Note: Descriptions are shown in the official language in which they were submitted.
i3
~ he pres~nt invention relateYI to the 3imulta-
neou~ two-~ay data transmi~io~ on 2-wi.re circult 9 and
more particularly to the t0chnique of echo cancellation.
It i~ kno~n that a t~chnique allowing sl~ulta-
neoun two-~ay data tran~misslon on two-wire line~ for
~xample by means of modems (modulator-d.emodulators) i~
that of "echo cancellation". ~h~n the ~lodems o~ each of th~
two end~ emit in the ame frequency ban,d, to recei~e th~
u~eful ~ignal, the echo~ mu~t be cancelled. To thi~ end,
echo cancelling device~ furni~h an e~timation of the echo~
and ~ubtract lt fro~ the signal received.
These echo~ are u~ually compo~d of a local echo
due to the mismatching of the differential circuit ensurine
4 wire - 2 wire passRge on ~hich di~tant echo~ due to
reflectlons of the signal transmitted by rea~o~ of i~pedan-
ce mismatchings, are superpo~ed. TheBe l~tter echo~ thu~
bear, at recep*ion, frequency drift, i.e, th2 carrier fre-
quency of the dl~tant ~cho i~ no longer the ~ame a~ that
from the ~ignal emitted.
Thuc, uhen the ~ignal emitted i~ ~odulated by a
carrier frequency fc, the echo arri~en, ~odulated by the
same carrier ~requency ~c subjected to ,R phase ~hirt
~n ~h~re ~n Y 2 ~ ~ fnT ~ ~0, L\f being the ~r~quency
drift of the echo, T the ~ampling periodO To cancel ~uch
an ~cho, it must be reconstituted by modulatin~ it by
the same carrier frequency fc, with9 at instant nT, a
phase ~n which i8 an estimation of the ~Dha~e ~ of thi~
echo. At each saMpling instant nT~ the e~timated echo
--1--
~,i'
9~3
signal must, at reception, then be c;ubtracted from the
signal received.
In an echo cancelling device known in the
prior art, at emission, the signal is in the form of
a series An of complex data which are modulated by
means of a modulator by the carrier frequency fc. A
differential circuit ensures coupling of the line with
the emission and reception o~ the modem. rrhe signal
hn received at instant nT is composed of the useful
signal coming from the distant end and from an echo
signal gn which is written:
= ~ A k~ke ~(2~ fc ~n) with ~n = 2~ ~ fnT + ~o~l]
and where rk is the sampled impulse response returned
into baseband of the echo signal.
At reception before demodulation, by means
of a demodulator, there is subtracte!d from each sample
hn received a sample ~n of correction ~o furnish a
sample en due to a subtraction circuit, en and ~n
being complex. It will be noted that the signal hn is
a complex signal phase shifted by ~2 by means of a
separator circuit inserted at reception between the
differential circuit and the subtraction circuit.
This sample en is then applied to the input
of a demodulator. The known echo cancellation devices,
enabling the phase ~n of the echo to be compensated,
are formed by a first system for reactualization of the
- 2 --
~g495~
coefficients Ck, for example in accordance with the
algorithm of the gradient:
-~ (2~ fen T + n)
Cnk+l = Ck + ~ enAn_k [2]
where the coefficients Ck are eomplex, where 7 is a real
positive eonstant, and by a second system for reactua-
lization of the phase ~n of the echo, the phase ~n
also being reactualized in accordance with the algorithm
of the gradient
~ n+l ~n + (en n ) [3]
where ~n is the reconstituted echo and ~n~ its conju-
gated complex and d iS a real positive constant.
At the output of the cancellation device,
the reconstituted eeho is written:
,,
n k Ck A ke ~(2~1fcn T + n) [5~
Sueh an eeho eaneellation deviee has been
deseribed, for example, in an artiele whieh appeared
in IEEE TRANS. COM. 25, No. 7, July 77, pages 654 to
666.
Sueh phase reaetualization systems are aeeep
table for eompensating a slow phase variation or a very
slight frequeney drift. However, when the phase varia-
tion eomprises a eonsiderable frequeney drift, sueh
systems are highly insuffieient. In partieular, when
the useful signal-to-eeho ratio, the eeho bearing drift,
is weak, the phase eompensation is no longer acceptable.
