Transcript Chuong 1 - Gio Thieu Quan Tri Mang

Center for Information Technology
Chapter 02
Radio Frequency
& Antenna Fundamentals
(part 2)
Basic RF Math
2
Units of Power
Watt (W) is a basic unit of power. One watt is equal to one
ampere of current flowing at one volt.
Milliwatt (mW) 1W = 1000 mW
The reason to be concerned with milliwatts is because most of
the 802.11 equipment that transmits at power levels between 1
and 100 mW.
For indoor use, it is recommended that the output power is less
than 100 mW. For outdoor WLANs may use more power if they
are providing site-to-site links or are providing coverage to a
large outdoor area.
The FCC limits the total output power from the antenna to 4 W
for point-to-multipoint applications in the 2.4 GHz ISM band.
3
Units of Power
Decibel (dB)
The decibel is a comparative measurement value. It is a
measurement of the difference between two power levels.
1 bel is a ratio of 10:1 between two power levels. Therefore, a
power ratio of 200:20 is 1 bel (10:1) and 200:40 is .5 bels (5:1)
and 200:10 is 2 bels (20:1).
bels = log(P1/P2)
decibels = 10×log(P1/P2) = 10xlog(Pout/Pin)
The decibel is relative where the milliwatt is absolute.
The decibel is logarithmic where the milliwatt is linear.
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dB
The differences between values can
become extremely large or small
and more difficult to deal with. It is
easier to say that a 100 mW signal
decreased by 70 decibels than to
say that it decreased to .00001
milliwatts.
10log(100/0.00001) = 70dB
Comparison of Milliwatts
and Decibel Change
(relative to 1 mW)
5
10’s and 3’s Rules of RF Math
1. A gain of 3 dB magnifies the output power by two.
2. A loss of 3 dB equals one half of the output power.
3. A gain of 10 dB magnifies the output power by 10.
4. A loss of 10 dB equals one-tenth of the output power.
5. dB gains and losses are cumulative.
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10’s and 3’s Rules of RF Math
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Quiz 1
An RF signal of 2 Watts is applied to a 100-foot
antenna cable, however, only 1 Watt of transmit
power is actually developed at the input of the Ans = C
transmitting antenna. What is the resulting cable
loss, measured in dB?
A. 0.5 dB
B. 1 dB
C. 3 dB
D. 5 dB
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Quiz 2
A loss of -10dB yields a power ratio of?
A. 1:3
B. 1:10
C. 2:1
D. 10:1
Ans = B
-10dB = 10log(P1/P2)  log(P1/P2) = -1
P1/P2 = 10-1 = 1:10
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Quiz 3
A gain of +3dB yields a power ratio of?
A. 2:1
B. 3:1
C. 10:1
D. 1:10
Ans = A
+3dB = 10log(P1/P2)  log(P1/P2) = 3/10
P1/P2 = 103/10 = 1.995262  2:1
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Quiz 4
You have an access point that is transmitting
at 50 mW. The signal loss between the
access point and the antenna is –1 dB, and
the access point is using a 5 dBi antenna.
Calculate mW output by antenna.
A. 100 mW
B. 125 mW
C. 150 mW
D. 200 mW
dB
+10
-3
-3
Ans = B
mW
50
500
250
125
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Combination of 10s and 3s
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dBm
The dBm represents an absolute
measurement of power where the
m stands for milliwatts.
dBm references decibels relative
to 1 milliwatt such that 0 dBm
equals 1 milliwatt.
The formula to get dBm from
milliwatts is
dBm = 10xlog(PowermW)
The benefits of working with dBm values instead of milliwatts is
the ability to easily add and subtract simple decibels instead of
multiplying and dividing often huge and tiny numbers.
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Quiz
In terms of RF power, 1 Watt = ______ dBm.
A. 3
B. 10
C. 20
D. 30
dBm
0
10
20
30
mW
1
10
100
1000
Ans = D
dBm = 10xlog(PowermW)
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dBi
The dBi (the i stands for isotropic) represents a measurement of
power gain used for RF antennas.
It is a comparison of the gain of the antenna and the output of a
theoretical isotropic radiator.
An isotropic radiator is an ideal antenna that we cannot create
with any known technology. This is an antenna that radiates
power equally in all directions.
The dBi value must be calculated against the input power
provided to the antenna to determine the actual output power in
the direction in which the antenna propagates RF signals.
15
dBd
dBi is a calculation of directional gain compared to an isotropic
radiator, dBd is a calculation of directional gain compared to a
dipole antenna.
dBd is a value calculated against the input power to determine the
directional output power of the antenna.
The difference is that a dBd value is compared with a dipole
antenna, which itself has a gain of 2.14 over an isotropic radiator.
An antenna with a gain of 7 dBd has a gain of 9.14 dBi.
