which doesn’t mention the phrase “frequency hopping” directly, but certainly alludes to it. Entitled “Method of Signaling,” the patent describes a system that would enable radio communication “without any danger of the signals or messages begin disturbed, intercepted, interfered with in any way”.
Tesla’s patent details a system whereby transmitter and receiver are synchronized and hop between two channels (although the patent notes any number of channels could be used) by altering the carrier frequency in a predetermined sequence to avoid interference.
Such an interesting idea didn’t escape the military’s attention of course, and by 1915, the Germans were making use of primitive frequency hopping radio to stop the British eavesdropping on their conversations. If the British had done their homework, they could have found out the details of the technology by picking up a copy of Jonathan Zenneck’s book Wireless Telegraphy that was originally published in German in 1908, but translated into English the same year as the enemy started using frequency hopping on the front line.
Zenneck was a German physicist and electrical engineer who had got interested in radio by attending Tesla lectures on “wireless sciences”. Wireless Telegraphy includes a section on frequency hopping, and, as it became a standard text for many years, probably introduced the technology to a generation of engineers.
Beauty and brains
Tesla and Zenneck were certainly clever, but were not known for their looks. In contrast, Hedwig Eva Maria Kiesler, better known as Hedy Lamarr, was more famous for her beauty than her inventiveness. But the Hollywood actress was no slouch when it came to engineering and was credited with inventing a form of FHSS in U.S. patent no. 2,292,387 (although the patent was taken out in the name of Markey, who was Lamarr’s second husband).
Figure 1: Part of Hedy Lamarr and George Antheil’s patent that describes a mechanical system for frequency hopping spread spectrum technology to remotely control torpedoes.
Lamarr, together with co-inventor George Antheil, a pianist and Hollywood composer, came up with a system for radio control of torpedoes. The idea was not new, but Lamarr’s concept of frequency hopping to prevent the intended target from jamming the controller’s transmissions was. Antheil chipped in with a technique for synchronizing the frequency hopping between transmitter (in a plane flying above the torpedo) and the weapon itself.
It’s probably no coincidence given Antheil’s musical training that the system used 88 different carrier frequencies, equal to the number of keys on a piano.
Lamarr and Antheil’s patent, enigmatically entitled “Secret Communication System” was granted on August 11, 1942. The radio-controlled torpedo idea never took off because in part, according to Antheil, the patent mentions a piano as part of the explanation of how it works and the senior staff at the U.S. Navy – without bothering to read the whole patent - thought that an actual piano would have to be fitted inside the weapon.
Fortunately, the concept was eventually taken up - using electronics rather than the mechanical system originally proposed – for secure military communications in the 1960s. Unfortunately for the inventors, this happened just a few years after their patent had lapsed.
Many modern patents in FHSS technology refer to the Lamarr-Antheil document as the basis of the field and today’s Bluetooth low energy and proprietary protocols owe much to the beautiful actress’ amazing intellect. References:
1. "Legacies: Jonathan A. Zenneck,” IEEE History Center.
2. “American Heritage of Invention & Technology,” Spring 1997, Volume 12/Number 4.This article is courtesy of Nordic Semiconductor's ULP Wireless Q. Back issues and free subscription to the newsletter can be found at http://www.nordicsemi.com/News/ULP-Wireless-Quarter