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Smart Antennas for Wireless System

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Title: Smart Antennas for Wireless System


1
Smart Antennas for Wireless System
ReferenceIEEE Personal Communications,1996.10
JACK H. WINTERS
ATT LABS-RESEARCH
ReporterYi -Liang Lin
Advisor Prof .Li-Chun Wang
2
Smart Antennas for Wireless System
  • Impairment
  • Smart Antenna Techniques
  • Diversity
  • Smart Antennas

Multibeam antenna
Adaptive array antenna
  • Application
  • Range Increase
  • Capacity Increase

3
Impairment
Multipath fading
  • Received signal amplitude and phase vary with
    antenna location,direction and polarization and
    with time

Delay spread
  • Difference in propagation delay among multiple
    path
  • Intersymbol interference can occur
  • Limit max data rate

Co-channel interference
  • Cellular system divide the available frequency
    channels into channel sets,using one channel set
    per cell with frequency reuse (frequency reuse
    factor 7 )
  • The number of channel set decreases ?
    interference increases
  • For given interference(channel set), cell size
    ?capacity

4
Smart Antenna Techniques
  • Permit greater coverage and capacity at each BS
  • The signals received by multiple antenna elements
    are weighted and combined to generate an output
    signal

5
Smart Antenna Techniques
  • An array provides an increased antenna gain of M
    plus a diversity gain against multipath fading
    which depends on the correlation of the fading
    among the antennas
  • Define Antenna gain as the reduction in required
    receive signal power for a given average output
    signal-to-noise ratio(indep of environment)
  • Define Diversity gain as the reduction the
    required average output signal-to-noise ration
    for a given BER with fading(only with multipath
    fading)

6
Diversity
Spatial Diversity
  • Antennas are separated far enough for low fading
    correlation
  • Separation depends on angular spread

Handset , BS indoor ?angular spread
360quarter-wavelength
Outdoor system with high BS ? few degrees 10-20
wavelength
Polarization Diversity
horizontal
Low correlation
vertical
? Only double diversity , Small profile
Angle Diversity
  • Antenna profile is small

7
Diversity
  • Adjacent narrow beams are used and have low
    fading correlation
  • With small angular spread,adjacent beams can have
    received signal levels more than 10dB weaker than
    the strongest beam,resulting in small diversity
    gain

8
Diversity
Four antenna diversity options with four antenna
elements
a) 7 ??spatial
b) 1/2??angularpolarization OR
7??spatialpolarization
c) 1/2??angular
4.2dB 2.9dB 4.4dB 1.1dB
10-2BER compare to two element array
9
Diversity
  • Diversity gain achieved in BS (uplink) to
    compensate for the higher transmit power of the
    BS on downlink

Selection diversity (Selecting the antenna with
the highest signal power)
using
Maximum ratio combining (weighting and combining
the received signal to maximize the signal
-to-noise ratio)
  • The main limitation on the handset antennas is
    typically not the handset size(diversity) , but
    the cost and power consumption of the receiver
    electronics for each antenna

10
Smart Antenna
  • Today cellular system,each BS uses three separate
    sets of antennas
  • for each 120? sector ,with dual receive
    diversity in each sector

Each sector uses a different frequency to reduce
co-channel interference, handoffs between sectors
are required
Performance ? narrower sector ? handoff
  • This leads us to smart antennas without handoffs
    between beams

Multibeam antenna
Smart antenna
Adaptive array antenna
Multibeam antenna ?different pattern
Antenna element
Adaptive array ?similar pattern
11
Multibeam antenna
  • Multiple fixed beam in a sector (four 30?beams
    cover a 120?sector)
  • M-beam antenna provides

M-fold antenna gain
Some diversity gain by combining the received
from different beams(angle diversity)
Or achieve dual diversity by using a second
antenna array (orthogonal polarization or
space far from first)
  • The same beam as on the uplink can be used for
    the downlink ,
  • thereby providing antenna gain (not diversity
    gain) on the downlink
  • These antennas have nonuniform gain with respect
    to angle due to
  • scalloping , 2dB less gain
  • ( decrease in gain between the beams due to the
    beam pattern of each)

