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Old May 2nd 08, 06:46 PM posted to rec.radio.amateur.antenna
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Default Disadvantages of using AM for DSSS/FHSS Spread Spectrum?

wrote:
The invention of spread spectrum is generally credited to George
Antheil and Hedy Lamarr (yes, the actress) and their patent of 1942.


What would be the difference between very wide band
suppressed carrier FM and spread spectrum?
--
73, Cecil
http://www.w5dxp.com
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Old May 2nd 08, 07:05 PM posted to rec.radio.amateur.antenna
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Default Disadvantages of using AM for DSSS/FHSS Spread Spectrum?

Richard Harrison wrote:
Jim Pennino wrote:
"The invention of spread spectrum is generally credited to George
Antheil and Hedy Lamarr (yes the actress) and their patent of 1942."


Yes, I believe Lamarr and her husband encoded several tones in sequence
using a player-piano roll. The system was used in WW-2. Poor old Tesla
only invented the a.c. power system, wireless power transmission, and
demonstrated his remote control of a boat using spread spectrum to avoid
interference more than 40 years before Hedy Lamarr`s patent. T.A. Edison
did his best to deny Tesla credit for anything. That`s commerce.


Best regards, Richard Harrison, KB5WZI


George Antheil and Hedy Lamarr were never married. Antheil was a
neighbor.

The didn't encode "serveral tones in sequence on a player-piano roll",
they used a piano roll to produce frequency hopping between 88 different
frequencies with the intention of making radio guides torpedoes hard
to detect or jam.

The first practical use (after technology caught up with the concept)
was by the Navy during a blockade of Cuba in 1962 based on the work
started in 1957 by Sylvania Electronic Systems.

While Tesla and several Europeans had their fingers in various frequency
hopping schemes, the general consensus is that Antheil and Lamarr were
the first originators of the basis for spread spectrum radio systems as
used today.

Both Antheil and Lamarr were given a special award in 1997 in recognition
of that fact.

Other than that, you mostly got it right.



--
Jim Pennino

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Old May 2nd 08, 08:59 PM posted to rec.radio.amateur.antenna
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Default Disadvantages of using AM for DSSS/FHSS Spread Spectrum?

In article ,
wrote:

What would be the difference between very wide band
suppressed carrier FM and spread spectrum?


To greatly simplify, spread spectrum is random frequency hopping to
discreate carrier frequencies and says nothing about the modulation
of the carrier.


That is one specific form of spread-spectrum transmission (FHSS or
frequency-hopping). This form tends to be used a lot in lower-cost
consumer electronic gear (older-generation cordless phones, for
example).

There are other forms of spread spectrum in common use, and in these
forms the spreading is tied to the modulation scheme to at least some
extent. Two examples:

- Direct-sequence spread spectrum (DSSS). A digital bitstream is
XORed with a high-speed pseudorandom "spreading sequence", and the
carrier is modulated by the resulting high-speed bitstream. The
basic carrier frequency going into the modulator doesnt't change
(it's fixed on a per-channel basis) but the energy is spread out
as a very broad set of sidebands. 802.11b WLAN uses DSSS.

- Orthogonal frequency-division multiplexing (OFDM), which uses a large
number of individual carriers, typically spaced at regular
intervals, each modulated at a relatively low rate with some
portion of the information being sent. Here, too, the frequency of
each individual carrier tends to be fixed. 802.11g, DMT (discrete
multitone) DSL modems, and the venerable Telebit Trailblazer
phone-line modem use OFDM.

Very-wide-band FM (i.e. FM with a high modulation index) carries
little of its energy at the carrier frequency - most of it is in the
regularly-spaced sidebands, located at offsets from the carrier which
are multiples of the modulating frequency/frequencies. In this sense,
its spectrum use would be not dissimilar to that of an FDM spread
spectrum (although it would generally not be OFDM because the
sidebands aren't spaced in an orthogonal manner).

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!


