On Mon, 28 Apr 2008 14:26:35 -0700, "Green Xenon [Radium]"
<glucegen1@excite.com> wrote:

>Most cell phones and wireless internet routers, modems, and access
>points that use spread spectrum usually broadcast and receive their data
>on FM-radio waves. Just out of curiosity, I ask, why not use AM?

I'll ignore cellular modulation methods because you already beat that
to death in sci.electronics.design. I'll assume that you've had a
brain transplant and are now capable of understanding various answers
to your marginal questions.

802.11 wireless modulation is pure FM for 1 and 2Mbits/sec data rates.
5.5 and 11 add AM to the FM modulation, thus yielding higher bits/Hz
modulation density, which is a measure of spectral efficiency.
Therefore, 802.11 wireless data does use some AM.

6Mbits/sec thru 54Mbits/sec use OFDM, which are 48 orthogonal
(non-overlapping) QAM (quadrature amplitude modulation) carriers. Each
carrier is modulated by both FM and AM. Each carries a part of the
data stream which when reassemble, yields even higher bits/Hz
modulation density and efficiency. Quadrature implies phase
modulation (or FM if you prefer). The AM in QAM implies that it's
also amplitude modulated.

>Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
>received using the AM radio waves in the UHF spectrum [i.e. spread info
>for transmission throughout the UHF band and receive AM radio waves
>throughout all UHF frequencies]. What would be the disadvantages of this?

The FCC would have you arrested and returned to wherever you escaped
from. Modulation methods and technology have evolved over the years
to satisfy many conflicting requirements. Pure AM is one of the
oldest, and has the advantage of simplicity. Everything else about it
is inferior to other modulation methods. FAA/FCC regulations
protecting obsolete technology has insured that it's still in use in
the aviation and broadcast business.

Pure AM has a number of disadvantages, depending on what you're trying
to accomplish. The big one is that it's grossly inefficient in both
the utilization of power and spectral efficiency. The carrier hogs at
least half the available power. AM has its place, but there are
better ways.

I'll skip all the intermediate modulation schemes and jump directly to
the present. Microprocessors have become so cheap, that it's now
economical to do DSP (digital signal processing) in every radio.
Digital has the huge advantage of offering error correction, noise
immunity, bandwidth compression, and high spectral efficiency. Schemes
have been devised that will extract useful audio or data from signals
that are well below the thermal noise floor. You can't do that with
AM (or FM). If you attempted to do Wi-Fi using pure AM technology,
the data thruput would be horrible and/or the error rate would be
hideous.

>Normally DSSS and FHSS are transmitted/received on FM radio waves.

DSSS are pure FM at 1 and 2Mbits/sec data rates. 5.5 and 11Mbits/sec
are a mixture of AM and FM. OFDM is neither pure AM or FM. It's QAM
modulation, a form of PSK (phase shift keying), with multiple
sub-carriers to reduce the effects of frequency selective fading.

>So I
>ask what would be the disadvantages of using AM instead of FM for this?

Lousy spectral efficiency, difficult linearity requirements, slow AGC,
excessive bandwidth, high error rate for data, and it's not
fashionable. I suppose there's some benifits to resurrecting stone
age technology, so that we don't forget our past, but I wouldn't want
to live like a cave man. That's essentially what you're suggesting by
reverting to pure AM technology.

>AM radio tends to be more vulnerable to unwanted magnetic disruptions
>than FM, however this only affects analog reception.

Magnetic? Wave a magnet around your cherished AM BCB (broadcast band)
radio. Hear anything different? How are magnets causing disruptions?

>Digital reception
>on AM should be unaffected even by the strongest-interfering analog RF
>magnetic signal. Right?

No. Magnets don't affect radio waves much. There is Faraday
rotation, which causes EM (electro-magic) waves to change polarization
in the presence of a magnetic field. That has some effect on
microwave signals and is used to good advantage in RF circulators and
isolators. However, for RF below about 1GHz, it's a non-issue.

So what are you proposing this time? Cellular service at HF
frequencies? Ultra high index of modulation AM schemes? Long range
cellular where you would have only a few simultenous users on a given
continent? Laws to limit acronyms to two letters?

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558