An antenna is a device of converting the guided waves present in a
waveguide into radiating waves traveling in free space or vice versa.
The art of antenna design is to ensure this process takes place as
efficiently as possible, with the antenna radiating as much power
from the transmitter into useful directions, particularly the direction
of the intended receiver, as can practically be achieved.
Applications: Communications, Broadcast, Radar, Radiometry,
Necessary Conditions for Radiation
a group of charges in uniform motion (or stationary
charges) do not produce radiation.
Radiation occurs when the velocity of the charges is
changing in time.
When the charges are reaching at the end of the wire
and reversing direction, producing radiation.
When the speed of the charges remains constant, but
their direction is changing, thereby creating radiation.
The charges are oscillating in periodic motion, causing a
continuous stream of radiation.
What is Antenna?
Radiate and receive radio wave ,convert high
frequency current to electromagnetic wave when
transmitting, and convert electromagnetic wave
to high frequency current when receiving
blah bl ah
Its main purpose is to convert the energy of a guided wave into the
energy of a free-space wave (or vice versa) as efficiently as possible,
while in the same time the radiated power has a certain desired pattern
of distribution in space.
Antennas and Propagation
– Challenges and Opportunities?
Size, cost and bandwidth in wireless terminals
Degradation due to proximity
– Human body effects and RFID
High gain antennas for space application
– Galileo GPS system, Space project in China
Demands imposed by new technologies
– UWB radios (3.1 -10.6 GHz)
Radio channel characterisation
– Smart antenna/Beam forming, MIMO system
UWB (Ultra Wide Band)
Wireless Personal Area Network
Difficult to intercept
Short range communication (up to
High data rate >100Mbps
ITS Radar System
Adaptive Cruise Control(ＡＣＣ) Radar
Control a car speed using detected distance, relative speed and
direction of obstacle
Angle resolution: less than 3 degrees
Scan angle: +/-8-12 degrees
Short Distance Radar
Detected an obstacles around car
Measurement resolution: 15cm
ITS Infrastructure Radar
Observation of obstacles on the road or traffic status
RAdio Detection And Ranging
Transmit radio wave and receive reflected wave
Time delay: distance
Doppler shift: relative speed
Angle of arrival: direction
Type of Radar
Detection of distance and relative speed
Pulse Radar, FM-CW Radar
Detection of direction
Mechanical scan, Beam switch
Mono pulse, Beam forming
Principle of Radar
• Space division to reduce interference
at both each terminal and base station
• Optimal antenna directivity is best
calculated on real-time basis.
AAA (Adaptive Array Antennas)
MIMO (Multi Input Multi Output)
• Space Division Multiplexing in the same
space using the same frequency band
• Capacity increase of number-of-antenna-
• Adaptive signal processing required to
establish each independent channel Four times channel capacity
using the same frequency band
Multi-Antenna Systems greatly improve Spectral Efficiency.
Small, lightweight, low power
High frequency operation
Converting and transferring information
High data rates
Robust to noise and interference
Supports many users
Connectivity and high speed
Energy and delay constraints
Software-Defined Radio (SDR)
Is this the solution to the device challenges?
Wideband antennas and A/Ds span BW of desired signals
DSP programmed to process desired signal
Today, this is not cost, size, or power efficient
Compressed sensing may be a solution for sparse signals
In antenna analysis we need to find the electric
and magnetic fields generated by some current
Magnetic vector potential
Electric vector potential and duality theorem
Auxiliary Potential Functions
Path 1: Direct solution of Maxwell’s equations tend to be difficult and
Path 2: Introduction of auxiliary vector potential functions allow
simpler and compact solutions.
An antenna radiation pattern or antenna pattern is a
mathematical function or a graphical representation of the radiation
properties of the antenna as a function of space coordinates.
Inmost cases, the radiation pattern is determined in the far field
region and is represented as a function of the directional
Radiation properties include power flux density, radiation intensity,
field strength, directivity, phase or polarization.
Amplitude field pattern is a trace of the received electric
(magnetic) field at a constant radius.
Amplitude power pattern is a graph of the spatial variation of the
power density along a constant radius.
Often the field and power patterns are normalized with respect to
their maximum value, yielding normalized field and power
Coordinate system for antenna analysis
For an antenna, the
field pattern( in linear scale) typically represents a plot of the
magnitude of the electric or magnetic field as a function of the
power pattern( in linear scale) typically represents a plot of the
square of the magnitude of the electric or magnetic field as a
function of the angular space.
power pattern( in dB) represents the magnitude of the electric or
magnetic field, in decibels, as a function of the angular space.
