OFDM synch oniza ion scheme o Powe
Line Telecommunica ions (PLT)
V.Baena, J.G anado, M.A.Agui e, A.To alba, L.G.F anquelo
Dp o de Ingenie ía Elec ónica. Uni e sidad de Se illa.
Camino de Los Descub imien os s/n. 41092-Se illa (SPAIN)
E-mail: {baena, joaquin, agui e, o alba, leopoldo}@g e.esi.us.es
Abs ac
This pape p esen s a new scheme o OFDM ime
and equency synch oniza ion wi h applica ion in
Powe Line Telecommunica ions (PLT).
Simula ion esul s show an excellen beha io , e en
o he low alues o SNR in he synch onize inpu
inhe en o PLT.
The synch onize has been p o o yped on an FPGA
p io o be in eg a ed in he single-chip PLT sys em.
Keywo ds: OFDM ime and equency synch oniza-
ion, Powe line communica ion, PLT, FPGA.
1. In oduc ion
The basic idea o mul ica ie modula ion (MCM) is
o di ide he a ailable spec um in se e al sub chan-
nels. In a classical FDM (F equency Di ision
Mul iplexing) sys em, na ow-band signals a e gen-
e a ed independen ly, assigned o a ious equency
bands, pa allel ansmi ed, and sepa a ed by il e s
a he ecei e [1].
In an OFDM (O hogonal F equency Di ision Mul-
iplexing) sys em, he in o ma ion is pa allel mapped
in o N-QAM (Quad a u e Ampli ude Modula ion)
signals and mul iplexed using a FFT (Fas Fou ie
T ans o m) [2].
This echnique has been conside ed o b oadband
applica ions including bo h wi ed and wi eless ap-
plica ions. In he case o adio ansmissions, OFDM
is used in no malized Digi al Te es ial Video
B oadcas ing (DTVB) and Digi al Audio B oadcas -
ing (DAB), bo h s anda ds om ETSI. In addi ion,
ETSI-BRAN amily o ecommenda ions (B oad-
band Radio Access Ne wo ks echnical body) has
selec ed his ansmission echnique o
HIPERLAN/2 (High Pe o mance Local A ea Ne -
wo k) [3].
Wi ed applica ions, such as ADSL (Asymme ic
Digi al Subsc ibe Line) o HDSL (High-bi - a e
DSL), employ OFDM echniques (also called DMT:
Disc e e Modula ion Technique) o deli e high bi
a es o he end use . [4].
Recen ly OFDM has been sugges ed o Powe Line
Telecommunica ions (ETSI-PLT Technical Body
and [5]). This echnology will be able o p o ide a
new local b oadband access as well as indoo da a
ne wo king using o dina y powe lines ins alled in
e e y home and o ice.
This pape p esen s an algo i hm o p o ide bo h,
coa se and ine synch oniza ion, o an OFDM sys-
em in Powe Line Telecommunica ions (PLT). The
ou line o his pape is as ollows:
Sec ion 2 desc ibes he p inciples o classical OFDM
sys ems. I will summa y he key aspec s o he
OFDM modula ion, as well as i s ad an ages and
d awbacks. Sec ion 3 illus a es he e ec s o syn-
ch oniza ion e o s in OFDM. Sec ion 4 p esen s a
synch oniza ion scheme o p o ide ime and e-
quency synch oniza ion in a PLT anscei e , and
some simula ion esul s. Sec ion 5 shows a ha dwa e
implemen a ion on an FPGA and, inally, in Sec ion
6 some conclusions a e d awn.
2. OFDM desc ip ion
The i s OFDM sys em was p oposed in 1971 by
Weins ein and Ebe [6]. Since powe ul silicon
echnology was no a ailable a his ime, he de el-
opmen o OFDM based sys ems was ce ainly
delayed un il nowadays.
In an OFDM sys em (see igu e 1), he incoming
in o ma ion signal S is pa allel mapped using an
o dina y cons ella ion o ob ain complex samples. N
o hese complex samples Xk,p (k = 0,.., N-1) a e
ans o med by an iFFT o cons i u e he p- h
OFDM symbol (see equa ion 1). As a esul , xn.p is a
disc e e base band sequence o N-QAM ca ie s.
