PERFORMANCE OF
A
LINEAR INTERFERENCE CANCELLER FOR
A
DS/
CDMA SYNCHRONOUS SYSTEM BASED
ON
THE EKF DELAY ESTIMATOR
Pila
Diaz,
Ram6n
Agus i
Dep . o Signal Theo y and Communica ions
inke si a Poli kcnica de Ca alunya
C/
So Eulilia d’hzizu s/n Mhdul D4
08034 Ba celona, Spain
Tel. 34-3-4017201, Fax 34-3-4017200
e-mail
:
[email p o ec ed]
Abs ac : Since he con en ional de ec o o en ails
o p oduce eliable decisions o
a
CDMA
channel,
se e al new mul iuse de ec o s ha e been p oposed
la ely. In his pape , we p opose bo h
a
simple linea
scheme o in e e ence cancella ion, which exhibi s good
pe o mance, and a synch onism scheme based on he
Ex ended Kalman Fil e
(EKF)
o achie e synch oniza ion
among he di e en use s a he base s a ion, o
a
di ec -
sequence code-di ision mul iple-access
(DSICDMA)
synch onous sys em. The in luence o he synch onism
scheme on he pe o mance o he in e e ence cancelle
is
analyzed unde Rayleigh mul ipa h ading.
I. INTRODUCTION
In
mobile communica ion sys ems
o
nex gene a ion,
he inc ease in capaci y will be one o he mos impo an
equi emen s
o
be
ul illed.
In
o de o imp o e he capaci y
in a DS/CDMA cellula sys em, which is basically limi ed by
in e e ence, known in o ma ion on in e e ing signals can be
employed o elimina e hem a he base s a ion. The op imum
demodula o o signals ha sha e he same channel in ime
and equency, desc ibed in [l], is based on a mul iuse join
de ec ion scheme. This op imum ecei e is, howe e , oo
complex o be implemen ed in
a
eal mobile communica ion
sys em, since i s g ows exponen ially in complexi y as he
numbe
o
simul aneous use s o be de ec ed inc eases.
0-7803-2742-XI95
$4.00
0
1995
IEEE
An
al e na i e scheme based
on
in e e ence
cancell a ion echniques is p esen ed he e o
a
synch onous
DS/CDMA cellula sys em, which employs Gold codes
as
sp eading sequences. The use o quasi-o hogonal sequences
as
he Gold codes in a synch onous DS/CDMA sys em
imp o es he pe o mance ob ained wi h an asynch onous
access
as
a
esul o he good beha iou
o
hese sequences
in e ms o pe iodic c oss-co ela ion. Ne e heless, in a
mul ipa h en i onmen , he quasi-o hogonali y be ween use s
in he same cell is des oyed by he mul iple p opaga ion
pa hs coming a he ecei e .
In
o de
o
keep
on
his quasi-
o hogonali y, i is necessa y o eso o a cancella ion
scheme ha elimina es he in e e ing p opaga ion pa hs, as
shown
in
Fig.
1.
In his case, he complexi y o he ecei e
only g ows linea ly
as
he numbe
o
simul aneous use s
wi hin
a
gi en cell inc eases. Despi e i s simplici y, i is
shown in [2] ia some compu e simula ion ha his scheme
imp o es signi ican ly he beha iou
o
he con en ional
In e e ence
cancelle
l
a
a
b
Fig.
1
A
DS/CDMA
ecei e
wi h
an in e ie ence cancella ion
scheme
68
ecei e in e ms o capaci y.
To
suppo his synch onous access among he di e en
mobiles a he base s a ion, a simple p ocedu e based
on
a
delay es ima ion scheme o low complexi y is illus a ed in
Fig.
2.
The p ocedu e we p opose consis s o es ima ing he
channel delay o e e y mobile a he base s a ion, and
acco ding o he ime e e ence e ained
a
he base, send he
app op ia e command o each mobile in o de o make hem
adjus hei ansmission ime o his e e ence. I his
adjus men is done
in
s eps o
A
seconds, le
us
no e ha he
synch onism command o be sen o e e y use would be
made
up
o only one bi .
The synch onism scheme his p ocedu e is based
on
consis s o a channel delay es ima o based
on
he EKF,
which is illus a ed in Fig.
3.,
and designed o wo king
p ope ly in acking
[3].
