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