[MLB-WIRELESS] History of Spread Spectrum with beginners intr o

[Tony Langdon]

> I believe this is the case with pretty much any radio 
> transmission. The
> doppler effect can play havoc with frequency modulated signals for the
> very reasons you mentioned (from my understanding). This is most
> apparent in extreme speeds though (jet planes/etc). 

Actually, Doppler shift is most severe on narrow bandwidth transmissions
such as CW (Morse Code).  SSB is particularly severely affected, because of
the need for a precise frequency relationship between the received signal
(which may be Doppler shifted) and the locally inserted carrier (which has
no Doppler shift) used to demodulate it.

Other modes such as AM and FM are much less affected by Doppler shift - as
the bandwidth increases, the signal is less affected, because less of the
total signal power falls outside the receiver's bandwidth.

A few examples.

At orbital speeds (low earth orbiting satellites - 27,000 km/h), the Doppler
is roughly as follows:

29 MHz - +/- 600 Hz  (SSB and Morse must be compensated)

145 MHz - +/- 3.5 kHz (AM needs to be compensated.  voice grade FM distorted
but workable without compensation)

435 MHz - +/- 10 kHz (Voice grade FM needs to be compensated for Doppler)

10 GHz - +/- 200 kHz (all modes except very wideband data/video need Doppler
compensation)

At highway speeds (100 km/h)

1.2 GHz - +/- 100 Hz (SSB workable with odd pitch shifts)
10 GHz - +/- 800 kHz (SSB requires Doppler compensation, AM/FM/etc work
fine).

As for data clocking in WLANs, this depends on how adaptive the clock
recovery loop is.  The good news is that the clock is often recovered from
the transmitted signal, meaning the recovery loop will tend to track some
degree of frequency shift.

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