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PROPAGATION ASPECTS FOR SMART ANTENNAS IN WIRELESS SYSTEMS

JACK H. WINTERS

AT&T Labs - Research

Red Bank, NJ 07701-7033

jhw@research.att.com

July 17, 2000

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OUTLINE

• Antenna types• Potential gains• Propagation issues• Measurements needed• Conclusions

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Smart Antennas

Smart Antenna Definition: Phased array or adaptive array antenna which adjusts to the environment.

SIGNAL OUTPUT

SIGNAL

INTERFERENCE

INTERFERENCEBEAMFORMER

WEIGHTS

SIGNAL OUTPUT

BEAM SELECT

SIGNAL

BE

AM

FOR

ME

R

Adaptive Antenna ArrayPhased Array

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Smart Antennas

Rooftop Base Station Antennas

11.3 ft

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

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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

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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

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• Fixed (or steerable) beams

• Consider cylindrical array with M elements (/2 spacing)

- Diameter (M / 4) feet at 2 GHz

•With small scattering angle ( = 4):

- Margin = 10log10M (dB)

- Number of base stations = M-1/2

- Range = M1/4

• Disadvantages:

- No diversity gain (unless use separate antenna)

- With large scattering angle , gain is limited for beamwidths

PHASED ARRAYS

Base Station

Mobiler

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Fixed Multibeam Antenna

Range Increase for IS-136

• 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

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INTERFERENCE NULLINGLine-Of-Sight Systems

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

User 1

User 2

User 1

Signal•••

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INTERFERENCE NULLINGMultipath Systems

User 1

User 2

User 1

Signal•••

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

Spatial Diversity: S/I = 0dB, AAA with 4 antennas vs. REF with 2 antennas

- ADJACENT INTERFERER

0

-0.5

-1

-1.5

-2

-2.5

-3

-3.5

-410 20 300

AAA(avg.)REF (avg.)

AAA (data)·REF (data)

TheoryLaboratory Results

BE

R (

log)

SNR (dB)

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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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MIMO Channel Testing

TxW1

TxW2

TxW3

TxW4

LO

Synchronoustest

sequences

Rx

Rx

Rx

Rx

• Record complex correlation of each transmit waveform on each receive antenna, C4x4

• Compute CHC correlation matrix to determine potential capacity and predict performance

• Compute fading correlation across receive array

LO

Mobile Transmitter Test Bed Receiver with RooftopAntennas

Transmit Antenna Configurations

Space diversity

Space / polarization diversity

Space / pattern diversity

Space / polarization / pattern diversity

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• 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

Measurements Needed:

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• 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

Conclusions