Beyond 3G - The Potential for LAN-like Data Rates with ...personal.stevens.edu/~bmcnair/selected_topics/Oct29_class_notes.pdfOct 29, 2002 · for LAN-like Data Rates with Cellular-like

EE/CpE440: Current Topics10/29/02

EE/CpE 440

Current Topics in EE/CpE

Beyond 3G - The Potentialfor LAN-like Data Rates

with Cellular-like Coverage

Bruce McNairBurchard 206201-216-5549

[email protected]://koala.ece.stevens-tech.edu/~bmcnair

October 29, 2002


Outline

• Background/Motivation for 4G

• Theory & Practical Considerations

• Prototype Implementation

• Experimental Results

• Future Directions


Outline







Fourth Generation (4G) Wireless Access

Home LANsHome LANsCable networksCable networks

intranetsintranets

1G/2G1G/2G

POTSPOTS

voice/analog data

sophisticated wired data networking demand ↑demand for mobility ↑

reliance on mobile computing/PDAs ↑

TheInternet

TheInternet

3G 4G

Need for sophisticated,high-speed wireless data⇒


Design Considerations for a 4G System

Low start-up bandwidth,flexible frequency usage,coordinated parameters

Crowded spectrum,Interference potential

Co-existence with present services

Packet mode operationMulti-user operation, Spectral efficiency

Internet access

High peak data rate,Bandwidth requirementsSpectrum allocation

Multimedia applications,low latency

Asymmetrical capacity,smart antennas,channel coding,

Power consumption, size, transmit power, link budget

Portable devices, reasonable battery life

TradeoffsRestrictionsDesirable Characteristics







Internet access

High peak data rate,Bandwidth requirementsSpectrum allocation












Internet access

High peak data rate,flexible assignment

Bandwidth requirementsSpectrum allocation












Internet access

High peak data rate,flexible assignment

Bandwidth requirementsSpectrum allocation






• User perceived data rate capacity of a shared packet channel is the unused capacity of the channel• Larger peak-capacity channels look like larger capacity dedicated channels for the same average level of utilization


4G WirelessA Direct migration path from 3G:

An existing 3G network provides: • signalling• control• 384 kb/s uplink• downlink for lower bandwidth traffic• ongoing compatibility with 3G terminals

4G WOFDM provides untetheredcable modem-like user experience:

• 10+ Mb/s peak user rate downlink• 384 kb/s EDGE uplink (asymmetric traffic rates)• power efficient design for portable terminals


Outline







Limiting Factors in Mobile Wireless Communications

• Noise, SNR

• Multipath fading

• Interference

• Multipath dispersion

• Frequency selective fading

• Doppler shift


Noise, SNR

Transmitter

Receiver

kTB noise

• Thermal noise is proportional to receiver bandwidth• -174 dBm/Hz • Increased by noise figure of receiver

• Transmitter signal attenuated by distance, obstacles• Square law attenuation in free space, ~3.5 power in terrestrial environment

• Link budget limits high-speed operation

Excess receivernoise


Multipath fading

• Reflected signals take longer path than direct signal• Delayed reflections create constructive/destructive interference

• With no line-of-sight path, fading characterized by Rayleigh distribution– 10% of the time, signal is in 10 dB fade– 1% of the time, signal is in 20 dB fade

• It is impossible to design sufficient margin to operate through deepest fades

Transmitter

Receiver


Interference

• Restricted spectrum availability creates greater interference potential

• Interference sources:– Co-channel operations– adjacent channel operations– Intermodulation products

Transmitter

Receiver

Interferer


Multipath Dispersion

• Delayed versions of signal interfere with each other– Equivalent to intersymbol interference on a baseband wireline system

• Multipath is modeled as a delay profile - signal delay and average amplitude with randomly varying instanteous signal level

• For comparison purposes, typical indoor and outdoor delay profiles have been standardized:

– Typical Urban, Hilly Terrain, Mountainous Terrain profiles– Exponential delay profile

Transmitter

Receiver

t


Multipath Dispersion/ Frequency Selective Fading

• Delayed versions of signal interfere with each other– Equivalent to intersymbol interference on a baseband wireline system

• Multipath is modeled as a delay profile - signal delay and average amplitude with randomly varying instanteous signal level

