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Number 4 on your FeedbKk card
Spread-Spectrum
A fascinating mode-and legal for hams to use!
by Andre Kesteloot N41CK
F
irst . a trivia quiz for World
War II buffs:
Question: What did movie star
Hedy Lamarr in Hollywood and
German General Rommel in Libya
have in common?
An.n,'a: Spread-spectrum! General
Rommel used a spread-spectru
m ($5) communicat ion link between
Ge rmany and Derna. Libya ;
and in 194 1 actress Hedy Lamarr
obtained one of the original patents
in the US on frequency-hopping
spread -spectrum.'
If nothi ng else. the above should
tell you that spread- spectrum is not
new by any stretch of the imag ination.
Why is it. then. that 48 years
after the beginning of World War
II. spread-spectrum still evokes in
many the idea of stealth and sec recy
, rather than spectru m management?
The Spread-Spectrum
Transmtsslon
To qualify as "spread-spectrum," a transmission
must meet the following criteria: (a)
the band width must be independent of the
modulating signal; (b) the bandwidth must be
much larger than those for traditional mod u­
lation schemes ; and (c) data recovery must be
achieved by sy nchronizing a code at the rece
iver end ofthe link.
Although there are many types of spreadspectrum
modulation schemes , o nly two
are authorized by the FCC for radio ama-
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Photo A. An 55 transmitter. A modified Hamtronics 440 MHz
exciter drives a final amplifier (/lo t shown in photo). TIu>board to
the right includes a pseudo-noise generator and pre-scaler divider
elwin.
teurs: frequency-hopping (FH) and dirccr-sequcnce
(OS).
Frequency-Hop ping
In fr equency-hopping, the transmitter
transmits fo r a sho rt time on frequency
Fl , then hops to frequency F2. then F3,
etc. Although the hopping pattern appears
to be random. it is in fa ct predetermined
(pseudo-random). Co mmunicat ion
can only take place if the receiver knows
the hopping seq ue nce and sy nchronizes
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its hopping to the transmitter's.
If we take. for instance. a hopping
seq uence of 127 discrete frequencies
, and we arc faced with
interference on discrete freq uencies
F3, F16 . and FS7 . re liable
transmission o f information will
still take place 124/127 ofthe time.
Because of the redundancy in human
speec h, this system should be
particularly attractive to those operati
ng in heavy interference .
nlrect-Seqcence
In direct sequence . generally a
ca rrier (say 446 MHz) is mixed in a
doub ly-balanced mixer with a
pseudo-random sequence (clocked
at. say. 2.78 MHz). In the process,
the carrier will be canceled and replaced
with a noise-like spectrum
of a bandwidth several Hz above
and below the original carrier.
Of the enc rgy transmitted , however
, 90 ,;{; will be concentrated in a
band between 443.12 MHz (446-2.78) and
448.78 MHz (446 + 2 .78) in this example.
At the receiver end, only a minor rise in the
noise-floor will appear around your frequency
(see Figure I), but after succes sful despreading.
the original carr ier will be recovered
(see Figure 2).
Advantages a nd Dirracu lties
Someo fthe advantages of spread-spectrum
arc:
Betta frequency spectrum utilization,
Since a non-correlated receiver hears nothing
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Figure I, A Direct Sequence (DS) spread-spectrum signal. reception.
Only u minor rise in the noise-floor willappeararoundyourfrequency.
12 73Amateur Radio . June, 1989
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Figure 2. The recovered signal after successful de-spreading.