12
Multibeam antenna
  • Problem locking wrong beam due to multipath or
    interference
  • and provide limited interference
    suppression

Since they cannot suppress interference if it is
in the same beam as the desired signal
13
Adaptive array
  • The signals received by the multiple antennas are
    weighted and combined to maximum the
    signal-to-interference-plus-noise ratio
  • Adaptive arrays have advantages

M-fold antenna gain without scalloping
M-fold diversity gain with sufficient low fading
correlation
  • These array cancel N interference with M
    antenna(MgtN)
  • and achieve M-N fold diversity gain
  • (MltN is possible)

Requiring a receiver for each antenna
  • This is cost of

Tracking the antenna weight at fading at fading
rate versus beam switching every few seconds
with the multibeam antenna
14
Adaptive array
Multipath LOS environment
15
Adaptive array
  • The adaptive antenna array weight and combines
    the signal to
  • enhance desired signal reception and null
    interference

A main beam in the direction of the desired
signal
It generate an antenna pattern
A null in the direction of interference
  • Under this conditions, with the number of
    antennas much greater than the number of arriving
    signal rays , it is easier to express the array
    response in terms of a small number of of angles
    of arrival, rather than the received signal phase
    at each antenna

MUSIC ESPRIT algorithms for improved
performance
  • M array antennas can form up to M-1 nulls to
    cancel up to M-1
  • interference
  • Such angular domain methods can be useful in some
    wireless
  • situations with near-LOS( BS in flat rural with
    high many antennas )

16
Adaptive array
however
  • The signal s arrive from each user via multiple
    paths and angles of
  • arrival , it becomes impossible to form an
    antenna pattern (above)
  • since the number of required nulls would be
    much greater than the
  • number of antenna
  • To provide diversity gain ,

the antennas at a BS can be spaced many
wavelengths apart , which results in many
grating lobes (many repetitions of the antenna
pattern in the field of view)
And with dual polarization antennas there is a
different pattern for each polarization
Antenna pattern is meaningless
17
Adaptive array
  • No matter how many paths each signal uses , the
    result is a given phase and amplitude at each
    antenna for each signal
  • There is an array response for each signal , and
    the performance of
  • the array depends on the number of signals ,not
    the number of the
  • path, with analysis in the signal space domain
    rather than the angular
  • domain
  • The hold true as long as the delay spread is
    small if not delayed
  • version s of the signals must be considered as
    separate signal (below)

18
Adaptive array
  • An important feature of adaptive arrays in
    multipath environments
  • the ability to cancel interferers independent of
    the angle of arrival
  • With multipath , objects around the antennas act
    as a huge reflecting
  • antenna (with the actual antennas acting as
    feeds )
  • which permit the receiving array to separate
    the signals
  • In particular , if the receiving antennas are
    spaced far enough apart
  • such that beams can be formed which are smaller
    than the angular
  • spread, the signals from two closely spaced
    antennas can usually be
  • separated using adaptive array combining
    techniques
  • The number of signals that can be separated
    increases with
  • the number of receive antennas , the angular
    spread and
  • the density of the multipath reflections within
    the angular spread

?multipath can be beneficial
19
Adaptive array
  • With delay spread , the array treats delayed
    versions of the
  • signals as separate signals
  • An adaptive array with M antennas can eliminate
    delay spread
  • over (M-1)/2 symbols or cancel M-1 delayed
    signals over any delay

20
Application
IS-136 TDMA
  • 3users/channel,162symbols/timeslotDQPSK
    modulation ?48.6kb/s
  • Equalizer is neededhandle delay spread up to one
    symbol
  • Synchronization sequence14symbol/timeslot

Equalizer training Determine adaptive array
weight
Used to
  • With rapid fading ,the channel change
    significantly across a timeslot

? the adaptive array weights must be adjusted
across the time slot the equalizer is
relatively simple(using MLSE)
GSM TDMA
  • 8users/channel,156.25symbols/timeslotMSK
    modulation?270.833
  • Because higher data rate ?delay spread over
    several symbols

