Rohling Pr

OFDMbased Systems and related Multiple

Access Schemes

Antalya, July 2005

Hermann Rohling Technical University of Hamburg-Harburg

Department of TelecommunicationsEißendorfer Straße 40

D-21073 Hamburg

Department for Telecommunications 2

TUHHTechnical University of Hamburg-Harburg

Department of TelecommunicationsProf. Dr. rer. nat. H. Rohling

UltrasoundCellular NetworksCellular Networks

UltrasoundBroadband MobileCommunications

UltrasoundSelf-OrganizingWireless Networks

UltrasoundUltrasoundMulti Sensor Systems

Automotive Radar


Overview• Requirements of 4G Systems

• The Broadband Radio Channel

• OFDM Basics

• OFDM System Building Blocks• Modulation: Coherent, Incoherent, Adaptive• Channel Estimation: Pilot-based, Blind• Channel Coding• Synchronisation

• OFDM for Multi-User Communications

• OFDM System Design and Performance

• Advanced OFDM Techniques• Joint Layer Optimization• MIMO• Cellular Environment: Synchronisation, Radio Resource Management


Evolution of Mobile Communications

Technique

Data Rate Systems

1G Analogue < 300 bps AMPS, NMT, …2G Digital 9.6k – 64kbps GSM, PDC, IS-95, …3G CDMA 64kbps -

2MbpsW-CDMA, TD-CDMA, …

4G ??? 2M - 20Mbps ?

1970 1980 1990 2000 2010

1G

2G

3G

4G

Research Deployment1969 1981

1982

1991 2002

Today

1990


Requirements for Future Systems

Mobility

Data rate [Mbps]

stationary

pedestrian

vehicular

0.1 1 10 100

3rd Generation(IMT-2000)

WirelessLAN

4th Generation


Packet-based Data Streams

circuit data packet data


General Requirements on Future Systems

• High spectral efficiency

• Support of high user mobility

• High flexibility to deal with a broad range of user and traffic scenarios

• React to changing transmission environments by a high adaptivity


The Broadband Radio Channel


Multipath Propagation (Power Delay Profile)

Transmitter

Receivert

h(t)

[dB

]

max

Propagation paths

1,1

2,23,3


The Linear Time-Invariant (LTI) Radio Channel

• Behaviour of multipath propagation with no movement is characterized by a LTI-system in the equivalent lowpass domain:

Radio Channel )(trT)(tsT )(Th

• The channel impulse response is given by

L

lllTT hh

1,)(

L

l

fjlTT

lehfH1

2,)(

• HT(f) denotes the channel transfer function

dthstr TTT )(


Narrowband Channel

• Channel transfer function is assumed to be constant over the signal bandwidth B

0

t

thT

max

STB 1 constfHT for 2Bf with

f

fHT

2B

2B

• Symbol duration TS is much larger than the maximum channel tap delay:

No frequency-selective fading! No Inter Symbol Interference (ISI)!

maxST


Broadband Channel

• Symbol duration TS is much smaller than the maximum channel tap delay

maxST

• Channel transfer function HT(f) fluctuates over signal bandwidth B

f

fHT

2B

2B Frequency-selective fading, ISI occurs!

• Quasi time-invariance during a small time interval( → coherence time TC)


Frequency Selectivity ISI

• Frequency Selectivity • Inter Symbol Interference

0 1 2 3 4 5 6 7 8 9 10-30

-20

-10

0

10

|H(f)

| / d

B

sTS 1max

0 1 2 3 4 5 6 7 8 9 10-30

-20

-10

0

10

|H(f)

| / d

B

f / MHz

sTS 55max f / MHz

h

dB0

dB30ST ST2 ST3 ST4 ST5

h

dB0

dB30ST ST2 ST3 ST4 ST5

0

0


User Mobility (Doppler Profile)

Frequency-fD,max +fD,max

Independent propagation paths

v

Line-of-sight path

Mobile with omni-directional antenna

Doppler profile

“Jakes Doppler Profile”

Angle of incident


The Time-Variant Radio Channel

• Behaviour of multipath propagation with movement is characterized by a linear time-variant system in the equivalent lowpass domain:

Radio Channel )(trT)(tsT ) ,( TT Tr t h ts t d

),( thT

• The time-variant channel impulse response is given by

,1

( , )L

T l ll

T h th t

2,

1

( , ) ljT

Lf

T ll

h tt eH f

• HT(f,t) denotes the time-variant channel transfer function


Time-Variant Transfer Function

Channel Parameters

B = 20 MHz

Non Line-of-Sight (NLOS)

Exp. Power Delay Profile

max = 0.8 ms

Jakes Doppler Profile

fD,max = 15 Hz

3 km/h @ 5.5 GHz


How Broad is Broadband ?|H(f)|

Frequency B/2-B/2

The sampling time of a broadband system T=1/B is much smaller than the maximum multipath delay of the channel max

1/B << max

The channel transfer function |H(f)| fluctuates over the system bandwidth B

Frequency selective fading Inter symbol interferences (ISI)


OFDM Basics


Single-Carrier Transmission with increasing Data Rates

h(t)

max

Example 1: Data Rate = 90 kbps BPSK, Bandwidth = 90 kHz

ISI effects 90% of a single Symbol Easy to equalize!

Data 1 Data 2 Data 3 Data 4

max = 10 s

Symbol Duration

Example 2: Data Rate = 1 Mbps BPSK, Bandwidth = 1 MHz

ISI effects 10 adjacent Symbols! Equalization becomes very complex!

h(t)

max D3 D4 D2D1


Multi-Carrier Transmission: Basic Idea Bandwidth is splitted in N narrowband subchannels

dB0

dB30

dB10

dB20

dB10

fH

f

f

Each subcarrier is flat faded. Channel influence can be described by a complex valued factor for each subcarrier

Example: Splitting of a broadband channel into N=32 subchannels

„narrowband“subchannel


Multi-Carrier Transmission: Advantage

Serial Transmission (Single Carrier):

• Maximum multipath delay max >> Symbol duration TSC

Inter-Symbol Interferences (ISI) Complex time domain equalizer

t

h(t)

max

t

s(t)T

Parallel Transmission (Multi-Carrier):

• Maximum multipath delay max << Symbol duration TOFDM

No Inter-Symbol Interferences (ISI) Simple frequency domain equalizer


Multi-Carrier Transmission: Comparison

• Data Rate: 10 MBit/s• BPSK transmission Bandwidth B=10 MHz• Multipath channel with maximum delay max = 10 s

Single-CarrierSymbol duration depends directly on system bandwidth:

TSC = 0.1 s = max /100

ISI extends over 100 symbols!