• For comparison purposes, typical indoor and outdoor delay profiles have been standardized:

– Typical Urban, Hilly Terrain, Mountainous Terrain profiles– Exponential delay profile

• Through the Fourier Transform, delay profile can be studied in frequency domain

Transmitter

Receiver

t


Doppler Shift

• Relative motion of transmitter/receiver creates an apparent frequency offset due to Doppler shift

• Offset is proportional to speed and frequency

• At a 2 GHz carrier frequency, 60 mph creates a 200 Hz Doppler shift


4G Wireless:The Challenge:

Doppler frequency shift

Noise, multipath fading,interference impair signal

time

-40 dB

0 dB

Signal power

Multipath dispersionimpairs high symbol rates

time

Channel response

path loss

multipath fading, dispersion

Frequency selective fadingimpairs single carrier systems

frequency

Channel response

0 dB

-40 dB

shadow fading

interference

“kTB” noise


Modulation Considerations• Parameters that can be modified to transmit information:

– Frequency– Amplitude– Phase

• FSK has a constant envelop, allowing use of power efficient non-linear amplifiers, but it isn’t spectrally efficient

• ASK requires linear amplification

• PSK is the modulation method of choice for most wireless communications systems


Modulation parameter tradeoffs

• Data rate = (symbol rate)*(bits per symbol)

• To attain high data rates:– Transmit high symbol rates, or– Transmit many bits per symbol





• High symbol rates are limited by time dispersion, creating intersymbol interference

• Large number of bits per symbol -> dense constellations -> susceptibility to noise and interference

• What is the solution?





• High symbol rates are limited by time dispersion, creating intersymbol interference

• Large number of bits per symbol -> dense constellations -> susceptibility to noise and interference

• What is the solution?

• Keep symbol rate low to avoid dispersion, keep constellation simple to deal with noise. Just transmit multiple carriers.

-> OFDM (Orthogonal Frequency Division Multiplexing)


OFDM Basics

tone spacingftNt tones

Operatingbandwidth, fB

f

t

CyclicsuffixNC/2

CyclicprefixNC/2

Rampdown

NR

RampupNR

2B R C G FN N N N N= + + +

2F F

B F C R G

N NN N N N N

η = =+ + +

Block efficiency

Tolerance todelay spread C Ct N≈ ∝

Tone spacing vsactive block time

1t

F

ft

=

Total bandwidth B t tf N f=

Raw capacityfor M-ary tone

modulationtN M

GuardNG

NF FFT samples

OFDM block, NBsample time t

block length tB

tFtC/2 tC/2


coder/intlvr

mod IFFTsignal/ctrl

pilots

PAPRwindow

cyclic ext

filter

dataintf

D/A filtercontroller

transmitter

RFRFRF

OFDM Transmitter


filter rotate FFT

ch est demod decode/deintlv

A/D

∆t est

∆f est

filter

dataintfcontroller

receiver

RFRFRF

OFDM Receiver


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 µs

4 µs

FFT

52=48+4 tones64 point FFT

Key 802.11a Physical Layer Parameters:

Symbol duration: 4 µsGuard interval: 800 ns

Subcarrier spacing: 312.5 kHzBandwidth: 16.56 MHz

Channel Spacing: 20 MHz20 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


OFDM Basics 2: 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 µs

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

1705

231-280 µs

7-56 µs

4.464 kHz

7.6 MHz

~700 MHz

5-10 Mb/s

QPSK,16QAM

1/2, 2/3, 3/4

640

200 µs

40 µs

6.25 kHz

4 MHz

2 GHz


DD/P

DD/P

D

f

t

WOFDM frame = 20 msec

one to four

1 MHz slots

burst = 1 msec

OFDM/TDMA Options for 4G

• Full peak data rates are achievable

• Dynamic Packet Assignment to base stations, mobiles is an option

• Portable terminals can process only relevant traffic for power savings


Fourth Generation WirelessAn OFDM-based Solution:

• Current OFDM applications - high-speed wired and wireless digital systems:

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

• Rapid ongoing advances in general purpose DSP technology directly benefit OFDM’s straightforward signal processing

