Post on 02-Jan-2016
description
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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
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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