enic 2002 -ofdm tutorial - luc deneire 1 wireless local networks are emerging wireless lan...

ENIC 2002 -OFDM tutorial - Luc Deneire 1

Wireless Local Networks are Emerging

Wireless LAN

Hiperlan-2, IEEE802.11a, MMAC

ENIC 2002-OFDM tutorial - Luc Deneire 2

Wireless OFDM Transceivers

Luc Deneire

[email protected]

Laboratoire I3S

http://www.i3s.unice.fr/


“Future” Broadband Wireless Networks will be OFDM based

100M

10M

1M

100k

10k

802.11b

802.11aHiperlan-II

MMAC

BluetoothHomeRF

time

Spread spectrum

OFDMlink adaptation6 to 54 Mbit/s

for multimediacommunication

1999 2000 2001 2002


What you will learn ...

Indoor Propagation

Basic OFDM concepts

OFDM performance

Adaptive loading

The Hiperlan-2 OFDM system

Implementation of Hiperlan-2 Transceivers

Crest factor reduction


Indoor Propagation Model


Office tables Metal cupboards

Multipath

RX

TX

Direct path

Multipath channel in an office room.


The channel : a collection of delayed, attenuated and dephased diracs

Channel

Power delay profile

∑−

=

−=1

0

)()(N

kk

jk teth k τδβ θ

∑−

=

−==1

0

2* )()()()(N

kkk tththtp τδβ

P

t


Channels differ in time and frequency behaviorCoherence bandwith - delay spread

•spreading in time

•widening of impulse response due to multipath

Coherence time - doppler spread•spreading in frequency

•doppler effect of moving transmitter and/or receiver


Delay spread


∑

∑−

=

−

=

−= 1

0

2

1

0

2)(

N

kk

N

kkk

RMS

β

βτττ

∑

∑−

=

−

== 1

0

2

1

0

2

N

kk

N

kkk

β

βττ

Delay spread measures the “length” of the channel

RMS delay spread is measure of the amount of dispersion

10 to 100ns correspond to paths 3 to 30m


Coherence bandwidth is where the channel is “similar”(correlated)

Autocorrelation of channel response

Bcoh is defined as f for which

{ });();(5.0);( * tththtc Δ+Ε=Δ τττϕ

2

1

)0(

)(=

ΦΦ

c

c f

{ } (.))();();(5.0),( *cc FouriertffHtfHf ϕττ =+Δ+Ε=ΔΦ


Coherence bandwith is inversely proportional to delay spread

rmscohB

τ1

≈ rmsτ

)(τϕ c)( fc Φ


Compared to the coherence bandwith •W Bcoh frequency selective channel

•W Bcoh frequency nonselective channel

Frequency selective fading…. where bandwith is large ….

Bcoh

W

Bcoh

W


Frequency selective channelsintroduce Inter Symbol Interference

Incoming signal

Outcoming signal

Channel impulseresponse


Coherence Time (10-50 ms indoor):

time in which a channel is “stable”

dc B

t1

)( ≈dB

S(f)

Signals sent at these instantssee uncorrelated channels

)( tc ΦFourier


Subsequent symbols see different channels in fast fading Tb (t)c fast-fading channel

Tb (t)c slow-fading channel


Propagation overviewSummary of channel properties

Fre

qu

ency

dis

per

sio

n

cohS

dTX

tT

BB

>>

<<or

cohS

dTX

tT

BB

<<

>>or

RMSS

cohTX

T

BB

τ>>

<<or

RMSS

cohTX

T

BB

τ<<

>>or

Slo

w fa

ding

Low

Dop

pler

Fas

t fad

ing

Hig

h D

oppl

er

Time dispersion

Frequency flatShort channel

Frequency selectiveLong channel

ISI-free and

flat-fading

channel

ISI and

flat-fading

channel

ISI-free and

fast-fading

channel

ISI and

fast-fading

channel

Using the previousmeasures oncharacteristics wecan place radiochannels in fourgroups.

NOTE that theclassificationis in relation tothe transmissionbandwidth/symbol-time.

Using the previousmeasures oncharacteristics wecan place radiochannels in fourgroups.

NOTE that theclassificationis in relation tothe transmissionbandwidth/symbol-time.

This is wherewe will try tofit the sub-carriers inOFDM.


Question

Assume a wireless system making use of BPSK modulation at 10Mbps.

The system is used indoor. There are two signal paths between Tx and Rx with a relative distance of 10m.

How many symbols are affected by the channel?

What happens if the relative distance becomes 100m? What if the datarate becomes 100Mbps?


