Chapter 4 Flashcards
a material that allows electrical current to flow through it
Conductor
passive conductor used to transmit electromagnetic waves through space
Antenna
Simplest antenna is a bare metal wire called
Found on older automobiles and cellular phones
The length of the antenna should be at least one-quarter of the wavelength of the electromagnetic wave that is sending or receiving
whip antenna
a system used to create and transmit RF signals
Labeled by the FCC
Specifically designed to send out electromagnetic waves (intentional)
Also designed to radiate out, or send, a signal (radiator)
Intentional Radiator
source of RF waves that have the same magnitude or properties in all directions
No preference in the direction of radiation
Isotropic radiator
amount of power that a theoretical isotropic radiator can generate
Includes any antenna amplification (gain)
Equivalent Isotropically Radiated Power (EIRP
passive gain of power that is funneled from an antenna compared to that of an isotropic radiator sent in all directions
Focusing the signal in a specific direction is known as a passive gain (no additional power is added)
Decibels isotropic (dBi)
most basic type of antenna
Dipole
compares the antenna gain against that of a dipole antenna
Decibels dipole (dBd)
most common type of antenna for a WLAN
Radiates its signal out horizontally in all directions equally
Omnidirectional antennas
orientation of radio waves as they leave an antenna
Antennas must be polarized alike to be efficient
Antennas in a horizontal position will not communicate well with an antenna in a vertical position
Polarization
Ability of an access point to examine multiple copies of a received transmission and then select the best signal
Selecting the best signal is known as switching
Antenna diversity
the AP can transmit on the antenna that most recently received the strongest incoming signal
Transmit Diversity
focuses energy in one direction
Primarily used for short and medium range remote wireless bridge networks
Semidirectional antennas
divides the coverage area into different sectors and gives each sector its own antenna
Sectorized antenna
multiple antennas that can be customized to send an optimal signal
Each antenna does not have to have the same power
Antenna array
sends a narrowly focused signal beam long distances
Generally reflective devices that are shaped like a concave disk
Used for outdoor long-distance point-to-point wireless links, such as connecting buildings that are up to 25 miles apart
Not used for indoor WLANs
Highly-Directional Antennas
used to illustrate an antenna’s radiation pattern
Antenna radiation chart
represents the horizontal coverage
Azimuth chart
used to show the vertical coverage
Elevation chart
a measurement of a transmission’s width
Beamwidth
theoretical straight line between a transmitter and the receiver
An object that is close to the path of the RF transmission can impact the signal
RF line of sight
elliptical area immediately surrounding the visual line of sight for RF transmissions
Steps for mitigating obstructions in the Fresnel zone include:
Raising the antenna mounting point on the existing structure
Increasing the height of an existing power
Building a new structure that is tall enough to mount the antenna
Locating a different mounting point on another building or tower for the antenna
Cutting down trees
Fresnel zone
A wireless device with multiple receive (Rx) antennas can improve transmissions by either:
Selecting the stronger incoming signal
Combining the individual signals at the receiver
system that uses one radio chain for each antenna
that each antenna can simultaneously transmit and receive signals
IEEE made MIMO the heart of 802.11n
Estimated the MIMO alone contributes 40% to the increase in speed
802.11n standard is sometimes called HT (MIMO) for High Throughput Multiple-Input Multiple-Output
Multiple Input Multiple Output (MIMO)
a MIMO technique of sending the same transmission out from different antennas that will take different paths
Can increase reliability of an RF signal
Each transmission will take different paths (called spatial paths)
It is unlikely all paths will degrade the signal in the same way
Spatial diversity
sends a normal version of the signal along with a shifted version of the same signal
Cyclic Shift Diversity (CSD)
Instead of sending the same data out through multiple antennas, first splits up the data and sends it out over multiple antennas
spatial multiplexing
The algorithm a MIMO AP uses when it receives multiple copies of a signal from a non-MIMO device
The AP will receive multiple copies of the multipath signal on each of its Rx antennas
MRC multiplies each received signal by a weight factor that is proportional to the signal amplitude
Maximal ratio combining (MRC):
An option for reducing outside signal interference by using complex antenna systems to allow for different directions and bandwidths
Transmit Beam Forming (TxBF):
a TxBF technique in which the receiver makes a series of computations and sends them to the transmitter, which then uses them to configure how to make the best transmissions
Explicit feedback
information that is computed by the receiver and sent back to the transmitter for use in antenna configuration
Implicit feedback