To convert from dBd to dBi, just add 2.14.
To convert from dBi to dBd, just subtract 2.14.
0 dBd = 2.14 dBi.
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SNR
Background RF noise, which can be caused by all the various
systems and natural phenomena that generate energy in the
electromagnetic spectrum, is known as the noise floor.
The power level of the RF signal relative to the power level of
the noise floor is known as the signal-to-noise ratio or SNR.
The higher the
ratio, the less
obtrusive the
background
noise is.
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RSSI
The received signal strength indicator is an arbitrary
measurement of received signal strength defined in the IEEE
802.11 standards.
Cisco uses a range of 0–100 in their devices, and most Atherosbased chipsets use a range of 0–60.
The RSSI rating is also arbitrarily used to determine when to
reassociate and when to transmit. In other words, vendors will
decide what the lowest RSSI rating should be before attempting
to reassociate to a basic service set with a stronger beacon signal
18
Link Budget
Link budget is an accounting of all components of power, gain,
loss, receiver sensitivity, and fade margin. This includes the
cables and connectors leading up the antenna, as well as the
antennas themselves. It also includes free space path loss.
A link budget is used to predict performance before the link is
established.
- Show in advance if it will be acceptable
- Show if one option is better than another
- Provide a criterion to evaluate actual performance
19
Receive Sensitivity
The minimum signal strength
needed at the receiver.
The receive sensitivity is not
a single dBm rating; it is a
series of dBm ratings required
to communicate at varying
data rates.
Ex: The lowest number in
dBm, which is −94dBm is the
weakest signal the radio can
tolerate.
20
System Operating Margin
The SOM is the amount of received signal strength relative to the client
device’s receive sensitivity.
The SOM = link budget, is the calculation of the amount of RF signal
that is received minus the amount of signal required by the receiver.
Ex: If we have a client device with a receive sensitivity of −94dBm and
the card is picking up the wireless signal at −65dBm, the SOM is the
difference between −94 and −65.
Therefore, the link budget is: SOM = RS − S
where S is the signal strength at the wireless client device and RS is the
receive sensitivity of the client device.
The resulting SOM is 29dBm. This means that the signal strength can be
weakened by 29dBm and the link can be maintained.
21
Link budget calculation
The receive sensitivity of both bridges is −94dBm. The
calculations are as follows:
Link budget calculation 1: 100mW = 20dBm
Link budget calculation 2: 20dBm−3dB+7dBi−83dB = −59dBm
Link budget calculation 3: (−94dBm) − (−59dBm) = 35 dBm
SOM = 35 dBm
22
Fade Margin
By including a few extra dB of strength in the required link
budget, we can provide a link that will endure longer.
This extra signal strength is fade margin.
We do not add to the link budget/SOM dBm value, but instead
we take away from the receive sensitivity.
For example, we may decide to work off of an absolute receive
sensitivity of −80dBm instead of the −94dBm supported by the
Cisco Aironet card mentioned earlier. This would provide a fade
margin of 14dBm. SOM in this case is 21 dBm.
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Intentional Radiator
The intentional radiator is the point at which the antenna is
connected.
The signal originates at a transmitter and may pass through
connectors, amplifiers, attenuators, and cables before reaching
the antenna. These components amplify or attenuate the signal,
resulting in the output power at the intentional radiator before
entering the antenna.
The FCC sets the rules regarding the power that can be delivered
to the antenna and radiated by the antenna. The FCC allows 1
watt of output power from the intentional radiator and 4 watts of
antenna output power in a point-to-multipoint link in the 2.4
GHz ISM band.
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EIRP
The equivalent isotropically radiated power (EIRP) is the output
power from the intentional radiator plus the directional gain
provided by the antenna.
Power radiated out of antenna of a wireless system
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Example 1
We have a wireless bridge that generates a 100 mW signal. The
bridge is connected to an antenna using cable that creates 3 dB of
signal loss. The antenna provides 10 dBi of signal gain. In this
example, calculate the IR and EIRP values.
26
Solution
10xlog100mW = 20 dBm
IR = 20 dBm - 3 dB = 17 dBm
EIRP = 17 dBm + 10 dB = 27 dBm
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Quiz 1
An access point is emitting a 100 mW signal that is
connected to a length of cable with a 3dB loss. If
the cable is then connected to a +9dBi antenna, Ans = C
what is the EIRP from the antenna in dBm?
A. 20 dBm
B. 23 dBm
dBm = 10xlog(PowermW)
C. 26 dBm
D. 29 dBm
dBm
0
10
20
mW
1
10
100
20dBm – 3dB + 9dBi = 26 dBm
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Quiz 2
An access point that emits a 1000
mW signal is connected to a cable
and its connectors with 10dB loss.
The cable is then connected to a
3dB gain antenna. What is the
resulting in mW from the antenna?
