Radio Physics for Wireless Devices and Networking
By Ron Vigneri
Rev: 8-26-13
Update of 10-8-2008 Article
Note: This material is applicable to any electromagnetic wave (broadcast) technology
including: Wi-Fi, radio, television, cellular telephone, radar, etc. Someone adding this
knowledge to his skill set is a good candidate for wireless computer networking,
cellular telephone, and/or related industries.
The Radio Physics of Wi-Fi
The standard for wireless LANs (WLANs) was completed in 1997 with the release
of the IEEE 802.11 specification which became the first major step in the development
of wireless network technologies. The spread of the technology is similar to
that of cellular telephone. Wireless technology is one the hottest items in
networking for the home, office, and even wide area (WAN) applications like
high speed Internet access. This is the case with the recent broadband
wireless deployments by AT&T, Verizon, Sprint, and others. Let’s see the physics
involved in everyday wireless technology in this lesson.
Radio Wave Physics
A radio wave is an electromagnetic wave that can propagate (travel) through
air, water, walls, some objects, and the vacuum of outer space. The wave is
an electric field and an associated magnetic field at right angles
to each other. Both these fields can vary periodically in amplitude and frequency.
The fields vary perpendicularly to the direction of propagation of the radio
wave. The fields, and hence, the radio wave can be generated by applying an
alternating current (or voltage) to a dipole (two conductor) antenna. The frequency of the
alternating current for the exercises in our study will be considered to be 2.4 gigahertz
(2.4 GHz), which is an unlicensed frequency regulated by the FCC. In the present Wi-Fi
standards another unlicensed frequency for use is 5 GHz .
So, electromagnetic waves consist of the propagation of oscillating electric
and magnetic fields shown in the following diagram. Note that
as the radio wave propagates (radiates) out from the dipole antenna source we
considered. It will decrease in amplitude (the “height” of the fields in the
diagram) as it travels farther from the source due to loss factors.
Fig. 1 Electromagnetic Wave
In the above illustration, the frequency of the electromagnetic wave can be
determined from the time period (T). The time period between the start and end
of one cycle of the waveform is the wave period, T. The frequency of an
electromagnetic wave is related to the period by the formula,
f = 1/T
where f = frequency in Hertz
T = time period in seconds
From that relationship, the period for a wave with a frequency of 2.4 GHz is
0.4166 x 10-9 (billionths of a second or nanoseconds) and that is very fast.
From famous physicists, Maxwell and Hertz, the frequency and wavelength of
an electromagnetic wave are related to the velocity of light by the equation
Frequency (f) x Wavelength (l) = Velocity of Light (c)
Which can be expressed as
f x l = c = 3 x 108 meters per second
where f = frequency in Hertz
l = wavelength in meters
c = 3 x 10E8 meters per second (E = exponent, here 10 raised to the 8th power)
Frequency is measured in cycles per second, which has been named a Hertz and
is abbreviated as Hz. A gigahertz would be one billion Hertz, represented by
1 GHz, with G meaning giga or 109. The frequency of 2.4 GHz, utilized
in the IEEE 802.11b and 802.11g standards, has a wavelength of 0.125 meters, or 12.5
centimeters or about 4.92 inches. The wavelength of an IEEE 802.11a standard frequency
of 5 GHz would be about 2.36 inches. The proposed new Wi-Fi specification 802.11n
utilizes both 2.4 and 5 GHz frequencies in some configurations.
The Federal Communications Commission (FCC) regulates the frequency assignments
for use in the United States. This paper will focus on the 2.4 GHz frequency band from
2.4000 to 2.4835 GHz is a band that can utilized without an FCC license. It is a public,
unlicensed area of the electromagnetic spectrum that is utilized for 802.11b WLAN
operation. In other words, we will be using the unlicensed 2.4 GHz band for our wireless
network examples.
The following table shows the US frequency bands for the 802.11 2.4GHz assignments.