~ 9~s~3
In fact, it wo~ld necessitate adopting, in phase
reactualization algorithm [3], too
- 3a -
4~
great a coefficient a, which would deteriorate per~ormance~
in the ab~ence of frequency drlft.
The inventors of th~ present in~ention, M~r~.
COLIN de YERDIERE, LANGLOIS and ~r~. ~LCCHI; the latter
working ~lth th~ CNRS~ Laboratoire deJ Slgnau~ ~t Systèm~
(L.2 .S. ), have sought to overcome the~ drawback~ of the
pha~e reactualization de~ices to cancel echo in order to
allo~ a co~siderable frequ~ncy drift o~` the echo to be
corrected.
The in~ention ~entially con~i~t~ in di~po~ing
a firYt phane Yariation ~tem for tha 810w pha~e fluctua-
ti~n un and a second phase ~ariation ~y~te~ for the rapid
pha~ fluctuatio~ w~, the total pha~e varlation ~n bei~g
: obtalned at tha output of an addition circuitp said addition
circuit receiving th0 Bignals Un and wn from ~aid flr~t
and Qecond ~y~tem~.
According to ~ first feature, at each sampling
: in~tant nT, from a mag~itud~ d~, the magnitude dwn repr~-
~enting tha lma~inary part of th~ product Or con~ugated
complex echo correction 6nff by the data oi~nal en obtained
at tha output of the echo cancellation d~Yice, after elimi-
nation of said echo, ~aid product being multipli0d by a
coeff1ci~nt of proportionality, th~ p~ha~e co~pensation
sy~tem according to the in~ention oompr:ise~ mean~ for addlng,
at each ~ampling in~tant, the 810~ pha~e variation un
with the rapid ~ha~e variation wn, ~aid addition mean~
~urni~hing at the output the naturel pha~ ~n of th~ 0cho,
~aid ~lo~ variatioll un being obtained from ~aid incr~a~e
--4--
S3
dw by fir~t mean~, ~aid rapid variation bein~ obtained
from 3aid increa~e dwn by second ~ean~.
Moreover, ~aid fir~t mean~ furni~hin~ un at th~
output co~pri~e mean~ for adding the f.luctuation~ un lY
of the preceding sampllng in~tant, w1t]h the magnitud0
T.Fn, T being equal to the 8~mpllng pe:riod, ~aid ~agnitud0
Fn belng obtalned by ~ean~ for a~dition of the ~agnitude
Fn 1~ of the preceding ~ampling in~tant, ~ith a ma~itud~
dFn, ~aid 0agnitude dFn bei~ obta~ned by maan~ for addit1on
of ~aid increa~e dwn, weighted by a con~tant ~ , with th~
magnitud~ d~n_l of the pr0ceding ~a~pl:Ln~ in~tant, ~aid
~agnitude d~n 1 al~o belng w~ightsd by anoth~r ~o~stant
(l-S), ~ and S bei~g real pssit1Ye conl~tant3.
According to a preferred e~bodiment of ~he inven-
tio~, the con~tant i~ equal to 1.
Accord~ng to a ~urther feature of the in~æntion,
said ~eco~d mean~ furni~hi~ the rapid fluctuation~ wn
comprise ~an~ for adding aaid increa~e dWn ~lth th~ ma-
gnitudc wn 1 of *he preceding samplins in3tant.
The in~ention enable~ echo~ b~aring a considerable
fr~uency drift to be ea~ily corrected"
The in~ention will be more roadily under~tood o~
reading the followlng de~cription wlth referenc~ to th~
accompanyi~g drawing~, in wh~ch :
Fig. 1 iB a general repre~en1;atlon of a ~nown
echo cancellation device.
Fig. 2 ~how~ a known ~yRtem o~ reactualization
D~ the phase of an scho canccllation de~Yice.
9S3
Fig. 3 shows a decomposition according to the
invention of the echo phase fluctuations.
Fig. 4 shows the phase correction loops accor-
ding to the invention.
Fig. 5 is a preferred embodiment of the inven-
tion of Fig. 4.