Answer

Datarate 10Mbps

•Tsymbol=100ns

Distance 10m•delay = distance / c = 10 / 3.108 s = 30ns

•delay / Tsymbol = 0.3

For 100m

•delay / Tsymbol = 3

For 100m, 100Mbps

•delay / Tsymbol = 30


What to do against ISI?

Wideband signals:•channel delay = many symbol periods

•heavy distortion of the received signal.

Several techniques can be applied to reduce or get rid of ISI in wideband signal transmission •equalization,

•spread-signal modulation,

•OFDM


An Equalizer is a costly filter

f

f

f

Signal (channel) spectrum

Equalizer

Equalized signal

t

t

t


OFDM avoids ISI by slowing pace needs linear amp + sync

Symbols of high bit rate signal are distributed over a large number of subcarriers. • Low symbol rate per carrier.

• Individual carrier signals see flat fading (no ISI).

Promising technique for future high bit-rate applications.

However, it suffers from a number of problems: • a very linear amplifier in the transmitter is required to prevent

signal distortion,

• accurate synchronization in the receiver is needed,

• in the transmitter and receiver real-time discrete Fourier transform (DFT) operations have to be computed.


OFDMbasic principles


OFDM is Multi-Carrierand lowers the symbol rate : less ISI

f1

f2

fnf

...T / sec

T/n / sec


OFDM : Overlapping spectra to save bandwith

(b)

(a)


Overlapping spectra are orthogonalto enable proper reception of individual carriers

(a) (b)

ijj

T

i dttftf δ=∫ )(sinc)(sinc0

Orthogonality to avoidinter carrier interference:

signal design + frequencies


Recent applications of OFDM high-bit-rate digital subscriber lines

(HDSL; 1.6 Mbps), asymmetric digital subscriber lines

(ADSL; up to 6 Mbps), very-high-speed digital subscriber lines

(VDSL; 100 Mbps), digital audio broadcasting (DAB), high definition television (HDTV)

terrestrial broadcasting, WLAN (6-54Mbps) indoor communication

(IEEE802.11a/g, ETSI Hiperlan/2)


Advantages of OFDM

OFDM deals with multipath At low COST (implementation)

OFDM enables adaptive loading : Bit rate RISES with SNR ON EACH carrier

OFDM is robust against narrowband interference, Inteference affects only part of the

carriers.


Disadvantages of OFDM

sensitive to frequency offset and phase noise.

large peak-to-average power ratio, ==> low power efficiency of the RF amplifier.


Parameters for designing an OFDM Systemnumber of subcarriers,

guard time,

symbol duration,

subcarrier spacing,

modulation type per subcarrier,

the type of forward error correction coding


Choice of parameters is influenced by system requirements

available bandwidth,

required bit rate,

tolerable delay spread and

Doppler values


SerialTo

Parallel

QAMData

exp(-jπNs( -t ts)/T)

(expjπ(Ns-2)( -tts)/T)

+ OFDMSignal

OFDM modulator block diagram

OFDM modulation can be realized with IFFT

An OFDM signal consists of a sum of subcarriers which are modulated by using Phase Shift Keying (PSK) or Quadrature Amplitude Modulated (QAM).


Example of 4 subcarriers within one OFDM symbol.

Time domain view of OFDM

All subcarriers have the same phase and amplitude, but in practice the amplitudes and phases may be modulated differently for each subcarrier.


The OFDM spectrum fulfills Nyquist’s criterium for an inter-symbol interference free pulse shape


Impact of channel on OFDM ReceptionMultipath channel spreads energy of

one symbol into adjacent symbol. Results in ISI between symbols

Solutions•make symbols longer by using more carriers,

ISI neglegible. But, negative impact due to coherence time, FFT size and latency

•use guard interval between symbols


Principle of guard interval


CP CP

Transmitters and receivers... through the channel ...

( )thch( )ts

( )tn

( ) ( ) ( ) ( )tnthtstr ch += *

t

CP CP

( )ts ( )tr

t

}

chT

}

chTt}

sampLT

As long as the CP is longer than the delay spread of thechannel, the CP will absorb the ISI.

Channel Noise


Guard time reduces ISI

The most important reasons to do OFDM is the efficient way it deals with multipath delay spread. By dividing the input data stream in Ns subcarriers, the symbol duration is made Ns times larger, which also reduces the relative multipath delay spread - relative to the symbol time - by the same factor.

To eliminate intersymbol interference almost completely, a guard time is introduced for each OFDM symbol.


What to transmit during guard interval?guard time > delay spread

•multipath components from one symbol cannot interfere with the next symbol.

The guard time could consist of no signal at all. However, in that case the problem of inter carrier interference (ICI) would arise. ICI is cross-talk between different subcarriers, which means they are no longer orthogonal.


