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?
>
>
> Thanks,
>
> Radium

Hi,

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.

Regards,
John