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Smart Antennas for Mobile Wireless Systems

Jack H. Winters

May 6, 2003

jack@jackwinters.com

jwinters@motia.com

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OUTLINE

• Smart Antennas

• Adaptive Arrays

• MIMO

• System Applications

• Radio Resource Management

• Conclusions

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Smart Antennas

Smart Antennas can significantly improve the performance of wireless systems

• Higher antenna gain / diversity gain Range extension and multipath mitigation

• Interference suppression Quality and capacity improvement

• Suppression of delayed signals Equalization of ISI for higher data rates

• Multiple signals in the same bandwidth Higher data rates

Switched Multibeam versus Adaptive Array Antenna: Simple beam tracking, but limited interference suppression and diversity gain

SIGNAL OUTPUT

SIGNAL

INTERFERENCE

INTERFERENCEBEAMFORMER

WEIGHTS

SIGNAL OUTPUT

BEAM SELECT

SIGNAL

BE

AM

FOR

ME

R

Adaptive Antenna ArraySwitched Multibeam Antenna

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COMBINING TECHNIQUESSelection:

• Select antenna with the highest received signal power

• P0M = P0M

Output

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COMBINING TECHNIQUES (CONT.)

• Weight and combine signals to maximize signal-to-noise ratio (Weights are complex conjugate of the channel transfer characteristic)

• Optimum technique with noise only

• BERM BERM (M-fold diversity gain)

Maximal ratio combining:

W1

WM

Output

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OPTIMUM COMBINING (ADAPTIVE ANTENNAS)

• Weight and combine signals to maximize signal-to-interference-plus-noise ratio (SINR)

- Usually minimize mean squared error (MMSE)

• Utilizes correlation of interference at the antennas to reduce interference power

• Same as maximal ratio combining when interference is not present

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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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• 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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CDMA with Adaptive Array

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Range Increase with CDMA Signals

Single beam for all RAKE fingers results in range limitation with angular spread for multibeam antenna (phased array)

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Range Increase with CDMA Signals - Different Beams per Finger

log10 (M)0 1 2 31

2

3

4

5

6

7

Nor

mal

ized

R

ange

Adaptive Array

Phased Array

Theory

5 SpacingFIXED SECTORS, 0=60°

10°0=3°

20°45°

60°

60°45°20°10°

3°3-fold Diversity

3M-fold Diversity

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ANTENNA AND DIVERSITY GAINAntenna 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

- 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

• Antenna diversity gain may be smaller with RAKE receiver in CDMA

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DIVERSITY TYPES

Spatial: Separation – only ¼ wavelength needed at terminal

Polarization: Dual polarization (doubles number of antennas in one location

Pattern: Allows even closer than ¼ wavelength

4 or more antennas on a PCMCIA card

16 on a handset

Even more on a laptop

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ADAPTIVE ARRAYS FOR TDMA BASE STATIONSAT&T Wireless Services and Research - Field Trial with Lucent

7/96-10/96

Field trial results for 4 receive antennas on the uplink:

• Range extension: 40% reduction in the number of base stations can be obtained 4 to 5 dB greater margin 30% greater range

• Interference suppression: potential to more than double capacity

Operation with S/I close to 0 dB at high speeds greater capacity and quality

24 (12 ft)3 (1.5 ft)

3 (1.5 ft)

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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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Multiple-Input Multiple-Output (MIMO) Radio

• With M transmit and M receive antennas, can provide M independent channels, to increase data rate M-fold with no increase in total transmit power (with sufficient multipath) – only an increase in DSP

– Indoors – up to 150-fold increase in theory

– Outdoors – 8-12-fold increase typical

• AT&T measurements show 4x data rate & capacity increase in all mobile & indoor/outdoor environments (4 Tx and 4 Rx antennas)

– 216 Mbps 802.11a (4X 54 Mbps)

