Page 52 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes gets onto the power planes and is coupled to the line drivers via the VCC and ground pins. The
Rf is then coupled to the cable through the line driver as common node noise. Since the noise
is common mode, shielding has very little effect, even with differential pairs. The RF energy is
capacitively coupled from the signal pair to the shield and the shield itself does the radiating.
At higher frequencies, usually above 500 Mhz, traces get electrically longer and higher above
the plane. Two techniques are used at these frequencies: wave shaping with series resistors and
embedding the traces between the two planes. If all these measures still leave too much RFI,
sheilding such as RF gaskets and copper tape can be used. Most digital equipment is designed
with metal, or coated plastic, cases.
Switching power supplies can be a source of RFI, but have become less of a problem as design
techniques have improved.

Most countries have legal requirements that electronic and electrical hardware must still work
correctly when subjected to certain amounts of RFI, and should not emit RFI which could interfere
with other equipment (such as radios).

3.2.5 Multipath

In wireless telecommunications, multipath is the propagation phenomenon that results in radio
signals reaching the receiving antenna by two or more paths. Causes of multipath include
atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and
terrestrial objects such as mountains and buildings.
The effects of multipath include constructive and destructive interference, and phase shifting of
the signal. Destructive interference causes fading. Where the magnitudes of the signals arriving
by the various paths have a distribution known as the Rayleigh distribution, this is known as
Rayleigh fading. Where one component (often, but not necessarily, a line of sight component)
dominates, a Rician distribution provides a more accurate model, and this is known as Rician
fading.

In facsimile and television transmission, multipath causes jitter and ghosting, seen as a faded
duplicate image to the right of the main image. Ghosts occur when transmissions bounce off a
mountain or other large object, while also arriving at the antenna by a shorter, direct route, with
the receiver picking up two signals separated by a delay.
In radar processing, multipath causes ghost targets to appear, deceiving the radar receiver. These
ghosts are particularly bothersome since they move and behave like the normal targets (which
they echo), and so the receiver has difficulty in isolating the correct target echo. These problems
can be overcome by incorporating a ground map of the radar’s surroundings and eliminating all
echoes which appear to originate below ground or above a certain height.
In digital radio communications (such as GSM) multipath can cause errors and affect the quality
of communications. The errors are due to intersymbol interference (ISI). Equalisers are often used
to correct the ISI. Alternatively, techniques such as orthogonal frequency division modulation
and rake receivers may be used.

Notes In a Global Positioning System receiver, Multipath Effect can cause a stationary
receiver’s output to indicate as if it were randomly jumping about or creeping. When
the unit is moving the jumping or creeping is hidden, but it still degrades the displayed
accuracy.

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