Note that only three channels do not overlap in frequency. That is why the preferred
channels for use in the US are: Channels 1, 6 and 11.
Channel No.
1
2
3
4
5
6
7
8
9
10
11
Frequency, MHz
2412
2417
2422
2427
2432
2437
2442
2447
2452
2457
2462
Table 1 Channel Frequencies
Remarks
No spectrum overlap
No spectrum overlap
No spectrum overlap
A very confusing aspect is the fact that a single channel Wi-Fi signal actually
electromagnetically spreads over five channels in the 2.4 GHz band resulting in only
three non-overlapped channels in the U.S. The 2.4000-2.4835 GHz band is divided into
13 channels each of width 22 MHz but spaced only 5 MHz apart, with channel 1 centered
at 2412 MHz. The Wi-Fi channel width is +/-11 MHz from the center frequency.
IEEE Spec.
802.11b
802.11g
802.11a
802.11n
802.11ac (new)
Frequency,
GHz
Typical Data
Data Rate,
Rate, Mbps
Mbps
2.4 GHz
max
2.4
3
11
2.4
23
54
5
23
54
2.4, 5
50
600
2.4, 5
200
1750
Table 2 Wi-Fi Specifications
Typical Range,
meters
35
35
30
70
100
Now is a good point to discuss the Table 2 Wi-Fi Specifications. Since this article was
first written in 2008, the 802.11n specification became standardized, and a new
specification, 802.11ac is being prepared for final release. The 802.11ac spec represents a
significant increase in performance and devices are now (2013) on the market. The
802.11ac Gigabit Wi-Fi spec supports larger channels at 40, 80, and 160 MHz channel
widths instead of 20 MHz widths. This yields increased peak performance and bandwidth
for wireless clients. Planning of channel assignment and widths on 11ac devices requires
a channel plan prior to a WLAN deployment. The full discussion of 802.11ac is beyond
the scope of this article, but a basic discussion is presented at the end of this article in an
Addendum because the future of Wi-Fi lies within this advanced technology.
Radio Frequency (RF) Power
A typical radio system will consist of a transmitter with a transmitting antenna
sending radio waves through some media to a receiving antenna connected to a
receiver. The radio system transmits information (data packets within a radio
frequency modulation scheme) to the transmitter. The RF signal containing the
data packets is transmitted through an antenna which converts the signal into
an electromagnetic wave. The transmission medium through which the electromagnetic
wave propagates is free space. The electromagnetic wave is intercepted by the
receiving antenna which converts it back to an RF signal that is the same as
the transmitted RF signal. The received RF signal is then demodulated by the
receiver to yield the original information.
Because of the wide range of power levels in RF signals, the measurement of
power is expressed in decibels (dB) rather than the Watt as the electrical unit
of power. For analyzing a radio system, the dBm convention is more convenient
than the Watts convention. The RF power level can be expressed in dBm (the subscript
“m” meaning the power is expressed in milliwatts) using the relation between dBm and
Watts as follows:
PdBm = 10 x Log Pmw
where PdBm = power in decibels
Pmw = power in milliwatts
Some examples are: 1 Watt = 1000 mW;
PdBm = 10 x Log 1000 = 30 dBm
500 mW;
PdBm = 10 x Log 500 = 27 dBm
100 mW;
PdBm = 10 x Log 100 = 20 dBm
50 mW;
PdBm = 10 x Log 50 = 17 dBm
30 mW;
PdBm = 10 x Log 30 = 14.8 dBm
15 mW;
PdBm = 10 x Log 15 = 11.8 dBm
Please note that whenever the power is halved that the dBm value decreases
by 3 dBm. This type of number is a logarithm, which is the exponent expressing
the power to which a fixed base number must be raised to produce a given number.
We are using a base of 10 for our logarithms.
Note: Refer to a Logarithm Table.
