clay@claysturner.com wrote:
> On Apr 28, 5:26 pm, "Green Xenon [Radium]" <gluceg...@excite.com>
> 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?
>>
>> Thanks,
>>
>> Radium
>
> Cellphones moved away from FM when they dropped analog service. IDEN
> phones use a 4 subcarrier 16-QAM, but they are channelized and not
> spread spectrum.


Yeah but even if the signal being transmitted/received is digital, it's
carrier wave is still analog. Right?

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.