We manufacture a radio modem (RS232/422 to RF and back) which
works in the 2.4GHz ISM band. The following is quoted from the
the integration guide for the radio part.

Begin quote

1.1. Why spread spectrum?

The radio transmission channel is very hostile, corrupted by
noise, path loss and interfering transmissions from other
radios. Even in a pure interference-free environment, radio
performance faces serious degradation through a phenomenon
known as multipath fading, a problem particularly prevalent for
indoor installations, which results when two or more reflected
rays of the transmitted signal arrive at the receiving antenna
with opposing phase, thereby partially or completely canceling
the desired signal. In the frequency domain, a multipath fade
can be described as a frequency-selective notch that shifts in
location and intensity over time as reflections change due to
motion of the radio or of objects within its range. At any
given time, multipath fades will typically occupy perhaps 5% of
the band, which means that from a probabilistic viewpoint, a
conventional radio system faces a 5% chance of signal
impairment at any given time due to multipath.

Spread spectrum reduces the vulnerability of a radio system to
both interference from jammers and multipath fading by
distributing the transmitted signal over a larger region of the
frequency band than would otherwise be necessary to send the
information. This allows the signal to be reconstructed even
though part of it may be lost or corrupted in transit.

End quote

This device uses frequency hopping spread spectrum. Not direct
sequence.


Ruben

> >I haven't seen anybody mention the advantage of eliminating multi
> >path fading problems with SS.
>
> Afaik no commercial band permitting SS is wide enough to allow the SS
> to 'overcome' standing wave 'holes' easily. Space diversity works much
> better (at least two antennas at least at the base station). The SS
> can be equated to space diversity antennas at a given distance
> (expressed in lambdas). Since the band is narrow, this distance will
> be small wrt. what can be achieved by simply putting the second
> antenna N/4 lambdas (or some other well chosen number) away from the
> first. To achieve the same effect with SS you would need a bandwidth
> of 25%.  Only the military can afford that sort of thing or so I
> think.
>
> Peter
>
==============================
Ruben Jvnsson
AB Liros Electronic
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
==============================

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