java - Removal of white noise from a signal, in frequency domain using low pass filter -

environment

hi, have 128 point array on plotting looks this

i want remove noise signal , make signal like

for plan convert signal in frequency domain , use low pass filter.

note - able convert raw signal in frequency domain gives me 2 arrays of length 128(one *frequency plot* , 1 *phase plot*). able recover original signal using these 2 arrays. (dft & idft).

problem

now want filter noise signal. using java programming language , this link/ this link provides library implementing iir filters. don't know cutoff frequency should use?

also if filter coefficients using above how implement filter?(please explain theoretically)

i want use butterworth lowpass filter.

so, linear filtering alone won't suffice; took signal picture, converted sequence of numbers, assigned arbitrary time scale, , plotted in time , frequency domain (plot 1-3):

then tried find filter @ least cancels of noisy peaks. notice how smoothed out edges of rectangular pulse in plot 4.

you might, however, apply threshold after filter , desired signal shape.