convolution using dft
Linear convolution of 2 arbitrary non-finite discrete-time sequences in Matlab is usually done using the function conv(a,b), where a,b - operands of convolution. It's not difficult to notice, that this computation is absolutely equivalent to polynomial multiplication (the coefficients of these polynomials are places in the row vectors a and b).
DFT is not exactly the linear convolution, it's the circular convolution. The main differences between these 2 types of convolution is the size of the final vector. Provided you evaluate the circular one it's equal to the size of the vectors, whereas concerning the linear one it's equal to length(a)+length(b)-1.
With respect,
Dmitrij