4G Wireless Access based on Wideband OFDM

The Evolution of TDMA to 3G & 4G Wireless Systems

Nelson Sollenberger

AT&T Labs-Research

Wireless Systems Research Division

AT&T Wireless Services

• TDMA

– European GSM over 250 million

– North American TDMA ~ 50 million

– Japanese PDC ~ 50 million

• CDMA

– North American CDMA ~ 60 million (including S. Korea)

AT&T serves over 14 million subscribers with digitalTDMA technology and some remaining analog technology,and provides packet data service with CDPD technology

Other TDMA operators - Rogers AT&T - Cingular (SBC & BellSouth) - throughout Mexico, Central & South America

Cellular Telephony Handsets

Nokia5160

EricssonPD 328

MotorolaStarTAC®ST7790 Phone

Nokia8860

Various TDMA phones available today

TDMA parameters

• 30 KHz channels (like analog & CDPD)

• 20 msec speech frames

• 24.3 kbaud symbol rate

• 3 time-slots/users

• 7.4 kbps ACELP speech coding

• 1/2-rate channel coding on important bits interleaved over 2 bursts in 40 msec

• Differential pi/4-QPSK modulation

TDMA Capacity Roadmap

Reuse N = 7 N = 5 N = 4

Dual band base •Operation at 800 or 1900 MHz.

Calls can be set up on either frequency band and handed between them to manage traffic

•Additional spectrum at 1900 MHz adds directly to capacity of cell

Smart Antennas •Base station antennas systems that

use digital signal processing to cancel interference

2000 2001 2002

Base Station Power Control •Base stations only transmit power required to

reach mobile with adequate signal quality resulting in lower interference

Dynamic Channel Assignment •Network automatically assigns radio frequencies

to cell sites for more efficient utilization of frequencies

Discontinuous Transmission•Mobiles transmit only during when user is speaking.

Lowers interference in the system and increases talk time

IS-136 Smart Antenna Test Bed

•Reuse of 3/9 to 4/12, instead of 7/21, approximately 2x capacity

•Two dual polarization uplink antennas, downlink multibeam antenna with 4 - 30° beams

•Shared linear power amplifier unit with Butler matrices

•Real-time downlink power control with beam tracking

Wireless Data Terminals

Nokia 9110

3COMPalm VII

Nokia3G visionSierra PCMCIA

CDPD Modem

The new Ericsson R380 phone, which features wireless data functions

WIRELESS COMPUTING

WIRELESSGROWTH

INTERNETGROWTH

RF & DIGITALTECHNOLOGY

MOBILESOFTWARE

- web access- e-mail- file transfer- location services- streaming audio & video

datarate

1 M

384 k

64 k

9.6 k IS-136

IS-136+

EDGE

WidebandOFDM

Macrocellular Wireless Data Evolution& AT&T’s Roadmap

CDPDGSM

IS-95

GPRS

IS-95+

WCDMA

1995 2000 2005

PDC

5 M

HDR

EDGE TechnologyEnhanced Data-rates for Global Evolution

• Evolutionary path to 3G services for GSM and TDMA operators

• Builds on General Packet Radio Service (GPRS) air interface and networks

• Phase 1 (Release’99 & 2002 deployment) supports best effort packet data at speeds up to about 384 kbps

• Phase 2 (Release’2000 & 2003 deployment) will add Voice over IP capability

GPRS Airlink• General Packet Radio Service (GPRS)

• Same GMSK modulation as GSM

• 4 channel coding modes

• Packet-mode supporting up to about 144 kbps

• Flexible time slot allocation (1-8)

• Radio resources shared dynamically between speech and data services

• Independent uplink and downlink resource allocation

EDGE Airlink• Extends GPRS packet data with adaptive

modulation/coding

• 2x spectral efficiency of GPRS for best effort data

• 8-PSK/GMSK at 271 ksps in 200 KHz RF channels supports 8.8 to 59.2 kbps per time slot

• Supports peak rates over 384 kbps

• Requires linear amplifiers with < 3 dB peak to average power ratio using linearized GMSK pulses

• Initial deployment with less than 2x 1 MHz using 1/3 reuse with EDGE Compact as a complementary data service

GPRS Networks• consists of packet wireless access network and IP-based backbone

• shares mobility databases with circuit voice services and adds new packet switching nodes (SGSN & GGSN)

• will support GPRS, EDGE & WCDMA airlinks

• provides an access to packet data networks

– Internet

– X.25

• provides services to different mobile classes ranging from 1-slot to 8-slot capable