Fig. 1 illustrates an echo cancelling device
known in the prior art. At emission, the signal is
in the form of a series An of complex data which are
modulated by means of a modulator 1 by the carrier
frecIuency fc~ A differential circuit 2 ensures cou-
pling of the line with the emission and reception of
the modem. The si~nal hn received at instant nT is
composed of the useful signal coming from the distant
end and from an echo signal gn which is defined in
accordance with the aforesaid rela-t:Lon [1~ .
At reception before demodulation, by means of
the demodulator 4, there is subtracl,ed from each sample
hn received a sample ~n of correction to furnish a
sample en due to the subtraction circuit 6, en and ~n
being complex. It will be noted that the signal hn is
a complex signal phase shifted by ~2 by means of a sepa-
rator circuit 7 inserted at reception between the diffe-
rential circuit,2 and the subtraction circuit 6.
This sample en is then applied to the input
of the demodulator 4. The known echo cancellation
devices 5, enabling the phase ~n of the echo to be
-- 6 --
.~
i3
compensated, are formed by a first system 51 for reac-
tualization of the coefficients Ck~ for example in
accordance with the aforesaid algorithm [2] and by
a second system 52 for reactualizat:lon of the phase 0n
of the echo, the phase ~n being reactualized in accor-
dance with the aforesaid algorithm [3] .
At the output of the cancellation device 5,
the reconstituted echo ~n is defined in accordance
with the aforesaid relation [5]
Fig. 2 shows the known systems of reactua-
lization of the phase ~n of an echo cancellation device,
which comprises a circuit 21 for mu]tiplication of the
estimated conjugated complex value -n~ reconstituted
for the preceding sample by the value en furnished by
the subtraction circuit 6 of Fig. 1. The circuit 22
receiving the result of multiplication of the circuit
21 furnishes at the output the pure imaginary part of
this magnitude which is then multiplied by ~ by means
of the multiplication circuit 23. ~ is an appropriately
selected constant extracted from a memory 24.
The magnitude furnished at the output of the
circuit 23 is added with the value ~' furnished at
n-l
the output of a circuit 25 delaying by a sampling pe-
riod T. The result obtained at the output of the addi-
tion circuit 25 is ~n' value of the phase shift of the
echo for each sampling instant nT according to a known
estimation of first order. This est:imation is written
by the relation ~n n-l n
- 6a -
i~,.!
The invention consists in decomposing the
phase ~n into two components un and wn, one, un,
coxresponding to a slow drift phenomenon capable of
decreasing or
- 6b -
~¢~953
increa~ing, the other, w , corr~ponding to a rapld pheno-
~enon, and 1n extracting from the rapidi pheno~enon at each
~amplin~ in~tant 9 the re~idue~ of ~low phenomenDn~ The
pha~e ~n~ at instant nT, i~ then ~riSt~n a~ the ~um of th0
compon~nt~ un and wn.
Referring to ~ig. 3, the compcnent u repre~e~t~
a m0an value o~ tha phase ~hil~t u repr0s~nt~ ~mall ampli~
tude variation~ about this mean drift value~
T~o different ~y~t~mA of reactualization o~ each
of the~e t~ component~ are di3po~ed accordlng t~ the ln-
~ention.
The component ~ 1~ reacSualized acc~rding to
the in~entl~n by mean~ of a phase loop Df the first ord~r
whil3t the co~ponent un i~ reactualized acc~rdin~ to the
invention by mean~ of ~ phas~e loop of a higher ordierO
One ftr~tly form~ the increa~e dwn ~rom
the conjugated comple~ r~constituted echo and ~rom ~n~ th~
compl~ v~lue of the u~eful signRl afte~r ~ubtractio~ of
the echo. To thl~ end, a multlplication circuit 31 reoei~
~ing en and ~n~ furnishe~ a magnitude alc the ~nput of a
circult 32 whlch allo~ only the imaginary part at the out-
put to pasc. Thi~ magnitude i~ mulSipll~d by a by ~ean~ of
a multiplication circuit 33 furnishing at the output dw 9
the increaae of thc rapid componentO The! con~teLnt ~ 1B
extracted from a memory 34.
