Moxa ANT-WSB-ANM-05 Manuel D’Utilisation
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11
Understanding Industrial WLAN – IEEE 802.11
All of the above phenomena results in multipath propagation so not all signals arrive at the receiver antenna at
the same time due to obstacles that change the signal paths. Whether you are setting up an outdoor or indoor
application, multipath can severely affect received signal quality because the delayed signals are destructive to
the main signal. The multipath issue can usually be compensated by antenna diversity at the RF level and/or by
OFDM at the baseband level.
the same time due to obstacles that change the signal paths. Whether you are setting up an outdoor or indoor
application, multipath can severely affect received signal quality because the delayed signals are destructive to
the main signal. The multipath issue can usually be compensated by antenna diversity at the RF level and/or by
OFDM at the baseband level.
Modulation and Spread Spectrum
The following chart categorizes different digital modulation techniques:
Digital modulation
linear
Constant envelope / nonlinear
Combined / hybrid
Spread spectrum
BPSK
BFSK
MPSK
PN
DPSK
MSK
M-ary QAM
DSSS
QPSK
GMSK
MFSK
FHSS
π / 4
QPSK
ODFM
As you can see, there are many RF modulation techniques. However, our discussion is limited only to the
techniques that pertain to the 802.11 standard, namely FHSS, DSSS, and OFDM.
techniques that pertain to the 802.11 standard, namely FHSS, DSSS, and OFDM.
FHSS (Frequency Hopping Spread Spectrum)
This modulation technique is one of the techniques
used in spread spectrum signal transmission. It is also
known as Frequency-Hopping Code Division Multiple
Access (FH-CDMA). Spread spectrum enables a signal
to be transmitted across a frequency band that is much
wider than the minimum bandwidth required by the
information signal. The transmitter “spreads” the energy,
originally concentrated in narrowband, across a number
of frequency band channels on a wider electromagnetic
spectrum. Some of the advantages include:
- Improved privacy
- Decreased narrowband interference
- Increased signal capacity
used in spread spectrum signal transmission. It is also
known as Frequency-Hopping Code Division Multiple
Access (FH-CDMA). Spread spectrum enables a signal
to be transmitted across a frequency band that is much
wider than the minimum bandwidth required by the
information signal. The transmitter “spreads” the energy,
originally concentrated in narrowband, across a number
of frequency band channels on a wider electromagnetic
spectrum. Some of the advantages include:
- Improved privacy
- Decreased narrowband interference
- Increased signal capacity
Diffraction (Shadow Fading)
Signal strength is reduced after experiencing diffraction. Obstacles
causing diffraction usually possess sharp edges such as the edges of
buildings. When EM waves encounter an obstacle with sharp edges that
cannot be penetrated, the EM waves wrap around the obstacle to reach
the receiver.
causing diffraction usually possess sharp edges such as the edges of
buildings. When EM waves encounter an obstacle with sharp edges that
cannot be penetrated, the EM waves wrap around the obstacle to reach
the receiver.
Scattering
When EM waves encounter many small obstacles (smaller than wave
length), the EM waves scatter into many small reflective waves and
damage the main signal, causing low quality or even broken links. Such
obstacles include rough surfaces, rocks/sand/dust, tree leaves, street
lights, etc.
length), the EM waves scatter into many small reflective waves and
damage the main signal, causing low quality or even broken links. Such
obstacles include rough surfaces, rocks/sand/dust, tree leaves, street
lights, etc.
Reflection
When EM waves run into large obstacles such as the ground, walls,
or buildings, they reflect and change their direction and phase. If the
reflected surface is smooth, the reflected signal will likely represent the
initial signal and not be scattered.
or buildings, they reflect and change their direction and phase. If the
reflected surface is smooth, the reflected signal will likely represent the
initial signal and not be scattered.