Radiation Pattern Lobes
Various parts of a radiation pattern are referred to as lobes, which may
be sub-classified into major or main, minor, side, and back lobes.
A radiation lobe is a “portion of the radiation pattern bounded by
regions of relatively weak radiation intensity.”
Radiation Pattern Lobes (2)
A major lobe (also called main beam) is defined as “the radiation
lobe containing the direction of maximum radiation.” the major lobe
is pointing in the θ = 0 direction.
A minor lobe is any lobe except a major lobe.
A side lobe is “a radiation lobe in any direction other than the
A back lobe is “a radiation lobe whose axis makes an angle of
approximately 180o with respect to the beam of an antenna.”
Minor lobes usually represent radiation in undesired directions, and
they should be minimized.
The level of minor lobes is usually expressed as a ratio of the power
density in the lobe in question to that of the major lobe. This ratio is
often termed the side lobe ratio or side lobe level.
Side lobe levels of −20 dB or smaller are usually not desirable in
most applications. Attainment of a side lobe level smaller than
−30 dB usually requires very careful design and construction.
In most radar systems, low side lobe ratios are very important to
minimize false target indications through the side lobes.
Isotropic, Directional, and Omnidirectional Patterns
An isotropic pattern (an ideal point radiator) is a hypothetical
lossless antenna having equal radiation in all directions.
A directional antenna is one “having the property of radiating or
receiving electromagnetic waves more effectively in some directions
than in others.
Omnidirectional antenna (a dipole) is the one having an essentially
nondirectional pattern in a given plane and a directional pattern in
any orthogonal plane.
For a linearly polarized antenna, performance is often described in
terms of its principal E- and H-plane patterns.
The E-plane is the plane containing the electric field vector and the
direction of maximum radiation.
The H-plane is the plane containing the magnetic-field vector and
the direction of maximum radiation.
the x-z plane (elevation plane; φ = 0) is the principal E-plane and the
x-y plane (azimuthal plane; θ = π/2) is the principal H-plane.
Radiation Power Density
The quantity used to describe the power associated with an
electromagnetic wave is the instantaneous Poynting vector defined as
The total power crossing a closed surface can be obtained by
integrating the normal component of the Poynting vector over the
entire surface. In equation form
Average Radiation Power Density
we define the complex fields E and H which are related to their
instantaneous counterparts and by
So the poynting vector or power density can be written as
The time average Poynting vector (average power density) can be
the average power radiated by an antenna (radiated power) can be
The radial component of the radiated power density of an antenna is
where Ao is the peak value of the power density, θ is the usual
spherical coordinate, and is the radial unit vector. Determine the
total radiated power.
For a closed surface, a sphere of radius r is chosen. To find the total
radiated power, the radial component of the power density is integrated
over its surface. Thus
Radiation Power Density for an isotropic source
An isotropic radiator is an ideal source that radiates equally in all
directions. Although it does not exist in practice, it provides a
convenient isotropic reference with which to compare other
Because of its symmetric radiation, its Poynting vector will not be a
function of the spherical coordinate angles θ and φ.
In addition , it will have only a radial component. Thus the total
power radiated by it is given by
and the power density by
which is uniformly distributed over the surface of a sphere of radius r
Radiation intensity in a given direction is defined as “the power
radiated from an antenna per unit solid angle.”
In mathematical form it is expressed as
The radiation intensity is also related to the far-zone electric field of
The total power is obtained by integrating the radiation intensity,
over the entire solid angle of 4π. Thus
For the problem of Example 1, find the total radiated power.
which is the same as that obtained in Example 1.
Isotropic source radiation intensity
For an isotropic source U will be independent of the angles θ and φ,
as was the case for Wrad. Thus can be written as
or the radiation intensity of an isotropic source as
Half-power beamwidth (HPBW) is the angle between two vectors,
originating at the pattern’s origin and passing through these points of
the major lobe where the radiation intensity is half its maximum, – 3dB
First-null beamwidth (FNBW) is the angle between two vectors,
originating at the pattern’s origin and tangent to the main beam at its
base. It is very often approximately true that FNBW/2≈ HPBW.
