Disadvantages of using AM for DSSS/FHSS Spread Spectrum?

Hi:

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?

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?

Normally DSSS and FHSS are transmitted/received on FM radio waves. So I
ask what would be the disadvantages of using AM instead of FM for this?

AM radio tends to be more vulnerable to unwanted magnetic disruptions
than FM, however this only affects analog reception. Digital reception
on AM should be unaffected even by the strongest-interfering analog RF
magnetic signal. Right?

Digital cell phones do use FM. Even if the signal being
transmitted/received is digital, it's carrier wave is still analog.
Right? IDEN phones use a 4 subcarrier 16-QAM [not spread-spectrum] but
still on an FM main carrier.

AFAIK, there is no such thing as a digital carrier wave. The carrier
wave is always analog just like a cable link is also always analog. The
signal transmitted through the analog medium maybe digital, though.

If a PCM signal [digital] is transmitted on an AM carrier wave [analog],
the AM wave's peak-to-peak amplitude will vary according to the PCM
signal in the following manner:

1. A positive amperage of the PCM signal will cause the AM carrier
wave's peak-to-peak amplitude to increase while a negative amperage
[i.e. going below the x-axis when graphed] will cause a decrease the AM
carrier's peak-to-peak amplitude.

2. A increase in frequency of the PCM signal will cause the AM carrier's
peak-to-peak amplitude to vary more rapidly while a decrease in the PCM
signal's frequency will cause the AM wave's peak-to-peak amplitude to
vary more slowly.

That's how I understand it. The PCM signal is digital but causes a
measurable affect on the analog AM carrier wave. Upon reception, an
AM-demodulator can retrieve this PCM signal and play it back. I could be
wrong though.

I am aware that FM demodulation produces a handy 'capture' effect. If a
weak and a strong signal are present together, then the recovered
baseband signal from the weaker FM signal is greatly reduced. This is
useful in rejecting interference from adjacent stations on the same
frequency.

With AM, the demodulated baseband signals are present in amplitudes that
are proportional to their RF amplitudes. In addition, the AM carriers
will 'beat' together to produce an additional and unwanted
tone-modulation of the received RF signal. But if the signal is digital,
won't it remain immune to EMI/RFI [analog disruption] even if received
on AM? DSSS and FHSS are digital. So I would think that the analog
magnetic interferences wouldn't affect them.

Also, doesn't FM have the disadvantage in that it hogs more bandwidth
than AM?


Thanks,

Radium

Green Xenon [Radium] wrote:
> Hi:
>
> 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?
>
> 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?
>
> Normally DSSS and FHSS are transmitted/received on FM radio waves. So I
> ask what would be the disadvantages of using AM instead of FM for this?
>
> AM radio tends to be more vulnerable to unwanted magnetic disruptions
> than FM, however this only affects analog reception. Digital reception
> on AM should be unaffected even by the strongest-interfering analog RF
> magnetic signal. Right?
>
> Digital cell phones do use FM. Even if the signal being
> transmitted/received is digital, it's carrier wave is still analog.
> Right? IDEN phones use a 4 subcarrier 16-QAM [not spread-spectrum] but
> still on an FM main carrier.
>
> AFAIK, there is no such thing as a digital carrier wave. The carrier
> wave is always analog just like a cable link is also always analog. The
> signal transmitted through the analog medium maybe digital, though.
>
> If a PCM signal [digital] is transmitted on an AM carrier wave [analog],
> the AM wave's peak-to-peak amplitude will vary according to the PCM
> signal in the following manner:
>
> 1. A positive amperage of the PCM signal will cause the AM carrier
> wave's peak-to-peak amplitude to increase while a negative amperage
> [i.e. going below the x-axis when graphed] will cause a decrease the AM
> carrier's peak-to-peak amplitude.
>
> 2. A increase in frequency of the PCM signal will cause the AM carrier's
> peak-to-peak amplitude to vary more rapidly while a decrease in the PCM
> signal's frequency will cause the AM wave's peak-to-peak amplitude to
> vary more slowly.
>
> That's how I understand it. The PCM signal is digital but causes a
> measurable affect on the analog AM carrier wave. Upon reception, an
> AM-demodulator can retrieve this PCM signal and play it back. I could be
> wrong though.
>
> I am aware that FM demodulation produces a handy 'capture' effect. If a
> weak and a strong signal are present together, then the recovered
> baseband signal from the weaker FM signal is greatly reduced. This is
> useful in rejecting interference from adjacent stations on the same
> frequency.
>
> With AM, the demodulated baseband signals are present in amplitudes that
> are proportional to their RF amplitudes. In addition, the AM carriers
> will 'beat' together to produce an additional and unwanted
> tone-modulation of the received RF signal. But if the signal is digital,
> won't it remain immune to EMI/RFI [analog disruption] even if received
> on AM? DSSS and FHSS are digital. So I would think that the analog
> magnetic interferences wouldn't affect them.
>
> Also, doesn't FM have the disadvantage in that it hogs more bandwidth
> than AM?
>
>
> Thanks,
>
> Radium
Ive no idea why you hv cross-posted this to uk.broadband, which is about
IP over wires, not radios..

