the implementation of estimation and correction of carrier frequency offset of cofdm system in dab...

7/25/2019 The Implementation of Estimation and Correction of Carrier Frequency Offset of COFDM system in DAB Receiver

1/4

The Implementation

of

Estimation and

Correction of Carrier Frequency Offset

of

COFDM system

in

DAB Receiver

Lingh i Huang,

and

Zaiwang Dung

Department of Electronics Engineering

Tsinghna University, F.RChin a

Abstract: Estimation of the carrier frequency offset is a

key pan of COFDM receivers.

An

novel Iiardware

sclienie of Carrier Frequency Offset Estiimtion and

Corrections of CO FDM system in DAB (digita1 audio

, broadcast) Receiver

is

i inpleniented on P G A . I t has

been embedded in the mnonolitluc DAB c h a d decoder

and it also can be utilized in other OFDM system after

soiiie inodfication

Kcyvord:

CFO(Camer frequency offset). COFDM

(Coded Orthogonal Frequency Division Multiplexing),

CORDIC(C0rdina

re

Rot.ar io i is DIgita l Compute)

1.

I n t r o d u c t i o n

OFDM is

a

bandwidth efficient signaling scheme

for digital conununications that

was

first proposed

by

C11;ing~~'I For its robustness against the effects of

inultipath propagation n4uch are main obshuctions

of

mobile reception and rea liz~ tio n of single frequency

nenvork. OFDM is widely adopted.

But the transpolt systems of OFDM are very

sensitivity to frequency offset.

To

obtain responsible

deniodulated signils. the carrier of the receiver must

keep consistent with the transmitter's.

Tliere're

many

carriers in OFDM signal. And the

frequency offset

k

engendered

in

channel includes

two

palt:

Y

=

( 1 )

\Vlicre,fic is the distance behveen contenn inous

carries. I is integer and n is decinial fraction less than 0 5

I, . is called coarse frequency offset wldcli covers

integer multiple of the distance beh5,een carries. And o

fs is called fine frequency offset which covers the offset

less thin

half

of the distance beh-een contenninou s

canies .

Channel decoder

Ld -o rd la t Er limnl ion

of

frequencyoffsn

(a)

C h a d dmoder

+=?z

R F

A I D

co*.Xtioon of

A I

Figun:

I

Many m ethods have been proposed on estimation of

frequency offset. and they can be classified into

hVO

class

by weather the transmitter signal contains the signal

specially set for frequency estiination. The correction of

the Frequency offset can also be c lassified into hv o class.

one is depict as figure l(a) that adjust the local-oscillator

to make the carriers accordant. and another is depict

as

figure

l b)

hat adjust the phase of the signal input to the

OFD M decoder

to

make frequency syncluonization

Tlus

paper

introduces the hardware sclieiiie

of

Carrier Frequency Offset Estimation and Corrections,

and

it

is according

to

ETS 300 401 DAB. It has been

employed in the ASIC design of DA B receiver.

0-7803-7889-X/03/$17.00@2003 IEEE.

890


2/4

2 .

Estimation of frequenc y offset

In

OFDM,

to avoid tlie inter-symbol interference

The

DAB

trammitter

signal includes specialized (ISI).

the

guard interv al is emp loyed wluclm is the sam e

as

sigilal called

phase

reference syiiibol. Correlating the the backend of signal of the symbol. So it can also be call

phase reference symbol with

the

standard phase as cyclic prefix. And u'e can

use

it to calculate the fine

rcfcrence symbol stored in local receiver can easily

get

frequency offset.

tlie coarse frequency offsct. but the estimation of fine r ( f )

is

the

guard in te n d of symbol received. I is

offset

is

coinplcx. the duration of a synibol which

is

the r eciprocal of

, . _ _ _ _ _ _ _ _ _ _ _ - - - - _ - _ _ _ _ _ _ _ _ _ _

t rim i r qu n yPiimstl

To estimate the

coarse offset should

calculate the sigiml

in frequency doinain

wluch

work after

FFT decoder. And to

estimate the fine

offset should make

the distance betaeen carries. that is f

=

lir . And

r r-T,,)

s

the baclrend

signal o f th e

.symbol. In

i d e a

I ctraniiel condition:

) ( I )

? / -T

)

( 2 )

If there is rrequency offset. the

two

sigmil

hive

pliase

diiierence. concerning

I )

that is

(

=

-7 ewd (3)

The

phase difference implicates

the

infomation

of

frequency offset.