Directivity of an antenna in a given direction is the ratio of the
radiation intensity in this direction and the radiation intensity
averaged over all directions. The radiation intensity averaged over
all directions is equal to the total power radiated by the antenna
divided by . If a direction is not specified, then the direction of
maximum radiation is implied.
If the direction is not specified, it implies the direction of maximum
radiation intensity (maximum directivity) expressed as
1) Isotropic source: D = 1 or 0 dB
2) Hertzian dipole: D = 1.5 or 1.76dB
3) λ/2 dipole: D = 1.64 or 2.14 dB
The total antenna efficiency eo is used to take into account losses at the
input terminals and within the structure of the antenna.
Such losses may be due to:
reflections because of the mismatch between the transmission line and the
losses (conduction and dielectric)
In general, the overall efficiency can be written as
where antenna radiation efficiency, which is used to relate the
gain and directivity
voltage reflection coefficient at the input terminals of the antenna
VSWR = voltage standing wave ratio =
Gain G of an antenna is the ratio of the radiation intensity U in a given
direction and the radiation intensity that would be obtained if the power
accepted by the antenna were radiated isotropically.
The gain is dimensionless quantity, which is very similar to the
directivity D. When the antenna has no losses, then
the total radiated power (Prad) is related to the total input power (Pin ) by
The radiated power is related to the input power through a coefficient
called the radiation efficiency.
which is related to the directivity
The maximum of the gain is related to the maximum directivity
Absolute Gain Gabs
we introduce an absolute gain Gabs that takes into account the
reflection/mismatch losses (due to the connection of the antenna
element to the transmission line), and it can be written as
where eo is the overall efficiency. Similarly, the maximum absolute gain
G0abs is related to the maximum directivity D0 by
If the antenna is matched to the transmission line, that is,
the antenna input impedance Zin is equal to the
characteristic impedance Zc of the line then the two
gains are equal (Gabs = G).
The Frequency Bandwidth of an antenna is the range of frequencies
within which the performance of the antenna characteristic conforms to a
Antenna characteristics, which should conform to certain requirements,
might be: input impedance, pattern, beamwidth, polarization, side lobe level,
gain, beam direction, radiation efficiency.
Often, separate bandwidths are introduced: impedance bandwidth, pattern
The FBW for broadband antennas, the bandwidth is expressed as the ratio
of the upper-to-lower frequencies of acceptable operation.
Broadband antennas with FBW (like 40:1 or greater) have been designed in
recent years. These are known as frequency independent antennas.
For narrowband antennas, the FBW is expressed as a percentage of the
frequency difference over the center frequency of the bandwidth.
Polarization of an antenna in a given direction is the polarization of the
wave transmitted (radiated) by the antenna.
Vertical polarization Vertical polarization Dual polarization
Omni antenna directional antenna directional antenna
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«»¯¶¨ Ï òÌ ì Ï ß Ë
«»¯¶¨ Ï òÌ ì Ï ß
Polarization may be classified as linear, circular, or elliptical.
An antenna is said to be vertically polarized (linear) when its electric
field is perpendicular to the Earth's surface.
An example of a vertical antenna is a broadcast tower for AM radio
and FM radio.
In a circular polarized antenna, the plane of polarization rotates in a
circle making one complete revolution during one period of the
wave. If the rotation is clockwise looking in the direction of
propagation, the sense is called right-hand-circular (RHC). If the
rotation is counterclockwise, the sense is called left-hand-circular
Circular polarization is most often use on satellite
Furthermore, due to the position of the Earth with respect to the
satellite, geometric differences may vary especially if the satellite
appears to move with respect to the fixed Earth bound
station. Circular polarization will keep the signal constant
regardless of these anomalies.
Antenna equivalent circuit
Equivalent circuit of the transmitting antenna
The radiation resistance is defined as the equivalent resistance
which would dissipate a power equal to that radiated.
The input impedance of the antenna is
ZA = antenna impedance at terminals a –b (ohms)
RA = antenna resistance at terminals a –b (ohms)
XA = antenna reactance at terminals a –b (ohms)
General, the antenna resistance has two terms:
Rr = radiation resistance of the antenna
RL = loss resistance of the antenna
For the infinitesimal dipole
For a free space medium
The above equation becomes
Radiation resistance of dipoles
1) Hertzian dipole:
2) Short dipole:
Eg: l=0.1λ, Rr=2Ω
3) λ/2 dipole:
The imaginary part of the input impedance is approximately j42.5 Ω .
