Enhancements to Wireless Local Area Networks

AT&T Labs - Research

Enhancements to Wireless Local Area Networks

Jack H. Winters

Division Manager

Wireless Systems Research Department


Middletown, NJ

[email protected]

July 27, 2001


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


OUTLINE

• Current Systems

• Current Trends

• Strategy Proposal

• Technical Issues


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


Data rate: 1, 2, 5.5, 11 Mbps (adaptation to our needs for 1 Mbps only)Modulation/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 at -30 dBcChannel spacing: 5 MHz

Total of 14 (but only the first 11 are used in the US)Number of channels:Carrier accuracy: ±25 ppm

Transmission modes:(dynamic rate shifting)

CCK

1 s11 chips

Barker

727 ns8 chips

CCK


Unlicensed national infrastructure (U-NII)

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, 3/4*Subcarriers: 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 (22 MHz at -20 dBc)

Channel spacing: 20 MHz

20 ppm

FFT size: 64

Carrier accuracy:Carrier accuracy @5.8GHz: 114 kHz

BPSK QPSK QAM16 QAM64

6 12 24R=1/2

48R=2/3

9 18 36 54R=3/4

User data rates (Mbps):

* optional

Frequency band:

Number of channels:


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


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)

• Australia

Bay Area 802.11b Access Point Map


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


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

– I-mobile, CC/PP, location based

Cellular Wireless

EnterpriseHome

Public

Internet

Wireless LAN’s


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)


Physical Layer Enhancements


100 kbps

1 Mbps

10 Mbps

100 Mbps

3G Wireless~ 2GHz

BlueTooth2.4GHz



Peak Data Rate

Range


$ 500,000

$ 1000

$ 100

$ 500

$ 100

$ 10



Mobile Speed

Enhanced



• 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



• Physical Layer research

– Experiments:

• 20 MHz MIMO channel measurements

• Smart antennas in 802.11b/a

• 216 Mbps MIMO 802.11a

• 4G streaming downlink


Smart AntennasSmart Antennas

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)

SIGNAL

INTERFERENCE

INTERFERENCEBEAMFORMER

WEIGHTS

SIGNAL OUTPUT


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)


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


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


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

MIMO Field Test Results


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


Standards Evolution


100 kbps

1 Mbps

10 Mbps

100 Mbps

3G Wireless~ 2GHz

BlueTooth2.4GHz

802.11a +



Peak Data Rate

Range


$ 500,000

$ 1000

$ 100

$ 500

$ 100

$ 10



Mobile Speed


Coding rate: 1/2, 2/3, 3/4Subcarriers: 52 - insufficient for high data rates in wide area

Pilots subcarriers: 4 - insufficient if number of subcarriers increased

Symbol duration: 4 s - too short for efficient wide area operationGuard interval: 800 ns - too short for wide area operation

Subcarrier spacing: 312.5 kHz - too large for narrow channelsBandwidth: 16.56 MHz - too large for spectrum available

Channel spacing: 20 MHz

G

3.2 s

4 s

FFT

Carrier accuracy: 20 ppm - leads to too much carrier errorCarrier error @5.8GHz: 114 kHz - too much for narrower channel spacing,

even at 1.9 GHz

Issues:

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

52=48+4 tones64 point FFT

FFT size: 64 - too small for number of carriers in crowded spectrum


OFDM tradeoffs

802.11aDVB-T

2k mode4G

Datarate

6, 9, 12, 18, 24, 36, 48, 54 Mb/s

Tonemodulation

BPSK, QPSK,16QAM, 64QAM

Codingrate

1/2, 2/3, 3/4

Nt 52

tB 4 s

tB-tF 800 ns

ft 312.5 kHz

fB 16.56 MHz

fop ~5 GHz

4.98-31.67 Mb/s

QPSK, “16QAM,” “64QAM”

[1/2, 2/3, 3/4, 5/6,7/8] + RS(204,88)

1705

231-280 s

7-56 s

4.464 kHz

7.6 MHz

~500 MHz

2.56-8.96 Mb/s

QPSK,16QAM

1/2, 2/3, 3/4, 7/8

640

200 s

40 s

6.25 kHz

4 MHz

~2 GHz



• 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

Documents

Enhancements to Wireless Local Area Networks