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The first patent with a possible
solution for the detection and jamming problem came in 1942 from a highly unlikely
source, a movie actress, Hedy Lamarr and a composer, George Antheil.
In it they proposed a system for guiding torpedoes using radio controls.
Obviously, this had to be done in a way which could neither be detected or
jammed in order to be effective.
Lamarr and Antheil generously turned
their patent over to the government to aid in the war
effort. The government responded in turn by paying the inventors nothing
and pretty much ignoring the idea at least for torpedoes. The military
apparently did at some point see great
potential for the spread spectrum communication but kept much of their work on it
top secret.
Today military communications can not
only be rapidly encrypted for secrecy, but are also much harder to detect
or jam. Thanks to well developed technologies like spread spectrum broadcasting,
an enemy will typically not even be able to detect a broadcast let alone jam it or understand what it says.
There's more than one way to do spread
spectrum broadcasting and it's beyond the scope of this article to
describe all of them. So let's focus on one called direct sequence. In it, the
information to be broadcast
is typically digitized, that is it's broken into a series of
positive and negative voltage pulses representing binary digits (see Fig. 1). Simultaneously, a
pseudo random
number generator (PRNG) creates a similar series of seemingly random pulses at a much higher frequency than the pulses
containing the information (see Fig. 2). The higher frequency pulses look
shorter than the lower frequency information pulses since more of the
higher frequency ones occur in a given period of time.
The information and PRNG pulses are
then combined (see Fig 3). This can be thought of as a type of
multiplication of negative and positive values of one. When the voltages in the pulses
are both negative, combining them is like multiplying two values of
negative one together. The pulse becomes positive one. Combining a negative
and positive pulse is like multiplying a negative and positive value of
one. The pulse becomes negative one. The output of this process looks
random .
A typical AM radio signal would be
broadcast in a narrow frequency range. Adding the random pulses spreads the
signal over a much larger range. Since the energy in the broadcast is
now divided among a range of frequencies in a random manner, it's hard to
distinguish the signal from normal random background noise.
After the signal is received, it's
decoded by combining it with a second pseudo random pulse series (PRS).
This pulse series has to be identical to the first pseudo random pulse
series and in perfect sync with it. The second PRS is combined
in the same way as the first one. In other words it is the same
multiplication-like process. If the original information pulse had a value
of negative one and it was multiplied by a random pulse of negative
one the result would be positive one. Multiply this a second time by negative one and
the output is restored to the original value of negative one. Hence, the
information is restored to look as though it were never modified (see Fig.
4).
If the second PRS is even
slightly out of sync with the first one, the "decoded" signal will still
look random as shown in Fig. 5. What's more, the random appearance
does not tend to look any better until the first and second PRSs are almost perfectly in sync. Hence it's difficult to recover the
transmitted information without knowing the settings for the PS pulse
series and how it was generated.
The PRNG used in spread spectrum
broadcasting is often a linear feedback shift register (LFSR). While it's
beyond the scope of this article to describe how these units work, they
are used since they can be built from relatively simple low cost digital
components. Like all PRNGs, LFSR units are not true random number
generators. They create extremely long number sequences which seem random
and can generally pass at least some statistical tests for randomness.
However, the sequence will eventually repeat itself.
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