A. 100 mW
B. 200 mW
C. 300 mW
D. 500 mW
Ans = B
dB
-10
+3
mW
1000
100
200
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RF Signal
and Antenna Concepts
30
Visual LOS - RF LOS
If we can physically see something, it is said to be in our visual
line of sight. This LOS is actually the transmission path of the
light waves from the object we are viewing (transmitter) to our
eyes (receiver).
RF LOS is more sensitive than visible LOS to interference near
the path between the transmitter and the receiver.
More space is needed for the RF waves to be seen by each end
of the connection. This extra space is called the Fresnel zone.
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The Fresnel Zone
The Fresnel zones are a theoretically infinite number of
ellipsoidal areas around the LOS in an RF link.
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Beamwidths
Beamwidth is the measurement of
how broad or narrow the focus of
the RF energy is as it propagates
from the antenna along the main
lobe.
The main lobe is the primary RF
energy coming from the antenna.
Beamwidth is measured both vertically and horizontally
The beamwidth is a measurement taken from the center of the
RF signal to the points on the vertical and horizontal axes
where the signal decreases by 3 dB or half power.
33
Beamwidths
Beamwidth measurements give the propagation pattern of an
antenna, they are less than perfect in illustrating the actual areas
that are covered by the antenna.
For more useful visual representations: reference Azimuth and
Elevation charts.
34
Azimuth and Elevation
Azimuth and Elevation charts provide a visualization of the antenna’s
propagation patterns.
The Azimuth chart shows a top-down view of the propagation path
The Elevation chart shows a side view of the propagation path
35
Isotropic Radiator
The isotropic radiator is a fictional device or concept that
cannot be developed using today’s technology.
We cannot currently create an antenna that propagates RF
energy equally in all directions.
dBi is a measurement of the gain of an antenna in a particular
direction over the power level that would exist in that direction
if the RF energy were propagated by an isotropic radiator. In
other words, dBi is a measurement of the difference between the
power levels at a point in space generated by a real antenna
versus the theoretical isotropic radiator.
The Sun is often used as an analogy of an isotropic radiator.
36
Polarization
Antenna polarization refers to the physical orientation of the
antenna in a horizontal or vertical position.
The electric field forms what is known as the E-plane.
The magnetic field forms what is known as the H-plane.
The E-plane determines the polarization of the antenna, since it is
parallel to the antenna. Therefore, if the antenna is in a vertical
position, it is said to be vertically polarized. If the antenna is in a
horizontal position, it is said to be horizontally polarized.
Vertical polarization means that most of the signal is being
propagated horizontally, and horizontal polarization means that
most of the signal is being propagated vertically.
37
Antenna Diversity
Antenna diversity is a feature that allows the device to receive
signals using two antennas and one receiver.
The reason to use antenna diversity is that a device transmit a
signal and it may arrive at a receiving device from multiple
angles with multiple signal strengths.
The device supporting antenna diversity will look at the signal
that comes into each antenna during the frame preamble of a
single frame and choose the signal that is best on a frame-byframe basis. The best frame preamble will determine which
antenna is used to receive the rest of the current frame.
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Antenna Diversity
39
Antennas
& Antenna Systems
40
Omnidirectional/Dipole Antennas
Omnidirectional antennas, the most popular type being
the dipole antenna, are antennas with a 360-degree
horizontal propagation pattern.
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Omnidirectional Antenna Usage
Omnidirectional antennas provide coverage on a horizontal
plane with some coverage vertically and outward from the
antenna. This means they may provide some coverage to floors
above and below.
To reach people farther away horizontally: use higher gain
To reach people farther up or down vertically: use lower gain
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Semidirectional Antennas
Semidirectional
antennas
are
antennas that focus most of their
energy in a particular direction.
Patch, Panel, and Yagi
semidirectional antennas.
are
Patch and panel antennas usually
focus their energy in a horizontal arc
of 180 degrees or less, whereas Yagi
antennas usually have a coverage
pattern of 90 degrees or less.
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Semidirectional Antennas
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Highly Directional Antennas
Highly directional antennas are antennas that transmit with a
very narrow beam. These types of antennas often look like the
satellite dish. They are generally called parabolic dish or grid
antennas. They are mostly used for PtP or PtMP links.
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Sectorized and Phased-Array Antennas
A sectorized antenna is a high-gain antenna
that works back-to-back with other
sectorized antennas.
A phased-array antenna is a special
antenna system that is actually composed
of multiple antennas connected to a single
processor. The antennas are used to
transmit different phases that result in a
directed beam of RF energy aimed at client
devices.
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MIMO Antenna Systems
Multiple-Input MultipleOutput (MIMO) can be
described as any RF
communications
system
that has multiple antennas
at both ends of the
communications link being
used concurrently.
The proposed 802.11n
standard
will
include
MIMO technology.
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