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