∑
−
=
−
=1
0
2
,,
N
k
j
N
kn
pkpn eXx
π
n = 0,1,..,N-1 (1)
A gua d in e al, called Cyclic P e ix (CP), wi h M
samples, is added o he ou pu o he iFFT in o de
o a oid possible ISI (In e Symbol In e e ence).
FFT
Y
N-2,
p
Y
N-1,
p
Y
0,
p
Y
1,
pSignap
demappe .
.
.
R
0,
p
R
2,
p
R
N-2,
p
R
N-1,
p
S/P
y
N-2,
p
y
N-1,
p
y
0,
p
y
1,
p
S/P
CP
S/P
iFFT
X
0,
p
X
2,
p
X
N-2,
p
X
N-1,
p
Sp
x
0,
p
x
2,
p
x
N-2,
p
x
N-1,
p
P/S
CP
Signap
mappe
.
.
.
S
0,
p
S
2,
p
S
N-2,
p
S
N-1,
p
u
n
h
c
( )
y
n
ADC
Fs
Fs
DAC I-Q
mod
Fc
I-Q
demod
Fc
Fig. 1. OFDM sys em o e iew
This cyclic p e ix is a copy o he las M samples o
he OFDM symbol ha is p e ended o be ansmi -
ed and i is a pu e sys em o e head ha educe he
base band bandwid h equi ed by a ac o
=N/(M+N). η
The disc e e base band signal is analogue con e ed
(DAC-Fs) and up mixed o he channel equency Fc.
The esul ing signal is exp essed in equa ion 2,
whe e is he absolu e ime, T is he OFDM symbol
du a ion, N is he numbe o sub ca ie s and Xk,p is
he cons ella ion poin ca ied by he k- h sub ca ie
o he p- h OFDM symbol. The indi idual spec a
a e now o hogonal sinc unc ions and he o al
bandwid h B is di ided in o N equidis an na ow
band sub channels.
∑∑
∞+
−∞=
−
−=





+−








=
p
N
Nk
F
T
pT j
pk
c
eX z
12/
2/
2)(2
,
)(
π
η
π (2)
The mos impo an ad an age o OFDM sys ems
when compa ed o single ca ie sys ems is ob ained
in b oadband applica ions o e equency selec i e
channels ( adio channels, powe line channels, e c).
Equaliza ion in OFDM is educed o a simple mul i-
plica ion o each sub ca ie by a complex ac o ,
whe eas equaliza ion in single ca ie ansmission
may no be easible o in oduces la ge delays.
OFDM p oduces much g a e immuni y o impulse
noise and as ades due o i s long symbol ime. In
addi ion, he cyclic p e ix inclusion educes ISI,
e en when using an Analogue F on End (AFE) wi h
la ge o de FIR il e s.
Di icul ies ega ding OFDM a e: peak- o-mean-
powe a io ha equi es an ex emely high linea
ampli ie o educe OOB (Ou -O -Band) In e e -
ence, and he equi emen o accu a e ime and
equency synch oniza ion.
3. Synch oniza ion e o s in OFDM
Time and equency synch oniza ion be ween ans-
mi e and ecei e a e o c ucial impo ance in
e ms o sys em pe o mance [3].
A equency misma ch be ween ansmi e and
ecei e causes a los o o hogonali y ha will e-
duce he use ul signal ampli ude and will lead o
In e Ca ie In e e ence (ICI). Bo h impai men s
cause an impo an BER deg ada ion. OFDM sys-
ems a e o de s o magni ude mo e sensi i e o
equency o se and phase noise han single ca ie
sys ems [7].
A ime o se in he FFT ime window es ima ion
causes phase o a ion in equency domain. The
ou pu symbol wi hin he OFDM symbol is o a ed
by a di e en angle. F om subca ie o subca ie ,
he angle inc eases p opo ionally o he equency
o se . In OFDM sys ems wi h cohe en de ec ion
his o a ion has o be p ope ly co ec ed. Howe e ,
unde non-cohe en de ec ion, his inc emen al o se
does no dec ease sys em pe o mances since he
in o ma ion is ca ied in phase o se s be ween con-
secu i e symbols.
I he es ima ed s a posi ion o he FFT window
loca es wi hin he da a in e al, he sampled OFDM
symbol will con ain some samples ha belong o
o he OFDM symbol. The phase o a ion imposed by
OFDM symbol synch oniza ion e o can hus be
co ec ed by app op ia ely o a ing he ecei ed
signal, bu he dispe sion o signal cons ella ion
caused by ISI o ms a bi e o BER loo due o he
p esence o un eco e able samples.