However, the whole post suggests that you really do not understand
modulation of RF signals at all.

In the limit, any modulation regime you use, results in changes in the
amplitude/frequency spectrum: broadcast FM only HAS a better S/N ratio
and capture effect BECAUSE it occupies so much spectrum.

Given the availability of complex signal processing at relatively cheap
price, choosing a modulation method is no longer technology bound: it is
a matter of using a given spectrum in whatever way suits the application
best, and spread spectrum, frequency hopping, phase modulation or
multiple channels all smear into each other as the data rate goes up.

For example, consider - say - a modulation in which say 8 digital bits
correspond to 8 different frequencies. As the data is sent, the
frequency 'hops'' from one frequency to another.

However,that is exactly the same as having one transmitter that is
frequency modulated, or 8 transmitters that are amplitude modulated, on
different frequencies.

Modulation schemata are chosen with an eye to the type of interference
likely to be encountered: multipath for example makes a mockery of phase
shifts, and makes single frequency working liable to peaks and nulls. A
wider spectral energy helps defeat multipath, and single frequency
interference sources, especially with some digital schema imposed on top.

That's why its done.

Cellular phones and WiFi type schemata are there to provide adequate
quality at low to medium data rates, per channel. And degrade gracefully
under increased levels of interference from e.g. other users of the
spectrum, whilst still maintaining absolute channel security.

The Natural Philosopher wrote:


> In the limit, any modulation regime you use, results in changes in the
> amplitude/frequency spectrum: broadcast FM only HAS a better S/N ratio
> and capture effect BECAUSE it occupies so much spectrum.


But wouldn't a digital system using AM be unaffected by the unwanted
EMI/RFI that would normally hinder analog AM?


>
> For example, consider - say - a modulation in which say 8 digital bits
> correspond to 8 different frequencies. As the data is sent, the
> frequency 'hops'' from one frequency to another.
>
> However,that is exactly the same as having one transmitter that is
> frequency modulated, or 8 transmitters that are amplitude modulated, on
> different frequencies.


I prefer the former over the latter. I've never heard of analog EMI/RFI
signal interfering with digital signal reception. So I'd have to say
multiple AM transmitters on different frequencies would yield better
results than one FM transmitter.


> Modulation schemata are chosen with an eye to the type of interference
> likely to be encountered: multipath for example makes a mockery of phase
> shifts, and makes single frequency working liable to peaks and nulls. A
> wider spectral energy helps defeat multipath, and single frequency
> interference sources, especially with some digital schema imposed on top.
>
> That's why its done.


But wouldn't multipath interference be as much of an issue in FM as in AM?