To acquire maximum power transfer, this reactance has to be
removed by matching (that is shortening) the dipole.
Radiation efficiency e takes into account the conductor-dielectric
(heat) losses of the antenna
Antenna Equivalent Areas
These are used to describe the power capturing characteristics of the
antenna when a wave impinges on it.
The effective area (aperture) in a given direction is the ratio of the
available power at the terminals of a receiving antenna to the power flux
density of a plane wave incident on the antenna from that direction, the
wave being polarization-matched to the antenna.
Ae = effective area (effective aperture) (m2)
PT = power delivered to the load (W)
Wi = power density of incident wave (W/m2)
The effective aperture is the area which when multiplied by the incident
power density gives the power delivered to the load.
Antenna Equivalent Areas (2)
Under conditions of maximum power transfer (conjugate matching), Rr + RL =
RT and XA = −XT , the effective area reduces to the maximum effective aperture
In fact, under conjugate matching only half of the captured power is delivered
to the load; the other half is scattered and dissipated as heat. So we need to
define the scattering, loss and capture equivalent areas.
The scattering area, As is given by
The loss area, AL is given by
The capture area, AC is given by
In general, the total capture area =Effective area + Scattering area + Loss Area
the aperture efficiency, εap is given as
Antennas for Mobile Communication Systems
The dipole and monopole are two of the most widely used antennas
for wireless mobile communication systems
An array of dipole elements is extensively used as an antenna at the
base station of a land mobile system
The monopole, because of its broadband characteristics and simple
construction, most common antenna element used for portable
equipment, such as cellular telephones, cordless telephones,
automobiles, trains, etc.
The radiation efficiency and gain characteristics of both of these
elements are strongly influenced by their electrical length which is
related to the frequency of operation.
In many applications it is necessary to design antennas with very directive
characteristics (very high gains) to meet the demands of long distance
Enlarging the dimensions of single elements often leads to more directive
To enlarge the dimensions of the antenna, without necessarily increasing the
size of the individual elements, is to form an assembly of radiating elements in
an electrical and geometrical configuration.
This new antenna, formed by multi-elements, is referred to as an array.
In most cases, the elements of an array are identical. This is not necessary,
but it is often convenient, simpler, and more practical.
The individual elements of an array may be of any form (wires, apertures, etc.).
Antenna arrays application
There are a plethora of antenna arrays used for personal, commercial,
and military applications utilizing different elements including dipoles,
loops, apertures, microstrips, horns, reflectors, and so on.
Arrays of dipoles, is an array that is widely used as a base-station
antenna for mobile communication.
The one in a classic array of dipoles, referred to as the Yagi-Uda
array, and it is primarily used for TV and amateur radio applications.
The array of dipoles, which is referred to as the log-periodic antenna,
which is primarily used for TV reception and has wider bandwidth than
the Yagi-Uda array but slightly smaller directivity.
Construct large (high gain) apertures from a number of
small (low gain)
Number of identical (usually) elements connected to one
source or receiver
Antenna arrays configuration
a) Fixed excitation giving fixed radiation pattern
b) Variable amplitude and/or phase giving controllable
Pattern multiplication rule
Using the normalized field pattern of a single element and the
The normalized field pattern of the array can be found as their product
The concept illustrated above is the so called pattern multiplication rule
valid for array of identical elements, where the excitation magnitudes,
the phase shift between elements and displacement between them are
not necessarily the same. The total pattern, therefore, can be
controlled via the single-element pattern, or via the AF.
The AF, in general, depends on:
Number of elements
Relative excitation magnitudes
Two element array example
Example: An array of two horizontal infinitesimal dipoles located at a
distance d = λ/4 from each other. Find the nulls of the total field for
phase differences: 0 and pi/2 if the excitation amplitudes are the same.
An array of identical elements with identical magnitudes and with a
progressive phase is called a uniform array. The progressive phase is β.
Phase terms of partial far-fields:
Array factor (2)
Example 2: N=10, β =0, d= λ/2
1) As N increases, main beam narrows (c.f. aperture);
2) Lager N, more sidelobes. (N-1) lobes in 2π period;
3) Minor lobe width = 2π/N, Main beam width 4π/N;
4) Sidelobe level decreases with N.
Ordinary end-fire array(2)
If the element separation is bigger than d=n/2, then in
addition to the end-fire maxima there also exist
maxima in other directions – grating lobes.
