Wireles Networks The Definitive Guide.pdf - Csbdu.in

usually buy some cable to separate the antenna from the PC Card, which can be useful for a base
station. However, be careful—coaxial cable (especially small coaxial cable) is very lossy at these
frequencies, so it's easy to imagine that anything you gain by better antenna placement will be lost in
the cable. People have experimented with building high-gain antennas, some for portable use, some for
base station use. And commercial antennas are available—some designed for 802.11 service, some
adaptable if you know what you're doing.
9.3.1.2 Amplifiers
Amplifiers make signals bigger. Signal boost, or gain, is measured in decibels (dB). Amplifiers can be
broadly classified into three categories: low-noise, high-power, and everything else. Low-noise
amplifiers (LNAs) are usually connected to an antenna to boost the received signal to a level that is
recognizable by the electronics the RF system is connected to. LNAs are also rated for noise factor,
which is the measure of how much extraneous information the amplifier introduces. Smaller noise
factors allow the receiver to hear smaller signals and thus allow for a greater range.
High-power amplifiers (HPAs) are used to boost a signal to the maximum power possible before
transmission. Output power is measured in dBm, which are related to watts (see the sidebar).
Amplifiers are subject to the laws of thermodynamics, so they give off heat in addition to amplifying
the signal. The transmitter in an 802.11 PC Card is necessarily low-power because it needs to run off a
battery if it's installed in a laptop, but it's possible to install an external amplifier at fixed access points,
which can be connected to the power grid where power is more plentiful.
This is where things can get tricky with respect to compliance with regulations. 802.11 devices are
limited to one watt of power output and four watts effective radiated power (ERP). ERP multiplies the
transmitter's power output by the gain of the antenna minus the loss in the transmission line. So if you
have a 1-watt amplifier, an antenna that gives you 8 dB of gain, and 2 dB of transmission line loss, you
have an ERP of 4 watts; the total system gain is 6 dB, which multiplies the transmitter's power by a
factor of 4.
Decibels and Signal Strength
Amplifiers may boost signals by orders of magnitude. Rather than keep track of all those
zeroes, amplifier power is measured in decibels (dB).
dB = 10 x log 10 (power out/power in)
Decibel ratings are positive when the output is larger than the input and negative when the
output is smaller than the input. Each 10-dB change corresponds to a factor of 10, and 3-dB
changes are a factor of 2. Thus, a 33-dB change corresponds to a factor of 2000:
33 dB = 10 dB + 10 dB + 10 dB + 3 dB = 10 x 10 x 10 x 2 = 2000
Power is sometimes measured in dBm, which stands for dB above one milliwatt. To find the
dBm ratio, simply use 1 mW as the input power in the first equation.
It's helpful to remember that doubling the power is a 3-dB increase. A 1-dB increase is
roughly equivalent to a power increase of 1.25. With these numbers in mind, you can
quickly perform most gain calculations in your head.
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