Le ( ) be he ecei ed signal, c( ) he sp eading code
assigned o he use o in e es and
2,
he channel delay o be
es ima ed. The i s block o he delay es ima o p ocesses
he ecei ed signal in o de o imp o e he signal- o-
in e e ence powe a io, ex emely low in CDMA
en i onmen s. The second block is he
EKF,
which is
esponsible o he es ima ion o he delay in oduced by he
mobile channel.
11.
DESCRIPTION
OF THE
CANCELLATION SCHEME
Fig.
4
shows he in e e ence cancella ion scheme
p oposed o a synch onous DS/CDMA sys em: cancella ion
is ca ied
ou
be o e combining he di e en p opaga ion
pa hs a he Rake ecei e .
us0
slgnal
,A
I,
/
4
_-
--7
h,
'(2
73'
~~Smcommand
Mobile
L
---
I
Fig.
2.
Synch onism scheme
o
a
DSiCDMA
synch onous
sys em.
Fig.
3.
Synch onismscheme based on he Ex ended
Kalman Fil e .
Le G( ) be he Gold sequence assigned o he k- h
use ,
L
he numbe o p opaga ion pa hs a he ecei e
inpu ,
M
he numbe o a ms a he Rake ecei e , K he
numbe o simul aneous use s in a gi en cell and
S
he
a e age ecei ed powe in absence o mul ipa h. The "use
k es ima o " in Fig.
4
compu es
s,( )
(n=O,.
.
.
,L-1)
om he
ecei ed signal, ( ):
whe e
i ,
and
dk
a e espec i ely he es ima e o he n- h
p opaga ion pa h complex ampli ude and he es ima e o he
i- h ansmi ed bi , bo h co esponding o he k- h use , and
ec ,( ) ep esen s a squa e wa e o m o du a ion
T.
Le n( ) ep esen he in e e ence gene a ed by o he use s
in he adjacen cells. When pe ec synch onism be ween
use s wi hin he same cell and pe ec es ima es o he
channel impulsional esponse and ansmi ed bi s a e
assumed, i can be shown ha ,,,( )
(i
=
1,.
. .
,K; m
=
0,.
.
.
,M-
1)
a e cancella ion akes he exp ession
[2]:
I
.
id
'I
Fig.
4.
Block diag am o he in e e ence cancelle .
69
Tha is, in a cellula mobile sys em limi ed by in e e ences,
a linea cancelle such as his p oposed he e would be ideally
limi ed in capaci y by he in e e ing signals coming om
adjacen cells.
In p ac ice, howe e , pe ec es ima es a e no a ailable
and pe ec synch onism be ween use s canno be
accomplished
so,
h oughou some ex ensi e simula ions, we
will show in sec ion
IV
ha he pe o mance o he cancelle
scheme in a eal en i onmen lies e y closed o his o he
ideal case.
111.
SIMULATION
MODEL
In o de o assess he pe o mance
o
he p oposed
cancella ion scheme o a
DSiCDMA
CBPSK synch onous
sys em based on he
EKF
delay es ima o , he comple e
ansmission chain has been simula ed o
K
di e en use s
wi hin a same cell. Such a chain is shown in Fig.
5,
whe e
sn( ) ep esen s he complex en elope
o
he signal
ansmi ed by he k- h mobile:
(3)
The se
o
Gold codes assigned o each use , cL( )
(k=l,
...,
K),
has been ob ained om wo PN sequences
o
leng h
N=
127
chips, gene a ed om he cha ac e is ic
polynomials
211
and
277
(oc al) espec i ely
[4].
Fig.
5
Block
diag am
o
he
simula ed
DS/CDMA
qynch onous
sys em
Le hk( ) be he low-pass equi alen impulsional
esponse
o
he channel o he k- h use :
The channel has been modelled wi h an exponen ial Powe
Delay P o ile (PDP), wi h
a
Delay Sp ead no malised o he
bi in e al equal o
D,,=0.01,
as
usual in an indoo
en i onmen .
Conce ning he low-pass equi alen Dopple Powe
Spec um o each channel p opaga ion pa h, he model
"class"
[SI,
gi en by he exp ession
hac been assumed, whe e
d
is
he maximum Dopple
equency. dT,=0.0005 has been se in ou simula ions o
model he channel a ia ions in an indoo en i onmen .
Finally, conce ning compu a ion o he bi and he
channel impulsional esponse, which a e needed o pe onn
cancella ion, a Rake ecei e and a il e ma ched o he
co esponding Gold sequence ha e been conside ed
espec i ely.