In order to avoid grating lobes, the maximum spacing
between the element should be less than λ/2.
Binomial array (2)
Making use of Pascal’s triangle
The relative excitation amplitudes at each element of an (N+1)-element
array can be determined. Such an array with a binomial distribution of
the excitation amplitudes is called a binomial array. The current
(excitation) distribution as given by the binomial expansion gives the
relative values of the amplitudes.
It is immediately seen that there is too wide variation of the amplitude,
which is the major disadvantage of the BAs. The overall efficiency of
such antenna would be very low. Besides, the BA has relatively wide
beam. Its HPBW is the largest as compared to this of the uniform BSA.
Helix antenna (2)
Broadside (normal) mode (l<<λ):
Used in mobile handsets
-Compact Axial (end-fire) mode (D, S ~ λ):
-Wide frequency bandwidth Used as high gain antennas
Yagi-Uda Array of Linear Elements
Another very practical radiator in the HF (3–30 MHz),
VHF (30–300 MHz), and UHF (300–3,000 MHz)
ranges is the Yagi-Uda antenna.
This antenna consists of a number of linear dipole
elements, one of which is energized directly by a feed
transmission line while the others act as parasitic
radiators whose currents are induced by mutual
A common feed element for a Yagi-Uda antenna is a
folded dipole. This radiator is exclusively designed to
operate as an end-fire array, and it is accomplished
by having the parasitic elements in the forward beam
act as directors while those in the rear act as
N – element Yagi-Uda array
-- cheap and easy
-- relative high gain
TV receiver antennas
Yagi-Uda array (3)
If the parasite is shorter than
the driver, but place on the
-- the similar effect of the
main beam enhancement.
The parasite – director
Three element Yagi-Uda
The combination of driver,
reflector and director.
Achieving high gain: 9dB
Even higher gain can be
obtained with more
Aperture antennas are most common used at
They may take the form of a waveguide or a horn
whose aperture may be square, rectangular, circular,
elliptical, or any other configuration.
Aperture antennas are very practical for space
applications, because they can be flush mounted on
the surface of the spacecraft or aircraft.
Their opening can be covered with a dielectric material
to protect them from environmental conditions.
This type of mounting does not disturb the
aerodynamic profile of the craft, which in high-speed
applications is critical.
An uniform rectangular aperture
Uniform fields across the aperture:
The far-field is obtained as:
Principle patterns for:
An uniform rectangular aperture (2)
Directivity of an uniform fields aperture
It can be derived that:
The power radiated is obtained as:
Where μ is the intrinsic impedance
Radiation pattern of a horn
The radiation pattern is
also measured on E-plane
and H-plane, as
There are back lobes
associated with a
The radiation is highly
- D0 = 15dB
The effective antenna aperture is the ratio of the available power at
the terminals of the antenna to the power flux density of a plane wave
incident upon the antenna, which is polarization matched to the
antenna. If there is no specific direction chosen, the direction of
maximum radiation intensity is implied.
It can be proved that:
Often microstrip antennas are also referred to as patch antennas.
The radiating elements and the feed lines are usually photoetched
on the dielectric substrate. The radiating patch may be square,
rectangular, thin strip (dipole), circular, elliptical, triangular, or any
Square, rectangular, dipole (strip), and circular are the most
common because of ease of analysis and fabrication, and their
attractive radiation characteristics, especially low cross-polarization
Microstrip dipoles are attractive because they inherently possess a
large bandwidth and occupy less space, which makes them
attractive for arrays.
Linear and circular polarizations can be achieved with either single
elements or arrays of microstrip antennas.
Arrays of microstrip elements, with single or multiple feeds, may also
be used to introduce scanning capabilities and achieve greater
General review of mobile terminal antennas
Most of the new mobile terminals (e.g. GSM phones)
have built-in antennas which are not extruded from the
exterior of the terminal.
– Inverted F-antenna (IFA)
– Planar Inverted F-Antenna (PIFA)
Summary patch antennas
- Thin low profile; light weight; simple to make; conformal; low cost;
robust; can be integrated with circuits.
- Narrow bandwidth (can be virtue or improved)
- Low efficiency and low power
- Spurious feed radiation (can be improved)
- Poor polarisation purity (can be virtue)
- Aircraft, spacecraft, satellite and missile
- Radars – vehicle , phased array, etc
- Wireless communications: Smart/active antennas, PIFA for mobile
General review of mobile terminal antennas
1984 – Earliest mobile communications system
- Typical cellular phone volume 600cc, weight
- Antenna used was half-wavelength monopole
General review of mobile terminal antennas 2
1999 – the volume of the cellular
handset has been reduced to less
than 60cc and weight less than 60g.
