Last week on November 9th, it would have been Hedy Lamarr’s 108th Birthday. Better known for her leading roles in Hollywood, this bombshell was actually the brains behind the technology we have used for Wi-Fi and Bluetooth.
Despite being a famous actress and arguably one of the most beautiful women in the world, her passion was science and she spent all her free time between filming tinkering with ideas and inventions.
Without her innovative mind and natural affinity for scientific problem solving, we may not be using the Wi-Fi, Bluetooth and other RF technologies we rely upon each and every day.
Along with composer George Antheil, Hedy Lamarr patented FHSS (Frequency Hopping Spread Spectrum) back in August of 1942. The inspiration for their idea was to prevent signal jamming on RF-controlled torpedoes.
Hedy Lemarr heard about the possibility of United States Navy torpedoes being veered off-course from enemy signal jamming – Leading her to the idea of a frequency-hopping RF guidance system that would be much more difficult to jam and interfere with.
Despite the brilliance of the idea, it was unfortunately never used for its intended purpose, with senior officials taking the view that Hedy would be more helpful in the war effort selling kisses as a pin-up.
Despite Hedy and George attempting to patent their RF transmitter device that emulated player-piano capabilities, their ingenious frequency hopping idea was immediately classified by the US government and was not used in time to help with World War II.
15 years later, the idea of spread spectrum and FHSS was further developed and used for the first time during the Cuban Missile Crisis between US ships blockading Cuba.
In the 1970’s, FHSS was declassified but with their original patent now expired, neither Hedy or George made any money from their technology or the subsequent developments.
What does FHSS have to do with Wi-Fi?
Most of the early Wi-Fi deployments used frequency-hopping technology, with FHSS being one of the original technologies used for RF communication. Legacy Wi-Fi radios using RF communications used the 2.5GHz ISM band. Most frequency-hopping legacy Wi-Fi radios were made between 1997 and 1999.
How does FHSS work?
In simple terms, Frequency Hopping Spread Spectrum works by using a small frequency carrier space to transmit data, then hopping to another small frequency carrier space and transmits data, then to another frequency, and another and so on.
In more specific terms, the frequency and period of time used for each data transmission is precise and determined by the dwell time. FHSS will transmit data for a set period of time (dwell time) on a certain frequency. After that time, it will move to another frequency and again transmit for only the dwell time before moving on again.
The hopping used by FHSS radios is predetermined with a set hopping sequence. Rather than transmitting on one set channel of frequency space, a pattern of hops (or subchannels) is predefined so that it is hopping through a series of small carrier frequencies. Once a series of hops has been completed, it then repeats.
The Institute of Electrical and Electronics Engineers (the IEEE) is the largest technical professional association, nurturing, developing and advancing global technologies. The standard of 802.11 from the IEEE specified that each hop must be 1 MHz in size, and then arranged in a predefined sequence.
These sequences consisted of at least 75 hops but no more than 79 (in North America and the majority of Europe). Some other countries used much less hops, e.g. France had a sequence of 35 and Japan used 23.
In order for a transmission to be successful, both the FHSS transmitters and receivers had to be synchronised on the same carrier hop at the same time. An FHSS access point could be used to configure a hopping sequence, with the information then being delivered via 802.11 beacon management frame to a client station.
The dwell time is also specified by the local regulatory body. A maximum dwell time of 400 milliseconds per carrier frequency during any 30 second time period was set by the Federal Communications Commission. They regulate international communications through cable, radio, television, satellite and wire to promote connectivity. However, typical dwell times are usually between 100 and 200 milliseconds.
The IEEE 802.11 standard also specified a maximum bandwidth of 79 MHz. This means that the maximum number of hops possible for a hop sequence would be 79. Based on a hop sequence of 75 hops, with a dwell time of 400 ms, it would take approximately 30 seconds to complete one FHSS hop sequence. As we mentioned above, once completed, the sequence is then repeated.
Remember that the original aim of this technology was to prevent signals getting jammed and US Navy torpedoes being veered off course by the enemy. Due to the Wi-Fi FHSS transmissions jumping inside a frequency range of 79 MHz, a narrowband signal or noise would only disrupt a small range of frequencies. This means that it would only cause a minimal amount of throughput loss.
The effects of interference can also be diminished through decreasing the dwell time. The longer the dwell time, the greater the throughput as the radio is transmitting data throughout the dwell time and the less often the transmitter has to waste time hopping to another frequency. The shorter the dwell time, the more frequent the transmitter has to hop, which decreases throughput.
Do we still use FHSS for Wi-Fi?
For Wi-Fi we have moved on to other RF technologies like OFDMA (orthogonal frequency-division multiple access) rather than FHSS.
However, we do still use FHSS for devices using Bluetooth and other radio transmitters.
If it wasn’t for the brains of Hedy Lamarr, we might have never seen the Wi-Fi and Bluetooth technology that we all know and love. If you’re interested to know more about Hedy Lamarr and the story of how her brains far exceeded her renowned beauty, have a look for the documentary ‘Bombshell – The Hedy Lamarr Story’ (2017). You can rent or buy it via Amazon Prime and other streaming platforms.
As we all know with technological advances, there are many great and innovative minds out there. Perhaps even if Hedy and George hadn’t come up with their frequency-hopping idea, someone like-minded would have come up with a different path on the route to Wi-Fi and RF communication as we know it today.
But the story of sex symbol and scientist Hedy Lamarr is always an interesting one for us Wi-Fi geeks. So Happy Birthday and Thank You to her!
Image from https://en.wikipedia.org/wiki/Hedy_Lamarr