Effect of multipath with zero signal in the guard time; the delayed subcarrier #2 causes inter carrier interference (ICI) on subcarrier #1 and vice-versa.

FFT Integration Time = 1/Carrier SpacingGuard Time

OFDM Symbol Time

Delayed subcarrier #2

Subcarrier #1

Part of subcarrier #2 causingICI on subcarrier #1


Cyclic extension in guard

Delayed replicas of OFDM symbols have integer number of cycles in FFT interval

No ICI if guard is longer than signal delay

FFT Integration Time = 1/Carrier SpacingGuard Time / Cyclic Prefix

OFDM Symbol Time

Guard time with cyclic extension


Example of an OFDM signal with 3 subcarriers in a 2-ray multipath channel. The dashed line represents a delayed multipath component.

FFT Integration Time Guard Time

OFDM Symbol Time First arriving path

Reflection

Reflection delay Phase Transitions

No crosstalk (ICI) between carriers, but distortion per carrier.Freq domain equalization needed.


Implementation complexity of OFDM vs single carrier modulation

OFDM has the ability to deal with large delay spreads with a reasonable implementation complexity. Frequency domain equalizer needed.

In a single carrier system, the implementation complexity is dominated by equalization, which is necessary when the delay spread is larger than about 10% of the symbol duration.


Implementation complexity of OFDM vs single carrier modulation (cont.)

For Single carrier systems with equalizers, the performance degrades abruptly if the delay spread exceeds the value for which the equalizer is designed and because of error propagation, the raw bit error probability increases so quickly that introducing lower rate coding or a lower constellation size does not significantly improve the delay spread robustness.

For OFDM, there are no such nonlinear effects as error propagation, and coding and lower constellation sizes can be employed to provide fallback rates that are significantly more robust against delay spread. This enhances the coverage area and avoids the situation that users in bad spots cannot get any connection at all.


OFDM Performance


OFDM Performance: AssumptionsThe impulse response of the channel

is shorter than the cyclic prefix

Transmitter and receiver are perfectly synchronised

Channel noise is additive, white and Gaussian

The fading is slow enough to consider the channel constant during one OFDM symbol


OFDM Performance: Transmitter

IDFTIDFTPtoS

PtoS

AddCyclicPrefix

AddCyclicPrefix

s(t)x0,k

x1,k

xN-1,k

s0,k

s1,k

sN-1,k

HtrHtr

xk


OFDM Performance: Channel

HchHch +

n(t)

s(t)r(t)

( ) ( ) ( ) ( )( ) chch

ch

Tttth

tntsthtr

><=

+⊗=

and 0for 0with

Cyclicprefix IFFT

ChannelInput

ChannelOutput

Tch


OFDM Performance: Receiver

StoP

StoP

DFT

DFT

RemoveCyclicPrefix

RemoveCyclicPrefix

r(t)y0,q

y1,q

yN-1,q

HreHre

q(N+v)T+pTr0,q

r1,q

rN-1,q

yq

( ) ( ) ( )( )( )

index symbol : index,carrier : index, sample :

2exp

)1

0,,

,

qnp

N

npjry

pTTvNqrr

trthtr

N

pqpqn

qp

re

∑−

=

⎟⎠

⎞⎜⎝

⎛−=

++′=

⊗=′

π

r’(t)


OFDM Performance: Combined ModelCombine transmit, channel and

receive filters

Received Signal:

( ) ( )fNfHfN

fHfHfHfH

re

rechtr

.)(

)().().()(

=′

=

( ) ( )( ) ( )

( )( )

( )pTTvNqn

mTTvNkpTTvNqhsr

tnmTTvNkthstr

k

N

vmkmqp

k

N

vmkm

++′+

−+−++=

′+−+−=′

∑ ∑

∑ ∑∞

−∞=

−

−=

∞

−∞=

−

−=

)(

)(.

.

1

,,

1

,


OFDM Performance: Combined Model Impulse response of h < v

Substitute in received signal( ) qp

p

vpmqmqp nmTpThsr ,,, . ′+−= ∑

−=

( )( )

pvpmkq

Np

vmTTvmNkpTTvNq

)...( and

1...0for

)(0

−==⇒

−=

≤−+−++≤


OFDM Performance: Combined ModelExpand sm,q

Vector Notation

( )

( )

2exp1

.2exp2exp1

.2exp1

,

1

0,

,

1

0 0,

,

1

0,,

qp

N

nnqn

qp

N

n

v

zqn

qp

p

vpm

N

nqnqp

nhN

npjx

N

nzThN

nzj

N

npjx

N

nmTpThN

nmjx

Nr

′+⎟⎠

⎞⎜⎝

⎛=

′+⎟⎠

⎞⎜⎝

⎛−⎟

⎠

⎞⎜⎝

⎛=

′+−⎟⎠

⎞⎜⎝

⎛=

∑

∑ ∑

∑ ∑

−

=

−

= =

−=

−

=

π

ππ

π

( ) qqq nxhr ′+= .IDFT


( )( )( )( )

qq

qq

qq

qq

nxh

nxh

nxh

ry

+=

′+=

′+=

=

.