– 1.5 Mbps EDGE

– 19 Mbps WCDMA

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Rx

Rx

Rx

MIMO Channel Testing

W1

W2

W3

W4

LO

Synchronoustest

sequences

Rx

• Perform timing recovery and symbol synchronization

• Record 4x4 complex channel matrix

• Evaluate capacity and channel correlation

LO

Mobile Transmitters Test Bed Receivers with RooftopAntennas

Terminal Antennas on a Laptop

Tx

Tx

Tx

Tx

Rooftop Base Station Antennas

11.3 ft

Prototype Dual Antenna Handset

Mobile Transmitters

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MIMO Antennas

Base Station Antennas

Laptop Prototype • Antennas mounted on 60 foot tower on 5 story office building

• Dual-polarized slant 45 1900 MHz sector antennas and fixed multibeam antenna with 4 - 30 beams

• 4 patch antennas at 1900 MHz separated by 3 inches (/2 wavelengths)

• Laptop prototype made of brass with adjustable PCB lid

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• Measured capacity distribution is close to the ideal for 4 transmit and 4 receive antennas

MIMO Field Test Results

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Current Systems

10 feet 100 feet 1 mile 10 miles

100 kbps

1 Mbps

10 Mbps

100 Mbps

3G Wireless~ 2GHz

BlueTooth2.4GHz

802.11a5.5GHz Unlicensed

802.11b2.4GHz Unlicensed

Peak Data Rate

Range

2 mph 10 mph 30 mph 60 mph

$ 500,000

$ 1000

$ 100

$ 500

$ 100

$ 10

$/Cell $/SubHigh performance/price

High ubiquity and mobility

Mobile Speed

UWB3.1-10.6 GHz

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Wireless System Enhancements

10 feet 100 feet 1 mile 10 miles

100 kbps

1 Mbps

10 Mbps

100 Mbps

3G Wireless~ 2GHz

BlueTooth2.4GHz

802.11a5.5GHz Unlicensed

802.11b2.4GHz Unlicensed

Peak Data Rate

Range

2 mph 10 mph 30 mph 60 mph

$ 500,000

$ 1000

$ 100

$ 500

$ 100

$ 10

$/Cell $/SubHigh performance/price

High ubiquity and mobility

Mobile Speed

Enhanced

UWB3.1-10.6 GHz

In 1999, combining at base stations changed from MRC to MMSE for capacity increase

Downlink Switched Beam Antenna

INTERFERENCE

SIGNAL

SIGNALOUTPUT

BEAMFORMERWEIGHTS

Uplink Adaptive Antenna

SIGNALOUTPUT

SIGNAL

INTERFERENCE

BE

AM

FO

RM

ER

BEAMSELECT

Smart Antennas for Cellular

• Key enhancement technique to increase system capacity, extend coverage, and improve user experience in cellular (IS-136)

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Cellular Data

• CDPD (US) < 10 kbps

• GPRS = 30-40 kbps

• EDGE/1xRTT = 80 kbps

• WCDMA = 100 kbps (starting in Japan, but not for several years in US)

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Data rate: • 1, 2, 5.5, 11 MbpsModulation/Spreading: • Direct Sequence Spread Spectrum (DSSS)

• DBPSK, DQPSK with 11-chip Barker code (1, 2 Mbps) (this mode stems from the original 802.11 standard)• 8-chip complementary code keying (CCK) (5.5, 11 Mbps)• optional: packet binary convolutional coding (PBCC), 64 state, rate 1/2 CC (BPSK 5.5 Mbps, QPSK 11 Mbps)

Barker

Key 802.11b Physical Layer Parameters:

Chip rate: 11 MHzFrequency band: Industrial, Scientific and Medical (ISM, unlicensed) 2.4 - 2.4835 GHz

Bandwidth: 22 MHz - TDDChannel spacing: 5 MHz

Total of 14 (but only the first 11 are used in the US), with only 3 nonoverlapping channels

Number of channels:

Transmission modes:(dynamic rate shifting)

CCK

1 s11 chips

Barker

727 ns8 chips

CCK

WLANs: 802.11b

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Unlicensed national infrastructure (U-NII), 5.5 GHz

Total of 12 in three blocks between 5 and 6 GHz

Data rate: 6, 9, 12, 18, 24, 36, 48, 54 MbpsModulation: BPSK, QPSK, 16QAM, 64QAM

Coding rate: 1/2, 2/3, 3/4Subcarriers: 52

Pilot subcarriers: 4

G

3.2 s

4 s

FFT

52=48+4 tones64 point FFT

Key 802.11a Physical Layer Parameters:

