1. Smart Antennas for Wireless System
Reference:IEEE Personal Communications,
JACK H. WINTERS
AT&T LABS-RESEARCH
Reporter:Yi -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 Delay spread Co-channel interference
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 360°quarter-wavelength
Outdoor system with high BS  few degrees 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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

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(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

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/timeslot+DQPSK modulation 48.6kb/s
Equalizer is needed:handle delay spread up to one symbol
Synchronization sequence:14symbol/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/timeslot+MSK modulation
Because higher data rate delay spread over several symbols

21. 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

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 TDMA systems 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 > 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