Thursday, August 2, 2007

A Hollywood Beauty and CDMA

Business India, January 25 - February 7, 1999

Reaching out with spread spectrum

A 60-year-old idea patented by a Hollywood actress is revolutionising wireless technology.

Shivanand Kanavi

Tom Cruise and Nicole Kidman, the famous Hollywood star couple, are deeply upset. Recently, a man used a commonly available frequency scanner to find out what frequency their cellular phones were using. He then proceeded to not only snoop into their private conversation but tape it and sell it to a tabloid in the US. The episode brought into the limelight the lack of privacy in a cellular phone call. However, if they were using a cell phone based on spread spectrum technology like CDMA then such snooping in would not have been possible.

Spread Spectrum technology assures a high level of security and privacy in any wireless communication. It has come into the limelight in the past decade and especially in the past five years, after a US company, Qualcomm, demonstrated its successful application for cellular phones. Since SS technology can be used for secure communications, which cannot be jammed or snooped into, the US military has done extensive research and development on it since the 1960s and 1970s. Ironically, this hi-tech, and revolutionary concept in radio communication was patented by a Hollywood diva, Hedy Lamarr, nearly 60 years ago.

Hedy Lamarr--Unlikely inventor
Though gorgeous and glamorous, she was not a bimbo. Hedy Lamarr hit headlines as an actress with a nude swimming scene in her Czech film Ecstasy(1933). Later she was married to a rich pro-Nazi arms merchant, Fritz Mandl. To Mandl she was a trophy wife, whom he took along to many parties and dinners, to mingle with the high and mighty in politics, military and arms trade of Europe. Little did he suspect that beneath the beautiful exterior lay a sharp brain with an aptitude for technology. Hedy was able to pick up quite a bit of the technical shop-talk of the men around the table.

When the war began, Hedy, a staunch anti-Nazi escaped to London. There she convinced Louis Mayer of MGM studios to sign her up. Mayer, having heard of her reputation after Ecstacy, advised her to change her name from Hedwig Eva Marie Kiesler to 'Hedy Lamarr' and to act in "wholesome family movies", which she promptly agreed to.

As the war progressed and US entered it after Pearl Harbour, Hedy informed the US government that she was privy to considerable amount of Axis war technology and she wanted to help. The Defence Department had little faith in her claims and advised her to sell war bonds. Hedy, however was unrelenting. She, along with her friend George Antheil, an avant garde composer and musician, patented their 'secret communication system' (1941) and gave the patent rights free to the US military. The patent discussed a design to provide jamming free radio guidance systems for submarine launched torpedoes based on the frequency hopping spread spectrum technique. It consisted of two identical punched paper rolls. One roll, which was located in the submarine, changed the transmission frequency as it was rotated and the other embedded in the torpedo aided the receiver in hopping to the appropriate frequency. The enemy jammer would be thus left perennially guessing the guiding frequency.

The idea though ingenious was too cumbersome as it involved mechanical systems and was hence not applied by the US Navy. However, in the late 1950s as electronic computers appeared on the scene, the US Navy revived its interest in Hedy's ideas. Subsequently, with the development of microchips and digital communication, very advanced secure communication systems have been developed for military purposes using spread spectrum techniques. In the telecom revolution of the 1990s, these techniques have been used to develop civilian applications in cellular phones, wireless in local loop, Personal Communication Systems and so on. The unlikely inventor showed that if you have a sharp brain even party hopping could lead to frequency hopping!

Spread Spectrum
Instead of using one fixed frequency, what if the transmitter keeps jumping from one to another in a random fashion? Then by the time the "enemy", who wants to snoop in, or who wants to jam the transmission, finds the frequency with a high-speed scanner, the frequency would have changed. As long as the hopping does not have a pattern that can be detected and the receiver knows the exact sequence of hopping then both snooping and jamming would be impossible. Thus a user does not use a channel but many users can use a band as long as their sequences do not dash. This technique is called frequency hopping spread spectrum. Hedy's idea belonged to this set.

Another set of techniques belongs to the direct sequence SS, where a signal is mixed with a strong dose of noise and transmitted. Only the receiver knows the noise that has been added and hence he subtracts the same from the received signal, thereby recovering the transmitted signal. This technique works best when the added noise is very powerful. (In reality, noise means a completely random jumble of power output in all frequencies. In this case the jumble is not totally random but only the receiver is privy to it hence it is called pseudo-noise). Normally noise is acquired during transmission, like the static hiss and other crackling sounds in a radio during a storm. Hence, every radio engineer tries to broadcast the signal at a high power so that at the receiver's end the signal to noise ratio is high enough for the message to be intelligible. After all, the received power falls inversely as the square of the distance from the transmitter while the "noise" level does not. Direct sequence SS turns this situation upside down; it needs a weak signal and a strong noise to be effective. An everyday example of this can be seen at a cocktail party. When the -wise level is very high, there is maximum privacy for conversation between neighbours because both ears discount the same background noise while the third person hears only the noise! Qualcomm's CDMA belongs to this set of direct sequence techniques.

Breaking out of channelsSS techniques violate all conventional wireless wisdom. Traditional wireless systems however sophisticated, are based on the channel separation principle. That is, each user has to use a fixed radio frequency or a small part of the spectrum exclusively. In the very early days of wireless itself, when two pioneers, Marconi and De Forest, were vying to show off the superiority of their respective radios, the problem of interference appeared that has dogged wireless to this day. Both of them were then trying to sell their ideas to the public and went on to broadcast the first-ever live commentary of a yachting event. However they found to their dismay that their broadcast frequencies were too close to each other. This led to interference or unintentional jamming and all that the listeners could receive was garbled sound. Hence the principle, "only one transmitter can use a frequency at any time". Since the radio frequency spectrum is limited, the allocation of frequencies has become a major regulatory job. It is done by the International Telecom Union at the international level and bodies like the Wireless Planning Committee in India and the Federal Communications Commission in the US at a national level.

Channelisation is ingrained into the thinking of radio engineers. They strive for better transmit filters to contain the transmiSSion in a narrow channel. They strive for better receive filters to reject any interference that may assault their receiver. They strive for hyperstable frequency synthesisers to keep the carrier tuned as sharply as possible. Because of scarcity of spectrum, radio engineers continuously look for ways to narrow bandwidth by channel splitting, various multiplexing techniques, better coders and modulators-demodulators (modems) and so on. Thus each cellular operator in India who has been given about 12.4 MHz of spectrum can accommodate about 200 simultaneous users in each cell. However SS techniques developed in the past decade have demonstrated that even in a mobile environment they can accommodate 10-20 times more users than analog cellular systems and four-seven times more users than traditional digital systems, though they violate the basic concept of channelisation. In fact, SS techniques work better and hence more efficiently in wider bandwidths. To a traditional radio engineer, SS enthusiasts appear as wild-eyed hippies.

Similarly, a major problem in mobile phones is what is called "multipath", that is the same signal gets reflected by various geographical features and reaches the receiver at different times leading to fading in and out of voice. Multipath is a frequency dependent effect hence it does not affect SS based systems as the broadcast is not at one frequency but a whole bunch of them in a wide band.

Having proven its superiority over traditional wireless technology, SS is becoming more and more popular especially in fixed wireless applications. Most of the new basic telephone operators who are using wireless in local loop to connect their exchanges with customers will be using CDMA. Even the next generation of traditional cellular phones- 3G, will incorporate this technology.

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