21
Application
  • Synchronization sequence26symbol/timeslot
  • Channel does not change significantly over a time
    slot
  • weight need only be calculated per frame
  • Equalizer is more complex

IS95 CDMA
  • Multiple simultaneous users in each 1.25MHz
    channel
  • with 8kb/s per user, spreading gain 128
  • RAKE receiver
  • which combines delayed version of the CDMA
    signal

Provide diversity gain
Overcome the delay spread problem
  • The CDMA spreading codes can provide the
    reference signal for adaptive array
    weight calculation

22
Range Increase
  • With small angular spread

Adaptive array
M element
Provide M fold increase in antenna gain
Multibeam antenna
This increase range M1/? and reduce the number of
BS by M1/? for given area ?propagation loss
exponent
  • Diversity gain

Adaptive array with spatial diversity for given
array size diversity gain increases with angular
spread ?greater range (decreases fading
correlation)
Multibeam (angel diversity ) provides only small
diversity gain ?diversity gain is limited
23
Range Increase
  • Furthermore

The antenna gain of the multibeam antenna is
limited by the angular spread
The multibeam antenna cannot provide additional
antenna gain when the beamwidth is less than the
angular spread because smaller beamwidths
exclude signal energy outside the beam
24
Range Increase
TDMA systems
25
Range Increase
  • Multibeam antenna

The range is limited to the predicted range
limitation
The range improvement is degraded due to the
angular spread for M less than the theoretical
value corresponding to the range limitation
  • Adaptive array

The range exceed the no-diversity theoretical
range for all angular spreads ,due to antenna
diversity
The diversity gain increases with M,angular
spread, antenna spacing which decreases fading
correlation but does not increase for angular
spread greater than about 20
26
Range Increase
  • Range increase applies to upink

For IS136, GSM ,IS95
Since downlink freq is different from the uplink
freq the same adaptive array techniques cannot be
used for transmission by the BS
Multibeam antenna can be used
to achieve diversity gain transmit diversity must
be used
But
Or handset have multiple antennas
These techniques may provide less gain on the
downlink than on the uplink ,this may be
compensated for by the higher transmit power of
the BS as compared to the handset
TDMA systems
27
Range Increase
  • CDMA with RAKE receiver

Additional diversity gain of adaptive is much
smaller
Antenna gain limitation is much less
?adaptive array provide only a slightly larger
range increase than multibeam antennas
  • Multibeam require less complexity

?preferable
Multibeam antenna
CDMA
may be preferable for
adaptive array
TDMA
(particularly large angular spread)
28
Capacity Increase
CDMA
  • Capacity(bit per sec,hertz,base station)depends
    on

Spreading gain
Corresponding number of equal power co-channel
interference
  • Multibeam antenna with M beams
  • reduces the number of interference per beam by a
    factor of M
  • and thereby increases the capacity M-fold
  • Adaptive array provide only limited additional
    interference
  • suppression because the number of interference
    gt antennas
  • Multibeam are less complex ?preferred in CDMA

29
Capacity Increase
TDMA
  • TDMA are limited in capacity by a few dominant
    interference
  • Multibeam antenna reduces the probability of the
    interference
  • being in the same beam as the desired

?permits higher capacity through greater
frequency reuse (particularly small angular
spread)
  • Adaptive array can cancel the dominate
    interference with just
  • a few antenna

?M-element array permits greater than M-fold
increase (independent of angular spread)
4 element adaptive array ?permit frequency reuse
in every cell
for 7fold increase in capacity
4-beam antenna ?permit a reuse of three or four
for doubling of
capacity (small angular)
30
Capacity Increase
Adaptive array ? uplink only
  • TDMA system

Multibeam antenna ? downlink
(less effective in reducing interference)
  • This problem is worse in I-136

Because the handsets require a continuous
downlink, the same beam pattern must be used for
all three user in a channel ?reduce the
effectiveness of multibeam antenna against
interference
  • Interference on uplink is typically worse than
    downlink

The signal from interfering mobile could be
stranger than desired mobile mobile At the
mobile the signal from an interfering BS should
not be stronger since the mobile chooses the BS
with the strongest signal
BS are typically more uniformly spaced than the
mobiles
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