Transcript Document
Little Wireless and Smart
Antennas
Jack H. Winters
[email protected]
2/26/04
Slide 1
February 26, 2004
OUTLINE
• Smart antennas
• Implementation issues
• Appliqué
• Conclusions
Slide 2
February 26, 2004
Smart Antennas for WLANs
Smart
Antenna
AP
Smart
Antenna
AP
Interference
Smart Antennas can significantly improve the performance of 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
Slide 3
February 26, 2004
Implementation Issues
Adaptive Antenna Array
Switched Multibeam Antenna
SIGNAL
BEAMFORMER
SIGNAL
BEAM
SELECT
SIGNAL
OUTPUT
SIGNAL
OUTPUT
INTERFERENCE
INTERFERENCE
BEAMFORMER
WEIGHTS
Smart antenna is a multibeam or adaptive antenna array that tracks the wireless
environment to significantly improve the performance of wireless systems
Adaptive arrays in any environment provide:
• Antenna gain of M
• Suppression of M-1 interferers
In a multipath environment, they also provide:
• M-fold multipath diversity gain
• With M Tx antennas (MIMO), M-fold data rate increase in same channel with same total transmit power
Slide 4
February 26, 2004
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
Measurements (e.g., AT&T) 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
Slide 5
February 26, 2004
WEIGHT GENERATION TECHNIQUES
For Smart Antenna: Need to identify desired signal and
distinguish it from interference
•
•
•
Weight
Generation
Blind (no demod): MRC – Maximize output power
Interference suppression – CMA, power inversion, power
out-of-band
Non-Blind (demod): Training sequence/decision directed reference signal
MIMO needs non-blind, with additional sequences
Slide 6
February 26, 2004
Digital vs. Analog Implementation
• Analog Advantages:
• Digital requires M complete RF chains, including M A/D and D/A's,
versus 1 A/D and D/A for analog, plus substantial digital signal
processing
• The cost is much higher for digital
• An appliqué approach is possible - digital requires a complete
baseband
• Digital Advantages:
• Slightly higher gain in Rayleigh fading (as more accurate weights
can be generated)
• Temporal processing can be added to each antenna branch much
easier than with analog, for higher gain with delay spread
• Modification for MIMO (802.11n) is easier than with analog
Slide 7
February 26, 2004
Appliqué
Wireless
Transceiver
RF
Appliqué
(Spatial
processing
only)
RF
Baseband/MAC
Processor
Processor,
Host Interface
• Conforms to 802.11 standard (blind beamforming with MRC)
• Appliqué configuration requires minimal modifications to legacy
designs
Slide 8
February 26, 2004
Smart Antenna WiFi (PCMCIA
Reference Design)
PCMCIA - CARDBUS Interface
Appliqué Architecture Plug-and-Play to legacy designs
Legacy Transceiver
Baseband/MAC
RF
Processor
Processor
Motia
Smart Antenna
RF Chip
Partners: Intersil/Globespan,
Maxim/TI, RFMD, Atmel
Slide 9
February 26, 2004
802.11b Packet Structure
Time permits weight generation
20
µs
96 symbol Short Preamble
Preamble
SFD
PHY H
56 Barker 16 Barker 24 Barker
BPSK
BPSK
QPSK
MPDU
Data from MAC
Barker
BPSK/QPSK
CCK 5.5/11Mbps
192 symbol Long Preamble
Preamble
128 Barker
BPSK
SFD
16 Barker
BPSK
Slide 10
MPDU
PHY H
Data from MAC
48 Barker
BPSK
Barker
BPSK/QPSK
(CCK 5.5/11Mbps)
February 26, 2004
802.11b Performance with Fading
Achieves a 12 to 14 dB gain over a single
antenna
Performance Comparison - All four data rate
0.8
0.7
802.11 spec
11Mbps Baseline
2Mbps Baseline
5.5Mbps Baseline
1Mbps Baseline
11Mbps 1-ant
5.5Mbps 1-ant
2Mbps 1-ant
1Mbps 1-ant
Poly. (1Mbps Baseline)
Poly. (2Mbps Baseline)
Poly. (5.5Mbps Baseline)
Poly. (11Mbps Baseline)
Expon. (11Mbps 1-ant)
Expon. (5.5Mbps 1-ant)
Expon. (2Mbps 1-ant)
Expon. (1Mbps 1-ant)
0.6
FER
0.5
0.4
0.3
0.2
0.1
y = 4.1054e-0.1845x
0
-10
-5
0
5
Theoretical for short packet
Slide 11
10
15
20
25
SNR (dB)
February 26, 2004
30
802.11b Beamforming Gains with 4
Antennas
Performance Gain over a Single Antenna in a
Rayleigh Fading Channel
2 Antenna
Selection
Adaptive
One Side
Adaptive
Both Sides
6.1 dB
12.8 dB
18.0 dB
2X to 3X Range + Uniform
Coverage
Theoretical
Bound Both
Sides
22.2 dB
3X to 4X Range + Uniform
Coverage
Slide 12
February 26, 2004
802.11n
Requirements for 802.11n:
–
–
–
Requires MAC changes and may require MIMO:
–
>100 Mbps in MAC
>3 bits/sec/Hz
Backward compatible with all 802.11 standards
4X4 system (?)
Next standards meeting in Orlando
Slide 13
February 26, 2004
802.11n Process
Slide 14
February 26, 2004
Summary and Conclusions
Current research is finding ways to implement
smart antennas in a variety of commercial
systems:
–
–
–
–
Reusing same silicon where possible to reduce cost
Minimizing modifications to existing systems
Staying within the standards
Meeting each system’s unique requirements
Slide 15
February 26, 2004