Signal Attenuation
An RF signal will fade (decrease in or lose power) as it propagates through
a medium or media. The media could consist of two layers of sheetrock plus
fiberglass insulation and wood framing plus air (a gas) through which an RF signal
propagates, going from one antenna to another. This attenuation (fading) is
expressed in decibels which can be converted to milliwatts. The units of power only need
be expressed in the same units (watts or milliwatts) in the relation
PdB = -10 x Log (Pout / Pin )
where Pin = the incident power level at the input of the attenuating media
Pout = the output power level at the output of the attenuating media
PdB = the attenuation loss expressed in decibels (dB)
A diagram for attenuation is shown below.
Fig. 2 Signal Attenuation
For example: If half the power is lost due to attenuation Pout= ½ Pin), the attenuation in
dB is -10 x Log (½) = -3 dB.
Path Loss
The Path Loss is the power loss of an RF signal traveling (propagating) through
space or obstructions. It is expressed in dB and depends upon:
The distance between the transmitting and receiving antennas.
The Line of Sight clearance distance between the receiving and transmitting
antennas.
The height of the antenna.
The loss in passing through walls or objects between antennas.
Fig. 3 Path Loss
Using the loss value for a sheetrock wall (listed in Table 3 presented later in this
lesson) the path loss would be:
Path Loss = Pl = 5 dB
We will use the path losses in the analysis of received RF signal strength in following
sections of this lesson. Different materials and combinations of materials have different
loss values which can be added directly using decibels to evaluate losses.
Free Space Loss
The Free Space Loss is an attenuation of the electromagnetic wave while propagating
through space. We will consider the loss to be the same in air as in the vacuum
of space. It is calculated using the following formula:
Free Space Loss = 32.4 + 20 x Log FMHz + 20 x Log RKm
where FMHz = the RF frequency expressed in MHz = 2,400 MHz for 802.11b systems
RKm = the distance in Kilometers between the transmitting and receiving antennas.
The formula at 2.4 GHz is:
Free Space Loss = 100 + 20 x Log RKm
In the following figure, The distance (D) can be expressed in kilometers or
miles, as we will discuss later in this section and consider the conversion
factors between kilometers and miles.
Fig. 4 Free Space Loss
The Free Space Loss is not usually a factor in the home and office wireless
network, but can be a factor in linking separate buildings, and definitely should
be included in a discussion of wireless link parameters. To calculate the loss
in units of miles and megahertz, the equation becomes:
Free Space Loss = 36.6 + 20Log10(Frequency in MHz) + 20Log10(Distance in Miles)
Antenna Characteristics
Isotropic Antenna
An Isotropic Antenna is an idealized, theoretical antenna having equal radiation
intensity in all directions. The Isotropic Antenna is used as a zero dB gain
reference in antenna gain (directivity) calculation.
Antenna Gain
The Antenna Gain is actually a measure of directivity and is defined as the
ratio of the radiation intensity (power) in a given direction to the radiation
intensity that would be obtained in the same direction from an Isotropic Antenna.
Antenna Gain is expressed in dBi (in other words, it is referenced to an isotropic
radiator). Some of the considerations in placing (mounting) antennas include
down-tilt angle (if any), beamwidth and aiming, and polarization. Most home
and office antenna mountings align the antenna with no down-tilt, especially
if it is an omni-directional antenna. Directional antennas may be mounted with
down or up-tilts depending upon the area of coverage desired in a high or multi-floor
level building. A diagram illustrating antenna tilt geometry follows.
Fig. 5 Antenna Tilt Angle Definition
The antenna in the above diagram has an axis that aligns with the electric
field vector of the RF signal, which is usually set in a vertical plane (aligns
with gravity vector at any point on the planet). In some point-to-point wireless
network designs, pairs of antennas may be rotated 90 degrees so that the electric
field variation is in the horizontal plane. The plane in which the electric
field variation (vector) aligns is known as the plane of polarization.