• radio resources shared dynamically between speech and data services

Compact vs Classic• Classic

– 4/12 reuse– continuous downlinks on first 12 carriers– 2.4 MHz x2 minimum spectrum

• Compact– 1/3 reuse in space– frame synchronized base stations– reuse of 4 in time for control channels– partial loading for traffic channels– discontinuous downlinks– 600 KHz x2 minimum spectrum

EDGE Channel Coding and Frame Structure

464 bits1 data block

Convolutional CodingRate = 1/3Length = 7

Puncture Interleave

Burst N

Burst N+1

Burst N+2

Burst N+3 Burst Format

8PSK Modulate

1392 bits 1392 bits

348 bits/burst

348 bits468.75 bits

156.25 symbols/slot

0 1 2 3 4 5 6 7

8 Time Slots

1 Time Slot = 576.92 µs

Tail symbols

3

Data symbols

58

Tail symbols

3

Data symbols

58

Training symbols

26

Guard symbols

8.25

Modulation: 8PSK, 3 bits/symbolSymbol rate: 270.833 kspsPayload/burst: 348 bitsGross bit rate/time slot: 69.6 kbps - overhead = 59.2 kbps user data

20 msec frame with 4 time-slots for each of 8 bearers

EDGE Modulation, Channel Coding & Bit Rates

Scheme Modulation Maximum

rate [kb/s]

Code Rate Family

MCS-9 59.2 1.0 A

MCS-8 54.4 0.92 A

MCS-7 44.8 0.76 B

MCS-6 29.6 0.49 A

MCS-5

8PSK

22.4 0.37 B

MCS-4 17.6 1.0 C

MCS-3 14.8 0.80 A

MCS-2 11.2 0.66 B

MCS-1

GMSK

8.8 0.53 C

EDGE Link Throughput

9

EDGE Compact System Performance

0102030405060708090

100

0 10 20 30 40 50 60 70

Probability throughput < = X per timeslot

X (kb/s)

26 users/sector at 3.5 kbps average load per user

0102030405060708090

100

0 1000 2000 3000 4000 5000

Probability packet delay < = X

X (msec)

% %

0

50

100

150

200

250

300

9 18 27 36 45

single-slot

Multi-slot

Average User Throughput (kb/s)

EDGE Classic Multi-slot Gain

Ave. # of users per sector

EDGE Evolution

• Best effort IP packet data on EDGE

• Voice over IP on EDGE circuit bearers

• Network based intelligent resource assignment

• Smart antennas & adaptive antennas

• Downlink speeds at several Mbps based on wideband OFDM and/or multiple virtual channels

0

5

10

15

20

25

30

35

40

45

50

55

Baseline Enhanced

Nor

mal

ized

voi

ce c

apac

ity

(Erl

ang/

Sit

e/M

Hz)

GSM IS-136 EGPRS/GMSK/F EGPRS/8PSK/H

30 29

50

35

11

7

20

10

7.2 MHz Spectrum

* 1/3 reuse* no shadow fading change due to mobility*Signal-based power control is assumed for baseline EGRPS*SINR-based power control & LI-DCA assumed for enhanced

VoIP over EDGE Bearer Performance• Focused on GMSK full-rate & 8PSK half-rate EDGE channels with

dedicated MAC & random frequency hopping for 7.4 kbps voice coding

*This assumes 30 mph vehicle speed for micro fading* SINR-based power control with adaptive target

Aggressive frequency re-use High spectrum efficiency

Increased co-channel interference

Downlink Switched Beam Antenna

SIGNALOUTPUT

INTERFERENCE

SIGNAL

SIGNALOUTPUT

BEAMFORMERWEIGHTS

Uplink Adaptive Antenna

SIGNAL

INTERFERENCE

BE

AM

FO

RM

ER

BEAMSELECT

Smart antennas provide substantial interference suppression for enhanced performance

Smart Antennas for EDGE• Key enhancement technique to improve system capacity and user experience• Leverage Smart Antennas currently in development/deployment for IS-136 & GSM

EDGE Smart Antenna Processing

Dual Diversity Receiver Using DDFSE for Joint ISI and CCI Suppression

Deinter-leaver

ViterbiDecoder

Soft Output

OutputData

Receiver

Feed-forwardFilter

Symbol Timingand Recovery

DDFSEEqualizer

EqualizerTraining

Rx Rx Filter

Rx Rx FilterFeed-forward

Filter

• Simulation results show a 15 to 30 dBimprovement in S/I with 2 receive antennas

• Real-time EDGE Test Bed supports laboratory and field tests to demonstrate improved performance

