Andrea Goldsmith

7/31/2019 Andrea Goldsmith

1/99

Wireless Communications:Trends and Challenges

ANDREA J. GOLDSMITH

Dept. of Electrical EngineeringStanford Universityhttp://ee.stanford.edu/~andrea

810.1-Cimini-7/98


2/99


3/99810.35-Cimini-7/98

[R. Katz, "Does Wireless Data Have a Future?", Plenary Talk, INFOCOM '96]

Seamless Multimedia Networks

with Mobility and Freedom from Tethers

WIRELESS DATA VISION

TAXI

Region

Campus

City

In-Building

laptops, PDAs


4/99

VOICE VERSUS DATA VERSUS VIDEO

810.80-Cimini-7/98

Wired Networks Trying to Integrate

(ATM, SONET, Multimedia Services)

Voice Data Video

Delay < 100 ms < 100 ms

Packet Loss < 1% 0


5/99

WHAT IS THE FUTUREOF WIRELESS DATA?

USA market

1995 20000

10

20

30

40

50

60

70

80

90

100

Internet users

paging subs

laptop users

cellular + PCS subs

annual laptop sales

dedicated wirelessdata subs

822.008-Cimini-9/97

*Estimates as of 1996

millions


6/99

THE ISSUE IS PERFORMANCE

810.81-Cimini-7/98

"The mobile data market has been slow to take off,but progress is being made. The most formidableobstacle to user acceptance remains performance."

I. Brodsky, "Countdown to Mobile Blast Off",

Network World, February 19, 1996

Link Performance: Data Rate and Quality

Network Performance: Access, Coverage, Reliability,QoS, and Internetworking

RadioPort

NetworkInterface

Signaling/RoutingMobilityControl

MobilityControl

Protocols

Radio Link

MobileMultimedia

Terminal

WirelessInterface

RadioProtocols& Modem

RadioProtocols& Modem

NetworkAdaptation& Control

RADIO ACCESS SEGMENT MOBILE NETWORK SEGMENT

BROADBAND WIRELESSNETWORK


7/99

GAP BETWEEN WIRED AND

WIRELESS NETWORK CAPABILITIES

10.79-Cimini-7/98

WIDE AREA CIRCUIT SWITCHING

User

Bit-Rate

(kbps)

14.4digitalcellular

28.8 modem

ISDN

ATM

9.6 modem

2.4 modem2.4 cellular

32 kb

PCS

9.6 cellular

wired- wireles

bit-rate "gap"

1970 20019901980YEAR

LOCAL AREA PACKET SWITCHING

User

Bit-Rate

(kbps)

Ethernet

FDDI

ATM100 M

Ethernet

Polling

Packet

Radio

1st gen

WLAN

2nd

gen

WLAN

wired- wirelessbit-rate "gap"

1970 200019901980.01

.1

1

10

100

1000

10,000

00,000

YEAR

.01

.1

1

10

100

1000

10,000

100,000


8/99


9/99

RADIO ENVIRONMENT

810.83-Cimini-7/98

Path Loss Shadow Fading Multipath

Limit the Bit Rateand/or Coverage


10/99

where Pr is the local mean received signal

PATH LOSS MODEL

Different, often complicated, models areused for different environments.

A simple model for path loss, L, is

The path loss exponent = 2 in free space;2 4 in typical environments.

power, Pt is the transmitted power, d is thetransmitter-receiver distance, f is frequency,

and K is a transmission constant.

822.011-Cimini-9/97

Pr 1

Pt f2d

= KL =


11/99

SHADOW FADING

810.84-Cimini-7/98

The received signal is shadowed byobstructions such as hills and buildings.

This results in variations in the local meanreceived signal power,

Implications

nonuniform coverage increases the required transmit power

Pr (dB) = Pr (dB) + Gs

where Gs ~ N(0, s ), 4 s 10 dB.2

R P = Pr0


12/99

MULTIPATH

Constructive and Destructive Interferenceof Arriving Rays

Received

Power

Delay Spreadt

dB With Respectto RMS Value

0 0.5

0.5

1.5

-30-20

-10

10

0

1t, in seconds

0 10 3020x, in wavelength

810.85-Cimini-7/98

h(t) = aiej id(t-ti)Si


13/99

DELAY SPREADTIME DOMAIN INTERPRETATION

large

T

smallT

0

11

T 2T

Channel Input

Channel Output

0 T 2T

0 T 2T

810.88-Cimini-7/98

Two-ray model= rms delay spread

2

Delay

Received

Power

T small negligible intersymbol interference

large significant intersymbol interference,which causes an irreducible error floorT


14/99

PHYSICAL LAYER ISSUES

Link Performance Measures

Modulation Tradeoffs

Flat Fading Countermeasures

Delay Spread Countermeasures

810.91-Cimini-7/98


15/99

LINK PERFORMANCE MEASURESPROBABILITY OF BIT ERROR

810.92-Cimini-7/98

The probability of bit error, Pb, in a radioenvironment is a random variable.