So the antenna polarization can be vertical or horizontal. If multiple wireless
networks are operating near one another, even on separate channels, interference
can sometimes be eliminated by changing the polarity of one set of network antennas.
Signal interference from many sources (including 2.4 GHz microwave ovens) can
sometimes be eliminated by a change in antenna polarization, as well as physical
location.
Another consideration in down-tilt antenna mounting is reflecting off surfaces
that the main lobe contacts. In a home or office with walls, ceilings, and floors
to bounce (reflect) the RF signal, aiming is important. Try to minimize the
reflections by keeping the angle of incidence as perpendicular (normal) to surfaces
as possible. Low angles of incidence cause more trouble than normal incidence
for RF signals.
These considerations are very important when designing outdoor RF signal links
where distances of miles between antennas exist. Even in modest home and office
link distances, these geometries should be considered. The following diagram
presents a tilted antenna configuration.
Fig. 6 Antenna Aiming
Radiation Pattern
A Radiation Pattern is the spatial energy distribution of an antenna. The spatial
distribution can be shown in rectangular or polar coordinates. The spatial distribution
of a practical antenna exhibits main lobes or lobe, and side lobes. The antenna
manufacturer will specify the radiation pattern for an antenna. The following
illustration shows the main lobe containing most of the RF signal power (energy),
and side lobes containing less RF signal power. The RF signal power radiates
outward from the antenna in all the lobes. This spreads the energy in the RF
signal ever wider which means that a receiving antenna farther away from the
transmitting antenna will receive a lower RF signal power level than a closer
located receiving antenna.
Fig. 7 Antenna Pattern
Side Lobes
Radiation lobes in directions other than that of a main lobe(s) are known as
Side Lobes. The antenna manufacturer will specify the radiation pattern
for an antenna. See the previous illustration. Side lobes can transmit enough
RF signal power to allow connection between other antennas.
Omnidirectional Antenna
An Omnidirectional Antenna radiates and receives equally in all directions
within a “pancake” shaped volume (spatial distribution). The antenna manufacturer
will specify the radiation pattern for an antenna. See the following illustration.</p>
Fig. 8 Omni Antenna
Directional Antenna
The radiation pattern of a Directional Antenna is predominantly in one direction.
The antenna still has side lobes, but the main lobe contains most of the radiated
and received power. The antenna manufacturer will specify the radiation pattern
for an antenna. Refer to the previous Antenna Radiation Power diagram as an
example of a directional antenna radiation pattern.
Antenna Beamwidth
The Antenna Beamwidth is defined as the RF Power included angle of a directional
antenna. The definition is the angle between two half-power (-3 dB) points on
either side of the main radiation lobe. The antenna manufacturer will specify the radiation
pattern for an antenna. Refer to the previous illustrations.
System Characteristics
Receiver Sensitivity (Ps)
The receiver sensitivity is the minimum RF signal power level required at the
input of the receiver for satisfactory system performance. This parameter is
usually specified by the radio equipment manufacturer. <b>Ps</b> in dBm is the
receiver sensitivity.
Effective Isotropic Radiated Power (EIRP)
The EIRP is the antenna transmitted power, which equals the RF signal output
power minus antenna cable loss plus the transmitting antenna gain. The equation
is:
EIRP = Pout – Ct + Gt
where Pout = transmitted output RF power to antenna in dBm
Ct = transmitter cable attenuation in dB
Gt = transmitting antenna gain in dBi
Effective Received RF Signal Power (Si)
The effective received signal power can be calculated using the following equation:
Si = EIRP – Pl + Gr –Cr = Pout – Ct + Gt – Pl + Gr – Cr
Where Pl = Path loss in dB
Gr = receiving antenna gain in dBi
Cr = receiver cable attenuation in dB
Example: Wireless System Link Analysis
Frequency = 2.4 GHz
Pout = 4 dBm (2.5 mW)
Tx and Rx cable loss for 10 meter cable type RG214 (0.6 dB/meter)
Ct = Cr = 6 dB
Tx and Rx antenna gain
Gt = Gr = 18 dBi
Distance between antennas RKm = 3 Km
Pl = 100 + 20 x Log(RKm) = 110 dB
Receiver sensitivity Ps = -84 dBm
Calculate:
EIRP = Pout – Ct + Gt = 16 dBm
Si = EIRP + Gr – Cr = 16 – (110) = -82 dBm
Analysis of the above result: The received signal power (Si) is above the sensitivity
threshold of the receiver (Ps), so the link should work. However, Si should
be at least 10 dB higher than Ps. In this case, the signal is only 2 dB higher
and we really should consider another loss factor, Signal Fading. A better system
solution would be to increase the transmit RF signal power to Pout = 10 dBm, which is a
power of 10 milliwatts.
