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How Hollywood's Hedy Helped Heighten Handhelds (hackaday.com)
szczys writes: Hedy Lamarr is a household name for the wrong reason. Her name is known as a Hollywood actress, but her legacy is in your pocket and reaches far more people than her movies. She was a brilliant thinker who plied her skills during World War II, developing technology that could help to win the war. Her patent wasn't used at the time, but is a foundation of spread-spectrum which is used in the radio modules of your cellphone: WiFi, Bluetooth, GPS, and others. This frequency hopping concept sat unused for decades before being added to the most ubiquitous of wireless connectivity methods.
(For those who don't get it.)
Lamarr invented "frequency hopping" while CDMA cellular and GPS use "direct sequence". Frequency hopping is just what it sounds like: a narrowband transmitter is continually retuned to a different radio channel. Unless the receiver tuning follows the same sequence at the proper times it cannot receive the transmission.
Direct sequence XORs a narrowband signal with a high speed pseudorandom "chip" sequence, and the receiver undoes this operation by XORing it again by the same sequence properly synchronized in time. It closely resembles a keystream-type encryption system, though the "keystream" is not necessarily secret. The main difference is that direct sequence is a wideband signal while, at any instant, a frequency hopped signal is still narrowband.
Each method has advantages. Frequency hopping can be especially resistant to strong narrowband jamming, so it's a favorite of military systems (Lamarr's intended use). Direct sequence is easier to use with coherent modulation so it tends to use transmitter power more efficiently, and it can often provide precise timing and positioning as a side benefit. Or, in the case of GPS, as its primary purpose.
While CDMA mobile phones were very important in the 1990s and 2000s, it is now being replaced with LTE (Long Term Evolution), which uses OFDM - Orthogonal Frequency Division Multiplexing. So do many other modern terrestrial digital communication systems including DSL, HD Radio, DVB-T (but not ATSC), WiFi and DRM (Digital Radio Mondiale).