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SMART ANTENNAS FOR WIRELESS COMMUNICATIONS

JACK H. WINTERS

AT&T Labs - Research

Red Bank, NJ 07701-7033

jhw@research.att.com

September 9, 1999

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AGENDA

• Background• Use of adaptive antennas• Key research issues• Conclusions

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Smart Antennas

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

From site visits: “Smart antenna technology is the most (or one of the most) important technology for 4G cellular systems” (speakers at NTT, Filtronics, etc.).

SIGNAL OUTPUT

SIGNAL

INTERFERENCE

INTERFERENCEBEAMFORMER

WEIGHTS

SIGNAL OUTPUT

BEAM SELECT

SIGNAL

BE

AM

FOR

ME

R

Adaptive Antenna ArrayPhased Array

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• Power:– Higher data rates need higher power for the adequate reliability and

perhaps higher frequencies (which have higher propagation and circuit losses)

• Interference suppression:– For higher capacity with frequency reuse

• Multipath mitigation:– For more reliable and robust communications

Future Systems Will Need Greater:

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• Power:– Phased arrays provide narrow pencil beams with higher gain,

particularly at higher frequencies, as do adaptive arrays

• Interference suppression:– Phased arrays reduce probability of interference, and adaptive

arrays suppress interference

• Multipath mitigation:– Diversity (spatial, polarization, and angle)

Smart Antennas Provide:

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• From site visits, phased arrays and adaptive arrays are considered and researched about equally

• Phased arrays:– Point-to-point– Some macrocellular base stations (e.g., GSM)– On satellites and on satellite terminals

• Adaptive arrays:– Indoor systems– Microcells– Some cellular base stations (e.g., ANSI-136)– Terminals

Smart Antennas Use:

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• Cost:– Power

• 50% of power is in RF electronics, adding more antennas increases power usage (decreases terminal battery life)

– RF electronics and A/D are expensive

– Low cost phased array

– Cost limits the number of antenna elements that can be used

– Solutions being studied:• Optical beamforming for large phased arrays

• Integrated antennas

• Limited introduction

• Size:– Antenna on a chip

– “Antenna-less” terminals

Key Research Issues:

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• Diversity:– Multiple antennas for diversity

– Spatial (separation)• Difficult on handset

• Difficult for point-to-point or base stations with small angular spread

– Polarization (dual diversity only)

– Angle (direction/pattern)• Multibeam antennas (depends on angular spread)

• Antennas on handset where hand placement is critical

• Minimize headloss

Key Research Issues:

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• Tracking– Angle-of-arrival with phased arrays

– Weights with adaptive arrays

– Signal processing horsepower is not considered a significant research issue for tracking in most future systems

– Better algorithms required:• AOA with MUSIC/ESPRIT today, but need to be robust with angular

spread, obtain higher resolution

• Adaptive array needs better subspace tracking methods:– higher data rates => longer equalizers => longer training sequences =>

greater overhead

Key Research Issues:

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• Spatial-Temporal Processing:– Equalization of intersymbol interference (delay spread at high data

rates) with cochannel interference suppression• Better architectures needed

– Space-Time coding• interference suppression

• tracking

– BLAST/MIMO technology• M-fold increase in capacity with M antennas without increase in power

or bandwidth

• Extension to outdoors with tracking and multipath richness issues

• Cochannel interference suppression

Key Research Issues:

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• Hooks in standards (interdisciplinary research - protocols):– Incorporate features so that smart antennas are possible

– 2G system problems:• ANSI-136: continuous downlink

• IS-95: common downlink pilot

– 3G hooks:• WCDMA: Downlink pilots dedicated to each user

• EDGE: Signals from base stations that need to be broadcast to all users restrict smart antenna usage

– 4G: Any packet/multimedia access to all users is a critical issue for smart antennas

Key Research Issues:

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• Vertical Integration/Interdisciplinary Approach– Protocols for smart antennas - pilots and broadcast signals

– Interaction with frequency hopping, power control, discontinuous transmission, adaptive channel assignment

– RF matching (multiband antennas)

– Adaptation to hand position

– Limited introduction with nonuniform traffic

– Ad hoc networks

– Channel characterization

– Base versus terminal antenna (complexity) tradeoffs

– Transmit diversity with space-time coding

Key Research Issues:

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• Future system needs: higher data rates/better coverage/ubiquitous users and systems

• Smart antennas are seen as a critical component to satisfy these needs - significant research in nearly all wireless companies visited

• Substantial research to reduce cost, develop technologies, and integrate into future systems is required to achieve these goals

Conclusions