Typically only one of these measures isuseful, depending on the Doppler frequency

and the bit rate.

average Pb, Pb

Pr [Pb > Pbtarget] outage, Pout=


16/99

LINK PERFORMANCE MEASURESEFFICIENCY

810.72-Cimini-7/98

Spectral Efficiency a measure of the data rate per unit

bandwidth for a given bit error

probability and transmitted power

Power Efficiency

a measure of the required received

power to achieve a given data rate

for a given bit error probability andbandwidth

Throughput/Delay


17/99

GOALS OFMODULATION TECHNIQUES

810.74-Cimini-7/98

High Bit Rate

High Spectral Efficiency

High Power Efficiency

Low-Cost/Low-Power Implementation

Robustness to Impairments


18/99

DIGITAL MODULATION

810.94-Cimini-7/98

Any modulated signal can be represented as

Linear versus nonlinear modulation impact

on spectral efficiency

Constant envelope versus non-constantenvelope hardware implications with impact

on power efficiency

s(t) = A(t) cos [ ct + (t)]

s(t) = A(t) cos (t) cos ct

- A(t) sin (t) sin ct

amplitude

in-phase

quadrature

phase or frequency


19/99

LINEAR MODULATION TECHNIQUES

810.95-Cimini-7/98

s(t) = [ an g (t-nT)]cos ct - [ bn g (t-nT)] sin ct

I(t), in-phase Q(t), quadrature

LINEAR MODULATIONS

CONVENTIONAL

4-PSK

(QPSK)

OFFSET

4-PSK

(OQPSK)

DIFFERENTIAL

4-PSK

(DQPSK, /4-DQPSK)

M-ARY QUADRATURE

AMPLITUDE MOD.

(M-QAM)

M-ARY PHASE

SHIFT KEYING

(M-PSK)

M 4 M 4M=4(4-QAM = 4-PSK)

Square

onstellations

Circular

Constellation

n n


20/99

SIGNAL CONSTELLATIONS

Tradeoffs

Higher-order modulations (M large) are more spectrallyefficient but less power efficient.

M-QAM is more spectrally efficient than M-PSK butalso more sensitive to system nonlinearities.

M-QAM (Square Constellations)

16-QAM

4-PSK

an

bn

M-PSK (Circular Constellations)

16-PSK

an

bn4-PSK

810.96-Cimini-7/98


21/99

PULSE SHAPING

810.75-Cimini-7/98

Rectangular pulses are spectrally inefficient

pulse shaping

intersymbol interference (ISI)

non-constant envelope

Nyquist pulses


22/99

RAISED COSINE PULSE SHAPING

G(f)

12T

0

= 1

= 0

= 0.5

f12T

-1T

- 1T

810.97-Cimini-7/98

30

256 QAM

64 QAM

16 PSK16 QAM

8 PSK

4 PSK

20

10

0 0.25 0.750.5 1.0

Cosine Rolloff Factor,

RelativeP

eak

InstantaneousP

ower(dB)

g(t)

0 4T3T

= 1

= 0 = 0.5

= 0, 0.5

tT 2T


23/99

DEMODULATION

for the reference signal.

reference.

Coherent detection requires a coherent phase

difficult to obtain in a rapidly fading

environment

increases receiver complexity

Differential detection uses the previous symbol

eliminates need for coherent reference

entails loss in power efficiency (up to 3 dB)

Doppler causes irreducible error floor,

typically small for high bit rates

810.133-Cimini-7/98


24/99

FREQUENCY SHIFT KEYING

810.98-Cimini-7/98

Continuous Phase FSK (CPFSK)digital data encoded in the frequency shift

typically implemented with frequency

modulator to maintain continuous phase

nonlinear modulation but constant-envelope

Minimum Shift Keying (MSK)

minimum bandwidth, sidelobes large

can be implemented using I-Q receiver

Gaussian Minimum Shift Keying (GMSK)

reduces sidelobes of MSK using a

premodulation filter

used by RAM Mobile Data, CDPD,and HIPERLAN

s(t) = A cos [ ct + 2 kf

d( ) d ]

t


25/99

SPECTRAL CHARACTERISTICS

822.013-Cimini-9/97

QPSK/DQPSKGMSK

(MSK)

1.0

1.0 1.5 2.0 2.50.50

-120

-100

-80

-60

-40

-20

0

10

0.25

B3-dBTb = 0.16

Normalized Frequency (f-fc)Tb

PowerSpectralDensity(dB)


26/99

BIT ERROR PROBABILITYAWGN CHANNEL

QPSK is more spectrally efficient than BPSK with thesame performance.

M-PSK, for M>4, is more spectrally efficient but requiresmore SNR per bit.

There is ~3 dB power penalty for differentialdetection.

