Enhancements to Wireless Local Area Networks
Jack H. Winters
Division Manager
Wireless Systems Research Department
AT&T Labs - Research
Middletown, NJ
jhw@research.att.com
July 27, 2001
AT&T Labs - Research
Goal
•
Wireless communications, anywhere, in any form
•
In any form:
– high-speed data (Internet)
– voice
– audio (music)
– video
•
Anywhere:
– home
– buildings (office)
– pedestrian
– vehicles
•
Secure wireless virtual office
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OUTLINE
• Current Systems
• Current Trends
• Strategy Proposal
• Technical Issues
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Current Systems
Peak Data Rate
High performance/price
100 Mbps
802.11a
5.5GHz Unlicensed
10 Mbps
802.11b
$/Cell
$/Sub
$ 500,000
$ 500
$ 1000
$ 100
$ 100
$ 10
2.4GHz Unlicensed
1 Mbps
BlueTooth
100 kbps
2.4GHz
High ubiquity and mobility
3G Wireless
~ 2GHz
10 feet
2 mph
100 feet
10 mph
1 mile
30 mph
10 miles Range
60 mph Mobile Speed
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Barker
Barker
1 ms
11 chips
CCK
CCK
727 ns
8 chips
Key 802.11b Physical Layer Parameters:
Data rate:
Modulation/Spreading:
Transmission modes:
(dynamic rate shifting)
Chip rate:
Frequency band:
Bandwidth:
Channel spacing:
Number of channels:
Carrier accuracy:
1, 2, 5.5, 11 Mbps (adaptation to our needs for 1 Mbps only)
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)
11 MHz
Industrial, Scientific and Medical (ISM, unlicensed) 2.4 - 2.4835 GHz
22 MHz at -30 dBc
5 MHz
Total of 14 (but only the first 11 are used in the US)
±25 ppm
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3.2 ms
FFT
52=48+4 tones
64 point FFT
G
4 ms
Key 802.11a Physical Layer Parameters:
Data rate:
Modulation:
Coding rate:
Subcarriers:
Pilot subcarriers:
FFT size:
Symbol duration:
Guard interval:
Subcarrier spacing:
Bandwidth:
Channel spacing:
Frequency band:
Number of channels:
Carrier accuracy:
Carrier accuracy @5.8GHz:
6, 9*, 12, 18*, 24, 36*, 48*, 54* Mbps
BPSK, QPSK, 16QAM, 64QAM*
1/2, 2/3, 3/4*
User data rates (Mbps):
52
BPSK QPSK QAM16 QAM64
4
R=1/2
6
12
24
64
R=2/3
48
4 ms
R=3/4
9
18
36
54
800 ns
312.5 kHz
16.56 MHz (22 MHz at -20 dBc)
20 MHz
Unlicensed national infrastructure (U-NII)
Total of 12 in three blocks between 5 and 6 GHz
20 ppm
114 kHz
* optional
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Current Trends
•
•
Enterprise and Home users are all potential public WLAN users when they are
away from the office or home.
Players: MobileStar, WayPort, AerZone, …
– Soon to cover over 400 hotels & 50 airports US, Canada, UK
– $2.50/quarter-hour
– $15 ~ $60/month (depending on minutes cap) (struggling to define pricing)
– MobileStar outsources deployment to IBM Global Services
•
•
Jan 2001: Starbucks+MSN plans to install WLANs in all 3,000 stores
WayPort and Dell team to give customers wireless public Internet Access
– http://www.wayport.com/
•
Spontaneous appearance of neighborhood/residential access sites via consumer
broadband wire-line connections
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Community 802.11b LANs
•
North America
– Bay Area Wireless User Group
– Equip2rip (Oahu, HI)
– Guerrilla.net (Boston)
– Pdx Personal Telco
– pdxwireless.org (Portland, Oregon)
– SBAY.ORG Wireless Network (San
Francisco Bay Area)
– Seattle Wireless (Seattle)
– Seattle Wireless Internet Project
– SFLAN (San Francisco)
– Xlan (Seattle)
•
Europe
– Consume (London, UK)
– Elektrosmog (Stockholm and Gothenburg)
– Wlan.org.uk (UK)
– Wireless France (France)
– Wireless MediaPoli (Helsinki)
•
Bay Area 802.11b
Access Point Map
Australia
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Possible Strategies
• Broadband Residential Access
– Provide 802.11b’s to selected cable modem customers or
pole locations for universal wireless high-speed data
coverage (1 mile radius) with access to other homes in
neighborhood
– Since cable modem is at 1.5 Mbps and 802.11b is at 11
Mbps, provide fiber to these selected homes or poles
(economical for selected homes)
• Broadband Business Access
– Fiber to building access points (e.g., floors)
– Extend to residences for virtual offices
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Internet Roaming
•
Seamless handoffs between WLAN and WAN
– high-performance when possible
Cellular Wireless
– ubiquity with reduced throughput
•
•
management/brokering of consolidated
WLAN and WAN access
adaptive or performance-aware applications
– I-mobile, CC/PP, location based
Internet
Wireless LAN’s
Enterprise
Home
Public
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Technical Issues
•
•
•
•
•
•
•
•
•
•
Voice
Music streaming
Video streaming
Secure virtual office
Universal coverage
Range (delay spread)
Mobility
High data rates
Capacity (interference)
Key constraint: Stay within existing standards/standard evolution (enhance
performance within standards and drive standards evolution)
AT&T Labs - Research
Physical Layer Enhancements
Peak Data Rate
