Hi,
I have several questions when I try to implement raised root cosine
filter.
1. I have read several books about digital communications, such as
that of Bernard Sklar. They normally interprete the ISI effect in a
simple one channel situation. If I want to use Matlab to simulate a
communication system using I,Q modulation, such as QPSK, where should
I put the RRC filter at? I only consider the simple I, Q channel
seperately, not include the complex I,Q filtering. From what I
understand, it seems that it should be between the following
functions.
In the modulation process:
z = modmap(x,Fd,Fs,method,...);
y = dmodce(z,Fd,Fs,[method,'/nmap'],...);

In the demodulation process: 
z = ddemodce(y,Fd,Fs,[method,'/nomap'],...);
rec_sig = demodmap(z,Fd,Fs,method,...);

2. The RRC filter in Matlab is like as:
[y, t]     = rcosflt(x, Fd, Fs, 'filter', num, den);

This function and the above mod/dem functions have Fd, Fs. How to
determine these values? Fd should be the same for all the functions?
If one function has the value Fs, the next stage function has the Fs
which is integer times of the previous Fs?

3. If the question 1 I suppose is correct, i.e. the RRC filter is
before the dmodce() function, the RRC filter can improve the abrupt
spectrum widening of QPSK(180 degree in the worst case)? I know RRC is
used for reducing ISI. But it also can be thought as  a low pass
filter, is it right?

Your comments are highly appreciated.