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The idea behind spread-spectrum transmission is a simple concept: disguise a narrowband transmission within a broadband transmission by spreading or smearing the narrowband component within the broadband carrier. This is done using either of two basic techniques: frequency hopping, in which a control signal directs the two communicating devices to hop randomly from frequency to frequency to avoid eavesdropping; or direct sequence, in which the signal is combined with a random noise signal to disguise its contents under the command of a control signal that knows how to separate the signal wheat from the noise chaff.
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Frequency Hopping Spread Spectrum (FHSS)
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The development of FHSS is one of those stories that is worth knowing about simply because it is so much fun to tell and so remarkable in the telling. During World War II there was considerable angst among Allied forces over the Axis Powers ability to thwart radio-controlled torpedoes launched from submarines by jamming the radio signals that guided
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Figure 6-16 Central switch authority commands end points to hop as instructed
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them to their targets. To that end, a pair of industrious entrepreneurs filed patent number 2,292,387 titled, Secret Communications System. The inventors were orchestra composer George Antheil and electrical engineer Hedy K. Markey, better known as early film star Hedy Lamarr. The technique described in their patent application is remarkably simple: A central authority (the base station, shown schematically in Figure 6-16) communicates with the two communicating endpoint devices, instructing them to hop randomly from frequency to frequency at randomly selected times and for random intervals, typically on the order of 5 ms. Or less. Only the base station (think of an orchestra conductor) and the two devices know when and where to jump and for how long. To an outsider looking in that is, any device wishing to eavesdrop on the conversation the hopping process appears completely random. It isn t, however: The base station knows precisely what it is doing and when it is doing it, so the hopping behavior is actually pseudorandom, and the technique is often referred to as a pseudorandom hopping code. This technique is most commonly used in CDMA cellular systems, and is (as you might expect) used extensively in secure military communications systems.
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DSSS, sometimes called noise-modulated spread spectrum, is a very different technique than its FHSS cousin. In frequency hopping, a
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conversation jumps from frequency to frequency on a seemingly random basis. In noise modulation, the actual signal is combined with a carefully crafted noise signal that disguises it. The otherwise narrowband signal, shown in Figure 6-17, is spread across a much wider channel typically on the order of 1.25 MHz. The bits in the data stream are combined with noise bits to create a much broader signal, and as before, only the base station and the communicating devices know the code that must be used to extract the original signal from the noise. The code is sometimes called a chipping code, and the technique is referred to as producing a chipped signal. One way to think about this technique is as follows. Imagine that you have just been appointed Ambassador to South Africa (Congratulations!). As part of your predeparture indoctrination you have learned that the country has 11 official languages: Afrikaans, IsiNdebele, IsiXhosa, IsiZulu, Northern Sotho, Sesotho, Setswana, SiSwati, Tshivenda, Xitsonga, and English. Upon arriving in the country, you attend a reception in your honor, and in attendance are dignitaries from the many regions of South Africa, all speaking their local language over glasses of mampoer and South African wine. The room is filled with people and the cacophony of eleven distinct languages is simply noise to your ears. Suddenly, however, from across the room, emerging from the general din, you hear someone calling to you. From across the ballroom your newly appointed aide de camp is talking to you in English at a normal level, yet you understand her perfectly.
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