To

correlate them then

R(,)= , j ( , ) . . r ( r - T , , ) = C e . ; p ( i z ~ f l ~ ~ s r , ) 4)

\\here C

is

real nuniber wluch equal

to

square of

the

amplitude of the

OFDM synrbol.

I n practical. the cunnilation is adopted to depress

interfere.

; = E R ( , ) = c v ( f )

. r t -

T,,)

=

~Ccxp(jZm&T,; ,) Kesp( j2mf ,7 ; , )

(5)

3. System Airhitectuix and Scheme

doniain just after

AD o the system is a half closed loop system for the

coarse frequency offset estiination is closed loop but the

fine offset estinwtion not. The system structure is

depicted in figure

2.

The system control and the

calculation of angle or phase in the fine frequency offset

estimation and the frequency correction

are the

most

difficulties.

3.1 Coarse frequency offset cstiniatiou

Using the c a m. signal of phase reference

synibol

decoded froin

FFT

to correlate with the standard plme

reference symbol. we can get coarse frequency offset

estimation witch less than 8liHz.And the offset large

than 8kHz can obtain froin the envelope demodulation in

frequency doma in

3.2 Fine frequency offset estimation

To get

the

fine frequency offset,

u'e

should calculate

the

fonnula 5). This acluevement is coiuposed

of

die

datain part. the corrclation part. the angle compute part

and

the

system control. The input includes two

orlhogonal signals

I

and

Q

crea ted from A D and

the

syn ~b ol tart signal.

891


3/4

2.2.1 The'inddta read control module

Range can

be

extended to *?rand

the

correspondiug

frequency offset can be extended to +jOOHz (mode

I).

The essence idea

of

CODIC

algoritluii

is

to

approach

the

ai111

by rotate

k a r c t a n ( l i 2 ) . The

pllaase error

is

confined

In formula (4)

or

( 5 ) .

we

need buffer

to

transfer the

data. In DAB mode I.

T=

2048, wlucli mean 2048

sample data (not including cyclic prefix). To perform the

correlation.

the

buffer has to

be

more tliaii 2048. In our

iinplementation,

the

donble port

sram is cmployed which the

s a n e

time is the transmit .

behveen DIQ and backend

decoder.

L d ot le

# m e =

pII-(Imp,,p, tJ,

-

ighl nrle

In DAB system. the time

s~nch ronizatio n can obtained

from the null symbol. so the

Figue 4

start position of each

symbol

can be ascertained and

we

can reduce

the

conelation qu antih form this

that

we do not

cokelate all

the

data but select p.m of the

cyclic prefix and

thc

c oms pond ing

pan

of

the

synbol. For both the front part of

tlie

q c l i c prefiz and tlic backend

of

the symbol

could

bc

dis tuh ed by multi-path effect and

mobile receiving condition.

we

can select

the pan

to

conelate

b?-

tlie channel condition.

But in pnctic e. fo r simpleness. firm position

is

chosen. \\~luc h

s

the m iddle part (256 samples) of the

c ~clic prefix

504

samples) and

it's

corresponding part.

All these tasks

are

achieved in control module.

R I p n r l a

l n l L ro i rc l r l o

Figurs

5

by the last rotate. For example. in a

IO

stage CORDIC.

the phase error is less than 0.004(radian).

Some regulations are applied to make it calculate

the arc iangent. The flow clia is shown in figure 4 and

its circuit design in figure 5 . After tlie CORDIC. the

condate outcome is rotated to

a

real nuniber. and lhe

angle it rotxed is

the

p l w e we want.

Pay

attention

on ihe coefficient the original complex iiumber Ius been

magnified.

It

is

a f ix

nlultiple:1.647. so

the

complex

iiumber should

be

eztcnd before CORDIC to avoid

oveflow. The allgle rotate on each

stage

is - ~ X I ~ ~ , ( I , ~ ' ) .

wlucli is easily implemented by shift in Iurdware design.

3.2.2 Angle and

Phase

Calculate

Calculating the angle and

pluse of

the outcome

of

correlator is the key p m in the estimation of frequency

offset.