High performance/price
100 Mbps
802.11a
5.5GHz Unlicensed
10 Mbps
802.11b
2.4GHz Unlicensed
1 Mbps
$/Cell
$/Sub
$ 500,000
$ 500
$ 1000
$ 100
$ 100
$ 10
Enhanced
BlueTooth
100 kbps
2.4GHz
High ubiquity and mobility
3G Wireless
~ 2GHz
10 feet
2 mph
100 feet
10 mph
1 mile
30 mph
Range
10 miles
60 mph
Mobile Speed
AT&T Labs - Research
Physical Layer Enhancements
•
Physical Layer research
– Smart antennas for range/capacity enhancement (keeping
within standards, using TDD)
– Smart antennas using MIMO for 216 Mbps 802.11a
– Equalizers for delay spread robustness
– Adaptive coding/modulation, dynamic packet assignment,
power control (using cellular techniques in WLANs)
– Modification of 802.11a (a+) for the outdoor environment
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Physical Layer Enhancements
•
Physical Layer research
– Experiments:
•
•
•
•
20 MHz MIMO channel measurements
Smart antennas in 802.11b/a
216 Mbps MIMO 802.11a
4G streaming downlink
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Smart Antennas
SIGNAL
SIGNAL
OUTPUT
INTERFERENCE
INTERFERENCE
BEAMFORMER
WEIGHTS
Smart Antennas significantly improve performance:
• Higher antenna gain  Range extension (50 to 100% greater coverage)
• Interference suppression  Quality and capacity improvement (>2x)
• MIMO capacity increase (with smart antennas at Tx/Rx)
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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 transmit power
(with sufficient multipath)
AT&T measurements show 4x bit rates & capacity increase in full mobile &
indoor/outdoor environments (4 Tx and 4 Rx antennas)
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MIMO Channel Testing
Test Bed Receivers with Rooftop
Antennas
Mobile Transmitters
W1
Tx
W2
Tx
W3
• Perform timing
recovery and
symbol
synchronization
Rx
• Record 4x4
complex channel
matrix
Rx
Tx
• Evaluate capacity
and channel
correlation
Rx
Tx
W4
Synchronous
test
sequences
Rx
LO
Terminal Antennas
on a Laptop
LO
11.3 ft
Prototype Dual
Antenna Handset
Rooftop Base Station Antennas
Mobile Transmitters
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MIMO Antennas
Base Station Antennas
Laptop Prototype
• 4 patch antennas at 1900 MHz separated
by 3 inches (/2 wavelengths)
• Laptop prototype made of brass with
adjustable PCB lid
• 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
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MIMO Field Test Results
• Measured capacity
distribution is close to the
ideal for 4 transmit and 4
receive antennas
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Delay Spread Robustness
• When path length differences approach data rate, ISI
degrades performance:
– 802.11b/a can only tolerate about 200 ns rms of delay spread
– Outdoor environment can have several microseconds of
delay spread
• => Enhance receiver with equalizer in 802.11b and 802.11a
AT&T Labs - Research
Standards Evolution
Peak Data Rate
High performance/price
100 Mbps
802.11a
5.5GHz Unlicensed
10 Mbps
802.11b
$/Cell
$/Sub
$ 500,000
$ 500
$ 1000
$ 100
$ 100
$ 10
2.4GHz Unlicensed
1 Mbps
BlueTooth
100 kbps
2.4GHz
High ubiquity and mobility
3G Wireless
~ 2GHz
10 feet
2 mph
100 feet
10 mph
1 mile
30 mph
Range
10 miles
60 mph
Mobile Speed
AT&T Labs - Research
3.2 ms
FFT
G
4 ms
52=48+4 tones
64 point FFT
Issues:
Data rate:
Modulation:
Coding rate:
Subcarriers:
Pilots subcarriers:
FFT size:
Symbol duration:
Guard interval:
Subcarrier spacing:
Bandwidth:
Channel spacing:
Carrier accuracy:
Carrier error @5.8GHz:
6, 9, 12, 18, 24, 36, 48, 54 Mbps
BPSK, QPSK, 16QAM, 64QAM
1/2, 2/3, 3/4
52 - insufficient for high data rates in wide area
4 - insufficient if number of subcarriers increased
64 - too small for number of carriers in crowded spectrum
4 ms - too short for efficient wide area operation
800 ns - too short for wide area operation
312.5 kHz - too large for narrow channels
16.56 MHz - too large for spectrum available
20 MHz
20 ppm - leads to too much carrier error
114 kHz - too much for narrower channel spacing,
even at 1.9 GHz
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OFDM tradeoffs
802.11a
4G
DVB-T
2k mode
Data
rate
6, 9, 12, 18, 24,
36, 48, 54 Mb/s
2.56-8.96 Mb/s
4.98-31.67 Mb/s
Tone
modulation
Coding
rate
BPSK, QPSK,
16QAM, 64QAM
QPSK,16QAM
QPSK, “16QAM,”
“64QAM”
1/2, 2/3, 3/4
1/2, 2/3, 3/4, 7/8
[1/2, 2/3, 3/4, 5/6,
7/8] + RS(204,88)
Nt
52
640
1705
tB
4 ms
200 ms
231-280 ms
tB-tF
800 ns
40 ms
7-56 ms
ft
312.5 kHz
6.25 kHz
4.464 kHz
fB
16.56 MHz
4 MHz
7.6 MHz
fop
~5 GHz
~2 GHz
~500 MHz
AT&T Labs - Research
Physical Layer Enhancements
•
•
•
•
•
Smart antennas for range/capacity enhancement (keeping
within standards, using TDD)
Smart antennas using MIMO for 216 Mbps 802.11a
Equalizers for delay spread robustness
Adaptive coding/modulation, dynamic packet assignment,
power control (using cellular techniques in WLANs)
Modification of 802.11a (a+) for the outdoor environment
AT&T Labs - Research