OFDMLarge Number of Subcarriers: N = 1000OFDM symbol duration: TOFDM = TSC N = 10 max

Required guard interval: TGuard max = 0.1 TOFDM

ISI free transmission!


OFDMOrthogonal Frequency Division Multiplexing


f

S n (f)

OFDM Transmission Technique


OFDM Transmission Technique - Transmitter

• Time-continuous signal of the ith OFDM block

mcs

mcsN

k

ftkjkii T

iTtrecteS

Nts

,

,1

0

π2,

1

• Time-discrete signal of the ith OFDM block

1

0

π2,,

1 N

k

tfnkjkiini eS

Ntnss

1

0

π2

,,1 N

k

Nnkj

kini eSN

s

NNT

Ttf mcs

mcs

11 ,

,

(IDFT)

with


OFDM Transmission Technique - Channel

Transmitted signal:

Received signal:

1

0

π2

,,1 N

k

Nnkj

kini eSN

s

Influence of the linear time-(in)variant radio channel

1

0

π2

,,,1 N

k

Nnkj

kikini eHSN

r

Linear Channel

orthogonalsubcarriers

orthogonalsubcarriers

Frequency| H(f )|

linear operations

Eigenfunctions of the channel

Subcarrier-wise Channel Transfer Factors


OFDM Transmission Technique - Receiver

• Received time-continuous signal of the ith OFDM block

)()()( tnthtstr iiii • Time-Domain:

)()()()( fNfHfSfR iiii

SynchronisationDemodulator FFT tri)( fRi

• Frequency-Domain:

• Received time-discrete signal of the ith OFDM block

nininini nhsr ,,,, • Time-Domain:

nininini NHSR ,,,, • Frequency-Domain:


Why Guard Interval?

Time

1

Path 1Data 1 Data 2

Path 2Data 1 Data 2

Path 3 Data 1 Data 2

Path PData 1 Data 2

P

FFT window

Without

Guard Interval

With

Guard Interval

Time

1

Path 1GI Data 1 GI Data 2 GI

Path 2GI Data 1 GI Data 2 GI

Path 3 GI Data 1 GI Data 2 GI

Path PGI Data 1 GI Data 2 GI

P

FFT window

Data 3

Data 3

Data 3

Data 3


OFDM Spectrum

Frequency

OFD

M S

pect

rum

Subcarrier spacingf

kk-1 k+2k-2 k+1

fkfT

fkfTTfGk

sin)(


OFDM Spectrum

10

0

-10

-20

-30

-40-100 -50 0 50 100 150 200

Spec

tral

Pow

er D

ensi

ty [

dB]

f / f


Single-Carrier vs. Multi-Carrier Systems

Data Rate (System Bandwidth)

Max

. Mul

ti-P

ath

Del

ay

Single-Carrier

Multi-Carrier

Digital Radio Mondial

xDSL

DVB-T

WLAN

4G


OFDM Based Systems

Wireline WirelessBroadcast:

Digital Audio Broadcasting (DAB) Digital Video Broadcasting (DVB-T) Digital Radio Mondial (DRM) Terrestrial repeaters for U.S. satellite Digital Audio Radio Service (SDARS) …

Communications:

HIPERLAN/2 IEEE 802.11a IEEE 802.16 Home RF …

Communications:

Digital Subscriber Line (xDSL) Power Line Communications (PLC) Cable TV Network (CATV / MMDS) …


OFDM

System Structure


OFDM System Structure

Rn.K-1

Bitstream

Pilot symbols

inverseFFT

P

S

Guardint. DA

Channel

+AWGN

ADWindow

P

S

FFT

De-mapping

Demodu-lation

Depunct.Deinterl.

Viterbidecoding

Bitstream

Channel EstimationEqualisation

Sn.0

Sn.K-1

sn,m

en(t)

rn(t)

rn,m

Rn.0

Transmitter

Receiver

PuncturingInterleaving

Modulation

Mapping

ChannelCoding

Synchronization


Digital Modulation Schemes

Rn.K-1

Bitstream

Pilot symbols

inverseFFT

P

S

Guardint. DA

Channel

+AWGN

ADWindow

P

S

FFT

De-mapping

Demodu-lation

Depunct.Deinterl.

Viterbidecoding

Bitstream


Sn.0

Sn.K-1

sn,m

en(t)

rn(t)

rn,m

Rn.0

Transmitter

Receiver


Modulation

Mapping

ChannelCoding

Synchronization


Representation in Signal Space

Constellation diagram

TsRe

TsIm

• Real part („inphase component“) and imaginary part („quadrature component“) of the baseband signal can be depicted in one two-dimensional diagram


• Modulation symbols usually plotted as fixed points in diagram

• Constellation diagram is very convenient for representation of linear modulation schemes

1011 0010 0011

0000 0001

0100 0101

0110 0111

1010

1001 1000

1101 1100

1111 1110


Amplitude Shift Keying (ASK)

Bandpass signal Example: 4-ASK

t

0

ST

ts


Trs

Tis

n

SnT nTtpIts

Baseband signal

0

tsTr

t

t

0

tsTi

t

10 11 01 00 11

00 01 11 10

21rect

STttp


Phase Shift Keying (PSK)

Bandpass signal Example: QPSK

t

0

ST

ts


Trs

TisBaseband signal

0

tsTr

t

t

0

tsTi

t

1

-1

1

-11-1

10 11 01 00 11

00 11

10

01

n

SnT nTtpIts

21rect

STttp


Amplitude and Phase Shift Keying (APSK)

Bandpass signal Example: 8-APSK

t

0

ST

ts


Trs

TisBaseband signal

0

tsTr

t

t

0

tsTi

t

101

110

011 111 001 100

010111100

101

001000 011

n

SnT nTtpIts

21rect

STttp


Coherent Modulation

cknkn

kn

knkn

kn

knckn DdecS

HN

SHR

D ,,,

,,

,

,,

ˆˆˆ

, , , ,n k n k n k n kR H S N

Received symbol on subcarrier k :

The equalization requires only a simple complex multiplication with the inverse channel transfer factor:

Channel transfer factors have to be estimated !