810.99-Cimini-7/98

Pb

BPSK, QPSK

DBPSK

DQPSK

010-6

2

5

2

5

2

5

2

5

2

5

10-1

10-3

10-4

10-5

10-2

2 4 6 8 10 12 14

For Pb = 10-3

BPSK 6.5 dB

QPSK 6.5 dB

DBPSK ~8 dB

DQPSK ~9 dB

b, SNR/bit, dB


27/99

BIT ERROR PROBABILITYFADING CHANNEL

Pb is inversely proportion to the average SNR per bit.

Transmission in a fading environment requires about18 dB more power for Pb = 10

-3.

0.100-Cimini-7/98

DBPSK

AWGN

BPSK

010-5

2

5

2

5

2

5

2

5

2

5

1

10-2

10-3

10-4

10-1

5 10 15 20 25 30 35

Pb

b, SNR/bit, dB


28/99

BIT ERROR PROBABILITYEFFECTS OF DOPPLER SPREAD

Doppler causes an irreducible error floor when differentialdetection is used decorrelation of reference signal.

The implication is that Doppler is not an issue for high-speed

wireless data.

data rate T Pbfloor

10 kbps 10-4s 3x10-4

100 kbps 10-5s 3x10-6

1 Mbps 10-6s 3x10-8

The irreducible Pb depends on the data rate and the Doppler.For fD = 80 Hz,

0

0 60504030201010-6

10-5

10-4

10-3

10-2

10-1

10

DQPSK

Rayleigh Fading

No Fading

fDT=0.003

0.002

0.001

0

QPSK

Pb

b, SNR/bit, dB

[M. D. Yacoub, Foundations of Mobile Radio Engineering, CRC Press, 1993]

10.101-Cimini-7/98


29/99

BIT ERROR PROBABILITYEFFECTS OF DELAY SPREAD

810.102-Cimini-7/98

The rms delay spread imposes a limit on the maximum bit ratein a multipath environment.For example, for QPSK,

ISI causes an irreducible error floor.

Maximum Bit RateMobile (rural) 25 sec 8 kbpsMobile (city) 2.5 sec 80 kbpsMicrocells 500 nsec 400 kbpsLarge Building 100 nsec 2 Mbps

[J. C.-I. Chuang, "The Effects of Time Delay Spread on Portable RadioCommunications Channels with Digital Modulation," IEEE JSAC, June 1987]

+

x

+

+

+

+

+

x

x

x

x

x

10-210-4

10-3

10-2

10-1

10-1 100

BPSKQPSKOQPSKMSK

Modulation

Coherent Detection

IrreducibleP

b

T=

rms delay spreadsymbol period


30/99

SUMMARY OFMODULATION ISSUES

Tradeoffs

linear versus nonlinear modulation

constant envelope versus non-constant

envelope coherent versus differential detection

power efficiency versus spectral efficiency

Limitations

flat fading

doppler

10.103-Cimini-7/98

delay spread


31/99

HOW DO WE OVERCOME THELIMITATIONS IMPOSED BY THE

RADIO CHANNEL?

Flat Fading Countermeasures

Fade Margin

Diversity

Coding and Interleaving

Adaptive Techniques

Delay Spread Countermeasures Equalization

Multicarrier

Spread Spectrum

Antenna Solutions

10.104-Cimini-7/98


32/99

DIVERSITY

Independent signal paths have a low probabilityof experiencing deep fades simultaneously.

The basic concept is to send the sameinformation over independently fading radio

Independent fading paths can be achieved byseparating the signal in time, frequency, space,polarization, etc.

10.105-Cimini-7/98

The chance that two deep fadesoccur simultaneously is rare.

0

0

-20

-40

-60

-80

-1004 8 12 16 d

ReceivedSignalPower

(dBm)


33/99

DIVERSITY COMBINING TECHNIQUES

Selection Combining: picks the branch with thehighest SNR.

Equal-Gain Combining: all branches are coherentlycombined with equal weights.

Maximal-Ratio Combining: all branches are coherentlycombined with weights which depend on the branchSNR.

810.106-Cimini-7/98

Combiner

Output

1 2 3 M


34/99

DIVERSITY PERFORMANCE

The output SNR with Maximal-Ratio Combining improveslinearly with the number of diversity branches, M thecomplexity becomes prohibitive.

There is dramatic improvement even with two-branchselection combining.

10 dB reduction in required SNR for 1% outage less transmitted power or higher bit rates or largercoverage area

5

2

10-6

10-5

5

2

10-4

5

2

10-3

5

2

10-2

5

2

10-1

5

10 15 20 25 30 35 40

Pb

b, SNR/bit, dB

M = 4

M = 2

M = 1

Maximal

RatioCombining

( )1margin

Pout

-400.01

0.02

0.05

0.1

0.2

0.5

1.0

2.0

5.0

10.0

20.030.040.050.060.0

99.99

99.999.598.0

90.080.070.0

-30 -20 -10 0 10

MaximalRatio

EqualGain

Selection

10log

M = 2

22.014-Cimini-9/97


35/99

CHANNEL CODING

Channel coding reduces Pb by introducing redundancy

in the transmitted bit stream.