DFT.

.IDFTDFT

DFT

OFDM performance: Combined ModelCalculate yq:


2 4 6 8 10 12 1410

-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Eb/No

Pe

idealN=256,v=16

OFDM Performance: AWGN

⎟⎠

⎞⎜⎝

⎛ +=

N

vNloss 10log


OFDM Performance: Rayleigh ChannelRayleigh received SNR per bit PDF:

Calculate BER() with standard formula.

Calculate E{BER}:

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛−=

exp

MP

{ }( ) ( )( )∫

∫=

dP

dPBERBERE


OFDM performance: Rayleigh Channel

5 10 15 20 25 3010

-4

10-3

10-2

10-1

100

Eb/No

Pe


Coded OFDM

Block codes

Convolutional codes

Concatenated codes

Trellis coded modulation of the carriers

transmittransmitStoP

StoP

errorcoding

errorcoding channelchannel receivereceive

PtoS

PtoS

errordecoding

errordecoding


Coded OFDM: interleaving

Time interleaver: block interleaver

Frequency interleaver:permutation over carriers

transmittransmitStoP

StoP

errorcoding

errorcoding

frequencyinter-leaver

frequencyinter-leaver

timeinterleaver

timeinterleaver


Summary

Channel charactersitics•delay spread (50ns) - coherence bandwidth

- frequency (non) selective fading

•coherence time (50ms) - doppler spread- fast fading Vs slow fading

OFDM: orthogonal frequency division multiplexing•split high speed serial data in Nc lower rate parallel

streams

• less impact from ISI because symbols are longer

•guard interval with cyclic prefix is used to overcome almost all ISI


Summary 2

OFDM alone has bad performance on fading channel.

Additional technique needed to exploit diversity:•error coding,

•adaptive loading.

5 10 15 20 25 3010-4

10-3

10-2

10-1

100

Eb/No

Pe


Adaptive Loading


Adaptive Loading: Principle

Estimate the attenuation per carrier and the noise power.

Adapt power and bit distribution per carrier to the measured frequency response.

Achieves capacity (if waterfilling

distribution of power and infinite

number of carriers).


Reference symbol x(k) is an arbitrary BPSK sequence of length N.

The reference sequence s(i) is a n-fold repetition of the IFFT of the reference symbol.

The reference sequence is distorted in the channel:

Take the FFT of each received FFT-symbol separately, for the jth symbol resulting in:

)()()()( inihisir +⊗=

Adaptive Loading: Channel Estimation

)()().()( kNkxkHky jj +=


Multiply yj(k) with the reference symbol:

Average over the n FFT-symbols:

Apply low-pass filter over carriers to obtain Hf(k).

∑ ∑= =

+==n

j

n

j

jj kNkxn

kHkzn

kH1 1

)().(.1

)()(.1

)(~

Adaptive Loading: Channel Estimation

)().()()( kxkNkHkz jj +=


Adaptive Loading: Noise EstimationSame reference sequence.Uses filtered channel estimation to

estimate noise signal:

Average over all carriers (assuming white Gaussian noise):

∑=

=N

ia iNN

1

)(~~

∑=

−=n

jf

j kxkHkyn

kN1

2|)().(~

)(|.1

)(~


Adaptive Loading: Loading AlgorithmsHughes-Hartogs:

•Maximises the datarate for a given BER.

•Allocate bit by bit, each time selecting the carrier with the smallest additional transmit power for a requested BER.

•Algorithmic complexity: O(RT x Nc) with RT the number of assigned bits.