Differential Modulation

Differential modulation in time direction:

}1,,0|{ /2, MieB Mijkn M-DPSK signal constellation:

ncknkn

knknkn

knknknkn

kn

knnckn

DB

NHSNHBS

RR

D

,,

,1,1,1

,,,,1

,1

,,

decˆ

Incoherent demodulation:

knknkn QSS ,,1,


Differential Modulation: Example 8-DPSK


Differential Modulation in OFDM Systems

Time

Freq

uenc

y

Freq

uenc

y

Time

Time direction Frequency direction

Time

Freq

uenc

y

Time and frequency direction


Differential Modulation in OFDM Systems

Performance of differential modulation depends on the correlation of symbols

f/BC, t/TC

Cor

rela

tion

Func

tion

-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.10.95

0.96

0.97

0.98

0.99

1

1.01

Frequency correlation function

Time correlation function

When is differential modulation in frequency direction better ?f

m

ax

0.2 0.4 0.6 0.8 1.0

0.1

0.2

0.3

0.4

0.5

1.2

FREQ

TIME

0.00.0

TS fD,max

BER

maxmax, 2 ffT DOFDM


Higher Order Differential Modulation M-DAPSK

Amplitude Bits

Phase Bits

Amplitude Factor a

M < 16 1 M -

M = 16 2 8 2.0

M = 32 2 16 1.6

M = 64 4 16 1.4

M = 128 4 32 1.3

1,0,1,0| pa

PjA NPNAea

DAPSK signal constellation:

ma

ma

z-1

AmplitudeMapping

MappingPhase

S

MappingPhase

Modified

MappingAmplitude

SDifferentialEncoding

n

n

nnQ

B

am +m p(b , ... , b )m +1a n

am +m p(b , ... , b )m +1a n

(b , ... , b )1

(b , ... , b )1

n

n


Performance of Differential Modulation

10-4

10-3

10-2

10-1

100

12 14 16 18 20 22 24 26 28 30

Bit

Err

or R

ate

S/N [dB]

64-DAPSK, quasi-coherent64-DAPSK, non-coherent

64-QAM, coherent


OFDM gives the opportunity to use:

• different modulation schemes for each subchannel

• different power for each subchannel

Adaptive Modulation

|H(k)|

b(k)

10

1

0.1

k

2

0

4

6

10 30 50 9070 110 130 150 170 190

Adaptive Modulation

Adaptation to the channel transfer function using subchannel specific modulation schemes and power


Adaptive Modulation

Subcarriers

Ave

rage

SN

R

BPSKQPSK16QAM64QAM256QAM

Not used due to low SNR

Algorithms: Chow, Cioffi and Bingham: capacity maximization

Fischer: Error probability minimization

Grünheid: simple blockwise loading algorithm

Hughes-Hartogs: sets target rate R, intensive searching


Adaptive Modulation

-6 -4 -2 0 2 4 6 8 1010

-5

10-4

10-3

10-2

10-1

100

SNR (dB)

BE

R

adapt modulation

fixed modulation

1.5 dB

• Adaptive modulation (average 2 bits per subcarrier)

Bit loading by Fischer Algorithm


Channel Estimation

Rn.K-1

Bitstream

Pilot symbols

inverseFFT

P

S

Guardint. DA

Channel

+AWGN

ADWindow

P

S

FFT

De-mapping

Demodu-lation

Depunct.Deinterl.

Viterbidecoding

Bitstream


Sn.0

Sn.K-1

sn,m

en(t)

rn(t)

rn,m

Rn.0

Transmitter

Receiver


Modulation

Mapping

ChannelCoding

Synchronization


Pilot-Based Channel Estimation

PilotsVirtual pilots by time interpolationDesired transfer factor

OFDM symbols

Time Interpolation

Frequency Interpolation

Two-Dimensional Interpolation

Sub

carr

iers

Nf

Nt

kn

knkn S

RH

,

,,

Least-Squares Estimation:


Interpolation Methods

Linear interpolation

Second order interpolation

Low pass interpolation

Spline cubic interpolation

Time domain interpolation


Pilot-Based / Blind Channel Estimation

Time

Freq

uenc

y

ntf

nfTS

Time

Freq

uenc

y

Current OFDM symbol

FFT Soft/Hard- Decision Decoding

Interpolation / Estimation

Remove Modulation

Hk,i

Rl,j

Sk,i

A priori known pilot symbols

Rk,i =Sk,i Hk,i +Nk,i

„Decision Directed“ „Pilot Based“


Decision-Directed Channel Estimation

DecoderDiv Demod

Hard Decision

Div

FFT

Mod

, filter

kneqY,

knR ,

kneqY,

kH ,1~

knH ,ˆ

knH ,~

, 1, 1, ,ˆ(1 )( )n k n k n k n kH H H

, 1, , 1,ˆ(1 ) ( )n k n k n k n kH H ,

,

1,

ˆn k

n keq

n k

RY

H

kneq

knkn Y

RH

,

,,

ˆ

Subcarrierwise channel estimation with , filtering


Decision-Directed Channel Estimation

-0.1-1.0 0.1 1.0

1.0

0.1

-0.1

-1.0

Locating a reliability area in the constellation diagramm (QPSK)

, 1, 1,n k n k n kH H

, 1,n k n k

If received symbols are inside the reliable area:

, 1, 1, ,ˆ(1 )( )n k n k n k n kH H H

, 1, , 1,ˆ(1 ) ( )n k n k n k n kH H

If received symbols are outside the reliable area: (=0, =0)

Optimal values =0.1, =0.008

Subcarrierwise channel estimation with , filtering


Channel Coding

Rn.K-1

Bitstream

Pilot symbols

inverseFFT

P

S

Guardint. DA

Channel

+AWGN

ADWindow

P

S

FFT

De-mapping

Demodu-lation

Depunct.Deinterl.