DD/P

DD/P

D

f

t

WOFDM frame = 20 msec

one to four

1 MHz slots 10 codewords (transport units)/burst

burst = 1 msec

Tone-by-tone flexibility

OFDMOFDM

Adaptivemodulation

Smartantennas Per user

tone allocation

Bandwidthcontrol

Resistanceto interference

Time/frequencycoding Inherent resistance

to dispersionRobustness

Simple underlyingmodulation

System flexibility

Per base stationtone allocation

Dynamic packetassignment

6.25 kHzN*160 tones

N*1 MHz

f


Outline







Base station

4G Wireless Research Prototype

Channel simulator

Mobile station

• prototype designed with general purpose DSPs for flexibility

• two-branch receiver diversity implemented at 1900 MHz

• performance measured on typical mobile outdoor channels

• robust performance demonstrated


OFDM Prototype

Pentek A/D Pentek Node Controller

Custom Clock Board

Custom D/A

RF

• PCS band operation• 384 kb/s end user capacity in 800 kHz • with OFDM• DSPs programmed (mostly) in C • Experiment with 2-way diversity,

synchronization

RF interface

Pentek ‘C40 DSPs

Sundevelopment

platform

bmcnair6/17/99


E

E

D

C

B

A

RF 2

A/D

A/D

Sampleclock

FFT

FFT

RSdecode

BERcount

AGC

RF 1Eras.det

monitor control

DSP-> RFinterface

Synth,RF init

D/AMonitorscope

demod

demod

∆testimate

∆ωestimate

Differential Equal Gain Diversity OFDM Receiver

G

G

G

G

framing

G

G

G

G

G

G

1

2

5

5

2

V

window

window

Hardware Architecture

Static gain adjust

Out of frame

A/D initialization

2.166 MHz Ts

512 FFT, 189 tones Differential in time demod, equal gain combining

RS(63,31) with 16 erasures

625 samples/288.46 µsec block

∆t estimation:differential in frequency

∆f estimation:differential in time acrosscyclic extension


FBA C

rotate

FFT

WaitISR

WriteGRAM

rotate

FFT

StartDMA

updatecount

StartDMA

updatecount

INT INT

D

signalB

WaitA

writeGRAM

WaitA| B

ReadGRAM

demod

signalC

SignalD

RSdecode

dropsampl

dropsampl

frame

BER

∆test

∆ωest

Wait C

AGC

sum

checkctl

writeRF

E

Dropsamples

Sumout

FFT A

FFT B

signalC

startDMA

INT wait forinput

get r_ptrdecodeinput

displayoptions

Set r_ptrwrite ctlstatus

controlstatus

outputvectors

InitializeA/D

writeclock

∆ω

InitializeRF

Process flow

CampISR

Markblock

WaitB

signalA

Alone?Y

N

SignalE

WaitD

erasedetect

descramb

ReadGRAM

OFDM Receiver - DSP Software Architecture


Datainterface

Coder ModulatorInverse

FFTD/A RF

OFDM transmitter

RF A/D FFT

DemodulatorErasure

detection DecoderDataIntf

RF A/D FFTOFDM receiver

• Fast Fourier Transform (FFT) forms basis of OFDM complexity advantage

• Multiple individual carriers retain antenna diversity advantage

• Good performance is maintained with high fading rates (i.e., high vehicle speeds)and channels with large delay spread (e.g., Hilly Terrain)

Internet/intranet

Internet/intranet


800 kHz

RF A/D FFT

DemodulatorErasure



Two equal rays 5 µsec delay


RF A/D FFT

DemodulatorErasure



“Typical Urban” channel

800 kHz


Outline







+atten

+atten

noise

atten

splitter

atten

noise

LPFLPF

TXBaseband

TXRF

LPF

LPF

TwoBranchFader

LPF

LPF

TwoBranch

RXRF

RXBaseband

RXBaseband

Experimental setup for OFDMreceiver performance measurements

• Simulated channels: AWGN, flat fading, two-ray, GSM models• Tested one- and two-branch receiver• Excellent repeatability and agreement with simulation and theory


Theoretical versus measured performance: one-branch receiver in AWGN

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

7 8 9 10 11 12

SNR

FE

R

measured

(47,31) ν=8

(63,31) ν=16


Simulated vs. Measured Results

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

5.00 7.00 9.00 11.00 13.00 15.00 17.00 19.00 21.00

SNR (dB)

