Transcript SMART ANTENNAS

For 3-G Systems
Tara Larzelere
EE 497A
Semester
Project
WHAT IS A SMART
ANTENNA?
 An array of antenna elements connected to a digital
signal processor
 Enhances the capacity of a wireless link through a
combination of diversity gain, array gain, and
interference suppression.
 Therefore more users can be accommodated in a
single cell with a higher data rate per user.
 Two Main ways to form antenna arrays
1. diversity gain
2. beamforming
Smart Antenna Analogy
 Ears represent beamforming
antennas
 Many “ears” collect signals and add them to
magnify sound producing a clear signal then
together to create a magnified signal.
 Brain represents signal processor
 Tunes out other conversations so that your
brain can only hear the me the speaker
Why do we need smart
antennas?
1) Multi-path Fading -unpredictable set of
reflections of a signal which are out of phase
with each other
2) Delay spread- result of multi-path fading,
difference in propagation among received
paths causing inter-symbol interference
3) Co-channel interference – a result of
frequency reuse. The desired signal contain
not only the desired forward channel from the
current cell, but also signals originating in
more distant cells
Beamforming vs diversity
 For diversity, we rely essentially on the
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statistical independence of the signals at
the different antenna elements.
Implemented in cases where fading is
common
Beamforming the antenna elements are
close together presenting coherence
between the antenna signals
The closeness of the antenna elements
allows us to form an antenna pattern with a
single main beam that enhances the
desired signal and suppresses the
interference.
Mathematically speaking, the difference
between diversity and beamforming is not
dramatic
Three Basic Ways
to Provide
diversity gain
1. Spatial
2. Polarization
3. Angle Diversity
 In radio communications, the
ratio of the signal field
strength obtained by two or
more different elements
combining to the signal
strength obtained by a single
signal path
 For Diversity gain with M
antenna elements an antenna
gain of M times can be
achieved.
SPACIAL DIVERSITY ARRAY
 equally separated antennas for low
fading correlation
 Simplest implementation: switches
to whichever antenna is currently
receiving the strongest signal.
 More complex systems: signal
combining based on weighting
according to the signal-to-noise ratio
of each of the received signals or
according to equal weighting where
signals are co-phased and summed.
 Not as commonly used because a
single antenna requires space and
cable runs, and increases
installation and maintenance costs
significantly.
Polarization
Diversity
.
 For polarization diversity
horizontal and vertical
polarization is used
 Can only double diversity
gain
 Polarization diversity is
known to be effective in
urban areas
Angle Diversity
 which implements
different beam patterns
on the different antennas
 This combination results
in a higher overall gain
pattern
 Allows the same 120
sectored base station
support more users by
changing the degree of
separation
Beamforming Networks
 Most conventional smart antenna
systems use a concept known as
beamforming, where the signal
energy is focused in a particular
direction toward the receiver
increasing SNR
 First, we estimate the directions of
arrival of all multi-path components.
 Then determine whether the signal
from a certain direction comes from
a desired user or from an interferer
(with the help of user ID)
 Algorithms compute the weights for SNR optimization and use this
pattern for reception of the data.
 The associated antenna pattern can be used to exhibiting nulls toward
interfering signals
 If chosen only one of the desired multi-path components used
 A main lobe towards individual users and attempts to treject
interference or noise from outised the main lobe.
Switched beam systems
 A fixed beamforming network
is implemented
 A switch is used to select the
best beam to receive a
particular signal
 Simple to implement, offering
many of the advantages of
more elaborate smart antenna
systems at the fraction of the
cost and complexity
 An RF switch and control
logic is used to select a
particular beam with the
greatest strength
Adaptive Array
 greater performance improvement is possible than are attainable
using switched beam systems.
 Complex array signal processing is implemented which is the
brain of the system utilizing an adaptive array algorithm
 Feedback through the system is used to input an adaptive
algorithm that adjusts or adapts a weight vector to maximize the
quality of the signal received,
Adaptive vs Switched Beam
Switched Beam Technology
Adaptive Antenna Arrays
Give high gain, narrower bandwidth
The antenna array can also combine
multi-path signals using space
diversity techniques
Is a relatively low technology approach so
has lesser cost and complexity
Antenna beams adaptively track signal
direction; a null can be placed in
the direction of an interferer
Requires only moderate interaction with
the base station receiver
May have greater capacity increase
when compared to switched
beams
Is unable to take advantage of path
diversity by combining coherent
multi-paths
More intensive signal processing
needed via DSPs
Intra-cell hand-offs between beams have to
be handled
Is more expensive to install than a
switched beam antenna
Using CDMA with Smart
Antenna Technology
 In CDMA systems all cells use the
same carrier frequency. Users are
distinguished by their different codes.
 The number of users within one cell
is limited mainly by the interference
that each user generates for all other
users.
 By pointing the main beam of the
adaptive antenna pattern to the
desired user, smart antennas
improve the SNIR for one user
without increasing interference for the
other users.
 Note that even an increase of the
capacity by only a factor 2 is highly
desirable, since it means that twice
as many customers can access the
network
Notable Capacity Increase
when using adaptive array
technology
These systems of antennas include a
LARGE NUMBER of techniques that
attempt to enhance the received
signal, increase range coverage,
suppress all interference, and
increase capacity.
QUESTIONS????