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Old May 2nd 08, 09:28 PM posted to rec.radio.amateur.antenna
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Default Disadvantages of using AM for DSSS/FHSS Spread Spectrum?

Cecil, W5DXP wrote:
"But frequency hopping is only one form of spread spectrum."

Yes. No more than two frequencies are required to switch between, though
the transition produces more frequencies than the originals. Carson`s
rule is a close approximation of the required bandwidth and is used by
the FCC to determine bandwidth:
BW= 2(Peak Deviation + Highest Mod. Freq.)

FSK or frequency shift keying uses only two frequencies to represent
ones and zeros. Switching produces FM and is a form of spread spectrum
transmission defined as:
A communications technique in which many different waveforms are
transmitted in a wide band. Power is spread thinly over the band so
narrow-band radios can operate within the wide band without
interference. FSK is often done within the audio frequency band with no
radio necessarily used in transmission.

Best regards, Richard Harrison, KB5WZI

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Old May 2nd 08, 09:31 PM posted to rec.radio.amateur.antenna
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Default Disadvantages of using AM for DSSS/FHSS Spread Spectrum?

In article ,
Cecil Moore wrote:

FM usually occupies about twice the bandwidth of AM.


Thanks for adding "usually" to your original statement.
I was just pointing out that if the FM peak deviation
is equal to the maximum modulation frequency, then the
FM signal occupies the same bandwidth as AM.


I believe that you're mistaken on this point, Cecil.

As I understand it, the spectrum of a frequency-modulated carrier
will have energy at the carrier frequency (except at certain very
specific modulation indices), and energy at offsets from the carrier
which are equal to the modulating frequency and all of its harmonics.

It's entirely possible (and in fact common) for an FM signal to have
energy at frequencies which are further away from the nominal carrier
frequency, than the maximum instantaneous deviation of the carrier
would suggest. For example, a carrier which is modulated with a 1000
Hz tone, but only to the level of having a maximum peak deviation of
500 Hz from nominal, will still have sideband energy out 1000 Hz and
2000 Hz away from the nominal carrier frequency.

This is *really* counter-intuitive, and I haven't been able to fully
wrap my brain around the question of just how the math works... but
the math says that it's true, and my own spectrum measurements show
that it's true.

An FCC-approved NBFM phone signal, which has a modulation index of no
more than 1.0 at the highest modulating frequency, *will* have energy
out further than the peak deviation would suggest. There will be some
amount of sideband energy located out at twice the highest modulating
frequency, and a bit at three times.

The levels of these further-away-from-nominal-carrier sidebands will
be relatively low - they don't start to become appreciable until you
get to a higher modulation index.

An AM signal being modulated by the same intelligence signal would not
have any energy out at the multiples, *if* it was generated and
transmitted in an entirely-distortion-free manner.

My guess is that in practice, NBMF (per FCC regs), and ham-grade AM,
probably have very similar bandwidths. All it would take for the AM
signal to be spread out as far as the FM signal, would be a bit of
nonlinear distortion in the audio chain, mixer, or amplifier chain...
this will create a second-harmonic sideband.

I did an introductory presentation to a local hamclub last year (with
some math, admittedly simplified and approximated and covered up with
moderate amounts of hand-waving) which explains some of these
concepts. I included some plots of actual measured spectra, generated
by an HP signal generator and captured using an old Systron Donner
spectrum analyzer.

http://www.radagast.org/~dplatt/hamr...modulation.pdf

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
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Old May 2nd 08, 10:03 PM posted to rec.radio.amateur.antenna
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Default Disadvantages of using AM for DSSS/FHSS Spread Spectrum?

Dave Platt wrote:
In article ,
Cecil Moore wrote:

FM usually occupies about twice the bandwidth of AM.

Thanks for adding "usually" to your original statement.
I was just pointing out that if the FM peak deviation
is equal to the maximum modulation frequency, then the
FM signal occupies the same bandwidth as AM.


