Transcript 無投影片標題 - National Chiao Tung University

Smart Antennas for Wireless System
Reference:IEEE Personal Communications,1996.10
JACK H. WINTERS
AT&T LABS-RESEARCH
Reporter:Yi -Liang Lin
Advisor: Prof .Li-Chun Wang
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Smart Antennas for Wireless System
•Impairment
•Smart Antenna Techniques
-Diversity
-Smart Antennas
Multibeam antenna
Adaptive array antenna
•Application
-Range Increase
-Capacity Increase
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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 3
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
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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)
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Diversity
Spatial Diversity
-Antennas are separated far enough for low fading correlation
-Separation depends on angular spread
Handset , BS indoor angular spread 360°quarter-wavelength
Outdoor system with high BS  few degrees 10-20 wavelength
Polarization Diversity
horizontal
vertical
Low correlation
 Only double diversity , Small profile
Angle Diversity
-Antenna profile is small
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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
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Diversity
Four antenna diversity
options with four
antenna elements
a) 7 spatial
b) 1/2angular+polarization OR 7spatial+polarization
c) 1/2angular
4.2dB
2.9dB 4.4dB
1.1dB
10-2BER compare to two element array
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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
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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
Antenna element
Multibeam antenna different pattern
Adaptive array
similar pattern
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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
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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
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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(M>N)
and achieve M-N fold diversity gain
(M<N 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
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Adaptive array
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Multipath LOS environment
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
It generate an antenna pattern the desired signal
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 15
situations with near-LOS( BS in flat rural with high many antennas )
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
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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)
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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
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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
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Application
IS-136 TDMA
-3users/channel,162symbols/timeslot+DQPSK modulation 48.6kb/s
-Equalizer is needed:handle delay spread up to one symbol
-Synchronization sequence:14symbol/timeslot
Equalizer training
Used to
Determine adaptive array weight
-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/timeslot+MSK modulation270.833
-Because higher data rate delay spread over several symbols
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Application
-Synchronization sequence:26symbol/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
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Range Increase
-With small angular spread
M
element
Adaptive array 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
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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
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Range Increase
TDMA systems
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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
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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
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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)
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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 > antennas
-Multibeam are less complex preferred in CDMA
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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
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for doubling of capacity (small angular)
Capacity Increase
Adaptive array
-TDMA system
 uplink only
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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