Adaptive Loading: Loading AlgorithmsChow et al.:

•Assign the bits according to:

•Algorithmic Complexity: O(Niter x Nc + 2Nc)

∑ =

Γ

⎟⎟⎠

⎞⎜⎜⎝

⎛

+Γ+=

iTimargin

i

i

inm

ii

RR

iSNR

iR

SNRR

guarantee y toiterativel determined

usage actual theandcapacity channel ebetween th difference the

carrier for ratio noise tosignal the

carrier for rate data the

with1logarg

2

γ

γ


Adaptive Loading: Loading AlgorithmsFischer et al.:

•Minimizes BER for a constant data rate RT

(application) and a constant total transmit power ST

•Assign the bits according to:

iR

iN

D

N

N

DD

RR

i

i

Di

D

ll

Ti

carrier for rate data the

carrier for power noise equivalent the

iteration specificin carriers ofnumber

with

)(log1 1

2

∏=⋅+=


Adaptive Loading: Loading Algorithms

•Iterate until all Ri 0 (exclude negative rate carriers)

•Quantize Ri and assure

•Adapt power per carrier to compensate for quantization

T

D

ii RR =∑

=1


Adaptive loading: Operation


Adaptive Loading: Performance


Adaptive Loading: OFDMA PrincipleEstimate for each user the

attenuation per carrier and the noise power.

Assign carriers to users based on these estimates.

Optionally, adapt the power and bit distribution per carrier to the measured frequency response.


Adaptive Loading: OFDMA Operation


Hiperlan/2 case study


Hiperlan/2 Positioning - Mobility vs. Bitrate

Mbps1 10 1000,1

Ou

tdo

or

Stationary

Walk

Vehicle

Ind

oo

r

Stationary/Desktop

WalkMo

bili

ty

HIPERLAN/2

User Bitrates

LAN

W-CDMA/ EDGE

Bluetooth


W-CDMA/EDGE

Hiperlan/2 Positioning - Cost vs. Bitrate

Mbps1 10 1000,1

Use

r C

ost /

bit

HIPERLAN/2

User Bitrates

LANBluetooth

Low

Medium

High

Very Low

$


Hiperlan/2 protocol architecture

Physical Layer

Convergence Layer

Control Plane User Plane

DLC Control SAP

DLCConnection

Control

AssociationControl

RadioResource

Control

RLC

DLC User SAP

Error Control

Radio Link Controlsublayer

Medium Access Control

Data Link Control -Basic Data Transport Function

One instance per MAC ID

One instance per AP

One instance per DLCUser Connection,identified by DUC ID(MAC ID + DLCC ID)

Higher Layers

Scope ofHIPERLAN/2standards

CL SAPs


Basic MAC frame structure

BCH FCH DL phase UL phase RCHs

MAC-Frame MAC-Frame MAC-Frame MAC-Frame

ACH

SCH

DiL phase

SCH LCH LCH SCH LCH... ... ...

2ms

DL to one MT

One DLC connection

One PDU train mapped one PHY burst


Hiperlan-2 Transmitter PHY Model

scramblerscrambler FECcoder

FECcoder interleaverinterleaver mappermapper OFDM

mod

OFDMmod

burstformatter

burstformatter

radiotransmitter

radiotransmitter


Data scramblerS(x) = X7 + X4 + 1

n4n3n2n1: frame counter, first 4 bits of broadcast channel (BCH)

Initialization sequence

n4 n3 n2 n11 1 1

Scrambled PDU train

out

PDU train in

X7 X6 X5 X4 X3 X2 X1


FEC coder

Y

XChannel coded

PDU trainScrambled PDU train

Append six tail bits

Convolutional encoder

Puncturing P1 with serial

output

Puncturing P2

Output data X

Output data Y

Input data Tb Tb Tb Tb Tb Tb


Data interleaver

Block interleaver with size equal to number of bits in OFDM symbol

Two step permutation

1. adjacent coded bits (k) mapped onto non adjacent subcarriers (i)

i = (NCBPS / 16) (k mod 16) + floor(k/16)

2. adjacent coded bits (i) mapped alternately onto LSB and MSB of constellations (j)

j = s*floor(i/s) + (i+ NCBPS - floor(16*i/ NCBPS)) mod s

s = max(NBPSC / 2, 1)


Mapper

Gray coded constellation mapping

10 0011 0001 0000 00

10 01

10 11

11 0101 0100 01

10 1011 10

00 11

+301 1000 10

11 11

-3

-3

+3

01 11

-1

-1 +1

+1

16QAMb1b2b3b4

Normalization to achieve same average power for all constellations•BPSK: 1

•QPSK: 1/sqrt(2)

•16QAM: 1/ sqrt(10)

•64QAM: 1/ sqrt(42)


Modulation Coding rateR Nominal bit rate[Mbit/s]

