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subroutine sync4d(cd0,i0,ctwk,itwk,sync)
! Compute sync power for a complex, downsampled FT4 signal.
include 'ft4_params.f90'
parameter(NP=NMAX/NDOWN,NSS=NSPS/NDOWN)
complex cd0(0:NP-1)
complex csynca(2*NSS),csyncb(2*NSS),csyncc(2*NSS),csyncd(2*NSS)
complex csync2(2*NSS)
complex ctwk(2*NSS)
complex z1,z2,z3,z4
logical first
integer icos4a(0:3),icos4b(0:3),icos4c(0:3),icos4d(0:3)
data icos4a/0,1,3,2/
data icos4b/1,0,2,3/
data icos4c/2,3,1,0/
data icos4d/3,2,0,1/
data first/.true./
save first,twopi,csynca,csyncb,csyncc,csyncd,fac
p(z1)=(real(z1*fac)**2 + aimag(z1*fac)**2)**0.5 !Statement function for power
if( first ) then
twopi=8.0*atan(1.0)
k=1
phia=0.0
phib=0.0
phic=0.0
phid=0.0
do i=0,3
dphia=2*twopi*icos4a(i)/real(NSS)
dphib=2*twopi*icos4b(i)/real(NSS)
dphic=2*twopi*icos4c(i)/real(NSS)
dphid=2*twopi*icos4d(i)/real(NSS)
do j=1,NSS/2
csynca(k)=cmplx(cos(phia),sin(phia))
csyncb(k)=cmplx(cos(phib),sin(phib))
csyncc(k)=cmplx(cos(phic),sin(phic))
csyncd(k)=cmplx(cos(phid),sin(phid))
phia=mod(phia+dphia,twopi)
phib=mod(phib+dphib,twopi)
phic=mod(phic+dphic,twopi)
phid=mod(phid+dphid,twopi)
k=k+1
enddo
enddo
first=.false.
fac=1.0/(2.0*NSS)
endif
i1=i0 !four Costas arrays
i2=i0+33*NSS
i3=i0+66*NSS
i4=i0+99*NSS
z1=0.
z2=0.
z3=0.
z4=0.
if(itwk.eq.1) csync2=ctwk*csynca !Tweak the frequency
z1=0.
if(i1.ge.0 .and. i1+4*NSS-1.le.NP-1) then
z1=sum(cd0(i1:i1+4*NSS-1:2)*conjg(csync2))
elseif( i1.lt.0 ) then
npts=(i1+4*NSS-1)/2
if(npts.le.16) then
z1=0.
else
z1=sum(cd0(0:i1+4*NSS-1:2)*conjg(csync2(2*NSS-npts:)))
endif
endif
if(itwk.eq.1) csync2=ctwk*csyncb !Tweak the frequency
if(i2.ge.0 .and. i2+4*NSS-1.le.NP-1) z2=sum(cd0(i2:i2+4*NSS-1:2)*conjg(csync2))
if(itwk.eq.1) csync2=ctwk*csyncc !Tweak the frequency
if(i3.ge.0 .and. i3+4*NSS-1.le.NP-1) z3=sum(cd0(i3:i3+4*NSS-1:2)*conjg(csync2))
if(itwk.eq.1) csync2=ctwk*csyncd !Tweak the frequency
z4=0.
if(i4.ge.0 .and. i4+4*NSS-1.le.NP-1) then
z4=sum(cd0(i4:i4+4*NSS-1:2)*conjg(csync2))
elseif( i4+4*NSS-1.gt.NP-1 ) then
npts=(NP-1-i4+1)/2
if(npts.le.16) then
z4=0.
else
z4=sum(cd0(i4:i4+2*npts-1:2)*conjg(csync2(1:npts)))
endif
endif
sync = p(z1) + p(z2) + p(z3) + p(z4)
return
end subroutine sync4d
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