Jack WintersHanks ZengAshutosh Dixit

EDGE 2-Branch Smart Antenna PerformanceLaboratory Tests

EDGE MCS-5 with Interference Suppression in aTypical Urban Environment

Blo

ck

Err

or

Ra

te

Signal-to-Interference Ratio (dB)

20 dB SNR

Laboratory results show a 15 to 30 dB improvement in S/I with 2 receive antennas

Improvement with Terminal Diversity and Interference Suppression: User Experience

010

2030

405060

7080

90100

0 10 20 30 40 50 60 70

No Diversity

SimpleDiversityInterferenceSuppression

Prob. (throughput <=X) (%)

X (kb/s)

Typical user throughput increased from 30 to 45 kbps per time-slot

Prototype Dual AntennaHandset

External Whip

Internal Patch

Multi-cell EDGE Compact Simulation- 1/3 reuse- 18 users per sector- 3.5 kbps average load per user

• spectrum - 500 MHz to 3 GHz• 3G EDGE/WCDMA network for uplink, downlink, control and signalling

• 4G WOFDM high speed downlink “a wireless cable modem”

• Complement to EDGE/UMTS

• High peak data rates (up to 10 Mb/s) in a 5 MHz channel

4G Wireless: One View

Path Loss and Fading Challenge

Delay Spread

Rayleigh Fading

Path Loss

rapid fading of 20 to 30 dB(power varies by 100 to 1000 timesin level at rates of about 100 times per second)

path loss up to ~ 150 dB(that is a 1 followed by 15 zeroes)

Reflected signalsarrive spread outover 5 to 20microsecond

Cellular Interference Challenge

0.001

0.01

0.1

1

-5 0 5 10 15 20 25

1|3 reuse

2|6 reuse

3|9 reuse

4|12 reuse

7/21 reuse

Signal to Interference ratio in dB

Cum

ulat

ive

Pro

babi

lity

Each base station is equippedwith three 120 degree directionalantennas to reduce interference& improve capacity

AT&T Labs-Research Work on 4G

• Smart antennas

• Multiple-Input-Multiple-Output Systems

• Space-Time Coding

• Dynamic Packet Assignment

• Wideband OFDM

MIMO Radio Channel Measurements

• Multiple antennas at both the base station and terminal can significantly increase data rates with sufficient multipath

• Ability to separate signals from closely spaced antennas has been demonstrated indoors and in AT&T-Lucent IS-136 field trial

• Lucent has demonstrated 26 bps/Hz in 30 kHz channel with 8 Tx and 12 Rx antennas indoors

• AT&T has performed measurements on 4 Tx by 4 Rx antenna configurations in full mobile & outdoor to indoor environments

MIMO Channel Measurement System

Transmitter

• 4 antennas mounted on a laptop

• 4 coherent 1 Watt 1900 MHz transmitters with synchronous waveform generator

Receive System

• Dual-polarized slant 45° PCS antennas separated by10 feet and fixed multibeam antenna with 4 - 30° beams

• 4 coherent 1900 MHz receivers with real-time baseband processing using 4 TI TMS320C40 DSPs

MIMO Measured Channel CapacityPotential Capacity Relative to a Single Antenna System

• Capacity increase close to 4 times that of a single antenna is possible with 4 transmit and 4 receive antennas

• Capacity for pedestrians is similar to mobile users

Performance Measure

• Complex channel measurement: H = [ H ij] for the ith transmit and jth receive antenna

• Capacity (instantaneous and averaged over 1 second) for 4 TX by 4 RX:

C = log2(det[I + (/4)H†H]) = log2(1 + (/4)i)

where is the total signal-to-noise ratio per antenna and

i is the ith eigenvalue of H†H

• To eliminate the effect of shadow fading, the capacity is normalized to the average capacity with a single antenna:

Cn = log2(1 + (/4)i) / (1/16) log2(1 + Hij)

Multiple Input Multiple Output Wireless• RX diversity - HF, terrestrial microwave, cellular….• TX frequency offset diversity & simulcasting for paging - 70’s• Adaptive array processing in military systems• TX diversity - 80’s

– frequency offset (channel decoding combining)– delay (equalizer combining)

• Optimum combining for cellular (multipath channels) - 80’s• Space-division multiple access - 80’s & 90’s

– angle-of-arrival based– multi-path based (supports co-location & multi-channels per user)

• MIMO - 80’s & 90’s– Multiple spatial channels using adaptive antenna arrays– BLAST - successive interference cancellation combined with coding– Space-Time coding

Space-Time Coding

How do you enhance TX delay diversity ( a repetition code)?