Symbol duration: 4 sGuard interval: 800 ns

Subcarrier spacing: 312.5 kHzBandwidth: 16.56 MHz - TDD

Channel spacing: 20 MHz

FFT size: 64

:

BPSK QPSK QAM16 QAM64

6 12 24R=1/2

48R=2/3

9 18 36 54R=3/4

User data rates (Mbps):

Frequency band:

Number of channels:

WLANs: 802.11a (g in 2.4 GHz band)

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Smart Antennas for WLANs

• TDD operation (only need smart antenna at access point or terminal for performance improvement in both directions)

• Interference suppression Improve system capacity and throughput– Supports aggressive frequency re-use for higher spectrum efficiency, robustness in the ISM band (microwave

ovens, outdoor lights)

• Higher antenna gain Extend range (outdoor coverage)• Multipath diversity gain Improve reliability• MIMO (multiple antennas at AP and laptop) Increase data rates

APSmart

Antenna

Interference

Smart Antennas can significantly improve the performance of WLANs

APSmart

Antenna

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Internet Roaming

• Seamless handoffs between WLAN and WAN

– high-performance when possible

– ubiquity with reduced throughput

• Management/brokering of consolidated WLAN and WAN access

• Adaptive or performance-aware applications

• Nokia GPRS/802.11b PCMCIA card

• NTT DoCoMo WLAN/WCDMA trial

Cellular Wireless

EnterpriseHome

Public

Internet

Wireless LAN’s

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Smart Antennas

• Adaptive MIMO

– Adapt among:

• antenna gain for range extension

• interference suppression for capacity (with frequency reuse)

• MIMO for data rate increase

• With 4 antennas at access point and terminal, in 802.11a have the potential to provide up to 216 Mbps in 20 MHz bandwidth within the standard

• In EDGE/GPRS, 4 antennas provide 4-fold data rate increase (to 1.5 Mbps in EDGE)

• In WCDMA, BLAST techniques proposed by Lucent, with 19 Mbps demonstrated

• In UWB, smart antennas at receiver provide range increase at data rates of 100’s Mbps

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Enhancements

• Smart Antennas (keeping within standards):

– Range increase

– Interference suppression

– Capacity increase

– Data rate increase using multiple transmit/receive antennas (MIMO)

• Radio resource management techniques (using cellular techniques in WLANs):

– Dynamic packet assignment

– Power control

– Adaptive coding/modulation/smart antennas

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Radio Resource Management

• Use cellular radio resource management techniques in WLANs: Adaptive coding/modulation, dynamic packet assignment, power control

• Use software on controller PC for multiple access points to analyze data and control system

• Power control to permit cell ‘breathing’ (for traffic spikes)

• Dynamic AP channel assignment

– Combination of radio resource management and smart antennas yields greater gains than sum of gains

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Cell Breathing in WLAN Systems

• Measure traffic load for each access point• Shrink overloaded cell by reducing RF power• Expand others to cover abandoned areas

AP

AP

AP

AP

AP

AP

AP

AP

AP

AP

AP

AP

AP

AP

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Adaptive Channel Assignment

3 1 2

2 3

2 3

1

1

3 1 2

2 2

3 3

3

2

Initial Assignment After one iteration

• Assign channels to maximize capacity as traffic load changes

Cochannelinterference

High trafficload

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Smart AntennasSmart Antennas

Smart Antennas significantly improve performance:

• Higher antenna gain with multipath mitigation (gain of M with M-fold diversity) Range extension

• Interference suppression (suppress M-1 interferers) Quality and capacity improvement

• With smart antennas at Tx/Rx MIMO capacity increase(M-fold)

SIGNAL

INTERFERENCE

INTERFERENCEBEAMFORMER

WEIGHTS

SIGNAL OUTPUT

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Conclusions

• We are evolving toward our goal of universal high-speed wireless access, but technical challenges remain

• These challenges can be overcome by the use of:

– Smart antennas to reduce interference, extend range, increase data rate, and improve quality, without standards changes

– Radio resource management techniques, in combination with smart antennas, and multiband/multimode devices