Signal Fading
RF signal fading is caused by several factors including: Multipath Reception,
Line of Sight Interference, Fresnel Zone Interference, RF Interference, Weather
Conditions.
Multipath Reception – The transmitted signal arrives at the receiver
from different directions, with different path lengths, attenuation, and delays.
An RF reflective surface, like a cement surface or roof surfaces, can yield
multiple paths between antennas. The higher the antenna mount position is from
such surfaces, the lower the multiple path losses. The radio equipment in the
802.11 specifications utilizes modulation schemes and reception methods such
that multiple path problems are minimized.
Line of Sight Interference– A clear, straight line of sight between
the system antennas is absolutely required for a proper RF link for long distances
outdoors. A clear line of sight exists if an unobstructed view of one antenna
from the other antenna. A radio wave clear line of sight exists if a defined
area around the optical line of sight is also clear of obstacles. Remember that
the electric and magnetic fields are perpendicular to the direction of propagation
of the RF wave. In setting up wireless networks in buildings, propagation of
the RF signal through walls and other items is a fact of life. If you recall
the signal attenuation discussion earlier, we can evaluate the related losses.
A following table presents loss values for typical items through which we want
our networks to transmit and receive.
Fresnel Zone Interference – The Fresnel (FRA-nel) Zone is a circular
area perpendicular to and centered on the line of sight. In radio wave theory,
if 80% of the first Fresnel Zone is clear of obstacles, the wave propagation
loss is equivalent to that of free space.
Fig. 9 Fresnel Zone
The equation for calculating the first Fresnel Zone utilizes distances to a
point in the line of sight with a possible obstruction in the path is:
FZ = 72.1 x sq. root (D1 x D2) / (f x Rm )
where
f = frequency in GHz
Rm = distance between antennas in miles
D1 = first distance to obstruction in miles
D2 = second distance to obstruction in miles = Rm – D1
FZ = radius of Fresnel Zone in feet from direct line of sight
We will calculate a Fresnel Zone radius later in this discussion. In the home
and office network in a building, the Fresnel Zone calculation is usually unnecessary
because of all the wall/ceiling/floor pass- through considerations for any RF
signal path. But in outside RF signal paths (links), the Fresnel Zone calculations
can be very important from quarter mile distances and longer.
My experience with tall loblolly pines on a project is a good case in point.
A wireless link was designed and setup for two medical facilities (two-story
structures) in Wilmington, NC which were located 0.5 and 0.75 miles from an elevenstory hospital. There was no direct line of sight between the two medical facilities, but
there was from both buildings to the hospital roof. After securing proper approvals, an RF
signal link was setup from each building antenna to hospital roof-mounted antennas.
Even though there was a good visual path from one building to the hospital roof,
some very tall, very scrawny loblolly pines were infringing into the Fresnel
Zone radius that was calculated for the link. It was just a few branches with
the wide-spaced loblolly needles, but we had to top the trees to obtain a satisfactory
signal-to-noise ratio for dependable communication. It is amazing how much
microwave (2.4 GHz) energy those long needles absorbed, reflected, deflected, and/or
scattered.
