Spread Spectrum: The Technology that Came in from the Cold (War)

1990s cell phone

If you have shopped recently for a cordless telephone, you may have noticed that some makers now offer “scrambling conversation protection.” On some of the boxes, you can see “spread spectrum” touted as a method of securing signals to assure that others cannot listen in on your conversations. 

Chances are, few consumers have heard of spread spectrum; much less do they understand what it means. Perhaps in a few years, however, as yet another complex communication technology becomes commonplace, the phrase will roll off our tongues, whether we understand it or not, as we make decisions about buying telephones and cellular telephone services. 

Radio signals travel through the ether with no protective escort. Receiving them without an invitation, as spies or even neighborhood eavesdroppers might, or disrupting them, as military foes try, were both pretty easy tasks at one time. An early success at selective jamming of radio signals impacted the 1944 Battle of the Bulge. American B-24 airplanes flew over the battle area carrying the high-powered radio jammer Jackal (officially named AN/ART-3) for the first time. Radio signals transmitted on AM (amplitude modulation) frequencies successfully jammed German radio communications while not interfering with overlapping FM (frequency modulation) signals from American transmitters. Despite this dramatic success, radio jamming and protecting technologies received less attention generally during World War II than did enemies’ attempts to listen into each others’ messages.

The Cold War, though, drove U.S. efforts toward gaining control over who could block whose messages. Jamming the signals directing radio-guided enemy missiles loomed large as a newly important problem. Conversely, learning how to prevent the jamming of “friendly” signals, such as Radio Free Europe broadcasts and one’s own missiles’ guidance signals, presented another serious problem. The Allies’ perceived need for jam-proof communications heightened in 1949, when the Soviet Union detonated its first nuclear bomb, and again later, when civil war exploded in Korea. 

Although the origins of spread spectrum techniques can be traced in one form or another back to the 1920s, a key document that highlights the post-World War II roots of spread spectrum techniques is the Project Hartwell report. Project Hartwell was one of a series of secret studies conducted at the Massachusetts Institute of Technology (MIT) in 1950 on behalf of the armed forces. MIT’s Research Laboratory of Electronics (RLE) was a major research facility which conducted top-notch secret work in support of the Korean War, including securing communications.  A joint laboratory of the MIT Physics and Electrical Engineering Departments, the RLE continued much of the fundamental electronic research of MIT’s World War II-era Radiation Laboratory. The Signal Corps, the Air Force, and the Office of Naval Research jointly funded the new laboratory, with the Signal Corps overseeing the arrangement. Former Radiation Laboratory employees filled research positions at the RLE, which occupied the top floor of a temporary structure on the MIT campus, known as Building 22, erected earlier for the Radiation Laboratory.

Appendix G to the Project Hartwell report laid out several methods for securing radio communications including two important, then new, telecommunications advances: transmitted-reference (TR) spread spectrum, and code-division multiple-access (CDMA).  Four principal contributors provided the material for Appendix G, including Harald T. Friis and Ralph K. Potter, who were researchers in antennas and information transmission systems, respectively, at Bell Telephone Laboratories. Potter had been a prime innovator of the remarkably successful and sophisticated ultra-secret “X System,” dubbed by its sponsor, the Army Signal Corps, “Sigsaly.”  The other two principal contributors to Appendix G, Jerome Wiesner and Edward E. David, were later appointed Science Advisers to Presidents Kennedy and Nixon, respectively. 

Transmitted-reference (TR) spread spectrum and code-division multiple-access (CDMA) were both introduced in Appendix G, but the document left both unattributed. A good bit of speculation and effort has sought to attribute these innovations to MIT physics professor Jerome Wiesner. Unfortunately, Wiesner’s own secret laboratory notebooks were lost or stolen around the time he became President Kennedy’s Special Assistant for Science and Technology. Many sources concur that such credit is most likely due Wiesner, but a definitive determination to that effect is yet to be made. Other participants included Yuk Wing Lee, Robert Fano, and Wilber Davenport. The latter directed the doctoral dissertation of yet another participant in the research, Robert Price, who was not only a student employee of the RLE, but who also had an Industrial Fellowship in Electronics from Sperry. Price has continued to contribute significantly to both creating and writing the history of communications technologies; regarding the latter, he has recently completed a book-length manuscript on the World War II overseas superscrambler, Sigsaly.

So, the next time you look at a cordless telephone and see “spread spectrum” promoted as a method of securing your phone calls, remember this as the communications technology that came in (to your home) from the Cold War.

Representation of changes in wave behavior with direct-sequence modulation
Representation of changes in wave behavior with direct-sequence modulation

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