MATLAB: How to do this more efficiently

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Hello,
I have this piece of code in MATLAB:
for kk=0:N-1
    for mm=0:N-1
        for pp=1:Np
            for qq=1:Np
                if ((kk*Ts+tau(pp)))<=(mm*Ts+tau(qq))) && ((kk+1)*Ts+tau(pp))>(mm*Ts+tau(qq)))
                   thetaSR=(((kk+1)*Ts+tau(pp)))-((mm*Ts+tau(qq)))); 
                   F_SR_MR(kk+1,mm+1)=F_SR_MR(kk+1,mm+1)+conj(H(pp))*H(qq)*(thetaSR*exp(1i*pi*fc**thetaSR)*sinc(fc*thetaSR));
                end
which obviously is not very efficient. How can I re-write it more efficiently?
Thanks

Best Answer

It isn't necessary to do the inequality test for every possible pair of kk and mm values. Since kk and mm are integers and Ts must surely be a positive number, your pair of inequalities is logically equivalent to
 kk-mm == d
where d = floor((tau(qq)-tau(pp))/Ts). Therefore you can simply add the appropriate vectors along corresponding diagonals of F. For large N, doing it this way should save quite a bit of computation time.
F=zeros(N);
for pp=1:Np
  for qq=1:Np
    d = floor((tau(qq)-tau(pp))/Ts);
    kk = max(d,0):min(N-1,N-1+d);
    mm = kk-d;
    theta=(d+1)*Ts-(tau(qq)-tau(pp));
    ix = d+1+(N+1)*mm;
    F(ix) = F(ix)+conj(H(pp))*H(qq)*theta.*exp(1i*pi*fc*theta).*sinc(fc*theta);
  end
end
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