dw iB wrltten :
dw~ _ ~ ~ (en ~-n ) L6 J
--7--
5~
According to the invention, there are ~xtracted
from thi1 ~agnitude dwn the reBidu~ of 910w phenomenon
u . To this end, dw i~ applied to the :Lnput o~ a flr0t
rcactualization aystem aff~ctin~ s~era:l proces~ing~ of
the ~lgnal dwn written mathematically :
n = (1 -~ ) dF~ 1 ~ ~ B d~
F ~ d~
n n-l
u z u ~ T F
n n-l n
~here ~ a~d ~ ~re appropriately chosen con~tant~ and T i~
equal to the ~ampling period,
Referring to Flg. 4, thi~ fir~t ~e~ctualizat$on
~y3tem i~ compo~ed of a first circuit 3~3 for ~ultiplication
of the ter~ d~ by a ~uitably choae~ csn~tant ~ extracted
fro~ a ~amory 39. Thi~ magnltude ~S dwn i~ applied~ at th~
outp~lt of the clrc~it 38g to the input of an add~tion cir-
cu~t 40, Thia addition circuit 40 furni~lhe~ at the output
a magnitude d~ . The other lnput of thi~; addition circuit
~0 receive~ the magnltude dFn 1 of prec~,ding ~a~pllng
instant (n-l)T, contain~d in a register 41 delaying the
8a~pling tl~e T9 ~ald magnit~de d~n 1 bei~g we~ghted by an
appropriately cho~en coefficient (l-b). A m~ltipllc~ti~n
circult 42 effect~ this weighting of dF 1~ the con~tant
1 -~ b0-lng e~tracted ~rom a me~ory 43.
The term dF la then applied to the input of a
c~rcuit Is4 for addition of ano~her loop effecting a fllte-
ring, ~ritten aa follow~ :
5~3
F = ~ ~ dF
n n-1 n
The output of the circuit 44 furni~he3 in f~ct
F whilst the other input of circuit 44 r~c3i~ th~ term
F 1 contained i~ th~ ~hlft regiat~r 45 d~l~yi~ by
3ampling ti~e T.
The ter~ F~ Dbtained at th2 output ~r the circuit
44 i~ ~pplied to th~ input of a MultlplicQti~l~ circult 46
~hich multiplie~ it by the ~agnitude T ~qu~l ~o th~ 3~m
plin~ pericd o~tract~d from a memorr 47~ Th~ output of th~
~ultiplication circuit 46 i8 co~nected to the input of an
additi~n cir~ult 48 whlch furnishes ~t the output th~ ~low
phase fluctuations u~O The ~ther lnput ~D~ the ~ddition
circult 48 r~caiYe~ the term un 1 ~tored in a shift regi~-
ter 49 delaying by a uampling time T. ~li~ new loop thu~
affect~ a~othsr flltering~ written A~ ~13110W~:
u = u ~ 5` F`
n n-l n
The co~ponent un furnl~had by the ~ddit~on
clrcui* 48 i3 applied to the lnput of an ~ddition cirouit
37 ~here it iu added to the other component ~n corre~pon-
dl~g to the rapid fluctu~tion~. Yha addi.tion circuit 37thu~ furnishe~ at the output the pha~a ~ e3timat~d for
~a~ple n.
The compnnent wn, pra~ent on th~ ~ther input of
the addition circuit 37 i~ alao obtained~ from the ~ncrea~
dwn, but by ~ean~ of a ~imple filtering. Thl~ magnitude
dw i~ applled to the input ~f an addition circult 35
which effecta the sum of thi~ magnitude dwn with tha
value ~n 1 furnished at the preceding ~amplin~ in~tant
3'~ 3
and conserved by means of the ~hift regi~ter 36 delaying
by timR T.
The ra~id phenomenon ~ i~ thu~ reactualized
in accordance with the algorithm of gradlent :
~n = Wn-1 ~ d~n X~o~lng that d~
Thu9 9 the ~lo~ phenomenon i~ reactualiz~d
according to the invention by m~an~ of a pha~e corr~cti~n
loop of the third order whil~t t~e rapid phenomo~on i~
r~act~allzed by ~ean~ of a pha~e correction loop of the
firYt order.
Th~ choic~ Or ~h~ con tant~ S r~ult~ fro~
a compromise bet~een the rapidity ~f ~cqulring th~ ~tima-
tio~ of the fr~q~ency drift and the acclspted de~radatio~
in the ab~ence of frequency drift o~ the echo~ ~owe~er,
i~ usually ohoaen to be greater tha~ 1~.
