DSP-based ionosphe ic adio-link using DS-
CDMA
and on-line channel es ima ion
J.L.Pijoan,
J.C.
Soco o,
J.A.
Mom,
F.
Ta es
Abs ac -In
his pape , a new blind mul iuse de ec ion
algo i hm is p esen ed. I can bo h cancel mul iuse
in e e ence and es ima e he mul ipa h channel esponse in a
blind way. The me hod has been specially concei ed o low
cohe ence bandwid h channels such as he ionosphe ic
channel and exhibi s e y low compu a ional equi emen s.
Real- ime measu emen s om a ully digi al
HF
adio-link
a e p esen ed ha con i m he eliabili y
o
he me hod
o
he ionosphe ic channel.
Index
e m-HF
communica ions, blind channel es ima ion,
mul iuse de ec ion,
CDMA,
ionosphe ic links.
I. INTRODUCTION
Ionosphe ic communica ions a e a good choice o low-
a e, long-dis ance links, due o hei low cos ,
independence o sa elli e in case o mili a y con lic and
low equency ca ie s ha allow di ec analog- odigi al
con e sion. Howe e , he ansmission powe equi ed
is
high and na ow bandwid hs a e a ailable. Mo eo e , he
ionosphe ic channel p esen s slow a ying mul ipa h
ading wi h e y small cohe ence bandwid h
(cs
1
KHz),
na owband in e e ence wi h low signal- o-in e e ence
a ios
(SIR)
and poo signal- o-noise a ios
(SNR)
[l].
To o e come hese limi a ions, a digi al DSP-based Di ec
Sequence Sp ead
Spec um
(DS-SS)
ionosphe ic adio-link
be ween Huel a and Ba celona has
been
de eloped using a
new low complexi y me hod (called
Dual Vec o Minimum
Ou pu Ene gy
-
DWOE)
o es ima e he channel
esponse in a blind-way when a ading
occu s
[2].
DVMOE
is a new MOE-based algo i hm
[3]
wi h addi ional
cons ain s ha allows
bo h
mul iuse in e e ence
cancella ion (p esen ing
Nea Fa esis ance)
and
mul ipa h channel es ima ion. DVMOE
has
been es ed in
a eal- ime
HF
adio-link and i s easibili y and eliabili y
in he ionosphe ic channel has
been
p o ed.
J.L.Pijoan,
J.C.
Soco o and
J.A.Mo an
a e
wi h
he
Depa men
o
Communica ions
and
Signal
Theo y,
La
Sdle
School
o
Enginee ing,
Ramon
Llull
Uni e si y
(URL),
Pg.
Bonano a
8,08022
Ba celona
(Spain).
F.
Ta es
is
wi h
he
Depa men
o
Signal
Theo y
and
Communica ions,
Poly echnic
Uni e si y
o
Ca aIunya
(VPC),
Campus
No d
UPC,
08034
Ba celona
(Spain).
The es o he pape
is
o ganized
as
ollows. The adio-
link is b ie ly desc ibed in
sec ion
11.
The sys em model
is
p esen ed in
sec ion
I11 and he o mula ion
o
DVMOE
is
in oduced in
sec ion
IV. Simula ions and measu emen s
om he eal- ime link a e p esen ed in
sec ion
V. Finally,
sec ion
VI
con ains he conclusions.
U.
DESCRIPTION
OF
THE
LINK
A
digi al
HF
adio-link
has
been
de eloped' wi h
h ee
main a ge s. Fi s , o ha e a lexible digi al pla o m able
o implemen any kind o communica ions scheme.
Second,
o e alua e he ionosphe ic channel
by
measu ing hose
pa ame e s o in e es om he
digi al
anscei e poin o
iew, i.e., he ime delay sp ead and he Dopple sp ead.
The las goal is o implemen a digi al sp ead-spec wn
ionosphe ic adio-link, aking ad an age o
SS
capabili ies,
such as obus ness agains na ow-band in e e ence,
mul ipa h ading and low
SNR.
Mul iple signa u es
can
be
used
o di e en use s and o inc ease he bi a e
o
a
single use as well. The main ea u es o he sys em
a e
lis ed below:
-
Simplex communica ions. The ansmi e is placed in
Huel a and he ecei e in Ba celo~. The dis ance
be ween bo h ci ies is
800
Km.
Ca ie equency:
4-8
MHz.
in
his
band, e lec ion is
almos gua an eed du ing all he day. Highe
equencies a e only a ailable in day ime.
In o ma ion a e:
cs
1 Kbps pe signa u e. Sp eading
ac o
is
31
and
da a
modula ion is DPSK. All hese
pa ame e s a e con igu able.
Full digi al implemen a ion.
A
di ec analog- odigi al
con e sion
(50
Msamplds) o he ecei ed
signal
and
a digi al downcon e sion
is
pe o med. The baseband
signal
is
hen p ocessed by 3 DSPs wo king in pa allel.