In conclusion, he use o a synch oniza ion scheme
which a oids OFDM symbol es ima ion e o , will
lead o an e ec i e dec ease in he leng h o he
cyclic p e ix, educing i s o e head. In his case, he
cyclic p e ix leng h can be educed un il he loo
le el imposed by he mul i-pa h and ading channel
ea u e.
The mos impo an synch oniza ion me hods in
OFDM a e p esen ed in [8]-[15].
In o
C
P eamble
BA
Fig. 2. Bu s s uc u e
4. Synch oniza ion algo i hm
A ime equency synch oniza ion scheme o bu s
based ansmissions is p oposed he e. I will be
in eg a ed in o a PLT sys em.
As o he bu s based ansmission sys ems, he
OFDM ame is s uc u ed as shown in igu e 2.
The p eamble is depic ed in igu e 3 and consis s o
h ee di e en sec ions (A, B and C).
Sec ion A is conside ed o p eamble de ec ion, gain
adjus men (no mally pe o med by an ex e nal
P og ammable Gain Con olle ), and coa se iming
es ima ion. Fine equency and ime uning a e done
in he B sec ion. The C-Field is ese ed o channel
es ima ion.
Sec ion A
B
32
B
32
B
32
Sec ion B
B
32
C
32
C
64
C
64
Sec ion C
A
16
A
16
A
16
A
16
A
16
-A
16
Fig. 3. Heade de ail desc ip ion
An au o-co ela ion scheme has been selec ed o
p eamble de ec ion and coa se iming synch oniza-
ion ( igu e 4). The ecei ed signal is delayed by he
co ela ion delay D (16 samples). Conjuga e com-
plex samples o he delayed e sion a e mul iplied
by he ecei ed samples. Resul ing p oduc s a e eed
in o he mo ing a e age block, whose window size
is W=64 samples, and hen hey a e pos -p ocessed
o h eshold de ec ion and maximum sea ch in o de
o ind he co ec iming.
D (.)*
Mo ing
A e age
|(...)|
2
Mo ing
A e age
|(...)|
Recei ed
da a
X(i)
Y(i)
X(i)/
Y(i) R
(
Fig. 4. Au oco ela ion scheme
Figu e 5 shows he au o-co ela o ou pu o sec ion
A in p esence o AWGN inpu noise. No e ha a
h eshold alue is equi ed a) o minimize he p ob-
abili y o alse p eamble de ec ion wi hin he da a
ield o a MAC (Medium Access Laye ) ame, and
b) o il e ou small peaks in he au o-co ela o
ou pu due o inpu noise. Howe e , a la ge h esh-
old alue dec eases he p obabili y o co ec
p eamble de ec ion wi hin he p eamble ield o a
MAC ame. A e exhaus i e simula ion, i has been
ound ha a h eshold alue o 0.55 ep esen s a
good comp omise.
Noise and mul i-pa h signal p opaga ion o e powe -
line channels p oduces b oade peaks a he au o-
co ela o ou pu , educing he accu acy o he im-
ing eco e y p ocess. This coa se synch oniza ion
can educes he bu den o he ine iming p ocess.
Simula ions esul s show ha he maximum e o
in oduced by he coa se iming p oces, when he
peak o he R(i) signal is abo e he h eshold alue,
is only +/-4 samples.
Fine iming is achie ed by using ma ched il e s.
Since he maximum e o is +/-4 samples, a bank o
nine ma ched il e s is equi ed, one o each possi-
ble sample delay. I has o be no ed ha wi h he
ou B- ields o he p eamble only h ee complex
mul iplica ion and h ee addi ions pe sample a e
needed, a much lowe load han he 32 complex
mul iplica ions and he 32 addi ions needed i a
c oss co ela ion is done o e all he incoming sam-
ples. Figu e 6 shows he il e s ou pu s in he case o
AWGN inpu noise.
Due o equency de ia ion be ween ansmi e and
ecei e oscilla o s, he ecei ed base band signal
has a ime a ying phase componen which de e io-
a es he OFDM demodula ion.