Viterbidecoding

Bitstream


Sn.0

Sn.K-1

sn,m

en(t)

rn(t)

rn,m

Rn.0

Transmitter

Receiver


Modulation

Mapping

ChannelCoding

Synchronization


Channel Coding

Flat bit error rate curve in the Rayleigh channeldue to faded subcarriers

Channel Coding

Questions:• choice of appropriate codes

(spreading codes, block codes, convolutional codes, turbo codes)• optimal modulation schemes (coded modulation)• metrics for soft-decision decoding • decoding techniques

BERB P S K , R ayleigh-K analB P S K , A WG N -K anal

S /N

1 e-3

1 e-0

1 e-1

1 e-2

1 e-5

1 e-4

0 3 05 1 5 2 0 2 51 0

BPSK, Rayleigh channelBPSK, AWGN channel


Soft-Output Demodulation of Coherent Signals

knknknkn NSHR ,,,,

2

2,,,

22,, 2

1

knknkn SHR

knkn eSRp

kknknkn

kknkn

knknkn

SHR

SRp

SRPS

2,,,

,,

)(,,,

)(minarg

)(maxarg

maxargˆ

2

,,,, knknknkn SHR

Received modulation symbol:

Posteriori PDF is Gaussian:

Maximum Likelihood Sequence Estimation (MLSE):

Metrik information fed to the Viterbi decoder:


Soft-Output Demodulation of Incoherent Signals

2

2,,

2

2,,2

1

knkn

ep knkn

k

knknknkn H2

,,

2

,, )(minarg

2,1,1

2

2,,

222

2

2,,

,1

,,

,1

,,

2

2,,

2

1

ln,ln

knknknkn

w

VW

w

knkn

kn

knkn

kn

knkn

SHSH

eVWp

SS

VRR

W

w

knkn

2

,1

2

,

2,

2,,

2,,,

2,

2,,

11

1,)()()(

)(minargˆ

knkn

knknknknknkn

kknknkn

RRRIVWd

RIdB

2,

2,, knknkn RId

Metrik information for DAPSK fed to the Viterbi decoder:

Differential phase modulation (DPSK):

Differential amplitude modulation (DASK):

DAPSK:


Concatenation of Coding and Differential Modulation

Convolutional Coding Interleaver Differential

Modulation

Convolutional Coding Interleaver Differential

CodingNon-Diff.

Modulation

Concatenated Code Turbo-Decoding


Differential Modulation with Turbo Decoding

AWGN channel

8-DPSK (bzw. 8-PSK)

Convolutional code:[171]8 [133]8

Block-Interleaver: 3066 Bits

OFDM: 1024 subcarrier


Synchronization

Rn.K-1

Bitstream

Pilot symbols

inverseFFT

P

S

Guardint. DA

Channel

+AWGN

ADWindow

P

S

FFT

De-mapping

Demodu-lation

Depunct.Deinterl.

Viterbidecoding

Bitstream


Sn.0

Sn.K-1

sn,m

en(t)

rn(t)

rn,m

Rn.0

Transmitter

Receiver


Modulation

Mapping

ChannelCoding

Synchronization


Synchronization

IFFT10100011

FFT10100011

coding /modulation

demodulation/decoding

P/S

S/P

cyclicextension

synchronizationframe

D/A

A/D

channel

windowing

clocksynchronization

frequencysynchronization


Guard Interval based Technique

GI DATA GI GI

()*

Moving sum Phase Fractional

frequency offset

Exploit the correlation introduced by the guard interval:

| | argmax Time offset

OFDM symbol

tSliding window

Maximum likelihood estimator !


OFDM

for

Multi-User Communications


OFDM for Multi-Use Communications

Coding +Interl. S/P

Mapping

? IFFTSn,iAdd

Guard

Coding +Interl. S/P

Mapping

Coding +Interl. S/P

Mapping

User K

User 1

User 2

NCL

Dk,l

For a given OFDM system find a suitable multiple access scheme that maps the user data to a modulation block !


OFDM Multiple Access Schemes

t

f

t

f

User / Code

f

t

OFDM-FDMA OFDM-TDMA

OFDM-CDMA


OFDM-TDMAS

ubca

rrie

rs

OFDM Symbols

1NC

1

NS

User

Use

r Dat

a

Advantages: No multiple access interferences (MAI) Incoherent or coherent modulation Adaptation to channel characteristics High coding gain due to diversity Robust against estimation errors No MAI in case of synchronisation errors Easy implementation

Disadvantages: Performance of „normal“ OFDM system

Principle:Every user allocates all subcarriers in a certain number of time slots (OFDM symbols) in each OFDM modulation block


OFDM-FDMAS

ubca

rrie

rs

OFDM Symbols

1NC

1

NS

User Data

Use

r

Advantages: No multiple access interference Incoherent or coherent modulation Adaptation to channel characteristics

• Select good subcarriers• Bitloading on selected subcarriers

Robust against estimation errors

Disadvantages: Stronger requirements on carrier

frequency synchronisation between users in the uplink

Principle:Every user transmits on a certain number of OFDM subcarriers during all time slots of the OFDM modulation block


FDMA Transmitter

IFFTSi,k

Coding +Interl. S/P

Mapping Di,k

Frequency Interleaver

Coding +Interl. S/P

Mapping Di,k

Frequency Interleaver

FDMA Multiple Access

User 2

User 1

How shall the subcarrier of each user be selected ?


Time-Frequency Block

t

f

T

f

bf

aT

To allow the utilization of subcarrier by different users define a time-frequency modulation block consisting of b subcarriers in a OFDM symbols:


OFDM-FDMA Resource Allocation

Subcarrier|H

| [dB

]

Subcarrier

|H| [

dB]

Subcarrier

|H| [

dB]

Time

Freq

uenc

y

User 1

User 2

User K

?

Independent multi-path channels


f

|H(f)| t

OFDM-(FH-)FDMA

If no channel information is available the TDMA/FDMA concept can be used to implement a frequency hopping scheme.


OFDM-TDMA/FDMA

|SNR1(f)| | SNR2(f)|t

f

bf

aT

With a OFDM-TDMA/FDMA multiple access scheme frequency bands can be assigned to users with highest SNR in that band

Multi-User diversity


OFDM-CDMA

Principle:Every user transmits on all OFDM subcarriers during all OFDM symbols of an OFDM modulation block using an orthogonal code (e.g. Walsh-Hadamard).