This s uc u e allows he sys em o be ex emely
lexible and e e y modi ica ion
is
educed o
a
so wa e ma e .
-
-
-
'
Suppo ed
by
he
esea ch
p ojec
CICYT
TIC97-0787-CO2-01
inanced
by
he
Spanish Go e nmen .
860
lo&
Medi e anean
Elec o echnical
Con e ence, MEleCon
2000,
Vol.
I1
0-7803-6290-X/00/$10.00
02000
IEEE
A
de ailed desc ip ion o he high
speed
acquisi ion and
p ocessing ha dwa e
can
be
ound in [4]. The sys em can
ope a e in
wo
modes:
-
Channel e alua ion mode.
A
es signal is ansmi ed
e e y minu e 24 hou s a day in o de o measu e he
sca e ing unc ion o he channel.
This
p ocedu e does
no equi e eal- ime ope a ion and is desc ibed in
de ail in
[
11.
Real- ime ope a ion. In o ma ion is ansmi ed and
ecei ed in eal- ime. PN acquisi ion in low
SN'R
en i onmen wi h na ow-band in e e ence and long
adings
has
o
be
accu a ely designed
[5].
Some
mul ipa h channel es ima o s ha e been de eloped o
eco e
in
a blind way when a ading
occu s.
-
111.
SYSTEM
MODEL
Le
us
conside a CDMA sys em wi h
K
synch onous use s
h ough a mul ipa h channel. The ecei ed baseband signal
can
be
modeled as:
k=l
n=-m
whe e
A,,
6,
a e he ampli ude and bi o use
k,
Tb
is he
bi du a ion and
n( )
is
a whi e gaussian noise wi h
uni
powe
spec al
densi y.
wk
( )
is he ecei ed wa e o m o
use
k
whe e {sk(1),sk(2)
...
sk(N);sk(i)
=
l}is he PN sequence
o use
k,
h( )is he complex alued channel esponse
(conside ed
equal
o all use s),
N
is he p ocessing gain
and
T,
is he chip in e al. A e chip-ma ched il e ing,
he wa e o m is sampled a chip- a e. The ecei ed
disc e e- ime signal in one symbol in e al is:
whe e
S,
and
S
a e
he ma ices o delayed eplicas o
he cu en and p e ious symbols and
h
is he - ap
mul ipa h channel esponse. Fo sho dis ance ionosphe ic
links,
L
is qui e long compa ed o he
symbol
du a ion,
so
a
no negligible pa o he symbol is co up ed
by
he
p e ious one. The obse a ion in e al is ixed o N in
o de o allow eal- ime implemen a ion, and a
Nea -Fu
esis an blind channel es ima o , able o deal wi h la ge
alues o
L,
is needed.
IV. FORMULATION
OF
DVMOE
Le
us
conside use
1
as he use o in e es . The p oposed
ecei e is:
6,
(n)
=
sgnp
(S
l,h
+
x))
(4)
whe e 6 and
x
a e calcula ed ollowing he minimum
ou pu ene gy c i e ion:
subjec o he cons ain s:
(Sl1612
=
1
and
SI$
I
x
.
When he ou pu ene gy is minimized, 6 is an es ima ion
o he channel esponse and
sl16
+
x
is
o hogonal
o he
mul iple access in e e ence. This me hod can be seen as an
ex ension
o
[3],
conside ing he ecei ed wa e o m as a
linea combina ion o delayed eplicas o he PN sequence.
I has been called
Dual Vec o
MOE
(DVMOE) because
he minimiza ion o
wo
ec o s has o be pe o med.
A
s eepes descen app oxima ion can be calcula ed by:
Th ee impo an cha ac e is ics
should
be emphasized.
Fi s ,
as
he nominal signa u e
is
no ixed bu depends
on
ec o
6,
he s a iona y solu ion does no su e om he
misma ch
p oblem. Second, DVMOE beha es no only as a
mul iuse de ec o , bu also as a blind
Nea Fa
esis an
channel es ima o able o deal wi h a highe numbe o
use s han o he me hods ([6],[7]) o a gi en channel
leng h. The basis o he me hod is he linea independence
among ecei ed ec o
Sllh
and he es o
2K-1
ec o s
om
(3).
Fo
L
<
N
/3,
a
co ec
es ima ion can be
achie ed o a numbe o use s
K
<N-L+l.
Fo
N
I3
<
L
<
N
,
he con ibu ion
o
p e ious bi s becomes
mo e sigm ican and 2K
<
N
-
L
+
1.
Finally, DVMOE is
a low complexi y me hod. I can wo k wi h an obse a ion
in e al o leng h N and only adds
4NL+L
ope a ions pe
symbol mo e
han
he blind de ec o om
[3].