In the earlier wireless link analysis example using the 3 Km distance between
antennas and assuming a mid-path constriction (D1 = D2), the Fresnel Zone is
calculated as follows using common conversion factors for US standard measurements.
Convert 3 Km to miles by dividing by a conversion factor of 1.6 kilometers per mile,
which yields using f = 2.4 GHz:
Rm = 3Km / 1.6Km/mile = 1.88 miles
D1 = D2 = 0.94 mile
FZ = 31.9 feet
The 80% Fresnel Zone radius for Free Space Loss equivalence would be obtained
by multiplying FZ by 0.8, which yields a radius of 25.5 feet. So the clear path
concentric cylinder around your systems line of sight for the distances and
frequency analyzed would be 51 feet in diameter at the middle of the RF link.
System Operating Margin (SOM)
SOM (System Operating Margin), also known as fade
margin, is the difference of the receiver signal level in dBm minus the receiver
sensitivity in dBm. It is a measure of the safety margin in a radio link. A
higher SOM means a more reliable over the air connection. We recommend a minimum
of 10 dB, but 20 dB or more is better for reliable, high bandwidth connections.
Fig. 10 Signal Operating Margin
SOM is the difference between the signal a radio is actually receiving vs. what it needs
for good data recovery (i.e. receiver sensitivity). By using the transmit and receive RF
signal power, the cable losses, the antenna gains, and the free space losses as considered
in this lesson, we can calculate the SOM. Thus we have a method for designing and
analyzing RF signal links used in wireless networking.
Rx Signal Level = Tx Power - Tx Cable Loss + Tx Antenna Gain – Free Space Loss
+ Rx Antenna Gain - Rx Cable Loss
SOM = Rx Signal Level - Rx Sensitivity
We can modify the SOM expression to consider attenuation losses due to transmission
through walls, etc., in an actual building wherein a home or office network would be
installed. It is simply adding more loss terms to the SOM equation. But first we will have
to consider the level of losses through various materials. The signal attenuation loss for
2.4 GHz transmission through the following structures can be included in the Rx Signal
Level equation for each pass-through in the straight line signal path (line of sight). The
dB loss values will be subtracted from the transmitted signal power to reflect the loss of
passing through the material structures.
Structure
Clear Glass Window
Brick Wall
Brick Wall next to a Metal Door
Cinder Block Wall
Sheetrock/Wood Frame Wall
Sheetrock/Metal Framed Wall
Metal Frame Clear Glass Wall
Metal Screened Clear Glass Window
Metal Door in Office Wall
Wired-Glass Window
Metal Door in Brick Wall
Table 3 Transmission Losses
Loss, dB
2
2
3
4
5
6
6
6
6
8
12
The loss for each structure passed through should be included in the calculations of Rx
Signal Level and SOM. The minimum SOM suggested is 15 dB, but a 25 dB margin
should be used in all designs as the real world losses are almost always higher than the
theoretical. The loss factors for walls or objects can be measured by using a wireless
signal source (router, access point, etc.) with output measured before and after the object.
Conclusion
Using the contents of this lesson any wireless network can be designed or analyzed.
All of the content of this article was presented to lead up to the ability to
understand and apply all the factors that comprise a wireless network's Effective
Received RF Signal Power (Si) and the System Operating Margin (SOM).
These two parameters are central to the design, analysis, and performance of any wireless
network.
That said, most Wi-Fi systems are not formally designed with Si or SOM analyses, but
rather Wi-Fi components are selected from available products in a price range of interest.
An on-site wireless survey using wireless devices including any intended antennas and a
laptop, tablet, or smartphone with an app to read signal power levels can be setup around
the site and signal tested. The system is then configured, installed and tested. Sometimes
it works satisfactorily and sometimes not. If not, the above radio physics topics can be
utilized to analyze the problem and then fix it. Good and bad signal level measurements
can be utilized to add access points, repeaters, higher gain antennas, etc., to obtain
reliable area coverage.