Block and convolutional codes acheive this improvementat the expense of increased signal bandwidth or a lowerdata rate.

Bit error probabilityAWGN channel

822.015-Cimini-9/97

Fading causes burst errors. If the fading is slow enoughrelative to the symbol rate, coding will not be effective.

For Pb = 10-6

Uncoded 10.5 dB

Hamming 10.0 dBBCH 6.5 dB

Conv. 5.0 dBPb

010-7

10-2

10-4

10-5

10-6

10-3

2 4 6 8 10 12 14

5

2

5

2

5

2

5

2

5

2

Uncoded

Hamming(7,4,1)

BCH(127,64,10)

Conv.1/2 rate

(k=7)

BPSK

b, SNR/bit, dB


36/99

CODING PERFORMANCEFADING CHANNEL

2810.17-Cimini-7/98

Pb performance for the IS-136 rate-1/2 convolutionalcode on a simulated mobile radio channel (hard-

decision decoding).

Negligible coding gain if fading is slow comparedto bit rate interleaving

Iyengar and J. Michaelides, "Performance Evaluations of RLPs (Radio Link Protocols)

r TDMA Data Services," ITIA Contribution TR45.3.2.5/93.03.30.10, Chicago, March 30, 199

1

10-1

10-2

10-3

10-4

Pb

8 10 12 14 16 18 20

Uncoded50 km/hr

Coded1 km/hr

Coded8 km/hr

Coded50 km/hr

Coded100 km/hr

b, SNR/bit, dB


37/99

CODING PERFORMANCEFADING CHANNEL

Pb performance for the IS-136 rate-1/2 convolutionalcode on a simulated mobile radio channel (softdecision decoding).

810.18-Cimini-7/98

Iyengar and J. Michaelides, "Performance Evaluations of RLPs (Radio Link Protocols)

r TDMA Data Services," ITIA Contribution TR45.3.2.5/93.03.30.10, Chicago, March 30, 199

1

Uncoded50 km/hr

Coded1 km/hr

Coded8 km/hr

Coded50 km/hr

Coded100 km/hr

10-1

10-2

10

-3

10-4

10-5

Pb

8 10 12 14 16 18 20

b, SNR/bit, dB


38/99


39/99

Trellis Codes reduce Pb without bandwidth expansion

through joint design of the channel codeand signal constellation

can be designed with built-in time diversity

Turbo Codes exhibit enormous coding gains interleaving inherent to code design

very complex with large delays not well-understood for fading channels

ADVANCED CODING TECHNIQUES

810.19-Cimini-7/98


40/99

CODING PERFORMANCE TCM

b1,R=2/3

10-1

10-2

10-3

10-4

10-5

10-6

10 12 14 16 18 20 22

Es/N0 (dB)

Pb

Uncoded4 PSK

UngerboeckCode

R=2/3, M=4

LSB

b2,R=2/3

b3,

R=2/3

MSB

8PSK TCM

810.69-Cimini-7/98


41/99

Adaptive Modulation

Automatic Repeat Request

ADAPTIVE TECHNIQUES

810.21-Cimini-7/98


42/99

Power and/or data rate adapted at transmitter tochannel conditions

Potential for large increase in spectral efficiency

Can be combined with adaptive compression

requires reliable feedback channel and accuratechannel estimation

increases transmitter and receiver complexity

810.22-Cimini-7/98

ADAPTIVE MODULATION

AdaptiveModulationand Coding

PowerControl

Demodulationand Decoding

ChannelEstimate+

Delay

TRANSMITTER RECEIVER

FEEDBACK CHANNEL

noise

Channel


43/99

Method of "self-adapting" the data rate tothe channel conditions

Used in combination with error-detecting code

Variations of ARQ used in Mobitex and CDPD

Types: Stop-and-Wait, Go-Back-N, Selective-

Repeat

22.023-Cimini-9/97

power and spectrally inefficient

impacts higher layer protocols

necessary for meeting stringent Pb

requirements or data

AUTOMATIC REPEAT REQUEST (ARQ)


44/99

DELAY SPREAD COUNTERMEASURES

Signal Processing at the receiver, to alleviate the problems

caused by delay spread (equalization)

at the transmitter, to make the signal less

sensitive to delay spread (multicarrier,spread spectrum)

Antenna Solutions change the environment to reduce, or

eliminate, the delay spread (distributedantenna system, small cells, directive

antennas)

22.024-Cimini-9/97


45/99

EQUALIZER TYPES AND STRUCTURES

The goal of equalization is to cancel the ISI

or, equivalently, to flatten the frequency response.