Half-wavelength monopole antenna
is too bulky for the small size
Normal mode helical antenna
(NMHA) has been widely used in
GSM mobile handset. The total
length of this antenna is only 4-
15%of a wavelength
General review of mobile terminal antennas 3
Another popular small
antenna extruded from the
handset is meander printed
This antenna is printed on a
small piece of flexible board
which is rolled on a core.
can be accomplished by
connecting two or more ¼
wavelength meanders in
parallel with each tuned to
its own frequency.
General review of mobile terminal
Some examples of PIFA in GSM phones:
Nokia 3210 Nokia 6210 Nokia 8210
General review of mobile terminal
Diversity antennas have been employed in the PDC
(Personal Digital Cellular) system in Japan.
Reflector antennas, in one form or another, have been in use since
the discovery of electromagnetic wave propagation in 1888 by Hertz.
However the fine art of analyzing and designing reflectors of many
various geometrical shapes did not forge ahead until the days of
World War II when numerous radar applications evolved.
Demands of reflectors for use in radio astronomy, microwave
communication, and satellite tracking resulted in spectacular
progress in the development of sophisticated analytical and
experimental techniques in shaping the reflector surfaces and
optimizing illumination over their apertures so as to maximize the
The use of reflector antennas for deep-space communication, such
as in the space program and especially their deployment on the
surface of the moon, resulted in establishing the reflector antenna
almost as a household word during the 1960s.
Although reflector antennas take many geometrical configurations,
some of the most popular shapes are the plane, corner, and curved
reflectors (especially the paraboloid).
Evolution of antennas to achieve high directivity & gain
Gain: 0 – 1.57 dB ~ 10s dB >30dB
By putting a reflector behind a non-directional source. The
larger the reflector, the greater the directivity. Eg. 304/100m
diameter radio telescopes.
Prime focus parabolic reflectors
The geometry of the parabolic reflector has two valuable features:
1) All rays leaving the focal point F, are collimated along the
reflector’s axis after reflection.
2)All path lengths from the focal point to the reflector and on to
the aperture plane are the same and equal to 2F .
Parabolic reflectors (2)
1) All rays leaving the focal point O are collimated along the
reflector’s axis after reflection.
2) All path lengths from the focal point to the reflector and on to
the aperture plane are the same and equal to 2F .
The maximum achievable gain for an aperture antenna is
This gain is possible only if the following is true: uniform
amplitude and phase distribution, no spillover, no ohmic
losses. In practice, these conditions are not achievable, and
the effective antenna aperture is less than its physical
where εap≤1 is the aperture efficiency,
Ap is the aperture area of the antenna.
Example: 1) Effelsberg dish (100m), G = 84dB at λ=1cm (West Germany)
2)Arecibo dish (304m), G = 77dB at λ= 7cm (Puerto Rico)
Dual reflector antennas
The dual-reflector antenna consists of two reflectors and a feed
antenna. The feed is conveniently located at the apex of the main
reflector. This makes the system mechanically robust, the transmission
lines are shorter and easier to construct (especially in the case of
The virtual focal point F is the
point from which transmitted
rays appear to emanate with a
spherical wave front after
reflection from the
Single Input Single Output (SISO)
This is the traditional method of accessing
the radio channel.
Each transmitter has a single antenna, as
does each receiver.
This method is used as the baseline against
which the performance of all multiple antenna
techniques is compared.
Multiple Input Single Output (MISO) -
MISO is also known as transmit diversity.
Each transmit antenna transmits essentially the same stream of
The multipath environment impacts upon the transmitted signal
resulting in the arrival of time displaced replicas of the same
signal at the receiver.
Used to improve the signal to noise ratio at the receiver and thus
the reliability of data transmission.
It is usual to apply antenna-specific coding to the signals prior to
transmission to increase the diversity effect.
Transmit diversity does not increase data rates as such, but
rather supports the same data rates using less power or, allows a
higher order modulation scheme to be used if sufficient
improvement in SNR is experienced at the receiver.
