Difference in filter coefficients by MATLAB and by formula

[saurabhmimani]

matlab filter coefficients

Hi all,
i was designing a FIR root raised cosine filter to use for my project of QPSK transmitter for pulse shaping purpose, first thing is I found following impulse response for rrc filter:-

(taken from wikipedia)
what I was planning to do is find h[nT] and than convolution it with input sequence.
where T is sampling time
Am I doing right or I should modify my approach as there is no mention about cutoff frequency or control on stopband attenuation.


After it I designed a FIR rrc filter using FDA tool of MATLAB, I found out the coefficients using FDA toll than I went in help and found that it uses following formula-

**broken link removed**

and i found coefficients using this formula but they are not matching , why is this and also using the formula i am not getting appropriate response.

Can somone help me why this is happening.

Thank You.

 

[ux]

fir matlab coefficient

I used a formula similar to wiki. It is correct. But you must also use the window to control out-of-band radiation. The window also brings ISI, therefore need a compromise between the number of coefficients and the strength of interference.
Result FDAtool or simulink RRC FIR block also correct, perhaps a bad formula in Matlab нelp or you are somewhere wrong. If calculated coefficients correct there would be no ISI in the model of the transmitter-receiver, check it.

 

[saurabhmimani]

fir coefficients in matlab

Thanks for reply.
I also wants to know using this formula:-
**broken link removed**
(from wikipedia)

How can I control cutoff frequency or stopband attenuation?

 

[ux]

cut off frequency with filter coefficients

How can I control cutoff frequency or stopband attenuation?

Сutoff frequency is given by Ts and roll-off factor(see Proakis Digital Communications), stopband attenuation is given by window and number of coefficients(see FDAtool).