Advantages: Processing gain due to frequency

diversity Robust against interferences

Disadvantages: Multiple access interferences Only coherent modulation possible No adaptation to channel

characteristics

Sub

carr

iers

OFDM Symbols

1NC

1

NS

User Data

Use

r


Performance Results for the Downlink BER performance comparison between OFDM multiple access

techniques (QPSK, R=1/2)

-6 -4 -2 0 2 4 6 8 1010

-5

10-4

10-3

10-2

10-1

100

SNR (dB)

BE

R

2.5dB5dB

OFDM-FDMA

OFDM-TDMA

OFDM-CDMA


OFDM System Design

and Performance


OFDM System Design

fc

vff CarrierD 03.0max,

max,max

103.05D

S fT

The maximum Doppler frequency sets the upper limit for the OFDM symbol duration:

The overhead of the guard interval sets the lower limit on the OFDM symbol duration:

Requirements for OFDM symbol duration:

sTs S 5.235.8Example: Hiperlan/2 ETSI-E channel model and 250 km/[email protected]

ST 2.0max


OFDM System Parameters

Parameter Value

System Bandwidth B = 20 MHzMaximum Delay max = 5 s

Coherence Bandwidth Bc = 200 kHz

Carrier Frequency fC = 5.5 GHz

Maximum Speed vmax = 200 km/h

Maximum Doppler Frequency fDmax = 1 kHz

OFDM Symbol Duration TS = 25.6 s

Guard Intervall Duration TG = 6.4 s

Total OFDM Symbol Duration TOFDM = 32 s

FFT Length NC = 512 (1024)

Guard Intervall Length NG = 128 = NC /4 (NC /8)

Subcarrier Spacing f = 39063 kHz (19531 kHz)Modulation Technique 16-QAM, 16-DAPSKCode Rate R=1/2User Data Rate 32 MBit/s

2

1

Cha

nnel

Pr

oper

ties

OFD

M S

yste

m

Para

met

ers


Performance Results for the Downlink

1e-05

1

0 5 10 15 20

BER

SNR [dB]

TDMATDMA (Adapt. mod.)CDMA (MMSE SUD)

Adapt. FDMAAdapt. FDMA (Adapt. Mod.)

Adapt. FDMA/CDMA

1e-04

1e-03

1e-02

1e-01

1e-05

1

0 5 10 15 20

BER

SNR [dB]

TDMATDMA (Adapt. mod.)CDMA (MMSE SUD)

Adapt. FDMAAdapt. FDMA (Adapt. Mod.)

Adapt. FDMA/CDMA

1e-04

1e-03

1e-02

1e-01


Advanced OFDM Techniques


OFDM-FDMA Scheme for the Uplink of a Mobile

Communication System


Multiple Users Uplink from Mobile Terminal (MT) to Base Station (BS) Single Cell Environment Sharing of Bandwidth by a specific OFDM-FDMA-Scheme

System Overview

OFDM-based Uplink Scheme

User

User

Userm

1M

0

1User

BSMT


Mobile Terminal

Two parts of the MT‘s OFDM-Structure are considered: Spreading matrix Equidistant subcarrier allocation

Encoder +

ModulationSpreading

Subcarrier

AllocationIDFT GI

,i nD ,i kS ,i ns

,i nD

,i kS

,i ns

: Modulation Symbols

: Transmit Symbols (Freq. Domain)

: Transmit Symbols (Time Domain)

Mobile Terminal


Subcarrier Allocation

General Observation:

Discrete Spectrum with equidistantly spaced non-zero values

Periodic Time Signal

IDFT

IDFT-Processing in an OFDM-system (ith OFDM-Block):1

2 /, ,

0

1 Nj nk N

i n i kk

s S eN

This effect is used in the considered subcarrier allocation scheme:

SubcarrierAllocation

,i nD

IDFT

,i kS ,i ns


Subcarrier Allocation

SubcarrierAllocation

The subcarriers are allocated equidistantly:

→ This leads to a periodic transmit time signal

1st period Mth period

,i nD

IDFT

,i kS ,i ns

, ,IDFTi n i ks S

Subcarriers

Mag

n. User 1

M

,0iS ,1iS , 1i LS

,0is ,1is , 1i Ls ,0is ,1is , 1i Ls ,0is ,1is , 1i Ls


Spreading

Second design element: Spreading is applied to the user‘s subcarriers

,i nD ,i kS ,i ns

Spread

Multiplication of modulation symbols with an orthogonal,

unitary matrix

32

3 92 2

/ 2

2 3

3

1 1 1 1

11

1

jj j

j j j

j jj

e e ee e e

e e e

,i nD

1 1 1 11 1 1 11 1 1 11 1 1 1

Well known examples:

Walsh-Hadamard Discrete Fourier


Spreading Matrix

,0 ,0

,1 ,1

, 1 , 1

i i

i i

i L i L

S DS D

S D

DFT

,i nD ,i kS ,i ns

Spread

In the considered OFDM-FDMA system,

only DFT-matrices are applied for spreading:

Joint application of DFT-spreading and equidistant subcarrier allocation leads to a greatly simplified system


,0 ,0 ,0 ,0

,1 ,1 ,1 ,1

, 1 , 1 , 1 , 1

IDFT IDFT DFT

i i i i

i i i i

i L i L i L i L

s S D Ds S D D

s S D D

Combination of Spreading and Subcarrier Allocation

In effect, the DFT of the spreading matrix and the IDFT-processing

in the OFDM-transmitter cancel out each other:

Consequence:

The DFT-spreaded OFDM-FDMA system is equivalent to

a single-carrier-system with guard intervall


Transmit signal is an M-times repetition of modulation

symbol vector :

Combination of Spreading and Subcarrier Allocation

Together with equidistant subcarrier allocation:

,i ns

iD


,0is ,1is , 1i Ls ,0is ,1is , 1i Ls ,0is ,1is , 1i Ls

,0iD ,1iD , 1i LD ,0iD ,1iD , 1i LD ,0iD ,1iD , 1i LD

identical


Combination of Spreading and Subcarrier Allocation As a result of combined spreading and subcarrier allocation, three components in the transmitter cancel out each other:

,i nD ,i kS ,i ns

Spread SubcarrierAllocation IDFT

,0iD,1iD

, 1i LD

,0iD,1iD

, 1i LD

,0iD

, 1i LD

,0iD,1iD

DFT

IDFT

,0iS,1iS

, 1i LS

1st

period

Mth

period

, 1i LD


Multiple Users

Other users allocate a shifted, but also equidistant subset of subcarriers:

