Transcript No Slide Title
PROPAGATION ASPECTS FOR SMART ANTENNAS IN WIRELESS SYSTEMS JACK H. WINTERS
AT&T Labs - Research Red Bank, NJ 07701-7033 [email protected]
July 17, 2000 1
OUTLINE
• Antenna types • Potential gains • Propagation issues • Measurements needed • Conclusions 2
SIGNAL Phased Array
Smart Antennas
Adaptive Antenna Array SIGNAL BEAM SELECT SIGNAL OUTPUT INTERFERENCE INTERFERENCE BEAMFORMER WEIGHTS SIGNAL OUTPUT Smart Antenna Definition: Phased array or adaptive array antenna which adjusts to the environment.
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Smart Antennas
11.3 ft Rooftop Base Station Antennas Prototype Dual Antenna Handset Prototype Smart Antenna for Laptops
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Potential Gains:
ANTENNA AND DIVERSITY GAIN
Antenna Gain
: Increased average output signal-to-noise ratio - Gain of
M
with
M
antennas - Narrower beam with /2-spaced antenna elements
Diversity Gain
: Decreased required receive signal-to-noise ratio for a given BER averaged over fading (requires multipath) - Depends on BER - Gain for
M
=2 vs. 1: •5.2 dB at 10 -2 BER •14.7 dB at 10 -4 BER - Decreasing gain increase with increasing
M
- 10 -2 BER: •5.2 dB for
M
=2 •7.6 dB for
M
=4 •9.5 dB for
M
= - Depends on fading correlation 5
Potential Gains:
• Range increase: Antenna gain of
M
plus
M-
fold multipath diversity gain • Capacity increase: Suppress up to
M
-1 interferers for higher frequency reuse • Data rate increase: MIMO increase with
M
spatial channels 6
Propagation Issues:
• Angular spread : • Decreases gain of multibeam antennas (phased arrays) • Increases diversity gain of adaptive arrays (reduces correlation) • Makes interference suppression independent of AOA • Increases data rate gain with MIMO (reduces correlation) • Delay spread • Limits data rate w/o equalization • Increases diversity gain with equalization • Multipath richness • Determines maximum data rate with MIMO 7
PHASED ARRAYS
• Fixed (or steerable) beams • Consider cylindrical array with
M
elements ( /2 spacing) - Diameter (
M
/ 4 ) feet at 2 GHz •With small scattering angle ( = 4): r Mobile - Margin = 10log 10
M
(dB) - Number of base stations =
M
-1/2 - Range =
M
1/4 • Disadvantages: Base Station - No diversity gain (unless use separate antenna) - With large scattering angle , gain is limited for beamwidths 8
Range Increase for IS-136
Fixed Multibeam Antenna
• Increases gain for better coverage • Range increase is limited by angular spread • No spatial diversity gain • Can be used on downlink or uplink
Adaptive Array
• Range increase independent of angular spread • Diversity gain increases with antenna spacing • Can be used on uplink with fixed multibeam downlink 9
INTERFERENCE NULLING
Line-Of-Sight Systems User 1 • • • User 1 Signal User 2 Utilizes spatial dimension of radio environment to: • Maximize signal-to-interference-plus-noise ratio • Increase gain towards desired signal • Null interference: M-1 interferers with M antennas 10
INTERFERENCE NULLING
Multipath Systems User 1 • • • User 1 Signal User 2 Antenna pattern is meaningless, but performance is based on the number of signals, not number of paths (without delay spread).
=> A receiver using adaptive array combining with
M
antennas and
N-
1 interferers can have the same performance as a receiver with
M-N+
1 antennas and no interference, i.e., can null
N-
1 interferers with
M-N+
1 diversity improvement (
N
-fold capacity increase).
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INTERFERENCE SUPPRESSION
-
ADJACENT INTERFERER Spatial Diversity: S/I = 0dB, AAA with 4 antennas vs. REF with 2 antennas 0 -0.5
AAA(avg.) REF (avg.) AAA (data) ·REF (data) Theory Laboratory Results -1 -1.5
-2 -2.5
-3 -3.5
-4 0 10 SNR (dB) 20
MIMO CAPACITY INCREASE
• With
M
antennas at both the base station and mobiles,
M
independent channels can be provided in the same bandwidth if the multipath environment is rich enough.
• 1.2 Mbps in a 30 kHz bandwidth using 8 transmit and 12 receive antennas demonstrated by Lucent (indoors).
• Separation of signals from two closely-spaced antennas 5 miles from the base station demonstrated by AT&T/Lucent.
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Mobile Transmitter W 1 Tx W 2 W 3 Tx Tx W 4 Synchronous test sequences LO Tx
MIMO Channel Testing
Test Bed Receiver with Rooftop Antennas Rx Rx Rx Rx
•
Record complex correlation of each transmit waveform on each receive antenna, C 4x4
•
Compute C H C correlation matrix to determine potential capacity and predict performance
•
Compute fading correlation across receive array LO Transmit Antenna Configurations Space diversity Space / polarization diversity Space / pattern diversity Space / polarization / pattern diversity
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Measurements Needed:
• Multipath richness for MIMO: – 2-fold for rooftop to rooftop (fixed wireless) – At least 4-fold for outdoor – As high as 150-fold indoor • Delay spread • Angular spread • Polarization 16
Conclusions
• Propagation environment influences smart antenna architecture and wireless system gains: – Large angular spread decreases effectiveness of multibeam antennas but increases adaptive array’s range, interference suppression, MIMO data rate increase – Large delay spread limits data rate, but with S-T processing or OFDM increases diversity gain – Multipath limits range, but with MIMO increases data rate • Additional measurements in wide range of environments still needed 17