Fig. 5. Au o-co ela o ou pu . A sec ion
100 200 300 400 500 600 700 80
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1R(i), AWGN, SNR=15dB
Sample index
R(i)
−5 −4 −3 −2 −1 0 1 2 3 4
5
0
0.5
1
1.5 Fine iming, AWGN, SNR=15dB
Co ela o index
Co ela o s ou pu s
The p ocess o he es ima ion o equency de ia-
ion is depic ed in igu e 7 and i is done in pa allel
wi h he ine iming p ocess. S a ing wi h he i s
sample o he i s B- ield, he incoming signal is
delayed by he delay D (32 samples). The conjuga e
complex samples o he delayed signal a e mul i-
plied by he ecei ed samples. The ou pu is hen
a e aged o e 96 p oduc s (un il he end o he B-
ield). S aigh o wa d analysis shows ha he angle
o he esul Y is p opo ional o equency de ia ion.
Fig. 6. Ma ched il e s ou pu . B sec ion.
F equency co ec ion can be easily pe o med by
complex mul iplica ion o he in-phase and quad a-
u e n- h inpu sample wi h , whe e ∆ω is he
es ima ed equency de ia ion.
nj
eω∆
D(.)*
A e age
Recei ed
da a
Fig. 7
5. Ha dwa e implemen a ion
P e ious sec ions desc ibed a new scheme o ime
and equency synch oniza ion wi h applica ion o
PLT. A ha dwa e implemen a ion o his scheme,
called Hype synch Module (HSM), has been done
using VHDL. HSM has been implemen ed ying o
sa e as much silicon a ea as possible while i ing i
in o he sys em clock equi emen s.
FIFO 16
MULT
CONJ
DinRe{13}+DinIm{13}
MOD
2
FIFO n
n{32:128}
SUM
NEG
ModDin
2
{27}
FIFO n
n{32:128}
SUM
NEG
Value{27} IncValue{27}
SUM+
ACUM
Acum
MOD
2
ENERGÍA POT2 ENEGÍA
2
DIV
ModAcum
n
FsmSinc oA
NewDa LoadFIFO16
PhaseB
PhaseA LoadFIFON1
LoadFIFON2
Cl Sinc oA
LoadFi o16
LoadFi oN2
LoadFi oN1
Fig. 8 Sec ion A a chi ec u e
Fi s o all, ega ding possible ime-sha ing o ha d-
wa e esou ces, coa se and ine synch oniza ion wi h
sec ions A and B o he p eamble a e made in di e -
en ime in e als, sha ing ha dwa e esou ces such
as memo ies, adde s and mul iplie s. Second, hose
a i hme ic ope a ions which a e oo slow o which
consume la ge silicon a ea, such as squa e oo and
di isions, a e a oided, by using powe -o - wo cod-
ing o he in e nal signals. Finally, pipeline
echniques has been in oduced in o de o educe
delay chain and op imize c i ical pa hs. Figu es 8
and 9 shows a b ie desc ip ion o he solu ion
adop ed o bo h blocks and ha dwa e esou ces
consumed. They show hei own memo y blocks, bu
in he inal sys em he module is sha ed. Fo a oid-
ing squa e oo s all he quan i ies a e squa ed.
Be o e being in eg a ed on he inal chip, HMS has
been p o o yped on an Xilinx Vi ex-XV300 FPGA
( o m HADES-1 Sys em [16]) using Founda ion 3.3i
The comple e sys em equi ed no mo e han 50.000
sys em ga es excluding memo ies, and wo king a 33
MHz.
The whole sys em (Base Band P ocesso ) has been
also in oduced using he same en i onmen and has
been a ed o 80.000 sys em ga es a 33MHz. The
p o o ype has been un using a XSV-800 boa d and
he on-chip memo y o s imuli injec ion. The esul s
has been compa ed wi h Ma lab high le el simula-
ions, and all he quan iza ion e o s and o he
e ec s has been alida ed he FPGA emula ion ap-
p oach.
Sinc oB Algo i hm
Da aIn Re{13},Im{13} X
COEFICIENT
MEMORY(32)
Dec0 Dec1 Dec2
FIFO32
+
Acumula o Bank
RegAng
COMPARATOR
F O
Rom and
Sma Acumula o
COSINE(N*F O )
SINE(N*F O )
SEQUENCE
COMPENSATION
Fig. 9 Sec ion B a chi ec u e
6. Conclusions
Powe line communica ion is now possible wi h
signi ican ad an ages o e con en ional cable in-
dus y because i uses he exis ing elec ic powe
in as uc u e. Howe e , he physical medium is
ha d, equi ing inno a i e solu ions. In his pape , a
new scheme o ime and equency synch oniza ion
o OFDM bu s ansmission is p esen ed as well as
i s ha dwa e implemen a ion on an FPGA. Simula-
ion esul s a e p o ided and show he e ec i eness
o he p oposed solu ion, and an implemen a ion has
been made using mode a e ha dwa e esou ces.