Subcarriers

Mag

nitu

de User 1

User m( ),0m

iS ( ),1m

iS ( ), 1m

i LS

m

The frequency shift leads to a phase rotation of the transmit symbols in the time domain:

( ), ( )m

i kS k m ( ), exp( 2 / )m

i ns j nm N


Resulting Multi-User System

All this results in a very simple signal processing in the transmitter in a multi-user uplink system

0...je 1...je 2 /j nm Ne

M-times repetition


GI

( ),0m

is ( ),1m

is ( ), 1m

i Ls

Phase Rotation

( ),0m

iD ( ),1m

iD

( ),0m

iD ( ),1m

iD ( ), 1m

i LD ( ),0m

iD ( ),1m

iD ( ), 1m

i LD ( ),0m

iD ( ),1m

iD

( ), 1m

i Ns ( ),m

i Ls

L Modulation symbols

Transmit signal in time domain

( ), 2m

i Ns

( ), 1m

i LD

( ), 1m

i LD

...je...je

( ),1m

is( ),0m

is


( 1),0M

nD

Receiver Structure

Receiver is equivalent to conventional OFDM-FDMA receiver

with additional despreading

(0),0nD

(0),1nD

(0), 1n LD

IDFT

DFT

(0),0

(0), 1

0 00 00 0

n

n L

G

G

( 1),0

( 1), 1

0 00 00 0

Mn

Mn L

G

G

(0),0nS

(0),1nS

(0), 1n LS

( 1),0M

nS ( 1),1M

nS

( 1), 1M

n LS

IDFT( 1),1M

nD

( 1), 1M

n LD

(0)nR

( 1)MnR

,i nr

DespreadOne-Tap EQ

User 0

User M

-1

Detection for M users


Equalization

The cyclic prefix in the transmit signal prevents ISI

→ Frequency domain equalization can be done by means

of a one-tap equalizer( ) ( ) ( ),0 ,0 ,0

( ) ( ) ( ), 1 , 1 , 1

0 00 00 0

m m mn n n

m m mn L n L n L

S G R

S G R

In order to avoid high noise amplification in deep spectral fades,

the coefficients are calculated from the MMSE-criterion:( ),m

n kG

( )*,( )

, 2( ),

1

mn km

n km

n k

HG

HSNR

are the channel coefficients of the k th subcarrier at time n( ),m

n kH


Bit Error Performance

The receiver structure is equivalent to a spreaded OFDM-FDMA

system and therefore, the same BER-Performance will be observed

As an example, the performance of an uncoded system is evaluated

System parameters: 256 subcarriers 20 MHz bandwidth 16 users

16 QAM Modulation 64 samples guard interval

Channel parameters: (WSSUS) Exponentially decreasing power delay (0 - 3.2µs) 30 uncorrelated paths Rayleigh fading No Doppler-shift


Bit Error Performance

0 5 10 15 20 2510

-5

10-4

10-3

10-2

10-1

100

SNR [dB]

BE

R

ZFMMSE

The performance figure gives a result for the introduced

system in comparison with Zero-Forcing equalizaton.

An arbitrary user is considered, because in general all users

experience the same average performance


PAR-Reduction

-2 -1 0 1 2

-2

-1

0

1

2

Real

Imag

-2 -1 0 1 2

-2

-1

0

1

2

Real

Imag

Due to the duality to a single-carrier system, the Peak-to-Averageratio (PAR) is smaller compared to conventional OFDM-systems:

For comparison: Complex envelope of two DFT-spreaded

transmission systems with (both QPSK)

a) random subcarrier allocation

b) equidistant subcarrier allocation

(a) (b)


Benefits of this Uplink Scheme

The OFDM-FDMA scheme reduces to a single-carrier system with guard interval

Reduction of transmitter complexity

Low PAR due to single-carrier equivalence

Same BER-Performance as conventionally spread OFDM-FDMA


Channel Prediction Requirements

Multiple Access Scheme Channel Prediction RequiredOFDM-TDMA No

OFDM-FDMA with Adaptive Modulation Yes

OFDM-FDMA with Spreading No

OFDM-CDMA No


Joint Optimization of Layers


Joint Optimization of PHY and DLC

PHY ModeSelection

StatisticalPER Data

SNRMeasurement

PHY

DLC

Higher Protocol Layers

Channel

Transfer Function

SNR

Soft Bits

DLC


PHYPrediction

Channel

PHY ModeSelection

PER / Goodput

PHY

Channel


No Link Adaptation

Conventional Link Adaptation

by DLC

Joint Link Adaptation by

DLC/PHY


SNR Thresholds for HL/2 PHY Mode Selection

BPSK, R=3/4

QPSK, R=3/4

16-QAM, R=3/464-QAM, R=3/4

QPSK, R=1/2

16-QAM, R=9/16

BPSK, R=1/2

4SNR [dB]

0.001

0.01

0.1

1

-4 -2 0 2 6 8 10 12 14 16 18 20 3028262422

LCH-

PER

BPSK, R=1/2QPSK, R=3/4BPSK, R=3/4

16-QAM, R=9/16

16-QAM, R=3/4

64-QAM, R=3/4

PER threshold 10-2

ETSI A Channel Model


16-QAM, R=3/4 PERs for an ETSI A Channel

ETSI_A, samplesETSI_A, avg.

AWGN

14 2524232221201918SNR [dB]

1716151312111090.001

0.01

0.1

1

LCH-

PER

PER for individual ETSI A channel realizations vs. average PER

Use PHY layer information about channel to optimize link adaptation !


OFDM-TDMA - Downlink


Delay Oriented Throughput

SNR [dB]

conventional scheme

64-QAMR=3/4

R=2/416-QAM

16-QAMR=9/16

QPSKR=1/2BPSK

R=1/2

5

4

3

2

1

0-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30-4

optimised scheme

Avg.

Thr

ough

put [

bits

/mod

sym

]


MIMO

Multiple Input - Multiple Output


Higher Data Rates needed

Fixed

Indoor

Pedestrian

Vehicular

High SpeedVehicular

Mobility&Range

Data Rate (Mbit/s)0.01 0.1 1 10 100 1000

UMTS

3G

WLAN/HL2GPRS

EDGE

2.5G

GSM

2G

Data rates greater than 100 MBit/s

??


