One of the principle requirements for wireless communication is that the transmitted
EM wave must reach the receiver with ample power to allow the receiver
to distinguish the wave from the background noise. An analogy can be made to
human hearing: when someone is talking to you, they must talk loud enough for
your ears to pick up the sound, and it has to be loud enough and clear enough
for your brain to be able to recognize and translate the sounds into individual
Just as your ears and brain require a minimum volume and clarity level to be
able to discern what is being said, a radio receiver also requires a minimum
power level of received signal in order to discern and recreate the transmitted
modulating wave. Signal strength for EM waves is usually measured in watts, or
more specifically, a logarithmic ratio of the signal strength divided by 1 milliwatt.
This logarithmic ratio is called decibels above 1 milliwatt (dBm).
Another common property used to describe signal strength is the S/N ratio.
The S/N ratio does not describe the absolute power in the signal, but instead
describes the power of the signal in comparison to the power of the background
noise.The higher the S/N ratio, the better or more powerful the signal. Looking
back at the hearing analogy: someone talking to you in a quiet room would be
able to whisper and you would still be able to hear; however, if there was a lot of background noise, say at a rock concert, that person would have to yell in order
for you to hear.The same concept applies to RF wireless communication. Since
the S/N ratio accounts for the level of background noise, it is a very valuable and
widely used indicator of signal strength.
Different modulation and encoding technologies require different minimum
S/N ratios to function. Most digital modulation schemes require a lower S/N
ratio than analog modulation schemes, because the receiver of a digitally modulated
carrier wave only has to distinguish between certain levels that represent a
logical 1 and a logical 0. Even in the presence of a lot of noise, the receiver is
able to distinguish between the predefined threshold levels and then regenerate
the digital square wave. In contrast, the receiver of an analog-modulated signal
has an infinite number of levels that must be distinguished and maintained. It
cannot assume that the received signal was supposed to be a 1 or a 0 and regenerate
the signal—it must receive the signal, demodulate it, and pass the resulting
representation of the original signal to the next processing device, such as an
amplifier.Therefore, any noise added to an analog signal during propagation will
alter the original signal. When the power of the modulated RF signal is several
times greater than the power of the background noise, the added noise will not
be noticed or can be reduced by filtering. On the other hand, when the noise is
nearly as powerful as the signal, the resulting demodulated signal will be noticeably
different than the original modulating signal.This is commonly called static.
The S/N ratio is an important aspect of network design.There are engineering
rules established by vendors, specific to their equipment, to provide a set of guidelines
for your design. Depending on the geographic span of your design, the S/N
ratio may warrant devices to amplify or regenerate the transmitted signal.
Attenuation, discussed in the following section, is another important consideration
in wireless networking.This will dictate the acceptable span between
antennas.The engineering rules mentioned earlier also include the attenuation
parameters acceptable by specific equipment.