A
eal- ime
implemen a ion
has
been success ully es ed and esul s a e
p esen ed in nex sec ion.
I
is
easy
o show
ha
Vh.J=SzVx.J,
so
lVL.JI
<<
lVx.JI
.
Hence,
px
<<
p,,
o ensu e small excess
e o s. Inc easing he numbe o ac i e
use s
o he
Nea
Fa
a io will make bo h g adien s highe ,
so
lowe
con e gence
speed
will
be
achie ed since smalle s eps
p,
lo i
Medi e anean Elec o echid Con e ence, MEleCon 2000, Vol.
I1
86
1
and
p,,
will ha e o
be
used.
This ac will lead o sligh ly
lowe con e gence
speed
han
o he subspace me hods
([61,[71).
.
SIMULATIONS
AND
REAL
TESTS
A
numbe o simula ions ha e
been
pe o med in o de o
p o e he obus ness o DVMOE in on o he numbe o
use s,
Nea Fu
and channel leng h. Gold sequences o
leng h 31 and a 15- ap complex alued mul ipa h channel
esponse ha e
been
used,
so
one
hal
o
he obse a ion
in e al is co up ed by p e ious
symbols.
All
he
in e e ing use s a e
5
dB
s onge
han
he use o in e es .
Figu e 1 shows he e olu ion o he channel es ima ion
e o o di e en numbe o
use s.
An
es ima ion e o
lowe
han
0.1 is achie ed in 1000 i e a ions wi hou
inc easing he il e size
i
he numbe
o
use s
is lowe
han
5.
io0
I
500
1Mx)
1500
Zoo0
25W
Bi s
Figu e
1.
Con e gence speed
o DW0E.
L=I5.
NF=5
dB
In igu e 2, he s a iona y solu ion is ob ained and
BER
e sus
%/No
is e alua ed wi h a 12- ap mul ipa h channel.
Fo a numbe o use s lowe
han
5,
he MMSE solu ion is
achie ed in a blind way.
Apa
om simula ions, he me hod
has
been
es ed in he
eal- ime digi al ionosphe ic adio-link desc ibed in
sec ion
II.
In o de o e alua e DVMOE as a channel es ima o , a
e e ence o he ins an aneous channel esponse is needed.
One
o
he in e e ing use s (i.e. use
2)
is sen wi h a
powe
20
dB
highe
han
he es o
use s
and he bi s and
signa u e
a e
known a he ecei e . Hence, he e e ence
can
be
calcula ed by:
whe e
(
)-'
s ands
o pseudoin e se.
...................
.........................................
............................
---_
--__
'-
io"
-
4
ioa
-
I
5
6
7
0
9
io
10aJ
EbNo
Figu e
2.
In luence
o
he numbe
o
use s.
L=I2.
h?F=5
dB
The es ima ion e o can now
be
de ined
as:
e
=
I
k-h,I2
Figu e
3b
shows he e olu ion o he es ima ion e o
e sus ime in a
eal
ansmission wi h
wo
use s.
Bi a e
is
600
bps wi h DPSK modula ion, p ocessing gain is 31
and sampling a e is
4
sampledchip.
Noise
powe is
measu ed be o e ansmission and
SNR
a he il e inpu is
shown
in igu e 3a.
In
seconds
2,4
and
7,
he chel
es ima ion e o g ows due o an inpu signal ading. Once
he
SNR
o he use o in e es eaches a ce ain le el
(W
dB),
DVMOE is able o sel - em e in a blind way in a ew
i e a ions. A e aged BER alues
be ween
IO5
and
lo5
(wi hou coding) ha e
been
meam ed om a
g ea
numbe
o ansmissions in Me en si ua ions.
VI.
CONCLUSIONS
A
new blind
MOE
algo i hm (DVMOE) o
DS-CDMA
sys ems
has
been
p esen ed. I is able o cancel mul iuse
in e e ence and o es ima e he mul ipa h channel
esponse a he same ime, showing a
good
adeo
be ween
channel leng h and maximum numbe o
use s
wi hou inc easing he obse a ion in e al.
DVMOE
exhibi s e y low compu a ional cos and i has
been
success ully es ed in a eal- ime
HF
adio-link.
862
10*
Medi e anean Elec o echnical Con e ence, MEleCon
2000,
Vol.
II
0
1
2
3
4
5
6
7
8
9
10
Time
(s)
(a)
2
I I
I
I
’I
1-
-
0
ULU
yc -
0
1
2
3
4
5
6
7
8
9
10
Time
(s)
(b)
Figu e
3.
E olu ion
o
signal
o
noise a io (a) and es ima ion e o
(b)
in a eal ansmission.
Day:
99/11/18
Time:
11:04
UT
VII.
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[4]
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10*
Medi e anean Elec o echnical
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MEleCon
2000,
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I1
863