How to increase the Data Rate?

More Bandwidth ?

April 2000: UMTS license auction in the UK: 5 licenses à 10MHz pairs are auctioned off for 40.109 Euro

August 2000: UMTS license auction in Germany: 6 licenses à 10MHz pairs are auctioned off for 50.109 Euro, almost 900 million Euro per MHz pair

Bandwidth is limited and expensive


How to increase the Data Rate?

Higher bandwidth efficiency by using higher-order constellation diagrams

But capacity cannot be larger than Shannon Limit

(p = 10 )b-5

16-QAM

64-QAM

16-PSK32-PSK

QPSK

8-PSK

25 3020151050.5

1

2

5

10

20

in dB

Shannon limit for AWGN capacity

Ban

dwid

th e

ffici

ency

R/B

[(b

it/s)

/ H

z]

E / N0b


MIMO - Multiple Input Multiple Output

s1

s2

sn

Transmitter

r1

r2

rm

Receiver

n Transmit antennas, m Receive antennas



In flat fading channels:

s1

s2

sn

Transmitter

r1

r2

rm

Receiver

n Transmit antennas, m Receive antennas



s1

s2

sn

Transmitter

r1

r2

rm

Receiver

For flat fading channels, the MIMO radio channel is written as the Channel matrix

The transmission is written as a matrix multiplication:


Cellular Environment


OFDM Application Systems

OFDM-based Systems Single Cell Cellular Networks

BroadcastingDAB

DVB-TDRM

Multi-Frequency Network (MFN)

Single Frequency Network (SFN)

Interactive Communication

HiperLAN/2IEEE 802.11a

MFN: HiperLAN/2

TUHH

TUHH has firstly proposed such a system!


Conventional Cellular OFDMA Network

BS

BS

BS

MT

Time

Freq

uenc

y

DL UL

Band filter

Modulation block

• Different cells use different resources

• Cells have to be separated by filters

• Independent operation of cells


Conventional vs. Self-Organized Management

1

3

2 4

2 4

7 5

6

76 5

73 6 5

42 3

High flexibility

Suitable for any user distribution

Conventional

Resource are clustered with reuse factor 7

SO-RRM

Resource are shared with reuse factor 1

Low flexibility

Optimal for uniform user distribution

Resources


BS

BS

BS

MT

Time

Freq

uenc

y

DL UL

• Different cells can access all resources

• Cells need not to be separated• Terminals have to be

synchronized• Propagation delay is

compensated by OFDM

Band filter

OFDM-based Cellular Single Frequency Network


Synchronisation Concept for a Selforganised SFN

Main Task: Decentralised, self-organised synchronisation of the cellular network

DL Data UL Data

BS

SY

NC

Uplink

Res

ourc

es

Time

MT S

YN

C

Downlink

All MTs synchronize to the BS in “their” cell

All BS synchronize to MTs in “adjacent” cells

DL Data UL Data

BS

SY

NC

Uplink

Res

ourc

es

Time

MT S

YN

C

Downlink

All MTs synchronize to the BS in “their” cell

All BS synchronize to MTs in “adjacent” cells

Two dedicated Sync signals

- preamble in downlink for Mobile Terminal synchronization

- postamble in uplink for Base Station synchronization


Sync Signal Structure

Frequency

One pair of pilot subcarriers used by all MTs of single BS

Distinct pairs of pilot subcarriers used by MTs of different BS

Separate subsets of pilot subcarriers by guard bands

“0” “0” “0”“0”

Frequency

One pair of pilot subcarriers used by all MTs of single BS

Distinct pairs of pilot subcarriers used by MTs of different BS

Separate subsets of pilot subcarriers by guard bands

“0” “0” “0”“0”


Sync Signal Properties

Sync signals are transmitted with maximum transmit power Sync signals have much higher SNR compared to the data

transmission

2048 30CN Gain dB


Estimation Procedure

RX

Pw

r

Subcarriers Subcarriers

RX

Pw

r

I

Im

Re

Im

Re

Phase difference between adjacent subcarriersof same

symbol = time offset

Phase difference between same subcarrierof consecutive symbols =

frequency offset

Frequency offset estimate

- OFDM symbol duration - OFDM subcarrier spacing

FFT #1 FFT #2RX

Pw

r

Subcarriers Subcarriers

RX

Pw

r

I

Im

Re

Im

Re

Im

Re

Im

Re

Phase difference between adjacent subcarriersof same

symbol = time offset

Phase difference between same subcarrierof consecutive symbols =

frequency offset

Evaluate each detected pair of subcarriersseparately to obtain a time and frequency offset

estimate for each cell

Evaluate each detected pair of subcarriersseparately to obtain a time and frequency offset

estimate for each cell

- OFDM symbol duration - OFDM subcarrier spacing

FFT #1 FFT #2

1 *1 2tan { ( ) . ( )}

2lff R l R l

2 ( )R l1( 1)R l

1( )R l

11 1tan { ( 1). ( )*}

2S

lT R l R l

Time offset estimate


System Overview

All BSs and MTs share the whole resources and can access them at any time

No BS controller – instead: radio resource management (RRM) using a self-organized dynamic channel allocation (SO-DCA)

Each BS can observe MTs located in its own cell and in adjacent cells Challenges: Interference from adjacent cells


Short Range Scenario

Cell size: 30m (office) or 100m (outdoor) Low mobility (less than 10km/h) Proposal : OFDM-FDMA based Synchronization in time and frequency

BS

BS

BS

MT

BS

BS

BS

MT

DL ULDL ULDL UL

Frequency

Time


Wide Area Scenario

Cell size: 400m till 2km High mobility (till 250km/h) Proposal : OFDM-TDMA based Synchronization only in time

BS

BS

BS

MT

BS

BS

BS

MT

Time

FrequencyDL UL


Simulation Parameters

Parameters Value

System bandwidth B = 100 MHz

Number of subcarriers N = 2048

Subcarrier spacing F = B/N = 48.8 KHz

Symbol duration Ts = 1/F = N/B = 20.48 s

Guard interval length NG = 80

Guard interval duration 0.8 s Number of cells NBS = 19

Cell radius R = 100 m

Path-loss coefficient 2.5

Shadowing deviation 4 dB

SNR at propagation distance R 20 dB

Average number of MTs per cell 7

Channel model 802.11n


Network Model Cellular network with identical cell radius MTs are uniformly located Quantitative results is counted only in central cell