A
da a ame has been designed in o de o
allow o es ima ion o he channel impulsional esponse
du ing he ecep ion o he known bi s in he p eamble, as
shown in Fig.
6,
and es ima ion o he ansmi ed bi s du ing
he ecep ion
o
he in o ma ion block.
Q
and
D
ha e been
dimensioned o an indoo en i onmen
(Q=lO
bi s and
D=90
bi s) acco ding o
[6].
I .
RESULTS
AND CONCLUSIONS
The pe o mance o he linea in e e ence cancelle has
been analyzed unde he assump ion o an open-loop powe
con ol and
an
EKF-based synch onism. Ex ensi e
simula ions
o
he whole sys em ha e allow us o s udy he
cancelle beha iou unde ealis ic si ua ions in e ms o
capaci y and analyze he EKF-based delay es ima o in luence
70
I
P eamble
I
In anla lon
i~
-
+
---___-___1
/
Q
bi s
D
bi s
Fig.
6
Da a i ame s uc u e employed in he simula ion
o he sys em
on he cancelle pe o mance.
The mean bi e o a e o a ixed numbe o
simul aneous use s has been aken
as
igu e
o
me i o
e alua e sys em pe o mance. Some ep esen a i e igu es
ha e been buil om he esul s we ha e ob ained in ou
simula ions.
Among
hese igu es, we ha e chosen Fig.
7
and
Fig.
8
ha illus a e he mean BER in e ms
o
he sys em
capaci y o
an
ideal synch onous access and o a
synch onous access based
on
he
EKF
delay es ima o . The
numbe
o
a ms in he Rake ecei e has been assumed o be
M
=
2
in bo h cases.
F om he wo
o
hem, we can conclude ha he use o
he cancelle s uc u e esul s in an impo an capaci y
imp o emen wi h ega d o he single-use ecei e , i.e. a
simple Rake ecei e , which may be e en g ea e in he
p esence o channel encoding.
In
addi ion, he synch onism
scheme based on he
EKF
is obus enough wi h e y li le
impac
on
he cancelle pe o mance. This is he eason why
0.1
1-
I
-+-wi h
ca celle
J
15
25
35
45
55
NUMBER
OF
c,iYJiTANFOLIS
IJSERS
Fig.
7.
Bi E o Ra e a he ou pu
o
he cancelle
o
an
ideal synch onism.
w
c
13.81
CY
0
E
CL
w
-
G7
_.
~.m!
LL
z
+no
cancelle
D.
Mbal
i
&wi !]
cancelle
5
15
25
35
45
55
NUMBER
OF
SIMULTANEOUS
USERS
Fig.
8.
Bi E o Ra e a he ou pu
o
he cancelle o
an EKF-based synch onism.
i seems o be a good candida e o synch onising he
di e en use s a he base s a ion
o
a synch onous sys em in
Rayleigh mul ipa h en i onmen s.
V.
REFERENCES
S.
Ve du, "Minimum P obabili y o E o o Asynch onous
Gaussian Mul iple-Access Channels",
IEEE
T ans.
on
In o ma ion
Theo y,
Vol.
IT-32,
Janua y 1986.
P.
Diaz, R. Agus i, "Analysis o
a
Linea In e e ence Cancelle in
Cellula DSiCDMA Sys ems", in P oceedings
o
he
43 d
IEEE
VTC,
June
1994.
P. Diaz, D. Henche, R. Agus i, "A PN Code Delay Es ima o Based
on
he Ex ended Kalman Fil e
o
a
DSiCDMA Cellula Sys em",
in P oceedings
o
he
4 h
In e na ional
Symposium
on
PIMRC,
Sep embe 1993.
R. de Gaudenzi, C. Elia, R. Viola, "Bandlimi ed Quasi-Synch onous
CDMA: A No el Sa elli e Access Technique
o
Mobile and Pe sonal
Communica ion Sys ems",
IEEE
Jou nal
on
Selec ed A eas in
Communicu ions,
Vol.
10,
No.
2,
Feb ua y
1992.
Cos
207,
"Digi al
Land
Mobile Radio Communica ions",
In o mu ion Technologies
und
Sciences
(CECJ,
1989.
P.
Diaz, R. Agus i.
"On
he In luence
o
he Channel Impulsional
Response Es ima ion on he Pe o mance
o
a
Linea In e e ence
Cancelle
o
a DSiCDMA Sys em", in P oceedings
o
he 5 h
In e nu ional
Symposium
on
PIMRC,
Sep embe
1994.
71