FER

measured: 2rx, 200Hz, 20usecmeasured: 2rx, 200Hz, 5usecmeasured: 2rx, 40Hz, 20usecmeasured: 2rx, 40Hz, 5usecsimulation: 2rx 200Hz 20usecsimulation: 2rx 200Hz 5usecsimulation: 2rx 40Hz 20usecsimulation: 2rx 40Hz 5usec


FER vs SNRRS(63,31) with erasure detection (ρ=16)

OFDM receiver, 2 ray

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

5.00 7.00 9.00 11.00 13.00 15.00 17.00 19.00 21.00

SNR

FE

R

1RX 40Hz 2usec

2RX 40Hz 2usec

1RX 40Hz 5usec

2RX 40Hz 5usec

1RX 40Hz 10usec

2RX 40Hz 10usec

1RX 40Hz 20usec

2RX 40Hz 20usec



one- and two-branch receiver, GSM delay profiles

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

5.00 7.00 9.00 11.00 13.00 15.00 17.00 19.00 21.00

SNR

FE

R 1RX HT 40Hz

1RX MT 40Hz

1RX TU 40Hz

1RX BU 40Hz

2RX HT 40Hz

2RX MT 40Hz

2RX TU 40Hz

2RX BU 40Hz


FER vs SNRRS(63,31) with erasure detection (ρ=16)two-branch receiver, GSM delay profiles

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

5.00 7.00 9.00 11.00 13.00 15.00 17.00 19.00 21.00

SNR

FE

R

2RX HT 200Hz

2RX HT 100Hz

2RX HT 40Hz

2RX BU 200Hz

2RX BU 100Hz

2RX BU 40Hz

2RX BU 1Hz



OFDM receiver, 2 ray

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

5 7 9 11 13 15 17 19 21

SNR

FE

R

1RX 1Hz 5usec1RX 5Hz 5usec1RX 100Hz 5usec1RX 200Hz 5usec2RX 1Hz 5usec2RX 5Hz 5usec2RX 40Hz 5usec2Rx 100Hz 5usec2RX 200Hz 5usec

.


SNR required for 10% FER,two-branch receiver

SNR required for 10% FER,one-branch receiver

Fading rate: 1 Hz 5 Hz 40 Hz 100 Hz 200 Hz

AWGN 9.4 dB

Bad Urban 13.4 dB 13.5 dB 14.4 dB 18.3 dB

2 µsec 13.5 dB ~21 dB

5 µsec 13.1 dB 13.1 dB 13.1 dB 14.0 dB ~21 dB

10 µsec 13.2 dB

20 µsec 14.5 dB 14.0 dB >21 dB

Typical Urban 14.4 dB 14.8 dB 15.5 dB >21 dB

Hilly Terrain 15.8 dB 17.3 dB >>21 dB

Mountainous Terrain 16.0 dB >>21 dB

Channel:

Flat 17.1 dB 17.1 dB 19.1 dB >21 dB

Performance Summary

Fading rate: 1 Hz 5 Hz 40 Hz 100 Hz 200 Hz

AWGN 7.0 dB

Bad Urban 9.3 dB 9.4 dB 9.7 dB 10.8 dB

2 µsec 8.9 dB 11.9 dB

5 µsec 8.1 dB 8.6 dB 8.9 dB 9.2 dB 10.4 dB

10µsec 9.1 dB

20µsec 9.0 dB 9.5 dB 11.6 dB

Typical Urban 10.4 dB 10.5 dB 10.6 dB 12.0 dB

Hilly Terrain 11.2 dB 11.6 dB 14.2 dB

Mountainous Terrain 10.4 dB 13.1 dB

Channel:

Flat 11.2 dB 10.9 dB 11.2 dB 12.7 dB


Outline







What is the Future of Wireless Communications?

1. Will there be an evolution to 4G as there has been from 1G-2G-3G?• Who can afford the nation-wide investment of a new network?• Will the top-down evolution continue?

2. Will there a grass roots deployment of Wireless LANs with extensions to outdoors, high-speed mobility?

• Will next generation wireless grow like the Internet did, bottom-up?• Who will deploy the infrastructure to make this happen?• Will 4G wireless be ubiquitous like cellular or localized like cable modem/DSL

service?