[J. G. Proakis, "Adaptive Equalization for TDMA Digital Mobile Radio,"IEEE Trans. on Veh. Tech. , May 1991]

Equalizer

Nonlinear

ML SymbolDetector

DFELinear MLSE

TransversalTransversal

ChannelEstimator

Types

Structures

LatticeTransversal Lattice

10.107-Cimini-7/98


46/99

LINEAR EQUALIZER

Equalizer

Heq(f)1

Hc(f)

Channel

Hc(f)

n(t)

10.108-Cimini-7/98

A linear equalizer effectively inverts the channel.

The linear equalizer is usually implemented as atapped delay line.

On a channel with deep spectral nulls, this equalizerenhances the noise.

poor performance on frequency-selective

fading channels


47/99

DECISION FEEDBACK EQUALIZER

The DFE determines the ISI from the previously detectedsymbols and subtracts it from the incoming symbols.

This equalizer does not suffer from noise enhancementbecause it estimates the channel rather than inverting it.

The DFE has better performance than the linear

equalizer in a frequency-selective fading channel.

The DFE is subject to error propagation if decisions aremade incorrectly.

Decisions are made on coded symbols. no coding gain

822.025-Cimini-9/97

Hc(f)Forward

Filter

n(t)

x(t)

DFE

Feedback

Filter

+

-

x(t)^


48/99

MAXIMUM LIKELIHOODSEQUENCE ESTIMATION

10.109-Cimini-7/98

MLSE has theoretically optimum performance.

It requires knowledge of the channel parameters and

the noise distribution.

The implementation complexity grows exponentially

with the length of the channel impulse response

not practical for high bit rates.


49/99

EQUALIZER ISSUES FORHIGH-SPEED WIRELESS DATA

The number of required equalizer taps, N, is proportionalto the delay spread.

The equalizer taps must be adapted at the highestDoppler rate.

The length and periodicity of the training sequence

impacts the spectral efficiency.

There is a tradeoff between speed of convergence

and complexity.

Number ofAlgorithms Multiply Convergence Advantages Disadvantages

(for DFE) Operations

Least Mean 2N + 1 ~10-100N Low computational Slow convergence,Square (LMS) complexity depends on

channel

Kalman 2.5N2

+ 4.5N ~N Fast convergence, HighRecursive Least good tracking ability computational

Squares (RLS) complexity

Square Root 1.5N2

+ 6.5N ~N Better stability Highthan Kalman computational

complexity

Fast Kalman 20N + 5 ~N Fast convergence Could beand good tracking unstable

22.026-Cimini-9/97


50/99

EQUALIZER PERFORMANCE

Pahlavan has shown that, for 30-meter cells ( = 50 ns), 20 Mb/scan be achieved using a DFE with 3 forward taps and 3 feedback taps.

[K. Pahlavan, S. J. Howard, and T. A. Sexton, "Decision Feedback Equalizationof the Indoor Radio Channel," IEEE Trans. on Commun., January 1993]

810.110-Cimini-7/98

BPSK

25

10-6

10-5

10-4

10-3

10-2

10-1

1

30 35 40 45 50

no equalizerDFE

10 Mbps

5

110

.1

.15

1

SNR (dB)

Pb

Target Pb

BPSK

1

10-4

10-3

10-1210-810-4

10-2

10-1

1

no equalizerDFE

16 Mbps

4,16

8

1

1

8

.1

.1,4

Pout


51/99

MULTICARRIER MODULATION

The transmission bandwidth is divided into manynarrow subchannels which are transmitted inparallel.

Ideally, each subchannel is narrow enough sothat the fading it experiences is flat no ISI.

10.111-Cimini-7/98

RF

Transmitter

R/N b/s

D(t)

f0

f1

fN-1

dN-1

(t)

QAM filter

R/N b/sQAM filter

R/N b/s QAM filter

(t)d0

(t)d1

Bandlimitedsignals

f0 f1 f2

f0

fN-1

f1

N-1

Receiver

RF

filter

QAM

QAM

QAM

filterf1

f0

filterfN-1


52/99

OFDM RECEIVER STRUCTURE

Subchannel Separation choose fn = f0 + n f, with f =

integrate over NT, then d(m) = d(m)

1

NT

^

Efficient FFT Implementation

A guard interval can virtually eliminate ISI(or, interblock interference) lower spectral

or power efficiency.

10.113-Cimini-7/98

parallelto

serialconverter

QAM

f0

f1

fN-1

d(0)Receiver

d(1)

d(N-1)

RF


53/99

WHAT TO DO WITHBAD SUBCHANNELS?

Coding Across Subchannels works bestwith large delay spread

Frequency Equalization requires accuratechannel estimation

Adaptive Loading requires reliablefeedback channel and accurate channel

estimation

10.114-Cimini-7/98


54/99

MULTICARRIER MODULATIONISSUES FOR HIGH-SPEED

WIRELESS DATA

Minimal training is required.

Time-varying fading, frequency offset, and timing

mismatch impair the orthogonality of the

subchannels.

