[HCARC] Fwd: [CTDXCC] WWII Wireless Pioneer

[Kerry Sandstrom]

Yes, Don, an interesting story that has been around a long time.  Yes, Hedy 
Lamarr does have a patent on a frequency hopping scheme,  but that may be 
where it ends.  There were several patents in the 20's and 30's that were 
also related to spread spectrum.  There are actually a couple different 
types of spread spectrum.  The three most common are frequency hopping 
(FHSS), direct sequence (DSSS) and a combination of both those types. 
Although the ideas for spread spectrum occurred before WWII, and there were 
even a couple primitive systems back then, it really didn't become practical 
until the development of communication theory by Claude Shannon at Bell Labs 
in the 40's.  A true spread spectrum system requires a psuedo-random 
spreading code, forward error correction coding and interleaving.  The 
spread spectrum signal occupies a bandwidth many (like several thousand) 
times greater than the original data.  In a cell phone your 3 kHz bandwidth 
voice occupies anywheres from ~100 kHz for GSM to ~5 MHz for CDMA.  CDMA is 
strictly a DSSS system while GSM I think uses a combination of techniques. 
Commercial broadcast FM is similar to a spread spectrum signal in that it 
occupies about 5 times the required bandwidth, however it fails in that the 
spreading function is not psuedo-random but is related to the data, the 
program.  It does provide the improvement in signal to noise ratio that 
spread spectrum does and for the same reason.  You can trade bandwidth for 
signal-to- noise ratio.  If anyone is interested in learning more about 
spread spectrum, IEEE Transactions on Communications for May 1982 is a 
special issue on spread spectrum and has an excellent article titled "The 
Origins of Spread-Spectrum Communications" which is a history of the 
development of spread spectrum and describes several of the early systems. 
Its 30+ pages long and, incidently, doesn't mention Hedy Lamarr.

Kerry