I believe that you're mistaken on this point, Cecil.


Yep, you're right. A modulation index of about 0.6
yields approximately the same bandwidth for AM and FM.
--
73, Cecil http://www.w5dxp.com
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Old May 2nd 08, 11:15 PM posted to rec.radio.amateur.antenna
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Default Disadvantages of using AM for DSSS/FHSS Spread Spectrum?

Dave Platt wrote:
In article ,
wrote:


What would be the difference between very wide band
suppressed carrier FM and spread spectrum?


To greatly simplify, spread spectrum is random frequency hopping to
discreate carrier frequencies and says nothing about the modulation
of the carrier.


That is one specific form of spread-spectrum transmission (FHSS or
frequency-hopping). This form tends to be used a lot in lower-cost
consumer electronic gear (older-generation cordless phones, for
example).


There are other forms of spread spectrum in common use, and in these
forms the spreading is tied to the modulation scheme to at least some
extent. Two examples:


- Direct-sequence spread spectrum (DSSS). A digital bitstream is
XORed with a high-speed pseudorandom "spreading sequence", and the
carrier is modulated by the resulting high-speed bitstream. The
basic carrier frequency going into the modulator doesnt't change
(it's fixed on a per-channel basis) but the energy is spread out
as a very broad set of sidebands. 802.11b WLAN uses DSSS.

- Orthogonal frequency-division multiplexing (OFDM), which uses a large
number of individual carriers, typically spaced at regular
intervals, each modulated at a relatively low rate with some
portion of the information being sent. Here, too, the frequency of
each individual carrier tends to be fixed. 802.11g, DMT (discrete
multitone) DSL modems, and the venerable Telebit Trailblazer
phone-line modem use OFDM.

Very-wide-band FM (i.e. FM with a high modulation index) carries
little of its energy at the carrier frequency - most of it is in the
regularly-spaced sidebands, located at offsets from the carrier which
are multiples of the modulating frequency/frequencies. In this sense,
its spectrum use would be not dissimilar to that of an FDM spread
spectrum (although it would generally not be OFDM because the
sidebands aren't spaced in an orthogonal manner).


What part of "To greatly simplify" are you having problems understanding?


--
Jim Pennino

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Old May 3rd 08, 09:16 AM posted to rec.radio.amateur.antenna
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Default Disadvantages of using AM for DSSS/FHSS Spread Spectrum?

Richard Harrison wrote:
. . .
Terman says about twice the bandwidth is required for FM as compared
with AM on pages 589 and page 590 of his 1955 opus.
. . .


You really should try to understand the context of the various
quotations from Terman.

In his _Radio Engineering_, Third Edition (1947), he points out that
"When the modulation index is less than 0.5, i.e., when the frequency
deviation is less than half the modulating frequency, the second and
higher order side-band components are relatively small, and the
frequency band required to accommodate the essential part part is the
same as in amplitude modulation." This is, of course, what is considered
to be narrow band FM.

Unlike an AM signal with its one pair of sidebands containing replicas
of the modulating signal, any FM signal contains an infinite number of
pairs of sidebands. However, as Terman and any other communications text
points out, the relative strengths of some of those sidebands can be
made to be very small by the choice of modulation index. In the case of
NBFM, all but the first pair are small. That first pair are spaced the
same distance from the carrier as AM sidebands, so the bandwidth is
essentially the same as for AM.

You can, of course, increase the modulation index which increases the
bandwidth by increasing the amplitudes of higher order sideband pairs,
making wideband FM. The advantage of doing this is that you can improve
the signal/noise ratio of the received signal as a trade for the
increased bandwidth.

So FM can be as narrow in bandwidth as AM, or any greater bandwidth, all
depending on the modulation index. Saying that "twice the bandwidth is
required for FM as compared with AM" is simply not a correct statement
and, if said by Terman, was taken out of context which surely qualified it.

Roy Lewallen, W7EL
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