Frequency Sync in a Cell Inside a cell, after 20 frames, frequency synchronization between all

MTs and their BS is correctly achieved Frequency offset is about 0.5% of the subcarrier spacing

0 20 40 60 80

-0.2

-0.1

0

0.1

0.2

0.3

Rel

ativ

e fre

q. o

ffset

to B

S [

f/S

ub.s

paci

ng]

Frame

Convergence of MTs to their BS frequency


Time Sync in a Cell Inside a cell, after 10 frames, time synchronization between all MTs and

BS is achieved Time offset is about 8% of the guard interval

0 20 40 60 80-0.5

0

0.5

Frame

Convergence of MTs to their BS timing

Rel

ativ

e tim

e of

fset

to B

S [

t/S

ymbo

l Dur

atio

n]


Frequency of all BSs within the network converge after 20 frames Frequency offset is about 1% of the subcarrier spacing

0 20 40 60 80

-0.2

-0.1

0

0.1

0.2

0.3

Convergence of other BSs to ref. BS frequency

Frame

Rel

ativ

e fre

q of

fset

to s

ub. s

paci

ng [

f/

F

]

Frequency Sync in Cellular Network


Time Sync in Cellular Network Timing of all BSs within the network converge after 20 frames Time offset is about 10% of the guard interval

0 20 40 60 80-0.5

0

0.5

Convergence of other BSs to ref. BS timing

Frame

Rel

ativ

e tim

e of

fset

to s

ymbo

l dur

atio

n [

t/Ts

]


Result Animation


Result Animation


Data Transmission

If the target BER = 10-5, the highest PHY mode ½ 256-QAM can be used with SNR > 22dB

OFDM im Rayleigh Kanal

3 dB @ 0.5Bit/s/Hz

8 dB @ 1Bit/s/Hz

13 dB @ 2 Bit/s/Hz

18 dB @ 3 Bit/s/Hz

22 dB @ 4 Bit/s/Hz


Frequency Offset Accuracy Frequency offset accuracy of 5% is sufficient for a data

transmission (SINR > 22 dB with frequency offset of 5%)


Synchronisation Concept - Conclusion Synchronization concept is proposed, using Sync signals in preamble

and postamble.

A Sync signal is transmitted with the maximum power by two of subcarriers and three OFDM symbols.

Time and frequency synchronization can be carried out simultaneously at the receiver.

Simulation results shows synchronization in OFDM-based cellular networks is feasible


Self-organized Radio Resource Management

Characteristic system features:

Resource allocation is done independently by each base station, without any information exchange.

SINR is calculated from estimated signal power and co-channel interference.

Resources with the highest SINR values are allocated.

PHY mode are selected based on the SINR values.


Interference Measurement for Resource Allocation

MT 1

MT 5

MT 2

MT 4

MT 3

BS

BS

BSBS

BS

BS 2

BS 1

Resource

RX

Pow

er MT1MT2

BS 1

Resource

RX

Pow

er

MT3

MT4MT5

BS 2


Interference Measurement for Resource Allocation Interference are measured continuously and averaged over time. DL interference values are measured at MT.

UL interference values are measured at BS. Co-channel interference is taken as the maximum value from

UL/DL measurements.

TMAC

DL UL

Framel l+1 l+2l-1... ...

ÎDL

k

MT measurements

ÎUL

k

BS measurements

NR NR

11


Signal Power Measurements

Signal power is measured exclusively in a reserved “Signal Measurement Slot”.

An “SM-slot” is only for new users, containing all subcarriers in one OFDM symbol.

The estimated value is the average received power over all subcarriers.

Downlink Uplink

t

f MAC frame

Signal Measurement Slot


SINR Calculation and Resource Ranking

1. Balance between UL and DL

2. Signal power estimation

3. SINR calculation

4. Ranking of resources based on their SINR values

, ,max( , )k k UL k DLI I I

1

0

1 N

kk

S SN

kk

SSINRI N


19 cells, 100 users, uniform user distribution inside each cell OFDM-FDMA, 128 subcarriers, synchronized in time and frequency Hotspot fraction: the probability that a user is located inside central cell

5 users in central cell

0

10

20

30

40

50

60

[dB

]

Subcarrier

Received power at the central AP

SignalInterference

Cellular Scenario



0

10

20

30

40

50

60

[dB

]

Subcarrier


SignalInterference


0

10

20

30

40

50

60

[dB

]

Subcarrier


SignalInterference

Hotspot fraction:

30%

Hotspot fraction:

60%

Hotspot Snapshots


Two processes are included: User concentration towards the central cell User scatteration from the central cell

Hotspot Demonstration


Dynamic Channel Allocation

BS

BS

BS

MT

Enable dynamic channel allocation (DCA) technique

Measure co-channel interference between adjacent cells

Always assign resources with minimum interference

Reuse all resources dynamically in adjacent cells


Digital and Analog Hardware

Aspects in OFDM Systems


Digital and Analog Hardware Aspects in OFDM Systems

Introduction Analog / Digital

Analog Hardware Aspects

Digital Hardware Aspects

OFDM Demonstrator

Performance Estimations


Introduction Analog / Digital

DigitalAnalog

- FPGA / ASIC- D/A-, A/D-Converter

- IQ-Modulator, -Demodulator- Amplifiers- Antennas


Summary

• Efficient mitigation of multipath propagation

• Excellent performance in coded systems

• Link adaptation techniques in OFDM makes the system very flexible and powerful

• Choice of multiple access scheme allows adaptation to channel and user requirements

• New aspect: cellular environment


Aspects of Future Systems

• Modulation Techniques: Coherent vs. IncoherentAdaptive Modulation

• Channel Coding: Coded ModulationTurbo Codes

• Network Aspects: Single Frequency NetworksAd-Hoc Networks

• Multiple Access: TDMA, FDMA, CDMA

• Dynamic „Bandwidth“: Dynamic ChannelDynamic Packet Allocation

• Diversity

Flexibility

Documents

Rohling Pr