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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