Large peak-to-average power ratio is a serious

problem when transmitting through a nonlinearity.

possible solutions: nonlinear coding,

clipping and filtering

10.115-Cimini-7/98


55/99

CURRENT AND PROPOSEDAPPLICATIONS OF OFDM

Asymmetric Digital Subscriber Line

Digital Audio Broadcasting

Wireless LAN

Digital Terrestrial Television

High Speed Cellular

10.116-Cimini-7/98


56/99

SPREAD SPECTRUM

10.117-Cimini-7/98

Spread spectrum increases the transmit signalbandwidth to reduce the effects of flat fading,

ISI and interference.

SS is used in all wireless LAN products in the ISM

band required for operation with reasonable power

levels

minimal performance impact on other systems

IEEE 802.11 standard

There are two SS methods: direct sequence andfrequency hopping.

Direct sequence multiplies the data sequence

by a faster chip sequence.

Frequency hopping varies the carrier

frequency by the same chip sequence.


57/99

DIRECT SEQUENCESPREAD SPECTRUM

Modulator

Transmitter

Channel

Spreading(PN) Code

Tc


58/99

RAKE RECEIVER

When the chip time is much less than the rms delay spread,each branch has independent fading equivalent to

diversity combining.

When the chip time is greater than the rms delay spread,the paths cannot be resolved no diversity gain.

10.119-Cimini-7/98

CoherentCombiner Demodulator

DataOutputReceivedSignal

sc(t)

sc(t-Tc)

sc(t-2Tc)

sc(t-TM)


59/99

PERFORMANCE OF RAKE RECEIVERFADING CHANNEL

810.27-Cimini-7/98

0.5

10-1

10-2

10

-3

10-4

10-5

Pb

0 5 10 15

b, SNR/bit, dB

Rayleigh

DPSK

AWGN

RAKE


60/99

SPREAD SPECTRUM ISSUESFOR HIGH-SPEED WIRELESS DATA

810.120-Cimini-7/98

Hardware Complexity synchronization high processing speeds for high

bit rates RAKE receiver

High Required Bandwidth to AccommodateSpreading

Spread spectrum is difficult athigh bit rates and not reallyneeded.


61/99

Goal: Reduce (or eliminate) delay spread

Distributed Antenna System

Very Small Cells antenna in every room

Sectorization

Directive Antennas/Beam Steering

Omnidirectional Sectorized Directive

90

270

180

150

120

30

300240

210 330

60

0

90

270

180

150

120

30

300240

210 330

60

0

90

270

180

150

120

30

300240

210 330

60

0

ANTENNA SOLUTIONS

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DISTRIBUTED ANTENNA SYSTEM

00

10

.5

1

20 30 40 50

RMS Delay Spread (ns)

Distributed

Monopoles

CentralMonopole

Pr

obabilityAbscissa

Exceeded

[A. A. M. Saleh, A. J. Rustako, Jr., and R. S. Roman, "Distributed Antennas

for Indoor Radio Communications," IEEE Trans. on Commun., December 1987]

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EXAMPLES OFPERFORMANCE IMPROVEMENTS

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High-Speed Narrowbeam Antenna Experiment[P. F. Driessen "Gigabit/s Indoor Wireless Systems withDirectional Antennas," IEEE Trans. on Comm., August 1996]

directional antennas (15 beamwidth) at both ends ofLOS link

no equalization 622 Mbps BPSK transmission without errors

Sectored Antennas [G. Yang and K. Pahlavan, "ComparativePerformance Evaluation of Sector Antenna and DFE Systems

in Indoor Radio Channels," Proc. of ICC '92] 6 sectors at base and mobile

best combination chosen for Pout = 0.01, 5 Mbps with omni, 25 Mbps with

sectored antenna

Six Sector Antennas

Target Pb

Pout

30 Mbps

20 Mbps

10 Mbps

10010-1 10-210-3 10-410-510-610-710-810-9 10-10-1210-1110-10

.0005

.002

.001

.005.01

.02

.0001

.001

.01

.1

1

Omnidirectional Antennas

out

30 Mbps20 Mbps

10 Mbps

5 Mbps3 Mbps2 Mbps

1 Mbps

Target Pb

10010-1 10-210-3 10-410-510-610-710-810-9 10-1310-1210-1110-10


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SUMMARY OF COUNTERMEASURES

Diversity

Coding and Interleaving

Adaptive Techniques

Equalization

Multicarrier

Spread Spectrum

Antenna Solutions

These techniques can be combined.

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COMBINED EQUALIZATION ANDSECTORED ANTENNAS

[G. Yang and K. Pahlavan, "Comparative Performance Evaluation of Sector

1

.1

20 40 60

.01

.001

.0001

Square room length (meter)

Pout

Omni

Omni+DFE

Sector

Sec+DFE

Pt = 100 mWRb = 20 Mbps

1

.1

20100 30 40 50

.01

.001

.0001

Rb (Mbps)

Pout

Omni

Omni+DFE

Sector

Sec+DFE

30mx30m

Antenna and DFE Systems in Indoor Radio Channels," Proc. of ICC '92]

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CHANNEL ACCESS ISSUES

Multiple Access

Random Access

Frequency Reuse

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MULTIPLE ACCESS TECHNIQUES

Frequency Division (FDMA)

Time Division (TDMA)

Code Division (CDMA)

Hybrid Approaches

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FDMA

The total system bandwidth is divided intochannels which are allocated to the differentusers.