Coded bits persub-carrier

NBPSC

Coded bits perOFDM symbol

NCBPS

Data bits perOFDM symbol

NDBPS

BPSK 1/2 6 1 48 24BPSK 3/4 9 1 48 36QPSK 1/2 12 2 96 48QPSK 3/4 18 2 96 72

16QAM 9/16 27 4 192 10816QAM 3/4 36 4 192 14464QAM 3/4 54 6 288 216

OFDM Modulation parameters

Different modulation schemes allow variable bitrate and QoS


OFDM modulation parameters

Parameter ValueSampling rate fs=1/T 20 MHzUseful symbol part duration TU 64*T

3.2 μsCyclic pr efix dura tion TCP 16*T

0.8 μs (mandatory)8*T0.4 μs (optional)

Symbol inte rva l TS 80*T4.0 μs (TU+TCP)

72*T3.6 μs (TU+TCP)

Nu mbe r of data sub -carrie rs NSD 48Nu mbe r of pilot sub -carrie rs NSP 4To tal numbe r of sub -carrie rs NST 52 (NSD+ NSP )

Sub -carrie r s pac ing Δf 0.3125 MHz (1/TU)

Spa cing be tween the t wo outmos t sub -carrie rs 16 .25 MHz (NST*Δf)

Copy

TCP TU

Data nCP

Allows delay spread of 250ns

Compromise betweencoherency time and bandwidth


OFDM sub-carrier frequency allocation and guard interval

D0,n D4,n D5,n D17,n D18,n D23,n D24,n D29,n D30,n D42,nD43,n D47,n

P0,n P1,n P2,n P3,n

DC

-26 -21 -7 0 7 21 26

Copy

TCP TU

Data nCP

IFFT


Burst formatter

Five different PHY bursts•broadcast burst

•downlink burst

•uplink burst with short preamble

•uplink burst with long preamble

•direct link burst

Each burst consists of preamble followed by payload data


Broadcast burst preamble

Enables• frame time synchronization

•automatic gain control

•carrier frequency synchronization

•channel estimation

Preamble has low PAPR (3dB) so non linearities of PA do not affect AGC

Copy

tPREAMBLE=16.0μs

Section 1 Section 2 Section 3 5*0.8μs=4.0μs 5*0.8μs=4.0μs 2*0.8μs+2*3.2μs=8.0μs

A IA A IA IA B B B B IB CP C C


Time synchronization based on auto correlation of A and B-fields



Correlation window

conjMoving average



Correlation window

conjMoving average


Frame time synchronization

Robustnessagainstmultipath,CFO

Accuracy Implementaitoncost

Autocorrelation High low low

Crosscorrelation low high high


Automatic gain control

Based on non-coherent energy measurement at baseband• (input . input*)

Gain is kept constant during burst because signal is non constant envelope


Carrier Frequency SynchronisationEstimate CFO (f) with C-field

Apply correction to the input datastream

Cyclicprefix C C

conj.

angle

Tf ..2 = πφ


Channel estimation and equalization based on C-fieldcompensate with a rotator per

carrier, i.e. a frequency domain equalizer.

a

Known transmitted

received

Cyclicprefix C

X

Cyclicprefix data...

αje−

training symbol data symbols


Radio transmission

Frequencies•8 bands between 5.15 and 5.35GHz with

23dBm EIRP

•11 bands between 5.47 and 5.725 with 30dBm EIRP

•20MHz carrier spacing


Spectrum Allocation at 5 GHz

5.200 5.300 5.400 5.500 5.600 5.700 5.800 5.9005.100

Europe

USA

Japan

Freq./GHz

5.350

5.150

5.150 5.250

5.350 5.725 5.825

5.150 5.470 5.725

Outdoor 1W EIRP Indoor 200 mW EIRP

Indoor 200 mW / Outdoor 1 W EIRP

Max mean Tx power

Outdoor 4 W EIRP

Max peak Tx power

DFS & PC DFS & PC

DFS: Dynamic Frequency Selection

PC: Power Control


Hiperlan-2 receiver block diagram

descrambler

descrambler

FECdecoder

FECdecoder

deinterleaver

deinterleaver

demapper

demapper

radioreceiver

radioreceiver

timedomain sync &AGC

timedomain sync &AGC

FFTFFTfreq

domain EQU

freqdomain

EQU


Main differences between H2 and 802.11aProtocol

•802.11a: MAC with CSMA/CA

•H2: centralized resource allocation

Preamble•802.11a: only 1 preamble similar to long uplink

preamble of H2, because only a single type of packets exists

Modulation modes•802.11a does not foresee 27Mbps, instead it

has modes of 24Mbps, 48Mbps


Implementation of Hiperlan2

transceivers


The implementation of OFDM modems is a challenge Is the implementation of the

IDFT/DFT a showstopper?

What do we need extra in an OFDM modem?

What performance can be expected?

How difficult is the radio front-end?