Oue

method is tlut

take

the image pan of the

corrclate outcome

a s it's

angle. Tlus method can work

ivcll \ \ l ien the

anglc is

\-e? small. but when

the

angle

is

not closed to

zero. it

will create lager e m r and it is

unacceptable especially for

an

unclosed loop system.

So

the accnmte angle is obligaton.

The CORDIC algoritluii is introduced into this 3.2.3 correction of frequency

offset

calculation I t needs less hardware overhead to calculate Figure l(a) show the tnditional iiietliod

to

the arc tangent. If qoadrant judge

is

a dded . the D y ~ i n i c c on tc t ion f re quency offset.

Tlus

method required the

892


4/4

local oscillator to be high precision high stability and

adjustable. It is llard

to

realized. In t l is implementation

the digiial correction method sh oa ~ n figure 1 @) k

eniployed. The principle of digital correction is to

counteract the frequency offset before OFD M decode by

digital signal process. Tllat is

~ - J W W ( I )

r(t)exp(j2mfsCT,)x exp(-j2mfxT,)

7 )

11

can be acldeved by Direct-Digital Synthesizer

(DDS) and multiple module. But the inultiple need large

area and thc DDS should look up table. DDS ~vluc li eed

sync-datain

niodule

on

4 M.

/Compilation Logic Cells Registers

82 71

sync-correlate 373

sync-correct

I

667

400

sync-compute

5 . Conclusions

An novel hardware design of C a m c r

Frequency Offsei Estimation aiid

s 1 m . a Corrections DAB recciver is impleoieuied

and verify on FPGA . Th e siinulaiion result

Phase? P h r v l P h S d

Figure

complex control and

its

precision is confined

to

the table

memot?;. The CORDIC can

also

work here. It is just the

counterreaction of the estimaiion. By accuinulate

the

frequenq offset we can get

the

phase wlucli

the

indata

should rotate. Aiid

p

modulate the pliase each stage to

make it zero

a t

the end. and the complex number after

process ciin be send

io FFT

to decode.

To

reduce the

work clock o n the coiirinlance work.

tlie

pipeline

stnicture is employed. Figure 6 depict the 9 stage

pipcline. Tlierc

also

c a n choose

a

tmde-off behveen area

aud speed.

Each stage required deferent precision .

to

reduce

the design resource.

the

wordlength of each stage is

defcreni. Stage 1 is Rbit and the last stage is 13bit. Tlus

change make the

gates

reduced about 1 0

4. 1ml)lementation

Tlds system is designed from top

to

down. All the

design is described in VHDL in register transfer

leuel(RTL). The design Iias been siuiulated on Modelsiui

and been placed and rouied on FPGA-complie aud

QUARTUS.

It s

donmload

on

EPlK100QC208-3 of

ALTERA's ACEX series FPGA. The resource of the c l i p

has been

used 62% aiid the design is about 50.000

gates.

All inodole work on 2 M clock except the indata control

and verification on FP GA prove th is design

is practicable.

Reference:

I .

2 .

2.

4 .

5

6 .

7 .

C l m g

R W.Synthesis of band-limited ort1iogon;il

signals for multi-channel data transuussion[J I.Bell

Syst

Tec h J.19GG.45( 12):1775-1 796

Cliang R W.Gibbey ,R A.A tlieoreiical study

of

perfonnance of an ortliogoilal tuultiplering

d a r ; ~

transmission sclieiue [I]. IEEE Trans

Coinuiunication Technolopy.

1968. G(S):529-5dO.

EBU. ETS 300 401. "Radio Broadcasting Systems:

Digital Audio Broadcasting DAB) o mobile.

portable and fixed receivers". 1997

Jack E.Volder. The CORDIC trigonomctriz

teclmique.IRE Trans.on Electronic Coiuputins.

1959.8 : 330-334

Hui Lu , "A rcsearch

on

tlie

DAB

receiver based ou

the sofnvare radio mechanism and tlie

syncluoniwtion algorithm". doctor dissert:ition of

Tsiogliua univ.. 3.2000

Lei X u, "design on the synchionization systeiii or

digital audio broadcasting". niaster dissertation of

Tsingliua u n i ..6.2002

Yonping Zhou. Hua

Guan.

Z a i w n g D o n g

controllable digital frequency

Synlliesizci'.

teclmiquc repolt of T singhua Uiiiv.; 9.1999

893

the implementation of estimation and correction of carrier frequency offset of cofdm system in dab...

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