ADDENDUM:
IEEE Specification 802.11ac Discussion
The standard method to denote 5 GHz channels has been to always use the 20 MHz
center channel frequencies for both 20 MHz and 40 MHz wide channels. Starting with
802.11n, 40 MHz channels were referenced as the primary 20 MHz channel plus an
extension channel either above or below the primary channel. An example would be a 40
MHz channel consisting of channel 36 (primary) + 40 (extension above).
802.11ac allows larger channel widths. Instead of continuing to reference the 20 MHz
extension channel(s), the reference is the center channel frequency for the entire 20, 40,
80 or 160 MHz wide channel. The valid channel numbers for various channel widths are:
Channel Width, MHz
20
40
80
160
Valid Center Channel Numbers
36, 40, 44, 48, 52, 60, 100, 104, 108, 112, 116, 120, 124,128,
132, 136, 140, 144, 158, 161, 165, 169
38, 46,54, 62, 102, 110, 118, 126, 134, 142, 151, 159
42, 58, 106, 122, 138, 155
50, 114
Table 4 5 GHz Channel Frequencies
Channel numbers increment by one for every 5 MHz increase in frequency. This will
probably be easier to reference through the following graphic. In the graphic below,
identify the center of each 80 MHz and 160 MHz channel block, follow it up to the 20
MHz IEEE channel numbers, then split the difference between the two 20 MHz channel
numbers that it falls between. For example, the 80 MHz channel block is centered
between channels 40 and 44; splitting the difference gives us channel 42. The center
frequency is calculated in MHz as 5 MHz bandwidth multipled by the channel number
added to 5000 MHz.
Figure 11 Unlicensed 5 GHz Band Channels
We will restrict our consideration to channels that are available 802.11ac devices at this
time (indicated in the yellow highlighted boxes in Figure 1. They are presented in the
following table.
Channel No.
Frequency, MHz
36
5180
40
5200
44
5220
48
5230
149
5745
153
5765
157
5785
161
5805
165
5825
Table 5 5 GHz Channel Center Frequencies
The advent of using multiple antennas, multiple streams, and multiple radios in wireless
routers and access points ushered in by the MIMO (Multi-In Multi-Out) architectures that
are in the IEEE 802.11n and 802.11ac specifications have significantly advanced the
technology. All the radio theory in this article still applies to all the latest radio
specifications. The 802.11ac specification presents a significant increase in performance
capability by somewhat complicated means, which yield greatly improved throughput
rates over earlier specifications. Wireless devices should be configured with frequency
selections that minimize possible interference. And mixing the types of devices will yield
throughput speeds of the lowest level of device that is connected. For example, a 802.11g
device on an 802.11ac connection slows everything down to 11g bandwidth operation.
The 802.11ac specification is designed to offer speeds up to 1.75 Gbps, double the
802.11n standard. It allows for up to 8 multiple input, multiple output (MIMO) streams
and multi-user MIMO. The 802.11n stopped at 4 streams. And 802.11ac utilizes a
technique called “beamforming”, which directs a concentrated wireless signal to a
specific area if the wireless router or base station supports it along with a device capable
of talking to the other device. It is a good idea not to mix 11ac devices from different
manufacturers before the final 11ac spec is released. Hardware running 802.11ac is not
expected to be widespread until 2015. It usually takes 4-5 years to reach final IEEE
specification release and then become a full standard. In the 802.11ac specification
advances were: the air interface concepts of advanced 802.11n technology were expanded
in wider RF bandwidth (up to 160 MHz); more MIMO spatial streams (up to 8); multiser MIMO; and high density modulation (up to 256-QAM). Keep in mind that 11ac
transmitters and receivers require the same number of antennas on each device to be able
to utilize all the advanced MIMO technology.
All the concepts, analysis, and equations presented in this article are applicable to the
analysis and design of 802.11ac systems. It involves the same radio physics as before.