In half-duplex operation the DFT/IDFT can be shared

(D)PSK or QAM Mo-

dulation

(D)PSK or QAM Mo-

dulationIDFTIDFT

CyclicExten-

sion

CyclicExten-

sion

(D)PSK or QAM Demo-

dulation

(D)PSK or QAM Demo-

dulationDFTDFT

RemoveExten-

sion

RemoveExten-

sion

Equa-lizer

Equa-lizer

Time &Carrier

Synchro

Time &Carrier

Synchro

shared


DFT/IDFT implementation based on hierarchical decompositionDFT/IDFT definition:

hierarchical decomposition:•decimation in frequency (DIF)

•decimation in time (DIT)

1,...,1,0 and

110,2

expwith

1 and

1

0

1

0

−=

−=⎟⎠

⎞⎜⎝

⎛−=

== ∑∑−

=

−−

=

Nk

,...,N,mNj

W

WXN

xWxX

N

N

k

mkNkm

mkN

N

mmk

π


DIF of a DFT results in two smaller DFTs

( )

( ) ( )

( )

( )m

NmNmmmr

N

N

mmr

mNmmmr

N

N

mmr

N

mmN

km

mkN

mkN

N

mmN

kN

Nmk

N

N

mmk

WxxxWxX

xxxWxX

xxW

WxWWxX

. with

with

1.

22

12

012

22

12

02

12

0 2

12

0 2

2.

12

0

⎟⎠⎞⎜

⎝⎛ −=′′′′=

+=′′=

⎟⎠⎞⎜

⎝⎛ −+=

+=

+

−

=+

+

−

=

−

=+

−

=+

−

=

∑

∑

∑

∑∑


We get a recursive DFT structure based on butterfly operations

N/2-taps FFT

x0

x1

xN/2

xN/2+1

xN-1

...

...

...

...N/2-taps

FFT...

...

W0

-

+

+

+

butterfly operation

105ENIC 2002 -OFDM tutorial - Luc Deneire 105

Radix-4 decomposition:

Remap indexes according to radix-2:

Apply these steps recursively

( )

( )( )

( ) ( )∑ ∑

∑

= =++

+

−

=++

=

=

1

0

1

0242

224

121121211

14/

0412112121142

2 1

111

221

21211212211

2

22

21211

...

BFwith

.,,,,BF

u u

kunuNuN

nkkN

ukuk

N

n

nkNkkk

WxWWW

,k,k,n,nn

WkknnnX

Recursive Radix Algorithm is based on radix-4 decomposition

∑ ∑−

= =++ =

14/

0

3

0444

2 1

11

214

2221

21..

N

n n

nknn

nkN

nkNkk WxWWX N


The basic structure is a radix-4 butterfly

x r

x N/4+r

x N/2+r

x 3N/4+r

WN1

WN3

WN2

0NW

-j

-


Optimizing wordlengths results in a considerable gain


OFDM Modem: 256 point (I)FFT 0.5μ CMOS, TLM,

3V

50 MHz

Throughput 195 kFFT/s

2.5 * 2.5 mm2

31000 gates

384 bytes RAM


Implementation: (I)FFT

Organisation #points clock Power Tech Area #transistorsCNET 8192 20 MHz 600 mW 0.5 μm 100 _mm 1.500.000CNET 2048 20 MHz 300 mW 0.5 μm 100 _mm 1.500.000CNET 1024 20 Mhz - 0.5 μm 40 mm_ 21 Kcells

Stanford Uni .v 1024 16 MHz 9.5 mw 0.7 μm 50 mm_ 460.000Stanford Uni .v 1024 173 MHz 845 mW 0.7 μm 50 mm_ 460.000Macquari e Uni .v 16 50 MHz 80 mW 0.6 μm 6 _mm 70.000

IMEC 256 50 MHz - 0.35 μm 20 mm_ 200.000


Fast acquisition is crucial in a burst mode system

Cyclicprefix

OFDMdata

sequence

symboltiming

sequence

carrieroffset

sequence

OFDMdata

sequence...

OFDMdata

sequence

Time-domainacquisitionsequence

OFDMreferencesymbol

OFDMdata

symbol...

OFDMdata

symbol


Time synchronisation is based on repetition of known sequence

COS-1 COS-2

Frame start

TS1 TS1 ….TS1TS2 TS2 TS1 TS1

RTS ATS

…. ….


