802.11 ac vs. Antennas Antenna Characteristics and Line of
Sight Paths Fundamentals of Wireless LANs version 1.1
Slide 2
2 IEEE 802.11ac IEEE 802.11ac is a wireless networking standard
in the 802.11 family (which is marketed under the brand name
Wi-Fi), developed in the IEEE Standards Association process,[1]
providing high-throughput wireless local area networks (WLANs) on
the 5 GHz band.[1] The standard was developed from 2011 through
2013 and approved in January 2014.[1][2] This specification has
expected multi-station WLAN throughput of at least 1 gigabit per
second and a single link throughput of at least 500 megabits per
second (500 Mbit/s). This is accomplished by extending the air
interface concepts embraced by 802.11n: wider RF bandwidth (up to
160 MHz), more MIMO spatial streams (up to eight), downlink
multi-user MIMO (up to four clients), and high-density modulation
(up to 256-QAM).
Slide 3
3 IEEE 802.11ac
Slide 4
4 Wireless AC is the next generation wireless technology
Wireless AC draft 802.11ac technology* was developed to optimize
video streaming experiences. Providing Gigabit Wi-Fi speeds allows
content to download faster and large video or music files to sync
more quickly. With an increasing number of Wi-Fi devices in the
home leading to greater Internet consumption, this new wireless
draft 802.11ac standard will help you meet your digital lifestyle
demands. 802.11ac, the emerging standard from the IEEE, is like the
movie The Godfather Part II. It takes something great and makes it
even better. 802.11ac is a faster and more scalable version of
802.11n. It couples the freedom of wireless with the capabilities
of Gigabit Ethernet.
Slide 5
5 IEEE 802.11ac Wireless LAN sites will see significant
improvements in the number of clients supported by an access point
(AP), a better experience for each client, and more available
bandwidth for a higher number of parallel video streams. Even when
the network is not fully loaded, users see a benefit: their file
downloads and email sync happen at low lag gigabit speeds. Also,
device battery life is extended, since the devices Wi-Fi interface
can wake up, exchange data with its AP, and then revert to dozing
that much more quickly. 802.11ac achieves its raw speed increase by
pushing on three different dimensions: More channel bonding,
increased from a maximum of 40 MHz with 802.11n up to 80 or even
160 MHz (for speed increases of 117 or 333 percent, respectively).
operate in the less crowded 5-GHz band.
Slide 6
6 IEEE 802.11ac Denser modulation, now using 256 quadrature
amplitude modulation (QAM), up from 64QAM in 802.11n (for a 33
percent speed burst at shorter, yet still usable, ranges). More
multiple input, multiple output (MIMO). Whereas 802.11n stopped at
four spatial streams, 802.11ac goes all the way to eight (for
another 100 percent speed increase). The design constraints and
economics that kept 802.11n products at one, two, or three spatial
streams havent changed much for 802.11ac, so we can expect the same
kind of product availability, with first- wave
Slide 7
7 IEEE 802.11ac 802.11ac products built around 80 MHz and
delivering up to 433 Mbps (low end), 867 Mbps (mid-tier), or 1300
Mbps (high end) at the physical layer. Second-wave products may
promise still more channel bonding and spatial streams, with
plausible product configurations operating at up to 3.47 Gbps.
802.11ac is a 5-GHz-only technology, so dual-band APs and clients
will continue to use 802.11n at 2.4 GHz. However, 802.11ac clients
operate in the less crowded 5-GHz band. Wireless networking devices
have antenna to maximize signal strength. Lets look at how antenna
directivity works.
Slide 8
8 Antenna Directivity Antennas radiate wireless power Accept
wireless signal energy from the transmission line connected to a
transmitter Launch that wireless energy into free-space
Slide 9
9 Antenna Directivity Antennas focus wireless energy like a
flashlight reflector (focusing element) focuses light from a
flashlight bulb. Without the focusing element, the bulb radiates
light energy in all direction. No direction receives more light
than any other direction.
Slide 10
10 Antenna Directivity Light energy from an unfocused
flashlight bulb is similar to the wireless energy radiated from a
theoretical isotropic antenna. Like a light bulb, an isotropic
antenna radiates wireless energy equally in all directions and does
not focus the energy in any single direction. Theoretical Isotropic
Antenna
Slide 11
11 Antenna Directivity A flashlight focuses the light into a
beam that comes out the front of the flashlight. The flashlight
(reflector) does not amplify the power or total amount of light
from the bulb. The flashlight simply focuses the light so all of it
travels in the same direction.
Slide 12
12 Antenna Directivity By focusing the light, the flashlight
provides more directivity (beam focusing power). An antenna
provides directivity for the wireless energy that it focuses.
Depending upon the design of the antenna, antennas focus and
radiate their energy more strongly in on favored direction. When
receiving, antennas focus and gather energy from their favored
direction and ignore most of the energy arriving from all other
directions.
