[SOLVED] What is the raised cosine filter and what is is used for?

[syeda amna]

What is the raised cosine filterr?

hi
i have a question about raised cosine filters.
what are raised cosine filters?
what are the use of raised cosine filter or any speciality?
during modulation if we increase the roll of rate and group delay what will happen?
thanls

 

[Milad-D]

they are used to reduce the isi and also smoothing the sharp edges of digital baseband signals before modulation to reduce out of band frequency content of them.

 

[laughlatest]

Re: What is the raised cosine filterr?

hi
i have a question about raised cosine filters.
what are raised cosine filters?
what are the use of raised cosine filter or any speciality?
during modulation if we increase the roll of rate and group delay what will happen?
thanls

They are used to transforming baseband digital signal into pulse waveform
which is more appropriate for transmission through the channel.
Therefore, they are also called as (the most important type of ) pulse-shaping filter. And sometimes they are called (one type of) Nyquist filter, because they meet the 1st Nyquist criterion (for Zero-ISI).
The pulse waveform is the impulse response of the filters.
The most important criteria for pulse waveform selection(hence pulse-shaping filter) are:
(1) Zero ISI; (2) Insensitivity to sampling timing error;
(3) Implementation complexity of the filter;
In practical, the raised cosine filters is divided into two parts, each of which
is called Square Root Raised Cosine filters, and implemented in Tx side and Rx side respectively.
For SRRC, roll-off factor(alpha) decides the relations of the signal bandwidth and the channel bandwidth: SBW*(1+alpha) = ChBW. Therefore, the greater the roll-off factor, the greater channel bandwidth is needed for the signal
transmission. The merit are: (1) Insensitivity to, for example, sampling timing error; (2) Filter implementation complexity.
With regard to the group-delay,
usually, SRRC is implemented as FIR filter, so the group delay is decided by the number of taps. So, the greater the group delay, you will get better filter characteristics, for example, smaller passband ripple, greater stopband attenuation. Of course, larger group delay will bring about larger end-to-end transmission delay of the link.