Initial carrier frequency com-pensation is in time-domain


Centralized data re-ordering minimizes memories

IFFTFFT

equalizer demapper r=2 (7kb)

refsym

mapperr=2

(4kb)

SYNCr=1

(8kb)

r=2 (4kb)RAM 1 RAM 2

SSR

r=2 (8kb)

: Tx datapath : Rx datapath : reference signal


Adaptive equalizer has low implementation cost

Gain control

yi

Refsym RAM

COEF RAMS

conjugate

Decide & rotate

Integrate & dump

conjugate

y’i

mode (feedback, reference)

mode (single, average)


Programmability is a must for OFDM modems

Parameter Options

Number of carriers 64, 128, 256

Guard interval 0:4:28Modulation QPSK (BPSK)

Equalizer modes REF-FF, SC-FB, AC-FBreference sequence

Spectral mask Complex, per carrier

FFT clipping 5-8b, MSB or LSB aligned

Spreading 1, 2, 4, 8; code sequence

Acquisition sequence, length,confidence factors

Number of zerocarriers

Low: 0:1:3, left and rightHigh: 0:2:30, left and right


Token flow control: integration = communication

Token flow produced: Soft initial token arrival time specification: Tmin < T < Tmax

=> boundary check only=> supports IP block strategy

‘smart’receiver

‘smart’senderTrigger at ‘1’

1

0

1

0

Repeatedtoken

Singletoken


Clock gating is essential for low-power operation

sleep RXSYNC

RXdemod

AREQ ACQ

EOB

2%6%

19%73%

6%

TRX

CFO

‘IEEE’ mode @ 50 MHz Power GOPS #acc/s Gbit/s

Tx mode 670 mW 3.8 1.32 G 21.3Rx mode 570 mW 6.8 1.28 G 20.3Sleep 150 mW n/a n/a n/a


Putting it all together

0.35μ CMOS, 3.3V, 5LM

50MHz

144pins

210 kgates

10 RAMs

20mm2

670 mW


Improved equalizer for better performance with QAM

QAM64 BER performance

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

12 13 14 15 16 17 18

SNR per bit (dB)

bit

err

or

rate

Improved equalizer

Standard equalizer


Second generation OFDM Modem supports QAM-64

0.18μ CMOS 5LM

20MHz

160pins

500 kgates

19 RAMs

25mm2


Crest Factor Reduction


Crest Factor: Definitions

Peak-to-Average Power Ratio:

Crest factor

Example

( ){ }

N

sE

sPAPR

km

km

=

= 2,

max2

,

PAPRCF =

dBCFdBPAPRN 12,23256 ==⇒=


We get a peak amplitude if all tones add in phase

0 50 100 150 200 250 3000

10

20

30

40

50

60

70


Large peaks do not occur very often (Gaussian distribution)

4 5 6 7 8 9 10 11 12 13 140

2000

4000

6000

8000

10000

12000

PAPR(dB)


The Crest Factor has major impact on implementationLarge CF with non-linear power

amplifier•in-band distortion

•spectral spreading

Lineair power amplifier or operated with large back-off:•expensive

•power inefficient


PAPR is bottleneck for low cost, low power front-end design

PAPR<19Large back-off (4)low efficiency (2%)30W DC power for 600mW RF Power

Pin

Pout

IFFT

.

.

.

.

.

.

channel ...

FFT

.

.

.

PA


IFFT

.

.

.

H(n) ...

.

.

.FFT

.

.

.FFT

.

.

.IFFT

.

.

.

H(n) ...

.

.

.FFT IFFT

.

.

.

.

.

.Equ

Single carrier Tx with freq domain processing


Several techniques try to reduce the Crest FactorClip the signal

•clipping noise = in-band distortion

•spectral spreading filtering required peak regrowth

Reduce the probability of clipping by:•coding of input data

•selected mapping

•partial transmit sequences


Crest Factor Reduction: Coding

Map the transmitted sequence into a larger sequence with limited PAPR.

Good performance with little overhead

Approaches:•Look-up tables Only applicable for small

number of carriers and constellation sizes.

•Pseudo-noise codes

Current work: systematic codes that also offer error correction


Crest Factor Reduction: Selected Mapping

Options for Ai:

• random rotation vectors

•M-sequences

log2(D-1) bits of side information are needed

IDFTIDFT

Selectionof best

yk

Selectionof best

yk...

yk,1

xk

IDFTIDFT

IDFTIDFT

x

x

x

A1

A2

AD

yk,2

yk,D

yk


Crest Factor Reduction: Partial Transmit Sequence

Select Ai from set of size W

(D-1).log2(W) bits of side information are needed.

IDFTIDFT

++

...

xk

IDFTIDFT

IDFTIDFT

x

x

A1

AD-1

ykPartition

intoSub-

blocks

PartitionintoSub-

blocks

Optimize ykOptimize yk


Crest Factor Reduction: Performance

enic 2002 -ofdm tutorial - luc deneire 1 wireless local networks are emerging wireless lan...

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