Acknowledgemen s
This wo k has been inanced by he Eu opean
Commission unde he IST V p ojec INSONET (IN
home and Soho NETwo king h ough he mains
ne wo k).
Re e ences
[1] H. Rohling, T. May, K. B üninghaus and R. G ünheid.
“ B oad band OFDM adio ansmission o mul ime-
dia applica ions”. P oc. O he IEEE ol. 87, no. 10,
oc obe 1999.
[2] John .A.C. Bingham: "Mull ica ie modula ion o
da a ansmission: an idea whose ime has come".
IEEE Communica ion Magazine, ol 28, no. 5, pp 5-
14, May 1990.
[3] T. Kelle and L. Hanzo. “Adap a i e mul ica ie
modula ion: a con enien ame wo k o ime-
equency p ocessing in wi eless communica ions”.
P oc. O he IEEE, ol. 88, no.5, May-2000.
[4] J.S. Cho, J.C. Tu and J.M. Cio i, “A disc e e mul i one
anscei e sys em o HDSL applica ions”. IEEE J.
Selec ed a eas Comm., ol.9, pp 895-908., Aug. 91.
[5] A. G. Bu , P.A. B own. "Applica ion o OFDM o
Powe line Telecomunica ions". P oceedings o 3 d
ISPLC, Landcas e . 1999.
[6] S.B. Weins ein and P.M. Ebe , “Da a amsmission by
equency di isión mul iplexing using he Fou ie
ans o m”. IEEE T ans. Comm. Tech. Vol com-19,
pp 628-634, Oc -1971.
[7] T. Polle , M. Van Bladel and M. Moeneclaey. “ BER
Sensibili y o OFDM sys ems o ca ie equency o -
se and Wiene phase noise”. IEEE T an. On Comm. ,
ol. 43, no 2-3-4, Feb-Ma -Ap 1995.
[8] P. H. Moose. “A echnique o o hogonal equency
di ision mul iplexin g equency o se co ec ion”.
IEEE T ans. Commun., ol. 42, pp. 1590-1598, Oc .
1994.
[9] G. San ella. “A equency and symbol synch oniza ion
sys em o OFDM signals: a chi ec u e and simula ion
esul s”. IEEE T an. On Veh. Tech., ol 49, no 1, Jan-
2000.
[10] T.M. Schmidl and D.C. Cox. ”Robus F equency and
Timing Synch oniza ion o OFDM”. IEEE T an. On
Comm., ol 45, no. 12, Decembe 1997.
[11] F. Claβen and H. Mey , “F equency synch oniza ion
algo i hms o OFDM sys ems sui able o communi-
ca ions o e equency delec i a channels”. P oc.
VCT-94, pp1655-1659.
[12] M. Spe h, F. Claβen and H. Mey . “F ame synch oni-
za ion o OFDM sys ems in equency selec i e ading
channels”. P oc. VTC-97. Phoenix. May 1997.
[13] M. Sandell, J-J an de Beek and P. Bo jesson, “Tim-
ing and equency synch oniza ion in OFDM sys ems
using he cyclic p e ix”. P oc. In . Symp. Synch oniza-
ion, Essen, Ge many. 1995. pp 16-19.
[14] D. Lands ön. "Synch oniza ion in OFDM sys ems".
Licen ia e in Enginee ing Thesis. Ma ch 1999. De-
pa men o Applied Elec onics. Lund Uni e si y
(Sweden).
[15] P. J. Lang eld, K. Dos e . “OFDM Sys em sych oni-
za ion o Powe line Communica ions”. P oc. o ne
2000 In e na ional Symposium on Powe Line Com-
munica ions and i s applica ins. ISPLC’00 pp 15-
22.Lime ick. I eland
[16] M.A. Agui e, J.N. Tombs, A. To alba and L.G.
F anquelo “HADES-1: A Rapid P o o yping En i on-
men based on Ad anced FPGA’s”. Accep ed o
DCIS’01. Opo o 2001.