Smart Antennas for Wireless System

1. 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

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 360°quarter-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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270.833 • 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 • 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