Slide 13
Antennas exhibit directivity by radiating most of their power
in one direction. Major or Main Lobe Main direction of the power
from the antenna Minor or Side Lobes Small amount of power in other
directions Nulls Where no power is radiated 13 Antenna Radiated
Patterns Main Lobe Side Lobes Front Back Top View Null
Slide 14
Antennas provide the same directivity for transmitting and
receiving. Antennas radiate transmitter power in the favored
direction(s) when transmitting. Antennas gather signals coming in
from the favored directions(s) when receiving. 14 Antenna Radiated
Patterns Main Lobe Side Lobes Front Back Top View Null
Slide 15
15 Antenna Radiated Patterns When selecting antennas, remember:
When receiving, antenna directivity not only gathers incoming
signals from the favored direction, but also reduces noise,
interference, and unwanted signals coming in from other directions.
Patch Antenna (Directional Antenna)
Slide 16
16 An omnidirectional antenna radiates equally well in all
horizontal directions around the main lobe, surrounding the antenna
like a donut. More later Antenna Radiated Patterns Dipole Antenna
(Omnidirectional Antenna) Top View (H)Side View (V)
Slide 17
17 Antenna Gain Antenna gain Measurement of the power in the
main lobe of an antenna and comparing that power to the power in
the main lobe of a reference antenna. Gain - This refers to the
amount of increase in energy that an antenna appears to add to an
RF signal. Measure in dBi or dBd dBd d is the gain measured
relative to the gain of a dipole reference antenna. dBi i is the
gain measure relative to the gain of a theoretical isotropic
antenna. More later Like a flashlight, there is always a tradeoff
between gain, which is comparable to brightness in a particular
direction, and beamwidth, which is comparable to the narrowness of
the beam. (coming)
Slide 18
18 The dBi is a unit measuring how much better the antenna is
compared to an isotropic radiator. An isotropic radiator is an
antenna which sends signals equally in all directions (including up
and down). An antenna which does this has an 0dBi gain. The higher
the decibel figure the higher the gain. For instance, a 6dBi gain
antenna will receive a signal better than a 3dBi antenna. Antenna
Gain +21 dBi or about 100 times the signal strength when comparing
it to an isotropic antenna Top View
Slide 19
19 Antenna Gain A dBd unit is a measurement of how much better
an antenna performs against a dipole antenna. As a result a dipole
antenna has a 0dBd gain. Note: Wireless power never stops exactly
on a sharp line like the lobe drawings show, but tapers off. More
later Dipole antenna
Slide 20
20 Antenna Beamwidth Beamwidth The width of the main beam (main
lobe) of an antenna. Measures the directivity of an antenna The
smaller the beamwidth in degrees, the more the antenna focuses
power into its main lobe. The more power of the main lobe, the
further the antenna can communicate.
Slide 21
21 Antenna Beamwidth 15 dBi 12 dBi -3 dBi Beamwidth is a
measurement used to describe directional antennas. Beamwidth is
sometimes called half-power beamwidth. Half-power beamwidth is the
total width in degrees of the main radiation lobe, at the angle
where the radiated power has fallen below that on the centerline of
the lobe, by -3 dB (half-power). 15 dBi
Slide 22
22 Remember, wireless power does not stop and start exactly
along a straight line, but declines gradually with distance. The
smooth outlines of the main lobes show the approximate intensity of
the wireless power at various distances away from the antenna. The
dotted lines pass through the half-power points the points on each
side of the center of the main lobe where the wireless power is
one-half as strong as it is at the center of the lobe.
Slide 23
Line-of-Sight (LOS)
Slide 24
24 Line of Sight When a wireless signal encounters an
obstruction, the signal is always attenuated and often reflected or
diffracted. It is important to try and obtain a wireless
line-of-sight whenever possible, especially in a wireless WAN
environment (outdoor connections between building or different
parts of a campus). A wireless LOS typically requires visual LOS
plus additional path clearance to account for the spreading of the
wireless signal (Fresnel Zone coming). Diffracted Signal Attenuated
Signal
Slide 25
25 Visual LOS There is a difference between visual LOS and
wireless LOS. This is because of the difference in wavelengths. The
wavelength of visual light is very small. For example, the
wavelength of a green light is only about 1/50,000 th of an inch
Remember, the wavelength of a 2.4 GHz WLAN signal is about 4.8
inches. I see you!And, I see you! 1 Mile
Slide 26
26 LOS A lightwave and a wireless wave are similar. Both are
forms of electromagnetic radiation. Both must obey the same laws of
physics as they propagate. Wireless signals are like lightwaves
that you cannot see. 1 Mile
Slide 27
27 LOS The shorter the wavelength of an electromagnetic wave,
the less clearance it needs form objects that it passes as it
travels between two points. The less clearance it needs, the closer
it can pass to an obstruction without experience additional loss of
signal strength. The clearance distance is known as the Fresnel
Zone. 1 Mile
Slide 28
28 LOS The green light has a shorter wavelength so only needs a
fraction of an inch to avoid additional attenuation. A 2.4 GHz
(802.11b/g) wireless signal has a larger Fresnel zone and needs to
clear the building by quite a few feet (about 10 feet in this
example). 1 Mile
Slide 29
29 Fresnel Zone Fresnel zone (pronounced frA-nel; the s is
silent). Provides a method for calculating the amount of clearance
that a wireless wave (or light wave) needs from an obstacle to
avoid additional attenuation of the signal.
Slide 30
30 Fresnel Zone Fresnel Zone = 72.1 * SqrRoot (dist1Mi *
dist2Mi / FreqGHz * DistanceMi) At least 60% of the calculated
Fresnel Zone must clear to avoid significant signal
attenuation.