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Code Space

Time

Frequency


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TDMA

Time is divided into slots which are allocatedto the different users.

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Code Space

Time

Frequency


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CDMA

Time and bandwidth are used simultaneously bydifferent users, modulated by orthogonal or semi-orthogonal codes (e.g. spread spectrum).

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Code Space

Time

Frequency


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IMPLICATIONS FOR HIGH-SPEEDWIRELESS DATA

Perform well with continuous stream traffic butinefficient for bursty traffic

ComplexityFrequency Division < Time Division < Code Division

Multiple Data Rates multiple frequency bands

multiple timeslots

multiple codes

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ALOHA

Carrier-Sense Techniques

Reservation Protocols

Implication for High-SpeedWireless Data

RANDOM ACCESS TECHNIQUES

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ALOHA

Data is packetized.

Retransmission is required when packets collide.

Pure ALOHA send packet whenever data is available a collision occurs for any partial overlap of

packets

Slotted ALOHA send packets during predefined timeslots avoids partial overlap of packets

Comments inefficient for heavily loaded systems capture effect improves efficiency combining SS with ALOHA reduces collisions

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.40

.30

.20

.10

0 0.5 1.0 1.5 2.0 3.0

G (Attempts per Packet TIme)

S(Throughputper

PacketTime

)

Slotted Aloha

Pure Aloha


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CARRIER-SENSE TECHNIQUES

Channel is sensed before transmission to determineif it is occupied.

More efficient than ALOHA fewer retransmissions

Carrier sensing is often combined with collisiondetection in wired networks (e.g., Ethernet).not possible in a radio environment

Collision avoidance is used in current wireless LANs.(WaveLAN, IEEE802.11, Spectral Etiquette)

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Wired Network

Busy Tone

Wireless Network


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DemandBased Assignment a common reservation channel is used to

assign bandwidth on demand reservation channel requires extra bandwidth very efficient if overhead traffic is a small

percentage of the message traffic

Packet Reservation Multiple Access (PRMA) similar to reservation ALOHA uses a slotted channel structure all unreserved slots are open for contention a successful transmission in an unreserved

slot effectively reserves that slot for futuretransmissions

RESERVATION PROTOCOLS

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EXAMPLES

ARDIS slotted CSMA

RAM Mobile Data slotted CSMA

CDPD DSMA/CD - Digital Sense Multiple Access collisions detected at receiver and

transmitted back

WaveLAN CSMA/CA

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Retransmissions are power and spectrallyinefficient.

ALOHA cannot satisfy high-speed data

throughput requirements.

Reservation protocols are also ineffectivefor short messaging.

Delay constraints impose throughputlimitations.

IMPLICATIONS FOR HIGH SPEEDWIRELESS DATA

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Reuse Distance (D)distance between cells using the same

frequency, time slot, or code

smaller reuse distance packs more usersinto a given area, but also increases their

co-channel interference

Cell Radiusdecreasing the cell size increases system

capacity, but complicates the networkfunctions of handoff and routing

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DESIGN CONSIDERATIONS


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CHANNEL ASSIGNMENT

Fixed Channel Assignment (FCA) each cell is assigned a fixed number

of channels

channels used for both handoff andnew calls

Reservation Channels with FCA each cell reserves some channels for

hand off calls

Channel Borrowing a cell may borrow free channels fromneighboring cells

Dynamic Channel Assignment

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Interference Averaging (CDMA)

Interference Reduction

(power adaption, sectorization)

Interference Cancellation(smart antennas, multiuser detection)

Interference Avoidance(dynamic resource allocation)

METHODS TO IMPROVESPECTRUM UTILIZATION

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Ad-Hoc Networks

Each node generates independent data.

Source-destination pairs are chosen at random.

Routing can be multihop.

Topology is dynamic Generally a fully connected network with

different link SNRs

Can allocate resources dynamically (rate, power,

BW, routes,)


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NETWORK ISSUES

Network Architectures

Mobility Management

Network Reliability

Internetworking

Security

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NETWORK ARCHITECTURES

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Implications for High-Speed Wireless Data single hop versus multiple hops

static versus dynamic topology

single points of failure

Hierarchical/Tree

Star

Ad-Hoc


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NETWORK CONTROL

Centralized RAM Mobile Data

CDPD

Altair

Distributed/Peer-to-Peer WaveLAN

Implications for High-Speed Wireless Data less channel estimation required with

centralized control increases efficiency

of packet transmission

centralized control provides more efficient

resource management with setup-time overhead

an extensive infrastructure is not required for

distributed control

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MOBILITY MANAGEMENT

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Location Management identification and authentication

home and visitor location data bases (cellular)

discovery and registration (Mobile IP)