OFDM tradeoffs

802.11aDVB-T

2k mode4G

Datarate

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

Tonemodulation



Nt 52

tB 4 µs

tB-tF 800 ns

ft 312.5 kHz

fB 16.56 MHz

fop ~5 GHz

4.98-31.67 Mb/s


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

1705

231-280 µs

7-56 µs

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 µs

40 µs

6.25 kHz

4 MHz

~2 GHz


Extending the WLAN Standards

802.11aDVB-T

2k mode‘4G’

Datarate

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

Tonemodulation



Nt 52

tB 4 µs

tB-tF 800 ns

ft 312.5 kHz

fB 16.56 MHz

fop ~5 GHz

4.98-31.67 Mb/s


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

1705

231-280 µs

7-56 µs

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 µs

40 µs

6.25 kHz

4 MHz

~2 GHz

Investigations show that a hybrid 802.11a format with modifications for outdoor operation is practical

Limitingparameters


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 µs - 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 µs

4 µs

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


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


Coding rate: 1/2, 2/3, 3/4subcarriers:

G

230.4 µs

FFT

Data rate: 1.66, 2.5, 3.33, 5, 6.66, 10, 13.33, 15 MbpsModulation: BPSK, QPSK, 16QAM, 64QAM

832 = 52*16

204.8 µs


Changes for high-mobility operation:

FFT size: 2048 = 64*32Symbol duration: 230.4 µs = 3.2*64 + .8*32

Guard interval: 25.6 µs = .8*32Subcarrier spacing: 4.833 kHz = 312.5/64

Bandwidth: ~5 MHz ª 16.56/4Channel Spacing: 5 MHz ª 20/4Carrier accuracy: .5 ppm for 5 GHz, 1 ppm for 2.4 GHz

Carrier error @5.8GHz: 2.9 kHz, 1.9 kHz @ 1.9 GHz

Pilot subcarriers: 64 = 4*16BPSK QPSK QAM16 QAM64

1.66 3.33 6.66R=1/2

13.33R=2/3

2.5 5 10 15R=3/4



Coding rate: 1/2, 2/3, 3/4subcarriers:

G

230.4 µs

FFT

Data rate: 1.66, 2.5, 3.33, 5, 6.66, 10, 13.33, 15 MbpsModulation: BPSK, QPSK, 16QAM, 64QAM

832

204.8 µs


802.11a

FFT size: 2048Symbol duration: 230.4 µsGuard interval: 25.6 µs

Subcarrier spacing: 4.833 kHzBandwidth: ~5 MHz

Channel Spacing: 5 MHzCarrier accuracy: .5 ppm for 5 GHz, 1 ppm for 2.4 GHz

Carrier error @5.8GHz: 2.9 kHz, 1.9 kHz @ 1.9 GHz

Pilot subcarriers: 64


1.66 3.33 6.66R=1/2

13.33R=2/3

2.5 5 10 15R=3/4


802.11a+6, 9*, 12, 18*, 24, 36*, 48*, 54* Mbps

BPSK, QPSK, 16QAM, 64QAM*1/2, 2/3, 3/4*

524

4 µs800 ns

312.5 kHz16.56 MHz

20 MHz20 ppm

64

114 kHz

G

3.2 µs

4 µs

FFT



6 12 24R=1/2

48R=2/3

9 18 36 54R=3/4



Coexistence of Simulcast/Individualized Traffic

Pure simulcast has marginal differentiation from broadcast

Pure individualized transmissionhas marginal differentiation

from cellular

Opportunity space is forcustomizable

simulcast - but existing systemsdon’t efficiently address this option

Interactive & location-basedservices are a natural extension

of customization


Simulcast Dispersion

Typical UrbanRMS delay spread

is about 1 µsec

Simulcast dispersion for 5 km radius cells is about 15 µsec at the 90th percentile

Most radio link schemes have been optimized to deal with intracell delay spread only

Documents

Beyond 3G - The Potential for LAN-like Data Rates with ...personal.stevens.edu/~bmcnair/selected_topics/Oct29_class_notes.pdfOct 29, 2002 · for LAN-like Data Rates with Cellular-like