Multiple Antennas increase System Capacity

• MIMO (BLAST & space-time coding) techniques increase bit rate and/or quality on a link by creating multiple channels and/or enhancing diversity

• Switched/steered beam antennas for base stations and interference suppression/adaptive antennas for terminals reduce interference, increasing system capacity

OFDM for 4G Wireless

~ 6 kHz

~ 800 tones

~ 5 MHz

• OFDM is being increasingly used in high -speed information transmission systems:

- European HDTV- Digital Audio Broadcast (DAB)- Digital Subscriber Loop (DSL)- IEEE 802.11 Wireless LAN

5 MHz channels~ 6 KHz tones~ 13/26 MHz sample rate2048 FFT size (160 usec OFDM blocks)256/512 sample OFDM block guard timeQPSK & 16-QAM modulation adaptive modulation/coding1 to 2 msec time-slots in 20 to 40 msec frames

Mobile OFDM parameters: ex.

OFDM Characteristics• High peak-to-average power levels• Preservation of orthogonality in severe multi-path• Efficient FFT based receiver structures• Enables efficient TX and RX diversity• Adaptive antenna arrays without joint equalization• Support for adaptive modulation by subcarrier• Frequency diversity• Robust against narrow-band interference• Efficient for simulcasting• Variable/dynamic bandwidth• Used for highest speed applications• Supports dynamic packet access

OFDM Robust Channel Estimation

FFT

FFT

synch wordremovedata

receivedsignals

IFFT FFT...

.

.

.

.

.

.

.

.

.

. . . . . .

data

Estimator 1

Estimator 2

2-branchmaximal-ratiocombining

WOFDM 2-Branch Diversity Performance

0.001

0.01

0.1

1

-1 0 1 2 3 4 5 6 7

CC, k=9CC, k=3RS

Spectrum Efficiency

Efficiency: IS-136 0.04; IS-95 0.07; GSM 0.04

Source: G. J. Pottie, IEEE Personal Communications, pp. 50-67, October 1995

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

5 7.5 10 12.5 15

Synch CDMA

Dynamic ChannelAllocation withPower Control

Dynamic ChannelAllocation

SNR (dB)

Efficiency

Dynamic Packet Assignment

1. Mobile locks to the STRONGEST base

2. Mobile sends measurements of path losses for nearby bases to serving base

3. Serving base forwards measurementsto nearby bases

4. Bases assign channels to all packets/mobiles

5. Bases forward channel assignment info to nearby bases

~ 50 % improvement in performance % improvement in performance

Wideband OFDM Staggered Frame

Frame20 ms

1 2 4 1 2 4 .....

Superframe 80 ms

Superframe 80 ms

Control Slots Control Slots .....

3 3

4 ms

5 Blocks 5 Blocks 5 Blocks

group A group B group C group D

16 resources in 1 msec time-slots

1B 2 B

Sync & data

20 OFDM Blocks

data

5 Blocks

2 B

data

WOFDM Performance with Dynamic Packet Assignment & 5 MHz of Spectrum

0

20

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120

0 500 1000 1500 2000 2500 3000 3500

MR, No beam-formingIS, No beam-formingMR, Four beams per sectorIS, Four beams per sector

Ave

. Use

r P

acke

t Del

ay (

mse

c)

Throughput per site (kb/s)

OFDM Experimental Program

• Baseband signal processing based on commercial off-the-shelf DSP hardware with some custom designed components• Sony-provided 1900 MHz transceivers• Real-time performance measured through RF channel fading simulator• Phase 1 parameters:

- >384 kb/s end user data rate - 800 kHz downlink bandwidth- GSM-derived clocks (2.166 MHz sample rate with 512 FFT)- 3.467 kbaud - 189 OFDM tones with 4.232 kHz tone spacing- differential detection- Reed-Solomon channel coding

RF A/D FFT

DemodulatorErasure

detectionDecoder

DataIntf

RF A/D FFTOFDM receiver

“Typical Urban” channel

800 kHz

Summary: Key Features of 4G W-OFDM• IP packet data centric• Support for streaming, simulcasting & generic data• Peak downlink rates of 5 to 10 Mbps• Full macro-cellular/metropolitan coverage• Asymmetric with 3G uplinks (EDGE)• Variable bandwidth - 1 to 5 MHz• Adaptive modulation/coding• Smart/adaptive antennas supported• MIMO/BLAST/space-time coding modes• Frame synchronized base stations using GPS• Network assisted dynamic packet assignment