Routing fixed data bases (connection-oriented)

Mobile IP (connectionless)

tree (virtual connection)

Handoff

transmissions may be delayed or dropped impacts higher layer protocols

multi-homing inefficient use of resources

overhead and delay impact throughput

suboptimal (triangle) routing delay

inefficiency and higher congestion


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NETWORK RELIABILITY

End-to-End connection is composed of many

wireless/wired hops.

widely varying data rates

high BERs on some/all hops

large, varying latencies

user mobility causes hop characteristics

to vary

Problem with reliability protocols like TCP. wireless losses mistaken for congestion

bulk losses cause timeouts large round-trip time variances and

asymmetric channels

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APPROACHES TONETWORK RELIABILITY

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Local (link-layer) solutions Forward error correction does not work well in fading

ARQ introduces large latency

End-to-end solutions Difficult to distinguish if packet loss due to congestion

or link quality

Difficult to design for changing hop characteristics

End-to-end performance guarantees are difficultto make

Potential solutions Hierarchical/layered coding of voice/video/images

Different Quality-of-Service classes

Application awareness Local solution with end-to-end awareness

Requires interaction between all layers


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QUALITY OF SERVICE (QoS)

Traffic dependent performance metrics required fortype of data transmitted

bandwidth

latency

likelihood of packet (message) loss

Categories guaranteed

predictive

best effort

Implications for high speed wireless data QoS performance generally based on switched,

fiber-optic, wired networks

wireless links have high Pb and high latency due

to link layer retransmission and unpredictable

link bandwidths

QoS guarantees and predictions are difficult to

make for wireless networks it is not clear

that the best effort is good enough for most

applications

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INTERNETWORKING

TCP/IP Compatible with existing wired networks

Works well over large range of wired subnet

performance

TCP has problems operating over wireless links

Wireless ATM ATM is emerging standard for multimedia

transmission over wired networks

ATM protocol based on links with 10-10 BER

and Mbps/Gbps data rates

high overhead in packet structure

QOS guarantees

Not clear that ATM protocol can be modified

for wireless links

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STANDARDS AND FUTURE SYSTEMS

Bluetooth

Wireless LANs

High-Speed Digital Cellular (3G)

4G Cellular

Wireless "Cable" Multichannel Multipoint Distribution

Service (2.2 GHz) Local Multipoint Distribution

Service (28 GHz)

Satellite Networks- Iridium, Globalstar, Others

HomeRF

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BLUETOOTH

Cable replacement RF technology

Short range (10 meters)

2.4 GHz band

1 Data (700 Kbps) and 3 Voice channels

Supported by over 200 telecommunications

and computer companies

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802.11b: standard for 2.4 GHz ISM band

Frequency hopped spread spectrum

1.6 Mbps data rates, 500 foot range

Star or peer-to-peer architecture

802.11a extends rates to 10-70 Mbps

Extensions trying to add QoS

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802.11 Wireless LANs


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HIPERLAN

Types 1-4 for different user types- Frequency bands: 5.15-5.3 GHz, 17.1-17.3 GHz

Type 1

- 5.15-5.3 GHz band- 23 Mbps, 20 MHz Channels- 150 foot range (local access only)- Protocol support similar to 802.11- Peer to peer architecture

- ALOHA channel access

Types 2-3

- Wireless ATM- Local access and wide area services- Standard under development

- Two components: access andmobility support

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HIGH-SPEED DIGITAL CELLULAR

North American Digital Cellular CDMA (IS-95) enhancements TDMA (IS-136) enhancements IS-136+ 32-64 kbps IS-136HS 384 kbps

GSM General Packet Radio System (GPRS) Enhanced Data Rates for GSM Evolution

(EDGE)

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WIDEBAND CDMA (3G)

The W-CDMA concept:

4.096 Mcps Direct Sequence CDMA

Variable spreading and multicode operation

Coherent in both up-and downlink

= Codes with different spreading,giving 8-500 kbps

f

t

10 ms frame

4.4-5MHz

High ratemulticode user

Variable rate users

...

.P

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W-CDMAKEY TECHNICAL FEATURES

High bit-rate services require wideband

Flexibility for different services

Optimized for packet data transfer

Capacity and coverage gain from frequencydiversity

Built in support for adaptive antenna arrays

multi-user detection hierarchical cell structures transmitter diversity

Low infrastructure cost (many users/transceiver)

BS synchronization not required

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The desire for mobility coupled with the demandfor Internet and multimedia services indicate abright future for wireless data.

Current products and services have unsatisfactoryperformance for high-speed wireless data applications.

The inherent limitations of the radio channel can besignificantly reduced using signal processing andarchitectural techniques, at the expense of costand complexity.

The network-level design must take into accountthe physical layer limitations of the wireless channel,as well as the impact of user mobility.

SUMMARY

Documents

Andrea Goldsmith