The performance of transmit diversity can be enhanced if the
receiver is able to feedback parameters to be used by the
transmitter to adjust the balance of phase and power used for
Single Input Multiple Output (SIMO)
SIMO uses one transmitter and two or more
receivers and is usually referred to as receive
It is particularly well suited for low SNR
There is no improvement in the data rate as only
one data stream is transmitted, but coverage at
the cell edge is improved due to the lowering of
the usable SNR.
Multiple Input Multiple Output (MIMO)
MIMO requires two or more transmitters and two
or more receivers.
Multiple data streams are transmitted
simultaneously in the same frequency and time,
taking full advantage of the multiple paths in the
For a system to be described as MIMO, it must
have at least as many receivers as there are
Multiple Input Multiple Output (MIMO)
Adding receive diversity (SIMO) to Tx diversity (MISO) does
not create MIMO, even though there are now two Tx and two
Rx antennas involved.
If N data streams are transmitted from fewer than N antennas,
the data cannot be fully descrambled by any number of
receivers since overlapping streams results in interference.
However, by spatially separating n streams across at least N
antennas, N receivers will be able to fully reconstruct the
original data streams provided the crosstalk and noise in the
radio channel are low enough.
One other crucial factor for MIMO operation is that the
transmissions from each antenna must be uniquely
identifiable so that each receiver can determine what
combination of transmissions has been received.
This identification is usually done with pilot or reference
The spatial diversity of the radio channel means that MIMO has
the potential to increase the data rate.
The figure below shows a simplified illustration of spatial
In this example, each transmit antenna transmits a different data
Single User MIMO (SU-MIMO)
This is the most common form of MIMO and can be applied in
the uplink or downlink.
The primary purpose of SU-MIMO is to increase the data rate to
a single user. There is also a corresponding increase in the
capacity of the cell.
The figure shows the downlink form of 2x2 SU-MIMO in which
two data streams are allocated to a single UE.
The two data streams (red and blue) are precoded in such a
way that each stream is represented at a different power and
phase on each antenna.
The two mixed data streams are then transmitted from each
antenna. The transmitted signals are further mixed by the
The purpose of the precoding is to optimise the transmissions
to the characteristics of the radio channel so that when the
signals are received, they can be more easily separated back
into the original data streams.
Multiple User MIMO (MU-MIMO)
MU-MIMO is used only in the Downlink .
MU-MIMO does not increase an individual user’s data rate but
does offer cell capacity gains.
In the figure, the two data streams originate from different UE.
The two transmitters are much further apart than in the single
user case, and the lack of physical connection means there is no
opportunity to optimise the coding by mixing the two data
However, the extra spatial separation does increase the chance
of the eNB picking up pairs of UE which have uncorrelated
paths. This maximizes the potential capacity gain.
MU-MIMo has an additional important advantage: the UE does
not require the expense and power drain of two transmitters, yet
the cell still benefits from increased capacity.
Co-operative MIMO (Co-MIMO)
The essential element of Co-MIMO is that two separate entities
are involved at the transmission end.
Two eNB collaborating by sharing data streams to precode the
spatially separate antennas for optimal communication with at
least one UE.
When this technique is applied in the downlink it is sometimes
called network MIMO.
The most advantageous use of downlink Co-MIMO occurs when
the UE is at the cell edge. Here the SNR will be at its worst but
the radio paths will be uncorrelated, which offers significant
potential for increased performance.
Co-MIMO is also possible in the uplink but is fundamentally more
difficult to implement as no physical connection exists between
the UE to share the data streams.
uplink Co-MIMO is also known as virtual MIMO.
Co-MIMO is not currently part of the release 8 LTE specifications
but is being studied as a possible enhancement to LTE in release
9 or release 10 to meet the goals of the ITU’s IMT 4G initiative.
Beamforming uses the same signal processing and antenna
techniques as MIMO but rather than exploit de-correlation in the
Beamforming aims to exploit correlation so that the radiation pattern
from the transmitter is directed towards the receiver.
This is done by applying small time delays to a calibrated phase
array of antennas. The effectiveness of beamforming varies with
the number of antennas.
With just two antennas little gain is seen, but with four antennas the
gains are more useful.
Turning a MIMO system into a beamforming system is simply a
matter of changing the pre-coding matrices. In practical systems,
however, antenna design has to be taken into account and things
are not so simple.
It is possible to design antennas to be correlated or uncorrelated;
for example, by changing the polarization. However, switching
between correlated and uncorrelated patterns can be problematic if
the physical design of the antennas has been optimised for one or