Fig. 5 3ho~e a preferred embodi~e~t of the
in~ention for which ~= 1. In thi3 way 7 the 810w pha~e
variation loop i~ ~implified ~n~ the al~Sorithm3 are
written aa follows :
2 0 ~ a u~ +
w a w ~ dw
n n-l n
dWn = a ~ ( ~ n en) ,> L
u ~ u + T.F
n n-l n
n-l + T d n "
~here u ~epresent~ the pha3e correction, which i5 the s~me
as elimin~ting the filtering of the term dF O Referrin~
_ 1 ~
to Fig. ~, the addition circuit 44 receive~ at it~ input
directly the term ~ dwn which lt adds with Fn 1~ furnii~h~d
by th~ delay T circult 45 to furni~h at the output the
ter~ ~n at the input of the multiplicatlon circuit 460
In thi~ way, only the loop formed by th~ delay oircuit 41
and the m~ltipllcation circuit 42 iB eli~inated. ~he alow
pha~e variation reactuallYiation ~yste~ pa~s~ fro~ the
third to the ~econd order.
Simulation~ of tha embodiment of ~lg. 5 hav~
made it po~ibl~ to obtain very ~ati~ifactory pha3e reac-
tualizRtion re3ult~, to cancel echo. In the pr~ferred
~y~tem Or ~ig. 5 9 the value~ of ~ and ~, in permanent
regime, vary f~om 10 3 to 10 for and 10 to 10 3
for ~.
It i~ ~imply nece~ary that t:he coefficient
be le8~ than ~ to ha~e an integration and ~ufficient
precl~ion on the e3ti~ation of the frequency drift of the
echo. On the contrary, if ~ i~ too ~mall, the ti~e neces-
~ary for obtaining a good estimiation ~f the frequellcy
drlft ~ill be too great. For ~ery lo~ value~ Or ~9 the
e~tlmation of the dri~t not b~ing obtai:n~d a~ r~pidly9
the quality of ~imultaneous two-way tra:ni~iislon 16
degradsd.
The pre~ent ~y~t~ of r~sactua.lization of the
phas~ ~ accordin~ to thc invcntion la Is~entially conc~r
ned wlth an increa~e dwn form~gd from thls data ~ignal en
after cancellation of the echo in the comple~ do~ain.
However, the pha~e reactualization ~y~ t~3m according to the
invention is also applicable on reconstitutlng only the
~eal part of en.
The lncrea~e d~n i~ then ~ritt~n :
dw~= ~ sn ~ ( ~n
and Fig~. 4, 5 and eq~ations [7~ rem~ll unchang~d.
The pre~nt system for reactu~li2ation of pha~e
~ n according to thc ln~ention i3 appli.ed bDth to a ~i~nal
A in ba~ band a~ shown in ~i~. 1 and to a ~lgnal A
in pa~ band in which case modulation occurs, in th~
canc0llation device, a little different;ly. In f~ct9 lt i~
known that, when the cancellation deYic:e 5 operate~ i~ base
band a~ ~ho~n in ~ig. l, thc coefficiemt~Cn are formed by
a reactu~lization y~te~ 51 d~rectly fro~ the cu~plex data
~ignal A . Modulation by a carrier ~ign.al fc pha~e ~hifted
by a pha~e ~n furnished by the pha~e reactuallz~tion
By3te~ according to the in~ention therefor~ occur~ at the
output of the ~y~tem for raactual1zatlon of the coefficientsO
~ oweYer, t~a phase comp2n~ation ~y~tem accordin~
*o tha inventlon would ~l~o be applied to an echo canc~l-
lation dc~ice functioning in pa~ band, The ~y~te~ ~orreactu~ ation of the coef~icient Cn t:hen r~c~ive~ thc
d~ta ~ignal A~ already modulated by the carrler fr~q~ency
fc and the phase shift of ~ furni~hed lby the pha~e
co~pen~ation sy~tem according to the in~vention 10 effected
vnly at the output of the ~y~tem ~or reactu~lizRtion of
the ~oefficientR.
-12-
